Cellulose Composites: Processing and Characterization 9783110768787, 9783110768695

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Pawan Kumar Rakesh, J. Paulo Davim (Eds.) Cellulose Composites

Advanced Composites

Edited by J. Paulo Davim

Volume 15

Cellulose Composites Processing and Characterization Edited by Pawan Kumar Rakesh and J. Paulo Davim

Editors Prof. Pawan Kumar Rakesh National Institute of Technology Uttarakhand Srinagar Garhwal 246174 Uttarakhand India [email protected] Prof. Dr. J. Paulo Davim Department of Mechanical Engineering University of Aveiro Campus Santiago 3810-193 Aveiro Portugal [email protected]

ISBN 978-3-11-076869-5 e-ISBN (PDF) 978-3-11-076878-7 e-ISBN (EPUB) 978-3-11-076884-8 ISSN 2192-8983 Library of Congress Control Number: 2022945746 Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the internet at http://dnb.dnb.de. © 2023 Walter de Gruyter GmbH, Berlin/Boston Cover image: gettyimages/thinkstockphotos, Abalone Shell Typesetting: Integra Software Services Pvt. Ltd. Printing and binding: CPI books GmbH, Leck www.degruyter.com

Preface The development of cellulose composites is a very challenging task due to the different physical and mechanical properties of natural fibers and biodegradable matrices. The chemical compositions of natural fibers like cellulose, hemicellulose, lignin, and wax vary depending on the types of fibers and their processing routes. The aim is to discuss factors that are directly dependent on the physical, chemical, and mechanical properties of cellulose composites that may influence the selection of composite materials for product development. The chemical treatments of natural fibers are one of the important processing techniques before reinforcing them into matrix materials. This book emphasizes (a) the influence on processing parameters in understanding the composite fabrication process, (b) deliberations on laminate joining, and (c) failure prediction of composite laminates, highlighted with a suitable diagram. This is a reference book for the subject “composite materials and processing.” We would like to thank all the contributors for their support. The editors acknowledge Dr. Christene Smith and Melanie Götz, De Gruyter, for their continuous support throughout the editing process. Finally, we would like to thank the god almighty “Ladesar Bhagwan.” Dr. Pawan Kumar Rakesh NIT Uttarakhand

https://doi.org/10.1515/9783110768787-202

Prof. J P Davim University of Aveiro

Contents Preface

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List of contributors

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Rennan F. S. Barbosa, Rafaela R. Ferreira, Lucas R. Gonçalves and Derval S. Rosa 1 Biodegradable polymer-based natural fiber composites

1

Ayfer Dönmez Çavdar, Sevda Boran Torun, Emrah Peşman, Naile Angin, Murat Ertaş and Fatih Mengeloğlu 2 Hybrid thermoplastic composite reinforced natural fiber and inorganic filler 21 Chuanwei Lu, Chunpeng Wang, Qiang Yong, Fuxiang Chu and Jifu Wang 3 Processing of non-all-wood cellulose-based composites 77 Jorge S. S. Neto, Henrique F. M. de Queiroz and Mariana D. Banea 4 Recent developments in nanofiller-modified natural fiber composites

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Lalit Ranakoti, Manoj Kumar Gupta, Dharamvir Mangal and Pawan Kumar Rakesh 5 Recycling of polymers and its application 137 Pawan Kumar Rakesh, Lalit Ranakoti and Manoj Kumar Gupta 6 Mechanical properties of chemically treated cellulosic fiber-reinforced polymer composites 151 Garima Raghav and Pawan Kumar Rakesh 7 Numerical simulation on lap joint configurations of glass fiber-reinforced polyester composites with natural fillers 163 Index

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List of contributors Derval S. Rosa Center for Engineering, Modeling, and Applied Social Sciences (CECS), Federal University of ABC (UFABC), Santo André, Brazil [Postal address: Av. dos Estados, 5001. CEP 09210-210. Santo André – SP – Brazil] e-mail: [email protected] Rennan F.S. Barbosa Center for Engineering, Modeling, and Applied Social Sciences (CECS), Federal University of ABC (UFABC), Santo André, Brazil Rafaela R. Ferreira Center for Engineering, Modeling, and Applied Social Sciences (CECS), Federal University of ABC (UFABC), Santo André, Brazil Lucas R. Gonçalves Center for Engineering, Modeling, and Applied Social Sciences (CECS), Federal University of ABC (UFABC), Santo André, Brazil Ayfer Dönmez Çavdar Department of Forest Industry Engineering, Faculty of Forestry, Karadeniz Technical University, 61080 Trabzon, Turkey Sevda Boran Torun Department of Material and Material Processing Technologies, Arsin Vocational School, Karadeniz Technical University, 61900 Trabzon, Turkey Emrah Peşman Department of Forest Industry Engineering, Faculty of Forestry, Artvin Coruh University, 08100 Artvin, Turkey Naile Angin Department of Forest Industry Engineering, Faculty of Forestry, Bursa Technical University, 16310 Bursa, Turkey

https://doi.org/10.1515/9783110768787-204

Murat Ertaş Department of Forest Industry Engineering, Faculty of Forestry, Bursa Technical University, 16310 Bursa, Turkey Fatih Mengeloğlu Department of Forest Industry Engineering, Faculty of Forestry, Kahramanmaraş Sutcuimam University, 46100 Kahramanmaraş, Turkey e-mail: [email protected] Jifu Wang Institute of Chemical Industry of Forestry Products, Chinese Academy of Forestry, Nanjing 210042; Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, International Innovation Center for Forest Chemicals and Materials, Nanjing Forestry University, Nanjing 210037, China e-mail: [email protected] Chuanwei Lu Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, International Innovation Center for Forest Chemicals and Materials, Nanjing Forestry University, Nanjing 210037, China Chunpeng Wang Institute of Chemical Industry of Forestry Products, Chinese Academy of Forestry, Nanjing 210042; Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, International Innovation Center for Forest Chemicals and Materials, Nanjing Forestry University, Nanjing 210037, China Qiang Yong Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, International Innovation Center for Forest Chemicals and Materials, Nanjing Forestry University, Nanjing 210037, China

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Fuxiang Chu Institute of Chemical Industry of Forestry Products, Chinese Academy of Forestry, Nanjing 210042; Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, International Innovation Center for Forest Chemicals and Materials, Nanjing Forestry University, Nanjing 210037, China Mariana D. Banea Federal Center for Technological Education of Rio de Janeiro (CEFET/RJ), Av. Maracanã, 229, 20271-110 Rio de Janeiro, Brazil Jorge S.S. Neto Federal Center for Technological Education of Rio de Janeiro (CEFET/RJ), Av. Maracanã, 229, 20271-110 Rio de Janeiro, Brazil Henrique F.M. de Queiroz Federal Center for Technological Education of Rio de Janeiro (CEFET/RJ), Av. Maracanã, 229, 20271-110 Rio de Janeiro, Brazil e-mail: [email protected] Pawan Kumar Rakesh Mechanical Engineering Department, National Institute of Technology Uttarakhand, Srinagar Garhwal, Uttarakhand, India Lalit Ranakoti Mechanical Engineering Department, Graphic Era Deemed to be University, Dehradun, Uttarakhand, India

Manoj Kumar Gupta Mechanical Engineering Department, H.N.B Garhwal University, Srinagar Garhwal, Uttarakhand, India Dharamvir Mangal Mechanical Engineering Department, Gautam Buddha University, Greater Noida, Uttar Pradesh, India e-mail: [email protected] Pawan Kumar Rakesh Department of Mechanical Engineering, National Institute of Technology Uttarakhand, Srinagar Garhwal, Uttarakhand, India Lalit Ranakoti Mechanical Engineering Department, Graphic Era Deemed to be University, Dehradun, Uttarakhand, India Manoj Kumar Gupta Mechanical Engineering Department, H.N.B Garhwal University, Srinagar Garhwal, Uttarakhand, India e-mail: [email protected] Pawan Kumar Rakesh Department of Mechanical Engineering, National Institute of Technology Uttarakhand, Srinagar 246174, Uttarakhand, India Garima Raghav Department of Mechanical Engineering, National Institute of Technology Uttarakhand, Srinagar 246174, Uttarakhand, India e-mail: [email protected]

Rennan F. S. Barbosa✶, Rafaela R. Ferreira, Lucas R. Gonçalves and Derval S. Rosa✶

1 Biodegradable polymer-based natural fiber composites Abstract: The increase in global environmental impacts has driven the development of eco-friendly and biodegradable materials for the next product generation. The use of synthetic fibers presents ecological concerns, while natural fiber application as a reinforcing no-material in composite development has stood out as a viable alternative. Natural fibers present attractive properties such as biodegradability, renewability, high specific strength, and specific modulus that make this material suitable for composites development in different areas such as packaging, automotive, sports, aerospace, medical devices, and so on. However, some challenges still limit its broad application, including poor interfacial adhesion between matrix and natural fibers, poor compatibility between non-polar matrix and natural polar fiber, poor moisture absorption, fire resistance, low impact resistance, low thermal stability, and low durability, which need to be addressed before processing. Furthermore, during the manufacture of this type of composite, the processing conditions, fiber loading, and inherent fiber properties must be evaluated, as all these factors affect the properties of the composites and may promote defects in the products. The wide variation in the characteristics of composites based on natural fibers presents a significant challenge to understanding the properties of these systems and ways to optimize them. This review seeks to infer, analyze, and optimize the characteristics of composite materials reinforced with natural fibers in relation to different types and sources of natural fibers, processing, modification techniques, physical, and mechanical behavior toward sustainable products. Therefore, this review aims

Acknowledgments: The authors thank the Federal University of ABC (UFABC) and the São Paulo Research Foundation (FAPESP) (2020/13703-3 and 2021/08296-2), and National Council for Scientific and Technological Development (305819/2017-8). The authors also thank the technical support of the Multiuser Experimental Center of UFABC (CEM-UFABC), CECS (UFABC), and REVALORES for assistance. ✶ Corresponding authors: Rennan F. S. Barbosa, Derval S. Rosa, Center for Engineering, Modeling, and Applied Social Sciences (CECS), Federal University of ABC (UFABC), Santo André, Brazil, Av. dos Estados, 5001. CEP 09210-210. Santo André – SP – Brazil, e-mails: [email protected]; [email protected] Rafaela R. Ferreira, Lucas R. Gonçalves, Center for Engineering, Modeling, and Applied Social Sciences (CECS), Federal University of ABC (UFABC), Santo André, Brazil

https://doi.org/10.1515/9783110768787-001

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to better understand the behavior of green composites and promote the increased use of renewable resources in advanced materials. Keywords: Natural fibers, hybrid composite, mechanical performance, polymer composite

1.1 Introduction Global plastic production has grown considerably, with a market responsible for the annual production of approximately 450 million metric tons [1]. Plastic materials are composed of polymeric structures and combine good mechanical properties, inert characteristics, low density, and low cost, making them highly attractive in different applications, with commonly observed single-use applications. However, despite the convenience that these materials present, their inadvertent use has promoted unexpected consequences to the environment, raising global attention about these materials [2]. Due to their inert properties, the polymeric materials usually applied are nonbiodegradable, which imposes ineffective end-of-life (EoL) options associated with recent price rises of raw material that have considerably impacted the plastic industry [3]. Conventional synthetic polymers have become a disadvantage in the EoL phase, due to their accumulation in the environment for a long time [4]. Furthermore, due to the scarcity of petroleum resources and emerging environmental concerns, a synergistic motivation has been encouraged in the development of new products that conciliate sustainable properties and reduce petroleum dependence [5]. This process is also stimulated by the growing consumer awareness and concerns, disseminated by green marketing that promotes new guidelines on recycling, social influence, and changing cognitive values, leading consumers to prefer ecologically correct products [6]. Therefore, there is an urgency to develop ecologically correct technologies applied in various sectors to minimize environmental impacts among the polymeric materials [7]. Biopolymers are renewable and eco-friendly products that usually present biodegradable properties and can be used to replace petroleum-based polymers [8]. However, these materials usually present lower impact strength, tensile strength, permeability, and thermal stability when compared to conventional materials [9]. Thus, the best approach to improve the properties and commercial importance of biopolymers is the incorporation of micro or nano reinforcements [10], resulting in eco-friendly polymeric composites or biopolymer composites [11]. These biopolymer composites offer attractive opportunities for automotive applications [12], aeronautical [13], biomedical [14], and food packaging [1]. Compared to pristine polymers, these composites are lighter and have improved rheological, thermal, mechanical, and barrier biodegradation properties [15].

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Abdillah e Charles [16] developed arrowroot starch/iota-carrageenan films at different concentrations. The results showed that the developed composites presented superior mechanical properties than pristine cassava starch film and improved barrier properties, tensile strength, and swelling properties [16]. Khoo, Chow, and Ismail [17] compared the properties of sugarcane bagasse fiber cellulose nanocrystals (SBFCNC) and microcrystalline cellulose-derived cellulose nanocrystals (MCC-CNC), evaluating the effects of both materials after incorporation into poly(lactic acid) (PLA). They observed that CNC types influence the tensile, thermal, and UV protection properties of PLA nanocomposites. In this study, the performance of SBFCNC was superior to that of MCC-CNC in terms of tensile properties and UV protection for PLA bio nanocomposites, demonstrating its potential as bio-based nanofiller for PLA [17]. Research in this field has indicated that biodegradable polymer composites are receiving improved applications due to their excellent mechanical properties, compatibility, and biodegradability [18, 19]. Thus, the main objective of this chapter is to highlight the main properties of biopolymers, along with the incorporation of ecologically correct micro or nano reinforcements, the modifications that reinforcements can undergo to enhance their properties accompanied by conventional and less common processing techniques for the manufacture of biocomposites. This chapter also discusses the main advanced applications of such biocomposites to meet the sustainable demands of advanced materials.

1.1.1 Polymers and their properties A polymer is a macromolecule with high molecular weight that is composed of repeating structural units (called monomers) that, usually, are connected by covalent chemical bonds. Polymers can be classified as synthetic, semisynthetic and natural. Synthetic polymers are synthesized in the laboratory by a process called polymerization, in which the monomers react under a controlled environment originating the polymer chain. Some examples of synthetic polymers are nylon, polyethylene, polystyrene, synthetic rubber, polyvinyl chloride, and Teflon, among others. On the other hand, natural polymers are found in nature, generally from plants and from animal sources. The modification of natural polymers by chemical treatment to change their properties leads to semisynthetic polymers [20]. Nowadays, every industry aims to reduce fossil fuel-based materials, leading to a crescent need for environment-friendly sustainable materials and their development [21]. Most polymers used today are synthesized from fossil fuels and usually are nonbiodegradable. With this in view, natural polymers have gained attention in recent years, since they are abundant, renewable, and easily accessible. Furthermore, natural polymers are more attractive than synthetic polymers and semisynthetic polymers because they are inexpensive, biocompatible, capable of chemical

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modifications, readily available, and biodegradable [20, 22]. These characteristics make the natural polymers excellent candidates for a wide range of applications in medicine [22], pharmacology [20], and industries including packaging [18], cosmetics [23], electronics [24], agriculture [25], water treatment [26], and biosensors [27]. The term biopolymer is widely used to refer to natural polymers; however, this term is, in some cases, wrongly applied. Some terms such as biopolymers, bio-based polymers, bioplastics, and biodegradable polymers are ambiguous and may confuse the broad audience. Biopolymers usually refer to the source for their products and are usually biodegradable, but they do not include synthesized biodegradable polymers. On the other hand, biodegradable polymers are related to the polymer property and may include synthesized or non synthesized polymers. Biodegradable or nonbiodegradable polymers produced from renewable resources are bio-based polymers. Finally, bioplastics do not necessarily have to come from biological sources, but they may be biodegradable, like polycaprolactone (PCL) and polybutylene succinate (PBS), petroleum-based polymers that are biodegradable [22]. Based on their definition, biopolymers can be divided into two groups: biopolymers from animals and from plants. The production of biopolymers from both sources occurs through their enzymatic activities, an essential process for their metabolism. Different naturally produced polymers include polysaccharides, proteins, lipids, microbial polyesters, polyphenols, and essential oils [22]. Among these polymers, the group that stands out is the polysaccharides, the most abundant macromolecules in the biosphere and one of the main structural elements of plants and animals [28]. They are found in abundance in nature from sources such as algae, plants, and animals and they can also be produced using recombinant DNA techniques [20]. Examples of polysaccharides obtained from animals are chitin, alginates, carrageenan, psyllium, and xanthan gum, while polysaccharides obtained from plants include hemicellulose, cellulose, pectin, glucomannan, agar, starch, and inulin [29]. The most important biopolymers from plants are the components of the natural plant fiber. They are present in the cell walls of plants, ensuring the necessary mechanical properties for plant activities [30]. For most plant fibers, the values of specific strength and stiffness are comparable to those of synthetic fibers. Moreover, when comparing the densities, synthetic fibers have higher densities (≈2.5 g/cm3) than those found in plant fibers (≈1.4 g/cm3) [30, 31]. With weight an essential factor for most structural applications as they have high strength and a low density, natural plant fibers have excellent potential for application as reinforcements in composite materials [31]. In addition, they are environment-friendly due to their characteristics such as CO2 neutrality, biodegradability, renewability, and recyclability, unlike synthetic and petroleum-based fibers, which pollute the environment and are not biodegradable [32].

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1.1.1.1 Composite materials Composite materials can be described as a mixture of two or more materials to obtain a final material with better properties than each constituent material. These materials are formed by a matrix that incorporates reinforcement arrangements, which can be polymeric, metallic, or ceramic. The matrix is called the continuous phase, and the reinforcement is called the discontinuous phase; generally, the composites are composed of one or more discontinuous phases distributed in a continuous phase. The number of possible combinations of materials to produce composite materials is practically infinite, allowing different formulations. The properties of these formulated materials depend on many factors, such as matrix nature, types of reinforcements, relative amounts of these elements, and the processing technique [33–35]. The most commonly used composites are known as large diffusion composites, representing more than 95% of current production. The principal areas that consume these materials are the transport industries, mainly the automobile and the electrical construction. These materials are usually fiberglass and unsaturated or phenolic polyester resin, nonbiodegradable synthetic materials with negative environmental impacts [33]. In the twenty-first century, the biggest challenge faced by researchers is finding a way to reduce the environmental impacts caused by these hazardous materials [36]. Given this scenario, natural fibers are gaining attention as an alternate to conventional fibers, in the production of composite materials [37]. Studies have shown that natural fibers, due to their mechanical properties comparable to those found in synthetic fibers, such as high strength, high stiffness, and low density, can be used in the production of natural fiber hybrid composites, in order to replace synthetic fiber-reinforced composites used in structural or semistructural applications [38]. In addition, they are eco-friendly materials because of their characteristics such as biodegradability, renewability, and recyclability [31]. Thus, these composites have been gaining ground in many areas such as automobile, construction, aircraft, medical, and electronics industries [21, 37]. Studies related to the production of natural fibers-reinforced composites to reduce weight in the transport area are increasing, mainly in the automobile and aerospace industries [21]. The use of a polymer composite reinforced with natural fibers for automotive application was analyzed by Ramasubbu and Madasamy. They compared the mechanical properties and water absorption capacity of three composites: sisal/kenaf fiber reinforced with epoxy matrix hybrid composite (HC), sisal fiber composite (SFC), and kenaf fiber composite (KFC). The results obtained showed that HC presented better properties when compared to the other composites and could be used for automobile applications without great loads [39]. Sarikaya, Çallioğlu, and Demirel evaluated the influence of using different types of fibers as reinforcement on the mechanical properties of composite materials with epoxy resin matrix. The composite materials were produced using three

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types of natural fibers: birch, palm, and eucalyptus. In this study, it became clear that the type of fiber used as reinforcement has great influence on the strength characteristics of the composites [21]. Otto et al. [40], also evaluated the influence of different types of natural fibers on the development of composites with polyurethane (PU). Composites were made by substituting up to 20% w/w of the polyethylene glycol with one or the mixture of three natural fibers: sugarcane bagasse, sisal, and rice husk. The evaluation of their mechanical properties indicated that sugarcane, rice husk, and sugarcane/rice husk hybrid composites were superior to the sisal composites. These materials have shown potential for application in shock absorbing and padding materials, being an economical and sustainable alternate to conventional PU composites [40]. Ashik, Sharma, and Gupta compared the efficiency of composite materials with epoxy resin matrix reinforced with two types of fibers: coconut fiber and fiberglass. Mechanical and water absorption tests were realized, which indicated that the composites reinforced with coconut/glass fiber showed greater water absorption when the percentage of coconut fiber was higher, and higher flexural strength was also observed. The results indicated that incorporating these types of fiber can improve strength and work as an alternate to the use of glass fiber [41].

1.1.1.2 Green reinforcements In composite materials, reinforcements, also known as fillers, are structures that modify the mechanical, electrical, or thermal properties of composites, when included in the matrix. They can be of organic or inorganic origin. Currently, most industries use inorganic materials as reinforcements or fillers. The main and most widespread material used as reinforcement is glass fibers, representing 95% of the applications [33]. The rubber industry, for example, uses hazardous nonbiodegradable materials as fillers: precipitated silica, clay, and calcium carbonate [42]. Green reinforcement refers to eco-friendly materials used as reinforcements or fillers in composite materials. These materials are mostly natural fibers from different plants such as jute, sisal, isora, coir, wheat bran, pineapple leaf, silk waste, bamboo, agave, kenaf, oil palm, hemp, and banana [42]. The natural fiber itself can be considered a composite material, with a matrix composed of hemicellulose and lignin reinforced by cellulose microfibrils. The cellulose microfibrils provide the natural fiber with excellent mechanical properties like high strength and stiffness. Furthermore, natural fibers have a low density than synthetic fibers and are biodegradable, inexpensive, and renewable [30, 31]. Due to these characteristics and the need to increasingly reduce the use of nonbiodegradable materials, the use of natural reinforcements/fillers is growing. Several works related to the production of composites reinforced with green reinforcements are found in the literature [42]. Marto and Othman evaluated the use

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of bamboo as soft clay reinforcement in embankment construction. They monitored the settlement and lateral movement of three built embankments: Embankment on Bamboo-Geotextile Composite (BGC)-reinforced, Embankment on High-Strength Geotextile (HSG)-reinforced, and control or unreinforced (UR) embankment. The results showed that the BGC-reinforced embankment showed better results than the others embankments (HSG and UR), decreasing deformation and increasing soil stability [43]. Pereira et al. [44] analyzed the influence of partial replacement of Portland cement by rice husk ash (RHA) to enable the use of green coconut husk fiber as reinforcement for cementitious matrix. The results obtained indicated that RHA as a substitute for Portland cement helped maintain the mechanical behavior of the green coconut fiber, providing a lightweight composite with better mechanical performance [44]. Dominic et al. evaluated the use of cellulose nanofibers (CNFs) extracted from Cuscuta Reflexa (a parasitic plant) as fillers in natural rubber (NR). The study indicated that using CNF as reinforcements in NR improved its processing properties and performance. Therefore, CNF could be potential green filler for developing natural rubber composites [42]. Zhu et al. used waste rubber particles and solid waste fly ash from industrial and mining enterprises to produce a new type of grout material in coal seam floor reinforcement. Tests conducted indicated that the material presented good physical and chemical properties, meeting the requirements for the strength and impermeability of the grouting floor [45].

1.2 Natural fibers The natural fibers are composed of structures that present a small diameter compared to their length, resulting in structures with a high aspect ratio (length/diameter). They may be obtained from plant, mineral, and animal sources, but the plant fibers are the most abundant fiber observed, with higher source availability. These fibers are attractive since they possess renewability, low density, high strength, and elasticity modulus and are biodegradable. These fibers may be obtained from different plant structures like steam (rice, bamboo, corn, wheat, bagasse), leaf (abaca, pineapple, sisal), seed (wider, kapok and cotton), xylem (flax and bast), bark (rosella, jute, hibiscus, and ramie), and fruit (palm and coconut) [46–48]. They have a cell wall that presents a complex polysaccharide structure mainly composed of cellulose, hemicellulose, and lignin. They also possess pectin, waxes, water-soluble substances, and oil, in lower content. The content of each component of the fiber cell wall depends on plant species, cell function, stage of development, and environmental factors [49]. The structure and the chemical constituents of the plant fiber are usually combined in a complex structure, where the fibers are composed of rigid crystalline

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cellulose nanofibrils combined into microfibrils. In this structuring, cellulose forms slender rod-like crystalline microfibrils surrounded by a hemicellulose matrix. Meanwhile, lignin works as a kind of “cement,” solidifying the cell wall [30]. Cellulose is the most abundant component in fiber composition and is responsible for ensuring fiber with high tensile strength and Young’s modulus, due to its crystalline structure [50]. The presence of lignin impacts the crystalline arrangement and reduces the fiber tensile strength. Chemically, cellulose is a hydrophilic glucan polymer formed by glucose units linked together through β (1→4) bonds, with high moisture absorption capacity due to free hydroxyl surface groups [51, 52]. Lignin is the second most abundant biopolymer in the fiber cell wall, responsible for improving its structural support. It is linked via ester bonds with hemicellulose and is composed of three precursors as active functional groups, namely coniferyl alcohol (G), p-coumaryl alcohol (H), and synapyl alcohol (S). The dominant linkage in lignin is aryl ether linkage (β-O-4), with about 50% [31]. The higher lignin content usually decreases the fiber’s mechanical strength properties but may improve UV degradation and char formation [31]. Hemicellulose is the third most abundant polysaccharide in the cell wall that presents branched structures and acts as a linkage between cellulose and lignin structures, promoting a network structure. Due to this interaction, the higher hemicellulose content usually impacts cellulose crystallinity and is responsible for reducing fiber mechanical properties [53]. Azwa et al. and coworkers indicated that hemicellulose improves moisture sorption, thus impacting fiber biodegradation [54].

