Unsaturated Polyester Resins: Fundamentals, Design, Fabrication, and Applications 0128161299, 9780128161296

Citation preview

Unsaturated Polyester Resins Fundamentals, Design, Fabrication, and Applications

Unsaturated Polyester Resins Fundamentals, Design, Fabrication, and Applications

Edited by

Sabu Thomas School of Chemical Sciences, Mahatma Gandhi University, Kottayam, India International and Interuniversity Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India

Mahesh Hosur Department of Materials Science Engineering, Tuskegee University, Tuskegee, AL, United States

Cintil Jose Chirayil Department of Chemistry, Newman College, Thodupuzha, India

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2019 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-12-816129-6 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisition Editor: Edward Payne Editorial Project Manager: Fernanda A. Oliveira Production Project Manager: R. Vijay Bharath Cover Designer: Greg Harris Typeset by MPS Limited, Chennai, India

List of Contributors Basim Abu-Jdayil Chemical and Petroleum Engineering Department, United Arab Emirates University, Al Ain, United Arab Emirates; Emirates Center for Energy and Environment Research, United Arab Emirates University Al Ain, Abu Dhabi, United Arab Emirates M. Alagar Centre of Excellence for Advanced Materials, Manufacturing, Processing and Characterisation (CoExAMMPC), Vignan’s Foundation for Science, Technology and Research (VFSTR), Vadlamudi, India Andrea C. Alexander Center for Polymers and Advanced Composites, Auburn University, Auburn, AL, United States Nima Alizadeh Center for Polymers and Advanced Composites, Auburn University, Auburn, AL, United States; Department of Chemical Engineering, Auburn University, Auburn, AL, United States Sandro C. Amico Post-Graduation Program in Mechanical Engineering, UFRGS, Porto Alegre, Brazil; Post-Graduation Program in Mining, Metallurgical and Materials Engineering, UFRGS, Porto Alegre, Brazil M. Arous Department of Physics, LaMaCoP, Faculty of Sciences of Sfax, University of Sfax, Tunisia Anjali A. Athawale Department of Chemistry, Savitribai Phule Pune University, Pune, India Maria L. Auad Center for Polymers and Advanced Composites, Auburn University, Auburn, AL, United States; Department of Chemical Engineering, Auburn University, Auburn, AL, United States D. Bachtiar Structural Materials and Degradation Focus Group, Faculty of Mechanical and Manufacturing Engineering, Universiti Malaysia Pahang, Pahang, Malaysia Nil Ratan Bandyopadhyay Dr. M. N. Dastur School of Materials Science and Engineering, Indian Institute of Engineering Science and Technology, Shibpur, Howrah, India Mattia Bartoli Department of Applied Science and Technology (DISAT), Politecnico di Torino, Torino, Italy Dibakar Behera School of Applied Sciences (Chemistry), KIIT University, Bhubaneswar, India Samantha A. Bird Center for Polymers and Advanced Composites, Auburn University, Auburn, AL, United States

xix

xx

List of Contributors

Bhabatosh Biswas Dr. M. N. Dastur School of Materials Science and Engineering, Indian Institute of Engineering Science and Technology, Shibpur, Howrah, India Daniel H. Builes Research and Development Center-Andercol, Andercol S.A.S, Medellı´n, Colombia Shalini Chaturvedi Samarpan Science and Commerce College, Gandhinagar, India Cintil Jose Chirayil Department of Chemistry, Newman College, Thodupuzha, India ´ Angelica Colpo Post-Graduation Program in Mechanical Engineering, UFRGS, Porto Alegre, Brazil Pragnesh N. Dave Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar, India Virginia A. Davis Department of Chemical Engineering, Auburn University, Auburn, AL, United States Eduardo A.W. de Menezes Post-Graduation Program in Mechanical Engineering, UFRGS, Porto Alegre, Brazil S. Devaraju Polymer Composites Lab, Division of Chemistry, Department of Science and Humanities, Vignan’s Foundation for Science, Technology and Research (VFSTR), Vadlamudi, India Hom Nath Dhakal School of Mechanical and Design Engineering, Advanced Materials and Manufacturing (AMM) Research Group, University of Portsmouth, Portsmouth, United Kingdom John R. Ebdon Institute for Materials Research and Innovation, University of Bolton, Bolton, United Kingdom Marco Frediani Department of Chemistry “Ugo Schiff”, University of Florence, Sesto Fiorentino, Florence, Italy Leandro Friedrich Post-Graduation Program in Mechanical Engineering, UFRGS, Porto Alegre, Brazil Cincy George Department of Chemistry, Newman College, Thodupuzha, India Z.M. Hafizi Advanced Structural Integrity and Vibration Research (ASIVR), Faculty of Mechanical and Manufacturing Engineering, Universiti Malaysia Pahang, Pahang, Malaysia M.H.M. Hamdan Structural Materials and Degradation Focus Group, Faculty of Mechanical and Manufacturing Engineering, Universiti Malaysia Pahang, Pahang, Malaysia

List of Contributors

xxi

Med Ben Hassen College of Engineering, Industrial Engineering Department, Taiba University, Saudi Arabia; Department of Textile Engineering, Textile Engineering Laboratory, HITS of Ksar Hellal, University of Monastir, Tunisia Mahesh Hosur Department of Materials Science Engineering, Tuskegee University, Tuskegee, AL, United States Sikiru Oluwarotimi Ismail Manufacturing, Materials, Biomedical and Civil Division, School of Engineering and Technology, Hutton Building, University of Hertfordshire, Hertfordshire, United Kingdom Keilash C. Jajam Department of Mechanical Engineering, Auburn University, Auburn, AL, United States Jose James Department of Chemistry, St. Joseph’s College, Moolamattom, Idukki, India; International and Interuniversity Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India; School of Chemical Sciences, Mahatma Gandhi University, Kottayam, India J. Jamiluddin Structural Materials and Degradation Focus Group, Faculty of Mechanical and Manufacturing Engineering, Universiti Malaysia Pahang, Pahang, Malaysia J. Jayapriya Department of Applied Science and Technology, A.C. Tech., Anna University, Chennai, India Baljinder K. Kandola Institute for Materials Research and Innovation, University of Bolton, Bolton, United Kingdom Ekta Khosla Department of Chemistry, Hans Raj Mahila Maha Vidyalaya, Jalandhar, India Qiong Li Key Laboratory of Bio-Based Polymeric Materials Technology and Application of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, P.R. China; University of Chinese Academy of Sciences, Beijing, P.R. China Songqi Ma Key Laboratory of Bio-Based Polymeric Materials Technology and Application of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, P.R. China Ricardo Ballestero Mendez Center for Polymers and Advanced Composites, Auburn University, Auburn, AL, United States; Department of Chemical Engineering, Auburn University, Auburn, AL, United States Smita Mohanty School for Advanced Research in Polymeric Materials (SARP) LARPM Central Institute of Plastics Engineering & Technology (CIPET) - IPT, Bhubaneswar, India

xxii

List of Contributors

Sanjay K. Nayak School for Advanced Research in Polymeric Materials (SARP) LARPM Central Institute of Plastics Engineering & Technology (CIPET) - IPT, Bhubaneswar, India Med Amin Omri Department of Physics, LaMaCoP, Faculty of Sciences of Sfax, University of Sfax, Tunisia Shivkumari Panda School of Applied Sciences (Chemistry), KIIT University, Bhubaneswar, India Jyoti A. Pandit School of Chemistry, Dr. Vishwanath Karad MIT World Peace University, Pune, India V. Ramamurthy Department of Biotechnology, PSG College of Technology, Coimbatore, India M.R.M. Rejab Structural Materials and Degradation Focus Group, Faculty of Mechanical and Manufacturing Engineering, Universiti Malaysia Pahang, Pahang, Malaysia Emmanuel Richaud PIMM, UMR 8006, ENSAM

CNRS

´ Paris, France CNAM, HESAM Universite,

Luca Rosi Department of Chemistry “Ugo Schiff”, University of Florence, Sesto Fiorentino, Florence, Italy Sushanta K. Samal School for Advanced Research in Polymeric Materials (SARP) LARPM Central Institute of Plastics Engineering & Technology (CIPET) - IPT, Bhubaneswar, India S.M. Sapuan Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, Selangor, Malaysia N. Saranya Department of Applied Science and Technology, A.C. Tech., Anna University, Chennai, India Aruni Shajkumar School for Advanced Research in Polymeric Materials (SARP) LARPM Central Institute of Plastics Engineering & Technology (CIPET) - IPT, Bhubaneswar, India Arijit Sinha Dr. M. N. Dastur School of Materials Science and Engineering, Indian Institute of Engineering Science and Technology, Shibpur, Howrah, India J.P. Siregar Structural Materials and Degradation Focus Group, Faculty of Mechanical and Manufacturing Engineering, Universiti Malaysia Pahang, Pahang, Malaysia Pavle M. Spasojevic Faculty of Technical Sciences, University of Kragujevac, Cacak, Serbia; Innovation Center of Faculty of Technology and Metallurgy, University of Belgrade, Belgrade, Serbia

List of Contributors

xxiii

Agnieszka Tercjak Materials+Technologies Group (GMT), Department of Chemical and Environmental Engineering, Faculty of Engineering Gipuzkoa, University of the Basque Country (UPV/EHU), Donostia-San Sebastian, Spain C. Tezara Department of Mechanical Engineering, Faculty of Engineering and Quantity Surveying, INTI International University, Negeri Sembilan, Malaysia George V. Thomas Department of Chemistry, St. Joseph’s College, Moolamattom, Idukki, India Sabu Thomas School of Chemical Sciences, Mahatma Gandhi University, Kottayam, India; International and Interuniversity Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India Hareesh V. Tippur Department of Mechanical Engineering, Auburn University, Auburn, AL, United States A. Triki Department of Physics, LaMaCoP, Faculty of Sciences of Sfax, University of Sfax, Tunisia Jacques Verdu PIMM, UMR 8006, ENSAM

CNRS

´ Paris, France CNAM, HESAM Universite,

Xiwei Xu Key Laboratory of Bio-Based Polymeric Materials Technology and Application of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, P.R. China; School of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou, P.R. China Jin Zhu Key Laboratory of Bio-Based Polymeric Materials Technology and Application of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, P.R. China

CHAPTER

UNSATURATED POLYESTER RESINS, BLENDS, INTERPENETRATING POLYMER NETWORKS, COMPOSITES, AND NANOCOMPOSITES: STATE OF THE ART AND NEW CHALLENGES

1

Anjali A. Athawale1 and Jyoti A. Pandit2 1

Department of Chemistry, Savitribai Phule Pune University, Pune, India 2School of Chemistry, Dr. Vishwanath Karad MIT World Peace University, Pune, India

1.1 INTRODUCTION Unsaturated polyesters (UPs) are synthetic copolymers having applications as fibers, plastics, composites, and coatings. Depending on the choice of monomers, initiators, curing agents, additives, and modifiers used, different varieties of products can be produced exhibiting a wide range of chemical and mechanical properties. The low cost involved in their production makes them attractive. Their main application is as matrices in the composite industry. Among the composites, fiber glass
reinforced composites are of prime importance.

1.2 TYPES OF UNSATURATED POLYESTER RESINS Based on their structure, unsaturated polyesters resins (UPR) can be classified as: (1) ortho resins; (2) iso resins; (3) bisphenol A fumarates; (4) chlorendics; or (5) vinyl ester (VE) resins.

1.2.1 ORTHO RESINS Ortho resins are also referred to as general-purpose polyester resins and are based on orthophthalic acid, namely, phthalic anhydride (PA), maleic anhydride (MA)/fumaric acid, and glycols. PA is relatively cheap and provides rigidity to the backbone. However, it has limited thermal and chemical resistance and processability. Among the glycols, resins formed using 1,2-propylene glycol (PG) are more important in comparison to other glycols. The pendant methyl group in PG lowers the crystallinity of resin and improves its compatibility with commonly used reactive diluents (such as Unsaturated Polyester Resins. DOI: https://doi.org/10.1016/B978-0-12-816129-6.00001-6 © 2019 Elsevier Inc. All rights reserved.

1

2

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

styrene). Neopentyl glycol or hydrogenated bisphenol A yields resins with high heat and chemical resistance.

1.2.2 ISO RESINS Iso resins are prepared using isophthalic acid, MA/fumaric acid, and glycol. They are relatively expensive and have considerably high viscosities. Hence, they require a large proportion of reactive diluent, which also imparts improved water and alkali resistance to cured resins. They find applications as gel barrier coats in marine environments since they have better thermal and chemical resistance and mechanical properties.

1.2.3 BISPHENOL A FUMARATES These are synthesized using ethoxy-based bisphenol A and fumaric acid. Though expensive, they exhibit superior chemical properties as well as corrosion resistance as compared to ortho and iso resins. The presence of bisphenol A in the backbone renders a higher degree of hardness and rigidity and improved thermal performance. Due to the reduced number of interior chain ester groups, their hydrolysis resistance is best among commercial unsaturated resins.

1.2.4 CHLORENDICS Chlorine/bromine-containing anhydrides or phenols are used for preparing chlorendics. They exhibit flame resistance along with good chemical and corrosion resistance. For example, the reaction between chlorendic anhydride/chlorendic acid and MA/fumaric acid and glycol yields resin with better flame retardancy than general-purpose UPR. Other monomers used include tetrachloroor tetrabromophthalic anhydride. The bromine content must be at least 12% in order to obtain a self-extinguishing polyester.

1.2.5 VINYL ESTER RESINS VE resins contain unsaturated sites only at the terminal position as bisacryloxy or bismethacryloxy derivatives of epoxy resins. They are prepared through the reaction of acrylic acid or methacrylic acid with epoxy resin (e.g., diglycidyl ether of bisphenol A (DGEBA), epoxy of the phenol novolac type, or epoxy based on tetrabromobisphenol A). These resins were first commercialized in 1965 by Shell Chemical Company under the trade name Epocryl [1]. In 1966 Dow Chemical Company introduced a similar series of resins for molding purposes under the trade name Derakane resins [1]. The viscosity of neat resins is high; hence, reactive diluents (e.g., styrene) are added to obtain solutions with lower viscosities (100
500 poise). Notable advances in VE resin formulations include low-styrene-emission resins, automotive grades with high tensile strength and heat deflection temperature, hybrid grades that balance performance and economy, and materials for corrosion resistance.

1.3 SYNTHESIS OF UNSATURATED POLYESTER RESINS

3

1.3 SYNTHESIS OF UNSATURATED POLYESTER RESINS UP is often synthesized as a viscous liquid through the melt condensation of an aromatic dicarboxylic acid such as phthalic acid or anhydride with polyhydric alcohol and unsaturated dicarboxylic acid or anhydride. The viscosity of the reaction product/oligoester (OER) is reduced using a reactive diluent such as a vinyl monomer, usually styrene. Free radical copolymerization between styrene and the double bonds of UP results in a rigid three-dimensional cross-linked structure, which is a heterochain thermoset type of polymer. Methyl ethyl ketone peroxide (MEKP) is a standard catalyst that initiates the curing reaction in combination with a cobalt or cobalt-amine activator system/accelerator at room temperature. Other free radicals used for curing UPRs include benzoyl peroxide (BPO) or cumene hydroperoxide [2]. After synthesis, an inhibitor is added to the resin to provide a long storage life, fast cure, and to minimize catalyzed or uncatalyzed drift, undesirable colors, odors, or side effects. Hydroquinone, 4,4-dihydroxybiphenyl, and substituted catechols are some examples of inhibitors [3]. Attempts have been made by various researchers to tailor the mechanical, thermal, corrosion, and fire resistance properties of UPRs for various applications. Parker et al. suggested the use of isophthalic acid for improved mechanical properties and corrosion resistance [4]. A two-stage synthesis process was patented by Watanabe et al. to address the necessary improvements using dimethyl terephthalate instead of isophthalic acid [5]. Styrene, vinyl toluene, tert-butylstyrene, chlorostyrene, and diallyl phthalate have been used as reactive diluents. The effects of various concentrations of anhydride (PA and MA) on mechanical properties were reported by Thomas et al., with 60%
70% MA showing the best mechanical properties [6]. They also synthesized various formulations by varying both the anhydride and the alcohol concentration. A mixture of 60% MA with PA yielded an UPR with the best mechanical properties. However, resin with a higher proportion of PA was found to be tough and flexible. Similarly, diethylene glycol (DEG) increased the toughness, impact strength, and flexibility, which was lost on standing. Optimal properties are observed with a 20/80 ratio DEG/PG resin together with an equimolar amount of MA and PA [7]. UPRs have also been synthesized from bio-derived diesters of unsaturated diacids such as itaconic, succinic, and fumaric acids with various diols and polyols to afford resins of M n B480
477,000 and glass transition temperature (Tg) of between 230.1 C and 216.6 C with solubilities differing based on the starting monomers used [8]. Yoon et al. regenerated UPR after recycling cured UPR. The recycled UPR exhibited a faster curing rate than that of neat resin. A comparison of the mechanical properties of the neat resin and the mixtures (neat resin and recycled) revealed that although the properties of neat resin were superior, those of the mixtures were dependent on composition and were found to be suitable for many applications [9]. Different proportions of cobalt (Co) curing agent were used (0.05%
1%). An increase in the concentration of Co from 0% to 1% led to a decrease in curing time. This was reduced to half in the presence of 0.05% Co [10]. The effect of volume ratios of curing agents, viz., cobalt octoate as an accelerator and MEKP as an initiator, on gelation time and exotherm behavior of a UPR has also been studied. The gelation of the resin was found to correspond with the onset of an increase in temperature during resin curing. The gelation time was found to vary inversely with the concentrations of accelerator and initiator [11]. The viscosity of the liquid system was found to decrease with increasing temperature, but increased at the curing temperature. The quality of the cured UPR

4

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

was predicted on the basis of its fragility parameter (Mc). In the UPR
MEKP system, the smaller the Mc the larger the Tg and the better the heat resistance [12]. The curing behavior of UPR was studied using an experimental and theoretical model by Kosar and Gomzi [13]. The kinetic behavior of the curing system was investigated using both dynamic and isothermal measurements and a good agreement was established between the two (in terms of presented kinetic parameters and reaction heat). Heat generated from the cure reaction was measured in molds of cylindrical shape. The difference in heat conductivity between glass and copper was the main reason for the greater heat generated in the glass mold. Control over resin shrinkage of residual monomers is an important concern in low-temperature molding processes. The presence of low-profile additives (LPAs) can reduce the shrinkage of UPR/ styrene resins under proper processing conditions, but may increase the residual styrene content. A systematic study was carried out to investigate the effect of the initiator system and reaction temperature on the sample morphology, final resin conversion, and resin shrinkage of UPR with LPA. The results showed that the final conversion of the resin system could be improved by dual initiators, with the effect being prominent at low temperatures. The study on shrinkage control reported that good LPA performances were achieved at low (35 C) and high (100 C) temperatures, but worse performances were observed in the intermediate temperature range (e.g., 60 C
75 C) (Fig. 1.1). The final shrinkage is influenced by the effect of temperature on the morphology, the relative reaction rate in the LPA-rich and UPR-rich phases, and microvoid formation [14]. The sample morphology shows a two-phase cocontinuous structure at 35 C (Fig. 1.2). One is a particulate phase (LPA-rich) having loosely packed spherical particles with diameters ranging from 1 to 5 mm. The other phase is a flake-like region (UPR-rich) with domain sizes ranging from 10 to 20 mm. At a curing temperature of 60 C, a similar two-phase structure is observed, but it is no longer cocontinuous. The particulate region is smaller and becomes the dispersed phase with a domain 4

Volume shrinkage (%)

3 2 1 0 –1 –2 –3 35

60 75 Temperature (ºC)

100

FIGURE 1.1 Volume shrinkage of UP/St/LPA systems cured at different temperatures (3.5% LPA, 0.5% Co Oct., 1.3% MEKP, 0.4% TBPB, 300 ppm BQ).

