Applications of High Energy Radiations: Synthesis and Processing of Polymeric Materials 9811990476, 9789811990472

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Materials Horizons: From Nature to Nanomaterials

Subhendu Ray Chowdhury   Editor

Applications of High Energy Radiations Synthesis and Processing of Polymeric Materials

Materials Horizons: From Nature to Nanomaterials Series Editor Vijay Kumar Thakur, School of Aerospace, Transport and Manufacturing, Cranfield University, Cranfield, UK

Materials are an indispensable part of human civilization since the inception of life on earth. With the passage of time, innumerable new materials have been explored as well as developed and the search for new innovative materials continues briskly. Keeping in mind the immense perspectives of various classes of materials, this series aims at providing a comprehensive collection of works across the breadth of materials research at cutting-edge interface of materials science with physics, chemistry, biology and engineering. This series covers a galaxy of materials ranging from natural materials to nanomaterials. Some of the topics include but not limited to: biological materials, biomimetic materials, ceramics, composites, coatings, functional materials, glasses, inorganic materials, inorganic-organic hybrids, metals, membranes, magnetic materials, manufacturing of materials, nanomaterials, organic materials and pigments to name a few. The series provides most timely and comprehensive information on advanced synthesis, processing, characterization, manufacturing and applications in a broad range of interdisciplinary fields in science, engineering and technology. This series accepts both authored and edited works, including textbooks, monographs, reference works, and professional books. The books in this series will provide a deep insight into the state-of-art of Materials Horizons and serve students, academic, government and industrial scientists involved in all aspects of materials research. Review Process The proposal for each volume is reviewed by the following: 1. Responsible (in-house) editor 2. One external subject expert 3. One of the editorial board members. The chapters in each volume are individually reviewed single blind by expert reviewers and the volume editor.

Subhendu Ray Chowdhury Editor

Applications of High Energy Radiations Synthesis and Processing of Polymeric Materials

Editor Subhendu Ray Chowdhury Bhabha Atomic Research Centre Mumbai, India

ISSN 2524-5384 ISSN 2524-5392 (electronic) Materials Horizons: From Nature to Nanomaterials ISBN 978-981-19-9047-2 ISBN 978-981-19-9048-9 (eBook) https://doi.org/10.1007/978-981-19-9048-9 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

This book is dedicated to, Dr. Pradeep Kumar Pujari, Former Director of Radio Chemistry and Isotope Group (RC & IG), Bhabha Atomic Research Centre, Mumbai, India, whose style of bringing out the best in others with human touch has always inspired many including me. This book is one outcome of this.

Foreword

This volume looks at the polymer synthesis and modification by high energy radiations. This is a very useful field in materials science and engineering nowadays. For decades, researchers from different parts of the world have been reporting new inventions and applications in this area. Development of many desired polymeric materials, starting from single to multicomponent polymeric systems (blends, composites, and nanocomposites), and desired improvements of polymer properties are being reported. But there are very few comprehensive collections, where the learners can gather wide range of information on the above discussed topic. The chapters in the book Applications of High Energy Radiations: Synthesis and Processing of Polymeric Materials from experienced researchers from various parts of the world shall represent data addressing relevant developments, challenges and applications in this field, which shall promote and guide further potential research for the societal benefit. I appreciate the implementation of expertises of editors, which generates the idea of compiling such topics belonging to interdisciplinary and multidisciplinary fields. Basic interactions of high energy radiation with polymers are very essential and have been brought out very nicely in some chapters. Uniquely, chapters are devoted to the development of materials for potential applications, namely few, biomedical, shape memory, aviation, space and nuclear applications, textile materials, tyres, construction materials, etc. Due to combination of properties by simple means, i.e. polymer blending, polymer composites, polymer nanocomposites preparations have become very popular for last couple of decades. Dispersion and compatibilityoriented several challenges are overcome in these multicomponent systems by high energy radiation-assisted crosslinking, grafting, curing and polymerization. Achievements of nanoscale dispersion of filler in polymer matrix and strong interface generation in nanocomposites by high energy radiation are also discussed here. Moreover, like radiation-assisted bond formation leading to crosslinking, grafting, curing, and polymerization, radiation-assisted bond breaking also has been utilized nicely in this field for recycling non-biodegradable polymers. Some chapters are written on this, which makes this volume a complete hand book for high energy radiation and polymeric materials.

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This book will be useful to students, researchers and professionals associated with polymer science and technology and radiation technology. In my judgement, the editors and authors of this volume have performed a commendable task. Bengaluru, India June 2022

Prof. C. N. R. Rao F.R.S Linus Pauling Research Professor and Honorary President, Jawaharlal Nehru Centre for Advanced Scientific Research

Preface

Polymers, due to having broad spectrum of properties, play essential and ubiquitous roles in everyday life. Rapidly, polymeric materials are replacing non-polymeric materials such as metals, glasses and composites. Therefore, demands of various kinds of polymeric materials with wide range of properties are increasing with time. To meet this demand, various means of synthesizing and modifying polymers are being implemented. High energy radiation-assisted route in this list is an attractive means for sustainable development because of various reasons, such as it is a room temperature process (requiring less energy), devoid of residue, high throughput, economic and environment friendly. There are plenty of literatures on this topic from various parts of the world. Many principles on radiation-assisted polymer synthesis and modification have also been established. These shall be helpful for students, researchers and industry people, if these are organized systematically. Therefore, it is our effort to bring the experiences of scientists and academicians in a common domain, which will discuss fundamentals of high energy radiations, processing and synthesis of polymeric materials by high energy radiation and then applications and challenges. Basically, high energy radiation can form covalent bonds in polymers through free radical generations as well as it can break covalent bonds at higher dose. This first one leads to grafting, curing, polymerization and crosslinking, and the second one, i.e. breaking covalent bonds, has been utilized for recycling non-biodegradable polymers by an environment-friendly way. Among the high energy radiation, gamma and electron beam radiations have been used successfully in various polymer industries. This book contains wide variety of chapters covering various kinds of methods of synthesis and processing of polymeric materials targeting various applications. Some chapters are dedicated on radiation-assisted development of various kinds of polymeric materials for specific applications such as tyre, mechanical joints, shape memory materials, materials for biomedical applications, space and aviation, nuclear

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Preface

applications, rubber latex and textile. The list includes Chaps. 2 to 7, 9 and 14. On the other hand, Chaps. 11 and 12 give an overview on radiation-assisted recycling of polymeric materials. Some other chapters summarize the development of polymeric multicomponent systems (blends, composites and nanocomposites). In this way, this book covers from fundamentals of high energy radiation to successful synthesis and modification of wide varieties of polymeric materials and their applications. We would like to thank all authors for contributing timely and high-quality chapters to make this project successful. We have enthusiastically pursued this project to present meaningful and exciting opportunities for beginners and experienced researchers in this filed. Hope this effort shall be useful. Mumbai, India

Subhendu Ray Chowdhury

Contents

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Application of Radiation Curing on Properties and Performance of Polymers and Polymer Composites . . . . . . . . . . . Suman Kumar Ghosh and Narayan Chandra Das Electron Beam Radiation Technology Application in the Tyre Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pratip Sankar Banerjee, Jagannath Chanda, Prasenjit Ghosh, Rabindra Mukhopadhyay, Amit Das, and Shib Shankar Banerjee Electron Beam Irradiation-Induced Compatibilization of Poly (Lactic Acid)-Based Blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ashish Kumar and Venkatappa Rao Tumu

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Radiation Curing of Fiber Reinforced Polymer Composite Based Mechanical Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Mohit Kumar, J. S. Saini, and H. Bhunia

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Thermally Stimulated Shape Memory Character of Radiation Crosslinked Polyolefinic Blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 Tuhin Chatterjee and Kinsuk Naskar

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Radiation Processed Emerging Materials for Biomedical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 Bhuwanesh Kumar Sharma, Manjeet Singh, Snehal Lokhandwala, Shrikant Wagh, Subhendu Ray Chowdhury, and Sudip Ray

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Effect of High-Energy Radiations on High Temperature-Resistant Thermoplastic Polymeric Composite for Aviation, Space, and Nuclear Applications . . . . . . . . . 219 K. Sudheendra, Jennifer Vinodhini, Mohan Kumar Pitchan, G. Ajeesh Mannadiar, and Shantanu Bhowmik

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Recent Developments of the Radiation Processed Hybrid Organic–Inorganic Polymer Nanocomposites: Expected and Unexpected Achievements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 Shalmali Hui and Santanu Chattopadhyay

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Radiation Processing of Natural Rubber Latex . . . . . . . . . . . . . . . . . . . 279 Neethu Varghese, Siby Varghese, and Sabu Thomas

10 Development of Multi-component Polymeric Systems by High Energy Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 Bhuwanesh Kumar Sharma, Atanu Jha, Rohini Agarwal, Subhendu Ray Chowdhury, and Suprakas Sinha Ray 11 Polymer Recycling by Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347 Kingshuk Dutta and Jaydevsinh M. Gohil 12 Radiation-Induced Degradation of Polymers: An Aspect Less Exploited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373 C. V. Chaudhari, K. A. Dubey, and Y. K. Bhardwaj 13 Electron Beam Radiation-Assisted Preparation and Modification of Thermoplastic Elastomer Blends . . . . . . . . . . . . . 409 K. Rajkumar, M. S. Banerji, P. K. Das, and Santosh Jagadale 14 Recent Advances in Electron Beam Processing of Textile Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 457 Amol G. Thite, Kumar Krishnanand, and Prasanta K. Panda

About the Editor

Dr. Subhendu Ray Chowdhury Ph.D. is currently a ‘Senior Scientist’ in Bhabha Atomic Research Centre (BARC), Mumbai, India. He is also a visiting faculty of Institute of Chemical Technology (ICT) [Formerly UDCT], Matunga, Mumbai, India. Dr. Ray Chowdhury obtained his Ph.D. in Materials Science from Indian Institute of Technology (IIT), Kharagpur, India. After completion of his Ph.D., Dr. Ray Chowdhury worked for seven years in various world renowned laboratories in USA, including the Department of Materials Science and Engineering, Cornell University and Department of Materials Science and Engineering, Pennsylvania State University. He has done pioneering work in the field of interface and surface engineering of multicomponent polymeric systems, application of radiation technology in materials science, sustainable natural polymer and synthetic polymer based superabsorbent for environmental remedy and water repellent breathable fabric. He has published more than 70 papers in International journals, couple of book chapters and more than 50 conference papers in radiation-assisted materials development through processing, modification and synthesis. He has supervised many Ph.D. and postgraduate students. He obtained couple of patents and transferred several technologies to Industries. His accolades and achievements include National Award for Technology Innovation 2019 by Ministry of Chemicals and Fertlizers, Government of India, CSIR research fellowship through NET (National Eligibility Test), etc. He has been involved in RCA/UNDP project on “Electron Beam Applications in Materials Science” in 2013 xiii

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as an ‘International Expert Committee member’ from India. He also served as a ‘Category A, International Faculty’ on Materials Science and Electron beam for environmental remedy (RCA/UNDP PROJECT) in July 2017, Republic of Korea (South Korea). He is a regular reviewer of more than 25 reputed international journals. As an experienced researcher he uses to deliver talks as key note speaker, invited speaker and chairs many sessions in international and national conferences.

Chapter 1

Application of Radiation Curing on Properties and Performance of Polymers and Polymer Composites Suman Kumar Ghosh and Narayan Chandra Das

1 Introduction With the progress of industrialization, pollution is a major concern for mankind. Radiation technology plays an important role to make a pollution-free world. The last few decades witnessed the extensive utilization of nuclear radiation as a strong energy source in various applications. Radiation technology gives entry into the polymer field through several chemical reactions such as polymerization, crosslinking and grafting. These all-important processes can be preceded by radiation technology [1–3]. The radiation processes dominate over conventional processes because of many advantages such as no need for catalysts or additives to start the chemical reaction, also the quality of the product can be controlled, energy-saving, clean process and saving of human assets. At low radiation dose chains, breakage of polymer molecules takes place. Also, at high radiation doses, free radicals are formed in the polymers which can form crosslinking of polymers [4–6]. Another advantage of radiation curing over conventional curing systems is that radiation curing takes place at ambient temperature, so the release of toxic material reduces. Therefore, radiation curing process is known as an environmental-friendly technique [7]. When exposed to high-energy radiation, the polymer molecules absorb energy and become electronically excited or ionized. Now, the excited molecules can initiate chemical reactions producing reactive products causing crosslinking and degradation reactions. Generally, irradiation produces low-energy photons and electrons which are responsible for the structural modification of polymers [8]. Radiation-initiated reactions in polymer and polymer composites can be categorized into three types: chain scission, grafting and crosslinking or curing. Crosslinking occurs through intermolecular bond formation in the polymer chains. Radiation curing does not require the presence of unsaturation or any reactive group in the polymer macromolecules. But the degree of crosslinking S. K. Ghosh · N. C. Das (B) Rubber Technology Centre, Indian Institute of Technology, Kharagpur 721302, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. R. Chowdhury (ed.), Applications of High Energy Radiations, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-9048-9_1

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changes with radiation dose proportionally. Curing mechanism changes with the types of polymer material [9]. The universally accepted mechanism involves the breakage of a C-H bond to form an active radical on the polymer chain leaving a hydrogen atom, subsequently distracting another hydrogen atom from another polymer molecule to form an H2 molecule. Finally, these two radicals unite to form a C–C bond crosslink. As a result of crosslinking, molecular mass increases with radiation dose which leads to the formation of a three-dimensional polymer network [10, 11]. Radiation curing by electron beam, gamma rays, beta-rays, X-rays and ionizing radiation is extensively utilized not only in the plastic field but also in the case of rubber and rubber composites. Radiation curing involves modifications and improvements in the physical, optical, mechanical, electrical, chemical and structural properties of the polymer composites. Thermal stability also improves on radiation curing [12]. In the case of crystalline polymers, the degree of crystallinity changes with radiation dose. Some studies showed that crystallinity increases along with improvement in physical, chemical and electrical properties with radiation dose. UV radiation curing is extensively used in the packaging industry and industrial coatings (flooring, outdoor building parts) [13]. The fiber-reinforced polymer composites are often cured by electron beams leading to the formation of several aerospace, water vessels and ground vehicle components. Compared to conventional heat curing, EB/X-ray curing possesses reduced or lesser crosslinking time. The radiation curing process changes the molecular structure of the polymers and improves the performance properties of the polymers and polymer composites. Curing or crosslinking promotes molecular weight enhancement that generally enhances properties improvement [14]. Application of gamma radiation curing on polymer or polymer composite improves mechanical stability, hardness, storage modulus, etc., and decreases elongation at break, elasticity and solubility. The radiation curing of polymers is used in several fields such as biomedical, food packaging, coating, textile, aerospace, electrical and nuclear plant [15–17].

2 Types of Radiation Curing Used in Polymer Systems Radiation curing involves the bond breakage of corresponding radiation-sensitive polymer molecules by the application of high-energy radiation such as ultraviolet, Xray, gamma ray and electron beam. Radiation curing is initiated by ionic or free radical intermediates which are formed during the decomposition of radiation-sensitive polymers on irradiation. Compared to other conventional curing methods, radiation curing possesses some superiority over thermally induced curing, including resin stability improvement, energy efficiency, fast curing speed, handling flexibility, etc. Electron beam curing involves accelerated electrons which supply initiation energy of the curing process by the breaking-up of the radiation-sensitive polymer. Gamma rays and X-rays curing of advanced polymer composites were investigated since the 1980s and have been used especially for thick composites (up to 300 mm). Compared to

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Fig. 1 Electromagnetic (EM) wave spectrum

electron beam curing dose rate of X-ray and gamma-ray curing are low resulting in a longer curing time [18]. UV curing is extensively utilized for the curing of polymer coating and thin films as this curing method is much more cost-effective, energysaving and environment friendly. But due to absorption behavior passing through matter, UV curing is limited to glass fiber-reinforced transparent composites with limitations of thickness and open mold processes [19]. Figure 1 shows types of radiation in the EM wave spectrum.

3 Radiation-Induced Polymerizations Ionizing radiation of different types is extensively used to initiate polymerization reactions. Electron beam curing is the most advantageous and established process to induce polymerization reactions as it is pollution-free and requires low energy. The radiation-induced polymerization is rapidly used for different applications such as coating, paints, adhesives and printing. The electron beam is often used for crosslinking of the vinyl monomers which are utilized for coating applications. This crosslinking involves the formation of free radicals by bond scission in the monomer units. Here, irradiation leads to a free radical mechanism that can be initiated by solvated electrons. Electron beam irradiation can directly facilitate the cationic polymerization of several monomers. Several reports on this cationic polymerization by E-beam radiation are available in the literature [20–22]. Cationic polymerization induced by irradiation is often used for coating applications. In a typical work, Crivello et al. employed E-beam radiation to polymerize bisphenol A diglycidyl

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ether monomers at an irradiation dose of 100–300 Mrad. Photo-induced polymerization was used earlier, but this process possesses some disadvantages such as the insufficiency of suitable photo initiators. This issue was solved by using alternating polymer methods like irradiation. UV curing is now commonly used for cationic polymerization of epoxide monomers. Not only in coating application, but UV-induced polymerization is also currently applied in stereolithography. Both UV and electron beam curing for polymerization methods have some advantages like low energy, high speed, low environmental impact, etc. The e-beam-induced cationic polymerization method is also widely used to crosslink the epoxy resins for the curing of fiberreinforced polymer composites. The resultant composite materials exhibit very good mechanical properties and chemical stability owing to the high penetration of E-beam in thick and opaque epoxy resins. Also, good processability and high performance of these composites can be achieved. E-beam-induced cationic polymerization of 1-propenyl and vinyl ethers is a very efficient method for the curing process in the field of adhesives and printing inks. The onium salt-based photo initiators are often used for promoting radiation-induced cationic polymerization of several monomers such as vinyl ether and epoxides. Besides free radical polymerization process of acrylate and methacrylate monomers is also induced by radiation. The polymerization induced by irradiation takes place at high reaction rates. Electron beam-initiated polymerization techniques become a very attractive alternative to produce highperformance structural composites. E-beam-induced curing polymerization is often used for acrylate monomers. In some cases, the temperature has a strong influence on reaction efficiency [23]. Defoort et al. elaborated a model to establish a relation between processing conditions such as time, temperature, dose rate, and sample volume during polymerization [8]. They studied the kinetics of E-beam-induced free radical polymerization, and the corresponding polymerization profile is modeled to fit experimental parameters as shown in Fig. 2. Gamma irradiation is also used to polymerize epoxide groups containing few monomers and oligomers [24]. Rp =[M]0 (1 − π ){exp [C1 (T − Tref )/(C2 + (T − Tref ))][AD0.5 exp (−Ea /RT)] + (1 − exp [C1 (T − Tref )/(C2 + (T − Tref )])BD}

(1)

where Rp , is the instant polymerization rate at time t and temperature T, p is the fractional conversion of the monomer at instant t, C1, C2 are the constants of a Williams Landel Ferry (WLF)-like model depicting the behavior of the system. Tref is a conversion-dependent variable. A typical electron beam-induced polymerization of acrylate monomer and corresponding network formation with carbon fiber to produce structural composite is schematically represented in Fig. 3.

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Fig. 2 Experimental (symbols) and modeled (continuous line) conversion profiles for the EBinitiated polymerization of an EPAC-based resin (fit by means of Eq. (1) with A = 3, B = 0.0032, C1 = 6.0, C2 = (0.88 D1) + 96, E1 = 4.3 kJmol−1 ) [8]

Fig. 3 Surface modification and free radical polymerization of acrylate base polymer to produce a high-performance composite with fibers and fillers [8]

4 Effect of Radiation Curing on Properties and Performance of Polymer and Polymer Composites 4.1 Gamma Irradiation of Polymer and Their Composites In the last few years, treatment and curing of polymers by gamma irradiation has been increased due to modifications and improvements in their performance and different properties such as structural, optical, electrical, physical and chemical. Compared to another radiation curing, the gamma radiation curing method is extensively used for polymer and polymer composites. This curing system is also suitable for thick material (thickness up to 300 mm). Gamma irradiation produces electron and lowenergy photons which are responsible for the curing process [25]. This curing system is extensively utilized not only for thermoplastics but also for rubber, thermoplastics, elastomers and their composites. Upon curing, different properties and performance of the polymer and their composites improve. This improvement strongly depends

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on the radiation dose used. Several properties of composites change with the dose of gamma radiation [26].

4.1.1

Plastics and Their Composites

Gamma radiation is extensively used in the plastic field to cure plastics and their composites. The resultant polymeric system exhibits improved mechanical stability, storage modulus and hardness, but elongation at break, solubility and elasticity of the material decreases. This change in performance and properties depends on the curing rate and radiation dose used. Polyethylene is one of the popular plastics used extensively. Sometimes polyethylene is cured by gamma radiation to get proper properties requirements depending upon its applications. Krupa et al. investigated the thermo-mechanical properties of linear low-density polyethylene (LLDPE) cured with gamma radiation. They also studied the effect of the dose of gamma irradiation on these properties and the performance of the thermoplastic composites. LLDPE was cured with different doses of gamma radiation such as 50, 150 and 250 kGy. The melting point, crystallization temperature and enthalpy of melting were studied from the thermal analysis. Results obtained for gel content show that upon gamma irradiation the gel content of the LLDPE composite increases. LLDPE with an irradiation dose of 50 kGy exhibits a gel content of 28 which enhances to 55 for 250 kGy dose of gamma irradiation. From the DSC measurements, it can be observed that melting enthalpy increases with irradiation dose. But the onset melting temperature and melting point decrease with a rise in irradiation dose. The degree of crystallinity reduces as crosslinks are not included in crystallites and some defect arises [27]. The TGA results revealed that after gamma irradiation thermal stability is improved. Gamma irradiation leads to the formation of more dense and compact three-dimensional structure in the LLDPE matrix which finally improves thermal stability. Both the degradation temperatures owing to 5 and 10% weight loss and temperature of the inflection point of the LLDPE polymer increase with an increase in the dose of radiation curing. Mechanical properties have strong influence on radiation dose. With an increase in radiation dose, Young’s modulus increases but yield stress does not change significantly with radiation crosslinking. However, other mechanical properties like stress at break, elongation at break and elongation at yield strongly rely on gamma irradiation dose. Stress at break and elongation decreases with radiation dose. When the irradiation dose is lower (50 kGy), this reduction is small but for the higher dose of irradiation, this reduction is more prominent. SEM images shown in Fig. 4 demonstrate the rupture mechanism in the sample [27]. As radiation crosslinking proceeds, the rupture in the LLDPE surface is observed along with different lines. During irradiation, the brittleness of LLDPE increases with an increase in the degree of crosslinking. For LLDPE cured with 150 and 250 kGy dose of irradiation, some crazes are formed which are very clear in Fig. 5 and these crazes are formed perpendicular to the direction of tensile stress applied. But the surface of LLDPE crosslinked with 50 kGy gamma irradiation does not exhibit any crazes.

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Fig. 4 SEM images of a non-irradiated LLDPE and LLDPE irradiated with radiation doses of b 50 kGy, c 150 kGy and d 250 kGy [27]

Gamma radiation technology is not only applicable in a single plastic system, but also it is extensively applied in plastic/plastic blend systems and their composites. Zaman et al. examined the effect of gamma radiation on the performance properties of polyethylene (PE)/polypropylene (PP) blend and corresponding jute fiber-reinforced composites. The best results are obtained for 80/20 PP:PE blend-based jute fabricfilled composites. Gamma radiation of 250–1000 krad was applied to the blend and composite systems at a rate of 300 krad/h. The tensile strength (TS) and bending strength (BS) show an increasing trend up to 500 krad and then decrease up to 1000 krad. Maximum TS and BS obtained for the composite are 76 and 79 MPa, respectively. The tensile strength and bending strength improved to 22 and 23% higher compared to the net polymer matrix. The water take-up by the composites against soaking time is higher for untreated composites than gamma radiation-treated composites. The electrical properties of the irradiated composites were also measured and compared with those of untreated composites. The temperature dependency of various electrical properties such as electrical conductivity and loss tangent dielectric constant of the resultant composites were found. The non-irradiated composites exhibit higher transition temperatures compared to irradiated composites. Dielectric constant and loss tangent were found to increase with irradiation, whereas electrical

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Fig. 5 SEM micrographs of a non-irradiated LLDPE and cured LLDPE with an irradiation dose of b 50 kGy, c 150 kGy and d 250 kGy (higher enlargement) [27]

conductivity of the composites decreases with radiation. They concluded that gamma radiation technology could be an effective strong source to improve the dielectric and mechanical properties of jute fabric-reinforced polymer composites [28]. Gamma radiation curing is also often used in resin systems. In a typical work, influence of processing temperature on thermal properties and dynamic mechanical properties of an epoxy resin during gamma irradiation was investigated. Onium salt was used as the cationic initiator. The epoxy system was cured at 150 kGy at different processing temperatures which were maintained constant during the application of gamma radiation. The calorimetric analysis was performed to determine the extent of irradiation-induced curing of epoxy composites. The results show that the calorimetric curves as shown in Fig. 6 exhibit two exothermic peaks owing to the thermally activated reaction during radiation curing. Increment in irradiation temperature leads to a reduction in the first peak area, but the area of the second peak remains the same. Also, the results show that all the resin systems undergo increasing crosslinking density exhibiting latent reactivity [29]. DMTA results show the generation of a heterogeneous structure with different crosslinking densities confirmed by the presence of several relaxation peaks in tan δ versus temperature curves. The processing temperature affects both the crosslinking

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Fig. 6 a Thermal analysis for epoxy systems irradiated at 150 kGy and different constant temperatures, b DMTA analysis for epoxy systems irradiated at 150 kGy [29]

density and sensitivity. As processing temperature increases, the tan delta value decreases (Fig. 7) and the relaxation process occurs at a higher temperature. Irradiation at lower temperatures leads to an enhancement in the modulus in between the relaxation temperature. This study suggests that to complete the curing process of epoxy resin, a post-thermal treatment by irradiation is very important.

4.1.2

Rubbers and Their Composites

Gamma irradiation of rubbers and their compounds are extensively used in various fields such as nuclear plants, wires and cable industry, textile and coating. EPDM rubber is one of the most widely used rubbers in wires and electrical cable coating. Gamma irradiation is often used for EPDM compounds used in the nuclear plant. In a typical work, prepared EPDM compounds were irradiated with different gamma

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Fig. 7 DMTA analysis for epoxy resin irradiated at 150 kGy at several processing temperatures and thermally post-cured at 175 °C for 2 h [29]

radiation doses. Swelling behavior and mechanical properties were measured and compared for both irradiated and non-irradiated samples [30]. Results indicated that tensile at break values reduced proportionally to the dose applied. This reduction is more prominent for radiation doses more than 100 kGy. This gives an indication of chain scission with further polymer degradation. Results showed that irradiation leads to crosslinking of polymer chains up to 25 kGy dose of gamma radiation. Further, the materials become worsen due to degradation. Different curing systems such as sulfur, peroxide and radiation are extensively used for rubber compounds. Different curing system yields different properties of the resultant compounds. Radiation curing especially gamma radiation curing of rubbers leads to some advantageous properties. Several works have been done on a comparison study of the effect of different curing systems on the performance and properties of rubber and their composites. In typical research work, a comparative study of calorimetric properties of nitrile butyl rubber (NBR) and styrene butadiene rubber (SBR) vulcanizates cured by different curing systems such as sulfur, peroxide and gamma radiation has been done. The thermal properties were performed by TGA analysis and evaluated based on maxima peak in DTG curve and temperature corresponding to 50% of weight loss. The effects of the various coagent and other additives on the thermal stabilities of rubber vulcanizates were also investigated, and the results are depicted in Fig. 8. The thermal stability of various radiation-cured NBR vulcanizates with SBR vulcanizates was evaluated and compared for both cases. The compressed sheets were irradiated with dose of 100 and 150 kGy and dose rate of 11 kGy/h in air. Carbon black and silica were used as fillers in rubber vulcanizates [31]. In comparison with peroxide and sulfur-cured vulcanizates, gamma radiationcured SBR and NBR vulcanizates show better thermal stability. Among all the investigated formulations, NBR formulations with 45 phr filler and cured at 100 kGy gamma radiation exhibit the highest thermal stability. Several studies have been done on

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Fig. 8 Thermal properties of gamma radiation-cured SBR vulcanizates as a function of coagent (type SR-633) concentration filled with a 45 phr silica, b 45 phr HAF carbon black and cured at 150 kGy [31]

rubber filler interaction during irradiation. In such work, the effect of gamma irradiation on silica filler and silica-silicone rubber interface was studied. Characterization techniques such as ESR and NMR spectroscopy were employed to study these effects. ESR study confirmed the existence of paramagnetic species in the silica matrix for all samples exposed to gamma irradiation. Results from NMR and the toluene-swell study showed that polymer-filler interaction decreases at a low gamma dose. At a high gamma dose, NMR shows the presence of segmental chain dynamics through polymer–polymer crosslinks. Through crosslinking reaction silica-polymer interfacial interaction increases confirmed from ammonia modified swell study. Finally, this can be concluded that the surface of silica is sensitive to gamma radiation suggested by both proton and silicon NMR studies [32]. Gamma radiation technology is also used in rubber/rubber blend systems to improve their performance and properties to be applicable in various fields. This radiation has a strong influence on the properties of the rubber blend systems. During irradiation, several organic molecules such as carboxylic acid, ethers and lactones are formed. Gamma radiation also influences the aging behavior of rubber blend compounds cured by sulfur or peroxide. Several works have been done on the effect of gamma radiation on various properties of rubber blend vulcanizates with different filler loading. Milena et al. investigated the influence of gamma radiation curing on the aging behavior of sulfur-cured NBR/CSM rubber blend and NR/CSM rubber blend systems filler with carbon black of variable loading. The resultant elastomeric composites were subjected to various radiation doses (100–400 kGy) in the presence of oxygen. After radiation accelerated aging mechanical properties of the vulcanizates were evaluated. During irradiation, various oxidation products such as acids, alcohols, esters, ethers, lactones and anhydrides are formed in materials. Production of these molecules was confirmed from FTIR spectra [33].

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S. K. Ghosh and N. C. Das

Fig. 9 Effect of the radiation dose on the modulus of a NR/CSM blend and b NBR/CSM blend reinforced with different content of carbon black [33]

Gamma irradiation has a strong influence on carbon black-filled rubber composites. With the increase in black loading as well as radiation dose modulus at 100% elongation, tensile strength and hardness increase but elongation at break is found to decrease with carbon black loading and radiation dose. At a lower radiation dose (less than 100 kGy), only there are small changes in the rubber macromolecules’ structure. But more than 100 kGy doses of gamma irradiation lead to chemical modification of the corresponding rubber blend samples. Figures 9 and 10 show the resultant mechanical properties of rubber compounds after irradiation. Figure 11 shows SEM images of fractured surfaces of corresponding rubber blend systems filled with a fixed amount of carbon black (60 phr). It can be seen that some small micro-craters are formed during irradiation in NBR/CSM blend. Due to the poor rubber/filler interaction, these micro-craters are detached from the fractured surface. In the case of NR/CSM blend, the relatively good rubber/filler interaction provides some resistance to fracture. The final properties of the blends are strongly influenced by the filler distribution in the polymer matrix. At a lower irradiation dose (100 kGy), the structure of rubber macromolecules changes slightly, but a higher dose of gamma irradiation (300 and 400 kGy) leads to serious chemical modification of the rubber blends.

4.1.3

Plastic/Rubber Blends (Thermoplastic Elastomers)

Before practical applications, polymers should have enhanced performance properties. Rubbers having poor mechanical properties should be vulcanized before use to get improved performance and properties such as mechanical and thermal. For the last three to four decades, blending of polymers has gained much attention because blending a new polymeric composite material with specific properties is suitable for some certain application. Rubber/plastic blends are very common and useful polymeric materials. Radiation-induced curing is extensively used for rubber/plastic blends or thermoplastic elastomers to induce network formation in the material.

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Fig. 10 Variation of tensile strength of a NBR/CSM and b NR/CSM rubber blends. Hardness of c NBR/CSM and d NR/CSM rubber blends containing various amounts of carbon black as a function of irradiation dose [33]

Fig. 11 SEM images of irradiated NR/CSM, NBR/CSM rubber blends contained with carbon black (60 phr) [33]

These radiation especially gamma radiation-cured blends are extensively utilized in the cable, packaging industry, and automotive industries with improved performance properties [34]. NBR is well-known rubber extensively used in the automotive industry, and HDPE is a commodity plastic mostly utilized in the packaging

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S. K. Ghosh and N. C. Das

industry. Blends of these two polymers exhibit improved performance properties when exposed to irradiation. Elshereafy et al. prepared NBR/HDPE blend with a different blending ratio of HDPE content ranging from 20 to 80 wt%. These blends were exposed to gamma irradiation curing at various radiation doses. The effects of irradiation doses on various properties such as physico-mechanical and thermal properties were evaluated [35]. Results show that tensile strength improves with radiation dose and HDPE content. Improvement in mechanical properties is a function of irradiation dose. With the increase in radiation dose and HDPE content, the heat shrinkability of the blend increased. The thermal stability of rubber vulcanizates (100/60) NBR/HDPE blend system was improved when compared to that of raw NBR due to the formation of crosslinking on radiation curing. Figure 12 represents the effect of gamma irradiation on the mechanical properties of NBR/HDPE blends. The yielding behavior of irradiated rubber composites is considered as an important mechanical property. Gamma radiation has a strong influence on the yielding behavior of polymer composites. In a typical work, some researchers have investigated the yielding behavior of HAF carbon black or white precipitated SiO2 filled 60/40 wt% MDPE/EPDM blend cured by gamma rays. The yielding behavior of this blend was measured by estimating yield stress, yield strain and cold drawing percentage. Gamma irradiation with different radiation doses up to 200 kGy was

Fig. 12 Effect of irradiation dose on a tensile modulus at 50% elongation, b elongation at break and c hardness of NBR, HDPE and their blends [35]

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applied with and without different additives. Results show that yield stress increases with an increase in radiation dose, but both cold drawing percentage and yield strain decrease with radiation doses. The composite containing 10 wt% of SiO2 exhibited the maximum yield stress when exposed to 150 k Gyradiation dose [36]. Manas et al. investigated the thermo-mechanical properties of thermoplastic elastomers (TPE) exposed to gamma irradiation and obtained significant changes in both properties. After irradiation at 66 kGy tensile strength has been increased by 35%. With increase in radiation, dose elongation at break decreased. The Young modulus increased gradually as a function of gamma irradiation dose. There is an obvious enhancement in the thermal stability of the thermoplastic elastomer after gamma irradiation. The resistance to creep of the crosslinked thermoplastic elastomers (TPE-Es) increased as a function of gamma radiation dose [37].

4.2 Effect of Electron Beam Curing on Properties and Performance of Polymer Composites Electron beam curing of polymer composites extensively used in aerospace is an efficient alternative to conventional thermally induced processing of these structural materials. This radiation processing can influence the process parameters, and accordingly, mechanical and other performance properties of these structural composite materials can be obtained. The mechanical performance and residual stress during E-curing are also dependent on dose of irradiation used. Few researchers investigated the processability and performance of electron beam-cured polymer composites and compared with the thermal curing process. Carbon fiber-reinforced epoxy composites are extensively used in aerospace engineering. E-beam-induced processing is also used for these composites like thermal treatment [38]. In a typical work, Raghavan investigated the mechanical performance and residual stress of fiberreinforced composites after the electron beam curing process. Also, the evolution of irradiation and the dependence of properties obtained with irradiation dose and dose per pass were investigated in that work. The effect of curing on glass transition temperature during curing was also studied. The irradiation dose of 20, 40, 100, 160 and 260 kGy was applied for the curing of IM7 carbon fiber-reinforced epoxy composites. Simultaneously, these composites were also cured thermally at a temperature of 150 °C cure levels of up to 73% to compare the results with those for radiation curing. He also investigated the cure kinetics for both types of the curing process. The storage modulus of these composites is found to be increased with an increase in irradiation dose, and the corresponding DMA plots shift to a higher temperature. He also observed multiple peaks in both loss tangent and loss modulus curves. A glass transition temperature (T g ) of above 40 °C was obtained after the curing process. But due to this glass to rubber transition, reversible heat flow can be separated from non-reversible heat flow while performing MDSC measurements. The glass transition temperature of the composites obtained from MDSC is more

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S. K. Ghosh and N. C. Das

accurate than T g measured from DMA analysis. DMA was also used to measure T g of thermally induced composites, and the maximum peak position of Tan δ in the DMA plot corresponds to the glass transition temperature. When the cure level was increased beyond 70%, a quick increase in glass transition temperature is achieved. The T g of the post-cured composites after radiation curing is found to be in the range of 205–217 °C. Both moduli increase with the increase in cure levels. The modulus obtained is higher for thermally cured composites (SB and SC) than electron beam cures samples at certain cure levels, but for other levels, both moduli are found to be higher for E-beam irradiated SA composites. With an increase in cure dose, failure strain of composite decreases. Transverse strength of radiation-cured composites increases up to 24 MPa and then decreases with further increase in the degree of cure. The residual stress obtained is compressive, and the effect of cure shrinkage and experimental temperatures on residual strain was also analyzed. The curvature of composites increases with a cure dose of up to 50%; then, it decreases for SA and SC composites. Residual strain for radiation-cured composites is higher than thermally cured composites at a higher degree of cure. A maximum tensile residual stress of around 75 MPa is achieved for E-beam irradiated composite [38]. The curing process induced by irradiation is an exothermic reaction. This reaction exotherm is strongly dependent on the structure of the monomer, radiation dose, other parameters and composite formulation. The performance properties and crosslinking density of a wide range can be observed in the irradiated resins. Few research works proved that the physical properties of the composites can be improved after postthermal treatment after radiation curing. The electron beam curing process is extensively used for epoxy resins. Zhang et al. investigated the effects of heat treatment on crosslinking density and performance properties of electron beam-cured epoxy resin based on bisphenol A. Also, they studied how the curing parameters influence the properties of the epoxy composite. A 5 MeV electron beam energy and 100 Gy/s irradiation dose were used for the curing of resin samples. The heat treatment leads to the formation of more gel fraction of the radiation-cured samples, and the increment in gel fraction is found to be inversely proportional to the dose of irradiation applied. Glass transition temperature rises after the heating process, and this enhancement in T g is related to the increment in crosslinking density. As the temperature of the heat treatment process increases, glass transition temperature and storage modulus improve due to the enhancement of reaction capability and high crosslinking density of the resins. Post-heat treatment at 250 °C improves both these two parameters. Also, an α-relaxation peak is observed in the tan delta curve of the resin heated at this temperature. The glass transition temperature and storage modulus at the high temperature of the epoxy resin cured at a sufficient dose can be higher than those for conventional heat-cured materials. The dose of the initiator used can also influence the post-heat treatment. The gel fraction increases with the temperature of the heating process of the resin cured at a radiation dose of 50 kGy. Also, crosslinking density of the sample rises with the initiator dose. When the crosslinking density is continuously increasing after the radiation process, the alpha-relaxation peak is shifted to a lower temperature value when the initiator dose is increased. Post-heating process of resin after electron beam curing can also be influenced by the molecular weight of the resin

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materials [39]. Zhang et al. investigated the effect of the molecular weight of epoxy resin on the heat treatment process after the electron beam was cured with 50 kGy. Gel fraction changes with the molecular weight of the resin at a fixed irradiation dose and initiator dose. The increment in gel fraction for low molecular weight is owing to the higher mobility of active groups so that thermal curing reactions can take place more quickly. Resins with a high polydispersity index may exhibit higher gel fraction after heat treatment. The distribution of molecular weight can also influence the heat treatment process of the E-beam-cured epoxy resin. Different crosslinking densities of the resin with wide molecular weight distribution exhibit two glass transition peaks in the tan delta curve. The storage modulus and crosslinking density of electron beam-cured epoxy resins after the heat treatment process are found to be higher with higher polydispersity index. Higher energy and depth of penetration of electron beam lead to rapid curing of the polymer composites of relatively thick samples. The material parameters such as fiber length, radiation dose, initiator concentration and processing temperature strongly influence the final properties of the fiber-reinforced polymer composites after electron beam curing. In a typical work, thermoplastic elastomeric blend of LDPE/EPDM and LDPE/SBR was prepared and irradiated with an electron beam with different radiation dose. The effects of radiation curing on mechanical properties, crosslinking parameters and hot set properties were measured as a function of radiation dose. Electron beam-cured LDPE/EPDM blend exhibited the highest gel content percentage. In the case of LDPE/SBR blend, there was an improvement in tensile strength up to 150 kGy of radiation dose, but only slight improvement of this property was observed for LDPE/EPDM blend up to 100 kGy. Above this radiation dose, tensile strength decreased. Hot set properties improved for LDPE/EPDM blend [40]. In another work, Raghavan et al. investigated the influence of irradiation dose, processing temperature and initiator concentration on the results related to performance properties of the weave fabric-reinforced epoxy composites. The initial dose of the electron beam was 50 kGy for the curing process. The curing process involves the occurrence of primary alpha-reaction, and this is strongly dependent on radiation dose. Also, secondary beta-reaction shows a weak dependence on the radiation dose used, and this secondary reaction is strongly influenced by initiator concentration. When the dose was increased up to 100 kGy, the extent of the radiation curing reaction increases rapidly. As the radiation dose was further increased, the extent of the curing process approaches a plateau value which is corresponding to the incomplete cure for resin by around 30% and around 22% for the composite at a temperature of 25 °C. The limitation of diffusion and the secondary beta-reaction are the primary causes of the incomplete curing process. When the processing temperature has increased, the extent of the curing reaction increases at a certain dose of radiation. The DSC measurements confirmed that thermal curing has also happened during electron beam curing of epoxy resin and corresponding composites. When the dose was increased from 0 to 30 kGy, the onset temperature decreased from 150 to 50 °C. As a result of this post-thermal treatment, the modulus of the cured resin decreases by 10%, whereas the glass transition temperature of the electron beam-cured epoxy resin at a radiation dose of 200 kGy increases from 130 to 370 °C. In this work, Raghavan

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S. K. Ghosh and N. C. Das

et al. also compared the service temperature and the storage modulus of the electron beam-cured resin and 100% thermally cured resin. The mechanical properties are found to be better for electron beam-cured resin than thermally cured resin [41]. The low-energy electron beam is also used to cure and fabricate some composite parts. Using low-energy E-beam for composite fabrication may be an efficient method for curing to reduce the cost. While the electron beam is used for curing the polymer resin and polymer/fiber composites at low energy, in some cases penetration depth and uniformity of irradiation may not be achieved properly. In that case, both sides curing of the samples can eliminate these issues. Zhao et al. studied the curing characteristics of carbon fiber-reinforced epoxy composites cured by the electron beam of low energy (below 125 keV). In this work, diaryliodonium hexafluoroantimonate was used as a cationic initiator and an electron beam was applied at the 80–150 kV voltage. A radiation dose of 30–300 kGy was used for the curing process. DSC was employed to study the degree of curing reaction and glass transition temperature and compared for different radiation dose. It was found that for one side curing of composite the electron beam energy continuously decreases with an increase in depth of penetration and penetrates up to a thickness of 0.125 mm. For both side irradiation, the dose becomes higher in the center than the bottom of the superposition curve for electron beam irradiation of 150 keV. The superposition curve of irradiation dose shows a straight line for the irradiation sample from both sides. The curve clearly depicts that the same irradiation dose is obtained for both the center area and surface and bottom of the prepreg sample. Also, the results indicate that the irradiation occurs much uniformly. The experimental results for dose distribution with the thickness of the prepreg are in accordance with the results calculated from the superposition curve. From the DSC analysis, it was found that the degree of curing reaction increases rapidly with a radiation dose up to 90 kGy. When the dose is increased and higher than 150 kGy, the degree of curing process increases slowly and tends to a definite value. A maximum radiation dose of 300 tends to the degree of curing of around 65% and glass transition temperature of 50 °C. When the temperature was below 140 °C, no exothermic peak is found in the DSC curve for a low dose of irradiation. But with an increase in temperature, the iodonium salt gets decomposed by heating which initiates the curing reaction. That’s why an exothermic peak appears on the DSC curve which further disappears at 260 °C. The critical dose of irradiation is found to be 50 kGy to induce the chemical crosslinking reaction. Results indicate that 50 kGy radiation dose is the minimum curing dose to obtain a composite of high-quality surfaces while using low-energy electron beam curing process. The thermal effect on the electron beam curing process is very low for low-energy beams compared to a high-energy electron beam. The degree of curing reaction and glass transition temperature for epoxy composites becomes lower for low-energy electron beam curing process than the high-energy electron beam curing process owing to the lower temperature of the prepreg sample. Post-curing by the thermal process was applied to improve the glass transition temperature and the degree of curing reaction for the electron beam-cured samples. When a radiation dose of 50–250 kGy was applied for curing and subsequent post-thermal curing, the T g and curing degree improve significantly. The glass transition temperature increases up to 166.5 °C for

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19

50 kGy dose of irradiation and 170.4 °C when the dose is increased up to 250 kGy. So the results indicate that post-curing for short time after low-energy electron beam curing can significantly influence the degree of curing reaction and glass transition temperature for the composites. The low-energy electron beam of 125 keV with post-thermal curing of a short time can be a promising method for manufacturing polymer composites [42]. Electron beam irradiation is extensively used to fabricate various rubber and thermoplastic elastomeric products for electrical, cable, coating and packaging applications. In this case, the radiation dose can be controlled easily. The electron beam irradiation process reduces the scrap and waste formation and maintains the purity of the product. For rubber samples, this irradiation process can take place at ambient temperature without any thermal degradation. Also, it excludes the formation of unwanted hazardous by-products and no solvent is required. As a consequence of these advantages, electron beam irradiation is extensively used in rubber, rubber/rubber and rubber/plastic blend systems. Performance properties of these rubber materials improve significantly after the irradiation process than normal conventional curing. Dutta et al. investigated the effect of E-beam curing on physic-mechanical, thermal and performance properties of ethylene vinyl acetate (EVA)/thermoplastic polyurethane (TPU) blend with 80/20 and 70/30 compositions [43]. In this work, they used low-energy electron beam for the curing process at an irradiation dose between 25 and 200 kGy at ambient temperature. The blends were prepared in Haake internal mixer at 180 °C with a rotor speed of 70 rpm. They also studied the effect of radiation curing on gel content, crosslinking density and dynamic mechanical properties of the blends. Upon irradiation, they achieved a drastic improvement in the mechanical properties of the blend systems. Irradiation with only 25 kGy dose increases tensile strength by 50 and 25% for 70/30 and 80/20 EMA/TPU blends. Tensile strength of both blends increases sharply up to 100 kGy of electron beam, but beyond this radiation dose, T.S. increases slightly for 150 kGy and then decreases for 200 kGy radiation dose. This significant enhancement in the tensile strength of blends can be ascribed to the formation of crosslinking networks through the matrix induced by radiation. Also, during tensile stretching, chances of crack formation and crack propagation are reduced by the interfacial interactions of the electron beam with polymer. A slight enhancement in elongation at break of sample is found for low radiation dose (25 kGy). With the increase in radiation dose elongation at break decreases further for both two blends. This reduction in elongation at break value is owing to the formation of a three-dimensional network thus increasing crosslinking density which restricts the mobility of the polymer chains. Modulus at 100% strain and 100% elongation improves upon irradiation. When an irradiation dose of 200 kGy was applied, a significant improvement of around 66 and 68% in modulus is achieved for 70/30 and 80/20 blends. The tension set for both cases reduces upon the application of an electron beam to the blends. This improvement in tension set is owing to the enhancement of elastic recovery of the polymer chains by radiation-induced crosslinking with the rise in electron beam dose. As the radiation dose increases, the hardness of the blends increases continuously. Non-irradiated 80/20 EVA/TPU blend exhibits a hardness of 25 (Shore D) which increases to 31.6 for 200 kGy radiation

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S. K. Ghosh and N. C. Das

dose, whereas for the 70/30 blend system it improves from 25.5 to 32.4 (Shore D). The irradiation process leads to the formation of crosslinking network which contributes to the enhancement of the hardness value. Also, the electron beam-cured blends show good resistance to the local deformations which finally increases the hardness value. It was obtained from the results that a radiation dose of 100 kGy reports the best combination of mechanical and physical properties. To study the viscoelastic deformation in the blend system upon irradiation, they employed DMA (dynamic mechanical analysis) technique. Corresponding damping characteristics and stiffness were analyzed in a temperature range of −80 °C to +60 °C. Results show that storage modulus improves with rise in irradiation dose up to 150 kGy and increases from 1533 to 2060 MPa for a 70/30 blend. In this same condition, the modulus increases from 1650 to 2160 MPa for the 80/20 EVA/TPU blend. At a higher dose of electron beam radiation (200 kGy), storage modulus reduces slightly due to the partial oxidative degradation which leads to the rupture of the crosslinking network. Therefore, stiffness of the blends reduces to some extent. Tan delta curves result in the glass transition temperature of the blend components. The glass transition temperature of the blend shifts to a higher temperature value with an increase in radiation dose. But only very little change of around 4 °C is obtained as crosslinking networks restrict the molecular movement, and therefore, more energy is required for glass–rubber transition in the blend systems. Better compatibility of the blend components is strongly suggested by the reduction in broadening of tan delta peak for all blends. This fact also supports the morphology and improvement in the mechanical properties. The melting temperature of the blends is found to be decreased with rise in electron beam dose. Approximately 7 °C reductions in melting point are observed when radiation of 200 kGy was applied. They also studied the crystallization temperature of the EMA/TPU blend from DSC analysis. It is observed that the crystallization peak for EMA phase temperature decreases continuously as a function of radiation dose. The formation of crosslinking network inhibits the crystal growth, and hence, more undercooling is required to crystallize the EVA phase. The morphological analysis depicts the formation of a two-phase structure for both non-irradiated and electron beam-cured blends. The minor TPU phase can be etched out from the blend by THF solvent even after irradiation. This strongly indicates that the minor TPU phase could not able to form a sufficient crosslinking network. Morphology of the blends also reveals that some interfacial crosslinking pathway is also formed between EVA and TPU components. FTIR analysis was also used to investigate the structural changes that occurred after the electron beam irradiation process. Results indicate the formation of interfacial crosslinks along with chemical crosslinks in the blend systems. The thermal stability of the blends after the irradiation process improves significantly which is observed from the TGA (thermo-gravimetric analysis). Temperature corresponding to 50% weight loss, maximum degradation temperature and heat resistant index continuously shift to higher temperature values after electron beam curing. For a radiation dose of 100 kGy, the decomposition temperature corresponding to the 5% weight loss increases from 323 °C to 340 °C and 322 °C to 335 °C for 80/20 and 70/30 EVA/TPU blends, respectively. The construction of dense crosslinking networks through matrix gives the blends more thermal stability and reduced the

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chances of further weight loss. After 100 kGy of radiation dose, thermal stability reduces slightly further up to 200 kGy radiation due to the occurrence of some oxidative degradation reactions in the polymer chains. Also, high dose of radiation tends to rupture of macromolecular chains slightly, and accordingly, polymer chains become easier to degrade thermally. The gel content of the electron beam-cured EVA/TPU blend is found to be increased as a function of irradiation dose. The formation of densely crosslinked networks in the blend systems is also strongly suggested by the enhancement in gel content with radiation dose. The gel content increases from 55 to 84 and 54 to 83 as the radiation dose is increased from 25 to 150 kGy for both blends, respectively. Then, it increases marginally with further increase in electron beam dose. The 80/20n EVA/TPU blend exhibits higher gel content than the 70/30 blend at a particular radiation dose. The swelling tests result in the continuous improvement in crosslinking density of both two blends upon increasing the radiation dose. When high dose of the electron beam is applied to the blend systems, more free radicals are formed which finally lead to the formation of denser and higher crosslinking networks in the polymer matrix. As the 70/30 EVA/TPU blend has lower EVA content than the 80/20 blend; therefore, the latter blend exhibits higher crosslinking density than the former. When the rubber, plastic or their blends are crosslinked by an electron beam to be applied to the cable insulation, then electrical properties especially the surface resistivity of these materials must be considered important. These materials should hinder the leakage of electric current in this application field. Dutta et al. also investigated the volume resistivity of the EVA/TPU blends as a function of radiation dose. When the electron beam dose was increased up to 150 kGy, the volume resistivity of the blends increases with the rise in radiation dose for both 80/20 and 70/30 EVA/TPU blends. Above 150 kGy radiation dose, volume resistivity decreases further with the increase in irradiation dose. The volume resistivity enhances from 3.2 × 1014 to 17 × 1014 Ω cm and 2.3 × 1014 to 10 × 1014 Ω cm for 80/20 and 70/30 EVA/TPU blends, respectively. The formation of dense crosslinking networks acts as barrels to restrict the movement of electrical charge in between the macromolecular chains. Neat EVA exhibits better electrical resistance than neat TPU. Therefore, more improvement in electrical resistance is achieved for 80/20 EVA/TPU blends than for EVA/TPU blends of 70/30 composition. While applying as cable sheathing materials, thermoplastics or elastomers should have sufficient oil resistance when exposed to the external environments. The samples were in IRM oil for seven days, and the oil resistance of both non-radiated and electron beam irradiated blends was measured and compared. Results show the remarkable improvement in the oil resistance properties of all EVA/TPU blends on the application of electron beam radiation. The swelling of oil constantly decreases with rise in radiation dose. As the radiation dose increases to 200 kGy, a 57% reduction in oil swelling is obtained for the 80/20 blend. The higher degree of crosslinking and the possible formation of dense crosslinking networks through polymer matrix hinder the penetration of oil into the polymer blends for swelling. So, the electron beams irradiated polymer blends with enhanced physic-mechanical, thermal, electrical and oil resistance properties can be suitable to be applied in cable sheath application [43].

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Zhang et al. investigated the dependency of curing characteristics of carbon fiberreinforced polymer composites on the dose rate of low-energy electron beam irradiation. They applied an electron beam up to 500 kGy. It is found that E-beam irradiation has a great influence on the interlaminar shear strength of the composite laminates. This interlaminar bonding strength improves upon irradiation due to the formation of higher chemical crosslinking networks [44]. The thermal and corrosion resistance of clay-reinforced unsaturated polyester nanocomposites was investigated by Salehoon et al. [45]. Also, they studied the effect of electron beam curing on these properties of polymer nanocomposites. Clay was varied from 1 to 7 wt% to prepare nanocomposites. The nanocomposite samples were irradiated with radiation dose of 100, 500 and 1000 kGy. The curing process was confirmed by FTIR analysis, whereas electrochemical corrosion resistance was also measured for cured samples. The dose of electron beam irradiation has a strong influence on the density of the polyester composites. The density of the nanocomposites increases continuously up to 500 kGy of radiation dose. It is mainly due to the crosslinking reaction in the polyester matrix and the formation of crosslinking network which leads to the compression of polyester chains. The density of the composite materials gets reduced on the exposure of 1000 kGy dose of electron beam owing to the occurrence of chain scission reactions on exposure of higher radiation dose. The density of the thermoplastic composite increases from 1.165 to 1.206 g/cm3 when the dose of the electron beam is increased from 0 to 500 kGy. The thermal stability of the nanocomposites for different organoclay loading improves significantly on electron beam irradiation. The cured composite with 5 wt% of nanoclay concentration shows the best thermal resistance than other cured composites. DSC thermographs are represented in Fig. 13, and corresponding results are provided in Table 1. In Table 1, T i and T f represent the initial and final temperature of the curing process and Δ H c is the heat of curing. To measure the corrosion resistance of the polyester nanocomposites, the sample was exposed to irradiation with different radiation dose. Corresponding irradiated samples were placed in acidic and alkaline environments. The samples cured up to 500 kGy show little chemical resistance. Beyond 500 kGy, the composites possess higher E corr and Rp values and lower I corr values compared to the neat polymer. The enhancement in chemical resistance up to 500 kGy dose of the electron beam can be attributed to crosslinking reaction in the polymer chains and the formation of crosslinking networks in the polymer matrix. The TGA plots are shown in Fig. 14, whereas the thermal stability result obtained from TGA is summarized in Table 2. At 1000 kGy dose of irradiation, chain scission may take place which reduces chemical resistance of the polyester nanocomposites by promoting damage process by chemicals. The water absorption properties of the resultant nanocomposites improve on electron beam irradiation [45]. The results obtained for the chemical stability of the EB irradiated polyester nanocomposites in acidic environment are given in Table 3.

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Fig. 13 DSC curves of a Neat UP resin and b–e UP resin with 1, 3, 5, 7 wt% of nanoclay loading during electron beam curing, respectively [45]

Table 1 DSC results for UP resin with different clay concentrations during electron beam curing

Sample

T i (°C)

T f (°C)

Δ H c (mJ/g)

UP0

121

199

187

UP1

120

196

170

UP3

117

193

128

UP5

118

194

136

UP7

119

196

165

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S. K. Ghosh and N. C. Das

Fig. 14 TGA and thermograms of a nanoclay, b–f UP resin with 0, 1, 3, 5, 7 wt% of clay concentration, respectively [45] Table 2 TGA data for polyester nanocomposites Sample

T onset (°C)

T offset (°C)

Δ T (°C)

Residual mass (%) at 600 °C

Nanoclay

238

364

126

80

UP0

362

421

60

12.32

UP1

354

428

74

11.92

UP3

339

430

91

11.67

UP5

331

428

97

8.24

UP7

351

425

74

11.9

Table 3 Results of polarization test in acidic environment for electron beam irradiated UP3 nanocomposite

Irradiation dose (kGy)

E corr (mV)

I corr (nA/cm2 )

RP (MΩ cm2 )

0

−297

0.0055

427.3

100

−110

0.0045

560.5

500

−2

0.0017

827.3

1000

−64

0.0025

91.5

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4.3 UV Radiation Curing on Properties and Performance of Polymer and Their Composites To transform liquid resin into solid materials, UV curing is one of the most efficient processes. UV radiation-cured polymer composites exhibit good heat resistance, chemical resistance and solvent resistance. UV radiation-induced curing is often used to prepare polyacrylate/clay nanocomposites at room temperature. Also, this curing method is extensively used for vinyl ether and epoxide-based resins [46]. In a typical work, Keller et al. studied the mechanical and viscoelastic behavior of UVcured polyurethane-acrylate/clay nanocomposites. Firstly, they dispersed organoclay in the acrylic resin followed by stirring at 50 °C. Then, the filled resin was exposed to UV radiation which initiates the crosslinking polymerization. The nanocomposites were cured by UV radiation into 2-mm-thick films. The structural and morphological properties of these UV-cured composites were also investigated by several characterization techniques. FTIR was also used to determine the degree of conversion during the UV curing process. DMA was employed to study their viscoelastic properties and glass transition temperature upon UV irradiation. Also, the weather resistance of radiation-cured composites and the effects of UV irradiation on weather resistance were investigated. Structural characterization from X-ray diffraction analysis shows that clay galleries become widen so that UV curable resin materials can easily penetrate inside these anisotropic layered silicates. The interlayer spacing inside the clay structure is found to be increased from 1.3 to 2.9 nm. The nanoclay particles are arranged together inside the UV-cured composites in a disordered fashion. TEM analysis also reveals that nanoclay particles are dispersed in the UV-cured acrylate nanocomposites in both intercalated and isolated patterns for composite containing 3 wt% of nanoclay loading. UV irradiation leads to the polymerization of polyurethane-acrylate resin rapidly. When UV light intensity was increased from 90 to 500 mW cm−2 , the polymerization reaction becomes faster and complete. The intensity of UV light plays an important role in the rate of the polymerization reaction. In the case of thick composite samples, the UV-induced polymerization reaction of ultrafast and exothermic occurs [46]. In a typical work, a thick sheet of 2.5 mm thickness was achieved within 5 s by a UV-induced polymerization reaction of acrylate polymer. ATR infrared spectroscopy was used to investigate whether the deep through-curing is achieved or not in a composites sample of 2 mm thickness. It is observed that upon exposure to UV radiation of 380 mW cm−2 , the polymerization for the resin containing organoclay proceeds efficiently. For composite, a reaction conversion up to 80% is achieved within a few seconds. From the morphological analysis, it can be observed that anisotropic layered silicates form a skeleton-like structure within the matrix phase, which finally leads to the enhancement of the properties of the resultant nanocomposites. Compared to conventional micro-composites these UV-cured nanocomposites exhibit improved permeability properties, higher thermal resistance, excellent mechanical properties and high flame retardancy. The tensile properties of the UV-cured acrylate nanocomposites strongly depend on the nanoclay dispersion in the polymer matrix. Also, the impact resistance and scratch

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resistance of the nanocomposites are dependent on the network structure formed by layered silicates. Results show that the tensile strength and elongation at break improve significantly upon irradiation. The mechanical properties of the UV-cured nanocomposites increase with the increase in nanoclay concentration. The hardness also increases with nanoclay loading for UV-cured nanocomposites. The viscoelastic properties of the nanocomposites cured by UV radiation were determined by the DMA technique. The Young modulus is enhanced on irradiation, whereas there is no significant effect on the glass transition temperature of the polymer nanocomposites. The nanocomposites exhibit greater flexibility and improved impact resistance. From the results of the optical properties of the nanocomposites, it can be seen that the transparency of these materials is reduced with an increase in clay loading. The particle size of the nanofillers plays an important role in achieving good optical properties of the UV-cured polymer nanocomposites. The UV-cured nanocomposites used for coating application show good moisture resistance. The moisture resistance of the organoclay-filled polyurethane-acrylate polymer composites was also measured at 100% humidity at ambient temperature. The results show significant improvement in this behavior. The gas barrier properties of the UV-cured nanocomposites are found to be increased with an increase in nanoclay loading. UV curing is also used for colored additives [47]. In a typical work, Atif et al. investigated the effect of UV curing on the thermo-mechanical and performance properties of the colored epoxy composites. A UV light of 33 Mw/cm−2 was used to irradiate the samples. The thermal behavior, gel content and dynamic mechanical characteristics of the composites on UV irradiation were investigated. Results indicate that UV curing has limitations in the processing of the thick and colored specimens. But UV curing of these colored composites leads to better filler dispersion in the polymer matrix which finally enhances the thermo-mechanical properties of the composites. Also, better dispersion of colored filler in the matrix phase leads to the formation of a smooth surface of the resultant composites [48]. UV-induced radiation curing is also used for the irradiation of modified unsaturated polyester-based composites. The performance of these composites is strongly influenced by UV irradiation. In a typical work, Shi et al. studied the effect of UV curing on the synthesis and properties of glass fiber-reinforced polyester-based composites where unsaturated polyester was modified by epoxy acrylate groups. The efficiency of the UV-induced crosslinking reaction for polyester laminated with different comonomers ratio was investigated in terms of mechanical properties. Besides tensile properties, they also investigated the water absorption of polyester laminates. Also, the effect of different photo initiators on UV irradiation of the polyester laminates was studied. They found that among the different photo initiators used to induce crosslinking reaction BDK shows more efficiency in the glass fiber-reinforced polyester composite laminates. This photo initiator produces radicals that can initiate the polymerization reaction steadily. The Mylar film shows no absorption in the UV range implying lower oxygen permeability. So this film can protect the polyester laminated from the atmospheric oxygen molecules and UV light does not interfere with the laminates during the photocuring process. The UV radiation-induced crosslinking of the modified unsaturated-based composites includes three major steps of reaction in the presence of a photo initiator.

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Different types of chemical bonding are formed due to the presence of acrylate double bonds in the polyester chains. The performance properties of the polyester laminates are strongly influenced by the degree of crosslinking. Tensile strength and flexural modulus are found to be increased with irradiation time initially and then reach a constant value with further increase in irradiation time. After 10 s of UV irradiation, optimum properties are obtained indicating the completion of the crosslinking reaction at cure time. Improvement in mechanical properties is due to a high degree of crosslinking. The percentage of water absorption decreases continuously with irradiation time, and for the highest irradiation time, it becomes almost constant. Both the tensile properties and the water-resistance properties improve rapidly at the start of UV irradiation. This enhancement is mainly due to the occurrence of crosslinking reactions at high rates as the multifunctional monomers are higher in concentration. The molecular weight of polyester and the glass fiber content can have a strong influence on UV irradiated polyester laminates. Three variations in molecular weight of polyester and glass fiber content are made. The low molecular weight polyester (UP1) becomes more responsive to UV irradiation. The higher rate of polymerization and high crosslinking density leads to a better improvement in mechanical properties of the UV-cured samples. Also, good compatibility between acrylate monomer and the UP1 polyester increases the performance efficiency of the UV-cured laminates. Also, both the mechanical strength and modulus of irradiated composite decrease for polyester of higher molecular weight. As the glass fiber content in the polyester matrix increases, both flexural and mechanical properties improve. Maximum tensile strength and flexural strength are achieved for 55% of glass fiber content and then decrease with further fiber content. As glass fiber proves sufficient strength of the laminates initially but at higher fiber content, the resins are not sufficient in amount to form good bonding with fiber resulting formation of voids in the UV-cured composite laminates. The reaction rate of UV-induced crosslinking reaction can be enhanced by the addition of a reactive diluent monomer. This typical study shows that tensile strength and modulus improve and reach a maximum value with 30% of TMPTA content, whereas impact strength increases gradually. Beyond 40% of TMPTA content mechanical properties of the cured polyester composites decreases. Different types of monomers can also influence the mechanical behavior of the laminates. It is observed from the results that the UV-cured composites exhibit the best combination of mechanical properties with TMPTA for all times of irradiation. The extent of curing decreases for different comonomers used in the following order: acryl ether > acrylate > allylic monomer. The UV-cured carbon fiber-reinforced modified unsaturated polyester composite-based laminates show excellent tensile and water-resistance properties and therefore can be used in techno-commercial applications [49]. UV curing of polymers is extensively utilized in various application fields, especially in the coating industry. UV-cured polymers exhibit superior heat and chemical resistance as a result of high crosslinking density. Clay is an important filler combined with a polymer to prepare polymer–clay nanocomposites. Sometimes polymer–clay nanocomposite is synthesized with the help of UV radiation curing. In a typical work, organophilic clay (3 wt%) contained resin was cured by UV radiation curing to synthesize polymer nanocomposites. The main aim of UV curing was to

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enhance the performance of the final product properties. These highly crosslinked polymer nanocomposites showed very good resistance to organic solvents, moisture and weathering. The mechanical and other properties like the surface gloss of the resultant nanocomposite are strongly influenced by UV irradiation [50].

4.4 Microwave Irradiation of Polymer and Polymer Composites Microwave irradiation is currently used in the processing of polymer composites to couple the polymer materials with electromagnetic energy. As carbon nanotubes can absorb microwave efficiently, therefore carbon nanotubes filled polymer composites exhibit very good interfacial bonding. Microwave irradiation can enhance the performance properties of various polymer composites. Qu et al. prepared bucky papers (BPs) containing polyethylene film with the help of microwave irradiation and studied the effect of irradiation on the mechanical, thermal and structural performance of the resultant composites [51]. A controlled treatment of microwave heating to the PE/BPs composites was applied. Deionized water was used as the carrier and the time of the microwave irradiation was varied from 1 to 30 s. The tensile strength of the polyethylene/bucky paper composites is higher than neat polyethylene. Nonirradiated composites exhibit a tensile strength of 21 MPa which is much higher than PE. This indicates the strong reinforcement effect of BPs in polyethylene matrix. The microwave irradiation leads to more increment in tensile strength of the resultant composites. The tensile strength enhances with the increase in pulse time. The microwave-heated PE composite shows a tensile strength of 34 MPa. Results indicate that microwave irradiation effectively enhances the tensile properties of PE/BPs composites. DMA measurements of PE composites provide storage modulus of the irradiated samples. This storage modulus can reflect the stiffness of the material which is related to the load-bearing capacity of the composite. From the results, it can be said that the storage modulus is strongly influenced by microwave irradiation and this dependence is more prominent in the glassy state. The storage modulus obtained for microwave-irradiated PE/BPs composites is about 1778 MPa which is 3 times higher than the storage modulus of neat polyethylene. This drastic improvement in storage modulus leads to the improvement in stiffness of the composites. Therefore, it can be said that microwave heating assisted PE/BPs composites to show excellent storage modulus as well as stiffness. The reinforcing mechanism of BPs in the polymer matrix was also investigated to study the effect of microwave irradiation on the structural performance of polyethylene composites. Morphology of the fractured surface of the composites shows the uniform distribution of BPs in the matrix phase. Microwave irradiation can lead to the penetration of polyethylene chains into bucky papers (BPs) effectively. Structural characterization by FTIR analysis reveals that the interfacial adhesion between PE and BPs has been improved upon irradiation. Raman analysis indicates that microwave radiation cannot affect the molecular

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structure of the composite. At higher temperatures, microwave-assisted oxidation of polyethylene molecules occurs to some extent. Therefore, it can be said that the chemical structure of PE/BPs is changed partially by microwave irradiation. The mechanical properties and SEM images of the bucky papers and polyethylene/bucky papers composites are shown in Figs. 15 and 16, respectively. The effect of microwave irradiation on the melting and crystallization behavior of the polyethylene composites was also investigated. As microwave irradiation precedes the degree of crystallization of PE/BPs increases. This increment in the degree of crystallization contributes to the mechanical properties of the composites. So, microwave irradiation is very effective to synthesize stronger and stiffer composites of PE/BPs. Microwave treatment also leads to the improvement in the thermal stability of polyethylene composites [51]. Microwave-assisted metal nanoparticles production and supported on RGO (reduced graphene oxide) nanosheets is a very effective method to decorate metal nanoparticles (Ag, Cu, etc.) on the surface of RGO sheets. The microwave irradiation method is also employed to synthesize polymer composites loaded with this type of nanofillers. Alsharaeh et al. prepared polystyrene/RGO/AgNPs nanocomposites via bulk polymerization method using two different approaches [52]. Firstly, the mixture of polystyrene filled with graphene, silver nitrate and hydrazine hydrate was treated and reduced by microwave irradiation. Secondly, RGO/AgNPs were prepared by microwave, and subsequently, styrene monomers were bulk polymerized to obtain PS-RGO/AgNPs nanocomposites. Several characterization techniques were employed to study their structural, morphological and thermal behavior and compared the results for both nanocomposites. From the DSC analysis, it is observed that the glass transition temperature of the PS-RGO/AgNPs nanocomposites increases by 59 °C compared to neat PS. For the other nanocomposite, T g increases by 51 °C than pure polystyrene. The thermal stability improves on

Fig. 15 a Stress–strain plots and b storage modulus as a function of temperature for PE and BPs-reinforced polyethylene nanocomposites [51]

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S. K. Ghosh and N. C. Das

Fig. 16 FESEM micrographs of BPs and irradiated PE/BPs composites [51]

microwave irradiation significantly in both cases. The results indicate a strong interfacial interaction between the nanoparticles and the polymer. The thermal stability is better for R-(PS-GO)/AgNPs nanocomposites compared to other nanocomposites. So, this study strongly suggests that microwave irradiation can produce PS filled with RGO/AgNPs nanocomposites which exhibit enhanced thermal stability with better filler dispersion and morphology compared to other composites synthesized without microwave irradiation. Microwave is often used to irradiate graphene-contained polymer composites. In a typical work, poly(vinyl alcohol)/graphene composites

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were prepared by Afzal et al. with the help of microwave irradiation [53]. They studied the effect of microwave treatment on the thermal performance of PVA and PVA/graphene composites. The nanocomposite samples were exposed to microwave radiation three different times with an interval of 5 min at a power of 200 W (2450 MHz frequency). DSC and TGA were employed to study the thermal stability and crystallization behavior of the prepared nanocomposites. The effect of irradiation on structural changes of the nanocomposites was also studied. But they were able to irradiate only neat PVA and 1% PVA nanocomposite, not the other composites. As graphene exhibits higher thermal conductivity, therefore when microwave passes through the composites with higher graphene content, overheating takes place. This finally leads to the burning of the PVA composites in the presence of air. From the FTIR analysis, as shown in Fig. 17, it is observed that the intensity of vinyl and carboxylic groups increases after 5 min of irradiation which indicates the formation of crosslinking networks. PVA undergoes some structural arrangements and ruptures on microwave irradiation. Raman analysis reveals that microwave irradiation leads to the formation

Fig. 17 FTIR spectra of a PVA and PVA/graphene composites, b PVA and irradiated PVA samples, c PVA and PVA/graphene irradiated nanocomposites [53]

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S. K. Ghosh and N. C. Das

of some structural defects in the graphene as shown in Fig. 18 which may increase the interaction between the graphene sheet and polymer chains through the covalent bond formation. After 5 min irradiation, the intensity of the D band increases in the Raman spectra strongly suggesting the formation of defects in graphene structure. The irradiation-induced structural defects lead to graphene exfoliation and strong interfacial interaction with polymer macromolecules via chemical bond formation. This gives a strong indication of the increased thermal stability of the nanocomposites. When irradiation time is further increased, the I D /I G ratio decreases which is an indication of the transformation of graphene structure from nanocrystalline to amorphous at higher irradiation time. Hence, thermal stability can be decreased due to weak interaction between graphene sheets and the PVA polymer chains in this case. SEM micrographs show that after 5 min of microwave irradiation the surface of the PVA polymer exhibits some erosion indicating the formation of crosslinking in composite (P5), but after 15 min irradiation the degradation takes place (for P15). Also for sample G1 microwave irradiation leads to the formation of a smooth and continuous surface after 5 min. The higher time of irradiation can rupture the polymer chains of G1 composite due to the degradation of macromolecular chains. Therefore, the thermal stability and the crystallinity percentage decrease in this case. The addition of graphene nanosheets in PVA decreases the crystallinity of the resultant nanocomposites as graphene sheets hinder the periodic arrangement of polymer chains and restrict their dynamic movement. But after microwave irradiation, the crystallinity percentage of PVA is almost the same as that of the irradiated sample. But the melting point of the polymer chains reduces slightly on irradiation. However, for 5 min of irradiation, melting temperature of PVA increases by 9% and then decreases upon further irradiation. Lower time of irradiation leads to chain scission and formation of shorter PVA chains and some defects in the graphene structure. So it is very easy to get arranged the PVA chains and better interaction between graphene and PVA chains [53]. Thermal properties obtained from DSC analysis of

Fig. 18 Raman spectra of a non-irradiated and b microwave-irradiated PVA nanocomposites [53]

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non-irradiated and irradiated samples are represented in Fig. 19 and corresponding results are provided in Table 4. Morphological analysis by SEM of the surface of the samples was done and corresponding micrographs are shown in Fig. 20. The crosslinking of polymer networks and degradation on the higher span of irradiation break the crystalline phase which finally reduces the melting point. The

Fig. 19 a–b DSC endotherm at 10 °C/min for PVA and PVA/graphene composites, PVA and microwave-irradiated PVA samples; (a, –b, ) DSC endotherms for PVA and PVA/graphene nanocomposites at different temperatures [53]

Table 4 Percentage crystallinity and melting temperature of pure and irradiated samples Sample

Crystallinity%

T m (°C)

Sample

Crystallinity%

T m (°C)

P

55

229

G1 (5M)

55

229

P (5M)

56

228

G1 (10M)

49

227

P (10M)

56

228

G1 (15M)

49

225

P (15M)

54

226

G5

43

231

G1

46

230

G10

41

232

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Fig. 20 SEM surface images of a P, b G1, c G5, d G10, a, P (5M), b, P (10M), c, P (15M) and d, G1 (5M) [53]

thermal stability of the PVA composite (G1) increases for 5 min of irradiation owing to the formation of crosslinking networks and better dispersion of graphene in the PVA matrix [53]. TGA curves for non-irradiated and microwave-irradiated samples are represented in Fig. 21 and the results are summarized in Table 5. For the higher span of microwave irradiation, thermal stability decreases but due to the formation of crosslinking structure temperature corresponding to 50% decomposition and maximum decomposition temperature is not much affected. They fitted the resulted data with the Avrami model and found that the Avrami model is well satisfied by the experimental data. Also, the incorporation of graphene leads to an enhancement in crystallization rate. Microwave irradiation is also used to produce clayreinforced polymer composites. Microwave-induced melt intercalation of nanoclay in the polymer matrix shows some advantages such as cost reduction, shorter

Fig. 21 a TGA plots and b DTG plots for PVA and PVA/1% graphene composite, respectively [53]

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Table 5 TGA data for neat and irradiated nanocomposites Sample

T onset (°C) T 50% (°C) T max (°C) Sample

T onset (°C) T 50% (°C) T m (°C)

P

116

110

P (5M)

334

316

G1 (5M)

306

281

100

326

279

G1 (10M) 107

305

281

P (10M) 100

325

276

G1 (15M) 106

302

280

P (15M) 98

323

276

G5

105

299

276

303

279

G10

104

294

268

G1

106

processing time and increment in ionic conductivity value over conventional thermal treatment processes [54]. Aranda et al. synthesized microwave irradiation-assisted poly(ethylene oxide)/clay nanocomposites and investigated their electrical properties. They used three different nanoclays, namely montmorillonite, laponite and hectorite to prepare PEO/clay nanocomposites through melt intercalation occurred by microwave irradiation process. Different parameters like irradiation time, power, relative humidity and amount of reagents were used to influence the electrical behavior of the nanocomposites differently. The effect of these parameters on the properties and performance of the PEO/clay nanocomposites irradiated by microwave was investigated in this work. To study the structural, morphological, thermal and electrical behavior of the resultant nanocomposites, different characterization techniques were employed. Microwave irradiation with a power of maximum 700 W and 2450 MHz frequency was used for radiation heating of the nanocomposites. The microwaveassisted melt intercalation of clay-reinforced polyethylene nanocomposites becomes more easier and faster process compared to conventional melt intercalation methods. It is observed that at 55% relative humidity and microwave irradiation at 525 W, sufficient intercalation does not occur below irradiation time of 5 min. Beyond that irradiation time, the PEO intercalation occurs which is confirmed from the XRD analysis in which interlayer spacing of clay platelets increases and shifts to lower Bragg’s angle. Also, some degree of disorder in the clay structure is observed depending on the exposure time of the microwave. So this behavior indicates that to produce melting of the PEO, a minimum time of irradiation is required. The clay stacking will be more ordered for a longer period of microwave irradiation. Irradiation time of 10 min was used to improve the electrical properties of the composites. The ratio of PEO and clay has a strong influence on the degree of intercalation. PEO/clay ratio in between 20:100 and 40:100 leads to the melt intercalation process. With the increase in PEO content, more disorder in the nanocomposite structure is observed. If the water content in the clay interlayer is more, then intercalation occurs easily. Also, PEO with higher crystallinity, e.g., poly(ethylene glycol), leads to the formation of well-intercalated composites. But PEO of higher molecular weight tends to lower the stacking order of the clay. Electrical characterization indicates that the nanocomposites show almost pure ionic conductivity where charge carriers are positive ions that are actually interlayer cations. However, the nanocomposites do not show high conductivity for using in practical applications as electrolytes in the batteries [55].

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5 Conclusion Radiation curing is extensively used in various polymer fields such as plastics, rubbers and their blends as well as their composites. This irradiation has a strong influence on the performance properties of the polymer systems. Herein, the effects of radiation curing on various properties and performance of plastics, rubbers and plastic/rubber blends are discussed separately. The discussions focus on the different irradiation systems used in these fields such as gamma, UV electron beam and microwave. The effects of radiation curing on the properties and performance of the polymer and their composites are discussed in this chapter.

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34. Shoushtari Zadeh Naseri A, Jalali-Arani A (2016) Study on the morphology, static and dynamic mechanical properties of (styrene butadiene rubber/ethylene propylene diene monomer/organoclay) nanocomposites vulcanized by the gamma radiation. J Appl Polym Sci 133:1–9. https://doi.org/10.1002/app.43581 35. Elshereafy E, Mohamed MA, El-Zayat MM, El Miligy AA (2012) Gamma radiation curing of nitrile rubber/high density polyethylene blends. J Radioanal Nucl Chem 293:941–947. https:// doi.org/10.1007/s10967-012-1801-3 36. Paran SMR, Naderi G, Ghoreishy MHR, Dubois C (2018) Essential work of fracture and failure mechanisms in dynamically vulcanized thermoplastic elastomer nanocomposites based on PA6/NBR/XNBR-grafted HNTs. Eng Fract Mech 200:251–262. https://doi.org/10.1016/j. engfracmech.2018.07.018 37. Nanda M, Tripathy DK (2010) Influence of different curing systems on the physico-mechanical and rheological properties of CSM rubber. Polym Polym Compos 18:417–427. https://doi.org/ 10.1177/096739111001800802 38. Raghavan J (2009) Evolution of cure, mechanical properties, and residual stress during electron beam curing of a polymer composite. Compos Part A Appl Sci Manuf 40:300–308. https://doi. org/10.1016/j.compositesa.2008.12.010 39. Zhang ZG, Sui G, Zhong WH, Sun ZJ (2002) Effect of heat treatment on thermomechanical properties of electron beam-cured epoxy resins. Polym Polym Compos 10:467–480. https:// doi.org/10.1177/096739110201000607 40. Ali Z, Youssef H, Afify T (2008) Structure–properties of electron beam irradiated and dicumyl peroxide cured low density polyethylene blends. Polym compos 29(10):1119–1124 41. Atkinson D (2007) Electron beam curing. Flexo Tech 21:29–30. https://doi.org/10.1351/gol dbook.et07615 42. Zhao X, Duan Y, Li D et al (2015) Investigation of curing characteristics of carbon fiber/epoxy composites cured with low-energy electron beam. Polym Compos 36(9):1731–1737 43. Dutta J, Chatterjee T, Dhara G, Naskar K (2015) Exploring the influence of electron beam irradiation on the morphology, physico-mechanical, thermal behaviour and performance properties of EVA and TPU blends. RSC Adv 5:41563–41575. https://doi.org/10.1039/c5ra03381k 44. Zhang J, Duan Y, Ming Y, Wang B (2019) Investigation of irradiation dose rate on curing characteristics of carbon fiber/polymer composites by low-energy electron beam. Polym Adv Technol 30:179–186. https://doi.org/10.1002/pat.4457 45. Salehoon E, Ahmadi SJ, Razavi SM, Parvin N (2017) Thermal and corrosion resistance properties of unsaturated polyester/clay nanocomposites and the effect of electron beam irradiation. Polym Bull 74:1629–1647. https://doi.org/10.1007/s00289-016-1793-1 46. Keller L, Decker C, Zahouily K et al (2004) Synthesis of polymer nanocomposites by UVcuring of organoclay-acrylic resins. Polymer (Guildf) 45:7437–7447. https://doi.org/10.1016/ j.polymer.2004.08.040 47. Mgbemena CO, Li D, Lin MF et al (2018) Accelerated microwave curing of fibre-reinforced thermoset polymer composites for structural applications: a review of scientific challenges. Compos Part A Appl Sci Manuf 115:88–103. https://doi.org/10.1016/j.compositesa.2018. 09.012 48. Atif M, Yang J, Yang H et al (2016) Effect of novel UV-curing approach on thermo-mechanical properties of colored epoxy composites in outsized dimensions. J Compos Mater 50:3147– 3156. https://doi.org/10.1177/0021998315616174 49. Shi W, Rånby B (1994) UV curing of composites based on modified unsaturated polyester. J Appl Polym Sci 51:1129–1139. https://doi.org/10.1002/app.1994.070510619 50. Decker C, Keller L, Zahouily K, Benfarhi S (2005) Synthesis of nanocomposite polymers by UV-radiation curing. Polymer (Guildf) 46:6640–6648. https://doi.org/10.1016/j.polymer.2005. 05.018 51. Qu B, Zhuo D, Wang R et al (2018) Enhancement of mechanical properties of buckypapers/polyethylene composites by microwave irradiation. Compos Sci Technol 164:313–318. https://doi.org/10.1016/j.compscitech.2018.06.002

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52. Alsharaeh EH, Othman AA (2014) Microwave irradiation synthesis and characterization of RGO-AgNPs/polystyrene nanocomposites. Polym compos 35(12):2318–2323 53. Afzal HM, Shehzad F, Zubair M et al (2020) Influence of microwave irradiation on thermal properties of PVA and PVA/graphene nanocomposites. J Therm Anal Calorim 139:353–365. https://doi.org/10.1007/s10973-019-08419-x 54. Bogdal D, Prociak A, Michalowski S (2009) Synthesis of polymer nanocomposites under microwave irradiation. In: Proceedings of 12th international conference on microwave and high frequency heating, AMPERE 2009 126–129. https://doi.org/10.2174/138527211793979835 55. Aranda P, Mosqueda Y, Pérez-Cappe E (2003) Electrical characterization of poly (ethylene oxide)–clay nanocomposites prepared by microwave irradiation. J Poly Sci Part B: Poly Phys 41(24):3249–3263

Chapter 2

Electron Beam Radiation Technology Application in the Tyre Industry Pratip Sankar Banerjee, Jagannath Chanda, Prasenjit Ghosh, Rabindra Mukhopadhyay, Amit Das, and Shib Shankar Banerjee

1 Introduction The scientific community in the post-World War II era witnessed a booming drive toward radiation processing techniques of polymers. Radiation sources like X-rays, gamma, ultra-violet (UV), and electron beam (EB) are noteworthy mentions in this aspect [1]. In the last few decades, utilization of electron beam radiation sources has been in vogue for various plastic materials including polyethylene (PE) and polyvinyl chloride (PVC). The curing and polymerization of these materials for coating, adhesives, and cable applications have been gradually gaining popularity worldwide [2, 3]. However, the focus has not only been restricted to plastic materials. A significant amount of study has been dedicated to the effect of the EB radiation process on P. S. Banerjee · S. S. Banerjee (B) Department of Materials Science and Engineering, Indian Institute of Technology Delhi, New Delhi 110016, India e-mail: [email protected] P. S. Banerjee e-mail: [email protected] J. Chanda · P. Ghosh · R. Mukhopadhyay Hari Shankar Singhania Elastomer and Tyre Research Institute, Hebbal Industrial Area, Plot No. 437, Mysore 570016, Karnataka, India e-mail: [email protected] P. Ghosh e-mail: [email protected] R. Mukhopadhyay e-mail: [email protected] A. Das Research Division Elastomers, Leibniz Institute of Polymer Research Dresden, Hohe Straße 6, 01069 Dresden, Germany e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. R. Chowdhury (ed.), Applications of High Energy Radiations, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-9048-9_2

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elastomers. Even though the ionizing nature of both EB and gamma rays can induce ‘radiation chemical reactions’ when incident on a polymer, EB radiation techniques are more prolific in terms of radiation processing. This inclination toward EB radiation is due to the aspects like high energy utilization efficiency, high processing performance, high directivity and steerability in the acceleration direction, and intensity control [1, 3, 4]. Furthermore, the high energy efficiency of EB ensures a shorter induction time requirement for desired changes in a polymer in comparison with gamma radiation due to the rapid energy transfer capability of EB source to polymer [3]. The most important aspect of radiation chemistry of polymers involves the curing or cross-linking process, whereby a 3-D network structure results from intermolecular bond formation between the polymer chains [5, 6]. Curing is an indispensable facet of elastomeric goods manufacturing, and tyres are the largest example of the elastomeric product in this regard. The earliest study on the application of radiation technology in the tyre industry was implemented in the 1950s by Firestone Co. Ltd., and by the next twenty years, they had set up the world’s first ‘radiation prevulcanization’ tyre production line. This pre-curing technique was primarily utilized to infuse adequate green strength into the rubber compound before final vulcanization, which in turn reduced the product failure and production cost. However, this technology has evolved over the years and has become a critical aspect in the radial tyre sector as it has positive impacts on productivity, as well as on the reduction of tyre mass without any deterioration of performance. Nowadays, this technology is gaining further impetus, and many tyre manufacturing companies have invested in the same [7]. The conventional curing techniques in the tyre industries include sulfur or peroxide curing. These technologies require various compounding ingredients like accelerators, activators, etc., and the quality of the finished products depends widely on factors like the effective dispersion of compounding ingredients, temperature control, scorch prevention, and so on. Although the aforementioned technologies have definite advantages of their own, the incorporation of various compounding chemicals renders instability to these products [1]. In addition, the presence of harmful chemical ingredients poses a challenge of environmental hazard due to emission and contamination problems that cannot be ignored. The ‘Green Drive’ set with the aim of environmental protection demands a cleaner, greener, energy-efficient, and eco-friendly technology for product manufacturing. EB curing technique provides a viable alternative to the conventional curing processes. Even for elastomeric components of considerable thickness such as tyre carcass, the penetrating power of EB beneficiates the efficient curing while maintaining a balance of product thickness and strength. Moreover, the EB radiation cure does not require the incorporation of compounding ingredients as it involves an ionization mechanism facilitated by the incident radiation. The absence of compounding ingredients renders this process clean and fast and imparts high purity to the product [1, 5, 6, 8, 9]. Nowadays, the major application of EBR technology is the procuring of various tyre components including tread, body ply and belt skims, sidewall, and inner liner. Studies by Mohammad and Walker on the effect of EBR curing on the halobutyl inner liners of radial tyres revealed an improvement of fatigue resistance of the bromobutyl

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inner liner compounds thereby enhancing the service life. However, this effect cannot be generalized as chlorobutyl rubber compounds had an adverse effect on the aging properties [7]. Similarly, different effects of EBR on different tyre rubbers have been observed. For example, natural rubber (NR) and styrene-butadiene rubber (SBR) are probably the most common tyre rubbers based on their widespread usage in tyre tread formulations. Upon exposure to EBR, the green strength and tack of both NR and SBR have shown to increase. The effect of improved tack can be attributed to the fact that the EB curing process is not a surface phenomenon henceforth preserving the inherent tackiness of rubber [7]. In addition, studies by Basfar and Silverman revealed the improved ozone resistance of SBR vulcanizates during a mixed curing technique employing a combination of sulfur and EB dose. The reduced amount of sulfur in the compound preserves the ozone-resistant residual vinyl groups which are otherwise present in lower concentrations in fully sulfur-cured SBR copolymers [10]. Thus, it can be safely said that a critical analysis of the effect of EBR on various tyre rubbers and their mechanical properties, cross-linked network structure, dynamic properties, and filler interaction which have predominant effects on the overall tyre performance would not go amiss. Despite all the advantageous aspects of EBR technology, the investment cost associated with it can be an important hindrance for tyre manufacturers. An estimated amount of one million dollars might be required for a complete installation of a 1 MeV, 50 kW electron accelerator combined with a shielding vault and transportation equipment for conveying products in and out of the radiation chamber [3]. That being said, ease of operation and maintenance coupled with high energy efficiency, ease of reaction control, control over product molecular weight, and fast throughput rates attracts tyre industries for a prospect of greater profitability and better product performance. The most common industrial sectors apart from the tyre sector which is known to employ EBR technology include bio-medical, textile, ink and coatings, wire, cables, polymeric foams, etc. [4, 9]. In this chapter, the primary focus is vested on the basic principles of EBR and its applicability in the tyre industry.

2 Principles of EB Irradiation The fundamental concept of EB implies an electron stream being accelerated across a potential difference under a vacuum of 10–6 torr. The electrons are generated by a process of thermionic emission, whereby metal is heated using tungsten coils. The source of electrons is either an electron injector composed of LaB6 or an electron gun [4, 9]. The accelerated electron energy of industrial EB accelerators can vary from as low as 150 keV in areas requiring low penetration depth to as high as 10 MeV. An entire system comprising this EB generation and subsequent irradiation on a target material upon passing through a window ensuing in a chemical reaction is known as the ‘Electron Beam Processing System (EPS)’ [9, 11]. The major components of a typical EB accelerator comprise of:

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i.

A power supply of high voltage which generates DC current which in turn creates the acceleration potential for electron generation and acceleration. The acceleration tube, for accelerating electrons to their full potential energy. A scanning chamber where the accelerated electrons get uniformly distributed throughout the target irradiation area. A process control system for maintaining all production variables in tandem and controlling all devices. A transportation unit for conveying the target material in and out of the scanning chamber.

ii. iii. iv. v.

The key features of any such EPS system are the high voltage DC supply, electron acceleration unit, and a vacuum unit for maintaining an ultra-high vacuum in the acceleration and the scanning unit [4, 7]. There are various important parameters in the EB irradiation process which are discussed in the next section.

2.1 Dosimetry During the EB radiation process, the material response is dependent upon the absorbed dose, dose rate and dose distribution. These parameters together comprise the dosimetry study. The amount of energy deposited per unit mass of the material exposed to the EB is known as the absorbed dose. The unit of absorbed dose is ‘rad’ or ‘Gray’ (Gy), where 1 Gy is the dose required to absorb 1 J of energy by 1 kg of a material. The inter-relation between the different units is given by 1 Gy = 100 rad = 1

s J = 1W − = 10−4 Mrad kg kg

(1)

If all the thermal energy is retained in an irradiated sample, a 2.4 °C rise in temperature will be encountered with a dose of 1 Mrad in water. The dose rate is defined as the absorbed dose per unit time (kGy/s or kGy/min) and it can be expressed as DR = K ·

I R

(2)

where DR is the dose rate, K is the stopping power of the electrons which depends upon the electron energy and the irradiated material density, I is the beam current, and R is the irradiation field area [7, 9, 12]. The maximum dose is experienced inside the body of the material rather than at the surface and this penetration depth depends on atomic weights for inorganic heavy materials (Fig. 1a) and on H-content for organic materials (Fig. 1b). Increasing H-content increases the electron density thereby decreasing the beam penetration due to inelastic electron–electron collision. For commercial rubbers loaded with inorganic fillers, the depth dose distribution

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Fig. 1 Depth-dose distribution curve for a inorganic samples showing an increase in maximum dose penetration depth with reducing atomic weight, b organic samples showing an increase in maximum dose penetration depth with decreasing H-content [13] (Reprinted with permission from Ref. [13], copyright (1979), Radiation Physics and Chemistry)

curve lies between the curves for Al and Carbon shown in Fig. 1a [13]. The EB penetration depth for a normal angle of incidence is also directly proportional to electron energy and inversely related to the material density [12, 14]. The dosimetry studies on an industrial scale are performed by relative dosimeters which are calibrated against some standard, the absolute dose of which is determined calorimetrically or from ionization chamber data. In such dosimeters, spectrophotometric analysis of the change in OD (optical density) at a specific wavelength upon electron beam irradiation accounts for the dosage measurements. Examples of industrial dosimeters are Far West Tech (FWT) thin-film dosimeter system (use range 0.5–200 kGy), blue cellophane (use range 5–120 kGy), nylon, polyvinyl butyral dyed with triphenylmethane cyanide, etc. [7, 12].

2.2 Electron Energy Utilization Efficiency The electron energy utilization efficiency is defined as the fraction of the incident EB power absorbed by the irradiated material. This efficiency is a function of the accelerator voltage, product thickness, scanning area, and scanned mass. By approximately matching the accelerator energy to the product thickness, it is possible to maximize the thickness efficiency. However, as the thickness of the irradiation window and the air layer matches the depth for the maximum dose in the target, the energy efficiency drops to zero even at the optimum product thickness. In this context, we can also mention another term known as the ‘beam current utilization efficiency’ which is quantified as the ratio of the EB current intercepted by the target to the total beam current. However, this beam current utilization efficiency is more common for flat sheets of homogeneous materials and a smaller quantity of target, and both voltage and current ratings have to be estimated separately. Hence, for the processing of

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bulk materials, it is more convenient to do a beam power calculation using mass throughput rates where the electron energy utilization efficiency comes into play [12, 13].

2.3 Processing Capacity and Yield The processing capacity of an irradiation process is expressed in terms of either mass throughput (kg/h) or area throughput rate (m2 /h). They are expressed as:   A p m2 Fi I =k· t h D

(3)

where k is the area processing coefficient, F i is the beam current utilization efficiency, A I is the total beam current, and D is the absorbed dose at any depth of the material. t p 2 is the area throughput rate. The k-value ranges between 0.85 and 1.76 Mrad m /min and it is a slowly changing inverse function of beam energy. M The expression for mass throughput rate, t p (kg/h), is given by: Mp Fe Fi P = 360 t D

(4)

where the average dose distribution value within the irradiated material is given by D (Mrad), F e is the electron energy utilization efficiency, and P is the beam power of the accelerator in kW [12, 13]. During an irradiation process, the yield of any reaction that takes place is expressed by the G value. For example, for a scission reaction: G(scission) = Yield of polymer chain scission for every 100 eV of absorbed dose (5) Similarly for a cross-linking reaction: G(X ) = yield of crosslinking points for every 100 eV of absorbed dose

(6)

Literature evidences can be obtained where G(S) and G(X) are evaluated by sedimentation, velocity, gel-permeation chromatography (GPC), and dynamic light scattering (DLS) measurements which actually quantify the effect of irradiation on molecular size distribution [15].

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3 Tyre Rubbers Since its first introduction by Dunlop in 1888, pneumatic tyres have been a critical engineering development for smoothening the concept of surface transport. Tyres used for vehicular transport include passenger car radial (PCR) tyres, truck bus radial (TBR), belted bias, and solid tyres. Operating temperature can also classify tyres into different categories such as summer tyres, winter tyres, and all-season tyres. Whatever the tyre type and build, the most important raw material for tyre manufacturing is elastomers or rubbers [12, 16]. Approximately 70–80% of the tyre mass is comprised of rubbers and the remaining part is composed of various compounding ingredients like reinforcing and non-reinforcing fillers, accelerators, activators, curatives, processing aids, etc., and also steel cords or nylon fabric which imparts a final shape to the tyre [17]. In a tyre, natural rubber (NR) is the most fundamental material which contributes around 30% of the tyre weight. The remaining polymeric composition is filled with different types of synthetic rubbers such as styrene-butadiene rubber (SBR), polybutadiene rubber (BR), synthetic polyisoprene (IR), and isobutylene-isoprene rubber or butyl rubber (IIR) and its halogenated derivatives [12, 16]. The chemical structure of NR is expressed as cis-1,4-polyisoprene (Fig. 2a) and it is obtained in latex form by ‘tapping’ the bark of Hevea brasiliensis trees.