1.2.1 Cellulose and nanocellulose Since cellulose is responsible for the superior mechanical properties of natural fibers, there is research interest in its isolation for new applications. Cellulose may be extracted using different approaches, and the chemical methods have stood out. Within these the use of sulfuric acid, followed by chlorination, alkaline treatment, and bleaching [55] are highlighted. Another method is using sodium bisulfate solution and alkaline treatment [54]. Additionally, acid-chlorite treatment using sodium chlorite acidified with glacial acetic acid has also been used to isolate cellulose [56]. Each method is responsible for removing the amorphous content in fiber structure, including lignin, hemicellulose, and wax. These extraction methods are responsible for producing microcellulose (MCC), composed of microfibrils with amorphous and crystalline regions linked by van der Waals interactions and hydrogen bonding [57], with crystallinity degrees usually ranging from 55% to 80%. MCC is widely used in the pharmaceutical, food, beverage, and cosmetics industries. These applications are highlighted for the hygroscopicity that MCC possesses due to free hydroxyl groups on its surface.

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Composite materials using MCC have also been investigated due to the intrinsic mechanical properties of cellulose. However, some challenges associated with the moisture absorption, poor wettability, incompatibility with polymeric matrices, and thermal sensitivity [58, 59] are still present in this area. To overcome these drawbacks, the development of cellulose nanostructures (CNS) has been investigated, since it aligns the material’s intrinsic properties with the high surface area and aspect ratio of nanomaterials. Moreover, even at low content, these nanomaterials can considerably increase the properties of composite materials, thus becoming suited for new advanced applications. The isolation of CNS is performed using acid hydrolysis, enzymatic hydrolysis, or mechanical treatments. Different morphologies and crystallinity and aspect ratios may be obtained, based on the isolation method used, as illustrated in Figure 1.1. Acid hydrolysis is the most often adopted method and is performed usually using strong acids like sulfuric acid in concentrated solutions. During this process, the acid is responsible for the consumption of the amorphous regions within cellulose, promoting the formation of a stable colloidal dispersion of CNS composed of the crystalline regions that are not consumed, and present a low aspect ratio known as cellulose nanocrystals (CNC) or nanowhiskers. Although it is a well-established method, it presents a high volume of acid waste that may promote severe environmental concerns, and thus, new alternatives have been investigated. Enzymatic hydrolysis is performed using a bundle of enzymes responsible for promoting the cellulose chain scission. The great advantage of this process is that it is performed in mild conditions; however, it requires a longer reaction time. Moreover, the cost is still a problem for applications of a larger scale. Mechanical treatments include different methods of isolation such as ultrasonication, ball milling, and high-pressure homogenization, among others. These methods usually promote fiber defibrillation, resulting in CNS with an elongated shape known as cellulose nanofibers, with a high aspect ratio. These structures present a higher crystallinity degree than MCC but lower than CNS obtained from acid hydrolysis, since some amorphous regions are still present after treatment.

1.2.2 Surface modification Although CNS may be employed directly to develop nanocomposites, they usually present incompatibility at the polymeric interface associated with polarity difference between the matrix and the filler. This effect is observed because each glucose unity in the cellulose chain possesses three hydroxyl groups available for interaction that are responsible for strong hydrogen bonds between chains, and may promote cellulose agglomeration. Moreover, the polymeric chain presents a hydrophobic nature that has little interaction with the cellulose nanostructures, restricting its enhancement properties.

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Figure 1.1: Different types of cellulose nanostructures and their different properties.

To address this problem, different treatments may be employed, such as silylation, esterification, etherification, oxidation, carbamation, and polymer grafting. These treatments are performed to react to the hydroxyl surface groups present at CNS surface and are responsible for including different chemical structures that may increase CNS dispersion properties and interaction with the polymeric matrix. Esterification, for instance, promotes the inclusion of an ester structure that reduces the hydrophobic character of CNS. The most employed esterification reaction is acetylation that uses acetic acid and anhydride, which are responsible for including acetyl groups at the CNS surface. The modification process is investigated by the degree of substitution (DS) that indicates the average number of modified hydroxyl groups from cellulose structure. Silylation is responsible for including alkoxy silane at the CNS surface, enhancing fiber wettability with polymeric matrix [60]. Carbamation introduces isocyanate groups like 2,4-diisocyanate toluene (TDI), hexamethylene diisocyanate (HMDI) at the CNS surface, and methylene diphenyl diisocyanate that may be used in as chemical linkers in further reactions [61]. Polymer grafting is another approach to increase CNS stability and enhance the interaction with the polymeric matrix. This method is based on introducing a polymeric chain into the cellulose surface, which can be performed by polymerization at the CNS surface or binding using a coupling agent. In this way, different structures may be attached to promote better interaction [62].

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1.3 Composite processing Once the natural fibers at nanoscale present attractive properties, there is a great interest to incorporate these nanofillers as functional materials in plastic matrices. This procedure reduces consumption of synthetic polymeric materials, promoting an alternate use of renewable raw materials, reducing associated costs and environmental impacts with the development of products with higher biodegradable character, as illustrated in Figure 1.2.

Figure 1.2: Structural properties found in natural fiber composites with biopolymers.

Polymer composites containing fibers can be manufactured by the conventional polymer processing technique, which is interesting for scale-up of production. Thus, the methods for preparing fiber-based composites discussed here include: melt blending (extrusion and injection molding), coating and roll-to-roll coating and casting, and solvent casting [63].

1.3.1 Extrusion The extrusion process is one of the most widely applied processing methods in the industry, since its production meets the industry needs and allows good homogeneity of fillers in the polymeric matrix. However, when working with cellulose nanostructure, the main difficulty is associated with its low dispersibility, due to the hydrophilic nature of cellulose and presence of strong hydrogen bonds that promote aggregation. To solve these problems, the chemical modifications previously discussed have been widely employed [64]. Extrusion involves a combination of several

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operations, including mixing, conveying, heating, kneading, shearing, and shaping. The polymeric material is melted due to the combined effect of shear effect and thermal energy, and physical-chemical reactions may occur if the conditions are not properly adjusted. The extrusion is a process with low cost, high productivity, and energy efficiency. It is also attractive, since it allows the production of many kinds of products [65]. Recently, several works have highlighted the use of twin-screw extrusion to produce materials CNFs with high solid contents (10–20% by weight) in a process that is economically viable [66–68]. Rol et al. [66], measured the energy consumption to produce CNFs by twin-screw extrusion from different pre-treated pulps. The authors evaluated different feed and processing conditions and observed that the twin-screw extrusion processes could be used to produce CNF with high solid contents. This procedure is attractive since it presents a low energy demand and can be easily scaled up for industrial production of nanocellulose. However, the CNFs produced depend heavily on the number of passages through extrusion [66]. Rocha et al. [69], developed biodegradable PLA composites, evaluating three different cellulosic residues as fillers: pine, maçaranduba, and sugarcane bagasse, and the effect of starch coating. The different formulations were prepared in a corotating twin-screw extruder, with an L/D ratio of 40, with nine heating zones with temperatures of 145, 180, 200, 200, 210, 210, 210, 200, and 180 °C and with a screw speed of 130 rpm. The authors observed increased mechanical properties, with better interaction for the fillers coated with starch, indicating that the material offers potential for application in several areas, such as furniture or packaging [69].

1.3.2 Injection molding Injection molding process is a widely desired method, as it shows scalable potential to develop different products. It is important to carefully adjust the molding parameters that include injection temperature, injection pressure, injection speed, mold temperature, waiting time, and cooling time [70]. Pappu et al. [79] manufactured a hybrid biopolymer composite by reinforcing PLA with sisal and hemp fiber through injection molding. The methodology for injection molding was the variation of the process temperature, being at 160 °C (food zone); 180 °C (mixing zone); 190 °C (reaction zone), and 200 °C (die exit zone) and maintaining a ram pressure of 30 kN. The authors observed that the developed composites showed good mechanical performance and environment-friendly nature, which could be explored in different applications like automotive, packaging, electronics, interiors, and agricultural.

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1.3.3 Coating and roll-to-roll coating Cellulose coatings are an alternative to barrier composites. In this process, liquid dispersion containing cellulose and other functional components are applied on the surface of a polymeric substrate. A uniform layer is then obtained, removing the excess liquid using a blade [71]. Montero et al. [72], prepared active films of poly(butylene adipate-co-terephthalate) (PBAT) loaded with CNF, incorporated with cinnamon essential oil (EO). The system solution was applied into a glass plate and extended using a wire extender. The authors investigated the component interactions, observing that the EO showed physical interaction with the PBAT matrix that impacted its molecular conformation. The results obtained indicate that the films present high thermal stability with potential for application in the food industry [72].

1.3.4 Casting The solvent casting method is usually used to develop biopolymer films on a laboratory scale [73]. This method requires polymer solubilization in a suitable solvent and usually requires agitation over time. The obtained solution may then be spread in a suitable substrate, to allow solvent evaporation. After this process, a thin film is obtained [74]. It is worth mentioning that the use of nonpolar polymers requires the use of solvents like chloroform, hexane, and acetone, among others; however, the CNC presents a hydrophilic nature that shows little affinity in this system. This lack of compatibility between the system components may promote aggregation effects that result in inadequate dispersion and a weak interface interaction [75]. Sucinda et al. [76] used different concentrations of cellulose nanowhiskers (NWCs) to reinforce PLA, using the solvent casting method. The adopted methodology consisted in the dispersion of NWCs and PLA into different chloroform solutions. These solutions were then mixed until a viscous solution was obtained. After dispersion, the solution was poured into a glass petri dish to allow chloroform evaporation. The obtained films showed good dispersion, high crystallinity, and tensile strength, showing that the PLA/NWC bio nanocomposite films present compatible properties for the packaging field [76].

1.4 Challenges Industry application of fibers usually explores the use of synthetic fibers, since their composites present very precise and homogeneous properties. Natural fibers, on the other hand, show an additional challenge associated with its homogeneous dispersion in the matrix [32, 77]. Factors such as species, harvest, soil conditions,

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climatic conditions, plant tissue, and treatment method affect the properties of the natural fibers [31, 77]. Furthermore, the difference in polarity between the matrix (nonpolar) and natural fibers (polar) makes fiber-matrix adhesion difficult, due to the repulsive behavior of the hydrophobic matrix. This results in nonuniform dispersion of the fibers in the polymeric matrix, impacting the stress-transfer behavior and promoting composites with poorer mechanical properties. To overcome this barrier, one alternative is to promote modification at fiber surface and to use chemical “coupling” agents that can increase fiber-matrix adhesion and improve composites properties. However, this chemical treatment increases the processing cost [32, 77, 78]. Natural fibers present hydrophilic nature that makes them susceptible to water absorption in humid environments, causing the fibers to swell and their dimensions to become unstable. This modifies the physical and mechanical properties of the composite and increases its susceptibility to rot, due to the decrease in microbial resistance [77]. Due to this, the application of composites containing natural fibers for the packaging sector is troublesome, since long-term storage becomes very complicated. In addition to these difficulties, the packaging industry faces a challenge with the high cost of biodegradable materials compared to petroleum-based materials. For instance, biodegradable polymers like PBS, PBAT, and PHAs have commercial value of approximately $3.5/kg, which is more expensive than nonbiodegradable polymers like polyethylene and polypropylene that usually present a cost of around $1.2–1.3/ kg [71, 77]. An additional challenge relates to the thermal properties of natural fiber composites, because at elevated temperatures (above 200 °C), the fiber’s main components start degrading and promote change in the composite properties. Unlike synthetic polymers, which have unique properties and specific types of structure, natural polymers have many variations in their characteristics, according to their origin. Thus, before developing composite materials using natural fibers, it is necessary to individually evaluate their thermal properties and behavior, preventing losses in their mechanical properties at certain processing temperatures [32].

1.5 Final considerations This chapter discusses the field of natural fiber and composites that can serve as promising materials in developing new environment-friendly products. Natural fibers present several available sources in each country, making them an abundant and low-cost material. The varied polysaccharide content of each fiber source impacts its mechanical, thermal, moisture absorption, and biodegradable properties. The development of eco-friendly composites is an attractive option, since it reduces the consumption of petroleum-based materials. The extraction of cellulose from

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natural fibers allows the obtainment of micro and nanomaterials that possess low density, high aspect ratio, and improved mechanical properties. Additionally, fiber modification is a strategy used to reduce moisture content and increase the interfacial interaction between matrix and reinforcement. The standard processing techniques employed for polymer production may be explored for the development of these eco-friendly composites, making them economically competitive, with potential to supply industry demands. In future years, a deeper understanding of how to control the extraction and functionalization of natural fibers to control composite properties and develop new products that meet the demands of modern society and align environmental aspects will be a hot topic to expand the applications of these materials.

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Ayfer Dönmez Çavdar✶, Sevda Boran Torun, Emrah Peşman, Naile Angin, Murat Ertaş and Fatih Mengeloğlu

2 Hybrid thermoplastic composite reinforced natural fiber and inorganic filler Abstract: This chapter summarizes the recent studies on natural and synthetic short fiber-filled thermoplastic-based hybrid composites. This review will provide information on the manufacturing processes and the structure, and the properties of hybrid thermoplastic composites, in separate sections. It supplies data on physical, mechanical, morphological, rheological properties, and thermal and flammability characterizations of select hybrid composites. This chapter underlines the importance of hybrid thermoplastic composite in achieving predetermined material performances. Recent studies have proven that the hybridizations of natural fiber and inorganic filler might provide exceptional and promising features. Keywords: Natural Fiber, Inorganic Filler, Hybrid Thermoplastic Composites, Polymer matrix

2.1 Introduction The dictionary meanings of composite and hybrid are quite similar in that both contain a combination of two or more materials in a predetermined geometry and scale to serve a specific engineering purpose. Composites consist of a filler/reinforcing phase, a matrix/continuous phase, and an interface phase separating them. The first phase is responsible for the principal load/stress-bearing elements and the second phase is responsible for maintaining the filler in a desired orientation and location as well as

✶

Corresponding author: Ayfer Dönmez Çavdar, Department of Forest Industry Engineering, Faculty of Forestry, Karadeniz Technical University, 61080 Trabzon, Turkey, e-mail: [email protected], https://avesis.ktu.edu.tr/adonmez/publications Sevda Boran Torun, Department of Material and Material Processing Technologies, Arsin Vocational School, Karadeniz Technical University, 61900 Trabzon, Turkey Emrah Peşman, Department of Forest Industry Engineering, Faculty of Forestry, Artvin Coruh University, 08100 Artvin, Turkey Naile Angin, Department of Forest Industry Engineering, Faculty of Forestry, Bursa Technical University, 16310 Bursa, Turkey Murat Ertaş, Department of Forest Industry Engineering, Faculty of Forestry, Bursa Technical University, 16310 Bursa, Turkey Fatih Mengeloğlu, Department of Forest Industry Engineering, Faculty of Forestry, Kahramanmaraş Sutcuimam University, 46100 Kahramanmaraş, Turkey

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serving as a load/stress transfer medium and protecting the fibers from environmental damages. The overall performance of composites is mainly affected by the composition of these phases [1, 2]. These composites could be natural or synthetic based. Usually, the nature of the matrix materials determines the name of the composites – thermoset-based composites or thermoplastic-based composites. Reinforcement can be short and continuous fiber, woven, short fiber mat, whiskers, or particles [2]. Fiber-filled/reinforced composites can sustainably provide lighter and tough materials in large quantities. These composites can provide different physical and chemical properties at the micro and macroscopic scale than other materials. These properties have generated increased interest among the engineering and structural industries. The automotive industry is one of the leading sectors utilizing composite materials to improve the weight to strength ratio of materials. In addition, the military, marine, aerospace, and civil engineering industries also benefit from fiber-reinforced composites [3]. Natural fibers and synthetic (manufactured) fibers are two distinct groups of fibers used in composite manufacturing. Synthetic fibers are more expensive than natural fibers but are mostly preferred for composite-related engineering industries. The general natural fibers are produced from animals or plants and they are not the primary choice for structural applications in composite reinforcement industries. Natural fibers have good mechanical properties apart from being lightweight. The other advantages of natural fillers over their synthetic counterparts are low cost, acceptable specific strength properties, low density, and biodegradability [3, 4]. Natural fibers-based polymer composites are widely used in transportation (automobiles, railway coaches, and aircraft) industries, building and construction industries (ceiling panels, partition boards), packaging, and in consumer products engineering applications such as electronic devices, sporting goods, etc. [5–8]. There is a growing interest in replacing synthetic fibers with natural ones in engineering applications to reduce the burden placed on the environment, since natural fibers have advantages over synthetic ones, being eco-friendly and bio-renewable [3, 9]. Plant maturity, the part of the plant that is harvested, the harvesting season, rain, sun, harvesting region, and the condition of the soils govern the mechanical properties of natural fibers [3, 10]. Their cellular structure and the low density of natural fibers provide excellent thermal and acoustic insulation properties over rock wool or glass fiber [3, 11]. Natural fiber-reinforced composites are critical for sustainability but the conditions mentioned earlier cause variabilities in their characteristic properties, which is one weakness of natural fibers. In some applications that require predetermined properties, just natural fiber-reinforced composites cannot compete with synthetic fiber-reinforced composites [3]. Hybridization can overcome the weak aspects of both natural and synthetic fibers while continuing to be environmentally friendly. Hybrid composites exhibit unique properties that cannot be attained from single fiber composites [12, 13]. Hybrid composites generally refer to a combination of filler/ reinforcer-type – natural or synthetic material – combining two fillers/reinforcers in

2 Hybrid thermoplastic composite reinforced natural fiber and inorganic filler

23

one matrix or a single filler/reinforcer in a mixture of matrices. Filler/reinforcer for hybrid composites can be natural-natural, natural-synthetic, or synthetic-synthetic, receiving increasing attention from researchers and the industry for structural applications due to their tailored mechanical and impact properties [3]. Several review papers are written on the manufacture and property determination of hybrid polymer composites with different fillers/reinforcers and with various matrices (thermoset or thermoplastic-based with single or multiple matrices). An overview of the recent advances in both interlaminar and intralaminar hybrid natural fiber composites for natural-synthetic and natural-natural fiber combinations in polymeric matrix was presented by Neto et al. [14]. The focus was on thermoset polymer composites and a limited information on thermoplastic polymer composites was also presented. Jawaid et al. [8] have documented a comprehensive review of the thermo-mechanical properties, the environmental aspects, and the applications of natural fibers and nano clay-based hybrid nanocomposites. Special emphasis was on the studies on bamboo, kenaf fiber and nano clay reinforced thermoset, and thermoplastic composites. Bichang’a et al. [2] also summarized papers on natural and synthetic filler-reinforced polymer composites. Most studies that were discussed had thermoset polymer matrices and continuous fiber mats. Singh et al. [15] reviewed publications on glass-fiber-filled hybrid composites. They also summarized a paper on composites that use natural fibers and glass fibers, mainly in thermoset matrices. In another study, Prince et al. [16] reviewed the thermoset and thermoplastic hybrid composites filled with only natural fibers. The dynamic mechanical behavior of natural fibers-reinforced polymer matrix composites was presented in a review paper by Haris et al. [17] and Venkategowda et al. [18]. Shahinur et al. [19] reviewed papers on the characterization and performance of jute fibers and the enhancement of physical, mechanical, thermal, and tribological properties of polymeric materials. They reviewed papers having synthetic or bio-based composites with thermoplastic or thermoset plastic matrices and jute fibers in a variety of forms such as particle, short fiber, or woven fabric. Similarly, Radzi et al. [20] summarized publications on properties, fabrication, and the potential applications of bamboo fiber-reinforced thermoset and thermoplastic composites. A review by Rangappa et al. [21] presented a summary of the progress in the research on natural fibers and their composites. Critical analyses were presented on the various research efforts directed toward improving the properties of natural fiber-reinforced composites. In another review study, papers on the tribological properties of thermoset and thermoplastic composites, having natural and synthetic fillers, were summarized [22]. Gogoi et al. [23] presented a comprehensive understanding of the existing and latest polyolefin-based CF-reinforced composites in their review paper. Some information on hybrid composites was also presented. Saba et al. [24] summarized the literature and presented information about the different classes of natural fibers, cellulosic fiber-based composites, nanocomposites, nanofillers, and natural fiber-/nanofiller-based hybrid composites, with specific focus on their applications. New aspects of nanotechnology

24

Ayfer Dönmez Çavdar et al.

were discussed in the development of hybrid composites for a sustainable and greener environment. Swolfs et al. [4] explained the basic mechanisms of the “hybrid effect” and described the state-of-the-art models to predict them. Farzana et al. [25] reviewed the papers on jute fiber-based reinforced composites and their applications. Mousavi et al. [26] focused on the evaluation of bamboo fiber as a reinforcer in different thermoset and thermoplastic polymer matrices, including polyester, epoxy, phenolic resins, and poly(lactic acid) and polypropylene.

2.1.1 Natural fiber In recent years, the natural fibers in the production of plastic-based composite materials began to attract attention due to their low cost, ease of manufacture, low density, biodegradability, high specific resistance and modulus of elasticity, easy surface modification, easy availability,, renewable and biodegradable, and their usage as a reinforcing agent instead of glass fibers or carbon fibers. Natural fiber has many excellent characteristics such as fewer health and safety risks, easy formability, specific mechanical properties that are competitive with glass fiber composites, CO2 balance, good thermal and acoustic insulation properties, and it is recoverable [27]. It has become necessary to seek different solutions such as the use of natural fiber [28]. From this point of view, composite materials containing plant fibers are products that best address the concept of “environmental friendliness” due to their efficient use of resources and their biodegradable properties, and hence are increasingly relevant. In recent years, these materials have also been of interest to the automotive and plastic industries. To improve the material properties, the plastics industry has turned to the production of plastic composites that are reinforced with natural fibers, instead of inorganic materials such as glass, mica, talc, calcium carbonate, or carbon fiber, which it has used for many years [29], particularly for door panels, seat backs, and interiors in the automotive industry. Intensive research has begun on the use of plastic composites that are reinforced with natural fibers in headliners, instrument panels, and interior parts [30]. Some researchers currently use natural fiber as a reinforcing/filler in the polymer matrix [31, 32, 186]. At the same time, natural fibers can be added to a polymer matrix at a very high rate at process temperatures where they will not degrade [33, 34]. Natural fibers are biodegradable and recyclable, and they can easily be converted into thermal energy during combustion, without leaving residues, causing lesser pollution [35]. They can be fiberized more easily than synthetic fibers, and cause less environmental issues. The production of synthetic fibers is essentially dependent on fossil fuels and it requires almost 10 times more energy than production of natural fibers [36, 37].

2 Hybrid thermoplastic composite reinforced natural fiber and inorganic filler

25

The tendency of natural fibers to cluster during production, their low operating temperature requirement ( Nanoclay > neat PP Tensile strength and fatıgue performance enhanced samples with nano-phased clay

Recommended References optimum loading ratio []

Up to  phr

[] / the soundproof applications

–% by mass

[]

Tensile strength nanophase and fatigue clay  wt.% strength Talc  wt.% coefficient decreased in samples with talc

[]

2 Hybrid thermoplastic composite reinforced natural fiber and inorganic filler

29

Table 2.1 (continued) Inorganic filler type

Polymer matrix

Enhancing properties

Talc //

PP

Toughness and hardness

✶

Nano calcium HDPE carbonate /// wt.% nanoclay //// wt.%

Increased elasticity of modulus and indentation modulus for both fillers ✶ The increased strain at break with increasing Nanoclay, more elastic-plastic behavior

Glass beads .–%

piezoelectric properties

HDPE

Worsening properties

Recommended References optimum loading ratio

✶ slight  wt.% decrease in tensile /flexural strength and moduli ✶ increased gaps and voids, depending on talc content

[]

the strain at Nano CC  wt.% [] break Nanoclay wt.% decreased with CC increase; more brittle-like mechanical behavior

 wt.%

[]

2.1.2.1 Ceramic based inorganic fillers Glass fiber Glass fiber is one of the most preferred materials in the automotive and aviation industries to reduce vehicle mass and provide strength properties [63]. Although it has lower strength properties than carbon fibers, glass fibers are preferred as a reinforcement material because they are cheaper and are less brittle. Glass fibers are silica-based (50–60% SiO2) and are used as thermal insulation materials for technical purposes [low thermal expansion coefficient and high thermal conductivity due to the inorganic compounds they contain (calcium, boron, sodium, aluminum and iron oxides)]. In general, glass fibers, with their high corrosion resistance and dimensional stability, exhibit better thermo-mechanical performance compared to asbestos and organic fibers [64]. Glass fibers offer many advantages such as durability, chemical

30

Ayfer Dönmez Çavdar et al.

resistance, tensile strength, bending strength, impact resistance, and thermal insulation when used as a reinforcer in a polymer matrix [65]. Talc Talc (Mg3Si4O10(OH)2) is a traditional inorganic filler used for reinforcing polyolefins due to its low cost and high aspect ratio [66]. It is a metamorphic mineral formed when magnesium-based minerals like serpentine, pyroxene, amphibole, and olivine undergo metamorphism in the presence of carbon dioxide and water [67]. At the same time, talc is in great demand for the synthesis of polymeric composites due to its excellent blending nature, thermal resistance, superior electrical resistance, and chemical inertness [68]. Mica Mica is the common name of a complex aluminum silicate family, including muscovite, phlogopite, biotite, and lepidolite. Its crystals can be easily cleavaged to very thin sheets/films/plates as well. Each mica crystal has a different color. Many researchers have evaluated mica as a reinforcer in thermoplastic matrices, including polyolefins, thermoplastic elastomers, and engineering thermoplastics so far. The generally accepted conclusion is that increase of mica causes an increase in heat deflection temperature, dimensional stability, and other mechanical properties, except for impact strength and elongation at break, and results in a decrease in warping deflection [69–73]. In addition, the interfacial adhesion between the inorganic filler, the organic matrices and the filler shape; the aspect ratio; the orientation of the filler; and the filler ratios in the matrices play a crucial role in the reinforced thermoplastic composites achieving superior mechanical properties [47, 53, 69–73]. To enhance the filler dispersion and the adhesion between the mica filler and matrix, the key coupling agents or interfacial agents used are -p-phenylene-bis-maleamic grafted atactic polypropylene (aPP-pPBMA), maleic anhydride grafted polyolefins (i.e., PP, HDPE), and organo- functional silanes and organo- titanates [69, 71]. Calcium carbonate Natural and synthetic calcium carbonate, called Ground Calcium Carbonate (GCC) and precipitated calcium carbonate (PCC), respectively, are among the most commonly used inorganic fillers in polymer applications. GCCs have high aspect ratio, cost less, and are abundant in nature. Their crystalline forms are calcite, aragonite, and vaterite. Calcite is typically the most used filler for reinforced polymer applications, especially for flexible and rigid PVCs, and polyolefins. The crystalline forms are natural and can be economically produced from limestone, chalk, and marble. PCCs, on the other hand, involve a more expensive manufacturing process compared to GCCs but can generate very low particle size crystals, with different forms

2 Hybrid thermoplastic composite reinforced natural fiber and inorganic filler

31

and shapes. They can be coated with stearic acid (C18) or fatty acid to enhance the properties of the composites [74].