1.3 SYNTHESIS OF UNSATURATED POLYESTER RESINS

5

FIGURE 1.2 Morphology of St/UP/LPA samples cured at different temperatures (3.5% LPA, 0.5% Co Oct., 1.3% MEKP, 0.4% TBPB, 300 ppm BQ).

size of less than 20 mm, while the flake-like region forms the continuous phase. On increasing the temperature to 75 C and 100 C, the size of the particulate region is further reduced. The various morphological structures result in different interface areas, strongly affecting the shrinkage control. Commercial UPRs contain 30%
40% styrene by mass. The miscibility of resin and styrene depends on the resin composition. Phase separation is reported with an increase in styrene concentration. Thermal stability and mechanical properties are governed by the phase behavior of the mixture and can, therefore, be controlled by styrene content [15]. Dynamic mechanical analysis (DMA) tests have shown phase separation in cured resin with high styrene concentrations. Tg is also dependent on styrene concentration together with thermal stability and mechanical behavior [16].

1.3.1 LOW-STYRENE-EMISSION UNSATURATED POLYESTER RESIN Styrene has remained a preferred reactive diluent for adding to UP due to its cost and availability. It controls the viscosity and facilitates the curing of polyesters at room temperature. However, the use of styrene is associated with serious health problems such as respiratory diseases and skin

6

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

irritation. It is carcinogenic and also attacks the central nervous system on exposure over a long period of time, leading to possible headaches and depression. The minimization of styrene volatilization or its elimination using alternative monomers is being attempted to overcome these problems. The volatilization of styrene is reduced by paraffinic waxes which act as a barrier. However, the wax layer needs prior removal to avoid problems of adhesion to other parts. The ambient concentration of styrene vapor can be reduced using spray guns that can monitor the amount of resin sprayed. Since alternatives such as vinyl toluene, alpha-methylstyrene, and diallyl phthalate suggested for styrene also involve health hazards, Poillucci and Hansen proposed the use of bioderived limonene oil and petroleum-derived vinyl neodecanoate and vinyl laurates as other substitutes for styrene, but they exhibit limited chemical compatibility. The styrene content was reduced by 50% using trimethylolpropane diallyl ether [17]. Mariani used various cross-linking agents such as 2-hydroxyethyl acrylates (HEA) or a mixture constituted of diurethane diacrylate and styrene or HEA for frontal curing of UPR derived from the reaction of MA and 1,2 propanediol [18]. Zang et al. reported a benzyl end-cap-UP resin with low styrene emission using benzyl alcohol as the end-capper [19]. For nonhalogenated resins, a marked restriction in styrene emission is achieved by including long-chain alpha-olefins with 18
40 carbon (C) atoms without the addition of wax. These olefins on their own will not usually provide such a marked restriction in styrene emission, but will allow for the incorporation of a waxy compound in an amount sufficiently large to achieve the desired styrene emission restriction without incurring the expected disadvantages associated with the incorporation of such large amounts of waxy compound [20].

1.3.2 STYRENE-FREE COMPOSITIONS FOR CURABLE COATINGS When UPRs are used as coatings, styrene-free compositions are favored since volatile emissions by such compositions are expected to be low. An example of such a formulation consists of one comonomer selected from the (meth)acrylates of cycloaliphatic alcohols and optional comonomers could be tetrahydrofurfuryl (meth)acrylate, methoxypolyethylene glycol, mono(meth)acrylate, ethylene glycol dimethacrylate, and di(ethylene glycol) di(meth)acrylate while the curing can be done by radiation and/or through the peroxide or thermal routes. More specifically, curing can be performed by adopting a process comprising at least one step of radiation and/or peroxide curing [21]. Styrene-free UPR coatings cured by infrared radiations are described as containing an unsaturated ether component as well as saturated monohydric alcohol along with dicarboxylic acid and dihydric alcohol [22]. Also, radically curable styrene-free coatings are claimed to be composed of compounds containing a (meth)acryloyl group and/or vinyl ether groups along with paraffin, a plasticizer, and carbamic acid [23]. Styrene-free compositions are reported by using various reactive diluents singly or in combination such as 2-hydroxyethyl methacrylate, 2-hydroxy propyl methacrylate, 2-HEA, 2-hydroxypropyl acrylate, and related compounds [24]. UPR can also be obtained as a reaction product of at least one diol having 2
8 C atoms, one monoalcohol with at least one allylic unsaturation, and at least one saturated aliphatic monoalcohol having 4
10 C atoms or one aromatic monoalcohol having 7
10 C atoms. The coating or molding composition of such a UPR is curable by radiation and/or through the peroxide or thermal routes [25]. McAlvin reported UPR derived from biologically renewable resources and recycled materials,

1.3 SYNTHESIS OF UNSATURATED POLYESTER RESINS

7

which are styrene-free and ultralow volatile organic compound (VOC) resins that provide matrix materials to produce more ecologically friendly composites [26]. A styrene-free UPR forming a stable dispersion in water has been reported. The modification was done by introducing polar hydrophilic groups such as carboxylic and sulfonic groups (sodium 5-sulfonatoisophthalic acid) into the resin molecule, which ensure good tolerance to water. Styrene has been replaced with the glycerol monoethers of allyl alcohol and unsaturated fatty alcohols as reactive built-in cross-linking monomers for resin modification [27].

1.3.3 MODIFICATION OF UNSATURATED RESIN FOR VISCOSITY CONTROL UPRs have replaced sheet metal in many applications such as in the automotive, electric, and home appliance industries as a consequence of their properties such as being light weight, having high strength, and their noncorrosive nature. UPR composite products are manufactured by compression molding in the form of sheet molding compounds (SMCs) or bulk molding compounds (BMCs), through injection molding in the form of BMC, resin transfer molding, casting, and hand layup. Chiu et al. attempted to develop UPR systems exhibiting viscosity profile properties such as rapid increase during maturation/thickening and mold filling so that they can be handled easily, have good fiber carrying characteristics, and long-term stability. For good material flow, a significant reduction in viscosity is required during molding which facilitates the complete filling of the mold as well as the complete wetting of the filler and other ingredients in the system by the UPR [28]. Fig. 1.3 shows the ideal viscosity profiles for SMCs and BMCs during molding. Chemically, thickening or “maturation” occurs by linking up various UPR molecules together to form polymer chains of considerably higher molecular weights. Usually, this is done by adding a Stable viscosity

Fiber carrying

Mold detail reproducibility

Easy handling Pressure Viscosity

Fast thickening

I

Shrinkage and dimentional control

II

III

IV

Time

FIGURE 1.3 Ideal viscosity profile for SMCs or BMCs during molding: (I) thickening; (II) storage; (III) mold filling; and (IV) curing.

8

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

di- or multifunctional compound to the system which couples two or more polyester molecules together via their terminal hydroxyl and/or carboxyl groups. As UPR molecules usually contain more than two functional groups, the actual product formed is a complex network of interconnected polymer chains rather than discrete individual chains. Compounds used for thickening UPRs are known as “thickening agents” or “maturation agents.” Two types of compounds are used as thickening agents. The first type comprises Group IIA metal oxides and hydroxides, for example, MgO [29]. Maturation with this type of agent occurs via the formation of ionic bonds through the reaction of MgO with the carboxylic acid end groups of polyester molecules. The other type of maturation agent is diisocyanate [30]. Diisocyanates operate by forming covalent bonds, specifically urethane linkages, with the terminal hydroxyl groups of polyester molecules. Each type of maturation agent has its own advantages and disadvantages. The maturation process with MgO-type agents is slow. They form ionic bonds which weaken at elevated temperatures encountered during molding. This results in a reduced compound viscosity and hence the desired material flow. Diisocyanate maturation agents exhibit rapid thickening. The covalent bonds formed with isocyanate-type thickeners do not weaken at molding temperatures and hence material flow is more difficult. MgO-type maturation agents are highly sensitive to humidity after maturation, whereas diisocyanates are not. A thermally breakable di-keto group can be introduced onto the UPR molecule before curing through salt formation. This group, along with the salt, may break at elevated temperature in most UPR molding operations and therefore reduce the compound viscosity upon heating; hence the desired amount of material flow is realized. Modified resins are further thickened with MgO or diphenyl diisocyanate. This exhibits a fast viscosity rise during molding and a stable viscosity during room temperature storage [28]. Molded articles made with conventional UPRs often exhibit poor surface finishes. This is probably due to shrinkage of the UPR during the molding operation. LPAs are used to overcome this problem. Along with LPAs, good material flow during molding is also necessary to obtain finishes of the highest quality. The reduced material flow encountered when diisocyanates are used as thickeners reduces the effectiveness of LPA in these systems, which in turn may lead to significant finish problems. One proposal to overcome this limitation is to use a combination of both MgO-type and diisocyanate-type thickeners in the same system [31].

1.4 UNSATURATED POLYESTERS RESIN BLENDS Polymer blends are made by the physical mixing of two or more different polymers or copolymers to produce a mixture with desirable mechanical and physical properties. Usually, the Tg of cured UPRs are high and their brittleness presents an obstacle for their use in engineering applications [32]. The mechanical, physical, and thermal properties of UPR can be improved by blending with other polymers or by reacting them with different additives or modifiers which generally form a second dispersed phase after the resin is cured. Blends show the demanded performance at low cost.

1.4.1 UNSATURATED POLYESTERS RESIN
ELASTOMER BLENDS The addition of elastomeric phases to UPRs usually improves their overall ductility over a wide range of temperatures, toughness, and impact resistances. Elastomers are blended with UPR before

1.4 UNSATURATED POLYESTERS RESIN BLENDS

9

curing by physical and chemical methods. When blends are formed by the physical mixing of two or more polymers at least 5% of another polymer is necessary to form a blend. If the component polymers are miscible, a single-phase blend is obtained. If they are immiscible, a multiphase blend is formed. Even if rubber additives are soluble in uncured resin, phase separation during curing is advantageous since these blends are tougher than homogeneous blends [33,34]. The presence of elastomeric domains increases the absorption and dissipation of mechanical energy. Various mechanisms proposed for toughening by blending with rubber include the debonding of the rubber matrix interphase, energy absorption by rubber particles, matrix crazing, shear yielding, and combining shear yielding and crazing [32,33]. Essential characteristics of elastomers for toughening are [35]: 1. The presence of a sufficient number of polar groups to enhance solubility in the resin. 2. The elastomer should have a slow rate of cross-linking compared to the UPR used to facilitate the distribution of discrete elastomeric particles during cross-linking. 3. The weight of the elastomer should be relatively high. 4. The major part of the elastomer should be thermodynamically incompatible with resin.

1.4.1.1 Unsaturated polyesters resin
natural rubber blends Natural rubber is an elastic material present in latex from rubber trees. Its easy availability, low cost, and excellent physical properties such as good resilience, high tensile strength, superior resistance to tear and abrasion, good tack, and self-adhesion have led to its use in preparing blends. On the other hand, it has poor age resistance and oil resistance. Blends based on UPR from recycled poly(ethylene terephthalate) (PET) wastes with varying percentages (0%
7.5 wt.%) of liquid natural rubber (LNR) have been prepared by Hisham et al. They are found to exhibit good compatibility compared to commercial resins, but show higher Tg. A blend with 2.5 wt.% LNR rendered the highest strength and best dispersion of elastomer particles while commercial resin required 5% of LNR to achieve optimum properties [36]. Studies on the influence of the source of water and immersion time on the mechanical properties of UPR
natural rubber blends have revealed increases in the impact strength and strain rates and decreases in the Young’s modulus of polymer blends under identical conditions [33]. Hybrid proton exchange membranes as an alternative for Nafion in polymer electrolyte membrane fuel cell (PEMFC) were developed by Jimenez et al. UPR
natural rubber blends were prepared and subjected to the process of vulcanization and TiO2 was added as an inorganic load. The blends exhibited higher Young’s moduli and strains compared to commercial Nafion membranes. The water uptake as well as ion exchange capabilities of the vulcanized membrane was found to be superior [37]. Natural rubber latex (NRL) when blended with UPR in the presence of dispersion aids such as sodium lauryl sulfate (SLS), toluene, and ammonia led to an improvement in impact strength. However, the flexural strength decreased with NRL content in the blend (Fig. 1.4). The impact strength was highest when NRL and toluene were 15 phr and 20 wt.%, respectively [38].

1.4.1.2 Unsaturated polyesters resin-synthetic rubber blends Synthetic rubbers have also been used for blending with UPRs. Binary polymer blends of UPR and different weight ratios (0%, 5%, 10%, and 15%) of nitrile butadiene rubber (NBR) have been prepared by mechanical mixing using toluene as a solvent. However, they showed poor mechanical

10

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

FIGURE 1.4 Impact strength of pure UPR and UPR/NRL blends using different dispersion aids: (A) using SLS as a dispersion aid; (B) using toluene as a dispersion aid; and (C) using liquid ammonia as a dispersion aid.

1.4 UNSATURATED POLYESTERS RESIN BLENDS

11

properties, except for their strain rates which were higher. The wear rates of the blends were found to decrease with increasing NBR content [39]. Cherian and Thachil prepared blends of UPR with elastomers bearing reactive functional groups such as hydroxy-terminated polybutadiene, epoxidized natural rubber, hydroxy-terminated natural rubber, and maleated nitrile rubber. These elastomers show better compatibility with resin and impart superior toughness, fracture resistance, and impact resistance as compared to unmodified elastomers [34]. The authors also synthesized UPR blends using two different strategies for incorporating rubber. The first method involves the dissolution of masticated elastomers such as natural rubber (NR), styrene butadiene rubber (SBR), NBR, butyl rubber (IIR), and chloroprene rubber (CR) in styrene followed by blending with UPR, while in the second method, elastomers are modified with MA and then dissolved in styrene and blended with UPR to get maleated elastomers. Blends having elastomers modified with MA show improved mechanical properties (toughness, impact resistance, and tensile strength) compared to unmodified rubbers (Tables 1.1 and 1.2). The performance of nitrile rubber is found to be far superior in

Table 1.1 Summary of Properties of UPR Modified With 0%
5% Elastomers Maximum Improvement Achieved (%)/Elastomer Concentration (%) Property

UPR

NR

SBR

NBR

CR

IIR

Tensile strength (MPa) Modulus ( 3 102 MPa) Elongation at break (%) Toughness (MPa) Impact strength ( 3 1022 J/mm2) Hardness shore D Abrasion loss (cc) Water absorption (%)

33.3 14.1 2.25 0.36 1.21 88 0.37 0.21

57.8/2.5 24.2/2.5 44.4/2.5 136/2.5 150/2.5 2 0.6/1 32.4/5 90.5/5

53.9/2 15.1/2 33.8/2 111/2 107/2 2 0.6/1 18.9/5 38.1/5

83.4/2.5 8.6/2.5 88.9/2.5 286/2.5 239/2.5 0/1 21.6/5 47.6/5

40.8/2.5 21.3/2.5 24.4/2.5 97.2/2.5 90.1/2.5 2 1.1/1 27.0/5 57.1/5

16.4/2 2.9/2 7.1/1 41.7/2 50.4/2 2 1.7/1 37.8/5 76.2/5

Table 1.2 Summary of Properties of UPR Modified With 0%
5% Maleated Elastomers Maximum Improvement Achieved (%)/Maleated Elastomer Concentration (%) Property

UPR

NR

SBR

NBR

CR

IIR

Tensile strength (MPa) Modulus ( 3 102 MPa) Elongation at break (%) Toughness (MPa) Impact strength ( 3 1022 J/mm2) Hardness shore D Abrasion loss (cc) Water absorption (%)

33.28 14.1 2.25 0.36 1.21 88 0.37 0.21

84.0/2.5 24.8/2.5 58.7/2.5 214/2.5 203/2.5 2 0.6/1 37.8/5 100/5

63.0/2 10.8/2 58.2/2 161/2 116/2.5 2 0.6/1 18.9/5 42.9/5

97.8/2.5 8.9/2.5 95.6/2.5 303/2.5 247/2.5 0/1 27.0/5 57.1/5

71.5/2.5 15.2/2.5 64.9/2 184/2.5 136 /3 2 1.1/1 32.4/5 71.4/5

35.0 /2 6.0/2 33.8/2 88.9/2 66.9/2 2 1.1/1 48.6/5 90.5/5

12

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

comparison to all other rubbers [32]. Toughening agents like carboxyl and vinyl terminated nitrile rubbers as well as urethane rubbers have also been used for preparing blends. The incorporation of flexible polyorganosiloxane segments in the UPR network enhances its flexibility [34]. The thermal stability of toughened UPR
NBR improved on reinforcing it with slag powder. The sample modified with stearic acid showed better mechanical properties. A fire resistance test showed reduced mass loss when exposed to direct open flame [40]. Suspene et al. observed an improvement in the compatibility of a UPR
carboxyl-terminated butadiene-acrylonitrile rubber (CTBN)blend by exchanging 10% CTBN for epoxy-terminated nitrile rubber (ETBN) in a blend with 5 phr of rubber. In the resulting triblock copolymer a decrease in particle size of the dispersed rubbery phase from 12 to 5 μm was observed and the interfacial tension between UPR and CTBN is also reduced. The impact behavior of the triblock copolymer was enhanced due to a reduction in failures caused by the presence of large particles [41].