Fig. 2 Chemical structure of a cis-1,4-polyisoprene or NR, b cis-1,4-polybutadiene or Buna, c styrene-butadiene rubber or Buna-S, d stereoisomers of BR, e IIR and its halogenated derivatives

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After tapping, the latex undergoes various processing steps such as coagulation and filtration and finally results in a dry rubber. The most common grades of NR used in tyre industries include the Technically Specified Rubber (TSR), which was first introduced as Standard Malaysian Rubber (SMR), followed by Standard Indonesian Rubber (SIR), Indian Standard Natural Rubber (ISNR), and so on. Before the advent of these grades, the rubber was prepared as sheets or crepes. Apart from TSR grades, granulated or crumb rubber also became popular due to the advantageous aspects of processing ease, cleaning ease, lesser processing times, and better product uniformity [12, 16, 18]. The largest application of NR lies in the commercial vehicle tyres. The tyre component where NR is the principal ingredient is the carcass of both radial and bias tyres. The superior fatigue resistance and low heat buildup of NR also facilitate the use of NR compounds in the sidewall of PCR tyres. NR is also used in combination with synthetic rubbers like SBR and BR in tyre tread formulation. However, in winter tyres, NR finds limited use in tyre treads due to poorer wet skid resistance and wear and tear characteristics. In hotter areas with rugged terrains, NR is preferable due to high molecular weight and strain-induced crystallization which gives rise to high cutgrowth resistance and low heat generation as well. The most celebrated example of such tyres is off-the-road tyres (OTR). Furthermore, the adhesion quality or inherent ‘tack’ of NR renders it suitable to be combined with steel cord enabling us to prepare belt and bead compounds with NR [18]. Being a natural material, NR has a ‘nervy’ structure which can result in processing difficulties. Hence, mastication is an important step in NR processing which might not be necessary for synthetic rubbers as they are tailor-made and have controlled molecular weight. Another disadvantage faced in NR-based compounds is the tendency to undergo hardening and oxidation upon prolonged use. Un-vulcanized NR gum compounds possess high green strength and tack which are often utilized to impart similar properties to compounds made up of synthetic rubbers. Altogether, the excellent mechanical and tensile properties, combined with high resilience, toughness, low heat buildup, and low hysteresis loss, depict the robustness of NR compounds. Furthermore, being a natural product, the ‘carbon neutrality resulting in low CO2 emission and high fuel-efficient tyres have ushered the recognition of NR as significant tyre rubber. Literature studies reveal attempts toward NR curing using EB radiation processes which eventually pose technical advantages such as improved tensile properties, short curing times, and a controlled degree of cross-linking [16, 18–20]. The earliest synthetic rubber to be developed was isoprene having the exact chemical structure of NR. Gradually, another popular tyre rubber BR was synthesized. As BR was synthesized by the polymerization of a 1,3-butadiene monomer unit in presence of sodium (Na) metal as a catalyst, it is abbreviated as ‘Buna’ (Fig. 2b). In the same line, with the target of improving the mechanical properties of BR, styrene was copolymerized with butadiene to produce SBR (Buna-S) [21] (Fig. 2c). BRs are nowadays synthesized in a solution either using an alkyl-lithium or a Ziegler–Natta catalyst as the initiator. Butyl-lithium is arguably the most popular organolithium catalyst and it results in the generation of polymers having 40–50% cis content and 5–10% vinyl isomers. However, Ziegler–Natta catalyst systems result

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in even higher cis content using rare-earth elements. However, nowadays, the use of lanthanide catalyst systems based on lanthanum (La), neodymium (Nd), samarium (Sm), etc., is more preferred due to the extremely high cis content of the polymers achievable. For example, Nd-catalyzed polymers have approximately 97 wt% cis content and merely 0.5 wt% vinyl content. In addition, they are also reported to have very high molecular weight, narrow molecular weight distribution, and superior abrasion resistance [12, 22–24]. The different stereoisomers of BR are shown in Fig. 2d. The high cis content of BR is normally desired due to improved processability, and it imparts high green strength, tack, and adhesion to fabric to the compound. BR is normally blended with SBR in tyre tread formulations which results in improved abrasion and tear resistance. Oil extended (OE) SBR/BR blends are most common in winter tyres due to higher wet grip and traction which beneficiates driving safety. Apart from tyre treads, blends of BR and NR are widely utilized in sidewalls of tyres. The excellent abrasion resistance, low rolling resistance, and high resilience imparted by BR are only achievable in low vinyl samples. High vinyl samples are disadvantageous since it raises the T g of the polymer by rendering the chains stiffer resulting in lower thermal stability [16, 22, 25]. The most well-known grades of SBR in the tyre industry for a long time are emulsion grade SBR or E-SBR and solution grade SBR or S-SBR. E-SBR, as the name suggests, is synthesized by the emulsion polymerization routes by using some surfactants and organic fatty acids which in the end leads to a decrease in the purity of the product and the control on the microstructure; for example, side branching is much lower as compared to S-SBR. The solution polymerized version is more trending these days in the PCR tyres owing to their better control over the rubber molecular weight, molecular weight distribution, branching, as well as improved physical properties like low rolling resistance and high wet skid resistance [24, 26]. Furthermore, E-SBR grades are classified into ‘hot’ and ‘cold’ polymerized grades based on the difference in the polymerization temperatures. In the cold polymerization process, a reaction temperature between 5 and 10 °C and using redox initiator system results in higher molecular weight and much lower branching as compared to the ‘hot’ process where the polymerization proceeds at ~50 °C. Cold SBR imparts better processability due to lesser branching and also better dynamic properties, better tread wear behavior, and abrasion resistance. S-SBR is synthesized in a hydrocarbon solvent using some alkyl-lithium initiator, e.g., butyl lithium [21, 27]. Literature evidence suggests that S-SBR cures faster than E-SBR and also has higher modulus and lower hysteresis loss [26, 28]. Furthermore, S-SBR compounds exhibit a greater polymer–filler interaction with silica fillers arising from the high vinyl content in their microstructure. This attribute is reflected in a greater bound rubber content (BRC), lower heat buildup (HBU), and a greater cross-link density for S-SBR-silica vulcanizates as compared to E-SBR-based systems [29]. Various parameters which are the determining factors in SBR copolymer properties include styrene-to-butadiene ratio, vinyl content, branching, and molecular weight distribution. For a typical SBR sample, the rebound resilience decreases, and

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modulus increases with increasing styrene content. Finally, SBR being most well known as the ‘tread rubber’ can be used in blends with BR and NR. However, as compared to NR, SBR lacks in tack and green strength, and without reinforcing agents, they cannot achieve the physical and mechanical properties required for tyre rubber. Due to a lesser percentage of unsaturation in the main chain, they require a larger quantity of curatives for the generation of adequate cross-linking points. The enhanced abrasion and tear resistance and improved cut-initiation resistance are also some important criteria for SBR to be used in tyre treads [21, 27]. EB irradiation technology-based modification of E-SBR has already been attempted by Bandzierz et al. reporting an increase in the cross-link density with an increasing amount of radiation. This was accompanied by an increase in the tensile strength and a decrease in the percent elongation at break (%EAB) implying constrained deformability [30]. Another noteworthy elastomer in the tyre industry is the butyl rubber or IIR. They are copolymers of isobutylene and 2–3% of isoprene which is introduced to provide some sites for cross-linking in an otherwise saturated backbone. The halogen derivatives can be generated by adding a halogen solution to the rubber solution in an inert solvent, resulting in chlorinated IIR (CIIR) or brominated IIR (BIIR) depending on the type of halogen added. Chemical structures for butyl rubber and its halogenated derivatives are provided in Fig. 2e. For both, butyl and halobutyl rubbers, the isoprene units permit sulfur vulcanization. However, the halogenated rubbers also provide pathways for metal oxide (ZnO) curing. Furthermore, the sulfur curing rate in halobutyl rubber is much more rapid as compared to non-halogenated samples [21, 31]. The most important application of halobutyl and butyl rubbers lies in the inner liner of tyres, which is the innermost tyre layer. The purpose of this layer is to retain the inflating air, maintain the inflation pressure, and resist air and moisture permeation into the tyre carcass. The inherent high air impermeability of these rubbers fulfills this purpose which arises from the hindered segmental motion of the chains due to the mutual interference of the methyl groups of the isobutylene units. This hindered segmental motion resists the air passage through the polymer chains via the diffusion route. These rubbers possess adequate heat resistance and flex life but lack green strength which can be enhanced by blending with NR [7, 16, 21]. Mohammad et al. reported an improvement in green strength of halobutyl inner liner compounds using EB irradiation without any compromise in tyre performance.

4 EB Irradiation-Induced Reactions in Tyre Rubbers In general, EB irradiation-induced reactions are of three categories: 1. Cross-linking 2. Chain scission 3. Grafting. However, unless a monomer is added, polymers like elastomers typically exhibit cross-linking and scission [1]. The precursors of these reactions can be ionic entities

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or radicals which are generated by a two-step process of primary and secondary reactions. The primary reaction involves ionization and excitation, i.e., the generation of an excited chemical species, followed by the secondary reaction whereby the excited species undergoes dissociation into a radical ion and a radical [7]. The irradiated polymers yield a stable product by incorporating these reactive species [7]. Directing the high-energy irradiation to an elastomer brings about predominant events of hydrogen abstraction eventually leading to hydrogen evolution, radical generation via chain scission, cleavage of double bonds, and molecular decomposition. The chemical yields of each of these events quantified as G values are given as [7, 32–36]: G(R · ) → Free radicals generated per 100 eV

(7)

G(M) → Molecular decomposition per 100 eV

(8)

The major requirement in the tyre industry is the cross-linking of the elastomer matrix. For effective cross-linking of polymers using EB irradiation, the criterion is the presence of at least an α-H atom in the repeat unit, the absence of which will potentially impart degradation tendency. The presence of the α-H atom beneficiates the hydrogen abstraction resulting in a resonance-stabilized radical which then gradually generates more radicals on subsequent polymer chains [37]. In common tyre rubbers like NR, SBR, BR, etc., the presence of these α-H atoms ensures the cross-linking tendency in presence of radiation [12]. In the case of NR and synthetic polyisoprene, the free radical formation occurs not via α-H abstraction, but there are quite a few possibilities where chain scission and double bond cleavage even result in reactive sites eventually leading to a cross-linked structure. Reaction mechanism of free radical generation by EB irradiation in the isoprene unit as proposed by Mohammad et al. [7] is shown in Fig. 3a.

Fig. 3 a Different modes of radical generation by EB irradiation in cis-polyisoprene units [7], b EB-induced cross-linking mechanism for NR/SBR blends as reported by Jing et al. [40]

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As evident from Fig. 3a, the tendency to form C–C cross-links facilitates better high temperature performances such as better hot tear strength as well-improved mechanical properties [38]. This C–C cross-linking proceeds by the addition of short-lived free radicals generated to unsaturated C=C bonds [19]. The improvement of mechanical properties was also reported by Manshaie et al. for NR/SBR systems whereby the higher tensile strength was exhibited by EB-cured system. This can be attributed to the presence of C–Sx –C or polysulfide cross-links which are weaker and thermolabile as compared to C–C and C-S linkages [38]. This is evident from the bond energies as the C–C bond energy is around 85 kcal/mol, while the S–S bond energy is around 57 kcal/mol [39]. The higher tensile strength and high hardness achieved, however, come with an obvious reduction of %EAB [38]. The mechanism of EB cross-linking of NR/SBR vulcanizates proposed by Jing et al. [40] is shown in Fig. 3b. Hence, the presence of α-H in NR, as well as BR, is a positive aspect in terms of cross-linking tendency in EB irradiation. But due to a higher degree of unsaturation in the backbone of NR, radiation damage should also be kept in mind particularly in stressed samples and in the presence of oxygen. This high degree of unsaturation is also present in BR, and hence, this homopolymer can cross-link readily and has poor radiation resistance [41]. Additionally, studies on the temperature dependence of cross-linking of BR revealed an increase in the cross-linking yield. However, below the T g of the polymer, it remains constant [3]. The situation is not quite similar in the case of SBR. Although the main chain backbone has both unsaturation and α-H atoms due to the presence of butadiene units, the radiation resistance is generally high in the case of SBR. Therefore, both types of radiation-induced changes, i.e., cross-linking and chain scission, are low for SBR and this type of stability arises predominantly from the aromatic ring. The resonancestabilized rings either absorb the majority of the ionizing energy or dissipate it, or the radical centers generated at the butadiene units get scattered by the ring, imparting a global protective effect over the polymer. The bulky ring also renders the polymer chains stiff with reduced flexibility, thereby the mobility of the micro-radicals is hindered and the chances of radical recombination are low [30, 41, 42]. Typical ionization radiation-induced reaction for SBR elastomers as proposed by Delman et al. [43] is shown in Fig. 4a. Thus, even though the cross-linking of SBR is ideally possible using EB irradiation technology, the radiation-resistant nature implies a higher dosage value of irradiation is needed [30]. But the higher dosage has some upper limits as the degradation possibility of butadiene units also has to be kept in mind [41]. In the case of IIR, the isobutylene units give rise to complication as it is fully saturated. Hence, for even sulfur vulcanization, the addition of a few isoprene units to induce unsaturation as potential sites for cross-linking is essential [31]. In the case of irradiation curing, the quaternary carbon atoms in these full isobutylene saturated units result in chain scission as the dominant phenomenon. This is attributed to the absence of resonance stabilization of the generated radicals formed by hydrogen abstraction. Therefore, these species may undergo C–C bond cleavage in the main chain, resulting in an unsaturated but more stable structure due to the possibility of

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Fig. 4 a Scheme of radiation-induced reaction in SBR as proposed by Delman et al. [43]. b Radiation-induced cleavage of C–C bonds for saturated main chain in case of butyl rubber

resonance stabilization [8, 37]. Such a possible cleavage of an isobutylene unit of butyl rubber is given in Fig. 4b. Thus, the addition of isoprene units is highly crucial to providing α-H atoms as potential cross-linking and radical generation sites. Otherwise, the compound will undergo rapid degradation as the rubber becomes a tarry and viscous liquid even at relatively low dosages of irradiation [41]. The addition of isoprene units, which provide sites for cross-linking, does not effectively reduce the scission yield. That can be achieved only through halogenation. Halogenation increases the reactivity of isoprene units and results in cross-linking as the dominant reaction over chain scission. This is evident from the enhancement in the green strength and Mooney viscosity of halobutyl rubbers in the initial stages of the process [7]. The tendency of cross-linking and degradation of the aforementioned tyre rubbers can be closely understood from the cross-linking (G(X)) and chain scission (G(S)) yield values as enlisted in Table 1.

54 Table 1 Approximate cross-linking and chain scission yields for different tyre rubbers [12, 44]

P. S. Banerjee et al. Rubber

G(X)

G(S)

NR

1.1

0.22

PBR

3.8

–

SBR

2.8

0.39

Polyisobutylene

0.05

1.5–5.0

Butyl rubber

0.4–0.6

3.9–6.1

It is also possible to modify sulfur vulcanized rubber compounds using EB radiation. Since conventional and semi-efficient sulfur vulcanization leads to predominantly thermolabile and weak (bond energy ~57 kcal/mol) S–S bonds [38], the EB irradiation breaks down most of the weak –Sx – bonds. These broken bonds result in the generation of sulfur radicals that eventually lead to the formation of mono- or di-sulfidic bonds according to the scheme suggested by Jing et al. shown in Fig. 5. The mono- and di-sulfidic bonds imply improvement of tensile and tear strength of the product [40].

Fig. 5 Modification of sulfur cross-links driven by EB irradiation [40]

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Another important aspect involved in the EB irradiation or any type of radiation processing is to ensure radiation protection to some extent. This is because, although, the target is to introduce radiation-induced chemical changes which might be favorable for the end application, and the excessive reaction needs to be avoided. For instance, excess cross-linking will lead to a hard and brittle material, completely unsuitable for tyre applications; similarly, excess chain scission will also render the polymer unfit. To avoid these circumstances, protective agents are added to reduce the degree of cross-linking and chain scission, in both the presence and absence of O2 [12, 45]. The antirad ability arises from the presence of a conjugated structure which can decrease the energy of the accelerated electrons as well as labile hydrogens present in their structure leading to radical stabilization of the polymer chain, thereby preventing further propagation [9]. Typical examples of antirad compounds include N-cyclo-hexyl-N , -phenyl-pphenylenediamine, 2-napthylamine, 1,4-napthoquinone, 2-napthol, tris-(p-amino phenyl) cyanurate, o, o, , o,, -tri (4-amino-3-pentadecyl phenyl)-thiophosphate, etc. [45, 46]. Baumann attempted to determine protecting agent effectivity for NR compound filled with 50 phr carbon black and reported the protecting nature was more visible in a vacuum than in the presence of O2 . In the absence of O2 , he reported an approximate decrease in the degree of cross-linking by a factor of 2 for all the protective agents used. Furthermore, the protective mechanism of these compounds was also postulated by Baumann as shown below [45]: R · + AH → R H + A· R · → Activated polymer radical AH → Activated molecule containing a labile H - atom A· → Protective agent free radical This generated protective agent-free radical may either undergo rearrangement or dimerization with a similar radical to stabilize itself. In the presence of O2 , they may get oxidized itself; hence, the reaction shown above will be inhibited, and consequently, the rate of cross-linking will not decrease [45]. Ghatghe et al. also performed protective agent performance studies using amine-based antirads on SBR (synaprene-500) tread stock in air. The vulcanization recipe was found to be able to prolong the tread life as well as protection in both fully cured and under cured states [46].

5 Analysis of Electron Beam-Cured Tyre Compounds Almost all modifications of tyre manufacturing processes and compounding ingredients are done with the target of optimizing the ‘magic triangle of tyre properties’ [47] that include rolling resistance, abrasion resistance, and wet traction. Trying to improve one of these properties might lead to the sacrifice of the other; for example,

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improving abrasion resistance would lead to poorer rolling resistance and wet traction [48]. These aforementioned properties of a tyre and its performance are interconnected through a chain of variables like the compound composition [49], cross-link density [50], green strength [2], and polymer–filler interaction [51] which in turn also affects the glass transition behavior [2, 50], mechanical properties [47], etc. Furthermore, tyre manufacturing industries these days also have to keep the environmental aspects in their mind. Particulate emissions are directly related to the abrasion resistance, and the compounding ingredients, as well as filler incorporated into the compound, are the prime contributors to environmental pollution. A cleaner and greener technology that requires much less quantity of compound ingredients would be beneficial in this aspect and EB irradiation technology can be the go-to option substituting conventional curing techniques entirely in the future [51]. For an in-depth understanding of the effect of EB irradiation on the final tyre performance, in this section, we are going to look into some of the individual aspects of the rubber compound which would play a significant role in altering the overall tyre characteristics.

5.1 Cross-Link Density Cross-linking of rubbers is an elementary reaction involved with tyre manufacturing. Studies involving the effect of EB irradiation on the cross-link density of tyre rubbers are available widely in the literature. The quantification of cross-link density of irradiated rubber compounds, which is the number of network chains present per unit volume, can be done by using the Flory–Rehner equation [52], which is expressed as:   2  ln 1 − v , + ν , + χ ν ,  ν=− , Vs (ν , )1/3 − ν2

(9)

where ν is the cross-link density, ν , is the rubber volume fraction obtained from the swelled sample after immersing it in a solvent for a given amount of time, χ is the Flory parameter or the polymer–solvent interaction parameter, and V s is the solvent-molar volume. This technique of cross-link density determination is also commonly known as the equilibrium swelling study of the sample. Now, another empirical equation for determining the cross-link density of the vulcanizates is given by the Mooney–Rivlin model which relates to the stress–strain properties of the sample. Unlike the Flory– Rehner theory, this technique is applicable to both swollen and unswollen or dry samples. The Mooney–Rivlin equation, derived from the ‘rubber elasticity’ theories and ‘Affine deformation’ models [1, 53], is expressed as:

2 Electron Beam Radiation Technology Application in the Tyre Industry

σ 2C2   = 2C1 + −2 λ λ−λ

57

(10)

σ is the stress acting on the rubber sample given by F/A0 , where A0 is the crosssectional area of the undeformed rubber compound, λ is the extension ratio given by (1 + ε), ε is the strain acting on the sample. The elastic constants are given by C 1 and C 2 . However, this equation is only valid for dry rubber samples. For swollen samples, the equation changes to: 1/3

σ Vr 2C2   = 2C1 + −2 λ λ−λ

(11)

where V r is the fully swollen rubber volume fraction. The intercept and the slope of the plots of the left-hand side of the above equations versus 1/λ give C 1 and C 2, respectively. The constant C 1 directly determines the cross-link density from the equation: n=

C1 RT

(12)

where n is the physically manifested cross-link density and R is the universal gas constant and the absolute temperature is given by T [53]. The Mooney–Rivlin method is also not devoid of its limitations. This method, similar to the Flory–Rehner model, will result in the evaluation of elastically effective cross-links, i.e., both physical entanglements and chemical cross-links [54]. The Mooney–Rivlin model is also invalid for large strains being effective only for 30–150% tensile elongation values [53]. Now, it is well known that during irradiation cross-linking of rubbers, predominantly (C–C) cross-links are produced. To obtain a sufficient degree of precure and cure, the dosage generally ranges between 1 and 200 kGy [55], although we can go to higher dosages depending on rubber type and conditions. Basfar et al. studied the effect of cross-linking on carbon black-filled NR and SBR samples irradiated between 20 and 200 kGy radiation dosages in the presence of coagents. The observation was an increased tensile strength of radiation-cured NR with increasing radiation dosages, and at around 200 kGy, the tensile strength was near about similar to sulfurand peroxide-cured systems depending on the compound formulation. This increase in tensile strength, however, is accompanied by an increased deformation stiffness since the predominant cross-links generated are C–C links which are short in length and have hindered mobility. This might ultimately have some detrimental effects on the mechanical properties of the NR sample. For SBR samples, the situation was not identified as the report points toward a marked improvement in tensile properties and a decrease in the elongation at break when compared with sulfur- and peroxide-cured samples. Here, we may comment that with the formation of a greater concentration of C–C links, the abrasion resistance and heat resistance should increase but the flex life would decrease since the flexibility of C–Sx –C links is higher [38, 56, 57].

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Banik et al. studied the effect of EB irradiation on sulfur-cured rubber samples for irradiation doses as high as 1500 kGy. The NR specimens exhibited a decrease in the %EAB, and tensile strength attributed to the increase in the gel fraction. In addition, the glass transition temperature (T g ) increases as the macromolecular chain mobility is restricted at higher irradiation dosages [58]. This result can also be explained based on the expression of the theoretical glass transition temperature of the cured sample (T gc ), given by: Tgc = Tg + K c ρ

(13)

where K c is the characteristic constant, T g is the glass transition temperature of the uncured sample, and ρ is the cross-link density obtained after curing [50]. The distinct change and effects of cross-linking upon the transition temperature are readily available in literature evidences when sulfur cross-linked NR was compared with EB-cured NR. Plots of temperature-dependent shear moduli of both kind of samples provide evidence of shift in the glass transition temperature with increasing degree of cross-linking. For moderate degrees of cross-linking or considerably low irradiation dosages, the transition temperature only shifts by ~10 °C with curing, whereas for curing with ~30% sulfur, this shift is around +110 °C. With increasing radiation dosages, this shift shows marked rise as the cross-link density increases. Temperature-dependent measurements of rubber elasticity for the determination of cross-link density can also be used to theoretically predict the shear modulus values given by the expression: Mc =

RTg ρ G

(14)

where M c is the average molecular weight between 2 cross-link points and is only valid if M c is not too low. R is the universal gas constant and ρ is the density in gm/cc, and the shear modulus of the elastomeric region is given by G (dyne/cm2 ) [59]. Another important tyre rubber, SBR loaded with an irradiation sensitizer (TMPTA) or 1,1,1-trimethyl propane triacrylate, when irradiated with EB showed a gradual increase in the cross-link density between 50 and 150 kGy dosages. Beyond that, a decrease in cross-link density was observed which may be attributed to the simultaneous and competing effects of cross-linking and chain scission, which eventually leads to the destruction of a few C–C links [60]. Similar observations are also reported by Shaltout, as he studied commercial SBR (SBR-1502) samples upon irradiation between 50 and 250 kGy dosages. Similar trends were also observed by Bandzierz et al., and it was also reflected in the thermal stability measurements. Below 150 kGy, the thermal stability increased as the C–C links are known to be higher heat resistant, but due to scission effects beyond 150 KGy, the thermal stability gradually decreases [61]. BRs are also widely studied for their cross-link densities upon EB irradiation. Out of different isomers of BR, the highest degree of cross-linking is reported to be achieved for vinyl isomers, intermediate for cis, and lowest for trans [62]. It is,

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Fig. 6 a Initial modulus plots of networks formed by bulk cross-linking as a function of EB irradiation dosage for BR samples. (Sample nomenclature: PB-096 refers to a M w of 96 × 10−3 as measured in GPC) [63] (Reprinted with permission from Ref. [63], copyright (1979), American Chemical Society). b Charlesby–Rosiak plot of experimental data obtained for EB-irradiated SBR samples. c Gel fraction of the SBR samples as a function of irradiation dosage [30] (Reprinted with permission from Ref. [30], copyright (2018), Radiation Physics and Chemistry)

however, already established in the earlier section that the cross-linking is much more beneficiated in the case of BR as compared to SBR due to the stabilizing effect of the benzene ring in the styrene units. In addition, the studies of Pearson et al. also consider the possibility of polymerization of the vinyl isomers thereby resulting in the functionality of four or more [62]. Dossin et al. studied the effects of EB irradiation on BR samples and the cross-linking of the samples is reported in terms of the initial modulus (G0), calculated from the phantom theory of rubber network. The assumption taken is that the initial modulus is contributed by both chemical (Gc) and topological (Ge) contributions, and the plot (Fig. 6a) shows an increase in the modulus values in a range up to 600 kGy radiation values. This plot brings certainty to the increasing cross-linking degree with increasing radiation dosages [63]. One thing that can be safely commented on is the fact that, as far as the crosslinking reaction is considered, EB irradiation, if used with control over the scission yields, is a potential and viable replacement for S-curing and other conventional curing techniques. It can be expected that majority of the tyre manufacturing industries can have their production times shortened as the curing times required in EB irradiation are lower as reported by Chakraborty et al. for tread compounds [2]. Furthermore, improvement of material behavior and cost efficiency are some of the other crucial aspects which are to be discussed in subsequent sections [2, 55].

5.2 Gel Content The sol–gel study is also a well-established technique for determining the degree of cross-linking and scission of cured samples. The studies of Flory were the starting point toward the analysis of cross-linking via sol–gel fractions where he first proposed

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the relation between the free chain ends of a polymer to the sol or the un-crosslinked fraction [64]. This work was extended using Flory’s assumption by Miller and Macosko to establish a relation between cross-link density and gel content. Their study started with a polymerization process which commenced as a finite chain and reached the advent of an infinite network structure at the gel point. A third-order equation for a tri-functional network according to their studies is expressed as:  3 1/3 X crd = X gel Wgel = C Wgel 1 − Wsol

(15)

where X crd is the cross-link density of both the gel and the sol, X gel is the cross-link density in the gel, W gel and W sol are the weight fractions of sol and gel components, respectively, and C is a constant. The cross-link density in the sol is assumed to be zero. The equation is dependent on the network functionality and the power changes accordingly [64, 65]. It is quite obvious that the sol fraction gradually decreases as the cross-linking reaction proceeds, and the crucial assumption in these studies is to assume an initially linear polymer undergoes cross-linking and scission simultaneously [66]. The gel fraction studies obtained from swelling of irradiation-cured rubber samples in suitable solvents provide a certain idea about the effect of irradiation dosages. A direct theoretical model of the sol–gel study was that the irradiation dosages can be performed by the Charlesby–Pinner equation given as: S + S 1/2 =

2 p0 + q0 q0 Uw,o D

(16)

where S is the sol fraction, i.e., the dispersed regime comprising molecules of finite molecular masses [67], U w.0 is the pre-curing weight average degree of polymerization, p0 and q0 are the chain scission and cross-linking density per unit radiation dose, respectively (kGy−1 ), and D is the irradiation dosage (kGy). Upon plotting (S + S 1/2 ) versus 1/D, we obtain a linear plot if assumptions of a random initial molecular weight are maintained, i.e., Mw /Mn = 2. Furthermore, the equation is valid if and only if there is a simultaneous participation of both scission and cross-linking reactions [68]. However, deviation from linearity is to be expected as the assumption of Mw /Mn = 2 is not always valid [30]. Henceforth, the modified Charlesby–Pinner equation or the Charlesby–Rosiak equation is to be utilized, which is expressed as: S + S 1/2 =

   Dv + D g p0 p0 + 2− q0 q0 Dv + D

(17)

where Dv is the virtual irradiation dose employed as a correction for Mw /Mn = 2 and Dg is the dose required for the real polymer to achieve gel point [69]. The plot of (S + S 1/2 ) versus 1/D using the Charlesby–Rosiak equation for SBR samples after solvent extraction in toluene for sol–gel analysis is shown in Fig. 6b and c. In Fig. 6b, we can observe a p0 /q0 < 1 indicating the greater contribution of cross-linking rather

2 Electron Beam Radiation Technology Application in the Tyre Industry Table 2 Effect of EB irradiation on the gel content of tyre tread samples [2]

61

100 kGy irradiation dose acting on 15-mm-thick tread Gel (%) samples Up to 2 mm

70

Up to 4 mm

70

Up to 6 mm

70

Up to 8 mm

55

Up to 15 mm

40

than scission, and in Fig. 6c, the gel content plotted as a function of radiation dosage exhibits a gradual rise in gelation implying cross-linking at higher radiation dosages. Above approximately 150 kGy, the gel content gradually reaches a plateau which can be attributed to the fact that with rising gel percent, there are no more available sites for cross-linking [30]. Irradiated NR and SBR latex were studied by Mitra et al. for dosages between 2.5 and 20 kGy. As expected, and already mentioned in the previous discussion, the cross-link density increases significantly for higher radiation dosages as indicated by the rising gel fraction. Furthermore, the processing and rheological behavior of the samples were observed and a decrease in die swell values from their virgin counterparts. This reducing nature of die swell upon gelation is apparently independent of the rubber itself and it is greatly influenced by the cross-link density of the gels, as it reduces the ability of the polymer to relax at the die exit [70]. Similar irradiation studies of Chakraborty et al. on tread compounds also establish the aforementioned facts. In addition, the effect of compound thickness on the gel content was also reported and shown in Table 2. The gel content remained unchanged for 100 kGy irradiation dosages on sample thicknesses up to 6 mm. However, beyond that the gel content decreases due to failure of the EB to effectively penetrate the samples [2]. In this regard, literature evidences stating a possibility of achieving uniform cure up to 6-mm-hick material having density similar to that of water might be contextual [3].

5.3 Green Strength and Tack For tyre shaping, building, and imparting dimensional stability, adequate green strength and tack are prerequisites. Tyre is composed of numerous components and inter-adhesion between each of those components is essential to prevent separation, gauge retention, and tearing [71, 72]. The green strength is defined as the tensile strength and the resistance to failure of uncured rubbers. Adequate green strength is also an important criterion for the determination of the tackiness of a compound. Tack is defined as the resistance to separation or bond breakage between two rubber pieces after their pieces have been in intimate contact for a short duration [72, 73]. The governing criteria for adequate green strength of an uncured compound depend on many factors such as molecular weight and degree of branching governed

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by the presence of stereoregularity [74], gel content [75], strain-induced crystallization [76], and cohesive strength [77]. The high molecular weight and ‘nervy’ structure, as well as the non-rubber constituents of NR, attribute to the strain-induced crystallization behavior which in turn gives rise to significantly high green strength in NR than in synthetic rubbers [72, 76]. In tyre industry, these two properties are no less essential than any other aspect. The importance has ever increased with the advent of radial tyres as during their construction; the un-vulcanized rubber compounds between the steel cords may even be subjected to strains measuring around three times their original dimensions [73, 78]. However, they are also crucial for bias tyres for maintaining the proper bias angle and the rubber-fabric placement during the forming and molding operations [55]. Furthermore, the pressure experienced by the carcass on the building drum must not push the cord fabric of the body plies through the rubber. The rubber can withstand such pressure only if it has adequate green strength. Same pressure withstanding capacity is essential for pneumatic tyres to withstand inflation pressures [55]. In all of these tyres, the importance of building tack lies in the fact that all the rubber components should be capable to hold together and impart long-term creep resistance before the curing operation [73]. Now, the tack is of two types, namely autohesive and adhesive tack. Both of them have their dependencies on contact time, contact pressure, temperature, molecular diffusion, molecular weight, and additives in the compound and their influence [73, 77]. The autohesive tack is applicable when both parties in contact possess the same chemical composition [73] and is arguably essential when holding different rubber components together. In the adhesive tack case, the two components or materials have dissimilar chemical compositions [73]. This is expected to be a governing criterion for adequate rubber-fabric or cord interaction, and the cohesive strength of the weaker substrate, in our case the rubber compound, will be even more important [77]. Molecular interdiffusion is an important criterion for autohesive tack as the molecules must diffuse through the interfaces and get entangled to attain a cohesive strength comparable to the green strength of the rubber. Therefore, low molecular weight of the polymer will be important; otherwise, the diffusivity will be hindered [73, 79]. However, the molecular weight dependency on tack is applicable for synthetic rubbers. For NR, although the diffusivity is minimal due to the extremely high molecular weight, the strain-induced crystallization of these species ensures a high tackiness. A slight level of interdiffusion of chains across two NR pieces gives rise to sufficient entanglements to ensure a high degree of strain-induced crystallization which prevents separation of the pieces [73, 79]. For the adhesive tack scenario, a similar molecular weight dependency is also observed and the number of interdiffusion chains can be quantified by the reptation theory expressed as [71]: G ∼ t 1/2 M −1 R 1/2

(18)

where t is the contact time between the test pieces, M is the molecular weight, and R is the testing rate. Furthermore, for samples of different chemical compositions, the diffusing species might experience resistance from the other phase. Bhowmick et al. depicted this in their findings as it was found that the diffusivity of chains from

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NR to EPDM is much lower as compared to the opposite movement. This can be attributed to the lower molecular weight of EPDM [71]. EB irradiation curing, as we have already seen, has a positive impact on the gel content. Hence, it can be safely assumed that the same amount of dosage should impart adequate green strength via procure, particularly to synthetic rubbers. However, care should be taken on the irradiation dosage as excessive curing will be detrimental to the tackiness of the material. The tackiness gets lost as cross-linking occurs since the cohesive strength, in that case, will increase by sacrificing the chain mobility. Mohammad et al. extensively studied the effect of irradiation pre-curing on the green strength and tack of inner liner compounds. Inner liner compounds are the innermost layers of the tyre carcass, prepared mainly from butyl and halobutyl rubber compounds, and are responsible for protecting the carcass from air and moisture penetration. For radial tyre application, a 100–300% elongation range for a green butyl compound should suffice. Halobutyl rubbers lack green strength and tack as compared to NR. It has been reported that EB irradiation-induced partial cross-linking imparts adequate green strength to the compound without affecting the tyre performance [7]. For BIIR, upon radiation pre-curing, the green strength value reaches a maximum at ~25 kGy followed by gradual reduction whereas for CIIR the values continuously increase with increasing radiation dosage. This can be attributed to the balance between cross-linking and chain scission for BIIR while a dominant crosslinking for CIIR induces scorch. Furthermore, the effect of EB irradiation on tack on these compounds was studied. The initial reduction in tack for both specimens can be attributed to sample handling followed by a subtle rise in tack values finally resulting in gradual decrease as curing sets in. For BIIR, the optimal pre-curing dosage value was found to lie between 10 and 15 kGy [7]. Hunt and Alliger studied different tyre compounds irradiated with EB. Table 3 shows the effect of EB irradiation precure on inner liner stock composed of butyl rubber, SBR 1502 and NR. As expected, the green strength gradually increases with increasing irradiation dosages. Similarly, a chafer strip compound composed of NR and SBR 1502 is also shown in Table 3. Between radiation dosages 0 and 200 kGy, the green strength also increased progressively with increasing cross-linking. Thereby, it can be said that the green strength which synthetic rubbers lack with respect to NR can be compensated via radiation pre-curing. This improvisation can prove beneficial as it can definitely reduce the raw material cost parallelly providing benevolent positive aspects such as better aging resistance [55]. Table 3 Effect of EB irradiation of different tyre components [7] Inner liner

Radiation dosage (kGy)

Peak Lbs

0

Green Strength

50

100

150

200

3.0

15.3

20.0

22.9

33.8

22.8

35.1

42.1

52.0

58.2

Chafer Strip Green Strength

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P. S. Banerjee et al.

Table 4 Variation of green strength of tyre compounds upon EB irradiation [2] Property

CIL 0 KGy

CTR 0 KGy

CIL 50 KGy

CTR 50 KGy

CIL 100 KGy

CTR 100 KGy

CIL 150 KGy

CTR 150 KGy

Green Strength (kg/cm2 )

46

–

91

–

135

–

138

–

Chakraborty et al. performed similar tests on NR and NR/SBR blends for tyre formulation and also on tread and inner liner samples, and their results are perfectly coherent with the reports of Mohammad and Walker. The green strength increases progressively accompanied by an initial rise in tack values followed by gradual reduction for reasons explained earlier. In addition, for tread and inner liner samples designated as CTR and CIL for tread and inner liner compounds, respectively, the effect of EB irradiation is provided in Table 4 [2]. Hence, a close control over the radiation dosages is absolutely essential for attaining an adequate green strength while maintaining sufficient building tack.

5.4 Mechanical Properties At this point, we have some fundamental understanding of the effect of EB irradiation on different facets such as cross-linking density, green strength, tack, and gel content of rubber compounds. These properties will have some impact on the mechanical properties of these materials. However, among the wide array of mechanical properties, we will be directly going forward to look into the most crucial and relevant ones which have a direct effect on the tyre performance. Literature evidence of the effect of EB irradiation on different static mechanical properties vide. Tensile strength, %EAB, hardness, etc., are readily available at our disposal. Studies reveal that the tensile strengths of NR and NR/SBR samples gradually increase with increasing radiation dosages which is only expected as the degree of curing and the gel content gradually increases. This results in the formation of stiffer C–C bonds having bond energy (~85 kcal/mol) as compared to C–S links. However, some studies reveal deviation from this trend as the tensile strength value reaches a peak followed by gradual reduction with increasing radiation. One of the explanations for this phenomenon can be due to the formation of excessive C–C cross-links with increasing radiation dosages that render the rubber compound so stiff that it becomes less compliant to the applied load. This leads to a reduction in the tensile strength [2, 56]. Furthermore, we can also comment on the possible explanation of the tensile strength reduction because with increasing dosages, if the chain scission yield value exceeds that of cross-linking it might lead to the deterioration of the mechanical properties. The data reported by Chakraborty et al. about the static mechanical properties of different rubber compounds at different radiation

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dosages are shown in Table 5. The stark contrast to the data reported by Chakraborty et al. can be observed if we take a look at Fig. 7a as reported by Manshaie et al. for NR/SBR blends. In this case, we see that even up to 400 kGy radiation dosages, the tensile strength of both aged and unaged rubber samples increases progressively. However, another important observation is that the change in the tensile strength for aged rubber samples is a bit lower as compared to sulfur-cured ones. This is indicative of greater thermal stability of the EB-cured samples as the formation of stable C–C links hinders the macromolecular rotation and greater temperature is required for such motion [38]. Figure 7b similarly gives us an idea about the %EAB for both aged and unaged NR/SBR blends as a function of irradiation dosages. The formation of a network structure and subsequent chain stiffening is the fundamental cause behind the reduction of %EAB. This kind of result is quite universal and can be found quite widely in the literature not only for NR/SBR blends but almost for all kinds of rubbers and rubber blends [2, 30, 38, 56]. The possible domination of chain scission at higher dosages of EB irradiation is also reflected in the tensile strength curve for radiation-cured E-SBR samples as shown in Fig. 7c [30]. The inherent static mechanical properties of SBR have room for improvement and the green compound has a tensile strength of ~0.5 MPa which can be significantly improved by blending with NR. The blending imbibes some strength into the sample via the inherent green strength and strain-induced crystallization behavior of NR [30, 80]. But whatever the blend we are using, the radiation dosages have to be optimized to obtain a nice synergy between different Table 5 Effect of EB irradiation dosages on the static mechanical properties of different rubber compounds [2] Radiation dose (KGy)

Tensile strength (kg/cm2 )

%EAB

NR-based sample 0

216

540

20

220

520

50

232

500

100

220

480

0

217

530

20

231

520

50

245

500

100

221

470

0

256

600

20

209

500

50

209

450

100

232

500

NR + SBR (70:30)

NR + BR (70:30)

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Fig. 7 Comparison between a Tensile Strength and b %EAB of sulfur and EB-cured NR/SBR blends [38] (Reprinted with permission from Ref. [38], copyright (2011), Radiation Physics and Chemistry). c Tensile strength of EB-irradiated SBR samples as a function of cross-link density. The line is the curve was added as a guide to the eye and to suggest a position of the maximum of the curve [30] (Reprinted with permission from Ref. [30], copyright (2018), Radiation Physics and Chemistry)

Table 6 Tensile Strength and %EAB of irradiated halobutyl-based inner liner compounds [7] Inner liner compound Dosage (kGy) Bromobutyl rubber

%EAB Chlorobutyl

0

7.5

10.0 15.0 20.0 25.0 30.0 50.0

8.7

7.5

7.6

7.5

7.5

7.2

7.3

420 400 390

380

Tensile strength, MPa 8.4

390

400

390

390

6.4

6.4

6.3

6.0

6.0

5.7

350 320 310

300

300

290

280

280

Tensile strength, MPa 7.0 %EAB

6.7

properties. Greater care has to be taken in the case of butyl and halobutyl samples as these rubbers, due to their saturated backbones, are already prone to chain scission. Table 6 shows the results obtained for the tensile strength and %EAB for bromobutyl and chlorobutyl-based inner liner compounds, and we can see a steady reduction of both with increasing radiation dosages. Although the tensile strength of inner liners is not of great importance, a minimal level of strength is necessary for holding the air pressure and having adequate service life [7]. Furthermore, the –Sx – type cross-links, although weaker, contribute to the higher tensile strength in a vulcanizate as compared to the EB-cured components. Hence, the tensile and tear strength of compounds are bound to decrease gradually for EBcured samples as the cross-linked bonds get shortened and the stress transmission of the chain segments is hindered [40]. Despite the significance of the static mechanical properties, tyres have to perform desirably under dynamic loading conditions on actual road surfaces, and hence, the dynamic mechanical properties are critical. The dynamic mechanical properties are basically governed by the viscoelastic properties of the rubber compound. As it is pretty well known that the rubber chain entanglements contribute to the viscous behavior while the free chains and the cross-links contribute to the elastic nature [81]. The fundamental understanding of dynamic mechanical properties is obtained by applying a sinusoidal load to the rubber compound to mimic the cyclic loading

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condition experienced during tyre operation. The modulus values experienced in such an analysis are twofold: the elastic modulus (E , ) and the loss modulus (E ,, ). The elastic modulus or the storage modulus is the strain response of the material that is elastic and it is basically in phase in the applied strain, and the loss modulus lags by some phase angle δ arising from the viscous dissipation effect. This gives rise to a damping factor or tan δ expressed as: tan δ =

E ,, E,

(19)

This damping factor is a critical term essential for determining innumerable tyre performance behavior such as rolling resistance and wet grip. These experiments can be performed not only by strain sweep experiments but also by frequency and temperature sweeps to achieve a wholesome picture of the compound’s attributes [82]. From the studies of George et al., the elastic modulus of SBR/NR blends is observed to decrease with increasing temperature due to the reduction of the compound stiffness. Again, the loss modulus exhibits maximum at temperatures corresponding to the glass transition temperature of the phases present, and hence, the dependence of glass transition temperature on the dynamic properties is observed [80]. However, the analysis of the dynamic properties and the effect of EB irradiation could become cumbersome and difficult to understand unless we pick some targeted areas where these properties become relevant. The dynamic properties are not only affected by glass transition temperature, but for filled rubber compounds, the polymer–filler interaction also plays a crucial part [83]. Henceforth, it would be convenient to pick up some of these individual aspects and try to get an idea of how the dynamic properties change in an actual tyre operation.

5.5 Polymer-Filler Interaction Whenever we are studying tyre performance aspects and rubber compounds to be used in the tyre industry, the polymer–filler interaction studies cannot be ignored. Fillers play a crucial role in determining the actual performance behavior of the tyre. Different kinds of filters exist in the tyre industry such as reinforcing fillers, non-reinforcing fillers, and cheapening agents. But for our current study, we will be solely focusing on the reinforcing ones, i.e., carbon black and silica [84]. Carbon blacks are the most widely used and most common reinforcing fillers in the tyre industry. Different grades of carbon black are in use, and apart from the N880 and N-990 thermal blacks, all of them are considered to be reinforcing and help in improving tensile strength, stiffness, and wear and tear resistance of the rubber compound [85]. The basic source of polymer–filler interaction in the carbon black filled rubber is the surface adsorption whereby a thin rubber film forms on the carbon

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black surface resulting in the formation of ‘bound rubber.’ The bound rubber formation is a direct result of the surface activity of the filler, and a greater surface activity would imply a greater polymer–filler interaction [86]. Another aspect of carbon black reinforcement is the ‘occluded rubber’ where small rubber domains get detached from the bulk rubber phase and get entrapped in the aggregate structure of the carbon black. The occluded rubber loses its rubber-like properties and becomes a part of the filler. At low strains, these filler networks remain intact and the occluded rubber domains remain shielded from the impending stresses [84, 87]. Another important reinforcing filler, which the modern tyre industries prefer, is precipitated silica. However, incorporating silica into a rubber compound is extremely challenging due to the polarity difference between the two. Therefore, silane coupling agents are employed which infuses a silanization reaction and thereby chemically modifying the silica surfaces. This also beneficiates the formation of chemical links between the rubber and the filler, and the rubber-filler interaction is much higher as compared to carbon blacks [88]. Without modification of silica, the hydrogen bonding between the filler particles promotes very high filler-filler interactions, imparting a very high storage modulus at low dynamic strains but leading to a rapid decrease in the modulus values at high dynamic strains which is commonly termed the Payne effect [88]. To reduce the Payne effect, a strong polymer–filler interaction is essential and EB irradiation can play a crucial role by imparting surface modifications to the filler to enhance the surface activity and formation of strong bonds between the rubber and the filler. Furthermore, EB irradiation-induced strong C–C cross-link networks prevent the polymer–filler separation at high strain values [82, 88, 89]. However, the use of EB-modified fillers in a tyre compound formulation is yet to gain popularity on the industrial scale. Wu et al. reported the interaction of NR filled with irradiationmodified carbon blacks which show the presence of greater concentrations of oxygencontaining functional groups on the surface of carbon black particles as compared to unmodified ones (Fig. 8). The irradiation modification of carbon black particles was performed by subjecting the particles to 50 kGy of irradiation dosages in each pass on a conveyor belt. The total irradiation dosage on the sample was determined by the number of passes and the dosages varied from 100 to 700 kGy [89]. A particle size reduction of carbon black is also affected which would lead to a significant increase in the specific surface area and thereby increase the bound rubber content with increasing irradiation dosages. However, continuous increase of the irradiation dosage is also not desirable as at around 600 kGy dosages the particle size uniformity of the particles decreases thereby reducing the bound rubber content. Expected results are also observed in the dynamic mechanical analysis as the irradiated carbon blackfilled NR resulted in the reduced storage modulus due to lesser elastic filler-filler interaction but also reduced the loss modulus as the bound rubber content is greatly enhanced. Thus, within a controlled dose limit, the polymer–filler interaction can be enhanced via EB irradiation but excess irradiation dosages bring about detrimental effects [89]. Shanghumaraj and Bhowmick studied the effect of EB irradiation on radiation-modified dual-phase fillers and mixed results are obtained. Although the irradiation-modified fillers reduced the Payne effect and also reduces the tan δ values at high temperatures, in the presence of trimethylolpropane triacrylate

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Fig. 8 Schematic diagram showing modification of carbon black producing more oxygencontaining groups and better uniformity by EB irradiation [89] (Reprinted with permission from Ref. [89], copyright (2015), Radiation Physics and Chemistry)

(TMPTA) in the formulation, the EB irradiation generated a secondary structure of fillers which increased the Payne effect [82]. Thus, we can say that not only irradiation dosage but also compound formulation also has to be studied extensively before successively implementing EB technology. Furthermore, the formation of stiff C–C networks via EB irradiation curing also reduces the mobility of the polymer chains and thereby prevents the breakage of the rubber-filler three-dimensional networks which impart a high storage modulus to the compound [40, 90]. Finally, with all these understanding we obtained till now, we will straightaway dive into the next most important aspect of tyre manufacturing technology which is the ‘magic triangle’ of tyres in order to actually implement an understanding of EB irradiation on real-life tyre performance.

6 Impact on the ‘Magic Triangle’ of Tyres ‘Magic Triangle’ study of tyres is not merely a study to optimize abrasion resistance, wet grip, and rolling resistance, but also essential for developing a sustainable, clean and green, and efficient technology [50, 91, 92]. Every aforementioned property and even those outside the scope of this chapter directly or indirectly affects the magic triangle. A typical ‘magic triangle’ diagram encompassing the three determining aspects of tyre performance is shown in Fig. 9.

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Fig. 9 Magic triangle of tyre

6.1 Rolling Resistance The first and foremost topic of discussion will be the rolling resistance which correlates with the fuel economy of the vehicles. This is a crucial property of a tyre and the target remains to reduce the rolling resistance within a permissible limit to enhance the fuel economy and energy requirements during vehicle operation [84, 93]. The energy which is lost by a tyre for every unit of distance traveled is termed the ‘rolling loss,’ and the higher the rolling loss implies a higher rolling resistance of a tyre [93]. The prime factors contributing to the tyre rolling resistance are the heat buildup, polymer–filler interactions, hysteresis loss or damping factors, etc. [40, 94]. Furthermore, during tyre operation, running speed and temperature also have some important effects on the rolling resistance values. In dynamic conditions, the temperature of a tyre is bound to increase via viscous heat dissipation, and this effect coupled with a decreasing tan δ value at high speeds effectively results in an initial reduction in the rolling resistance. After a certain speed, a gradual rise in the rolling resistance is exhibited with a further rise in the temperature and speed due to rapid strain cycles and centrifugal forces, finally resulting in tyre failure [94, 95]. The filler-filler interactions also have an important role to play in this aspect as the filler particles tend to agglomerate into a three-dimensional framework. This network is elastic and as it breaks down, more will be the damping effects at higher strain values indicating a higher rolling resistance [84, 94, 95]. The most common laboratory predictor of rolling resistance is the tan δ value at 60 °C. Jing et al. reported tan δ for tread samples composed of NR/SBR vulcanizates at varying EB irradiation dosages. The post-cured compression molded samples were subjected to 50 kGy of irradiation dosages in pass accounting for total dosage values between 100 and 600 kGy. It is evident from Fig. 10 that at radiation dosages ~300 kGy, tan δ actually decreases which may be attributed to the stronger network formation as well as the radiation-induced modifications of carbon black surface. Such modifications prevent the breakdown of the polymer–filler network structure thereby reducing the losses [40]. Similar results of greater filler–polymer interactions reduce the damping at high temperature thereby reducing the rolling resistance as reported by Shanghumaraj et al. They reported that EB modification of fillers reduces the tan δ value and enhanced storage modulus at higher temperature (~70 °C), which is indicative of

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Fig. 10 tan δ versus temperature curve for post-cured EB-irradiated NR/SBR vulcanizates [40] (Reprinted with permission from Ref. [40], copyright (2013), Radiation Physics and Chemistry)

lower rolling resistance. This phenomenon can be attributed to the greater polymer– filler interaction as the EB irradiation induces surface modification of the fillers resulting in an increased bound rubber volume [82].

6.2 Abrasion Resistance The second crucial aspect of the ‘magic triangle’ is abrasion resistance. This property finds its importance since tyres have to run in multifarious operating conditions and have to be in dynamic contact with the road surface. As the tyre runs, it is in a mode of frictional sliding arising from the tangential friction forces acting on it via repetitive loading. This leads to the wear of the compound through the initial detachment of small rubber particles [96]. This process is followed by the formation of abrasion ridges on the polymer surface resulting in large particle debris. Finally, this results in fatigue crack growth in the rubber due to dynamic loading mechanisms [97]. Here, it is quite expected that NR samples would be more resistant to such fatigue crack growths due to strain-induced crystallization. Synthetic rubbers like BR and SBR lack in this aspect. Thomas reported this exact phenomenon when he studied the dynamic cut growth for NR samples [98]. Another mode of abrasion is the mechanochemical mode or smearing, common for filled rubbers which have their chains stiffer and reinforced due to the reinforcing effects of the filler. Hereby, a general decomposition of the molecular network into a low molecular weight material takes place. BR is inherently resistant to smearing mechanisms and the property can also be widely improved by incorporating reinforcing fillers like carbon black [97].

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Table 7 Abrasion volume loss for EB-cured NR/SBR blends [40] Dose (kGy)

0

100

200

300

400

500

600

Abrasion volume (cm3 )

0.1169

0.1049

0.0996

0.1038

0.1084

0.1014

0.0965

The abrasion resistance is calculated based on abrasion loss or DIN volume loss, lower the value of which implies a greater abrasion resistance. The volume loss is expressed as [40, 96]: V =

Δ m × S0 ρ×S

(20)

where V is the volume loss in mm3 , ρ is the sample density (mg/mm3 ), S 0 is the nominal abrasive power value (200 mg), and S is the average abrasive power (mg). Table 7 shows the abrasion volume loss of NR/SBR blends for varying EB irradiation dosages. The loss volume decreases with increasing irradiation dosages which can be attributed to the greater bond energy of the C–C links, increase in the cross-link density, and enhanced polymer–filler interaction [40]. Similar findings were also reported by Chakraborty et al. as they performed such tests on commercial tyre treads [2]. Here, it should be mentioned that tyre treads are the primary candidates to succumb to abrasion via wearing as they come in direct road contact. Hence, adequate abrasion resistance is a prerequisite for optimal tyre performance [97]. Henceforth, although in the previous section it was established that sulfur cross-links contribute to the tensile strength, the wear and tear resistance is preferably beneficiated by the stronger C–C links from which it can be proposed that EB irradiation can be benevolent for tyre longevity.

6.3 Wet Grip The final aspect of the ‘magic triangle’ and which in tandem with the previous two governs the overall tyre performance is wet skid resistance or wet grip. This factor is directly correlated to the driving safety in wet road conditions, and it is dictated by the traction generated between the tyre and the road [50, 94]. In the case of wet or icy road conditions, wet skid resistance is an important requirement for tyre operation. For example, in icy road conditions, near about 0 °C, the ice surface is unstable and will lead to a reduction in the friction between the tyre and the surface. This is because, at 0 °C, the cohesive energy of ice is lower than the tyre to ice adhesive energy [94]. For the assessment of a tyre wet grip, we generally analyze the tan δ or damping factor values at 0 °C as the grip factor is more critical at low temperatures and icy road conditions. Here, contrary to what we saw in the rolling resistance case, the tan δ value should be high to ensure an adequate tyre to surface grip [50, 83, 99]. In this regard, the EB irradiation dosages have to be critically controlled as excessive

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C–C links reduce the chain mobility and thereby the loss of energy or tan δ [50, 90]. EB irradiation effects on the wet skid resistance by Jing et al. give us a clear idea that wet skid resistance is not affected adversely with increasing dosages. However, when safety is a concern, we should consider every minute fluctuation very closely. Therefore, it is reported that below 200 kGy dosages, the wet skid resistance remains constant, but with increased dosages, the value drops. One of the reasons for this is already mentioned above. Another contributor is the polymer–filler interaction which is enhanced due to stronger networks at higher irradiation dosages thereby reducing the wet grip [40]. Ahmadi-Shooli and Tavakoli performed similar studies on SBRENR blends for 100 kGy irradiation dosages. They reported for such low irradiation dosages; the wet grip shows significant improvement as compared to sulfur-cured samples due to lower degrees of cross-linking. Furthermore, the rolling resistance exhibits significant reduction while maintaining the wet skid which is essentially the requirement of every tyre industry [90]. Finally, we can comment that magic triangle optimization is the most tricky and challenging aspect of tyre manufacturing as a control of the three aforementioned properties has to be maintained simultaneously. For example, if we attempt to reduce the abrasion loss by inducing greater percentages of C–C cross-links, the wet skid resistance is going to be poorer for reasons explained earlier. Similarly, reducing rolling resistance will improve the mileage and fuel efficiency, but it would again adversely affect the wet grip. Again, focusing on the wet grip will also lead to greater wear and tear of the tyre. Jing et al.’s study gives us a great deal of understanding in this regard as we experimentally proved that at lower irradiation dosages the three properties remain at an optimal level while maintaining synergy between them [40].

7 Conclusion EB irradiation technique has a great potential to be benevolent for tyre manufacturing. The environmental aspect and a drive toward a cleaner and greener technology can be successfully addressed as this technology is a quick and energy-efficient process. Furthermore, the requirement of numerous chemical ingredients in the conventional curing processes can also be avoided in this technology. Being a cost-efficient and economic process, tyre industries have derived good benefits from this technology [7]. The positive impacts of irradiation processes on the properties of tyre compounds such as cross-link density, gel content, green strength and tack, and static and dynamic mechanical properties cannot be overlooked. Based on the performance requirements and the aforementioned beneficial aspects, there lies a tremendous possibility for EB irradiation technology to become a utilitarian technology. In addition, a profound impact on the magic triangle of tyre performance is also expected, though extensive experimental studies targeted toward the effect of EB irradiation on the tyre performance are still required. Whatever be the results of such a study, the authors believe that this technology, in every possibility, can be the one to conventional curing technologies existing today.

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51. Manoharan P, Naskar K (2016) Exploring a highly dispersible silica–elastomer composite for tire applications. Journal of Applied Polymer Science 133(24) 52. Salmazo L et al (2016) Influence of the irradiation dose in the cellular structure of natural rubber foams cross-linked by electron beam irradiation. Ind Crops Prod 89:339–349 53. Sombatsompop N (1998) Practical use of the Mooney-Rivlin equation for determination of degree of crosslinking of swollen NR vulcanisates. J Sci Soc Thailand 24(3):199–204 54. Manik S, Banerjee S (1969) Determination of chemical cross-links in rubbers. Die Angewandte Makromolekulare Chemie: Applied Macromolecular Chemistry and Physics 6(1):171–178 55. Hunt J, Alliger G (1979) Rubber-application of radiation to tire manufacture. Radiation Physics and Chemistry (1977) 14(1–2):39–53 56. Basfar A, Abdel-Aziz M, Mofti S (2002) Influence of different curing systems on the physicomechanical properties and stability of SBR and NR rubbers. Radiat Phys Chem 63(1):81–87 57. Stephens HL (1999) The compounding and vulcanization of rubber. In: Rubber technology. Springer, pp 20–58 58. Banik I, Bhowmick AK (2000) Effect of electron beam irradiation on the properties of crosslinked rubbers. Radiat Phys Chem 58(3):293–298 59. Heinze H et al (1962) The temperature shift of the second order transition of natural rubber by crosslinking. Rubber Chem Technol 35(3):776–793 60. Wang Q, Wang F, Cheng K (2009) Effect of crosslink density on some properties of electron beam-irradiated styrene–butadiene rubber. Radiat Phys Chem 78(11):1001–1005 61. Shaltout N (2009) Effect of electron beam irradiation and degree of boric acid loading on the properties of styrene-butadiene rubber. React Funct Polym 69(4):229–233 62. Pearson DS, Skutnik BJ, Böhm GG (1974) Radiation crosslinking of elastomers. I. Polybutadienes. Journal of Polymer Science: Polymer Physics Edition 12(5):925–939 63. Dossin LM, Graessley WW (1979) Rubber elasticity of well-characterized polybutadiene networks. Macromolecules 12(1):123–130 64. French DM (1977) Crosslink density from sol-gel contents. Journal of Macromolecular Science—Chemistry 11(3):643–666 65. Miller DR, Macosko CW (1976) A new derivation of postgel properties of network polymers. Rubber Chem Technol 49(5):1219–1231 66. Langley NR (1968) Elastically effective strand density in polymer networks. Macromolecules 1(4):348–352 67. Alemán J et al (2007) Definitions of terms relating to the structure and processing of sols, gels, networks, and inorganic-organic hybrid materials (IUPAC recommendations 2007). Pure Appl Chem 79(10):1801–1829 68. Charlesby A, Pinner S (1959) Analysis of the solubility behaviour of irradiated polyethylene and other polymers. Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences 249(1258):367–386 69. Olejniczak J, Rosiak J, Charlesby A (1991) Gel/dose curves for polymers undergoing simultaneous crosslinking and scission. International Journal of Radiation Applications and Instrumentation. Part C. Radiation Physics and Chemistry 37(3):499–504 70. Mitra S et al (2010) Electron beam crosslinked gels—preparation, characterization and their effect on the mechanical, dynamic mechanical and rheological properties of rubbers. Radiat Phys Chem 79(3):289–296 71. Bhowmick AK, De PP, Bhattacharyya AK (1987) Adhesive tack and green strength of EPDM rubber. Polym Eng Sci 27(15):1195–1202 72. Amnuaypornsri S, Sakdapipanich J, Tanaka Y (2009) Green strength of natural rubber: the origin of the stress–strain behavior of natural rubber. J Appl Polym Sci 111(4):2127–2133 73. Hamed G (1981) Tack and green strength of elastomeric materials. Rubber Chem Technol 54(3):576–595 74. Grechanovskii V, Ivanova L, Poddubnyi IY (1973) Nature of the cohesional strength of synthetic cis-polyisoprene SKI-3 and carbon black compounds based on it. Rubber Chem Technol 46(5):1234–1238

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75. Grechanovskii V, Poddubnyi IY, Ivanova L (1974) Molecular structure and macroscopic properties of synthetic cis-poly (isoprene). Rubber Chem Technol 47(2):342–356 76. Kawahara S et al (2002) Effect of gel on the green strength of natural rubber. Rubber Chem Technol 75(4):739–746 77. Bhaumik T, Gupta B, Bhowmick A (1987) Tack and green strength of blends of bromobutyl and EPDM rubbers. I. Unfilled gum blends. The Journal of Adhesion 24(2–4):183–198 78. Buckler E et al (1978) Green strength in emulsion SBR. Rubber Chem Technol 51(5):872–887 79. Skewis JD (1966) Self-diffusion coefficients and tack of some rubbery polymers. Rubber Chem Technol 39(2):217–225 80. George SC et al (2000) Styrene–butadiene rubber/natural rubber blends: morphology, transport behavior, and dynamic mechanical and mechanical properties. J Appl Polym Sci 78(6):1280– 1303 81. Ulmer J (1996) Strain dependence of dynamic mechanical properties of carbon black-filled rubber compounds. Rubber Chem Technol 69(1):15–47 82. Shanmugharaj A, Bhowmick AK (2003) Dynamic mechanical properties of styrene-butadiene rubber vulcanizate filled with electron beam modified surface-treated dual-phase filler. J Appl Polym Sci 88(13):2992–3004 83. Roland CM (1990) Dynamic mechanical behavior of filled rubber at small strains. J Rheol 34(1):25–34 84. Zhang P, Morris M, Doshi D (2016) Materials development for lowering rolling resistance of tires. Rubber Chem Technol 89(1):79–116 85. Waddell WH, Evans LR (1996) Use of nonblack fillers in tire compounds. Rubber Chem Technol 69(3):377–423 86. Wolff S (1996) Chemical aspects of rubber reinforcement by fillers. Rubber Chem Technol 69(3):325–346 87. Drozdov AD, Dorfmann A (2002) The payne effect for particle-reinforced elastomers. Polym Eng Sci 42(3):591–604 88. Sarkawi SS, Dierkes WK, Noordermeer JW (2012) Natural rubber-silica combinations for low rolling resistance truck tyre treads. Rubber world 2012:26–31 89. Wu Y et al (2015) Improved dynamic properties of natural rubber filled with irradiationmodified carbon black. Radiat Phys Chem 111:91–97 90. Ahmadi-Shooli S, Tavakoli M (2019) A comparative study of the dynamic-mechanical properties of styrene butadiene rubber/epoxidized natural rubber dual filler nanocomposites cured by sulfur or electron beam irradiation. Journal of Macromolecular Science, Part B 58(7):619–633 91. Le Gal A et al (2008) Modelling of sliding friction for carbon black and silica filled elastomers on road tracks. Wear 264(7–8):606–615 92. Sarkar P, Bhowmick AK (2018) Sustainable rubbers and rubber additives. J Appl Polym Sci 135(24):45701 93. Schuring D, Futamura S (1990) Rolling loss of pneumatic highway tires in the eighties. Rubber Chem Technol 63(3):315–367 94. Grosch K (1996) The rolling resistance, wear and traction properties of tread compounds. Rubber Chem Technol 69(3):495–568 95. Hall DE, Moreland JC (2001) Fundamentals of rolling resistance. Rubber Chem Technol 74(3):525–539 96. Tabsan N, Wirasate S, Suchiva K (2010) Abrasion behavior of layered silicate reinforced natural rubber. Wear 269(5–6):394–404 97. Gent A, Pulford C (1983) Mechanisms of rubber abrasion. J Appl Polym Sci 28(3):943–960 98. Thomas A (1958) Rupture of rubber. V. Cut growth in natural rubber vulcanizates. Journal of Polymer Science 31(123):467–480 99. Takino H et al (1997) Viscoelastic properties of elastomers and tire wet skid resistance. Rubber Chem Technol 70(4):584–594

Chapter 3

Electron Beam Irradiation-Induced Compatibilization of Poly (Lactic Acid)-Based Blends Ashish Kumar and Venkatappa Rao Tumu

1 Introduction Numerous polymeric materials are available in nature. Lignin (major constituent in all plants), cellulose, starch (in corn, potatoes), natural rubber (poly-cis-isoprene), proteins, and DNA are a few examples. The degree of sophistication of natural processes to synthesize these materials is far beyond the conventional synthetic polymerization processes, such as polymerizing amino acids into protein at ambient temperature and pressure. These natural materials cannot serve humankind directly for commodity and engineering applications as conventional petrochemical-based plastic materials. This is because conventional thermoplastic polymers/plastics have more heat stability than their natural counterpart and are usually processed via molten state into any complex desired shape as per applications; however, natural polymeric materials such as lignin, cellulose, and starch cannot be heated into the molten state due to thermal degradation or decomposition. There are ample advantages of the conventional polymer/plastics over their natural counterparts. These include low cost, simple chemical structure, good barrier properties for end-use applications, ease to process into complex shapes, excellent performance, etc. [1, 2]. However, petrochemical-based non-biodegradable polymers/plastics have acute environmental issues that severely affect the life of wild species and also spoil the scenes due to their virtually endless applications. The increasing insecurity of petrochemical resources, increasing oil prices, and growing concerns about greenhouse gas emissions are thought-provoking issues. Hence, in the past few decades, the synthesis of bio-based A. Kumar (B) Department of Physics, Indian Institute of Technology Roorkee, Roorkee 247667, Uttarakhand, India e-mail: [email protected] V. R. Tumu Department of Physics, National Institute of Technology, Warangal 506004, Telangana, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. R. Chowdhury (ed.), Applications of High Energy Radiations, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-9048-9_3

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Fig. 1 Production of biopolymers from different routes [4]

and biodegradable polymers from renewable resources has gained great interest in both industries and academia. Further, as schematically represented in Fig. 1, the biopolymers, based on their source of origin, can be classified into three major categories [3, 4]: natural polymers, which are obtained by direct extraction from biomass, viz. cellulose, lignin, starch, and proteins; synthetic polymers which are retrieved by microbial fermentation of biomass and then extraction, viz. poly (hydroxyalkanoates) (PHA); and synthetic polymer synthesized from natural monomers, viz. poly (lactic acid) (PLA). Moreover, synthetic biopolymers can be processed by conventional processing equipment and are known as bio-plastic. The synthetic biopolymers/plastics are not necessarily biodegradable. The biodegradable polymer/plastic must be mineralized completely by the action of microorganisms in the natural environment (i.e., landfill) and returned to the environment within a short time ( 50%) with improved crystallinity showed

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Fig. 7 TEM micrographs of PLA/EGMA blends stained by RuO4 vapor [a (95/5) PLA-L/EGMA prepared with screw rotation speed of 30 rpm, L95-30, b (80/20) PLA-L/EGMA prepared with a screw rotation speed of 30 rpm, L80-30, c (80/20) PLA-L/EGMA prepared with a screw rotation speed of 200 rpm, L80-200, and d (80/20) PLA-H/EGMA prepared with a screw rotation speed of 200 rpm, H80-200]. The figure is adapted from [38], copyright © 2008, with the permission of Elsevier

an increment in the tensile modulus and a severe decrement in the elongation at break. Wang and Hillmyer [46] have reported the PLLA/ LDPE (Low-density polyethylene) binary and PLA/LDPE/PE-b-PLLA ternary blends. In the first step, they synthesized a diblock copolymer PE-b-PLLA from L-lactide and PE-OH (hydroxylterminated PE) through the ROP process. The weight ratio of PLLA and LDPE was fixed (i.e., 80:20) in the prepared binary blend. Moreover, PE-b-PLLA block

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copolymer was further incorporated into a prepared binary blend with a weight percentage that varied from 2 to 10 wt% to prepare PLLA/LDPE/PE-b-PLLA ternary blend. They have analyzed the effect of different concentrations of added block copolymer on the morphology and other properties of the prepared blend. The morphology results showed that the PLLA/LDPE binary blend exhibited a large average particle size, i.e., 25.7 μm of dispersed LDPE phase, and wide size distribution (4–150 μm) due to poor compatibility of the blend components. The PLLA/LDPE blend showed a decrement in the tensile modulus, tensile strength, and impact resistance; however, it showed an increment in the elongation at break. The addition of 2 wt% PE-b-PLLA copolymer to the PLLA/LDPE blend significantly reduced the particle size of the dispersed LDPE phase to 3.5 μm. It narrowed down the particle distribution (1–15 μm). This is because the PE-b-PLLA block copolymers acted as a compatibilizer and increased the interfacial adhesion of the blended polymers. As the wt% of block copolymer increases, the average particle size of the dispersed phase further decreases. The PLLA/LDPE/PE-b-PLLA ternary blend has shown increment in the impact strength, elongation at break, and slight decrement in the tensile modulus and tensile strength. Choi et al. [47] prepared a PLLA/PCL blend with a fixed weight ratio of 70:30. They further incorporated a random copolymer PLLA-co-ECL and block copolymer PLLA-b-ECL into PLLA/PCL blend and studied their effect on the morphology and degradation properties. It was noticed that the size of the dispersed PCL phase in the blend decreased noticeably from 10 to 3 μm with the incorporation of a 5 phr PLLA-co-ECL copolymer. This indicates that the PLLA-co-ECL copolymer acted as a compatibilizer and enhanced the miscibility of the PLLA/PCL blend. In the case of PLLA-b-ECL, the size of the dispersed PCL phase decreased with increasing block copolymer content up to 10 phr. Hence, both copolymers significantly enhanced the dispersed PCL phase and PLLA matrix compatibility in the resulting blend. Oyama [38] proposed the super though PLA/poly (ethylene-co-glycidyl methacrylate) (PEGM) blend, which was synthesized by a reactive blending of PLA with PEGM. He noticed a significant enhancement in the deformation at break and notched impact strength. It was asserted that during melt blending, the PLA-g-PEGM graft copolymers are formed at the interface, which compatibilized the resulting blend. It was further reported that the annealing of PLA/PEGM blend at 90 °C for 2.5 h increased the impact strength 50 times than neat PLA, which confirmed that the matrix crystallization has a significant contribution to toughening. The effect of crystallization of the polymer matrix on the impact toughness of the PLA/PCL blend was reported by Bai et al. [48]. The concentration of nucleating agent and dispersed PCL phase varied to achieve a wide range of matrix crystallinity (10– 50%) in the PLLA/PCL blend. The effect of different mold temperatures in injection molding on the crystallinity of prepared blends was also studied. The impact toughness of the resulting blend was tailored by controlling the crystallization of the PLLA matrix. They reported that the PLLA/PCL blend exhibits poor crystallinity in injection molding at both mold temperatures, i.e., 50 and 130 °C. However, incorporating the nucleating agent into PLLA and PLLA/PCL blend significantly increased the matrix crystallinity up to 50% at a mold temperature of 130 °C. The remarkable

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enhancement in the impact toughness up to 28.9 kJ/m2 was achieved for PLLA/PCL blend with a highly crystallizing PLLA matrix. It was also noticed a linear relationship between matrix crystallization and toughness. Further, the crystallization of the PLLA matrix has a significant contribution to improving the impact toughness rather than interfacial adhesion or average size and distribution of dispersed PCL phase. The various heat-resistant polymers and copolymers, viz. poly (carbonate), nylon, poly (oxymethylene), and poly (acrylonitrile–butadiene–styrene), etc., were also blended with PLA to improve its heat resistance property, as well as impact toughness [42], had used and blended. For instance, Zhang et al. [49] synthesized a ternary blend of PLA, poly (3-hydroxy butyrate-co-hydroxyvalerate) (PHBV), and poly (butylene succinate) PBS, which showed the balance stiffness and toughness along with improved heat deflection temperature (HDT) ~72 °C. The directly extracted biopolymers such as lignin and starch had also employed to make fully bio-based and biodegradable PLA blends for single-use applications. These biopolymers were used as cheap fillers to reduce the cost of PLA because these polymers are abundant in nature. However, blending these biopolymers with PLA is not straightforward because it results in poor miscibility of the resulting blend components, which leads to a dramatic loss of the mechanical and thermal properties of PLA. Hence, various approaches, i.e., chemical modification, use of plasticizers, use of compatibilizers and copolymers, were exploited to increase the miscibility of these polymers into the PLA matrix, resulting in a compatible blend with improved mechanical and thermal properties [50, 51]. Zhang and Sun [52] synthesized PLA and wheat starch blend compatibilized by maleic anhydride (MA). An initiator, L101, was employed to start the reaction among compatibilizer and polymers. The PLA/Starch blend was compatibilized with MA and L101, which showed significant improvement in the mechanical properties compared to the virgin PLA/starch blend. This signifies that the compatibilizer significantly improved the interfacial bonding between both polymer phases. Ferrri et al. [53] reported PLA and thermoplastic starch (TPS) blend is compatibilized with maleinized linseed oil. The weight percentage of TPS was fixed, i.e., 30%. However, compatibilizer, i.e., maleinized linseed oil, was added as parts per hundred from 0 to 8 phr. They found that the small amounts of compatibilizer positively improved the compatibility of matrix and dispersed phase as well as ductility of the resulting blend due to its combined compatibilization-plasticization effect. The PLA/TPS blend with 6 phr maleinized linseed oil exhibited the elongation at break up to 160% and high Charphy’s impact strength 9.5 kJ/m2 . The decrement in the tensile strength, modulus, and T g was noticed due to the plasticization effect of a compatibilizer. Xiong et al. [54] compatibilized the incompatible PLA/Starch blend by utilizing epoxide soybean oil (ESO). The compatibility of the resulting blend was increased by replacing the starch with MA-grafted starch. This is because the reaction possibility of ESO with MA-g-starch was more as compared to virgin starch. During the melt blending, the formation of an actual compatibilizer, i.e., PLA-ESO-Starch copolymer, leads to better compatibility and improved properties of the resulting blend.

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Li et al. [55] investigated the mechanical and thermal properties of the PLLA/lignin blend. The lignin was incorporated at four levels, i.e., 10, 20, 30, and 40 wt%, to prepare the PLLA/lignin blend. They noticed the decrement in the tensile strength, elongation at break, and thermal degradation temperature as the lignin contents reached 20 wt% in the resulting blend; however, Young’s modulus remained intact. The results also asserted the existence of intermolecular interaction among PLLA and lignin molecules. It was stated that the direct blending of PLLA and lignin would mix the blend components at a macroscopic level, not at the molecular level. Labidi et al. [30] reported the chemical modification of lignin by using the acetylation process and its blending with PLA. They found that the acetylated lignin has shown better miscibility and good compatibility with PLA in the PLA/Lignin blend as compared to unmodified lignin. Hence, significant improvement in the initial degradation temperature, hydrolytic degradation, and contact angle was noticed in the PLA/lignin blend having acetylated lignin. However, its mechanical properties decreased as the concentration of acetylated lignin increased from 5 to 20%. Chung et al. [56] have synthesized lignin-lactide copolymer and reported that it could be employed as a dispersion modifier in PLA/lignin-lactide composite. They observed improvement in the mechanical properties at a low lignin concentration, i.e., 0.9 wt%. However, the decrement in the mechanical properties was noticed as the concentration changed to 4.4 wt%. Recently, Liu et al. also synthesized and characterized dodecylated lignin-g-PLA graft copolymer and used it for significant toughening of PLA [57]. They incorporated this graft copolymer into PLA and improved elongation at break, tensile strength, and Young’s modulus at low lignin % (w/w) (i.e., 1.8%). However, a significant decrement was observed in the elongation at break, as the concentration of lignin-g-PLA graft copolymer was increased, with the increment in lignin % (w/w) (i.e., 4.5%).

2.2.1

E-beam Irradiation-Induced Compatibilization of PLA Blends

The E-beam irradiation-induced compatibilization of PLA blends is a green and straightforward physical approach to increase the interfacial adhesion between blend constituents through crosslinking and in situ formed graft copolymers [58–61]. This is because it does not entail cumbersome steps, viz. acetylation, grafting, copolymerization, etc., and toxic substances as in chemical compatibilization processes. The E-beam irradiation can be done on a large scale; hence, this technique could be used at an industrial scale to manufacture compatibilized PLA blends for diverse applications.

Pre-Irradiation of Polymers or Fillers Prior to Blending The pre-irradiation of polymers or fillers with high energy radiations (i.e., E-beam or gamma) is a simple, sustainable technique to modify their surfaces. This technique

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could also be employed to alter the physicochemical properties of additive polymers or fillers before blending with PLA. The E-beam irradiation of polymers could be executed for different purposes, viz. functionalization, free radical generation, grafting, crosslinking, etc. The yields of these processes depend on the irradiation doses and irradiation environment (i.e., in the air or in the inert gases). For instance, E-beam irradiation of polymers like PLA, polyethylenes (PEs) and lignin, etc., in the air gives rise to oxidative degradation of these polymers [26, 58, 62]. E Adem at el. reported radiation compatibilization of polyamide-6/polypropylene (PA6/PP) blends by E-beam pre-irradiation of polypropylene in the air [63]. The obtained results indicate that the pre-irradiation of PP leads to oxidative degradation of PP, not crosslinking. The oxidative degradation of PP due to E-beam irradiation resulted in the formation of peroxide and hydroperoxide groups, followed by the formation of polar groups. These induced polar groups of pre-irradiated PP help to improve the compatibility with the PA6 polar groups. The compatibility of PA6/PP blends was increased with increasing absorbed doses up to 100 kGy. The best results were acquired at a dose of 100 kGy. The E-beam irradiation compatibilization of PLA/lignin (PLA/LG) blends by pre-irradiation of lignin was also reported in the literature [58]. The lignin was irradiated to E-beam with three different doses, i.e., 30 kGy, 60 kGy, and 90 kGy, prior to blending with PLA. The compatibilized PLA/LG blends were prepared by blending pre-irradiated lignin in different percentages with PLA along with triallyl isocyanurate (TAIC) as a crosslinking agent. The pre-irradiation of lignin leads to the formation of poly-conjugated and peroxy radicals in the lignin matrix (Fig. 8). The generated free radicals are stable at high temperatures up to 453 K. During melt blending, these free radicals have initiated the reaction between pre-irradiated lignin and TAIC molecules. The resulting molecules were further reacted with PLA chains to form PLA-TAIC-Lignin crosslinked structure (Fig. 8). The in situ formed PLA-TAIC-Lignin crosslinked molecules act as an interface between PLA and preirradiated lignin and improve their compatibility. The compatibilized PL/LG blends having E-beam irradiated lignin were shown better mechanical, thermal and degradation properties as compared to immiscible PLA/LG blends having unirradiated lignin. This can be noticed in Table 2. PLA/LG-5% 30 kGy blend having 5 wt% of preirradiated lignin (number represents wt% of lignin in the blend) has shown significant improvement in the yield strength, notched impact strength, and elongation at break as compared to PLA/LG-5% blend having unirradiated lignin. Further, PLA/LG-20% blend with 20 wt % lignin has inferior mechanical properties. However, PLA/LG20% 30 kGy blend with 20 wt% pre-irradiated lignin has good mechanical properties. The compatibilized PLA/LG-5% 30 kGy blend have shown better thermal properties (i.e., glass transition temperature T g = 56.2 °C, melting temperature Tm = 152.0 °C) as compared to immiscible PLA/LG-5% blend (i.e., T g = 54.5 °C, T m = 148.1 °C). The compatibilized PLA/LG blends having pre-irradiated lignins are more stable to hydrolytic degradation as compared to PLA/LG blends with unirradiated lignins. PLA/LG-5% 30 kGy blend has shown 30.13% less weight loss as compared to pure PLA after 28 days of degradation assay. The pre-irradiation of lignin to higher doses, i.e., 60 and 90 kGy, leads to the formation of excess free radicals [64]. The

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Fig. 8 Possible reaction mechanism for in situ formations of PLA-TAIC-Lignin crosslinked structure Reprinted from [58], Copyright © 2018 with the permission of Elsevier

presence of excess free radicals leads to the self-crosslinking of lignin molecules and hampers the formation of PLA-TAIC-Lignin crosslinking structures. Hence, the compatibility of PLA/LG blends decreases with the addition of pre-irradiated lignin with high absorbed doses, which results in a decrement in the mechanical properties as absorbed doses of E-beam irradiated lignin increase from 30 to 90 kGy (Table 2). The decrement in thermal and hydrolytic degradation properties was also noticed as the absorbed doses of E-beam irradiated lignin increased from 30 to 90 kGy. Hence, 30 kGy is the best-absorbed dose for synthesizing PLA/LG miscible blend. The compatibilization of PLA and poly (butylenes adipate-co-terephthalate) (PBAT) was done by using E-beam pre-irradiated PLA as a compatibilizing agent [65]. The 5% E-beam pre-irradiated PLA along with EcovioTM were incorporated into PLA/PBAT blend to enhance the miscibility of constituents’ phases. The E-beam pre-irradiation of bamboo power prior to blending with PLA was also

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Table 2 Mechanical properties and Heat deflection temperature (HDT) of neat PLA, PLA/LG blends having unirradiated, and E-beam irradiated lignin Sample name

Yield strength (MPa)

Tensile modulus (MPa)

Elongation at break (%)

Notched Flexural izod impact modulus strength (MPa) (kJ/m2 )

HDT

PLA

65.00 ± 0.25

2300 ± 25 5.76 ± 0.44 2.65 ± 0.32 3767 ± 21 52.60 ± 0.45

PLA/LG-5%

28.00 ± 1.22

2933 ± 32 1.00 ± 0.04 1.28 ± 0.02 4052 ± 37 56.55 ± 0.35

PLA/LG-20%

6.00 ± 0.34 1422 ± 272 0.58 ± 0.19 0.97 ± 0.04 4305 ± 83 53.50 ± 0.35 50.00 ± 0.25

2001 ± 68 7.32 ± 0.34 2.89 ± 0.03 3516 ± 67 54.40 ± 0.30

PLA/LG-20% 37.00 ± 4.86 30 kGy

2826 ± 70 1.49 ± 0.35 2.31 ± 0.33 3802 ± 59 54.85 ± 0.15

48.00 ± 0.38

2021 ± 96 4.79 ± 0.77 2.41 ± 0.14 3414 ± 57 54.56 ± 0.55

PLA/LG-20% 22.81 ± 1.06 60 kGy

2767 ± 9 0.73 ± 0.20 2.15 ± 0.36 3807 ± 67 53.50 ± 0.60

43.81 ± 1.07

2292 ± 69 3.36 ± 0.37 2.06 ± 0.51 3714 ± 55 55.55 ± 0.55

PLA/LG-20% 25.75 ± 3.05 90 kGy

2366 ± 61 1.21 ± 0.17 2.29 ± 0.57 3681 ± 40 54.40 ± 0.20

PLA/LG-5% 30 kGy

PLA/LG-5% 60 kGy

PLA/LG-5% 90 kGy

Reconstructed from [58], Copyright © 2018 with the permission of Elsevier

reported to synthesize compatibilized PLA/bamboo powder composites [66]. The compatibilized PLA/bamboo composites having E-beam irradiated bamboo power showed good mechanical, thermal, and hydrolytic degradation properties compared to immiscible PLA/bamboo composites having unirradiated bamboo powder.

In Situ Compatibilization of PLA Blends Through E-beam Irradiation Generally, polymer blends are thermodynamically immiscible and consist of coarse morphology. The E-beam irradiation could be used to enhance the miscibility of polymer blends by minimizing the interfacial tension and increasing the adhesion between the blend components. The direct irradiation of polymer blends to E-beam does not lead to the changes only at the interphase (i.e., interfacial crosslinking) but also in the bulk of the blend’s constituents (i.e., chain branching, chain scission, and crosslinking). So, it is not straightforward to find the cause, (i.e., changes due to the compatibilization effect or changes in the constituent bulk), which changes the thermo-mechanical properties of a polymer blend. Several reports are there on the compatibilization of PLA blends through E-beam irradiation. The immiscible PLA/poly (E-caprolactone) (PLA/PCL) was compatibilized by E-beam irradiation

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Fig. 9 SEM micrographs of the PLA/PCL (80/20) blend for cryofracture surfaces irradiated at a 0 kGy and b 10 kGy and tensile fracture surfaces irradiated at c 0 kGy and d 10 kGy. Reprinted from [67], Copyright © 2013 with the permission of Elsevier

in the presence of glycidyl methacrylate (GMA) as a comatibilizing agent [67]. Ebeam irradiation of PLA/PCL blends in the presence of GMA (3 phr) results in crosspolymerization of PLA and PCL through medium GMA. E-beam irradiation-induced interfacial cross-copolymerization has enhanced the compatibility of blend components. This E-beam-initiated interfacial cross-copolymerization reduced the size of the PCL dispersed phase and filled the gap between PLA matrix and dispersed phase (Fig. 9b, the morphology of cryofractured surface of E-beam irradiated PLA/PCL blend having GMA). Hence, there are partial junctions exist (denoted by arrows in Fig. 9b) between the dispersed PCL phase and continuous PLA matrix, which is absent in the unirradiated PLA/PCL blend having GMA (3 phr) (Fig. 9a). Further, E-beam compatibilized PLA/PCL blends showed better mechanical and rheological properties as compared to unirradiated PLA/PCL blend. The mechanical properties such as tensile strength and tensile modulus increased with increasing irradiation dose up to 20 kGy and then decreased at higher doses. However, elongation at breaks of PLA/PCL blend decreased upon E-beam irradiation. In case of the PLA/PCL blend irradiated at 20 kGy dose, the increment in tensile strength and modulus was noticed at 140% and 30%, respectively, compared to the unirradiated PLA/PCL blend (Fig. 10). E-beam compatibilized PLA/PCL blend exhibited good interfacial adhesion, which is adequate to transfer the tensile load from one phase to another phase of the blend, as noticed in Fig. 9c. Figure 9c and d represents the tensile fractured surfaces of unirradiated and E-beam irradiated blends. The unirradiated blend shows the matrix PLA was significantly deformed by tensile load, but

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Fig. 10 Effect of E-beam irradiation dose on the mechanical properties of the PLA/ PCL blend, a tensile strength at break, b modulus, and c elongation at break. Adapted from [67], Copyright © 2013 with the permission of Elsevier

the dispersed PCL particles were slightly deformed and embedded in the stretched holes (Fig. 9c). The E-beam irradiated blend shows inseparable and highly elongated morphology in which it is tedious to distinguish the elongated dispersed PCL phase from the elongated PLA matrix (Fig. 9d). Figure 9d also shows the many tiny holes instead of stretched big holes. Jeon et al. also reported the compatibilization of PLA/PCL blends by E-beam irradiation in the attendance of GMA. They reported that the impact strength, thermal and rheological properties of blends were improved by E-beam irradiation-induced compatibilization [68]. The E-beam irradiation was also employed for the compatibilization of PLA/Starch composites in the presence of reactive compatibilizer GMA [69]. The E-beam irradiation of PLA/Starch composites in the presence of GMA initiates the graft copolymerization of PLA and starch through mediator GMA. This E-beam-induced graft copolymerization minimized the phase separations and improved the interfacial adhesion between PLA and starch and, hence compatibilized the PLA/starch composites. The E-beam compatibilized PLA/Starch composites showed an improvement in the rheological properties and modulus compared to unirradiated PLA/Starch composites and pure PLA. The effect of E-beam irradiation on the mechanical properties of PLA/PCL blends in the presence of crosslinking

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Fig. 11 Gel fraction of different crosslinked sc-PLA samples with different radiation doses. Adapted from [70], Copyright © 2007 with the permission of Elsevier

agent TAIC was reported by Maninowski [60]. It was observed that irradiation introduced the crosslinking among the same polymer chains and between both polymers and minimized the phase separation. This leads to improvement in the flexural and tensile strength and modulus of the PLA/PCL blend. However, the irradiation also leads to partial degradation of the PLA phase or both PLA and PCL phases; as a result, elongation at breaks and impact strength of irradiated PLA/PCL blends were decreased. Quynh and his research group reported equimolar stereo blends of poly(llactide)/poly(d-lactide) (sb-PLA) [70]. The concentration of TAIC crosslinking agent in the properly mixed sb-PLA/TAIC blends (sc-PLA) was controlled by sc-CO2 treatment. Further, sc-PLA samples were irradiated to an electron beam with various irradiation doses to achieve different crosslinking density in the materials. They had analyzed and reported that the sc-CO2 treatment made the sc-PLA samples softer and reduced their mechanical properties like Young’s modulus and tensile strength. However, these mechanical properties of sc-PLA samples were significantly improved by radiation crosslinking at a suitable dose. This is because the crosslinking network in the crosslinked sc-PLA samples restricted crystallization, enhanced the rigidity, and lowered brittleness, making the sc-PLA crosslinked samples tougher. The gel fraction study revealed that the gel fraction of crosslinked sc-PLA samples was drastically increased with irradiation dose, and it was maximum at 30 kGy dose (Fig. 11). There was no further increment in the gel fraction of crosslinked samples from 30 to 50 kGy. Thus, 30 kGy is a suitable irradiation dose, and sc-PLA3 (with 8% TAIC) irradiated at 30 kGy has improved mechanical properties and the best thermal properties. The electron beam had also been employed to compatibilize poly (lactic acid) (PLA)/poly (ethylene-co-glycidyl methacrylate) PEGM)/Hexagonal boron nitride (HBN) blend-composites [26]. It was reported that the addition of HBN particles in the PLA/PEGM blend led to improvement in the heat deflection temperature

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Fig. 12 Highly magnified SEM micrographs for the notched impact fractured specimens of unirradiated and E-beam irradiated PLA/PEGM/HBN 5 phr blend-composite. The scale bar is 2 μm. Adapted from [26], Copyright © 2018 with the permission of Elsevier

of resulting blend-composites without affecting their other properties. However, a significant decrement in the mechanical and thermal properties was noticed for blend-composites with high HBN particle concentration, i.e., 5 phr and 10 phr (part per hundred). Figure 12 shows that the blend-composite with a high concentration of HBN particles exhibits the phase separation and voids formation due to the agglomeration of HBN particles, leading to less interfacial adhesion. Hence, blend-composite specimens with high HBN concentration were shown poor performance upon mechanical and thermal testing. The E-beam irradiation of PLA/PEGM HBN blend-composites with 5 phr and 10 phr HBN showed a remarkable increment in the mechanical and thermal properties. It can be attributed to electron beaminduced compatibilization of irradiated blend-composites, which can be asserted by seeing the uniform phase morphology with better interfacial adhesion and disappearance of voids in the case of E-beam irradiated PLA/PEGM/HBN 5 phr blendcomposite (Fig. 12). Further, the E-beam irradiation of blend-composites also helps to speed up their hydrolysis. So, the electron beam can be employed to compatibilize PLA/PEGM/HBN blend-composites and optimize their thermo-mechanical and degradation properties for various commercial and medical applications. The E-beam irradiation-induced crosslinking of PLA/poly (butylenes terephthalate-co-adipate) (PLA/PBTA) blends in the presence of TAIC was

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also reported [71]. The E-beam irradiation of PLA/PBTA blends in the presence of TAIC increased the gel fraction as the percentage of TAIC increased but did not show any remarkable enhancement in the mechanical properties. Recently, the effect of electron beam irradiation on the thermo-mechanical properties of blends of PLA and poly (butylenes adipate-co-terephthalate) (PBAT) was studied [72]. The E-beam irradiation was performed from 25 to 100 kGy. It was found that E-beam irradiation decreased the molecular weight of PLA and hence, severely reduced the thermal and mechanical properties of PLA when subjected to high irradiation doses. However, the E-beam irradiation did not influence the molecular weight of PBAT and PLA/PBAT blends. So, PBAT and PLA/PBAT blends could be considered good to resist E-beam up to 100 kGy. Hence, it can be summarized that the electron beam irradiation is useful for in situ compatibilization of PLA-based blends and blend-composites by crosslinking of polymer chains or graft copolymers formed during irradiation.

3 Conclusion The blending of selective polymers with PLA is a more adaptable and economical method to tailor PLA properties. However, PLA is thermodynamically immiscible with most polymers. Hence, the compatibilization of PLA blends with E-beam irradiation is a sustainable and efficient technique. It does not entail tedious steps and toxic substances as chemical compatibilization methods do. This technique is newly introduced to modify the properties by altering the structure of polymers or by enhancing the miscibility of polymer blends and composites. The E-beam irradiation could be used to modify the physicochemical properties of polymers before blending to improve their compatibility in the resulting blends. The direct irradiation of PLA blends along with additives could be exploited to initiate in situ cross-copolymerization among the blend components or to generate in situ graft copolymers of PLA blend components which act as an interface between the blend constituents and improve their compatibility. There are few reports which used the E-beam irradiation to compatibilize the PLA blends. Hence, this simple and green technique should be further explored to compatibilize the PLA blends with target properties for desired applications.

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Chapter 4

Radiation Curing of Fiber Reinforced Polymer Composite Based Mechanical Joints Mohit Kumar, J. S. Saini, and H. Bhunia

1 Introduction Fiber reinforced composites have a large range of application in different fields like aerospace, automobile, civil, etc. These composites are required to be cured which can be done using various different techniques such as electron beam (EB) curing, gamma ray curing, X-ray curing and ultraviolet (UV) curing. Among all, EB curing emerges as an innovative technology that have shown significant properties in the polymer matrix composites due to its high controllability, fast curing, higher efficiency, low energy consumption, room temperature curing, low cost and environmental friendliness [1–4]. In polymer matrix composites, electron beam curing process uses high-energy electron beam that initiates the polymerization and crosslinking in the polymers. In late 1970s, the Aerospatiale French-based company was the first who introduced the concept of electron beam curing of polymer composites [5]. Many researchers from Canada, United States, Japan, China and Europe have acquired huge research findings in this field. Lot of investigations was conducted by Singh and Saunders [6–9] on the effect of EB curing on the epoxy acrylate composites, fiber and its sizing. Beziers et al. [10] worked on the filament wound EB cured epoxy

M. Kumar Mechanical Engineering Department, Chandigarh University, Mohali, Punjab 140413, India J. S. Saini (B) Mechanical Engineering Department, Thapar Institute of Engineering and Technology, Patiala, Punjab 147004, India e-mail: [email protected] H. Bhunia Chemical Engineering Department, Thapar Institute of Engineering and Technology, Patiala, Punjab 147004, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. R. Chowdhury (ed.), Applications of High Energy Radiations, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-9048-9_4

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acrylates and bismaleimide composites, since 1970s. The EB curing of acrylatebased composites under irradiation, undergoes free radical mechanism, and are not able to match the properties of advanced composites due to high moisture absorption and high shrinkage. Therefore, a suitable resin system was required for composite applications. Criverllo et al. [11] had developed the epoxy resin which undergoes cationic polymerization using cationic initiators such as onium salts. Due to the excellent properties and processing characteristics of epoxy resins, these are widely used in the polymer matrix composites [1]. Different variety of epoxy resins have shown excellent radiation reactivity and become the prime resin matrix in EB curable composites [1, 12–14]. From 1994 to 1997, Department of Energy and Defense along with other industrial agencies signed Cooperative Research and Development Agreement (CRADA) which potentially worked for EB curing of polymer matrix composite technology. Various developments and optimization processes of polymer matrix composites has been performed within the CRADA to meet the performance of thermally cured composites [14–17]. With the positive aspects of research and development in the EB cured cationic polymer systems, these systems were potentially applied in the curing of fiber reinforced polymer (FRP) composites [12, 18–30]. For FRP composites, numbers of fibrous reinforcement are available such as carbon, Kevlar, glass and aramid fibers, which are to be cured with EB technology. Different studies have been investigated on the carbon fiber reinforced composites for optimizing the processing parameters and their mechanical properties. Nishitsuji et al. [23] compared the thermal and EB curing processes for epoxy composites and found that time duration was reduced by 20 h for electron beam curing process. For electron beam curable resins, the T g value was high. Raghavan [24] evaluated the influence of process parameters (i.e., dose and dose rate) on the mechanical properties, evolution of cure and residual stresses. It was found that shorter dose rate and prolonged exposure ensured higher degree of cure. Vautard et al. [25] used electron beam curing process and UV radiation process to investigate the influence of fiber surface modification on the adhesion strength of carbon/acrylate composites. Many researchers found that the degree of cure by electron beam (EB) radiations was not 100% and further post-curing may be required after total irradiation dose of 150–250 kGy [31]. Pitarresi et al. [28] observed the behavior of carbon/epoxy composites with electron beam cured, thermally cured and EB cured followed by post-curing on short beam shear test and found that post-curing helps in achieving the mechanical properties equivalent to the thermal curing. Spadaro et al. [29] evaluated the crosslinking degree of EB irradiated carbon/epoxy composites and examined the non-uniformity in the structure. Further evaluation was done on post-curing technology, which gave uniformity in the structure using dynamic mechanical thermal analysis. Zhao et al. [30] used double sided low electron beam below 125 keV irradiation source to examine the carbon/epoxy composites for curing behavior and interlaminar shear strength (ILSS) properties. The ILSS properties showed that 50 kGy irradiated samples with post-curing at 160°C for 30 mins produced better results. The similar investigations were done by Zhang et al. [32] with double sided irradiation process of 150 keV irradiated source for estimating the effect of irradiation dose.

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These EB cured carbon fiber based advanced polymer composites serves in various applications such as aerospace, automobile and sports industries. In structural applications, these composites are typically joined using mechanical fasteners, adhesive bonding or combination of both techniques [33, 34]. But due the advantages of easiness to detachability and disassembly, the mechanical joints are preferred for joining of large structural components. These joints required holes in the composite structures that are highly susceptible to large stress concentration which reduces the load carrying capacity and is often responsible for the unexpected composite failure [35]. So, taking care of the stress concentration it’s important to investigate the load carrying capacity of the joints. There are three basic modes of failure in the mechanical joints, i.e., Net-tension, Shearing and Bearing mode of failures. These modes of failure are shown in Fig. 1 [36]. Numerous investigators have worked upon the failure mechanism of mechanical joints that are effected by parameters such as width to diameter (W/D) ratio, edge to diameter (E/D) ratio, ply orientation, stacking sequence and addition of nanofillers. Karakuzu et al. [37] examined the bearing strength and failure modes using different geometric parameters in single and double pin loaded glass/vinyl ester composites and found that load bearing capacity have severe effect on the E/D ratios. Aktas et al. [38] used Yamada Sun failure criteria to calculate the failure modes and bearing strength in glass/epoxy composites loaded with single and double serial pin joints. Zhang et al. [39] studied the effect of elevated temperatures in carbon/epoxy composite pin joint on damage progression using finite element model. The results indicated that at elevated temperatures joint strength gets reduced. Singh et al. [40] used a statistical approach to optimize the parametric combinations for double pin joints prepared from glass/epoxy/nanoclay composites.