2.1.2.2 Carbon-based inorganic filler Carbon-based fillers as reinforcers have attracted worldwide attention in the manufacture of advanced composite materials with superior electrical, mechanical, and thermal performance. Carbon fibers have been the most studied among carbon fillers since their invention in 1860. Besides, carbon powder, carbon black, carbon nanotube, carbon nanofiber, carbon nanodots, fullerenes, graphite, graphene oxide, graphene nano phthalate, graphene nanoribbons, graphene-like materials, reduced graphene oxide, and bio-based carbon materials are mostly hot topics in researches on the manufacturing process of functional carbon nanocomposites for electrical vehicles, sensing applications, aerospace, and automotive industries [75–77]. Although these nano-fillers have many superior properties, they may not be able to achieve the expected improvement in composite properties due to clustering problems in the polymer matrix. In addition, especially for CNTs, their safety in terms of human health has not been fully proven.

2.1.2.3 Metal-based inorganic fillers Metal hydroxides are often incorporated into thermoplastic polymer matrices as passive flame retardants. Mg (OH)2 and Al(OH)3 hydroxides stand out, especially because they have high heat absorption capacity, less smoke emission, and hence are nontoxic and environmentally friendly [78, 79]. Mochane’ survey has addressed the recent studies on the effect of metal hydroxides in thermoplastic composites, the aspect of their particle size, modification techniques, synergistic effect with other fillers, and the effect of coupling or dispersing agent(s). It has been emphasized that metal hydroxides increase the fire retardancy of composite materials at loadings between 30% and 60%, but deteriorate their mechanical properties. To get the desired properties without any significant decrease in the metal hydroxide-filled composites, researchers have adopted different methods: i) use compatibilizers such as MAPP (PP-grafted maleic anhydride), POE-g-MA (poly (ethylene octene) grafted-maleic anhydride) [80]; ii) surface treatment with silane and silicone oil [81], with triethoxysilane and polymethyl-vinyl silicone rubber [82]; iii) use nanosize Mg (OH)2 [83]; and iv) incorporating with other fillers such as graphene [84] and thermally reduced graphite oxide [85].

32

Ayfer Dönmez Çavdar et al.

2.1.2.4 Recycled performance of inorganic filler-reinforced thermoplastic composites A major apprehension in the recycling of plastic materials is the ability of the recycled plastic materials to retain sufficient technological properties without a significant loss, compared to those of virgin materials. Recycling of plastic composites involves re-heating, which leads to the degradation of plastic and its filler, in some situations. The polymer is subjected to thermal and mechanical stresses during extrusion or injection. The high number of thermal processes, especially at the higher melting points of the polymer, causes a change in its molecular weight and meltflow index (viscosity). According to Bueche’s mechanical degradation theory, if the broken chains are located near the center of the macromolecule, with multiple extrusion processes, degradation might occur and result in a loss of thermal and mechanical strengths of the polymer composites [86–88]. Bahlouli et al. [88] stated that molecular characteristics as well as the rheological and mechanical properties of the thermoplastic composites play a crucial role when evaluating secondary recycle processing of the composites. However, Afif et al. [67] confirmed that the characteristics of the finished PP/talc composites that are re-produced with a hot-melt mixture were close to the characteristics of materials using virgin materials with the secondary recycled process. The aspect ratio of the fillers also affects the recyclability process of the reinforced composites. When a filler with a high aspect ratio was used, the properties of the composite deteriorated due to the breaking down of the filler during multiple injection or recycling processes. In the case of fillers with low aspect ratio, such as calcium carbonate, silica, and dolomite, no significant loss of composite properties was detected [89].

2.1.3 Classification, types, and some properties of natural fibers and inorganic fillers Natural fibers can be divided into six categories [90]; bast fibers (hemp, ramie, flax, jute, and kenaf); leaf fibers (sisal, abaca, and pineapple); seed fibers (cotton, coir, and kapok); core fibers (kenaf, hemp, and jute); grass and reed fibers (wheat, corn, and rice); and all other types (roots and wood). Images of select natural fibers are presented in Figure 2.2. The mechanical properties of some natural fibers are presented in Table 2.2. Table 2.3 also shows mechanical properties of some mineral and synthetic inorganic fillers. Advantages and drawbacks of natural fibers are given in Table 2.4. A comparison of natural and synthetic fibers is shown in Table 2.5. Table 2.6 summarizes the results of inorganic filler- and natural fiber-filled thermoplastic composites.

2 Hybrid thermoplastic composite reinforced natural fiber and inorganic filler

33

Figure 2.2: Hemp bast fiber (a), banana stalk fiber (b), sisal fiber (c), chestnut cupula (d), beech fiber (e), and lignin (f).

2.1.4 Classification of the hybrid thermoplastic composites Basically, composites can be classified in two different ways. The first approach is based on the matrix material such as ceramic matrix composites (CMC), metal matrix composites (MMC), and polymer matrix composites (PMC). Secondly, it can be classified based on the reinforcing material structure. They are particulate composites, fibrous composites, and laminate composites. Particulate composites are formed by a matrix and a dispersed phase in the form of particles. Particles are either indiscriminately oriented in composites or oriented in a desired way. When oriented as desired, the dispersed phase of materials consists of two-dimensional flat plates (flakes) that are arranged parallel to each other. Fibrous composites are categorized as short-fiberreinforced composites and long-fiber-reinforced composites. A matrix is reinforced by a dispersed phase in the form of discontinuous fibers whose lengths are 100 times their diameter (in short-fiber-reinforced composites). In composites, fibers are either randomly oriented or directed in a specific direction. The matrix of long-fiber-filled composites is reinforced by a dispersed phase in the form of continuous fibers. Fibers can be either unidirectional or bidirectional (woven) oriented. In laminate composites, multilayer composites are fiber-reinforced composites made up of many layers with variable fiber orientations.

34

Ayfer Dönmez Çavdar et al.

Table 2.2: Mechanical properties of some natural fibers. Fiber type

Density (g/cm)

Diameter Tensile (µm) strength (MPa)

Elastic modulus (GPa)

Elongation (%)

References

Flax

.–.

–

–,

–

.–.

[]

.

–

–,

.

.–.

[]

.

–

–,

.

.–.

[, , ]

.–.

–

–,

.–

.–.

[]

.–.

–

–

–

.–.

[]

.–.

–

–

–

.–.

[]

.–.

–

–

–.

.–.

[, , , ]

.–.

–

–

–

–.

[]

.



–

.

.–.

[]

Betelnut

.–.

–

–

–

.–.

[]

Kenaf

.–.

–

–,

–

.–.

[]

.

–

–

.

.–.

[]

–

–

.

[]

Jute

. .–.

–

–,

.–

.–

[, ]

.–.

–

–

.–

–.

[]

.

–





.

[]

Henequen

.

–

–

.–.

.–.

[]

Oil palm

.–.

– –

.–.

–

[, ]

.–.

–

–

–

.–

[]

.

–





–

[]

.

–

–

–.

.

[]

.–.

–

–

.–

.–

[]

.

–

–

.–

–.

[]

.

–

–

.–

–

[, , , ]

.–.

–

–

.–

–

[]

.–.

–

–

–

–

[]

Hemp

Bagasse

Isora

2 Hybrid thermoplastic composite reinforced natural fiber and inorganic filler

35

Table 2.2 (continued) Fiber type

Density (g/cm)

Diameter Tensile (µm) strength (MPa)

Elastic modulus (GPa)

Elongation (%)

References

Ramie

.

–



.

.

[]

.–.

–

–,

–

.–

[]

.

–

–

.–

.–.

[, , ]

–.

–

–,

.–

.–

[]

.–.

–

–

–

.–.

[, ]

.–.

–

–

–

.–.

[]

.–.

–

–

–

–

[]

.–.

–

–

–.

.–.

[, ]

.

–





.–

[]

.–.

–

–

–

–

[, ]

.

–

–

–

–

[, , ]

.–.

–

–

.–

–.

[]

Nettle

.

–

–,

.–

.–.

[]

Açai

.

–

.–.

.–.

–

[]

Cotton

.–.

–

–

.–.

–

[]

.–.

–

–

.–.

–

[]

.–.

–

–

.–.

.–

[, ]

.–.

–

–

.–.

–

[, , ]

Curaua

.

–

–,

.–

.–.

[, ]

Abaca

.

–

–

.–.

.–.

[, , ]

.

–

–

.–

–

[]

.–.

–

–

.

.

[]

.

–

–,

–

.

[, ]

Sugarcane bagasse

.–.

–

–

.–.

.–.

[]

.

–





–

[]

Coconut

.

–

–

–

–

[]

Bamboo

Banana

Coir

Pineapple

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Ayfer Dönmez Çavdar et al.

Table 2.2 (continued) Fiber type

Density (g/cm)

Diameter Tensile (µm) strength (MPa)

Elastic modulus (GPa)

Elongation (%)

References

Sisal

.–.

–

–,

.–

.–

[]

.–.

–

–

–

.–.

[]

.

–

–

.–

–

[]

Table 2.3: Mechanical properties of some mineral and synthetic inorganic fillers. Fiber type

Density (g/cm)

Carbon

.–. –

E-Glass

Elastic modulus (GPa)

Elongation References (%)

,–,

–

.–.

[, ]

.–. –

,–,

–

.–

[, ]

.

–

,–,



.

[, ]

.

–

,–,



.

[, , ]

.



,

.

.

[]

.

–

,



.

[, , , , , ]

.



,

.

.

[]

Glass fiber

.



,



.

[]

Aramid

.

–

,–,

–

.–.

[, , ]

Carbon

.

–

,

–

.–.

[, , ]

.



,–,

–

–

[]

Quartz

.



,



–

[]

Kevlar 

.

.

,



.

[]

Kevlar 

.

.

,



.

[]

Basalt

.

–

,



–

[]

Boron

.

– ,



–

[]

S-Glass

Diameter Tensile strength (µm) (MPa)

37

2 Hybrid thermoplastic composite reinforced natural fiber and inorganic filler

Table 2.4: Advantages and disadvantages of natural fibers [5, 111, 112, 195]. Advantages

Disadvantages

Relatively low cost

Dimensional instability

Renewable source

Moisture adsorption that results in swelling of the fibers

Production needs little energy, zero CO footprint/low hazard manufacturing processes

Low thermal resistance

Low density, low specific weight

Lower durability

High specific mechanical properties

Poor fire resistance

High stiffness

Variability

Good acoustic insulation properties

Anisotropic behavior

Good thermal isolation

Discontinuous

Less abrasive damage

Low strength properties(especially its impact strength)

Table 2.5: A comparison of natural and synthetic fibers [112, 113]. Type of fiber

Mechanical properties

Moisture resistance

Thermal sensitivity

Source

Natural

equable

Low

high

endless low

good

low

High

low

limited

equable

high

Synthetic high

Manufacture Recoverability Price

high

Table 2.6: The effects of natural fiber and inorganic filler addition on the mechanical, physical, thermal, morphological, and recyclability properties of thermoplastic composites [Adapted from [5, 114] Cavdar & Boran, 2016. The reinforced thermoplastic composites

Natural Fiber

Inorganic Filler

Mechanical properties

Tensile strength

+/–

+

Tensile of modulus

+

+

Elongation-at-break

–

–

Flexural strength

+/–

+

Flexural of modulus

+

–

Impact strength

–

–

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Ayfer Dönmez Çavdar et al.

Table 2.6 (continued) The reinforced thermoplastic composites

Natural Fiber

Inorganic Filler

Physical properties

Light-weight

–

+

Dimensional stability

–

+

Hardness

+

+

Thermal stability

+

+

Char residue at  °C

+

+

Glass transition temperature

+

No change

Melting point

No change

–

Surface roughness

+

+

Gaps /voids in the polymer matrix

+

+

+

–/+

Thermal properties

Morphological properties

Recyclability

Likewise, hybrid composites can be classified based on filler types. Hybrid filler composites are classified into three types such as hybrid natural fiber composites made up of different natural fibers (two or more types), hybrid inorganic filler composites that include two or more inorganic fillers, and natural fiber/inorganic filler hybrid composites. This chapter covers natural fiber-/inorganic-filled hybrid composites.

2.2 Manufacturing processes of the hybrid thermoplastic composites A proper and optimum manufacturing method should be able to transform the raw materials into the desired composite according to the targeted shape, size, and mechanical/physical properties in an easy and feasible way [115]. Thermoplastics and additives are commercially sold in different forms such as pellets, powders, flakes, etc. Therefore, the form of both the thermoplastic material and the additive has led to the application of different composite production techniques. As an advantage, thermoplasticity gives the material the chance to easily apply these several methods. However, a sensible selection of the appropriate production method needs a detailed study on all parameters, including expected material properties, manufacturing cost, and environmental concerns [116]. Basically, to produce a hybrid thermoplastics composite mixture, mold, heat, and pressure are needed. At the beginning of the process, feeding the homogenous mixture to the system increases the success of the final product [117]. Thermoplastic

2 Hybrid thermoplastic composite reinforced natural fiber and inorganic filler

39

composite can be produced by extrusion, blow, or injection molding. Each of these processes can be used together or separately but for all techniques, it is essential that the filler/fiber in the composite is well-dispersed. Additionally, other techniques such as thermoforming, compression molding, stamping, and tape winding are frequently used.

2.2.1 Extrusion The extrusion method is mostly preferred for small-medium size manufacturing because it is slightly more economical and feasible than other methods. In the basic working steps of extrusion, the powder or granular material is fed into the system and compressed to expel the air inside then, melted and homogenized, and finally the shaped composite is given out. The basic parts of an extruder are illustrated in Figure 2.3. Extruders are usually classified by the number and the properties of the screw. The most common types of extruders are single-screw and twin-screw extruders. In general, a twin-screw extruder is used in the manufacture of inorganic filler or natural fiber-added thermoplastic composites (i.e. wood or glass fiber-based composites) because it offers a better dispersal. The other significant parameters in the extrusion process are the diameter of the screw (D), the length/diameter ratio of the screw (L/D), and the rotational speed. As the L/D ratio increases, the composition homogeneity increases. Hence, longer screws (L/D ⁓ 36–42) are preferred, especially for the manufacturing of anisotropic material-added hybrid thermoplastic composite. Provided the working principle of the process remains the same, the parts of the extruder can be modified, new parts such as blower can be integrated, and the extrusion direction can be changed according to the desired final hybrid product [116]. Although extrusion is a widely used technique, there are some chronic problems that occur frequently during the extrusion process. Some of these problems come from the nature of the process, for instance melt fraction or die-swelling caused by the friction between the die and the melted polymer. Another problem is the clogging of the extruder during composite manufacturing. This situation is usually caused by the nonhomogeneous melting and the feeding of the reinforcement material into the process, without being sized to a sufficiently small size. To prevent this problem, it is recommended that all materials that are fed into the process should be adequately milled.

40

Ayfer Dönmez Çavdar et al.

Figure 2.3: The basic parts of an extruder.

2.2.2 Injection molding Finished goods can be manufactured with the injection molding technique, and it is the most preferred method for mass-production. This technique is suitable for producing custom-sized parts or parts of complicated shapes. The basic principle of this process is to shape the molten polymer by injecting pressure into a mold. The essential zones and components of the injection molding device are given in Figure 2.4. The main objectives of the plasticizing unit are to melt the polymer homogeneously and transport the molten blend with the help of screws. Clearly, the screw performs similar tasks as in the extrusion process. However, different from the extrusion, the screws have the ability to move backwards characteristically to allow injection [118]. The melted hybrid composite blend is injected with high pressure into the mold. At this step, the screw acts as a piston and it is responsible for regulating the injection pressure. After the injection process, the exit of the screw is closed, the screw moves back and continues to accumulate the melted hybrid composite blend for a new injection. In the molding stage, the melted composite blend is kept under pressure for cooling and solidification. At the end of the process, the two halves of mold are separated and the shaped composite is ejected by pushing it forward. The mechanical properties of the hybrid composites are enhanced by adding continuous fiber reinforcements, but these materials cannot be easily integrated into the

2 Hybrid thermoplastic composite reinforced natural fiber and inorganic filler

41

Figure 2.4: The essential zones and components of the injection molding device [Osswald & Menges, ©Carl Hanser Verlag, Munich [118]].

mass-production process and hence their usage with injection is usually restricted. Moreover, during the injection process, the continuous fibers are damaged due to the friction between the fibers [119]. If injection molding is desired to produce continuous fiber-reinforced thermoplastic composites, it is recommended to setup a pre-extrusion process. Rabbi et al. [120] reported that injection molding is suitable for short fibers, while extrusion process is suitable for both short and long fibers. Shrinkage is a perennial problem in molding of thermoplastics. With the addition of natural or inorganic fibers to the pure polymer, the chain structure of the polymer is slightly disrupted, and anisotropy occurs in the material. This implies that cooling or solidification during molding could be heterogeneous, and it may result in shape deflect. Hence, it is critical that the reinforcement materials are compatible with each other as well as the thermoplastic polymer for the success of the injection molding process [117]. Moreover, Chaitanya & Singh [121] reported that natural or inorganic fibers within injection-molded composites exhibit a complicated movement and dispersion behavior. As a general expectation, the fibers that accumulate near the mold walls have a tendency to move in the direction of the melt flow whereas those near the core or the cavity center exhibit randomized distribution. This phenomenon affects the mechanical behavior of the injection-molded fiber-reinforced thermoplastic composites and, usually, this is clearly visible under the electron microscope. Some researchers go beyond the traditional or customary practices and try the above-mentioned methods by combining or by ignoring some steps. As mentioned before, an extrusion step is conventionally recommended before injection molding. In some cases, this precept may change, and the extruder may not need to be used. For instance, in a study of the production of synthetic fiber-reinforced hybrid composites,

42

Ayfer Dönmez Çavdar et al.

the length of synthetic fibers fed directly into the injection molding process remained approximately 3 times longer than with the conventional technique (extrusion followed by injection molding). Thanks to the relatively long fiber length, mechanical properties of the hybrid composite improved [122].

2.2.3 Compression molding Compression molding is the familiar and traditional way in the manufacture of the composites. In brief, it is a closed-type molding process that is aided by high pressure. The granular or powder form of the composite blend is placed between the two hot plates of the mold. Pressure is applied by bringing the hot plates closer and it is then left to cool [115]. This easy technique was often used in the manufacture of automotive parts because it allows the processing of both thermoset and thermoplastic polymers with/ without fiber reinforcement [118]. However, its use in the thermoplastic industry has decreased over time. Firstly, it takes a long time for the plates to be heated and cooled. The waiting period is an undesirable situation, especially for the automotive industry. Since there is an external heating in this process, it is difficult to achieve homogeneous melting, and it may result in shape or color defects. Therefore, it is not suitable for use in precise and custom-sized shaping. Ultimately, compression molding devices are generally not fully automatic and require labor and employee costs [116].

2.2.4 Thermoforming In this technique, the raw material mixture is not used directly, and usually an extruded composite mixture is preferred. In this process, the laminated material is heated to above its softening point. It is then shaped and cooled for solidification [117]. Frequently, vacuum is assisted during formation. While external infrared heaters are used in the heating phase, air or water spray can be used for cooling. The method can be modified and used for packing foods such as milk products or honey, but there are some restrictions that limit its use with thermoplastics. Principally, in order to have an effective thermoforming process, the matrix polymer should have a wide rubber region. The rubber region refers to a specific area between the glass transition temperature and the melting temperature of thermoplastics. Amorphous thermoplastics (e.g. PVC, PS) are very suitable for thermoforming whereas semi-crystalline thermoplastics (PP, PE etc.) are difficult to shape with this method [117]. With the addition of fiber reinforcement to the composite blend, the thermoforming process becomes more difficult; so it is recommended to avoid customized complex shaping [116].

2 Hybrid thermoplastic composite reinforced natural fiber and inorganic filler

43

2.2.5 Filament or tape winding Filament/tape winding is briefly described as impregnated polymer composites that are heated and wound around a mandle until it reaches the desired thickness. The most important step in this method is providing an effective heating. Funck & Neitzel [123] reported that the thermoplastic filament winding could be cheaper and economically more feasible than thermoset winding. Another advantage of this method is that it allows the use of a high rate of reinforcement – up to 70%. Thus, the hybrid thermoplastic composites can have superior mechanical properties. However, the percentage of fiber is relevant in the enhancement of mechanical properties, especially when using natural fibers as a reinforcement material [116]. The most essential point that determines the wiping quality of the fiber-reinforced hybrid composite is the ratio of voids and the air inside. Nowadays, scientists have been trying to minimize the void rate, and recommend making the preliminary modeling studies online to determine the optimum reinforcement/thermoplastic ratio [124].

2.2.6 Others Other manufacturing techniques such as stamping, foaming, coating, pultrusion, calendaring, draping, and lay-up are also encountered. However, these methods are generally for rough shaping and the product homogeneity is not the desired level. For this reason, they are generally not used alone, but by integrating with other methods. Furthermore, the composite industry has gone further than the traditional methods, and has veered toward modern and risk-controlled manufacturing techniques. Currently, prototype and e-manufacturing techniques are becoming prominent. Thanks to these digital and software supported methods, limited and overcomplicated parts can be designed and manufactured easily without requiring huge production costs. Moreover, computer simulation of hybrid thermoplastic composite manufacturing processes allows for trial and error, and offers a chance to detect and solve problems before mass production [125].

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Ayfer Dönmez Çavdar et al.

2.3 The structure and properties of hybrid thermoplastic composites 2.3.1 Physical properties The physical qualities of each natural fiber, such as fiber structure, strength, size, and defects, are crucial. There are various physical properties of each natural fiber that must be understood before the fiber can be exploited to its full potential [90]. The hydrophilic character of natural fibers is due to the existence of hydroxyl groups of cellulose and hemicellulose. Crystalline cellulose content, cellular aspect ratio, cell number, and microfibrillar angle are parameters that govern the properties of natural fibers. High fiber strength requires higher cellulose content and lower microfibrillar angle. Fibers are frequently distinguished by their aspect ratio [112, 126, 188]. The distribution of fiber dimensions in a sample can be determined by supposing a cylindrical shape and measuring the length and diameter of the individual fibers. The natural fiber density is lower than the cell wall density (1.5 g/ cm3) due to their hollow structure [91, 112, 127]. The hydrophilic character of fibers poses a significant challenge to their usage as polymer reinforcement. The biggest problem during the production of polymer composites is the interfacial bonding between the fiber and the matrix because of their different characteristics (hydrophilic fiber and hydrophobic polymer) [12]. Surface modification of the filler or adding coupling agents can improve the fiber-matrix bonding [128]. The absorption of water by plant fibers adversely affects the composite’s performance [129]. During the material’s life cycle, water absorption causes a volume change in the fibers within the composite. On the other hand, vaporization of the water trapped inside fibers when the polymerization process of the matrix is above 100 °C may occur, resulting in their shrinkage. In addition, internal stresses at the fiber/matrix interface are caused by swelling and shrinkage of the fibers surrounding the matrix. The phenomena can lead to matrix damage and significant reductions in the properties of the composite [111, 130]. In contrast to natural fibers, inorganic fillers have been used to enhance the mechanical and physical properties of hybrid polymer composites. Cavdar et al. [131] reported that inorganic fillers increased the dimensional stability of the natural fiber hybrid composites. It was stated that the alkali treatment of natural fibers enhanced the mechanical, physical, and thermal properties of the hybrid PLA composites and also reduced the thermal expansion coefficient [128]. [185] also claimed that alkali treatment of natural fiber (sugar palm yarn fiber) and hybridization with glass fiber improved the dimensional stability. Furthermore, Jumaidin et al. [130] reported that the addition of sugar palm fiber lowered their water uptake. This explains why the hybrid composites’

2 Hybrid thermoplastic composite reinforced natural fiber and inorganic filler

45

dimensional stability has improved thermal, mechanical, and physical properties of seaweed/sugar palm fiber-hybrid composites. Additionally, silane and MAPP-treated kenaf/glass hybrid composite showed reduced water absorption. With the presence of a coupling agent, the adhesion between the polymer matrix and the filler(s) improved with the narrowing of the interfacial gaps and by blocking the hydrophilic groups. The water saturation level in fibers with a ratio of 30% treated kenaf/70% treated glass composite was attained after 55 days of soaking. The presence of silane on the glass fiber improved the interfacial bonding between the filler and the matrix. The silane coupling agent forms a waterproof coating on the surface of the glass fiber, thus penetration of water into the composite samples is prevented [132]. This study reveals that each filler needs a bonding agent that is appropriate for its chemical structure for hybrid composites to attain superior mechanical performance. Another study on the hybrid composite filled with bamboo and glass fibers provided similar results on the performance of water uptake [133]. In the study of Panthapulakkal & Sain [134], water absorption of glass fiber/ hemp /PP-hybrid composites increased, depending on the soaking duration, until equilibrium conditions are reached. The water absorption of composites is directly related to the hydrogen bonding of water molecules to the free OH groups. However, the addition of glass fiber, up to 15 wt.%, decreased the tendency of water uptake in the short hemp/PP matrix. Water absorption and thickness swelling of the wood flour/ glass fiber/PLA hybrid composites increased with increase of the amount of NF in the polymer matrix, owing to its chemical composition. On the other hand, the water absorption of the hybrid PLA composites reduced considerably with the increase of the glass fiber content in the polymer matrix [135].