1.4.2 UNSATURATED POLYESTERS RESIN
PHENOL FORMALDEHYDE RESIN BLENDS UPRs are highly flammable and produce large quantities of smoke and toxic gases on burning. The flame resistance of UPR can be improved by adding flame-retardant additives or by blending it with other polymers such as phenolic formaldehyde resins (PF). The addition of additives usually leads to unfavorable reactions between the polymer and additives resulting in the deterioration of the mechanical properties of polymers to some extent. Blends of UPR and PF show good fire retardant abilities due to the high charring tendency of PF. PF is known to generate less toxic gases and smoke and leave a large amount of carbon residue [42]. Among phenolic resins, resoles and novolacs are important; on curing, they produce highly cross-linked thermally stable network structures, which on exposure to high heat or fire, char, thus producing relatively low levels of combustible volatiles [43]. Kandola et al. used ethanol-soluble epoxy and allyl-functionalized phenolic resoles to overcome the incompatibility of UPR and PF resins resulting from their chemical structures and curing mechanisms. They demonstrated an increase in compatibility with functionalization. Allyl-functionalized resole exhibited the best compatibility with UPR. A mechanism has been proposed for their decomposition and interactions and their effects on flammability based on thermal behavior and infrared spectroscopic analysis of volatile degradation products [44]. Mahadar et al. blended UPR with resole to produce materials with good flame retardancy. The blends showed good compatibility when compounded with kenaf fiber, which is a natural fiber. Although the thermal stability of the blends was improved, the mechanical properties were found to be slightly inferior [42]. Novolac resin was modified with free-radically curable methacrylate groups (M-Nov) with styrene to give a material with a higher Tg, better thermal and thermo-oxidative stabilities, and better flame retardancy than cured UPR, with an approximately 20% lower modulus at room temperature (RT) [43]. An alternative modification of novolac with the vinylbenzyl group to obtain a homogeneous, free-radically cocured phenolic/UPR blend with better flame retardancy than those made using M-Nov has also been attempted. The cured vinylbenzylated novolac and its cocured blends with UPR exhibited superior flame retardancy in comparison to cured UPR and have potential applications as matrix resins in glass-reinforced composite laminates, especially for marine structures [45].

1.4 UNSATURATED POLYESTERS RESIN BLENDS

13

1.4.3 UNSATURATED POLYESTERS RESIN
EPOXY RESIN BLENDS UPR and epoxy resin are miscible with each other and show good compatibility. Hybrid polymer networks (HPNs) based on UPR and epoxidized phenolic novolacs (EPNs) have been prepared through reactive blending. EPN shows good miscibility and compatibility with UPR. The blend shows substantially improved toughness and impact resistance along with better thermal stability. Blends with 5 wt.% of EPN exhibit the highest tensile strength [46,47]. HPNs were also synthesized by coreacting UPR with epoxidized cresol novolac and DGEBA. Cocured blended resins showed substantial improvements in toughness and impact resistance along with thermal stability and damping properties. The performance of the blends with EPN was found to be superior [47]. A new bioresin was produced by Mustapha et al. by blending UPR with epoxidized palm oil (EPO) in 10, 20, and 30 wt.%. The addition of EPO in UPR resin lowered the Tg at 20 wt.% loading of EPO, Tg decreased by 6 5 C, and the storage modulus decreased by about 20% in comparison to UPR. However, the impact properties increased with the amount of EPO added. EPO provides a rubbery phase and absorbs the energy applied by the impact loadings. Bio-based thermoset UPR blended with EPO may reduce the dependency on conventional composite matrix systems made from petrochemicals [48]. UPRs were prepared by reacting bisphenol A epoxy resin with various glutaconic acids using a base catalyst. They were functionalized by treatment with acryloyl chloride to yield acrylated polyesters (APEs). Blending of these APEs were carried out with styrene monomers. In comparison to APEs, these blends exhibited high curing temperatures, slow degradation of products (i.e., low weight loss), good chemical resistances, and good mechanical and electrical strengths [49].

1.4.4 UNSATURATED POLYESTERS RESIN
ESTERS BLENDS Ardhyananta blended VE and UPR containing aromatic benzene rings (10%
80 wt.%). The UPR/ VE blends were prepared by mechanical blending and cured at room temperature using 4% of MEPK in the absence of an accelerator. The mechanical and thermal properties of the blends were found to be superior [50]. Polymer blends of unsaturated polyether ester resins and dicyclopentadiene polyester resins yield cured thermoset resins having high tensile and flexural strengths. The coaction resulting from blended polymers provides an economic way to improve both the stiffness and strength of cured polyether ester resins [51]. Styrenated polyester resins were blended with poly(vinyl acetate) (PVAc). A cocontinuous phase morphology was observed in blends containing PVAc with concentrations $ 6% and styrene levels $ 40%. An increase in the styrene content from 20% to 80% resulted in the sharpening of the principal dynamic loss peak, and the peak temperature reached a maximum at a concentration of 40%. The change from particulate PVAc to cocontinuous structure was associated with a sharp drop in GBc and KBc. Parallel studies have shown this transition to be important in “low-profile” behavior [52].

1.4.5 UNSATURATED POLYESTERS RESIN
POLYSACCHARIDE BLENDS Modified UP was blended with cellulose and ethyl cellulose (5%
25%) at ambient conditions in the presence of MEKP as a curing agent. The blends showed compatibility with modified UP as a

14

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

result of the polar
OH groups in their structure. The results indicate that cellulose increases the impact strength, hardness, and dielectric constant and decreases the bending of the blends, while ethyl cellulose causes an increase in the impact strength, hardness, and bending but a decrease in the dielectric constant of the blends [53]. The work done by Salih et al. involves the blending of UPR with starch powder (0
3 wt.% fraction) with particle sizes less than 45 μm. The blends were further irradiated by UV acceleration. The UV irradiations had a noticeable effect on most of the mechanical properties of the blends. The mechanical properties were found to be a function of particle size and the dispersion of starch powder in the UP matrix. A significant decrease was observed in the ultimate tensile strength and elongation percentage with increasing weight fractions of starch powder, while the modulus of elasticity of the blend showed a significant increase. Other mechanical properties of the blends such as hardness, impact strength, fracture toughness, and flexural strength also increased with increasing weight fractions of starch powder (1%), except the flexural modulus at 1.5%, followed by a decrease at higher percentages of starch [54].

1.4.6 THERMOPLASTIC BLENDS Xanthos and Wan reported the melt blending of polypropylene (PP) with a nonconventional low molecular weight UPR (5:3 PP/UPR wt. ratio) in the presence of organic peroxide by reactive processing. The reacted blend exhibited a finer and more uniform morphology and different properties. The results indicate the possibility of the formation of block and/or graft PP/UPR compatibilizing copolymers [55]. UPR blends of different compositions were prepared with two different thermoplastics, polystyrene (PS) and polycarbonate (PC), by mixing solutions of the polymers in chloroform. A miscibility study of these solution blends was carried out using simple and inexpensive techniques. The UPR/PS blend was found to be miscible while the UPR/PC blend was immiscible [56]. Hydrogen bonding interactions between the two components in poly(ethylene oxide) (PEO)/ OER blends and PEO/cross-linked UPR blends were understood by Fourier transform infrared spectroscopy (FTIR) study. These hydrogen bonding interactions are responsible for the miscibility of the blends. The crystallization kinetics and morphology of PEO in the PEO/UPR blend was found to be dependent on cross-linking. At the crystallization temperature, the overall crystallization rate of PEO in the PEO/UPR blend was larger than that in PEO/OER blend [57]. Li et al. used an improved nuclear magnetic resonance (NMR) method to measure the interphase thickness and to interpret the phase behavior, miscibility, heterogeneous dynamics, and microdomain structure of a thermoset blend of UPR with a cotriblock polymer of PEO-block-poly(propylene oxide)-block-PEO (PEO-PPO-PEO). The results indicated that thermodynamic interaction between the block copolymer and the cross-linked thermoset resin is a key factor in controlling the phase behavior, domain size, and interphase thickness of these blends [58]. Although poly(ε-caprolactone) (PCL) was found to be miscible with uncured polyester/OER, it is partially miscible with crosslinked polyester resin (PER). The miscibility of PCL and OER/PER was found to be a consequence of intermolecular hydrogen bonding between the components of the blend. The importance of the contribution of entropy to the miscibility of thermosetting polymer blends is also shown from FTIR results. The spherulitic morphology of the blends was remarkably affected by cross-linking. Birefringent spherulites were observed in uncured PCL/OER blends, whereas a distinct pattern of extinction rings, which was absent both in the pure PCL or in the uncured PCL/OER blends, was apparent in the cross-linked PCL/PER blends [59].

1.5 INTERPENETRATING NETWORKS

15

1.4.7 SIMULATION STUDIES ON BLENDS Ruffier et al. performed a simulation to show the connection between voids scattered inside the UPR
polyvinyl acetate blend and the blend phase separation mechanism [60]. The effect of curing temperature on the morphology of UPR/styrene/PVAc blends with 5% and 10% PVAc cured between 75 C and 150 C was studied. A cocontinuous phase separated structure resulted for 10% PVAc. An insignificant change in morphology with curing temperature was observed for this composition [61]. A computer simulation model to analyze the reaction injection molding process of polyurethane and UP blends has also been proposed [62]. Mezzenga et al. modeled the free energy of mixing during polymerization in blends of UPR, styrene, and allyl ether functionalized hyperbranched polyesters. They combined the Flory
Huggins theory and gel permeation chromatography (GPC) molecular weight measurements during modeling. The cure behaviors of UPR, phenol, and UPR/ phenol blends were detected and simulated using differential scanning calorimetry (DSC) and DMA. Cure behavior was used to calculate and predict the cure rate, cure temperature, conversion, and changes in the Tg along with various cure orders in order to obtain the optimum parameters for processing [63]. With dynamic scanning, isothermal DSC procedures, and Borchardt
Daniels dynamic software, cure data for the UP resin were obtained; 90% of the conversion rate at 100 C being achieved after 15 minutes. However, for the phenol and UPR/phenol blends, gradually increasing the temperature was found to be the best for curing according to the DSC and DMA test results [64].

1.5 INTERPENETRATING NETWORKS OF UNSATURATED POLYESTER RESIN These are a relatively new type of polymer blends which consists of two or more cross-linked polymers in which at least one network is synthesized in the presence of the other. Although different modification processes are reported, the formation of interpenetrating polymer networks (IPNs) is a promising method.

1.5.1 UNSATURATED POLYESTERS RESIN
POLYURETHANE INTERPENETRATING POLYMER NETWORKS Mutually permeable semi-IPN-type networks consisting of UPR and polyurethane resin (PUR) (semi-IPN UPR/PUR) have been prepared using a new method of adding PUR to styrene. Both resins form dispersed phases with heterogeneous microstructures. PUR seems to affect the mechanical properties significantly, but the effect ceases on increasing the PUR content above 10%. The dynamic elasticity modulus depends only on composition [65]. IPNs with four different types of UPRs (commercially available unsaturated polyester (UPE), partially end capped UPE, OH-free, and having acetate linkages at the end) and PU were cured with UV. The reaction sequence was found to be an important factor in determining the phase mixing, phase morphology, and hence, the

16

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

mechanical properties of the IPNs. The simultaneous reaction of the two reacting systems resulted in a cocontinuous structure that provided enhanced tensile properties and impact strengths [66]. A series of BaTiO3 fiber and nanopowder unfilled and filled IPNs composed of polyurethane (PU) and UPR were prepared using a simultaneous polymerization process. The damping behaviors and degree of phase separation of the unfilled and filled IPNs were strongly dependent on the PU/ UP component ratios, types of filler, and the amount of nanopowder added. The filled IPNs exhibited synergistic effects on damping properties. Performing a polarizing process enhanced the properties. The temperature ranges exhibited excellent consistency of maximum dielectric loss and dielectric constant with damping loss factor [67].

1.5.2 UNSATURATED POLYESTERS RESIN
ACRYLATE INTERPENETRATING POLYMER NETWORKS Polyester
poly(ethyl acrylate-co-styrene) IPNs were synthesized using a two-step in situ sequential technique. Both semi- and full IPNs were synthesized. Poly(ethyl acrylate-co-styrene) acts as the rubbery phase and polyester as a hard phase. With increasing proportions of ethyl acrylate in the IPN, the elongation at break, toughness, and molecular weight between cross-links was higher, but tensile strength, modulus, tear strength, and density were lower. The full IPNs showed higher tensile strength, modulus, tear strength, density, and hardness, but lower elongation at break and toughness compared to semi-IPNs. The semi-IPNs showed higher toughness and elongation. The extent of cross-linking of the elastomer had a determining role in the mechanical property profile. The diameter of the domain depended on the amount of elastomer added [68]. Acrylate-modified PUR resin was first prepared and then added to UPR to obtain an IPN that could be cured at RT. An improvement in miscibility led to higher degree of penetration and entanglement, thus resulting in improved mechanical properties [69]. A series of semi-IPNs based on different compositions of an acyclic PET oligomer and UPR were prepared with styrene as a cross-linker, MEKP as a catalyst, and cobalt naphthenate as a promoter. The mixture was cured at RT. The tensile strength of the IPNs decreased, whereas the elongation at break increased with the concentration of PET oligomer [70].

1.5.3 UNSATURATED POLYESTERS RESIN
EPOXY RESINS INTERPENETRATING POLYMER NETWORKS Simultaneous IPNs based on epoxy (DGEBA) and UP were prepared using m-xylene diamine and BPO as curing agents. Single Tg suggested good compatibility of epoxy and UP. Interlock between the two growing networks led to a retarded viscosity increase. The hydroxyl end groups in the UP catalyzed the curing reaction of epoxy; leading to rapid increase in viscosity. Entanglement affected the cracking energy absorption and was reflected in an improvement in toughness [71]. The Tg of simultaneous IPNs was found to increase with the EP (epoxy polyester) content (10%
90 wt.%). IPNs containing higher EP contents exhibited higher values of tan δ(max.) (Fig. 1.5) and lower cross-linking densities in the rubbery state probably due to the plasticization effect of the EP component along with the heterogeneous network structure [2]. Studies on the curing kinetics of simultaneous UP/DGEBA IPNs showed lower total heat of reaction compared to that observed while curing pure resins. This could be an effect of network interlock that could not be compensated

1.5 INTERPENETRATING NETWORKS

17

0.7 0.6

100EP 10UPR:90EP 30UPR:70EP 50UPR:50EP 70UPR:30EP 90UPR:10EP 100UPR

tan δ

0.5 0.4 0.3 0.2 0.1 0 –135

–80

–25

30 85 140 Temperature (ºC)

195

250

FIGURE 1.5 tan δ versus temperature of BPO/THPA-cured IPNs, BPO-cured UPR, and THPA-cured EP.

completely by an increase in curing temperature. Incomplete curing in isothermal mode is caused by both network interlock and the vitrification of DGEBA. The rate constant for 50/50 of UP/ DGEBA was higher while the activation energy was lower presumably due to the catalytic environment provided by the hydroxyl end group of UP in the IPN [72,73]. A series of IPNs with excellent flame-retardant and damping properties were developed. The flexibility and range of thermal transition increased as the content of UPR increased in the IPNs while the homogeneity decreased. The heat resistance, damping, and mechanical properties were all improved simultaneously with the addition of plate-shaped carbon black (CB) into the UPR/epoxy IPNs [74]. Shin and Jeng also prepared UPR/epoxy IPNs. A series of IPNs based on UP/epoxy were developed. Phase separation was observed when the UPR content was higher than 30%. The best miscibility for IPN was obtained for a composition with similar amounts of hydrogen donors and carbonyl group [75]. From a kinetic study of EP/UPR it was found that the heat resistance of UP was enhanced with the addition of a flame-retardant or epoxy resin [76]. A series of translucent, compatible IPNs were prepared by Shaker et al. using an elastomeric amine-cured epoxy and UPR. A 45% increase in toughness was observed for one of the compositions. This was a reflection of the homogeneous distribution of the rubber component [77]. SemiIPNs of epoxy and UPR have been synthesized with different proportions of UPR (0%
50%). Trimethylenetetramine was used as a room temperature curing agent. IPNs with 11.1% of UPR exhibited improved mechanical properties. The blends were further modified by aromatic amines such as benzidine and diphenyl amine. The mechanical properties of the blend modified with diphenyl amine were found to be superior [78].

1.5.4 UNSATURATED POLYESTERS RESIN
PHENOL AND UNSATURATED POLYESTERS RESIN
NYLON INTERPENETRATING POLYMER NETWORKS IPNs of UPR and several phenolic resoles have been reported by Avendano et al. These IPNs were found to show both physical and chemical compatibility as they cocure such that they result in

18

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

cocontinuous IPNs. The participation of the allyl groups of resole in the cross-linking process of IPNs could be confirmed from the solid-state 13C-NMR spectra [79]. Novel semi-IPNs of UPR and nylon have been produced by mixing different amounts of Nylon 66 oligomers (residues of industrial Nylon 66 polymerization) into UPR and heating followed by cross-linking. Nylon 66 was obtained from industrial waste. Three important aspects of this work include (1) the possibility of producing new materials with improved impact strengths, (2) the plastifying effect of Nylon 66 oligomers on the UP resin, and (3) ecologically more important, the feasibility of reutilizing waste materials for producing engineering materials with tailored properties [80].

1.6 UNSATURATED POLYESTERS RESIN COMPOSITES Composites are heterogenous materials made up of two or more chemically distinct constituents. The basic components are a reinforcement and a matrix. Each of these should have appropriate characteristics and function both individually and collectively so that composites attain the desired superior properties. The reinforcement contributes to the strength and modulus to the composite, while the main role of the matrix is to transmit and distribute stresses in the reinforcement. Reinforcements are of two types, namely particulate and fibers. Commercially, glass fiber (GF)reinforced polyester composites are important due to their high strength-to-weight ratio, low cost, and easy manufacturing methods. In comparison to particulate-filled composites, many fiber-filled composites are anisotropic with tremendous strength in one direction; although uniaxially oriented fiber composites have very high moduli in one direction, the other moduli are low. Therefore to get good properties in atleast two or three directions, fibers can be randomly oriented such that composites are nearly isotropic in a plane. In the case of fibers as a reinforcement, it also provides protection against both fiber aberration and fiber exposure to moisture or other environmental conditions.