Fig. 1 Basic modes of failure [36]

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In numerous applications of marine and civil sectors, these FRP composite joints have concerns about their long-term durability under harsh environmental conditions, such as ultraviolet (UV) radiation, moisture, elevated temperature, alkalinity and fire [41–43]. One of the primary causes of FRP composite degradation is moisture absorption, which results in the epoxy swelling, hydrolysis of resin, microcracking and debonding of the fiber/matrix interface [44–46]. Another source of FRP deterioration is the UV radiation, which is attributed to the physical and chemical changes in the constituent resin because of the series of complex processes characterizing UV radiation and oxygen [47, 48]. UV radiation may also lead to photochemical degradation that is initiated by the interaction of light photons with polymeric molecular chain. At elevated temperatures, the degradation in FRP composites begins when the glass transition temperature (T g ) of the resin material is reached [49]. From the previous investigations, it is noted that the combined action of UV radiation, moisture and elevated temperature accelerates the degradation of polymeric materials. The study on the effect of UV radiation and condensation on carbon fiber-epoxy composites was conducted by Kumar et al. [50]. It was revealed that combined environmental factors operate in a synergetic way that causes substantial loss of the epoxy matrix, resulting in the reduction of tensile strength by 29% after 1000 h of aging duration. Stewart and Douglas [51] explored the performance of FRP composites under accelerated aging conditions and found that the loss of mechanical properties were due to the degradation of the epoxy matrix. The epoxy swelling, plasticization, hydrolysis and chain scission reactions are the main mechanism that was responsible for the degradation of tensile, flexural and fracture toughness properties. Similarly, Yan et al. [52] studied the combined effect of UV radiation and water spraying on mechanical properties of flax fabric reinforced epoxy composite after 1500 h of aging. The test results showed a 29.9%, 34.9%, 10% and 10.2% reduction in tensile strength, tensile modulus, flexural strength and modulus properties, respectively. Batista et al. [53] examined the effect of UV, elevated temperature and moisture on the polyphenylene sulfide (PPS)/carbon fiber composites. The results revealed that UV exposure for a short duration increases the compressive strength, while long duration promoted deterioration in the strength properties. The increase in strength was related to the stiffening effect due to crosslinking whereas reduction indicates the formation of microcracks due to photo-oxidation and excessive embrittlement. Bazli et al.[54] evaluate the mechanical properties of glass/polymer pultruded profiles under similar conditions of UV radiation, moisture and elevated temperature for 1000, 1500, 2000 and 3000 h of aging. The result showed a decrease in the mechanical properties with aging. However, the rate of reduction was slight lesser during 1000 h, rapidly increased during 1000–2000 h, and again slow down during 2000– 3000 h. The maximum reduction in tensile, bending, and compression strength was 28, 34 and 23% after 3000 h of exposure time. Korach and Chiang [55] performed accelerated aging of carbon/vinyl ester composites using multiple environmental exposure chambers (UV radiation, salt-fog spray, moisture and temperature) for 800 h, to determine the effect of saltwater on mechanical properties. It was found that the conditions involving the UV radiation exhibited a significant reduction in the flexural strength by 21 and 24%, processed in UV-moisture and UV-salt spray chambers.

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Rios et al. [56] used acoustic emission techniques to evaluate the mechanical, thermal and morphological behavior in the polyurethane/epoxy/fiberglass composite plates during the accelerated aging process. Nicholas et al. [57] assessed the accelerated aging effect on the microstructure and the impact behavior of glass/polyurethane composite. The sample was exposed for 250, 500, 750 and 1000 h in the accelerated weathering environment. The results revealed that there is no significant change in impact properties and the material showed resistance to the UV radiations. Panaitescu et al. [58] conducted the accelerated aging for automotive parts made of glass/polyurethane composites. It was found that the combined effect of high temperatures and liquid immersions produced a significant impact on the material in terms of delamination and chemical alterations. Dogan and Arman [59] investigated the effect of hygrothermal and UV radiation on the glass/epoxy composites. The results showed reduction of tensile strength by 24 and 10% after exposure to 70 °C (70% humidity) with and without UV radiation for 1500 h. Number of efforts have been made for establishing the radiation curing technology for advanced composite materials. Large amount of research was focused on the interfacial strength development between the fiber/polymer interface by using different fiber sizing, optimization (dose, dose rate, % of cationic initiator, etc.) and post-curing techniques. As per the literature review, limited amount of research is focused on the load carrying capacity of the mechanical joints prepared from EB cured carbon/epoxy composite laminates and also some facts are quite intact about the performance of composite joints under the accelerated environmental conditions as whole structural integrity is dependent on the composite joints. In the present work, the failure modes and failure loads of mechanical joints prepared from carbon fiber/epoxy composites were determined using different geometric combinations of W/D and E/D ratios. Also, the performance of EB cured carbon/epoxy composite joints under accelerated weathering conditions to evaluate the bearing failure loads of the composite joints. The results were obtained both experimentally and numerically. The numerical model was developed using progressive damage along with the Hashin damage criteria.

2 Materials and methods The following section gives the details of materials and preparation of composite laminates, which are being used in the present chapter.

2.1 Materials A polyacrylonitrile (PAN) based, 200 gsm woven carbon fiber was used as reinforcement, which was supplied by CFW Enterprises Pvt Ltd, New Delhi, India. The woven carbon fiber has advantages of high strength and stiffness to the weight ratios,

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Table 1 Material properties [62] Materials

Properties

PAN-based woven carbon fabric, CFW Enterprises Pvt Ltd., New Delhi, India

Areal weight: 200 g/m2 Density: 1.8 g/cm3 Tensile strength: 4000 MPa Tensile modulus: 240 GPa Elongation: 1.7 % Poisson ratio: 0.3

L-12, Bisphenol A based epoxy resin, Atul Ltd., Gujarat, India Density: 1.1–1.2 g/cm3 Viscosity: 9000–12000 m Pas Cationic photoinitiator, bis(4-methylphenyl)iodonium hexafluorophosphate, Sigma-Aldrich, Missouri, United States

Purity: 98% Melting point: 175–180 °C

temperature and corrosion resistance properties and low density which make them favorable over other fiber materials [60, 61]. In matrix part, the diglycidyl ether bisphenol A (DGEBA)-based thermoset epoxy resin is used as matrix, which was supplied by Atul Industries, Gujarat, India. A cationic photoinitiator of iodonium salt, i.e., bis(4-methylphenyl) iodoniumhexafluorophosphate was procured from SigmaAldrich, Missouri, United States, which on reaction with EB rays decompose into protonic acid H+ and further reacts with the oxygen atom of the epoxy group. The material used in the present work along with their properties are given in Table 1 [62].

2.2 Manufacturing of EB Cured Carbon/Epoxy Composite Laminates The preparation of composite laminates consists of three main phases, i.e., the hand layup technique for combining laminas, radiation curing by exposing mold under the electron beam accelerator and post-curing of irradiated laminates. Firstly, the bisphenol A-based epoxy resin was mixed with 1.5 wt.% of bis(4-methylphenyl)iodonium hexafluorophosphate initiator through homogenization process (15 min) followed by sonication process (30 min). Then this mixture was used for the preparation of composite laminates with carbon fabric as reinforcement using hand layup technique. For getting same properties in longitudinal and lateral directions, stacking sequence of [0/90]6 was taken. The prepared laminate was kept in the aluminum molds, which were pressed using clamps along the sides so that the constant pressure was maintained to obtain better surface finish and uniform thickness. The 4 MeV pulsed horizontal accelerator, shown in Fig. 2 [62] was used for curing of laminates. The machine is located at Bhabha Atomic Research Center (BARC), Mumbai, India. The molds were placed on the conveyor belts, under the electron accelerator gun. During the process, the conveyor speed was maintained at 3 cm/s, pulse current at 100 mA and frequency at 10 Hz,

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Fig. 2 Horizontal 4 MeV pulsed electron beam accelerator machine [62]

for total irradiation dose of 150 kGy. After irradiation, the composite laminates were post cured at 100 °C for 30 min. The final thickness of cured composite laminate obtained was 2 ± 0.2 mm [62].

2.3 Mechanical Characterization For mechanical characterization, i.e., tensile, shear and compression, the specimens were prepared from the EB cured carbon/epoxy composite laminates. The mechanical properties were found as per ASTM D3039, D732 and D695 standards. The experimentally obtained properties of carbon/epoxy composite specimens are given in Table 2 [62]. The tensile properties obtained through EB cured composite specimens were matched with other used parameters. The thermally cured laminates were prepared by hand layup technique in follow up with curing at room temperature (24–48 h) for gel formation and then curing using compression molding at 150 °C temperature and 3.5 MPa pressure for 45 min [63]. The mechanical properties of thermally cured, EB cured without post-curing and EB curing followed by post-curing composite specimens were shown in Table 2 [64]. It was found that thermally cured composites have maximum mechanical properties. The reduction in the properties in EB cured composites is due to the higher void content present in the specimens and also the low degree of cure compared to thermally cured composites [28]. Furthermore, the increased mechanical properties after post-curing of EB cured composites attributed

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Table 2 Mechanical properties of carbon/epoxy composite laminates [64] Properties

Symbol (units)

Thermally cured (± SD)*

EB cured (without post-curing) (± SD)*

EB cured (with post-curing) (± SD)*

Tensile modulus (longitudinal)

E 1 (GPa)

77.85 ± 0.70

68.26 ± 0.70

73.12 ± 0.80

Tensile modulus (transverse)

E 2 (GPa)

77.85 ± 0.70

68.26 ± 0.70

73.12 ± 0.80

Tensile strength X t (MPa) (longitudinal)

606 ± 10

535 ± 15

570 ± 13

Tensile strength Y t (MPa) (transverse)

606 ± 10

535 ± 15

570 ± 13

Compressive strength (longitudinal)

X c (MPa)

315 ± 15

250 ± 11

287 ± 12

Compressive strength (transverse)

Y c (MPa)

315 ± 15

250 ± 11

287 ± 12

Shear strength

S(MPa)

125 ± 05

100 ± 06

113 ± 04

Elongation

%

1.36 ± 0.04

1.10 ± 0.01

1.32 ± 0.05

34–40

0.34–0.67

2.34–2.67

Processing time Hours * Standard

Deviation

to the enhanced crosslinking between the epoxy matrix and the fibers [29]. Inspite of that, the advantages of EB curing such as less curing time, environment friendly nature (no hardener), processing complex structure, high controllability, implementation in mass production [12], interestingly consents to further investigations on EB curing with post cured composites.

2.4 Preparation of Mechanical Joints The mechanical joints were prepared from EB cured carbon fiber/epoxy composite laminates. The following subsections describe the preparation of two types of mechanical joints; pin joints and bolted joints, for which the performance assessment was carried out.

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Fig. 3 The geometry of single hole pin joint configuration [63]

2.4.1

Pin Joints

The single pin joint composite specimens were prepared from the carbon fiber/epoxy composite laminates using different geometric combinations of width to diameter (W/D) ratio and edge to diameter (E/D) ratio, both varying from 2 to 5. The single pin joint configuration consists of a rigid pin inserted into the carbon fiber/epoxy composite specimens having a length (L), thickness (t), width (W ), edge distance (E) and hole diameter (D = 4 mm), as shown in Fig. 3 [63].

2.4.2

Bolted Joints

The bolted joint composite specimens were prepared from the carbon fiber/epoxy composite laminates as per ASTM D5961 standard having geometric parameters of W/D ratio and E/D ratio fixed to 6 and 5, respectively. The hole diameter used to insert the M4 size bolt was fixed to 4 mm. The bolt torques of different levels (0, 2 and 4 Nm) were used.

2.5 Accelerated Aging Conditions The combined cyclic exposure of UV radiation, moisture and temperature was given to the EB cured carbon fiber/epoxy composite specimens. The test was conducted in the QUV accelerated weathering chamber manufactured by Q-Lab Corporation, USA as per ASTM G154. The composite specimens were irradiated using 8 UV-A lamps, generating UV flux of 0.68 W/m2 at the wavelength range of 340 nm, which is the most detrimental range for polymer degradation. The cyclic exposure of 8 h of UV at 60 °C and 4 h of condensation at 50 °C at (relative humidity of 100% in

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condensation cycle) was imposed on the composite specimens for 250, 500, 750 and 1000 h of duration. For uniform exposure at both sides, the specimens were turned over after every 24 h.

3 Characterization The different techniques used for the characterization of prepared composite laminates and their joints have been given in the following subsections.

3.1 Scanning Electron Microscopy (SEM) The structural morphology of the composite specimens was analyzed using SEM (JEOL 6200 Scanning Electron Microscope, JEOL Pty. Ltd. USA) at an accelerated voltage of 20 kV. Before visualization, the specimens were prepared by gold coating using MP-19020NCTR Neocoater to prevent the charging problem of material.

3.2 Thermal Properties The thermogravimetric analyzer (TA Instruments TGA Q-500 series, USA) was used to determine the thermal stability of the composite specimens. The samples of 5–10 mg were placed in an aluminum pan and then heated from 30 to 600 °C at a rate of 10 °C/min.

3.3 Chemical Properties The chemical structure changes in specimens before and after the aging process were examined by Fourier transform infrared (FTIR) spectroscopy on Perkin Elmer Spectrum spectrometer using attenuated total reflectance (ATR) mode. The FTIR test was conducted on prepared specimens after 0, 250, 500, 750 and 1000 h of aging time. The range and resolution of spectra were taken as 400 to 4000 cm-1 and 2 cm-1 , respectively.

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3.4 Mechanical Properties The tensile, flexural, compression and shear properties of manufactured composite laminates were evaluated using ASTM D3039, D790, D695 and D732 standards, respectively, before and after the aging under different conditions. Five specimens for each configuration were tested on the Zwick Roell make universal testing machine (UTM) of 100 kN capacity. The crosshead speeds of 2, 2, 1.3 and 2 mm/min were used for tensile, flexural, compression and shear testing, respectively. The performance of joints was evaluated on the Zwick Roell make universal testing machine (UTM) having a capacity of 10 kN at a crosshead speed of 2 mm/min.

4 Performance Evaluation of Joint Specimens The failure analysis was done to evaluate the performance of pin joint and bolted joint composite specimens. In pin joint composite specimens, the failure loads and failure modes were estimated under the effect of different geometric parameters, i.e., W/D and E/D ratios from 2 to 5, respectively. The dimensions of the specimens with combinations of different geometric parameters are shown in Table 3 [62]. Table 3 Combinations of different geometric parameters [62] S.No.

Hole diameter, D (mm)

Ratios W/D

1

4

2

2

4

3

4

4

E/D

W (mm)

E (mm)

2

8

8

2

3

8

12

2

4

8

16

4

2

5

8

20

5

4

3

2

12

8

6

4

3

3

12

12

7

4

3

4

12

16

8

4

3

5

12

20

9

4

4

2

16

8

10

4

4

3

16

12

11

4

4

4

16

16

12

4

4

5

16

20

13

4

5

2

20

8

14

4

5

3

20

12

15

4

5

4

20

16

16

4

5

5

20

20

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The failure loads of pin joint composite specimens were evaluated under tensile loading on Zwick Roell make UTM machine with a capacity of 10 kN and crosshead speed of 2 mm/min. The testing setup used for pin joint composite specimens is shown in Fig. 4 [62]. The failure of the pin joint is much simpler than the bolted joints where the lateral constraints are involved in terms of compressive forces. In many engineering applications like civil, aircraft and marine sectors, bolted joints are susceptible to the aging environment. To outspread the scope of the mechanical joints, failure analysis was done for performance evaluation of the bolted joints under different aging environmental conditions. The tests were performed on the bolted joint composite specimens, before and after the aging process. The unaged and aged joint composite specimens were clamped into the fixture through fasteners as shown in Fig. 5 [62] and given a tightening bolt torque using a calibrated torque wrench having a variable torque and a least count of 0.5 Nm. The fabricated fixture consists of steel plates, M4 size shoulder bolts and washers. The material properties and dimensions of different components of the fixture are given

Fig. 4 Test setups for analysis of failure loads a UTM machine and b specimen in steel fixture [62]

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Fig. 5 Schematic design of composite specimen and fixture [65]

in Table 4. Three different bolt-tightening torques (0, 2 and 4 Nm) were used for the composite joints before testing. The bolt torques of 0, 2 and 4 Nm correspond to 0, 2500 and 5000 N preload values. Through testing, the maximum torque limit was found to be 9 Nm which produces a bearing stress of 149.2 MPa under the washer, having an outside diameter of 2D+3 mm. To avoid unwanted damage to the laminate surface, the maximum torque limit was not exceeded beyond 9 Nm. Finally, the complete assembly (joint specimens along with fixture) was clamped in the Zwick Roell make UTM machine having a capacity of 10 kN and tested under tensile load with a crosshead speed of 2 mm/min. The UTM setup along with the fixture and specimen is shown in Fig. 6 [65].

4.1 Bolted Joint Under Accelerated Aging Conditions The bolted joint specimens prepared from the EB cured carbon fiber/epoxy composite laminates were exposed to the aging environment in the accelerated weathering chamber for different durations. After aging, the bolted joint specimens were clamped into the fixture through fasteners and given a tightening bolt torque using a calibrated torque wrench. Three different bolt-tightening torques (0, 2 and 4 Nm) were used for the bolted joints before testing. The testing fixture was prepared as per ASTM

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Table 4 Specifications of the different parts of the fixture [65] Components

Dimension/size (mm)

Material properties

Steel plates

Length = 120 Width = 24 Thickness = 5

Stainless steel (SS 304): Density= 8 g/cm3 Tensile strength= 515–750 MPa Tensile modulus= 193 GPa Yield strength= 205 MPa

Bolts

M4, Diameter = 4

Alloy steel: Tensile strength= 19,0000 psi Yield strength= 17,0000 psi

Washers

Internal diameter = 4.3 External diameter = 9 Thickness = 0.8

Stainless steel (SS 304): Density= 8 g/cm3 Tensile strength= 515–750 MPa Tensile modulus= 193 GPa Yield strength= 205 MPa

Fig. 6 UTM machine setup along with fixture assembly [65]

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D5961, which consists of steel plates and fasteners (shoulder bolts (M4), lock nuts and washers). The specification of the components of testing fixture is given in Table 4. Five joint specimens for each configuration were tested under tensile loading before and after the aging process on the Zwick Roell make UTM machine having a capacity of 10 kN at a crosshead speed of 2 mm/min. The UTM setup along with the bolted joint specimen is shown previously in Fig. 6.

5 Results and Discussion The following section provides the results obtained from the pin joint specimens and the bolted joint specimens prepared from EB cured carbon fiber/epoxy composite laminates, after normal and accelerated aging conditions

5.1 Pin Joints 5.1.1

Effect of Geometric Parameters on Pin Joints

The influence of different geometric parameters, i.e., E/D and W/D ratios, on the bearing performance of EB cured carbon/epoxy composite pin joint specimens was evaluated under tension on the German manufactured Zwick Roell, UTM machine of 10 kN capacity. The results were obtained with a crosshead speed of 2 mm/min that applied a gradually increasing load with gear arrangements and electric motor with no backlash. The trends obtained in single pin joint configuration using different geometric parameters for carbon/epoxy composite laminates are shown in Fig. 7 [62]. For each parametric combination, five specimens were tested, further average values were taken and graphs was generated as shown in Fig. 7. According to the theoretical approach, the bearing, shearing and net-tension modes of failure exists in the failure phenomenon. The occurrence of failure in net-tension and shearing modes were catastrophic type. Whereas the bearing mode of failure is not immediate and follows the progressive type of damage failure. The trends show the dependency on the W/D and E/D ratios and by selecting the proper geometrical parameter values, the net-tension and shearing failure modes can be evaded. The mode of failure, which occurs as a sudden reduction in the graph values is a net-tension mode of failure. If the graph shows dropping of the curve in a zig-zag manner than it represents the shearing mode of failure. Moreover, the bearing mode is indicated by the progression in zig-zag manner. The trends in Fig. 7 shows that at lower W/D and E/D ratios, catastrophic and premature shear mode of failure occurs [62]. These modes of failure are caused by immoderate tensile and shear stresses present at hole surface area and the margin between the pinhole and side edges is very small. Net-tension and shearing need to be avoided by proper design

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Fig. 7 Graphs for different joint configurations of EB cured carbon/epoxy composite laminates under tensile loading [62]

due to its catastrophic failure characteristic. So, the bearing mode of failure were selected that occurs at higher values of W /D and E/D ratios. Bearing failure mode was caused by compressive stresses that occurs due to accumulated delamination damage and micromechanical buckling in the laminate [66]. For W/D ≥ 3 and E/D ≥ 3, complete bearing failure mode was observed which is desirable for the applications. It was also observed that at W/D = 2, E/D ≥ 3 and W/D ≥ 3, E/D = 2, mixed failure mode occurs, i.e., bearing followed by net-tension and shearing, which shows an indication for the initiation of bearing mode. The actual images of some selected joint geometries were presented in Fig. 8 showing different modes of failure in carbon/epoxy composite specimens, where B, N and S stands for bearing, net-tension and shearing mode of failure [62]. Table 5 represents the failure modes and failure loads for different pin joint configurations for EB cured carbon/epoxy composite joints. It can be seen from the table that the overall increment in ultimate failure load for desired bearing failure modes at W/D = 3–5 and E/D = 3–5 were found to be in the range of 12.4–16.7% in composite joints [62].

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Fig. 8 Failure modes in EB cured composite specimens for selected geometric combinations a W/D = 2, E/D = 3, b W/D = 2, E/D = 5, c W/D = 4, E/D = 2, d W/D = 5, E/D = 4 [62]

5.1.2

Numerical analysis

In the fibrous composite material, the structural component undergoes the number of local damages before it loses its complete mechanical strength. The ultimate failure in the composite material is considered as the complete loss in the different segments of the composite material. The present work deals with the progressive damage analysis of the EB cured composite pin joint using Hashin damage criteria for the prediction of damage evolution and damage initiation. Ansys software was utilized to design a model of desired joint configuration. The initial values of mechanical properties for the material were taken as per Table 1. The design of working model involves of three key steps (stress analysis, failure criterion and the material degradation rule) which together caused a progressive damage as shown by the algorithm [67] shown in Fig. 9. In whole progressive mechanism, the load is increased gradually up to occurrence of first fiber damage. Further, materials properties degrade as per damage evolution law. According to this law, the load transferred to other elements of the specimen may or may not show the specific increment in the bearing load. The increment in load shows the load bearing capacity of that element in the specimen and decrement shows that the further damage occurred in the other element. The different joint geometries with varying values of W/D and E/D ratios for EB cured composite materials were modeled in Ansys ACP pre-module. The model is

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Table 5 Experimentally obtained failure modes and failure loads for EB cured carbon/epoxy composite pin joints [62] S.No.

Hole diameter D (mm)

W/D ratio

E/D ratio

EB cured carbon/epoxy composite pin joint Failure mode

Failure load (N)

1

4

2

2

N

s

2

4

3

N

1900

3

4

4

N

2150

4

4

5

4

6

4

3

5

B→N

2210

2

S

2100

3

B

2250

7

4

4

B

2390

8

4

5

B

2450

9

4

2

B→S

2310

4

10

4

3

B

2430

11

4

4

B

2540

12

4

5

B

2620

13

4

2

S

2430

14

4

3

B

2520

15

4

4

B

2610

16

4

5

B

2730

5

Fig. 9 Algorithm for the progressive damage analysis of composite pin joints [67]

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Fig. 10 Wireframed mesh around the pin hole showing side edge and free edge [62]

designed with the stacking sequence of [0/90]6 with each layer of 0.20 mm thickness. The wireframed view representing the mesh around the pin hole and boundary conditions are as shown in Fig. 10 [62]. To determine the failure modes and failure loads, Hashin failure criteria was used for the fiber and the matrix failure. The material properties of that element degraded where the failure mechanism occurs [68]. The Hashin damage criterion works upon the four failure equations for fiber and the matrix. Table 6 shows the criteria along with the reduced material properties [62]. The mesh density plays an important role in numerical analysis to predict the results. Therefore, the convergence study was carried out to evaluate the mesh density around the hole boundary. The results from one joint configuration given in Fig.11 show that the error reduced to < 1% with variation in the mesh density. In Fig.11, numerically predicted failure load is the quantity utilized to calculate the error. The error is the difference between the previously predicted failure load and the new Table 6 Hashin damage criterion and associated degradation rule [62] Failure criterion Tensile failure in fiber (σ1 ≥ 0) Compression failure in fiber (σ1 < 0) Tensile failure in matrix (σ2 ≥ 0) Compression failure in matrix (σ2 < 0)

Equations  2   2 σ1 + τS12 = 1 At

Reduced material properties

σ1 Ac

=1

E 1 , V12 , V21 → 0



2

σ2 Bt

 σ2 2 2S



Bc 2S

+ +

2

 τ12 2

E 1 , V12 , V21 → 0

=1

E 2 , G12 , V12 , V21 → 0

+  − 1 σB2c = 1

E 2 , G12 , V12 , V21 → 0

S

 τ12 2 S

Where σ 1 = longitudinal stress; σ 2 = transverse stress; S = shear stress in plane; At = stress limits in tension along longitudinal direction; Bt = stress limits in tension along transverse direction; Ac = stress limits in compression along longitudinal direction; Bc = stress limits in compression along transverse direction

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Fig. 11 Convergence study on failure load [62]

predicted failure load by increasing the number of elements. The number of elements is increased till the difference between the previous predicted failure load and the new predicted failure load (i.e., error) is < 1% [62]. In the present work, failure analysis was done on the collective damage of all the layers of the pin joint specimen. The failure modes were estimated on the basis of 8f , the angle of first failure node. The bearing mode is considered if 8f lies between 0° to 15°, shearing mode is considered if 8f lies between 30° to 60° and the net-tension mode is considered if 8f lies between 75° to 90°. The net-tension, shearing and bearing modes of failure from composite specimens for random joint configurations are shown in the Figs. 12, 13 and 14 [62]. The joint configuration with geometric parameters of W/D = 2 and E/D = 4 for EB cured composite specimen showing net-tension mode of failure under tensile loading of 3300 N is presented in Fig. 12. It can be seen from the figure that the failure progresses firstly near the hole boundary and further spreads in the direction of side edges as load is increased, justifying the failure in net-tension mode. The shearing mode of failure occurred for joint configuration with geometric combination of W/D =3 and E/D =2 for EB cured composite specimen under tensile loading of 3200 N, shown in Fig. 13. It can be seen that the damage progresses toward the free edge of the pin joint, as from the hole boundary, free edge distance is smaller than the side edge corresponding to shearing failure mode. Similarly, bearing mode of failure is obtained from the failure analysis for joint configuration with geometric combinations of W/D =5 and E/D =5 for EB cured

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Fig. 12 Net-tension mode of failure in EB cured composite joint for W/D =2 and E/D =4 at different intervals of applied load [62]

composite specimen under tensile loading of 3800 N, shown in Fig. 14. The numerically predicted ultimate failure loads with different geometric parameters are plotted in Fig. 15 [62]. Comparing the experimental values of ultimate failure loads with the numerical values, it was observed that the predictions are within 9% of the acceptable difference, which shows the good agreement among the obtained results.

5.2 Bolted Joints 5.2.1

FTIR Analysis

The FTIR analysis of EB cured composite specimens was performed before and after exposure to different accelerated aging conditions. The ATR-FTIR spectra of the carbonyl region and the hydroxyl region for specimen is shown in Fig. 16 [64]. Previous research have found that carboxyl and hydroxyl groups were produced due to hydrogen abstraction from the polymer backbone and degradative oxidation

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Fig. 13 Shearing mode of failure in EB cured composite joint for W/D = 3 and E/D = 2 at different intervals of applied load [62]

reactions following chain scission [69]. In Fig. 16b (carbonyl region), the peaks at 1653 cm−1 signify the carbonyl stretching vibrations, at 1716 cm−1 was related to the ester groups, at 1510 cm−1 was related to N–H stretching, at 1435 cm−1 was related to fluorenone type structure, at 1010, 1155 and 1215 cm−1 was related to the 1,4-distributed benzenes, at 925 cm−1 was related to the diphenyl ketone group and at 850, 765 and 680 cm−1 was related to the aromatic C-H out of plane vibration of two adjacent hydrogens [70]. According to the intensity of each group, several carbonyl categories (diphenylketone, fluorenone, esters) were formed, illustrating the deterioration via chain scission reaction and the formation of low molecular products after exposure to UV radiation. Figure 16 represents the hydroxyl region of specimen showing the change in the hydroxyl group spectrum from 3700 to 2800 cm−1 . The peaks at 3440 cm−1 attributed to O–H stretching of carboxylic acid and at 2935 and 2860 cm−1 related to the C–H stretching of the monomer units. The higher peaks in the hydroxyl region attributed to the moisture absorption during the condensation exposure step [64].

5.2.2

Thermogravimetric Analysis

Typical thermograms of the EB cured composite specimens is shown in Fig. 17 [64]. The thermal stability at each aging condition in terms of the onset temperature (T onset ),

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Fig. 14 Bearing mode of failure in EB cured composite joint for W/D = 5 and E/D = 5 at different intervals of applied load [62]

maximum decomposition temperature (T max ) and residual mass for specimens are summarized in Table 7. The results revealed that the specimens before and after exposure show a one-stage decomposition process independent of the exposure time, whereas the amount of residue at 630 °C strictly depends on the exposure time [71]. It can be seen from Table 7 [64] that for unaged, higher onset temperature is at 300.7 °C. With the increase in accelerated aging time, a significant drop in the onset temperature was observed (257.5 °C). For short aging exposure, no changes were observed in the weight loss. The T max of the specimens decreased with exposure time from 368.7 to 349.2 °C. The percent residue of composite specimen 71.2% at 630 °C. Evidently, this trend continued after 1000 h of accelerated aging exposure, although the amount of residue drops significantly [64].

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Fig. 15 Comparison of experimental and numerical predicted failure loads [62]

5.2.3

Effect of Accelerated Aging on the Mechanical Properties of the Composites

The mechanical strength and modulus properties (in tension, compression and shear) of composite specimens were determined before and after exposure to 1000 h accelerated aging environment and the results are presented in Fig. 18a–c [64]. Five specimens for each configuration were tested and average results were plotted in the form of bar graphs (strength) and blue lines (modulus) in Fig. 18. It was found that for aging upto 250 h, the strength and modulus properties increased showing the stiffening effect due to crosslinking formation in the epoxy matrix [53] due to combined exposure of UV, elevated temperature and moisture. Another cause for improvement is the presence of unreacted cationic photoinitiator under UV radiations and temperature that increases the degree of cure in the composite specimens. After 250 h of aging, the reduction in the strength and modulus properties began which directed toward the initiation of degradation mechanism as seen from results after 500, 750 and 1000 h of aging (Fig. 18). It can be said that in this phase epoxy degradation was more prominent than the relative stiffening effect. The combined effect of UV radiation and condensation at elevated temperature accelerates the degradation mechanism in number of ways. At higher aging durations, the diffusion of chemical agents and moisture increases because of the pathways provided by the microcracks developed on the surface due to UV exposure. Another mechanism is

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Fig. 16 ATR-FTIR spectra of EB cured composite specimen showing changes in a hydroxyl region and b carbonyl region for unaged and accelerated aged specimens [64] Fig. 17 TGA thermograms of EB cured composite specimens before and after accelerated aging [64]

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Table 7 TGA results of EB cured composite specimens before and after accelerated aging [64] Exposure time (h)

Neat specimens T onset (o C)

T max (o C)

Residue (%)

Unaged

300.7

368.7

71.2

250

299.4

364.3

70.8

500

283.1

358.2

67.9

750

269.7

353.0

64.7

1000

257.5

349.2

62.4

the enhancement of photo-oxidation reactions due to the presence of moisture. In the condensation cycle, the soluble photo-oxidation reaction products were removed due to the exposure of surface to UV radiation which set up a new surface on which further degradation occurs [50]. Furthermore, the epoxy swelling due to absorbed moisture, plasticization, chain scission reactions and thermally induced microcracks is the source of deterioration in the composite specimens. For improved perception, the strength retention graphs of EB cured composite specimens, before and after aging are presented in Fig. 19 [64]. It was found that in aging time of 250 h, there was a slight increase in tensile, compressive and shear strength of 2.8%, 2.4% and 4.3%, respectively, for composite specimens as compared to unaged specimens. After 250 h, the mechanical properties got reduced and in 1000 h of aging time, the tensile, compressive and shear strength retentions in composite specimens were 89%, 82% and 83.5%, respectively.

5.2.4

SEM Micrographs

For material morphological investigations, SEM images were taken before and after the accelerated aging process from the surface of composite specimens and from the tensile fractured specimens. The surface micrographs of composite specimen before and after exposure to accelerated aging conditions are shown in Fig. 20 [64]. It was found that before exposure to aging conditions, unaged composite specimens had homogenous surfaces and there was no degradation and microcracks observed on the surface. The SEM micrographs for a fractured surfaces of composite specimens before and after the aging conditioning are shown in Fig. 21 [64]. Generally, fiber pull-out, matrix cracking, fracture of fiber and debonding are the general failure mechanisms of the FRP composites [56]. For unaged composite specimens, the dominant failure mechanism is the fiber fracture as shown in Fig. 21a, b which promotes the excellent mechanical properties of the material. The generation of this failure mechanism is due to the applied tensile stress. Under tension, the failure of material with an increased level of fiber fracture showed the excellent interfacial bonding between the fiber and the epoxy matrix,

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Fig. 18 Strength and modulus properties of EB cured composite specimens a tensile, b compressive and c shear [64]

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Fig. 19 Strength retentions of EB cured composite specimens a tensile strength, b compressive strength and c shear strength [64]

Fig. 20 Surface morphology of neat composite specimens a unaged, b after 1000 h of accelerated aging [64]

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Fig. 21 SEM micrographs obtained from tensile fracture specimens of neat composite specimens a, b unaged and c, d aged for 1000 h [64]

which in turn required higher tensile stress for material failure. Therefore, higher mechanical properties can be obtained with more fiber fracture. From Fig. 21c, d, it can be seen that after 1000 h of aging exposure, severe degradation was observed showing the fiber pull-out and matrix cracking failure mechanisms. Large fiber pull-out in tensile fractured surface showed the decrease in interfacial strength, followed by poor mechanical properties. This is due to the increased crosslinking (by exposure to UV radiation) that leads to embrittlement and creates microcracks, which result in debonding at the fiber/matrix interface. The widening of microcracks accelerates the degradation rate by exposing the new surface to UV radiations and increased moisture diffusion [54]. It was observed that surface morphology of pulled-out fibers changes from wavelike to relatively smooth, i.e., less adhesion of the resin to fibers. The accelerated weathering effect leads to interphase shrinkage due to crosslinking of an epoxy matrix, which weakens the interfacial bonding between epoxy and fibers.

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Performance of Bolted Joints Under Accelerated Aging Conditions

Figure 22 [64] shows the tensile testing results for unaged and accelerated aged EB cured composite bolted joint specimens at different bolt torques. As can be seen from Fig. 22, the curves for bearing response start with a linear movement and after reaching the first failure load, it continues moving in a zig-zag pattern showing the bearing mode of failure. The highest peak of a curve in the graphs is taken as the ultimate failure load for a particular condition. The mechanism of bearing failure in bolted joints undergoes different stages before final failure. In each stage, a set of shear cracks were produced in hand tightening bolt torque (0 Nm) through washers. This is due to the damage growth in individual plies of the joint specimen. The generated shear cracks propagate toward the outer surface of the specimens as lateral constraints opposes delamination by inhibiting the inward movement of cracks. Furthermore, the damaged material was supported by these lateral constraints that provides the additional failure load even after significant damage has occurred under the washers. With increase in load, the propagation and accumulation of damage continued in the joint specimen. Proceeding from the first set, a second set of shear cracks was formed. This shear crack growth mechanism continued until the damage propagated beyond the washers. At this point, joints lose its capacity to withstand higher failure loads due to the absence of lateral supports. But in the case of higher bolt torques (2 and 4 Nm), the shear crack formation was hindered as transverse expansion of damaged material was restrained. Although bolt torque does not alter the nature of the bearing failure mechanism under lateral constraints but hinders the material damage from formation of visible shear cracks. Hence, the increase in the ultimate failure load in bolted joints at higher bolt torques attributed to the frictional forces between the washers and the specimens [72]. The load versus displacement curves (Fig. 22) revealed that the ultimate failure load values decreased with an increase in cyclic exposure of UV radiation and condensation at higher temperatures. It was observed that with varying bolt torque from 0 to 2 Nm and 2 to 4 Nm, the slope of the curves shows increasing trends. This was due to the increased lateral compressive forces through washers, which increase the joint stiffness. But with increased exposure time under an accelerated environment, the slope of the curves decreases and varying bolt torques become less effective. The epoxy swelling due to absorbed moisture, plasticization, chain scission reactions and thermally induced microcracks are the source for degradation at severe aging exposures. The bar graph for bolted joint specimens for unaged and accelerated aged conditions, at different bolt torques are shown in Fig. 23 [64]. It was found that the ultimate failure load values increased drastically at 250 h of aging exposure (Fig 23) in the joint specimens. The increased values attributed to the fact that exposure to UV radiations, moisture and temperature under this condition give rise to a stiffening effect due to crosslinking formation in the epoxy matrix [53]. Another cause of improvement is the presence of unreacted cationic photoinitiator

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Fig. 22 Load versus displacement graphs for EB cured composite bolted joint specimens a unaged, b 250 h, c 500 h, d 750 h and e 1000 h of accelerated environmental aging [64]

under UV radiations and temperature that increases the degree of cure in the joint specimens. It can be seen from Fig. 23 that there was a significant loss in the ultimate failure load values after 250 h of aging exposure due to severe degradation in the bolted joint specimens. During 250–1000 h of aging exposure, the relative stiffening effects lag behind the other degradation factors (such as plasticization, epoxy swelling and photo-oxidation reactions) that deteriorate the joint specimens and were further responsible for lower ultimate failure loads. Another reason was the excessive embrittlement that gives rise to generation of microcracks under cyclic exposure at elevated temperatures. The bolt torque contribution can also be seen from Fig. 23, which reflects toward positive aspects of using bolt torque under accelerated conditions. It was found that for 0, 2 and 4 Nm bolt torques, the overall increase in ultimate failure loads at 250 h of aging exposure was 5.5%, 4.4% and 4.8%, respectively, for bolted joint specimens.

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Fig. 23 Bar graphs showing ultimate failure loads of EB cured unaged and aged joint specimens at different bolt torques [64]

Similarly, the overall decrease in ultimate failure loads in bolted joint specimens for 0, 2 and 4 Nm bolt torques at 1000 h of aging exposure were 12.1%, 20.6% and 21.1%, respectively.

5.2.6

Numerical Analysis

To predict the failure loads and failure modes in the composite bolted joint specimens, the finite element analysis was performed using static structural module in ANSYS software. In-situ properties of the composite specimen were considered for numerical analysis. The progressive damage analysis along with Hashin damage criteria was used for failure analysis. Generally, in the joint specimens the drilled hole has high stress concentration around the hole boundary and may give premature results after evaluation of failure. So, to predict the real-time failure load in the composite bolted joint specimen, the characteristic curve method has been used [73]. The characteristic curve radius (rc ) is given by Eq. (1) and characteristic curve is portrayed in Fig. 24 [64].

rc =

D + Rot + (Roc − Rot )cosθ 2

(1)

where hole diameter is represented by D and the compressive and tensile characteristic lengths are represented by Roc and Rot , respectively. The tensile and bearing tests were performed numerically on the open hole composite specimen to obtained the Roc and Rot values. The specimen was subjected

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Fig. 24 Characteristic curve [64]

to symmetrical tensile load (P). The mean tensile strength ((σ t )mean ) was obtained using Eq. (2). (σ t )mean =

P (W − D)t

(2)

where W and t are the width and thickness of the composite specimen. While moving in the transverse direction, the distance from the hole edge to the point where equivalent stress is equal to the mean tensile strength is the tensile characteristic length (R ot ). The hole specimen was subjected to the compressive load (P). The mean bearing strength ((σ b )mean ) was obtained using Eq. (3) (σ b )mean =

P D×t

(3)

The distance from the hole edge to the point where equivalent stress is equal to the mean bearing strength is the compressive characteristic length (R oc ). In the present work, the numerically calculated values of compressive characteristic length (R oc ) and tensile characteristic length (R ot ) are 1.062 mm and 2.291 mm, respectively. For damage progression in the bolted joint specimen, the Hashin damage criteria was used using equations as mentioned in Table 6 [64]. The maximum value among the Hf and Hm in tension and compression is termed as maximum failure index (FImax ) which is used to determine the failure loads. Failure modes can be determined depending upon the failure location (θ ) of the

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bolted joint on the characteristic curve. The failure in the joint occurs when the value of maximum failure index reaches the reference value on the characteristic curve. Eqs. (4) and (5) are used to determine the failure modes and failure loads in the bolted joint specimens. Bearing: 0◦ ≤ θ ≤ 15◦ , Shearing: 30◦ ≤ θ ≤ 60◦ ,

(4)

Net - tension: 75◦ ≤ θ ≤ 90◦ The failure load of the bolted joint specimens is calculated using Eq. (5). Failureload =

P FImax

(5)

Loads and Boundary Conditions The finite element analysis was performed on the composite bolted joint specimens clamped into the fixture with boundary conditions, applied loads and contact regions as shown in Fig. 25 [64]. An arbitrary tensile load (P) is applied along the x direction to the fixture while the composite bolted joint specimens is fixed at the end. To allow working of bolt pretension (bolt torque), the motion is permitted in z direction and constraint in x, y directions. In all contact regions, frictional contacts are used except the contact between the bolt and nut, which represents the locked condition and given bonded contact. It is assumed that the coefficient of friction remains unaffected after aging conditions and taken as constant in all joint specimens. The meshing is done using multizone mesh method with all quad elements. A refined mesh is used in the region around the hole boundary to increase the accuracy of results. The meshed composite bolted joint specimen with fixture is shown in Fig. 26 [64]. The analysis has been done in two steps. Firstly, the bolt pretension is given to the joint specimen and secondly, the tensile load is applied while given bolt pretension is locked. The progressive damage contour plots of an unaged neat composite bolted joint specimen at 0 Nm and 4 Nm bolt torque are shown in Figs. 27 and 28 [64]. With the gradual increase in applied load, the damage initiates at the hole boundary and progresses radially toward the free edge of the joint specimen. At 0 Nm bolt torque (Fig. 27), the damage spreads in the narrow zone and travel toward the free edge, showing less effect of the bolt torque as there is no lateral constraints. But at 4 Nm bolt torque (Fig. 28), the damage was distributed over wide area instead of going straight toward the free edge, showing that lateral constraints tend to control the damage propagation and distributed the loads in the washer’s contact area. The failure in the joint is measured when the damage status is unity on any point of the characteristic curve.

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Fig. 25 Loads and boundary conditions in the composite bolted joint specimens [64]

Fig. 26 Meshing of composite bolted joint specimen [64]

Figure 29 [64] represents the comparison of numerically predicted progressive failure damage with the experimental obtained damage showing similar trends at 0 and 4 Nm bolt torque of the composite specimen. The ultimate failure loads for composite bolted joint specimens under accelerated aging condition at varying bolt torques were determined numerically and are shown in Fig. 30 [64]. These results were compared with the experimental obtained results and found good correlation between the results.

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Fig. 27 Progressive damage contour plots of unaged composite bolted joint specimen with bolt torque of 0 Nm at a 20%, b 40%, c 60%, d 80% and e 100% of applied load [64]

Fig. 28 Progressive damage contour plots of unaged composite bolted joint specimen with bolt torque of 4 Nm at a 20%, b 40%, c 60%, d 80% and e 100% of applied load [64]

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Fig. 29 Numerical and experimental obtained damage plots obtained at bolt torque for unaged bolted joint configuration a 0 Nm, b 4 Nm [64] Fig. 30 Numerically predicted ultimate failure loads under accelerated aging exposure of 0, 250, 500, 750 and 1000 h for composite bolted joint specimens at varying bolt torques [64]

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6 Conclusions In the present study, the preparation of composite material was done using the radiation curing method, which have number of advantages. The bearing response of carbon fiber/epoxy composite pin joints were estimated that were cured using radiation-curing process. The experimental study was validated through numerical analysis. The increasing geometric parameters, i.e., W/D and E/D ratios showed the great impact on enhancement of failure loads for pin joints. Net-tension and shearing failure modes occurs at lower values of E/D and W/D. Sudden failure is seen in these failure modes. At E/D ≥ 3 and W/D ≥ 3, the occurrence of bearing failure mode was seen corresponds to higher load carrying capacity. Ultimate failure loads increase with increased values of geometric parameters. The accelerated aging shows an impact on the properties of EB cured composite bolted joints. On aging for 250 h, the EB cured composite specimen shows a slight increase in tensile, compressive and shear strength by 2.8%, 2.4% and 4.3%, respectively, as compared to unaged specimens. After 250 h, the mechanical properties get reduced and after aging for 1000 h, the tensile, compressive and shear strength retentions for EB cured composite specimens are 89%, 82% and 83.5%, respectively. The chemical structural changes have been detected through FTIR results showing higher peaks in carbonyl region (600–1800 cm−1 ) and hydroxyl region (2800–3700 cm−1 ) for EB cured composite specimens. The composite bolted joints are enormously affected by accelerated weathering condition after long exposure time. After 1000 h of aging exposure (at 0 Nm bolt torque), the overall reduction in ultimate failure load is 12.1% for bolted joint specimens. Bolt torque gave a positive effect on the performance of joints. It is concluded that as compared to aged specimens, the bolt torque effect is more prominent in unaged specimens. But at higher exposure time (1000 h) under accelerated environment, the bolt torque effectiveness gets reduced by 13%. The numerically predicted results have shown good correlation with the experimental ones. Acknowledgment This work was financially supported (No. 35/14/10/2017-BRNS with RTAC) by Bhabha Atomic Research Centre (BARC), Trombay, Mumbai (India). Authors are really thankful to the BARC team for their technical and financial support.

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Chapter 5

Thermally Stimulated Shape Memory Character of Radiation Crosslinked Polyolefinic Blends Tuhin Chatterjee and Kinsuk Naskar

1 Introduction 1.1 Shape Memory Polymers (SMPs) and Its Background SMPs belong to the technologically important class of stimuli-responsive materials and can change their shape in a predefined way from a less-constrained shape/configuration to a strained temporary shape/configuration and then again revert to the memorized shape/configuration upon triggering by an external stimulus, such as heat, light, electricity, and magnetic field [1, 2]. The term “shape memory polymers” consists of three words, such as “shape”, “memory”, and “polymers” as shown in Scheme 1. The term “memory” helps the material to remember its past. Thus it can be stated that shape memory materials (SMMs) are a promising class of smart materials that can remember their past after being severely and quasi-plastically distorted [3, 4]. It means SMMs have the capability to memorize their permanent shape and to be programmed and fixed for one or many temporary shapes under specific conditions of temperature and stress, while spontaneously recovering their original permanent shapes from temporary deformations upon exposure to an external stimulus as shown in Scheme 2. The term “shape-memory” was first proposed by Vernon in 1941 [5]. However, the importance of SMPs was not recognized until the 1960s, when crosslinked polyethylene (PE) was used for making heat-shrinkable tubes and films. More efforts to T. Chatterjee · K. Naskar (B) Rubber Technology Centre, Indian Institute of Technology, Kharagpur, West Bengal 721302, India e-mail: [email protected] T. Chatterjee Department of Chemical Engineering, University of Groningen, Groningen, The Netherlands © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. R. Chowdhury (ed.), Applications of High Energy Radiations, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-9048-9_5

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Scheme 1 Schematic representation of shape memory materials

Scheme 2 Schematic representation of shape memory behavior

develop SMPs began in the late 1980s, accelerating in the 1990s and making significant progress only in the past 5–10 years. The main focus of the earlier works in shape memory polymers was the development of new materials for various end-use applications. In that case, the fundamental inquiry was not so much and few thermomechanical properties were tuned only. Recently, these materials have gained interest in the academic field for more tailored applications. Unlike the gradual and linear response of regular polymers to external stimuli, SMPs exhibit a significant change even in presence of a small magnitude of external stimulus, and the response of SMPs to external stimuli is very rapid and nonlinear in nature. Depending upon the response of SMPs to external stimulus, SMPs can be of various categories which are shown below in Scheme 3 [6, 7]. During the past two decades, SMPs have stimulated research interest from both academia and industries. Some real-world applications have been developed for potential uses, such as functional textiles, smart customer products, active aircraft equipment, adaptive biomedical devices, and interactive electronic apparatuses.

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Scheme 3 Classification of various SMPs

1.2 Basic Principles of SMPs Before moving to the fundamental aspects of the SMP mechanism, key terms and techniques which are used to describe and characterize the SMP behaviors are summarized below [8–10]. Transition temperature ( T trans ): T trans which could be the melting temperature (T m ) or glass transition temperature (T g ) is the temperature around which a shape memory material changes from one state to another state. Shape fixing components: In an SMP network, shape fixing components are defined as domain or net point which maintains the dimensional stability during deformation and recovery. These net points are generally either physically crosslinked (physical entanglement or H-bonding) or chemically crosslinked. Shape fixing components: Usually, polymer chains in the SMP networks act as shape switching components which can switch from one state to a different state in response to the stimuli. Shape deforming temperature ( T d ): Temperature at which SMP is strained to a temporary shape. T d has significant impact on the shape memory behavior of an SMP. Shape fixing temperature ( T f ): Working temperature at which the temporary shape of a deformed SMP is fixed. Generally, T f is lower than the T trans . Shape recovery temperature ( T r ): It is defined as the temperature at which an SMP is triggered to recover its permanent shape from its deformed shape. Usually, T r is higher than T trans .

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Fig. 1 Schematic representation of cyclic thermomechanical test and sample deformation during shape memory cycle [11]

Cyclic thermomechanical test: Fig. 1 depicts the most used quantitative analysis of the shape memory characteristics of an SMP. The test can be performed either in a stress-controlled mode or in a strain-controlled mode using mechanical testing equipment attached to a temperature control unit. In a stress-controlled test, a predefined stress and temperature ramping are applied to the SMP, and the strain is recorded over time. In a strain-controlled test, a predefined strain and temperature ramping are applied to the SMP, and the stress is recorded over time. The driving force for shape recovery of an SMP is the recoiling of polymeric chains from a strained configuration (temporary state) to a less-ordered configuration (“memorized” state), namely, entropy elasticity [12, 13]. The “memorized” state could be the most relaxed, equilibrated configuration as the material was prepared. At a higher temperature (T > T trans ) during the deformation dislocation of the net points and alteration of the original orientation of the chain, segments take place. This results in the formation of new sets of local chain-chain interactions. Then this temporary or dormant shape can be fixed by cooling the material below the transition temperature (T < T trans ). For fixation of the temporary shape, the interactions between the newly formed chains should be strong enough so that they can overcome the elastic recoiling tendency of the chain segments. Again triggering to the higher temperature (T > T trans ) allows the material to revert to the permanent shape and shape recovery takes place.

1.3 Class of SMPs Net points which provide the dimensional stability to an SMP network are either covalently or physically crosslinked. The switching components may be either amorphous or semicrystalline reversibly responding to the temperature changes. Depending upon the presence of switching components and net points SMPs can be divided into four categories [14, 15]: (a) Chemically crosslinked net points with amorphous switching components.

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(b) Chemically crosslinked net points with semicrystalline switching components. (c) Physically crosslinked net points with amorphous switching components. (d) Physically crosslinked net points with semicrystalline switching components. The first two categories belong to the thermoset SMPs whereas the last two belong to the thermoplastic SMPs. In the case of thermoplastic SMPs, localized crystalline domains that are formed due to strong chain-chain interactions or chain entanglements act as a physical crosslinking site. Thermoplastic SMPs have several advantages, and these are: (a) easy moldability with various configurations or shapes, (b) a high range of shape deformation, and (c) a higher probability of blending with additives and other polymers. The main drawback of thermoplastic SMPs is the probability of vanishing of the original network points means the physical crosslinks during the deformation as the physical crosslinks depend upon temperature. This results in poor incomplete shape recovery in the case of thermoplastic SMPs. In contrast to thermoplastic SMPs, chemically crosslinked SMPs mean thermoset SMPs show superior shape memory behavior in terms of higher strain recovery and strain fixing ratios. Thermoset SMPs also possess higher shape recovery stress and the rate of strain recovery is also very fast. The higher degree of crosslinks network formation allows the thermoset SMPs to exhibit the above-mentioned properties. The presence of chemical crosslinks restricts the reprocessability to a new configuration or shape which is the main drawback of the thermoset SMPs. The difficulty to reprocess a thermoset SMP is not a matter of concern, where the SMPs are designed for a one-time application. Thus it can be stated that the shape memory properties are a combination of mechanical and thermal properties. Therefore, based on the appropriate choice of the processing programs, optimization through rational molecular and network designs, and development of a successful, application-driven design SMPs can be done that will exhibit the combined properties of a thermoplastic and thermoset SMP.

1.4 Molecular Mechanism of SMPs A polymer that exhibits the shape memory behavior should be stable in the deformed state above its transition temperature. It means the polymer should be stable in the temporary state and that gives the shape fixity character which is relevant for the particular application. SMP can achieve the shape fixity function in presence of the network chains that can act as a kind of molecular switch and the chain flexibility should depend upon the temperature. Above the transition temperature (T > T trans ), the network chains become flexible, while below the transition temperature (T < T trans ), the flexibility of the network chain is restricted to some extent. In explaining the mechanism of shape memory effects of SMPs, numerous kinds of programming models and structures and programming models have been discussed. The molecular mechanism of programming the temporary form and recovering the permanent shape of thermal-sensitive SMPs is demonstrated in Fig. 2 where the

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Fig. 2 Schematic representation of the molecular mechanism of the thermally induced SME for a covalently crosslinked polymer with T trans = T m [16]

melting transition temperature (T m ) is considered the transition temperature for the polymer network. If the melting point (T m ) is chosen as a thermal transition for the fixation of the temporary shape, strain-induced crystallization of the switching segment can be started by cooling the material. Crystallite formation upon cooling restricts the tendency of recoiling of the chain segments although always incomplete crystallization takes place. Hence, it prevents the instantaneous recovery to the lessconstrained permanent shape as shown in Fig. 2. Thereafter return to the permanent shape occurs due to the presence of covalent net points in the case of chemically crosslinked SMPs. In the case of physical crosslinked SMPs, the permanent shape is stabilized by the phase which exhibits higher thermal transition [17, 18].

1.5 Conventional SMPs The shape memory phenomenon can be demonstrated for different polymer systems having different morphology and molecular structure. In general, the usual and common shape memory polymer systems include crosslinked polyethylene (PE), polycyclooctene and ethylene–vinyl acetate copolymer (EVA), graft copolymers

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of PE and nylon 6, polynorbornene, trans polyisoprene, styrenic polymers, acrylic polymers, polymers based on epoxy and thiol-ene, segmented polyurethane (PU), and ionomers of segmented PU [19–22]. In addition, few biopolymers like poly(3hydroxyalkanoate)s (PHAs), copolymers of sebacic acid, or dodecanedioic acid monomers development have been taken place to explore the shape memory effect. The mechanical properties of those biopolymers are poor, and the shape memory character is not also highly desirable which requires further modulation [23].

1.6 General Concept of Shape Memory Polymer Blends The blending of two or more structurally and functionally different polymers with suitable proportions has been found a common technique and frequently applied to develop a product with improved mechanical properties using inexpensive polymers. While synthesizing a completely new polymeric material with all desired properties is a daunting task and consumes high cost, long time, expensive raw materials, and huge efforts, effective blending provides a low-cost alternative with improved technical properties playing a key role in polymer and elastomer industries meeting specific purposes [24]. To improve the properties or to obtain new functions of SMPs, SMP blends are a matter of concern [25]. The preparation of SMPs blends is mainly for five aims: (a) (b) (c) (d) (e)

To improve shape recovery stress and mechanical properties. To decrease shape recovery induction time by increasing thermal conductivity. To create new polymer/polymer blends with shape memory effect (SME). To tune switch temperature, mechanical properties. To fabricate shape memory materials sensitive to electricity, magnetic, light, and moisture.

From literature, it has been found that researchers have focused on a few polymer blends that could exhibit shape memory behavior. In most cases, the type of blends described usually consists of an amorphous polymer and a crystalline polymer and the two components should be melt-miscible. A group of researchers has studied the heat shrinkability behavior of grafted low-density polyethylene/polyurethane elastomers [26], and it is suggested that the interchain crosslinking between the grafted polyethylene and the elastomer improves their shrinkability. Similarly, heat shrinkability study and amnesia rating of electron beam-modified thermoplastic elastomeric films from blends of ethylene–vinyl acetate copolymer and polyethylene have been studied by another group of researchers [27]. Further, a novel type of shape memory polymer blend that consisted of two immiscible components, an elastomer, and a switch polymer was reported. The elastomer could be a rubber or thermoplastic elastomer, and the switch polymer could be amorphous or crystalline. Styrene–butadiene–styrene tri-block copolymer (SBS) was chosen as the elastomer, and poly (εcaprolactone) (PCL) was used as the switch polymer [28]. Similarly, several studies have been carried out in detail on shape memory polymer blends which include

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the blend based on polyethylene and polycyclooctene, polypropylene/metallocene catalyzed ethylene 1-octene copolymer, polyvinylidene fluoride (PVDF)/polymethyl methacrylate (PMMA), polyvinylidene fluoride (PVDF)/polyvinyl acetate (PVAc), Polylactic acid (PLA)/polyvinyl acetate (PVAc), and PCL/PLLA polyurethane [29–32], etc. In this context, the author has mainly focused on the shape memory behavior of radiation crosslinked polyolefinic blends which are discussed below in detail.

2 Various Polyolefinic blends 2.1 SMP of Polyethylene/Polycyclooctene Blends In the year 2012, Radusch et al. focused on the multiple shape memory behavior of electron beam crosslinked polyethylene (PE)/polycyclooctene (TOR) blends [33]. In this study, the author was mainly concerned with the development of SMP materials based on covalently crosslinked polyolefinic blends and therefore PCO containing trans-polyoctenamer (TOR) which is thermodynamically incompatible with polyethylenes and has a low viscosity in the melt was chosen as another blend component. Binary or ternary blends based on high-density polyethylene (HDPE), ethylene octene copolymer (EOC), and PCO containing trans-polyoctenamer (TOR) were prepared by the melt mixing method, and the compositions are given in Table 1. Then the samples were allowed to crosslink in presence of high-energy electrons at a radiation dose of 200 kGy. Differential scanning calorimetry (DSC) study reveals that especially the HDPE/TOR and EOC/TOR binary blends exhibit discrete melting and crystallization peaks compared to the HDPE/EOC binary blends. DSC thermogram shows that the melting temperature of HDPE, EOC, and TOR is near about 130 °C, 92 °C, and 40 °C, respectively. Discrete melting point peaks for the HDPE/TOR and EOC/TOR binary blends suggest the presence of two separate phases of crystalline TOR and crystalline EOC or HDPE. This also supports the thermodynamic incompatibility between the two phases. Following that scanning electron microscopy (SEM) study also pointed Table 1 Compositions of various blends [33]

Sample designation

Content (%) HDPE

EOC

TOR

50HDPE/50TOR

50

–

50

50HDPE/50EOC

50

50

–

50EOC/50TOR

–

50

50

30HDPE/70TOR

30

–

70

20HDPE/40EOC/40TOR

20

40

40

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Table 2 Shape memory characteristics of various blends [33] Sample designation

Shape memory behavior Shape fixity (%)

Shape recovery (%)

50HDPE/50TOR

97.8

96.2

50HDPE/50EOC

96.9

97.5

50EOC/50TOR

98.8

94.5

30HDPE/70TOR

99.5

96.8

20HDPE/40EOC/40TOR

98.3

97.1

out the formation of two-phase morphology for the HDPE/TOR and EOC/TOR binary blends, and it also indicates that TOR forms the continuous matrix phase for the binary blends. SEM analysis also demonstrated the dispersion of HDPE in the continuous TOR matrix phase in the droplet form or in the extended particle form of varying sizes from 1 to 50 μm. In the case of the ternary 20HDPE/40EOC/40TOR blend also the formation of two-phase morphology was noticed. This is due to the partial compatibility between the HDPE and EOC phases, and therefore, HDPE and EOC appear in the same contrast. Shape memory study revealed that the binary blends of HDPE/TOR and EOC/TOR exhibit pronounced triple-shape memory behavior and the ternary 20HDPE/40EOC/40TOR blend demonstrated some blurred indistinct quadrupleshape memory behavior. In contrast binary blend of HDPE/EOC evinced no such triple-shape memory behavior which correlates with the absence of phase-separated structure supported by the DSC and SEM study. Overall the shape memory behavior of the binary and ternary blends was high in terms of shape fixity and shape recovery and the values are enlisted in Table 2. In this respect, one important thing is to mention that Radusch et al. have also explored the multiple shape memory behavior and thermal–mechanical properties of peroxide crosslinked blends of linear and short-chain branched polyethylenes [34– 36]. They reported that electron beam crosslinked HDPE/EOC/TOR blend showed a pronounced multiple shape memory behavior only after a one-step programming process, while multiple shape memory behavior of peroxide crosslinked EOC/HDPE blends was noticed only after a multiple-step programming process.

2.2 SMP of Polyethylene/Polypropylene Blends Ran et al. studied the dual shape memory effect of thermoplastic SMPs based on lowdensity polyethylene (LDPE) and polypropylene (PP) with various compositions prepared by melt compounding [37]. Binary blends comprising PP and LDPE of different compositions were prepared at 180 °C and the compositions are given in Table 3.

158 Table 3 Sample designation and compositions of different blends [37]

T. Chatterjee and K. Naskar Sample designation

LDPE (wt%)

PP (wt%)

PP00

100

–

PP10

90

10

PP20

80

20

PP30

70

30

PP50

50

50

PP70

30

70

PP80

20

80

PP90

10

90

PP100

–

100

Subsequently, the blends were irradiated at 5, 10, 15, 25, 50, and 100 kGy radiation doses. Now considering the phase morphology of the various non-irradiated blends the author argued that based on the composition the morphology of the blends changed from a droplet structure to a co-continuous structure and back to the droplet one. More specifically, the PP10 and PP80 blend exhibited typical sea-island architectures, while the PP30 and PP50 showed co-continuous phase morphologies. From the gel content study, it was noticed by the author that gel fraction percent was extremely low when PP content exceeded 50 wt%. From this, it was also suggested by the author that during the electron beam irradiation, the degradation predominates over the crosslinking process. To get an idea about the shape memory behavior of the non-irradiated and irradiated LDPE/PP blends the programmed samples were heated at 130 °C above the melting temperature of LDPE. It was noticed by the author that in the case of PP-rich blends (say PP70) both in the non-irradiated and irradiated conditions there was no change in shape due to their large brittleness. Only the irradiated LDPE-rich blends (say PP30) showed the satisfying shape memory effect. Even the non-irradiated LDPE-rich blends were almost melting and became extremely soft at 130 °C. Thus it was suggested that the physical crosslinks of PP crystalline domains were not stable enough to quantitatively held the permanent shape in the thermomechanical cycle when these materials were in their deformed temporary shape. Thus it was demonstrated that the formation of a radiation crosslinked network in the case of LDPE/PP blends could be beneficial for the pronounced shape memory capability. Therefore, considering the shape memory effect of irradiated LDPE/PP blends it was viewed that the shape fixity and shape recovery ratios of the blends irradiated at 10 kGy radiation dose showed a consistent trend when the PP content did not exceed 30 wt%. At the same radiation dose with the increase of PP content, both the shape fixity and shape recovery decreased tremendously. Therefore, the author finally focuses on the shape memory performance of the PP30 blend as a function of radiation dose. It was reported by the author that with the increase of radiation dose from 5 to 100 kGy, the shape recovery ratio increased

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significantly from 17.5 to 93.8%. The scenario of shape fixity was reversed, i.e., the shape fixity ratios decreased slightly from 85.9 to 78.6% with the increase of radiation dose from 5 to 100 kGy.

2.3 SMP Blends of EOC-EPDM In the year 2011, Le et al. first demonstrated the manufacturing, morphology, and properties of carbon-black filled EOC-EPDM blends as a shape memory material [38]. In this work also the author used the peroxide as the crosslinking agent. Considering this work Naskar et al. first studied the shape memory characteristics of a novel heterogeneous polyolefinic blend system based on ethylene octene copolymer (EOC) and ethylene propylene diene terpolymer (EPDM) rubber. Both the base polymer contains a higher content of ethylene. Polyolefin elastomer, EOC (Engage 8440), with an ethylene content of 77 wt% and comonomer content of 23 wt% had a melting temperature of 93 °C. On the other hand, ethylene propylene diene rubber (EPDM) (Keltan 5508), with an ethylene content of 70 wt% and an ethylidene norbornene content of 4.5 wt%, had the Mooney viscosity, ML(1 + 4) at 125 °C of 55.

2.3.1

SMP of Un-crosslinked EOC-EPDM Blends

In the first case, Naskar et al. studied the shape memory behavior of EOC-EPDM blend in the un-crosslinked state, and therefore, five different blends having different blend ratios were prepared [39]. In this case, also binary blends of EOC and EPDM were developed by the melt mixing method and the sample designation is enlisted in Table 4. The literature review suggested that both the base polymer had a glass transition temperature (T g ) well below the room temperature and the difference between these two polymers in terms of T g was not so wide that resulted in a single T g value in the DSC thermogram. The single T g obtained from the DSC thermogram also indicates a higher degree of compatibility between the two-phase. However, the small shift in T g values (more or less 1 °C) was noticed due to the difference in blend composition. Apart from the glass transition temperature, two major transitions were observed Table 4 Blend designation and composition [39]

Designation

EOC (phr)

EPDM (phr)

P25 R75

25

75

P40 R60

40

60

P50 R50

50

50

P60 R40

60

40

P75 R25

75

25

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above the room temperature. The first one was above 31 ºC corresponds to the melting of the crystalline domain of the EPDM phase, and the second one was above 93 ºC related to the melting of the crystalline domain EOC phase. Glass transition temperature (T g ), melting temperature (T m ), and crystallization temperature (T c ) of the two base polymer and that of various blends are demonstrated in Table 5. Considering the condition of cyclic thermomechanical test condition and also followed by DSC melting endotherm of EPDM crystal (T m ≈ 32 °C), shape memory study of the various blends in un-crosslinked state was carried out at 60 °C only. Typical SME cycle of the different blends in strain-controlled mode is shown in Fig. 3. From the cycle itself, it was viewed that EPDM rich blend, i.e., P25 R75 blend shows higher shape fixity behavior and the EOC-rich blend, i.e., P75 R25 blend possess the lowest value of shape fixity. It was reported that P25 R75 blend exhibits the highest value of shape fixity of 86.7% and P75 R25 blend shows the lowest value of shape fixity of 74.4%. Intermediate blends (P40 R60 , P50 R50 , and P60 R40 ) possess the shape fixity values in between. Shape fixity ratio of the different blends after cooling is depicted in Fig. 4. Table 5 Glass transition temperature (T g ), melting temperature (T m ), crystallization temperature (T c ), and percentage crystallinity of soft segment of the virgin polymers and the various blends [39] Sample designation

T g (°C) of the blend

T m (°C) of EPDM

T m (°C) of EOC

T c (°C) of EPDM

T c (º C) of EOC

EPDM

− 40.23

32.30

–

19.73

–

P25 R75

− 39.39

31.75

94.12

14.91

68.22

P40 R60

− 38.11

31.64

93.71

15.68

72.64

P50 R50

− 37.17

31.59

94.22

16.10

75.00

P60 R40

− 36.12

31.25

94.55

16.72

76.58

P75 R25

− 35.11

31.28

94.53

17.14

79.80

EOC

− 33.67

–

96.44

–

81.82

Fig. 3 Shape memory cycle of various blends in un-crosslinked state [39]

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Fig. 4 Shape fixity values of the different blends [39]

It was suggested by the author that as the shape memory test was performed at 60 ºC (above the melting of EPDM crystal), the EPDM phase is primarily responsible for the thermoelastic behavior of the blends. On the other hand, the EOC phase acts as a hard domain and therefore provides adequate stiffness and reinforcement. To explain the superior shape fixity character of P25 R75 blend the author suggested one plausible explanation which is as follows. At first, the chains of the hard and soft phases get oriented along the direction of stretching at 60 ºC. Thereafter cooling of the material to room temperature in the stretched condition allows the formation of crystals in the EPDM phase, and the crystallinity of the EPDM phase restricts the relaxation of the blends upon unloading hence the sample length is not anticipated to change much more. The higher the crystallite formation in the EPDM phase higher is the shape fixity value of the blend. Thus P25 R75 blend shows the highest value of shape fixity and P75 R25 blend shows the lowest value. Unconstrained shape recovery of the sample from the temporary shape to the permanent shape happened in presence of heat only. Reheating the sample again at 60 °C allowed to recover the permanent shape of the sample. Reheating of the sample allows the melting of the crystallites, formed during cooling of the specimens in the stretched condition, and once the melting starts the sample gets relaxed from the stretched condition to the original non-deformed state. In this study to assess the shape of recovery behavior, a recovery study was carried out by the author both in the heating chamber and also in hot water. The shape recovery ratio of the blends is shown in Fig. 5. From the figure, it was noticed that shape recovery ratio (RR) was highest for EPDM rich P25 R75 blend and for P75 R25 blend it is the lowest one. It was postulated that higher proportion of EPDM results higher shape fixity (f ) due to formation of higher level of EPDM crystallites and the melting of all the crystallites in presence of heat also implies highest percentage of shape recovery. The most interesting fact was to note that the shape recovery of the specimens increased drastically in the presence of hot water than that of shape recovery in hot chamber which may be due to instantaneous heat transfer to the specimens from the hot water to the specimens whereas rate of heat transfer in the heating chamber to the specimens was much lower [40, 41]. Therefore, shape recovery ratio for the P25 R75 blend in the hot chamber was

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Fig. 5 Shape recovery of various blends in different condition [39]

81.41%, whereas shape recovery ratio was 92.10% for P25 R75 blend in contact with hot water. Typical shape memory effect of the polymer blends in hot water was shown in Fig. 6. From Fig. 6, it can be observed that in hot water more or less complete recovery of the sample was taking place. Sample recovery in hot water was higher than recovery in heating chamber and also instantaneous because of fast heat transfer from the hot water to the sample.

Fig. 6 Photo series demonstrating the shape memory behavior of the blend system: a initial sample; b softening of the sample in hot water; c fixation of the temporary shape by emerging in cold water; d fixed temporary shape; e immersion of the temporary sample of the sample in hot water for recovery to its initial length; f–i recovery of the sample in different stages; and j sample after final recovery [39]

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Tension set values of the blends are shown in Fig. 7 which clearly demonstrates that with increase of EOC content in the blend, percent tension set gradually increases. It indicates that with increase of EOC content, elastic nature of the blend gradually reduces while the viscous behavior increases, on the other hand. Therefore, P25 R75 blend (containing 25% EOC and 75% EPDM) shows a set value of 8.2% whereas P75 R25 blend shows 18.3% set value due to containing highest amount of EOC. This also indicates that P25 R75 blend exhibits highest elastic behavior than that of other blends. It is well-known that, the force required maintaining a constant strain that is applied on a viscoelastic material, gradually decreases with time and this phenomenon is called “stress relaxation.” Therefore, during tension set test change of stress value after each 30-s interval was noted down as the decay of stress value. Figure 8 depicts the relaxation phenomenon of the various blends at room temperature. From Fig. 8a, it can be noticed that the decay in stress (stress relaxation) is relatively lower for the blend containing lower fraction of EOC. With gradual increase Fig. 7 Tension set values of the blends [39]

Fig. 8 a Stress relaxation versus time curves. b Relaxation ratio versus time of the different blends in tension mode at room temperature [39]

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of EOC fraction, decay of stress becomes higher. It means P75 R25 blend relaxed at the fastest rate whereas stress relaxation for P25 R75 blend is the slowest one. This result also supports the tension set behavior of the blends, and it also indicates that highest EOC-containing blend possesses highest viscous nature which results faster decay of stress value. Table 6 demonstrates the decay of stress value of the various blends at room temperature. Therefore, decay of stress for P25 R75 blend is 0.54 MPa only whereas decay of stress is of 1.58 MPa for P75 R25 blend. Another interesting phenomenon, relaxation ratio of the blends with respect to time at 100% strain at room temperature has been plotted in Fig. 8b. From the figure, it can be observed that relaxation ratio is highest for the P75 R25 blend and minimum for the P25 R75 blend. It is a well-known fact that lower relaxation ratio results low hysteresis loss ratio and also indicates the higher recovery of a sample from the temporary shape to permanent shape. Considering this factor, it can be stated that SMP blend which shows low relaxation ratio will exhibit better shape recovery behavior. This also suggests that the P25 R75 blend (low EOC-containing blend) will show the better shape recovery behavior. From the shape memory study, it can be seen also that P25 R75 blend possesses highest shape recovery among all the blends which supports the relaxation ratio results of the different blend system. On the other hand, P75 R25 blend shows highest relaxation ratio, and it also shows poor shape recovery behavior. Typical tensile stress–strain plots of the blends at room temperature as well as at 60 °C are shown in Fig. 9a, b. It clearly demonstrates that modulus at 100% strain of the various blends at room temperature is higher than that of the modulus at 60 °C. This phenomenon is simply due to the melting of the crystal of the EPDM phase at 60 °C. Table 7 shows the tensile modulus value of the various blends at room temperature and also at 60 °C. Percentage reduction in tensile modulus of the blends is also given. Percentage reduction in tensile modulus is the maximum (66.67%) for P25 R75 blend and minimum (44.70%) for P75 R25 blend. Higher the EPDM content in the blend, higher is the percentage reduction in tensile modulus due to complete melting of the EPDM crystals at 60 °C. Low modulus at room temperature and at 60 °C in case of P25 R75 blend also supports the better shape recovery of the blend. Stress relaxation study has also been performed during shape memory testing. It means that decay of stress also takes place during cooling of the specimen from Table 6 Initial stress values, final stress values, and decay of stress values of the various blends [39]

Sample code

Initial stress (MPa)

Final stress (MPa)

Decay of stress (MPa)

P25 R75

1.92

1.38

0.54

P40 R60

2.57

1.86

0.71

P50 R50

2.86

2.06

0.80

P60 R40

3.34

2.14

1.20

P75 R25

4.10

2.52

1.58

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Fig. 9 Typical stress–strain plot of the SMP blends at a room temperature and b at 60 °C stretching up to 100% of strain [39]

Table 7 Tensile modulus of the blend at room temperature and at 60 °C [39] Sample Name

Modulus (MPa) at 100% strain at room temp

Modulus (MPa) at 100% strain at 60 °C

Reduction in modulus (%)

P25 R75

1.89

0.63

66.67

P40 R60

2.48

1.11

55.24

P50 R50

2.64

1.36

48.48

P60 R40

3.37

1.60

46.74

P75 R25

3.87

2.14

44.70

60 °C to a temperature which is below the transition temperature holding at 100% of strain. Stress relaxation phenomenon and the relaxation ratio study are also carried out, and it has been depicted in Fig. 10. Here also the same phenomenon of stress relaxation was observed like the stress relaxation phenomenon observed at room temperature. Decay of stress is higher for the EOC-rich blend. With increase in EPDM content, stress relaxation phenomenon gradually decreases as depicted in Fig. 10a. On the other hand, Fig. 10b reports the relaxation ratios of the blends with respect to time. In this case, the relaxation ratio is the minimum for the lowest EOC-containing blend means for P25 R75 blend and maximum for P75 R25 blend system that contains 75% EOC. Thus based on the postulation of relaxation ratios upon the shape recovery behavior, it can be stated that P25 R75 blend gives better shape recovery behavior while the P75 R25 blend exhibits poor shape recovery, and all the results of relaxation ratio behavior support the shape memory behavior of the EOC/EPDM blend systems. However, shape recovery of different blends in heating chamber varies with respect to time. The shape recovery of different blends as a function of time has been shown in Fig. 11.

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Fig. 10 a Stress relaxation versus time curves b Relaxation ratio versus time of the different blends in tension mode at 60 °C [39]

Fig. 11 Recovery with respect to time for various blends [39]

For all the blends, recovery time was not more than 10 min, i.e., within 10 min recovery of various blends are complete. Only there were the differences in percentage of recovery. The figure depicts that maximum percentage of recovery of all the blends has taken place within 6 min. It can be observed that the fastest shape recovery is shown by P25 R75 blend, whereas shape recovery is slowest in case of P75 R25 blend. As a result, percentage recovery of P25 R75 blend after 2 min is 62% whereas it is 37% in case of P75 R25 blend. Following all the above-mentioned shape memory characteristics, stress–strain behavior of the shape memory ECO-EPDM blends is depicted in Fig. 12. Inset is the mechanical properties of the pristine polymers. Neat EPDM exhibits tensile strength of 8.0 MPa and elongation at break of 1337% whereas pristine EOC shows a tensile strength of 30.3 MPa and elongation at break of 1230%. From the graph, it can be clearly viewed that in spite of having higher ethylene content EPDM still shows rubbery behavior whereas EOC shows behavior like thermoplastics. Neat

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Fig. 12 Stress–strain curves of the EOC-EPDM blends [39]

EOC shows necking phenomenon which is absent in case of EPDM which can be clearly seen from Fig. 12. Necking phenomenon is also observed in case of blends. With increase of EOC content, tendency to show necking phenomenon is also higher. For this reason, P75 R25 blend (contains 75% of EOC phase) shows maximum necking behavior whereas P25 R75 (containing 25% EOC phase) shows least necking behavior. In between this two terminal blends; necking phenomenon gradually reduces with decrease of EOC content from P60 R40 to P40 R60 . In case of blends, with increase addition of EOC phase tensile strength steadily increases. Tensile strength (T.S.) for P25 R75 blend is 15.7 MPa whereas for P50 R50 blend 22.5 MPa followed by 24.3 MPa for P75 R25 blends. On the other hand, elongation at break stepwise decreases with increasing amount of EOC. Like tensile strength, modulus values of the blends show the same trend. Tensile strength (T.S.), elongation at break (%EB), modulus at different percentages (100, 200, 300%) of strain, and hardness of the pristine polymers and blends are enlisted in Table 8. Table 8 Mechanical properties of the virgin polymers and their blends [39] Sample code

T.S. (MPa)

EB (%)

M100 (MPa)

M200 (MPa)

M300 (MPa)

Hardness (Shore A)

Hardness (Shore D)

EPDM

8.0

1337

1.0

1.4

1.7

54.1

10.2

P25 R75

15.7

1240

1.7

2.2

2.7

72.2

17.5

P40 R60

19.6

1207

2.5

2.8

3.3

75.6

18.2

P50 R50

22.5

1145

2.8

3.3

3.9

81.2

23.3

P60 R40

23.3

1107

3.0

3.6

4.2

83.1

28.5

P75 R25

24.3

1097

3.8

4.4

5.1

85.3

33.1

EOC

30.3

1230

5.4

5.6

6.4

91.4

36.8

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Table 9 Designation of the various blends after the electron beam radiation [16] Blend ratio

Irradiation dose (kGy) 25

50

75

100

(EOC:EPDM) (25:75)

P25 R75 EB25

P25 R75 EB50

P25 R75 EB75

P25 R75 EB100

(EOC:EPDM) (50:50)

P50 R50 EB25

P50 R50 EB50

P50 R50 EB75

P50 R50 EB100

(EOC:EPDM) (75:25)

P75 R25 EB25

P75 R25 EB50

P75 R25 EB75

P75 R25 EB100

2.3.2

SMP of Radiation Crosslinked EOC-EPDM Blends

In the second stage, Naskar et al. explored the effect of electron beam radiation upon the shape memory behavior of EOC-PEDM blends. In this case, the author developed the blends having three different blend ratios (25:75, 50:50, 75:25) and the blends were irradiated at various radiation doses of 25, 50, 75, and 100 kGy [16]. Ultimately, twelve samples were prepared, and the details of the samples are given in Table 9. It was found that in presence of electron beam radiation chances of crosslink formation in EOC phase was higher rather than the EPDM phase [42]. As a result, extent of crosslink formation will also be higher in the EOC phase rather than that of EPDM phase. The higher extent of crosslink formation was supported by the higher value of gel content and also by the morphology study shown in Fig. 13. From the phase images of P25 R75 EB25 , P50 R50 EB25, and P75 R25 EB25, it can be clearly viewed that at low radiation dose (say 25 kGy) EOC phase primarily gets crosslinked whereas in EPDM phase crosslink formation is less. In case of P25 R75 EB25 blend, EOC phase (25 parts) is almost fully crosslinked which results proper dispersion and distribution of crosslinked EOC particles in EPDM matrix, and the particle sizes of EOC vary from 0.05 to 0.25 μm. On the other hand, for P50 R50 EB25 and P75 R25 EB25 blend also, at low radiation dose (25 kGy) crosslink formation in EOC phase has been occurred mostly. For P50 R50 EB25 blend although EOC phase (50 parts) is getting crosslinked fully, tendency of coalescence of crosslinked EOC particles leads to the formation of EOC aggregate and agglomerate that results larger sizes of EOC particles. On the other hand, P75 R25 EB25 blend also shows similar trend like P50 R50 EB25 blend but the presence of higher fraction of EOC phase results high chances of coalescence of the EOC particles, and it leads to the formation of fully agglomerate structure of crosslinked EOC particles which is very much clear from the figure. On the other side, at higher radiation dose (say 100 kGy), crosslink formation in EPDM phase has also taken place together with the crosslink formation in EOC phase. From the phase images of P25 R75 EB100 blend, it can be clearly noticed that with increase of radiation dose, crosslink formation in EPDM phase has been occurred. Formation of crosslinks in EPDM phase results the presence of much more hard phases (yellowish region) which is very much clear from the figure. From the figure,

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Fig. 13 AFM phase images of the irradiated blends [16]

one phenomenon can also be noticed that coalescence formation of crosslinked EPDM particles has taken place due to the presence of higher fraction of EPDM phase. For P50 R50 EB100 and P75 R25 EB100 blend also, EPDM phase gets crosslinked with increase of radiation dose and that is shown in the figure.

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From the DSC thermogram, it was also noticed that after the treatment of electron beam irradiation there were significantly changes in the melting and crystallization behavior of the two phases. It was clearly seen that with increase of electron beam dose from 25 to 100 kGy, melting temperature of both the EPDM and EOC phase came down to lower temperature (near about 3 °C) which is maybe due to the breakage of the crystallinity of both the EPDM and EOC phases. It is a well-known fact that when a semicrystalline polymer is exposed to electron beam radiation two parallel reactions used to take place and the reactions are: (a) formation of higher degree of crosslinks network in the polymer backbone and (b) breaking of the crystalline region of the polymer which results eventually the reduction in percentage of crystallinity [43]. Therefore, with increase of radiation dose from 25 to 100 kGy, formation of higher degree of crosslinks network inside the polymer backbone took place and on the other side breakage of the crystallinity of both the EPDM and EOC phases occurred that results the reduction in the percentage crystallinity of both EPDM and EOC phases. The reduction in percentage crystallinity of the blends was also supported by the X-ray diffraction analysis study. Now considering the factor of shape fixity behavior of the samples, it can be assumed that the shape fixity behavior is primarily dependent upon the growth of the crystallite formation in the soft phase during stretched condition. Higher the crystallite formation in the soft EPDM phase better will be the shape fixity value of the blends. It has been already discussed that during crosslinking by electron beam generally two parallel phenomena used to take place inside the sample: (a) formation of crosslinks with higher electron dose and (b) secondly percentage reduction of crystallinity of both soft and hard phases due to breakage of crystallinity after crosslinking. As a result, higher the radiation dose higher will be crosslink formation in the soft (EPDM) and hard (EOC) phases and higher will be the reduction in the percentage crystallinity of the samples. Therefore, with increment of radiation dose from 25 to 100 kGy, crystallinity of the soft phase (EPDM) and hard phase (EOC) will gradually reduce and as the crystallinity of soft phase has the prime impact on the shape fixity value of the blends, shape fixity values will gradually reduce that is shown in the SME cycle of the various blends. Typical SME cycle of the different blends radiated at different doses in strain-controlled mode is shown in Fig. 14. Now considering the shape memory behavior of the electron beam crosslinked blends it was observed that with increase of electron beam radiation dose from 25 to 100 kGy the shape fixity value of the radiation crosslinked blends became lower. This was due to the breakage of the crystalline structure of the soft EPDM phase upon exposure to electron beam radiation. It was seen that after the treatment of electron beam radiation for all three blends (P25 R75 , P50 R50 , P75 R25 ) shape fixity value reduced as compared to its un-crosslinked blend with increase of radiation dose. In case of P25 R75 blends, the reduction in shape fixity value was the maximum from 86.7% for un-crosslinked P25 R75 blend to 75.4% for P25 R75 EB100 blend. On the other hand, for P75 R25 blends decrease in shape fixity value was the minimum from 74.4% for P75 R25 to 68.5% for P75 R25 EB100 which is illustrated in Table 10. On the other side from the table, it was also viewed that shape recovery of all the radiation crosslinked blends improved compared to its un-crosslinked blend system.

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Fig. 14 Typical SME cycle of the electron beam crosslinked a P25 R75 , b P50 R50, and c P75 R25 blends [16]

For crosslinked P25 R75 blends after exposure to electron beam shape recovery of the sample improves from 81.4% for un-crosslinked P25 R75 blend to 95.1% for electron beam crosslinked P25 R75 EB100 blend. On the other side, for crosslinked P50 R50 and P75 R25 blends also shape recovery increases from 72.9% to 88.9% and 68.8% to 84.0%, respectively, from its un-crosslinked blend to the blend crosslinked at highest radiation dose. This improvement in shape recovery for all the crosslinked blend system clearly reveals that the formation of crosslinks in the polymer backbone which acts as a net point helps the crosslinked blend for better recovery. Thus the author finally argued that in case of radiation crosslinked polymeric system not only the melting of the crystallites has impact upon the shape recovery of the sample but also the chemically crosslinked network formed in the sample has also significant impact on the shape recovery behavior of the sample. Decay of stress value of different radiation crosslinked blend of three different blend ratios at room temperature is depicted in Fig. 15a, c, e, respectively.

172 Table 10 Shape memory properties of electron beam crosslinked EOC-EPDM blends [16]

T. Chatterjee and K. Naskar Blend ratio

Blend designation

Shape fixity (%)

Shape recovery ratio (%)

(EOC:EPDM) (25:75)

P25 R75

86.7

81.4

P25 R75 EB25

83.4

84.6

P25 R75 EB50

80.1

88.0

P25 R75 EB75

78.1

91.5

P25 R75 EB100

75.4

95.1

P50 R50

78.7

72.9

P50 R50 EB25

76.1

79.9

P50 R50 EB50

74.2

82.1

P50 R50 EB75

72.8

86.1

P50 R50 EB100

70.5

88.9

P75 R25

74.4

68.8

P75 R25 EB25

73.7

73.0

P75 R25 EB50

72.6

76.5

P75 R25 EB75

70.8

80.5

P75 R25 EB100

68.5

84.0

(EOC:EPDM) (50:50)

(EOC:EPDM) (75:25)

From the graphs, it can be clearly seen that after crosslinked by electron beam radiation the initial stress value of the crosslinked blends increases compared to its un-crosslinked blend and this has happened for all the three different blend ratios. This also implies that formation of crosslinks increases the elastic nature of the blends and side by side the modulus value also increases that is very much clear from the figure. Considering P25 R75 blends, it can be stated that with increase of radiation dose from 25 to 100 kGy stress decay phenomenon gradually decreases which clearly indicates that the stress relaxation of un-crosslinked P25 R75 blend is faster whereas stress relaxation for P25 R75 EB100 is the slowest one. It also supports the formation of highest degree of elastic network (formation of crosslinks) in presence of electron beam. It means at higher radiation dose (100 kGy) highest degree of crosslinks formation takes place into the sample which increases the elastic behavior of the P25 R75 EB100 blend whereas for un-crosslinked P25 R75 blend decay of stress value is highest due to its highest viscous nature. Thus among the different radiation crosslinked P25 R75 blend, P25 R75 EB100 blend possesses highest degree of network structure that enhances its elastic behavior. Similar trend was seen for the other blend systems also. Here also, due to the formation of crosslinks network structure that enhances the elasticity of the polymer network decay of stress value comes down [44, 45]. Although after electron beam curing for all the three blend ratios elasticity of the network increases due to the formation of crosslinks, lowest stress decay phenomenon for P25 R75 EB100 blend

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Fig. 15 Stress relaxation versus time curves a, c, e and relaxation ratio versus time curves b, d, f of P25 R75 , P50 R50, and P75 R25 blends respectively [16]

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supports the highest elastic nature of that blend that supports the lowest tension set value and better elastic recovery of that blend system. In a similar way, relaxation ratio of the blends with respect to time at 100% strain at room temperature has been plotted in Fig. 15b, d, f, respectively. Following all the three figures, it can be observed that for all the three different blend ratios, relaxation ratio is highest for its un-crosslinked blend whereas it gradually reduces with increase electron beam dose (from 25 to 100 kGy) and it becomes the lowest one for highest radiation dose (100 kGy) crosslinked blend. It is a well-known matter of fact that lower relaxation ratio of the blends implies lower hysteresis loss ratio and also indicates the highest recovery of the sample from its temporary shape to the permanent shape. Thus considering the above factor, it can be argued that the blend with lowest relaxation ratio will exhibit better recovery from its temporary shape to its permanent shape. Therefore, for all the three blend ratios, highest radiation dose crosslinked blend (P25 R75 EB100 , P50 R50 EB100 , P75 R25 EB100 ) will show highest shape recovery from its temporary shape compared to other radiation dose crosslinked blend and its un-crosslinked blend also. Overall considering all the three Fig. 15b, d, f it can be noticed that relaxation ratio is lowest for P25 R75 EB100 blend, and it suggests that P25 R75 EB100 blend will exhibit highest shape recovery among all the blends and this result also supports the shape recovery study of the different blends. In this regard, the author also focused on the rheological study and temperature scanning stress relaxation (TSSR) study of the un-crosslinked and radiation crosslinked EOC-EPDM blends and the author found a correlation of the shape memory character of the blends with the above two studies [46, 47].

2.4 SMP Blends of Two Alpha Olefins After pursuing the research on the development of shape memory blend based on EOC-EPDM blend, Naskar et al. also focused on the shape memory character of the blends of two alpha olefins (EOC 8440 and EOC 8200) with dual network structure [11]. EOC 8440 with comonomer content of 23 wt% and percentage crystallinity of 27% possess the melt index of 1.6 dg min−1 at 190 °C and 2.16 kg load. On the other side, EOC 8200 with comonomer content of 38 wt% and percentage crystallinity of 19% possess the melt index of 5.0 dg min−1 at 190 °C and 2.16 kg load. Keeping the blend ratio (50:50) same, shape memory behavior of the blends of two different alpha olefins was studied in details in both un-crosslinked and radiation crosslinked (irradiated at 25, 50, 100 kGy) state. Final sample is enlisted in Table 11. It was well reported that chain scission and crosslinking process always co-exist when a polymeric material is exposed to radiation. When the chain scission process predominates over the crosslinking phenomenon, there is deterioration of mechanical strength, thermal stability of the polymeric materials. Side by side there is an increment in melt flow rate. On the other hand, if the crosslinking phenomenon

5 Thermally Stimulated Shape Memory Character of Radiation … Table 11 Designation of electron beam irradiated various alpha olefinic blends [11]

Blend ratio (50:50) (EOC 8440:EOC 8200)

175

Irradiation dose (kGy) Designation 0

EB00

25

EB25

50

EB50

100

EB100

predominates over the chain scission process, there is the improvement of mechanical strength, thermal stability of the polymeric materials, and increment in viscosity also. The difference in percentage crystallinity and comonomer content plays the vital role during the network formation by the application of the electron beam. Although the blends of two different EOC were irradiated at various radiation doses and both the EOC phase was exposed to radiation, it is a well-known fact that copolymer with higher branching density, i.e., higher octene content will attain the higher level of crosslink density [48, 49]. Therefore, it can be argued that in presence of electron beam higher level of crosslinks, formation will take place in EOC 8200 compared to the EOC 8440 as EOC 8200 possesses higher level of branching. This can be explained as follows. EOC 8200 with higher octene content possesses higher number of tertiary carbon atoms and the radicals which is formed on tertiary carbon atoms upon exposure of electron beam is much more stable than the other ones. Therefore, number of radical formation on tertiary carbon atom is higher for EOC 8200 than the EOC 8440 as shown in Fig. 16. Higher the number of radical formation, higher will be the formation of crosslinks network for EOC 8200 than the EOC 8440, and this was supported by the higher value of gel content for EOC 8200. It was demonstrated by the author that at 100 kGy radiation dose, the gel content value for EOC 8200 is 88%, and it is of 79% only for EOC 8440. This gel content value clearly indicates the better radiation crosslink ability of EOC 8200 copolymer with higher comonomer content or higher octene content rather than the EOC 8440 copolymer. The differences in network formation also reveal the significant impact upon the shape memory behavior of the blends. In this case, considering the DSC thermogram shape memory test of the blends was carried out at 70 °C, i.e., above the melting endotherm of EOC 8200 crystal which was 46 °C only. From the DSC thermogram, one interesting phenomenon was noticed that after crosslinking melting endotherm of EOC 8200 shifts to higher temperature, i.e., from 46.8 °C for the un-crosslinked one to 50.0 °C for the highest radiation crosslinked EB100 blend and the depth of the melting endotherm of EOC 8200 was also reduced. On the other hand, melting endotherm of EOC 8440 comes down from 95.6 °C for un-crosslinked one to 91.3 °C for highest radiation crosslinked EB100 blend. Thus looking at this phenomenon the author argued that may be the crosslinking phenomenon was predominating for EOC 8200, and the chain scission process was dominating for EOC 8440 during the electron beam treatment.

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Fig. 16 Schematic representation of the crosslinking mechanism of two olefins [11]

Above all from the SME cycle of the various blends itself it was noticed that the stress value gradually increased for the radiation crosslinked blends with increase of radiation dose. On the other hand, it was also noticed that after the electron beam treatment the shape fixity character that is primarily dependent upon the growth of the crystallite formation in the EOC 8200 phase deteriorated. Higher the radiation dose lower was the shape fixity value. The shape fixity value for the un-crosslinked blend was 94.10%, and it reduced to 88% for the highest radiation crosslinked (EB100 ) blend shown in Fig. 17. This reduction in the shape fixity value for the radiation crosslinked blends is due to breakage of crystallinity of soft EOC 8200 phase upon application of electron beam radiation. On the other hand, after crosslinking the scenario of the shape recovery character of the blends was opposite that can be clearly seen from the figure. Shape recovery behavior gradually improved with the application of radiation dose. Improvement in shape recovery character after the electron beam treatment clearly intimates the effect of crosslink formation upon it. As a result, the percentage shape recovery increases from 78.5% for un-crosslinked blend to 86.6% for highest radiation crosslinked EB100 blend. During the shape memory cycle, the change of sample length is pictorially depicted in Fig. 18. It clearly shows how the dumbbell-shaped sample of initial length (stage a) gets fixed to a fixed sample length (stage b) upon cooling during the shape memory cycle and finally it recovers from the fixed sample length to recovered length (stage c).

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177

Fig. 17 Shape fixity and shape recovery ratio values of un-crosslinked and radiation crosslinked blends [11]

Fig. 18 Pictorial depiction of the sample length of a dumbbell-shaped specimen during the shape memory cycle: a initial sample length, b fixed sample length, and c recovered sample length [11]

In general, the recovery of the polymer should be fast means it should come back to the original shape as soon as possible upon triggering of the external stimulus. From the figure, it can be noticed that the final recovery percentage for all the blends never reaches to 100%. But inside the heating chamber under stress-free condition, shape recovery of the various radiation crosslinked blends varies with time. Shape recovery time for all the blends was not more than 15 min, i.e., within 15 min recovery of all the blends gets completed. Only there was difference in percentage of shape recovery with time for different blend system. Percentage shape recovery as a function of time for different radiation crosslinked blends is illustrated in Fig. 19. This figure clearly illustrates the difference in shape recovery with time for various blends. After 5 min, the recovery of the un-crosslinked P2 R2 blend and highest radiation crosslinked P2 R2 EB100 is of 53.5% and 75.8%, respectively. After 5 min, the shape recovery for blends treated at 25 kGy and 50 kGy radiation doses is also higher than the un-crosslinked one. Fastest recovery of the crosslinked blends also clearly signifies the effects of the crosslinks formed upon the shape recovery behavior after the treatment of electron beam. Higher the radiation dose, higher will be the shape recovery and the shape recovery induction time will also be less. It means,

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Fig. 19 Recovery as a function of time for various blends [11]

formation of higher degree of crosslinks enhances the percentage shape recovery of the blends, on the one side, and on the other side, it also reduces the shape recovery induction time. To understand the creep behavior of the un-crosslinked and radiation crosslinked P2 R2 blend, strain versus time plot of various P2 R2 blends at room temperature and at 70 °C is demonstrated in Fig. 20a, c, respectively, and the corresponding creep compliance value has also been plotted in Fig. 20b, d, respectively. From Fig. 20a, it can be clearly noticed that upon application of the constant stress (0.5 MPa) instant deformation (primary creep) of the blends takes place due to the elastic nature of the blends. After that there was a steady and continuous increase of strain as a function of time with the subsequent attainment of steady state creep (secondary creep). Upon removal of stress, there is sudden drop in strain which is followed by a smooth decrease because of the elastic recovery of the blends. Viscoelastic nature of the polymer always results creep recovery below 100%. Figure 20a clearly demonstrates that there is a significant difference in both primary creep and secondary creep value among the un-crosslinked and radiation crosslinked blends. With increase of radiation dose, the substantial reduction in creep deformation for the radiation crosslinked blends compared to the un-crosslinked one clearly suggests the formation of crosslinked network structure that significantly lowers the creep behavior [50, 51]. Consequently, the crosslinked network structure improves the dimensional stability of the crosslinked P2 R2 blends also compared to its un-crosslinked blend. After electron beam crosslinking, lower response to creep behavior and better dimensional stability of the crosslinked blends also indicate the superior shape recovery behavior of the radiation crosslinked blends. At the same time, creep compliance values plotted in Fig. 20b reveal that there is a fall in the magnitude of the creep compliance value. Fall in the magnitude of the creep compliance for the crosslinked blends compared to the un-crosslinked one signifies the improved creep resistance behavior for the crosslinked blends, and consequently, it also supports the improved shape recovery behavior for the crosslinked blends. At 70 °C similar scenario in creep behavior was also observed as shown in Fig. 20c. At 70 °C the crystalline part of EOC 8200 copolymer will be in the molten condition

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Fig. 20 Strain versus time plot and creep compliance value of various P2 R2 blends at a and b room temperature and c and d at 70 °C [11]

as it has the melting point near about 46 °C obtained from the DSC curves. As the EOC 8200 phase is in the molten condition at 70 °C, creep deformation of the P2 R2 blend at 70 °C is much higher compared to the creep deformation at room temperature. In presence of electron beam both the polymer will get crosslinked (more or less depending upon the molecular structure). Especially the crosslink formation in EOC 8200 phase will show pronounced effect upon the creep response study at 70 °C as EOC 8440 has the melting point of 95 °C. Therefore, EOC 8440 has the same effect on the creep behavior at 70 °C like the effect on creep response at room temperature. Thus the crosslinks formed in EOC 8200 phase predominantly governs the creep response of P2 R2 blend. From Fig. 20c, it can be viewed that compare to the crosslinked blend, primary creep and secondary creep value of the un-crosslinked one is much higher. With increase of irradiation dose more the crosslink formation lower is the value of primary creep and secondary creep for the crosslinked blends, and consequently, creep recovery is higher for the crosslinked blends than that of the un-crosslinked one as shown in Fig. 20c. Similarly, there is a drastic reduction in the creep compliance value also for the crosslinked blends than the un-crosslinked one as shown in Fig. 20d. Creep compliance value reduces to 73.24% for P2 R2 EB100 blend than that of the un-crosslinked P2 R2 blend and this subsequent reduction in creep compliance also satisfies the better resistance to creep behavior for the crosslinked

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blends, and consequently, it also supports the better shape recovery behavior of the crosslinked blends.