2.3.2 Mechanical and rheological properties Filling materials, for instance nanoclay, carbon nanotubes or nanofibers, silicates, and ceramic powder short fibers are widely used to improve the mechanical properties of thermoplastic composites, such as tensile and flexural properties, stiffness, and dimension stability. In addition, artificial fibers like carbon/graphite fibers, glass fiber, and aramid are preferred in special industrial applications that require high performance [136, 137]. It is hopefully expected that positive changes occur in the mechanical properties with the addition of natural fibers or inorganic fillers to the thermoplastic polymer matrix. However, these changes depend on too many parameters, such as the chemical properties (organic or inorganic) of the selected fiber, fiber length, compatibility between the filler and the polymer, the percentage of filler presence, fiber distribution, manufacturing technique, etc. For example, the use of talc generally improves the composite‘s mechanical behavior, such as crystallinity, strength, and stiffness, but it may harm deformation properties [138].

46

Ayfer Dönmez Çavdar et al.

Inorganic fillers are frequently used to enhance the mechanical properties and the dimensional stability of the reinforced composites. Each of the inorganic additive materials, such as glass fiber, metals, inorganic minerals, carbon black, etc. gives positive mechanical properties to the material, and many interesting and remarkable results have been shared in literature. Studies have shown that the best mechanical properties, especially the tensile strength value, are obtained by adding 30% talc to the PP matrix, However, it should be taken into consideration that using additives at these rates will cause agglomeration and pore problems [138]. Moreover, high inorganic filler loading and non-homogeneous mixing weakens the surface interaction between the additive and the matrix. Furthermore, the high amount of filler may restrict the mobility of the chains and also influence the crystallization kinetics. In a study comparing the mechanical properties of PP-based composites with the addition of the same amount of different additives, it was reported that the highest flexural strength values were obtained for talc and the lowest values for CaCO3. Kaolin provides relatively less improvement in all the mechanical properties of the samples than others, due to its agglomeration [139]. Fu et al. [140] investigated hybrid glass fiber-reinforced thermoplastic composites that are produced via an optimized injection-overmolded technique, and reported three times higher flexural strength values for hybrid composites. Hybridization is essential to minimize the individual disadvantages of inorganic and organic reinforcement materials, and to provide better mechanical properties [141]. According to [142], hybridization may improve the mechanical behavior due to two reasons: a) Hybridization ensures significant advantages thanks to having two fibers with similar lengths but with different diameters in a thermoplastic matrix. The diameter diversity of the fibers increases the possibility of adhesion between the fibers and the matrix via increasing the effective area. b) Even if the low elongation fiber breaks during processing, the load continues to be carried by the high elongation fiber. Thus, stress is transferred from the matrix to the fibers, and the mechanical properties are strengthened.

Figure 2.5: Randomly and cross oriented glass fibers in the PLA matrix.

2 Hybrid thermoplastic composite reinforced natural fiber and inorganic filler

47

The manufacturing technique of the composite is another factor affecting the mechanical properties. During manufacturing, the fibers are exposed to heat and pressure, and therefore there may be deformation or breakage in the fibers. An appropriate production method should be selected for each reinforcement/polymer combination to obtain the desired mechanical properties. [143] investigated the effects of injection molding and blow film extrusion methods on PP/talc nanocomposites and pointed out that the biggest difference between the two methods is the orientation of the fibers. Because there is a molding step in the injection process, though the fibers close to the walls of the mold are oriented parallel, there was a random distribution in the other regions. In blown film extrusion, the fibers are usually oriented parallel to each other. Thus, best mechanical values were observed in the samples produced by the blow film extrusion method. Moreover, Caylak et al. [135] reported a similar problem for natural fiber-/glass fiber-reinforced hybrid PLA composites. According to the study, a decrease in the flexural strength value was observed as a result of the wood fiber and glass fiber not being homogeneously dispersed apart form being oriented randomly in the hybrid composites produced by the extrusion method (Figure 2.5). On the other hand, during extrusion and followed by the hot-press process, the polymer chains degrade and recrystallize, which may cause a decrease in crystallinity.

2.3.2.1 Flexural properties Flexural strength (FS) refers to the maximum bending stress maintained by composites when under bending load. FS is an important performance feature and determines the intended use of the material [141]. It is known that the flexural properties of the reinforced thermoplastic composites are higher than those of the neat polymer. Particularly, the modulus significantly increased with the incorporation of a natural fiber and an inorganic filler [131, 144, 145]. Ertas et al. [145] investigated the effects of halloysite nanotube (HNT) on the performance of organic fiber-added PLA composites. Maleic anhydride grafted onto PLA (MA-g-PLA) was used to enhance the interfacial bonding between the hydrophobic/ hydrophilic phases of the NF/HNT/PLA composites. It is reported that the flexural strength (FS) of the hybrid composites improved by using MA-g-PLA. It acted as a bridge between the PLA and the fillers, resulting in increased adhesion between them. The FS values of the NF/HNT/PLA composites did not change remarkably in the presence of HNT. However, the flexural modulus of elasticity (FMOE) of all the hybrid composites was better than neat PLA (nearly 40% improvement). This result was attributed to the increased ductility effect and rigidity of HNT. A study [40] examined

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the impact of boron-based fire retardants (BbFRs) and NF (spruce wood flour) loading on mechanical, fire, and thermal performances of HDPE composites. Spruce wood floor was impregnated with boric acid/borax/mixture solution. It is reported that while the FS values of the samples treated with boric acid decreased, an increase was observed in the other samples treated with the borax and mixture solution. This study shows that pre-treatment of the natural fiber with the mixture solution of boric acid and borax before hybrid composite manufacturing gave more positive results on FS than by directly adding them as solid dust form in the polymer matrix. The flexural modulus of the composite was enhanced by 57% when the fiber addition increased from 20% to 40%. Cisneros-López et al. [146] used recycled-PLA (RPLA] and neat PLA together (wt.% RPLA is 30), and microcrystalline cellulose was added as a natural fiber to this matrix. The prepared mixtures were produced by a 3D printer as well as by the injection molding method and the mechanical properties of the composites were compared. According to the results of the study (Figure 2.6), the mechanical properties of the samples produced via 3D printer were remarkably lower than those produced by injection molding. The reason for over a 30% decrease in the properties is believed to be the voids that occurred in the samples due to the variations in the 3D printer.

2.3.2.2 Tensile properties Tensile strength is an important output of mechanical tests and it is used for the mechanical characterization of materials. Cavdar et al. [131] investigated the mechanical properties of inorganic based-fire retardants (FRs) and microcrystalline cellulose (MCC)-filled high-density polyethylene (HDPE) composites. Mono ammonium phosphate (MAP), ammonium zeolite (AZ), or natural zeolite (NZ) were used as a fireretardant with 10 wt.%. Based on the results of the study, it was reported that the

Figure 2.6: Flexural properties of 3D-printed and injection-molded samples: (a) flexural modulus and (b) strength. Different letters (a, b) refer to significant differences (p C = C < bond emerged. These results indicated that the DA reaction had occurred, and the dynamic cross-linked thermoset elastomer was also successfully prepared.

Figure 3.29: FT-IR of spectra of (a) cellulose graft copolymer EC-g-P(LMA-co-FMA700), (b) Dynamic cross-linked thermoset elastomer EC-g-P(LMA-co-FMA700) + ESOM. Reproduced with permission from C. Lu, X. Guo, C. Wang, J. Wang and F. Chu, Carbohydrate Polymers, 2020, 242: 116404. ©2020 Science Direct [46].

The formation of the dynamic cross-linked network was also confirmed by swelling experiment. As shown in Figure 3.30, after immersion in DCM for 30 min, EC-g-P (LMA-co-FMA700) was dissolved, and the EC-g-P(LMA-co-FMA700) + ESOM was only swollen. During the swelling process, the penetration of dichloromethane into the cross-linking network led to an increase of the size of spaces between molecules, which caused the swelling of the thermoset elastomer. The swelling rate was about 2.7 g/g. This result further confirmed the generation of the cross-linked network in the thermoset elastomers.

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Figure 3.30: Swelling experiment of cellulose graft copolymers, before and after DA reaction. Reproduced with permission from C. Lu, X. Guo, C. Wang, J. Wang and F. Chu, Carbohydrate Polymers, 2020, 242: 116404. ©2020 Science Direct [46].

3.3.2.5 Thermal and mechanical properties of thermoset elastomer Similar to the previous reports [52–54], dynamic mechanical analysis (DMA) was carried out to investigate the thermal mechanical properties of the grafting copolymer EC-g-P(LMA-co-FMA700) and thermoset elastomers EC-g-P(LMA-co-FMA700) + ESOM. As shown in Figure 3.31, the glass transition temperatures (Tg) of EC-g-P(LMA-coFMA700) and EC-g-P(LMA-co-FMA700) + ESOM were 45 °C and 31 °C, respectively. The storage modulus of the EC-g-P(LMA-co-FMA700) dropped sharply with the increase in temperature. The storage modulus of thermoset elastomer was lower than that of EC-g-P(LMA-co-FMA700) at low-temperature region due to the internal plasticization effect of ESOM. Furthermore, the range of Tg of EC-g-P(LMA-co-FMA700) + ESOM was significantly broadening, which was associated with the increase of chain entanglement restraining the movement of polymer chains, and resulted in the relaxation of the polymer chain segment during the heating process [55, 56]. Monotonic stress–strain was used to measure the mechanical properties of cellulose graft copolymers and thermoset elastomer. As shown in Figure 3.32 and Table 3.2, the FMA monomer content remarkably influenced the mechanical properties of cellulose graft copolymers. And increasing FMA content contributed to the increase of the stress at break, whereas the elongation at break decreased with the increase of FMA content. Particularly, the mechanical strength of EC-g-P(LMA-coFMA800) reached 4.59 MPa, and the stress–strain curve also showed a stress yielding phenomenon. For the thermoset elastomers, the stress–strain curves displayed significant elastomeric characteristics, owing to the internal plasticization of ESOM, which could enhance the toughness and flexibility of elastomers. Furthermore, the mechanical strength of thermoset elastomer was also enhanced, which was associated with the increase of chain entanglement density.

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Figure 3.31: DMA curves of EC-g-P(LMA-co-FMA700) and EC-g-P(LMA-co-FMA700) + ESOM at a heating rate of 3 K/min. Reproduced with permission from C. Lu, X. Guo, C. Wang, J. Wang and F. Chu, Carbohydrate Polymers, 2020, 242: 116404. ©2020 Science Direct [46].

Figure 3.32: (Left) Monotonic stress–strain curves of cellulose graft copolymer, (Right) Dynamic cross-linked thermoset elastomers at a stretch rate of 50 mm/min. Reproduced with permission from C. Lu, X. Guo, C. Wang, J. Wang and F. Chu, Carbohydrate Polymers, 2020, 242: 116404. ©2020 Science Direct [46].

The step cyclic tensile tests were performed to evaluate the elastic behavior of thermoset elastomer. As shown in Figure 3.33, the thermoset elastomer showed excellent elastic property, the elastic recovery (ER) values were increased with the increase in stretching strain, and the ER values were above 90% at the strain of 140%. It should be noted that the elastic recovery value of the thermoset elastomer was higher than that of the cellulose graft copolymer, which was associated with the internal plasticization effect of ESOM.

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Table 3.2: Molecular weight and mechanical property results of cellulose graft copolymer and thermoset elastomers Reproduced with permission from C. Lu, X. Guo, C. Wang, J. Wang and F. Chu, Carbohydrate Polymers, 2020, 242: 116404. ©2020 Science Direct [46]. Mn/(g/mol) c

PDI c

Stain at break/%

Stress at break/MPa

EC-g-P(LMA-co-FMA)

. × 

.

.

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EC-g-P(LMA-co-FMA)

. × 

.

.

.



.

.

.

Entry

Sample name

a a

. × 



a

EC-g-P(LMA-co-FMA)



b

EC-g-P(LMA-co-FMA) + ESOM

–

–

.

.



b

EC-g-P(LMA-co-FMA) + ESOM

–

–

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.



b

EC-g-P(LMA-co-FMA) + ESOM

–

–

.

.

a: cellulose graft copolymer before DA reaction; the number behind FMA is the molar ratio of FMA in feeding, and the total molar ratio of LMA and FMA is 1,000; b: dynamic cross-linked thermoset elastomers; c: measured by GPC.

Figure 3.33: Nominal stress–strain curve for (Left) EC-g-P(LMA-co-FMA700), (Right) DA cross-linked thermoset elastomer during the step cyclic tensile deformation, and the value of elastic recovery. Reproduced with permission from C. Lu, X. Guo, C. Wang, J. Wang and F. Chu, Carbohydrate Polymers, 2020, 242: 116404. ©2020 Science Direct [46].

3.3.2.6 Shape memory property of thermoset elastomer The formation of the dynamic cross-linked network via DA reaction determined the permanent shape of the thermoset elastomers. The P(LMA-co-FMA) chain with broad Tg was used as a switch domain for shape memory behavior. The thermal triggered shape memory property of the thermoset elastomers was investigated by stress-controlled DMA. As shown in Figure 3.34, when the sample was heated to 80 °C, 0.12 MPa stress was applied. The thermoset elastomer was stretched, and the

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stress was maintained, until the sample was cooled to 0 °C. However, the strain was released and the elastomer recovered to its original shape, while the sample was reheated to 80 °C. The shape fixity ratio and the shape recovery ratio were measured to be 99.4% and 96.2%, respectively.

Figure 3.34: DMA curve of shape memory programming for dynamic cross-linked thermoset elastomer EC-g-P(LMA-co-FMA700) + ESOM. Reproduced with permission from C. Lu, X. Guo, C. Wang, J. Wang and F. Chu, Carbohydrate Polymers, 2020, 242: 116404. ©2020 Science Direct [46].

The representative photos of the shape recovery process are shown in Figure 3.35. The spiral-shaped elastomer was bent at 80 °C, and fixed at 0 °C. When the elastomer was reheated to 80 °C, the spline recovered to its original shape within 9 s. It is worth noting that the thermoset elastomer did not show obvious damage even after several shape recovery processes.

Figure 3.35: Photos of shape recovery process of dynamic cross-linked thermoset elastomer EC-g-P (LMA-co-FMA700) + ESOM at 80 °C. Reproduced with permission from C. Lu, X. Guo, C. Wang, J. Wang and F. Chu, Carbohydrate Polymers, 2020, 242: 116404. ©2020 Science Direct [46].

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3.3.2.7 Self-healing property of thermoset elastomer The self-healing property of the cellulose-based thermoset elastomers with dynamic cross-linked network was also investigated, according to the previous works [48, 49, 57]. DSC analysis was firstly carried out to evaluate the thermal reversible characteristic of the dynamic cross-linking structure. As shown in Figure 3.36, the heating curve exhibited two distinct endothermic peaks corresponding to rDA reaction in the temperature region of 100 to 190 °C, namely, one peak at 126 °C and the other at 165 °C. These results are associated with the two kinds of adducts (endo and exo adducts) formed during the DA reaction. The peak at 126 °C corresponded to the endo adducts, and the peak at 165 °C was assigned to the exo adducts [58, 59]. Furthermore, the cooling curve also showed a broad and weak exothermic peak corresponding to DA reaction in the range of 90–180 °C. This result demonstrated the thermal reversible characteristic of the dynamic cross-linking network.

Figure 3.36: DSC curves of dynamic cross-linked thermoset elastomer EC-g-P(LMA-co-FMA700) + ESOM at a heating rate of 20 K/min. Reproduced with permission from C. Lu, X. Guo, C. Wang, J. Wang and F. Chu, Carbohydrate Polymers, 2020, 242: 116404. ©2020 Science Direct [46].

The change of solubility of elastomer in DMF confirmed the thermal reversibility of the DA reaction. As shown in Figure 3.37, the thermoset elastomer swelled at 40 °C resulting from the presence of the cross-linked network. And, the film could be dissolved with heating the solvent to 130 °C, suggesting the disappearance of the cross-linked network due to the occurrence of retro-Diels-Alder (rDA) reaction. Interestingly, the solvent became a gel state after cooling to 40 °C for 72 h, resulting from the reoccurrence of DA reaction and reconstruction of the cross-linking network structure.

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Figure 3.37: Photos of solution property of dynamic cross-linked thermoset elastomer (Left) Dynamic cross-linked thermoset elastomer swollen in DMF at 40 °C; (Middle) the solvent heated to 130 °C for 10 min; and (Right) the solvent cooled to 40 °C for 72 h. Reproduced with permission from C. Lu, X. Guo, C. Wang, J. Wang and F. Chu, Carbohydrate Polymers, 2020, 242: 116404. ©2020 Science Direct [46].

The thermoset elastomer EC-g-P(LMA-co-FMA700) + ESOM with a dynamic crosslinking network was used to explore the self-healing property. As shown in Figure 3.38, the EC-g-P(LMA-co-FMA700) + ESOM film showed a clear scratch on the surface. Then, the film was heated at 130 °C for 5 h, and the changing of the crack was observed by an optical microscope at different times. It could be observed that it took at least 5 h to heal the crack. Moreover, the film needs to be heated at 40 °C for 72 h to repair the cross-linking network by DA reaction.

Figure 3.38: Photo of self-healing process of DA cross-linked thermoset elastomer EC-g-P(LMA-coFMA700) + ESOM. Reproduced with permission from C. Lu, X. Guo, C. Wang, J. Wang and F. Chu, Carbohydrate Polymers, 2020, 242: 116404. ©2020 Science Direct [46].

The self-healing property of the EC-g-P(LMA-co-FMA700) + ESOM/CNTs conductive elastomer composite was also investigated. As shown in Figure 3.39, the circuit with a battery was assembled by the conductive elastomer composite, and it successfully lighted an LED bulb. However, the LED bulb faded, while the elastomer composite was cut into two separate pieces. Subsequently, hot-press at 130 °C for 15 min was carried out for the healing of the damage of the elastomer composite film. It was found that LED lighted up again, indicating successful healing of the damaged film. This result demonstrated that the service life of a conductive elastomer composite

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could be prolonged by the self-healing property. These conductive elastomer composites show potential application in flexible strain sensors.

Figure 3.39: Photos of electrical self-healing process of conductive elastomer composite film: (a) original sample; (b) the sample cut into two separate pieces; (c) the sample healed by hot pressed at 130 °C for 15 min; and (d) the bulb was lighted up again. Reproduced with permission from C. Lu, X. Guo, C. Wang, J. Wang and F. Chu, Carbohydrate Polymers, 2020, 242: 116404. ©2020 Science Direct [46].

Figure 3.40 shows the mechanism of the self-healing performance. The reversibility of dynamic covalent cross-linked network formed by DA reaction imparts the selfhealing property to this thermoset elastomer. During the self-healing process, the occurrence of rDA reaction at 130 °C made the chemical cross-linking network disappear, which accelerated the molecular chain mobility and facilitated the flow of molecular chain towards the crack to heal the damage. Subsequently, the reoccurrence of the DA reaction reconstructed the cross-linking network by cooling the elastomer composite film to 40 °C, by which the damage was completely healed.

Figure 3.40: The mechanism of self-healing process. Reproduced with permission from C. Lu, X. Guo, C. Wang, J. Wang and F. Chu, Carbohydrate Polymers, 2020, 242: 116404. ©2020 Science Direct [46].

3.3.2.8 Recyclability of thermoset elastomer Generally, the recycling of conventional cross-linked polymers was difficult, owing to the permanent covalent cross-linked network, which caused serious environmental

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problems. Inspired by the previous reports on the utilization of dynamic covalent bonds to fabricate the recycled materials [50, 60, 61], the thermoset elastomers with the dynamic cross-linking network also had the recyclability. As shown in Figure 3.41, the small pieces of thermoset elastomer were cut and hot-pressed at 130 °C for 15 min, obtaining a well-consolidated recyclable film. And, the mechanical property of the recycled thermoset elastomer was investigated after heating at 40 °C for 72 h.

Figure 3.41: Demonstration of recycling of dynamic cross-linked thermoset elastomers. Reproduced with permission from C. Lu, X. Guo, C. Wang, J. Wang and F. Chu, Carbohydrate Polymers, 2020, 242: 116404. ©2020 Science Direct [46].

The stress–strain curves of recycled thermoset elastomers were presented in Figure 3.42. After recycling, the mechanical strength of the thermoset elastomer was increased from 4.12 MPa to 5.22 MPa, but the strain at break was slightly decreased. The reason for this phenomenon was that the self-polymerization of furan groups further increased the cross-link density [59]. Compared with the EC-derived conventional cross-linked polymers [52, 55], these thermoset elastomers could reduce the waste of materials and show better sustainability due to the formation of dynamic crosslinking network.

3.3.2.9 Strain sensor based on the conductive elastomer composite The features of flexibility and elasticity of the thermoset elastomers allowed them a promising application in the strain sensor, and the conductive elastomer composite EC-g-P(LMA-co-FMA600) + ESOM/CNTs was applied to investigate the sensing performance. ΔR/R0 = |R0-R|/R0 defined the resistance change ratio upon tensile strain, where the R0 was the initial resistance [62, 63]. The dependence of the resistance change ratio corresponding to the tensile strain of strain sensor is shown in Figure 3.43. It could be observed that the increase in resistance was caused by the tensile deformation, and the ΔR/R0 showed a linear increase with the increase of strain. The

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Figure 3.42: Monotonic stress–strain curves of recycled thermoset elastomers. Reproduced with permission from C. Lu, X. Guo, C. Wang, J. Wang and F. Chu, Carbohydrate Polymers, 2020, 242: 116404. ©2020 Science Direct [46].

sensitivity (S) was calculated according to the equation S = δ(ΔR/R0)/δ(ε) [64, 65]. It could be found that the strain sensor displayed three obvious linear stages corresponding to the different sensitivity. The S to normal stretching was 1.744, 0.963, and 1.398 in the strain ranges of 0–7%, 7–28%, and 28–47.2%, respectively. As shown in Figure 3.43, the response time of the strain sensor was 277 ms, indicating the fast responsive feature of the strain sensor. In addition, it is worth noting that the mechanical strength of the strain sensor was 4.77 MPa, which was better than that of the previously reported stretchable conductive composite [30, 66–68].

Figure 3.43: (Left) Strain-resistive change curves for EC-g-P(LMA-co-FMA600) + ESOM/CNTs. (Right) The response time for the strain sensor. Reproduced with permission from C. Lu, X. Guo, C. Wang, J. Wang and F. Chu, Carbohydrate Polymers, 2020, 242: 116404. ©2020 Science Direct [46].

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Furthermore, the reliable sensing ability of the conductive elastomer compositederived strain sensor was demonstrated by cyclic loading-unloading (Figure 3.44). It could be observed that the ΔR/R0 of the strain sensor exhibited reversible changes during the 200 cyclic loading-unloading tests at the strain of 25%. Although the plastic deformation of the elastomer composites caused a slight increase of the baseline of ΔR/R0, the change value of ΔR/R0 still remained stable during the loading-unloading cycles after 200 cycles tensile deformation, suggesting the excellent sensing stability of the elastomer composite-derived strain sensor.

Figure 3.44: Resistive change versus time under cyclic tensile to 25% strain at a stretch rate of 50 mm/min for 200 cycles. Reproduced with permission from C. Lu, X. Guo, C. Wang, J. Wang and F. Chu, Carbohydrate Polymers, 2020, 242: 116404. ©2020 Science Direct [46].

In addition, the conductive elastomer composite, integrated with excellent mechanical flexibility and high sensing sensitivity, could also be used in wearable sensors. Theoretically, this conductive elastomer composite-derived wearable sensor would be in direct contact with the skin for monitoring human motion. Hence, the sensor was integrated into the second joint of the index finger that can bend with an angle (θ) from 0° to 90° [69, 70]. Bending fingers would induce a tensile deformation of the strain sensor, which would also be accompanied by a change in resistance. Thus, the finger motions in real-time could be distinguished by monitoring the changing of resistance. As shown in Figure 3.45, when bending the finger to 30°, 60°, and 90°, in turn, the ΔR/R0 exhibited stable and differentiated signal response, and the ΔR/R0 was 3%, 5.9%, and 8.6%, respectively, suggesting that the sensor featured with repeatability and accuracy has great potential application in detecting human bodily motions.

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Figure 3.45: Relative resistance changes versus time for the index finger bending to 30°, 60°, and 90° in turn at room temperature (Inset: the pictures of the bent states of the finger). Reproduced with permission from C. Lu, X. Guo, C. Wang, J. Wang and F. Chu, Carbohydrate Polymers, 2020, 242: 116404. ©2020 Science Direct [46].

3.4 Outlook and conclusions Cellulose-derived materials show potential for use as high-performance reinforcement in polymer composites, which creates opportunities to fabricate the high-tenacity and high-durable bio-based functional engineering materials that are applied in consumer, structural, and medical applications. Up to now, the potential of cellulose reinforced composites has only primarily been realized by film casting or template impregnation. However, the challenge is to translate the successful potential application of cellulose-reinforced composites to commercial availability through the development of cost-effective and scalable compatibilization and dispersion techniques. Recent results are encouraging and show that living radical polymerization is emerging as a powerful alternative to the use of cellulose. Particularly, living radical polymerization allows the introduction of well-defined polymers with precisely controlled structure, molecular weight, polydispersity, and functionalities onto the cellulose surface, which makes it possible to fabricate a series of functional cellulose composites to partially replace the composites derived from petroleum chemicals. This chemical modification technique holds significant potential for the largescale use of cellulose in functional composites.