1.6.1 UNSATURATED POLYESTERS RESIN
SYNTHETIC/GLASS FIBER COMPOSITES Commercial interest in GF-reinforced UPR composites is due to their high strength-to-weight ratio, and low cost. E-GF composites prepared using a hand layup technique with concentrations varying between 15 and 60 wt.% rendered excellent mechanical properties. Table 1.3 shows the improvement in the mechanical properties as a function of filler content [81]. Table 1.3 Effect of Glass Fiber of Fabricated Composites Contents on Tensile Strength No. Control sample GFRP 1 GFRP 2 GFRP 3 GFRP 4

Content (%)

Width (mm)

Thickness (mm)

Max. Load (N)

Yield Strength (MPa)

Tensile Strength (MPa)

0

19

5

2707.50

10.29

19.76

15 30 45 60

19 19 19 19

5 5 5 5

3504.30 4827.90 5370.35 7488.85

18.63 20.32 21.08 12.54

28.25 50.82 56.53 78.83

1.6 UNSATURATED POLYESTERS RESIN COMPOSITES

19

Belaid et al. carried out thermal ageing studies on polyester fiberglass composites and reported a strong effect on mechanical properties. The Young’s modulus decreased with aging time; from 6% after 30 days to 55% after 120 days [82]. Pedroso et al. achieved significant improvements in the texture, flexural strength, and impact resistance of sheets of UPR GF composites by pressing and heating the sheets at 40 C and 50 C during curing [83]. Studies on the immersion of GFreinforced composites (GFRP) in seawater revealed significant water absorption initially, while soluble material extraction was higher later. The tensile and bending strengths showed decrease with prolonged immersion. Serious corrosion of the interface was observed in micrographs [84]. Mechanical properties such as density, ultrasonic velocity, shear modulus, except Poison’s ratio and elasticity modulus were reported to increase with increasing concentrations of GF (5%
25%) after ultrasonic treatment at 26 KHz [85]. Ferreira et al. reported a higher char yield for an aluminized E-GF composite compared to that of an unmetallized E-GF composite [86]. Surface functionalized chopped cellulose fibers (CFs) when added in small amounts (1%
3 wt.%) further enhanced the strength of composites [87]. The incorporation of methylene spacers in the backbone of UPR enhanced the strength of oil palm empty fruit bunch (OPEBF) as well as fiberglass
UPR composites. Composites based on six methylene spacers showed the highest strength as compared to UP composites based on two methylene spacers [88]. Dagwa and Ohaeri prepared composites of UPR together with banana empty fruit bunch fibers, GFs, and OPEBF particles, which showed a decrease in flexural strength with increases in banana fiber content, while with GFs it increased. A ternary composite with 5 wt.% OPEBF and 10 wt.% banana fiber/10 wt.% GF showed a high impact strength, that is, 55.556 J/m2, representing a 1568.67% improvement over virgin UP. A binary composite with 15 wt.% banana fiber and 5 wt.% GF showed the highest hardness (3.55 HV), representing a 136.67% improvement. Hardness is seen to be influenced by increase in banana fiber content; therefore, banana fiber could be considered for applications requiring high impact strengths such as some parts of automobile vehicles (Fig. 1.6) [89].

FIGURE 1.6 Hardness test values for banana and glass fiber polyester composites.

20

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

TiO2 particulate-filled GF-reinforced polymer composites were prepared by Moorthy and Manonmani with two different fiber lengths, 3 and 5 cm, by hand layup method. The TiO2 content was varied from 10 to 40 wt.%. The combined reinforcement yielded better mechanical properties with increased fiber length and particulate material. Chemical resistance was more pronounced in the 5 cm fiber length composites [90]. The toughness of GF/UPR composites was improved by adding low-molecular-weight polyisobutylene (1
12 wt.%), which was grafted onto MA and glycidyl methacrylate through the novel solvothermal method to improve compatibility [91]. GF/URP exhibited considerable chromatic changes upon UV exposure. Although the mechanical properties were slightly poor, especially in the immersion and condensation chambers, the durability tests proved the generally good behavior of this material under aggressive conditions [92]. Microfibril cellulose (0.3 wt.%) when added to GF
UPR composites improved its mechanical properties. The composites were prepared by hand layup and vacuum bagging method. The impact strength was increased by 19.6% and flexural strength increased from 192.40 Mpa to 208.63 Mpa by hand layup method. The tensile strength increased from 10.24% to 19.62% for samples prepared by hand layup and vacuum bagging, respectively as reported by Vu et al. [93]. Jiang et al. modified carbon fibers with two different functional polyhedral oligomeric silsesquioxanes (POSS) monofunctional (methacrylolsobutyl) and multifunctional (methacryl) POSS and mixed these with UPR to form composites. They showed significantly increased lamellar strength (62 and 67 MPa, increase of 31.9% and 42.6%, respectively) and interfacial shear strength (IFSS). The impact energy was also higher for modified CF composites in comparison with CF/UPR composites [94].

1.6.2 UNSATURATED POLYESTERS RESIN
NATURAL FIBER/PARTICLE COMPOSITES 1.6.2.1 Fibers The use of natural fibers in composites is increasing due to their light weight, nonabrasive, combustible, nontoxic, low cost, and biodegradable properties. The only limitation is their poor mechanical properties, and to overcome this synthetic fibers are used. Osman et al. observed that alkali treatment and the length of kenaf fibers affect the mechanical properties of composites [95]. The effect of water absorption on the flexural properties of kenaf fiber composites significantly reduced with the incorporation of recycled jute fibers [96]. In the case of natural fibers, it is commonly observed that alkali-treated fibers render superior properties in composites. The structural features of sugar palm fibers were found to be affected by alkali treatment using NaOH and an enhancement in IFSS was observed due to the internal morphological changes of the sugar palm fibers. Also, the effect of sugar palm fiber for both untreated and that treated with NaOH on single fiber strength and IFSS has been studied [97]. Cho et al. revealed that henequen fibers when subjected to surface treatment (1% and 6% NaOH) using two different methods, namely soaking and ultrasonic, exhibit drastic changes in topography (increased surface roughness and area). The change is strongly dependent on the treatment method and media used. The IFSS between the fibers and the matrix of the composites was

1.6 UNSATURATED POLYESTERS RESIN COMPOSITES

21

appreciably improved by surface treatment. The topological and interfacial results were quite consistent with each other [98]. Coconut fibers (5
20 wt.%) treated with 10% NaOH provided a reinforcing property, tensile properties, and microhardness to the composites. A 10% loading gave the best reinforcing property, while a 15% loading exhibited the best microhardness [99]. Wood fibers also showed improved mechanical properties [100]. Chemically modified wood flour and wood fiber
UPR composites are reported to offer better performance under compressive loads [101]. Bagasse fiber
reinforced UPR composites gave optimum mechanical properties at 5
10 wt.% loadings [102]. The tensile and flexural properties of alfa fiber
reinforced composites improve with increasing concentrations of NaOH (1
7%) [103]. Acetic anhydride and styrene treatment of alfa fiber added into UPR increased the water resistance and mechanical properties of composites [104]. The thermal stability of modified polyester resin and jute and maize fiber composites is good [105,106]. Cat tail fiber
UPR composites exhibit improved tensile and flexural strengths, which increase with increasing fiber volume (0%
6.01%) [107]. An increase in water absorption with increasing fiber content (0
25 vol.%) in hemp fiber
UPR composites was observed by Rouison et al. Various fiber treatments were tested but none resulted in a substantial increase in the resistance to water absorption [108]. Studies of water absorption on the mechanical properties of hemp fiber (0
0.26 fiber volume)-reinforced composites have shown that the water uptake increased with increasing fiber volume simultaneously to decreases in tensile and flexural properties. Moisture-induced degradation was significant at elevated temperatures. The percentage of moisture uptake increased as the fiber volume fraction increased (0%
0.26%) due to the high cellulose content [109]. Balaji and Senthil Vadivu synthesized coir and cotton
reinforced UPR composites. The cotton fiber
reinforced UPR had better mechanical properties useful for packaging applications. Figs. 1.7
1.9 demonstrate the tensile, flexural, and impact strengths for samples (1
6) prepared with 80% polyester and coir and cotton fibers together with percentages varying from 20% to 0% at intervals of 4% and vice versa [110].

Tensile strength (MPa)

35 30 25 20 15 10 5 0 1

2

3 4 Sample no.

FIGURE 1.7 Tensile strength comparison of different composite materials.

5

6

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

Flexural strength (MPa)

22

18 16 14 12 10 8 6 4 2 0 1

2

3 4 Sample no.

5

6

5

6

FIGURE 1.8 Flexure strength comparison of different composite materials. 4.5 Impact strength (J)

4 3.5 3 2.5 2 1.5 1 0.5 0 1

2

3 4 Sample no.

FIGURE 1.9 Impact strength comparison of different composite materials.

Acetylating and cyano-ethylating treatment of luffa fibers enhanced the flexural strength and flexural modulus of the composites due to improved adhesion between the fiber and the matrix [111]. Comparative studies of the dielectric and electrical properties of chicken feather and kenaf fiber
reinforced UPR composites revealed a lower dielectric constant, dissipation factor, and loss factor for chicken fibers. The increase was higher at 40% fiber content. Experimental values were correlated with theoretical calculations [112]. Chemical modification was found to improve the adhesion of the fiber to the matrix, as well as the physicomechanical properties of sisal fiber, hemp fiber, and bamboo fiber
UPR composites [113
115]. Banana empty fruit bunch and sisal fiber
UPR composites also show superior mechanical properties [116,117]. The addition of flax fibers to UPR limits the role of styrene in network formation in the composite [118]. The effect of cold plasma and autoclave treatments on the mechanical properties of

1.6 UNSATURATED POLYESTERS RESIN COMPOSITES

23

flax fiber
reinforced UPR composites has shown that plasma treatment improves fiber/matrix adhesion while autoclave treatment reduces water solubility in the fibers [119].

1.6.2.2 Particles The mechanical properties were improved when wood ash and microcrystalline cellulose (MCC) were added as fillers into UPR. 5% wood ash provided the best tensile strength and elongation at break while MCC provided the best tensile modulus together with a slight improvement in impact strength. However, both fillers had adverse effects on the flexural strength and modulus [120]. Guar gum is a natural polysaccharide that has been explored for various applications. Shenoy and D’Melo have shown that the inclusion of guar gum and its derivatives results in composites with increased solvent resistance and mechanical properties [121]. Coconut shell and snail shell powders (5%
50%) were added to UPR. The coconut shell particles improved the tensile properties while the snail shell composites showed better thermal properties. The maximum improvement in tensile elongation of the composites was 375%, while that in microhardness was 125% over virgin UPR. The maximum improvement in tensile strength of the composites was 140% over that of virgin UPR [122,123]. A comparison of the mechanical properties of charcoal and snail shell (particle size 635 μm, 0
30 wt.%)-reinforced UPR composites indicated superior properties for snail shell composites over charcoal composites [124,125]. Odusanya et al. evaluated the properties of hybrid seashell/snail shell
reinforced UPR and found the highest resistance before breakdown at 30 wt.% reinforcement compared to individual fillers [126]. Among as received fly ash and surface-treated particles used as fillers to obtain composites with UPR, both exhibit improvement in mechanical properties but certain properties were better in surface-treated fly ash composites [127]. The moisture absorption properties of modified linen
UPR composites also improved by 30.3% compared to unmodified resin [128].

1.6.3 UNSATURATED POLYESTERS RESIN
SYNTHETIC PARTICLE COMPOSITES 1.6.3.1 E-glass Polymer composites absorb moisture; however, composites of aluminized E-glass and UPR exhibited significantly reduced water absorption at RT. At high temperatures, these show the opposite behavior due to their unstable nature [129]. The mechanical properties of E-glass nonwoven matreinforced UPR composites improved after heat treatment below 100 C. The maximum tensile strength (200.6 MPa) was obtained for samples treated at 90 C. Water uptake increased with time and degraded above 150 C [130]. The flexural modulus of composites (UPR glass/carbon) increased while the flexural strength and izod impact decreased with increasing fiber content. The dynamic storage modulus also increased with increasing carbon and CaCO3 content and postcure temperature. Thermogravimetric analysis (TGA)showed increased weight loss with increasing carbon content in an N2 atmosphere, while the opposite trend was observed in air [131].

1.6.4 UNSATURATED POLYESTERS RESIN
METAL/METAL OXIDE COMPOSITES The electrical and thermal conductivity of copper-filled UPR composites has been investigated by Yaman and Taga; dendrite-shaped copper (Fig. 1.10) was used and the effects of both size (Fig. 1.11, fractional size groups) and content were studied. The thermal as well as electrical

24

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

FIGURE 1.10 Dendrite-shaped copper particle ( 3 500).

FIGURE 1.11 Copper powder particle size distribution.

conductivity increased with increasing filler content and particle size. The maximum thermal conductivity of the composite obtained experimentally was 4.72 W/m/K, an increase of 21 times that of neat UPR. The models of Maxwell and Budiansky exhibit convergence to the experimental results, particularly for lower (below 37% volumetric) filler content [132].

1.6 UNSATURATED POLYESTERS RESIN COMPOSITES

25

The thermomechanical properties of UPR reinforced with ceramic Al2O3 particles (0
20 vol.% fractions) were improved [133]. UPR filled with 3 wt.% bentonite-modified silsesquioxanes (POSS) synthesized by Oleksy and Galina exhibited improved mechanical properties and flame resistance [134]. Composites of polyester resin reinforced with diorite, pulverized sandstone, and cornstalk were studied for their moisture absorption properties. The cornstalk
polyester composite was found to absorb more moisture than other composites. The diorite
polyester composite absorbed the least amount of moisture and the moisture decreased with increasing amounts of diorite filler [135]. Composites of UPR filled with silica (micro and nano), nano ZnO and chitin powder
filled UPR composites showed good solvent resistance together with better tensile properties. Hardness increased with the addition of silica, and ZnO, whereas it decreased in the presence of chitin [136]. Expanded polystyrene (EPS) as a waste material was incorporated as a filler into UPR. Tin and zinc oxides were added in different amounts (0
2 wt.%). The composites were fabricated as flat panel windows or glazing to replace glass. Results showed that both the EPS and metal oxide imparted higher flame retardancy to the composites, although each additive reacted differently with the polymeric matrix [137].

1.6.5 UNSATURATED POLYESTERS RESIN
GRAPHITE/CARBON COMPOSITES In the case of graphene and graphite UPR composites, each one has a different effect on the mechanical properties due to the difference in their chemical bonds. The amounts added were 0.05, 0.10, and 0.15 wt.%. Among the two, the storage modulus did not vary with graphene oxide (GO) content while graphite increased the storage modulus up to 65 C. The storage modulus, and loss moduli and Tg were higher for both composites compared to neat UPR (Figs. 1.12 and 1.13). The damping factor values were higher for UPR/graphene composites. Thermal degradation is not affected significantly. SEM showed graphene flakes, graphite particles, and dispersion degree [138].

FIGURE 1.12 Storage modulus and loss modulus versus temperature for polyester/graphene composites.

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

Polyester

Polyester+0.05 wt.% graphite

Polyester+0.01 wt.% graphite

Polyester+0.15 wt.% graphite

2750 2500 2250 2000 1750 1500 1250 1000 750 500 250 0

250 200 150 100 50

Loss modulus (MPa)

Storage modulus (MPa)

26

0 30

60

90

120

150

Temperature (ºC)

FIGURE 1.13 Storage modulus and loss modulus versus temperature for polyester/graphite composites.

Composites containing carbon (teak wood, ground nut, neem wood, and rose wood; 10
40 wt.%) prepared from environmental waste were fabricated using a casting technique. The composites are amorphous in nature as observed from X-ray diffraction (XRD). Teak wood carbon showed the best thermal stability. The inadequacy of the bond between the filler and the matrix leads to poor mechanical properties. A higher loading level leads to deterioration in properties. SEM showed crack initiation due to an increase in the agglomeration of particles [139]. Modeling studies of the curing process of two types of composites, Al-filled and CB-filled UPR, showed that the Al
UPR composite responded faster to heat input
induced curing and as such was able to cure faster than the polyester
carbon composite [140]. A bismaleimide
UPR composite exhibited an increase in the thermal index of the material, thus making it useful for high-temperature applications [141].

1.7 UNSATURATED POLYESTERS RESIN
NANOCOMPOSITES The main challenge in nanocomposite synthesis is achieving a uniform dispersion of the nanofiller thought the UPR matrix. Commercially available nanocomposites using UPR matrix are costly.

1.7.1 METAL/METAL OXIDE The results of nanocomposites of TiO2 (1% w/w)
UPR showed an interaction between the -OH groups on the TiO2 surface and the ester groups of the UPR leading to decreased crystallinity and hydrophilicity. Fig. 1.14A, C, E, and G shows micrographs of pure aromatic polyester (APE) and APE/TiO2 (1, 3, 5% w/w). A homogeneous distribution of the filler can be observed for the composite with 1% TiO2, whereas clusters are observed at higher concentrations of TiO2. Energydispersive X-ray spectra (EDS) of the nanocomposites (Fig. 1.14D, F, and H) evidenced an increase

1.7 UNSATURATED POLYESTERS RESIN
NANOCOMPOSITES

27

FIGURE 1.14 Micrographs of the pure polymer (A); APE with 1% (C); 3% (E); and 5% TiO2 (G). EDS spectra of the pure polymer (B); APE with 1% (D); 3% (F); and 5% TiO2 (H).

28

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

100

Weight (%)

80

(A) (B)

60

(C) 40

(D) (E)

20

(F) 0

100

200

300 400 500 Temperature (ºC)

600

700

800

FIGURE 1.15 TGA thermograms of UPR/alumina nanocomposites. (A) Pure polyester; (B) UPR/1% nanoalumina; (C) UPR/3% nanoalumina; (D) UPR/5% nanoalumina; (E) UPR/7% nanoalumina; (F) UPR/9% nanoalumina.

in the Ti peak compared to that of the pure polymer (Fig. 1.14B) and confirmed the incorporation of the TiO2 filler into the APE matrix. However, improvements in thermal properties and hardness are evidenced [142]. UP nanocomposites filled with nanoalumina (60
70 nm, 1
9 wt.%) were prepared by Baskaran et al. using a casting technique, which showed higher tensile, flexural, and impact strengths than pristine UPR. The storage modulus increased maximum up to 5 wt.% of filler loading level. The thermal stability of the nanocomposites was also higher with char yields increasing as a function of alumina concentration in the composites (Fig. 1.15) [143]. The effects of the particle size of nanoalumina and the concentration of the coupling agent used on the erosion resistance and mechanical and thermal properties of composites with UPR have been investigated. A higher particle size of nanoalumina and higher concentrations of the coupling agent showed increased erosion resistance along with overall increases in thermal and mechanical properties [144]. A new type of polyester-based composite material with enhanced flame retardancy has been developed by modifying the polymer with nanoalumina and microsilica particles. Experiments were based on Taguichi’s methodology. The material and processing parameters used had different effects on the properties. The composites showed better fire retardance when particles were added singly or in combination. Best fire resistance properties were obtained for a combination of 0
5% nano alumina or micro silica particles along with 1
15% phosphinate based flame retardant [145]. The results of Chen et al. have shown that surface-modified hydrophobic ZnO nanoparticles improved the thermal properties of composites with UPR. The tensile strength and bending strength increased by 91.4% and 71.3%, respectively, with a 3 wt.% addition of ZnO

1.7 UNSATURATED POLYESTERS RESIN
NANOCOMPOSITES

29

nanoparticles [146]. The mechanical properties of nanoiron particle (1
7 wt.%) reinforced epoxy/ polyester nanocomposites were investigated. Two highly dispersed nanoparticles, Fe2O3 and functionalized Fe2O3 (f-Fe2O3), were prepared using a chemical reduction method. The mechanical properties of the f-Fe2O3 composites were better than those of the Fe2O3 composites. The matrix became magnetically harder after the incorporation of nanoiron particles. Machine-generated results were compared and analyzed with system generated software analysis of variance (ANOVA) values. ANOVA seemed to reduce the P-values but the machine-generated values were greater than what were expected [147]. Rusmirovic et al. studied the mechanical properties of hybrid composite materials prepared using UPEs based on glycolyzates and chemically modified silica nanoparticles with vinyl reactive functionalities, namely vinyl, methacryloyl, and linseed oil fatty acid reactive residues were investigated. TEM confirmed that the silica nanoparticles formed domains of aggregates in the polymer matrix. The results reveal that the method of synthesis used for preparing UPR had a more pronounced effect on the dynamic mechanical properties as compared to the SiO2 particles [148]. Glass fiber reinforced composites when added with small amounts (1% and 2%) of fumed silica (FS) improved the tensile strength by 8% and 11%, respectively. The flexural strength also increased and the resin system underwent a transition from having a brittle to a ductile nature [149]. Surface-modified FS added to UPR exhibited a higher heat deflection temperature. The elastic modulus was enhanced while the tensile properties were unaffected. The strongest effect was found to be on the impact strength; modified silica resulted in a positive effect while unmodified silica had a negative effect [150]. Small amounts (1%
3%) of nanosilica (50
60 nm) when added to UPR improved the thermal and mechanical properties of the composites. An excess of 5% led to a decline in properties [151]. Nanocomposites based upon hexahydrophthalic anhydride-cured bisphenol A diglycidyl ether and layered silicates such as synthetic fluoromica (Fmica), purified sodium bentonite, and synthetic hectorite (5
10 wt.%) prepared by Zilg et al. showed enhanced toughness associated with the formation of dispersed anisotropic laminated nanoparticles consisting of intercalated layered silicates [152]. UP
POSS hybrid nanocomposites showed improvements in thermal properties in proportion to the proportion of functionalized POSS added [153].