3 Conclusions The overall review of the investigation and generation of shape memory polymers based on radiation crosslinked heterogeneous semicrystalline polymer blends reveals that such an approach has great future potential. As an alternative gateway to the expensive synthesis of block copolymers or complex interpenetrating networks, both the one-way and two-way multiple SM effect can be also achieved in covalent networks based on polymer blends, if a suitable phase morphology can be generated. From the review, it is also clear that for electron beam crosslinked networks multiple shape memory characters can be noticed in a one-step programming process while a multi-step programming process is required for the peroxide crosslinked network. In general for the electron beam crosslinked polyolefinic blends, after the application of the electron beam there was an improvement in shape recovery value due to the formation of net points or crosslinks while the reverse scenario was noticed in the case of shape fixity character because of the destruction of crystalline domain structure. The same scenario was noticed for the peroxide crosslinked heterogeneous polyolefinic blends also [52]. Although the radiation crosslinked polyolefinic blends are superior to peroxide crosslinked polyolefinic blends in terms of higher shape recovery behavior, free from foreign materials, etc., generally, SMP blends exhibit novel performance properties which are distinctly different from their pure counterparts. As a result, SMP blends can be suitable for various applications. It can be anticipated that, research in SMP blends will grow further due to the promising potential applications. Still, there are some shortcomings. In some cases, too low recovery stress of SMP blends confines their applications in various stages. Although shape recovery stress can be improved by incorporating other materials, it results in the low shape recovery ratios and deformable strain of the SMP blends. Ultimately, most of the shape recovery of the SMP blends is triggered by temperature, which also implies limited applications of SMP blends. In the future, further study is still needed for the development of multi-sensitive SMP blends that can respond to different parameters according to the applications. Acknowledgements The authors would like to acknowledge Board of Research in Nuclear Sciences, Department of Atomic Energy (DAE), Mumbai, India, for their kind cooperation during the electron beam treatment of the samples. Authors also would like to thank Dr. Y. K. Bhardwaj, (BARC, Mumbai) for his kind help and suggestion. Thanks to IPF Dresden, Germany, for the kind help in thermal characterization of the blends. The author would like to thanks to Mr. Syed Mushtaq and Mr. Pradip Das for their help during the thermomechanical test. Also thanks to Dr. Syed Mohammad Reffai, Dr. Partheban M., Dr. Pijush Mondal, Dr. Sayan Ganguly, Dr. Padmanabhan R., Dr. Asit Baran Bhattacharya, Dr. Sanjay Pal, and Dr. Anagha M. G.

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Chapter 6

Radiation Processed Emerging Materials for Biomedical Applications Bhuwanesh Kumar Sharma, Manjeet Singh, Snehal Lokhandwala, Shrikant Wagh, Subhendu Ray Chowdhury, and Sudip Ray

1 Introduction Radiation processing of polymeric materials has been well known as a clean and toxic chemical-free method for the synthesis of novel materials. The technique is renewable, non-toxic, and environmental friendly. Radiation-processed polymers

B. K. Sharma Department of Chemistry, SRICT-Institute of Science and Research, UPL University of Sustainable Technology, Ankleshwar 393135, India e-mail: [email protected] M. Singh Fuel Chemistry Division (FCD), Bhabha Atomic Research Centre, Trombay, India e-mail: [email protected] S. Lokhandwala Department of Environmental Science and Technology, SRICT, UPL University of Sustainable Technology, Ankleshwar 393135, India e-mail: [email protected] S. Wagh Department of Chemical Engineering, SRICT, UPL University of Sustainable Technology, Ankleshwar 393135, India e-mail: [email protected] S. R. Chowdhury (B) Isotope and Radiation Application Division (IRAD), Bhabha Atomic Research Centre, Trombay, India e-mail: [email protected] S. Ray New Zealand Institute for Minerals to Materials Research, 100 MacKay Street, Greymouth 7805, New Zealand e-mail: [email protected] Chemical and Materials Engineering, The University of Auckland, Auckland, New Zealand © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. R. Chowdhury (ed.), Applications of High Energy Radiations, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-9048-9_6

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show better property improvement regarding mechanical, thermal, chemical resistance, and biocompatibility as compared to chemical-processed polymers. Radiation technology has offered easy processing of both natural and synthetic polymers for useful healthcare applications where processed materials can be used for wound dressings, adsorbents of toxins, antibiotics, and antioxidant properties. One of the most versatile applications of polymeric materials has been studied in biomedical sectors. It is used at large level in medical, biotechnology, food and cosmetics sectors. Polymer applications include surgical devices, implants, and supporting materials (e.g. artificial organs, prostheses, and sutures), drug delivery systems, carriers of immobilized enzymes and cells, biosensors, components of diagnostic assays, bioadhesives, ocular devices, and materials for orthopaedic applications [1, 2]. Polymers which show physical and mechanical properties in bulk manner for implant are not reported for showing any blood compatibility. The best method to enhance the blood compatibility of the polymer is to undergo proper surface treatments on the polymer [3]. Mostly biopolymers such as polylactic acid (PLA) and poly-(E-caprolactone) (PCL) and synthetic biocompatible polymers have attracted more attention towards living systems (human body), biocompatibility, eco-friendly behaviour, and nontoxicity [4–6]. The technique of controlled degradation process of biopolymers has been considered for biomedical applications and environmental purposes for the last many years [7]. PLA, PGA, and PCL have played their role in healthcare applications, which play a proper biocompatible process. Nowadays, the common and suitable processing and sterilization technique is high-energy irradiation like electron beam, gamma, X-rays, UV radiation, microwave, and other high-energy radiation. Ionizing radiation imparts radiation-induced changes in the polymeric system which leads to crosslinking or scission of polymer main chain without any alternations polymeric properties [8]. The radiation sterilization methods for polymers have become an effective tool to illustrate the importance of radiation-processed polymers in our lives: forty years ago for making syringes and transfusion tubes but currently, they have been used to manufacture the artificial heart valve, blood vessels, and kidney membrane [9]. Radiation-crosslinked polymers blends may be useful for modification of surface morphology and crosslinking or chain scission in inter- or intra-phase of the two-component polymer system. An attempt to make radiation-processed blends of synthetic and natural polymer has come into focus for use in medical applications. The use of a certain amount of natural polymer (collagen) increases the biocompatibility of plastic masses used for medical devices. To study the “in-vitro” biocompatibility, the degree of the synthetic polymer (PVC) was bioactivated by collagen, compared to pure synthetic polymers. This technique of culturing was used in stabilized calf kidney epithelial cells [10]. Polyethylene and its blends are one of the most important plastics materials in the commercial market, and it has a broad application that can vary from food packaging to the medical industry. The irradiated polyethylene system usually can resist up to high temperatures (near the boiling point of water) with retaining its properties, which is useful for some applications, especially for medical use [11].

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Many varieties of synthetic and natural polymers which are biodegradable and biocompatible have been used to fabricate the stereo-regulated scaffolds, including synthetic polymers, such as poly-(lactide) (PLA), poly-(glycoside) and their copolymers, poly-(lactide-co-glycolide) (PLGA), poly-(E-caprolactone) (PCL), and natural polymers, such as collagen, protein, and fibrinogen [12]. Tissue engineering via radiation processing (TE) represents the repairing and re-establishing of the damaged tissue function “tools” like cells, scaffolds. In recent years, the radiation processing on tissue engineering (TE) scaffolds has been tried to improve, either using drugreleasing scaffolds or by incorporating drug delivery devices into TE scaffolds [13, 14]. In drug delivery, the open-loop-controlled devices through which the drug release process can be activated by an external stimulus with temperature changes, magnetism, ultrasound, electrical effect, and irradiation [15, 16]. Radiation sterilization has been approved as a safety procedure in tissue banking and engineering. For the radiation processing of tissue engineering, the soft tissue of human skin is treated for grafting onto a patient [17, 18]. The radiation sterilization of polymers of medical devices made of polymers has become the most suitable technique in radiation processing. The radiation sterilization of medical devices has been initiated by Johnson & Johnson Company (J&J) in 1956 [19]. A wide range of medical devices is sterilized using EB and X-ray processing. Some issues are mainly considered for using electron beams or X-rays derived from electron beams for medical device sterilization: (1) the materials used in the manufacture of the device; (2) when in the manufacturing operation will the device be exposed to radiation sterilization, and (3) the amount or degree of exposure needed to attain “sterility assurance” levels. Radiation-processed polymers have been known for great importance in human lives: Many years ago, radiation-processed polymers were limited to only syringes and transfusion tubes, but nowadays, they have got wide opportunities to make artificial heart valves, blood vessels, and kidney membranes. Recently, an attempt to blend synthetic polymer with natural polymer has become more interesting to increase bioactive characteristics for medical applications. The research effort has been carried out to obtain new categories of these materials through the blending of polyvinyl chloride with collagen for medical use [18, 19]. Ultra-high molecular polyethylene (UHMWPE) has attracted more attention towards radiation processing for medical purposes in the last decade. Electron beams of 10 MeV high-energy accelerators were first used for irradiation sterilization of packaged medical devices. Both in-house and service centre facilities have applied this type of equipment in manufacturing processes. When ionizing radiation is used for sterilization of medical devices, the question of compatibility arises between radiation and polymeric medical devices. Ionizing radiation not only kills microorganisms but also modifies material properties. Medical devices are made of many different materials, some of them are metals, but most devices are nonmetals, such as formed polymers, composite structures such as syringes, gloves, glucose solution bottles, and catheters; plastics such as polyvinyl chloride (PVC) are used to replace glass in some products, such as containers for saline solutions [19].

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2 Radiation Processing: Purpose High-energy radiations processing of polymers is used in wide range of industrial purpose to increase the mechanical, thermal, electrical insulation and chemical resistance of polymers and minimize toxic by-products. Electron beam processing shows reliability which is used as ionizing radiation for a particular commercial use. Gamma and electron beam irradiation have become the most popular and well-established methods for sterilizing polymer-based medical devices. The use of these techniques put significant alterations in the materials to be treated. The following reasons which are being concerned for radiation processing of polymer: 1. To alter the properties which are matched with end-use requirements, i.e. chemical, physical, thermal via chain crosslinking, etc. 2. Radiation grafting of polymer and copolymer for structural and functionalized modification. 3. To sterilize the medical product to minimize toxicity and contamination. 4. To study the degradation phenomenon of the polymer at a specified radiation dose. 5. Synthesis of polymers via radiation polymerization. 6. Synthesis of hydrogel, microgel, and nanogel for drug delivery and healthcare application. 7. To study the radiation stability of polymer blends and composites. 8. To establish structural property relationship in polymeric system. 9. To synthesize nanoparticle-reinforced nanomaterial for biomedical applications.

2.1 Advantages of Radiation Processing Over Conventional Chemical Method There are many ways to synthesize new functional polymeric materials or modify the surface properties of existing materials, but radiation initiation of the suitable chemical reactions has several advantages over classical initiation methods [4]: • The absence of additives (initiators and catalyst) is usually toxic materials that can contaminate the product • simple, efficient, clean, and environment-friendly process • The possibility of initiating the reaction at any temperature a wide variety of monomers and polymers that cannot be polymerized by classical chemical initiation • The possibility to control the degree of crosslinking and grafting • The possibility to control the depth of surface modification without altering the base properties • The simultaneous synthesis and sterilization • The possibility of simultaneous immobilization of bioactive materials without any loss in their activity

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• It usually allows combining the synthesis and sterilization in a single technological step, thus reducing costs and production time.

3 Types of Radiation Processing The radiation used in polymer materials is limited to radiation from high-energy cosmic ray, gamma rays, X-rays, UV radiation, and accelerated electrons. These types of radiation are called “ionizing” because their energy is high enough to remove electrons from atoms and molecules and convert them to electrically charged particles called ions [20, Fig. 1]. Electron beam (beta ray) and gamma rays are even more penetrating and more dangerous than heat, light, alpha ray, and non-ionizing micro and radio waves (Fig. 2). Electrons, X-rays, and gamma rays ionize the material they strike by stripping electrons from the atoms of the exposed material [20–22]. This ionized environment is very damaging to bacteria, viruses, and insects and can also change the chemical structure of materials. Irradiation is simply the act of applying radiation (or radiant energy) to some material. Irradiation by penetrating electrons, X-rays, and gamma rays ionizes materials rather than simply heating them. The term irradiation usually is not used to non-ionizing radiation, such as infrared, visible light, microwaves from cellular phones, or electromagnetic waves emitted by radio and TV receivers and power supplies. All kinds of radiation are being used to sterilize objects after use and produce medical instruments and disposables such as syringes and disinfect and sterilize food [23]. Small doses of ionizing radiation

Fig. 1 Ionizing and non-ionizing radiations spectrum [20]

Fig. 2 Penetration effect of alpha, beta, and gamma beam on matter

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(X-rays, gamma, and electron beam) may be used to kill bacteria in food or other organic material, including blood [22, 23].

3.1 Ultraviolet (UV) Radiation Processing Ultraviolet (UV) light is electromagnetic radiation with a wavelength lower than visible light but higher than X-rays, in the range between 400 and 200 nm. UV radiation is found in sunlight and is emitted by electric arcs and sources such as mercury lamps and black lights. A large fraction of UV, including all that reaches the surface of the earth, is known as non-ionizing radiation. The change in properties due to UV irradiation occurs from the ultraviolet photon’s power without to ionizing atoms enough. UV rays are usually used to polymerize monomer and oligomers into high molecular weight polymer via photopolymerization [24]. In photopolymerization, UV ray itself acts as photo-initiator (Fig. 3). UV radiation processing of natural polymer Ultraviolet radiation has also applications in medical sector such as treatment of skin conditions. The combination of UVA and UVB radiation has been much used treatment for psoriasis although this treatment is not frequently used now because the combination produces dramatic increases in skin cancer. In cases of skin infections, e.g. psoriasis and vitiligo, UV light with a wavelength of 311 nm is shown effective treatment. Ultraviolet light has been used to sterilize workplaces, products, and tools used in biology laboratories and medical facilities. Commercially available lowpressure mercury-vapour lamps emit about 86% of their light at 254 nm (nm), which coincides very well with one of the two peaks of the germicidal effectiveness curve (i.e. effectiveness for UV absorption by DNA). Natural polymer-like DNA shows effective absorption at a particular specified wavelength of UV light, and peptide bond of DNA polymer is broken by treatment of UV rays [25] (Fig. 4).

Fig. 3 Ultraviolet (UV) polymerization of liquid monomer [24]

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Fig. 4 UV light processing of natural DNA polymer [25]

Effect on synthetic polymer It is known that UV degradation of polymer falls under the category of photodegradation, e.g. UV degradation of polyethylene (PE), polypropylene (PP), polystyrene (PS), poly-(methyl methacrylate (PMMA) and polyvinyl chloride. UV irradiation causes the degradation in polymer chains which leads to decrease in tensile strength. The degradation in polypropylene by UV irradiation causes due to presence of quaternary carbon atom present in repeating manner. The impact of UV on polymers is used in nanotechnology, X-ray lithography, and medical facilities [26]. More recently, this technique has been used in the preparation of biomaterials with applications in important areas such as tissue engineering, biosensors, development of drug delivery systems, dental restorations, and surface modifications to control the material’s cell adhesion [27, 28]. Polymers which are synthesized by UV polymerization process show various applications like bone and cartilage tissue engineering. Non-Ionizing UV sterilization Ultraviolet light irradiation is also useful for the sterilization of surfaces and some transparent objects. However, some polymers such as polystyrene, polypropylene, and polyvinyl chloride are not able to sustain under UV exposure. UV-cured hydrogel are three-dimensional polymeric gel which is used in biotechnology and medical fields. 3D network hydrogel is capable for gene therapy, wound dressing, and drug delivery. Monomers and oligomers under UV exposure undergo chain crosslinking by free radical chain polymerization processes. Both monomer and oligomer can be crosslinked and cured due to presence of reactive functional group at their chain end. Under UV irradiation, polymer is synthesized by initiation, propagation, and termination. Photo-initiator plays important role to initiate polymerization by reaction amongst reactive functional group. The UV crosslinking process occurs by reaction between reactive functional groups in repetitive manner to form the 3D network polymer gel.

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3.2 Microwave Irradiation of Polymers Microwave energy is a form of electromagnetic radiation with a wavelength between 1 mm and 1 m (located between infrared radiation and radiofrequency). Microwave irradiation is a well-established technique for study and synthesize different organic compounds to correlate the structure and property relationship. Microwave irradiation is an alternative, efficient, selective, and fast volumetric heating method that increases the energy of molecules by directly interacting with their dipole moment [29]. In general, principles of microwave heating are based on the ability of polar substances to absorb and transform microwave irradiation into heat. Microwave irradiation has advantages over conventional heating method, e.g. homogenous and selective heating with higher heating rates, higher yield, and energy saving. The application of microwave synthesis of polymeric materials has various advantages, e.g. energy saving, lesser reaction time, reduction of waste, and CO2 emission. People have synthesized the organic compounds and polymeric materials by microwave irradiation technique with high efficient method [30, 31]. One of most advantages of microwave technique is synthesis of compound in rapid manner for medical, industrial, and domestic applications. Microwaves are electromagnetic radio waves having wavelengths ranging from one metre to one millimetre and frequencies between 300 MHz (0.3 GHz) and 300 GHz. Microwaves have applications widely in medical sector such as surgery and computed tomography (CT) [32–34]. Various polymers are synthesized by microwave technique for their medical applications. The surface modification of metallocene polyethylene (mPE) by microwave radiation has been studied for blood compatibility. The blood compatibility of mPE is improved for the fabrication of implants, blood-contacting devices, and artificial vascular prostheses [35, 36]. Advantages of microwave irradiation Non-contact heating. Energy transfer instead of heat transfer. Material selective and volumetric heating. Fast start-up and stopping.

3.3 X-ray Processing X-radiation or X-rays are generated by impinging accelerated electron upon materials. X-rays having broad spectrum of energies can be produced by bombarding accelerated generated electron beam upon high-atomic-number metal, such as tungsten or gold. Monoenergetic X-ray photons are produced by the electron interaction with orbital electrons; bremsstrahlung photons are produced by the interaction with the nucleus of an atom.

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Fig. 5 Comparative penetration of electron beam (β), gamma (γ ), and X-ray beam [37]

High energy of bremsstrahlung X-rays is penetrating ionization radiation. X-rays show highest penetrating power compared to electron beam and gamma [37] (Fig. 5). Such X-ray intensities from high power, high-energy industrial X-ray generators far exceed those of common medical X-ray equipment. X-radiation can now be used as an alternative to the use of radioactive isotopes in such areas as medical device sterilization. Penetration power of X-ray is much greater than highest energy produced by the electron beam systems (10 MeV), and penetration power of X-ray is comparable to gamma radiation. A variety of polymer products and materials may be irradiated with X-radiation to modify their characteristics and improve their economic value or for health-related purposes. Examples are single-use medical devices (sterilization), agricultural commodities (preservation), and various polymeric products (material modification). High-power X-ray technologies are commercially used in many other applications such as sterilization of medical devices and environmental applications [38, 39].

3.4 Electron Beam Processing (EBP) of Polymers Physical and chemical changes induced by the absorption of high-energy radiation are the important topic of both industries and universities research over the past sixty years. Material modification and polymer properties enhancement by the utilization of electron beams have been increased and well documented more than four decades. The plastic industries have been using e-beam processing (EBP) to improve thermal, chemical, barrier, impact, wear, and other properties of inexpensive commodity thermoplastics, extending their utility to demand the applications. EBP of crosslinkable plastics has produced the materials with improved dimensional stability, reduced stress cracking, higher service temperatures, reduced solvent and water permeability, and significant improvements in other thermomechanical properties [40].

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Electron beam processing is widely used for an industrial purpose for various kinds of polymer products. For commercial purpose, there are highly efficient EB accelerators with high output. The accelerators are typically described in terms of their energy and power. Low-energy accelerators have energy range from 150 keV to 2.0 MeV, and medium-level accelerators are known to have from 2.5 to 8.0 MeV. High-energy accelerators have beam energies beyond 9.0 MeV. The requirement of EB irradiation and its energy depends upon end-use application, e.g. curing, ink coating, crosslinking for packaging film requires the energy range of accelerator from 150 to 500 keV [40]. Electron beam accelerator generates ionizing electron beam radiation which demonstrates wide-range applications for radiation processing of the various polymeric materials. Electron beam accelerator does not possesses the harmful effect of radioactive isotope which shows its high reliability and durability for radiation processing [40]. The electron beam (EB) accelerator is widely applicable in industries for irradiation purpose to enhance the mechanical, chemical, thermal, and electrical insulation properties of polymeric compound. The electron beam irradiation is a clean and free from toxic by-product compared to gamma irradiation. The highcurrent E-beam accelerator has also beneficiary effect of conversing the electron beam to X-rays commercially to replace the gamma irradiation [41, 42].

3.5 Gamma Processing of Polymers Gamma radiation is one of most high energetic electromagnetic radiation which is widely used for polymer processing. Gamma radiation is known to possess very short wave length since it has high-energy frequency. Gamma radiation has more penetrating power than alpha and beta rays but less than X-rays [43, 44]. Due to high-energy nature, gamma radiation is also used in medical sector to destroy the body cancer cells. On Earth, gamma radiation is generated by nuclear explosions and the radioactive decay [45]. Hydrogels were synthesized by gamma in a wide range of different shapes and sizes from microgels to nanogels. The grafting of suitable monomers by gammainitiated polymerization was used for biocompatible surface preparation, haemodialysis membranes, and implant applications. Ionizing radiation like EB or gamma ray is an efficient tool for sterilization. A great part of single-use medical products is sterilized by radiation technique. The application of radiation for the formation of hydrogels for biomedical use offers a unique possibility to combine the formation and sterilization of the product in a single technological step. X-rays have a similar effective penetration as gamma rays which are derived from radioactive sources, mainly cobalt-60. X-rays have slightly higher dose rates than gamma rays and may not be as deleterious on some polymers, formulated PP, for example like gamma ray radiation [44–46].

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Effects of Electron beam (EB) and gamma irradiation on polymers High-energy radiation exposure to polymeric material causes homolytic scission in covalent bond and leads to formation of free radicals. These species turn towards recombination to give stable products, often through other free radicals and metastable species. The change in structure of polymer due to radiation exposure is influenced by irradiation conditions, e.g. exposure in oxygen environment leads to more structural degradation. EB and gamma irradiation cause many modifications in polymeric systems such as (i) crosslinking, (ii) polymerization, (iii) scission, and (iv) sterilization of medical disposables, and these technologies are applied nicely in industries. Irradiation to polymer blend causes the bond scission and crosslinking in either intra-phase or interphase depending upon amorphous and crystalline fractions. Those effects modify the blend properties [41, 43]. Various studies have been reported for highenergy radiation processing of different polymer blends to investigate the stabilization of the phases and property development. The irradiation of polymeric materials with ionizing radiation (gamma rays, X-rays, accelerated electrons, ion beams) leads to the formation of very reactive intermediates via a free radical mechanism. These intermediates can do several reaction ways, which leads to the formation of new bonds. These all transforms into chain scission, chain crosslinking, grafting of monomer, and radical polymerization in polymeric materials. The alternations depend on the structure the conditions of treatment of polymers before and after radiation exposure. Nowadays, the structural modification of polymers includes crosslinking, radiation-induced polymerization (graft polymerization and curing), and their degradation. Electron beam or gamma treatment of polymers can provide three different effects, which can result in improved mechanical properties and provide an improvement in processing characteristics. During irradiation, following phenomena are taken place simultaneously: Crosslinking: The formation of bonds between polymer chains, allowing them to link together (Fig. 6). Scissoring: The breaking of bonds in the main polymer chains. Branching: The alteration of the number and types of side chains from the polymer backbone. Electron beam and gamma treatment of polymers to crosslink or scission polymer chains or alter their branches are particularly effective because: • • • •

No special additives are required The process is carefully controlled and reproducible A wide range of polymers may be processed The process reduces cost.

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Fig. 6 Electron beam (EB) or gamma crosslinking in material

Improvements in property and end-use performance Through the irradiation of polymers, the improvements may be achieved by building up long-chain branching within the polymer as well as controlling the degree of crosslinking. Some of these improvements include: • • • • • •

Higher mechanical strength Improved chemical resistance Improved thermal properties Higher environmental stress crack resistance (ESCR) Better surface properties Excellent biocompatibility, non-toxicity, and biodegradability.

3.6 Neutron Beam Processing Neutron beam is generated in neutron source devices which contain compact linear accelerators and produce neutrons by fusing isotopes of hydrogen. By bombardment of accelerated deuterium, tritium, or a mixture of both into a metal hydride target containing deuterium, tritium, or a mixture of these isotopes, results fusion reaction in target. Neutron generators have applications in medicine, security, and

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materials analysis [45, 47]. Neutron beam processing on polymers has been studied by small-angle scattering. One of the longstanding issues in polymer science is the crystallization of polymers underflow; recent experiments on polymer crystallization underflow are using time-resolved depolarized light scattering, small-angle and wide-angle X-ray scattering, and small-angle neutron scattering in a wide spatial scale from 0.1 nm to several tens μm [48]. Neutron reflectometry is used to study surfaces, thin films, buried interfaces, magnetic films, multi-layered structures, and processes that occur at surfaces and interfaces. Small-angle neutron scattering (SANS) is a highly versatile technique for investigating a wide range of materials including polymers, emulsions, colloids, superconductors, porous materials, geological samples, alloys, ceramics, and biological molecules such as proteins and membranes. SANS technique is used to characterize the polymer blends & composites. Neutron reflectometry provides information on the composition, changes in surface characteristics over time, thickness, and interfacial roughness of the surface of films [49].

4 Classification of Polymeric System for Biomedical Application Both natural and synthetic polymers and their blends and composites have been used for radiation processing in different biomedical applications.

4.1 Single Polymer System 4.1.1

Natural Polymers

To gain importance with the rapid development of therapeutic protein products and carbohydrate-based delivery systems, various aspects of the bioprocessing method can impose tremendous stresses on macromolecular-based products. One of them is sterilization (thermal processing and irradiation) processing is commonly used to sterilize the biopolymers like carbohydrates and protein. The form of sterilization is irradiation by γ- or X-ray radiation [50]. Both have sufficient energy to produce ionization of material leading to disruption of the DNA of microbial and other contaminants to products. The disruption is either by direct cleavage of the DNA or secondary cleavage through the release of free radicals. Polysaccharides such as xyloglucans are not only used as mucoadhesive, hydrogels but also as direct therapeutic agents themselves [51]. Various biomolecules show active immuno-stimulation to make useful them for wound healing and tissue engineering applications. Poly(3-hydroxybutyrate) (P(3-HB)) and its copolymer, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P(3-HB)-co-3-HV), are biodegradable

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thermoplastic polymers generally referred to as poly-(hydroxyalkanoates) (PHA’s). Due to their characteristics, P(3-HB) and P(3-HB-co-3-HV) can be largely used in applications for medical devices due to their good mechanical, thermal, and biocompatibility characteristics [52–54]. FDA approval of poly-(4-hydroxybutyrate) (P4-HB) for surgical sutures and meshes has cleared the way for more specialized uses. In this context, the use of the sterilization method [EB or Gamma] for P(3-HB) and other PHA’s is of extreme importance [55]. However, irradiation of polymers may cause physicochemical changes such as embrittlement, discoloration, odour generation, and alterations in chemical resistance or melting temperature. These changes result, primarily, due to main chain scission and crosslinking of macromolecules generated by interactions with gamma rays. Irradiation of natural polymer chitosan with gamma or electron beam increases its antimicrobial activity, i.e. to resist the infection of diseases [1]. A polymer film from homogeneous monomer solution (Ethylene glycol dimethacrylate, tetra ethylene glycol dimethacrylate, triethylene glycol dimethacrylate, nona ethylene glycol dimethacrylate, and hydroxyethyl acrylate, HEA) has been synthesized for biomedical application. A homogeneous monomer solution containing these monomers and antibodies in the vessel was irradiated by low energy electron beam accelerator (1Mrad, 300 kV, 5 mA) [56–58]. The immobilized anti-AFP films obtained by this technique are applicable for the enzyme immunoassay of AFP (Anti-α-fetoprotein), tumour-associated antigen [59–61].

4.1.2

Synthetic Polymeric System

Microwave-assisted radiation processing of metallocene polyethylene (mPE) for surface modification has been studied for better biocompatibility in medical applications. Contact angle analysis has revealed a decrease in the contact angle of the treated samples which signifies an increase in hydrophilicity and better biocompatibility. Coagulation assays performed for estimating prothrombin time (PT) and activated partial thromboplastin time (APTT) showed an increase in the clotting time which further evidencing the improved blood compatibility of the microwavetreated surfaces. Microwave-assisted surface modification of mPE enhanced in blood compatibility. Improved blood compatibility of mPE may be thought of for application for the fabrication of artificial vascular prostheses, implants, and various blood-contacting devices [36]. Radiation-assisted sterilization of synthetic polymers at mild doses has been a common and effective method for irradiation for medical applications. Polypropylene (PP) is used in medical device manufacturers because of its stiffness and greater resistance to thermal distortion (T d ) than that of even the highest density (0.965) HDPE. However, when exposed to radiation, polypropylenes are known to chain scission [62]. Electron beams were first used at the very outset of irradiation sterilization, in the 1950s. High and mid-energy level accelerators, e.g. 10 MeV, 3.0–5.0 MeV are used to sterilize the various medical products such as medical operation theatre accessories, syringe, and catheters. Electron beam crosslinking of UHMWPE has improved the

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significant properties and performance of UHMWPE for medical implants compared to conventional gamma sterilized UHMWPE. These highly crosslinked UHMWPEs have great importance to reduce osteolysis in the longer term. PVC tubes and blood-containing bags and containers for saline solutions to replace the glass products are widely used in the hospitalization process. In the medical device area, there is a problem with the discoloration of PVC materials, such as tubing, bags, and other low-cost medical supply items, when exposed to radiation sterilization, e.g. EB or gamma sterilization at a certain dose. This issue has been overcome by the use of additives and alternatives which have been developed to replace the use of PVC in medical devices [63]. Rigid transparent medical devices can be fabricated from radiation-resistant materials aromatic structures, such as polystyrene (PS), polycarbonate (PC), and polyethylene terephthalate (PET). The manufacturers of PC and PET have developed specialty grades that minimize any discoloration from radiation exposure. Figures and show devices that are made from these rigid transparent plastics [64]. Flexible tubing and other products for medical devices are also made out of silicone elastomer.

4.2 Polymer Blends These newer materials are based on polyethylene blends include the optically clear metallocene-catalysed polyethylene (mPE) or polypropylenes (mPP) and composites of such materials to replace PVC for radiation processing of similar applications as shown in figure exposed to the electron beam and X-ray sterilization. The blend of polyethylene glycols (PEG) and polyvinyl alcohols (PVA) is used for radiation sterilization for suitable biomedical use. The ethylene unit of those polymeric materials shows affinity towards radiation-induced chain crosslinking. Poly-(vinyl alcohol) and poly-(vinyl pyrrolidone) (PVA-PVP) blends for wound dressing have been prepared by using the gamma-rays irradiation technique. The justification of application has been carried out by the water vapour transmission rate. PVA/PVP-blended hydrogel shows water vapour transmission rate (WVTR) from 50 to 200 g/m2 /h. The polymer blends can be considered a good barrier against microbes. It was found that the PVA/PVP blend was a very useful material that can meet the requirement [65]. PVA/PVP blend and its properties has been optimized to use in biomedical applications. The properties of the blends have also been studied in the context of their biomedical applications. The biocompatibility of materials based on blends of collagen and synthetic polymers such as polyvinyl pyrrolidone (PVP) or polyvinyl alcohol (PVA) or other natural polymers such as chitosan has been studied by UV-assisted radiation processing. The mechanical and surface properties of chitosan, poly(vinyl pyrrolidone) (PVP), and chitosan/PVP blends have been studied, and the influence of UV irradiation on these properties has been compared [66]. New materials based on blends of chitosan and PVP were irradiated with UV irradiation with a wavelength of 254 nm for different lengths of time as this radiation

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is usually used for the sterilization of biomedical materials. A non-toxic, biocompatible, pH-sensitive hydrogel system with the potential ability to immobilize and absorb proteins can be obtained from the blend of poly-(N-vinylpyrrolidone) and carboxymethylated chitosan. Blends can be prepared with electron beam irradiation at room temperature [67, 68].

4.3 Polymer Composite and Nanocomposite It has been seen that the use of natural fibres as reinforcement in polymeric composites is an important research field that has been growing in the last decades. The interaction of high energy radiation with natural fibres e.g. cellulose and lignin causes the chemical alteration in their structure by chain scission or crosslinking [69]. The incorporation of natural fibre in the blend system of PLA (Polylactic acid) and PCL (Polycaprolactone) has been tried for application in the biomedical field. Due to the high costs of PLA, the focus was initially on the manufacture of medical-grade sutures, implants, and controlled drug release applications. Synthetic biodegradable aliphatic polyester, i.e. polycaprolactone (PCL) has also the potential effect in the medical application used in the medical field as a biodegradable suture. The addition of natural fibre may bring cost reduction and increase biocompatibility and mechanical strength for a composite system in the relevant field [70]. Many researchers have developed polymer-drug composites with various solid forms such as disc, needle, and sphere and applied them to implantable drug delivery systems. However, recent medical doctors wanted a liquid from the polymer-drug mixture that can be injectable conveniently but solidified in the body after injection for continuous sustained release. The development of polymeric nanocomposite has also appeared as a potential candidate in healthcare applications. The electrospinning technique uses several different solvents to fabricate nanomaterials, for example to fabricate biomimetic nanostructured scaffolds for tissue engineering, the electrospinning of chitin/silk fibroin blend solutions in 1,1,1,3,3,3-hexafluoro-2-propanol was investigated [71]. The nanofibrous matrix made from a chitin/silk fibroin blend can be considered suitable for tissue engineering scaffolds with excellent cell attachment properties [72]. Another material for biomedical application can be produced as a film by mixing chitosan and silk fibroin. Blends of natural and synthetic polymers as a matrix for micro- and nanoparticles such as hydroxyapatite, silica, and aragonite incorporation have been tried to make a composite. For the development of new materials based on blends and composites containing two or more polymers and inorganic nanoparticles, many laboratories are working. New materials based on the blends of two polymers or biopolymers which contain nanoparticles can be used as an implant in the context of both hard and soft tissues. Nanostructured materials can be achieved by the intercalation of inorganic nanoparticles in a polymeric matrix.

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5 Preparation and Processing of Radiation-Processed Polymers Hydrogel As has been seen above that the use of ionizing radiation in two major areas: synthesis of polymers and gels for medical and biotechnological applications, and modification of surfaces to achieve specific functionality and/or to immobilize bioactive materials. Hydrogels are crosslinked polymer networks that possess the nature of swell in water and biological fluids. The crosslinks are usually formed by covalent or ionic bonds although physical crosslinks, such as entanglements, hydrogen bonds, hydrophobic, or van-der-Waals interactions, can also provide three-dimensional networks [72, 73]. Hydrogels can be synthesized either by radiation polymerization and crosslinking of a monomer or monomer mixtures (in bulk or solutions) or by crosslinking of polymers. For burn and wound dressing, hydrogels are commonly used materials. They prevent microbial contamination of the wound, inhibit the loss of body fluids, and provide free flow of oxygen to the wound and generally accelerate the healing process. The properties of the hydrogels (pore size, swelling-shrinking, etc.) can be controlled by the synthesis conditions. Properties of hydrogel are very much affected by the absorbed dose, dose rate, monomer(s) type and concentration (their ratio), and the irradiation temperature. By appropriate selection of these parameters, product can be prepared as per requirement for a specific application. Hydrogels can be considered as “classical” or “stimuli-responsive”, synthesized by gamma and electron beam irradiation, in a wide range of different shapes and sizes from nanogels and microgels to microgels [74]. The water content of hydrogels is usually very high due to good biocompatibility and the possibility of application in direct contact with living tissues. One of the most common applications is wound dressing. Radiation-synthesized hydrogels that were developed by a participant of this CRP are commercially produced in Poland and patented in several other countries. Collaborative research to develop new copolymer hydrogels with improved properties for wound dressing is continuing. Many synthetic and natural polymers have been used for the preparation of hydrogels. Biodegradable and affinity-based hydrogels were also studied.

5.1 Synthesis of Hydrogel by Radiation Processing Hydrogels are materials having permanent three-dimensional network of hydrophilic polymers that can able to fill water in the space between the polymer chains. Hydrogels are synthesized in many “classical” chemical ways. These include onestep procedures like polymerization and parallel crosslinking of multifunctional monomers, as well as multiple-step procedures involving the synthesis of polymer molecules having reactive groups followed by crosslinking, and the reaction of polymers with suitable crosslinking agents [73].

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Ionizing radiation has been long recognized as a very suitable tool for the formation of hydrogels [74, 75]. Easy process control, the possibility of joining hydrogel formation and sterilization in one technological step, no necessity to add any initiators, crosslinkers, etc., possibly harmful and difficult to remove, relatively low running costs, no waste—this makes irradiation method as a best choice in the preparation of hydrogels, especially for biomedical use. Hydrogels are typically synthesized by one of the two well-established procedures: (a) polymerization and simultaneous or post-polymerization crosslinking of hydrophilic monomers and (b) modification of hydrophilization of existing polymers with potential hydrogel properties. Hydrogels can be obtained by radiation technique in different ways, including irradiation of solid polymer, monomer (in bulk or solution) or an aqueous solution of the polymer. The first method, i.e. irradiation of hydrophilic polymer in a dry form [76], has some drawbacks; therefore, it may require special sample preparation (like pressing or melting) for obtaining homogeneous macroscopic hydrogels. Moreover, it requires usually much higher doses of ionizing radiation to obtain a gel compared to irradiation in solution, and it may be difficult to remove fully the oxygen, which can promote unwanted side reactions. Most common the technique of irradiation of monomer is applied [74, 75]. In this technique, polymerization takes place in the first stage, followed by crosslinking of the formed chains. This way is possibly most convenient when the chosen monomer is easily available, but its polymer is not. Since many of the monomers used are harmful or even toxic (usually contrary to the corresponding polymers), particular care has to be taken when using this method for the formation of hydrogels for biomedical use to ensure that either all the monomer has reacted or its unreacted residues have been fully extracted in a separate operation. Carenza has reviewed the inherent advantages of using high-energy radiation in the synthesis of hydrogels for biomedical applications [77]. The preparation of hydrogels by radiation treatment of aqueous solutions of hydrophilic monomers or polymers carries some advantages over conventional techniques. Typical examples of simple, synthetic polymers used for hydrogel formation by this method are poly(vinyl alcohol)-PVA, poly-(vinyl pyrrolidone)-PVP, poly-(ethylene oxide)-PEO, and poly-(Acrylamide, PAAm). Preparation For the preparation of acrylamide/maleic acid hydrogels, acrylamide (AAm) weighted 1 g is dissolved in 1 ml aqueous solution of 0, 20, 30, 40, 50, 60 mg maleic acid, respectively. For the preparation of poly-(n-vinyl-2 pyrrolidone/itaconic acid) hydrogels, 2 ml n-vinyl-2 pyrrolidone is dissolved 1 ml aqueous solution of 0, 60, 120, 180, 240 mg itaconic acid. To increase the amount of IA in the VP monomer and to prepare different network properties having hydrogels, 2 ml n-vinyl-2 pyrrolidone is dissolved in 1 ml aqueous solution of 60, 120, 180, and 240 mg itaconic acid and 0.25% (v/v, EGDMA/VP) crosslinking agent EGDMA added in the mixture. These solutions are irradiated at different doses in the air at ambient temperature in a 60Co gamma cell 220γ-irradiator [78]. Hydrogels are dried in a vacuum oven at 315 K to constant weight and subjected to Soxhlet extraction with water as solvent

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Uncrosslinked polymer and/or residual monomer are removed with this extraction from the gel structure. Extracted gels are dried again in a vacuum oven at 315 K to constant weight. Percentage gelation, i.e. percentage conversion of monomers into the insoluble network was based on the total weight of the VP or AAm and diprotic acid in the initial mixture.

5.2 Hydrogel for Therapeutic (Drug Delivery System) Use The therapeutic system for the local release of prostaglandins is based on hydrogel devices obtained by irradiation of N-vinylpyrrolidone (VP). The method of obtaining the therapeutic system is a three-stage procedure. It consists of radiation polymerization and crosslinking of VP, incorporation of prostaglandin into hydrogel matrix, and subsequent radiation sterilization of product. The polymerization is carried out in a special form, which enables obtaining the desired shape of devices. Placing the surgical silk thread in the monomer before irradiation makes it possible to obtain the rod with strongly fixed thread. Incorporation of prostaglandin into the devices is carried out by placing this rod inappropriate solution of the hormone. The sterilization is carried out in a cobalt-60 source with a dose of 25 kGy. It has been found that these therapeutic systems are highly useful and safe for women in childbirth.

5.3 Hydrogel for Contact Lens (Ophthalmic Sector) Poly-HEMA is a tolerable material, primarily the main component of contact lenses in medical fields. Poly-HEMA and its various combinations with other, both hydrophilic and hydrophobic, polymers are till now the most often used hydrogel materials for contact lenses in medical purposes. The market of contact lenses is completely subjected to the products of poly-HEMA. There are also a great number of various hydrogel compositions, which contain copolymers such compounds as poly-(vinyl pyrrolidone), poly-(vinyl alcohol), and poly-(methacrylic acid). Co-polymerization is mainly used to improve the mechanical properties of poly-HEMA and increase its oxygen permeability. In most countries, some contact lenses produced by radiation technology have been marketed [79].

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5.4 Preparation of Polymers via UV Radiation Processing for Contact Lens Poly-HEMA has been modified with many natural and synthetic substances and by various methods and has been applied in the production of contact lenses. The properties of poly-HEMA depend, amongst other things, on the method of synthesis, polymer content, degree of crosslinking, temperature, and final application environment. The synthesis can be carried out with simultaneous crosslinking by UV radiation. In one possible way of preparing poly-HEMA [80], 2-hydroxyethyl methacrylate (HEMA) as a monomer, polyethylene glycol dimethacrylate as the crosslinking agent, and benzoin isobutyl ether (BIE) as the UV-sensitive initiator were used. Deionized water (DI) in an amount appropriate to the desired concentration should be added to the system prepared for components. The final products are obtained in the form of films or membranes by treating them with UV radiation (4 lamps, 20 W, λ = 253.7 nm, 11 mm distance from the source, 20 min). Next, the film is immersed for 24 h in water until it is fully saturated to remove toxic or unreacted substances that could damage living tissue. Small-angle neutron scattering (SANS) Neutron beam radiation-assisted processing of prepared hydrogel has been seen widely. Small-angle neutron scattering is a versatile technique for investigating a wide range of polymers for contact lens applications (Fig. 7). SANS is a powerful technique to investigate structures on the nanoscale from 1 to 10 nm. When a neutron beam impinges on a sample, some neutrons scatter along a path that differs from the transmitted beam by as little as several hundredths of a degree. This “small-angle” scattering provides information about relatively large structural details on the molecular scale. This technique can provide shapes, particle sizes, and averaged distributions over a macroscopic sample. One of the main problems with successful extended wear contact lenses has been the inability of conventional hydrogels to prevent significant overnight corneal swelling caused by low oxygen permeability. To improve materials’ Fig. 7 Small-angle neutron beam scattered hydrogels for contact lens

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suitability for extended wear contact lenses, novel block copolymer materials with high oxygen permeability in combination with superior hydration properties need to be developed. To understand this, the molecular architecture of diblock copolymers with phase separation on the nanoscale (to ensure optical clarity) is being determined by SANS [49].

5.5 Radiation-Processed Polymeric System for Implant 5.5.1

Surface Modification

Radiation-processed polymeric materials either can be implanted in a living system for short or long-term applications or used externally. The acceptance and failure of a biomaterial depend mostly on its surface properties. The surface modification of these biomaterials has ranged from a simple treatment for cleaning, to specific treatments to improve biocompatibility or to enhance or prevent protein and cell interactions. Since radiation-induced processes can easily be limited to the surface area only, so they are specially fitted for these modifications. Gamma and electron beam irradiation, as well as radiofrequency glow discharge (RFGD), was used to tailor polymer surfaces to form non-fouling, protein-repellent surfaces for major applications as implants.

5.5.2

Radiation Grafting

Grafting of relevant monomers by gamma and electron beam-initiated polymerization was used for biocompatible surface modification for implant applications. Modified surfaces were prepared by electron beam-initiated grafting of stimuliresponsive polymeric hydrogels for use as potential surfaces modification for implantation (Fig. 8). Radiation grafting has been a well-known technique for modifying the surfaces of materials for many years [81]. Grafting adds low molecular weight moiety or monomer to a linear and high molecular weight formed polymer to alter properties. Grafting can be defined as the ability to attach or grow a different material onto the backbone of another. A chemical bond is then formed between the grafted moiety and the material. Low-energy EB processing is well suited for surface grafting functionalization. Radiation grafting has been used to enhance the biocompatibility of polymers. Hydrogel in the objective of implantation can be synthesized by radiation polymerization and crosslinking of a monomer or monomer mixtures (in bulk or solutions) or by crosslinking of polymers. The properties of the hydrogels (pore size, swelling-shrinking kinetics, water content, etc.) can be controlled by the synthesis conditions.

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Fig. 8 Preparation of hydrogel via modification of polymer by radiation grafting method [108]

It is widely reported that the absorbed dose, dose rate, monomer(s) type and concentration (their ratio), and the irradiation temperature affect the properties of hydrogels. By appropriate selection of these parameters, gels can be prepared as per use for a specific application [4]. Radiation-induced crosslinking of some hydrophilic polymers [poly-(acrylic acid, PAA), poly-(ethylene oxide, PEO), some polysaccharides] in aqueous solutions has been schematically illustrated. Such crosslinkings are useful for optimizing the results of promising biomedical applications of polymers in field like implants, controlled drug delivery, etc. [4].

6 Radiation Processing of Nanomaterials for Biomedical Applications Various kinds of radiations such as gamma rays, electron beams, UV, and microwave can be considered for irradiating molecules to initiate free radical-based reactions in different phases of materials (solids, liquids, or gases) to synthesize nanoscale materials. Those materials in an aqueous medium are widely used in the novel preparation of metallic nanoparticles (NPs) from precursor material. All high and low energy are capable to initiate the reactions in a solid or liquid state in an aqueous or non-aqueous medium to convert the micro-scale material into nanoscale [82–84]. To synthesize nanomaterial, the radiation acts to stabilize and crosslink the NPs of natural, synthetic polymers and biopolymers to prepare the NPs via radiation polymerization. The ionizing radiation has become a very useful technique for the synthesis of metallic NPs in a different manner using monomers, metal ions, and

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polymers. Radiation processing of nanomaterials with controlled radiation dose and rate leads to property development for synthesized materials with improved chemical, thermal, magnetic, optical, biochemical, and biocompatibility characteristics [84–86]. Those improved properties enable nano-synthesized materials in biomedical applications such as therapeutic use, drug delivery, antimicrobial, antibacterial material, diagnostics, and biomedicine use [85–87]. Nanoscience and technology for the development of nanomaterial by radiation technique have brought come together with various multi-disciplinary branches of science and technology such as chemical, biological, physical, and medical science. This concept has grown up the impact of biomedical application purpose of nanosized material for tissue engineering, micro- and nano-sensing chips for chemical and microorganisms, bio-transducers, and biosensors [82, 83]. Various forms of nanomaterials such as nanofilm, particles, and organic frameworks are well known to possess high surface area with optimized required application-based characteristics which make suitable them for drug delivery applications. The radiation processing of nanomaterials has several potential benefits over chemical-based and other nanoprocessing techniques where it reduces the use of toxic solvents and chemicals [84–86]. Nanomaterial processing by radiation technology has established a unique platform worldwide for the development and characterization of new nanoscale functional materials. Gamma radiation-grafted methacrylate filled with a silver (Ag) nanoparticle also has been synthesized for antibacterial applications (Fig. 9) [82]. The irradiation dose rate and level also play an important role to bring the material up to nanoscale level. The amount, shape, and size of synthesized NPs are depended upon types of radiation energy, dose rate, and dose level in a particular irradiation medium at a higher radiation dose; the size of nanomaterial or particle is reduced. Some investigations are performed with Cu and Pt NPs and have been seen as radiation dose increases from 50 to 250 kGy; the size of particles decreases from 56 to 35 nm [83]. A similar study also has been performed with Pt NPs where the size of particle decreases from 4.4 to 2.8 nm with radiation dose from 80 to 120 kGy, obtaining in both cases a Gaussian distribution on the NP size [82] (Fig. 10). Polymeric nanogels processed by gamma or electron beam radiation at a controlled dose also have been investigated for their use in biomedical applications Their characteristics have made them most useful and applicable compared to non-irradiated or crosslinked materials for therapeutic stimuli coating, antimicrobial, therapeutic, tissue engineering, encapsulation, and wound dressing [88–91]. Those radiationprocessed polymeric nanogels are stabilized due to their unique shape, structure, size, and their capability for binding and controlled release of the biomolecules. The radiation grafting of the hydrogel at the nano-level onto a cell surface makes it well resistant and compatible with the change in the temperature and pH of culture media [89–91].

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Fig. 9 Gamma radiation processing for the synthesis of nano-organic material [82]

7 Specifications to Meet at Applications Polymeric materials are widely used in the biomedical area. Although it is convenient to use synthetic polymers in the biomedical field, natural polymers are also required due to their biocompatibility and biodegradability. Polymers against radiation processing for specified requirements possess the following interesting and exploitable properties [92–94]. • Most abundant in nature (natural polymer) • Friendly with the living system, biocompatibility in the contact with blood, body fluids, and tissues. • Biodegradability (in the case of a natural system) • Good mechanical and thermal stability (Synthetic polymer system) for use in the medical area. • Optimum solubility parameter in water as well as organic solvents • Lower water vapour transmission rate (WVTR) characteristics • Unique structural properties

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Fig. 10 Gamma radiation polymerization of monomer in aqueous medium [82]

• The ability of radiation processing (Chain crosslinking and sterilization against gamma, EB, UV, neutron, etc.) • Compatibility of natural as well as synthetic polymers to each other for blending. • Miscibility of polymers to determine properties • Drug compatibility and diffusivity with radiation-processed polymers for drug delivery system. • Desirable oxygen permeability • Intermolecular interaction between polymers • The ability of polymeric surface for modification via grafting • Lower surface free energy • Nontoxicity, functionality, and sterilisable • Minimal foreign body reaction • An intrinsic antibacterial nature and antimicrobial activity • The ability to be moulded into various geometries and forms such as porous structures, suitable for cell ingrowth • The ability to absorb aqueous solutions without losing shape and mechanical strength, are commonly met in many natural constituents of the human body, like muscles, tendons, cartilage, etc.

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8 Applications The radiation interaction with various types of polymer results into an improvement in their end-use properties for wide-range areas of biomedical sectors, e.g. joint implant, heart valve, preparation of nano-pores for DNA sequence, surface modification for control drug delivery application. The radiation-processed polymeric materials have attracted both academic and industrial attention because they exhibit improvements in the above-mentioned properties required in the biomedical field. The radiation-processed polymeric materials show significant improvement in their properties which is suitable to use in biomedical applications such as drug delivery devices, tissue engineering scaffolds, implants, bone plates, wound healing and dressings, injectable formulations, immobilized enzyme, and cell bioprocesses. Radiation-processed hydrogel may be applied to the transient application, e.g. needles for vaccination or phlebotomy, cardiopulmonary bypass, and cardiac assist systems. Biopolymer collagen is the most abundant protein in animals, where it provides the principal structural and mechanical support [92–95]. It is a major structural protein, forming molecular strands that provide strength to the tendons, and vast, resilient which support the skin and internal organs. Bones and teeth are made of collagen with the mineral crystals like hydroxyapatite. Radiation-processed hydrogels have several biomedical applications, such as wound-care products, dental and ophthalmic materials, drug delivery systems, elements of implants, and constituents of hybrid-type organs, as well as stimuli-sensitive systems.

8.1 Radiation-Processed Hydrogel for Medical Use Always, every individual has a connection with radiation-processed materials at some time during his or her life. This contact may occur in several ways: permanent implantation, e.g. heart valves, total joint replacement, dental restoration, intraocular lenses, long-term application, e.g. fracture fixation devices, contact lenses, removable dental, prostheses, and haemodialysis systems. The cardiovascular and neurosurgical implants are involved in the primary class, e.g. cardiac valves, vascular grafts, pacemakers, and hydrocephalus shunts. The second category includes biomaterials before restoring function, such as joint replacement, fracture fixation devices, dental implants, and total hip replacement. The success rates vary appreciably in this category, ranging from excellent results to lesser success rates in other joints. Poly-(hydroxyethyl methacrylate, poly-HEMA) has been known primarily as the main component of contact lenses, but also in many other medical fields. PolyHEMA and its copolymers, both hydrophilic and hydrophobic, polymers are the most used hydrogel materials for medical purposes. UV crosslinked poly-HEMA has been applied in the production of contact lenses and dressings and for drug delivery and tissue engineering purposes. Polyurethane (PU) hydrogels are applied as drug carriers, in DDS, wound dressing manufacture, artificial kidney membranes,

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Fig. 11 Hydrogel dressing [92]

catheter coating materials, and contact lenses [92–94, 96]. Blends of poly-(vinyl pyrrolidone/ Polyethylene oxide), PVP/PEO hydrogels are sterile and non-toxic, which makes PEO an ideal addition to biomedical hydrogels designed as dressings. UV radiation-processed PVA hydrogels are used in contact lens production, cartilage reconstruction, and regeneration, artificial organs, drugs systems, and wound dressings, providing a humid environment beneficial for wound healing. Nano-composite from hydroxyapatite nanoparticles was introduced into the structure of PVA hydrogel with high adhesion to the living tissue with its good biocompatibility and lack of thrombogenicity, so it is applicable as bone defect and cartilage filler [97]. PEG (PEO)-based hydrogels are characterized by their high biocompatibility, lack of toxic influence on surrounding tissue, and solubility in water, which makes them good candidates for drug delivery systems and burn treatment applications (Fig. 11).

8.2 Synthetic Polymeric System: Medical Device Sterilization A very diverse range of medical devices is sterilized using EB processing. Table 1 lists examples of the diverse types of medical products that can be subject to radiation sterilization at a lower mild dose (10 or 25 kGy). Three issues are concerned for using electron beams or X-rays derived from electron beams for medical device sterilization:

212 Table 1 Medical disposable articles sterilized by ionizing radiation [93]

B. K. Sharma et al. Syringes (polypropylene))

Gloves

Catheters tube (PU)

Surgical gowns and drapes

Tubing Urine bags Bandages Polyethylene laminates

Hand towels Petri dishes Flexible PVC saline bottle PVC blood bags

1. The materials to fabricate the medical accessories and device. 2. When in the manufacturing operation will the device be exposed to radiation sterilization? 3. The degree of irradiation is required to maintain the level of “sterility assurance”. Non-disposable item is sterilized by high-energy radiation, e.g. hip and implants. Those implants are fabricated with metals assembled plastics. The biocompatibility of those fabricated materials is based upon the level and type of radiation to explore the application areas. The radiation sterilization of medical devices depends upon the compatibility between components. Ionizing radiation kills microorganisms at the cost of material properties. Medical devices are fabricated from various thermoplastics and their composites (PP, PE, PVC, PU, PC, PMMA, PET, PS, etc. [93]).

8.3 Augmenting Additive Manufacturing by Integrating with Radiation Technology for Biomedical Applications Recently, additive manufacturing, aka three-dimensional (3D) printing, has been found to be extremely useful in biomedical applications [98], especially for rapid prototyping of complex designs, material systems, and bespoke solutions. The polymers mentioned in the previous sections, namely PLA, polyethylene, and their blends, showed great promise for their suitability in this emerging advanced processing technology [99–101]. Even soft materials such as hydrogels can be directly printed for drug delivery applications [102]. The physicochemical properties and drug release performance of 3D-printed hydrogels can be enhanced and fine-tuned for specific pharmaceutical applications using a suitable crosslinking agent. For example, UV and gamma radiation under a nitrogen environment can be considered for crosslinking the PEO-based 3D-printed tablets [103]. Likewise, 3D bioprinting is used for regenerative purposes or to construct tissue models [104]. Crosslinking is one of the most significant influencing parameters for attaining the necessary biomechanical stability of printed structures and for developing biomimetic and sustainable 3D constructs. UV radiation-assisted photo crosslinking is commonly deployed in 3D bioprinting [105]. Another promising biomedical application is the fabrication of elastomeric engineered wound dressings or customized soft tissue scaffolds through 3D printing. Thus, biocompatible silicone elastomer can be UV-cured and 3D printed via thiol-ene

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photopolymerization between vinyl and thiol-functionalized polysiloxanes [106]. One of the critical aspects of crosslinking the polymers by radiation processing is optimizing the reaction conditions to achieve the desired end product performance while avoiding or minimizing any adverse degradation. Spectroscopic techniques offer practical inspection, diagnosis, and product troubleshooting for additive manufacturing, including investigating the crosslinking chemistry [107]. Hence, radiation processing, together with advanced characterization technologies, can be exploited in this fast-expanding advanced material production, bringing new opportunities for fundamental research and commercial uses.

9 Conclusion From this chapter, overall it can be concluded that radiation processing of different materials by different kinds of radiations such as electron beam (EB), gamma, Xray, UV, and neutron beam has opened the door to versatile opportunities for the biomedical sector. In the healthcare and biomedical sector, there has been grown up of requirement of radiation-processed new micro, nanomaterials, hydrogel, nanogel, and polymer blends and composites for tissue engineering, cell culture, implantation, organ-based transplantation in the living body, radiation-treated biosensors for living organ, ophthalmic, sterilization, implantation, and drug delivery system. The findings of various investigations found by researchers in different methods would be a tool for the understanding of radiation effect on various kinds of materials. Radiationprocessed hydrogel prepared from blends of natural polymer and synthetic polymer is a good decision to fabricate novel materials for their suitability in the biomedical sector. Surface modification or tailoring, e.g. grafting, blending, and composite materials of polymers via radiation processing brings the desired properties which meet the relevant application. The use of a certain amount of biopolymer may increase the biocompatibility of radiation-processed materials in the biomedical field; however, synthetic polymer products are also the potential candidate in radiation-assisted sterilization methods. From the published findings in this relevant area, it can be concluded that in future, big attention to expert and experienced holders in this area would bring the innovation of new products in the biomedical sector.

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Chapter 7

Effect of High-Energy Radiations on High Temperature-Resistant Thermoplastic Polymeric Composite for Aviation, Space, and Nuclear Applications K. Sudheendra, Jennifer Vinodhini, Mohan Kumar Pitchan, G. Ajeesh Mannadiar, and Shantanu Bhowmik

1 Introduction In recent years, investigating how irradiation affects polymer substances has been a popular field of polymer research. The effect of radiation exposure on the polymer chain can vary from constructive polymer chain crosslinking to polymer chain scission [1]. The nature and degree of the effect is primarily dependent on the inherent polymer structure property relationship of the base polymer [2]. The process of polymer chain alteration on exposure to radiation is multidirectional, with scission and crosslinking taking place at the same time. The dominant alteration process is dependent on the energy and density of ions and polymer materials. Table 1 highlights the characteristic behavior of certain polymers when exposed to radiation [3]. The effect of this radiation in space can be detrimental to the structures which are continuously exposed to the radiation from space in the absence of an atmosphere. Sufficient decay of the radiation occurs on the earth’s surface as the radiation passes through the atmosphere. The radiation mainly consists of the following components [4]: • Galactic cosmic rays (GCR)—85–90% protons, 10–13% helium ions (alpha particles), and 1% electrons, • Trapped radiation—(mostly electrons and pro tons) in the earth’s geomagnetic field (Van Allen radiation belts): K. Sudheendra · J. Vinodhini · M. K. Pitchan · G. Ajeesh Mannadiar · S. Bhowmik (B) Department of Aerospace Engineering, Amrita School of Engineering, Coimbatore, Amrita Vishwa Vidyapeetham, Coimbatore 641112, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. R. Chowdhury (ed.), Applications of High Energy Radiations, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-9048-9_7

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K. Sudheendra et al. Cross linking

Polyethylene Polyacrylates Polyvinylchloride Polysiloxanes Polyamides

Scission

Polymethacrylates Polymethylstyrene Polymethacrylamides Cellulose Polypropylene ether

• High-energy solar energetic particles (SEP)—charged particles composed mostly of protons, electrons, helium ions, and highly energetic particles in the heavy ion component • Secondary particles caused by incoming radiation that interacts with spacecraft materials in a nuclear reaction: • Internal craft radiation from on-board nuclear reactors designed for propulsion or auxiliary power.

1.1 Applications of Polymers in Radiation Environment Polymers and plastics are used in a variety of applications throughout the space industry. Some of these applications include: temperature blankets, thermal control paints, adhesives, lubricants, paints, circuit boards, coatings, and insulating coatings. These must meet the criteria for how much they can be used in space for the duration of the mission, such as [5]: • • • • • •

High vacuum; Outgassing; Harsh UV, X-ray, and gamma electromagnetic radiation; Charged particle radiation; Erosion from atomic oxygen; Wide temperature extremes.

The polymeric materials used on the spacecraft’s exterior would be an outlier if all of these conditions were met simultaneously.

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Thermal Blankets

Most spacecraft rely heavily on thermal blankets to keep the interior at a constant temperature.[6, 7]. The blankets are made of a layered polymer film that is either: • Incorporating carbon black filler to absorb sunlight; • Coated with a vapor-deposited aluminum covering to reflect sunlight. 1.1.2

Thermal Control Paints

Similarly to blankets, thermal control paint can be used to regulate the temperature inside a spacecraft. These paints come in two colors, black and white, and are made up of pigments mixed with either an organic polymer or an inorganic binder. Polyurethane is the most commonly utilized polymer binder for black paint. Polydimethylsiloxane or potassium silicate is the most frequent binders for white paints [8]. White paints primarily function by exhibiting a high level of emissivity and reflecting the heat back into the environment, and electrically inert mixtures can be obtained through the addition of metallic fillers. The incorporation of carbon black in black paints provides dual usage as it absorbs sunlight to keep the structure warm while also protecting the binder from UV radiation. The primary disadvantage of the control paints is their high degradation rate [9].

1.1.3

Adhesives

The most common method of joining in spacecraft is adhesive bonding using polymer-based adhesives. These adhesives have very steep operational temperature variability from 200 to −196 °C, which makes them an ideal choice for bonding [10, 11]. Two-part epoxy adhesives with the required fillers to obtain specific required properties are the most widely used adhesives. The capability of joining dissimilar surfaces, operational temperature limits, and tailorability makes polymer adhesives a preferred choice of material for adhesive bonding [12, 13].

1.1.4

Structural Applications

Composite materials made of high-modulus carbon fibers mixed in polymer resins have become the typical spacecraft building materials. They are generally made as laminates, with different carbon fiber orientations to obtain balanced multidimensional properties [14, 15]. Composites’ outstanding strength-to-weight and stiffness-to-weight ratios allow for reductions in spacecraft mass while simultaneously enhancing mechanical performance. Moisture absorption and impact resistance are also crucial aspects to consider when making a composite selection. Water absorption needs to be thought about in terms of both dimensional stability (the coefficient of moisture expansion) and

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outgassing, and even a small impact (also called “barely visible damage”) can reduce the compressive strength of a typical polymer composite by more than 50%. The metering truss of the Hubble Space Telescope is an excellent illustration of composite materials and construction. The new composite classes made from very high-modulus fibers and polycyanate resins have excellent water resistance, homogeneous curing, dimensional stability, and radiation resistance. Most commercial poly(cyanates) can absorb up to 35% of their weight in water before becoming saturated, and their moisture expansion (quasi-isotropic, in-plane) is less than 100 ppm. Despite their higher cost and more demanding fabrication techniques, composite rollers feature low mass, high specific stiffness, high specific (tensile) strength, and outstanding dimensional stability that are virtually unsurpassed by any other class of material. Over the past decade, composite materials have largely replaced aluminum, titanium, and steel in spacecraft with demanding mass and stability requirements [16, 17].

1.1.5

Nuclear Power Industry

The nuclear industry demands material capability of the highest order with exposure to extreme and prolonged radiation, temperature, pressure, and chemical conditions. Some specialty elastomers and high-performance polymers can operate under these conditions [18]. As a result, these materials are commonly used in hoses, tubing, gaskets, seals, insulation, and other critical components [19]. Table 2 lists the elastomers and polymers used in nuclear applications [20, 21].

2 High-Energy Radiations Radiation is defined as the form of energy given out by a source in the form of propagating waves. Radiation is classified at multiple levels based on its energy and ionizing capabilities. The categorization of the same is given in Fig. 1 [22, 23]. Table 2 Elastomers and polymers used in nuclear applications

Polymer/elastomer

Applications

Reinforced EPDM/chloroprene

Membrane valves

Fluorpolymers

Scales in valves

EPDM seals

Basin doors

PEEK

Self-actuated abeyance scale

Polychlorotrifluoroethylene

Tank liners

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Fig. 1 Distinction between ionizing and non-ionizing radiations

Fig. 2 High-energy particles penetration power

2.1 Ionizing Radiation The penetration capability of radiation is distinct owing to the differences in the inherent properties of the particles. The variability in the effect and penetrating power is shown in Fig. 2. The source of the radiation may be natural as well as artificial. The ionization capabilities and the subsequent effect of the radiation on the polymer material are a function of the substrate as well as the energy of the particle.

2.2 General Effect of Radiation on Polymers The effect of radiation on the material depends on the material as well as the nature of the radiation. Heavily charged particles pass through the material to capture electrons and transform into neutral atoms (proton to hydrogen atom, alpha particle to helium atom). A light-charged particle like an electron is captured by the polymer. A neutral particle either directly ionizes the material or changes the nucleus in a way that makes the material become radioactive. EM radiation such as gamma or UV rays leads to polymer chain linking or scission, leading to a change in the inherent property of the polymer material [24].

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2.3 Plasma Surface Modification Plasma is the fourth state of matter that makes up more than 99% of the space between the stars, most of which is not visible and remains unexplored. Plasma consists of energetic species in the form of neutral ionized gases containing ions, radicals, electrons, neutrons, photons, and other radiation. With the presence of such high-energy particles, the interaction of plasma with polymers leads to significant changes in its properties [25]. The bombardment of such high-energy particles on the surface of the material leads to characteristic changes at multiple levels, including physical and chemical aspects of the material [26, 27]. A particle (atom/molecule) is excited to a higher energy through an energy source such that it is capable of ionizing another particle. Under conducive conditions, sufficient ionizations are achieved to satisfy the plasma generation requirements [28, 29]. Different types of plasma can be achieved through a combination of different operational parameters and equipment. Every generated plasma has distinctive properties such as energetic species, temperature, density, energy, and uniformity. These properties can be achieved through alteration of input parameters such as pressure, gas type and composition, flow rate, energy source, distance between the cathode and anode, and potential difference [30–32]. Every combination of parameters leads to a peculiar alteration of the property of the surface [32, 33]. Polymers on exposure to plasma retain their bulk properties as the plasma modification technique is a surface-level alteration of properties. The changes that occur due to the treatment are restricted only to a few nanometers of the material, and thus the inherent properties of the material remain unaffected by exposure to high-energy particles from plasma. Surface etching may lead to minor weight loss but is highly effective in the creation of nano-roughness on the surface, thereby altering the wetting properties of the material at a physical level [34]. Therefore, plasma can be used to regulate the material to enhance the required surface properties without affecting the inherent capabilities of the material. The particular advantages of plasma treatment are listed below [35–39]: • • • • • • • •

Surface-level property modification Tailorable and controlled modification Physical and chemical modifications No solvents/chemicals involved and eco-friendly Alteration of wetting properties Multilayer films can be fabricated Scalable process Material degradation is minimal.

2.3.1

Plasma Gases

As discussed in the previous section, the effect of plasma on the material depends on a number of operational parameters. One of the primary parameters of influence

7 Effect of High-Energy Radiations on High Temperature-Resistant … Table 3 Effect of different plasma gases

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Plasma gases

Applications and effects

Reducing gases (H2 )

Removal of oxygen-centric functional groups

Oxidizing gases (O2 , air)

Introduction of oxygen-centric functional groups

Nitrogen

Surface cleaning and removal of oxide layers

Noble gases

Surface roughening and etching

Active gases (ammonia)

Introduction of amine groups

Fluorinated gases

Hydrophobic surface

Polymerizing gases

Direct polymerization

is the plasma gas. The selection of the gas during the plasma treatment is critical to ensure the desired properties are achieved. Plasma gas is known to have considerable influence on the changes in surface energy, adhesion, morphology, wetting, and biological aspects of the material post-modification. Hence, the role of the plasma gas becomes vital in ensuring the desired properties are achieved post-treatment. Table 3 describes the primary role of plasma gases in the enhancement of surface properties of the substrate subjected to plasma treatment [40].

3 High-Performance Polymers A number of high-performance polymer grades have been developed in recent years as possible materials for use in highly corrosive load-bearing applications in numerous industries, including very critical ones like the nuclear and aerospace industries. These polymers are designed to offer superior strength and stiffness with extreme chemical stability and good thermal properties. These polymers are primarily designed for operations at elevated service temperatures. There are various categories of high-performance polymers, a few of them are listed below in Table 4 [41, 42]. Table 4 High-performance polymers and their categorization

Amorphous

Semi crystalline

Polysulfone (PSU)

Polyphenylene Sulfone (PPS)

Polyether Sulfone (PES)

Polyetheretherketone (PEEK)

Polyether Imide (PEI)

Polyetherketone (PEK)

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3.1 Thermal Stability Thermal stability is an important characteristic of high-performance polymers. Further enhancement of the inherent superior properties can be achieved through the incorporation of reinforcing materials or by cross-polymerization to incorporate bulkier groups, which enhances the mechanical as well as thermal properties of the material due to reduced mobility between the chains. High-performance polymers achieve higher operation temperatures when compared to thermosets due to the incorporation of aromatics with oxygen groups (PEEK) or sulfonic groups (PPS, PES) or nitrogen groups (PAI or PEI). The resulting service temperature of the highperformance polymers would be around 250 °C [43]. The increase in thermal stability due to the incorporation of bulkier groups is due to the increased resistance to chain scission and subsequent oxidation [44]. Aromatics offer good resistance to thermal degradation as well as thermaloxidative degradation. Incorporation of aromatics increases the thermal stability but increases the rigidity, leading to difficulty in processing of the material. Therefore, high-performance polymers incorporate flexible chains in the polymer backbone, and therefore, diverse varieties of high-performance polymers are produced by altering the backbone of the base polymer chain.

3.2 Crystallinity High-performance polymers can be classified into two types, as shown in Table 4. Semi-crystalline polymers with incorporation of fillers or reinforcements can be used above their glass transition temperature due to the presence of a crystalline melting point. In addition to the thermal aspect, semi-crystalline high-performance polymers have excellent chemical resistance properties [45].

4 Case Study 1: Gamma Irradiation Effects on High-Performance Polymer The purpose of this study is to acquire a greater understanding of the influence the radiation, chemical, and thermal environments have on the mechanical and thermal properties of polyetherimide (PEI) composites. The experiments are carried out on specimens made of polyetherimide (PEI) and PEI reinforced with modified carbon nanofiber (CNF). The samples are exposed to 5 MGy of gamma radiation. This is the total amount of radiation that the body has been exposed to from all spent nuclear fuel up until the end of radioactivity [46].

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Fig. 3 Chemical structure of PEI

4.1 Materials The thermoplastic polymer polyether imide (PEI) is a high-performance amorphous polymer with outstanding inherent properties. The properties of PEI are on par with the properties of metals in certain aspects of mechanical properties. The primary advantage of PEI over metals or alloys is its inherent chemical stability in subjective corrosive environments. The aromatic ring with delocalized pi electrons is responsible for the chemical inertness of the polymer. The chemical structure of the polymer is given in Fig. 3. The processing temperature and radiation resistance capability make PEI an ideal candidate for radiation shielding applications. The matrix phase of polymer matrix composites is responsible for the environmental resistance of the structure. The degradation of the matrix phase would lead to degradation of the structure, eventually leading to a catastrophic failure. The reinforcement phase is responsible for the stiffness and strength of the composite, and the interface is responsible for the load transfer between the two phases. Thus, it is imperative that the matrix-reinforcement interaction be optimized to obtain the potential of the combination of materials. Because of the exceptionally high aspect ratio of some carbon nanoparticles, it is possible for these particles to be aligned with a single axis of the composite. Capabilities for manufacturing conducting polymers are made possible by their high axial thermal and mechanical properties. Traditional conductive fillers may be phased out in favor of carbon nanofillers due to their potential utility. The potential of carbon nanofiller-reinforced composites can only be realized provided the fillers are properly dispersed and distributed within the matrix phase without compromising the filler structure. To promote effective load transmission between the matrix and filler, good interfacial interaction between the filler and matrix phase is also necessary. Only, good interfacial adhesion between the filler and the resin can improve the composite’s mechanical properties. Surface modification techniques can be used to get the best possible interfacial contact (Fig. 4).

4.2 Exposure to Radiation and Corrosive Environment The study aims to comprehend the behavior of the material when exposed to radiation and a harsh chemical corrosive environment. The specimens are subjected to a cobalt 60 radiation source in a chamber at an emission rate of 76 kGy/day at 80 °C for

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Fig. 4 a Basic PEI specimen b CNF-reinforced PEI specimen

70 days, resulting in a cumulative dosage of 5MGy. The specimens were tested for chemical stability by being subjected to 100 °C for 200 h in pH environments of 1, 4, 7, 9, and 13.

4.3 Changes in Thermomechanical Properties Post-exposure to Radiation 4.3.1

Thermal Analysis

Figure 5A, b show DSC plots for unexposed and exposed PEI, as well as CNFreinforced PEI. Differential scanning calorimetry (DSC), a thermoanalytical method, measures, as a function of temperature, the difference between how much heat is needed to raise the temperatures of the sample and the reference. The results show no predominant changes in the thermal properties of PEI and PEI composites due to the exposure to aggressive environments. The glass transition is around 210 °C for PEI and PEI composites. The difference is not so significant between the reinforced and non-reinforced PEI due to the quantity of reinforcement in the composites. The glass transition temperatures of the polymeric samples are around 210 °C before and after exposure to aggressive environments. Figure 6a, b show TGA plots for PEI and PEI composites, respectively. There is no effect of the aggressive environment on the decomposition temperature of the material.

4.3.2

Mechanical Testing

The effect of exposure to radiation on the mechanical properties of the nanocomposites is quantified by performing tensile tests using the standard ASTM D638. Tensile tests are also used to correlate the presence of chain scission or chain linking due to the exposure to radiation, thermal, and chemical environments. The parameters from the tensile tests are shown in Figs. 7 and 8.

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Fig. 5 a DSC plots of PEI and PEI composites before exposure to aggressive environment, b DSC plots of PEI and PEI composites post-exposure to aggressive environment

Figure 8 clearly indicates that there is no significant loss in tensile strength under the influence of radiation and thermal environments. It is also observed that there is an increase in the tensile strength of PEI composites due to the influence of carbon nanofiber. Because of the exceptional strength of the nanofiller combined with the large surface area, CNF reinforced in PEI provides increased strength. Figure 9 shows that the elongation of the polymeric nanocomposites does not change significantly before and after exposure to radiation and elevated temperatures. The degree of mechanical deterioration of the polymers before and after exposure is determined by the elongation of composites. PEI and PEI composites have the same elongation at break. This discovery could possibly be explained by the polymers’ lack of a significant degree of chain scission as a result of radiation and thermal conditioning.

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Fig. 6 a TGA plots of PEI and PEI composites before exposure to aggressive environments, b TGA plots of PEI and PEI composites after exposure to aggressive environments

The exposed samples were exposed to aggressive chemical environments, and it was discovered that when polymeric samples were exposed to alkaline environments; their tensile strength was significantly reduced. The tensile strength of exposed samples under aggressive chemical environment is as shown in Fig. 9. Figure 9 shows that when a basic medium is present, the tensile strength of PEI that is exposed to it drops by a lot. There is also a decline in the tensile strength of PEI composites exposed to basic environments. The reinforcement of CNF compensates for the loss in mechanical properties of PEI. Chemical exposure ordinarily results in a softening effect or cracking and crazing of the thermoplastic. As softening of the PEI occurs, the percent weight increases while the tensile strength decreases. It can be noted that there is a 60% decrease in the tensile strength of PEI from 100 to 40 MPa when exposed to a basic environment. The presence of nanoparticles nullifies the effect of the basic environment on PEI composites by 42%. The tensile

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Fig. 7 Tensile strength of polymers

Fig. 8 Elongation of polymers at break

strength of the PEI composites is reduced from 135 to 78 MPa. Research has in the past given significant evidence for the reduced permeability due to the presence of nanomaterials in polymers.

4.4 Conclusion The study showed there were no significant changes in the thermal properties postexposure to an aggressive environment, highlighting the potential application of the

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Fig. 9 Tensile strength of PEI composites under chemical environment

material in nuclear waste storage applications. The mechanical properties suffered a reduction in the properties post-exposure to radiation and aggressive environments, but the design limits can be altered to ensure the operation ability of structures.

5 Case Study 2: Plasma Surface Modification of High-Performance Polymers The primary aim of the study is to understand the effect of plasma treatment on the surface of polytherketone (PEK) and carbon fiber (CF). The surface of the material was altered for 300 s, and the change in physiochemical and mechanical properties was investigated using several characterization techniques. The changes due to the treatment were quantified to understand the degree of enhancement [47].

5.1 Materials PEK is a high-performance thermoplastic polymer with superior properties that can retain properties at elevated temperatures. The strong aromatic-based structure ensures superior thermal properties along with a balanced chemical stability due to its crystallinity (Fig. 10). PAEK can be used for wear applications with the incorporation of fillers. PAEK inherently is characterized by low surface energy, leading to poor interfacial adhesion properties with the reinforcement. Thus, it is necessary

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Fig. 10 Structure of PAEK

to enhance the surface energy of the polymer to extract the utmost capability of the composites manufactured through the reinforcement of fibers within the matrix. The matrix-fiber interfacial interaction plays a major role in establishing the required property. The carbon fiber surface is graphitic in nature, which results in a high degree of inertness toward polymeric materials. Thus, surface modification is necessary to ensure that the carbon fiber surface is adequately activated for extensive interfacial interaction with the resin. Thus, surface modification alters the physiochemical aspect of the surface to enable better interfacial interaction.

5.2 Plasma Surface Modification PAEK film and carbon fiber were plasma treated using the pulsed DC magnetron sputtering system. The dumbbell-shaped stainless steel vacuum chamber houses the electrodes with a separation of 15 cm. The pressure within the vacuum chamber is evacuated to 5 × 10–2 mbar before the introduction of a specific gas at a flowrate of 12 L/min to achieve an operational pressure of 1.1 × 10–1 mbar. In this study, the plasma treatment was performed with argon. The power supplied to generate plasma was 90 W for argon plasma treatment, which is the initial point of plasma generation. The treatment time was optimized in the previous study and was fixed at 300 s. The glow obtained during both the treatments is shown in Fig. 11.

5.3 Changes in Properties Post-plasma Treatment 5.3.1

Wettability

The contact angle of deionized water on the PAEK film with different plasma treatment times is shown in Fig. 12. The untreated film had a contact angle of 82° while the same for the argon plasma-treated film for 5 min showed a contact angle of 58°. This result shows the enhancement effect of plasma treatment on the polymer film. The contact angle decrease shows an increase in the hydrophilic nature of the film post-plasma treatment. This increase in the wettability of the polymer film positively impacts the interfacial capability of the matrix-fiber region, ensuring better adhesion.

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Fig. 11 Argon plasma treatment on carbon fiber

Fig. 12 Images of sessile drop method on PAEK surface to quantify the contact angle with deionized water a untreated, b 1 min, c 2 min, d 3 min, e 4 min, f 5 min

5.3.2

Surface Topography

AFM analysis was used to analyze the morphological changes in PAEK and CF surfaces following exposure to argon plasma. Figure 13 shows comparative images of the carbon fiber surface pre- and post-argon plasma treatment. The figure highlights a change in the surface topography of the carbon fiber post-plasma treatment through the formation of deeper and denser longitudinal grooves along the fiber. The results from AFM show that there is a positive impact in terms of enhancement of surface roughness due to plasma treatment, and the

Fig. 13 AFM images of a untreated CF and b treated CF surface

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Fig. 14 Tensile strength value of untreated CF/PAEK and treated CF/PAEK

degree of enhancement of surface roughness varies with different plasma treatments. The results show that argon plasma is effective in positively enhancing the surface morphology. The increase in surface roughness due to the plasma treatment is attributed to the etching of surfaces by the plasma generated. The microgrooves formed on the surface of the fiber result in an increase in the interaction between the fiber and matrix due to enhanced mechanical interlocking owing to an increased area of contact in the interfacial region. The improved surface roughness parameters through plasma treatment positively impact the wettability of carbon fiber.

5.3.3

Tensile Strength

The results show that the argon plasma-treated laminate exhibited a mean tensile strength of 590 MPa while that of the untreated was 539 MPa. There was a 9.5% increase in tensile strength post-argon plasma treatment as shown in Fig. 14. The observations are in concurrence with the results from AFM, and the microgrooves have positively impacted the tensile properties of the laminate, enhancing the interfacial adhesion between the fiber and matrix.

5.4 Conclusion The interfacial adhesion between the fiber and matrix was considerably enhanced through argon plasma treatment. This enhancement is owed to the physiochemical enhancement of properties, as shown through enhancement of surface wetting and morphology, leading to an optimized property.

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6 Summary, Conclusions, and Scope for Future Work The application of polymers in environments of radiation is immense and varied. Thus, it is necessary to understand the types of radiation and their subsequent effects on polymers. Radiation can have a variety of effects on polymers, ranging from constructive chain crosslinking to destructive chain scission, depending on the structure. High-performance polymers are especially known to handle radiation effectively due to their inherent structural properties. Two specific cases have been specifically examined where high-performance polymers are examined by exposure to radiation in different forms, and the subsequent effect on the material is examined. In the first case, a high-performance polymer is subjected to aggressive environments and exposed to gamma radiation to examine its thermal and mechanical properties. No drastic changes are observed, leading to the conclusion that this material can be used in nuclear waste storage facilities. In the second case, plasma is used to constructively enhance the properties of a thermoplastic composite material. Thus, with these two cases, one can understand the potential of radiation-based polymer technologies. The technological advances that today’s industry needs can only be realized with a deep understanding of the effect of radiation on polymers and the adoption of the materials in potential applications.

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31. Pitchan MK, Bhowmik S, Balachandran M, Abraham M (2016) Effect of surface functionalization on mechanical properties and decomposition kinetics of high performance polyetherimide/MWCNT nano composites. Compos Part A Appl Sci Manuf 90:147–160. https://doi. org/10.1016/j.compositesa.2016.06.025 32. Jang J, Yang H (2000) Effect of surface treatment on the performance improvement of carbon fiber/polybenzoxazine composites. J Mater Sci 35:2297–2303. https://doi.org/10.1023/A:100 4791313979 33. Cools P, Astoreca L, Esbah Tabaei PS, Thukkaram M, De Smet H, Morent R, NDG (2020) Surface treatment of polymers by plasma. In: Surface modification of polymers: methods and applications, 1st edn 34. Liu D, Hu J, Zhao Y et al (2006) Surface modification of PBO fibers by argon plasma and argon plasma combined with coupling agents. J Appl Polym Sci 102:1428–1435. https://doi. org/10.1002/app.24287 35. Veazey D, Hsu T, Gomez ED (2017) Next generation high-performance carbon fiber thermoplastic composites based on polyaryletherketones. J Appl Polym Sci 134:19–21. https://doi. org/10.1002/app.44441 36. Sharma M, Bijwe J, Mitschang P (2011) Abrasive wear studies on composites of PEEK and PES with modified surface of carbon fabric. Tribol Int 44:81–91. https://doi.org/10.1016/j.tri boint.2010.09.005 37. Sharma M, Bijwe J, Mitschang P (2011) Wear performance of PEEK-carbon fabric composites with strengthened fiber-matrix interface. Wear 271:2261–2268. https://doi.org/10.1016/j.wear. 2010.11.055 38. Käppler I, Hund RD, Cherif C (2014) Surface modification of carbon fibres using plasma technique. Autex Res J 14:34–38. https://doi.org/10.2478/v10304-012-0048-y 39. Altuncu E, Üstel F, Esen SG, Karayel E (2016) Infuence of oxygen and nitrogen plasma treatment on polypropyleme (PP) bumper surface. J Achiev Mater Manuf Eng 77:18–34. https:// doi.org/10.5604/17348412.1229664 40. Jang J, Kim H (1997) Composites using plasma treatment. Polym C 18:125–132 41. Ramgobin A, Fontaine G, Bourbigot S (2019) Thermal degradation and fire behavior of high performance polymers. Polym Rev 59:55–123. https://doi.org/10.1080/15583724.2018.154 6736 42. Sikder A, Sikder N (2004) A review of advanced high performance, insensitive and thermally stable energetic materials emerging for military and space applications. J Hazard Mater 112:1– 15. https://doi.org/10.1016/j.jhazmat.2004.04.003 43. Meng Y, Hlil AR, Hay AS (1999) Synthesis and thermal properties of poly(arylene ether ketone)s containing phthalazinone moieties. J Polym Sci Part A Polym Chem 37:1781–1788. https://doi.org/10.1002/(SICI)1099-0518(19990615)37:12%3c1781:: AID-POLA7%3e3.0.CO;2-I 44. Díez-Pascual AM, Naffakh M, Marco C et al (2012) High-performance nanocomposites based on polyetherketones. Prog Mater Sci 57:1106–1190. https://doi.org/10.1016/j.pmatsci.2012. 03.003 45. Harris L (2011) A study of the crystallisation kinetics in PEEK and PEEK composites. University of Birmingham 46. Ajeesh G, Bhowmik S, Sivakumar V et al (2017) Effects of high energy radiation and thermochemical environments on polyetherimide composites: futuristic approach to nuclear waste storage. Int J Nucl Energy Sci Technol 11:80. https://doi.org/10.1504/IJNEST.2017.085091 47. Vinodhini J, Sudheendra K, Balachandran M, Bhowmik S (2021) Influence of argon plasma treatment on carbon fibre reinforced high performance thermoplastic composite. High Perform Polym 33:285–294. https://doi.org/10.1177/0954008320957062

Chapter 8

Recent Developments of the Radiation Processed Hybrid Organic–Inorganic Polymer Nanocomposites: Expected and Unexpected Achievements Shalmali Hui

and Santanu Chattopadhyay

1 Introduction During the last two decades, innovative materials are continuously produced for scientific and commercial requirements due to the very rapid growth and development of industrial activities all over the world. Polymer systems are widely used due to their unique attributes, e.g., ease of production, lightweight, and ductility. Polymer blending is a cost-effective and promising route to introduce and develop new polymeric materials where ease of processing and high performance-to-cost ratio are coupled together. Thermoplastic elastomers (TPEs) are special types of polymers, polymer blends, or composites in which the functional properties of conventional crosslinked thermoset elastomers and the satisfactory processing characteristics suitable for thermoplastic fabrication equipments, are combined together [1]. However, these materials possess certain disadvantages like poor set and chemical or oil resistance as compared to a crosslinked elastomer. These limitations can be significantly improved by a process called ‘dynamic vulcanization’, where the elastomeric phase, as well as its interface with the plastic domains, are crosslinked [2]. This improves a multitude of properties, although processibility is sacrificed to a certain extent. Dynamic vulcanization imparts compounding complications and trims down the purity of the TPE system, which restricts some of their uses in food packaging, pharmaceutical, and bioapplications. Here, one of the best possible ways is electron beam (EB) modification of these blends at a controlled radiation dose. The S. Hui (B) Department of Chemistry, Hijli College Affiliated to Vidyasagar University, Kharagpur, West Bengal 721306, India e-mail: [email protected] S. Chattopadhyay Rubber Technology Centre, Indian Institute of Technology, Kharagpur, West Bengal 721302, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. R. Chowdhury (ed.), Applications of High Energy Radiations, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-9048-9_8

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effect of irradiation on polymeric materials has become a promising field of research after the first nuclear chain reaction in 1942 [3, 4]. The process of EB irradiation is very fast and can be automatically controlled. Hence, the possibility of oxidative degradation of the materials is less as compared to γ–irradiation of a similar dose [5]. Another advantage of EB processing is that the EB has high penetrating power and high energy. Thus, in EB processing, most parts of the transferred energy can be directly converted into chemical energy. Chattopadhyay et al. recently have shown that the EB processing of polyolefin-based TPE blends at low and controlled radiation dose (20–50 kGy) is a potential alternative to dynamic vulcanization [6, 7]. At room temperature, EB radiation crosslinking in the solid state in absence of chemical catalysts can be efficiently used to upgrade the thermal, electrical, chemical, and mechanical properties of polymeric products as compared to conventional cross-linking processes. A three-dimensional network structure is formed through the combination of in situ-generated macroradicals by EB modification of polymers [8, 9]. However, the technical properties of TPEs are inferior due to the presence of a weak rubbery phase and an interface dispersed in a continuous plastic matrix. To widen the diversity of polymers, another well-known approach is to modify their technical properties by the addition of nanofillers. Here lies the importance of ‘Polymer Nanotechnology’. Nanotechnology is now recognized as one of the most promising fields of research in the twenty-first century. Polymer nanocomposites (PNCs) are polymers that are reinforced with rigid inorganic/organic particles having at least one dimension in the nanometer size range. Organic/inorganic nanocomposites combine the advantages of the inorganic material (e.g., rigidity, thermal stability, gas barrier property, electrical conductivity, magnetic susceptibility, hardness, abrasion resistance, superconductivity, optoelectronic property, piezoelectric, and optical property, etc.) and the organic polymer (e.g., flexibility, dielectric characteristics, ductility, and processability). These nanofillers include nanotubes, layered silicates (e.g., montmorillonite, saponite), nanoparticles of metals (e.g., Au, Ag), metal oxides (e.g., TiO2 , Al2 O3 ), polyhedral oligomeric silsesquioxanes (POSS), semiconductors (e.g., PbS, CdS) so on and so forth. The importance of this PNC technology stems from providing value-added properties not present in the neat polymer without sacrificing the polymer’s inherent processibility, low density, and mechanical properties. Any composite can be divided into three parts: the matrix, the reinforcing component, and the so-called interfacial region. The interfacial region is responsible for communication between the matrix and the reinforcing component (filler), and its conventionally ascribed properties differ from the bulk matrix because of their proximity to the surface of the filler [10]. The high specific surface area of nanofillers (due to lower size scale and higher aspect ratio) is one of the reasons why the nature of reinforcement is different in nanocomposites. The technical properties of thermoplastic polyolefin blend-based TPE systems can be modified significantly using fillers like nanosilica. Nanosilica is of particular interest due to the large surface area and its branched structure which provide for establishing interactions with specific matrices or chemical reactive groups.

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The polyolefinic TPE-based blends, derived from low-density polyethylene (LDPE) and ethylene–vinyl acetate (EVA), have a lot of potentials as packaging and heat shrinkable films and laminates or as electrical insulators. Scientifically, these blends are interesting as both these polymers have common ethylene segments, and hence can be co-crosslinked and compatibilized. In addition, due to the polar nature of silica, it can be selectively located in the polar EVA phase to a certain extent. Thus, by varying the blending sequence, the partitioning of nanosilica between the LDPE and EVA phases and their interfaces can also be controlled in the blend system. The radiation sensitivity of the polymeric materials is also strongly influenced by the presence of nanosilica. Thus, there is no need to add an external organic sensitizer to promote the radiation crosslinking efficiency because nanosilica particles themselves have the potential to act as radiation sensitizers as well as reinforcing agents. Application of irradiation at a moderately low and controlled dose can potentially be an effective way of upgrading the technical properties of filled TPE systems which is clean, simple, fast, energy-saving, environment-friendly, and economically beneficial. Hence, there is ample opportunity to study and characterize nanosilica-filled and radiation-modified LDPE/EVA TPE systems. The impact of EB irradiation on the structures and properties of polyethylene (PE) and EVA copolymer using different types of sensitizers has been reported in the literature [11–15]. It has been established that upon EB irradiation EVA has higher crosslinking efficiency than LDPE. In addition, at lower radiation doses EVA containing a higher VA content is useful for the crosslinking [16]. It is also well known that the gel fraction of pure EVA containing 45% VA content is 53%, whereas for pristine LDPE it is 17% at a 20 kGy radiation dose [17]. These LDPE/EVA blends have been widely studied from different angles, that is, structure-properties relationship, morphology, mechanical properties, rheology, thermal properties, and electrical properties, among others [18–47]. Thus, readers should refer to those articles to get general guidelines on the impact of EB irradiation, preparation, and properties of LDPE/EVA blends. In this chapter, we do not reproduce those general aspects of the LDPE/EVA blends. Instead, we wish to explore some expected and unexpected achievements in recent developments of the radiation-processed hybrid organic– inorganic PNCs based on the pristine silica nanoparticle-filled model (LDPE/EVA) TPE system. The main aim of this chapter is to selectively strengthen the weaker EVA phase and interface without much affecting the plastic phase so that the reprocessibility would be maintained at a tolerable level and technical properties would be uplifted. We highlight the detailed evaluation of bulk morphology of various samples using SEM, FESEM, and TEM. We also briefly touch on the simultaneous effect of the addition of nanosilica and controlled EB irradiation on the mechanical, rheological, thermal, and electrical properties of this model system. Finally, these properties have been correlated with the morphology of these systems. It has been established that in LDPE/EVA (45% VA) blend, the irradiation effect at a low radiation dose (20 kGy) is limited to the amorphous portion of the concerned blends. This particular dose gives the best compromise between the permanent set and reprocessibility. Above 50 kGy dose, the gel fraction increases, processibility decreases, and LDPE/EVA TPE system progressively becomes a thermoset [6]. On this basis, EB

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irradiation at lower dose levels (20 and 40 kGy doses) has been chosen by Hui et al. This may finally freeze the morphology with improved interfacial bonding. Thus, the reprocessing characteristics, as well as thermoplasticity of the TPE, can also be retained considerably. Thus here, onwards, these dose levels have been mentioned as the controlled dose in the present chapter.

2 Hypothesis of Bulk Morphology of Nanosilica-Filled Model LDPE/EVA TPE System Hui et al. have prepared a model LDPE/EVA blend in the molten condition in Brabender Plasticorder at 130 °C and 80 rpm rotor speed by varying two different sequences of addition of ingredients and amount of nanosilica (1.5, 3 and 5 wt%, respectively). In one of the compositions, coupling agent Bis [3–(triethoxysilyl) propyl] tetrasulfide (Si69) has been used along with 3 wt% nanosilicate to improve the interaction of hydrophilic nanosilica fillers with the polymer matrix which is more economical than modifying the fillers separately before mixing. This silane coupling agent is often used because it has a unique bifunctional structure. Its one end is compatible with the polymer matrix, whereas another end can react with the silanol groups on the silica surface. The chemical structure of Si69 is shown in Fig. 1. In sequence-1, first LDPE is mixed with EVA, then silica particles are added. In sequence-2, first EVA and silica are mixed to produce a masterbatch, then it is blended with LDPE. After the initial mixing, the mixtures are remixed at 130 °C to achieve more homogeneity. An electrically heated hydraulic press is used to mold the prepared nanocomposites. The mixtures are compression-molded between two Teflon sheets for 3 min at 150 °C with a pre-heat time of 1 min and with a load of 5 tons. The thickness of the molded films is 0.03–0.04 cm. To maintain overall dimensional stability, the moldings are cooled under compression. The details of the samples and their appropriate designations are given in Table 1. Hui et al. have assumed a hypothesis of the morphology (Fig. 2) and tried to correlate this with the microscopy results.

Fig. 1 Chemical structure of Si69

60

60

60

60

40

40

40

40

40

CS/3-1

CS/3-2

CS/3-Si69-2

CS/5-2

a

60

40

C

CS/1.5-2

2

2

5

2

3 + 10% Si69 w.r.t silica

1

2

–

Sequence of addition

3

3

1.5

0

SiO2 (wt%)

C—Control EVA (60 wt%) + LDPE (40 wt%) blend, S—Silica, R—Irradiation

60

EVA (wt%)

LDPE (wt%)

Composition

Table 1 Sample designationa

CS

CS/3-Si69-2

CS/3-2

CS/3-1

CS/1.5-2

C

0 kGy

CS/5-2/2R

CS/3-Si69-2/2R

CS/3-2/2R

CS/3-1/2R

CS/1.5-2/2R

C/2R

20 kGy

Radiated sample designation (ID)

CS/5-2/4R

CS/3-Si69-2/4R

CS/3-2/4R

CS/3-1/4R

CS/1.5-2/4R

C/4R

40 kGy

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Fig. 2 Schematic depiction and hypothesis

An EB accelerator (RDI-Dynamitron, DPC-2000 Control System, USA) having 2.0 MeV energy is used to irradiate (at 20 and 40 kGy radiation doses) the molded samples in the air at room temperature (25 ± 2 °C) under forced air cooling. The average EB current is (0.5–50) mA, and the accelerating voltage frequency is fixed at 100 kHz. The speed of the transport system is ~ 3 m min − 1 for the 20 kGy dose, and the sample is passed twice through the irradiation window for the 40 kGy dose.

3 Evaluation of Bulk Morphology by Microscopy Studies Since the bulk morphologies have a critical role to decide the radiation sensitivity of the composites, therefore, in this chapter the morphologies obtained by scanning electron microscopy (SEM), field-emission SEM (FESEM), and transmission electron microscopy (TEM) analyzes have been discussed followed by evaluating their radiation sensitivity.