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Jorge S. S. Neto, Henrique F. M. de Queiroz and Mariana D. Banea✶

4 Recent developments in nanofillermodified natural fiber composites Abstract: The use of natural (cellulose) fibers presents various benefits due to their many positive characteristics, such as biodegradability, low density, good acoustic isolation and low cost, when compared to the synthetic fibers. However, their main shortcomings are the variability in fiber quality and absorption of humidity due to the hydrophilic characteristic of the fiber, which leads to low interfacial adhesion between the fiber and the hydrophobic matrix, consequently leading to relatively low mechanical properties. Different techniques as reported in literature are used to surpass these shortcomings, such as chemical treatments of the fibers and hybridization techniques. One method explored in the literature to improve the properties of cellulose fiber-reinforced composites is the use of fillers (either on the fiber surface or into the matrix as the second reinforcing phase). The fillers can be organic, inorganic, metallic, and ceramic and they can be in macro-, micro-, or nanoscale. Generally speaking, microsized fillers are defined as those between 1 and 1,000 µm, while the nanosized fillers defined as those that have at least one dimension in the range of 1 to 100 nm. The fillers may contribute to water resistance (lowered hydrophilic nature) of polymeric composites, increased dimensional stability and shrinkage reduction, thermal stability, machinability, and vibrational damping. However, these properties depend on many factors such as shape, size, and orientation of filler reinforcement, filler content, filler/matrix/fiber adhesion, and agglomeration. Another important benefit derived from filler reinforcements is the possible improvement in fracture toughness of the composites, due to crack arrestment mechanisms such as crack bridging, microcracking, filler debonding, and shear banding. This chapter presents recent developments in the field of cellulose composites reinforced with nanofillers. The main challenges met in the production of nanofillerreinforced cellulose composites (agglomeration of nanofillers during manufacture, insufficient interfacial compatibility between fillers and the matrices, and degradation of the fillers during processing, etc.) are briefly discussed. Keywords: Natural fiber, mechanical properties, thermal properties, nanofiller, hybridization

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Corresponding author: Mariana D. Banea, Federal Center for Technological Education of Rio de Janeiro (CEFET/RJ), Av. Maracanã, 229, 20271-110, Rio de Janeiro, Brazil, e-mail: [email protected] Jorge S. S. Neto, Henrique F. M. de Queiroz, Federal Center for Technological Education of Rio de Janeiro (CEFET/RJ), Av. Maracanã, 229, 20271-110, Rio de Janeiro, Brazil

https://doi.org/10.1515/9783110768787-004

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4.1 Introduction Research on natural fiber-reinforced composites (NFRCs) is gaining ground in many industry sectors, the principal virtues of these materials being low density and cost [1–3]. Another relevant factor for the application of natural fibers in the industry is its ecology associated with lower energy consumption in its manufacturing process [4, 5]. For instance, the natural fibers cost around US$ 200–1,000/ton, while the synthetic (glass) fibers cost around US$ 1,200–1,800/ton [6]. NFRCs have good mechanical properties when compared to the pure polymer for certain applications [7, 8]. However, the main limitations of NFRCs are their hydrophilicity, variation in mechanical properties, low thermal stability, and low wettability with other materials [2, 3, 9]. Various techniques have been employed by researchers to overcome these shortcomings (i.e., chemical treatments of the fibers and the hybridization techniques) [9, 10]. One method explored in the literature to enhance the properties of cellulose fiber-reinforced composites is the use of fillers (either on the fiber surface or into the matrix as the second reinforcing phase). Nanofillers are classified into four types: organic (cellulose), inorganic (metals and metal oxides), carbon nanostructures (graphene, fullerenes and carbon nanotubes (CNTs)), and clays [9]. It was demonstrated in the literature that the fillers may contribute to water resistance (lowered hydrophilic nature) of polymeric composites, increased dimensional stability and shrinkage reduction, thermal stability, machinability and vibrational damping. However, these properties are a function of many factors such as shape, size and orientation of filler reinforcement, filler content, and filler/matrix/fiber adhesion and agglomeration [11–13]. It was shown in the literature that the physical nature of nanofillers plays an important role in dispersing them into the matrix. For instance, as-produced CNTs are held together in bundles or entanglements consisting of 50 to a few hundred individual CNTs by van der Waals force, and these bundles and agglomerates lead to reduced mechanical properties of composites as compared to theoretical predictions related to individual CNTs [14]. Therefore, difficulties in uniformly dispersing these particles in the polymer matrix were reported. It was shown that the homogenization of CNTs is not only a geometry issue (length and size of the CNTs), but also relates to the methodology to separate single CNTs from CNT agglomerates as well as stabilizing them in the polymer matrix to avoid further agglomeration [14]. One significant factor that affects the mechanical and thermal properties of polymer cellulosic composites modified with fillers is the filler content [15]. Higher filler content may lead to poor bonding and dispersion of fillers, which decrease the properties of the composites. Another relevant aspect to be taken into account is the interfacial adhesion between the fillers and the polymer matrix. It is well known that the capacity of load transfer depends on the filler/matrix interface; thus, a stronger interfacial adhesion

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is linked to higher mechanical properties of composites via an improved matrix/ filler load transfer mechanism. This chapter presents recent developments in the field of cellulose composites reinforced with nanofillers. The effects of nanofillers (natural and synthetic) on mechanical and thermal properties are presented. The main challenges met in the production of nanofillers-reinforced cellulose composites (agglomeration of nanofillers during manufacture, insufficient interfacial compatibility between fillers and the matrices, and degradation of the fillers during processing, etc.) are briefly discussed.

4.2 Effect of nanofillers on the mechanical properties of cellulose polymer composites There are many available techniques that focus on the enhancement of the mechanical properties of NFRCs. The addition of nanofillers to an NFRC represents one of the most promising ones, generating a multiscale reinforced composite (i.e., macroscale long fibers and nanoscale fillers) [16]. The positive effects on the mechanical properties range from increased impact resistance, fracture toughness in both Modes I and II, interlaminar shear strength, to improved thermal and electric conductivity, barrier properties and environmental ageing resistance, as well as antimicrobial effects [8, 17–20]. However, it is important to note that the positive effects of the nanofillers are highly dependent on many parameters such as filler size, shape, surface chemistry, weight fraction, and fabrication technique [21]. The filler’s specific surface area (surface area per unit weight) increases as size decreases, resulting in the highest surface area being found in nanoscale fillers. A larger surface area significantly affects the interfacial contact between the filler and matrix, leading to improved load transfer efficiency [22]. The scale also affects the filler shape, as nanoscale fillers tend to have a higher aspect ratio than microscale fillers. The high potential of nanofillers to improve the composite performance is also heavily influenced by the dispersion/distribution, which is a function of the fabrication method. This is because nanoscale filler reinforcements tend to agglomerate and form lumps, which usually act as stress concentrators and lead to crack origination sites [23]. Therefore, each type of nanofiller and its individual composite application will result in an optimum filler content whose distribution homogeneity can be assured by the mixing/fabrication method. Nanofillers may be synthetic, inorganic, or organic/natural [16]. The following subsections will present the effects of synthetic and natural nanofillers on the mechanical properties of NFRCs.

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4.2.1 Effect of synthetic nanofillers on the mechanical properties of cellulosic composites The effect of synthetic fillers on the mechanical properties of cellulose polymer composites has been reported [8, 13, 17–19, 24–28]. The diversity of the fillers used is high; however, the most commonly used synthetic nanofillers are: CNTs, MWCNTs, alumina, and titanium dioxide (TiO2), among other metal oxides [16]. It is clear that synthetic nanofillers have a significant effect on the mechanical properties of the reinforced NFRCs. This is due to the previously described general advantages afforded by nanoscale synthetic fillers. Due to their high specific surface area, good chemical interaction with the hydrophobic matrix, and crack arresting capabilities, a synergistic relationship is observed with NFRCs [24]. However, a careful determination of the fabrication and matrix/filler homogenization technique must be done in order to avoid filler agglomeration, which may lead to detrimental effects [8, 19]. Several authors have investigated the effect of synthetic nanofillers on the mechanical properties of NFRCs [8, 17–19, 24–26, 28–32]. For example, Prasad et al. [17] studied the effect of nano-titanium oxide in the mechanical, thermal, and water absorption properties of flax fiber-reinforced composites, and 50 nm nano-titanium oxide was used. A balanced satin weave flax fiber bidirectional was used along with an epoxy resin matrix. Four fabric layers were used at an approx. 28% volume fraction. High energy mechanical stirring (1,000 rpm) for 60 min followed by 45 min sonication was used to disperse the nano-titanium weight fractions in the resin, prior to fabrication (i.e., 0.5%, 0.7% and 0.9%, respectively). Compression molding technique was used to fabricate the composite plates. The tensile, flexural, impact, and ILSS (interlaminar shear stress) properties of the materials were assessed according to their respective ASTM standards. It was found that the optimum weight fraction for the nano-titanium modified composites was 0.7%. Significant improvements were reported regarding the mechanical and water barrier properties of the modified composites. Homogeneous dispersion was observed via the combined technique of mechanical stirring and sonication. Improvements of 10.95%, 20.05%, 10.45%, and 18.8% were reported for tensile, flexural, impact, and ILSS properties, respectively, compared to the unreinforced specimens. In a follow-up research, Prasad et al. [18] investigated the effect of nanotitanium-modified flax fiber-reinforced composites regarding the Modes I and II interlaminar fracture toughness, and 4H satin weave flax fiber fabrics were used with an epoxy resin. The compression molding fabrication technique was used. Mechanical stirring, followed by sonication technique, was used to disperse the nano-fillers in the resin. The weight fractions of the nanofillers used were 0.3%, 0.4%, and 0.5%, respectively. Four layers of fabric were used, and the pre-crack was controlled via the placement of a high-temperature polyimide tape (15 µm thickness) between the intermediate layers. Four layers of unidirectional carbon fibers were

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placed on the outer sides of the flax core to avoid mixed cracking during bending of the specimen arms. Significant improvements to the fracture toughness in both Modes I and II were reported as a function of the nanofiller reinforcement. The delamination resistance of the composite was enhanced. The Mode I toughness value (GIC) improved by 52% for the 0.4% specimens, and the 0.5% specimens presented an enhancement of the Mode II value (GIIC) of 73%, when compared to the control case, respectively (see Figure 4.1).

Figure 4.1: Effect of nano-titanium dioxide on Mode II fracture toughness of flax fiber-reinforced composites (reproduced with permission from [18]).

Recently, Prasad et al. [24] investigated the effect of coating flax fibers with nanotitanium oxide in epoxy resin fiber-reinforced composites. Satin weave bidirectional flax fibers were used, along with epoxy resin. Four layers of flax fiber fabrics were used. Nano-TiO2 was grafted onto the surface of the flax fibers. A sonicated silane and ethanol solution (1:10 ratio) was used to dip coat the fibers, which were subsequently washed with deionized water and ovendried. The nano-titanium oxide weight fraction range was 0.2% to 0.8%. The mechanical properties were assessed via tensile, flexion, and ILSS tests, according to their respective ASTM standards. The tensile properties improved by 22%, while flexural and interlaminar shear strength values improved by 24% and 16%, respectively, when compared to the control specimens (see Figure 4.2). Neto et al. [19] investigated the effect of MWCNTs (multiwalled carbon nanotubes) in the UV and water spray ageing process of natural fiber and hybrid composites. Jute

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Figure 4.2: Mechanical properties of nano-TiO2-coated flax fiber epoxy composites as a function of filler content: a) tensile strength; b) flexural strength; and c) interlaminar shear strength (reproduced with permission from [24]).

and glass fiber plain weave bidirectional fabrics were used, along with an epoxy resin matrix. The MWCNTs used were of 95% purity with 50–100 nm internal diameter and 5–10 nm internal diameter and an average size of 5–10 µm. The compression molding technique was used to fabricate the composite plates. The fillers were first stirred with the resin manually and then sonicated at 100 W. The filler weight fraction was 0.6%. The mechanical properties were analyzed via tensile and flexural tests according to ASTM standards. The samples were aged at 500 h and 1,000 h of exposure, respectively. It was reported that, for the pure natural fiber composites, an improvement in the mechanical properties was observed. However, for the hybrid composites, a slight decrease in tensile and flexural strength was observed. Queiroz et al. [8] studied the effect of multiscale hybrid natural/synthetic reinforcements in epoxy composites. Plain weave bidirectional jute and E-glass fiber fabrics were used, along with an epoxy resin. The macroscale fiber fabric stacking sequence was three layers of glass fabrics around a five-layer jute fabric core. Both

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micro and nanoscale synthetic fillers were used (i.e., micro glass and nano-TiO2). Microscale natural fillers were fabricated and used (i.e., micro jute and curauá fillers). The weight fractions studied were 1% and 3%. The dispersion methodology was manual stirring, followed by sonication of the fillers in the resin. The fabrication method of the composite plates was compression molding. It was reported that the macroscale hybridization technique significantly improved the general mechanical properties of the composites. The filler nature and scale was reported to be highly significant. The highest improvement observed was in the 3% nano-titanium specimens in tensile testing, when compared to the unreinforced macroscale hybrid composite. Sumesh and Kanthavel [25] investigated the effect of nano-TiO2 filler inclusion in mechanical and free vibration dampening behavior of hybrid natural fiber composites. Banana, sisal, and pineapple fibers were used, along with an epoxy resin. The fibers were 5 mm in length and underwent NaOH treatment at 5%. The nanoTiO2 was dispersed in the resin via mechanical stirring of a nanotitanium/acetone epoxy solution, followed by sonication. The mixture was then cooled, prior to hardener addition. The filler content investigated ranged from 1 to 4 wt%. The compression molding technique was used to fabricate the composite plates. It was reported that the tensile and impact properties of all composites were enhanced with the incorporation of 3% TiO2 nanofiller. The natural frequency of the composites also improved to a point, since at 4 wt%, agglomeration was reported and the natural frequency decreased. Maharana et al. [26] investigated the effect of fumed silica nanofiller addition and stacking sequence on the interlaminar fracture behavior of bidirectional juteKevlar hybrid nanocomposites. Bidirectional jute (J) and Kevlar fabrics (K) were used as well as an epoxy resin. Fumed nano-silica with an average size of 37.7 nm was used. PTFE sheets were used to create and control the pre-crack. The dispersion method was mechanical stirring at 1,000 rpm of an epoxy/acetone solution. The studied filler fractions were 0–4.5 wt% (1.5% intervals). The jute fabrics were alkalitreated at 5% for 1 h, and were then washed with distilled water and dried. Four layers of fabric were used, and the stacking sequence was JJJJ, JKKJ, KJJK and KKKK. A fiber weight fraction of 25% was maintained. The fabricated composites were tested in Modes I and II. It was reported that the addition of the nanofiller significantly enhanced the Mode I fracture toughness observed in the fiber bridging and mechanical bonding between the fiber and the matrix. A catastrophic transverse crack through the jute arms of the JJJJ-1.5 specimens was reported, and this led the researchers to not recommend this architecture, even though it represented the highest Mode I value. This is due to the brittle nature of the jute composite. However, the JKKJ-1.5 case was recommended given the significant improvement when compared to the KKKK-1.5 specimens in Mode I. Mode II was also significantly improved by nanofiller addition.

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Shen et al. [28] investigated the effect of carbon nanotube addition on the mechanical properties of natural fiber composites. Ramie fiber-woven fabrics were used to fabricate the epoxy composites. Carboxyl-functionalized multiwalled CNTs (MWCNTs) with outer diameters ranging between 8 and 15 nm and lengths of around 50 µm were used. The dispersion methodology was mechanical stirring of an epoxy/acetone MWCNT solution (weight contents of 0–0.6% with 0.2% intervals), followed by sonication. The hand lay-up fabrication method was used. Two different ply stacking sequences were used for the flexural and short-beam shear tests, namely, 17- and 40-ply composites, respectively. The composites were tested in flexural, ILSS, single notch bending (SENB), and Charpy impact tests, according to their respective ASTM standards. Dynamic mechanical analyzer (DMA) was used to characterize the storage modulus and loss factor in three-point bending. It was reported that the addition of CNTs significantly improved the ILSS, flexural strength, and modulus. The quasi-static fracture toughness was enhanced, while the impact energy decreased. Improvements were reported in the storage modulus and glass transition temperatures, to a certain extent. Raghavendra et al. [29] investigated the effect of alumina nanofiller (Al2O3) on the mechanical behavior of hybrid epoxy composites. Jute and glass bidirectional fabrics were used alongside epoxy resin. Al2O3 nanofiller, with a size range of 120–40 nm, was used. The weight fractions studied were 2, 4, and 6 wt%. The dispersion method was mechanical stirring. Four plies were used, and the hand lay-up technique was used to fabricate the plates. Five different stacking sequences were studied: GGGG, JJJJ, GJGJ, JGGJ, and GJJG. The properties of the composites were assessed via tensile and flexural testing according to ASTM standards. It was found that the GJGJ + 4 wt% nano- Al2O3 composites showed better flexural results, when compared with the epoxy/glass fiber composites, while GJJG + 4 wt% nano-Al2O3 composites showed better tensile results. The flexural strength was reported to improve significantly with nanofiller addition, with a range between 10% and 30% enhancement. The highest strength improvement was observed in the 4 wt% case. For the tensile tests, the improvement range was 5% to 15%, and the highest overall case was also the 4 wt%. Prasob and Sasikumar [30] studied the static as well as dynamic behavior of jute/ epoxy composites reinforced with ZnO and TiO2 fillers as a function of temperature. Jute fabrics (bidirectional plain weave) and epoxy resin were used to fabricate the composites. The fillers were reduced to nanoscale via a ball milling machine at 250 rpm. The filler was dispersed within the epoxy matrix via manual stirring. Six layers of jute fabric were used, and the filler weight fractions studied were 2%, 4%, and 6%. The average filler size was 100 nm. The fabrication technique used was the hand lay-up. The composites were analyzed in free vibration, damping factor, tension, compression, flexion, ILSS, as well as Charpy impact. The cryogenic temperature (-40 °C) was achieved via immersing the specimens in liquid nitrogen for 5 h. It was reported that the dynamic behavior of the composite has a trend of enhancement

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from 0 to 4 wt%. The natural frequency also presented an increase with the subzero temperatures. Significant dampening was also observed at 4 wt% of filler. The mechanical properties (tensile, compressive, flexural, and ILSS) improved as a function of filler addition, when compared to the unfilled case. The impact tests showed that higher energy was absorbed due to the presence of the fillers. The authors concluded that the matrix toughening, microcrack bridging through addition of the ZnO and TiO2 nanofillers, and good interfacial bond between the matrix and fiber contributed to better mechanical properties of the filled composites. The mode of failure was a combination of fiber breakage, fiber pull out, matrix cracking, matrix debonding, and matrix deformation. Foruzanmehr et al. [31] investigated the effect of nano-TiO2 grafting of flax fibers in the mechanical properties of poly-L-lactic acid (PLA) composites. Unidirectional flax fibers were used along with a commercial PLA matrix. The fibers were treated in two separate stages: dewaxing and oxidation. Dewaxing was done via boiling acetone under reflux for 45 min. Then, they were treated with a 5% sodium hydroxide solution for 40 min and then washed with distilled water. The pH was then stabilized, and the fibers were dried. Subsequently, the alcohol groups were oxidized to carboxylic groups by using TEMPO. A TiO2 thin film was coated onto the fibers via sol–gel technique with Titanium isopropoxide as a precursor. Six-ply composite plates were fabricated by the compression molding technique. The mechanical properties of the composites were assessed via tensile, ILSS, and Izod impact tests, according to their respective ASTM standards. The ILSS results reported that a significant improvement took place as a function of nano-titanium oxidation prior to grafting. Therefore, the interfacial strength was enhanced. This was also observed in the tensile results. Furthermore, the impact resistance was significantly higher when compared to the neat PLA case. To summarize, the main features contributing to the improvement in mechanical properties of NFRCs reinforced with nanofillers are matrix toughening, microcrack bridging through nanofillers, and the better interfacial bond between the fiber and matrix. In general, the failure mechanism is a combination of matrix deformation, fiber pullout, interface debonding, and matrix cracking.

4.2.2 Effect of natural nanofillers on the mechanical properties of cellulosic composites Similar to the synthetic nanofillers, natural nanofillers have also been used in the reinforcement of cellulosic composites. The most commonly used natural fillers are cellulose nanofibrils (CNF), cellulose nanocrystals (CNC), and nanoclays [16, 33]. Classical natural fibers may also be used as nanoscale phase reinforcements and may be produced via ball milling [34, 35]. Many advantages afforded by the synthetic nanofillers are also present in the natural ones. For example, the high specific

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surface area due to the nanoscale of the fillers offers high potential for stress transfer between the fillers and matrix. However, the natural nanofillers present a high level of hydrophilicity due to the cellulose, and, therefore, care must be taken to diminish the possible decrease in the properties given the hydrophobic nature of common matrix materials such as epoxy. This may be done by chemical treatments or with the use of coupling agents [33]. There are other interesting advantages due to the use of nanocellulose such as antimicrobial activity [36]. Several researchers have studied the effects of natural nanofillers in the mechanical properties of cellulosic composites [37–41]. Saba et al. [37] studied the effect of oil palm nanofiller in the mechanical properties of kenaf-reinforced epoxy composites. Kenaf nonwoven mats were used alongside an epoxy matrix. Nanoscale oil palm fiber (OPEFB) as well as montmorillonite (MMT) and organically modified montmorillonite (OMMT) nanoclays were used. The 3 wt% fillers were mixed with the resin via a high-speed mechanical stirrer. The composites were fabricated by the hand lay-up method. The mechanical properties of the composites were assessed via tensile and notched Izod impact tests according to their respective ASTM standards. It was found that the mechanical properties were significantly improved by the addition of the oil palm nanofiller (see Figure 4.3). It was noted that the fillers act as obstacles for crack initiation and propagation. Alamri and Low [38] investigated the effects of halloysite nanotubes in the mechanical properties of recycled cellulose epoxy fiber composites. Recycled cellulose fiber paper (RCF) and halloysite nanotubes (HNTs) were used as reinforcement for the epoxy composites. A high-speed mechanical mixer was used to mix the HNTs with the resin (1, 3, and 5 wt%). The RCF paper sheets were fully wetted with the resin and then compressed in a silicone mold, under constant pressure at room temperature for 24 h. RCF was approx. 52 wt%. The mechanical properties were analyzed via flexural and Charpy impact tests. It was reported that when the filler-reinforced resin is compared to the neat resin, significant improvements were observed. However, for the RCF/HNT/epoxy composites, the addition of the fillers did not present significant variation in the mechanical properties. Eom et al. [39] studied the effect of multiscale hybridization of silk nanocellulose fibrous composites. Silk cocoons and bamboo fibers were used. The silk fibers (SF) were degummed by boiling in an aqueous solution containing sodium carbonate, in order to remove sericin and other impurities, followed by washing several times with deionized water. They were then dried for 3 days and wound onto a small mandrel yielding 6-cm long fibers. CNF were obtained from the bamboo fibers. First, dewaxing of the fibers was done using a Soxhlet with benzene/ethanol mixture. Then, the lignocellulosic material was removed using a chlorite solution five times, until the sample became white. The residual hemicellulose, starch, and pectin were then removed with a potassium hydroxide solution. Then, the bamboo cellulose dispersion was prepared by adjusting the concentration of distilled water and sonicated at high energy to produce CNFs. A solution of distilled water and CNF with varying wt% (5, 10, and 15 wt%)

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Figure 4.3: Effect of nanofiller content on the mechanical properties of the composites, a) tensile strength; b) tensile modulus; and c) impact strength (reproduced with permission from [37]).

was sonicated, and, subsequently, SF was added and allowed to incubate for 10 min., in order to fully wet the SF with the CNF. The solution was then poured onto a mold and homogeneously distributed. After hand drying, the SF/CNF mixture was placed onto a hot press for 10 min at 120 °C to fabricate the composites. This represents a simple, short-term thermal bonding process. It was chosen due to its eco-friendliness and energy efficiency. The mechanical properties were analyzed via tensile and Izod impact tests, according to the ASTM standards. It was reported that significant improvements were achieved in the mechanical properties of the composite as a function of the CNF. The SF/CNF composite presented an enhancement in impact energy and tensile strength of 110 and 225%, respectively, when compared to the nonwoven SF.

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Fortea-Verdejo et al. [40] investigated the effect of nanocellulose as a binder in the production of flax fiber preforms. Short and loose flax fibers of approx. 25 mm in length were used along with chlorine free pulp. Bacterial cellulose (BC) was also used. NFC was produced from the pulp via a procedure described in a previous research effort [42]. Basically, the pulp was soaked in water for 10 days, followed by ultrafine friction grinding. Similarly, BC was extracted from commercially available nata de coco. The details of the extraction process may be found in Ref. [43]. Nonwoven flax fiber preforms were manufactured via a single filtration process, similar to a papermaking process. 36 g of short and loose flax fibers were added to suspensions containing 10, 20, and 30 wt% BC, NFC, or pulp. The suspensions were then vacuum-filtered and then, the excess water was removed via a Buchner funnel. This was followed by wet pressing with blot paper three times, under a pressure of 1.5 t. A final hot press (120 °C) completely dried and consolidated the nonwoven preforms. The mechanical properties of the fabricated nonwoven preforms was assessed via tensile and flexural tests. It was reported that BC and NFC were excellent binders for the flax nonwovens due to the high surface area. However, the pulp was not a good binder due to low surface area. Furthermore, lower porosity was observed in the preforms fabricated with BC and NFC, compared to pulp. This is attributed to the higher fiber diameter of the pulp. It was reported that the mechanical properties of the flax nonwoven preforms outperform those of conventional flax nonwovens that use thermoplastic polymers as binders and are directly comparable to conventional flax nonwovens. Finally, it was reported that the preforms did not disintegrate after being submerged in water for 21 days, due to the fact that the hydrogen bonds formed upon nanocellulose drying remain stable in water. Mohan and Kanny [41] studied the effect of nanoclay filler in sisal fiber-reinforced epoxy composites. Epoxy resin, montmorillonite nanoclay (Cloisite 30B), and unmodified clay (Na+ Cloisite) were used along with sisal fiber in the form of chopped strand mat. The clays were washed in acetone and dried. The dispersion of the clay in the resin (1, 3, and 5 wt%) was done via a magnetic stirrer, prior to the vacuum-assisted resin infusion molding technique. Tensile tests were carried out according to an ASTM standard. It was observed that the tensile properties were positively affected by the nanofiller, up to 5 wt%; beyond this, the properties were negatively affected due to filler agglomerations. To conclude, the applications of natural fillers are still limited, in general, owing to their lower mechanical properties, despite their good specific mechanical properties at lower density. However, their low cost and low content level required make them efficient as filler reinforcements.