1.7.2 OTHER INORGANIC FILLERS A nanosized calcium carbonate (CaCO3) filler (1
9 wt.%) with particle sizes between 50 and 60 nm was dispersed in a UPR matrix using a casting technique. Uniform dispersions were obtained below 5 wt.%, which enhanced their mechanical properties [154]. A nanocomposite gel coat system prepared using UPR with aerosil powder, CaCO3, and organoclay (1
3 wt.%) showed improved mechanical and water barrier properties. Improvements of 55%, 25%, and 30% were observed in tensile modulus, flexural modulus, and impact property while the Tg was slightly increased [155]. The addition of ramie cellulose nanofiber and CaCO3 as a reinforcement in a UPR matrix also resulted in the increased thermal stability and mechanical properties of the biocomposite as reported by Wahono et al. [156]. Al(OH)3 and Mg(OH)2 can be used as alternatives for fire retardant additives, which can improve the fire retardancy of composites. A combination of 40% Al(OH)3 and 10% Mg(OH)2 improved the thermal stability of the composite by reducing the mass loss rate to 4.9%/min and

30

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

total mass loss to 77%, while the tensile strength decreased to 64% and the hardness improved to 64.5%. The results of the morphology and mapping of the composite showed that Al(OH)3 was well dispersed while Mg(OH)2 had the tendency to agglomerate [157]. Pereira et al. investigated the flame-retardant properties of layered double hydroxides (LDHs) such as adipate-LDH (A-LDH) and 2-methyl-2-propene-1-sulfonate-LDH (S-LDH) when added to UPR and significant reductions in the UPR flammability (by 46% and 32%) were indicated by incorporating 1 wt.% of A-LDH and 5 wt.% S-LDH, respectively, followed by enhanced char formation while evolved smoke remained unchanged [158].

1.7.3 MONTMORILLONITE Preparation procedures also affect the properties of composites due to the chemical and physical reactions involved [159]. UPR
montmorillonite (MMT) clay composites prepared by Suh et al. through in situ free radical polymerization also have improved thermal and mechanical properties as compared with neat UPR due to them having more homogeneous dispersion and optimum amounts of styrene monomer molecules inside and outside the MMT layers at 1 wt.% loading [160]. At an MMT content of only 1.5 vol.%, the fracture energy, Cs, of the nanocomposite was doubled to 138 J/m2 as compared to the 70 J/m2 of the pure UPR [161]. Hassan et al. investigated the possibility of using an ammonium polyphosphate (APP)-filled UP/ phenolic/MMT blending system to obtain nanocomposites exhibiting excellent flame retardancy, thermal stability, and mechanical properties. The optimum APP content in the composites was 30 phr where the char formation on the surface of the composites forced an insulating carbon layer and resisted further escalation of fires. The optimum content of APP and MMT were 30 and 3 phr, respectively, to achieve the best balance of properties based on flame retardancy, thermal stability, and mechanical performance [162]. Tunisian nanoclay/UPR nanocomposites also have better mechanical and thermal properties. The degradation temperature was increased by 78 C with the addition of organic modification [163]. Romanzini et al. showed that the chemical modification of MMT (Cloisite) with compatible silanes, vinyltriethoxysilane, and γ-methacryloxy propyltrimethoxysilane helps in preventing agglomeration and enhances the interaction between MMT and UPR and hence, improves the thermal mechanical and fire retardancy [164]. The chemical resistance of organically modified MMT
UPR composites under aqueous conditions in acetic acid, nitric acid, hydrochloric acid, sodium hydroxide, aqueous ammonia, and sodium carbonate shows maximum weight gain/loss with increasing clay content. The Tg value was found to be maximum for the composites with the maximum clay content [165]. The solvent resistance of UPR
MMT-filled nanocomposites was studied in acetic acid using an equilibrium swelling method. The composites showed low diffusion coefficients. The diffusion coefficient, sorption coefficient, and permeation coefficient increased with increases in temperature for all the samples [166].

1.7.4 CLAY COMPOSITES The effects of variables such as clay type, clay content, and prepolymer
clay mixing type on the mechanical properties of UPR/clay nanocomposites including tensile strength and percentage elongation have been addressed by Johnson et al. Unmodified kaolinite clay and vinyl silane
modified

1.7 UNSATURATED POLYESTERS RESIN
NANOCOMPOSITES

31

clay were used; the clay type and mixing method were found to have a profound effect on the mechanical properties showing good improvement [167]. PEO was used as a new modifier to replace traditional ionic surfactants which present the problem of disintegration at high temperatures. The clay galleries changed to intercalated state in the nanocomposites and the properties of the nanocomposites were improved significantly with only 1 phr loading of organoclay [168]. UPR reinforced with nanosized clay Cloisite 30B (C30B) and CB were prepared using hand layup and open-molding techniques. The amounts of filler added varied between 0 and 10 wt.%. Mechanical strength was superior for C30B compared to CB due to its higher surface area. At 4% filler content the mechanical properties were optimum [169]. Kusmono and Ishak reported an exfoliated structure upon the addition of clay to UPR/GF composites at 2% loading while at 6 wt.% it formed an intercalated structure. The optimum loading was 2%, where tensile strength, flexural strength, and flexural modulus were approximately 13%, 21%, and 11%, respectively. The highest values for impact and fracture were obtained at 4 wt.% loading [170]. Studies of influence on the volume shrinkage of nanoclay composites showed a decrease in volume change while at higher nanoclay contents the reaction rate increased and induction time decreased [171]. Dhakal et al. reported a strong correlation between the nanomechanical properties and interlayer d-spacing of clay particles in the nanocomposite system. The incorporation of 1, 3, and 5 wt.% of layered silicate nanoclay into UPR showed an improvement in hardness of 29%, 24%, and 14%, respectively. The elastic modulus increased from 5393 MPa for neat polyester to 6646 MPa (23% increase) with 5 wt.% nanoclay [172]. Composites with various amounts (1, 3, and 5 wt.%) of different nanofillers such as Nanofil 116, Cloisite 30B, and Laponite RD exhibited slightly enhanced thermal stability as Nanofil 116 and Laponite RD content increased, they also imparted good strength and stiffness. In contrast, 1 wt.% of Cloisite 30B (UPC1) showed a higher degradation temperature and thermal stability than those with 3 and 5 wt.%. The compressive strength was also maximum at 1 wt.% [173]. The addition of halloysite nanotubes into UPR improved the flexural properties of the nanocomposites in dry conditions and after water-methanol exposure. A significant increase in surface roughness was also observed [174].

1.7.5 BENTONITE Bentonites were chemically modified with a surfactant (quaternary ammonium salt) through a cation exchange reaction and added to UPR as reinforcements. They improved the thermal and mechanical properties of the nanocomposites. Nanocomposites loaded with 7 wt.% showed better mechanical properties compared to unsaturated polyesters (PEs) filled with micrometer clay (40 wt.%). According to Motawie et al., the electrical conductivity was also improved [175]. No detrimental effect was observed on the barrier properties (water absorption), which is important for several applications. Phosphonium salts can be used instead of ammonium salts [176].

1.7.6 NATURAL FILLERS Isora nanofibrils (INFs) with a length of 300 nm, width of 20 nm, and an aspect ratio of 15 extracted from Helicteres isora by steam explosion showed a network-like structure. The high

32

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

aspect ratio of the nanofibrils led to an improved network in the polyester matrix, hence, resulted in good water absorption (73% decrease) and mechanical properties of the nanocomposites. The volume fraction of the constrained region was highest at an INF loading of 0.5 wt.% while the tensile strength increased by 57%. A constrained model was proposed to understand the role of the constrained region in enhancing the mechanical properties. The Tg was higher by 10 C [177]. The water absorption properties were also improved due to interfacial adhesion which prevents the penetration of water. At 90 C, as the INF loading increased the water uptake also increased. The mechanism of diffusion was found to be Fickain-type [178]. Nanocomposites of UPR and cellulose nanocrystals (CNCs) prepared by Kargarzadeh et al. showed that the crystallinity index of the CNCs was reduced after surface treatment. However, it did not impact on the size and aspect ratio of the rod-like nanoparticles. The tensile strength and stiffness of the composites improved in the presence of silane-treated CNCs (STCNCs). Interestingly, the impact energy increased significantly with the addition of untreated CNCs. The viscoelastic behavior and thermal degradation for both the CNC- and STCNC-reinforced nanocomposites were improved. The water absorption behavior of the UPR was found to decrease upon the incorporation of CNCs, and a further reduction was observed with STCNCs [179].

1.7.7 CARBON FILLERS Acid and ammonia functionalized multiwalled carbon nanotubes (MWCNTs) were coated with iron oxide (III) and used to obtain polymer/MWCNT nanocomposites with different contents (0.05, 0.10, 0.15, 0.20, and 0.25 wt.%). The viscosity was optimum for MWCNTs in the range between 0.15 and 0.20 wt.%. Fe functionalized MWCNTs exhibited the best mechanical behavior, while the electrical conductivity increased by three or four orders of magnitude with unfunctionalized MWCNTs [180,181]. Seyhan et al. investigated critical aspects related to the processing of nanocomposites made of CNTs with and without amine (NH2) functional groups and polyesters. The composites were processed using three roll milling and sonication techniques. Styrene evaporation from the polyester resin system was a critical issue for nanocomposite processing. CNT/polyester suspensions exhibited shear-thinning behavior, while polyester resin blends behaved as Newtonian fluid. Nanotubes with amine functional groups were found to have better tensile strengths [182]. GO and its derivatives with vinyl and alkyl functional groups (modified GO; mGO) were synthesized and dispersed into UPR to prepare nanocomposites. The mGO was easily dispersed in the UPR compared to the GO, even without sonication. A 55% improvement in fracture energy was obtained with little change in flexural strength or modulus with only 0.04 wt.% mGO. This high effectiveness renders mGO economically viable. Scanning electron microscopy suggests that mGO particles interact with the propagating crack; the main toughening effect being crack pinning [183]. The strength of UPR
MWCNT composites rose with the rising content of MWNTs (0.1
0.5 wt.%). Composites were prepared by solution dispersion and casting methods. The microcrystallinity, stiffness, and strength of nanocomposites rose with the rising content of CNT. A noticeable improvement was observed in the Tg, melting temperature (Tm), and enthalpy (ΔHm). The microcrystallinity of nanocomposites increased with increasing CNT content [184]. The dispersion of MWCNT in UPR was investigated by rheology and optical microscopy. The results revealed a percolation threshold of 0.097 vol.%; which is very close to the theoretical value

REFERENCES

33

of 0.085 vol.% expected for individually dispersed MWNT with an average aspect ratio of 590. The dispersions formed an open network with a fractal dimension of 1.28. The results were compared with single-walled carbon nanotubes (SWNT) and PS-modified MWNT (MWNT-PS). The SWNTs formed stronger networks than those of the MWNTs, but the MWNT-PS networks were weaker than those of the unmodified MWNTs [185]. The influence of MMT nanofiller on the mechanical properties of glass fiber recyclate (rGF) (25
40 wt.%, coarse and fine) reinforced UPR composites, have been studied by Hannan et al. The results showed that the addition of MMT further enhanced the tensile strength (14% in 40rGF
3MMT compared to nonhybrid 40rGF), optimal concentration of MMT is 2%
3% and rGF is 25%. Fiber/resin adhesion is better for low concentrations of MMT. Coarse rGF composites contain relatively larger aspect ratios and hence have better tensile properties [186]. A study on the thermal decomposition kinetics of UP and UPR composites reinforced with 2, 4, and 6 wt.% toner carbon nanopowder (TCNP) revealed faster rates of decomposition of the composites. The activation energy, reaction rate constant, and thermodynamic properties were lower in these composites. This enhancement is attributed to the nanosized iron content in TCNP, which enhances the pyrolysis reaction [187].

1.8 FUTURE CHALLENGES Some of the challenges include reducing dependency on petroleum-derived UPRs using sustainable methods by synthesizing bio-based UPs or blending them with other bio-based oils/polymers. The development of a new variety of UPRs using bio-modifiers and synthesizing biocomposites to replace existing composite materials are also gaining importance. Synthesizing UPRs using monomers obtained by recycling polymer waste is also an important task. Obtaining UPRs with optimum properties from sustainable sources and through recycling is a real challenge. Halogen-free flameretardant UPR (with improved mechanical properties) molded products with superior surface quality is also an important research area. Tailoring low-styrene-emission/styrene-free UPR compositions is necessary to save the environment and to minimize health issues associated with styrene emissions. The effect of thermal aging on different properties of cured resin also needs to be addressed.

ACKNOWLEDGMENT The authors are grateful for assistance from Ms. Vandana Mooss, Mr. Prakash Rathod, Mr. Sudhaker Satpal, and Mr. Yadnesh Kesari in the compilation of this chapter.

REFERENCES [1] A.K. Kulshreshtha, C. Vasile, Handbook of polymer blends and composites, Comp. Mater. 1 (2002) 558. [2] M. Worzakowska, Thermal and dynamic mechanical properties of IPNS formed from unsaturated polyester resin and epoxy polyester, J. Mater. Sci. 44 (2009) 4069
4077.

34

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

[3] E.E. Parker, Inhibiters for unsaturated polyester resins, I & EC Product Res. Dev. 2 (2) (1963) 102
105. [4] E.E. Parker, Unsaturated polyesters, Reinforced Plast. Symp. 58 (4) (1966) 53
58. [5] T. Watanabe, M. Sato, W. Koga, Process for manufacturing unsaturated polyester Resins, U Patent 3901953; 19, 1975. [6] A. Ahamad, C.M. Lubi, A. Mohan, M. Safeer, E.T. Thachil, Synthesis of unsaturated polyester resin— effect of anhydride composition, Des. Monomers Polym. 4 (3) (2001) 261
268. [7] B. Cherian, E.T. Thachil, Synthesis of unsaturated polyester resin—effect of choice of reactants and their relative proportions, Int. J. Polym. Mater. Polym. Biomater. 53 (2004) 829
845. [8] C.S.M.F. Costa, A.C. Fonseca, J. Moniz, M. Godinho, J.F.J. Coelho, A.C. Serra, Going greener: synthesis of fully biobased unsaturated polyesters for styrene crosslinked resins with enhanced thermomechanical properties, Express Polym. Lett. 11 (2017) 885
898. [9] K.H. Yoon, A.T. DiBenedetto, S.J. Huang, Recycling of unsaturated polyester resin using propylene glycol, Polymer 38 (9) (1997) 2281
2285. [10] N.M. Nurazzi, A. Khalina, S.M. Sapuan, A.M.D. Laila, M. Rahmah, Curing behaviour of unsaturated polyester resin and interfacial shear stress of sugar palm fibre, J. Mech. Engg. Sci. 11 (2) (2017) 2650
2664. [11] R.R.P. Kuppusamy, S. Neogi, Influence of curing agents on gelation and exotherm behaviour of an unsaturated polyester resin, Bull. Mater. Sci. 36 (7) (2013) 1217
1224. [12] Z.X. Lin, X.B.X. Fang, W.X. Zhu, Liquid fragility of the unsaturated polyester resin, Sci. China Chem. 56 (6) (2013) 724
728. [13] V. Kosar, Z. Gomzi, Thermal effects of cure reaction for an unsaturated polyester in cylindrical moulds, Chem. Biochem. Eng.; Q 15 (3) (2001) 101
108. [14] X. Cao, L.J. Lee, Control of shrinkage and residual styrene of unsaturated polyester resins cured at low temperatures. I. Effect of curing agents, Polymer 44 (2003) 1893
1902. [15] C.P. Hsu, L.J. Lee, Free-radical crosslinking copolymerization of styrene/unsaturated polyester resins: 3. Kinetics-gelation mechanism, Polymer. 34 (21) (1993) 4516
4523. [16] E.M.S. Sanchez, C.A.C. Zavaglia, M.I. Felisberti, Unsaturated polyester resins: influence of the styrene concentration on the miscibility and mechanical properties, Polymer 41 (2000) 765
769. [17] R. Poillucci, C. Hansen, Reducing use of styrene monomers in unsaturated polyester Resins, in: 19th International Conference on Composite Materials. [18] S. Fiori, A. Mariani, S. Bidali, G. Malucelli, Synthesis and characterization of unsaturated polyester resins cured by frontal polymerization. e-Polymers No. 001, 2004. [19] S. Liu, J.R. Chen, Y. Zhang, End-cap modified unsaturated polyesters with low styrene emission, IOP Conf. Ser. Mater. Sci. Engg. 359 (2018) 1
4. [20] J.K. Fink, Unsaturated polyester resins, Justia Patent 4387171, 2013, 1981. [21] A. Perez, Styrene-free unsaturated polyester resin composition, European Patent Specification, EP 2058343 B, 2011. [22] J. Meixner, H. Clemens, A.Sickert, Process for the production of coatings based on unsaturated polyester resins by hardening under infra-red radiation, US Patent (19)5068125, 1991. [23] J. Meixner, W. Fischer, M.Muller, Radically curable coating compositions and their use for coating wood substrates, US Patent (19) 5470897, 1994. [24] J.E. McAlvin, D.A. Oakley, P.T. Hutson, D.J. Zwissker, T.J. Folda, Styrene-free unsaturated polyester resin compositions for coating applications, US Patent 7326752B2, 2008. [25] A. Perez, Styrene-free unsaturated polyester resin composition, European Patent Specification EP 2215136 B1, 2008. [26] J.E. McAlvin, Green composites through the use of styrene-free resins and unsaturated polyesters derived from renewable and recycled raw materials, Composites (2011) 1
10. [27] G. Rokicki, H. Wodzicki, Waterborne unsaturated polyester resins, Macromol. Mater. Eng. 278 (2000) 17
22.