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3.1 FESEM Studies To increase the contrast of the deformed rubbery phase and comparatively nondeformed plastic matrix, FESEM is performed on samples prepared purposely at room temperature to visualize the dispersion–aggregation of silica particles. EVA is more ductile than LDPE at room temperature. It is found that the average diameter of dispersed nanosilica particles varies from 12 to 200 nm. Besides this, aggregates of higher length scales also appear (Fig. 3a–e) [48]. In the case of CS/1.5–2, the nanosilica particles are finely dispersed in the polymer matrix. Besides it, quite a few aggregated structures along with distorted EVA domains are also observed (Fig. 3a). The nanosilica particles are dispersed in both phases as well as in the interface in a nano-to micro-scale range in CS/3–1, (Fig. 3b). Nevertheless, in this case, EVA phase is deformed to a lower extent (indicated by spherical lumps) as compared to CS/3–2. This indicates that in sequence-1, there is a possibility of intermixing both phases and strengthening the EVA phase [48]. This has also been explained in light of TEM observation in the following sect. For CS/3– 2, the EVA phase is deformed to a greater extent (Fig. 3c). This indicates that the dispersion of silica particles in the interface of CS/3–2 is not as good as that of CS/3– 1 [48]. For CS/5–2, visibly more aggregated nanosilica particles are distributed in the polymer matrix (Fig. 3d). For silane-loaded sample (Fig. 3e), a transformation of the shape of the deformed EVA phase is observed from elongated elliptical to cylindrical. This indicates that the coupling agent causes better interfacial interaction that leads to an improvement in the miscibility between the two phases [48]. However, after irradiation of the filled samples, the distinction between LDPE and EVA is not observed which indicates the improvement of interfacial strength. Therefore, bulk morphology, as expected, is not supposed to alter significantly upon irradiation.

3.2 TEM Studies TEM analysis has been performed by Hui et al. [48] for some selected samples to investigate the state of dispersion of silica particles in the bi-component polymeric matrix (Fig. 4a–c). Osmium tetroxide is used to stain the samples which offer higher scattering cross-sections to incident electrons by combining with EVA. So, the rubbery EVA phase of this model system appears as dark, and the LDPE phase appears as bright. In addition, the interface and intermixed (EVA and LDPE) portion of rubber/plastic phases appear as a light gray phase. The probability of the existence of the nanosilica particles in the area of EVA and LDPE phase and the interface is greater for sequence-1 than sequence-2 because of the occurrence of intermixing and partitioning of particles in the two different phases (Fig. 4a). On the other hand, most of the silica particles are present in the EVA phase both in the dispersed and aggregated state for sequence-2 (Fig. 4b). Few particles are observable there in the interface and the amorphous portion of the LDPE phase. Thus, in this case, the silica particles

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Fig. 3 FESEM photomicrographs of the microtomed surface of non-irradiated samples of LDPE/EVA TPE systems with various loadings of nanosilica particles: a CS/1.5–2 at ×160,190 magnification; b CS/3–1 at ×10,000 magnification; c CS/3–2 at ×10,000 magnification; d CS/5–2 at ×97,000 magnification; e CS/3-Si69-2 at ×15,000 magnification. Adapted with permission from [48]

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selectively reinforce the weaker EVA phase while maintaining the crystallinity of the LDPE phase undisturbed. Interestingly, in comparison with the untreated silica, the silane-treated silica particles are dispersed more uniformly within the polymer matrix (Fig. 4c). This indicates that the affinity of silica toward the more hydrophobic polymer matrix increases due to its surface modification by silane [43]. So, it is established from the microscopy results that what our hypothesis was initially about the morphology (Fig. 2) of these model TPE systems works perfectly here.

Fig. 4 TEM photomicrographs of non-irradiated samples of LDPE/EVATPE systems with various loadings of nanosilica particles: a CS/3–1; b CS/3–2; c CS/3-Si69-2. Adapted with permission from [48]

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3.3 SEM Studies Finally, the fractured surfaces of a few selected samples have been analyzed by SEM, as shown in Fig. 5a–d [48]. After failure, large hollows and damaged surfaces appear in all the samples. In the control blend(C) (Fig. 5a), the edges which indicate the stress are disseminated within fractures (mostly from the interfacial region). It has been established by Hui et al. that fragility increases after irradiation [48]. An increase in crosslink density is obvious from this result. This brittleness increases with an increase in radiation dose. A rougher appearance has been found for all the cracked surfaces of filled samples in comparison with the control sample. A resemblance has been observed for the irradiated fractured surfaces of CS/3– 1/2R and C/2R (Figs. 5b, c) by Hui et al. [48]. This is because of the intermixing phenomenon which results in random dispersion of nanosilica particles in both phases as well as in the interface, and finally causes a reduction in the modulus difference between EVA and LDPE (Storage modulus of LDPE is 6826 MPa and of EVA is 8275 MPa at −58 °C]. Hence, the fracture behavior is very closely similar to that of brittle single-phase material. For CS/3–2/2R (Fig. 5d) some spherical chunk– type mass is found on the irradiated surface which appears due to poor interfacial strength. The bulk morphology, as expected, is not supposed to alter significantly upon irradiation.

Fig. 5 SEM photomicrographs of cryo-fractured surfaces of irradiated and non-irradiated samples of LDPE/EVA TPE systems with various loadings of silica particles: a C; b C/2R; c CS/3–1/2R; d CS/3–2/2R. Adapted with permission from [48]

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4 FTIR Studies The FTIR spectra of pure silica, pure EVA, and 1.5 wt% nanosilica-loaded EVA in the region (400–4000) cm−1 is shown by Fig. 6 [48]. The peak at 1743 cm−1 of pure EVA is shifted to a lower frequency at 1731 cm−1 upon nanosilica loading. This causes the broadening of the >C=O absorption band (Fig. 6). This indicates that the >C=O group of EVA interacts with silica and forms hydrogen bonds with the –OH groups of silica (Fig. 7). In addition, the hydrogen bond formation is further evident from the shifting of the C–O stretching frequency at 1256 cm−1 of pure EVA to 1241 cm−1 together with peak broadening (Fig. 6). This trend continues in the case of 3 wt% loadings. With 5 wt% loading, overall peak broadening is observed due to the substantial scattering effect of aggregated nanosilica fillers. As a result, the peak shift is not observed clearly. Few multiple peaks appear in case of the control blend irradiated at different doses which are merged within the region 1750–1723 cm−1 (cyclic ketones and esters) (Fig. 8).This may be due to the fact that the macroradicals which are formed during irradiation process undergo cyclization [49]. Fig. 6 FTIR spectra of pure silica, pure EVA, and 1.5 wt% silica-loaded EVA. Adapted with permission from [48]

Fig. 7 Possible hydrogen bonding interaction between EVA and silica. Adapted with permission from [48]

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Fig. 8 FTIR spectra of control blend irradiated at different dose. Adapted with permission from [48]

5 Mechanical Properties The variations of mechanical properties of filled LDPE/EVA TPE systems with different radiation doses are shown in Fig. 9a, b [48]. Table 2 shows the effect of EB irradiation on the mechanical properties of these silica-filled samples at controlled radiation doses (20 and 40 kGy) [48]. In general, in comparison with the control sample, a dramatic enhancement in mechanical properties is observed for filled samples upon irradiation by Hui et al. [48]. The dispersion of silica particles and the selective reinforcement of the weak EVA phase and the interface are more substantial in all irradiated filled samples, whereas the improvement in mechanical properties is much less in the case of the control sample at the same dose. However, in comparison with the filled samples, the radiation dose has to be boosted at least twofold for the control sample to reach an equivalent enhancement. The role of silica to act as a radiation sensitizer is manifested from here. An optimal dispersion of silica particles occurs in the polymer matrix with 1.5 wt% loading which leads to an improvement in the radiation–sensitizing effect of silica particles. This is reflected in the mechanical properties and is also equally evident from morphological analyzes mentioned in the earlier sects. The percentage improvement in properties is more in CS/3-1/2R as compared to CS/32/2R. The occurrence of intermixing as well as more interfacial crosslinking in CS/31/2R is further established from this result. CS/3-2/4R exhibits higher high-and lowstrain moduli than CS/3-1/4R. This supports the fact that an optimum reinforcement occurs in the EVA phase in sequence-2 with 3 wt% loadings. The tensile strength is increased in all filled samples (except CS/3-Si69-2/4R). Most importantly, the elongation at break is improved in all irradiated samples including the control blend. This indicates the fact that because of the radiation-induced crosslinking effect the interfacial strength is improved along with maintaining the elastic property of rubber. As compared to the control sample, this development is much more important in all

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Fig. 9 Variation of mechanical properties of LDPE/EVA TPE system for sequence-2: a effect of radiation dose on the tensile strength of silica-filled system; b effect of radiation dose on the elongation at break of silica-filled system. Adapted with permission from [48]

irradiated silica-filled samples due to the significant strengthening of both phases and their interfaces as well. Noticeably, the crosslinking of individual phase, as well as crosslinking of EVA phase is established from the results of Table 2 where at a dose of 20 kGy, the elongation at break is more for CS/3-2 than for CS/3-1. But the elongation at break at a dose of 40 kGy is lower for CS/3-2 than CS/3-1. This result indicates that at a higher radiation dose, higher crosslink density is formed in silica-filled EVA matrix for sequence-2. Interestingly, Si69 reacts with silanol groups on the silica surface which leads to the formation of a dense or thick coupling layer at the interface and outside of the SiO2 surface. This may prevent its sensitivity towards EB radiation for CS/3-Si69-2. This supports the fact that silane acts as a radiation scavenger. Therefore, in this case, the fine dispersion of silica particles plays an important role in overall reinforcement. With 5 wt% loading, more radiation–crosslinked interfacial area is available due to

6.5 (66%)

6.9 (60%)

6.3 (32%)

7.5 (115%)

CS/3-1

CS/3-2

CS/3-Si69-2

CS/5-2

9.2 (163%)

6.0 (26%)

7.3 (83%)

7.4 (90%)

7.0 (86%)

6.2 (81%)

40 kGy

668 (48%)

622 (27%)

637 (23%)

612 (28%)

532 (11%)

519 (8%)

20 kGy

670 (49%)

619 (26%)

580 (12%)

609 (27%)

558 (17%)

520 (8%)

40 kGy

Elongation at break (%)

3.03 (22%)

3.25 (24%)

3.20 (22%)

40 kGy

3.24 (42%)

3.12 (11%) 3.42 (50%)

2.87 (3%)

3 (17%) 3.28 (28%)

3.11 (25%)

3.32 (27%)

2.73 (4%)

20 kGy

Modulus (100%)

3.88 (49%)

3.6 (13%)

3.52 (25%)

3.63 (26%)

3.89 (33%)

3.26 (12%)

20 kGy

4.06 (56%)

3.36 (5%)

3.84 (35%)

3.62 (28%)

3.79 (29%)

3.73 (29%)

40 kGy

Modulus (200%)

4.34 (49%)

4.12 (0.48%)

4.06 (30%)

4.19 (30%)

4.52 (39%)

3.56 (14%)

20 kGy

4.78 (64%)

3.88 (0%)

4.49 (43%)

4.27 (32%)

4.45 (37%)

4.36 (39%)

40 kGy

Modulus (300%)

Number given within parenthesis represents percentage of improvement as compared to the non-irradiated samples

6.6 (73%)

CS/1.5-2

a

4.6 (35%)a

C

20 kGy

Sample Code Tensile strength (MPa)

7.51 (34%)

7.36 (0.82%)

6.90 (11%)

6.83 (12%)

7.79 (32%)

6.62 (3%)

20 kGy

7.43 (33%)

5.86 (0%)

7.52 (21%)

6.10 (0%)

6.74 (14%)

7.41 (16%)

40 kGy

Modulus (3%)

10.0 (17%)

11 (12%)

11.0 (15%)

10.0 (29%)

13.0 (13%)

14.0 (17%)

20 kGy

Set (%)

Table 2 Effect of filler loading on the properties of filled LDPE/EVA TPE system at 20 kGy and 40 kGy. Adapted with permission from [48]

9.0 (25%)

12 (4%)

10.0 (23%)

9.0 (36%)

11.0 (27%)

12.0 (29%)

40 kGy

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the large mass of silica particles. As a result, maximum enhancement in properties by irradiation is achieved in this case. An improvement in the set properties is also observed for irradiated TPE systems. The improvement is much more prominent in filled samples (Table 2).

6 Reprocessibility Studies Reprocessibility studies have been performed by Hui et al. on the irradiated samples [48]. A fair reprocessibility is observed for the samples irradiated up to 40 kGy radiation dose. The dynamic shear viscosity data of all irradiated samples at 130 °C, 2.79% strain, and at a constant shear rate of 0.03 s-1 for three reprocessing cycles are shown in Table 3. Overall, the shear viscosity increases with an increase in radiation dose. A poor reprocessibility is observed for the control blend. This indicates that the control blend changes its morphology in various reprocessing cycles. In the case of CS/31, the apparent viscosity increases noticeably. This is due to the inferior thermoplasticity of this sample resulting from the intermixing of LDPE and EVA in the continuous matrix. An outstanding reprocessibility is observed in the case of CS/32, especially at 20 kGy radiation dose, because here silica particles are dispersed in the EVA matrix along with maintaining the crystallinity of the LDPE matrix [50]. So, the morphology remains invariant with the processing cycle. At 20 kGy dose, all the samples displayed moderately good reprocessing characteristics up to the third cycle, except the silane-containing sample. A similar trend is observed at 40 kGy dose. However, additional crosslinks are produced at a higher dose which leads to enhancement in the viscosity. At 40 kGy dose, CS/3-Si69-2 exhibits a fairly steady viscosity data at several processing cycles, while an inferior reprocessibility is observed at a 20 kGy dose. Table 3 Reprocessibility data for various irradiated films at constant shear rate of 0.03 s−1 . Adapted with permission from [48] Sample code

C CS/1.5-2 CS/3-1

Viscosity at 20 kGy radiation dose (Pa·s)

Viscosity at 40 kGy radiation dose (Pa·s)

Original

Original

1st cycle

2nd cycle

3rd cycle

1st cycle

2nd cycle

3rd cycle

7893

6792

12,299

9863

16,255

13,767

15,419

14,611

13,583

9912

12,666

12,760

15,159

14,318

14,318

14,310

9863

9912

11,931

10,646

14,685

13,951

15,970

15,786 13,256

CS/3-2

6523

5507

4589

4600

13,951

14,153

13,516

CS/3-Si69-2

5874

7159

8260

8652

9120

8767

8950

8962

12,299

9545

11,564

11,896

16,255

17,071

21,477

21,687

CS/5-2

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7 Rheological Properties Rheology means the interrelationship between the flow and deformation characteristics of matter. It is an emerging field of research in order to access the processing characteristics of polymers, polymer blends, and/or composites. The ultimate morphology has a potential impact on the end-use performance of the thermodynamically immiscible blend systems, which itself is significantly influenced by rheological behavior of the system [51–54]. The molecular characteristics [55–59], flow geometry [60– 63], and processing conditions [55–57, 64, 65] such as temperature, shear rate, or frequency have a large effect on the flow behavior of polymer melts. Therefore, the rheological properties of polymer blends specifically depend on nature of polymers, molecular weight and its distribution [66, 67], blend compositions, the interactions among components including interfacial tension [68–70], and blend morphology [68–73]. However, in case of filled polymer systems, this rheological behavior becomes more complex. Payne effect which is attributed to the breakdown of filler–filler networks at lower strain levels has a significant effect to control the mechanism of reinforcement of filled polymeric blends [74]. To understand the degree of polymer– filler interactions and the structure–property relationship of the polymeric materials linear and nonlinear viscoelastic properties in the molten state are typically used. As a result, rheology seems to be an inimitable technique for the study of PNCs. Over the past years, the use of high-energy irradiation in the field of processing of polymers is being endlessly extended [75].

7.1 Melt Viscosity by Capillary Flow—MPT Studies In this study, Monsanto Processibility Tester (MPT) is used to evaluate the melt flow characteristics of the nanosilica-filled LDPE/EVA TPE systems. The apparent shear stress (τapp ), apparent shear rate (γ˙app ) and apparent shear viscosities (ηapp ) are calculated using the following Eqs. [57]: τapp =

dc ΔP 4lc

(1)

γ˙app =

32Q π dc3

(2)

ηapp =

τapp γ˙app

(3)

where ΔP is the pressure drop across the length of the capillary; d c and l c are the diameter and length of capillary, respectively; Q is the volumetric flow rate of the material.

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Figure 10a shows the variation of the melt viscosity with the radiation dose over a range of shear rates at 120 °C [76]. The variation of the melt viscosity with the shear rate (at 120 °C) for silica-filled LDPE/EVA systems at 20 kGy radiation dose is shown in Fig. 10b [76]. Hui et al. have reported that as compared to the unirradiated samples, all the irradiated samples exhibit higher viscosity (e.g., the percentage of increase in the melt viscosity are ~64%, ~88%, and ~93% for C/2R, CS/3-1/2R, and CS/3-2/2R, respectively, at a shear rate of 12.25 s–1 in comparison with the unirradiated samples) [76]. Furthermore, because of the radiation-induced crosslinking effect [77], the melt viscosity has been reported to increase with the increase in the radiation dose as shown in Fig. 10a. An improved degree of crosslinking at a higher dose level is evident from a more noticeable rise in the viscosity with the radiation dose in the low shear region than in the high shear region where flow curves converge. For example, the development percentage of the melt viscosity at 40 kGy dose and 12.25 s–1 shear rate is ~104%, ~66%, and ~81% for C/4R, CS/3-1/4R, and CS/3-2/4R, respectively, in comparison with 20 kGy radiation dose (Fig. 10a). The pseudo-plastic or shearthinning nature of the samples is indicated by a decrease in the melt viscosity with the increasing shear rate [77, 78]. In this model system, silica particles play dual functions—as reinforcing filler [79] and as radiation sensitizer [48]. In general, crosslinking takes place upon irradiation at 20 kGy dose results in an improved interfacial bonding. In the case of CS/3-1/2R this crosslinking effect dominates over the reduction in crystallinity. So, its viscosity is higher than that of C/2R and CS/3-2/2R as well (Fig. 10b). It is well known that EVA is more prone to radiation crosslinking and silica acts as a radiation sensitizer. Thus, when 20 kGy dose is applied in sequence-2 then the crosslinking mostly takes place in silica-filled EVA domains, whereas a negligible amount of crosslinking may take place in the LDPE phase. As a result, under melt flow conditions the tendency of polymer molecules to slip past or roll over the nanosilica-filled and crosslinked rigid EVA domains becomes more in CS/3-2/2R than that is present in CS/3-1/2R. So, the apparent viscosity is decreased. In the case of CS/1.5-2/2R, the viscosity is very high due to the crosslinking and the silica reinforcement effect as well (Fig. 10b). In CS/5-2/2R, initially, viscosity is high due to more accessible radiation crosslinked interfacial area [48] (Fig. 10b). But after a certain shear rate, viscosity decreases due to the rupture of EVA domains with aggregated silica particles. Interestingly, in the case of CS/3-Si69-2/2R, the radiation scavenging effect of Si69 is evident from a lower viscosity than that of the other silica-filled samples (Fig. 10c). Here as compared to other filled samples (Fig. 10a) the percentage improvement of viscosity with radiation dose is not significant. The flow behavior index, n, and the consistency index, k are calculated by using the Power Law model as follows: n τapp = k γ˙app

(4)

by definition ηapp = τapp /γ˙app , therefore, n−1 ηapp = k γ˙app

(5)

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Fig. 10 a Effect of radiation dose on silica-filled LDPE/EVA TPE systems at 120 °C. b Apparent shear viscosity versus apparent shear rate of silica-filled LDPE/EVA TPE systems irradiated at 20 kGy at 120 °C. c Effect of radiation dose on CS/3-Si69-2 at 120 °C. Adapted with permission from [76]

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Table 4 Flow behavior index (n) and consistency index (k) at 110 °C, 120 °C and 130 °C of LDPE/EVA nanocomposites. Adapted with permission from [76] Sample ID

110 °C k(kPa·sn )

120 °C n

k(kPa·sn )

130 °C n

k(kPa·sn )

n

C/2R

42.67

0.34

20.78

0.37

19.23

0.37

CS/3–1/2R

58.63

0.29

28.03

0.34

26.16

0.34

CS/3–2/2R

49.11

0.32

26.17

0.33

17.59

0.40

Logarithmic form for Eq. (5) may be written as: log ηapp = log k + (n − 1) log γ˙app

(6)

The values n and k are calculated from the initial linear region from the log ηapp versus log (shear rate) plot observed at lower shear rates. Hui et al. have calculated the n and k by using this power law equation (Table 4). As n < 1, therefore a pseudo-plastic behavior is shown by all the composite systems. At 110 °C and at 120 °C, the factors like residual crystallinity, radiationinduced crosslinking, and ball bearing/rolling effect are used to explain the variation of ‘n’. However, both the ball bearing mechanism and the residual crystallinity have a negligible contribution at 130 °C. At this temperature, the k–value (approximately equal to zero shear viscosity) decreases because increase in temperature tends to increase the free volume of the system. Also, upon filler loading, the k–value increases because the free volume of the system decreases. Interestingly, as compared to CS/3– 1/2R, k is lower in CS/3–2/2R at 110 °C, 120 °C, and 130 °C. It further supports the fact that the ball bearing effect takes place in this particular sample prepared by sequence-2 which becomes more significant upon irradiation.

7.2 Oscillatory Shear Flow—RPA Studies: Frequency Sweep The rubber process analyzer (RPA) is used to measure the dynamic rheological (linear and nonlinear) viscoelastic properties of samples. The linear viscoelastic (LVE) region of the samples is initially determined by strain sweep at a test frequency of 0.500 Hz. Thereafter, the frequency sweep test is performed where the oscillating frequency is programmed to change in steps from 0.033 Hz to 33.333 Hz within the LVE, at constant strain amplitude of 2.79% and constant temperature conditions (130 °C). At both 20 and 40 kGy radiation doses, an increase in elastic response with increasing frequency is observed by all the filled and unfilled blends due to increasingly imposed restriction on polymers [80] as shown in Fig. 11a, b. Consequently, as compared to the unirradiated samples, the elastic modulus is higher in the irradiated samples [76]. The highest degree of elastic response is shown

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Fig. 11 Frequency sweep test at 130 °C and 2.79% strain in LDPE/EVA systems filled with various nanosilica loadings a Elastic modulus (G' ) as a function of frequency at 20 kGy b Elastic modulus (G' ) as a function of frequency at 40 kGy. Adapted with permission from [76]

by CS/3-1/2R. This is evident from its higher G' value in combination with comparable G'' value. The values of G' and G'' become similar at 40 kGy radiation dose. Interestingly, at both doses, lower elastic modulus is shown by CS/3-Si69-2. This is due to the radiation scavenging effect of Si69 as mentioned in the earlier sects. Overall, the radiation-induced crosslinking of the silica-filled TPE systems causes an increase in the elastic modulus (G' ) with increase in the radiation dose. Likewise, the loss modulus (G'' ) also increases with the test frequency (not shown here). This is due to the prerequisite of higher energy for molecular viscous response. At 20 kGy dose, the variation of the complex viscosity (η*) is shown as a function of frequency at 130 °C in Fig. 12. Evidently, the rheological behavior of the pseudoplastic fluids is exhibited by both unfilled and filled blends at both doses [81]. Overall, viscosity of the irradiated samples becomes higher as compared to the unirradiated samples [76], and the viscosity also increases with the increase in the radiation dose. The observations match with the results obtained from the capillary rheometer. Here, the viscosity of all the samples merges over the range of frequency studied at a radiation dose of 40 kGy (not shown here).

7.3 Comparison Between the Capillary and Dynamic Rheology: Effect of Frequency or Shear Rate Hui et al. have compared the oscillatory shear viscosity as a function of test frequency to the steady shear viscosity as a function of shear rate at 130 °C by using Cox–Merz concept [82] shown in Eq. (7): η∗ (ω) = η(γ˙ )

(7)

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Fig. 12 Complex Viscosity (η*) as a function of frequency in LDPE/EVA systems filled with various nanosilica loadings at 130 °C at 20 kGy. Adapted with permission from [76]

A good correlation has been derived between dynamic and capillary rheological results for the unirradiated samples by Hui et al. [79]. Interestingly, upon irradiation, a substantial difference between dynamic and capillary rheological data by these systems has been observed (Fig. 13a–c) [76]. The apparent viscosity (ηapp ) is higher than the complex viscosity (η*) in all the samples. The crosslinking effect primarily causes this deviation. The flow is localized and restricted in oscillatory experiment (RPA), whereas the capillary flow is not localized (longer range). It is rather translational motion involving polymer molecules as a whole. Henceforth, more significant effects of crosslinking are observed in capillary flow.

7.4 Morphology of Extrudates The analyses of the extrudate morphology (both at low and high shear rates) are done by TEM, and the representative photomicrographs of some selected samples are shown in Fig. 14a–d. The LDPE matrix and intermixed (LDPE–EVA) portion are indicated by the gray and the white regions, respectively. The inter-mixed portion of silica-filled EVA domains is represented by a dark phase. Most of the silica particles are dispersed arbitrarily in both LDPE and EVA phase as well as in the interface on the micrographs of CS/3-1/2R extruded at 612.5 s–1 shear rate (Fig. 14a). So, the occurrence of intermixing of both polymer phases (EVA and LDPE) is again proved here that becomes more prominent after extrusion. Ultimately, this type of morphology is a consequence of the change in the viscosity at various shear rates of this particular sample. The morphology of extruded (at 612.5 s–1 ) CS/1.5-2/2R is similar to that of CS/3-1/2R (Fig. 14b). But interestingly, the morphology of the extruded (at 612.5 s–1 ) CS/3-2/2R is entirely different (Fig. 14c). Here, almost all silica particles exist in the EVA domains. However, distortion of the spherical shape of domains is observed due to quicker quenching as it escapes the die

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Fig. 13 Superimposed plots of oscillatory viscosity and steady shear viscosity as a function of frequency or shear rate in LDPE/EVA systems filled with various nanosilica loadings on varying sequences: a C/2R b CS/3-1/2R c CS/3-2/2R. Adapted with permission from [76]

[76]. In the case of CS/5-2/2R, after extrusion at 612.5 s–1 shear rate, silica particles are dispersed randomly in the polymer matrix, and breakdown of the silica-filled EVA domain takes place (Fig. 14d). In addition, from the FESEM photomicrographs of the extruded samples (not shown here) at a high shear rate (1225.0 s–1 ), it has been reported by Hui et al. that the particle diameter spans from 30 to 443 nm [76]. In the case of extruded CS/52/2R, the silica-filled EVA domains break, and some silica particles are dispersed in the LDPE phase. However, the finer dispersion of filler particles is retained in EVA domains in the case of CS/3-2/4R at the same shear rate.

8 Recent Developments of the Radiation Processed Hybrid …

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Fig. 14 TEM photomicrographs of samples extruded at 612.5 s–1 : a extruded CS/3-1/2R at 12 k magnification; b extruded CS/1.5-2/2R at 10 k magnification; c extruded CS/3-2/2R at 20 k magnification; d extruded CS/5-2/2R at 8 k magnification. Adapted with permission from [76]

8 Thermal and Thermo-oxidative Degradation Studies Nowadays, the thermo-analytical tool is used widely for polymer characterization in research and industry. The stabilization and degradation study of polymers is an extremely vital field that gives insight into the thermal properties of polymeric materials. To control, accelerate, or delay the degradative pathways of polymers, it is crucial to realize their thermal degradation pathways and the effect of various additives on the degradation. Carrying out the degradation in a modest environment, controlled temperatures, and heating rates gives some extra mechanistic information. Thermogravimetric analysis (TGA) is an indispensable tool that is used to study the kinetics of thermal degradation as well as to determine a material’s thermal stability. Here, the weight change of a material is measured as a function of temperature or time. TGA is simple to operate, and the wealth of information can be derived from a simple thermogram. Therefore, nowadays it is extensively used in analytical polymer laboratories [83]. In general, LDPE shows single-step decomposition, whereas EVA shows twostaged decomposition. It has been reported that acetic acid is formed in the first step of degradation of EVA [84, 85] the amount of which is increased with increasing the

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vinyl acetate (VA) content in EVA. In the second step, degradation of main chains occurs. Similarly, the thermal degradation of LDPE/EVA blends shows two-staged decomposition and these blends exhibit superior thermal stability as compared to pure polymers [25]. In the case of thermo-oxidative degradation of unfilled LDPE/EVA blends, the deacylation of the VA groups in EVA is exhibited by the initial step (300– 380 °C). The second step (390–480 °C) involves the degradation of the hydrocarbon polyacetylene–ethylene chains [18]. For the study of thermogravimetric analysis, the samples are heated from ambient temperature to 600 °C under dynamic N2 and O2 atmospheres, respectively, with a steam line flow rate of 60 ml/min at various heating rates, e.g., 5, 10, and 15 °C/min.

8.1 Effect of Pristine Nanosilica on Thermal and Thermo–oxidative Degradation Characteristics of LDPE/EVA Systems It has been reported by Hui et al. that in the N2 atmosphere, the thermograms of all the nanosilica-filled LDPE/EVA TPE systems show two-staged decomposition. In comparison with the control blend, the filled samples do not exhibit any severe upgradation of thermal stability [86]. CS/3-2 shows only a modest degree of improvement. Interestingly, a remarkable change is observed in the O2 atmosphere (air) as compared with those in the N2 atmosphere. In the air, EVA is thermally more stable than LDPE. Moreover, the control blend shows higher thermal stability than that of the pure EVA and LDPE which results from the grafting reaction between EVA and LDPE [86]. Here, the control blend exhibits a synergism. In the case of all unfilled and filled samples, the first step (306–316 °C) involves the deacetylation of the VA group of EVA with the elimination of acetic acid and the formation of double bonds [18], whereas the second step (420–443 °C) corresponds to the further degradation of polyacetylene–ethylene chains formed in the first step accompanied with the degradation of LDPE [18]. Overall, the filled systems are thermally more stable in the air (O2 ) than the unfilled blend by delaying initial thermo-oxidative degradation. But at a higher temperature range (above 430 °C), silica acts as an acid catalyst and speeds up the degradation of filled systems [86]. CS/3-Si69-2 exhibits the maximum shift of onset of degradation (281 °C) [86] due to enhanced compatibility between two phases in presence of silane coupling agent, and fine dispersion of silica in both phases as well as in the interface [as evident from previous morphological analysis]. As a result, degradation is prohibited via controlled O2 diffusion through the polymer matrix. CS/3-1 shows higher thermal stability than CS/3-2 up to 20% conversion level which further proves the occurrence of intermixing that finally results in a strengthened silica-polymer interface and initial restriction of O2 diffusion [86]. Interestingly, in the higher temperature range, CS/3-1 shows lower thermal stability than CS/3-2 [86]. This is because a barrier effect of silica particles

8 Recent Developments of the Radiation Processed Hybrid …

263

delays O2 diffusion and decomposition products at a higher temperature which leads to retardation of thermal degradation. The onset of degradation increases with an increase in silica loading for samples prepared by sequence-2.

8.2 Effect of EB Irradiation on Thermo–oxidative Degradation Characteristics of Silica Filled Nanocomposites Figure 15 represents the TGA thermograms for a few selected irradiated samples in air at a heating rate of 10 °C/min. Upon irradiation, no noteworthy development in the onset of degradation temperature is observed in case of the control blend, while a slight improvement is found in case of CS/3-1 and CS/3-2 [86]. The onset of degradation for all the samples remains almost unchanged with the increase in radiation dose. The parameters obtained from TGA thermograms are reported in Table 5 [86]. Actually, upon EB irradiation, both filler–matrix interactions as well as crosslinking have a great impact on thermal stability. Both effects are comparable at 20 kGy dose, whereas the crosslinking effect is predominant at 40 kGy dose [48]. Irradiation has been performed under air where the polymer matrix is subjected to crosslinking reaction as well as oxidation reaction via a free radical process. In the present work, the possibility of oxidation is less because here very low and controlled dose of EB irradiation has been chosen by Hui et al. In the case of irradiated CS/3-1, crosslinking occurs in both phases as well as in the interface due to the occurrence of intermixing (evident in earlier Sects.). This crosslinking effect causes more constraints in the polymer matrix. Furthermore, the inorganic nanosilica particles Fig. 15 Typical TGA traces of C/2R, CS/3–1/2R and CS/3–2/2R in oxygen atmosphere (air). Adapted with permission from [86]

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Table 5 Characteristic parameters obtained from the thermo-oxidative degradation of nanosilicafilled irradiated LDPE/EVA systems in O2 atmosphere (air) at 10 °C/min heating rate. Adapted with permission from [86] Sample ID

Irradiation dose (kGy)

Tonset (°C)

T1 (°C)

T2 (°C)

C/2R

20

261 ± 1a

307 ± 1

404 ± 1

C/4R

40

261 ± 1

331 ± 1

446 ± 1

CS/3–1/2R

20

280 ± 2

326 ± 2

429 ± 2

CS/3–1/4R

40

280 ± 2

329 ± 2

423 ± 2

CS/3–2/2R

20

280 ± 1

324 ± 1

444 ± 1

CS/3–2/4R

40

283 ± 1

324 ± 1

428 ± 1

a

Standard deviation T1 = First decomposition temperature (°C) T2 = Second decomposition temperature (°C)

increase the radiation sensitivity [48]. This radiation sensitivity is much more prominent in CS/3-2/4R. This enhanced radiation sensitivity results in higher crosslink density in the nanosilica-filled EVA domains which ultimately causes optimum reinforcement of this weaker rubbery phase. However, in N2 atmosphere, the changes are not very significant.

8.3 Kinetic Methods for Degradation [87] Hui et al. have also done kinetic analysis to simulate the thermal degradation behavior of a material. The activation energies of degradation up to lower range of conversions have been determined by non-isothermal and isothermal kinetic analyzes. The reaction rate during degradation can be defined as the variation of degree of conversion (α) with time or temperature, and the conversion is calculated as: α=

W0 − Wt W0 − W f

(8)

where W 0 , W t and W f are the weight at the beginning of the degradation step, the actual weight at each point of the curve, and the final weight measured after the specific degradation process completed, respectively. All kinetic studies assume that the isothermal rate of conversion (dα/ dT ), is a linear function of a temperature–dependent rate constant k(T ), and a temperature-independent function of the conversion (α) i.e., dα = k(T ) f (α) dt

(9)

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where f (α) depends on the mechanism of the degradation reaction. The function k(T ) is usually described by the Arrhenius Equation: k(T ) = Ae− RT E

(10)

where A, E, R, and T are the pre–exponential factor (min−1 ) assumed to be independent of temperature, the activation energy (kJmol−1 ), the gas constant (8.314 Jmol−1 K−1 ), and the absolute temperature (K), respectively. Combination of Eqs. (9) and (10) gives the following expression: dα E = A f (α)e− RT dt

(11)

At constant heating rate, β' = dT/ dt and Eq. (11) may be written as the following: β'

dα E = A f (α)e− RT dT

(12)

Integration of Eq. (12) within the limits of an initial temperature, T 0 , corresponding to a degree of conversion α 0 , and the peak temperature, T p , corresponding to another degree of conversion α p , gives the following Eq.: ∫α p α0

dα A = ' f (α) β

∫Tp

e− RT dT E

(13)

T0

The integral function of conversion, g (α), is obtained if T 0 is low, and hence it is assumed that α 0 = 0 and also considering that there is no reaction up to a temperature of T 0, one gets: ∫α p g(α) = α0

A dα = ' f (α) β

∫Tp

e− RT dT E

(14)

0

Flynn–Wall–Ozawa method (FWO method) This method was suggested independently by Ozawa [88], Flynn, and Wall [89]. This is an isoconversional integral method based on the following Eq.: log β ' = log

0.457E AE − 2.315 − Rg(α) RT

(15)

where β ' is the heating rate. A, E, R, and T have their usual significance as described earlier. Thus, for α = constant, the plot of log β versus 1000/T obtained at several heating rates should give rise to a straight line whose slope can be used to evaluate

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Table 6 Mean activation energies of different irradiated nanocomposites obtained from various kinetic methods in O2 atmosphere (air). Adapted with permission from [86] Sample ID

C/2R

Mean activation energy, E (kJ/mol) FWO method

KAS method

67 ± 2a

65 ± 1

Sample ID

C/4R

Mean activation energy, E (kJ/mol) FWO method

KAS method

84 ± 3

83 ± 1

CS/3–1/2R

76 ± 1

73 ± 1

CS/3–1/4R

88 ± 1

84 ± 2

CS/3–2/2R

83 ± 2

77 ± 2

CS/3–2/4R

94 ± 1

89 ± 2

a

Standard deviation

the activation energy. Using this integral method, one can determine the activation energy even without the knowledge of reaction order at different levels of conversion. Kissinger–Akahira–Sunose method (KAS method). This is also an isoconversional integral method based on the following Eq. [90]: ln

E AR β' − = ln 2 T Eg(α) RT

(16)

where g(α) is the algebraic expression for integral methods as described earlier. For α = constant, the plot of ln β ' /T 2 versus 1/T, obtained from thermograms recorded at several heating rates yields a straight line whose slope allows evaluation of the activation energy. Using both FWO and KAS methods, the activation energies of decomposition of C, CS/3-1, and CS/3-2 (irradiated at both 20 and 40 kGy doses) have been calculated in O2 atmosphere, and the mean activation energy values are shown in Table 6 [86]. Overall, the activation energy of the irradiated samples increases with the increase in radiation dose. Interestingly, the activation energy of irradiated CS/3-1 (both at 20 and 40 kGy doses) is less in comparison with other irradiated samples. The similar trend is also observed in case of isothermal kinetic analysis (not shown here).

9 Electrical and Dielectric Properties Most polymers possess intrinsic electrical insulating properties. These properties are used to restrict and shield current in conductors and to sustain high electric fields without breakdown [91, 92]. The polymeric interfaces transport electrical charge [93]. Henceforth, it is of paramount importance to explore the effect of interfaces on the generation of charge carriers, their transport and storage in polymeric systems. For suitable selection of polymers to be used as insulating materials and capacitors it is often essential to investigate thoroughly their overall dielectric properties (e.g., dielectric constant and loss factor as a function of frequency and temperature). These are also important in order to achieve a complete understanding of the nature of their

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electrical responses [94]. In the field of dielectrics and electrical insulation, in 1994, the most important paper entitled “Nanometric Dielectrics” has been published by Lewis [95]. Since then, in this field, detailed study regarding polymeric matrices filled with inorganic nanoparticles, i.e., PNCs have gained a significant interest among the researchers. Tanaka et al. have reported manufacturing process, characterization, and applications of PNCs as dielectrics and electrical insulation where nanofillers improve the resistance to high-voltage environment, electrical and thermal properties [96, 97]. Chattopadhyay et al. and Borhani et al. have reported the electrical properties of EB modified TPEs based on LDPE and EVA [29, 30]. In this study, the dielectric and electrical properties of the nanocomposites are measured in the frequency range of 102 –106 Hz at different temperatures ranging from 30 to 110 °C. The dielectric constant or dielectric permittivity (ε' ) is determined by using the following Eq.: ε' = C p /Co

(17)

where C p is the observed capacitance of the sample (in parallel mode), and C o is the capacitance of the cell. C o = (εo A)/d', where d' and A are the thickness and area of the sample, respectively. εo (εo = 8.85 × 10–12 F/m) is the dielectric constant of vacuum. Usually, several polarization phenomena play an important role to generate the ε’ of a polymer when it is subjected to an oscillating electric field. In case of polar polymers like EVA the major portion of the total polarization is controlled by the orientation polarization. The presence of dipoles connected to EVA chains, its chemical construction, oxidation or diffusion of polar molecules from the environment cause this type of polarization in EVA. Thus, the value of ε' is higher in EVA than that of LDPE [98]. The β-relaxation of EVA is represented by 105 Hz of frequency which takes place due to the movement of acetate side chains [29]. The ε' of the control blend (C) lies in between those of EVA and LDPE. In case of C, the nature of variation in ε' with frequency is similar to that of pure LDPE. It again supports the fact that in this model LDPE (40)/EVA (60) blend, LDPE forms the continuous matrix and EVA forms the dispersed phase [98]. In one side nanosilica particles suppress the orientation polarization but on the other side these particles increase the interfacial polarization due to the Maxwell–Wagner effect [99]. For EVA, ε' decreases with frequency upon addition of nanosilica because here orientation polarization controls over interfacial polarization [98]. In case of filled polymer systems, the electrical behavior is controlled by the state of dispersion of fillers. In this model system, the presence of various interfaces in both sequence-1 and sequence-2 at ambient temperature is shown schematically by Fig. 16. It is clearly seen that due to the occurrence of intermixing (proved by FTIR and TEM analysis as reported in earlier sects.) the probability of existence of various interfaces (EVA–silica interface as well as LDPE–silica interface) is more for sequence-1 (Fig. 16). As a result, for sequence-1, the interfacial polarizations have a significant effect which results in its higher frequency dependency than that of samples prepared by sequence-2. In sequence-2, this intermixing phenomenon is less important. In this

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Fig. 16 Schematic depiction of morphology of C, CS/3-1, and CS/3-2 at ambient temperature. Adapted with permission from [98]

particular sequence, there is a more probability of existence of silica–EVA interface along with EVA–LDPE interface which leads to less ε' in this system.

9.1 Effect of Electron Beam Irradiation at Controlled Dose on the Dielectric Properties The effect of controlled EB irradiation on some selected samples at room temperature is shown by Fig. 17a [98]. It has been reported by Hui et al. that the ε' decreases for all the irradiated samples which is more pronounced in case of the filled samples [98]. This indicates the occurrence of radiation-induced crosslinking as well as the sensitizing effect of nanosilica particles (This is also evident in the earlier sects.). The average decrease of ε' is 5%, 13% and 18% for C/2R, CS/3-1/2R and CS/3-2/2R, respectively. Interestingly, more or less frequency independency is observed for all the irradiated samples. So, it may be inferred that controlled irradiation improves the interfaces which results in reduction in the interfacial polarization. The radiation sensitizing effect of nanosilica particles is further supported by a more significant decrease in ε’ value in case of CS/3–2/2R than that of CS/3–1/2R [48]. The variation of ε' of filled irradiated LDPE/EVA TPE systems against temperature at a frequency of 103 Hz is shown by Fig. 17b. Both the interfacial polarization and the ionic conduction phenomena mostly control the dielectric properties of these systems at higher temperature (above 90 °C). At 20 kGy radiation dose, radiation-induced crosslinking effect strengthens both phases (mainly EVA phase)

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Fig. 17 a Permittivity (ε' ) as a function of log frequency for C, CS/3–1 and CS/3–2 (Both unirradiated and irradiated at 20 kGy) at room temperature. b Variation of the permittivity with temperature for C, CS/3–1 and CS/3–2 (both unirradiated and irradiated at 20 kGy) at a frequency of 1000 Hz. Adapted with permission from [98]

and interfaces. Additionally, at higher temperatures the ε' decreases. This is due to the restriction of the chain mobility at the melting and softening temperatures that arises from the increase in gel fraction. The samples irradiated at 40 kGy radiation dose also display similar trends. The nature of their change in ε' versus temperature or frequency is marginally different from the samples irradiated at 20 kGy dose.

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9.2 Volume Resistivity (ρ υ ) and Electrical Breakdown Testing It is well known that in an insulator, the valence electrons are bound tightly, therefore it does not have free electrons to conduct electricity. The insulators also have large dielectric strength. Besides these, the most required characteristic of an insulator is its ability to resist the leakage of electrical current. Therefore, resistivity studies are very important for insulating materials. LDPE is an outstanding insulator (ρ υ is in the order of 1016 Ω cm), whereas the ρ υ of EVA is in the order of 1013 Ω cm. The ρ υ of C is in the order of 1014 Ω cm range lies in between LDPE and EVA. Table 7 shows the ρ υ values of nanosilica-filled TPE systems along with pure EVA and LDPE [98]. The resistivity is higher in case of samples prepared by sequence-2 than the samples prepared by sequence-1 as well as the control blend. The maximum ρ υ value is shown by CS/3-2. Overall, it is clear from the resistivity results that all the filled systems are good insulators. Interestingly, the ρ υ of all the samples increases when irradiated at 20 kGy dose. This is due to the improved crosslink density which enhances medium compactness and improves the interfacial defects [98]. Overall, as compared to C, the breakdown strength of all nanosilica-filled LDPE/EVA TPE systems is higher at room temperature as shown in Table 7 [98]. The interfacial area between the polymer matrix and the nanosilica filler has a large impact on insulation breakdown strength which becomes more significant in case of CS/3-Si69-2. However, upon irradiation the interfacial defects are minimized and crosslinking of individual polymers occurs. As a result of these, the irradiated Table 7 Volume resistivity and voltage breakdown strength of nanosilica-filled LDPE/EVA TPE Systemsa (both irradiated and unirradiated). Adapted with permission from [98] Sample ID

Volume resistivity (Ω m)

EVA

2.2 × 1011

Voltage breakdown (kV/mm) –

LDPE

6.5 ×

C

2.1 × 1012

44.0

CS/1.5-2

2.7 × 1012

47.0

1014

–

CS/3-1

5.8 ×

1011

47.9

CS/3-2

3.3 × 1012

48.1

CS/3-Si69-2

2.3 × 1012

51.1

CS/5-2

2.2 ×

1012

50.2

C/2R

2.2 × 1012

45.4

CS/3-1/2R

6.1 × 1011

49.5

CS/3-2/2R

3.7 × 1012

49.8

a

Per formulation, three volumetric electrical resistivity and breakdown voltage measurements are performed. The percentage error in the measurement of volume resistivity is found to be ±1.5%. The percentage error in the measurement of breakdown voltage is found to be ±2%

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samples undergo dielectric breakdown at voltages higher than those of the unirradiated samples. The breakdown strength of CS/3-2/2R is higher than that of CS/3-1/2R. This is because nanosilica particles in CS/3-2/2R are mostly confined in the EVA phase; subsequently, more crosslinking occurs in the amorphous EVA domains upon irradiation than the semi-crystalline LDPE matrix. This again supports the fact that nanosilica particles act as radiation sensitizer [48]. The crosslinked EVA domains prevent the electrical breakdown path inside the polymer matrix by acting as barriers which results in its higher breakdown strength.

9.3 Swelling-Deswelling Kinetics The kinetics of swelling of the samples is followed gravimetrically. The solvent intake (ws ) at each time being defined as follows: ws = [100 × (m t − m i )]/m e

(18)

where mi is the initial weight of the sample, mt is the weight of the sample at regular time intervals, and me is the weight of the sample at swelling equilibrium. The kinetics of deswelling is also followed gravimetrically. The percentage solvent retention (wt ) at each time being calculated as follows: wt = 100 − [100 × (m e − m t )/m e ]

(19)

The swelling–deswelling kinetic analysis of this system has been studied gravimetrically at different time intervals by Hui et al. [98]. Both MEK and EVA are polar. So, MEK tends to diffuse more into the polar EVA phase rather than that of non-polar LDPE matrix. The representative graphs are shown in Fig. 18a–d. The intermixing of both phases in sequence-1 is further reflected from its maximum solvation ingestion capacity (Fig. 18a) (also evident from earlier Sects.). This intermixing phenomenon helps MEK to diffuse into the polymer network of CS/3-1 more easily than CS/32 and the C as well. In sequence-2, solvent diffusion into the polymeric matrix is obstructed by nanosilica-reinforced EVA domains that leads to lower rate of swelling for CS/3-2 (Fig. 18a). However, the swelling results (Fig. 18a) are not well correlated with the dielectric results, but the deswelling results (Fig. 18b) are well correlated with the dielectric results [98]. This is because during swelling process all the interfacial factors do not contribute in a short span of time although all these factors contribute to a greater extent during deswelling. In presence of EB irradiation, both filler–matrix interaction and crosslinking of matrix have a significant impact on swelling. Thus, as compared to the unirradiated samples the percentage of swelling diminishes upon irradiation (Fig. 18c) [98]. Interestingly, after irradiation, the swelling results are well correlated with the dielectric results which are largely influenced by interfacial polarization. This is because when radiation is applied to these samples then interfacial defects are minimized due to the

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Fig. 18 a Variation of percentage of swelling with time for C, CS/3–1, and CS/3-2 at 25 °C. b Variation of percentage of deswelling with time for C, CS/3-1, and CS/3-2 at 25 °C. c Variation of percentage of swelling with time for C/2R, CS/3-1/2R, and CS/3-2/2R at 25 °C. d Variation of percentage of deswelling with time for C/2R, CS/3-1/2R, and CS/3-2/2R at 25 °C. Adapted with permission from [98]

formation of crosslinked network [48] as well as improvement of all the interfacial defects. Here, nanosilica particles act as radiation sensitizer [48]. The rate of swelling is less in CS/3-2/2R than that of CS/3-1/2R (Fig. 18c). This is because in comparison with sequence-1, the percentage of crosslinking is more in sequence-2 due to the presence of nanosilica filled EVA domains [98]. The rate of swelling of CS/3-1/2R is very close to that of C/2R because of the improvement of interfacial defects upon irradiation. The rate of deswelling of irradiated samples (Fig. 18d) follows the similar trend like unirradiated samples. On a whole, after irradiation, the movement of polymer chains is restricted specifically at the interfaces, i.e., the irradiated samples become stiffer because of the development of crosslinked network structure and sensitizing effect of nanosilica. As a result, the irradiated samples become less penetrable by the solvent.

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10 Conclusion and Outlooks In recent years, radiation processing—an excellent commercially viable technology has attracted pivotal importance in the era of modern polymer industries. The recent progress in polymer radiation chemistry has led to substantial improvements/modifications in the structure and properties of polymers, and several applications of commercial and economic importance. To widen the diversity of polymers, another well-known approach is to modify their technical properties by the addition of nanofillers. Controlled irradiation can potentially be an effective way of tailoring the technical properties of such PNCs which is clean, simple, fast, energysaving, environment-friendly, and economically beneficial. This chapter explores some expected and unexpected achievements in recent developments of the radiation processed hybrid organic–inorganic PNCs based on pristine silica nanoparticlefilled model (LDPE/EVA) TPE system. A brief overview of the simultaneous effect of the addition of nanosilica and controlled EB irradiation on the mechanical, rheological, thermal, and electrical properties of this model system has been discussed. The detailed evaluation of the bulk morphology of various samples has also been highlighted. Finally, these properties have been correlated with the morphology of these systems. When the pristine nanosilica particles are dispersed in the LDPE/EVA-based model TPE system, these particles alter the microscale morphology as well as the crystalline morphology of the blend systems. This is a strong function of the sequence and extent of nanosilica addition. At a very low level of loading, silica particles are dispersed at a micro and nanoscale level within the polymer matrix. As compared to sequence 2, more intermixing of both polymer phases (LDPE and EVA) occurs in the continuous matrix in sequence 1. When the samples are irradiated with EB irradiation at a low level of radiation dose (20 and 40 kGy, i.e., controlled dose) then both interfacial and filler–matrix adhesion is improved to a greater extent. The role of nanosilica particles as radiation sensitizers is evident from here. Interestingly, the compatibility between the two phases enhances upon the addition of a silane (Si69) coupling agent while the radiation sensitivity diminishes. FTIR analysis indicates a significant interaction between EVA and silica. The mechanical, rheological, thermal, electrical, and dielectric properties of this model nanosilica-filled TPE system are influenced remarkably by radiation dose, loadings of nanosilica, variation of sequences of addition of ingredients, and addition of Si69. The radiation sensitizing effect of nanosilica and radiation scavenging activity of Si69 are further reflected in these results. The controlled irradiation improves the set properties of these samples. It is obvious from reprocessibility studies that the TPE systems irradiated at controlled radiation dose can still be reprocessed. The melt viscosity of the irradiated silica-filled samples is higher than that of unirradiated samples. The factors like residual crystallinity, radiation-induced crosslinking, rolling effect, and improvements in interfacial bonding can explain the variation of viscosity with temperature. Interestingly, the dynamic and steady shear rheological properties do not follow a simple correlation. However, the extrudate

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morphology correlates well with the rheological behaviors. The thermal stabilities of the nanosilica-filled LDPE/EVA TPE systems are improved slightly upon controlled EB irradiation. The formation of crosslinked network structure with improved interfacial bonding is reflected in the dielectric results where ε' of the irradiated samples are less as compared to the unirradiated samples. The breakdown strength of all nanosilica-filled samples increases upon irradiation. The swelling–deswelling kinetic studies again support the effect of filler–polymer or polymer–polymer interfaces on the electrical properties. Furthermore, the dielectric results, volume resistivity, breakdown strength, and the swelling–deswelling kinetics are well correlated. Finally, there is a good correlation between the above-mentioned properties and the morphology of this model TPE system. Therefore, the simultaneous effect of nanosilica reinforcement and controlled irradiation can upgrade the technical properties and stabilize the micro and nanoscale morphology of these LDPE/EVA systems. The selective modification of these TPE systems with nanofillers combined with the EB processing at a controlled dose level can adjoin a new breadth to the science and technology of new generation TPEbased materials. Thus, this green technique can be potentially extended for uplifting of the technical properties of conventional TPE systems, and it will have a promising potential for future commercial use which is expected to give valuable insights into new research directions.

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82. Cox WP, Merz EH (1958) Correlation of dynamic and steady flow viscosities. J Polym Sci 28:619–622 83. Wang XL, Yang KK, Wang YZ et al (2003) Thermogravimetric analysis of the decomposition of poly (1,4-dioxan-2-one)/starch blends. Polym Degrad Stab 81:415–421 84. Mcneill IC, Jamieson A, Tosh DJ et al (1976) The thermal degradation of copolymers of vinyl acetate with methyl methacrylate and other monomers. Eur Polym J 12:305–312 85. Mcgrattan BJ (1994) Examining the decomposition of ethylene-vinyl acetate copolymers using TG/GC/IR. Appl Spectrosc 48:1472–1476 86. Hui S, Chattopadhyay S, Chaki TK (2010) Thermal and thermo-oxidative degradation study of a model LDPE/EVA based TPE system: Effect of nano silica and electron beam irradiation. Polym Compos 31:1387–1397 87. Vyazovkin S (2000) Kinetic concepts of thermally stimulated reactions in solids: a view from a historical perspective. Int Rev Phys Chem 19:45–60 88. Ozawa T (1965) A new method of analyzing thermogravimetric data. Bull Chem Soc Jpn 38:1881–1886 89. Flynn JH, Wall LA (1966) A Quick, direct method for the determination of activation energy from thermogravimetric data. Polym Lett 4:323–328 90. Kissinger HE (1957) Reaction kinetics in different thermal analysis. Anal Chem 29:1702–1706 91. Ku CC, Liepins R (1987) Electrical properties of polymers. Hanser, Munich, Vienna, New York 92. Blythe T, Bloor D (2005) Electrical properties of polymers. Cambridge University Press, Cambridge 93. Tripathi AK, Pillai PKC (1985) Thermally stimulated depolarization studies on laminated films of cellulose triacetate and polystyrene. In: Proceedings of the 5th International Symposium on Electrets, IEEE, New York, p 138 94. Mathes KN (1986) Electrical properties. In: Kroschwitz JI (ed) Encyclopedia of polymer science and engineering. Wiley, New York, p 507 95. Lewis TJ (1994) Nanometric dielectrics. IEEE Trans Dielectr Electr Insul 1:812–825 96. Tanaka T, Montanari GC, Mulhaupt R (2004) Polymer nanocomposites as dielectrics and electrical insulation-perspectives for processing technologies, material characterization and future applications. IEEE Trans Dielectr Electr Insul 11:763–784 97. Tanaka T (2005) Dielectric nanocomposites with insulating properties. IEEE Trans Dielectr Electr Insul 12:914–928 98. Hui S, Chaki TK, Chattopadhyay S (2010) Dielectric properties of EVA/LDPE TPE system: effect of nanosilica and controlled irradiation. Polym Eng Sci 50:730–738 99. Maxwell JC (1892) Electricity and magnetism, vol 1. Oxford University Press, Oxford, p 452

Chapter 9

Radiation Processing of Natural Rubber Latex Neethu Varghese, Siby Varghese, and Sabu Thomas

1 Introduction Latex is harvested by controlled wounding on the bark of the trees termed tapping, from which it flows out and is collected in parenchyma cells or more often in the tube structure known as laticifers. There are many plants capable of producing latex, most of which belong to dicotyledon families. Natural rubber is mainly produced from latex obtained from trees of the genus Hevea and species brasiliensis, of the family Euphorbiaceae. These species grow in the hot humid intertropical regions. The laticifers are also capable of apical intrusive growth in the cotyledons, inner seed coats, and young leaves. Laticifers also form from procambial cells in young plantlets which are found in the primary phloem in roots, shoots, and the veins of young leaves and later in flowers and fruits. When the cambium has formed, it produced a special laticiferous system in the secondary phloem. In Hevea, these secondary laticiferous vessels of the trunk are exploited by tapping the bark. The tree releases a large amount of latex at each tapping and can be exploited for more than a decade.

N. Varghese · S. Varghese (B) Advanced Centre for Rubber Technology, Rubber Research Institute of India, Rubber Board P.O., Kottayam, Kerala 686009, India e-mail: [email protected] S. Thomas School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala 686560, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. R. Chowdhury (ed.), Applications of High Energy Radiations, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-9048-9_9

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2 Natural Rubber Latex Natural rubber latex (NRL) is a natural product that has tremendous economic importance. Natural rubber latex is mainly used in applications for which its good film formation ability, high strength, excellent elasticity and flexibility, and antimicrobial protection make it the material of choice. The factors which gave considerable interest in NRL products are their low cost and biodegradability. Although the use of natural rubber latex is much less than that of the dry forms, considerable progress has been made in understanding the science and technology, and different latex products like gloves, balloons, catheters, teats, soothers, latex foam, elastic thread, adhesives, etc., are now available.

2.1 Major Constituents of the Fresh Hevea Latex Natural rubber latex is mainly obtained from the bark of Hevea brasiliensis by the process of tapping [1]. The latex that comes out of the tree is a white or slightly yellowish opaque liquid. Fresh latex is slightly alkaline or neutral. Upon storage, it becomes acidic due to bacterial action. The freshly tapped latex is a whitish fluid of density between 0.975 and 0.980 g /ml, pH from 6.5 to 7.0 and surface free energy from 40 to 45 ergs cm−2 . The rubber content of latex varies between 25 and 40% by weight, and this variation is owing to factors such as type of tree, tapping intensity, soil conditions, and the season. Latex consists of a suspension of fine rubber particles in an aqueous liquid or serum. The compositions of fresh rubber latex are given in Table 1. The main component of natural rubber latex is rubber hydrocarbon. This substance has hardly ever been obtained as perfectly pure. It is closely associated with resinous matter and protein. Freshly tapped latex of Hevea brasiliensis contains, in addition to rubber hydrocarbon, a large number of non-rubber materials suspended in latex. The major non-rubber constituents are proteins, lipids, and inorganic salts. The last two components occur entirely in the aqueous phase or serum. The lipids are nearly all on the surface or in the interior of the rubber particles, and the proteins are dispersed between the serum and rubber serum interface. Table 1 Typical composition of natural rubbers latex

Components

Percent by weight

Rubber hydrocarbon

30–40

Water

55–65

Proteinaceous substance

2.0–2.5

Fat and related components

1.0–2.0

Ash

1.0

Sugar

1.0–1.5

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The size of the rubber particles ranges from 0.02 to 3.0 μm, and the shape is mostly spherical, and the particles are strongly protected by a film of adsorbed proteins and phospholipids. The other particles in latex comprise lutoids and Frey-Wyssling particles. The lutoids are subcellular membrane-bound bodies ranging in size from 2 to 5 μm, containing a fluid serum known as B-serum, which is a destabilizer for rubber particles. Frey-Wyssling particles are spherical, larger, and yellow-colored [2, 3]. Methyl-l-inositol (quebrachitol), sucrose, and glucose are the major carbohydrates in latex. About 20% of the available proteins in latex are adsorbed on the rubber particles, an equal quantity found in the B-serum and the remainder in the latex serum. The adsorbed proteins and phospholipids impart a negative charge on the rubber particles, thereby improving the stability of the colloid. The lipids associated with rubber and non-rubber particles play a key role in the stability and colloidal behavior of latex. Latex also contains amino acids, nucleotides, and low molecular weight thiols [4].

2.2 Processing of Latex After tapping, the latex is collected from the tree; anticoagulant is treated to prevent premature coagulation and brought to factory. Ammonia is the most common anticoagulant used. Other anticoagulants are sodium sulfite and formaldehyde [5]. Latex continues to flow slowly for several hours after the initial collection. This latex is not collected but coagulates spontaneously in the collection cup. This is known as cup lump. A small amount of latex coagulates as thin film on the tapping cut to form tree lace. Some latex also drips to the ground to form earth snap. The coagulated materials known as field or natural coagulum constitute about 15–20% of the total crop and are collected on next tapping day.

2.3 Concentration of Latex About 12% of the world’s NR is processed in the form of latex concentrates. Concentration of latex increases the rubber content in the latex to 60% or more from an initial value of about 30–35% in the field latex. The process of latex concentration involves the removal of a considerable quantity of serum from field latex and thus making latex richer in rubber content [6]. Concentration of latex is crucial because of four reasons. (a) (b) (c) (d)

Economy in transportation. Preference for high DRC by the consuming industry. Better uniformity in quality. Higher degree of purity.

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Various processes have been anticipated for concentrating latex. Out of these, four have emerged as of special consequence. (a) Evaporation Evaporation methods yield a concentrate having properties altogether different from those of centrifugally concentrated latex. This is chiefly because in evaporation the only constituent removed from latex is water; in addition, certain non-volatile stabilizers must be used. Since all of the natural constituents of latex are reserved, such concentrates are referred to as whole latex, and the rubber obtained from the concentrate is whole latex rubber. Latex is evaporated by the relevance of heat, with or without the assistance of a vacuum, hot air currents over the evaporation surface, agitation, or other means of rushing the removal of water. (b) Creaming Particles dispersed in a fluid medium and subjected to a gravitational field tend to move relative to the dispersion medium if the density of the particles differs from that of the dispersion medium. In the case of natural rubber latex, the density of the rubber particles is less than that of the density of the dispersion medium. So the rubber particles tend to rise to the surface of the dispersion medium, this process is known as creaming. The process of concentration by creaming is a comparatively simple one. The ammonia preserved latex is placed in a tank; the solution of the creaming agent is added and thoroughly stirred; and the mixture is allowed to stand for at least two days until a concentration of the desired dry rubber content is obtained. The serum is then drawn off. Creaming agents: most commonly used creaming agents are ammonium alginate, sodium alginate, and tamarind seed powder. (c) Centrifugation Concentration by centrifugation was first realized by Biffin in 1898. This is a method currently used for the concentration of NR latex. About 90% of the NR latex concentrate used industrially is produced by centrifugation. The remaining 10% of NR latex is produced either by creaming or by evaporation. Centrifugation is in effect a type of accelerated creaming process, in which the motion of the particles is affected by a centrifugal field rather than the gravitational field. By analogy, the product obtained from the centrifugal concentrate is known as cream and the latex obtained as a bi-product known as skim. (d) Electro-decantation The electro-decantation process is based on the phenomenon of stratification resulting from electrodialysis of latex. The latex is placed in a cell having walls constructed of permeable diaphragms such as cellophane. On either side of the cell are electrodes in a conducting aqueous medium, as in a conventional electro dialysis cell. When a potential is imposed across the cell the latex particles move toward the diaphragm nearest the anode. If the potential is correctly adjusted, the particle is not coagulated at the diaphragm, but because of its density tends to move upward along the membrane wall. Frequent reversal currently prevents

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accumulation at the diaphragm and eventual coagulation. For maximum efficiency, several factors must be controlled including potential gradient, frequency of current reversal, the charge on particles, area of membranes, etc. Only creaming and centrifugation are the commonly used method for the concentration of latex.

2.4 Latex Compound and Its Processing NR latex alone is not enough for the manufacture of a commercial product. A large number of latex compounding materials are used for modifying the characteristics of raw latex and to impart desired service properties to the products. The different ingredients used in a latex compound are (i) surface-active agents, (ii) vulcanizing agents, (iii) accelerators, (iv) activators, (v) antioxidants, (vi) fillers, and (vii) special additives. The water-soluble materials are added as solutions, insoluble solids as dispersions, and immiscible liquids as emulsions. The particle size of the ingredients should be reduced to that of the rubber particles in latex for getting uniform distribution in the latex compound. Further, the colloidal stability of the dispersions and emulsions should be comparable to that of the latex for maintaining the colloidal stability of the final mix [5]. Need of vulcanization To increase the properties of NR, we have to crosslink different molecular chains together. This interlinking of polymer chains with or without bridging constituents is termed as vulcanization.

3 Vulcanization Natural rubber is sticky and nonelastic by nature. Vulcanization is a process that converts the predominantly thermoplastic or raw rubber into an elastic or hard ebonite-like state. This process is also known as “crosslinking” and involves the association of macromolecules through their reactive sites. The crosslinking imparts various properties to rubber. It improves its tensile strength, becomes more resistant to chemical attack, and is no longer thermoplastic. It also makes the surface of the material smoother and prevents it from sticking to metal or plastic, a chemical catalyst. It is a good insulator against electricity and heat. These attractive physical and chemical properties of vulcanized rubber have revolutionized its significance. This heavily crosslinked polymer has strong covalent bonds between the chain. The most important vulcanizing agent for rubber is sulfur because of ease of availability, low cost, and minimal interference with other compounding ingredients. Various concentrations of sulfur are used to manufacture different kinds

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of rubber compounds. During vulcanization, sulfide bridges are formed between adjacent rubber chains. Sulfur is being used as the most accepted vulcanizing agent since its discovery by Goodyear, about 160 years ago. Although rubber can be cured by sulfur alone, the process is very slow, and the properties obtained are not perfect. The chemical reaction between sulfur and the rubber hydrocarbon occurs mainly at the C=C (double bonds) and each crosslink requires 40 to 55 sulfur atoms (in the absence of an accelerator). The process takes around 6 h at 140 °C for completion, which is too expensive by any production standards. The vulcanizates thus produced are very prone to oxidative degradation and do not possess acceptable mechanical properties for practical rubber applications. These limitations were overcome through the inventions of accelerators which then became a part of rubber compounding formulations. To enhance the physical properties of NRL products, NRL must be pre-vulcanized before use.

4 Pre-vulcanization It is a well-known fact that the addition of sulfur, zinc oxide, and accelerator to natural rubber latex caused crosslinking of rubber within the individual latex particles. When such crosslinking is small and the latex product is post-cured by conventional means, the pre-cure step is often referred to as a maturing process. When the precure is considerable the latex is classed as pre-vulcanized latex, although additional crosslinking normally occurs during the drying of the finished article. Such latices are widely employed in the dipping and casting industries, where their use eliminates a compounding and post-vulcanizing operation. Pre-vulcanization is an important term used in latex technology, which is the process of crosslinking the rubber particles in latex stage without affecting the colloidal stability of the latex. Thus pre-vulcanized latex, in effect, is latex of vulcanized rubber. The appearance of pre-vulcanized latex is very similar to un-vulcanized latex, and the original fluidity of latex is retained during pre-vulcanization. During pre-vulcanization, crosslinking of the rubber molecules takes place inside discrete rubber particles dispersed in the aqueous phase of the latex without affecting their state of dispersion appreciably. The particles in the pre-vulcanized latex exhibit similar Brownian movement as in un-vulcanized latex and after pre-vulcanization particles have the same shape, size, and size distribution as those in the initial unvulcanized latex [7]. Pre-vulcanized latex is widely used for the manufacture of various dipped goods such as gloves, toy balloons, condoms, catheters, adhesives, latex foam, latex thread, textile combining, latex composites, and blends, since initial crosslinking of the rubber particle is possible during pre-vulcanization, and complete vulcanization is obtained by normal drying of the product. This enables the manufacturer to decrease the time required for an optimum cure in the circulating hot air oven.

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Advantages of pre-vulcanized latex (a) (b) (c) (d) (e) (f)

Higher output rate Lower energy requirement in latex dipping No need to have equipment to prepare these materials Stable latex viscosity and properties of latex film Latex could be utilized immediately after it is received Latex could still be further modified to suit customer’s requirements.

There are different techniques used for the pre-vulcanization of NRL. They are reaction of rubber particles with sulfur or peroxide and irradiation of latex with UV and gamma rays. The rate of pre-vulcanization reaction varies with different vulcanizing systems, and the extent of pre-vulcanization has an intense effect on the final vulcanized properties.

4.1 Sulfur Pre-vulcanized Natural Rubber Latex The possibility of vulcanizing the rubber molecules within the dispersed rubber particles in the latex was first investigated by Schidrowitz [8] in the early years of this century. Sulfur pre-vulcanized latex involves heating of latex with various compounding ingredients such as sulfur, activator, and accelerator until the required degree of crosslinking is achieved. Drying of pre-vulcanized latex produces crosslinked film without the need for further vulcanization. Generally, pre-vulcanization of rubber latex is carried out at 70–80 °C for 2–3 h duration.

4.1.1

Mechanism of Sulfur Pre-vulcanization of NRL

The chemical mechanism of vulcanization is not well understood and is still controversial. They explained the occurrence of pre-vulcanization on the basis that both sulfur and accelerator dissolve in the aqueous serum of latex before migrating into the rubber phase and crosslinking it. If this species is surface active, then the most obvious mode of transfer would be adsorption from the aqueous phase into the surface of the rubber particle. The sulfur-accelerator species after adsorption loses by dissociation some or all of the hydrophilic moieties which have rendered it sufficiently water soluble for transfer to the surface of the rubber particles. The lost moieties would then return to the aqueous phase. The residual sulfur-accelerator species at the particle surface would now be sufficiently hydrophobic to migrate into the interior of the rubber particle. When the active sulfurating species reach the surface there are two possibilities, first the diffusion of these reactants into the rubber and then crosslinking. It is also possible that crosslinking can take place faster than diffusion. The particles are crosslinked preferentially near their surface and reduce the mobility of the rubber chain at the surface making the effective fusion of the particles more

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difficult, and highly coherent film would not be expected. The rate of crosslinking is much greater than the rate of diffusion. There is no satisfactory explanation available for the reaction [9].

4.1.2

Comparison of Sulfur Pre-vulcanization of NRL and Sulfur Vulcanization of Dry NR

The results obtained by Loh [10] illustrate that sulfur pre-vulcanization of NRL occurs at much lower temperatures and is more facile than sulfur vulcanization of dry NR using an identical vulcanizing system. Similar results are obtained for artificial latex of synthetic cis-1, 4-polyisoprene, and the corresponding dry solid polymer. The unexpected facility of sulfur pre-vulcanization of synthetic polyisoprene latex compared to ammonia preserved NRL is due to the absence of non-rubber substances in them. The speed of the pre-vulcanization reaction seems to be associated primarily with the presence of water. Pre-vulcanization of NRL has two particular advantages over vulcanized dry rubber. (i) Pre-vulcanized latex tends to be very much stronger and elastic as the elastomer chains have not been degraded by the mechanical work needed to incorporate curatives into the dry material. (ii) Since even post vulcanization takes place at a relatively low temperature (below 120 °C) possible to incorporate coloring chemicals that could not survive the high temperature used for cost-effective conventional vulcanization (typically 100–200 °C). 4.1.3

Comparison Between Post-vulcanized and Pre-vulcanized Natural Rubber Latex

The TEM micrograph shows the greater number of inter-particle chemical crosslinks in post vulcanized film compared with pre-vulcanized latex film. The strength of the pre-vulcanized film is due to two reasons: intra-particle crosslink density (chemical crosslinking) and secondary valence force between the particles. The strength of post-vulcanized film is derived from these two factors and inter-particle crosslinks. This indicates that higher the intra-particle crosslink density in pre-vulcanized latex is sufficient to give it higher overall strength than post-vulcanized film [7, 11].

4.1.4

Drawbacks of Sulfur Vulcanization

Sulfur vulcanized natural rubber latex products such as examination gloves, surgical gloves, catheters may cause health problems in some of the users. This is due to the residual traces of accelerators or chemicals, which cause allergy (Type IV). The residual proteins cause allergy (Type 1). Moreover, nitrosamines are produced

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in latex products by this nitrozation reaction of secondary amines generated from antioxidants or accelerators. These accelerators are carcinogenic. Hence, it becomes necessary to develop a new technology to vulcanize NR latex to avoid the above health problems.

4.2 Peroxide Pre-vulcanization Pre-vulcanized latices prepared by using organic peroxide and/or hydroperoxide involving free radical crosslinking are referred to as peroxide vulcanized natural rubber latex. The process involves the heating of NRL in the presence of organic peroxide.

4.2.1

Mechanism

The organic peroxides undergo homolytic cleavage at elevated temperature. The resulting free radicals induce abstraction of hydrogen atoms from the polymer backbone. These in turn create free radicals on the polymer backbone, which allow the chains to form carbon–carbon crosslink with one another. These carbon–carbon bonds are stronger and more stable than those formed by sulfur crosslinking [12].

4.2.2

Drawbacks

Pre-vulcanization of natural rubber latex by heating in the presence of organic peroxide has been known for many years. This process requires heated pressure vessels. Due to the expensive pressure vessels, peroxide pre-vulcanized latex is made along with co-agent at low temperature. Peroxide vulcanization of latex was a simple process; there was little industrial interest, due to the relatively poor aging behavior of products due to remaining peroxide and inferior mechanical properties. Recently, peroxide pre-vulcanization was reinvestigated because of concern about the possible presence of carcinogenic nitrosamines in products made by conventional sulfur accelerator reaction.

4.3 Radiation Vulcanization Pre-vulcanized NRL prepared by using gamma radiation or high energy electron beams in place of sulfur in conventional process is known as radiation vulcanized NRL (RVNRL). Radiation vulcanization of NRL involves radiation-induced

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crosslinking of macroscopic particles of NR dispersed in the aqueous medium. Radiation is defined as electromagnetic wave that has sufficient energies to eject electrons from the materials by the collisions with the materials. The generated electrons are known as secondary electrons. This secondary electron collides with another molecule and ionizes and excites another molecule. Ionization and excitation are repeated until the radiation energy decreases to lower than the ionization energy of the molecule. The irradiated material is not radioactive. Advantages over conventional sulfur vulcanized NRL are • • • • • • • •

Absence of N-nitrosamines Very low cytotoxicity Less protein allergy response Degradability in the environment Transparency and softness Space and energy saving in the latex factory Low emission of SO2 and reduced formation of ashes when burned No contamination of effluent with ZnO.

4.3.1

Gamma Rays and Electron Beams

Gamma rays are waves having wavelength less than 3 × 10–11 m and frequency greater than 1 × 1019 Hz. Electrons having energy 0.32 MeV are also emitted when Co60 atom decays. The most widely used γ-ray emitter is Co60 , a radioisotope produced by the activation of Co59 in a nuclear reactor. Co59 + neutron → Co60 The advantage and disadvantage of gamma rays are high penetration power and high construction costs respectively. Patent to Kemp [13] discloses that 60% centrifuged NRL contained in sealed glass tubes were exposed to γ-rays from Co60 source contained within a heavy concrete shield. Tubes were withdrawn after various times of exposure such that the radiation absorption doses were, respectively, 8, 80 and, 400 KGy. The film tensile strength increases sharply with increasing radiation dose. Electron beams (EB) are generated by electron accelerators. Materials consist of electron and nucleus. The electrons that are thermally or electrically removed from the materials are accelerated in the strong electric field. Low construction costs and low penetrating power are the disadvantage and advantage of electron beam, respectively. Chirinos (2003) [14] investigates the applications of electron beam for the irradiation of natural rubber latex. The radiation vulcanization of natural rubber latex has been carried out with 250 keV. They studied the effect of dose rate or beam current on vulcanization dose of irradiated NR latex with low energy electron beam. They found that vulcanization dose decreases by increasing dose rate. Usually, low energy EB is used in industries for irradiation of surface layers and thin films because of low penetration and high energy output.

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Primary process P (polymer)

P++ e-

(2) Excitation

-

(3) Thermal stabilization of electrons

P -

e

e th

*

-

P + e th P

(1) Ionization

*

*

P

*

(4) Neutralization ·

R1 + R2

·

·

·

(5) Radical formation

R3 + H

(6) Radical formation

R· + R·

R-R

(7) Crosslinking

R· + P

RH + P

(8) Hydrogen abstraction

R· + O2

RO2·

(9) Oxidation

C=O, OH, COOH

(10) Degradation

Secondary process

RO2

·

·

·

R +M ·

RM

RM + nM

(11) Initiation of graft polymerization ·

RMn

(12) Propagation of graft polymerization

Scheme 1 Mechanism of radiation vulcanization

4.3.2

General Mechanism of Radiation Vulcanization

The primary reaction is ion pairs, and excited polymers are produced by irradiation of polymers. These ions, electrons, and excited polymers undergo further reactions to produce polymer radicals and other secondary species. Excitation is followed by dissociation of the resulting excited polymer into free radicals (Eqs. 5 and 6). Free radicals are responsible for a large variety of chemical changes induced by radiation. The recombination of polymer radicals is formed during the radiolysis of the polymer resulting in crosslinking (Eq. 7). If the irradiated medium contains polymerizable unsaturated molecules, the primary radicals add to the double bonds, thereby initiating the polymerization. Irradiation of polymers in the presence of monomer results in graft polymerization (Eqs. 11 and 12). The sequence of reactions is given in the following equation (Scheme 1).

4.3.3

Radiation Crosslinking of NR

The major constituents of NR latex are natural rubber and water. The NR molecules are dispersed in water as fine particles in NR latex. NR molecules and water molecules absorb radiation energy independently. The radiolysis of NR molecules produces NR radicals. Two types of allyl radicals are formed by the abstraction of a hydrogen atom from methylene group by the action of radiation. The hydrogen atom abstracts other hydrogen from vicinity, resulting in the formation of allyl radical and hydrogen gas

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CH 2- -CH2

-CH2 C

C

CH3

H

CH2

-CH2

H

.

C CH3 +

H

H

. C

C CH3

H

.

.

CH -

-CH2

-

+ H C

C

CH

+ H2

C CH3

Fig. 1 Formation of NR radicals by radiation

evolution. The addition of NR radicals to an unsaturated C=C bond provides the formation of crosslink. The radiation crosslinking efficiency is not high as expected. This may be due to the loose packing of cis structure of NR molecules and the presence of methyl groups. These make it difficult for an NR radicals to approach an unsaturated C=C bond. The mechanism is shown in Fig. 1.

4.3.4

Radiation Chemistry of Water

The radiolysis products of water diffuse into NR particles and react with NR molecules. The major reactions in the radiolysis of water are shown in Scheme 2. These reactions depend on temperature, impurities, and pH. Radicals and hydrated electrons are formed. These active species are involved in RVNRL [15].

4.3.5

Radiation Vulcanization of NR Latex

The high ammonia-type NR latex was used commercially and irradiated with gamma rays without an RV accelerator. The dose at which the maximum tensile strength (Tb) is called vulcanization dose (Dv). The standard international unit of dose is Gray (Gy), which represents J/Kg. Minoura et al. [16] studied the mechanical properties of irradiated NR latex film. The tensile strength increases with increasing dose to the maximum value at 250 kGy then decreases. The maximum value of tear strength is also obtained at a higher dose. Elongation at break value decreases with increasing dose.

9 Radiation Processing of Natural Rubber Latex Scheme 2 Radiolysis of water

291

H2 O

H2O+, e-, H2O*

H2O+ + H2O

H3O+ + OH

H2 O*

H2O

e- + nH2O

e-aq hydrated electron

e-aq + H2O

H + OH-

2e-aq

H2 + 2OH-

e-aq + H

H2 + OH

e-aq + OH

OH-

e-aq + H3O+

H + OH

2H

H2

H + OH

H2O

2OH

H2O2

OH + H2O2

H2O + HO2

2HO2

H2O2 + O2

The vulcanization dose of NR latex is around 250 kGy, which is too high to be used for the manufacture of products. Reduction of vulcanization dose is necessary for the manufacture of RVNRL products. Chemicals that reduce the vulcanization dose are called RV accelerators. The first RV accelerator proposed was carbon tetrachloride. In 1961, the addition of 5phr of carbon tetrachloride can reduce the vulcanization dose to 50 kGy [17]. It is subsequently released into the environment during the drying of films from the latex. Halogenated hydrocarbon depletes the stratospheric ozone layer and is a serious worldwide environmental problem. The main reason for the lack of interest from the industry was the tentative advantages of the products. The disadvantages of the products are (a) Low tensile strength (less than 20 MPa) (b) Poor aging properties (c) Inconsistent properties and not economic due to high dose requirements.

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Cost reduction and quality improvement of RVNRL products were achieved by using polyfunctional monomers. The divinyl benzene was investigated as an RV accelerator [18]. The accelerating efficiency of divinyl benzene was not satisfied for RVNRL. The accelerating efficiency depends on solubility in NR and reactivity with NR.

4.3.6

RV Accelerators

Radiation vulcanization of polymers can be improved by the addition of polyfunctional monomers (PFM). PFM is vinyl monomer containing more than two polymerizable C = C double bonds in a molecule. General mechanism of PFM consists of three steps and is shown in Fig. 2 [19]. The first step is radical formation of polymer by radiation. Second step is graft polymerization of PFM from the radicals, resulting in main chain containing many polymerizable C=C bonds. The third step is the polymerization of C=C bonds. Three dimensional networks called gels are formed. Another property of PFM improving the crosslinking is to mobilize polymer radicals. A large number of polymer radicals are formed by the radiation, but only a small fraction of the formed radicals contribute to crosslinking because they are separately formed. The mobility of polymer radicals is very low, while the mobility of PFM is high. Thus PFM is easily diffused to radical site and initiate graft polymerization. The propagating radicals combine with another polymer radical to form three dimensional crosslink. This suggests that PFM enhances the probability of crosslinking. Radical Radiation

Polymer

Graft polymerization Three dimensional network Fig. 2 Mechanism of graft polymerization of RVNRL

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4.3.7

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Graft Polymerization

Graft polymerization occurs when polymer radicals react with monomers. Graft copolymer consists of main chain polymer and branch polymer. There are two radiation-induced graft polymerization methods: the simultaneous irradiation method and the pre-irradiation method. In the pre-irradiation method, the main chain polymer is pre-irradiated and then reacts with the monomer. The trapped radicals on the main chain polymer initiate the graft polymerization. In the simultaneous irradiation method, a mixture of main chain polymer and monomer is irradiated. A typical example is NR latex which is irradiated in the presence of methyl methacrylate. The polymerization of the monomer initiates NR radicals that were formed by radiation [20]. The graft copolymerization of methyl methacrylate and styrene onto natural rubber in the emulsion process was carried out [21]. The rubber macroradicals reacted with methyl methacrylate and styrene monomers to form graft copolymers. The graft copolymerization of ethyl acrylate and methyl methacrylate mixture onto natural rubber latex using ceric ammonium nitrate as the initiator was carried out [22]. The grafting efficiency of graft-modified natural rubber decreased with an increase in methyl methacrylate. Grafting reactions between different acrylates such as methyl, ethyl, butyl, hexyl, and lauryl, and natural rubber latex (NRL) were performed in miniemulsion [23]. It is observed that polyacrylates with a longer alkyl chain length, i.e., ≥4 carbons, only produce stable dispersions. The grafting efficiency of BA on NRL is constant at ∼35 wt.% for BA contents from 16.4 to 22.7 wt.%, but for further increasing BA amount to 27 wt.%, a 3.7 times higher grafting efficiency of BA on NRL was observed.

4.3.8

Polyfunctional Monomers (PFM)

Polyfunctional polymers are commonly used to improve the radiation vulcanization of polymers. The PFM such as polyethyleneglycol dimethacrylates with the varying number of oxyethylene groups indicates that the percent absorption decreases with the increasing number of oxyethylene groups. Since the oxyethylene group is hydrophilic, increasing the oxyethylene group in PFM, more the hydrophilic PFM. Increasing the hydrophilicity of PFM, its solubility in NR decreases. In conclusion, the solubility of PFM in NR increases with decreasing the hydrophilicity. In addition, dimethacrylates are less hydrophilic than monoacrylates due to the presence of two methyl groups [24]. Trimethylolpropane triacrylate (TMPTA) is a trifunctional monomer and its accelerating efficiency is not too high [25]. But its reactivity is very high and the vulcanization dose is greater than 150 kGy. The reason for low accelerating efficiency is the low solubility of TMPTA and hydrophilic nature. The solubility was improved by the co-addition of ligroin [26]. But the co-addition did not improve accelerating efficiency. Another reason for low accelerating efficiency is that crosslink structure incorporated by poly-TMPTA into NR. It should be easily crystallized upon extension. However, the hard segment of poly-TMPTA in the crosslinking networks causes

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poor crystallization of the rubber. It concluded that trifunctional monomers have no marked enhancement of RVNRL. Makkuchi [24] investigates that hydrophobic monomers were more effective in accelerating the vulcanization than were hydrophilic monomers. This was ascribed to the high solubility of hydrophobic monomers in rubber particles. Among the hydrophobic monomers, neopentylglycol dimethacrylate (NPG) exhibited the highest efficiency in accelerating the vulcanization. Advantages of using NPG are (1) high colloidal stability of the irradiated latex and (2) high thermal stability of dried rubber film. Makuuchi and Hagiwara [27] reported on the effect of solubility and reactivity of various PFM as promoters for gamma radiation PVL of NRL. The NR latexcontaining neopentylglycol dimethacrylates are quite stable, while the viscosity of NR latex-containing neopentylglycol acrylates gradually increased and the latex was coagulated overnight. The coagulation was prevented by dilution of latex to a 50% total solid content. This suggests that monoacrylates tend to localize on the surface of NR particles and diacrylates dissolve in NR particles. The results suggest that the vulcanization dose of monoacrylates is lower than that of diacrylates because the radical polymerization of acrylates is greater than that of monoacrylate. The maximum tensile strength is obtained in a film containing monoacrylate. The reason is that the crosslinked structure formed with monoacrylate is favorable for the orientation of the rubber molecules during stretching. The methyl groups in poly-NPG, by which NR molecules, are combined restrict the orientation of rubber molecules. The lower tensile strength is also due to the heterogeneous distribution of the crosslinking in NR molecules. Makuuchi and Tsushima were studied the influence of ligroin (co-addition of solvent) to enhance the stability of RVNRL against diacrylates [26]. It was found that ligroin would cause environmental pollution in the factory. Monofunctional monomers were used to replace ligroin because monofunctional monomers might be polymerized during irradiation. Diacrylate can be used as an RV accelerator together with solvent (ligroin and 2-ethylhexyl acrylate). However, many diacrylates cause skin irritation. The skin irritation of diacrylates makes it difficult to use as an RV accelerator. The degree of skin irritation by chemicals is indicated by the Primary Skin Irritation Index (P.I.I). Generally, methacrylates are less irritating than acrylates, and large molecules of acrylates are less toxic. Since RVNRL is to be used as a raw material for the manufacture of latex articles that will contact the skin, a PFM with high P.I.I. cannot be acceptable for RVNRL. Among the difunctional monomers, 1, 9-nonanediol diacrylate (NDDA) has a relatively low P.I.I. of 1.9. The vulcanization dose is reduced to 20 kGy while using NDDA as RV accelerator [28]. No odor is an additional advantage of NDDA. There are many types of monofunctional monomers such as 2-ethylhexyl acrylate and n-butyl acrylate. The accelerating efficiency of monoacrylic monomers increases with decreasing the glass transition temperature of the polymer obtained from the monomer. This indicates that the flexibility of the polymer of the corresponding monomer is an essential factor to accelerate RVNRL. The flexible polymer is easy to reorient upon extension. This is the reason for the high tensile strength of RVNRL film

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containing 2-ethylhexyl acrylate or n-butyl acrylates. The highest RVNRL efficiency among monofunctional monomers is 2-ethylhexy lacrylates (2-EHA). But 2-EHA is not used as an RVNRL accelerator due to the very bad smell of RVNRL. This bad smell was caused by the un-polymerization of 2-EHA. The vapor pressure of 2-EHA is too low to remove 2-EHA by evaporation [26]. Another type of monomer is ether-type monofunctional monomers (phenoxy ethyl acrylates, nonyl phenoxy ethyl acrylate, methoxy ethyl acrylates, and ethoxy ethyl acrylates). Among the ether-type monofunctional monomers, phenoxy ethyl acrylate (PEA) is the best with regard to high tensile strength and low P.I.I. But the vulcanization dose of PEA is very high (32 MPa) [29].

n-Butyl Acrylate (n-BA) The main advantage of n-butyl acrylate as an RV accelerator is that the vapor pressure of n-BA is very high and it can be easily removed from the RVNRL latex. The other advantages are. • • • •

High accelerating efficiency No residue in the final dipped products Reasonable price Remaining n-BA in the RVNRL film can be easily removed because the vapor pressure of n-BA is high.

However, n-BA tends to coagulate the RVNRL. The instability is caused by nbutyl group. The viscosity of the NR latex is gradually increases by the addition of n-BA and then coagulates. A stabilizer should be added before the addition of n-BA to prevent coagulation. Potassium hydroxide is found to stabilize the NR latex against n-BA [30]. Potassium hydroxide (KOH) modifies the surface of NR particles by reacting with non-rubber adsorbents. The amount of KOH required to stabilize the latex is 0.2 phr KOH. The effect of KOH is the dissociation of weak acids on the surface of NR particles that generates strong repulsive force between particles. Haque et al. [31] investigate the effect of standing time of latex (period between the addition of n-BA and irradiation) on tensile strength. They found that tensile strength decreases with increasing storage period. This may be due to the reduction of concentration of n-BA by hydrolysis with KOH.

Reduction of n-BA During Storage N-BA is a good RVNRL accelerator in terms of physical properties and irradiation dose of RVNRL. The main disadvantage of n-BA is the bad smell. This is due to un-polymerized n-BA in RVNRL. The remaining n-BA causes environmental pollution in industry. Complete polymerization of n-BA is not possible because further irradiation causes reduced tensile strength.

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The effect of n-BA concentration on vulcanization dose and tensile strength shows that the vulcanization dose decreases with increasing concentration up to 6 phr and then no remarkable change occurs. The vulcanization dose obtained was 15 kGy. The maximum tensile strength at 15 kGy is 30 MPa at 5 phr [31]. The concentration of residual n-BA exponentially decreased with the vulcanization dose. Forty percent of n-BA remained at 20 kGy. Residual n-BA was decreased with increasing storage time. This means that residual n-BA was reduced from 40 to 11% in one week. No smell of n-BA exists in the RVNRL. KOH will catalyze the hydrolysis of n-BA. As a result of hydrolysis, acrylic ion and butanol are formed. n-BA + OH− = Acrylate ion + BuOH The concentration of n-BA decreases with increasing storage period and 20% of the n-BA had diminished in 10 h. BuOH is detected and its concentration increases with increasing storage period. This indicates that the decreasing the concentration of n-BA is due to the hydrolysis of n-BA. This means that storage of RVNRL is an effective method to reduce the residual n-BA without lowering the tensile strength. To accelerate hydrolysis of residual nBA, the RVNRL is heated. The effect of heating temperature at 80 °C on the decay of residual n-BA has been investigated. They found that heating treatment at 80 °C is more effective for reducing residual n-BA than to store at room temperature. There is no effect of latex heating on mechanical properties. As the temperature increases, the concentration of residual n-BA in the latex decreases. But the tensile strength slightly deceases with increasing heating temperature [32].

Co-additives of n-BA The vulcanization dose was decreased by the addition of co-vulcanizing agents such as peroxide. To reduce the radiation dose required for vulcanization of natural rubber latex, hydrogen peroxide was used as co-vulcanizing agent [33]. It was found that the vulcanization dose was reduced from 22 to 17 kGy within the addition of hydrogen peroxide. It was also found that there is no accelerating effect of hydrogen peroxide along with n-BA. The addition of hydrogen peroxide is a more practical method to reduce the vulcanization dose. The effect of gamma irradiation dose on hybrid radiation and peroxidation vulcanizations in improving the mechanical properties of radiation vulcanized natural rubber latex (RVNRL) was studied [34]. Irradiation of latex formulations based on 2.5 phr of hexanediol diacrylate (HDDA) as a sensitizer, 0.1 phr of tert-butyl hydroperoxide (t-BHPO) as a co-sensitizer, and 2.5 phr of Aquanox LP as an antioxidant at a minimum dose of 6 kGy can produce rubber film with a tensile strength of 27 MPa with crosslink percentage of 94%. The effect of radiation-induced vulcanization by peroxide on the mechanical properties of NR latex film was also investigated. Here, n-butyl acrylate (n-BA) was used as a sensitizer and t-butyl hydroperoxide (t-BHPO) as a co-vulcanizing agent during irradiation. It

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was found that the addition of t-BHPO is a more practical method to reduce the vulcanization dose required for natural rubber latex [35].

4.3.9

Factors Affecting RVNRL

(a) NR latex for RVNRL The selection of NR latex is one of the most important steps for obtaining good quality RVNRL products. The requirements for NR latex for RVNRL are biomedical safety, low vulcanization dose, and high tensile strength of the vulcanized film. The factors affecting RVNRL are NR latex properties and irradiation conditions. Commercial use of centrifuge latexes is high ammonia (minimum 0.7%) and low ammonia (maximum 0.3% ammonia) types. Commonly used LA-type latex is LA-TZ which consists of 0.2% ammonia, 0.013% tetramethyl thiuram disulfide (TMTD), 0.013% zinc oxide, and 0.50% lauric acid. A high ammonia latex concentrate is recommended for RVNRL. (b) Effect of concentration method The field latex has lower tensile strength than concentrated latex. Concentration by centrifugation method improved the tensile strength. This is due to the removal of all non-rubber components in the serum and small latex particles present in the centrifuged latex. Non-rubber components disturb the better cohesion of rubber particles during film formation of the field latex. Particle diameter increases with the increasing number of centrifugations. The total number of particles decreases with the increasing no. of centrifugations. The tensile strength increased with increasing the number of centrifugation [36]. Concentration by creaming is another method. This process involves the mixing of the creaming agent such as tamarind seed powder or ammonium alginate with field latex and allowing the field latex to divide into two layers, an upper layer of concentrated latex and a lower layer of serum containing very little rubber. The lower portion is removed. The mechanical properties of irradiated creamed and centrifuged latex were compared [37]. The report shows that there is no remarkable difference between tensile properties of both centrifuged and creamed RVNRL. These suggest that creamed latex is equally used for radiation vulcanization. (c) Effect of preservatives Due to bacterial action, field latex becomes acidic and tends to coagulate. To prevent coagulation, an anticoagulant was added to field latex. Ammonia is the most widely used preservative. HA is preserved with 0.7% ammonia, and LA contains one or more preservatives in addition to ammonia. The comparison between RVNRL of high ammonia and low ammonia-type latex shows that vulcanization dose is higher for LA-TZ. This means that TMTD in LA-TZ acts as a radical scavenger. Electron abstract from electron rich sulfur atom of TMTD by rubber radical is the scavenging reaction mechanism. Thus LA-TZ-type latex is not suitable for RVNRL [38].

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(d) Effect of VFA and MST Volatile fatty acids (VFA) are formed by the decomposition of carbohydrates in the NR latex by the attack of acid-producing bacteria. The major formed acid such as acetic acid and formic acid are strong coagulants of the NR latex, micro-flocculation tends to occur. A low VFA number indicates high tensile strength film. NR latex with a VFA number of less than 0.03 is recommended for RVNRL [39]. Mechanical stability time (MST) is a good index of the stability of NR latex. The addition of n-BA to NR latex with low MST causes micro-flocculation. More than 800 s of MST is desirable for RVNRL. MST is a good index of the stability of an NRL against the addition of an RV accelerator. Reduction in MST is obtained by the addition of hydroxyl apatite, or fumed silica was investigated [40]. This suggests that increased in MST is caused by adsorption of decomposition of the non-rubber ionic component by radiation, and fine inorganic particles absorb the stabilizing components. (e) Effect of Magnesium The effect of magnesium content shows that a large amount of magnesium content in latex shows low tensile strength 15. It is a well-known fact that magnesium content leads to low MST in RVNRL. Magnesium inactivates laurate ions by combining with it to form magnesium by dilaurate. A low MST causes the formation of a low MST causes the formation of micro-coagulum which affaects the stability of the latex resulting in low tensile strength. Magnesium is a good nutrient for bacteria. The magnesium content must be controlled to be less than 8 ppm in centrifuged latex. The magnesium content is reduced by adding diammonium hydrogen phosphate (DAHP) to the preserved field latex. The latex was undisturbed for about one day. The reaction product of magnesium and DAHP was precipitate. The sediment is then removed from preserved field latex [41]. (f) Effect of Manganese RVNRL films were prepared by the addition of manganese to natural rubber latex and irradiated with various radiation doses (0–20 kGy). The mechanical properties of the films increased markedly with the addition of manganese. The reason for the positive effect of Mn on NR vulcanization may be due to a transition character that readily forms covalent bonds with others [42]. 4.3.10

RVNRL Process

(a) Pre-treatments of RVNRL The latex is diluted to 50% total solids content by the addition of n-BA emulsion. The n-BA emulsion is prepared by mixing potassium laurate with n-BA and water in the ratio of emulsifier: water: n-BA = 1: 99: 100. Usually, used emulsifiers are polyoxyethylene oleyl ether and potassium laurate. The n-BA emulsion is slowly added to the NR latex while stirring for one hour. No effect

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(b)

(c)

(d)

(e)

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of emulsifier on the physical properties of RVNRL was observed. The formulated NR latex is irradiated for a specific period. After irradiation, antioxidant dispersion is added to the RVNRL. In RVNRL, an antioxidant should not be added to NR latex before irradiation. There are two reasons. One reason is the drop in the antioxidant activity due to the decomposition of antioxidants by radiation. The second reason is the reduction of vulcanization efficiency due to the radical scavenger effect of antioxidant. Dilution of RVNRL The total solid content of NR latex is around 60%. Dilution of NR latex is used for RVNRL to prevent the increase in viscosity caused by the addition of the RV accelerator. Total solid content decreases with increasing the vulcanization efficiency because the RVNRL is enhanced by hydroxyl radicals produced by the radiolysis of water. The effect of the total solid content of NR latex on the dosetensile strength relation was found that NR latex is susceptible to coagulation at a total solid content higher than 45%. The maximum tensile strength tends to decrease with increasing TSC of the latex. It is concluded that a low total solid content of NR latex irradiation causes an increase of NR latex volume to be irradiated and difficult to produce thick dipped products [15]. Vulcanization dose The physical properties of RVNRL depend on the vulcanization dose. A low vulcanization dose is equivalent to a large vulcanization rate. Tensile strength decreases with decreasing dose rate. According to the theory of radiation vulcanization, the initial molecular weight is inversely proportional to the dose for gel formation. This means that high molecular weight NR reduced the vulcanization dose. The theory of emulsion polymerization shows that increasing the number of NR particles with increasing the rate of polymerization in latex. This indicates that the vulcanization dose depends on the concentration method. This is because the particle size varies with the concentration method. The vulcanization is also reduced by increasing the concentration of radical scavengers in NR latex. Effect of oxygen and irradiation temperature The influence of bubbling nitrogen into the latex to exclude oxygen from the latex showed that the effect of oxygen is not remarkable. This indicates that NR latex contains only a small amount of oxygen [15]. Pre-vulcanization was carried out in two atmospheres: a normal air atmosphere and an inert argon atmosphere. The evidence shows that pre-vulcanization was insensitive to oxygen [43]. The effect of irradiation temperature on RVNRL has been investigated that irradiation temperature increases with increasing the viscosity of the latex. Crosslink density does not influence by temperature. But tensile strength of RVNRL irradiated at room temperature was higher than that of RVNRL irradiated at 60 °C. One of the advantages of radiation processing is energy-saving due to room temperature irradiation. Effect of maturation The maturation effect of NR latex shows that after a storage period of 9 weeks, vulcanization dose decreases and tensile strength increases [44]. The NR latex

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becomes stable after two or three months of storage. This is termed maturation. During the maturation period, complex reactions have occurred in the latex. These actions are hydrolysis of lipid and proteins. Another important aspect is microgel formation. The vulcanization dose decreases with decreasing swelling ratio. Low swelling ratios indicate a high degree of crosslinking and high gel content. The NR latex vulcanizes with low dose which consists of highly crosslinked NR. The increase in tensile strength is due to the formation of microgel. This suggested that high tensile strength can be obtained at a low vulcanization dose for NR latex consisting of high microgel content. Proteins are responsible for the formation of microgel. Contrary to the theory, tensile strength is independent of the content of proteins in HA latex. Proteins itself does not show any role in an increase in the tensile strength. Deprotienization caused by the decomposition of microgel results in the formation of a few rubber chains as star-type polymer. Once the microgel structure is formed by hydrogen bonds and chemical bonds by radiation, the decomposition of the microgel by deproteinization will not affect the physical properties of RVNRL [45]. (f) Tensile Strength The tensile strength of RVNRL depends on chemical crosslinking (intra-particle crosslinking) and physical crosslinking (inter-particle entanglement). Chemical crosslinking increases with increasing irradiation dose. Physical crosslinking (inter-particle entanglement) depends on free rubber chain ends on the surface of the latex film. These chains coalesce during film formation and contribute to the strength of the film. Increasing irradiation dose decreases the length of the free chain ends. Thus, the tensile strength of the film increases up to maximum level and then decreases with increasing dose [46]. The inter-particle entanglement occurs at the stage of coalescence during film formation [47]. The fundamental process involves the transformation of the particles in a stable latex dispersion into a continuous film. Over the years, various improvements to these early models have appeared, and the general picture that emerged recognizes, conceptually for an idealized model, the film formation process as being divided into three distinct stages. a.