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4.3 Effect of nanofillers on the thermal properties of cellulose polymer composites Thermal stability of natural fiber composites is a significant factor worth considering, since process temperatures play a vital role in composite fabrication. At a high temperature, natural fiber constituents (i.e., cellulose, hemicellulose, and lignin), begin to degrade, and the most significant properties (mechanical and thermal) of the composite are altered. Considerable research attempts are constantly being made, and a number of the shortfalls of NFRCs were addressed by recent advancements regarding fiber treatment and modification, evaluation of new natural fibers, and hybridization. The fiber modification techniques provide enhanced fiber–matrix interfacial adhesion, altered fiber roughness, and wettability and depend on the particular fiber/matrix used and the composite application. On the other hand, hybridization offers flexibility in fiber selection for the material properties in relation to the end-use application needs [44–46]. Several authors used the Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), and DMA analysis to determine the thermal properties of cellulosic composites modified with nanofillers [44, 47]. The glass transition temperature (Tg), crystallization temperature (Tc), melting temperature (Tm), thermal expansion coefficient (CTE), viscoelastic behavior, and thermal stability are the most common thermal properties determined by these methods. The thermal properties of composites modified with fillers are governed by several factors such as filler type, aspect ratio and dispersion, and interfacial interactions of the fillers with polymer matrix, among others [48]. In general, superior thermal properties can be achieved by adding nanofillers, at low reinforcement concentration, usually < 10 wt %. For instance, the improvements in thermal properties are attributed to the restriction imposed by the intermolecular bonding between the filler and the matrix, which limits the flexibility and mobility of chains. However, in some cases, diminished thermal properties are reported, mainly due to the filler agglomerations.

4.3.1 Effect of synthetic nanofillers on the thermal properties of cellulosic composites Several researchers investigated the thermal characteristics of natural fiber composites modified with synthetic nanofillers. It was shown that, in general, the use of synthetic nanofillers in cellulosic composites improve their thermal stability through their constraint effect on the polymer matrix segments and chains [3]. For instance, Prabhudass et al. [49] investigated the incorporation of multiwalled carbon nanotube (MWCNTs) in NFRCs. The configurations studied were bamboo + epoxy, kenaf + epoxy, bamboo + kenaf + epoxy, bamboo + kenaf + epoxy + MWCNT, bamboo + MWCNT + epoxy, and kenaf + MWCNT + epoxy. The authors used 5% NaOH to treat the natural

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fibers in order to improve the interfacial adhesion between fiber–matrix, whereas the method used to disperse the MWCNT was the catalytic chemical vapor deposition method (CCVD) and the one with a weight of 6 g of MWCNT. The results show that the hybrid composite reinforced by MWCNT presented an improvement in the glass transition temperature (Tg) and a 41% increase in the storage modulus (E’), because of the presence of MWCNT. Da Costa et al. [50] analyzed piassava fiber composites modified with graphene oxide (GO) in an epoxy matrix. The configuration studied was epoxy + 10% piassava, epoxy + 30% piassava, epoxy + 50% piassava, epoxy + 10% piassava + GO, 30% piassava + GO, and epoxy + 50% piassava + GO. The piassava fibers were immersed in 0.56 mg/mL for 30 min and agitated in a shaker for 30 min. The composite with 10% piassava + GO presented a storage modulus (E’) higher than the composite with 50% piassava + GO. However, the authors reported that the addition of GO did not present any changes in the glass transition (Tg) of the composite. Prasob et al. [30] studied NFRCs filled with nano-ZnO and nano-TiO2 fillers at various weight ratios of 2%, 4%, and 6% using the hand lay-up technique. They found that the nanofillers limit the water absorption capacity of composites, filling the voids found in the fiber–matrix interface. The composites modified with particles presented an increment in TIDT, when compared to the pure jute composite (328 °C for TiO2modified samples and 327 °C for JFRP + Glass, compared to 322 °C of JFRP). However, the glass transition temperature of the jute/epoxy composite filled with nanofillers presented an insignificant variation, when compared to the unfilled jute/epoxy composite. In a different approach, Wang et al. [51] modified flax fiber through grafting of nano-TiO2, using a silane coupling agent. The effects of the grafting on the mechanical and thermal properties of the flax fibers and flax fiber-reinforced epoxy composites were investigated. It was found that flax fiber grafting effectively enhances the storage modulus and glass transition temperature of the composites, when compared to the control fiber and other treated fiber-reinforced composites (see Figure 4.4). The author concluded that the increased rubbery modulus and Tg is due to the enhanced bonding between the flax fiber and the epoxy resin matrix. Da Silveira et al. [52] studied the influence of chemical treatment on the mechanical and thermal properties of hemp fiber-reinforced composites functionalized by graphene oxide (GO). The fibers were treated in an alkaline solution (NaOH) with 5 wt% and 10 wt% for 1 h. After the washing and drying process, the hemp fibers were immersed in a 5.9 mL solution of GO (13.6 mg/mL) and 500 mL of deionized water, totaling 1 wt%, for 1 h. The cases studied were HF (pure hemp), HF + GO(1 wt%), HF + AT5 (NaOH), HF + AT5 + GO, HF + AT10(NaOH), and HF + AT10 + GO. The composite HF + AT10 + GO presented a value of 318.5 °C for the starting temperature (Tonset), when compared to the composites: HF (304 °C), HF + AT5 (284.5 °C), and HF + AT10 (284.6 °C), respectively. In addition, the HF + AT10 + GO composite showed an increase in the glass transition temperature when compared to the pure hemp composite.

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Figure 4.4: Thermal analysis graphs of the flax fiber-reinforced composites: (a) storage modulus; and (b) loss factor (tan delta) (reproduced with permission from [51]).

Sumesh and Kanthavel [53] investigated the effects of incorporation of aluminum oxide (Al2O3) on the mechanical and thermal properties of hybrid composites. The configuration used was sisal/coir, sisal/banana, and banana/coir, and the percentage of Al2O3 used was: 0.1, 2, and 3 wt%. The manufacturing technique used was compression molding. The results showed that for the cases of sisal/coir, sisal/ banana composites with the addition of Al2O3, an increase in thermal stability was reported, when compared to the composite without nanofiller. Arulmurugan et al. [54] studied the effect of barium sulfate (BaSO4) on the thermal properties of an aloe vera/flax hybrid composite. The composite architectures

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studied were: four layers of aloe vera fiber (HNRP1), four layers of flax fiber (HNRP2), two layers of flax fiber + two layers of aloe vera fiber (HNRP3), four layers of aloe vera fiber + 5% of BaSO4 (HNRP4), four layers of flax fiber + 5% of BaSO4 (HNRP5), and two layers of flax fiber + two layers of aloe vera fiber + 5% of BaSO4 (HNRP6). Increasing BaSO4 in both pure as well as hybrid composites enhanced the thermal resistance of HNRP. Furthermore, the highest peak for the treated composites presents an increment with the application of BaSO4 in its structure. Kavya et al. [55] studied the effect of the incorporation of fly ash and TiC nanofiller on coir-reinforced composites. The studied cases were epoxy + coir + fly ash (CFE), epoxy + coir + TiC nanofiller (CTE), and epoxy + coir + TiC + fly ash (CFTE). The mechanical mixture was used for a uniform dispersion of particles within the epoxy matrix. The authors reported that the presence of TiC nanofiller increased the onset temperature for CTE composite, when compared to the other cases. The value found was 339.28 °C for CTE, while CFE and CFTE presented values of 328.84 and 327.73 °C, respectively. Moreover, the CTE composite showed an increase in Tg when compared to CFE composite, while, for the CFTE composite, only a slightly improved Tg was found.

4.3.2 Effect of natural nanofillers on the thermal properties of cellulosic composites In recent years, several scientists and various sectors of the industry are using natural fillers due to several advantages, such as ecological characteristics, low cost, less tool wear, etc. Moreover, the type of the natural fillers, type of matrices, processing techniques, and the drying process impact the thermal properties of the natural fiber composites modified with natural fillers [4, 15, 41, 56–58]. The main challenge is to disperse and distribute the natural fillers in the hydrophobic polymer matrices, due to hydrophilic nature of natural fillers. Various surface modifications of natural fillers through grafting to enhance the interaction between fillers and hydrophobic matrices have been reported in the literature [59]. Vivek et al. [4] analyzed the effect of bagasse ash filler on the mechanical and thermal properties of natural hybrid composites. The composites were manufactured using the vacuum bag-assisted resin transfer method. The cases studied were banana/flax (HBF), banana/kenaf (HBK), sisal/flax (HSF), and sisal/kenaf (HSK). Three percentages of bagasse (1, 3, and 5 wt%) with 350 nm of size, were incorporated in the composites by manual stirring. The authors reported that HBK composites with 3 wt% and HSK with 5 wt% showed high thermal stability. Dinesh et al. [57] studied the influence of rosewood dust and padauk wood dust fillers on the mechanical and thermal properties of the jute fiber-reinforced composites. The studied configurations were: W1 – rosewood, W2 – padauk, W3 – rosewood/padauk, and W4 – without filler. It was found that the thermal stability of the

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rosewood dust filler composites presented higher starting temperature values than the padauk wood dust composites. Mohana et al. [15] analyzed the incorporation of calcium carbonate (CaCO3) filler in coir/luffa hybrid cylindrical composites. The composites were fabricated by hand lay-up technique. The tests used in the study were: thermo-gravimetric (TG) and DMA. The percentages of CaCO3 used in the study were: 0, 2, and 4. It was found that the hybrid composite with 2% CaCO3 presented higher values of storage modulus (E’) when compared to the other cases, while the hybrid composite with 4% of CaCO3 showed a decrease in the values of E’ and Tg because of the nanofillers agglomeration, heterogeneity that affects the adhesion between the fiber–matrix. Moreover, the authors showed that the residue contents of the composites with CaCO3 presented values higher than the composites without CaCO3, as shown in Figure 4.5.

Figure 4.5: TGA of with and without calcium carbonate-filled C/L/C hybrid composites (reproduced with permission from [15]).

Mohan and Kanny [41] studied the effect of nanoclay filler on the thermal properties of sisal fiber-reinforced composites. The results show that the nanoclay filler positively affects the storage modulus (E’) and Tg, when compared to the composite without filler. Figure 4.6 shows that the sisal fiber + 5% nanoclay composite presented a Tg value of 70 °C, whereas the samples without nanoclay and microclay presented a value of Tg of 55 °C, respectively.

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Figure 4.6: DMA properties of unfilled and clay-filled composite (reproduced with permission from [41]).

4.4 Summary This chapter presents an overview of the main nanofiller materials, both synthetic and natural, used in the reinforcement of cellulosic composites with the aim of improving their mechanical and thermal properties. The most commonly used synthetic nanofillers are MWCNTs, metal oxides, and TiO2. Among the natural nanofillers, the

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wide application and use of cellulose nanofibrils and nanocrystals as well as nanoclays were reported. It has been clearly demonstrated that the nanofiller reinforcement technique with both synthetic and natural filler materials can have a significant positive impact on both the mechanical (i.e., tensile, flexural, energy absorption, and fracture toughness) and thermal (i.e., storage and loss modulus, as well as transition temperatures) properties of the filler-modified composites.

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Lalit Ranakoti, Manoj Kumar Gupta, Dharamvir Mangal and Pawan Kumar Rakesh✶

5 Recycling of polymers and its application Abstract: Plastics have now become an indispensable element in everyone’s life. It may be household goods or a part of a large shipment, which has not left any area untouched. One cannot even imagine a single day without coming in contact with the plastics. Because of its wide range of applications, plastics are now replacing metals in almost every sector. An increase in the production of plastics is due to the inflating demand of the consumers leading to “postconsumer” consequences. Carbon emission and deterioration of land, forest, and marine are some of the major aftermaths that take place due to the dumping of plastic wastes in the environment. To maintain the balance between consumer demand and nature, recycling of plastics is now being performed. Various operations have been implemented on used plastics to make them fresh. Thus, this chapter deals with the various types of recycling processes intended to be on plastics with relevant applications. Keywords: Recycling, polymers, recycled polymers

5.1 Introduction The existence of polymers in human life is dated since ancient times. Nature has been providing polymers to humans in the form of silk, rubber, cotton, shellac, and so on but scientific revolutions have made humans capable of making synthetic polymers [1, 2]. The first to propose the concept of repeating monomer present in the polymer was Herman Staudinger [3]. The commercial feasibility of synthetic polymer was started in the eighteenth century and its significance was appreciated globally in early 2000. Till then, polymers like epoxy, polyvinyl chloride (PVC), acrylonitrile–butadiene styrene, polymethyl methacrylate, polyolefins, polyurethane (PU), polystyrene (PS), polypropylene (PP), polyethylene (PE), and Nylon were synthesized and commercially

✶

Corresponding author: Pawan Kumar Rakesh, Mechanical Engineering Department, National Institute of Technology Uttarakhand, Srinagar Garhwal, Uttarakhand, India, e-mail: [email protected] Lalit Ranakoti, Mechanical Engineering Department, Graphic Era Deemed to be University, Dehradun, Uttarakhand, India Manoj Kumar Gupta, Mechanical Engineering Department, H.N.B Garhwal University, Srinagar Garhwal, Uttarakhand, India Dharamvir Mangal, Mechanical Engineering Department, Gautam Buddha University, Greater Noida, Uttar Pradesh, India https://doi.org/10.1515/9783110768787-005

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available [4]. Due to its several advantages, synthetic polymers were achieving new heights as plenty of stuff was being manufactured from it. Synthetic polymers were everywhere in the field of automobile, medical, agriculture, and aerospace. Its favorable properties such as lightweight, durability, strength, and resistance to corrosion made it very prevalent in every above-stated sector [5–6]. Even today, if we look around ourselves, we will find most of the things made of plastics. The top five plastic producing countries is shown in Figure 5.1. It is not just that the plastics are eligible entities but the limiting resources of materials especially metals have put the world in a state of encouraging the plastics not as much as they can but to exploit it. Despite such high demand and attractive properties, polymers are always coupled with the problem of post utilization and it is nothing but the waste which has been accumulating in the landfills [7]. Various reports confirm that tons of millions of plastic wastes are being dumped every year with a very low percentage of recycling or reuse causing serious problems of societal and environmental. Wastes dumped in lands deteriorate the fertility of soil and those dumped in the sea put marine life in danger. Increasing the unavailability of landfills for dumping is aggravating and making the condition more critical [8]. Solving these issues not just required recycling, but also suitable waste management. Waste management will help in proper dumping of plastic wastes in places where these could be easily recycled. The past recent years were dedicated to the recycling of plastic wastes in numerous ways [9]. Some of them may be named mechanical recycling and chemical recycling, which can be used as a recycling method depending upon the chemical structure, state, and future usage of plastic wastes. Thus, this chapter deals with a brief discussion on the plastic types, recycling methods, and post recycling applications. Plastic producing countries in 2019 volume of production (in million tonnes) 70 60 50 40 30 20 10 0 China

USA

India

Germany

Figure 5.1: Top five plastic producing countries.

Brazil

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5.2 Plastics The very basic definition of a polymer is the “combination of a long chain of hydrocarbons.” But plastics are not fully polymers although they are derived from polymer but formed only after combining with the additives [10]. Generally, based on the application, the polymer percentage varies in plastics. Practically, it is varied from a maximum of 100% to a minimum of 20% or less sometimes. If we look into the variety of plastics, then one would get shocked by knowing that more than 20,000 grades of plastics are available in the market [11]. To continue our discussion on recycling, we should first understand the fundamentals of plastics. Based on physical behavior, plastics are categorized into two groups: thermoplastics and thermoset plastics. Their molecular structure and the method of recycling applied to it differentiate them from one another [12]. Some of the common plastics and their applications are listed in Table 5.1. Table 5.1: Plastics and their uses. Plastics

Applications

References

Polystyrene

Food trays, cartons, and disposable cups and bowls

[]

Polypropylene

Plastic containers

[]

Low-density polyethylene

Plastic wraps

[]

High-density polyethylene

Water jugs, bleach, and shampoo bottles

[]

Polyvinyl chloride

Infrastructure components

[]

Polyurethane and polycarbonate Components of car and electronic parts

[]

Polyethylene terephthalate

[]

Soft drink bottles, detergent bottles, and containers

Thermoplastics melt on heating and solidify on cooling. To recover thermoplastics, reheating is required. Some of the common thermoplastics such as HDPE, LDPE, PS, and PP are available in huge amounts to fulfill the consumer needs for the manufacturing of various products like the packaging of food and medicines, cover tops of tanks, and travel bags. Carbonated bottles for drinking purposes are made from polyethylene terephthalate (PET), whereas cable coating, hoses, and pipes are made of PVC material [13]. On the other hand, thermosets cannot be reheated and it gets decomposed rather than melting. It is due to the cross-linking of the chain during the curing process which results in the formation of an extremely dense chemical structure and thus leads to brittleness and stiffness. Epoxides, phenolics, formaldehyde, and so on are some typical thermosets used in the manufacturing of household items like electrical insulation, pans, adhesives, and kitchenware [14]. Their service life is long but it is challenging to recycle them. Nevertheless, methods are being explored to recycle thermosets which are discussed in the coming section.

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5.3 Processing of plastics There are numerous ways for manufacturing of plastics in the final product. Injection molding, blow molding, film blowing, and extrusion are usually among the most preferred processes. Each process is used for a particular type of application only. The most typical process for a general idea of recycling is shown in Figure 5.2. Compounding is carried out in extrusion in which additives are added in the polymer which generally enhances the characteristics of base material, that is, polymer [15]. Usually, simple products are made by extrusion, for example, plastic sheets and profiles for windows. Complex products in very less time can be made from extrusion. Television housing, door handle for car, mobile case, and water bucket can be manufactured by extrusion process. Blow molding is preferred for a hollow structure, for example, bottles for drinking water and soft drink. Luggage bags and packing materials can be prepared by using film blowing. This is to be noted that input raw material in the form of pellets is common in all the processes [16].

Figure 5.2: Recycling process of plastic wastes.

During processing, plastics experience various physical and mechanical changes like heat, pressure, melting, deformation, and shear. Heating the polymer changes the phase from solid to liquid and makes it melt. Deformation of melt polymer occurs due to the applied pressure, and the rate of deformation increases with the increase in temperature. Shear arises due to the deformation of melt polymer and causes mechanical changes in the raw plastic [17]. Recycling of the plastic wastes may require all the processes in reverse which will not be economical and feasible but recycling is one channel to make the waste of plastic a reusable one. Several techniques of recycling and reprocessing of waste plastic have evolved during the time and some are under progress, making them fit for recycling the plastic wastes.

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It is not an easy task that we can recycle any plastic. There are many grades of plastics that are very difficult to recycle. Recycled plastics and their uses with the rate of recycling are shown in Table 5.2. Table 5.2: Use of plastics after the recycling and rate of recycling. Plastics

Use after recycling

Recycling Reference rate

Polystyrene

Stationary items, picture frames, and toys

%

[]

Polyurethane and polycarbonate

Automotive and electronic parts

%

[]

Polyvinyl chloride

Household goods, hangers, and cans

%

[]

Low-density polyethylene

Furniture, decking, and shipping envelopes

%

[]

Polypropylene

Plant pots, toothbrush handles, and paint cans

%

[]

Polyethylene terephthalate

Cheap carpets, fleeces, clothing, and soda bottles

%

[]

High-density polyethylene

Block board, park benches, and pipes

%

[]

5.4 Recycling of plastic wastes The process of converting plastic wastes into useful ones is known as recycling or reprocessing. Recycling has become the necessity of every country and it is practiced worldwide to avoid the harmful results of plastic wastes. Every year, around 10–15 million tons of plastic wastes are dumped into the ocean which if not ceased will not only affect marine life but can also create the environmental imbalance [18]. In recycling, any kind of plastic is picked, sorted, and chipped, and then it is sent for melting to form a pellet [35]. The total number of plastics produced and dumped in the ocean is shown in Figure 5.3

5.4.1 Primary recycling Converting clean and waste plastic into a new and usable form is known as mechanical recycling. The physical and mechanical properties remain intact in mechanical recycling. Most of the industries have the provision of recycling the post waste plastic due to which this process is also called closed-loop recycling or primary recycling [19]. Waste plastic has to undergo several primary processes before

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Values in million tonnes 24

150

300

Total plastic produced

Single used plastic

Plastic waste dumped in the ocean

Figure 5.3: Plastic production data of the world.

bringing it to mechanical recycling. These are a collection of similar types of plastics based on shape and size, manual sorting, and so on making mechanical recycling an expensive task [20]. For this reason, it is not in fashion in most of the industries. To make the waste plastic more homogeneous, crushing, shredding, and milling are carried out. This leads to the easy mixing of additives and fire retardants for further processes. Shaping the waste into a new form requires processes like extrusion, rotational molding, and heat pressing. Mostly, thermoplastics like PP, PE, PET, and PVC are recycled mechanically [7]. In summary, primary mechanical recycling delivers quick, impurities free, durable, and similar to virgin plastic.

5.4.2 Secondary recycling Due to the inexact content and purity of plastic wastes, it is therefore subjected to secondary recycling which includes separation and purification, unlike primary recycling. In secondary recycling, the molecular weight reduces due to the presence of water and acid. This also deteriorates the mechanical property of the plastic but vacuum drying can be used to overcome this problem. In some cases, compatibility issue arises due to the mixing of different grades of polymers, which is also a strong reason for the lowering of mechanical strength [21]. For instance, mixing of PET with PVC leads to the formation of PET lumps in the PVC phase resulting in degradation of property. Effective separation of different plastic wastes can be done by using Fourier-transform infrared and X-ray diffraction techniques. Laser sorting and electrostatic detection are some of the latest technologies that can also be used for separating different types of polymers [22]. The dissolution method is one special type of mechanical recycling in which organic solvent is added to the mixture of plastic waste followed by the addition of

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nonsolvent to form the precipitation. The common solvent used is toluene, xylene, benzyl alcohol, and dichloromethane. Sometimes, it becomes too complicated or expensive to carry out secondary recycling. In such a situation, plastics are directly converted to fuel by incineration [23]. To improve the process of mechanical recycling, compatibilization can be done. Fragments of copolymer are blended with the waste plastic to carry out the physicochemical interaction of compatibilizer and waste polymer. The product obtained is much denser than obtained in secondary and primary recycling due to the fine dispersion of compatibilizer in the polymer matrix. For a particular polymer, a specific compatibilizer is used. For instance, ethylene–propylene diene rubber (EPDM) is used for PP and PE; and malleated styrene–ethylene–butylene–styrene for PET [24].

5.4.3 Tertiary recycling Conversion of large polymer chains into small molecular chains by a chemical process is termed as tertiary recycling. The product obtained in this process is used as feedstock for the production of new polymer, chemicals, and fuels; hence, it is also called feedstock recycling. Chemical processes used in tertiary recycling are pyrolysis, gasification, hydrolysis, and hydrocracking [25]. Chemical recycling is mainly appropriate for the polymers that are formed by the polymer condensation (e.g., nylon, PU, and PET), provided the waste should be free of impurity and same grade.

5.4.3.1 Pyrolysis In this process, PU polymers can easily break down to produce gas and oils under the oxygen-free environment. It was first discovered by the German researcher in which the waste of electronics subjected to pyrolysis at temperature 700–900 °C produces oil and gas of high calorific value [26].

5.4.3.2 Hydrocracking This is similar to the pyrolysis process but accompanied by hydrogen gas. This process is also called hydrogenation. Due to the cracking of polymer, a state of saturation is achieved with the molecules of hydrogen which results in a saturated liquid and hydrogen in gaseous form. Generally, very high quality of crude oil is obtained in hydrocracking. It is to be noted that adequate pressure is required to suppress repolymerization [27]. As the products of hydrocracking are of high quality, it is generally not used as feedstock but processed for high-grade polymers.

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5.4.3.3 Gasification In this process, waste of plastic in the form of solid and liquid is converted to gas form through chemical reactions. This process is carried out in the presence of oxygen as carbon compounds are broken down in the form of CO and H2. The product obtained is synthetic gas which can be used for the production of electricity and other valuable products. The temperature required in this process is between 800 and 1,600 °C [28]. The carbon collected in the residue can also be used for producing useful gas.

5.4.3.4 Glycolysis This can be used in the case of the mixed industrial waste polymer. “Post-consumer” waste plastic including PUs mixed with diol compound (which contain two hydroxyl groups) under the application of high heat leads to a chemical reaction and form a polyol [29]. These are similar to the original polyol and can retain their functional properties for the countless applications.

5.4.4 Quaternary recycling This is also known as incineration in which energy is recovered from the plastic wastes. It is very common in European countries and gradually disseminating all across the world. Mixed and dirty plastic waste, which is uneconomical and impossible to recycle by any other method, can be processed by incineration [30]. Heat and electricity, which can be used for technological purposes, can be directly produced by the burning of solid plastic waste. The volume of plastic waste can be reduced to 1% by this method. In this process, waste harmful gases also get decomposed. The slag and ash formed in this process can be used in road construction. A general layout of incineration is shown in Figure 5.4.

5.4.5 Cross-linking of polymers In this process, chemical agents are added in the mixture of polymer blend during the processing to improve the mechanical properties of the resulting polymer. Interaction takes place between the polymer blend and the chemically active agents which results in the degradation of properties. Hardening of the blend by the chemical agents witnesses good incompatibility but loses thermoplasticity. This makes it very difficult for further recycling of the polymer. Cross-linking of polymer converts the thermoplastic into thermosets, which

Figure 5.4: Generation of electricity by incineration method.

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are difficult to reshape. However, mixing of such thermosets with the analogous thermoplastic with the help of chemical or thermal binding can lead to valuable end polymer. Chemical agents such as silane, peroxides, and erythritol-bis-carbonate are used to harden PE, while PP, PS, and PVC can be combined by the chemical compound called dicumyl peroxides [31]. The chemical agents are prepared in an aqueous solution of either urea with formaldehyde or melamine with formaldehyde. The radiation technique can also be used for the formation of a cross-link chain between different polymers which results in improved strength and improved recyclable ability of the polymer.