REFERENCES

35

[28] Y.Y. Chiu, R. Saito, L.J. Lee, Modification of unsaturated polyester resins for viscosity control, Polymer 37 (11) (1996) 2179
2190. [29] E.L. Rodriguez, On the thickening mechanism of unsaturated polyesters by alkaline earth oxides and hydroxides, J. Appl. Polym. Sci. 40 (1990) 1847
1855. [30] D.E. Fjare, Reinforced unsaturated polyester resin compositions, US Patent 2003/0220035 A1, 2003. [31] G. Allen, J.C. Bevington, Comprehensive Polymer Science, vol. 7, Pergamon Press, Oxford, 1989. [32] A.B. Cherian, E.T. Thachil, Toughening studies of an unsaturated polyester resin using maleated elastomers, Prog. Rubber Plastics Technol. 17 (4) (2001) 205
224. [33] A.M. Hameed, Effect of water absorption on some mechanical properties of unsaturated polyester resin/ natural rubber blends, Jordan J. Phys. 5 (3) (2012) 119
127. [34] A.B. Cherian, E.T. Thachil, Blends of unsaturated polyester resin with functional elastomers, J. Elastomers Plastics 35 (2003) 367
380. [35] C.K. Riew, Rubber-toughened plastics, Adv. Chem. Am. Chem. Soc. 222 (1989) 303. [36] S.F. Hisham, I. Ahmad, R. Daik, A. Ramli, Blends of LNR with unsaturated polyester resin from recycled PET: comparison of mechanical properties and morphological analysis with the optimum blend by commercial resin, Sains Malaysiana 40 (7) (2011) 729
735. [37] K.A.R. Jimenez, A.R. Jimenez, M.A. Morantes, Synthesis and characterization of proton exchange membranes from blend of unsaturated polyester resin and natural rubber, Int. J. Eng. Trends Tech. 4 (9) (2013) 4005
4009. [38] P. Pachpinyo, P. Lertprasertpong, S. Chuayjuljit, R. Sirisook, V. Pimpan, Preliminary study on preparation of unsaturated polyester resin/natural rubber latex blends in the presence of dispersion aids, J. Appl. Polym. Sci. 101 (2006) 4238
4241. [39] E.G. Daway, Study tensile strength and wear rate for unsaturated polyester resin and nitrile butadiene rubber polymer blend, Al-Khwarizmi Engg. J. 11 (2) (2015) 45
50. [40] A. Mousa, G. Heinrich, U. Wagenknecht, Toughened unsaturated polyester composites reinforced with slag material, Polym. Plastics Technol. Eng. 56 (2017) 1657
1664. [41] L. Suspene, J.F. Gerard, J.P. Pascault, Morphology of unsaturated polyester resin additive blends, Polym. Eng. Sci. 30 (24) (1990) 1585
1590. [42] M.M. Mahadar, A. Hassan, N.Y.M. Yunus, H.P.S. Abdul Khalil, M.H.M. Kassim, Thermal properties and mechanical performance of unsaturated polyester/phenolic blends reinforced by Kenaf Fiber, Adv. Mate. Res. 1134 (2016) 61
65. [43] B.K. Kandola, L. Krishnan, D. Deli, P. Luangtriratana, J.R. Ebdon, Fire and mechanical properties of a novel free-radically cured phenolic resin based on a methacrylate-functional novolac and of its blends with an unsaturated polyester resin, RSC Adv. 5 (2015) 33772
33785. [44] B.K. Kandola, L. Krishnan, D. Deli, J.R. Ebdon, Blends of unsaturated polyester and phenolic resins for application as fire-resistant matrices in fibre-reinforced composites. Part 2: effects of resin structure, compatibility and composition on fire performance, Polymer Degrad. Stab 113 (2015) 154
167. [45] B.K. Kandola, J.R. Ebdon, P. Luangtriratana, L. Krishnan, Novel flame retardant thermoset resin blends derived from a free-radically cured vinylbenzylated phenolic novolac and an unsaturated polyester for marine composites, Polymer Degrad. Stab 127 (2016) 56
64. [46] A.B. Cherian, E.T. Thachil, Epoxidized phenolic novolac: a novel modifier for phenolic unsaturated polyester resin, J. Appl. Polym. Sci. 100 (2006) 457
465. [47] A.B. Cherian, L.A. Varghese, E.T. Thachil, Epoxy-modified, unsaturated polyester hybrid networks, Eur. Polymer J. 43 (2007) 1460
1469. [48] S.N.H. Mustapha, A.R. Rahmat, A. Arsad, S.K. Yeong, Novel biobased resins from blends of functionalized palm oil and unsaturated polyester resin, Mater. Res. Innovations 18 (6) (2014). S6 326
S6 330. [49] D. Patel, Interacting blends of novel acrylated unsaturated polyesters based on DGEBA with styrene monomer, Int. J. Chem. Sci. 8 (2) (2010) 1278
1286.

36

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

[50] H. Ardhyananta, F.D. Puspadewa, S.T. Wicaksono, A.T. Wibisono, B.A. Kurniawan, H. Ismail, et al., Mechanical and thermal properties of unsaturated polyester/vinyl ester blends cured at room temperature, IOP Conf. Ser. Mater. Sci. Eng. 202 (2017) 012088. [51] L.S. Yang, Synergistic blends of unsaturated polyether ester resins and dicyclopentadiene polyester resins, European Patent Application EP 0684284 A1, 1995. [52] C.B. Bucknall, I.K. Partridge, M.J. Phillips, Morphology and properties of thermoset blends made from unsaturated polyester resin, poly(vinyl acetate) and styrene, Polymer 32 (2) (1991) 786
790. [53] F.S. Matty, J.K. Al-Kafaji, D.H. Badri, Preparation and characterization of unsaturated polyester material blended with cellulose and with ethyl cellulose, Eur. Chem. Bull. 2 (7) (2013) 477
480. [54] S.I. Salih, Q.A. Hamad, S.N.A. Jabbar, N.H. Sabit, Effects of irradiation by UV-acceleration on mechanical properties of polymer blends (polyester: starch)., Al-Nahrain J. Eng. Sci. 21 (1) (2018) 147
152. [55] M. Xanthos, C. Wan, Reactive melt modification of polypropylene/unsaturated polyester blends, Components (1982) 1
4. [56] S.K. Kim, J.T. Kim, H.C. Kim, K.Y. Rhee, J. Kathi, Miscibility of solution blends of unsaturated polyester resin with polystyrene and polycarbonate, J. Macromol. Sci. Part B: Phys. 49 (2010) 750
756. [57] H. Zheng, S. Zheng, Q. Guo, Thermosetting polymer blends of unsaturated polyester resin and poly(ethylene oxide). II. hydrogen-bonding interaction, crystallization kinetics, and morphology, J. Polym. Sci. Part A: Polym. Chem 35 (1997) 3169
3179. [58] X. Li, W. Fu, Y. Wang, T. Chen, X. Liu, H. Lin, et al., Solid-state NMR characterization of unsaturated polyester thermoset blends containing PEO-PPO-PEO block copolymers, Polymer 49 (2008) 2886
2897. [59] Q. Guo, H. Zheng, Miscibility and crystallization of thermosetting polymer blends of unsaturated polyester resin and poly(E-caprolactone), Polymer 40 (1999) 637
646. [60] M. Ruffier, G. Merle, J.P. Pascault, H. Boulec¸ane, N. Vincent, The shrinkage compensation of unsaturated polyester resins—polyvinyl acetate blends polymerization proceeds through fractal morphologies: characterization and simulation, J. Mater. Sci. 31 (1996) 4679
4687. [61] C.H. Liu, T.L. Yu, C.C. Chen, Effect of curing temperature on the morphology of unsaturated polyester resin blended with poly(vinyl acetate), Polym. Eng. Sci. 42 (3) (2002) 567
581. [62] J.H. Kim, S.C. Kim, Analysis of reaction injection molding process of polyurethane-unsaturated polyester blends. Part I: computer simulation, Polym. Eng. Sci. 27 (16) (1987) 1243
1251. [63] R. Mezzenga, B. Pettersson, J.A.E. Manson, Thermodynamic evolution of unsaturated polyester-styrenehyperbranched polymers, Polym. Bull. 46 (2001) 419
426. [64] H.T. Chiu, R.E. Jeng, J.S. Chung, Thermal cure behavior of unsaturated polyester/phenol blends, J. Appl. Polym. Sci. 91 (2004) 1041
1058. [65] P. Krol, J. Wojturska, Wspo´łprzenikajace sie sieci polimerowe z poliuretanu i poliestru nienasyconego, Polimery/Polymers 47 (4) (2002) 242
249. [66] Y.C. Chou, L.J. Lee, Mechanical properties of polyurethane-unsaturated polyester interpenetrating polymer networks, Polym. Eng. Sci. 35 (12) (1995) 976
988. [67] D. Tang, J. Zhang, D. Zhou, L. Zhao, Influence of BaTiO3 on damping and dielectric properties of filled polyurethane/unsaturated polyester resin interpenetrating polymer networks, J. Mater. Sci. 40 (2005) 3339
3345. [68] B. Das, T. Gangopadhyay, S. Sinha, Studies on polyester-poly (ethyl acrylate-co-styrene) interpenetrating polymer networks, Eur. Polym. J. 30 (2) (1994) 245
249. [69] D. Tang, Z. Jin, W. Cai, W. Di, Morphology and mechanical property of acrylate modified polyurethane/unsaturated polystyrene resin graft-IPNs, ID-1683. [70] A.A. Castillo, V.M. Castano, Semi-interpenetrating polymer networks produced with polyethylene terephthalate oligomer and unsaturated polyester resin, Polymer. Bull 32 (4) (1994) 447
453.

REFERENCES

37

[71] M.S. Lin, C.C. Liu, C.T. Lee, Toughened interpenetrating polymer network materials based on unsaturated polyester and epoxy, J. Appl. Polym. Sci. 72 (1999) 585
592. [72] M. Ivankovic, N. Dzodan, I. Brnardic, H.J. Mencer, DSC study on simultaneous interpenetrating polymer network formation of epoxy resin and unsaturated polyester, J. Appl. Polym. Sci. 83 (2002) 2689
2698. [73] M.S. Lin, R.J. Chang, T. Yang, Y.F. Shih, Kinetic study on simultaneous interpenetrating polymer network formation of epoxy resin and unsaturated polyester, J. Appl. Polym. Sci. 55 (1995) 1607
1617. [74] Y.F. Shih, R.J. Jeng, Carbon black containing IPNs based on unsaturated polyester/epoxy. I. Dynamic mechanical properties, thermal analysis, and morphology, J. Appl. Polym. Sci. 86 (2002) 1904
1910. [75] Y.F. Shih, R.J. Jeng, IPNs based on unsaturated polyester/epoxy: IV. Investigation on hydrogen bonding, compatability and interaction behavior, Polym. Int. 53 (2004) 1892
1898. [76] Y.F. Shih, R.J. Jeng, Thermal degradation behaviour and kinetic analysis of unsaturated polyester-based composites and IPNs by conventional and modulated thermogravimetric analysis, Polym. Degrad. Stab. 91 (2006) 823
831. [77] Z.G. Shaker, R.M. Browne, H.A. Stretz, P.E. Cassidy, M.T. Blanda, Epoxy-toughened, unsaturated polyester interpenetrating networks, J. Appl. Polym. Sci. 84 (2002) 2283
2286. [78] J.A. Pandit, A.A. Athawale, Epoxy-polyester IPNs modified with aromatic amines, J. Appl. Polym. Sci. 125 (2012) 836
843. [79] D. Deli, B.K. Kandola, J.R. Ebdon, L. Krishnan, Blends of unsaturated polyester and phenolic resins for application as fire-resistant matrices in fibre-reinforced composites. Part 1: identifying compatible, cocurable resin mixtures, J. Mater. Sci. 48 (2013) 6929
6942. [80] R.G. Avendano, A.A. Castillo, C. Vazquez, A. Del Real, V.M. Castano, Unsaturated polyester resin modified with nylon 66 oligomers, Int. J. Polym. Mater. 35 (1997) 21
27. [81] M.S. El Wazery, M.I. El Elamy, S.H. Zoalfakar, Mechanical properties of glass reinforced polyester composites, Int. J. Appl. Sci. Eng. 14 (3) (2017) 121
131. [82] S. Belaid, S.F. Chabira, P. Balland, M. Sebaa, S. Belhouideg, Thermal aging effect on the mechanical properties of polyester fiberglass composites, J. Mater. Environ. Sci. 6 (10) (2015) 2795
2803. [83] A.G. Pedroso, D.S. Rosa, T.D.Z. Atvars, Manufacture of sheets using post-consumer unsaturated polyester resin/glass fibre composites, Prog. Rubber, Plastics Recycl. Technol 18 (2) (2002) 111
125. [84] H. Gu, Behaviours of glass fibre/unsaturated polyester composites under seawater environment, Mater. Des. 30 (2009) 1337
1340. [85] M.H. Al-Maamori, R.M.A. Alradha, A.I. Al-Mosawi, Mechanical properties for unsaturated polyester reinforcement by glass fiber using ultrasonic technique, Int. J. Multidiscip. Curr. Res. (2013) 1
7. [86] J.M. Ferreira, O.A.Z. Errajhi, M.O.W. Richardson, Thermogravimetric analysis of aluminised E-glass fibre reinforced unsaturated polyester composites, Polym. Test. 25 (2006) 1091
1094. [87] H.Q. Ali, M.A. Raza, A. Westwood, F.A. Ghauri, H. Asgar, Development and mechanical characterization of composites based on unsaturated polyester reinforced with maleated high oleic sunflower oiltreated cellulose fiber, Polymer Comp. 2018 (2018) 1
8. [88] A.M. Issam, M.N. Shilla, H.P.S. Abdul Khalil, Effect of methylene spacers of unsaturated polyester resins on the properties of composites based on oil palm empty fruit bunches and fiberglass, Adv. Composite Mater 20 (2011) 1
12. [89] I.M. Dagwa, J.O. Ohaeri, Property evaluation of hybrid OPEBF/banana/glass fiber reinforced unsaturated polyester composites, Am. J. Mater. Sci. Eng 2 (4) (2014) 45
53. [90] S.S. Moorthy, K. Manonmani, Fabrication and characterization of TiO2 particulate filled glass fiber reinforced polymer composite, Mater. Phys. Mech 18 (2013) 28
34. [91] Y. Xu, W. Gong, Y. Su, R. Qi, M. Huang, Polyisobutylene-based modifiers for glass fiber reinforced unsaturated polyesters composites, Iran Polym. J. 21 (2012) 91
97.

38

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

[92] J. Correia, S. Cabral-Fonseca, F.A. Branco, J.G. Ferreira, M.I. Eusebio, M.P. Rodrigues, Durability of glass fibre reinforced polyester (GFRP) pultruded profiles used in civil engineering applications, Third. Int. Conf. Compos. Constr. (2005) 1
9. [93] C.M. Vu, D.D. Nguyen, L.H. Sinh, H.J. Choi, T.D. Pham, Micro-fibril cellulose as a filler for glass fiber reinforced unsaturated polyester composites: fabrication and mechanical characteristics, Macromol. Res. 26 (2018) 54
60. [94] D. Jiang, L. Xing, L. Liu, X. Yan, J. Guo, X. Zhang, et al., Interfacially reinforced unsaturated polyester composites by chemically grafting different functional POSS onto carbon fibers, J. Mater. Chem. A 2 (2014) 18293
18303. [95] E.A. Osman, A. Vakhguelt, I. Sbarski, S. Mutasher, Mechanical properties of Kenaf-unsaturated polyester composites: effect of fiber treatment and fiber length, Adv. Mater. Res. 311
313 (2011) 260
271. [96] E.A. Osman, A. Vakhguelt, I. Sbarski, S.A. Mutasher, Kenaf/recycled jute natural fibers unsaturated polyester composites: water absorption/dimensional stability and mechanical properties, Int. J. Comput. Mater. Sci. Eng. 1 (1) (2012). 1250010-1
1250011-17. [97] V. Vilay, M. Mariatti, R. Mat Taib, M. Todo, Effect of fiber surface treatment and fiber loading on the properties of bagasse fiber-reinforced unsaturated polyester composites, Compos. Sci. Technol. 68 (2008) 631
638. [98] D. Cho, S.B. Yoon, T. Drzal, Cellulose-based natural fiber topography and the interfacial shear strength of henequen/unsaturated polyester composites: influence of water and alkali treatments, Compos. Interfaces 16 (2009) 769
779. [99] T.U. Onuegbu, E.T. Umoh, N.C. Okoroh, Tensile behaviour and hardness of coconut Fibre-ortho unsaturated polyester composites, Global J. Sci. Front. Res. Chem. 13 (2013) 1
6. [100] H.H. Zhang, Y. Cui, Z. Zhang, Modified wood fiber enhances reinforced unsaturated polyester composites, Soc. Plastic Eng. Plast. Res. (2013) 1
2. [101] N.E. Marcovich, M.A.M. Reboredo, M.I. Aranguren, Mechanical properties of wood flour unsaturated polyester composites, J.Appl. Poly. Sci. 70 (1998) 2121
2131. [102] I.O. Oladel, Effect of bagasse fibre reinforcement on the mechanical properties of polyester composites, J. Assoc. Prof. Eng. Trinidad. Tobago. 42 (1) (2014) 12
15. [103] A. Benyahia, A. Merrouche, Z.E.A. Rahmouni, M. Rokbi, W. Serge, Z. Kouadri, Study of the alkali treatment effect on the mechanical behavior of the composite unsaturated polyester-Alfa fibers, Mech. Ind. 15 (2014) 69
73. [104] A. Bessadok, S. Roudesli, S. Marais, N. Follain, L. Lebrun, Alfa fibres for unsaturated polyester composites reinforcement: effects of chemical treatments on mechanical and permeation properties, Comp. Part A 40 (2009) 184
195. [105] R.B. Patel, K.S. Patel, R.N. Patel, K.D. Patel, Thermal and mechanical properties of modified polyester resin and jute composite, Der. Chem. Sin 5 (1) (2014) 47
54. [106] B.D. Saravana, K.G.C. Mohan, Thermal properties of maize fiber reinforced unsaturated polyester resin composites, in: Proceedings of the World Congress on Engineering, 2013. [107] J. Narsaiah, K.V.S. Phani, Tensile and flexural properties of cat-tail fiber reinforced unsaturated polyester composite, Int. J. Eng. Adv. Res. Tech. 2 (8) (2016) 28
32. [108] D. Rouison, M. Couturier, M. Sain, B. MacMillan, B.J. Balcom, Water absorption of hemp fiber/unsaturated polyester composites, Polym. Comp. 26 (2005) 509
525. [109] H.N. Dhakal, Z.Y. Zhang, M.O.W. Richardson, Effect of water absorption on the mechanical properties of hemp fiber reinforced unsaturated polyester composites, Comp. Sci. Tech. (2006) 1
10. [110] V. Balaji, K.S. Vadivn, Mechanical characterization of coir fiber and cotton fiber reinforced unsaturated polyester composites for packaging applications, J. Appl. Packag. Res. 9 (2) (2017) 12
19. [111] L. Ghali, S. Msahli, M. Zidi, F. Sakli, Effects of fiber weight ratio, structure and fiber modification onto flexural properties of luffa-polyester composites, Adv. Mater. Phys. Chem. 1 (2011) 78
85.