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Linear cumulative water loss with time of concentrated latex dispersion, with increasing restricted Brownian motion of the particles until they come into contact. b.

Further, slower evaporation of water leads to deformation and calescence of the soft deformable particles in the second stage. c.

At the end of this stage, the film is dry but particle contours are still discernible, the particles having deformed into a polyhedral structure. d.

At the final stage, inter-diffusion of polymer chains across the particle–particle interface occurs, if the film is at a temperature above T g of the latex particles, resulting in a mechanically continuous homogeneous film. The inter-particle entanglement mainly depends on following two processes: (a) Leaching and (b) drying of the NR film. Rubber particles in the latex are covered with water soluble non-rubber components that inhibit better fusion of the rubber particles. Leaching results in the removal of adsorbed components like extractable proteins and enhances the fusion of particles [48]. Heating also enhances fusion of rubber particles and increased chain entanglement of rubber molecules. The effect of leaching of RVNRL film shows that after leaching proteins are removed from the NR latex film but does not destroy the microgel structure. The maximum tensile strength of the film is obtained for leached NR latex film. Leaching and heating effect are shown in Fig. 3. It is concluded that heating without leaching is not sufficient to improve the tensile strength. Combining leaching and heating improves the mechanical properties of the film [46].

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Fig. 3 Leaching and heating effect of film

Leaching effect

Heating effect

4.3.11

Effect of Fillers on Tensile Strength of RVNRL

Usually, inorganic fillers such as carbon black and silica are used for the reinforcement of dry rubbers. However, these fillers do not enhance the tensile strength of RVNRL film. They investigate the effect of fumed silica on tensile strength. There was no significant effect on tensile strength. Improvement of tear strength was observed. Aqueous dispersions of layered silicates (sodium bentonite and fluorohectorite) were mixed with radiation vulcanized natural rubber latex (RVNRL) was studied [49] and shows that layered silicate nanocomposites, expressed good mechanical properties compared to the control. Fluorohectorite showed better properties compared to bentonite. TEM studies revealed that silicate layers were exfoliated, and a network was formed around the rubber particles. The NRL/CNTs were prepared by using solving casting method by dispersing carbon nanotubes in a polymer solution was investigated [50]. The physical and mechanical properties of natural rubber latex CNTs composites were improved due to the presence of nanosized CNTs in the natural rubber matrix. A range of radiation vulcanized natural rubber latex films was prepared using various concentrations of Diospyros peregrina fruit, which acted as a crosslinking agent and was studied [51]. They found that the presence of 15 phr fruit extract had a significant effect on increasing crosslinking density of the latex film and crystallization properties of the RVNR latex. The radiation effect on the vulcanization of natural rubber latex and the synergistic effect of urea addition along with sensitizer on the physical properties of RVNRL film have been investigated [52]. The results show that by the addition of 0.3 phr urea to the latex before irradiation, the crosslink density and tensile strength increase, respectively. Anand reports the synthesis of RGO by a green route, and its efficacy as a potential filler for radiation-vulcanized natural rubber latex (RVNRL) was explored. Compared with gum RVNRL, significant improvements in tensile strength and elongation at break were obtained for RVNRL-XGO nanocomposites at 1 wt% XGO loading, indicating increased polymer–filler interaction. The morphological results showed aggregation of filler particles at a concentration of 1.25 wt% [53]. Increasing the thermal aging properties of radiation vulcanized natural rubber latex with the addition of waste eggshell powder was investigated [54]. The results also suggested that WES could effectively replace commercial CaCO3 as reinforcing filler in RVNRL composites as their overall mechanical properties were similar or even slightly improved.

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The incorporation of cassava starch (CS) as biodegradable fillers with natural rubber latex (NRL) through a sulfur-free crosslinking technique using radiation pre-vulcanization natural rubber latex (RVNRL) in comparison with sulfur prevulcanized natural rubber latex (PVNRL) was studied. The total bond of S−C from SPVNRL contributes to high tensile strength compared to the C–C intermolecular rubber bond from the radiation vulcanization system [55].

4.3.12

Blending of RVNRL

The blending of NR latex is a common technique in the latex industry. Makuuchi reported the tensile strength of blended latexes having the same average dose of 250 kGy [56]. This means that the average dose is the same, but the tensile strength of blended latex is different. The tensile strength of blended latex increases with decreasing dose. This indicates that blending of low irradiated doses improves the physical properties of high irradiated latex. The effect of blending of highly vulcanized NR latex obtained by irradiation to 600 kGy into a properly vulcanized NR latex obtained by irradiation to 250 kGy. The tensile strength decreases with the addition of highly crosslinked NR latex. The tear strength increases to 25 N/mm when the highly crosslinked particles occupy 50% volume. However, results in a reduction of tensile strength by more than 10%. The blending of polymethyl methacrylate emulsion with RVNRL was investigated [57] and shows that homogeneous and transparent film was formed when 25, 50, and 75 ppm of PMMA were added to RVNRL. The maximum tensile strength was obtained at 50 ppm of PMMA. The effect of concentration of AE311 (copolymer of 2-EHA and styrene) on tensile strength and tear strength of film prepared from irradiated blended latex shows that the tensile strength decreases with increasing concentration, while tear strength increased at 2 phr of AE311. Blend rubber films were prepared by mixing styrene grafted rubber latex and natural rubber latex (NRL) with varying proportions by gamma radiation. The increase in tensile strength is insignificant, but the modulus increases from 5.61 to 7.46 MPa with the increased proportion of grafted rubber latex from 40 to 70% in the blend at this radiation dose [58]. Natural rubber latex (NRL) and methyl methacrylate (MMA) grafted rubber latex were blended in different ratios and irradiated at various absorbed doses by gamma rays. The maximum tensile strength and modulus at 500% elongation were obtained at an absorbed dose of 8 kGy [59]. This suggests the reduction of vulcanization dose by grafting polymer onto natural rubber latex. Maleic anhydride (MA) is an interesting monomer to be grafted onto natural rubber (NR) due to its potential as a compatibilizer of hydrophobic rubbers and polymers with higher polarity [60]. It was found that the highest impact strength of the blends was achieved when the grafting was carried out at the absorbed dose of 4 kGy.

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Surface Tackiness

Dipping factories encounter a serious problem with high tack of products. The tackiness of NR film originates in free rubber chain ends at the surface of the film. The tackiness is not dependent on the vulcanization method. It is not directly related to vulcanization. The tackiness of RVNRL film is reduced by surface treatment. (a) Effect of antitack agents There are many methods to reduce the tackiness of the film. Chlorination is employed in latex industries to reduce tackiness. But chlorine can be unhealthy and other materials rub off and can contaminate the surgical gloves. The most effective antitack agent is hydrophobic silicone oil. But small thin areas are formed by the addition of silicone oil. RVNRL film immersed in colloidal silica dispersion is also an effectively antitack agent. The tackiness was also reduced by heating the film in tap water. The tackiness of film immersed at different temperatures in tap water for various periods was reported [61]. It is found that tackiness is reduced by immersion for 10 min and at the high-temperature heating film. The tackiness of the film is not reduced by warming the distilled water. It was also found that the transparency of the film was reduced and the roughness of the film increased with decreasing tackiness. This is due to the formation of a thin layer of substances dissolved in tap water. Calcium carbonate was found to be responsible for the reduction of tackiness. The deposition of calcium carbonate on the surface of the film was analyzed. The reduction of surface tack is also obtained by using hydrogel coatings. Hydrogel is a water-containing gel. The polymer consisting of hydrogel is characterized by hydrophilicity and insolubility in water. The insolubility is caused by the presence of crosslink. Hydrogel coating for RVNRL products was developed by radiation-induced polymerization of the hydrophilic monomer. Hydrophilic monomers such as acrylic acid and hydroxyethyl methacrylate (HEMA) are suitable for applications requiring a low tackiness [62]. However, film-coated with acrylic acid produced a hard surface and peeling of the coated layer upon stretching. On the other hand, poor wetting of HEMA with film surface was observed 19. Wetting of HEMA is enhanced by blending with n-BA. An improved wetting of HEMA on the surface of rubber could be due to interaction by hydrogen bonding between water present in rubber film and the hydroxyl group of HEMA at the interface between RVNRL film surface and the monomer mixture. Water-soluble polymers such as polyvinyl alcohol (PVA) and polyethylene oxide (PEO) reduce the surface tackiness of RVNRL film [63]. The incorporation of WSP slightly decreases the tensile strength of RVNRL films, while a continuous increase in tear strength was observed. This implies that tear strength depends on the stiffness of the polymer. PVA is stiffer than PEO, resulting in the high tear strength of RVNRL film. The tackiness of the film continuously decreased with the increasing amount of WSP added to RVNRL. This could be explained by the migration of PVA which accumulates more on the surface than on the underside when RVNRL film was dried.

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After leaching, tackiness showed a slight increase on the surface. This is due to the removal of PVA from the surface during the washing process.

4.3.14

Aging Properties of RVNRL

The aging of NR is the phenomenon of degradation of the crosslinked structure. A highly crosslinked structure is insensitive to aging. The effect of aging of irradiation dose on the aging property was found that the tensile strength of lower dose irradiated NR latex film decreases by aging at 70 °C, while the tensile strength of higher dose, i.e., 500 kGy irradiated NR latex film increases by aging. Thus, higher dose irradiation is one of the methods to improve the aging property. The changes in tensile strength of RVNRL film and sulfur vulcanized latex film aged at 70 °C [50]. Radiation vulcanizate is more sensitive to oxidative aging than sulfur vulcanizate. This indicates that inhibitors for oxidative aging such as naturally occurring antioxidants and chemicals are used for sulfur vulcanizate. But the radiation vulcanizate contains only naturally occurring antioxidant that is leached out into ammonia solution during leaching [64]. The aging property of leached radiation vulcanizate is inferior to those of sulfur vulcanizate. The reason for the poor aging property of radiation vulcanizate is the absence of dithiocarbamates which as strong antioxidants. Thus, aging properties can be improved by the addition of antioxidants. Isolated proteins from the NR latex improved the aging property. Hevein was the most effective antioxidant [65]. The antioxidant property of Hevein may be attributed to its amino acid composition. The effect of sulfur-containing amino acids such as cystine, cysteine, and methionine on the aging property of RVNR latex shows that methionine exhibited a very good antioxidant property. Methionine is more hydrophobic than others. This hydrophobic property could be penetrated and interact with rubber molecules, thus becoming a more effective inhibitor against its oxidative aging [66].

Selection of Antioxidant The aging of NR is a result of auto-oxidation reactions. The reactions and effect of antioxidant can be simply shown in Scheme 3. There are two types of antioxidant that prevents chain reactions. (a) Radical acceptor-type antioxidant (AH) (b) Peroxide decomposer-type antioxidant (B). The radical acceptor-type antioxidant (hindered phenols and secondary alkylaryl and diaryl amines) deactivates the NR radicals by reactions (9)–(11). Peroxide decomposer-type antioxidants (sulfides, dialkyldithiopropionates, aryl, and alkyl phosphites, and metal salts of dithioacids) abstract oxygen from the peroxide and reduce the peroxide into alcohol that is not involved in the chain reactions.

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Initiation R

R·

(1)

R· + O2

ROO·

(2) Addition of O2 to NR radical Propagation

ROO· + RH

ROOH + R·

(3)

nROOH

RO· + ROO· + OH·

(4)

RO· + RH

ROH + R·

(5)

·OH +RH

H2O + R·

(6)

R·+ R·

R-R

(7)

R· + ROO·

-OH, -COOR, -COOH, -C=O

(8)

R·+ AH

RH + A·

(9)

ROO· + AH

ROOH + A·

(10)

ROO· + A·

ROOA

(11)

ROOH + B

ROH + BO

(12)

Termination

Antioxidant

Scheme 3 Mechanism of antioxidant

The efficiency of various antioxidants were measured as the retention of the gel fraction after boiling under oxidizing conditions. Two antioxidants such as TNPP and DAHQ were selected for RVNRL. Two antioxidants such as TNPP tris(2, 4di-tert butylphenyl)phosphate] and DAHQ 2, 4-di-tert butylphenyl)phosphate], were selected for RVNRL. The effect of both antioxidants was examined by measuring the mechanical properties. Retention of tensile strength was achieved in both cases. The appearance of aged film containing TNPP was better than that containing DAHQ antioxidant, i.e., this means better transparency and low discoloration. This suggests that RVNRL products protected by TNPP can be kept for a longer period once the RVNRL contact with humidity, they will deteriorate faster [67]. TNPP is an excellent antioxidant for RVNR latex [68]. But TNPP contains a small amount of nonylphenol and is recognized as an endocrine disruptor. It is reported that shortly it will be prohibited to import, export, and sell substances containing it. Syuhada Ramli studied the effects of different antioxidants (Aquanox LP, Irganox, and Wingstay L.) in the Radiation Vulcanization Natural Rubber Latex. Tensile strength for RVNRL with Aquanox was found to achieve the optimum strength between 25.4 and 27.13 MPa. The scanner electron microscopic (SEM) indicated more Aquanox molecules to penetrate and interact with the rubber molecule, thus becoming a more effective inhibitor against its oxidative aging.

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Natural Antioxidants Antioxidants such as hevein and methionine retarded the oxidative degradation of RVNRL film, but other sulfur-containing natural products were explored as natural antioxidants. The most abundant sulfur-containing natural product is keratin which is found in hair and feathers. Comparison of aging properties of RVNRL film protected from TNPP and keratin. The tensile strength retention after aging increases from 35% (control) to 60% with the addition of keratin [66].

4.3.15

Protein Allergic Response to RVNR Latex

With the recent increase and awareness of natural rubber latex protein allergies, all health care workers need to understand and recognize the need to decrease and/or eliminate latex exposure in the rubber gloves and other rubber products for those individuals who are allergic to natural rubber latex. The water-soluble proteins in the NR latex products should be removed as much as possible to reduce allergic reactions. The most common method is chlorination. This method involves immersing the glove in a dilution solution of free chlorine. The reaction between chlorine and NR surface results in the reduction of natural tackiness of NR. There is significant reduction occurs in the level of proteins in the product. However, the disadvantages of chlorine are the poor mechanical, physical, and aging properties of NR latex products. NR latex is subjected to radiation resulting in protein degradation. It was expected that denatured NR proteins by irradiation might not cause allergic reactions. However, the irradiated NR latex still exhibited a moderate allergenic response. The response was independent of irradiation dose. In practice, RVNRL products will not cause allergic reactions, providing the products are adequately leached to completely remove the extractable proteins. The nitrogen content of the original NR latex is 0.26%, it was reduced by leaching. The water-soluble and insoluble nitrogen content in non-irradiated latex is 27 and 73%, respectively. But nitrogen content is increased by irradiation at 20 kGy. This increase is caused by the formation of water-soluble ammonium salts of the radiolysis products of non-rubber components, probably proteins are attached to the rubber particles. Above 90% of nitrogen-containing components are removed by leaching. Irradiated latex had less protein content than non-irradiated latex due to the protein molecules getting broken under irradiation [69]. The changes in the EP content in the rubber phase and serum by radiation of field latex. EP in the rubber phase decreases with increasing irradiation dose. But, EP in the serum increases with increasing irradiation dose. The EP in the serum can be discharged by centrifugation. Centrifugation of RVNRL latex is not successful for this purpose. The main reason for a high amount of EP was detected in the RVNRL that was directly centrifuged at high TSC. An influential factor was the dilution of RVNRL. EP in the centrifuged latex decreased with decreasing TSC before centrifugation. The detectable protein was very low in

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the centrifuged latex at 30% TSC. The dilution affects the concentration of EP due to the change in ionic strength on the surface of the rubber particles [70]. Another method to minimize the EP is the water-soluble protein to RVNR latex. Extractable protein measurements showed that mixing of water-soluble polymer into RVNRL, followed by centrifugation and reduced the extractable protein from RVNRL. However, extractable protein could be further effectively removed from RVNRL by adding low-molecular-weight PVA (LMW-PVA) and PVP (LMW-PVP). Based on these results, a new process has been developed by the incorporation of LMW-WSP into RVNRL and centrifugation, followed by leaching. Fourier Transform Infrared measurements showed that most of the WSP was removed soluble proteins from RVNRL during the centrifugation process [71]. Natural polymers such as alginate, carrageenan, and starch also found the same effect. Chemically modified cellulose such as hydroxy ethyl cellulose (HEC) and hydroxyl propyl cellulose (HPC) are added to the RVNR latex. This cellulose is suitable for the reduction of protein in RVNRL. Because they are protein scavengers. The EP in non-irradiated latex contains 0.29 mg/g rubber after centrifugation. It was reduced to 0.05 mg/g rubber by the addition of HEC. No extractable protein was observed after centrifugation [72]. A new procedure for the production of protein-free pre-vulcanized NR latex from field latex. Mature field latex is radiation vulcanized and then centrifuged in the presence of water-soluble polymers. Another possibility is that a low dose like 2 kGy would irradiate the field latex, then a small amount of low molecular weight watersoluble polymer is added and finally centrifuged to DRC more than 60%.

4.4 Radiation by UV Vulcanization of NRL The pre-vulcanization plays an important role in the enhancement of mechanical, physical, and aging properties of dipped latex products. In industrial-scale processes, pre-vulcanization is usually carried out in the presence of sulfur or sulfur-donor agents. In addition, accelerators and activators are required that reduce the reaction time and reaction temperature [73]. Other crosslinking processes involve thermal curing with peroxides or hydroperoxides and curing with high energy radiation such as gamma rays or electron beams has become a well-established technique [74–76]. Besides the mechanical properties, the film formation properties are directly affected by the employed pre-curing system. Recent work has revealed that sulfur pre-vulcanization, as well as radiation vulcanization of natural rubber (NR) latex leads to uniformly crosslinked rubber particles [77]. In peroxide curing, only small NR particles exhibit a homogenous crosslink structure whereas a core–shell structure has been observed for larger particles [78, 79]. Compared to the conventional method of sulfur pre-vulcanization, the alternative of using ultraviolet (UV) light to initiate pre-vulcanization has important advantages: lower doses of vulcanizing chemicals are needed and reduce the residual chemical in the final product. In addition, the life cycle of the product does not involve

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emissions of sulfur dioxide, an environmental pollutant [80]. Ultraviolet lights are widely applied for modifying the properties of polymer bulk materials and polymer surfaces. The UV-based pre-vulcanization does not produce carcinogenic substances and the toxicity of the final product is lower than that of the conventionally made products. Therefore, UV-based pre-vulcanization is especially useful for making medical products as it reduces the chance of allergenic reactions occurring as a result of the use of the product. Allergenic reactions are associated with the sulfur- and nitrogen-containing chemicals used in the sulfur-based pre-vulcanization [81]. UV light is non-ionizing. UV irradiation has a lower operational cost and energy consumption compared to the other irradiation methods. Furthermore, the products made using UV pre-vulcanization show good skin compatibility and good physical and mechanical properties. UV irradiation does not generate any radiation residues. The UV light can vulcanize NR latex in an accelerator-free environment [82]. The photoinitiators used in UV-based curing include α-hydroxyphenyl ketone and 2,2-dimethoxy-2-phenyl acetophenone [83]. Commonly used photoinitiator that responds to UV light is Irgacure 1173 (2-hydroxy-2-methyl-1-phenyl-1-propanone). The coagents commonly used in UV-initiated crosslinking are thiols, amines, and acrylates. The pre-vulcanization of the NR latex using a photoinitiator in the presence of a polyfunctional thiol coagent has been reported [82]. In these studies, the dry rubber content (DRC) of the NR latex was kept constant, and a thiolene addition reaction was involved in the crosslinking. As the thiol coagent contained sulfur, the products made using this type of pre-vulcanization have the potential to cause type IV allergic reactions on contact with human skin and other tissue. Furthermore, the use of 40% DRC in the NR latex restricts the applications of the products made. Although a 40% DRC of the NR latex is suitable for making dip molding products such as medical gloves and condoms, the manufacture of many other products (e.g., dental dams, tubes, baby feeder teats) requires the use of 60% DRC. Similarly, amines may produce bad odors, are moisture sensitive, and can stain rubber products with their yellow color. In contrast, acrylates are colorless, less odorous, and free of sulfur. Therefore, acrylates are potentially useful coagents for making hypoallergenic products. The coagent used in vulcanization helps in improving the flexibility of the crosslinks in the network of the crosslinked polymer chains. Acrylate coagents have been used in UV-induced crosslinking of many double-bond-containing polymers. Acrylate coagents do not contain sulfur. The UV-mediated pre-vulcanization of NR by using diacrylate coagents with various lengths of aliphatic hydrophobic backbone chains was studied. The results showed that HDDA was a good coagent as it provided the highest tensile strength and crosslink density of PVNR films. This was due to its suitable alkyl chain length that made HDDA highly compatible with NR [83]. Falling film photoreactor To provide continuous irradiation of the liquid latex the pre-vulcanization is carried out in a falling film photo reactor. Falling film reactors are well known and are used in various industrial technologies including water waste treatment and preparative

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organic chemistry. Due to the low light transmissivity of NRL, the concept of the falling film process allows homogeneous irradiation of the reaction mixtures in thin films with thicknesses in the range of millimeters. Compared with thin-film reactors there is no contact between the reaction medium and the UV lamp. This new technology operates at room temperature and crosslinking is accomplished within minutes, allowing enormous cost-saving options compared with conventional sulfur vulcanization processes which are carried out at elevated temperatures for hours in batch processes [81]. UV pre-vulcanized NRL was used for the production of latex gloves by a conventional dipping process. The dipped latex articles display excellent mechanical properties as well as good stability against aging and gamma sterilization. Skin sensitization, irritation, and cytotoxic tests prove the good skin compatibility of UV crosslinked NRL gloves.

4.5 Radiation Vulcanization of Natural Rubber Latex by Caesium137 Source Radiation vulcanized natural rubber latex is a natural rubber latex vulcanized by using ionizing radiation such as gamma radiation or electron beam. Gamma radiation is better than electron beam due to its higher penetration power. The source of gamma radiation source is cobalt60 for the irradiation process. Advantages are as follows: Source activity is much longer than Co60 and irradiation facility requires less replenishment. Disadvantages are as follows: Longer half-life and lower gamma radiation energy. RVNRL prepared by using Cesium137 source from a gamma cell and to compare its tensile properties with RVNRL prepared by using Cobalt-60 source was studied. At radiation dose of 12 kGy, the tensile property of RVNRL prepared by Cesium137 source is comparable to those by Cobalt60 . At radiation dose below 12 kGy, the effectiveness of Caesium137 in radiation vulcanization is found to be better than that of Cobalt60 [84].

5 Conclusion In the radiation vulcanization of NRL process, radiation energy replaces the use of sulfur based process and produces a material that retains all properties of the conventional product. However, it has some additional remarkable qualities: the absence of carcinogenic nitrosoamines, extremely low cytotoxicity, absence of sulfur and ZnO, high transparency and softness. These properties are important for many products, particularly catheters, protective gloves and other medical and hospital supplies. At the early stages, radiation vulcanization was carried out without sensitizing agent.

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Accordingly, higher radiation dose was needed in order to achieve sufficient level of crosslinking between the rubber molecules. Later on, several kinds of sensitizers were used to reduce the radiation dose required for vulcanization. Monofunctional and polyfunctional monomers were also used by some investigators for this purpose. Makuuchi et al. obtained an optimum vulcanization dose of 15 kGy using n-butyl acrylate as sensitizer. The vulcanization dose was decreased by the addition of co-vulcanizing agent. The effect of radiation-induced vulcanization by peroxide on the mechanical properties of NRL film was studied. It was found that the addition of t-butyl hydroperoxide as co-vulcanizing agent is more practical method to reduce the vulcanization dose required for NRL. The blending of NRL is a common technique in latex industry. NRL and methyl methacrylate grafted rubber latex were blended in different ratios and irradiated at various doses by gamma rays. The maximum tensile strength and modulus at 500% elongation were obtained at an dose of 8 kGy. This suggests that reduction of vulcanization dose by grafting of polymer on to NRL. It was also reported that natural rubber nanocomposites produced by blending RVNRL with dispersions of layered silicates showed excellent barrier and after aging properties. With the recent increase and awareness of NRL protein allergies, it is very important for all health care workers to understand the need to decrease latex exposure in rubber gloves and other rubber products for those individuals who are allergic to NRL. As the non-rubber constituents and proteins in natural rubber latex get removed during radiation processing RVNRL films offer excellent transparency which makes the material suitable for the manufacture of baby teats. RVNRL was also prepared by using Caesium 137 source from a gamma cell and to compare its tensile properties with RVNRL prepared by using Cobalt 60 source was studied. At radiation dose below 12 kGy, the effectiveness of Caesium 137 in radiation vulcanization is found to be better than that of Cobalt 60.

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Chapter 10

Development of Multi-component Polymeric Systems by High Energy Radiation Bhuwanesh Kumar Sharma, Atanu Jha, Rohini Agarwal, Subhendu Ray Chowdhury, and Suprakas Sinha Ray

1 Introduction We are now in an age of nuclear energy and polymers. Scientists and technologists have marked this era as the ‘age of polymers’. Multi-component polymeric materials have attracted tremendous interest from academics and industries [1, 2]. The combination of two or more polymers can lead to the development of a novel multicomponent polymeric system with improved essential properties, like better mechanical properties, chemical inertness, higher thermal stability, etc. Multi-component polymeric systems majorly consist of polymeric blends, composites, and nanocomposites [1–3]. High energy radiation, namely gamma and electron beam radiation, has found to be well established and widely used tool for modifying the properties of polymers and/or multi-component polymeric systems [4–6]. Radiation technology is industrially preferred over chemical and other conventional techniques because of its beneficial characteristics, namely cheaper processing cost, room temperature

B. K. Sharma Department of Chemistry, SRICT-Institute of Science and Research, UPL University of Sustainable Technology, Ankleshwar 393135, India e-mail: [email protected] A. Jha · R. Agarwal · S. R. Chowdhury Isotope and Radiation Application Division (IRAD), Bhabha Atomic Research Centre, Trombay, India e-mail: [email protected] S. R. Chowdhury e-mail: [email protected] S. S. Ray (B) Centre for Nanostructures and Advanced Materials, DSI-CSIR Nanotechnology Innovation Centre, Council for Scientific and Industrial Research, Pretoria 0001, South Africa e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. R. Chowdhury (ed.), Applications of High Energy Radiations, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-9048-9_10

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operation, involvement of lesser chemicals, higher throughput, and better control over reaction parameters [5–8]. Gamma rays and electron beam (EB) radiation are the two most used high energy radiations, due to their successful applications in polymer science and technology [9, 10]. Both gamma and EB radiation-initiated modifications like crosslinking and grafting have been used in many applications [9–12]. Crosslinking of polymers, sterilization of medical products, curing of monomeric coatings, heat-shrinkable plastic tubing and encapsulations for industrial products, and preservation of foods are some of the widely practiced use of high energy radiation [13]. The purpose of the chapter is to provide insight of the recent developments on multi-component polymeric system namely blends, composites, and nanocomposites using radiation technology. A book chapter with detailed information on the development of polymeric systems using radiation technology can give valuable insight into the possible applications and new development and generate a scope for further studies.

1.1 Various Polymeric Systems: Multi-component Polymers Multi-component polymeric systems are characterized by the simultaneous presence of multiple phases; at least one of those should be a polymer. Multi-component polymeric systems are primarily consist of the polymer blend, polymer composites, and polymer nanocomposites. These multi-component polymeric materials have attained immense importance and interest from academics and industries in recent times due to their potential to incorporate unique properties for high-end applications [3, 4, 13]. A comprehensive understanding of these materials is very important. Blending is a physical mixture of two or more polymeric materials. Polymer blends can be broadly divided into three categories: miscible, partially miscible, and immiscible blends [14, 15]. Polymer blending is a very attractive way to obtain new materials, with combinations of properties of the constituting polymers. Composite is a mixture of polymer and inorganic/organic fillers or components with defined geometries (fibers, flakes, spheres, and particulates). Some examples of polymer composites are epoxy resin/carbon fiber, polymer/flax fiber, polymer/graphene composites, etc. [15, 16]. Recently, a new type of composites, namely nanocomposites, have also been developed and are defined as polymer composites with at least one dimension of the dispersed phase in the nanometer range. Examples are polymers filled with carbon nanotubes, polymer/organoclay nanocomposites, etc. [5, 16–19] (Figs. 1, 2 and 3).

1.2 Advantages of Multi-component Polymeric Systems • It is an easy, successful, and cost-effective method of developing polymeric material with combinations of properties.

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Fig. 1 a SEM image of the polymer blend and b Schematic representation of blending of polymers [17, 18]

Fig. 2 a AFM image of polyester/glass fiber polymer composite, b Schematic representation of the composite formation [19]

Fig. 3 a TEM image of LLDPE/clay polymer nanocomposite and b Schematic representation of blending of polymers [5, 20]

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• Properties can be tailored according to their end use by correct selection of the constituting polymers and composition. • Reduction of cost by diluting high-cost polymer with low-cost polymer. • Development of broad property range materials. • Recycle industrial/municipal plastics scrap.

2 General Aspects of High Energy Radiation High energy radiations are capable of ionizing the atoms or molecules of any material, called ionizing radiation. The ionizing radiation may be either electromagnetic radiations, e.g., X-rays, gamma (γ)-rays, or particulate radiations, e.g., electrons, beta-particles, alpha-particles, and protons. We will discuss the effects of gamma rays and electron beams on polymer in detail due to their successful applications in polymer science and technology [4–8].

2.1 Gamma and Electron Beam Radiation The major irradiating sources for modification of materials property include γ-rays from radioactive isotopes such as 60 Co and electron beams from electron accelerators gamma and EB radiation-initiated modifications like crosslinking, grafting, and controlled degradation have resulted in many useful applications [9–11, 13].

2.1.1

Interaction of Gamma Radiation with Polymer

Gamma radiation (7) (originating within the atomic nucleus such as from 60 Co, 137 Cs) is the most popular irradiation source. The typical energy of γ-rays is a few hundred kilos of electron volts (keV) to a few mega electron volts (MeV). γ-rays interact with matter by three main processes: the photoelectric effect, Compton scattering, and pair production. The interaction proceeds primarily through Compton scattering in the energy range from 100 keV to 1 MeV [10, 11, 13].

2.1.2

Interaction of Electron Beam with Polymer

Electron beam (EB) radiation is produced by accelerating a stream of electrons generated from the cathode through an electromagnetic or electrostatic field. Several types of electron beam accelerators are commercially available and are used as radiation sources for academic and industrial units. The four primary types of interaction of charged particles with matter are inelastic or soft collisions, hard collisions, inelastic

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collisions with a nucleus, and elastic collisions with a nucleus. Among these, inelastic collision is the major pathway for effective interaction with material [10, 13].

2.1.3

Mechanism of Electron Beam and Gamma Irradiation

Gamma and electron beam radiation primarily exchange their energy by two reactions, ionization and excitation. If the transferred energy is higher than the ionization energy of a particular orbital electron of the irradiating material, the electron is ejected, and the atom is ionized. When the energy is not high enough for ionization, the electron is raised to an upper energy level, resulting in excitation [10, 21–24]. The ions, secondary electrons, and excited molecules undergo further transformations, charges, and energy exchange, and react with the surrounding molecules, thereby producing free radicals and other reactive species, as shown in the reaction, Fig. 4. Free radicals are created either through the scission of the main molecular chain or through the dissociation of the C-H side chain of any polymer [10, 11].

Fig. 4 Various possible reactions of gamma and electron beam radiation

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2.2 Chain Crosslinking and Grafting The most important fundamental change in a polymeric system is crosslinking, where polymer chains are joined together, and an interconnecting network is formed. In the case of grafting, a new monomer or unlike polymer is grafted onto the base polymer chain. Some polymers can be degraded in a controlled manner by using radiation. These three possible reactions of a polymer matrix upon radiation exposure are schematically represented in Fig. 5.

2.2.1

Chain Crosslinking

Crosslinking is a process of forming a three-dimensional interconnecting network structure from a linear polymer. Crosslinks result from recombination between the formed radicals on the polymer chain. Crosslinking and chain scission are two competing processes that always coexist under irradiation. The overall effect depends on which of the two is predominant at a certain time [10, 13].

2.2.2

Factors Affecting Crosslinking

The response to radiation for different polymers is directly related to the chemical structures of the polymers. Polymers with more hydrogen atoms on the side chain (e.g., polyethylene) are prone to crosslink with radiation [11, 13]. Crosslinking can improve the mechanical properties, thermal properties, chemical and environmental resistance, and radiation resistance properties of polymeric materials [5, 6, 13]. Polymers which are crosslinked and which are degraded preferentially are listed in Table 1. G-values for crosslinking G(X) and for chain scission G(S) of polymeric materials are given in Table 2 [24]. Polymeric materials with G(S)/G(X) ratio below 1.00 are

Fig. 5 Schematic representation of various radiation-assisted reactions

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crosslinked upon exposure to radiation. For polyethylene (PE), the G(S) value is half of the value of G(X); hence, it gets cross-liked, and it is a highly commercially important cross-linkable polymer. Similarly, natural rubber and polybutadiene also have a very favorable ratio for crosslinking. The ratios for polymethyl methacrylate (PMMA), nylon, and polypropylene (PP) are not particularly favorable. The ratios for polytetrafluoroethylene (PTFE) and polyisobutylene are unfavorable for crosslinking and favorable for degradation. Polystyrene (PS) has a favorable ratio, but it is radiation stable because of the presence of aromatic compounds [23, 24]. Table 1 Predominant effect of radiation on common polymers [24] Polymeric material

Crosslinking G(X)

Scission G(S)

Ratio G(S)/G(X)

Low density polyethylene (LLDPE)

0.8–1.1

0.4–0.5

0.47

High density polyethylene (HDPE)

0.5–1.1

0.4–0.5

0.56

Polyvinylidene fluoride (PV

1.00

0.30

0.30

Polymethyl methacrylate (PMMA)

0.5

0.77

1.54

Polymethylacrylate (PMA)

0.5

0.04

0.07

Nylon-6

0.67

0.68

1.01

Nylon-6, 6

0.50

0.70

1.40

Polyvinyl acetate

0.30

0.07

0.23

Atactic polypropylene

0.27

0.22

0.81

Isotactic polypropylene

0.16

0.24

1.50

Natural rubber

1.05

0.1–0.2

0.14

Polybutadiene

5.3

0.53

0.10

Polytetra fluoroethylene

0.1–0.3

3.0–5.0

20

Polyisobutylene

0.5

5

10

Cellulose

Low

11

High

0.41

Polystyrene

Table 2 G-Values for crosslinking G(X) and chain scission G(S) of common polymers [24] Mainly crosslinking

Mainly scission

Radiation resistant

Polyethylene

Polyisobutylene

Polystyrene

Polyacrylates

Polymethacrylates

Polyethylene terephthalate (PET)

Polyvinyl chloride

Polymethylstyrene

Polysiloxanes

Polymethacrylamides

Polyamides

Polyvinylidene chloride

Polyacrylamides

Polytetrafluoroethylene

Ethylene vinyl acetate

Polypropylene ether Cellulose

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Fig. 6 Probable estimation of crosslinked polyethylene market size of USA 2014–2025 [25]

It is also interesting to note that the value of G(X) and G(S) change with the absorbed dose. Generally, at a very high dose, degradation is preferred over crosslinking; hence the value of G(S) increases at a higher dose than the value of G(X). Typical range of dose required for crosslinking is 50–200 kGy. As the dose rate of gamma irradiation is very low, the reaction/processing time for crosslinking is very high compared to EB. A longer processing time allows higher oxidative reactions in the case of gamma treatment; hence, degradation is greater for gamma treatment than EB treatment for the same dose (Fig. 6). The international crosslinked PE market size is projected to expand at a CAGR of 6.0% from 2019 to 2025. Surging demand from the plumbing industry, weather, heat, and moisture resistant wires and cables materials manufacturing sector are the key factors to assist the demand for crosslinked PE over the coming years.

2.3 Radiation Grafting Grafting is the covalent attachment of monomer/branched copolymer onto the base polymer matrix, where the components of the side chain are structurally different from that of the main chain. Radiation-induced grafting is performed using three common methods, Fig. 7.

2.3.1

Simultaneous or Mutual Irradiation

In this method, the base polymer substrate is irradiated in the presence of a reactive monomer, which may be present as vapor, liquid, or in the form of a solution. Radicals can generate polymers and/or monomers, resulting in grafting. This method requires comparatively lower dose. It is the simplest and very common method suitable for polymers that are sensitive to radiation [10, 13].

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Fig. 7 Schematic representation of various methods of grafting [26]

2.3.2

Pre-irradiation in Vacuum or Inert Atmosphere

In this method, a polymer is irradiated in a vacuum or the presence of inert gas. Then, the irradiated polymer is exposed to monomer at an elevated temperature to achieve grafting. This method is preferred when the reactive monomer is prone to homopolymerization [4, 10, 12, 13, 26].

2.3.3

Peroxidation (Pre-irradiation in Air)

In this method, the base polymer is irradiated in the air or in the presence of oxygen to yield stable peroxides or hydroperoxides. The peroxidized polymers are then stored at room temperature and reacted with the monomer in air or vacuum at elevated temperature to obtain successful grafting [4, 10, 13, 26].

2.3.4

Factors Affecting Grafting:

The property of the grafted polymer strongly depends on the extent of modification, i.e., grafting yield. Grafting yield depends on various reaction parameters, namely dose rate, total absorbed dose, monomer concentration, solvent type, reaction temperature, material thickness, and radiochemical yields (G value) of the materials involved. Radiation-assisted grafting is a powerful method for incorporating of functionality and tailored properties like surface polarity, pH responsiveness, electrical property, friction resistance, barrier properties, water repellence, etc., of a polymer in the existing form without affecting the properties of the bulk material [10, 13, 26, 27]. Some of the recent notable developments in the modification of polymeric systems using radiation-assisted grafting include [7, 8, 27–30], Table 3.

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Table 3 Notable differences of mutual and pre-irradiation methods of radiation-induced grafting [27] Parameters

Mutual method

Pre-irradiation method

Absorbed dose

Low (generally 10 kGy or less)

High (generally 100 kGy or more)

Dose rate

Low

Low/high

Irradiation time

Short

Long

Extent of homopolymerization

High

Low

Processing temperature

Ambient

Irradiation: ambient Grafting reaction: at high temperature

• Improvement of compatibility of polymeric blends/composites by tuning surface polarity/surface energy of the constituting polymers • Tuning of hydrophilicity and/or hydrophobicity of a polymeric surface • Improvement of blood compatibility of medical devices made from polymers • Rendering cell adhesion and cell growth on polymeric scaffolds used in tissue engineering • Developments of suitable membranes used in batteries, fuel cells, chromatography, and water filtration • Preparation of polymeric adsorbents/absorbents for metal ion separation, oily liquid separation from aqueous media. The selection of the radiation grafting method depends on the nature of the base polymer to be modified, the reactivity of the monomer, and the radiation source. Polymers which cannot withstand high radiation exposure (radiation sensitive/degradable polymers) can be grafted using the mutual irradiation method. Monomers which have a tendency of homopolymerization under radiation are not suitable to be grafted using the mutual irradiation method. Radiation resistant polymeric materials are suitable to be modified by all the grafting methods. Pre-irradiation technique requires a very high radiation dose (generally > 100 kGy); hence, polymers which are radiation sensitive (degrade at a high dose) are not suitable to be grafted using this method. The pre-irradiation technique is useful when access to irradiation source is limited. Polymers can be pre-irradiated (in the air or a vacuum) using an irradiation facility. They can be stored for a certain time and then can be used to initiate the grafting using a proper monomer and suitable solvent.

2.4 Effect of Radiation Source Radiation-assisted grafting can be carried out using gamma radiation as well as using EB radiation. In the case of gamma radiation (isotope sources), the dose rate is considerably low (kGy/h), whereas, for electron beams, it is very high (kGy/s).

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Consequently, for a fixed dose requirement, the irradiation time in a gamma source is much longer (in hours) than when using a high energy electron beam (in min or s). Gamma radiation source is preferable for the mutual grafting method as dose rate is less in case of gamma and reaction time is higher. Lower dose rate results in higher efficiency of grafting because generated radicals get enough time to react with a base material. At a higher dose rate, recombination predominated, leading to more homopolymerization. For the pre-irradiation grafting method, dose requirement is higher; hence, the preferred radiation is EB [21, 23, 31].

3 Radiation Processing of Polymeric Multi-component Systems Radiation processing of polymer multi-component systems such as polymer blends, composites, and nanocomposites by electron beam and gamma radiations has become very popular for their property development [32, 33]. The electron beam or gamma irradiation of polymer blends generates the 3D chain crosslink network either inter or intraphase to improve their end-use properties for the desired applications [34]. The polymer blend consists of two different immiscible phases, and its end-use performance depends upon the compatibility between its immiscible phases.

3.1 The High Energy Radiation Processing of Polymer Blends A polymer blend consists of two different polymers of different blend compositions, and each component of the polymer blend has a different degree of interaction with high energy radiation. The electron or gamma irradiation of polymer blend depends on many factors such as interaction with amorphous or crystalline zone, blend composition, immiscibility and miscibility between two polymers, and phase morphology of individual polymer in blend. The electron beam and gamma irradiation of polymer blends with a controlled dose cause the improvement in their enduse properties, e.g., mechanical properties, thermal stability, chemical resistance, electrical insulation, long-term environmental stability, processability, antimicrobial property, and surface properties for various long-term durable applications[35]. Both electron beam and gamma radiation processing of polymer blends have found many end-use application areas such as wire and cable insulation, electrical, automotive, industrial processing, seal and gasket, heat-shrinkable tube, food packaging film, and medical and biomedical sectors [33–36] (Fig. 8).

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Fig. 8 High energy radiation-induced chain crosslinking and scission in a polymer blend

3.1.1

Chain Crosslinking and Scission in Polymer Blends

The high energy radiation-induced processing of various thermoplasticsthermoplastic blends, elastomer-elastomer blends, and thermoplastic-elastomer (TPE) blends has created a platform where the chemical, optical, mechanical, electrical, environmental, and thermal properties of blends can be altered and improved for the desired applications [1, 35–37]. Radiation crosslinking of polymer blends has generated wide opportunities to use them for versatile applications since radiation crosslinking is clean, easy, rapid, environment friendly, and toxic free compared to chemical peroxide crosslinking. Gamma and electron beam irradiation causes the alternation in the polymer blend chemical structure, which results in the 3D chain crosslink network. Due to the formation of crosslinked structure, the end-use properties of blends are improved. The polymer blend materials processed by electron beam or gamma radiation are indeed useful and competent materials in the modern techno-marketing field [4]. The last three decades of research in polymer radiation technology have opened opportunities for numerous commercial aspects as well as economic importance [4]. During irradiation, an initiation in polymer backbone starts with alternation and bond scission via the formation of free radicals in backbone or side groups which later on all those radicals combine together to generate crosslink bonds in threedimensional manner [1, 35, 36]. It is investigated that chain crosslinking in polymer blend mainly occurs in the amorphous region. The polymer blend made of thermoplastic LDPE, LLDPE, and elastomers such as EPR, EPDM, EVA, and POE show good crosslink ability. The possibility of formation of chain crosslinking in the blend has been reported to occur either in intraphase or interphase of two different polymers. Not only the crosslinking but also the chain degradation in a product is based on the polymeric structure, radiation dose, and conditions [1, 36, 37].

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Factors Affecting the Radiation-Induced Crosslinking in Polymer Blends

The electron beam or gamma radiation crosslinking in polymer blends is influenced by numerous factors which decide their end-use performance for desired applications, such as the structure of the individual polymer, degree of crystallinity, amorphousness, impurities, presence and role of additives, irradiation conditions, dose level and rate, source and type of irradiation (electron beam or gamma irradiation), presence of oxygen, nature of ionizing radiation, post-irradiation structural effect, and the thickness of the material (Fig. 9). Various polymeric multi-component materials have been studied by electron beam and gamma irradiation for various end-use sectors for the last three decades. Researchers have irradiated various thermoplastic and elastomer blends, e.g., lowdensity PE (LDPE)/PP, LDPE/EPDM, PE/NBR), LDPE/EVA copolymer, PP/EPDM, PS)/PMMA, polydimethylsiloxane (PDMS)/polyolefin elastomer (POE), linear LDPE LLDPE/PDMS, silicone/EPDM, and EVA/EPDM and studied their mechanical, thermal, chemical, crosslinking behavior, electrical insulation, rheological, and morphological properties [1, 34–39]. Mehtap Sirin et al. [40] have prepared LDPE/PP blends in different blend ratios and irradiated by gamma radiation with 10, 30, 50, 70, and 100 kGy radiation doses. They have examined the tensile strength, % elongation, hardness, melting temperature, crystallization temperature, melting enthalpy (ΔHm), and degradation temperature of irradiated LDPE/PP blends. Chaudhari et al. [41] have examined the effect of electron beam irradiation up to 250 kGy on the mechanical, structural, and morphological behavior of PP/PE blends. They have found that incorporation of 20 wt% PE into PP followed by electron beam crosslinking at 250 kGy has

Fig. 9 Factors affecting the electron beam or gamma radiation crosslinking polymer blends

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increased the mechanical properties of PP/PE blends [41]. They have made a correlation between the tensile, hardness properties, and surface morphology of the electron beam crosslinked PP/PE blends and characterized by scanning electron microscope (SEM) and X-ray diffraction and Fourier-transform infrared spectroscopy (FTIR). The effect of electron beam crosslinking on the mechanical, dynamic mechanical, and electric insulation properties of ethylene octane copolymer (EOC) and poly dimethyl siloxane (PDMS) rubber blend has been studied [42]. In this work, authors have reported the role of blend composition in increasing the physio-mechanical and electrical insulation and decreasing the creep compliance due to electron beam irradiation [42]. Polymers are blended to develop their properties for advanced and highperformance applications. There are various methods to prepare the polymer blends, e.g., solution and melt processing. A significant improvement in their end-use properties has been obtained by the researchers through high energy radiation crosslinking at optimum dose. Hassan et al. [43] have prepared a thermoplastic vulcanizates of devulcanized rubber and polypropylene with in a weight ratio of 75/25 and studied the effect of gamma radiation and dicumyl peroxide crosslinking on the properties of the blends. They irradiated the blends at 25, 50, 75, and 100 kGy at a gamma radiation dose rate of 4 kGy/h. It was observed that the mechanical and thermal properties of the blend were improved after irradiation [43]. The effect of electron beam irradiation has also been examined on elastomerelastomer blends. The blend of millable polyurethane and ethylene acrylic elastomer has been prepared in different blend ratios. The optimized blend has been crosslinked by electron beam radiation to study the crosslinked network, mechanical, structural, thermal stability, and morphological properties [44]. Non-degradable waste recyclable tire material (RTR) filled with elastomer also has been treated with electron beam radiation for property enhancement. For recycling purposes, the waste tire material has been mixed with ethylene vinyl acetate (EVA) elastomer with crosslinking promoters such as trimethylol propane triacrylate (TMPTA) and tripropylene glycol diacrylate (TPGDA), followed by the electron beam irradiation from 50 to 200 kGy [45]. The crosslinking promoters have been used to accelerate the electron beam crosslinking EVA/waste tire blend. The presence of radiation sensitizers leads to enhancing the degree of crosslinking in RTR and the RTR/EVA blend. The effect of gamma radiation crosslinking on polypropylene (PP)/ethylene acrylic elastomer blends has been studied, and crystallization, thermal degradation, structural degradation, flow behavior, gel fractions, and dynamic rheological properties have been examined [46]. Due to gamma irradiation, the state of thermoplastic vulcanizates has been formed in PP/ethylene acrylic elastomer hybrid [46]. PP/EPDM blends have been well known for their high performance in automobile and industrial applications. The effect of electron beam crosslinking on PP/EPDM thermoplastic-elastomer blends also has been studied for biomedical applications. The PP/EPDM with mixing ratios of 80/20, 50/50, and 20/80 have been blended using an internal mixer at 165 °C, and a rotor speed of 50 rpm and specimens are prepared by compression molding. The blends have been crosslinked by a 3.0 MeV electron beam

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(EB) at different doses from 0-100 kGy in air and room temperature. EPDM elastomer shows high damping nature, crosslink-ability good impact strength, and electrical insulation, which is beneficial in PP/EPDM blends. With PP, the degree of crystallinity, tensile strength, and young’s modulus in PP/EPDM has been increased [47]. Elshereafy et al. [48] have prepared the HDPE/nitrile rubber (NBR) thermoplasticelastomer blends in different compositions and irradiated HDPE/NBR samples by gamma radiation from 50 to 250 kGy. The effect of radiation on blend crosslink ability was examined. The higher amount of HDPE in the blend at 250 kGy radiation dose resulted in higher gel content and mechanical properties. It was also found that the heat shrinkability of the blends was improved up to 60% when irradiated to a higher gamma dose [48]. In our earlier studies, the effect of electron beam irradiation on the degree of crosslinking, mechanical, crystallinity, thermal stability, and morphological properties of various thermoplastic-elastomer blends such as LDPE/EPDM and EVA/EPDM has been investigated. In our investigation, the thermoplastic-elastomer LDPE/EPDM blends in different compositions 80/20, 50/50, and 20/80 ratios have been prepared by melt processing [49, 50]. The compression molded specimens of LDPE/EPDM blends (LE82, LE55, and LE28) have been irradiated by electron beam at 50, 100, 150, and 200 kGy radiation dose. LDPE is a crystalline, opaque, and stiff thermoplastic, while EPDM has an amorphous, flexibility and ductile nature, and in heterogenous LDPE/EPDM blend, the characteristic properties of both LDPE and EPDM have been obtained [49]. Radiation crosslinked LDPE/EPDM blends have been very useful in commercial sectors, e.g., wire and cable insulation, heat-shrinkable tube, and gasket applications. In our investigation, due to the amorphousness and random coiled nature of elastomeric chains, EPDM shows more degree of crosslinking, i.e., % gel fraction, and as a consequence, the degree of increment in tensile strength, young’s modulus, bending strength, surface hardness, and damping properties of LDPE/EPDM blends containing higher EPDM concentration has been observed. LDPE contributes its higher mechanical strength, hardness, crystallinity, melting temperature, degradation temperature (thermal stability), and storage modulus to LDPE/EPDM blends [49, 50].

3.1.3

Effect of Crosslinking on the Structure–Property Relationship of Polymer Blends

The effect of electron beam irradiation and crosslink promoter on the structure and property relationship for EVA/EPDM blends has been studied [6]. Ethylene– vinyl acetate is a random copolymer of ethylene and vinyl acetate monomers which enables EVA to use in diverse applications. EVA is extensively used in automobiles, household appliances, agriculture, solar PV module encapsulant, sports, cable jacketing, hot melt adhesive, packaging, and waterproofing which are decided upon its vinyl acetate (VA) concentration. EPDM also possesses important properties which are used in electrical insulation, seal, gasket, wire and cable connectors,

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sports, waterproofing, and automobile sectors [6]. The change in VA content of EVA affects its structure–property relationships. The presence and concentration of VA segment decide the end-use performance and application of EVA elastomer. As the vinyl acetate increases in EVA copolymer, the amorphousness, flexibility, adhesion, softness, clarity, polarity, and ductility are increased. PE non-polar –C–H units EVA are responsible for electrical insulation, chemical resistance, mechanical strength, hardness, stiffness, and gas barrier properties. Based on vinyl acetate content, various EVA/EPDM blends containing EVA of various VA content 18, 28, 40, and 70% have been prepared with and without the addition of crosslinking promoter (trimethylol propane triacrylate; TMPTA) [6]. All EVA/EPDM blends have been irradiated by electron beam radiation to study the effect of EB crosslinking and vinyl acetate content on the properties of blends. The change in the structure, morphology and properties mechanical, crystallinity, thermal stability, and flame retardancy of EVA/EPDM blends has been investigated [6]. In EVA/EPDM blends, the degree of crosslinking increases with an increase in vinyl acetate (VA) in blends. As higher the VA content, the higher the amorphousness and chain coiling in EVA. The amount of VA content influences the structure–property relationship in EVA, which also decides its suitable applications as the end-use application of EVA depends upon its VA content concentration [6]. The presence of higher content of VA causes higher ductility and deterioration in crystalline regions in EVA.

3.1.4

Effect of Radiation-induced Crosslinking on Compatibility in Polymer Blends

Electron or gamma radiation crosslinking has been found to occur either at intraphase or interphase of an immiscible polymer blend. It has been investigated in various multi-component systems such as LDPE/EPDM, PP/EPDM, and LLDPE/PDMS. The electron beam crosslinking also plays an important role in altering the morphology of immiscible polymer blends. In LDPE/EPDM blends, it has been observed that the surface of an immiscible phase of LDPE/EPDM blends is transformed into smooth, finer, uniform, continuous, and compared to unirradiated blends [49]. A similar effect has been observed in EVA/EPDM system. In addition, with electron beam irradiation, there is also a predominant effect of vinyl acetate content on the alternation of the morphology of EVA/EPDM blends. As VA content increases, the immiscible and non-uniform fractured surface of EVA/EPDM has been observed as fine, smoother, and miscible morphology (Fig. 10) [6]. After electron beam irradiation, an irregular discontinuous path on the fractured surface almost disappeared and formed a continuous, uniform, and stable morphology (Fig. 6). This transformation may be due to the micro-radicals formed by EB irradiation [51]. These micro-radicals may interact at the boundary surface of the different immiscible phases to cause more interphase interaction to produce uniform and miscible surfaces [51, 52].

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Fig. 10 Effect of electron beam crosslinking on morphology of EVA/EPDM (18 and 40% VA content) blends at 100 kGy radiation dose [6]

3.2 The Radiation Resistant Polymer Blends Some polymeric materials are used in high energy radiation environment where polymer product obtains continuous exposure of radiation during end-use application. Mostly in space and nuclear sectors, the polymeric products get lifetime exposure to gamma radiation till end-use. The material which shows resistance power against gamma radiation where no significant alternations in mechanical and thermal properties can be observed. The radiation resistance of polymeric products can be described in terms of half value dose of % elongation. It can be defined as the retention

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of 50% of mechanical properties specially % elongation, compared to the original one after radiation aging [4, 53]. Extensive exposure of gamma radiation causes the deterioration in the polymer chains, which makes the product brittle and useless with loss of end-use properties. Various polymer blends have been reported to examine the resistance against gamma radiation. Silicone/EPDM blend has been irradiated by gamma radiation till 250 kGy, and the electrical performance of the blend has been studied. It was found that the blend showed higher volume and surface resistivity at a higher gamma radiation dose. After 250 kGy of gamma irradiation, silicone/EPDM blend showed an increasing trend in crosslinking [54]. The EVA/EPDM blends have been irradiated by gamma irradiation at various radiation doses (500, 1000, and 1500 kGy), and the effect of radiation dose and vinyl acetate content on the resistance of EVA/EPDM blends have been studied [55]. The effect of long exposure of gamma radiation has been studied on the % retention of tensile strength, % elongation, hardness, and compression set properties of EVA/EPDM blends. As VA content increases, the amorphousness in EVA increases. The coiled and amorphous structure enables EVA to sustain in a gamma radiation environment. Ethylene-propylene diene elastomer (EPDM) is also studied for high energy environments. It has been observed that the coiled and amorphous chains of EPDM can crosslink and are capable against gamma radiation exposure for long-term exposure [56]. It has been observed the radiation resistance of EVA/EPDM blend increase with an increase in vinyl acetate content and can sustain up to 1500 kGy dose of gamma radiation [55]. On the basis of performance and % retention, this blend has been suggested to use as gasket, seal, and O-ring for nuclear and high energy radiation environments.

4 Effect of High Energy Radiation on the Performance of Polymer Composites The electron beam and gamma radiation processing of filler or fiber-reinforced polymer composite multi-component systems has also been widely studied for various advanced applications [56]. The incorporation of filler reinforcements such as metal oxide, nanoclay, carbon black, carbon nanotube (CNT), mica, talc, and fiber reinforcements such as carbon fiber, glass fiber, and natural fibers in polymer (thermoplastic or thermoset) matrix causes the improvement in mechanical properties and thermal stability of individual polymer [56, 57]. The physical interaction of filler or fiber with a polymer matrix (thermoplastic or thermoset) enables the polymeric composite multi-component system to apply in aircraft, aerospace, high-temperature material, sports, nuclear and fuel sectors, agriculture, biomedical, implant, transportation, tire, automobile, and building construction applications [56, 57]. The electron beam or gamma irradiation to polymer composite imparts the benefit of adhesion at the interface of reinforcement and polymer matrix by crosslinking, which further

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Fig. 11 Effect of radiation processing on chain degradation and crosslinking of polymer composite

improves the mechanical, thermal, chemical, and morphology of polymer composites (Fig. 11). Many investigations have been reported on fillers filled and fiber-reinforced thermoplastics and thermoplastic-elastomer matrix irradiated by electron beam or gamma radiation. The interaction of polymer composite material with high energy gamma or electron beam depends on many factors such as type of reinforcement, e.g., filler or fiber, type of polymer matrix, reinforcement size, shape, and loading level to the polymer matrix, radiation source (EB or gamma), irradiation conditions, and radiation dose rate and level. Many investigators have studied the effect of high energy gamma or electron beam irradiation on the mechanical properties, thermal stability, dynamic mechanical, water absorption, and environmental properties of thermoplastic or thermoset matrix composites and nanocomposites for advanced applications [57]. The effects of both electron beam and gamma radiation processing on polymer composite and nanocomposites materials have been investigated for a wide range of application development. However, similar to polymer blends, the electron beam irradiation of polymer composites is also rapid, simpler, clean, and green technology operated at ambient conditions compared to chemical peroxide curing. These high energy radiation processing has advantages over the chemical approach for modifying, alternating and improvement in structure, nano-dispersion, properties, morphology, and adhesion of filler with polymer matrix [58]. Maria Daniela et al. have prepared the flax fiber-reinforced EPDM composite mixed with different concentrations and exposed to electron beam radiation for various doses (75, 150, 300, and 600 kGy) to study crosslink density, gel fraction, mechanical, thermal, and morphological properties [59]. Electron beam irradiation of EPDM composite caused an increase in mechanical strength, young’s modulus, and hardness and a decrease in % elongation and elasticity. Manal Shaker [60] has studied the effect of ionizing radiation on the characteristics of EPDM/UHMW-PE composites. In this study, the chopped ultra-high molecular weight PE (UHMW-PE)

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fibers have been added as a reinforcements to ethylene–propylene–diene terpolymers (EPDM) matrix in different concentrations [60]. M.M. Younes et al. [61] have studied the effect of electron beam irradiation on physio-mechanical properties of wood flour–PE composites [61]. In this work, agricultural lignocellulosic fiber (wood flour) waste has been mixed into a LDPE matrix from 10 to 20 wt% loading followed by electron beam irradiation at different doses (10–50 kGy) [62]. From their observation, the flexural strength, modulus of elasticity, and impact strength increase with the content of LDPE and electron beam irradiation dose. In addition to that, the EB irradiation enhances the water resistance, and physical and mechanical properties of LDPE composite materials from 10 to 50 kGy [61]. The curing-based polymerization method by electron beam irradiation also has become popular for the development of high-performance polymer composite materials. Many parameters have been examined on shrinkage on curing, wettability of carbon fiber or filler, and adhesion at fiber-matrix interface [62]. Salem et al. have studied the effect of gamma irradiation on carbon black loaded LDPE film. They have investigated the thermos-mechanical properties of composites at different gamma radiation doses (5–30 kGy) and carbon black concentrations. The authors have studied the alternation in properties of LDPE due to irradiation and carbon black loading. It has been found that LDPE composite with 7wt.% carbon black loading has shown good tensile properties and which can be suitable for packaging applications [63]. Recently, the effect of the gamma radiation on magnesium silicate hydrous (sepiolite) filled UHMW-PE composite with different concentration of sepiolite and gamma radiation doses [64]. The composites have been fabricated and irradiated with 25 kGy and 50 kGy gamma radiation doses. The sepiolite filler mixed UHMW-PE irradiated composite can be used for medical implants, defense armor, and bulletproof jackets [64]. In our earlier investigation, the effect of copolymer addition, magnesium hydroxide concentration, and electron beam crosslinking on the degree of crosslinking, mechanical, thermal, morphology of HDPE/EPDM multi-component material have been studied. In this investigation, the ethylene-octene (EOC; ENGAGE) copolymer has been used as a compatibilizer for immiscible HDPE and EPDM blends [65]. To improve some industrial important properties of HDPE/EPDM blend, the incorporation of magnesium hydroxide [Mg(OH)2 ] and electron beam (EB) crosslinking into the system is also followed into the system. The gel content is found to increase with radiation dose, EPDM content, and Mg(OH)2 dispersion. ENGAGE increases interestingly promotes the degree of crosslinking in compatibilized HDPE/ENGAGE/EPDM blends. The filler [Mg(OH)2 ] dispersion and crosslinked network formation maintain the compatibility of the ternary system confirmed by X-ray diffraction, differential scanning calorimetry, mechanical properties, and scanning electron microscope. The compatibilization between HDPE and EPDM, Mg(OH)2 dispersion, and EB crosslinking improve the mechanical, thermo-mechanical, flame retardant properties, and phase morphology considerably [65].

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4.1 High Energy Radiation Processing of Polymer Nanocomposite The radiation processing of nanocomposites has also opened opportunities for the use of irradiated nanomaterials in advanced applications. The addition of carbon nanotube (CNT) to polymer matrix also improves the thermal, mechanical, chemical, and morphological properties of nanocomposite. People have also studied different nano reinforcements such as nanoclay, CNT, and nano silica and alumina mixed thermoplastic and thermoset-based various nanocomposites for advanced highperformance applications. Grazyna Przybytniak et. al. [66] have prepared CNT mixed epoxy nanocomposites followed by curing with gamma radiation using cationic iodonium salt as an initiator. According to them, the mechanical properties have been improved by the addition of carbon nanotube. Good compatibility between carbon nanotube and epoxy matrix has been established [66]. Tarawneh et al. have prepared the nanocomposite of thermoplastic elastomer mixed with carbon nanotubes and montmorillonite [67]. In this work, the effect of the gamma radiation on different mechanical, conductivity, and thermal properties of the nanocomposites has been examined [67]. Nanohybrid formation using PE and inorganic nanoclay (layered silicate) is believed to increase the mechanical properties, dynamic mechanical properties, barrier properties, thermal properties, crystallinity, and insulation, properties of nanohybrid composite. The purpose of developing nanocomposites is to generate a large amount of nano interface by a small number of nanoparticles with a high degree of dispersion to get a better macroscopic property [4, 5]. Nanoclay filled LDPE nanocomposite compatibilized with gamma radiation grafted LDPE has been developed by Chowdhury et al. (Fig. 12) [5]. In this investigation, 10 wt.% of MAA-g-LDPE with 90 wt.% un-grafted LDPE has been with Closite-20A to develop the LDPE-based nanohybrids [5]. In their work, methacrylic acid (MAA) has been grafted onto LDPE by gamma radiation to develop MAA-gLDPE compatibilizer for LDPE and organically modified clay. The degree of grafting of MAA on the LDPE backbone was around 5–7 wt.%.

4.2 High-Performance Hybrid Clay Nanocomposite The fabrication of nanohybrids with different concentrations of LDPE (10 and 90 wt.%) and nano-clay (20A: 2, 5, and 8wt.%) is mixed for 4 min in a Brabender platicorder at 140 °C at a rotor speed 50 rpm. Specimens are characterized to prove the compatibility between the MAA-grafted LDPE matrix and nano-clay [5]. The MAA-g-LDPE hybrid nanocomposite has shown excellent performance for property development, e.g., improvement in mechanical strength, thermal stability, and morphology. Both tensile strength and young’s modulus of nanocomposite are improved with an increase in loading level of Closite-20A (g-LDPE 8 wt.%). The

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Fig. 12 Schematic diagram of gamma radiation grafted LDPE nano-clay composite [5]

thermal stability has also increased significantly. It is found that thermal degradation temperature (Td) increased by 25 °C for 2 wt.% nanoclay loaded the LDPE nanocomposite. The crystallographic study was performed by X-ray diffraction. The LDPE nanohybrid formation and interaction of nano-clay with MAA-g-LDPE are characterized by the wide angle X-ray scattering (WAXS) technique. Through WAXS, the increase of space gallery (d space) of clay has been confirmed by the intercalation of LDPE chains. The molecular-scale organic–inorganic nanohybrid formation is confirmed by transmission electron microscopy (TEM) where the dispersion of nano-clays in the LDPE matrix can be seen. Figure 13a, b, and c shows transmission electron microscopy (TEM) pictures of LDPE/20A (2 wt.%) and LDPE/20A (5 wt.%) nanocomposites. In Fig. 13a, clays in LDPE are seen as both single layers and stacks, suggesting a mixed intercalated/exfoliated morphology of LDPE– clay nanohybrids. Two main morphologies of polymer–clay nanocomposites are discussed (a) Intercalation: where tactoid (layered) structures of clays are maintained (b) Exfoliation: tactoid (layered) structures of clays are ruptured, and single layers are dispersed into the polymer matrix. In this study, the intercalated morphology dominates over-exfoliation (confirmed from TEM and XRD).

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Fig. 13 TEM of g-LDPE-based nanohybrids: a g-LDPE/20A (2 wt.%); b g-LDPE/20A (2 wt.%) at higher magnification; and c g-LDPE/20A (5 wt.%) [5]

The nylon-6-clay nanocomposites also have been fabricated to study the effect of dispersion of clay into nylon-6 matrix [68]. The effect of the electron beam (EB) crosslinking on the morphology of nylon-6-clay nanocomposites (nano-clay dispersion), crystallinity, as well as water absorption properties of nylon-6-clay nanocomposites has been investigated [68]. Correlations have been established among water absorption, clay dispersion, crystallinity, morphology, and degree of crosslinking in nylon-6-clay nanocomposites [68]. In this work, nylon-6 has been mixed with different proportions of nano-clay (3, 5, and 7 wt.% nanoclay). The molded samples are irradiated by electron beam (EB) using 2 MeV EB accelerator (Model: ILU6) in helium at ambient temperature by 10, 20, 30, and 40 Mrad at a dose rate of 10 kGy/pass. From gel content analysis, it is noticed that both the dose and clay loading influence the degree of crosslinking in nylon-6 (Fig. 14). The gel content is high due to high energy radiation-induced crosslinking [69]. This trend is observed in nylon-6 nanocomposites, i.e., an increase in dose increases the degree of crosslinking, and up to 20 Mrad, the increase is rapid, after which it becomes slow. Interestingly, in the presence of clay the degree of gel content, i.e., the degree of crosslinking, is hampered [68]. Nanoclay acts as a nucleating agent, which may change the spherulite size as well as the amount of crystallinity, which alters the properties of nylon-6 matrix. In addition, EB crosslinking, which occurs in the solid state, may also affect the crystallinity to some extent. In this study, the crystallographic behavior of the neat nylon and its nanocomposites have been investigated by XRD and DSC. However, one common property of nylon is that it is hygroscopic. It absorbs water at substantially higher levels than most other engineering plastics. Water has a plasticizing effect. Water molecules get into intermolecular spaces and as a consequence, an intermolecular force is reduced. As a result, processing and end-use performance (mechanical, dimensional, surface appearance, etc.) are seriously affected due to plasticizing effect of water [70]. In this investigation, the water absorption rate and maximum water content of nanocomposites are found to increase with a decrease in nanoclay content. But electron beam irradiation of nylon-6-nanoclay composite causes a decrease in

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Fig. 14 Variation of gel fraction with composition of blends and radiation dose [17]

the rate of water absorption. Thus, by EB crosslinking, the water absorption properties of nylon-clay nanocomposites can be tuned without losing the nanocomposite morphology.

5 Radiation Curing of Ink, Resin, and Coating Materials Radiation-assisted curing specially electron beam (EB) curing has been well accepted commercially in surface coating, printing, arts, and packaging applications. EB curing of resin is a process of crosslinking and polymerizing oligomers at substrate [71]. The electron beam coated resins are extensively utilized on surfaces of metal, glass, plastics, and fabrics to provide better adhesion. The use of electron beam cured ink and resins as adheres been raised significantly year by year at the commercial level, such as the adhesive coating on tape, printed ink, wood finishing, decoration, and packaging [72]. For coating of resin, it requires very less depth of penetration of radiation and EB curing is quite suitable for this application purpose. EB cured resins such as vinyl ethers, acrylic resin, urethane acrylate, and epoxy are widely being studied for coating at various substrates, e.g., wood, metal, plastic, glass, etc. [71–74]. EB radiation cured coating shows better improvement in properties such as chemical resistance, thermal resistance, corrosion resistance, durability, and flexibility compared to thermally cured resin. However, industrialists also have shown their interest in ultra-violet (UV) curing of resins and inks for thin layered coating

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application as minimum depth is required for penetration of radiation and most important, the cost of UV chamber equipment is quite lesser compared to EB accelerator [71, 73].

6 Electron Beam Versus Gamma Processing: Current Scenario In 1990s, mostly researchers and industrial experts used to irradiate the polymer blends by gamma radiation for their research and commercial purpose; however, later on, their interest shifted toward electron beam irradiation. Since gamma irradiation shows many drawbacks compared with electron beam irradiation. Electron beam technique is known to irradiate the polymeric material rapid, more convenient, clean, and in higher volume with large irradiation place compared with gamma irradiation [33, 34]. The gamma irradiation source (usually Co-60) in irradiation chamber emits gamma radiation of higher energy (1.33 MeV) which may cause more chain degradation rather than chain crosslinking. Apart from that, gamma chamber has less space for irradiation purposes compared with electron beam irradiation. Because of all this, for the current scenario, electron beam irradiation has become more popular for irradiation of polymer blends and composites for application development [33– 35]. The electron beam processing has been widely used to study the crosslinking effect in the polymer product irradiated at different radiation doses. By investigation, it has also been found that the electron beam crosslinking alters the morphology and establishes the compatibility between two immiscible phases of blend [34, 35].

7 Radiation-Processed Polymer Multi-Component Systems: High-Performance Applications Radiation technology for processing polymer blends and composites has several benefits over the chemical-induced process. The main application areas of radiationinduced polymeric multi-component systems include food packaging, aerospace, aircraft, wire and cable insulation, heat-shrinkable polymers, automobile sector, nuclear sectors, gasket, seals, O-ring, plastic foams, gaskets and seals, sports, agriculture, molded engineering products, biomedical implant, and hip and knee joint components, and prevulcanized radial tires, etc. (Fig. 15). In the twenty-first century, gamma and electron beam processed polymeric composite materials have gained importance in automotive sectors to reduce carbon emissions and lightweight products. Radiation-processed sheet molding compound (SMC) polymer composite materials are well-suitable for the automotive industry. The radiation cured SMC materials have replaced the metal and steel parts of the vehicles, which also results in the vehicle’s weight loss with an increase in fuel efficiency.

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Fig. 15 Various sector-wise applications of radiation-processed polymer composites

8 Conclusion The high energy radiation (electron beam and gamma) processing of polymeric multi-component systems has established a platform where their structure and properties can be altered for various high-performance applications. Individual polymeric materials (thermoplastic, thermoset resin, and oligomer gel) have been successfully functionalized by radiation grafting and polymerization techniques at an optimum dose to alter their structure and properties. Various polymeric multicomponent systems, e.g., polymer blends, composites, and nanocomposites, have been developed and studied the property development due to irradiation at different doses. The effect of radiation crosslinking and chain degradation on the mechanical, thermal, structural, chemical, and morphological properties has been well described for various thermoplastic-thermoplastic, thermoplastic-elastomer blends, thermoplastic-metal hydroxide, thermoplastic-nanoclay composite, and thermoset composite systems. Both gamma and electron beam processed polymeric multicomponent materials have been used in aircraft, aerospace, high temperature insulation, wire and cable insulation, heat shrinkable, automobile, transportation, ink and resin coating, food packaging, biomedical implant, hydrogel dressing, and tissue engineering applications.

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62. Przybytniak G, Nowicki A, Mirkowski K, Stobinski L (2016) Gamma-rays initiated cationic polymerization of epoxy resins and their carbon nanotubes composites. Radiat Phys Chem 121:16–22 63. Salem MA, Hussian A, EI-Ahdal AA (2003) Study of the effect of gamma irradiation on carbon black loaded low-density polyethylene films. J Polymer Mater 35:361–365 64. Bakhsh AA (2021) Gamma-ray modified polymer/clay composites: synthesis, characterization, and formulation optimization using multivariate calculus and graph theory. Energies 14:2724 65. Sharma BK, Chowdhury SR, Mahanwar PA, Sarma KSS (2017) ENGAGE compatibilized HDPE/EPDM blends: Modification of some industrially pertinent properties and morphology upon incorporation of Mg(OH)2 filler and electron beam crosslinked network. J Appl Polym Sci 134:44922 66. Coqueret X, Krzeminski M, Ponsaud P, Defoort B (2009) Recent advances in electron-beam curing of carbon fiber-reinforced composites. Radiat Phys Chem 78:557–561 67. Tarawneh MA, Saraireh SA, Chen RS, Ahmad SH, Tarawni MM, Yu LJ (2021) Gamma irradiation influence on mechanical, thermal and conductivity properties of hybrid carbon nanotubes/montmorillonite nanocomposites. Radiat Phys Chem 179:109168 68. Ray Chowdhury S, Francis S, Sarma KSS (2014) Electron beam modified nylon 6-clay nanocomposites: morphology and water absorption behavior. J Polym Eng, AOP, De Gruyter, pp 1–12 69. Charlesby A (1960) Atomic radiation and polymers. Pergamon Press, Oxford, pp 198–283 70. Brydson JA (1999) Plastic materials, 7th edn. New Delhi, Butterworth Heinemann 71. Christmas B, Kemmerer R, Kosnik F (1986) UV/EB curing technology: a short history. In: Seymour RB, Kirshenbaum GS (eds) High performance polymers: their origin and development. Springer, Dordrecht, pp 331–337. https://doi.org/10.1007/978-94-011-70734_32 72. Molenaar F, Buijsen P, Smit CN (1993) Adhesion of electron beam curable coatings on metal substrates. Prog Org Coat 22:393–399 73. Hargreaves I (2007) Ultra-violet and electron-beam curing systems. In: Leach RH, Pierce RJ, Hickman EP, Mackenzie MJ, Smith HG (eds) The printing ink manual. Springer, pp 636–677. https://doi.org/10.1007/978-1-4020-6187-5_11 74. Mehnert R (1996) Review of industrial applications of electron accelerators. Nucl Instrum Methods Phys Res Sect B 113:81–87

Chapter 11

Polymer Recycling by Radiation Kingshuk Dutta and Jaydevsinh M. Gohil

1 Introduction Recycling of polymers can be classified into four key groups: primary/close loop recycling, secondary/mechanical recycling, tertiary or chemical recycling and quaternary recycling (i.e., energy recovery) (Fig. 1). Usually, primary recycling is employed by industries to reprocess high purity well-identified polymer wastes that have been produced by various processes, in order to reduce polymer waste generation. Mechanical recycling is the most widely used for reprocessing of various thermoplastics wastes that have been used and subsequently produced by consumers and recyclers [1]. Depending upon the type of contamination present and polymers blend used, it requires different operation, such as separation and sorting of polymer wastes, grinding, washing, drying and melting, before reprocessing. In spite of its widespread use, mechanical recycling is limited due to the following two major reasons: (i) in this process, individual polymers respond differently based on their chemical structure, mechanical behaviours and thermal properties and (ii) mechanical processing is not possible for temperature-sensitive polymers, polymer composites and thermoset polymers (i.e., polymers that do not flow at elevated temperature). Nevertheless, crosslinked thermoplastic elastomers, such as vulcanized rubber tires, can be reprocessed by extrusion-assisted microwave radiation. It should be noted here that microwave radiation is a tertiary process that can be known as an eco-friendly

K. Dutta Advanced Polymer Design and Development Research Laboratory (APDDRL), School for Advanced Research in Petrochemicals (SARP), Central Institute of Petrochemicals Engineering and Technology (CIPET), Bengaluru 562149, India J. M. Gohil (B) Advanced Research School for Technology and Product Simulation (ARSTPS), School for Advanced Research in Petrochemicals (SARP), Central Institute of Petrochemicals Engineering and Technology (CIPET), Chennai 600032, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. R. Chowdhury (ed.), Applications of High Energy Radiations, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-9048-9_11

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recycling method, as it avoids the use of chemical reagents during the devulcanization (i.e., scission of C–S and S–S crosslinks) process. Tertiary recycling is used to convert polymer wastes into oils, hydrocarbons, monomers or oligomers, through thermochemical and biochemical processes in the presence of chemical reactants, such as catalysts, water and air. Chemical recycling involves breaking of chemical bonds; thus, this reprocessing can be employed for thermosetting plastics, rubbers and thermoplastic matrices under severe conditions (i.e., employment of either high temperature and pressure or low temperature and pressure). The key limitation of chemical/tertiary recycling is that it involves high cost of energy, and this restricts their widespread applications. Radiation-assisted chemical processes, such as microwave, are also being used as energy sources for polymer recycling in the presence of suitable chemical catalyst/reagent [2]. Energy recovery or quaternary recycling is employed for highly mixed and polluted polymer wastes (i.e., mixed wastes, biomedical polymer wastes, hazardous polymer packaging and highly crosslinked thermosets) that are very difficult to reprocess simply or involve high cost when done by other recycling processes. Conversion of high calorific polymer wastes into energy, which is produced in the form of steam/hot gas, heat and electrical power when incineration of polymer wastes is carried out at high temperature. However, incineration does not save as much energy as other recycling [3]. Ongoing research advancement in polymer recycling processes further provides insight into overcoming the limitations of conventional recycling by shifting the research focus towards (a) improvement in chemical recycling efficiency through development of highly selective catalysts, (b) decreasing the requirement for polymer sorting by designing compatibilizers and (c) expanding the recycling beyond thermoplastics and single-use polymer. Figure 2 shows some environmentally friendly radiation-based recycling processes that can be used during primary, secondary,

Fig. 1 Classification of polymer recycling operations

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Fig. 2 Some environmentally friendly radiation-based recycling processes that can be used during primary, secondary, tertiary or quaternary recycling of polymers

tertiary or quaternary recycling of polymers. Radiation aids polymer degradation, crosslinking, oxidation, grafting, branching, polymerization and free radical formation. These results in amelioration of properties and degradation repair of recycled polymers (or blends of different polymers), compatibility enhancement of polymer resins and inorganic fillers and aid to chain scission and chemical decomposition of polymers. To better understand the current interest of radiation-based plastics recycling and to get a clear picture of the position of this particular technique among the major recycling processes, Fig. 3 presents a direct comparison between the number of published documents, in the last 20 years, within the domains of ‘plastics recycling by radiation’ and ‘plastics recycling’. Figure 4 provides an insight into the major countries/territories publishing research on this specialized technique, as against the overall plastics recycling domain. Plastic recycling by the use of radiation technologies is an emerging field. In particular, gamma and electron beam radiation methods serve as complement to conventional mechanical- and chemicals-based recycling techniques. This chapter discusses irradiation-based methods, which can be used separately or in combination with the existing plastic recycling techniques for (i) sorting of polymeric waste (which is mechanically treated) by polymer type, (ii) breaking down of polymer chains into smaller fragments/compounds that can be utilized as a starting raw material for the development of novel plastic-based products, (iii) modification of plastics for amalgamation with other materials, such as additives, fillers, etc., for the preparation of new materials/products and (iv) conversion of plastic waste into fuel (i.e., oil, gas, etc.) and feedstock by radiolysis [4, 5].

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Fig. 3 Number of published documents, from year 2000 to July 2022, searched using the keywords ‘plastics recycling + radiation’ and ‘plastics recycling’. Credit Scopus, Accessed on: 26th July 2022

Fig. 4 Major countries/territories publishing research on radiation-based plastics recycling (from year 2000 to July 2022). Credit Scopus, Accessed on: 26th July 2022

2 Mechanistic Role of Radiation in Polymer Recycling It is very crucial to understand the key role of radiation processes in polymer recycling before discussing the applications. For that, it is necessary to know how the radiation phenomena-induced changes in plastic waste help in plastic waste sorting, identification and recycling. As discussed above in Fig. 2 about the applications of

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radiation technology in polymer recycling, the key mechanistic role of radiation in polymer recycling can be described as [4–6]: (i) Usually, upon irradiation of plastic waste by accelerated electron beam, electrons penetrate into the plastics and get trapped, causing ionization of the polymer molecules, which ultimately generates negative charge within the polymer due to trapped electrons and/or formation of anionic radicals. This phenomenon can be used for sorting, via electrostatic means, of plastic wastes containing polymers of similar chemical structures, such as lowdensity polyethylene (LDPE), high-density polyethylene (HDPE) and polypropylene (PP), which are otherwise very difficult to separate by conventional technique. However, use of antistatic agent and other additives in polymer matrix affects the effectiveness of electron beam radiation towards the generation of charged polymer. (ii) Further, treatment of plastics waste with oxygen or air during irradiation, or after the irradiation process, results in oxidation of the polymers. Usually, irradiation of plastics in oxygen-rich atmosphere results in the formation of peroxide radicals onto the surface and bulk of the polymer matrix, which ultimately generates polar bonds/groups on the surface plastic particle and bulk of the plastic. The extent of polymer oxidation depends upon the conditions of oxidation, such as irradiation time and dose rate, oxygen consumption, temperature, sample thickness and depth of penetration of the electron beam into the plastic waste, as well as the characteristics of the polymer, such as crystallinity, crosslink density and chemical structure. (iii) Oxidation of polymer also sometimes leads to degradation of polymer chains. Hence, oxidation of polymers by radiation and subsequent changes in physicochemical and mechanical properties of recyclates improve compatibility of plastic waste with other binders and fillers that is useful for reuse in construction and household materials, composite preparation and plastics’ modification through grafting and chemical processes. Moreover, oxidative degradation can be applied for chain scission, manufacturing of low molecular-weight petrochemicals and feedstock. (iv) Radical generation within plastics by radiation helps in improving the efficiency of heat energy absorption with the plastics that eventually leads to lowering of plastic processing temperature and heating power requirement, and to make heating more homogeneous and faster. These benefits of radiolysis are being utilized for pyrolysis of plastics waste for recovery of pyrolysis oil, liquid and fuels.

3 Application of Radiolysis in Polymer Sorting Once collection of polymeric waste is done, they are subjected to sorting. Although this step is tedious and very difficult to execute, it is extremely important (rather essential) towards the success of the overall management and recycling process of polymeric wastes. Sorting refers to separation of individual components present in polymeric wastes by a series of steps, starting from the elimination of foreign materials (such as glass and metals), followed by separation of polymeric materials (depending upon their types) for further processing (namely sizing, granulation, etc.) towards the formation of plastic recyclates. Broadly, sorting methods can be

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Fig. 5 A summary of the classification of polymeric recycling techniques

classified into manual and automated [7], and these are discussed below. Additionally, a summary of the classification of polymeric recycling techniques has been presented in Fig. 5. Sorting by manual technique involves identification and separation of components based on their appearance, colour, shape, trademark and identification code [8]. This technique is simple and cost-effective, but involves high time consumption, is labour intensive and is, therefore, economically unviable. In addition, this technique involves the element of human error. As a result, it is only employed in situations where the volume of plastic components is large enough, so that the involvement of effort and time gets justified [7]. Automated techniques of sorting can be further categorized into wet and dry methods. While the wet methods comprise of float-and-sink and froth flotation techniques, the dry methods are constituted of melting, mechanical, electrostatic, magnetic, air, marker/tracer and radiation techniques. The float-and-sink method works on the principle of density difference among the waste components. By this method, foreign materials, as well as different polymeric components, can be separated. The choice of the fluid used for density-based separation is an important factor, and it depends on the specific gravity of the components. This process is cost-effective and fast, but suffers from the fact that most polymers possess similar density range [8]. On the other hand, froth flotation technique, although similar to the float-and-sink technique, involves treatment of the materials with surfactant before separation in a fluid medium. In addition, this technique involves pumping of air into the separation system, so that air bubbles get adhered to certain plastic types (depending upon their surface wettability) and make them float on the separating fluid; while, the other materials settle at the bottom. The surface wettability of the plastic materials can

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be manipulated by flame treatment or chemical conditioning with wetting agents (such as methyl cellulose) [7]. Two of the major consumer polymers, namely poly (vinyl chloride) (PVC) and poly (ethylene terephthalate) (PET), can be separated by applying the froth flotation technique [8]. Also, this technique has shown efficiency towards separation of high-impact polystyrene from acrylonitrile butadiene styrene [7]. Melting-based sorting technique, which is of low operational cost, involves separation of two plastic types based on the difference in their melting points. For this technique to be successful, the melting points of the two polymers must be substantially different from each other. The polymer that gets softened first adheres to the heated belt that carries them, thus enabling separation [7]. Mechanical sorting is a relatively less explored technique for separation of polymers. Centrifugation is generally used as the mechanical means to separate polymers quickly. Also, hybridjig method, involving jigging and flotation in the presence of air bubbles applied to the material bed, has been used [9]. Differential ability of polymers towards electrostatic charging (generation of triboelectric charge) of their surfaces when subjected to an electric field has led to the technique of electrostatic sorting. This process is often used for segregation of polymers having close density ranges, for example, polyethylene (PE) and PP [7]. Again, sorting by application of air on grounded polymeric materials (size between 0.25 and 0.50 in.) has been employed to separate them based on their specific gravity. Magnetic sorting is applied for the removal of metallic contaminants present in polymeric wastes. Radiation-based sorting techniques include near-infrared, X-ray fluorescence and laser-aided processes [8]. Near-infrared process is a fast and accurate method for separation of polymeric wastes based on transmission of infrared light. Depending upon the chemical composition of a polymer, it absorbs infrared radiation of a certain wavelength, which, in turn, enables identification of the polymer types, followed by separation. This process is often applied for effective separation of poly(lactic acid) from carbon board present in streams of mixed packaging, as well as for identifying bottles made of PET. Nevertheless, this process fails when applied on dark-coloured polymers, polymers containing additives, residues and adhesives [10]. Similarly, based on the chemical composition and molecular structure of polymers, X-ray fluorescence process has been developed for sorting of polymers. In this process, generation of spectral fingerprints upon subjecting the polymers to X-rays leads to their identification and separation. This process is mostly employed for sorting of PVC mixtures of plastics by detection of chlorine atoms present in it [7, 8]. Another potentially promising process of radiation-based sorting technique is laser-induced identification, which involves beaming of a laser on the surface of polymers to identify them based on their properties [8]. Lastly, electron beam radiation has been made use of in sorting of polymeric wastes by taking advantage of the fact that such irradiation results in penetration of the incident electrons into the polymer, leading to generation excitation and ionization of the polymeric molecules [4]. In this process, the electrons that penetrate the polymer get trapped inside it, resulting in the formation of permanent negative charges. This process has been found to be

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superior to other charge-based separation techniques, namely chemical ionization and triboelectric charge generation. Tracer or marker-based sorting of polymeric wastes is a new technology that employs incorporation of readily detectable fluorescent pigments into the polymers or products for aiding in identification and separation after their service life. However, this process involves high-cost owing to the need of installation of separate dedicated marking and scanning facilities [8, 11]. Again, digital watermarking of identification codes into the packaging design can facilitate detection on high-speed sorting lines [11, 12].

4 Modification of Waste Polymers Through Radiolysis A major step towards realizing recyclability of waste polymers is to carry out structural modifications of the polymers after their end-of-service. These structural modifications not only give a changed identity to the waste polymers, but also enable their fruitful use in further applications [6]. For this purpose, the major types of irradiations employed are electron beam, gamma and microwave. For instance, Navratil et al. [13] subjected waste HDPE to irradiation from electron beams and used the radiationinduced crosslinked waste polymer powder to fill virgin LDPE powders. This resulted in reduced melt flow index, enhanced toughness and increased hardness. It was further observed that irradiation of polytetrafluoroethylene (PTFE) with electron beams led to scission of polymer chains that resulted in reduction of molecular weight and simultaneous enhancement of crystallinity [14]. Chulkov et al. [15] showed that electron beam irradiation can effectively cause partial fragmentation and depolymerization of PE, polystyrene (PS), PP, PET and expanded PS foam to form waxy and liquid products. Using γ -radiation, Lubna et al. [16] performed grafting of vinyl acetate monomer on films of recycled PET. This modification caused enhancement of tensile strength and thermal stability, with simultaneous reduction in elongation at break. The advantage of the use of γ -radiation was that it reduced the time of operation as well as consumption of chemicals, compared to conventional chemical techniques. El-Zayat et al. [17] further found that γ -irradiation can cause improvement in compatibilization between recycled HDPE matrix and fibrous sugarcane bagasse fillers. This improvement, in turn, resulted in enhancements in thermal stability and mechanical attributes, while reducing the water uptake tendency. The changes in thermal stability and water uptake of the composite were a direct consequence of the crosslink formation caused by the γ -irradiation. Pereira et al. [18] showed that it is possible to modify composites of recycled PP and ethylene-propylene-diene monomer (EPDM) rubber by exposing this composite to both gamma and ultraviolet irradiations. On the other hand, microwave radiation has found use in dechlorinating PVC [19], which was found to be a much faster (due to lower activation energy) and more efficient (i.e., higher dichlorination to total weight loss ratio) process compared to the usual heating

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process. It was realized that the microwave-induced dichlorination was more selective, which resulted in reduced weight loss. Utilizing a different approach, Suriapparao et al. [20] performed co-pyrolysis containing agro residues, namely sugarcane bagasse and rice straw, with waste PS and PP by employing microwave radiation, and subjected the pyrolysis vapours to catalytic upgradation in the presence of protonated Zeolite Socony Mobil-5 (HZSM-5) catalyst. It was noted that the upgraded bio-oil derived by this process possessed comparable attributes to light fuel oil, in terms of flash point, optimum density, viscosity and calorific value, coupled with a reduction in oxygen content. A schematic depiction of this entire procedure has been summarized in Fig. 6.

Fig. 6 Co-pyrolysis of sugarcane bagasse (B) and rice straw (RS) with PS and PP by employing microwave radiation, followed by upgradation in the presence of the catalyst HZSM-5. This resulted in the formation of upgraded bio-oil, which possessed comparable attributes to light fuel oil. Adapted with copyright permission from Suriapparao et al. [20]

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5 Recycling of Elastomers Elastomeric wastes represent a significant proportion of the ever-increasing polymeric waste generation in the global scenario. Out of the different categories of elastomeric wastes, end-of-life tires (composed of vulcanized rubber) amounts to a prime contributor to the total waste generation. These non-biodegradable wastes not only result in extreme deterioration of our environment, but also causes serious threat to lives of human as well as animal [21]. Hence, it is of ample importance to recycle the large quantity of elastomeric wastes that get generated around the world [22]. However, we must be very careful in choosing the right recycling procedure, keeping in mind and ensuring the sustainability and green aspects of the process. Devulcanization, by use of radiation, is considered as one of the most attractive processes that can be applied to elastomeric wastes, especially used tires [24]. This is because of the following pros: (a) physical nature of the process, involving no use of harmful chemicals; (b) achievement of high productivity as a result of applying high-energy input in a short time span; (c) continuous nature of the process; (d) simple modification prospects of the process parameters; and (e) uniform volumetric heating of the material [24]. Use of microwave electromagnetic radiation has been found to be very suitable for the devulcanization process of elastomeric wastes [25]. In a typical study, Aoudia et al. [23] conducted devulcanization of waste tire rubber by applying microwave radiation, after grinding the rubber into powder. The authors further used this devulcanized rubber powder to modify epoxy composites. It was found that devulcanized rubber powder as a filler performed better than vulcanized rubber powder, resulting in superior physicomechanical properties of the rubbermodified epoxy composites. In this process, it was discovered that degree of devulcanization was dependent on the applied electromagnetic energy up to an optimized value of 1.39 kJ kg−1 , which was found to be the most suitable in producing a regeneration phenomenon within the elastomeric waste powder. Improvement of mechanical performance of the devulcanized rubber-filled epoxy composites was attributed to the improved adhesion of the filler particles with the epoxy matrix (Fig. 7). Colom et al. [26] observed that such devulcanization causes thermo-oxidation of the rubber, resulting in reduction of its carbon black content due to carbon dioxide generation. In addition, the devulcanization step causes the important breakage of S–S bridges and C–S bonds (formed during the original vulcanization step), as well as reduction of structural groups, such as methine and methylene. The authors reported a strong dependence of the devulcanization degree on the silica content of the grounded tire rubber—higher the amount of silica present, more easily is the devulcanization achieved—indicating towards an improved efficiency of the microwave devulcanization procedure due to the catalytic role played by silica. Molanorouzi and Mohaved [27] reported that sizes of rubber particles and their size distribution of the grounded waste tire rubber plays a significant part in the process of microwave devulcanization, with a median value of 319 μm serving as the optimized value. Also, it was further discovered that the polysulfide bridges that exist in the vulcanized tire rubber get partially or completely broken during the devulcanization process; and in case of

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partial breakdown, monosulfide structures are dominant within the remaining sulphur crosslinks. de Sousa et al. [24] went a step further and studied the kinetics of the revulcanization of microwave irradiation-effected devulcanized waste tire rubber. They concluded that the presence of carbon black within the original tire rubber and the devulcanized rubber plays a major part in reducing the activation energy (E a ) scorch time (t s1 ) and optimized vulcanization time (t 90 ) of the revulcanization process, due to its catalytic effect (Fig. 8). Therefore, duration of microwave exposure plays a vital role in the revulcanization procedure, as it affects the carbon black content during the devulcanization step. Using γ -irradiation, Martínez-Barrera et al. [28] developed polymer concretes, composed of recycled tire fibres, silica and polyester resin. The concretes containing recycled tire fibres, as a partial replacement of silica, demonstrated higher mechanical properties compared to the concrete composed of polyester resin and silica. The effect of irradiation was found to be positively prominent in cases of flexural strength, compressive deformation and flexural deformation. However, in terms of compressive strength, the irradiated fibres produced lesser improvement compared to their non-irradiated counterpart. These positive results were attributed to the role played by the radiation in modifying the polymer chains in terms of crosslinking and scission, as well as changes in morphology and degree of crystallinity. However, the effect of gamma radiation was found to be more in case of the use of waste PET from PET bottles and waste polycarbonate (PC) from computer monitors compared to that on waste tire rubber [29]. Mészáros et al. [30], on the other hand, utilized electron beam irradiation in order to enhance the compatibility between grounded waste tire rubber, PE and ethylene vinyl acetate to form rubber-reinforced thermoplastic composites. The radiation was found to cause crosslinks that led to exhibition of enhanced compatibilization between the rubber and the thermoplastic phases, hardness, tensile strength, impact resistance and elongation at break by the composite material. Recycling of rubber products is difficult mainly owing to the high extent of crosslinking present in rubber in order to make it suitable for any application. Therefore, one approach to ensure easy recyclability of rubber products after use is to induce within it the desired rubbery properties with lesser number of crosslinks. In this respect, high-energy electron beam irradiation has been found to be especially useful. In comparison with conventional crosslinking methods, electron beaminduced irradiation possesses certain critical advantages, such as simple and fast processes and environmental friendliness. In addition, the degree of crosslinking can be precisely controlled in order to produce desired properties at lower degree of crosslinking. For example, Chowdhury et al. [31] used electron beam irradiationinduced crosslinking to develop nanoclay dispersed LDPE/elastomeric poly(ethylene vinyl acetate) hybrid nanocomposites that exhibited superior thermal stability, tensile strength, crystallinity and recyclability compared to conventionally crosslinked and non-crosslinked composites. Two similar studies, using electron beam radiationinduced crosslinking, have been reported by Datta et al. [32, 33]. They performed crosslinking of styrene–butadiene–styrene block polymer by means of electron beam irradiation and found out that the crosslinked block copolymer demonstrated good

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Fig. 7 Scanning electron micrographs of failure surfaces of epoxy composites filled with: a grounded tire rubber, and devulcanized grounded tire rubber treated with b 0.54 kJ kg−1 , c 0.772 kJ kg−1 , d 1.003 kJ kg−1 , e 1.157 kJ kg−1 and f 1.389 kJ kg−1 Adapted with copyright permission from Aoudia et al. [23]

mechanical and rheological attributes. Moreover, most importantly, the copolymer could potentially be recycled with minor loss of properties. Yasin et al. [34] found out that electron beam irradiation-induced crosslinks are more stable than sulphur crosslinks, which led to more stable blends of grounded waste tire rubber and EPDM rubber. Among other important studies in this domain, Zaharescu et al. [35] explored the possibility of using γ -irradiation for recycling and reuse of waste EPDM rubber and found it to be working positively when added to pristine EPDM rubber. Hassan et al. [36] successfully produced devulcanized waste tire rubber sheets and blended

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Fig. 8 Impact of the amount of carbon black content on the revulcanization reaction of the microwave irradiation-effected devulcanized grounded tire rubber. Adapted with copyright permission from de Sousa et al. [24]

them with HDPE in the presence of γ -radiation. This blend demonstrated significant improvement in tensile strength, hardness and onset temperature, owing to the effect rendered by radiation on the crystal size of PE phase within the blend. Similarly, El-Nemr et al. [37] developed a thermoplastic elastomer by blending devulcanized waste tire rubber with waste-expanded PS in the presence of γ -radiation, which exhibited better mechanical attributes (i.e., tensile strength, abrasion and hardness), rheological properties and flame retardancy in comparison with their non-irradiated counterpart.