5.5 Applications of recycled plastics As sustainability is concerned, the applications of recycled plastic have a very big role to play in the construction industry. A large proportion of recycled plastic is used as construction materials. Water drainage systems, duct pipe, decking, membranes for damping, channels, fencing, roof tiles, construction of bridges, pavement, and so on are some of the examples where recycled plastic can be used efficiently. Apart from the field of construction, recycled plastics have also scope in household products like decorations, ceilings, rooftop panels, and glazing. Conversion of plastic waste into assembly block is now very famous in underdeveloped countries due to the low input cost and high rate of productivity. It is water resistant, tough, and pest resistant. For this, a hand press is used which was designed in America in 2010. It is a very practical machine that even plastic films can be easily processed by this machine which is very difficult to recycle by other processes [32]. This has brought prosperity in the life of thousands of families by offering them an opportunity to build their durable shelter by the low-cost plastic waste. One such example can be seen in Swansea where 18 tons of waste plastic was used for constructing houses that were found to be corrosion resistant, airproof, and even tougher than concrete. The latest building material made from waste plastic called plans has been invented by UK-based firm and is expected to replace concrete, timber, and steel. Any kind of plastic can be used for making Iplas except PVC and thermoset. Recycled plastic obtained from drinking bottles can be used for the development of insulating plastic fiber having properties similar to high-density glass fiber. Besides, the plastic fiber also possesses high thermal resistance. Plastic waste is light and durable and can be recycled for making floor tiles, kerbstone duct pipe, and drainage pipe [33]. It has been seen that materials used as a surface covering and aggregate in construction and landscapes are now preparing from recycled waste plastic. Material for the resilient floor can be made by a waste of automobile tire. Waste plastic also finds its applications in sports ware, damping vibration system, and housing to animals to provide good comfort [34–35]. Carpets used in

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health centers, dancing studio, and care centers can be made by recycled rubber. Mulch for the plant beds and playgrounds can also be made from shredded recycled rubber. Recycled waste plastic can be recycled again after being used. It is costefficient and requires low maintenance also, thus proving it to be one of the best alternatives to nonplastic materials. Recycled plastics are also used for the generation of gas and fuel energy. This energy can be further used for the production of electricity. Waste plastic materials are burned in the chamber to recuperate the heat energy which is then converted to electricity. Many industries from the USA, South East Asia, and Europe have already been carrying out the energy recovery process from waste plastic.

5.6 Conclusion Accumulation of plastic waste in land and ocean is disturbing ecological and social balance. The need of the time is to recycle the plastic waste and making it virgin again. Several techniques have been evolved in the last two decades for the recycling of plastic waste. Various industries have been performing various methods of plastic recycling and forbidding the dumping of plastic waste. Thermoset plastics are tough to recycle but work has been going on in finding ways of recycling it. Applications of recycled plastic can be seen in construction, household, energy, and automobiles. Dumping of plastic has posed serious environmental and economic challenges that are being addressed with various recycling processes: making of goods and reusable commodities, application of recycled plastics in industries, corporate offices and IT offices, and making of reusable household commodities. Generation of electricity by the incineration process is being carried out to avoid the dumping of plastics. Further implementation of modern technology is being under development to tackle waste plastic efficiently and economically.

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[26] Constantinou A. A review on thermal and catalytic pyrolysis of plastic solid waste. Journal of Environmental Management 2017, 197, 177–198. [27] Mortensen P. M., Grunwaldt J. D., Jensen P. A., Knudsen K. G., Jensen A. D. A review of catalytic upgrading of bio-oil to engine fuels. Applied Catalysis. A, General 2011, 407(1–2), 1–19. [28] Zhang L., Xu C. C., Champagne P. Overview of recent advances in thermo-chemical conversion of biomass. Energy Conversion and Management 2010, 51(5), 969–982. [29] Nikje M. M. A., Garmarudi A. B., Idris A. B. Polyurethane waste reduction and recycling: From bench to pilot scales. Designed Monomers and Polymers 2011, 14(5), 395–421. [30] Makhale N. O. (2016). An investigation on attitudes and behaviours of residents towards recycling of municipal solid waste in Olievenhoutbosch, Centurion, in the Gauteng Province (Doctoral dissertation, University of Johannesburg). [31] Freegard K., Tan G., Morton R. Develop a process to separate brominated flame retardants from WEEE polymers–final report. Waste & Resources Action Programme (WRAP) 2006, 1–335. [32] Muniyasamy S., Ofosu O., Linganiso L. Z., Motaung T. E. Bioplastics: From the landfill to the market. WASTE-TO-PROFIT (W-T-P) 2018, 203–226. [33] Eneh A. E. Application of recycled plastics and its composites in the built environment. Best International Journal of Management, Information Technology and Engineering 2015, 3(3), 9, 16. [34] https://www.bbc.com/news/science-environment-42264788 [35] https://www.indiaspend.com/india-is-generating-much-more-plastic-waste-than-it-reportsheres-why/

Pawan Kumar Rakesh✶, Lalit Ranakoti and Manoj Kumar Gupta

6 Mechanical properties of chemically treated cellulosic fiber-reinforced polymer composites Abstract: Nowadays, the cellulosic fiber-reinforced polymer composites (CFRPCs) are gaining attention in the field of electrical and electronic items, automobiles, sports, structural applications, packaging, and utensils. CFRPCs have good tensile and flexural strength, have low density, are biodegradable, are of low cost, and are environmentally friendly. The natural fibers are cultivated around 150 million tons across the world. The major part of cultivated natural fibers is utilized in different forms such as to feed cattle, to cook food, and to build roof of huts. Due to its lacking technical properties, these fibers (jute, wheat straw, flax, hemp, kenaf, pineapple, banana, abaca, sisal, Sansevieria, date palm, coconut, silk, kraft, hardwood, bamboo, rice husk, pine, wood, cottons, and kapok) are not utilized in composite product development. The selection of suitable fibers for product development is based on fiber properties, that is, tensile strength of fiber, adhesion of the fiber and matrix, fiber reinforcement pattern (unidirectional fibers, plain weave, stain weave, and twill weave), and thermal stability of fiber. The adhesion of fiber and matrix will be improved by surface treatments. The manufacturing methods (such as hand layup, compression molding, injection molding, vacuum molding, pultrusion process, and resin transfer molding), already being implemented to make the composite laminates, also play an important role in the mechanical properties of composites. Keywords: Natural fibers, thermoplastics, processing techniques, surface treatments

6.1 Introduction In today’s scenario, the motivation behind product development is totally changed toward “fully green composites” which are fully biodegradable and environmentally friendly. The green composite materials are comprised of biofibers with the

✶ Corresponding author: Pawan Kumar Rakesh, Mechanical Engineering Department, National Institute of Technology Uttarakhand, Srinagar Garhwal, Uttarakhand, India, e-mail: [email protected] Lalit Ranakoti, Mechanical Engineering Department, Graphic Era Deemed to be University, Dehradun, Uttarakhand, India Manoj Kumar Gupta, Mechanical Engineering Department, H.N.B Garhwal University, Srinagar Garhwal, Uttarakhand, India

https://doi.org/10.1515/9783110768787-006

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biodegradable matrix. The green environment accomplishments include “biomaterials,” “green technology solutions,” “green energy conversion,” “green building,” and “green living standard” [1]. The cellulosic fiber-reinforced composites (CFRCs) have tremendous properties such as high tensile strength, low abrasive product, easy to manufacture, lightweight, biodegradable, recyclable, and low cost [1–9]. The application of biocomposites has been increased in the field of structural, vehicle applications [7], and in household equipment due to low density, and nonirritating and nonabrasive nature of cellulosic fibers [10–11]. The raw materials (matrix and fibers) of polymer composites are abundantly available in the world. The processing of natural fiber-based composites depends upon the properties of matrix and fiber materials. The natural fibers cannot be used directly as reinforcement into the matrix. The raw natural fibers need to be processed by surface treatment to remove impurities from the fiber surface. The processed fibers will have improved adhesion between the fiber and matrix, which directly influence the mechanical properties of fabricated composites [17–28]. The interfacial properties of reinforcement can be enhanced by surface treatment (mechanical, physical, and chemical treatment). The manufacturing technique also played an important role in the mechanical properties of CFRCs [29–31]. In this chapter, the aim is to describe the suitable matrix and reinforcement materials with surface modification techniques that will increase the mechanical properties of CFRCs.

6.2 Cellulosic fibers The cellulosic fibers are extracted from different parts of plants. The natural fibers need to be processed by different techniques to improve the bond between the matrix and fibers. The classifications of natural fibers are stem fibers (bamboo, jute, rice husk, flax, wheat straw, hemp, kenaf, and silk), leaf fibers (Sansevieria, pineapple, banana, sisal, and abaca), seed fibers (cotton and kapok), fruit fibers (date palm and coconut), and wood fibers (kraft, hardwood, and pine wood) [13–16]. The physical and mechanical properties (tensile strength, Young’s modulus, specific gravity, and specific modulus) of natural fibers are given in Table 6.1. The mechanical properties may vary depending on the fiber diameter, fiber length, fiber density, processing technique, extraction process, and environmental conditions [49]. The basalt fibers give the highest tensile strength (2,800 MPa), and betel nut fibers give the lowest tensile strength (166 MPa). In comparison to density parameters, densities of all fibers are almost similar (1.5 g/cm3), except ramie fibers, that is, 4 g/cm3. The chemical compositions of natural fibers are cellulose, hemicellulose, lignin, pectin, wax, ash, and moisture. The impurities like noncellulose, wax coat, hydroxyl coat, and inorganic materials of natural fibers are required to be removed by the different surface treatment processes (physical, chemical, and mechanical

6 Mechanical properties of chemically treated cellulosic fiber-reinforced

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Table 6.1: Physical and mechanical properties of cellulose fibers. Fiber type

Density (g/cm)

Cotton

.

Jute

.

Flax

Elongation (%) 

Tensile strength (MPa)

Young’s modulus (GPa)

Specific gravity

Specific modulus (GPa)



.

.

.

.





.

.

.

.

,



.



Ramie

.

.

,



.



Sisal

.





.

.

Bamboo

.

.





.



Hemp

.

.

,



.

.

Kenaf

.

.

,



.

.

Abaca

.





.

.

.

Oil palm

.





.

.

.

Betel nut

.





.

.

.

Bagasse

.

.



.

.

.

Coir

.

.



.

.

.

Banana

.





.

.

.

Pineapple

.

.

,

.

.



Henequen

.

.



.

–



Basalt

.

.

,



.

.

Curaua

.

.

,



–





methods). The different types of chemical treatments such as alkalization, acetylation, acrylation, bleaching, benzoylation, and isocyanates, permanganates, peroxides, and maleated coupling agents were used for removing impurities from fibers [32–35]. All chemical treatment processes are not suitable for different types of natural fibers. Salema et al. [18] studied the effects of fiber treatment on the oil palm and they found that the only mercerized and permanganate treatment processes improved the tensile strength of oil palm, while all other treatments (acetylated, peroxide, radiation, isocyanate, silane, acrylated, and acrylonitrile) degraded the tensile strength of fibers at the same concentration of treatment. Other methods remove lignocelluloses with impurities and hence degrade the tensile properties of oil palm. During the treatment, the fibers should be carefully handled to avoid surface damage. The tensile strength of treated and untreated fiber-based composites is shown in Figure 6.1. The tensile strengths of three different conditions (neat polylactic acid,

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polylactic acid reinforced with bamboo fibers, and NaOH- and silane-treated bamboo fiber-reinforced polylactic acid) were investigated and found that the NaOHtreated bamboo fiber-reinforced polylactic acid obtained higher tensile strength among all other composites [48]. Similarly, NaOH-treated ramie fiber-reinforced polylactic acid composites achieved higher tensile strength among other untreated and silane-treated ramie fiber-based composites. In case of polypropylene and highdensity polyethylene, the silane-treated bamboo fibers and henequen fibers give higher strength among other composites. Among three different matrix materials, such as polylactic acid, polypropylene, and high-density polypropylene, the tensile strength of PLA-based composites gives higher tensile strength due to good compatibility of fibers and matrix materials.

Figure 6.1: Effects of fiber treatments on polymer composites.

The chemical composition of different natural fibers (Zamioculcas zamiifolia, soy stem, Pongamia pinnata, Eichhornia crassipes, Phaseolus vulgaris, Perotis indica, Calotropis gigantea fibers) is mentioned in Table 6.2. The main strength of natural fibers depends on the cellulose and hemicellulose wt% present in the fiber. The role of lignin is to bind the fiber with matrix materials. The wax component is needed to be removed by suitable chemical treatment. The moisture content is reduced by heating the treated natural fibers under sunlight or oven at the temperature of 70 °C. The mechanical properties of natural fibers may be increased or decreased depending on the types of surface treatment applied.

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155

Table 6.2: Chemical composition of treated natural fibers. Treated fiber

Cellulose (wt%)

Hemicellulose (wt%)

Lignin (wt%)

Wax (wt%)

Moisture content (%)

References

Alkali-treated Zamioculcas zamiifolia

.

.

.

.

.

[]

Alkali-treated soy stem fiber

.

.

.

.

.

[]

Alkali-treated Pongamia pinnata

.

.

.

.

.

[]

Alkali-treated Eichhornia crassipes

.

.

.

.

.

[]

NaOH-treated Phaseolus vulgaris

.

.

.

.

.

[]

NaOH-treated Perotis indica

.

.

.

.

.

[]

NaOH-treated Calotropis gigantea fibers

.

.

.

.

.

[]

6.3 Matrix materials The environmental degradation time of polymer matrix materials is up to 450 years [4]. All thermoplastics and thermoset matrix materials (PS, polystyrene; PP, polypropylene; PLA, polylactic acid; EP, epoxy; PF, phenol formaldehyde) are not suitable with different types of reinforcement. The matrix materials do not adhere to all different types of natural fibers. Therefore, the selection of matrix with fibers is a big challenge for the composite industry. The mechanical properties of matrix materials with different reinforcement are given in Table 6.3. It was found that the treated kenaf fibers with polypropylene matrix give higher strength than the untreated fibers. The researchers and scientists have put efforts on PP [22] and PS matrix materials only due to low heat distortion temperature of the matrix [12]. The highest flexural strength (238.9 MPa) was reported in 5% NaOH-treated jute fiberreinforced vinyl ester composites [50]. The maximum tensile strength (165 MPa) was reported in the case of bamboo fiber-reinforced epoxy composites. Very limited research has been done on polylactic acid polymers due to their brittleness. The mechanical properties (tensile strength and flexural strength) of bamboo fibers (volume fraction = 30%) with different matrix materials are given in Table 6.4. The bamboo fibers with different matrix have obtained different mechanical strength. The bamboo fibers with polyethylene matrix materials have obtained higher tensile

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Pawan Kumar Rakesh, Lalit Ranakoti and Manoj Kumar Gupta

Table 6.3: Mechanical properties of matrix and fibers. Fiber

Matrix Fiber (wt%) Treatment type Tensile strength (MPa) Flexural strength (MPa)

Cotton

PS



% NaOH

Jute

PP



UN

VE



% NaOH

EP



PS Flax

.

.





–

.

UN







UN

.

.

PP



MAPP





Ramie

EP



UN





Sisal

PP



UN





Hemp

PP



UN





Kenaf

PP



UN





Abaca

PP



BDS





Oil palm

PS



UN





Betel nut

PS



% NaOH





UN

.

–

Bamboo

Sugarcane PS

.

EP

–

UN

.

.

PP



UN





PF



UN





EP

–

% NaOH





Pineapple

PP

.

UN





Date

EP



% NaOH



Coir

Banana

–

Note: PS, polystyrene; PP, polypropylene; EP, epoxy; PF, phenol formaldehyde; BDS, benzene diazonium salt; NaOH, sodium hydroxide; UN, untreated; MAPP, maleic anhydride–polypropylene.

and flexural strength when compared with other matrix materials. The lower tensile strength was found with polypropylene and high-density polyethylene matrix materials. The adhesion between bamboo fiber and rubber matrix is also improved by the bonding agent (phenolformaldehyde) [36–37]. The authors/researchers have also developed hybrid (cellulosic fibers with glass fibers) composites. Glass fiber lamina is used on the top and bottom sides of cellulosic fibers that will increase the strength in all directions as well as improve the surface finish [38]. The hybrid (sisal/glass) composites with alkali-treated have also increased the tensile strength as compared with the untreated fibers (Table 6.5). The

6 Mechanical properties of chemically treated cellulosic fiber-reinforced

157

treated fibers also decreased the elongation rate. A hybrid composite means two or more fibers are mixed with the matrix materials in the same manner. Table 6.4: Mechanical properties of bamboo fiber with matrix-based composites. Matrix

Tensile strength (MPa)

Flexural strength (MPa)

PP





MA-g-PP





EP





PE









HDPE

Note: PP, polypropylene; MAPP, maleic anhydride-grafted polypropylene; EP, epoxy; PE, polyethylene; HDPE, high-density polyethylene.

Table 6.5: Sisal/glass fiber hybrid composites. Treatment method Tensile strength (MPa) Young’s modulus (MPa) Elongation (%) Untreated





.

Alkali treated





.

6.4 Fiber–matrix interface The fiber–matrix interface is more critical during all types of loadings [39]. The interface plays a major role in between the adhesion of matrix and fibers. The cellulosic fibers are hydrophilic in nature and have efficient moisture susceptibility, resulting in fiber swelling and dimensional stability. Generally, the matrix materials are hydrophobic in nature. This incompatibility in matrix and fibers will give poor adhesion and has reduced the mechanical strength. To obtain more stress transfer, improvement of fibers and matrix at the interface is required. It is well known that the stress transfer in composites is dependent on the properties of the interface. The surface treatments (physical, chemical, physical–chemical, and mechanical) of fiber will be less hydrophilic and give good covalent bonding between the fiber and matrix, hence, improved the mechanical properties. It is very difficult to completely remove the moisture from fiber. However, it is necessary to do coating or encapsulation of fibers before reinforcing it into matrix to avoid moisture.

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6.5 Processing aspects The factors that are important during processing of composites are the length of fibers, the diameter of fibers, selection of matrix and fibers, and manufacturing techniques. The different types of manufacturing techniques such as hot pressing, pultrusion process, injection molding, compression molding, resin transfer molding, vacuum-assisted resin transfer molding, twin-screw extrusion, and casting were used to fabricate the composites. Depending upon the manufacturing techniques, different complex product geometries can be made. During hot compaction, it was found that more physical fiber damages take place as compared with other processes [40]. The optimizations of process parameters are necessary to obtain the highest mechanical properties.

6.6 Conclusions The following points have been drawn from this study of surface treatment on cellulosic fiber-reinforced polymer composites: a) The fibers should be carefully extracted from the raw materials to avoid damage. b) The fiber should be carefully treated by physical, mechanical, and chemical methods to increase the adhesion of the matrix. c) The selection of matrix and reinforcement is important before manufacturing of polymer composites. d) The manufacturing methods include fiber and matrix materials, processing time, and product geometry, which play an important role in the composite product development.

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[29] Murali Mohan R. K., Rao Mohana K., Ratna P. A. V. Fabrication and testing of natural fibre composites: Vakka, sisal, bamboo and banana. Materials & Design 2010, 31, 508–513. [30] Ariel S., Lucas C., Adrián C., Vera Á. Manufacturing and testing of a sandwich panel honeycomb core reinforced with natural-fiber fabrics. Materials & Design 2014, 55, 394–403. [31] Moothoo J., Allaoui S., Ouagne P., Soulat D. A study of the tensile behaviour of flax tows and their potential for composite processing. Materials & Design 2014, 55, 764–772. [32] Venkateshwaran N., Elaya P. A., Arunsundaranayagam D. Fiber surface treatment and its effect on mechanical and visco-elastic behavior of banana/epoxy composite. Materials & Design 2013, 47, 151–159. [33] El-Sabbagh A. Effect of coupling agent on natural fibre in natural fibre/polypropylene composites on mechanical and thermal behavior. Composites: Part B 2014, 57, 126–135. [34] Singha A. S., Rana Raj K. Natural fiber reinforced polystyrene composites: Effect of fiber loading, fiber dimensions and surface modification on mechanical properties. Materials & Design 2012, 41, 289–297. [35] Claudia M., Soldi G. V., Barra M. O. Influence of fiber surface treatment and length on physico-chemical properties of short random banana fiber-reinforced castor oil polyurethane composites. Polymer Testing 2011, 30, 833–840. [36] Hanafi I., Edyham M. R., Wirjosentono B. Bamboo fibre filled natural rubber composites: The effects of filler loading and bonding agent. Polymer Testing 2002, 21, 139–144. [37] Rao K. M. M., Rao K. M. Extraction and tensile properties of natural fibers: Vakka, date and bamboo. Composite Structures 2007, 77, 288–295. [38] Vijaya R. B., Junaid K. S., Niranjan R. R., Sathyanarayanan R., Elanchezhian C., Rajendra P. A., Manickavasagam V. M. Evaluation of mechanical properties of abaca–jute–glass fibre reinforced epoxy composite. Materials & Design 2013, 51, 357–366. [39] Shalwan A., Yousif B. F. Investigation on interfacial adhesion of date palm/epoxy using fragmentation technique. Materials & Design 2014, 53, 928–937. [40] Gu Y., Tan X., Yang Z., li M., Zhang Z. Hot compaction and mechanical properties of ramie fabric/epoxy composite fabricated using vacuum assisted resin infusion molding. Materials & Design 2014, 56, 852–861. [41] Tengsuthiwat J., Rapeeporn A. V., Laongdaw S. Thermo-mechanical characterization of new natural cellulose fiber from Zmioculus zamiifolia. Journal of Polymers and the Environment 2021. https://doi.org/10.1007/s10924-021-02284-2. [42] Vinod A., Sanjay M. R., Siengchin S., Fischer S. Fully bio-based agro-waste soy stem fiber reinforced bio-epoxy composites for lightweight structural applications: Influence of surface modification techniques. Construction and Building Materials 2021, 303, 124509. [43] Umashankaran M., Gopalakrishnan S. Effect of sodium hydroxide treatment on physicochemical, thermal, tensile and surface morphological properties of Pongamia pinnata L. Bark Fiber. Journal of Natural Fibers 2021, 18(12), 2063–2076. [44] Palai B. K., Sarangi S. K. Suitability evaluation of untreated and surface-modified Eichhornia crassipes fibers for brake pad applications. Journal of Natural Fibers 2021, 20, 1–14. [45] Babu B. G., Princewinston D., Saravanakumar S. S., Khan A., Aravind Bhaskar P. V., Indran S., Divya D. Investigation on the physicochemical and mechanical properties of novel alkalitreated Phaseolus vulgaris fibers. Journal of Natural Fibers 2020, 19, 1–12. [46] Prithiviraj M., Muralikannan R. Investigation of optimal alkali-treated Perotis indica plant fibers on physical, chemical, and morphological properties. Journal of Natural Fibers 2020, 19, 1–14. [47] Senthamaraikannan P., Kathiresan M. Characterization of raw and alkali treated new natural cellulosic fiber from Coccinia grandis L. Carbohydrate Polymers 2018. https://doi.org/ 10.1016/j.carbpol.2018.01.072.

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Garima Raghav and Pawan Kumar Rakesh✶

7 Numerical simulation on lap joint configurations of glass fiber-reinforced polyester composites with natural fillers Abstract: The mechanical performance of different lap joint configurations such as adhesive joint (ADJ), nut–bolt joint with adhesive, and riveted joint with adhesive has been investigated using finite element method (FEM). Modeling of glass fiberreinforced polyester (GFRP) with natural fillers such as coir husk, rice husk, and wheat husk has been done in ANSYS 19.0 software. The aim is to find out the effect of natural fillers on the mechanical properties such as maximum stress, strain, and failure load of different lap joint configurations under uniaxial tensile loading. Different lap joint configurations were modeled according to the ASTM D5868 standard with a 25 mm bonded length of polyester adhesive. Hashin damage criterion, cohesive zone model using traction separation law, and progressive damage models were used in the finite element simulations to characterize the damage initiation and damage evolution for the adhesive (polyester) and adherent (GFRP), respectively. The highest maximum shear stress (213 MPa) was found in the adhesive joint of GFRP/C composite. The shear damage variable shows maximum value of the damage (0.93) for the hybrid riveted joint. Adhesive joint of GFRP/R bears the maximum load among all the lap joint configurations. Keywords: GFRP composite, natural fillers, adhesive joints, hybrid joint, numerical simulation

7.1 Introduction Lightweight and strong materials are in great demand in many aspects of the automotive industry. Future technological advancements need a high strength/weight ratio and cost-effective materials. In the mobility sector, raising industry standards, increasing fuel efficiency, and lowering exhaust gas emissions requirements are boosting the demand for new and creative lightweight materials. Glass fiber-reinforced polyester (GFRP) composites are one of the alternative materials being explored as potential lightweight options for automobile body shells. The joining of GFRP laminates is very challenging to meet the standards for joint strength [1, 2]. A composite’s ✶

Corresponding author: Pawan Kumar Rakesh, Department of Mechanical Engineering, National Institute of Technology Uttarakhand, Srinagar 246174, India, e-mail: [email protected] Garima Raghav, Department of Mechanical Engineering, National Institute of Technology Uttarakhand, Srinagar 246174, India https://doi.org/10.1515/9783110768787-007

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mechanical characteristics are affected by the factors such as matrix material, fiber types, fiber orientation, fiber design, and fiber volume fraction. Several researches have been conducted by the FEM method to investigate the maximum failure load, stress–strain distribution, tensile strength, and failure analysis of adhesive joints and bolted joints. Mechanically fastened joint (MFJ) design is a difficult problem to solve since the results of the analysis are dependent on the bolt clearance and friction between the hole slot and the bolt surface. The nut–bolt joint can fail due to one of the three mechanisms: tension, shear, or bearing failure [3–5]. Until the catastrophic failure occurs, the tension failure condition is the same as the failure of an open-hole plate under tensile loading [6]. To identify three-dimensional (3D) stress distributions through the thickness of laminates in the bolt hole region, Ireman [7] used a 3D FEM study of composite hybrid bolted joints to verify the numerical model, elongations, stresses, and bolt load on the specimens, and a variety of factors were adjusted, that is, laminate stacking sequence, thickness, and clamping force. In general, there was a good match between computational and experimental data. For stress investigation of the singlelap hybrid joint (bolted with an adhesive joint) of the composite plate under uniaxial and lateral direction loads, Barut and Madenci [8] applied a semianalytical solution approach. Mindlin and Timoshenko’s beam theories were used to calculate the displacements of plate and bolt. The shear-lag model was used to represent the adhesive displacement field. The virtual work idea was used to create governing equilibrium equations. Paroissien et al. [9] proposed one-dimensional elastic scientific models to determine load transmission in a hybrid (bolted with adhesive) lap joint. They used an elastic–plastic technique to establish the integration of local equilibrium equations, and then they added a new element termed “bonded-bar” in FEM analysis. Harris et al. [10] devised a nonlinear FEM depending on the strain assumption to assess the distribution of stress in adhesive joints. Gunnion and Herszberg [11] investigated the effect of several factors on the performance of adhesive joints and concluded that adding an extra laminate with the glue might lower the peak stress. For load–displacement studies, Andruet et al. [12] designed specific adhesive elements and adherend using shell components. Anyfantis et al. [13] developed a novel traction separation criterion to analyze ductile adhesive layer failure. Cohesive Zone Model (CZMs) have been utilized to represent tiny interfaces between adherend and adhesive in the adhesive joints. Blackman et al. [14] studied the physical importance of the maximum stress in bonded composite topologies using CZMs. CZMs were employed by Li et al. [15] to predict the adhesive fracture of adhesively bonded joints. To simulate damage propagation in bonded joints, Moura et al. [16] employed cohesive and continuous mixed-mode damage models. Li et al. [17] and Luo et al. [18] used implicit FEM techniques to analyze the adhesive joint’s failure behavior. The same procedure was applied for shell element model delamination using CZM. Mccarthy et al. [19] studied the effects of bolt-hole clearance on the stiffness and bearing strength of single-lap composite joints with a single bolt (both protruding and countersunk).