REFERENCES

39

[112] K.S. Kiew, S. Hamdan, M.D.R. Rahman, Comparative study of dielectric properties of chicken feather/ kenaf fiber reinforced unsaturated polyester composites, Bio Resour. 8 (2) (2013) 1591
1603. [113] A.A. Salisu, M.Y. Yakasai, K.M. Aujara, Physico-mechanical properties of chemically modified sisal fibre reinforced unsaturated polyester composites, Int. J. Chem. Mol. Eng. 10 (1) (2016) 81
88. [114] R. Qiu, W. Liu, K. Li, Investigation of bamboo pulp fiber-reinforced unsaturated polyester composites, Holzforschung 69 (8) (2015) 967
974. [115] M.A. Sawpan, K.L. Pickering, A. Fernyhough, Analysis of mechanical properties of hemp fibre reinforced unsaturated polyester composites, J. Compos. Mater. 47 (12) (2013) 1513
1525. [116] M.H. Mohammed, B. Dauda, Unsaturated polyester resin reinforced with chemically modified natural fibre, IOSR J. Polym. Text. Eng. 1 (4) (2014) 31
38. [117] A.V. Ratna Prasad, K. Mohana Rao, G. Nagasrinivasulub, Mechanical properties of banana empty fruit bunch fibre reinforced polyester composites, Ind. J. Fibre Text. Res. 34 (2009) 162
167. [118] F. Gouanve, M. Meyer, J. Grenet, S. Marais, F. Poncin Epaillard, J.M. Saiter, Unsaturated polyester resin (UPR) reinforced with flax fibers, untreated and cold He plasma-treated: thermal, mechanical and DMA studies, Composite. Interfaces; 13 (4
6) (2006) 355
364. [119] S. Marais, F. Gouanve, A. Bonnesoeur, J. Grenet, F. Poncin-Epaillard, C. Morvan, et al., Unsaturated polyester composites reinforced with flax fibers: effect of cold plasma and autoclave treatments on mechanical and permeation properties, Composites: Part A 36 (2005) 975
986. [120] K. Karakus, I. Atar, I.H. Basboga, F. Bozkurt, F. Mengeloglu, Wood ash and microcrystalline cellulose (MCC) filled unsaturated polyester composites, Kastamonu universitesi Orman Faku¨ltesi Derg 17 (2) (2017) 282
289. [121] M.A. Shenoy, D.J. D’Melo, Evaluation of properties of unsaturated polyester/acetylated hydroxypropyl guar gum composites, eXPRESS Polym. Lett. 1 (9) (2007) 622
628. [122] S.O. Adeosun, E.I. Akpan, H.A. Akanegbu, Thermo-mechanical properties of unsaturated polyester reinforced with coconut and snail shells, Int. J. Compos. Mater. 5 (3) (2015) 52
64. [123] H. Salmah, M. Marliza, P.L. Teh, Treated coconut shell reinforced unsaturated polyester composites, Int. J. Eng. Technol. 13 (2) (2013) 94
103. [124] M.C. Ifeyinwa, B. Babatunde, U. Reginald, N.J. Nneka, Comparison of the mechanical properties of charcoal unsaturated polyester matrix composite and snail shell unsaturated polyester matrix composite, Int. J. Sci. Eng. Res. 5 (11) (2014) 208
213. [125] C.I. Madueke, B. Bolasodun, R. Umunakwe, Mechanical properties of tere-phthalic unsaturated polyester resin reinforced with varying weight fractions of particulate snail shell, IOSR J. Polym. Text. Eng. 1 (4) (2014) 39
44. [126] A.A. Odusanya, B. Bolasodun, C.I. Madueke, Property evaluation of sea shell/snail shell filler reinforced unsaturated polyester composite in comparison with seashell and snail shell filler reinforced unsaturated polyester composite, Int. J. Sci. Eng. Res. 3 (12) (2014) 80
90. [127] S. Zahi, Enhanced properties of fly ash-reinforced unsaturated polyester composites, Int. J. Mater. Eng. Innov. 4 (3/4) (2013) 241
257. [128] Z. Wenjie, H.U. Qiuxue, Z. Niaona, D. Chenjun, Z. Qi, X. Wei, et al., Moisture absorption and mechanical property of modified linen/unsaturated polyester composites, Acta Materiae Compositae Sinica 35 (1) (2018) 35
43. [129] O.A.Z. Errajhi, J.R.F. Osborne, M.O.W. Richardson, H.N. Dhakal, Water absorption characteristics of aluminised E-glass fibre reinforced unsaturated polyester composites, Compos. Struct. 71 (2005) 333
336. [130] M.M. Hossain, A.H.M.F. Elahi, S. Afrin, M.I. Mahmud, H.M. Cho, M.A. Khan, Thermal aging of unsaturated polyester composite reinforced with e-glass nonwoven mat, Autex Res. J. (2016) 1
6. [131] C.H. Chen, K.C. Lien, Pultruded hybrid fibre (glass/carbon) reinforced unsaturated polyester composites: mechanical and thermal properties, Polym. Polym. Compos. 14 (2) (2006) 155
164.

40

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

[132] K. Yaman, O. Taga, Thermal and electrical conductivity of unsaturated polyester resin filled with copper filler composites, Int. J. Polym. Sci. (2018) 1
10. Article ID 8190190. [133] A. Lect, A.F. Abbas, A. Lect, N.A. Betti, R.U. Abbas, Study thermomechanical properties of unsaturated polyester composite reinforced by ceramic particles (Al2O3), Int. J. Innov. Sci. Eng. Technol. 2 (6) (2015) 150
152. [134] M. Oleksy, H. Galina, Unsaturated polyester resin composites containing bentonites modified with silsesquioxanes, Ind. Eng. Chem. Res. 52 (2013) 6713
6721. [135] I. Osiki, T. Thamae, Moisture absorption of composites made from unsaturated polyester filled with pulverized sandstone, diorite and cornstalkm, Zimbabwe J. Sci. Tech. 10 (2015) 1
10. [136] K. Trinath, G. Ramanjaneyulu, Mechanical characteristics of micro and nano silica, ZnO and chitin powder filled unsaturated polyester composites, Indian J. Sci. Technol. 9 (S1) (2016) 1
5. [137] R. Mohamed, S.A. Syed Mustafa, M.N. Norizan, L.S. Amerudin, Unsaturated polyester/expanded polystyrene composite: thermal characteristics and flame retardancy effects, IOP Conf. Ser. Mater. Sci. Eng. 223 (012035) (2017) 1
8. [138] M. Bastiurea, M.S. Rodeanu, D. Dima, M. Murarescu, G. Andrei, Thermal and mechanical properties of polyester composites with graphene oxide and graphite, Digest J. Nanomater. Biostructures 10 (2) (2015) 521
533. [139] N. Prabu, M. Ahmed, S. Guhanathan, Studies on wood carbons/unsaturated polyester composites, MSAIJ 11 (3) (2014) 113
119. [140] A. Adeodu, C. Anyaeche, O. Oluwole, S. Afolabi, Modeling of conventional autoclave curing of unsaturated polyester based composite materials as production process guide, Int. J. Mater. Sci. Appl. 4 (3) (2015) 203
208. [141] R. Girase, R. Jaiswal, L. Chaudhari, S. Bhattacharya, D. D’Melo, Studies on unsaturated polyester composites for high-temperature applications, J. Vinyl Additive Technol. 18 (2012) 46
51. [142] L.M. dos Santos, C.L.P. Carone, S.M.O. Einloft, R.A. Ligabue, Preparation and properties of aromatic polyester/TiO2 nanocomposites from polyethylene terephthalate, Mater. Res. 19 (1) (2016) 158
166. [143] R. Baskaran, M. Sarojadevi, C.T. Vijayakumar, Unsaturated polyester nanocomposites filled with nano alumina, J. Mater. Sci. 46 (2011) 4864
4871. [144] R.A. Sharma, S. Swain, L. Chaudhari, S. Bhattacharya, Effect of coupling agent on PD resistivity of unsaturated polyester-alumina nano-composites, 10th Int. Conf. Prop. Appl. Dielectr. Mater.; IEEE (2012) 1
5. [145] M.C.S. Ribeiro, S.P.B. Sousa, P.R.O. No´voa, C.M. Pereira, A.J.M. Ferreira, Fire retardancy enhancement of unsaturated olyester polymer resin filled with nano and micro particulate oxide additives, IOP Conf. Ser. Mater. Sci. Eng. 58 (2014) 1
9. [146] H. Chen, X. Tian, J. Liu, Unsaturated polyester resin nanocomposites containing ZnO modified with oleic acid activated by N,N0 -carbonyldiimidazole, Polymers (Basel) 10 (362) (2018) 1
4. [147] G.N. Kumar, Y.V.M. Reddy, K.H. Reddy, Mechanical properties of nanoiron particles reinforced epoxy/polyester nanocomposites, IJMET 8 (2017) 175
184. [148] J.D. Rusmirovic, K.T. Trifkovic, B. Bugarski, V.B. Pavlovic, J. Dzunuzovic, M. Tomic, High performance unsaturated polyester based nanocomposites: effect of vinyl modified nanosilica on mechanical properties, eXPRESS Polym. Lett. 10 (2) (2016) 139
159. [149] D. Sequeira, J. Mascarenhas, D. Picardo, R. Dias, O. Sutari, Mechanical behaviour of fumed silica/glass reinforced polyester nanocomposites, Am. J. Mater. Sci. 5 (3C) (2015) 92
95. [150] M. Cakir, R. Simsek, A. Alparslan Celik, Effect of surface modification of fumed silica on mechanical properties of unsaturated polyester composites, Asian J. Chem. 27 (11) (2015) 4120
4124. [151] R. Baskaran, M. Sarojadevi, C.T. Vijayakumar, Mechanical and thermal properties of unsaturated polyester-silica nanocomposites, Nano Sci. Nano Technol. Indian J. 4 (1) (2010) 47
55.

REFERENCES

41

[152] C. Zilg, R. Mulhaupt, J. Finter, Morphology and toughness/stiffness balance of nanocomposites based upon anhydride-cured epoxy resins and layered silicates, Macromol.Chem. Phys. 200 (1999) 661
670. [153] S. Jothibasu, A. Chandramohan, A.A. Kumar, M. Alagar, Polyhedral oligomeric silsesquioxane (POSS) reinforced-unsaturated polyester hybrid nanocomposites: thermal, thermomechanical and morphological properties, J. Macromol. Sci. Part A: Pure Appl. Chem 55 (5) (2018) 433
439. [154] R. Baskaran, M. Sarojadevi, C.T. Vijayakumar, Mechanical and thermal properties of unsaturated polyester/calcium carbonate nanocomposites, J. Rein. Plast. Compos. 30 (2011) 1549
1556. [155] P. Jawahar, M. Balasubramanian, Preparation and properties of polyester-based nanocomposite gel coat system, J. Nanomater. (2006) 1
7. Article ID 21656. [156] S. Katoch, V. Sharma, P.P. Kundu, Synthesis and characterization of saturated polyester and nanocomposites derived from glycolyzed PET waste with varied compositions, Bull. Mater. Sci. 36 (2) (2013) 277
286. [157] A.H. Saputra, A. Arfiana, Synthesis and characterisation of unsaturated polyester resin/aluminium hydroxide/magnesium hydroxide fire retardant composite, Chem. Eng. Trans. 56 (2017) 1759
1764. [158] C.M.C. Pereira, M. Herrero, F.M. Labajos, A.T. Marques, V. Rives, Preparation and properties of new flame retardant unsaturated polyester nanocomposites based on layered double hydroxides, Polym. Degrad. Stab. 94 (2009) 939
946. [159] D.J. Suh, Y.T. Lim, O.O. Park, The property and formation mechanism of unsaturated polyesterlayered silicate nanocomposite depending on the fabrication methods, Polymer 41 (2000) 8557
8563. [160] S. Sen, H.B. Gundem, B. Ortac, Property enhancement in unsaturated polyester nanocomposites by using a reactive intercalant for clay modification, J. Appl. Polym. Sci. 129 (2013) 3247
3254. [161] X. Kornmann, L.A. Berglund, J. Sterte, E.P. Giannelis, Nanocomposites based on montmorillonite and unsaturated polyester, Polym. Eng. Sci. 38 (1) (1998) 1351
1358. [162] A. Hassan, L.Y. Hau, M. Hasan, Effect of ammonium polyphosphate on flame retardancy, thermal stability, and mechanical properties of unsaturated polyester/phenolic/montmorillonite nanocomposites, Adv. Polym. Technol. 36 (2017) 278
283. [163] F. Laatar, M. Gomez, M.R.B. Romdhane, E. Srasra, Preparation of nanocomposite by adding of Tunisian nanoclay in unsaturated polyester matrix: mechanical and thermal study, J. Polym. Res. 23 (239) (2016) 1
7. [164] D. Romanzini, A. Frache, A.J. Zattera, S.C. Amico, Effect of clay silylation on curing and mechanical and thermal properties of unsaturated polyester/montmorillonite nanocomposites, J. Phys. Chem. Solids. 87 (2015) 9
15. [165] Y.J.V. Ruban, S.G. Mon, D.V. Roy, Chemical resistance/thermal and mechanical properties of unsaturated polyester-based nanocomposites, Appl. Nanosci. 4 (2014) 233
240. [166] S. Katoch, V. Sharma, P.P. Kundu, Swelling kinetics of unsaturated polyester and their montmorillonite filled nanocomposite synthesized from glycolyzed PET, J. Basic. Principles Diff. Theo. Expt. Applicat. 15 (4) (2011) 1
28. [167] N. Johnson, P.R. Varma, M. George, K.E. George, Upgradation of unsaturated polyester resin using nanoclays and the effect of process variables on mechanical properties of polyester/clay nanocomposites, Int. J. Mech. Ind. Eng.; (IJMIE) 3 (2) (2013) 74
79. [168] T.D. Thanh, N.D. Mao, N.T.K. Ngan, H.T.C. Nhan, H.T. Huy, A.C. Grillet, Study structure and properties of nanocomposite material based on unsaturated polyester with clay modified by poly(ethylene oxide), J. Nanomater. (2012) 1
5. Article ID 841813. [169] N.N. Bonnia, A.A. Redzuan, N.S. Shuhaimeen, Mechanical and morphological properties of nano filler polyester composites. MATEC Web Conference 39:01008, 2016, pp. 1
5. [170] Kusmono, Z.A.M. Ishak, Effect of clay addition on mechanical properties of unsaturated polyester/glass fiber composites, Int. J. Polym. Sci. (2013) 1
7. Article ID 797109.

42

CHAPTER 1 UNSATURATED POLYESTER RESINS, BLENDS

[171] M.H. Beheshty, M. Vafayan, M. Poorabdollah, Shrinkage control and kinetics behaviour of clayunsaturated polyester nanocomposites, Iran Polym. J. 15 (10) (2006) 841
849. [172] H.N. Dhakal, Z.Y. Zhang, M.O.W. Richardson, Nanoindentation behaviour of layered silicate reinforced unsaturated polyester nanocomposites, Polym. Test. 25 (2006) 846
852. [173] J.N. Asaad, S.L. Abd-El-Messieh, N.E. Ikladious, Unsaturated polyester nanocomposites based on poly (ethylene terephthalate) waste using different types of nanofillers, Proc. J. Nanoeng. Nanosys. 228 (4) (2014) 174
183. [174] M.S. Saharudin, J. Wei, I. Shyha, F. Inam, Flexural properties of halloysite nanotubes-polyester nanocomposites exposed to aggressive environment, Int. J. Mater. Metall. Eng. 11 (4) (2017) 315
319. [175] M.A. Motawie, N.M. Ahmed, S.M. Elmesallamy, E.M. Sadek, N.G. Kandile, Unsaturated polyesters/ layered silicate nanocomposites: synthesis and characterization, IOSR J. Appl. Chem. 7 (10) (2014) 34
43. [176] R. Ollier, E. Rodriguez, V. Alvarez, Unsaturated polyester/bentonite nanocomposites: influence of clay modification on final performance, Compos. Part A 48 (2013) 137
143. [177] C.J. Chirayil, J. Joy, L. Mathew, J. Koetz, S. Thomas, Nanofibril reinforced unsaturated polyester nanocomposites: morphology, mechanical and barrier properties, viscoelastic behavior and polymer chain confinement, Ind. Crops. Prod. 56 (2014) 246
254. [178] C.J. Chirayil, M. Raj, Neenugeorge, L. Mathew, S. Thomas, Diffusion studies of nanofibril reinforced unsaturated polyester nanocomposites, Int. Ref. J. Eng. Sci. 6 (3) (2017) 52
58. [179] H. Kargarzadeh, R.M. Sheltami, I. Ahmad, I. Abdullah, A. Dufresne, Cellulose nanocrystal: a promising toughening agent for unsaturated polyester nanocomposite, Polymer 56 (2015) 346
357. [180] M. Murarescua, D. Dima, G. Andrei, A. Circiumaru, Synthesis of polyester composites with functionalized carbon nanotubes by oxidative reactions and chemical deposition, Digest J. Nanomater. Biostruct. 9 (2) (2014) 653
665. [181] M. Murarescu, D. Dumitru, G. Andrei, A. Circiumaru, Influence of MWCNT dispersion on electric properties of nanocomposites with polyester matrix., Int. DAAM Symp. 22 (1) (2011) 925
926. [182] A.T. Seyhan, F.H. Gojny, M. Tanoglu, K. Schulte, Critical aspects related to processing of carbon nanotube/unsaturated thermoset polyester nanocomposites, European Polym. J. 43 (2007) 374
379. [183] S. He, N.D. Petkovich, K. Liu, Y. Qian, C.W. Macosko, A. Stein, Unsaturated polyester resin toughening with very low loadings of GO derivatives, Polymer. 110 (2017) 149
157. [184] A.K.M.M. Alam, M.D.H. Beg, R.M. Yunus, Influence of carbon nano tubes on the thermo-mechanical properties of unsaturated polyester nanocomposite, IOP Conf. Ser. Mater. Sci. Eng. 78 (2015) 1
6. 012023. [185] E.E. Urena Benavides, M.J. Kayatin, V.A. Davis, Dispersion and rheology of multiwalled carbon nanotubes in unsaturated polyester resin, Macromolecules. 46 (2013) 1642
1650. [186] U.A. Hanan, S.A. Hassan, M.U. Wahit, R. Yusof, B. Omar, S.K. Jamal, Mechanical properties of recycled glass fibre reinforced nanoclay/unsaturated polyester composites, Int. J. Adv. Appl. Sci. 4 (3) (2017) 1
6. [187] S.A. Al-Bayaty, A.J. Farhan, Thermal decomposition kinetics unsaturated polyester and unsaturated polyester reinforcement by toner carbon nano powder (TCNP) composites, IJAIEM 4 (3) (2015) 139
146.