6 Upscaling/upcycling of High-Performance Polymers (or High-Temperature/Performance Polymers Processing) Radiation predominantly induces polymer chain scission (i.e., degradation) and/or network formation (i.e., crosslinking). Hence, based on the radiation resistance of the polymers, the most generic polymers that predominantly form crosslinks are PE, polyacrylates, PVC, polysiloxane, polyamides, PS, polyacrylamides and

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poly(ethylene vinyl acetate). On the other hand, the polymers that predominantly undergo chain scission are polyisobutylene, polymethacrylates, poly(methyl styrene), poly(methacryl amides), PVC, PTFE, polypropylene ether and cellulose [38]. In practice, polymers undergo chain scission and crosslinking during irradiation. However, both of these processes change with radiation conditions, such as radiation dose and temperature, and the overall supremacy of one process over the other depends on the polymer structure, energy of irradiation and composition of polymers. Usually, polymer backbones, substituted with more hydrogen atoms in the side chains, get crosslinked upon irradiation. For example, PE and polymers substituted with a methyl group, like poly(methyl acrylate) (PMA, disubstituted) and PTFE (halogen-substituted), will prefer degradation upon radiation. Polymers containing benzene ring either in the backbone or on the side chain, such as PC and PS, are typically resilient to irradiation. Thus, radiation imparts chain scission to polymers containing quaternary carbon atom and C–O ether bond in their backbones. Primarily, PTFE or Teflon possesses a number of unique characteristics, namely high thermal stability, insolubility in almost all organic solvents, super hydrophobicity and low coefficient of friction [39]. Besides, the fact that PTFE elastomers are difficult to grind and are highly stable under very rigorous environmental conditions, make it very attractive for O-ring, gasket and connector that are highly demanded in the automobiles, shipbuilding, aerospace, oils and chemicals industries. Recycling of PTFE is required to avoid environmental pollution in landfills, where it can remain as such for more than 500 years. Recycling of PTFE can be done by irradiation or mechanical pulverization of the PTFE scrap [40]. PTFE has poor resistance to high dose of ionizing irradiation; therefore, the irradiated PTFE waste gets converted to low molecular weight PTFE by degradation. The particle size and molecular weight of PTFE decrease upon increasing the irradiation dose. After irradiation, PTFE becomes very brittle; thus, upon further grinding or air-jetting, gets converted to micro-powder. Irradiation of PTFE is usually done by gamma or electron beam irradiation, either in the presence of air (which is faster) or inert atmosphere/vacuum (which produces stable radical, leading to a slow process). The C–C bonds along the PTFE chains get cleaved and turn into low molecular weight PTFE free-flowing micro-powder (having dimension in the range from 2 to 20 μm) at room temperature, upon the use of high-energy electron beam irradiation; while, γ -radiation often needs a higher temperature to produce a similar effect. Industries, such as Solvay Company (Italy), Shamrock Technologies (USA), Lubrizol Corporation (Germany), Kitamura Company Ltd. (Japan) and Jiashan Senga Tech Co. Ltd. (China), are presently involved in the production of PTFE powder of varying particle size and dispersive power. PTFE ultrafine-powder, having average sizes of particles 2 μm, is being used as a functional additive for application in engineering polymers, anticorrosive surface coatings, non-stick coatings, coils coatings, powder coatings and inks [39]. PTFE powder, of average particle size of few hundred micrometres, can be used for the preparation of sheets and pipes. The PTFE micro-powder can be functionalized by grafting, using vinyl monomers, to improve its dispersibility in various solvents as

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well as to enhance its compatibility with other polymers [41, 42]. Recycled micropowders of < 4 μm range have been employed as additives for inks, oils, lubricants, paints and coatings [43, 44]. An earlier study has demonstrated PTFE scrap recycling by subjecting it to an irradiation dose of 100 kGy, followed by subsequent incorporation of acrylonitrile butadiene rubber (NBR) and radiation-induced vulcanization, to produce a rubber composite for sealant application, exhibiting self-lubricating property at a temperature of 100 °C [45]. PC [poly(bisphenol A carbonate)] is another high-performance polymer (T g of about 150 °C) that is enormously used in electrical and electronic parts, media components (CD and DVD), automobiles accessories, glazing materials for building/construction industries, sports safety-tools, and recyclable containers for foods and drinks [46, 47]. Waste PC can be recycled through chemical route, such as alkaline hydrolysis, glycolysis, hydroglycolysis, methanolysis and aminolysis. These processes are performed under high temperature and pressure under severe reaction conditions, in the presence of chemical catalysts and reagents. Microwave radiation-assisted chemical route for hydrolysis of waste PC under mild alkaline conditions has been found very promising towards recovery of monomer bisphenol A [48]. This study revealed that microwave radiation facilitated PC decomposition at faster rate (requiring only 10 min. at 160 °C, using 10% NaOH), compared to the convectional thermochemical process that required 8 h to achieve > 80% decomposition. Similarly, microwave radiation-assisted catalytic glycolysis of PC, obtained from waste CDs, in the presence of ethylene glycol, resulted in the formation of bisphenol A [49]. Glycolysis of PC requires an optimum concentration of NaOH (of about 2%) and microwave radiation power (of about 600 W for 500 s) in order to recover bisphenol A. Bisphenol A can also be successfully recovered from waste PET by glycolysis and aminolysis under microwave radiation [50, 51].

7 Composite Recycling Most of the polymers can be successfully converted into composites by inclusion of different types of fillers. Radiation processes have often been used to produce fiberreinforced composites, using waste polymers and elastomers [52]. Fiber-reinforced polymer(s) (FRPs) have been widely used in automobile, medical, infrastructure, energy (such as wind turbine blades), sports, aerospace, electronics and defence sectors, as well as in under-water applications (such as bathtubs, boats and septic tanks) [53, 54]. Recycling of polymer composites, especially based on thermosets polymers, has been getting a lot of interest owing to the technical issues involved in separating thermoset polymers from the reinforcing matrix. With the objective of enhancing the recovery yields and the properties of recovered fibres, usually three techniques are being used: (a) mechanical recycling, for separating fibres and matrix by grinding; (b) thermal processing, in which a polymer matrix is separated by decomposition by means of heat or microwave irradiation in the presence of chemicals or catalysts; and (c) solvolysis process, in which a thermoset matrix is decomposed

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through chemical reaction at atmospheric pressure or supercritical conditions in an appropriate solvent, such as water or organic solvent [55]. The use of microwave radiation-assisted pyrolysis, as a replacement of the use of conventional heating source, increases the efficiency of heat transfer and reduces energy consumption of pyrolysis. Advantageously, recovered fibres from FRPs, by employing this method, retain higher mechanical strength than fibres recovered by conventional thermal recycling [56]. For instance, treatment of waste FRP by microwave irradiation (700 W, 2.45 GHz) in an inert argon atmosphere for 5 min, resulted in 100% decomposition of the resin matrix; and the recovered fibres exhibited almost similar tensile strength to that of the virgin carbon fibres [57]. Likewise, recycling of waste FRP at 500 °C for 30 min, under microwave irradiation in a nitrogen atmosphere, produced carbon fibres with similar properties to the virgin fibres [58]. The chemical/tertiary recycling of waste FRP is one of the challenging tasks for chemical transformations. Kamimura et al. [59] recovered phthalic anhydride and glass fiber by chemical recycling, assisted by the use of microwave radiation, from waste fiber-reinforced plastics, containing unsaturated polyester, PS and glass fiber. For this purpose, a mixture of waste FRP and ionic liquid was irradiated with microwave at 340 °C for 2 min. Finally, phthalic anhydride and ionic liquid was recovered from the crude oil by dissolution in a suitable solvent, and subsequently passing through alumina column. Zhao et al. [60] investigated the catalytic degradation of crosslinked epoxy resin-based FRP by microwave radiation, using diethylenetriamine as a solvent to recover glass fibres. Apart from that, microwave irradiation was able to decompose waste glass fiber-reinforced epoxy resin in the presence of H2 O2 and tartaric acid as the oxidizing agent and the acid catalyst, respectively [61]. This radiation-based chemical process was found to recover > 90% of the glass fibres, with retention of the fiber strength of > 90%, only after 3 min of microwave irradiation. Further, carbon and glass fibres, recovered by microwave radiation-integrated pyrolysis and chemical recycling, were successfully used for the preparation of novel FRP composites, using thermosets and thermoplastics [62]. There is a huge potential behind polymer upcycling, i.e., converting postconsumer polymer waste to high-value products [64]. Radiation-induced recycling is a versatile technique to transform waste polymer streams into valued resources through polymer modifications. These modifications are performed in order to introduce tailored functionalities, by selecting appropriate monomers for targeted applications. Grafting process has been studied for upcycling of PP waste powder by radiationgrafting method through application of γ -rays in the presence of monomer acrylonitrile, followed by subsequent amidoximation using hydroxylamine hydrochloride (Fig. 9) [63]. Amidoximated PP has the ability to act as adsorbent(s) for the removal/separation of Cu ions from wastewater streams. A similar approach had also been found useful for grafting of styrene-monomer to PP waste (in the form of powder), by employing γ -rays (Co-60 irradiator) having dose rate of 4.78 kGy h−1 (at room about 25 °C) [65]. It has been further observed that the sulfonation of PP-g-styrene resulted in the formation of sulfonated polymer ionomer, i.e., cationicexchange resin having ion-exchange capacity of about 3.56 me g−1 . Thus, upcycling of PP wastes into sulfonated ionomer, possessing comparable ionic properties to that

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formed by conventional radiation grafted system, has been rendered possible. PE is another polyolefin that produces a large volume of polymer wastes. Recent study demonstrates the preparation of porous sulfonated carbon from waste LDPE upon microwave irradiation of LDPE, soaked in the presence of sulfuric acid, followed by carbonization of sulfonated LDPE under argon atmosphere at 900 °C (Fig. 10) [66]. The resulting microporous sulfonated carbon was used in Li–S batteries, as an interlayer material to improve battery capacity. This approach can also be extended to other polymeric matrix for the purpose of producing high surface area conductive scaffold, having porous structure and negative surface charge. Advancement in microwave-aided pyrolysis at high temperature (about 1000 °C) has seen avoidance of the use of expensive catalyst and the involvement of costly and time-intensive sorting and pre-treating procedures. This continuous process has been successfully demonstrated at a laboratory scale continuous cracking of plastics wastes (and plant oils) into ethylene, PP and other value-added chemicals [67]. Furthermore, it was found that this experimental procedure is appropriate not only for thermoplastics, but also for thermosets, rubbers and practically for all types of

Fig. 9 A schematic of the simultaneous radiation grafting of acrylonitrile on PP waste (PPw) and its amidoximation. Adapted with copyright permission from ul Hassan et al. [63]

Fig. 10 Procedure to prepare the pyrolyzed porous sulfonated LDPE. Adapted with copyright permission from Kim et al. [66]

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polymer-based wastes in all forms and shapes that include fiber, composite, oily wastes, etc. Another postconsumer plastic waste, i.e., PET, in the form of drink bottles, fibres and films, account for major accumulation of polymer wastes. The preparation of high value-added products from used PET is a very viable option to overcome the limitations of the conventional mechanical/chemical recycling, such as high-energy requirement and pre-processing costs [68, 69]. Thus, upcycling of PET is required to enhance the transformation to the monomer, hydroxyethyl terephthalate (BHET), by glycolysis, and reduction of the reaction time by applying economical and eco-friendly approaches. Usually, high molecular weight PET can be converted to the monomer BHET in a short time period by employing microwave, electron beam and γ -irradiation during recycling; and the recovered monomer can be esterified using plant-based fatty acid (oil) to form bio-based polyol [70, 71]. This can be subsequently used for the manufacturing of polyurethanes and dispersion of polyurethane foams, as well as synthesis of polyesters, alkyd resins, epoxides and textile dyestuffs. Furthermore, past research studies have shown the possibility of utilizing recycled thermoplastic polymer wastes, consisting of LDPE, HDPE, PP, PS and PET, for the preparation of composites, using electron beam irradiation in the presence of compatibilizers [72].

8 Low- or High-Energy Radiation-Induced Pyrolysis of Polymer: Conversion to Chemicals and Fuels The word ‘pyrolysis’ has been originated from the Greek words ‘pyro-’ means ‘heat’ and ‘-lysis’ means ‘release’ that relates to ‘breaking down’ or ‘decomposition’. Pyrolysis is a high-temperature thermal-cracking procedure of breaking down of large polymer molecules in the absence of oxygen into low molecular weight molecules [73]. When it is conducted in the absence of catalyst it is termed as ‘thermal-pyrolysis’, ‘thermolysis’ or ‘thermal-cracking’, and it is termed as ‘catalytic pyrolysis’ or ‘catalytic cracking’ if it is carried out in the presence of catalyst. Thermal pyrolysis usually carried out using fossil fuel to heat plastics waste, hence it is highly energy intensive process. Ideally, high-energy irradiations such as gamma rays and electron beams can be used for polymer pyrolysis. Products of pyrolysis also depend upon the reaction conditions, radiation energy and type of polymer. This is due to fact that gamma rays have ability to penetrate intensely into plastic waste, but it comparatively possesses low dose-rate; while accelerated electron beam radiation produces high dose-rate. However, at the same time it possesses low penetration depth. These features of electron beam radiation are favourably useful for oxidation and degradation of polymer waste. Most of the high-energy radiation-based processes used in polymer processing and recycling are limited only for sorting and modification of polymer waste, while as of now only a few studies focus on the use of high-energy radiation for polymer pyrolysis.

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Recently, low-energy microwave-induced pyrolysis has been emerged as a new method for conversion of plastics waste to valuable chemicals, materials and fuels. Extent of heating of materials by microwave radiation depends upon the dielectric properties of materials, since plastics having poor dielectric strength, upon heating to their melting point results in substantial absorption of microwaves that eventually leads to poor control of pyrolysis process [74]. To overcome this limitation by taking advantage of plastics heating by microwave, plastics are mixed with microwaveadsorbent materials which absorbs microwave energy, and this thermal energy of microwave-adsorbents gets transferred to plastics by conduction mechanism [75, 76]. This process is often known as microwave-induced or microwave-assisted pyrolysis, as the plastics are indirectly heated by the rapidly heated highly microwaveabsorbent materials. This process is also widely being investigated to decompose various biomass [76–79]. Table 1 lists some recent studies involved in the microwaveassisted radiation processes for chemical recycling of plastic waste. As seen from table this new method found potential for production of value-added chemicals such as gasoline fuel, hydrocarbon liquid fuels, hydrogen and carbon-based nanomaterials. Chemical upcycling of polymer waste via low- and high-energy radiation-assisted catalytic and non-catalytic pyrolysis processes are emerging technologies for conversion of waste to valuable chemicals, fuels and carbon-based functional materials [87, 88]. Further, preparation of low-cost catalysts, designing of simple rector for highenergy irradiation and to improve conversion of polymer wastes to pyrolysis products, and development of efficient methods for recovery of various chemicals from pyrolysis oils are the key aspects for successful commercialization of radiation-based technology for polymer recycling.

9 Conclusion and Future Directions Herein, this chapter has briefly discussed different types of polymer recycling methods, as well as the recently emerging trends in polymer recycling using radiation technology. Radiation aids in modification of polymeric waste through radical and anionic radical formation, which in turn are used for segregation/sorting of a particular polymer from a mixed waste. Polymer oxidation, induced by controlled radiolysis, has found application towards making of compatible polymer blends and composites, improving interfacial surface adhesion among polymer molecules, and plastic adhesion with other non-polymeric compounds. Radiation technique is useful for reprocessing of thermoset plastics and halogen-bearing polymers, such as PVC and PTFE. Specifically, high-energy irradiation degrades and chemically alters polymer wastes, which can be amalgamated with construction materials like concrete and asphalt and can be transformed into liquid and gaseous fuels for recovery of energy. Optimization of plastic waste recycling processes is a global concern. Therefore, development of highly efficient and low-cost methodologies, having high recycling rate, is required to lower the impact of plastic waste. In this regard, radiation-based

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Table 1 Radiation-assisted pyrolysis for chemical recycling of plastics waste for value-added chemicals and fuels production Plastic waste

Pyrolysis condition(s)

Polyethylene (PE) + Palm oil

Quartz reactor; 900 W • Gas 1.4 wt%, solid 22.0 microwave power, 30 g wt%, liquid 6.6 wt% • Gas constituted 30.3 carbon coated wt% ethylene, 4.5 wt% aluminium oxide fiber, propylene, 3.3 wt% H2 , Temperature:1000 °C 15.5 wt% CO and 24.1 wt% CH4

PE

Continuous microwave-assisted pyrolysis reactor, ZSM-5 catalysts, Temperature: 620 °C

Waste medical plastics and Heating @24 °C/min; frying oil 20 min

Pyrolysis product(s) and yield

References [67]

• Gas 49 wt%, liquid 48.9 [80] wt%, wax 1.2 wt% • Gas contained H2 , CH4 and C2 -C4 hydrocarbons • Liquid contained aromatic and isomerized aliphatic • Liquid 81 wt% contained [81] C10–C28 hydrocarbons

Polyethylene terephthalate (PET) bottle scarp

Microwave absorber: Silicon carbides, Quartz reactor; 500 W microwave power, Temperature: 550 °C

• Solid 35.67 wt%, gas 40 wt%

[82]

PET bottle scarp + rice husk

Microwave absorber: Graphite, 450 W microwave power, Temperature: 600 °C, Quartz reactor

• Gas 9.9–68.9%, bio-crude 12–29%, char 15.7–31.7% • Gas contained H2, syngas etc

[83]

HDPE

Quartz tube rector, Catalysts/microwave susceptors, 1000 W microwave power, Temperature: 550 °C (heating @ 20 °C/min)

• 55.6 mmol H2 gplastic −1 , • 1560 mg C gplastic −1 gcatalyst −1 , composed of > 92wt% MWCNT

[84]

LDPE

Catalyst: NiO and HY zeolite

Oil 56.5 wt% of oil Gasoline fraction 93.8%

[85]

HDPE

Glass rector; γ-ray irradiation; Catalyst: ZSM-Zeolite Catalyst: Ferric oxide

• Liquid 77.98 wt%, Solid [86] 10.27 wt% and Gas 11.75 wt% (Zeolite) • Liquid 70.81 wt%, Solid 10.64 wt% and Gas 18.55 wt% (Ferric oxide)

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polymer recycling techniques hold potential for further development to satisfy the polymeric waste recycling needs. For large-scale applicability of radiation technology in polymer recycling, it is of foremost importance to ensure the feasibility of the processes in terms of cost, user-friendliness, safety as well as scaling up. In addition, applicability of radiation technologies to the recycling of a wide range of polymeric items is essential to justify their usage in a scaled-up setup. The way forward in the near future is to use these radiation technologies in combination with the conventional technologies for wide-scale and efficient polymer recycling applications.

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Chapter 12

Radiation-Induced Degradation of Polymers: An Aspect Less Exploited C. V. Chaudhari, K. A. Dubey, and Y. K. Bhardwaj

1 Introduction In essence, polymers are large molecules made up of similar or dissimilar building blocks called monomers. On the basis of their origin, they have been categorized as natural polymers and synthetic polymers. Natural polymers have been part of human life since time immemorial and easily undergo degradation on environmental exposure. Degradation is a change in properties like mechanical strength, molecular weight, shape, size, texture and color under any type of external stress. Degradation is as a result of change in chemical and/or physical structure of the polymer chain, which finally leads to a decrease in the molecular weight of the polymer [1]. Degradation of polymers, particularly natural polymers, is being relooked at again with renewed interest because of their unique characteristics like inherent biocompatibility, biodegradability and easy availability [2]. However, their finite supply, slow production rate, high cost, limited possibility of tailoring the properties, faster and uncontrolled degradation and poor mechanical properties have forced to look for their synthetic substitutes [3]. Other reason for exploring synthetic polymers was rapid demand for the manufactured products. Since the Second World War, synthetic polymers have made pervasive entry into our life and there is hardly any domain, be it industry, technology or science which remains untouched by polymers. Because of their versatile properties, ease of processing and low-cost polymers have replaced construction materials like wood, jute, brick, glass, concrete, metal and others. The C. V. Chaudhari (B) · K. A. Dubey · Y. K. Bhardwaj Radiation Technology Development Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India e-mail: [email protected] K. A. Dubey e-mail: [email protected] Y. K. Bhardwaj e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. R. Chowdhury (ed.), Applications of High Energy Radiations, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-9048-9_12

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plastic consumption worldwide has increased from ~1.5 MMT in 1950 to 359 MMT in 2018. Their impact on daily life is incalculable [4, 5]. Nevertheless, a widespread use of polymers, particularly for applications with linear consumption pattern, has contributed to ever increasing amount of solid plastic waste. It is estimated that polymer accounts for ~10% by weight and ~20% by volume of municipal solid waste. Most of this ends up in landfills causing negative impact on environment as well as loss of a valuable resource. The management of polymer waste has to some extent overshadowed the positive influence of synthetic polymers. Polymer waste management has added set of challenges which have to be dealt with effectively. For sustainable development, society, economy and environment all three have to be satisfied simultaneously [6]. Already efforts are on to mitigate the negatives of the synthetic polymers. A two-way approach has been proposed as solution. First is synthesis of biodegradable synthetic polymers preferably from renewable sources. In this direction, polymers like poly(caprolactone) (PCL), poly(lactic acid) (PLA), poly(glycolic acid) (PGA), polydioxanone (PDO), their copolymers and other polymers have been investigated with encouraging outcomes [7–10]. Second is reduce, reuse and recycle approach [11]. Reduction is the most desirable as it automatically leads to reduction in waste. Reuse and recycle aim at turning back waste into resource [12]. Recycling in each life cycle causes deterioration in properties of polymer, and finally after several life cycles of recycling operation, the properties deteriorate to such an extent that recycling does not exceed the benefit. It has been realized that the degradation of polymers (natural and synthetic) is unavoidable and all polymers degrade. Thus, comparatively new approach is to reuse the degraded polymer itself as a resource by accelerating degradation and tailoring the degradation to fragments of desired length. Several methods like thermal, chemical, biological radiolytic alone or in combination are being explored or investigated for degradation of polymers [13, 14]. Degradation of polymers using radiation has its unique advantages. This article reviews exclusively radiolytic degradation of polymers and applications of radiolytically degraded products for different applications.

2 Polymer Degradation Methods Polymer degradation is characterized by lowering of molecular weight through scissioning of the main-chain backbone. Depolymerization and random degradation are the predominant ways of chain scission.

2.1 Depolymerization Depolymerization is reversal of polymerization and converts the polymers into component monomers (smaller molecules) or mixture of monomers. The tendency of polymers to depolymerize depends on their ceiling temperature (T c ), the temperature

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at which the enthalpy of polymerization is same as the entropy gained by converting polymer into monomers molecules. At temperatures > T c , the depolymerization rate being greater than polymerization rate, formation of polymer is inhibited [15]. The monomer so produced can be repolymerized back to the polymer which is identical to initial virgin polymer. Thermal or thermochemical depolymerization of some synthetic polymers which are difficult to recycle has been investigated [16–19]. Though the concept of depolymerization seems fascinating, still it has been established for few polymers like poly(ethylene terephthalate) (PET) for other polymers, and the process is still in infancy mainly because of economics of the process [20]. Natural polymers are formed in nature during the growth cycles of all organisms, and they degrade and finally mineralize easily in nature. Their depolymerization has caught interest due to potential applications of depolymerized oligomers of specific molecular weight for healthcare applications and also as biofuels [21, 22]. Depolymerization of natural polymers has been possible because of specificity of microorganisms to degrade them to desired chain length [7, 23]. However, the depolymerization of natural polymers has been exploited not to get monomer but to get glucose source which finally leads to industrial products of interest. Synthesizing polymers which can be biodegraded has been the recent approach to mitigate the menace of synthetic polymers, but there is also the aim to degraded polymer in long run not to end up with monomer.

2.2 Polymer Degradation Ideally, it is expected that polymers would maintain their all-desired properties until they are disposed-off. But by now, there is fair realization that all polymers do degrade slowly under environmental stresses during their useful life time. All polymers are susceptible to thermal, photochemical, oxidative, hydrolytic and radiolytic degradation depending on their composition and environment. Thermal degradation is caused by exposure to elevated temperatures in the absence of any chemical agents. Photochemical degradation is caused by absorption of light wherein triplet O2 gets converted to singlet O2 , a highly reactive form of O2 , which effects spinallowed oxidations. In atmosphere, organic compounds degrade due to generation of highly reactive strong oxidizing species the OH radical on reaction of ozone with water. Biological degradation is caused by the action of enzymes on polymers. Microorganisms produce variety of enzymes capable of reacting with both natural and synthetic polymers. Chemical degradation is caused by solvolysis and mainly by hydrolysis to give lower molecular weight compounds. Hydrolysis takes place in acidic or alkaline water. Polymers are susceptible to attack by atmospheric O2 particularly at elevated temperatures. Ionizing radiation causes degradation due to breaking of the main chains of the polymer or of side chains with emission of gaseous products, as well as oxidation of the polymer [24]. Degradation which was initially being viewed as a curse is now being reviewed from point of it benefits. Utilization of degraded polymer would contribute reduce, reuse and recycle trilogy chain in

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each step. Thus, emphasis is now shifted from inhibit to accelerate the degradation of polymers of interest [6].

2.2.1

Factors Affecting Polymer Degradation

Factors Affecting Conventional Degradation Methods (Thermal, Photocatalytic, Oxidative, Hydrolytic) The degradation mechanism and susceptibility of polymer to undergo degradation depend on both chemical and physical characteristics of polymers as well as the surrounding environment. Polymer properties governing the degradation of polymer are as follows: (i)

Chemical structure of polymer: Chemical structure is fundamental to properties of polymers like glass transition temperature (T g ) of polymer crystallinity extent, water affinity etc. It has been observed that polymers with polar groups (−OH, > C=O) are more susceptible to chemical degradation and also polar molecular fragments are more prone to further degradation [25, 26]. Heat, light, presence of O2 , moisture and bioactive organisms further enhance the degradation of polymers (ii) Crystallinity: Higher the amorphous content of the polymer easily, it undergoes degradation. Because the well-ordered and packed structure of crystalline polymer or crystalline regions of semi-crystalline polymers are not accessible for initiate chemical reactions. (iii) Glass transition temperature (T g ): Polymers undergo transition from glassy to rubbery state across T g . As the segmental motion sets in at temperature > T g , chains are comparatively freer and polymer as a whole system is more conducive for gaseous and liquid ingress and hence to degradation. Factors Affecting Radiolytic Degradation of Polymers Polymers being long-chain molecules, final outcome of interaction of high energy radiation (gamma or electron beam) with polymers may be their degradation or cross-linking (entanglement) as shown in Fig. 1 [27]. Though all polymers undergo degradation and cross-linking simultaneously, the predominant effect depends on the chemical structure of constituent monomer, crystallinity and physical state of polymer during irradiation. The extent of effect (cross-linking/degradation) is a strong function of molecular weight and crystallinity of the polymer. However, as cross-linking and degradation are not intrinsic properties of polymers, a degradingtype polymer may act as cross-linking type and vice versa depending on the conditions. An empirical rule depending on polymer structure is that when each backbone C-atom is attached to at least one H-atom, polymer predominantly undergoes cross-linking while polymers with tetra substituted C-atom predominantly undergo

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Fig. 1 Radiation effect on polymers

Fig. 2 Polymer structure

degradation, i.e., in Fig. 2, below if R1=R2=H or if R1/R2=H, the polymer is predominantly cross-linking type, and if R1=R2=other than H, than it degrades at room temperature. For example, polyethylene (PE) predominantly cross-links while poly(tetrafluoroethylene) (PTFE) degrades on irradiation. Crystallinity is an important polymer feature which governs the net radiation effect on polymer. Highly crystalline polymer shows delayed effect on irradiation. As the radicals generated in the bulk of the crystalline region have to migrate to the interface of crystalline and amorphous domains through series of H abstraction reaction with the neighboring C-atoms for inter-chain interactions or other chemical reactions (Fig. 3). Also, higher the molecular weight of polymer more intense is the net effect of irradiation [28].

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Fig. 3 Radiation effect on semi-crystalline and amorphous polymers

2.3 Mechanism of Degradation 2.3.1

Mechanism of Conventional Degradation Processes

Prolonged exposure of polymers to natural environment may cause random-chain scission or side-group elimination. Obviously, random-chain scission causes significant decrease in molecular weight, while side-chain scission causes formation of trapped gaseous, which cause positive pressure and ultimately cracks, fissures and pinholes in polymer matrix. Degradation of polymer along with other changes at macro-level causes two very visible changes, namely embrittlement of polymer and development of color. The embrittlement, i.e., loss of elasticity of polymer on environment exposure begins with photodegradation which leads to thermo-oxidative degradation. UV light from sunlight provides the activation energy required to initiate the incorporation of oxygen atoms into the polymer. Combination of these processes finally causes the polymer to become brittle and to break into smaller fragments. These polymer chains further undergo degradation to low molecular weight fragment to be metabolized by microorganisms [29, 30]. Development of yellow color is as a consequence of generation of conjugated structure, entrapment of free radicals in crystalline region of polymers or due to generation of carbonyl groups [31–33]. In general, backbone structure, morphology and the types of impurities present in polymers determine the degradation probability, extent and kinetics of degradation.

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The degradation chemistry of most of polymers is well documented. For most of the polymers, it starts with formation of radicals and series of reactions follows. For a polymer “P,” the degradation mechanism has been illustrated in Scheme 1. In the presence of oxygen, polymer undergoes auto-oxidative degradation, thus enhancing the degradation kinetics (Scheme 2). Product of the side chain scission depends on composition of side chains. Sidechain removal may cause change in the structure of the parent polymer chain like cyclization or generation of unsaturation and conjugation. The change in chemical structure of parent polymer may further increase or decrease the polymer degradation susceptibility [34–36]. Degradation kinetics of polymers was first reported in 1930s,

Scheme 1 Polymer degradation through radical mechanism

Scheme 2 Auto-catalytic polymer degradation in the presence of oxygen

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but the topic still continues to fascinate different groups and degradation kinetics is now better understood depending on the mathematical modeling and simulation studies [37–39]. However, prime assumption in all these studies is that polymer degradation follows first-order kinetics.

2.3.2

Mechanism of Radiolytic Degradation of Polymers

As high energy radiation carries much more energy than binding energy (10–15 eV) of most labile electrons in polymers, the ionizations are highly probable. Only a fraction of energy deposited causes ionization, the remainder produces electronic excitation similar to that of photochemical excitation. However, the energy levels reached with ionizing radiation are much higher which eventually leads to its breakdown to free radicals (Scheme 3) [40]. The fate of ions produced during polymer radiolysis depends on the irradiating conditions, i.e., whether polymer is irradiated bare, slightly swelled with solvent or dipped in excess of solvent. The energy is proportionately distributed among the components being irradiated and the radiation chemistry of the components varies. From the point of view of radiolytic degradation of polymer, the bare polymer or polymer embedded with solvent (intentionally or unintentionally), the cations may undergo ion molecules’ reactions, may react with solvent or excite solvent molecules eventually leading to free radicals (Scheme 4). This chemistry takes place in solid polymers or solvents only the time of the sequence varies [41]. The ultimate effect of series of chemical reaction on radiolysis of polymer is formation of cross-links, branching or scission (degradation). Branching increases molecular weight (MW) similar to cross-linking; however, the increase in MW on branching is comparatively too less to cross-linking, and thus, generally the branching aspect is overlooked while discussing radiation effects on polymers. During irradiation of polymers, all these phenomena occur parallelly. The quantitative estimation of polymer modification induced by radiation is made in terms of G-value, which is number of molecules changed per 100 eV of energy absorbed. Polymers have been put in three categories depending on their radiation susceptibility. Polymers with C–Cl bond in polymers’ structure {poly(vinyl chloride), poly(chloroprene)}, aliphatic polyethers with ether linkages connecting the repeat units {cellulose, chitin, poly(oxyethylene), poly(butylene oxide)} and aliphatic polysulfones (R–SO2 –R' ) undergo dramatic changes on irradiation at doses of even

Scheme 3 Basic radiolytic processes in polymer irradiation

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+

PH2 + P

Trapped

Trapped

hole

electron

PH

PH

+

PH

Crosslinking +

_

e

Sol

Sol**

Sol

+

Sol + PH

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Free radicals

OR Degradation

_

Sol

Scheme 4 Radiolytic process in the presence of solvent

few kGy. Aliphatic polyesters, polyolefines and polyamides are moderately resistant, while polymers containing aromatic groups in their molecular structure, such as polyethylene terephthalate, polyethersulfones, polyphenyl ethers, polyphenylene oxides, polyphenyl ketones, aromatic polyamides, polysulfones, polyetherimides and epoxy resins, remain unmodified even at dose of ~1000 kGy [42, 43]. The presence of aromatic ring in polymer structure imparts significant radiation stability to the polymer which was noticed in early fifties, by earliest workers in the field [44]. Later, impact of ionizing radiation on benzene and its derivatives was studied through several decades, and it was believed that intrinsic radiation resistant of benzene is due to the conjugated π-system which is reflected in radiation stability of polymers with aromatic ring [45]. Later, it was suggested that the radiation resistance of polystyrene is due to the existence of dimeric associates of aromatic rings, that is, intermolecular interaction on radiolysis [46]. Recent radiolysis studies of benzene in rigid media have established that radiation resistance of benzene cannot be explained explicitly by its aromatic structure but is also determined by the matrix properties, which affect energy dissipation. Inefficient matrix cooling and low efficiency of intersystem crossing result in significant contribution of skeleton rearrangement particularly in such rigid media. These recent findings on benzene radiolysis have important implications for the radiation chemistry of various solid systems containing benzene and other aromatic and may further provide insight into radiation stability of aromatic polymers [47]. Fundamentals of radiolytic effects on family of polymers have been extensively reported elsewhere [48]. Here, a brief discussion on radiolytic degradation of two polymers which predominantly undergo degradation is given for completion of the manuscript.

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Scheme 5 Mechanism of radiolytic degradation of PTFE in vacuum

Mechanism of Radiolytic Degradation of Poly(Tetrafluoroethylene) (PTFE) In spite of its high chemical inertness, bare PTFE (also known as Teflon) in solid state is highly prone to radiation-induced degradation, unlike PE which undergoes cross-linking predominantly. The chemical structure of PTFE is same as PE, in which all H-atoms have been replaced by F-atoms. Vast difference in the radiationinduced changes in PE and PTFE caused lot of interest in investigating radiolysis of PTFE [49–53]. The reported literature agrees on the aspects that irradiation cases serious decrease in molecular weight of PTFE, turns it into brittle friable matrix and increases its crystallinity; and, the degradation is enhanced in the presence of oxygen [54–56]. The mechanism given in Schemes 5 and 6 is largely established for PTFE degradation. Utilization of high energy EB for Teflon degradation brings in other aspect, the temperature rise during degradation. Temperature rise beyond melting point of Teflon in the penetration area primarily due to low thermal conductivity of Teflon is reported. The degraded Teflon of wide molecular weight range results if the target Teflon is not cooled during irradiation while cooling results in higher molecular weight degraded Teflon with narrow molecular weight dispersion [50]. Radiation susceptibility of PTFE toward radiation in fact can be exploited to convert otherwise inert PTFE scrap to lower molecular weight polymer powder which finds wide industrial applications [57, 58].

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Scheme 6 Mechanism of radiolytic degradation of PTFE in air

Radiolytic Degradation of Cellulose (CellH) Among polysaccharide, cellulose is the most abundant natural polymer being prime structural component of plants. It undergoes natural erosion, and degradation was observed quite early by human beings. In modern times, degradation of cellulose is of interest in view of many products produced through it through different technological processes [59]. Radiation-induced degradation of plant raw materials is primarily aimed at the intensification of industrial hydrolysis of raw materials for production of desired products. The successful adoption of radiation for cellulose degradation is stemmed from the possibility of replacing ecologically hazardous chemical steps by more environment-friendly ionizing radiation. This encouraged studies in the field of radiation chemistry of cellulose and elucidation of radiation-induced chemical transformations in cellulose. The mechanism of radiolytic degradation of cellulose (briefly described below) is well understood particularly due to works of Ershov and co-workers [60]. Cellulose is highly crystalline polymer which degrades on irradiation, and the degree of degradation is strong function of temperature. The initial radiolysis reaction for cellulose is shown in Scheme 7. The monomer unit

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Fig. 4 Cellulose unit

of cellulose is pyranose rings in the chair conformation with axial hydrogen atoms (Fig. 4). Low temperature (77 K) studies revealed that electron generated on irradiation recombines with cationic center positioned on one of the monomeric units of cellulose. Neutralization causes excitation which finally leads to cleavage of weakest C–H bond at C(1) or C(4) position. The H-radical so generated further undergoes H-abstraction reaction with any of H-atom of monomeric ring. Analysis of hyperfine splitting constants established that radical at C(1) or C(4) can be generated only when glycosidic linkage in cellulose backbone cleaves [60]. The C(1) and C(4) radicals generated on mid-chain through series of H abstraction reaction are converted to terminal radicals. Fragmentation of macro-radical usually occurs via cleavage of a bond in β-position relative to the radical center [61]. The resulting terminal radical can be positioned either at same monomer unit which first underwent abstraction reaction {reaction 4(b) in Scheme 4} or at adjacent unit as shown in Fig. 5.

2.4 Quantitative Estimation of Radiolytic Degradation Polymers on irradiation undergo cross-linking and degradation simultaneously. However, the net effect observed is due to the predominant process happening during irradiation which depends on irradiation conditions as well as polymer morphology, structure and state. For quantitative estimation of cross-linking and degradation, Charlesby–Pinner equation (Eq. 5) relating sol fraction to absorbed dose is used [62]. Though slightly modified version of Charlesby–Pinner equation has been proposed, still the equation is fairly accepted for quantifying radiation effects on polymers [63, 64]. s + s 0.5 = β/α + 1/(α Pn D)

(5)

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Fig. 5 Radicals generated during radiolytic degradation of cellulose

Scheme 7 Initial radiolytic reactions in cellulose degradation

where s is sol fraction, β and α are the fractions of ruptured and cross-linked mainchain units per unit dose, respectively, Pn is the number average degree of polymerization, and D is the absorbed dose. A plot of (s + s0.5 ) against the reciprocal of absorbed dose (1/D) is a straight line, having an intercept equal to β/α on coordinate axis. For a polymer predominantly undergoing cross-linking, the value of intercept on coordinate is < 2, whereas for polymer predominantly undergoing degradation, the intercept value is > 2.

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3 Radiation-Induced Degradation of Polymers and Their Applications 3.1 Radiation-Induced Degradation of Synthetic Polymers 3.1.1

Radiation-Induced Degradation of Poly(Tetrafluoroethylene) (PTFE)

PTFE is chemically inert polymer with low coefficient of friction, good heat resistance and exceptional dielectric properties. These very exceptional properties are a hindrance for its scrap disposal as it does not undergo any deterioration under natural conditions. It predominantly degrades on irradiation losing its mechanical properties. The degradation kinetics is further enhanced in the presence of halogenated sensitizers [65]. Efforts have been made to utilize the PTFE scrap through its grafting to synthesize low-cost adsorbents for treatment of effluents or for applications that involve using PTFE in powder form [56, 66, 67]. The PTFE powder has been proposed as filler, as solid lubricant, for antistatic and water proof coatings. Cryogenic pulverization of PTFE is energy intensive, and the ground powdered PTFE does not show appreciable change in molecular weight and agglomerates when in use, while low molecular weight radiation degraded Teflon powder does not [68, 69]. PTFE micro-fine powder was used as filler by blending with elastomers’ and thermos’ plastics [70, 71]. We recently reported effect of radiolytically degraded PTFE microparticles (PTFEMP) and organoclay in PTFE-reinforced ethylene vinyl acetate (EVA) composites [72]. PTFEMP was produced by mechanically grinding radiolytically degraded (Co-60 γ-radiation) to asymmetric micro powder. Figure 6 shows distribution of particles segregated in sieves of different mesh sizes after sieving mechanically ground PTFE for 30 min. The lower size particles were obtained with increase in absorbed dose. The composites of ethyl vinyl acetate (EVA) containing PTFEMP and organoclay in different proportions were melt compounded and characterized. Mechanical properties demonstrated high synergy between fillers, leading to manifold increase in the modulus of dual filler filled composites in comparison to single filler systems. Figure 7 shows stress–strain profile of EVA/nanoclay (5%) nanocomposites loaded with different amounts of PTFEMP. Elongation at break decreased while stiffness increased with introduction of PTFEMP into the composite matrix. Inset of Fig. 7 shows variation in elastic modulus of EVA and EVA composites containing different amounts of organoclay. It is evident from the figure that both for EVA and EVA composites, modulus increased with addition of organoclay. However, the increase was much sharper and higher for ternary composites, indicating high synergy between PTFEMS and organoclay. High synergy between organoclay and PTFEMP was further exemplified by the fact that the increase in stiffness at 50 wt% (~30% volume fraction) PTFEMP loading was about 3.5 times higher in EVA composites than in pristine EVA. X-ray diffraction studies revealed around 10% intercalary expansion in organoclay, in the composites having high loading of PTFEMP, though the crystallinity of EVA did not change. The melt flow index of

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the composites decreased with addition of PTFEMP and organoclay both in binary and ternary composites (Fig. 8). The high synergy between PTFEMP and organoclay was attributed to the increase in the melt viscosity of the ternary composites and to the partial intercalary expansion of organoclay. 60

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200 kGy biodegrades comparatively slowly. The slower biodegradation of PLA irradiated to dose > 200 kGy has been attributed to cross-linking of PLA. In order to reduce the rigidity of PLA, its blending with other flexible biodegradable polyesters like poly(butylene adipate-co-terephthalate) (PBAT) has been investigated in order to extend its application in food packaging. The EB irradiation of thin films of these blends showed noticeable decrease in degradation and also of PBAT. The PLA:PBAT (42:58) blend showed almost similar radiation stability as pure PBAT [81].

3.1.5

Radiation-Induced Degradation of Poly(Caprolactone)

Polycaprolactone (PCL), a semi-crystalline polyester, is widely used for biomedical applications. It is also biodegradable, but unlike PLA is elastic in nature. Because of its elasticity, it finds application as biodegradable packaging also [82]. PCL is very much prone to radiation-induced degradation, and its biodegradability is further enhanced on irradiation. However, interesting finding has been reported regarding its composites with nano-ZnO. It has been reported that solvent-casted PCL-nano-ZnO composites’ films show marginal decrease in mechanical properties to an absorbed dose of 25 kGy, the dose recommended for radiation sterilization [83]. These results indicate potential of biodegradable PCL-based composites for radiation sterilization.

3.2 Radiation-Induced Degradation of Natural Polymers and Their Applications Radiation processing of natural polymers is of special interest to countries like India with large natural resources and having agriculture-based economy. Radiation processing of natural polymeric material largely remained unexplored as most of them have complex chemical structures and degrade when exposed to radiation. There non-toxicity, biodegradability and availability at low cost have forced scientific community which have a relook into their degradation. In the last two decades, particularly, there has been a rapidly expanding interest in study of processes and methods of degradation of natural polymers which can add value to degraded products. It is now being realized that radiation processing can also be beneficially utilized either to improve the existing methodologies used for processing natural polymers or to impart value addition to such products by converting them into more useful form like low molecular weight natural polymer.

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Radiation-Induced Degradation of Cellulose

Cellulose is a linear chain of ringed glucose molecules with flat ribbon-like conformation. The molecular weight of cellulose depends on its source, and the two anhydroglucose rings are bonded through an oxygen atom through 1-4 glycosidic linkage between two rings as shown in Fig. 12. The intra-chain hydrogen bonding between hydroxyl groups and oxygen of the adjoining ring molecules stabilizes the linkage and results in the linear configuration of the cellulose chain [84]. Radiation-induced chemical transformations in cellulose are provoked by a random H-abstraction from some pyranose unit with formation of radicals which finally cause main-chain degradation. Recent studies report that degradation of cellulose is highly dependent on temperature during irradiation [85]. In temperature range 290–370 K, the prevalence of H-bonding leads to minor main-chain (G ≈ 0.62) degradation and side-chain fragmentation with elimination of CO2 predominates. With rise in temperature (410 K), there is noticeable shift from side-chain fragmentation to main-chain degradation (G ≤ 3.10). It has been established that rise in temperature favors transfer of radical center to nearby chain due to highly mobile H-radical abstraction reaction and also species like formyl or formyloxyl are eliminated due to damage to pyranose end unit [85]. At temperatures 480–550 K, dehydration of damaged pyranose unit leads to formation of conjugated structure. Degradation of cellulose has been exploited for many industrial applications. Cellulose of natural origin is always associated with hemicellulose and lignin. For bioethanol production, cellulose and hemicellulose are sources of fermentable sugars, while presence of lignin poses hindrance to the conversion of these sugar polysaccharides into ethanol [86]. An expensive and energy-intensive step of pretreatment which reduces the recalcitrance of lignin (which makes feedstock more susceptible to saccharification) and reduces the molecular weight of cellulose to oligomeric length is involved in bioethanol production. Various physical, chemical, biological

Fig. 12 Structure of cellulose with intra and inter-hydrogen bonding

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or combined methods are employed to pretreat lignocelluloses. Radiation degradation has been effectively used for degrading cellulose to lower molecular weight oligomers by passing the corrosive chemical treatment step. The presence of lignin due to it aromatic structure imparts radiation resistance to the cellulose feedstock. Use of radiation (gamma or EB) for degradation of cellulose polysaccharide not only avoids the problem of corrosion associated with use of chemicals but also eliminates use of large volumes of solvents for recovery and recycling of these chemicals. Use of radiation allows controlled degradation of cellulose as well. Production of biofuels from cellulose also involves cellulose degradation. It has been established that as in case of bioethanol production, radiation-induced degradation of cellulose for biofuel production reduces or replaces use of ecologically hazardous chemicals, reduces the number of processing stages and decreases energy consumption. Irradiation enhances the degradation of cellulose upon subsequent thermal or enzymatic hydrolysis. The extent of radiolytic decomposition of cellulose is considerably affected by the conditions of the irradiation such as the temperature and the presence of various chemical species [87].

3.2.2

Radiation-Induced Degradation of Carboxymethyl Cellulose

Carboxymethyl cellulose (CMC) is an industrially important cellulose derivative with carboxymethyl group bound to the hydroxyl groups of the glucose unit. Among its industrial application, it is used as thickener and a binder in food industry, in oil industry as a lubricant for drilling and in the cosmetic industry as a stabilizer and a binder. Because of its non-toxicity, biodegradability and biocompatibility, CMC has also been widely used for biomedical application like bone graft. The radiation effect on CMC in solution form depends on the CMC concentration. At concentration < 5%, it degrades while at concentration > 5% undergoes cross-linking leading to increase in gel fraction and mechanical strength [88]. The CMC degradation in solution has also been established through studies on viscosity changes in CMC solution on irradiation. The viscosity of the CMC solution decreased with an increase in the radiation dose, but the extent of the degradation decreased with an increase in the CMC concentration. Addition of vitamin C as a radical scavenger to the solution was shown to effectively prevent the decrease of solution viscosity. Interestingly, irradiation at lower temperature (−70 °C) further inhibited the degradation. The irradiation at high-dose rate using EB caused less degradation than the gamma irradiation [89].

3.2.3

Radiation-Induced Degradation of Chitosan

Chitin is the second most abundant natural polysaccharide found in the outer skeleton of insects, crabs, shrimps and other marine animals. It is structurally similar to cellulose except that it has a β-linked 2-acetamido group in the 2-deoxy-d-glucose residue,

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Fig. 13 Structure of chitin and chitosan

unlike cellulose, which has hydroxyl group at position 2 of 2-deoxy-d-glucose (Fig. 13). Among natural polymers till recent times, industry focus was on cellulose and chitin remained an underutilized resource probably because of its limited solubility in common solvents. However, its derivative chitosan which can be easily obtained by alkali hydrolysis of chitin; under boiling conditions, it has been found to be a functional material of high potential for various fields [90]. Chitosan has emerged as a versatile matrix due to the presence of a reactive −NH2 group at position 2 and two hydroxyl groups at positions 3 and 6, respectively, of the 2-deoxy-d-glucose residue allows it to form chelate with many metal ions. The presence of an amino group also renders chitosan soluble in weak acids which enhances its commercial applications [91]. Chitosan belongs to the class of natural polymers that undergoes degradation on irradiation. The degradation dose for chitosan depends on irradiation conditions, molecular weight, state of chitosan (solid or solution) and presence of types of additives. Following the change in viscosity of chitosan solution of molecular weight 2.5 × 106 with absorbed dose, it was observed that viscosity of the solution decreases almost exponentially with absorbed dose indicating degradation of chitosan as shown in Fig. 14. Solid-state irradiation of chitosan shows similar results like other degrading polymers described above like enhanced degradation in the presence of oxygen and further post-irradiation degradation with time [92, 93]. The low molecular weight chitosan has found potential application as an antibacterial agent, plant growth promoter and in preservation of food stuffs. Therefore, several groups have tried controlled degradation of chitosan. Degradation of chitosan with molecular weight of 6.73 * 105 –1.2 * 106 Da to chitosan of 1.03 * 105 was successfully carried out while retaining its primary structure by simply irradiating solid flakes of chitosan dispersed in water with γ-ray dose of 50 kGy. Degradation to same extent could be achieved at much lower dose of 20 kGy in the presence of H2 O2 as sensitizer [94].

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Similar results, i.e., enhanced degradation in solution and further enhancement in the presence of H2 O2 have been reported by other groups also [95]. Another observation regarding the lower molecular weight chitosan produced in Yoksan et al.’s work was that it was comparatively more reactive than parent chitosan. A fantastic comparative study of different degradation methods for chitosan solution establishes that among ultrasonic, ultraviolet and gamma radiation, the gamma irradiation is most effective in its degradation [96]. Low molecular weight chitosan is an antibacterial agent which shows wide spectrum of antibacterial activity. Chitosan nanoparticles (CSNPs) were investigated for their antibacterial activity against ATCC strains of Escherichia coli and Staphylococcus aureus by zone inhibition and bacterial reduction method using four different concentrations of CSNP solutions. Antibacterial activity of CSNP was evident from these studies; the antibacterial activity increased with increasing concentration of CSNP. The antibacterial activity was more prominent against E. coli than that of S. aureus. A concentration of 10 mg/mL CSNP completed inhibited E. coli and for S. aureus proliferation [97]. Radiolytically degraded chitosan has been reported to be an effective plant growth promoter and inducer of stress tolerance in agricultural and horticultural plants and thus has a bearing toward sustainable agriculture and climate resilience. Application of chitosan (337.73 kDa) and oligo-chitosan (82.20 kDa) (obtained on irradiation to a dose of 100 kGy gamma dose) were applied as foliar spray on potato plants to analyze growth and stress tolerance inducing affects. Significant improvement in shoot height and number of nodes was observed after foliar spray of 50–75 mg/L. The chlorophyll, carotenoids, proline, reducing and total sugars content enhanced considerably. The antioxidant and defense enzymes also showed prominent increment [98]. Similar foliar application (40 mg/L) in case of fenugreek increased total alkaloid content

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by 34.9%, seed yield by 125.4% and trigonelline content by 17.8% [99]. Hossain et al. investigated effect of foliar application on radiation-processed chitosan as plant growth promoter and anti-fungal agent on tea plants. The results showed increase in productivity (about 38% based on fresh weight of tea leaves) and reduced total fungal count dramatically (> 100 times in contrast with the control). Study demonstrated that radiation-processed chitosan has positive impact on tea plants in terms of productivity and anti-fungal activity [100]. Chitosan degraded to optimum extent (dose 25 kGy) has been reported to show antioxidant activity coefficient 27 times higher activity than the non-irradiated chitosan. However, higher dose did not further improve the antioxidant activity [101].

3.2.4

Radiation-Induced Degradation of Sodium Alginate

Alginate is a natural polysaccharide from brown sea weeds with widespread applications in food, beverage, pharmaceutical and bioengineering industry. Chemically, it is copolymer composed of β-d-mannuronate (M) and α-l-guluronate (G) residues organized into blocks of homopolymeric segments of MM or GG as shown in Fig. 15. Its modified forms as grafted matrix, degraded alginate or cross-linked alginate are widely used for industrial applications. The alginate oligomers of tetramer to hexamer have particularly have shown special growth promotion effects in plants. Alginates undergo enzymatic and chemical hydrolysis to product of smaller chain length. Radiation-induced degradation of alginate in solution form causes darkening of solution which has been attributed to generation of conjugation on irradiation. The G-value of degradation in water is about 30 times more than that in solid form [102]. Radiation-induced degradation of alginate is enhanced in the presence of additives like potassium persulfate. Appropriate quantity of potassium persulfate has shown to decrease the dose requirement for degradation to same extent to 1/5th of initial

Fig. 15 Structure of alginate

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dose. The degraded alginate shows enhanced plant growth and seed yield in many crops [103].

3.2.5

Radiation-Induced Degradation of Carrageenan

Carrageenan belongs to the family of polysaccharides, found in many species of red algae. They are mixtures of water-soluble, linear, sulfated galactans. The main types of carrageenan are kappa (k-), iota (i-) and lambda (λ-) classified according to the number and position of sulfate groups. Oligomers of carrageenins have been of interest due to their promising antiherpetic, anti-HIV (human immune deficiency virus) activities and as anti-infectants. Irradiation causes maximum fragmentation of pendant groups in λ-carrageenan while κ-carrageenan undergoes maximum mainchain degradation [104].

3.2.6

Radiation-Induced Degradation of Starch

Starch is mainly composed of two types of homopolymer chains known as amylose and amylopectin. Amylose possesses a linear structure with α-(1-4) glycosidic linkage, while amylopectin possesses a branched structure with α-(1-4) as well as α-(1-6) glycosidic linkages as shown in Fig. 16. The proportion varies from 10 to 20% amylose and 80–90% depending on the source. The presence of hydroxyl imparts hydrophilicity to starch. The presence of water (0.12–0.14 weight fraction) decreases T g of starch to 60–80 °C and changes internal interaction and morphology of starch. Starch in thick paste form acts as thermoplastic which can be injection molded successfully [105]. The physicochemical, rheological, pasting, textural and morphological properties of starch (control or radiation degraded radiation) are hugely influenced by botanical source of starch. Starches of all origin undergo degradation on gamma or EB irradiation. The degradation is more prominent in solution form. Amylose content, gelatinization consistency, swelling, viscosity and textural parameters decrease in corn, potato, wheat and cow pea starch pastes on irradiation. Nevertheless, the decrease of amylose content and swelling is more in potato starch than in wheat starch. Also, the potato starch is more sensitive to shearing. An interesting revelation about corn starch degradation is that EB degradation results in degraded product of narrow molecular weight distribution [106–108]. Synthetic polymers are widely used as packaging material because of their durability and low cost. However, not only their disposal has emerged as a challenge for solid waste management, but also they negatively affect the quality of natural resources like water and soil. Blends of synthetic and natural polymers have gained considerable interest as nature-friendly alternative for synthetic packaging material. It has been observed that these partially biodegradable blends are cost effective, have good mechanical properties and display desired biodegradation behavior [109]. Starch has good potential as blending material for designing partially biodegradable

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Fig. 16 Structure of starch

blends; because of its low cost and easy availability, it is easily metabolized by a wide range of microorganisms and can be processed as a thermoplastic material after suitable plasticization. Thermoplastic starch (TPS) has been blended with host of synthetic polymers particularly low-density polyethylene (LDPE) for inducing biodegradation in LDPE. Major drawback of these blends has been deterioration in mechanical properties due to poor interfacial compatibility which to some extent has been overcome by using radiation for blend modification [110–112]. Dubey et al. degraded potato starch through gamma irradiation, and the starch degraded to desired extent was plasticized using glycerol–water mixture (RTPS). The RTPS was suitably blended with LDPE. Figure 17 shows effect of irradiation on molecular weight and melt flow index (MFI) of LDPE–RTPS blend. MFI of LDPE– RTPS blend was higher than that of LDPE–TPS blend, and it showed a sustained increase with increase in the radiation dose in the dose range studied, indicating that LDPE was better mixed with RTPS than TPS. Figure 18 shows the torque values derived from Brabender profiles at 30 rpm, 100 °C after 10 min of mixing for RTPS prepared from starch irradiated to different doses and for blending of LDPE and RTPS. Figure clearly shows that with increase in absorbed radiation, dose torque reduces substantially for TPS which was attributed to the lowering in the melt viscosity of TPS due to radiolytic degradation of starch. Nevertheless, the reduction in torque for LDPE–RTPS was not substantial. Figure 19 shows water uptake of virgin and biodegraded (kept for soil burial for 3 months) LDPE/TPS blends. An interesting

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observation was that water uptake by the biodegraded blends was in the range 7–4% even after 70–50% removal of starch, i.e., it was not in linear proportion to residual starch in the biodegradable blends. Disproportionate water uptake was assigned to formation of micro-voids and capillaries in the biodegraded blend matrix, which may eventually lead to an increase in the oxidative degradation of LDPE matrix. The use of RTPS leads to improved process ability of blends and makes the processes less energy intensive. Biodegradation of LDPE/RTPS was inhibited with increase in the dose imparted to starch, which was attributed to poor connectivity of starch domain in LDPE phase as well as to the increased crystallinity of the LDPE domain when RTPS is used [113].

4 Future Scenario Radiation is an effective tool to degrade polymers at room temperature. Using radiation for polymer degradation provides better control over degradation while avoiding the harmful thermal degradation effects. Most of the polymers undergo faster degradation in the presence of oxygen and water. The degraded synthetic polymers can be judiciously used as filler for various industrial applications for bringing down the cost of product without compromising on desired properties. Oligomers of natural polymers find application as plant growth promoter, in healthcare and food industry. Reuse of degraded polymer or used polymer with modification for useful application opens a new window to address the menace of polymer waste management. With further understanding of radiolytic degradation of other polymers, it can be optimistically said that in future, better ways and methods will be designed to use degraded polymers. Degradation of polymers would not be looked as negative but a

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phenomenon which essentially occurs in case of polymers but which can be better controlled as per needs.

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Glossary CMC carboxymethyl cellulose T c ceiling temperature CellH cellulose CSNP chitosan nanoparticles Da Dalton EB electron beam EVA ethylene vinyl acetate T g glass transition temperature kGy kilo Gray LDPE low-density polyethylene MFI melt flow index T m melting point MMT million metric ton PBAT poly(butylene adipate-co-terephthalate) PCL poly(caprolactone) PDO polydioxanone PE polyethylene PET poly(ethylene terephthalate) PGA poly(glycolic acid) PLA poly(lactic acid) PMMA poly(methyl methacrylate) PP poly(propylene) PTFE poly(tetrafluoroethylene) PTFEMP PTFE microparticles TPS thermoplastic starch ZnO zinc oxide

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Chapter 13

Electron Beam Radiation-Assisted Preparation and Modification of Thermoplastic Elastomer Blends K. Rajkumar, M. S. Banerji, P. K. Das, and Santosh Jagadale

1 Introduction A polymer blend is a mixture of two or more polymers together. Generally, blending aims to create a new material with different physical properties. In other words, the properties of the blends can be developed according to their end-use by the correct selection of the component polymers [1]. Elastomers are a class of polymeric materials with flexible nature and low intermolecular forces that allow them to have high degrees of elongation. The cross-links within the elastomers provide the ability to regain the original form after the applied stress, that is, either dynamic or static, is removed. In addition, elastomeric materials have very high toughness values; in other words, they can absorb very high energies under stress. Elastomers, in general, have other many superior properties. Examples of elastomers include natural rubbers, styrene-butadiene block copolymers, polyisoprene, polybutadiene, ethylene-propylene rubber, ethylene-propylene-diene rubber, silicone elastomers, fluoroelastomers, polyurethane elastomers, and nitrile rubbers. A thermoplastic is any plastic material that melts into a soft, pliable, and moldable form above a certain temperature and solidifies upon cooling. Examples of thermoplastics include polypropylene, polystyrene, nylon, acrylic, polyester, and Teflon. Thermoplastic elastomers (TPE), also referred to as thermoplastic rubbers, are a class of K. Rajkumar (B) · M. S. Banerji · P. K. Das · S. Jagadale Indian Rubber Manufacturers Research Association, (IRMRA), 254/1B, Rd Number 16U, Neheru Nagar, Wagle Industrial Estate, Thane West, Thane, Maharashtra 400604, India e-mail: [email protected] M. S. Banerji e-mail: [email protected] P. K. Das e-mail: [email protected] S. Jagadale e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. R. Chowdhury (ed.), Applications of High Energy Radiations, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-9048-9_13

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copolymers or a physical mix of polymers (usually plastic and rubber) and have properties of both plastic and elastomeric. Thermoplastic elastomers show advantages typical of both rubbery materials and plastic materials. The benefit of using thermoplastic elastomers is the ability to stretch to moderate elongations and return to their near original shape creating a longer life and better physical range than other materials. The principal difference between thermoset elastomers and thermoplastic elastomers is the type of cross-linking bond in their structures. Cross-linking is a critical structural factor that imparts high elastic properties. Thermoplastic elastomers (TPEs) are gaining importance in recent years because they offer several practical advantages over thermoset rubbers. They behave like vulcanized rubber but can be rapidly processed and fabricated like thermoplastics. Radiation energy is one of the most abundant forms of energy available in nature. Some natural substances generate radiation which can be destructive to life, but when harnessed, it can provide other forms of energy or can be used in a lot of industrial, medicines, and scientific applications. These radioactive substances produce ionizing radiation by radioactive decay and can be used for the betterment of society. There are four major types of radiation, i.e., alpha, beta, neutrons, and electromagnetic such gamma rays, ultraviolet (UV), infrared (IR), X-ray, and electron beam (EB). They differ in mass, energy, and how deeply they penetrate objects. It can therefore be expected that radiation processing, especially in the field of polymeric materials, will play an ever-increasing and important role in the industry. In today’s world, where the industry is more energy-conscious, the utilization of radiation to enable different processes is an extremely efficient way. With the continuing rapid advances in radiation processing technology, many novel products can be expected in future. These radiation-induced reactions are responsible for many useful applications in industry. While using radiation technology in the polymer field, there is no need for chemical catalysts or toxic chemicals, or extreme physical conditions like high temperature and pressure, and can be carried out at ambient temperature. Further, irradiated materials do not themselves become radioactive, output rates are very high, the treatment cost is competitive with the conventional chemical processes, and irradiated materials are usable immediately after processing and thus offer an alternative technique for developing new polymeric products. High-energy electron beam radiation is being used for many years for curing polymers and polymer blends.

2 Literature Review The effect of radiation on polymer materials is an area of rapidly increasing interest. Many industries need specialty polymers that exhibit a specific response upon exposure to radiation and need specialty technical applications [2–4]. Free radicals and ions are produced when the polymer is exposed to ionizing radiation [5]. Ionizing radiation of polymeric material leads to the formation of very reactive intermediates, free radicals, ions, and excited states. These intermediates can follow several reaction paths, which result in disproportion, hydrogen-abstraction, arrangements,

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and/or the formation of new bonds. The ultimate effects of these reactions can be the formation of oxidized products, grafts, scission of main chains (degradation), or cross-linking. In comparison with conventional methods, radiation processing is an energy-conserving and environmentally friendly technology. The number of electron accelerators in industrial use has been steadily increasing although radiation-based technologies face scrutiny by society and regulators [6]. Radiation processing technologies include radiation vulcanization of rubber latex, polymer recycling, production of heat-shrinkable polymers and fiber-reinforced composites, development of ion track membrane, and microdevice production [5–12]. The degree of cross-linking is proportional to the radiation dose. The dose rate of the reaction can be varied widely, and thus, the reaction can be better controlled. Variation of the degree of cross-linking within a product can be achieved in the case of the electron beam by a suitable choice of the electron energy, by multiple irradiations with different electron energies, and/or by using metallic masks, which absorb the electrons or reduce their energy before they reach the product surface. Hence, according to Charlesby [13], it is possible to manufacture products with different material properties from the same starting material. Thus, the unique advantage of radiation cross-linking is that cross-linked structures with improved mechanical, and chemical properties can be prepared from thermoplastics and elastomers without any chemical agent and heat. Czvikovszky [14] shows that injection moldable composites of short fibers and thermoplastics can be greatly improved through ionizing radiation. The interface is an important factor, especially in the case of injection moldable composites of short fibers and thermoplastics. Cook [15] and Cook et al. [16] say one of the largest applications of EB cross-linking is in the manufacture of heat-shrinkable products because of their higher and uniform degree of cross-linking, ease of handling, and excellent heatshrink/expansion properties and fast process with minimum scrap. RAYCHEM has pioneered this technology and is one of the largest users of electron beam cross-linking. Saunders et al. [17] and Berejka and Eberle [18] show the importance of EB curing of polymer matrix composites (PMC) and its progress as an emerging technology for the manufacture of a variety of structures and components for aerospace and automotive applications. Saunders et al. [19, 20] also suggest that EB curing of PMC is a considerably more energy-efficient process compared to thermal curing and initiatives taken by the aerospace industry for developing EB technology to make specific products like cryogenic fuel tanks, improved canopy frames for jet aircraft, and all-composite military aircraft. Spenadel [21] observed that low-density polyethylene (LDPE) cross-linking on irradiation with 3 MeV electrons in the air was enhanced by the addition of EPR (elastomeric copolymer of ethylene and propylene), EPDM (elastomeric terpolymer of ethylene, propylene, and a non-conjugated diene), TMPTMA (trimethylolpropane trimethylacrylate), and carbon black. This blend gives better corona resistance, reduced voids, more flexibility, and overall suitability for use in automobile fascia.

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The usefulness of cross-linked PE is well known but requires special treatment to heat-seal or fuse. Berejka [22] reported in a patent that on irradiation, blends of HDPE with conjugated diene butyl (CDB) polymers were cross-linked better and could be easily heat-sealed. Although HDPE required a 150 kGy dose, the blends required only 20–50 kGy dose for the same level of cross-linking. Bohm and Nelson [23] claimed in a patent that the morphology of heterogeneous blends containing radiation cross-linkable elastomers can be stabilized by irradiation to the gel dose level. The irradiated blends with the stabilized morphology can then be conventionally processed, including conventional curing. These blends include NR/EPDM, SB/EPDM, and BR/EPDM. This technique of stabilizing morphology has been a key to the production of modern tires. Stabilization of the blend morphology is important for the performance of PP/elastomer blends. Electron beam irradiation of PP/EPDM blends in air resulted in cross-linking of EPDM, degradation of PP, and cross-linking between the two phases [6, 24] which contributed to the stabilization of the blend morphology. In Japan, more than 92% of radial tires for passenger cars and more than 70% of truck tires are manufactured using this technology [25]. The tire produced by this technique is lighter than the tire produced using traditional technology [26]. Coran and Patel [27] patented a process for electron irradiation, of PE (10–75 wt%) blends with polychloroprene, CR (90–25 wt%). Irradiation improved such properties as tensile strength, stress at break, tension set, toughness, solvent resistance, and high-temperature behavior. The improvements were attributed to cross-linking of both PE and CR. The blends were useful for making such products as tires, hoses, belts, gaskets, and moldings. Coran and Patel [27] suggested that the effects would be similar for electron, gamma, and X-ray irradiation. Akhtar et al. [28] investigated gamma irradiation of HDPE/NR blends. The irradiated HDPE/NR blends (30–70 wt% HDPE) showed a broad minimum in tensile strength vs dose, at 100–250 kGy. According to the authors [28], irradiation had two effects on the mechanical properties of the blends: (i) It produced cross-links between carbon atoms, which increased the stiffness; and (ii) it reduced crystallinity. Ahmad and co-researchers [29] have examined the mechanical properties and physical properties of the natural rubber/low linear density polyethylene (NR/LLDPE) blends cross-linked by electron beam (EB) irradiation. They have performed the study on 60/40 NR/LLDPE blends with various doses of EB irradiation. The results showed that the tensile strength of the NR/LLDPE blend increased up to 250 kGy and started to decrease with further dosage to 300 kGy [8]. It was concluded that a radiation dose of 200 kGy was needed to achieve optimum tensile strength. Zurina and co-researchers [30] have studied the effect of EB irradiation on tensile, dynamic mechanical properties, thermal properties, and morphology of ENR-50, EVA, and ENR-50/EVA blend was investigated. All the samples were irradiated using a 3.0 MeV electron beam (EB) machine with doses ranging from 20 to 100 kGy. Results indicate that the gel fraction of ENR-50, EVA, and ENR-50/EVA blend increases with irradiation dose. Concerning tensile properties, it can be seen that EB

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radiation increases the tensile strength of all the samples, increases the elongation at break of ENR-50 and ENR-50/EVA blend, reduces the elongation at break of EVA, increases M200 (modulus at 200% strain) of ENR-50 and EVA, while decreases M200 of the ENR-50/EVA blend. For dynamic mechanical studies, it was found that EB radiation increases the Tg of all the samples due to the effect of irradiationinduced cross-linking. The compatibility of the ENR-50/EVA blend was also found to be improving upon irradiation. In the case of thermal properties, it was detected that Tm, Tc, and the degree of crystallinity of ENR-50/EVA blend increase with an increase in irradiation dose. This was due to the perfection in the crystal growth occurring upon radiation. Morphology changes play a major role in the changes in the properties. Das et al. [31] show that HNBR can be cross-linked by an electron beam at a low radiation dose, and irradiation has a significant influence on the storage modulus (E ' ) of HNBR. The increase in E ' is believed to be due to irradiation-induced crosslinking. FTIR spectra show that double bonds, carbonyl groups, and ether linkages form upon the electron beam irradiation of HNBR. Also, its shows [32] that it is possible to prepare high strength heat- and oilresistant thermoplastic elastomers based on 50/50 nylon 6 and HNBR blends through melt blending in an internal mixer and subsequent electron beam irradiation of extruded pellets. The morphology of the blends indicates a two-phase structure in which HNBR is dispersed in the continuous nylon phase. Injection molding of pellets, irradiated at low doses (below 8 Mrad), improves blend morphology resulting in good mechanical properties. The blends show good heat and solvent resistance and excellent resistance to oil swelling at elevated temperatures. The addition of black filler can alter the mechanical properties of the blends. Significant improvements in elastic recovery of the blends were observed with the incorporation of 10 phr SRF black. Das et al. [33] also studied blend of 30:70 and 70:30 Nylon6: HNBR. Blends were prepared through melt blending in an internal mixer, and subsequent electron beam irradiation of extruded pellets was studied. SEM studies of blends show that morphology induced during mixing and extrusion is altered and improved by injection molding as a consequence of different shear stresses. Excessive cross-linking of the HNBR phase of the pelletized blends adversely affects the blend morphology during subsequent injection molding, leading to inferior mechanical properties at higher radiation doses. Blends rich in nylon show excellent hot oil and solvent resistance. Significant improvement in elastic recovery and heat resistance was observed for blends with high rubber content. It was found that the blends rich in nylon had superior mechanical properties, hot oil, and solvent resistance, whereas blends with higher HNBR content had better set and heat resistance. The effect of radiation on interaction in these blends was also evaluated and was found to induce possible inter-chain cross-linking in the blends. The gel fraction, mechanical and dynamic mechanical properties, and reprocessability of EVA–LDPE blends were investigated by Chattopadhyay et al. [34] and concluded that the gel fraction of the films increases with an increase in the irradiation dose, monomer level, and EVA content of the blend. The tensile strength and modulus increase, while the elongation at break initially increases followed by a

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decrease with an increase in the radiation dose at a constant monomer level and blend ratio. The hysteresis loss ratio and permanent set decrease with an increase in the radiation dose due to an increase in the gel fraction. Radiation-induced cross-linking improves the interfacial adhesion of the PE–EVA blends. Theoretical calculations reveal that the blends up to PEVA46100 have predominantly PE as the continuous phase. The modulus increases, but the elongation at break decreases with an increase in the DTMPTA content of the 50:50 blend at a constant radiation dose, while the tensile strength increases steadily up to a 3 wt% DTMPTA level followed by a decrease. The hysteresis loss ratio and permanent set decrease with the monomer level due to an increase in the gel fraction. The tensile strength and modulus of the blends increase with the increase in the PE content of the blends, while the elongation at break decreases. The hysteresis loss ratio and permanent set decrease with an increase in the EVA content of the blends. The dynamic mechanical spectra of the blends indicate both their immiscibility and two-phase structure. The changes in cross-linking and molecular architecture are reflected in the dynamic mechanical properties, namely the tan delta and the storage modulus (E ' ) due to the variation of irradiation doses, DTMPTA levels, and blend ratios. Also, they observed from the reprocessability studies that the blends irradiated at 50 kGy and below can be reprocessed. The decrease in properties after the processing cycles is probably due to the change in molecular architecture at a high temperature of 150 °C. Although the processability of the compositions containing a greater amount of PE is better, they suffer from a poor permanent set. Chattopadhyay et al. [35] also studied the heat shrinkability of electron beamirradiated thermoplastic elastomeric films from blends of ethylene-vinylacetate copolymer (EVA) and low-density polyethylene (LDPE). They concluded that the influence of temperature, time, and extent of stretching and shrinkage temperature and time on percent heat shrinkage and amnesia rating have been evaluated, and these testing parameters have been optimized. The results are explained based on elastic recovery and crystallinity. Percent heat shrinkage and amnesia rating of the films are found to decrease as radiation dose increases progressively for a given blend ratio without any level of the sensitizer. For a constant radiation dose and blend ratio, percent heat shrinkage is found to decrease with the increase in DTMPTA level in the blend. At a fixed monomer level and radiation dose, percent heat shrinkage and amnesia rating decrease with the increase in EVA proportion in the blend. The crystallinity is decreased on stretching (40%) due to the destruction of the original ordered structure. It is further decreased to a considerable extent on shrinkage due to melting of crystallites and recoiling of chains. Heat shrinkage increases with the increase in percent crystallinity, but the amnesia rating follows the reverse trend. Heat shrinkage decreases with the increase in gel fraction of the blend. Also, films were prepared [36] from a blend of low-density polyethylene (LDPE) and ethylene–vinyl acetate (EVA) containing 45% VA and ditrimethylol propane tetraacrylate (DTMPTA). Electron beam initiated cross-linking of these films was carried out over a range of radiation doses (20–500 kGy), concentrations of DTMPTA

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(1–5 parts by weight), and blend compositions. EVA-LDPE films were cross-linked to a varying degree by irradiation and DTMPTA and were characterized by IR spectroscopy, gel fraction, XRD, DSC, and morphology. They concluded that oxidation and cross-linking dominate up to a 100-kGy irradiation dose, after which the scission and disproportionation become the major mechanism in the 50:50 PE and EVA blends without DTMPTA. At a constant irradiation dose, the deacetylation is suppressed due to the addition of DTMPTA in the blend. After 3 wt% of DTMPTA chain scission becomes the progressively dominating process. The gel fraction of the films increases with the increase in irradiation dose, DTMPTA level, and EVA content of the blend. The sol fraction of the control and the blend with and without DTMPTA fit into the modified type of Charlesby–Pinner equation. The XRD and DSC studies reveal that the crystalline portion of the blends is affected by irradiation only at higher radiation doses (200 kGy and above). SEM studies show that the PE forms the continuous matrix in the 50:50 blend; however, a co-continuous structure is observed in the 30:70 blend of PE and EVA. The morphology does not change after irradiation. However, the addition of 3 wt% DTMPTA causes the blend to form a co-continuous morphology. Ryszard et al. [37] studied isotactic polypropylene (PP) by blending it in an extruder with 0–50% addition of styrene-ethylene/butylene–styrene (SEBS) and styrene–butadiene–styrene (SBS) block copolymers. Granulated blends were irradiated with an electron beam (60 kGy) followed by processing (after 1 week) with an injection molding machine. Properties of samples molded from irradiated and non-irradiated granulate were investigated using DSC, WAXS, MFR, SEM, and mechanical and solubility tests. Based on the results of various measurements, it was shown that irradiation with an electron beam leads to considerable changes in the behavior and properties of PPSEBS and PP-SBS blends. The elastomer type, and especially the chemical structure of its elastic block connected with the presence of double bonds, plays a dominant role in the susceptibility of the system to radiation modification. It was shown that SEBS copolymer and its blends with PP are generally more resistant to the action of the electron beam, i.e., the property changes in this system are generally smaller in comparison with the SBS-based blends. However, irradiation improves commonly the ultimate properties of blends. It was already mentioned that the specimens used for property determinations were molded from irradiated (or non-irradiated) granulate 1 week after irradiation. Significant structure and property changes of irradiated blends, which probably take place during melting and homogenization in the molding process 1 week after irradiation, testify to the large persistence of macroradicals created in irradiated granulate. This observation is also consistent with the literature data. It was found [38–40] that various low molecular radiolysis products are trapped in polymers, and they can be activated during processing in the molten state. Concluding, it seems that the radiation modification of PP-SEBS and PP-SBS blends can be a useful tool for improving their properties. Chattopadhyay et al. [41] explained the effects of crystallinity, chain branching, and polarity of rubber and plastics on the mechanical and dynamic mechanical properties, morphology, and reprocessability of electron beam modified thermoplastic

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elastomeric films. The elastomeric components have been varied, e.g., metallocenebased polyolefins (engage of different grades) and EVA with varying amounts of vinyl acetate content. Engage, polyolefin elastomers (POEs), is ethylene octene copolymers produced by metallocene catalyst (stereospecific) having excellent controlled properties and rheology. By controlling the molecular architecture, engage provides an ideal balance of predictable and consistent performance in both processing and critical properties. EVA, on the other hand, is a random copolymer of ethylene and vinyl acetate, having excellent flexibility and stress corrosion crack-resistant performances [42]. This is the first report on the thermoplastic elastomers made from blends of engage. New thermoplastic elastomeric blends from metallocene-based polyolefins and EVA copolymers of different grades and LDPE have been prepared by electron beam modification and concluded that the extent of interaction between the elastomeric and the plastic phases increases with the decrease in chain branching and increase in crystallinity of the polyolefinic elastomers. On the other hand, LDPE and EVA are comparatively incompatible, and the incompatibility increases with the increase in vinyl acetate content (and polarity) of the blends. SEM studies indicated that in all cases, LDPE forms the continuous matrix due to its high MFI and low melt viscosity at high temperatures. This is further corroborated by the AFM investigation. Electron beam cross-linking is limited to the amorphous portion of the blends. Degree of cross-linking increases with an increase in amorphous nature and chain branching of the elastomeric phases. Significant improvements in mechanical and dynamic mechanical properties have been obtained due to electron beam irradiation in all the blends retaining its reprocessability characteristics. DTMPTA affects deleteriously the mechanical and dynamic mechanical properties probably by weakening the interface in the case of the blends of engage elastomers with LDPE. EVA being polar can take up DTMPTA easily during mixing, (polar–polar interaction), and hence, a further improvement in mechanical and dynamic mechanical properties has been obtained after irradiating the EVA-based blends.

2.1 EPDM-PP and EPDM–PE Blend System The preparation of thermoplastic elastomer composition by dynamic vulcanization in an internal mixer has been discussed by Gessler and Haslett [43]. This process was later used by Fisher [44] to prepare thermoplastic elastomers from blends of polypropylene (PP) and ethylene-propylene-diene elastomer (EPDM) by partially vulcanizing the elastomer phase. An organic peroxide was used to cross-link the elastomer in the presence of a thermoplastic PP. It was observed that the peroxide has a detrimental effect on thermoplastic PP. This discovery was further advanced by Coran and Patel [45–47], and Abdou-Sabet and Fath [48], through the use of preferred curatives, other than peroxides, to achieve improvement in properties which led to the successful commercialization of dynamic vulcanization technology.

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The dispersion of cured elastomer particles in the continuous thermoplastic phase is a key factor to determine the performance of the TPE. As the size of these particles decreases, the mechanical properties of the TPE increase. The commercial grades of EPDM-PP thermoplastic elastomer contain EPDM particles in the one to twomicrometer range. The degree of cross-linking of the soft elastomer phase is the other key to TPE performances. A highly cross-linked elastomeric phase gives exceptionally good properties, and a TPE should have a fully cured EPDM phase. With the increase in cross-linking density, the tensile strength, modulus, set properties, and fatigue resistance greatly improve. Santoprene is a commercial name of a class of TPEs made from EPDM/PP blends through dynamic vulcanization. Santoprene finished parts are gradually replacing EPDM vulcanizates in many applications. They are available in a broad hardness range, starting from 60shore A to shore D. The technological importance of these TPEs has led to the publication of several research papers over the last three decades on morphological development, morphological type, rheological behavior, and viscoelastic properties of these materials. Coran et al. [49] have reported morphological features and mechanical properties of TPEs based on EPDM/PP. Goharpey et al. [50, 51] studied the morphology development during dynamic vulcanization for EPDM/PP blends in an internal mixer and demonstrated that during dynamic vulcanization cured rubber particles of the agglomerate network in the PP matrix, which is governed by several parameters including blend composition, viscosity ratio, shear forces, and interfacial interaction between two phases. The twin-screw extruder can be used as an alternative to the internal mixer for the preparation of TPEs from polymer blends by dynamic vulcanization. Sararoudi et al. [52] have demonstrated that it is possible to produce a dynamically vulcanized thermoplastic elastomer from EPDM/PP blends in a twin-screw extruder, with properties similar to those TPEs prepared in an internal mixer. Coran et al. [46] have also described the development of TPEs by dynamic vulcanization of blends of EPDM and PE. The result indicates that EPDM–PE blends are also greatly improved by dynamic vulcanization, though the best effect of dynamic vulcanization is observed for EPDM/PP system. It is already established that both cross-linking and chain scission occur simultaneously when polymers are exposed to irradiation [5, 53]. The overall properties of the irradiated polymer ultimately depend on the process which predominates during the irradiation condition, and the same is mainly governed by several factors like the chemical nature of the polymer, irradiation condition like inert or oxidative, irradiation temperature, etc. The mechanism of cross-linking/chain scission is either free–radical or ionic. Electron beam curing of polymers is a relatively new technique and is gaining commercial importance in recent times. It has certain advantages over conventional curing processes such as it is quick, chemical-free, performed under ambient temperature, and can be controlled very precisely. A wide range of dose rates on the order

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of 100–10,000 rad/s, an increased penetration depth with accelerating voltages of up to 10 MeV coupled with the high-energy efficiency of radiation, available from high-power electron beam accelerators, has made electron beam radiation superior for polymer processing over other forms of ionizing radiation available like X-rays, gamma rays, etc. [54]. Many publications have been devoted to the action of the electron beam on polymers [55–60]. The effects of the electron beam on polymers also have been reviewed [61]. Electron beam radiation can be used to cure polymers very selectively by varying the radiation dose. The curing of polymers by electron beam forms carbon–carbon bonds between polymer chains. This process is now in use for curing heat-shrinkable tubes, tire components, wire, and cable. Rubbers like ethylene-propylene-diene terpolymer (EPDM) undergo curing and thermoplastics like polypropylene (PP), degrade when exposed to electron beam radiation in the air. Another thermoplastic, PE mainly undergoes cross-linking in presence of irradiation [62]. The presence of a small number of multifunctional monomers like trimethylolpropane trimethyacrylate (TMPTMA) promotes curing by radiation even at low doses. These monomers are efficient because they produce high yields of radicals during irradiation [60]. One disadvantage of TPEs made from EPDM/PP blends by dynamic vulcanization is that the presence of a cross-linked rubber phase makes these thermoplastic elastomers sometimes difficult to process. This processing problem has been reduced by using processing oils, processing aids, or by controlling the amount of cure. All these conventional approaches can deleteriously impact the mechanical elastomers that have improved processing characteristics with good mechanical properties. When a blend of EPDM and PP is irradiated by an electron beam, the mechanical properties of the blend are enhanced due to the curing of the EPDM component, and the processing behavior of the blend is improved as a result of chain scission in the PP component. Gisbergen et al. [63] have reported the improvement in the processability of PP/EPDM blends (EPDM up to 33%) by electron beam irradiation. The authors have observed that electron beam irradiation can be used for morphology fixation for rubber toughened PP with EPDM. Zaharescu et al. [64] have reported the gel formation in EPDM/PP blends by the action of gamma irradiation. The effects of electron beam irradiation and EVA (ethylene–vinyl acetate copolymer) content, on the radiation cross-linking, in LDPE/EVA bends have been studied by Mateev and Karagegeogiev [65]. Chattopadhya et al. [34] have reported the development of TPE from PE/EVA blends by electron beam irradiation. In this work, we report the development of TPE from EPDM/PP and EPDM/PE blends by the action of electron beam radiation. The effect of cross-link promoter like TMPTMA has also been studied.

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2.2 NBR–PP and NBR–HDPE Blend System Coran and Patel [66–68] have described the development of oil-resistant thermoplastic elastomers, based on NBR and PP by dynamic vulcanization. Since these two polymers are grossly incompatible with each other due to large differences in polarity, the use of a small amount of block copolymer, containing both polar and non-polar segments, as compatibilizer produces thermoplastic elastomers with much improved mechanical properties. The block copolymer acts as a macromolecular surfactant, which ensures better wetting between two polymers. The authors have stated several ways for achieving suitable block copolymers for NBR-PP blends such as PP functionalized with malic anhydride reacts with amineterminated liquid NBR; PP modified with dimethylol phenolic resin and the react with amine-terminated liquid NBR; maleic modified PP reacts with carboxy-NBR. Zhang et al. [69] reported the preparation of TPE of NBR and PP with excellent mechanical properties by using a new compatibilizer-glycidyl methacrylate grafted PP/amino compound. Geolast is a trading name for a TPE made from blends of dynamically vulcanized NBR and PP by melt–mixing technique. It is supplied in hardness ranging from 80 Shore A to 50 Shore D. It has the potential to replace NBR vulcanizates in many applications. Thermoplastic elastomers based on NBR and polyethylene (PE) by dynamic vulcanization were reported by Coran et al. [70]. The authors described the selection of polymer combinations for technologically useful thermoplastic elastomers. Thermoplastic elastomer compositions with good integrity were obtained when the elastomer, and the thermoplastic had similar surface energy. George et al. [71] have reported the development of thermoplastic elastomers from high-density polyethylene (HDPE) and NBR by melt blending technique. Since the system is incompatible, the authors have functionalized PP to improve chemical interaction between the blend partners. So far, no work has been reported on the effects of electron beam irradiation on NBR/PP and NBR/HDPE blends. The present work aims to prepare thermoplastic elastomer compositions, from blends of NBR/PP and NBR/HDPE with different blend ratios by electron beam irradiation. The present investigations also report the acceleration of cross-linking in these blends by the addition of TMPTMA.

3 TPEs Prepared by Dynamic Vulcanization Polymer blends in general are prepared by melt mixing, solution blending, or latex mixing techniques. Melt mixing avoids problems of contamination, solvent, and water removal and is sometimes preferred over other techniques. Dynamic vulcanization is the process of vulcanizing elastomer during its melt mixing with a thermoplastic. Blends with improved properties can be produced by partial or complete vulcanization of the elastomer phase.

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Electron beam radiation of blends can provide an interesting route for the preparation of thermoplastic elastomeric compositions having a broad range of properties for different areas of applications. High-energy radiation like electron beam can be used as an alternative to conventional dynamic vulcanization to prepare thermoplastic elastomers. In such cases, there is a possibility of the formation of linkages between molecules of the two polymers, which could reduce the interfacial tension and provide a material with improved ultimate properties. The presence of a small number of multifunctional monomers like TMPTMA, known as curing promoters, provides sufficient curing at a low radiation dose. The objective of this chapter is to put the highlight on development of thermoplastic elastomer from blends of thermoplastic and elastomer using electron beam irradiation in following three major areas of application: 1. Heat resistant (moderate to high) 2. Moderate heat and oil resistant.

3.1 Heat Resistant TPE Blends of EPDM-PP and EPDM-PE at different blend ratios were considered for the development of heat-resistant TPE using electron beam irradiation. Due to the similar surface energies of blend partners, these blends are compatible and do not require a compatibilizer. EPDM rubber shows very good heat and ozone resistance properties and is used to make various rubber goods for heat resistant and outdoor applications. The absence of unsaturation in the polymer backbones of thermoplastics PE, PP, and EPDM elastomer makes the TPEs derived from them very resistant to degradation by heat, oxidation, or ozone attack. Due to their non-polar nature, these TPEs are highly resistant to water, aqueous solutions, and other polar fluids like alcohols and glycols and show good electrical properties.

3.2 Moderate Heat and Oil-Resistant TPE Nitrile rubber (NBR) has good oil-resistant and moderate heat resistance and is generally used in oil-resistant applications. Polyolefins like PP and high-density polyethylene (HDPE) show high melting temperature, good mechanical properties, and processability characteristics. So the TPEs developed from blends of NBR and these polyolefins (PP and HDPE) couple the properties of NBR and the respective polyolefins. Although the large difference in surface energy of NBR and these polyolefins makes their blends grossly incompatible, the use of suitable compatibilizer can help and improve the blend properties.

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4 Material 4.1 TPEs from EPDM/PP Blend EPDM rubber used was EP96 (ML (1 + 8) 120°, 53: 73% ethylene content, 50% paraffinic oil extended) supplied by JSR, Japan; two different grades of PP used in this study (one high MFI and other low MFI) were obtained from IPCL, Baroda, India, (Kolylene AM120N, density 905 kg/m3 , melt flow index 12 g/10 min. and RIL, Mumbai (Repol HO07, DENSITY 905 kg/m3 , melt flow index 0.7 g/10 min.); TMPTMA was obtained from Sartomer, USA.

4.2 TPEs from EPDM–PE Blend EPDM rubber used was EP96 (ML (1 + 8) 120°, 53: 73% ethylene content, 50% paraffinic oil extended) supplied by JSR, Japan; LDPE used in this study was obtained from IPCL, Baroda, India, Koylene AM120N, density 905 kg/m3 , melt flow index 12 g/10 min; TMPTMA was obtained from Sartomer, USA.

4.3 TPEs from NBR/HDPE Blends The materials used for this study include nitrile rubber (NBR 230SL) with ACN content 35 wt% from JSR Co. Japan; HDPE, Indothene HD 50 MA 180 from IPCL, Baroda, having a density of 0.950 g/cc and melt flow index value of 18 g/10 min. Trimethylolpropane trimethacrylate (TMPTMA) was a product of Sartomer, USA.

4.4 TPEs from NBR/PP The materials used for this study include nitrile rubber (NBR 230SL) with ACN content 30 wt% from JSR Co. Japan; PP, HO20 from IPCL, Baroda, having a density of 0.950 g/cc and melt flow index value of 2 g/10 min. Trimethylolpropane trimethacrylate (TMPTMA) was a product of Sartomer, USA.

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5 Sample Preparation and Irradiation 5.1 EPDM: PP Blend Sample Preparation The EPDM rubber, the thermoplastic PP, and the curing promoter TMPTMA were mixed in a Brabender Plasticorder, PL 2207 having a mixture of 80 cm3 volume, at 170° and 80 rpm rotor speed for 6–7 min. The rubber to plastic blend ratios covered were 80/20, 70/30, 60/40, 50/50, and 40/60. The mixes were then sheeted out through an open mill. These mixed materials were then compression molded into sheets of around 1 mm thickness at 180° for 2 min at 5 MPa pressure in an electrically heated press. The hot molds along with sample sheets inside were then cooled under compression to room temperature to maintain the dimensional stability of the sheets. The levels of TMPTMA used were 2, 4, 6, and 8 phr. Blending of some selected compositions containing EPDM, PP, and TMPTMA was carried out in a Brabender Plasticorder, PL 2207 having a mixture of 350cm3 volume, at 170° and 80 rpm rotor speed for 6–7 min for studying the injection molding behavior of the blend. It was remixed in a Brabender single–screw extruder equipped with a rod die (3 mm) having L/D 20 and operating at temperature settings ranging from 170 to 185° at 60 rpm. After extrusion, the blende samples were quenched in water and subsequently pelletized into 3–5 mm long strands.

5.1.1

Irradiation of EPDM: PP Blend

Both compressions molded sheets and pellets were subsequently irradiated in the air at room temperature by an electron beam accelerator (Model ILU-6) at Bhaba Atomic Research Centre (BARC), Mumbai, India cure the rubber phase. The rubber phase can be cross-linked by using varying dosages of radiation to obtain a broad range of physical properties; at the same time, it is necessary to ensure that the composition can be processed by conventional plastic processing techniques like injection molding. Irradiation doses of 2 to 20 Mrad were used. The dynamic irradiation technique was utilized at a conveyor speed of 0.9 m min−1 . The acceleration energy and beam current used were 1.8 MeV and 1 mA, respectively. A radiation dose of 1 Mrad was applied in a single pass. If the required dose was 4 Mrad, the sample was subjected to four passes. Irradiated pallets were then injection molded into sheets of around 1 mm thickness on an Engel Injection–Molding machine with nozzle and barrel temperature in the range of 170–190 °C. The mold was kept at 20 °C and a 4-s injection time plus a 20-s cooling time was used. These compressions molded and irradiated sheets and injection-molded sheets were used to measure physical properties.

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5.2 EPDM: PE Blend Sample Preparation The EPDM rubber, the thermoplastic LDPE, and the curing promoter TMPTMA were mixed in a Brabender Plasticorder, PL 2207 having a mixture of 80cm3 volume, at 130 °C and 80 rpm rotor speed for 6–7 min. The rubber to plastic blend ratios covered was 70/30, 50/50, and 30/70. The mixes were then sheeted out through an open mill. These mixed materials were then compression molded into sheets of around 1 mm thickness at 140 °C for 2 min at 5 Mpa pressure in an electrically heated press. The hot molds along with sample sheets inside were then cooled under compression to room temperature to maintain the dimensional stability of the sheets. The levels of TMPTMA used were 2, 4, 6, and 8 phr.

5.2.1

Irradiation of EPDM: PE Blends

The compression-molded sheets were subsequently irradiated in the air at room temperature by an electron beam accelerator (Model ILU-6) at Bhabha Atomic Research Centre (BARC), Mumbai, India to cure the rubber phase. Irradiation doses of 2 to 20 Mrad were used. The dynamic irradiation technique was utilized at a conveyor speed of 0.9 m min−1 . The acceleration energy and beam currently used 1.8 MeV and 1 Ma, respectively. The radiation dose of 1 Mrad was applied in a single pass. If the required dose was 4 Mrad, the sample was subjected to four passes.

5.3 NBR: HDPE Blend Sample Preparation Melt blending of HDPE with NBR and TMPTMA was carried out in a Brabender Plasticorder, PL 2207 having a mixture of 350 cm3 volume, at 130° and 80 rpm rotor speed for 6 min. The NBR to HDPE blend ratios covered were 60/40 and 50/50. The mixes were then sheeted out through an open mill. It was remixed in a Brabender single–screw extruder equipped with a rod die (3 mm) having L/D 20 and operating at temperature settings ranging from 130 to 160 °C at 40 rpm. After extrusion, the blended samples were quenched in water and subsequently palletizes into 2–3 mm long strands. The level of TMPTMA used in the blend was 4 phr. Some pellets were injection molded into sheets of around 1 mm thickness on an Engel injection –molding machine with nozzle and barrel temperature in the range of 140–160 °C. The mold was kept at 20 °C and a 4-s injection time plus a 20-s cooling time was used. Pure NBR was masticated in a Brabender Plastocorder, PL 2207 at 60 rpm rotor speed and 60 °C temperature for 3 min. The stock was then hot pressed at 5 MPa at 150 °C for 2 min. in an electrically heated press to obtain a smooth sheet of 1 mm thickness for irradiation.

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Irradiation of NBR: HDPE Blends

Both molded sheets and extruded pellets were irradiated in the air at room temperature by an electron beam accelerator (Model ILU-6) at Bhaba Atomic Research Centre (BARC), Mumbai, India. Irradiation doses of 2, 4, & 6 Mrad were used for blends. The irradiation process has already been discussed before. Irradiation pellets were then injection molded into sheets under the same conditions as un-irradiated pellets for the measurement of mechanical properties.

5.4 NBR: PP Blend Sample Preparation Melt blending of PP with NBR and TMPTMA was carried out in a Brabender Plasticorder, PL 2207 having a mixture of 350 cm3 volume, at 165 °C and 80 rpm rotor speed for 5 min. The NBR to PP blend ratios covered were 60/40 and 50/50. The level of TMPTMA used in the blend was 4 phr. The mixes were then sheeted out through an open mill. It was remixed in a Brabender single-crew extruder equipped with a rod die (3 mm) having L/D 20 and operating at temperature settings ranging from 170 to 185 °C at 40 rpm. After extrusion, the blended samples were quenched in water and subsequently palletized into 2–3 mm long strands.

5.4.1

Irradiation of NBR: PP Blends

The extruded pellets were irradiated in the air at room temperature by an electron beam accelerator (Model ILU-6) at Bhaba Atomic Research Centre (BARC), Mumbai, India. Irradiation doses of 2, 4, and 6 Mrad were used for blends. The irradiation process has already been discussed before. Irradiated pellets were then injection molded into sheets of around 1 mm thickness on an Engel injection—molding machine with nozzle and barrel temperature in the range of 175–190 °C. The mold was kept at 20 °C and a 4-s injection time plus a 20-s cooling time was used.

6 Measurement of Properties, Results, and Discussion for EPDM: PP Blend The following test methods/procedures were used for measuring/calculating various properties.

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6.1 Measurement of Properties 6.1.1

Mechanical Properties

Tensile strength and elongation at break were measured at 25 ± 2 °C according to the ASTM D412 test method using dumb-bell-shaped test specimens which were punched out from injection-molded (compression and injection) sheets. The tests were carried out in a Zwick Universal Testing Machine (model 1445) at a crosshead speed of 500 mm/min. The hardness of the samples was obtained by ASTM D2240 and expressed in shore D units. The tension set was measured according to ASTM D 412 using dumb-bell specimens (Die C). The samples were elongated up to 100% and kept at that position for 10 min, then relaxed for 10 min.

6.1.2

Gel Content

The gel content was determined by solvent extraction. The samples, approximately 0.15 gm in weight, were extracted in boiling xylene for 24 h, and the extracted samples were dried to constant weight. The gel content was calculated as Gel content (%) = (W 2/W 1) × 100 And the soluble fraction (s) is calculated as S = 1 − (W 2/W 1) where W 1 is the weight of the sample before extraction and W 2 is the weight of the dried sample after extraction.