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Joint stiffness was decreased as clearance increased for finger-tight joints and vulnerable to significant damage in the bearing area due to a shear-fracture spreading from the bolt-hole edge. Steward [20] studied that hybrid lap joints were 1.5 and 1.16 times stronger than the bolted joint and adhesive joint, respectively. Monika [21] used an experimental and computational approach to investigate the influence of squeezing force on residual stresses surrounding the hole as well as the affected fatigue strength [22–23]. This work is focused to utilize agricultural waste materials (wheat husk, rice husk, and coir fiber) as fiber reinforcement in GFRP laminates to make different lap joint configurations. The polyester adhesives are used to join the adherend in a single-lap joint assembly. The effect of natural fillers such as coir fiber, rice husk, and wheat husk, on the maximum stress, strain, and strain energy under uniaxial tensile loading has been investigated.

7.2 Finite element modeling Modeling of different single lap joint configurations has been done according to the ASTM D5868 standard with 25 mm bonded length of polyester adhesive using the ANSYS 19.0 software under unidirectional tensile loading for investigating the mechanical properties of GFRP, GFRP/C, GFRP/R, and GFRP/W composites. GFRP (4 mm thickness) modeling was done with eight node quadrilateral elements as mentioned in Figure 7.1. The stacking sequence of adherend laminates is taken as (0/90/45/-45) s. An adhesive layer of 2 mm sandwiched in between two GFRP Joint

ANSYS Diagram

Actual Diagram

Configuration Adhesive joint

Nut-bolt joint

Rivet joint

Figure 7.1: Lap joint configurations: (a) adhesive joint, (b) nut–bolt joint, and (c) rivet joint.

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laminates. Adherend plates are jointed with a 4 mm bolt diameter in the nut–bolt joint and a 4 mm rivet diameter in the case of riveted joint with two drilled holes. The general surface-to-surface contact is taken for adherend and adhesive. Frictional contact has been taken between the adherend and nut–bolt surface, and the coefficient of friction is taken as 0.2 for the tangential behavior. The nut–bolt is made up of eight nodes of a solid element with steel. Steel shows the linearly elastic behavior and the GFRP composite shows the nonlinear behavior. The FEM model of different lap joint configurations of the adhesive joint and hybrid composite joint (nut–bolt with adhesive, riveted with an adhesive) is shown in Table 7.1. Mechanical properties of woven glass fiber and polyester resin are given in Table 7.2. Table 7.1: Mechanical properties of glass fiber. Density (ρf ), g/cm3 .

Young’s modulus (Ef ), GPa

Shear modulus (Gf ), GPa

Poisson’s ratio (vf )

Tensile strength (σ f ), MPa

.

.

.

,

Table 7.2: Mechanical properties of polyester resin. Properties Density (ρm ), g/cm3 Young’s modulus (Em ), MPa Poisson’s ratio (vm ) Value

.

,

.

The volume fraction of fiber can be calculated with the help of these formulas, vf , vm is the fiber and matrix volume, respectively wf =

ρf v f ρf vf + ρm ð1−vf Þ

(7:1)

The stress of a unidirectional lamina may be determined using the theoretical micromechanics response of the lamina, the material parameters of the fiber and matrix, and the volume fraction of the fiber and matrix. σ1 = σf vf + σm vm

(7:2)

σ1 is the composite’s average tensile stress, σf and σm are the fiber and matrix stresses, respectively, and vf , vm are the fiber and matrix volume, respectively. The longitudinal modulus for the composite, E1 E1 = Ef vf + Em ð1 − vm Þ

(7:3)

where Ef and Em are the Young’s modulus of the fiber and matrix, respectively. Equation (7.2) is known as the rule of mixture. v12 is primary Poisson’s ratio. v21 is

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minor Poisson’s ratio, and transverse modulus E2 , for example, and shear modulus are denoted as v12 = vf vf + vm vm

(7:4)

E2 =

Ef Em Ef vm + Em vf

(7:5)

v21 =

E2 v12 E1

(7:6)

G12 =

Gf Gm Gf vm + Gm vf

(7:7)

G12 is the in-plane shear modulus. These equations only give estimations of the elastic characteristics of a continuous fiber 0° lamina [24].

7.2.1 Boundary conditions All degrees of freedom (longitudinal, translational, and rotational) are restricted for one end of the bottom plate (clamped length = 30 mm), and a loading rate of 2 mm/ min is provided to one end of another top plate. It can move only in the x-direction of the tensile loading, and all degrees of freedom of translational and rotational are restricted.

7.2.2 Failure criterion The damage evolution of a composite laminate depicts the progression of damage from its onset to its final failure. In FEM analysis, material property degradation criteria are used with the 3D Hashin damage criterion as mentioned in Table 7.3. It predicts the anisotropic damage in elastic-brittle material. It consists of four progressive failure criteria: (a) fiber tensile failure, (b) fiber compression failure, (c) matrix tensile failure, and (d) matrix compression failure.

7.3 Results and discussion A hole must be drilled in the laminates and then an adhesive layer through which the mechanical fastener will be put at the joint position to produce a hybrid connection. A hole creates a stress concentration zone, which causes strains to build up around it. To increase not only the strength but also the margin of safety and fatigue resistance, mechanical fastening is combined with adhesive joining [25]. Over

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Table 7.3: Hashin failure criterion. Failure mode

Hashin criterion

Matrix tensile failure



Matrix compression failure



σ2 Yt

2  2 τ 12 + >1 S

σ2 Yc

2  2 τ 12 + >1 S

Fiber/matrix shear failure

 2  2 σ1 τ 12 + >1 Xc S

Fiber tensile failure

 2  2 σ1 τ 12 + >1 Xt S

Fiber compression failure

 2 σ1 >1 Xc

Xt, Xc, Yt, Yc, Sij (i,j = 1, 2, 3) are the fiber- and matrix-related failure strengths.

the lap length, cracks in the adhesive or at the adhesive-substrate interfaces are either stopped or delayed, preventing them from spreading.

7.3.1 Stress analysis The maximum shear stress values for single lap adhesive joint, nut–bolt joint with adhesive, and riveted joint with adhesive were calculated, that is, 213.3, 206.25, and 189.81 MPa, respectively. When compared to the hybrid joint near the border of the bonded zone under the same displacement, the adhesive joint shows the highest shear stress. The stress contour plot shows the maximum stress concentration on the overlap bonded area of the lap joint configuration as shown in Figure 7.2. The amount of stress that builds up in the adhesive is determined by the tensile load. One side of the plate experienced compressive loads, while the other side had tensile tensions. The stress distribution is determined by the model’s geometry as well as the adhesive and adherent materials. In the lateral direction, shear stress falls approaching the bonded portion of the composite laminate. When a tensile load is applied during a lap shear test, the stress concentration at the joint contact is raised, resulting in crack development [26]. When the load is increased, the fracture propagates until it reaches the hole’s stress concentration zone, resulting in premature joint failure [27]. Further loading causes adherend and adhesive separation at the overlap borders when the adhesive can no longer sustain the load after reaching the maximum yield stress.

7 Numerical simulation on lap joint configurations of glass fiber-reinforced

(a) Maximum Shear Stress of Adhesive Joint

(b) Maximum Shear Stress of Hybrid Nut-Bolt Joint

(b) Maximum Shear Stress of Hybrid Riveted Joint

169

Experimentally Failed Adhesive Joint

Experimentally Failed Hybrid Nut-Bolt Joint

Experimentally Failed Hybrid Riveted Joint

Figure 7.2: Maximum shear stress of GFRP: (a) adhesive joint, (b) hybrid nut–bolt joint, and (c) hybrid riveted joint.

7.3.2 Stress versus strain behavior The stress and strain behavior of GFRP composites during uniaxial tensile testing for single-lap adhesive joint, hybrid nut–bolt joint, and hybrid riveted joint was demonstrated as shown in Figure 7.3. The linear stress–strain relationship is shown for different lap joint configurations, which follows Hook’s principles. In comparison to the hybrid joint, the adhesive joint shows the greatest strain for the same loading rate (rivet with an adhesive joint; and nut–bolt with the adhesive joint). Tensile stress increases as strain increases for all lap joint configurations.

7.3.3 Effect of natural fillers The maximum shear strength of GFRP composite without filler is 213.3 N/mm2 for the adhesive joint. The GFRP/W composite for an adhesive joint has almost the same maximum shear strength as the GFRP/C have as shown in Figure 7.4. The

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(a)

(b) 450

400

Adhesive Joint

350

Nut-Bolt Joint

300

Equivalent Stress (MPa)

Equivalent Stress (MPa)

450

Rivet Joint

250 200 150 100 50

400

Adhesive Joint

350

Nut-Bolt Joint

300

Rivet Joint

250

200 150 100

50 0

0 0

0.002

Strain

0.004

0

0.006

(c)

0.002

0.003

0.004

0.005

0.006

Strain

(d) 450

450 Adhesive Joint Nut-Bolt Joint Rivet Joint

400 350

300

Equivalent Stress (MPa)

Equivalent Stress (MPa)

0.001

250 200 150

100 50 0

400

Adhesive Joint Nut-Bolt Joint

350

Rivet Joint

300

250 200 150 100 50 0

0

0.002

0.004

Strain

0.006

0

0.002

0.004

0.006

Strain

Figure 7.3: Stress versus strain behavior of lap joint configurations: (a) GFRP, (b) GFRP/C, (c) GFRP/R, and (d) GFRP/W.

Figure 7.4: GFRP with natural fillers: (a) equivalent stress and (b) equivalent elastic strain.

adhesive joint shows the maximum equivalent (von Mises) stress and maximum shear stress compared to the hybrid nut–bolt joint and hybrid riveted joint. GFRP/C shows the maximum equivalent (von Mises) stress and maximum shear stress among all the lap joints under different natural fillers. The equivalent elastic

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Figure 7.5: Stress versus total length of GFRP laminate.

strain is found to be maximum for GFRP composite filled with rice husk. The GFRP composite for adhesive joints has comparable equivalent elastic strain with GFRP/C. At a time step of 60 s, four layers of woven glass fiber laminates were subjected to uniaxial tensile loading. The damage appears to begin around the edge of the bonded joint and the hole and then spread outward in the direction of the fibers. Figure 7.5 depicts the stress distribution over the total length of the GFRP laminate. The greatest value observed at the bonded edge of the laminate.

7.4 Load versus deformation behavior It is obvious that matrix degradation occurs first around the drilled hole and at the end of the bonded area and subsequently spreads outward, as shown in Figure 7.6. During the tensile testing of GFRP laminates, the damage variable (failure index (FI)) is quite close to the experimental findings [28]. The fiber–matrix interface may debond when the matrix failure occurs, and the failure propagates further along with the fiber–matrix interface. The hybrid riveted joint appears to be significantly more conservative than the other lap joints. The curves in initial phase show the linear ascending relation between load and deformation and achieve the maximum load. In the hybrid joint,

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7 Adhesive Joint

Load (KN)

6 Nut-Bolt Joint

5 Rivet Joint

4 3 2 1 0 0

0.2 0.4 Total Deformation (mm)

0.6

Figure 7.6: Load versus deformation of GFRP laminate. Table 7.4: Comparison of maximum failure load of different lap joint configurations under experimental and numerical simulation. Joint types

Experimental

Numerical simulation

GFRP/C

GFRP/R

GFRP/W

GFRP/C

GFRP/R

GFRP/W

Adhesive joint (kN)

.

.

.

.

.

.

Nut–bolt joint (kN)

.

.

.

.

.

.

Riveted joint (kN)

.

.

.

.

.

.

adhesive material was always completely damaged before the bolt or the adherend materials as shown in Figure 7.6. The adhesive joint of the GFRP/R composite bears the maximum load, that is, 6.86 kN, that is, 7.68% greater than the maximum failure load obtained in the experimental analysis, as shown in Table 7.4. The hybrid nut–bolt joint and hybrid riveted joint of GFRP/C composite show the maximum failure load of 6.21 and 4.24 kN, respectively, under the same loading condition. Numerical simulation of nut–bolt joint of GFRP/C shows a 9% increment and the rivet joint shows an 11.7% increment in maximum failure load than the experimental analysis. The difference in maximum failure load between experimental and numerical simulation may be due to void, porosity, material or manufacturing defect, and residual stresses present in the composite. The nut bolt prevents the joint from prematurely failing because of its ductile failure properties.

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7.5 Progressive shear damage plot Generally adhesive joints are subjected to the four different types of failure load such as tensile failure load, shear failure load, peel failure load, and cleavage failure load. Damage (failure) occurs due to one or more of these types of failure load in the lap joint configurations because the adhesive joint attains the maximum shear strength under the tensile loading conditions [29]. GFRP composite material behaves as a linearly elastic material before failure initiation after that material degrades continuously and failure occurs, as shown in Figure 7.7.

Figure 7.7: Maximum shear damage variable for adhesive joint.

Some design parameters like adhesive thickness, adherend width, adherend thickness, and bonded length of the joint affect the simulation results such as strength and load-carrying efficiency of the lap joint. In unidirectional tensile loading shear damage variable (SDV) of the adhesive lap joint of GFRP composite is found to be 0.86, as shown in Figure 7.8. The adhesive was damaged earlier than the bolt or the composite material in all the cases. Hybrid joints were compared to the adhesively bonded joints, and the results show that adhesive joint bears the maximum load. In hybrid joints, due to hole drilling in the laminate, bonded cross-sectional area of the joint reduced and resulted in reduction of load bearing capacity. Hole drilling increases the debonding and delamination in the laminate. Maximum SDV for riveted joint with adhesive is shown in Figure 7.9. Further increasing the load or displacement, the material stiffness coefficient degrades as well as the FI rises. Material damage occurs when the FI approaches one. When the FI of a composite material reaches 1, it is considered to have failed. The

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Figure 7.8: Maximum shear damage variable for hybrid nut–bolt joint.

Figure 7.9: Maximum shear damage variable for hybrid riveted joint.

SDV of the GFRP composite adhesive lap joint is found to be 0.86 in unidirectional tensile loading conditions. The riveted joint has the maximum SDV, that is, 0.93, as shown in Figure 7.10. Matrix and fiber damage is computed to find the damage variable with the help of failure evolution law. Material stiffness degradation law predicts the damage variable (0 presents the undamaged and 1 presents the completely damaged).

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Figure 7.10: Damage variable of all lap joints.

7.6 Conclusion The GFRP composite with natural fillers (coir husk, rice husk, and wheat husk) and without filler was effectively modeled in ANSYS19.0 software, and simulations on the joining behavior of various lap joint configurations were conducted. The equivalent von Mises stress of the GFRP composite filled with coir husk adhesive joint is (426 MPa) greater than the hybrid joint (nut–bolt joint with adhesive or riveted joint with adhesive). A hybrid riveted joint with adhesive shows minimum equivalent (von Mises) stress than the nut–bolt joint with adhesive. The maximum shear strength of the GFRP/C adhesively bonded joint is greater than the hybrid joint (nut–bolt with adhesive joint or riveted with an adhesive joint). The adhesive joint of GFRP composite filled with rice husk bears the maximum load, that is, 6.81 kN which is 7.68% higher than the maximum failure load obtained in the experimental analysis. In the numerical simulation, the maximum failure load for the hybrid nut–bolt joint of GFRP/C shows 9% increment, and the hybrid riveted joint shows 11.7% increment in maximum failure load than the experimental analysis. The equivalent elastic strain is found to be maximum for GFRP composite adhesive joint (0.006) among all the composite lap joint configurations under the same displacement loading rate. The maximum SDV or FI found for GFRP composite hybrid riveted joint is 0.93, and the minimum SDV value found for the adhesive joint is 0.86. Disclosure statement: The authors declare that there is no conflict of interest.

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Index adhesion 30, 45–47, 54, 58, 61 adhesive joining 167 agglomeration 46, 56, 58–59, 61 aggregation 25, 61 alkali treatment 44 alkaline 128 All-cellulose composites 78 aloevera 129 application 38 aspect ratio 30, 32, 44 bagasse ash filler 130 ball milling 122 bamboo 127 Banana 121 bidirectional 121 biodegradability 22, 24–25 biodegradable 24 Biopolymers 2 bio-renewable 22 bonding 44–45, 47, 54, 58 brittle 27, 29 calcium carbonate 24, 30, 32, 59, 63, 131 Carbon 26, 31, 36 cellu 63 cellulose 6, 25, 44, 48, 53–54, 56, 63, 78 cellulose composites 78 cellulose graft copolymers 91 cellulosic 123 cellulosic fibers 152, 156–157 chemical modification 91 Classification 32–33 CNTs 118 coating 119 coconut fiber 6 coir 129 compatibility 45, 49 compatibilizer 49, 59, 61 composite 21–24, 27, 31–32, 38–40, 42–45, 47–61, 121 composite materials 5 conductive elastomer composite 107 crack initiation 124 crystallization temperature 127 curauá 121

https://doi.org/10.1515/9783110768787-008

dampening 123 deformation 140 degradation 32, 52–53, 57 degrade 127 delamination 58 density 22, 24–25, 27–28, 37, 44, 48, 56, 62 Diels-Alder reaction 99 dimensional stability 27, 29–30, 44 disperse 33, 116 dispersion 30, 41, 59, 61 DSC 50–51, 54–56, 62 dynamic behavior 122 dynamic crosslinked network 99 elastomer composite 80 electrical 26, 30–31 environmental 22–24, 38 Extrusion 11, 32, 39–41, 47, 49, 61 fiber 22–25, 29, 32–33, 36–47, 49, 51–60, 62–63 filler 21–26, 29–32, 37–39, 44–47, 49–51, 56, 58, 60–61, 116 filler agglomeration 118 filler content 116 flame retardant 31 Flammability 52, 57 flax 126 flexibility 50 Flexural 27, 37, 47–48 fly ash 130 fracture toughness 117 Gaps 38 Gasification 144 GFRP laminates 163, 165–166, 171 glass fiber 22, 24, 39, 44–47, 49, 54–55, 57, 59 glass transition temperature 127 glycolysis 144 grafting 128 graphene 31, 57, 63 graphene oxide 128 graphite 25, 31, 45 green composites 151 Green reinforcement 6

180

Index

halloysite 124 hand lay-up 122 hardness 25, 29 HDPE 27, 29–30, 48, 56, 59–62 Hemicellulose 8, 124 hemp 128 hybrid 21, 120, 156–157 Hybrid composite 22 hybrid joints 173 hybridization 22, 44, 46, 57–58, 121 hydrophilic 44–45, 47 hydrophilicity 116, 124 hydrophobic 44, 47 hydroxide 31 impact resistance 117 incorporation 47 injection 32, 39–41, 46–49 Inorganic 21, 25, 29, 32, 38, 46, 58 Inorganic filler 21, 25, 29 inorganic fillers 25–26, 29–32, 36, 44, 50 interface 157 interfacial 30, 44–45, 47, 54, 58, 61 interfacial adhesion 116, 127 Jute 119 kenaf 124 LDPE 26, 62 Lignin 8 lignocellulosic 124 living control radical (LCR) polymerization 91 load transfer 116–117 loading 26, 29, 46, 48, 56 LOI 52, 63 luffa 131 manufacturing 151–152, 158 MAPP 28, 31, 45, 53, 59, 61–62 mechanical 22–25, 28–32, 37–38, 40–52, 55, 58–59, 61, 63 mechanical properties 82, 100, 116 mechanical stirrer 124 mechanically fastened joint 164 Melting point 38 mica 24, 27, 30, 50 micro scale 117 mineral 25, 30, 32, 36

modification 24–25, 31, 44 modulus 24, 26–29, 36–37, 47–48, 50, 55, 57, 63 molding 39–42, 47–49 molecular chain 50 montmorillonite 124 morphological 37, 58, 61 multiscale 117 multi-walled carbon nanotube 127 MWCNTs 118 Nano-titanium dioxide 119 nano-titanium oxide 118 nanocellulose 124 nanoclay 25, 27, 29, 45, 49, 55 nanoclay filler 126 nanoclays 123 nanofibrils 123 nanofiller 23 Nanofillers 116 nanoscale 117 NaOH treatment 121 natural fiber 21–24, 32, 37–39, 44, 47–48, 58 Natural fibers 1, 2, 13–14, 151–152, 154–155 natural fiber-reinforced composites 116 natural fillers 163, 165, 170, 175 natural frequency 123 non-all cellulose composites 80 oil palm 124 optimum 38, 43 optimum weight fraction 118 padauk wood dust 130 particle size 30–31 photothermal conversion 87 physical 22–23, 37–38, 44, 50 piassava 128 pineapple 121 plain weave 122 plant fiber 24 plastic waste 146 pollution 24 polyester adhesive 163, 165 Polymer grafting 10 polymers 2 polyolefin 23 polypropylene 24, 30, 50, 52–55, 58–60, 62

Index

primary recycling 141–143 protrusions 58 Ramie 122 recyclability 32, 37, 107 recyclable 24 recycle 32 Recycled 124 recycling 137, 140–141 reinforce 38 reinforced 21–26, 30, 32–33, 38, 41, 43, 46–47, 49, 51, 53–54, 56–57, 59, 62 reinforcement 22, 25, 29, 39–44, 46–47, 49 reinforcer 22–24, 30 resistance 24, 29–30, 37, 49 rheological 32, 49–50 rosewood dust 130 self-healing property 104 SEM 58–61, 63 shape memory property 102 silica 121 silk 124 sisal 121, 126 sonication 118 specific surface area 118 spontaneous self-healing performance 84 step cyclic tensile 83 stiffness 27, 37, 45, 50 storage modulus 128 strain sensor 107 strength 22, 26–30, 32, 36–37, 44–49, 51, 59, 61 stress relaxation 81

181

sugarcane 6 surface treatment 31 sustainability 22 Synthetic fiber 22 synthetic polymers 138 talc 24, 28–30, 32, 45–47, 50, 59–61 Tensile 26–28, 36–37, 48–49 TGA 50–54, 57, 62 thermal 22–24, 26, 29–32, 37, 44, 48, 50–58 thermal mechanical properties 100 Thermal properties 38, 52 thermal stability 26, 52–57, 127 thermoelectric conversion 88 thermoplastic 21–26, 30–33, 37–39, 41–47, 50, 52, 57–58 thermoplastic elastomer 30 thickness swelling 45 TMA 50, 63 Toughness 29 UL94 52 Viscoelastic 50 Viscoelasticity 50 viscosity 32, 49–50 void 38, 43 waste management 138 water absorption 27, 44–45 water uptake 44–45 weight fraction 120

Also of interest Series: Advanced Composites J. Paulo Davim (Ed.) ISSN - Published titles in this series: Vol. : Hybrid Composites () Ed. by K. Kumar, B. S. Babu Vol. : Plant and Animal Based Composites () Ed. by K. Kumar, J. Paulo Davim Vol. : Glass Fibre-Reinforced Polymer Composites () Ed. by J. Babu, J. Paulo Davim Vol. : Polymers and Composites Manufacturing () Ed. by K. Kumar, J. Paulo Davim Vol. : Biodegradable Composites () Ed. by Ed. by K. Kumar, J. P. Davim Vol. : Wear of Composite Materials () Ed. by J. P. Davim Vol. : Hierarchical Composite Materials () Ed. by K. Kumar, J. P. Davim Vol. : Green Composites () Ed. by J. P. Davim Vol. : Wood Composites () Ed. by A. Alfredo, J. P. Davim Vol. : Ceramic Matrix Composites () Ed. by J. P. Davim Vol. : Machinability of Fibre-Reinforced Plastics () Ed. by J. P. Davim Vol. : Metal Matrix Composites () Ed. by J. P. Davim Vol. : Biomedical Composites () Ed. by J. P. Davim Vol. : Nanocomposites () Ed. by J. P. Davim, C. A. Charitidis