6.1.3

Differential Scanning Calorimetry (DSC) Studies

Melting and crystallization of the irradiated blends were studied with the help pf Perkin-Elmer DSC-7 instrument. The sample was heated at the rate of 20 °C/min up to 200 °C, then cooled, and re-melted at the rates of 20 °C/min. The peak maximum from the melting thermogram was considered as the melting temperature (T m ) and the peak area as the heat of fusion.

6.1.4

Air Aging Test

Aging studies were carried out in an air circulated Aging Oven, at different temperatures and durations. The samples were kept at room temperature for 24 h after aging, and the tensile properties and hardness were measured as described earlier.

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6.2 Results and Discussion Tables 1, 2, 3, 4 and 5 show the mechanical properties of compression-molded sheets of different blend compositions and the effect of irradiation. It is observed that the tensile strength increases with an increase in the radiation dose, reaches a maximum, and then decreases. EPDM has more tendency to cross-link, but PP mainly undergoes chain–scission under the effect of high-energy radiation [5, 31–72]. The initial increases in tensile strength may be attributed to the cross-linking of the EPDM phase of the blend, and as the radiation dose increases the degradation of the PP phase predominate, as a result, the tensile strength decreases at the higher dose. The addition of TMPTMA increases the tensile strength at each radiation dose. It is reported that TMPTMA produces large numbers of free radicals upon irradiation and greatly increases the cross-linking efficiency [5, 31–72]. At constant radiation, dose the tensile strength increases with TMPTMA level up to a certain value and then decreases. The optimum concentration of TMPTMA depends on blend composition. It has been observed that the optimum level of TMPTMA for tensile strength varies between 4 and 6 phr. Similarly, the optimum radiation dose for tensile strength varies between 2 and 4 Mrad. Since the PP phase provides a reinforcing effect to the blend, the tensile strength of blends increases with PP content. The elongation at break increases initially with radiation dose and TMPTMA level. It was noticed that the optimum level of TMPTMA for elongation at break varies between 2 and 4 phr, and the property reaches the peak value at 4 Mrad for almost all blends. The gel content (a measure of cross-link density) increases with radiation dose indicating that more and more EPDM molecules are cross-linked with radiation dose. The addition of TMPTMA increases gel content, which suggests that the additive acts as a cross-linking promoter in presence of radiation. An attempt was made to study the effect of the molecular weight of PP on the mechanical properties of irradiated EPDM/PP blends. For this purpose, a 60/40 blend of EPDM with PP of low MFI was made in the Brabender Plasticorder with different TMPTMA levels in the same way as described earlier, and the compression-molded sheets were irradiated followed by measurement of mechanical properties. Table 6 shows mechanical properties of compression molded and irradiated blends having PP with low MFI. It is observed that the tensile strength of these blends is higher than the blends with high MFI-PP. These blends also retain their mechanical properties especially elongation at break to a large extent at high radiation dose compared to their counterpart with high MFI-PP. This suggests the high radiation stability of low MFI-PP. The melting temperature and heat of melting data in the second heating run for irradiation blends (60/40EPDM/PP) with 4phr TMPTMA level are reported in Table7. The melting temperature of PP decreases on increasing the radiation dose, which indicates that the crystalline region is affected by radiation. The heat of melting which is a measure of crystallinity also decreases by increasing the radiation dose.

4.7 6.7 6.8 9.0 8.0 650 700 620 550 540 58

4.8 7.2 7.5 9.8 8.4 680 750 640 480 450 59

4.5 7.8 7.3 9.5 7.8 650 650 550 370 350 59

4.3 7.0 7.2 8.5 6.6 630 600 520 250 250 59

4 Mrad

8 Mrad

12 Mrad

20 Mrad

62

62

61

57

2

63

63

62

62

60

56

4

63

63

62

62

60

56

6

Hardness, shore A 0

60

8 59

6

4.2 4.2 4.0 4.0 3.8 550 550 560 530 530 57

4

4.5 5.5 6.0 7.4 7.0 600 650 600 590 590 58

2

0 Mrad

8

2 Mrad

6

% elongation at break

4

0

2

Tensile strength, MPa

0

Radiation dose and TMPTMA (phr) Properties checked at different TMPTMA (phr) and radiation dose

62

62

62

61

59

55

8

0

2

0

4

0

6

0

8

38 41.0 41.4 42

31.3 35

24.0 32.6 36

17.6 27.9 31

43

40

38

33

45

41

39

34.4

13.7 18.7 22.0 25.0 26.0

0

0

% gel content

Table 1 Properties of compression molded sheets of EPDM/PP (MFI—12 g/10 min), 80/20 blend and with different TMPTMA level

13 Electron Beam Radiation-Assisted Preparation and Modification … 427

6.3 7.8 8.4 10.2 8.0 450 480 470 390 320 78

5.7 8.0 8.3

5.1 7.7 7.9

8 Mrad

12 Mrad

20 Mrad

8.2 7.6 420 430 380 260 220 78

9.6 7.8 430 440 420 300 260 78

78

6.6 7.2 7.4 10.0 9.5 480 500 500 450 370 78

4 Mrad

80

80

80

77

76

2

81

81

81

80

78

75

4

82

82

82

82

80

75

6

Hardness, shore A 0

8.2 8.0 460 500 480 460 400 78

8

5.7 5.2 450 460 440 430 390 77

6

6.3 6.5 6.8

4

6.0 6.0 5.8

2

2 Mrad

8

0 Mrad

6

% elongation at break

4

0

2

Tensile strength, MPa

0

Radiation dose and TMPTMA (phr) Properties checked at different TMPTMA (phr) and radiation dose

81

81

82

82

78

74

8

0

2

0

4

21

0

6

40

33.4 37

31.5

22

0

8

42

39

42.5

40

33.5 34

28.5 31 28.8 33

23

31.3 36

30

21

14

11.0 17.6 18

0

0

% gel content

Table 2 Properties of compression molded sheets of EPDM/PP (MFI—12 g/10 min), 70/30 blendsand with different TMPTMA level

428 K. Rajkumar et al.

5.5 7.4

5.0 6.0

12 Mrad

20 Mrad

6.6

8.5 7.3

8.6 8.0 40

60

100 90

90

93

9.0 100 210 190 170 130 93

94

94

94

92

8.8

9.0 220 220 230 230 190 92

6.8 7.5

9.3

7.5 9.5 10.5 10.3 10.0 380 370 330 340 260 92

8 Mrad

90

4 Mrad

6.8 340 340 330 340 350 88

2 92

6.8

0

94

94

93

93

92

90

4

93

94

93

93

90

90

6

Hardness, shore A

9.5 11.2 340 400 360 350 410 91

9.8

6.9

8

7.4 9.2

6

7.0 7.0

4

2 Mrad

2

0 Mrad

8

0

6

% elongation at break

4

0

2

Tensile strength, MPa

Radiation dose and TMPTMA (phr) Properties checked at different TMPTMA (phr) and radiation dose

93

94

93

92

89

88

8

0

4

6 17 19 20

0

2

30

25

18

30

20.5

8

35 38 39

31 35 37

39.5

38.5

26 30 31.8 32.5

13.5 21 27 29

10

0

0

% gel content

Table 3 Properties of compression molded sheets of EPDM/PP (MFI—12 g/10 min), 60/40 blendsand with different TMPTMA level

13 Electron Beam Radiation-Assisted Preparation and Modification … 429

6

8

0

9.3

9.5

7.6

7.8

7.8 30

8.0 40 30

40 30

50 20

40

20

30

48

48

48

20 Mrad

8.2

40

8.0 10.0 10.1

110 150 70

12 Mrad

9.0 70

49

49

49

49

9.2

9.1 10.9 12.0 10.6 10.5 180 150 280 120 110 48

8.2 10.3 11.2

4 Mrad

8 Mrad

48

46

2

49

49

49

49

48

45

4

48

48

49

48

47

45

6

Hardness, shore D

9.0 150 170 150 140 140 46

4

9.4 11.7 11.8 11.0 10.7 200 200 240 180 180 47

10.0 10.5 11.0 10.0

2

2 Mrad

0 Mrad

8

0

6

% elongation at break

4

0

2

Tensile strength, MPa

Radiation dose and TMPTMA (phr) Properties checked at different TMPTMA (phr) and radiation dose

48

48

48

48

47

45

8

0

2

0

4

27

19

0

6

32

31.5

28

19.5

0

8

26 32 35.5 37.5 38

22 27 30.2 32

16 25 28.5 30

12 20 25

10 16 18

0

0

% gel content

Table 4 Properties of compression molded sheets of EPDM/PP (MFI—12 g/10 min), 50/50 blendsand with different TMPTMA level

430 K. Rajkumar et al.

9.8

20 Mrad

12.0

11.0

10.2

8 Mrad

12 Mrad

13.0

11.5

4 Mrad

12.3

11.2

11.8

13.7

12.1

12.5

2 Mrad

13.0

12.0

12.8

13.0

14.0

14.4

10.4

11.0

11.8

12.2

12.8

12.5

6

10.0

10.6

11.0

11.2

12.5

11.2

8 100

20

20

20

80

120

20

20

30

100

150

120

2

20

30

50

90

160

140

4

20

20

50

70

130

120

6

% Elongation at break

4

0

2

Tensile strength, MPa

0

Properties checked at different TMPTMA (phr) and radiation dose

0 Mrad

Radiation dose and TMPTMA (phr)

10

20

30

60

100

120

8

56

56

56

55

55

54

0

57

57

57

56

56

54

2

58

58

57

56

56

53

4

Hardness, shore D

57

58

57

56

55

53

6

57

58

57

55

53

8

13

11.5

9

6

3.5

0

26

24

19

16

13.5

2

% gel content

Table 5 Properties of compression molded sheets of EPDM/PP (MFI—12 g/10 min), 40/60 blendsand with different TMPTMA (phr)

30

28

28

23

17

4

33

30

30

24.5

17.5

6

33

32

31.5

25

18

8

13 Electron Beam Radiation-Assisted Preparation and Modification … 431

7.1

6.8

12 Mrad

20 Mrad

8.5

8.8

9.0

9.5

7.9

7.4

4 Mrad

8 Mrad

7.2 8.2

7.3

7.6

2 Mrad

7.5

8.6

9.0

11.0

10.5

8.8

7.9 9.5

8.7

10.0

11.3

10.9

6

9.8

10.0

10.5

11.2

10.2

8.5

8 350

230

240

250

310

340

250

250

270

320

350

340

2

170

250

340

330

330

350

4

% elongation at break

4

0

2

Tensile strength, MPa

0

140

230

340

340

350

350

6

Properties checked at different TMPTMA (phr) and radiation dose

0 Mrad

Radiation dose and TMPTMA (phr)

130

180

250

260

410

330

8

94

94

94

95

94

95

0

95

94

94

95

95

94

2

96

96

95

95

96

95

4

Hardness, shore A

Table 6 Properties of compression molded sheets of EPDM/PP (MFI—0.7 g/10 min), 60/40 blends with different TMPTMA level

96

95

95

95

95

95

6

95

95

94

94

95

95

8

432 K. Rajkumar et al.

13 Electron Beam Radiation-Assisted Preparation and Modification … Table 7 DSC results for EPDM/PP (high MFI), 60/40 blends with 4 phr TMPTMA

433

Radiation dose (Mrad)

Melting temperature (°)

Heat of melting (J/g)

0

164

34.0

2

163

31.5

4

161

30.0

8

160

28.2

12

157

26.5

20

153

26.0

The DMA spectra of a few samples of 60/40, EPDM/PP with 4 phr TMPTMA, are shown in Fig. 1. The damping (tan delta) curve for pure PP shows a transition at around 14.5° and is attributed to the glass transition temperature (Tg) of PP. The blends show two tan delta peaks corresponding to the glass transition temperature of EPDM (-44°) and PP. It is apparent from Fig. 1 that the Tg of the EPDM phase is shifted to a higher temperature upon irradiation. This observation is believed to be associated with the radiation-induced cross-linking of EPDM. The tan delta peak height also exhibits a decline upon irradiation, which further supports the occurrence of cross-linking induced by irradiation. This phenomenon is commonly observed for lightly cross-linked rubbers [73].

Fig. 1 Effect of irradiation on the temperature dependence of tan δ of 60:40 EPDM: PP blend and pure PP

434 Table 8 Injection molded sheets of irradiated pellets of EPDM/PP (MFI—0.7 g/10 min), 60/40 with 4 phr TMPTMA level

K. Rajkumar et al. Properties

0 Mrad irradiation dose

2 Mrad irradiation dose

4 Mrad irradiation dose

Tensile strength (MPa)

8.5

10.2

12.0

Elongation at break (%)

300

360

460

Hardness (shore 93 A)

94

94

48

35

32

Tension set (%)

Heat aging at 100° for 72 h Tensile strength (MPa)

9.0

11.4

13.5

Elongation at break (%)

180

280

360

96

96

Hardness (shore 94 A)

The 60/40, EPDM/PP (low MFI) was selected for studying injection molding behavior. Blending and subsequent palletization were done as described before, and the pellets were irradiated up to 4 Mrad dose. The level of TMPTMA used was 4 phr as it is already observed that this is the optimum level for this blend system. After irradiation, these pellets were injection molded, by the process described earlier, into around 1 mm thick sheets. The mechanical properties of these sheets are shown in Table 8. Some improvement in mechanical properties like tensile strength and elongation at break is observed for injection-molded sheets compared to compression-molded sheets. This may be attributed to remixing of the blend and improved particle orientation during injection molding. The irradiated samples show good tension set property also required for a thermoplastic elastomer. The heat resistance property of the TPE is also very good at 100-degree temperature as the tensile properties after aging are retained to a large extent. Table 9 presents a comparative study between some existing TPEs made by dynamic vulcanization (conventional technique), and some TPEs developed by electron beam radiation curing in this project. It has been possible to match (sometimes improved) the mechanical properties of conventional TPEs with the developed ones using electron beam radiation technique.

Existing (dy.vul.)

13.3

450

90

60/40

Properties

Tensile strength, MPa

% EB

Hardness, A

EPDM/PP ratio (w)

94

460

12.0

Developed (rad. Vul.)

80/20

65

640

4.7

Existing (dy.vul.)

Table 9 Comparative study of some developed TPE with existing material (TPE)

62

650

7.8

Developed (rad. Vul.)

80/20

55

400

5.2

Existing (dy.vul.)

59

650

5.5

Developed (rad. Vul.)

13 Electron Beam Radiation-Assisted Preparation and Modification … 435

436

K. Rajkumar et al.

7 Measurement of Properties, Results, and Discussion for EPDM: PE Blend 7.1 Measurement of Properties 7.1.1

Mechanical Properties

Mechanical properties were tested as described earlier in Sect. 7.1.

7.1.2

Differential Scanning Calorimetry (DSC) Studies

Melting and crystallization of the irradiated blends were studied with the help pf Perkin-Elmer DSC-7 instrument. The sample was heated at the rate of 20 °C/min up to 150 °C and then cooled and re-melted at the rates of 20 °C/min. The peak maximum from the melting thermogram was considered as the melting temperature (Tm) and the peak area as the heat of fusion.

7.2 Results and Discussion Tables 10, 11 and 12 show mechanical properties of compression-molded sheets of different blend composition and the effect of irradiation. It is observed that the tensile strength increases gradually with increase in the radiation dose, reaches a maximum, and then decreases. The addition of TMPTMA increases the tensile strength marginally at each radiation dose. At a constant radiation, dose the tensile strength increases with TMPTMA level, up to a certain value and then decreases. Due to the reinforcing effect of LDPE, the tensile strength of blends increases with LDPE content. The elongation at break decreases with radiation dose, and the addition of TMPTMA improves elongation at the break only at low radiation doses. It was noticed that the optimum level of TMPTMA for elongation at break varies between 2 and 4 phr. Unlike EPDM/PP blends, much improvement in mechanical properties is not observed by electron beam irradiation of EPDM/LDPE blends. The melting temperature and heat of melting data in the second heating run for irradiation blends (50/50, EPDM/LDPE) with 2 phr TMPTMA levels are reported in Table 13. The melting temperature of LDPE is not changed much by radiation dose, which indicates that the crystalline region is un-effected by radiation. The heat of melting which is a measure of crystallinity almost remained unchanged on increasing the radiation dose. This suggests the high radiation resistance nature of LDPE compared to PP.

5.2

3.8

12 Mrad

20 Mrad

4.5

5.6

6.0

6.2

6.0

5.6

4 Mrad

8 Mrad

4.4 5.7

4.9

5.2

2 Mrad

4.3

4.2

5.7

6.2

6.3

5.9

4.0

4.0

5.2

5.5

6.0

5.5

3.8

4.8

5.0

5.4

5.2

3.7

8 930

450

500

570

780

870

460

560

630

840

910

920

2

390

500

610

800

900

930

4

0

6

% elongation at break

4

0

2

Tensile strength, MPa

370

480

570

720

880

910

6

Properties checked at different TMPTMA (phr) and radiation dose

0 Mrad

Radiation dose and TMPTMA (phr)

Table 10 Properties of compression molded sheets EPDM/LDPE 70:30 blends with different TMPTMA level

340

450

550

700

860

910

8

67

67

66

65

64

61

0

66

66

66

65

65

61

2

66

66

66

65

64

60

4

Hardness, shore A

65

65

65

64

64

60

6

64

64

64

63

62

59

8

13 Electron Beam Radiation-Assisted Preparation and Modification … 437

8.0

6.0

12 Mrad

20 Mrad

6.2

8.5

8.9

9.3

8.2

8.8

4 Mrad

8 Mrad

7.1 8.3

7.1

7.9

2 Mrad

7.2

6.4

9.0

9.2

9.6

11.5

7.9

8.2

9.2

9.5

10.8

10.2

6

8.7

10.2

10.5

11.5

9.5

6.4

8 720

380

560

620

640

680

300

500

540

650

700

730

2

280

460

520

670

730

770

4

% elongation at break

4

0

2

Tensile strength, MPa

0

260

440

500

620

690

760

6

Properties checked at different TMPTMA (phr) and radiation dose

0 Mrad

Radiation dose and TMPTMA (phr)

Table 11 Properties of compression molded sheets EPDM/LDPE 50:50 blends with different TMPTMA level

250

420

480

580

650

760

8

74

81

81

81

79

77

82

82

82

80

78

74

2

82

82

82

81

78

74

4

Hardness, shore A 0

82

82

81

79

77

73

6

81

81

81

79

77

73

8

438 K. Rajkumar et al.

13.5

12.8

13.0

12.2

11.5

4 Mrad

8 Mrad

12 Mrad

20 Mrad

11.7

12.8

13.8

13.0

12.5

12

12.2

2 Mrad

10.5

12.5

13.5

13.7

13.5

12

9.8

11.5

12.0

12.2

11.8

11.5

9.5

10.2

11.4

12.0

11.6

11.2

8

280

350

400

450

510

580

300

370

410

450

520

580

2

250

310

390

460

530

590

4

% elongation at break 6

0

4

0

2

Tensile strength, MPa

Properties checked at different TMPTMA (phr) and radiation dose

0 Mrad

Radiation dose and TMPTMA (phr)

Table 12 Properties of compression molded sheets of EPDM/LDPP 30:70 blends with different TMPTMA

220

270

350

420

510

580

6

200

250

330

390

500

550

8

93

93

93

92

91

89

93

91

93

92

91

89

2

93

93

92

91

90

88

4

Hardness, shore A 0

92

92

91

90

89

88

6

91

91

90

90

88

87

8

13 Electron Beam Radiation-Assisted Preparation and Modification … 439

440

K. Rajkumar et al.

Table 13 DSC results for EPDM/LDPE, 50/50 blends with 2 phr TMPTMA

Radiation dose (Mrad)

Melting temperature Heat of melting (°C) (J/g)

0

100

17.8

2

101

17.9

4

101

18.0

8

101

18.0

12

100

19.0

20

99

17.0

8 Measurement of Properties, Results, and Discussion for NBR: HDPE Blend 8.1 Measurement of Properties 8.1.1

Insoluble Content

The insoluble content was measured by solvent extraction. The samples were extracted in both boiling xylene and dichloroethane, separately, for 24 h, and the extracted samples were dried to constant weight. The insoluble content was calculated as Insoluble content(%) = (W 2/ W 1) × 100 where W 1 is the weight of the sample before extraction, and W 2 is the weight of the dried sample after extraction.

8.1.2

Scanning Electron Microscopy (SEM)

Selected blend samples were fractured manually after freezing them in liquid nitrogen temperature. The fractured surfaces were then sputter coated with gold and examined for the phase morphology using a JEOL, JSM 5800 scanning electron microscope.

8.1.3

Mechanical Properties

Mechanical properties were tested as described earlier in Sect. 7.1.

13 Electron Beam Radiation-Assisted Preparation and Modification …

8.1.4

441

Air Aging Test

Aging studies were carried out in an air circulated aging oven, at 100 °C for 24 h. The samples were kept at room temperature for 24 h after aging, and the tensile properties and hardness were measured as described earlier.

8.1.5

Swelling Test

Swelling studies were performed as per ASTM D 471 test method. Rectangular test specimens measuring about 25 mm × 50 mm were cut from the injection-molded sheets, immersed in IRM 903 oil kept in jar and heated to 100 °C for 72 h. in an air circulated oven. The volume swell was calculated as Volume swell (%) = (M3 − M4)/d(M1 − M2) ∗ 100 where M1 & M2 are specimen weights in air and water, respectively, before immersion; M3 & M4 are specimen weights in air and water, respectively, after immersion; d is the density of the immersion liquid.

8.2 Results and Discussion 8.2.1

Insoluble Content (%)

NBR is soluble in dichloroethane, whereas HDPE is insoluble in this solvent. All irradiated blends were extracted with dichloroethane, and the results are shown in Table 14. The insoluble content increases with increasing radiation dose, indicating that more and more NBR molecules are cross-linked with radiation dose. A high insoluble content value at a low radiation dose (2 Mrad) indicates the high cross-linking efficiency of NBR upon exposure to electron beam radiation. Blends show significantly lower than expected values of % insoluble, based on the fraction of HDPE and % insoluble of pure NBR, especially at low irradiation dose (2 Mrad). The probable reason for low insoluble content may be the extraction of HDPE grafted NBR. The lowering in % insoluble value is more prominent for blends with high NBR content (60/40 NBR/HDPE). Morgan et al. [54] have reported reasonable grafting of acrylonitrile on polyethylene powder in presence of an electron beam at a dose, as low as 1 Mrad. The addition of TMPTMA to the 60/40 NBR/HDPE blend increases insoluble content without irradiation, suggesting gel formation in NBR during processing, and this gelation occurs likely due to thermal cross-linking in presence of TMPTMA. The % insoluble content for HDPE, extracted with xylene increases with radiation dose, which implies that HDPE undergoes predominantly cross-linking when exposed to the electron beam.

442

K. Rajkumar et al.

Table 14 Insoluble content (%) of irradiated samples % insoluble content

Sample

0 Mrad

2 Mrad

4 Mrad

6 Mrad

Solvent-dichloroethane NBR 100

2.2

91.5

93.8

95.2

NBR/HDPE 60/40

42

73.5

89.4

90.8

NBR/HDPE 60/40 (TMPTMA 4 phr)

50

84.8

91.9

94.7

NBR/HDPE 50/50

51.7

90

94.2

96.5

HDPE 100

100

100

100

100

0

0

4.5

31 (after 8 Mrad)

Solvent xylene HDPE 100

48 (after 12 Mrad)

Since, it is clear from % insoluble content values that NBR cross-links at a faster rate than HDPE, the blending of NBR with HDPE is likely to increase the radical yield and improve the irradiation cross-linking efficiency of HDPE.

8.2.2

Scanning Electron Microscopy

Scanning electron micrographs of fractured surfaces of 60/40 and 50/50 NBR/HDPE blend samples are shown in Figs. 2 and 3, respectively. The dispersed particles in the extruded cords (Figs. 2a and 3a) are irregular and large. Subsequent processing of the palletized blend, via injection molding, alters this morphology due to different shear stresses. After injection molding, the particles are oriented in the direction of injection, and the particle size decreases (Fig. 2b and 3b). It is also observed that phase dispersion improves after injection molding. Van Gisbergen et al. [24] have reported similar observations for 80/20, EPDM/PP blends. The authors have shown that the morphology of extrudate pellets with droplets in a matrix structure changes into a co-continuous structure after injection molding due to different shear stresses. A considerable difference in morphology was observed after injection molding for the extruded, pelletized 60/40 NBR, and HDPE blend irradiated with a dose of 4 Mrad as shown in Fig. 2c. Since the main reaction in HDPE and NBR as a result of electron beam irradiation is cross-linking, the material flow during injection molding is hindered leading to inferior phase morphology. Similar observation is noticed in case of 50/50 NBR:HDPE blend (Fig. 3c). A large increase in blend viscosity due to high cross-link density in both NBR and HDPE phases made it difficult to process the palletized blends, irradiated above 4 Mrad dose, in injection molding machine.

13 Electron Beam Radiation-Assisted Preparation and Modification …

443

Fig. 2 Scanning electron micrographs of 60/40, NBR/HDPE blend samples; a extruded cord before irradiation; b injection-molded sheet irradiated at 4 Mrad; c pellets irradiated at 4 Mrad and injection molded

8.2.3

Mechanical Properties

The tensile strength of irradiated sheets and pellets of NBR/HDPE blends (after injection molding) for blend ratio of 60/40 and 50/50 are shown in Figs. 4 and 5, respectively. As the radiation dose is increased, the tensile strength of the blend is improved due to the occurrence of irradiation-induced cross-linking. The further enhancement in the tensile strength with the addition of TMPTMA was also observed for 60/40 NBR/HDPE blend system (Fig. 4). This could be attributed to the increase in cross-link density with the addition of TMPTMA. Figure 6 depicts the effect of radiation dose on elongation at break of 60/40, NBR/HDPE blends. The elongation at the break of irradiated sheets increases substantially with increasing radiation dose. This increase in elongation at break is probably associated with an increase in interfacial interaction through radiationinduced grafting and cross-link formation between HDPE and NBR at the interface.

444

K. Rajkumar et al.

Fig. 3 Scanning electron micrographs of 50/50, NBR/HDPE blend samples; a extruded cord before irradiation; b injection-molded sheet irradiated at 4 Mrad; c pellets irradiated at 4 Mrad and injection molded

As a result, the possibility of crack initiation and propagation at the interface during stretching reduces. Elongation at break reaches a maximum at 4 Mrads and then decreases due to an increase in cross-link density in NBR and HDPE phases at a higher irradiation dose (at 6 Mrad). The addition of TMPTMA improves the elongation at the break property of the blend marginally. The 50/50 NBR/HDPE blends give lower elongation at break, compared to 60/40 NBR/HDPE blends (Fig. 7), at each dose level due to low rubber content. The injection-molded sheets of irradiated blend pellets show very low elongation at break value because of morphological changes brought about by the injection molding process leading to inferior phase dispersion. The tension set is decreased with radiation dose for both the blend systems (Fig. 8). The values corresponding to 0 Mrad represent tension set at break. The increase

13 Electron Beam Radiation-Assisted Preparation and Modification …

Fig. 4 Effect of irradiation on tensile strength of the 60/40 NBR/HDPE blend

Fig. 5 Effect of irradiation on tensile strength of the 50/50 NBR/HDPE blend

Fig. 6 Effect of irradiation dose on elongation at break of 60/40 NBR/HDPE blend

445

446

K. Rajkumar et al.

Fig. 7 Effect of irradiation dose on elongation at break of 50/50 NBR/HDPE blend

Fig. 8 Effect of irradiation dose on tension set of blended sheets

in cross-link density improves the elastic recovery and decreases the tension set value. As expected, blends with high rubber concentration show better-set resistance. Though, higher amount of rubber in the composition increases resistance to set, excessive amount can result in poor fabricability. A further improvement was noted when the blend was irradiated in presence of TMPTMA. The hardness of the blends increases gradually with radiation dose (Fig. 9), and blend with higher plastic content shows higher hardness.

8.2.4

Heat and Oil Resistance

The results of mechanical properties before and after air aging at 100° for 72 h are given in Tables 15 and 16 for irradiated sheets of 60/40 and 50/50 NBR/HDPE blends, respectively. The volume swell values in IRM 903 oil at 100° for 72 h for the same

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Fig. 9 Effect of irradiation dose on hardness of NBR/HDPE blend

samples are also given in the same table. It is clearly noticed that hardness increases after aging and tensile strength of the blends remain almost unchanged after aging. However, a significant drop in the elongation at break is observed after aging. It is believed that proper selection of antioxidant package can improve the retention of extensibility for these blends after aging. The results in Tables 15 and 16 show that irradiation has some effect on percent volume change of blend samples in IRM 903 oil. The volume swell percentage decreases gradually with irradiation dose indicating the growing probability of formation of three-dimensional network structures, which resist the penetration of oil into blend system. Table 15 Heat and oil resistance of 60/40 NBR-HDPE sheets irradiated after injection molding Irradiation dose, MRad

0 Mrad

2 Mrad

4 Mrad

6 Mrad

9

9.2

9.5

10

Unaged samples Tensile strength (Mpa) Elongation at break (%)

70

260

370

300

Hardness (shore D)

39

40

41

42

Heat aging at 100 °C for 72 h Tensile strength (Mpa)

10

10.8

10.5

10.5

Elongation at break (%)

40

110

150

120

Hardness (shore D)

41

43

44

45

18

15

14

Oil aging—at 100 °C for 72 h in IRM 903 Volume swell (%)

21.5

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Table 16 Heat and oil resistance of 50/50 NBR – HDPE sheets irradiated after injection molding Irradiation dose, MRad

0 Mrad

2 Mrad

4 Mrad

6 Mrad

Unaged samples Tensile strength (Mpa)

11.5

12.8

13.2

14

Elongation at break (%)

40

240

350

290

Hardness (shore D)

44

45

46

47

Heat aging at 100 °C for 72 h Tensile strength (Mpa)

12

13

13.5

14

Elongation at break (%)

30

130

160

120

Hardness (shore D)

46

48

49

50

22.5

20.5

19

Oil aging—at 100 °C for 72 h in IRM 903 Volume swell (%)

25

9 Measurement of Properties, Results, and Discussion for NBR: PP Blend 9.1 Measurement of Properties 9.1.1

Mechanical Properties

Tensile strength, % elongation at break, hardness, and tension set were measured as described earlier in Sect. 7.1.

9.1.2

Air Aging Test

Aging studies were carried as described earlier in Sect. 7.2.

9.1.3

Swelling Test

Swelling studies were performed as per ASTM D 471-98 test method as described before. Rectangular test specimens measuring about 25 mm * 50 mm were cut from the injection-molded sheets, immersed in IRM 903 oil kept in a jar, and heated to 100° for 72 h in an air circulated oven.

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9.2 Result and Discussion Tables 17 and 18 show the mechanical properties of injection-molded sheets of irradiated pellets of different blend compositions and the effect of irradiation. It is noted that the tensile strength decreases with an increase in the radiation dose, whereas the elongation at break increases initially at 2 Mrad irradiation and then decreases. The decrease in tensile strength may be attributed to the irradiation-induced degradation of the PP phase. Table 17 Mechanical properties of 60/40, NBR-PP irradiated pellets after injection molding Irradiation dose, MRad

0 Mrad

2 Mrad

4 Mrad

6 Mrad

14.5

13.5

13.2

12.8

Unaged samples Tensile strength (Mpa) Elongation at break (%)

70

250

210

130

Hardness (shore D)

40

43

45

46

Tensile set (%)

18(B)

53

44

15(B)

Heat aging at 100 °C for 72 h Tensile strength (Mpa)

15

14.5

14

13.5

Elongation at break (%)

50

160

130

70

Hardness (shore D)

42

46

48

48

19.5

12.5

11

Oil aging—at 100 °C for 72 h in IRM 903 oil Volume swell (%)

26

B indicates break

Table 18 Mechanical properties of 50/50, NBR-PP irradiated pellets after injection molding Irradiation dose, MRad

0 Mrad

2 Mrad

4 Mrad

6 Mrad

Unaged samples Tensile strength (Mpa)

18

15

14

13

Elongation at break (%)

50

220

170

110

Hardness (shore D)

45

48

50

51

Tensile set (%)

20(B)

58

48

17(B)

Heat aging at 100 °C for 72 h Tensile strength (Mpa)

17.8

15.8

15

14.2

Elongation at break (%)

40

130

90

50

Hardness (shore D)

47

51

53

24

22

Oil aging—at 100 °C for 72 h in IRM 903 oil Volume swell (%) B indicates break

30

19

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The initial increase in elongation at break is a result of the combined effect of irradiation-induced cross-linking of NBR and degradation of PP. Blends rich in NBR show higher elongation at break and better tension set properties after irradiation due to improved elasticity imparted through radiation cross-linking of the NBR phase. The blend composition shows moderate heat resistance and good oil resistance properties. The decrease in volume swell with radiation dose is a result of more and more cross-link formation in the NBR phase.

10 Conclusion From this investigation, it is revealed that electron beam irradiation of elastomerplastic blends, of different elastomer/plastic combinations can provide an interesting route for the preparation of thermoplastic elastomeric compositions having a broad range of properties for different areas of application.

10.1 EPDM/PP System (i)

It is possible to develop TPEs with good mechanical properties in the hardness range 55 shore A to 47 shore D. (ii) Blends containing 50% or more EPDM content show significant improvement in mechanical properties upon irradiation due to radiation degradation nature of PP. (iii) Use of PP with low MFI (high molecular weight) improves mechanical properties of blends after irradiation because of its higher radiation resistant nature. (iv) Addition of TMPTMA as multifunctional monomer improves mechanical properties of irradiated blends further. (v) The optimum radiation dose and TMPTMA level for a blend system to achieve a balance in mechanical properties are 4 Mrad and 4 phr, respectively. (vi) The irradiated pellets (TPEs) can be easily injection molded into good quality products. (vii) TPEs developed in this work show very close, even better in some cases, mechanical properties to existing TPEs made by dynamic vulcanization. (viii) These TPEs can find applications in mechanical rubber goods like grips, caps, plugs, connectors; industrial hoses and in automotive rubber products like weather strips, grommets, sleeves; electrical applications like wire and cable insulation and jacketing, plugs, etc.

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10.2 EPDM/LDPE System (i) This blend system also can be cross-linked by electron beam irradiation. (ii) Tensile strength increases with radiation dose initially, though elongation at break decreases with radiation dose. (iii) Tough unlike EPDM/PP system, these blends exhibit marginal improvement in mechanical properties, still they can be used as TPEs in some applications due to their high elongation at break even at a dose as high as 12 Mrad.

10.3 NBR/HDPE System (i) (ii) (iii) (iv)

(v) (vi)

Cross-linking of NBR/HDPE dissimilar blends is possible in presence of electron beam and thereby enhancement of properties. Addition of TMPTMA improves the mechanical properties further. Possibility of radiation-induced grafting has been observed for this blend system. Blend morphology has been improved after injection molding but irradiation before injection molding hampers flow during injection molding leading to inferior mechanical properties especially elongation at break. Curing by irradiation after molding or shaping is a better option for this system than curing before shaping. This blend composition shows moderate heat resistance and oil resistance as good as NBR after irradiation.

10.4 NBR/PP System (i) This blend system can be modified through electron beam processing. (ii) Tensile strength decreases with radiation dose due to degradation of PP phase. (iii) Significant improvement in elongation at break is observed upon irradiation at low doses. (iv) Irradiation cross-linking of NBR phase improves tension set property of the blends, required for a TPE. Unlike NBR/HDPE system, they can be processed after irradiation. (v) This blends composition shows moderate heat resistance and oil resistance and can be used in oil resistance application where requirement of elongation at break is not very high. With increasing demand for longer life from polymeric components especially in auto sector, efforts are directed toward development of hot oil, higher temperature resistant, and high strength thermoplastic elastomeric blend compositions.

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The work indicates that a number of TPE materials fabricable as thermoplastics and exhibiting good elastomeric properties can be produced in a range of hardness. As stated through this investigation, electron beam curing greatly improves the properties of some elastomer-plastic blends. However, in addition to radiation curing technological compatibilization by the incorporation of suitable compatibilizers can be tried to improve properties of some dissimilar blends. Acknowledgements We are thankful to Dr. S. Sabarwal and Mr. K. S. S. Sarma, BARC, Mumbai for valuable inputs, suggestions and carrying out irradiation. We are thankful to Board of Research in Nuclear Sciences, Department of Atomic energy, Government of India, for giving funding for the project work related to thermoplastic elastomer study. We are also thankful to Staff of Indian Rubber Manufacturers Research Association (IRMRA), Thane for their co-operation and help for carrying out the testing.

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Chapter 14

Recent Advances in Electron Beam Processing of Textile Materials Amol G. Thite, Kumar Krishnanand, and Prasanta K. Panda

1 Introduction Textiles are versatile materials. They provide humankind comfort, protection, and performance benefits such as thermal regulation, moisture management, soil release, easy care, self-cleaning, super absorption, medical, cosmetic, and odor-resistant properties [1]. Textiles are also used to prepare composites that have great use in daily life and as advanced materials. On the other side, the textile industry produces much wastewater, which pollutes our environment daily and is a hazard to aquatic life and human health. Environmental concern has compelled the industry and researchers to find newer methods to innovate products in an environmentally friendly way. Hence, radiation technology has emerged as a potential substitute for currently available conventional methods. It comprises ultraviolet, gamma, microwave, X-rays, and electron beams ionizing radiation to modify material properties [2]. Among the radiation techniques, the electron beam (EB) is one of the powerful sources of electromagnetic radiation, providing high-dose delivery at ambient temperature, consuming only electricity. An electron beam is produced in an electron beam accelerator by accelerating electrons originating from a heated tungsten wire through thermionic emission. These electrons accelerate in a vacuum-accelerating tube powered by DC or RF power. The electron beam eventually comes out of the tube (scan horn) into the air after passing through a thin titanium alloy metallic foil. Accelerating voltage (generally 100 keV–10 MeV) and beam current are two essential parameters of the electron beam. The beam energy is the same as the accelerating A. G. Thite (B) · P. K. Panda The Bombay Textile Research Association, LBS Marg, Ghatkopar (W), Mumbai 400086, India e-mail: [email protected] P. K. Panda e-mail: [email protected] K. Krishnanand Bihar National College, Patna University, Patna 800004, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. R. Chowdhury (ed.), Applications of High Energy Radiations, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-19-9048-9_14

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voltage, and power output (commonly 0.5–200 kW) is obtained by multiplying the accelerating voltage with the beam current. The penetration depth depends upon electron beam energy, density, and geometry of the irradiated material. For a 5 MeV EB machine, one can think of a 4.2 cm penetration depth on double-sided irradiation of unit density material. The absorption of EB radiation energy is described by gray (Gy = J/Kg) or Kilo-gray (kGy) dose [3]. The depth-dose profile of the electron beam indicates that the absorbed dose peaks at a few cm beneath the surface. It allows modification of plastics, polymers, and textiles as they have reasonable thickness and density [4]. The several advantages of this technology over the currently available radiation techniques are higher throughput due to high-dose rate and high power output, economical processing at ambient temperature, penetration depth controlled by accelerating voltage, and environmentally friendly process. Electron beam irradiation (EBI) processing often involves cross-linking, curing, grafting, oxidation, and degradation of polymers. Therefore, it has been utilized widely in the plastic and rubber industry. The textile industry has also used EBI and found potential applications in composite preparation, development of functional materials, biomedical applications, and treatment of textile chemical processing effluent [5–8].

2 Theoretical Basis of Electron Beam Radiation Processing of Textiles Textile materials are the assembly of fibers usually composed of long-chain polymers. Radiation processing of textiles eventually involves the treatment of polymeric materials with ionizing radiation to modify their physical and chemical properties to make property improvements and value addition [9]. The underlying mechanism for the radiation processing of textiles is that high-energy radiations transfer energy to the polymeric molecules, which undergo excitation or ionization. After the primary interaction, polymeric radicals get produced. Once radicals generate, they can experience several reactions, as shown in Fig. 1. These reactions are hydrogen abstraction, generation of the double bond, combining two radicals to produce cross-link or branching, chain scission (degradation), oxidation of polymeric chain, and the polymerization of a monomer leading to grafting or curing. These different radiation responses can primarily influence by the chemical structures of the polymers, the state of materials, the presence of external additives, and irradiation conditions such as temperature and atmosphere [5]. Abstraction of hydrogen occurs after polymer radiolysis, leading to cross-links or double bonds and hydrogen gas production. A network structure can form in crosslinking, which rapidly increases polymer solutions’ viscosity and reduces the melt flow. It also increases the thermal and mechanical stability of the polymer. However, cross-linking and degradation are two mutually competing processes. Both crosslinking and scission coincide during irradiation but to different extents. Again, it

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Fig. 1 Schematic of electron beam radiation and chemical reactions initiated after EB irradiation of textile polymer [10]

depends on the nature of the polymer, the state of materials, and the conditions of irradiation. Many synthetic polymers undergo cross-linking, especially those containing at least one aliphatic α-hydrogen (–CH–) in the main chain attached to the carbon. The tetra-substituted carbon chains predominantly undergo chain scission, leading to a drastic reduction in molecular weight. The degree of cross-linking/degradation is also influenced by the temperature of irradiation and the atmosphere of irradiation as well (air, nitrogen, argon, acetylene) [11]. Oxidation of the polymeric chain via peroxide radicals often leads to chain scission, resulting in the polymer’s degradation. It can be seen as discoloration or cracking in the structure. The application of radiation technology for processing natural-based polymers is limited due to predominant chain scission when irradiated in the air. Therefore, it is of prime importance to study the radiation response to the mechanical properties of that particular polymer so that optimum dose levels can be selected for post-modification, such as graft polymerization [12]. In grafting, a new monomer is polymerized on the main chain of the polymer to create a branched network. Since the branched chain has different properties, new functionalities may be introduced. Branches can also modify the viscosity of the polymer solution. In curing, simultaneous polymerization and cross-linking happen. It has applications in coating and composites.

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3 Factors Affecting Radiation Processing of Textiles 3.1 Absorbed Dose It is the amount of energy absorbed by the per unit mass of the substance. The requirements of absorbed dose are always different depending on the nature of the material, and it may be in the extensive range (0.1–1000 kGy). However, most polymerization, grafting, and cross-linking applications cover 10–150 kGy. A textile polymer’s radiation response can quantify by the G value, which reflects the number of sample molecules reacted per 100 eV of radiation energy. The crosslinking phenomenon can be measured by G(X), and the chain scission by G(S) value. The G(S)/G(X) ratio determines whether the sample is undergoing chain scission or cross-linking predominantly. It is calculated using the polymer’s sol-gel analysis and the Charlesby–Pinner equation [13] (Eq. 1). S + S 1/2 =

p0 1 + q0 q0 × D × μ1

(1)

where S is the sol-fraction, p0 and q0 are the fracture density and cross-linked units per unit EB dose in kGy, respectively. μ1 is the initial average degree of polymerization before irradiation, and D is the absorbed EB dose in kGy. The value of G(S) and G(X) both increase with the absorbed dose but G(S) very soon overtakes G(X), and at a very high dose, the result is in favor of chain scission. Moreover, the G(X) value depends upon the nature of the polymer, additives and chemical structure, and irradiation conditions.

3.2 Electron Beam Energy This energy plays an essential role in controlling energy penetration into the substance. The incident energy can linearly increase the penetration in the material.

3.3 Depth of Penetration The absorbed doses near the entrance surface are far higher in materials with higher electrons per unit mass. The depth of penetration into different densities of materials is determined by multiplying the penetration depth calculated from the curves by the water density to the substance density.

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3.4 Line Speed It displays the material processing rate under the beam. This factor significantly rises with the increase of the beam’s current and reduces with the width of the beam and absorbed dosage.

3.5 Effect of Chemical Nature of Polymer Radiation cross-linking predominantly occurs in the amorphous region of the polymer, where the chains move to bring radicals closer to each other. The mobility of chains is essential above the glass transition temperature only. Factors affecting Tg are chain stiffness, intermolecular forces, and pendant groups. G(X) value generally decreases with an increase in the Tg value. Increasing crystallinity decreases the amorphous region where cross-linking happens; hence G(X) value decreases. The yield of radicals is also dependent on chemical composition. The generation of radicals is dependent on bond dissociation energy. Scission of a side chain generates a polymer radical that can undergo cross-linking, but degradation occurs if the main chain has loose bonds. It was found that C-X bond dissociation energy (X = H, Cl, R) is lowest for the dissociation of X from a tertiary carbon, and they produce more significant numbers of radicals among primary, secondary, and tertiary substitution. Unsaturated C=C groups enhance radical generation. A methyl group substitution reduces the primary chain C–C bond energy, promoting polymer degradation upon irradiation. These groups also raise the Tg of the polymer. Since C–Cl bond energy is less than C–H, irradiation promotes cross-linking for partially halogensubstituted polymer. However, if the group generates strong cohesive forces and restricts chain mobility, then polymer degradation occurs. If there are phenyl groups, they can protect the molecules from radiation-induced reactions, and therefore, G value is significantly less for polystyrene. In polyesters, the presence of methyl substituents promotes degradation of the main chain like poly(hydroxybutyrate) and poly(lactic acid). However, if many CH2 groups are present, cross-linking occurs as in polycaprolactone and poly(butylene succinate). That is why poly(methylene oxide) undergoes degradation, but poly(ethylene oxide) undergoes cross-linking on irradiation. It was also found that some chemical groups or structures promote degradation, cross-linking, and some make it radiation-resistant [14, 15].

3.6 Effect of the Environment Oxygen makes peroxide radicals with carbon radicals. Therefore, the polymer chains undergo oxidation and degradation through the peroxide radical mechanism

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when irradiated in the air. Consequently, it is advised to carry out irradiation in an air-free or oxygen-free atmosphere.

3.7 Effect of Temperature As the temperature increases, the mobility of polymer chains also increases, bringing chains nearby. It will favor cross-linking. Polytetrafluoroethylene (PTFE) is known for radiation degradation at ambient conditions but undergoes cross-linking at temperatures above its melting point in an oxygen-free atmosphere.

4 Enhancement in Radiation Processing of Textiles As explained earlier, cross-linking occurs in the polymer’s amorphous region, where mobile polymeric radicals come near each other to form a cross-link. Therefore, factors that increase the yield of polymeric radicals increase the volume fraction of the amorphous region, increase the mobility of polymeric chains, and bring radical chains closer will increase the polymer’s G(X) value. These factors can categorize in the following way.

4.1 Increasing the Yield of Polymeric Radicals By (i) Addition of sensitizer: The sensitizers are compounds that quickly decompose on high-energy irradiation to generate radicals. It further reacts with polymer to create polymeric radicals. Examples of sensitizers are organic halides (carbon tetrachloride, chloroform, etc.) and water. Organic halides have a fragile bond with halogen, which undergoes homolytic cleavage to generate X* on irradiation and abstract hydrogen from the polymer creating polymeric radicals. Water is adequate as a sensitizer in an aqueous system only. (ii) Post-irradiation heat treatment: Heat treatment increases the mobility of chains. It can transfer of some of the radicals from the crystalline region to the amorphous area where cross-linking occurs. It has been found faithful in the case of cross-linking of PP, PVAc, PVDF, and EVOH.

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4.2 Enhancing the Possibility of Recombination of Polymeric Radicals By (i)

Compression: The application of high pressure can bring chains closer to the amorphous region, increasing the probability of collision and creating crosslinks. PVA degrades at normal pressure, but it can cross-link at high pressure. The advantage of compression is that it does not require any additives. (ii) High-temperature irradiation: Polymer chains show a sharp rise in mobility above the glass transition temperature. Above this temperature, polymers offer a high G(X) value. It is why the inability to cross-link PVC (Tg:85 °C) and PVA (Tg:87 °C) at room temperature. Poly(tetrafluoroethylene-cohexafluoropropylene) or FEP shows degradation when irradiated below its Tg value but cross-links at a higher temperature. PTFE cross-links above its melting temperature. Polyethylene sulfone and polysulfone also show enhanced cross-linking at higher temperature irradiation. (iii) Use of plasticizers: The role of plasticizers is to enhance the mobility of chains in the amorphous region by weakening the intermolecular forces between chains and thereby reducing the Tg of the polymer. Reduced Tg and enhanced chain mobility in the amorphous region favor cross-linking. Plasticizers which contain aromatic systems are less efficient in selecting crosslinking because of their radiation-protective nature. PVC undergoes crosslinking in the presence of non-aromatic-based plasticizers such as fatty acidesters and polyester-based plasticizers. Dioctyl phthalate and dibutyl phthalate are not practical because of their aromatic rings. EVA in LDPE, PEO in PMMA, and poly(propylene-co-vinylsilane) in PP promote cross-linking. (iv) Polyfunctional monomer: Polyfunctional monomers have multiple double bonds, which can cross-link upon irradiation. These PFMs penetrate the amorphous region of the polymer and initiate radical graft polymerization upon irradiation. Grafted propagating radicals are highly flexible and can combine easily to form a cross-link. These PFMs also called cross-link promoters, sensitizers, accelerators, or prorads. Examples of PFM are diacrylates, dimethylacrylates, triacrylates, and trimethyl acrylates. The elevated temperature can cause the polymerization of some of these PFMs. Therefore, heat-resistant PFMs such as triallyl cyanurate (TAC), triallyl isocyanurate (TAIC), and trimethallyl cyanurate (TMAIC) are more famous for the enhancement of cross-linking. The advantage of using PFMs is that they can cross-link PVC polyamides at room temperature, but it is said that TAC and TAIC are toxic (mutagen).

4.3 Addition of Fillers Fillers like carbon black, ZnO, MgO, and silica have a dramatic influence on radiation cross-linking. These fillers can influence the volume fraction of the crystalline and

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amorphous region of the textile polymer (superstructure), and sometimes they can form bonds with polymer molecules in the amorphous region.

5 Recent Advances in Electron Beam Radiation Processing of Textiles 5.1 Textile Fibers Textiles are very close to human life. For a long time, natural materials have been used for fabrics. With the development of polymer science in the twentieth century, manufactured fibers became an integral part of textile materials. Therefore, the detailed classification of textile fibers is shown in Fig. 2. The important textile fibers of natural origin are cotton, wool, silk, jute, flax, ramie, hemp, sisal, coir, and banana. They are either cellulosic or protein fibers. Some fibers are regenerated cellulosic fibers like viscose, modal, and lyocell. Polyester, polyamides, vinyl, cellulose acetate/triacetate, acrylic, olefin, and polyurethane are popular synthetic fibers. The high-performance specialty fibers used for technical textiles include aramids, carbon, and UHMWPE.

5.1.1

Conventional Textile Fibers/Yarns

Many researchers have studied the effect of electron beam radiation on the tensile strength of both traditional natural and synthetic fibers. Cellulosic yarns’ common trend is decreased tensile strength with increased radiation dose [12]. For silk and nylon, a decrease in yarn strength is noticeable only at high doses like 100 kGy [17]. For PET yarns, there is no noticeable change in tensile strength in the selected range of 0–100 kGy. It is consistent with the ability of aromatic rings in the polymeric chain to resist electron beam-induced modifications [18]. Further, loss of tensile strength after EB irradiation in cellulosic materials was found because of a decrease in the degree of polymerization and a decrease in crystallinity, as reported by Driscoll et al. [12]. Cotton being highly crystalline compared to viscose, the slower rate of decline in tensile strength was observed in it. According to Halabhavi et al., the crystallinity of P31 Bombyx mori silk fibers decreased with an increase in radiation dose resulted in an increased mechanical properties and thermal stability, suggesting the possibility of cross-linking of polymer chains [19]. It is in marked contrast to Shao’s observation of electron beam irradiation of mill-degummed 100% silk twill fibers. Shao’s group found that the breaking strength of electron beam-irradiated silk decreases with radiation dose and is noticeable only after 25 kGy [20]. The greenish tinge of the irradiated samples and loss of breaking strength at high doses were observed. It becomes fascinating to explain these different trends in the tensile strength of electron beam-irradiated silk materials. Therefore, a thorough investigation of microstructural properties is required for silk materials to elucidate their radiation response.

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Fig. 2 Classification of textile fibers [16]

The case of polyamide-6 is more enjoyable. According to Adem’s observation, his group found polymer chain scission dominant at room temperature below their glass transition temperature (Tg), while cross-linking was dominant above Tg. They also found a continuous decrease in stress and strain at the break while an increase in Young’s modulus and stress at yield with increased radiation dose [21]. However, chain scission dominates more in PA-66, while cross-linking dominates in PA-6. Crystallinity was found to be one of the essential micro-structural factors controlling the physical property, as it decreases with an increase in radiation dose. Crystallinity values varied slightly in this case except for the EBI samples above 100 kGy. It was found that these changes are consistent with cross-linking in the amorphous zone. The increased temperature predominates cross-linking over scission and analogous

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changes in mechanical properties [22]. A significant improvement in the dyeability, color fastness, and durability of polypropylene, polyester, and Nylon 6 was observed after EB irradiation in the air by 0–300 kGy. Recently, Payamara et al. [23] investigated the effect of electron bombardment with the energy of 10–40 keV and various periods into polypropylene (PP). PP exhibited significant changes in its physical and chemical characterization by EBI processing. The results showed an amenable modification of PP surface after EBI processing and improved wettability, dyeability, and printability through forming (–O–H) and (C=O) groups on the PP surfaces. Also, dyeing with basic dyestuff for treated fabrics showed a relative increase in color strength. The SEM micrographs of treated samples showed a rougher texture than the untreated with the rise of the absorbed dosage of electron bombardment.

5.1.2

High-Performance Textile Fibers/Yarns

(i) Aramid fibers Aramid fibers contain aromatic rings in which more than 85% amide bonds are directly connected with the two aryls. The specialty of these fibers is having high strength and modulus, light in weight, high-temperature resistance, and excellent corrosive resistance. These aramid fibers are called meta-aramid (Nomex) and para-aramid (Kevlar) based on the bonding position to the phenylene ring structure. For meta-aramid, the angle between the amide plane and phenylene plane is 30°, providing stability to the network. The crystalline structure of para-aramid is pseudoorthorhombic. Its chemical structure favors extensive hydrogen bonding, resulting in excellent physical and chemical properties. However, problems arise in the application due to high crystallinity, smooth surface, and lack of chemically active groups. Therefore, it is necessary to modify its surface or structure. The surface modification of Kevlar fibers was carried out via electron beam (EB)-induced post-grafting polymerization. Acrylic acid (AA) was introduced onto its surface by Xu et al. The effects of radiation dose on the degree of grafting were investigated, as shown in Fig. 3. This study reported that the degree of grafting was greatly affected by the homo-polymerization of AA at high adsorbed EB doses, leading to a decrease in the degree of grafting [24]. (ii) Carbon fibers Carbon fiber is known for its high-performance properties, such as high tensile strength and modulus. Due to this, it is widely used in aerospace, national defense, sports, medical devices, military, and other civil industries. It has a strong footprint in the energy and automotive sector. Demand for it is continuously increasing day by day. There is a need to produce high-quality carbon fibers at a lower cost in this context. The tensile strength of the highest-quality carbon fiber T1000 has only 5% of its theoretical value. Therefore, there is an excellent scope for scientists across the globe to improve the performance of carbon fibers.

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Fig. 3 Effect of absorbed dose on the degree of grafting (40 wt% AA, 0.4 wt% Mohr’s salt, 60 wt% DI water, temperature: 60 °C, reaction time: 17 h) [24]

The precursors of carbon fibers are classified into different types based on their source, i.e., polyacrylonitrile (PAN), pitch, and cellulose. Polyacrylonitrile is the most extensively used (almost 97%) precursor to manufacture high-performance carbon fibers. However, PAN sourced from fossil fuels possesses many toxic gases during carbonization and waste management problems after use. In addition, it is expensive, and its cost depends on the variations in crude-oil price. Therefore, to replace this, an environmentally friendly and low-cost precursor is required to develop highperformance carbon fibers. Cellulose is one of the amplest and oldest precursors used for carbon fibers. However, carbon fibers’ tensile strength and yield grew from this precursor lean-to-low. Despite these disadvantages, cellulose-based carbon fibers are considered instead of PAN-based carbon fibers. They offer advantages of an abundant supply of raw materials and economic and environmental profit. Therefore, there is much curiosity about developing cellulose-based carbon fibers with good mechanical properties and high yields. Lyocell is a cellulose-based regenerated fiber that exhibits higher mechanical and lower shrinkage properties than other rayon-based fibers. However, unlike rayon-based fibers, lyocell fibers require pre-treatment to enhance carbon yield and mechanical properties before thermal stabilization. Shin et al. [25] employed EB irradiation treatment instead of thermal treatment in the stabilization step for producing carbon fibers from a PAN precursor. The tensile strength of carbon fibers obtained from thermally treated and stabilized PAN fibers after electron beam radiation pre-treatment was as high as 2.3 GPa. However, the electron beam radiation dose was very high (1000 kGy). The high-performance carbon fibers from textile grade homo and copolymer PAN using continuous EB irradiation, stabilization, and carbonization were reported by M.R. Buchmeiser et al. They have reported 3.1 ± 0.6 GPa tensile strength 212 ± 9 GPa Young’s modulus, which exceeds commercially available Toray T300 carbon fiber. However, in this case, the used EB dose was also very high [26]. Therefore, to bring down electron beam radiation dose with good mechanical properties at high yields of lyocell-based

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Fig. 4. Illustration of the preparation of carbon fabric from lyocell fabric grafted with PAM using EBI, stabilization, and carbonization [27]

carbon fibers, H. K. Shin and his group grafted lyocell with polyacrylamide (PAM) using EBI during pre-treatment, as shown in Fig. 4. They studied the effect of PAM concentration and EBI doses on the tensile strength performance of the produced lyocell-based carbon fibers. Figure 5 evaluates the tensile strength of carbon fibers isolated from carbon fabrics obtained from lyocell grafted with PAM via EBI. Accordingly, the tensile strength of these produced carbon fibers increased from ~ 0.82 to 1.39 GPa. In addition, this study established the feasibility of EBI-induced PAM grafting on lyocell fabrics to produce high-performance carbon fibers with the highest tensile strength (1.39 GPa) at 0.5 wt% PAM. (iii) Ultra-high molecular weight polyethylene (UHMWPE) fibers Ultra-high molecular weight polyethylene (UHMWPE) fiber has excellent properties such as high work of fracture, low density, high tensile strength, high specific modulus, and good workability. It has led to its general use in manufacturing high-performance materials, such as marine vessel ropes, military protective materials, and aerospace materials. However, it is more prone to intermolecular creep due to the simple planar zigzag structure, small intermolecular forces, and lacking hydrogen bonding between macromolecules. This defect has dramatically affected the applications of UHMWPE fiber in many areas. So, many modifications to UHMWPE fiber are being explored with the help of electron beam irradiation. Okubayashi S. et al. grafted vinyl sulfonate and phenyl groups-based sodium pstyrenesulfonate (p-SSNa), 2-(4-Benzoyl-3-hydroxyphenoxy) ethyl acrylate (BHPE) and N-(4-Hydroxyphenyl) methacrylamide (HPMA) monomers on the UHMWPE fiber by electron beam irradiation and studied the influences of the monomer type and pre- and post-EB irradiation conditions on the dyeing ability with cationic and

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Fig. 5. Tensile strength of carbon fibers obtained from lyocell fabrics grafted with PAM via EBI [27]

dispersed dye. From the results, the grafting amount of p-SSNa on UHMWPE was found to be increased up to around 5% omf with increasing irradiation dose and the monomer concentration, while those of BHPE and HPMA, which bear phenyl groups, were up to 45% omf and 14% omf. With the increment of the grafting amount, the color depth K/S of p-SSNa grafted fabric, and dyed with cationic dye and methylene blue was increased up to 3.7. The maximum K/S of BHPE-grafted UHPE dyed with CI Disperse Blue 56 was 4.9 [28]. Further, the UHMWPE fibers were cross-linked with a trifunctional monomer, Trimethylolpropane trimethacrylate (TMPTMA), through supercritical CO2 pre-treatment and then irradiated under the electron beam. The performance of UHMWPE fibers changed to varying degrees after supercritical CO2 pre-treatment and electron beam irradiation. As the irradiation dose increased, the gel content first rose, then declined; the breaking strength continuously decreased; the elongation at break increased initially and then decreased. The creep rate originally decreased and then rose before finally falling slowly. The promoting effect of multifunctional monomers on the cross-linking of UHMWPE fibers is mainly through the transfer of macromolecular chains. It forms crosslinking networks between monomer polymers and macromolecules. This process describes the sensitized irradiation cross-linking mechanism of UHMWPE fibers in the presence of TMPTMA [29].

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5.2 Textile Fabric 5.2.1

Graft Polymerization

Conventional fabrics have been modified to deliver specific functions used as technical textile products. The manufacturing process involves developing functionality in the fiber itself and then weaving/knitting/braiding into textile products or modifying the conventional fabric through resin or graft polymerization. Graft polymerization introduces new functionality to the existing fabric, and this functionality is highly durable because it is covalently bonded. The graft polymerization can be carried out using chemical reagents and by using radiation treatment as well. The choice depends on the economy and environmental concerns. The graft polymerization of organic monomers on the surface of textile fabrics [30] is one of the promising methods in which any of the three main procedures can carry out: direct irradiation method, post-irradiation, and peroxidation method [31]. All are heterogeneous systems in which fabric is in solid form, and the monomer could be in neat liquid, vapor, or solution form. The fabric is immersed in the monomer solution in the simultaneous irradiation mode and then irradiated together in an inert atmosphere. Trunk polymer and simultaneous monomer irradiation are carried out in the direct method. A high dose of radiation is required. Apart from graft polymerization, lots of homo-polymerization also occurs. In a post-irradiation way, the polymer is irradiated alone in the inert atmosphere/vacuum to generate radicals used later to carry out graft polymerization when placed in a monomer solution. It is suitable for semi-crystalline polymers. One variation of this method is the peroxide method, in which irradiated polymer is exposed to an oxygen atmosphere to generate peroxides. These peroxides are thermal initiators for free radical polymerization. Post-irradiation and peroxide methods are carried out to better command the graft polymerization process. The peroxide method is even successful with amorphous polymers. However, oxidation can bring degradation of the trunk polymer. Hence, the dose of radiation should be low to prevent oxidative degradation. Control on graft yield requires the study of the effect of different factors which can affect the rate and degree of grafting. The important factors are as follows: (a) (b) (c) (d) (e)

Irradiation condition: atmosphere, dose, and dose rate Reaction temperature Solvent Monomer concentration Additives.

The high degree of grafting is realized by increasing the number of radicals on the trunk polymer, increasing the movement of monomer to the radical site, decreasing termination by the combination of extremists, and preventing homo-polymerization of monomers. Monomers are usually used in the solution form, but water can also be used as a solvent for hydrophobic monomers in the emulsion. Grafting in the emulsified monomers gives a higher degree of grafting, and the degree of grafting is

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even better when the stability of monomer suspension is poor. Further, the properties of grafted fabric depend on the condition of irradiation and the degree of grafting. Irradiation-induced reactions may alter properties. The grafted polymer will also have a different property from the original polymer, which can be observed in the stress-strain curve. Since grafting occurs in the amorphous region, it influences the crystalline area and will alter the melting point due to alternation in conformation. Therefore, due to the enormous advantages of radiation grafting for functional textiles, researchers have been widely attatracted to it. Vinyl monomers, methacrylic acid monomers, and their acid-ester derivatives are the most commonly used functional organic monomers due to their mild and efficient self-polymerization reactions [32]. Recently, radical graft polymerization of glycidyl methacrylate has been carried out by peroxidation method on cotton fabric by Krishnanand et al. [33] using 10 kGy electron beam radiation dose. Then it was followed by hydrazination of the epoxy ring for the electroless deposition of silver for electro-conductive cotton fabric, as shown in Fig. 6. Here, GMA may be a very interesting monomer with two different functional parts—the vinyl functional part that responds to radical polymerization. In contrast, the epoxy ring part is vulnerable to nucleophilic attack by many interesting nucleophiles. One such exciting nucleophile and a well-known reducer is hydrazine. It has been used here to chemically incorporate it within the cotton fiber chemical structure via the opening of the epoxy ring. Additionally, incorporated hydrazine can reduce metal ions from its salt solution. This process is understood as electroless plating. The electroless plating has deposed silver from the aqueous ammoniacal silver salt solution. In situ generated metal particles immediately help to stabilize the hydroxyl groups of cotton. The XRD pattern showed that in situ-produced Ag particles are nanoscale, with a typical size of around 41 nm. According to the TGA plot, metallic

Fig. 6. (i) Scheme of reactions (a) electron beam-induced graft polymerization of glycidyl methacrylate monomer onto the fiber, (b) opening of the epoxy ring with hydrazine, and (c) redox reaction, and (ii) SEM images: (a) Cotton fabric; (b) Cotton-g-GMA fabric; (c) Cotton-g-GMA after hydrazination; (d) metal deposited fabric [33].

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Fig. 7. (i) Current versus voltage plot of metalized fabric sheet placed over parallel metallic electrodes. (ii) Wash durability of the conductive fabric: (a) surface resistivity at different washing cycles; SEM of conductive fabric (b) before washing and (c) at the 20th washing cycle at two different magnifications, 100× and 1000× in the inset [33].

silver accounts for nearly 7.5% of the metalized fabric. These results from hydrazine derivatives already present in the fibers reducing metal ions from their salt solution. The surface electrical resistivity of cotton fabric was measured as per AATCC 76 standard operating procedure using parallel plate electrodes. The sample resistance is often obtained from the Current (A) versus Voltage (V) slope, as given in Fig. 7. The surface resistivity of the silver-deposited cotton fabric comes out to be 3.63 Ω/square against the plain; it was grafted, and hydrazine-modified fabric in the order of 109–10 Ω/square could be attributed to the contact network among metallic silver nanoparticles. The developed electro-conductive cotton fabric was found to be wash durable for 15 washing cycles carried out as per ISO standard 105-C10:2006 (E) test number A (1) (Fig. 6) that conforms to the strong adherence of silver nanoparticles to the fiber surface.

5.2.2

Synthesis of Nanostructures in Textile Fabric

As the success of nanostructures grows in modern society, it does importance of our ability to regulate their synthesis in precise manners, often with atomic precision, as it will directly affect the final properties of the material. Hence, it is crucial to have both deep insights, ideally with the real-time temporal resolution, and

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precise control during the fabrication of nanomaterials. The electron beam radiation primarily involves applying radiation to water to produce active free radicals such as hydrated electrons, hydrogen atoms, and hydroxyl radicals. The hydrated electrons are a robust reducing agent toward oxygen-containing functional groups, such as hydroxyl, epoxies, ketones, and carboxylic acids [34]. Thus, comprehensively, direct synthesis of zero, one, and two dimensional, and surface functionalized ZnO [35], SiO2 , Ag [36] Au [37], TiO2 [38]. Graphene oxide [39] nanostructures using only (or mainly) an electron beam is a relatively recent and up-and-coming manufacturing technique due to its high-speed, chemical reductant-free, efficient, eco-friendly, and easily scalable features. Silver nanoparticles have exciting properties. Their embodiment in the fabric results in enhancing UV protection, as perpetual exposure to UV light may lead to an acceleration of skin aging, photo-dermatitis (acne), erythema (skin redding), sunburn, increased risk of melanoma (skin cancer), eye damage and even DNA damage [40]. Therefore, protection from ultraviolet (UV) light is a desirable attributes needed for consumer textiles. Protection from this radiation can measure in terms of the ultraviolet protection factor (UPF). Higher UPF value means more protection of the material. The UPF value of textile fabric is depends on many aspects, such as its chemical nature, fabric construction, cover factor, presence or absence of dyes, delustrants, organic and inorganic-based UV absorbers, etc. The intensity of UV protection granted by these elements is influenced by the fabric’s washing, deformation, and moisture content [41, 42]. Silver nanoparticles have been generated using chemical, biological, and radiation approaches [43]. However, they were often synthesized using wet chemical methods involving the chemical reducing agent. Moreover, the reduction reactions are often difficult to control and contain many by-products. Alternatively, ionizing radiation methods have been employed excellently to generate metal nanoparticles. This method has significant advantages over wet chemical methods such as ambient temperature process, neat reaction system in terms of by-product formation, and better control over particle size and distribution [44]. Therefore, with these facts, electron beam (EB) radiation is one of the excellent and most efficient eco-friendly methods. It has a very high throughput of electrons in its beam path that can instantly reach water and substrate, generating many reactive species. These reactive species can interact with aqueous-alcoholic silver nitrate salt solution to produce metallic silver. Thite et al. developed ultraviolet radiation-protective cotton fabric by electron beam radiation-induced in-situ synthesis of silver nanoparticles [45]. In that study, initially, cotton fabrics were padded with 0.01%, 0.1%, and 1.0% (wt./vol.) concentration of an aqueous-alcoholic silver nitrate solution and later exposed to 10, 25, and 50 kGy EB doses in an inert atmosphere. The characteristics of the modified fabrics by SEM showed that the silver particles were homogeneously dispersed on the sample’s surface, and their dimensions fell in the nano range (Fig. 8). The percentage of synthesized silver nanoparticles content in modified cotton fabric was measured by ICP-OES spectroscopy testing. The synthesized silver nanoparticles content was more upon increasing solution concentration. The effect

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Fig. 8 SEM image of cotton fabric: (a) control, (b) EB radiation synthesized silver nanoparticles (1% wt./vol., 50 kGy), (c) after washing, and (d) EDS spectra of the treated sample. The images were taken at a magnification of 5 k× [45]

of electron beam dose was also similar, but the prominence of this effect is relatively straightforward at only a high concentration of silver nitrate solution. It could be understood from the interaction of an electron beam with water. Reactive products like hydrated electrons, hydrogen atoms, and hydroxyl radicals are generated [46]. In isopropanol, hydroxyl radicals were converted into another effective species called isopropyl radicals. Silver deposition occurs from the nitrate solution thanks to reducing hydrated electrons, hydrogen atoms, and isopropyl radicals. Increasing the concentration of silver nitrate means more significant numbers of silver ions are present in the system. Similarly, more reactive species might have been generated at a higher dose. The probability of getting reduced to silver metal increases rapidly with concentration. It will explain the noticeable difference in silver particle deposition at different doses at relatively higher concentrations. Silver particle deposition has also provided UVblocking in the UV-Vis region (Fig. 9). The negligible value of UV transmittance that is very close to 0% confirms the less reflection and UV rays blocking property of the treated fabrics rather than transmitting through it. The UPF value at 0.1% AgNO3 (wt./vol.) with 25 kGy EB dose resemble the UPF value of 270 obtained by Shateri-Khalilabad et al. at 0.02M AgNO3 [0.33% (wt./vol.)] solution concentration using chemicals as reducing agents [44]. Thus, the latter method utilized a higher silver nitrate concentration than the EB radiation method for getting approximately the same UPF value. At a low concentration of 0.01% AgNO3 , the UPF values of the fabrics were below the desired value of 40 (Fig. 10). After ten washing cycles, UV-blocking potency has remained very close to before washing samples, suggesting excellent durability toward aqueous washing.

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Fig. 9 Transmittance behavior of fabrics in UV–Vis region at 1.0% (wt./vol.) concentration of silver nitrate solution used for padding [45]

After washing, there was a minimal decrease in UPF value due to the removal of larger nanoparticles. In contrast, the smaller nanoparticles still adhere to the fiber surface. Hence, significant amounts of silver nanoparticles were still present on the surface fibers and in the yarn structure of the fabric, indicating good wash durability. The modified material was also potent for antibacterial activities against two various microorganisms, Staphylococcus aureus and Klebsiella pneumoniae. It was confirmed that UPF and antibacterial properties showed durability after ten standard washing cycles. This functional modification had no appreciable alter in the physical characterization of fabrics, such as bending length and air permeability. Another application of EBI on polyester (PET) was revealed by Zhang et al. [47]; using EBI, the authors developed functionalized QAS-HPs as an antibacterial agent for PET fabrics. The findings of the water contact angle proved that the modified fabric with QAS-HPs via EBI demonstrated significant improvement in wettability. According to the washing durability findings, the cross-linked QAS-HPs on the PET fabric showed efficient and good washing stability. Within 30 min of contact time, the treated PET fabrics had high antibacterial activity, inactivating 100% S. aureus and 90.84% E. coli. In this regard, this study presented a promising environmental finishing technique to produce a durable antibacterial polyester fabric depending on the numerous benefits of EBI on polymer modification.

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Fig. 10 UPF values of silver deposited cotton fabric (UW: unwashed, WC: washing cycles) [45]

5.2.3

Modification/Cross-Linking of Textile Fabric

Recently, Porubská et al. [48] investigated the effect of 0–400 kGy absorbed EB dose in the air on wool fabric. The results found that at a higher absorbed dose, the α-helix was shifted from the majority into the minority non-monotonously. There was only the creation of cystine dioxide and cysteic acid at more than 220 kGy. The improvement of the hydrocarbon groups showed a moderate design of cross-links at 250 kGy. The early stages of breaking the peptide chain were detectable at higher absorbed doses. The tensile break had no effect at all ranges of doses. However, the initial elongation improved; a monotonous reduction was found after that. Silk fiber is called as queen of textile fibers because of its elegance, luster, class, and comfort properties. It is also well-known for its remarkably high extension and energy absorption before break compared to other textile fibers. It gives comfort-ness and warmth-ness to the wearer. The triangular prism-like cross-sectional shape of the fibers allows refraction of light at multiple angles giving the shimmering appearance to the fabric. Despite having many advantages from all the corners, silk fabric creases and tenders after aqueous alkali soap washing. In addition, it loses its strength by 20% in wet conditions. Many organic solvents readily damage it. Microbes quickly grow and affect the silk polymer properties, whereas long exposure to light weakens it. Therefore, silk fabrics require protection from sunlight, harsh chemicals, heat, and washing. Efforts have been made to enhance its protection by cross-linking or coating silk. Cross-linking of silk with various cross-linking agents by wet chemical methods has been reported in the literature [49]. However, these cross-linking methods need high

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curing temperature and time with precise control over the reaction. In addition, crosslinking reactions by chemical processes often generate many by-products. Therefore, a method is essential to reduce the cross-linking time while operating at low temperatures. Alternate to chemical modes, electron beam radiation can be utilized effectively for cross-linking purposes. This approach has significant advantages over wet chemical methods, like the speedy process at room temperature and less by-product formation possible by tuning the radiation dose and its parameters. With this connection, EB radiation is highly efficient in generating many reactive species like hydrated electrons, hydrogen atoms, and hydroxyl radicals [43]. These reactive species could be utilized further for cross-linking or grafting purposes. Triallylisocyanurate (TAIC) is a well-known multifunctional chemical and radiation cross-linker. TAIC monomer has been successfully used for chemical [50] and radiation [51] induced cross-linking of polymer to improve its thermal and mechanical properties. Recently, Thite et al. investigated the influence of electron beam dose on the chemical stability of the cross-linked silk fabric [17]. In their study, the silk fabrics were padded with an aqueous methanolic-TAIC solution followed by electron beam irradiation in 100-300 kGy in an inert environment. The effect of radiation energy on the degree of cross-linking of silk fibers has been studied. The extent of cross-linking of silk by TAIC was determined by sol-gel content analysis by solvent extraction method concerning EB dose [52], shown in Fig. 11. A steady increase in gel content and a decrease in sol content were observed with an increase in EB dose. It can be elucidated that after EB irradiation, the number of free radicals may generate at polymeric chains and TAIC monomer, which the EB dose may decide. Consequently, a more significant number of free radicals developed as the EB dose increased. The free radicals generated at the TAIC monomer could

Fig. 11 Sol-gel contents of untreated and EB-irradiated silk fabrics [17]

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Fig. 12 Charlesby–Pinner plots of cross-linked silk fabrics at 5% error bars [17]

establish the covalent bond with activated species of molecular silk chains and form a secondary structure. Furthermore, this reaction continuously develops a threedimensional polymeric network structure called cross-linking. Simultaneously, the polymer molecular chain scission also occurs because of high EB radiation energy. Therefore, the degree of simultaneous cross-linking and molecular chain scission of EB-irradiated silk in terms of G value was found to be 0.36 from the intercept of the straight line in Charlesby–Pinner graph of S + S 1/2 against the inverse of the absorbed EB dose (D) as shown in Fig. 12. It shows that cross-linking of silk by TAIC is more prominent than the molecular chain scission during EB irradiation. Further, this degree of cross-linking to EB dose was demonstrated by a chemical stability study of silk fabric in 4% sodium hypochlorite, 5% sodium hydroxide, and 70% sulphuric acid solvents in which ordinary silk is readily damaged and dissolved. The photograph (Fig. 13) shows that untreated silk fabric had no chemical stability in all three solvents compared to EB-irradiated silk fabrics. The chemical stability of EB-irradiated and cross-linked silk fabrics increased with EB dose. This enhancement in the chemical strength of silk corresponds to the increase in cross-linking density with the EB dose, which is consonant with sol-gel content data.

5.3 Textile Material Reinforced Composites Fiber-reinforced composites (FRP) were made of fibers embedded in a polymer matrix. Fibers provide strength for load-bearing applications while the polymer

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Fig. 13 Photographs representing the chemical stability of silk fabrics in (a) 4% sodium hypochlorite (b) 5% sodium hydroxide and (c) 70% sulphuric acid [17]

matrix keeps its placement and orientation intact [53]. In thermoset FRP, the matrix is either oligomers or monomers cured to solidify and harden the matrix material [54]. The curing process may be accomplished by thermal, UV, or high-energy radiation methods involving X-rays, γ-rays, and electron beams. There are several advantages of curing carried out by high-energy radiation (electron beam) compared to thermal curing: [55, 56] (i) (ii) (iii) (iv) (v) (vi)

Rapid curing translates into higher throughput. Curing at near room temperature. Lower consumption of energy. Reduction in emission of toxic volatile organic components (VOC) thus beneficial for the environment and worker safety. Simplified tooling and material handling as curing is carried out at near ambient temperature. Better control of curing process in terms of location and degree of crosslinking.

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(vii) Lower cost of radiation curing. (viii) Since radiation curing does not contain an initiator, so the storage stability of resin becomes better. (ix) Reduced residual stress. Unsaturated oligomers used in electron beam cross-linking are epoxy acrylates, polyester acrylates, polyurethane, polyether acrylates, and acrylic acrylates. Reinforcing fibers could be carbon, glass, aramid, or other fiber that shows considerable radiation resistance during the irradiation process. The good part of the EB curing of composites is that it is compatible with classical composite fabrication technologies like prepregs, filament winding, resin transfer molding (vacuum-assisted), and pultrusion [57–59]. Properties of electron beam cured composites are assessed based on the following parameters, such as (a) void volume (%), (b) glass transition temperature, (c) water uptake (%), (d) resin shrinkage (%), (e) density, (f) tensile strength and tensile modulus, (g) compressive strength and compressive modulus, (h) flexural strength and flexural modulus, and (i) interlaminar shear strength (ILSS). The thickness of the resin matrix (density of 1.6 g/cm3 ), which could be cured with a 10 MeV accelerator, is roughly 2.1 cm on one side and 4.2 cm on two-sided irradiation [60–62]. In 1990, the Oak Ridge National Laboratory (ORNL) conducted experiments on curing epoxy-based resins to compare the two methods of curing: thermal curing and electron beam curing. Properties of EB cured composites had similar glass transition temperature flexural modulus but a low value of interlaminar shear strength (ILSS). However, they had low water absorption and low shrinkage. Lower interfacial shear strength is a matter of concern. Electron beam cured composites usually show 30–60% lower interfacial shear strength than their thermally cured counterparts. It is due to the new process of curing utilizing electron beam radiation. Rapid curing of resins at ambient temperature restricts the diffusion of curable resin in the fiber. The curing reaction slows down as the glass transition temperature of curing crosses the curing temperature. The curing temperature depends on the irradiation-induced rise and temperature rise due to the polymerization reaction. It has been found that high-temperature results in higher curing due to increased mobility of monomers and oligomers in the matrix. Thus, the ILSS property depends on (a) the chemical compatibility of fiber and matrix or may depend on the (b) maximal temperature attained during curing versus the glass transition temperature of the matrix, or (c) the thermal conductivity of the fiber employed [63]. The following strategies have been employed to improve ILSS property. (a) Use of sizing agents to improve adhesion of fibers to the matrix: Isocyanatebased sizing agents such as (6-Isocyanto n-hexyl) carboxyethyl methacrylate in an acrylic resin reinforced with carbon fibers substantially improves ILSS between fiber and matrix. This group has synthesized isocyanate urethane methacrylate and isocyanate urethane styrenic compounds as a chemical difunctional sizing agent for carbon fibers. (b) Fiber surface modification: Carbon fibers have been modified by plasma treatment, improving the adhesion property in carbon fiber-reinforced composites.

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(c) Thermal conductivity and heat capacity of fibers: Temperature attainment depends upon used fibers’ thermal conductivity and heat capacity. During the curing process, the temperature of the resin matrix will rise; some conductive fibers, like carbon fiber, will conduct this heat to cooler zones, while glass fibers are relatively inefficient. Higher temperature means higher conversion. The primary beneficiary of EB curing could be the automotive, aircraft, and aerospace industries, where faster curing is crucial for rapid recovery from damage caused by lightning, birds, and other foreign objects. Thus, reducing the downtime of operations, NASA has also explored EB curing composites for satellites and International Space Station. The US army has also developed EB curing technology for military applications. The interpenetrating network of epoxies and acrylates improved interlaminar strength. The automotive industry has utilized EB curing technology with pultrusion and thus increased production rates, reduced cost, and reduced VOC emissions while working at ambient temperature [64] (Table 1). Natural fiber-based composites have been prepared by electron beam crosslinking. Natural fibers of hemp, flax, banana, jute, palm oil leaf, etc., have problems in compatibility with hydrophobic resin, leading to poor adhesion between the fiber and resin. The mechanical properties of pineapple fiber-reinforced composite containing polystyrene polymer as the matrix was improved using trimethylolpropane triacrylate (TMPTA) or tripropylene glycol diacrylate and EB irradiation [65]. Although EB curing of fiber-reinforced composites is technically quite capable, the high equipment cost, shielding cost, and higher cost of EB curable resin are the main detriments to its commercial success. Companies in aerospace manufacturing have found processing cost-saving possible in 25–65%. This saving comes from the unique nature of EB curing technology, such as reduced curing time, allowing in-line manufacturing, ambient temperature processing, reduced tooling cost, and improved resin stability. Thus transportation and storage become easier; reduction in emission of volatile components and issues related to that such as delamination, outgassing and void formation, impact on environment and health issues; flexibility in product designing due to option of location of curing, degree of curing and depth of curing of the product. These advantages are already enjoyed by the aerospace industry, military aviation, space exploration, cryogenic tank manufacturing, and commercial aviation. It has also found potential applications in the transportation industry, sports industry, electronic industry, and military (groans and marine).

5.4 Textile Wastewater Treatment Water is life and a precious resource in the future as it becomes increasingly scarce. Water demand is growing at more than twice the world population’s rate. Over the past 100 years, the world population has increased threefold, while water consumption has risen seven times.

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Table 1 Examples of fiber-reinforced composites cured by electron beam radiation in industries Industry

Applications

Automotive industry Composite concept vehicles from Chrysler corporation: electron beam curable adhesive to join composite structure to the metal surface. The ambient curing process is beneficial due to the difference in plastic and metal thermal expansion Composite armored vehicle panels for the US military: for armor panel manufacturing, vacuum-assisted resin transfer molding, and electron beam curing have been employed Aircraft industry

Use of composite patches for cracks, corrosion repair for aircraft: Carbon fiber prepreg is used for patch repair by placing the patch over the damaged structure using a vacuum bag, thermal gluing using epoxy resin, and then electron beam curing. US Airforce has used this technology for F-18 Hornet, Canadian Acetek Composites Inc for the Airbus fleet. The technology provides advantages of rapid curing at ambient temperature and on-craft curing. It is to be noted that downtime of a commercial flight is very costly, costing half a million euros for every 8 h Windshield frame: US Airforce uses EB curing for the T-38 composite windshield frame. The windshield frame should have a very high impact-resistant property

Space industry

Manufacturing of cryogenic tanks: electron beam curing is the preferred choice here because of reduced internal stress in composite parts, lower micro-cracks, and good hydrogen barrier properties Lockheed and NASA have explored this technology to cure the whole wings of a space shuttle Satellite flywheel: AFS Trinity and NASA explored EB curing of composite structure prepared using filament winding method for satellite flywheel. 30% reduction in cost compared to thermal curing has been reported Manufacturing of satellite antenna: satellite antennas are enormous and should be lightweight and dimensionally stable to operating temperature. Composites are used to make them. Electron beam curing presents advantages of ambient temperature processing, low thermal coefficient of thermal distortion, and economic benefits

The textile industry is one of the most water-consuming industries that utilize a large amount of water and various chemicals during production. The water is mainly used to apply chemicals onto the fibers and rinse the final products. It takes approximately 2500–3000 l of water to manufacture a single cotton shirt. The bulk of this water is required to grow the cotton, followed by the wet chemical processing and finishing process. Therefore, about 200 L of water is used to process 1 kg of textile. It is the first consequence of water shortages already being felt in the textile industry. Further, textile wastewater is another major problem as it is a complex mixture of organic and inorganic compounds with large suspended solids and variable pH [66]. It contains highly suspended matters like fibers, fiber debris, fillers, coating

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materials, soluble particles, dyes, printing gums, fixing agents, bleaching agents, starches, detergents, etc., from various textile processing, as shown in Fig. 14. Wastewater from the textile and dyeing industry represents a significant environmental problem primarily due to the synthetic reactive dyes. Humans manufacture thousands of reactive dyes and use them annually [67]. Further, they are entirely soluble in water; their complex chemical structures contain low-biodegradable groups, so it is impossible to remove them by the conventional or biological treatment. Hence, the extensive amounts of textile wastewater from water bodies may undergo chemical and physical changes under sunlight and increase the biological oxygen demand, inhibiting plants’ photosynthesis and re-oxygenation process and destroying aquatic life. Therefore, textile wastewater can cause bleeding, skin

Fig. 14 Flow diagram for textile processing [67]

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ulceration, nausea, skin irritation, dermatitis, and even cancer; hence unsuitable for agriculture and residential uses [68]. Since textile effluent discharges various pollutants, its treatment methods also vary. Many effluent treatment processes for textile wastewater are reported and used commercially [69–71]. The conventional method mainly employs physical, chemical, and biological phenomena to treat textile effluents. Physical treatment methods such as adsorption and filtration will only transfer pollutants from one medium to another, so further treatment becomes necessary. Chemical coagulation/flocculation is frequently practiced to remove suspended solids and colloidal particles. The next stage of conventional wastewater treatment is the biological method, where microorganism helps to degrade the organic pollutants into simple substances. Bacteria and fungi are the most commonly used microorganisms to treat dye wastewater [68, 72]. Even though such methods are highly efficient, the main drawbacks of such practices are that the excess use of chemicals causes sludge generation and related disposal problems. It results in operating costs and lacks discoloration with certain dyestuffs. Bacteria and fungi also cannot degrade the complex structure of organic compounds. Therefore, conventional microbial and chemical treatments usually are not so effective in removing color from dyestuff [73]. Thus, the degradation of dyes and other hazardous chemicals can be achieved by performing tertiary coagulation, adsorption (activated carbon filtration), membrane filtration, ozonation, and advanced oxidation. Among the available methods, advanced oxidation processes (AOPs) [74] are developing fast where oxidation potential (E 0 ) is higher than that of oxygen (1.23 eV), such as hydrogen peroxide (E 0 = 1.78 eV), ozone (E 0 = 2.07 eV), and the hydroxyl radical (E 0 = 2.28 eV). The widely employed AOPs to eradicate dyes compounds include ozonation, sonolysis, Fenton’s reagent (Fe2+ /H2 O2 ), and photocatalytic oxidation (UV/TiO2 ) [75]. They are commonly used for de-coloration and pollutant degradation by generating hydroxyl free radicals. However, most techniques are limited by costs and difficulties associated with industrial operations. The performance of these processes is also not strong enough due to their selective preference toward removing some target pollutants in a realistic time and cost frame. Therefore, there is an urgent need to develop new technologies to treat textile wastewater that will consume appropriate amounts of chemicals and energy and effectively comply with effluent standards [76]. Ionizing radiation may be one of the most efficient and effective methods which create three short-life intermediates in the water: hydroxyl radicals, hydrated electrons, and the hydrogen atom [77], as shown in Fig. 15. It produces hydroxyl radical as one of the active species with higher oxidation potential, and the reaction time is less than milliseconds with a rate constant in the range of 107–1010 L/mol/sec. The hydroxyl radicals are unstable and highly reactive because of their oxidation potential (E 0 = 2.8 eV). Therefore, hydroxyl radical has an oxidation potential superior to other oxidizing agents [79]. These methods involve generating and using potent but relatively non-selective hydroxyl radicals in sufficient quantities to oxidize most of the complex chemicals in the effluent water and not require the chemicals to form the hydroxyl radicals. Electron beam radiation technology achieves highly effective hydroxyl radical generation

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Fig. 15 Water radiolysis: formation of free radical species in water employing ionizing radiation [78]

[80]. Then decomposition of pollutants is caused due to their reactions with highly reactive species formed from water radiolysis. They reduce the toxic compounds to non-toxic compounds and enhance wastewater’s biodegradability by containing non-biodegradable organic matter solo or combined with O3 , H2 O2 , Fe+2 /H2 O2, and TiO2 /H2 O2 , [80, 81]. EB irradiation has been developed to treat wastewater originating from textile units, particularly the dyeing and printing units that often contain various refractory non-biodegradable pollutants. Some experiments of the EB radiation treatment on actual textile and dyeing wastewaters are listed in Table 2. It can be used effectively to remove color, heavy metals, and water purification before or after the other treatment steps depending on the biodegradable nature of the pollutants [82, 83]. Most studies on EB irradiation on textile wastewater focused on a single pollutant. However, in real textile effluent, dye substances are present with other components viz, size, alkali, salts, pigment, urea, and di-ammonium phosphate, which does not often result in decolorization easily, as mentioned in the literature [84]. Therefore, electron beam radiation technology has been employed pre- and post-treatment on mixed effluent that comprises all constituents related to scouring, de-sizing, dyeing, and printing with known concentrations [77, 85]. Kim et al. [86] conducted an experiment using an EB accelerator (1.0 MeV, 40 kW) on real wastewater collected from Textile Industry Complex at Daegu, Korea. The raw wastewater parameters were total phosphorus of 2.5 mg/L, total nitrogen of 38 mg/L, pH of 11, a temperature of 42 °C, BOD5 of 1184 mg/L, and COD of 1805 mg/L. Total nitrogen, BOD5 , and COD are categorized as high-strength contaminants. The experiment also investigated the coupling of EB pre-treatment and aeration biological treatment for textile wastewater. The results show that EB radiation could accelerate the aeration efficiency with a low dose (1.0 kGy). For example, the BOD5 /COD ratio was incremented after irradiation. The results proved that EB could turn recalcitrant

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Table 2 Some experiments of the EB radiation treatment on actual textile and dyeing wastewaters S. No.

Employed Parameters EB-absorbed dose investigated

Changes in parameters

Remarks

References

1.

EB up to 1 kGy

BOD5 , COD

BOD5 /COD ratio improved from 0.68 to 0.79

Refractory organic substances converted into biodegradable intermediaries

[86]

2.

EB with coagulation (3 kGy)

BOD5 , COD, TOC, Color index

BOD5 : Aerated solution [87] 1620-700 with coagulation ppm; TOC: 1000-305 ppm

3.

EB up to 20 kGy

BOD5 , COD, TOC,

Increase in removal efficiency up to 30–40%

EB irradiation with biological treatment

[88]

4.

EB up to 20 kGy with biodegradation

BOD5 , COD, TOC,

Reduction in time by 50% with similar efficiency

Industrial-scale treatment

[89]

5.

EB up to 20 kGy

Color toxicity

55–96% removal; 33–55% reduction

Three distinct effluents from the textile industry

[90]

organic compounds into readily biodegradable compounds, enhancing efficiency and decreasing the cost of a further biological treatment step. Further, Khomsaton et al. [91] conducted similar experiments by using the EPS 3000 EB machine to treat combined textile (900–3000 mg/L COD, 100–150 mg/L BOD5 , and above 1000 ADMI unit color) and food processing wastewater (530– 1000 mg/L COD, 200–400 mg/L BOD5, and 50–100 ADMI unit color). The current and energy of EBs were 30 mA and 1.0 MeV, respectively, with the dose of 100 kGy irradiation at room temperature. The biological experiments after irradiation tests were conducted using an activated sludge process (DO > 4 mg/L). DO, pH, and MLSS were tested daily to ensure the system worked. The first system was treated for the mixture of non-irradiated textiles and food industry wastewater, and the second for the mix of irradiated textiles and food processing wastewater. Results show that COD removal efficiency and color unit after irradiation reached 62.4% and 379.3 ADMI compared to 29.4% and 899.5 ADMI with a single biological treatment. A new membrane bioreactor (MBR) combined with EB (10 MeV, 100 kW; Rhodotron TT 200, IBA, Belgium) was explored by Sun et al. [92] on real textile wastewater containing polyvinyl alcohol (PVA) at Suzhou, China. The influent contained 90–100 mg/L of PVA, 211.7 ± 26.3 mg/L of COD, and a pH of 8.1 ± 0.2 with around 0.1‰ nutrient solution. The influence factors of the EB system, such as current intensity, scan length, and flow velocity of the wastewater, were controlled

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from 0 to 10 mA, 0 to 100 cm, and 0 to 12 m/min, respectively. The coupling process included a batch EB pre-treatment and continuous MBR treatment, i.e., the irradiated wastewater from the EB was continuously transferred to the MBR. The sample was obtained from a real aeration tank in a textile wastewater treatment plant (Suzhou, China). The COD variation in the wastewater uses individual biological, individual EB, and EB combined biological treatment. At the stable operation stage (36–86 days), the 12 kGy EB process militates approximately 14% of COD compared to raw wastewater. The EB and activated sludge hybrid showed a better result; the COD removal capacities could reach 45% in 86 working days. The effect of irradiation dose and dye concentration was evaluated by Ting and Jamaludin [83] with the textile wastewater from Rawang integrated industrial park (RIIP), Malaysia. The natural dyeing wastewaters were a mixture of reactive dyes (80–90%) and dispersed dyes (10–20%). The pH of raw dyeing wastewater at RIIP was alkaline (ranging from 10.35 to 10.45), while COD, color, and TSS were 515 mg/L, 1900 color unit (CU), and 250 mg/L, respectively. Before irradiation, the raw wastewater was diluted by distilled water to the desired concentrations of color at 255 CU (abs = 0.072), 520 CU (abs = 0.147), 990 CU (abs = 0.280), and 1900 CU (abs = 0.539). The EB radiation used a 3.0 MeV and 30 mA electron accelerator (Nissin, Japan). The accelerator was controlled by an automatic control system that could alter the current intensity (5–20 mA), the flow velocity of the conveyor (0– 12 m/min), and the sample thickness was 3.0 mm. The irradiation desired dose was adjusted following EB’s current (mA) and the speed of the conveyor. The experiment was implemented on the Petri dishes in a batch system at the following doses: 0.5, 2.4, 8, 18, 41, 53, 108, and 215 kGy at room temperature. The results indicated that after 108 kGy, the decolorization efficiencies of all concentrations reached over 80%. S. Baride et al. from our organization (BTRA, Mumbai) conducted EB irradiation experiments on a mixed textile effluent containing scouring, de-sizing, dyeing, and printing with known concentrations [77]. The results are shown in Fig. 16. The results show that the de-sizing, dyeing, and printing processes mainly contribute to non-biodegradable contaminants in textile effluent. Further, the results showed that e-beam irradiation pre-treatment did not improve biodegradability of textile effluent even when it was irradiated up to 80 kGy. Contemporary e-beam irradiation as post-treatment to biologically treated samples could significantly enhance the biodegradability even at the shallow dose of 1 kGy, wherein hydroxyl radical played a very active role. On the other hand, electron beam radiation did not help enhance the biodegradability of printing effluent and textile wastewater containing printing constituents. The results showed that EB irradiation before the activated sludge process was ineffective in improving the biodegradability of raw textile wastewater even at a higher dose of 80 kGy. However, EB post-treatment significantly enhanced the biodegradability of biologically treated textile wastewater containing printing effluent constituents, with a lower amount of 1 kGy. Apart from this, the electron beam treatment seems to be a promising alternative for heavy metal removal from textile effluent. It depends strongly on the initial characteristics of effluent, such as organic, inorganic, biological composition, and pH, due to their competition for reactive specimens’ consumption produced by the irradiation process [93, 94].

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Fig. 16 Effect of activated sludge process on the degradation of simulated textile wastewater viz. scouring, de-sizing, dyeing, printing, and mixed effluent in terms of (a) COD reduction, (b) BOD reduction, and (c) BOD/COD ratio [77]

6 Future Scope The twenty-first century is when sustainability is the core of every research and development project. Electron beam processing has many technical, economic, and environmental advantages making it one of the green technologies available to scientists. Although there are many benefits to using EB radiation in processing textile materials, only some companies have been using these techniques in reality. The application of this technique in practice has been limited, primarily due to the high capital cost, space requirement in a series of product development, and high-energy consumption.

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Further, if the materials contain volatile organic compounds (VOCs), EB treatment may result in toxic by-products, resulting in additional care and recovery costs. Nevertheless, electron beam radiation-induced textile modifications and incorporation of newer functionalities in the corresponding textile material are still thrust among the research community. There is a growing demand for intelligent and technical textiles, including smart coating, energy storage textiles, stimuli-responsive and biomimetic textiles, etc. There is a tremendous scope to use electron beam radiation to develop these products. Electron beam radiation is interested in modifying and enhancing the performance of specialty technical textile fibers such as Kevlar, Nomax, Silicon carbide, UHMWPE, carbon, and many more. It can also modify the product to reduce water pollution and synthesize various nanomaterials in the textile substrate. The use of electron beam radiation may be explored for the interphase designing of micro and nanocomposites to improve the adhesion between polymer matrix and reinforced textile materials. It may be widely used to treat actual textile wet processing effluent at a common effluent treatment plant (CETP) to reduce the environmental chemical and biological burden.

7 Conclusions The use of EB technology is almost relatively new in textiles, but the developments in EB technology have helped achieve excellent performance in this field. It is an advanced, clean, solvent-free, time-saving, and being ecological approach with proper handling and operation properties. Hence, this technology has gained more attention as it appears to be a promising economically and environmentally sustainable alternative to traditional wet chemical processing. Currently, e-beam is extensively employed for the surface modification and functionalization of natural and synthetic high-performance textile fiber, fabrics, and fiber-reinforced composites. In addition, it is also now being considered an effective method to treat actual textile processing effluent. Overall, electron beam radiation technology in the textile field leads to the development of alternative sustainable, revolutionary advanced techniques and materials.

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