FURTHER READING B.N. Raju, K. Ramji, V.S.R.K. Prasad, Mechanical properties of glass fiber reinforced polyester ZnO nanocomposites, Mater Today Proc. 2 (2015) 2817
2825.

CHAPTER

UNSATURATED POLYESTER— MACROCOMPOSITES

2 S. Devaraju1 and M. Alagar2

1

Polymer Composites Lab, Division of Chemistry, Department of Science and Humanities, Vignan’s Foundation for Science, Technology and Research (VFSTR), Vadlamudi, India 2Centre of Excellence for Advanced Materials, Manufacturing, Processing and Characterisation (CoExAMMPC), Vignan’s Foundation for Science, Technology and Research (VFSTR), Vadlamudi, India

2.1 INTRODUCTION AND OVERVIEW The manufacturing of polymer composites is a tedious process that involves design and development, processing fabrication, testing, and cost-reduction. The appropriate processing techniques along with a balanced selection of matrix materials and reinforcements, including organic, inorganic, and hybrid materials, fiber weight percentage, fiber length, design, and other factors need to be taken in to consideration. For polymer matrices, either thermoplastic or thermosetting polymers with the appropriate functional additives are used. Fillers with different natures, continuous or discontinuous geometries, and random or oriented structures are considered. Reinforcements are classified into one-dimensional (e.g., roving, yarn), two-dimensional (e.g., mat, woven, and knitted fabric), and three-dimensional (e.g., braid, fabric). Composite materials comprise two or more different materials that are physically or chemically interacted together. Each of the components exhibits their own unique characteristic properties. Composites consist of two phases, namely polymer matrix phase and reinforcement (fillers). The polymer matrix phase is continuous and the reinforcements are either continuous or discontinuous depending on their nature. Reinforcements contribute to the strength of composites and matrices render the reinforcements intact with strong adhesion. Composite materials are considered as vital materials in the modern world due to their combined properties, namely their lightweight, good stiffness, high strength, high temperature stability, resistance to moisture, weather, flame, fire and corrosion, and good dielectric behavior, which are not possible with the individual components. They are widely used as materials in the fabrication/production of aircraft components, packaging for electronic and medical equipment, space crafts, and in construction, etc. [1
15]. Composites are combinations of two or more matrix materials and fillers with varying percentage weights of composition, in which the individual systems (components) retain their distinctive characteristics. These distinct components work together to give the essential properties including good mechanical strength and stiffness to composite systems.

Unsaturated Polyester Resins. DOI: https://doi.org/10.1016/B978-0-12-816129-6.00002-8 © 2019 Elsevier Inc. All rights reserved.

43

44

CHAPTER 2 UNSATURATED POLYESTER—MACROCOMPOSITES

Composites based on matrix

Metal matrix composites

Thermosetting composites

Polymer matrix composites

Ceramic matrix composites

Graphite/carbon matrix composites

Thermoplastic composites

FIGURE 2.1 Classification of composites based on the type of matrix.

2.2 CLASSIFICATION OF POLYMER COMPOSITES Based on the matrix phase, composites can be classified into four types as shown in Fig. 2.1, namely into: 1. 2. 3. 4.

metal matrix composites (MMC) polymer matrix composites (PMC) ceramic matrix composites (CMC) graphite/carbon matrix composites (GCMC)

Composites can also be classified based on the type of reinforcement as shown in Fig. 2.2, mainly including: 1. particulate composites (composed of particles) 2. fibrous composites (composed of fibers) 3. structural composites (composed of sandwich and laminates) Lastly, composites can be classified based-on the size of the reinforcements, namely into: 1. polymer nanocomposites (reinforcement size 1
100 nm) 2. polymer microcomposites (reinforcement size 0.1
10 μm) 3. polymer macrocomposites (reinforcement size .10 μm) The differences between macro- and nanocomposites are discussed in Table 2.1 in detail with suitable examples.

2.3 UNSATURATED POLYESTER RESIN Among the thermosets, unsaturated polyester resin (UPR) is the most adaptable and industrially important with the highest economic potential and was first commercially produced during the

2.3 UNSATURATED POLYESTER RESIN

45

Composites based on reinforcement

Fibrous composites

Thermosetting composites

Particulate composites

Thermoplastic composites

Structural composites

Sandwich composites

Laminate composites

Glass fiber Aramid fiber

Large particle composites

Dispersed particle composites

Polymer fiber Ceramic fiber Boron fiber Natural fiber Graphite fiber

FIGURE 2.2 Classification of composites based on reinforcement.

Table 2.1 Comparison Between Macrocomposites and Nanocomposites Macrocomposites

Nanocomposites

Short or long fibers reinforced in an organic polymer matrix. Fibers: glass, aramid, carbon, polymer, ceramic, boron, aluminum, silica carbide, natural fiber, and others.

Multiphase material comprising a matrix with a dispersed nanosized inorganic phase. Inorganics: nanosized particles/tubes/fibers, clay, silica including porous and nonporous (SBA-15 and MCM-41), POSS, mica, zeolite, carbon materials such as SWCNTs, MWCNTs, GO, porous and nonporous carbon particles, metal, metal oxide, and metal sulfide. The properties are influenced by the size and shape of the inorganic phase (generally in nanometer size between 1 and 100 nm) and polymer particle wetting behavior.

The properties are based on the existence of interfacial strength between the matrix and fiber.

1940s. UPRs are the most widely used thermosetting polymeric resins in the form of coatings and matrices in the composites industry, making up around three fourths of all the thermoset resins used in the industry. UPRs are useful because of their ability to be chemically modified with crosslinkers. UPRs have almost penetrated into all the sectors of the composites industry including for use in adhesives, coatings, sealants, and laminates, etc. The major advantages of these UPRs are

46

CHAPTER 2 UNSATURATED POLYESTER—MACROCOMPOSITES

their useful properties, namely their good mechanical properties, dimensional stability, resistant against chemicals, electricity, and weather, and amenable processing by conventional processing methods including their competitive cost [15
26]. UPRs are prepared by simple esterification reactions using dicarboxylic acid (saturated or unsaturated) or their anhydrides (typically isophthalic acid, phthalic anhydride (PA), and maleic anhydride (PA)) with bifunctional alcohols including mono-ethylene glycol, diethylene glycol, propylene glycol (PG), etc., over a temperature range of 210 C
230 C through polycondensation. Unsaturated group in the polyester skeleton is introduced via dibasic acid or its anhydride (maleic acid or MA). UPRs form extremely durable structures and coatings when cross-linked with a reactive vinylic monomer; styrene can be used as a reactive vinyl monomer. Reactive monomers (styrene or vinylic monomer) act as diluents and help reduce the viscosity of UPR for easy processing, which in turn improves the processing and cross-linking through free-radical polymerization. The blend of UPR and styrene and catalyst are poured into a mold, and then they are allowed to copolymerize them to form a cross-linked three-dimensional rigid network. Products like storage tanks, automobile parts, home appliances, automobile bodies, boat hulls, shower stalls, flooring, translucent panels, corrosion-resistant products, and construction components are produced using UPRs. UPRs are also reinforced with different fillers to produce kitchen counter tops and bathroom accessories through casting methods.

2.3.1 CURING OF UNSATURATED POLYESTER RESIN UPRs are polymerized by free-radical polymerization techniques; a small amount of suitable freeradical initiator is introduced into the UPR and the initiator generates free radicals under adverse conditions such as heat, UV or visible light, catalysts, and/or ionizing radiation. The initiator disassociates to form radicals and then initiates the free-radical polymerization of the UPR. However, for the curing of UPR at low or room temperature, an initiator itself is not possible and accelerators are needed to help activate the initiator at room temperature, which helps the room temperature curing ability of UPR for low temperature fabrication applications. Generally, styrene monomer is copolymerized with unsaturated double bond of UPR this will be influence the length and cross-link density of the final cured product. Cross-linking is essential in the commercial fabrication processing of UP composites. The mechanical properties of UP composites are enhanced through increasing in the cross-link density. Different vinylic monomers exhibit varying chemical structures and they convey their unique personalities to the polymeric backbone when they are copolymerized together. The properties of UPRs are determined by the type of monomer and quantity of monomer used, as well as the reaction conditions adopted for the copolymerization reaction.

2.4 CLASSIFICATION OF UNSATURATED POLYESTER RESIN Unsaturated polyester resins are classified on the basis of their physical and chemical properties as: 1. flexible UPR 2. chemical-resistant UPR 3. specialty UPR

2.4 CLASSIFICATION OF UNSATURATED POLYESTER RESIN

4. 5. 6. 7.

47

general purpose UPR resilient UPR electrical-resistant UPR flame-retardant UPR

UPRs are materials that have a wide range of applications and are constantly evolving with changes and modifications in accordance with the market demands for more technically tailored and environmentally sensitive products. Unsaturated polyesters are also subclassified according to their chemical composition and reactivity, the nature of the raw materials used namely diacids and diols, their properties namely physicochemical, thermal, mechanical, and weather and flame resistance behaviors, their methods of fabrication, and their fields of application.

2.4.1 FLEXIBLE UNSATURATED POLYESTER RESIN Flexible UPRs (Scheme 2.1) are prepared using linear chain dibasic acids such as adipic acid, sebacic acid, tall oil, fatty acids, and maleic acid/anhydride. UPR flexibility can be achieved using linear aliphatic long chain diols including 1,2-ethanediol, 1,4-butanediol, and 1,6-hexanediol, etc., which contribute to enhanced flexibility and high impact resistance at low temperatures. The combination of different glycols with varying chain lengths contributes to an enhanced chemical resistance, hydrolytic stability, and flexibility, and these UPRs are commonly used for decorative applications [27
29]. O O

O +

HO

n

O

OH

+ HO

H2 C

OH CH 3

O

CH 3

O

O

CH 3

*

CH O

C H2

C n H2

C H2

O H C

C CH CH 3

C H

O C

*

O n-4, 8

SCHEME 2.1 Preparation of flexible UPR.

2.4.2 CHEMICAL RESISTANT UNSATURATED POLYESTER RESIN Chemical resistant thermosetting UPRs are prepared using isophthalic acid, MA, and PG with styrene as a reactive diluent [29
31]. This type of UPR is mainly used to increase stability and

48

CHAPTER 2 UNSATURATED POLYESTER—MACROCOMPOSITES

chemical resistance by controlling the chemical structure of the UPR and is particularly intended for corrosion-resistant applications. Further, it also exhibits excellent mechanical properties in addition to better chemical resistance compared to those of other UPRs. Furthermore, the introduction of neopentyl glycol in combination with isophthalic acid can be used to make a high performance UPR with enhanced chemical and corrosion resistance. A neopentyl glycol portion in UPR imparts an improved hydrolytic and weathering stability as well as chemical and water resistance toward intended applications. Chemical resistant UPRs are also used for chemical storage tanks and pipelines, fuel tanks, and in the construction of ships and boats, etc.

2.4.3 SPECIALTY UNSATURATED POLYESTER RESIN UPRs for specialty applications can be obtained using different types of reactive monomers in the backbone of the UPR with the capacity for further chemical reactions and crosslinking, which in turn enhances the properties of the UPR, making it suitable for different applications. For example, UPRs with improved heat resistance and other properties can be prepared using triallyl cyanurate (Scheme 2.2) as a reactive monomer in place of styrene [29,32
34]. O OH O

O

O

+

+

O

H 3C

C H2

OH

O 160–190°C –H2O N

O O O

C H

H C

C O

O

CH

H2 C

H3 C O CH CH2 O

O

N +

O O

C

C

C

CH2 O

O

O

CH

H2 C

CH3

CH O O

O O

CH2 CH2

H2 CH O C CH C CH O O

SCHEME 2.2 Preparation of cross-linked UPR for specialty applications.

CH3

N O

O O

CH3

O

O

2.4 CLASSIFICATION OF UNSATURATED POLYESTER RESIN

49

2.4.4 GENERAL PURPOSE UNSATURATED POLYESTER RESIN General purpose UPR (Scheme 2.3) is prepared based on PG, PA, and MA and the same was reported by Hidenari et al. [35]. In an RB flask, a mixture of PG (1.25 mol), PA (0.5 mol), p-toluene sulfonic acid (0.2%), and xylene was added as a solvent. The reaction mixture was stirred mechanically and heated to 120 C under a nitrogen atmosphere. When the reaction mixture turned into a clear solution, it was allowed to cool to 80 C and then 0.5 mol of MA was added and the reaction was continued at about 200 C until an acid number of about 20 was reached. Water formed as a byproduct was removed continuously by azeotropic distillation. After the reaction, the solvent was completely removed by distillation and the temperature of the crude product was brought to 160 C, at which point a small amount of hydroquinone inhibitor was added to the reaction mixture and when the temperature dropped below the boiling point of styrene, the UPR was mixed with 35 wt.% of styrene [29,36]. The resulting UPR can be processed by hand layup, spray layup, or with casting. UPRs with excellent reinforcement wetting properties can be used for different applications.

O OH O

O

O

+

+

O

H3C

C H2

OH

O 160 –190°C –H2O O O

C H

H C

C

O

O

CH

H2 C

H3C O CH CH2 O

O O O

CH3

O O

C

C

C

CH2 O

O

CH

H2 C

CH3

CH CH2 CH2 CH CH2 O O

HC H2 CH O C CH C CH O O

SCHEME 2.3 Preparation of general purpose unsaturated polyester resin.

CH3

O

+

50

CHAPTER 2 UNSATURATED POLYESTER—MACROCOMPOSITES

General purpose UPRs are used for construction, automotive protective parts, marine vessel components, railway interior products, sanitary wares, pollution control equipment, rehabilitation components, etc.

2.4.5 RESILIENT UNSATURATED POLYESTER RESIN Resilient UPRs are made up of a combination of both rigid general-purpose UPR and flexible UPR. Such UPRs with low acid numbers can be obtained by first reacting isophthalic acid with glycol and MA. These types of resilient UPRs offer the high strength and good resilient properties including good resistance against weather and chemical able to used for a wide range of applications. Resilient UPRs possess good toughness behavior when compared to that of general purpose and other UPRs [29
34].

2.4.6 ELECTRICAL RESISTANT UNSATURATED POLYESTER RESIN Electrical resistant UPRs are prepared by dibasic acids including isophthalic acid, MA with neopentyl glycol, or tetrabromobisphenol-A instead of PA and PG. The introduction of various types of additives such as antimony trioxide, mica, kaolin, and calcium carbonate into UPRs improves the electrical resistant properties of the resulting UPRs [29,37
41] and these are used in electrical and electronic equipment including printed circuit boards (PCBs).

2.4.7 FLAME RESISTANT UNSATURATED POLYESTER RESIN Flame resistant UPRs are prepared using dibasic acids with the halogen functional group such as tetrachlorophthalic anhydride and/or tetrabromophthalic anhydride with dibromoneopentyl glycol or tetrabromobisphenol-A. The flame retardant behavior of UPR can be further enhanced by the introduction of triphenyl phosphate, antimony trioxide, or metal oxides, etc. These types of UPRs find applications in equipment, building panels, and navy boats [29,42
45].

2.5 UNSATURATED POLYESTER COMPOSITES The utility of thermoplastics for structural applications is limited due to their low creep resistance and their thermal stability. Hence, most structural components are fabricated with thermosetting polymers such as UPRs, phenolic resins, epoxy resins, novalac, and vinyl ester resins because of their ability to form 3D cross-linked network structures. UPRs are polymer matrix systems that are widely used in various industries, particularly in marine and construction industries, for example, in yachts and boats. Thermosetting polymer systems are generally low molecular weight monomers and/or prepolymers, which on polymerization produce highly cross-linked polymeric networks through simple heating or by reacting with curatives, catalysts, and or accelerators. Cross-linked neat UPRs have limited strength behaviors; hence, in order to improve their mechanical strength, they are normally strengthened with fibrous reinforcements. Fiberglass-reinforced UPRs are lightweight and have good durability, and are most widely used in building and construction, marine

2.7 CATALYSTS AND ACCELERATORS FOR UNSATURATED POLYESTER

51

components, and land transportation components. The resin matrix plays a major role, namely it binds the reinforcements together, sustains the structure of a component, and transfers the applied load throughout the fibrous reinforcements. The resin shelters the reinforced fibers from decomposition or degradation due to abrasion or weathering. UPR also significantly enhances the mechanical properties of structural composites, resists delaminating between plies of reinforcements, and hinders fiber buckling during compression. In the recent past, different types of composite materials have been developed with varying dimensions of reinforcements; accordingly, they are classified into nanocomposites, microcomposites, and macrocomposites. These composites exhibit superior physical, thermal, mechanical, electrical, flame retardancy, and gas-barrier properties. Among the different types of composites, polymer nanocomposites exhibit significantly improved properties and processing behavior due to their surface behavior and homogeneity.

2.6 REACTIVE DILUENTS For the fabrication of UPR composites, reactive diluents such as styrene, methyl methacrylate, α-methyl styrene, and diallyl phthalate are generally used to enhance their properties and to facilitate the fabrication process of composites for various specialty applications. The most commonly used reactive diluent is styrene, which is used with certain precautions.

2.7 CATALYSTS AND ACCELERATORS FOR UNSATURATED POLYESTER RESIN COMPOSITES The copolymerization of UPR initiated by a free-radical polymerization mechanism using styrene and other vinylic/allylic monomers with the maleate groups of the UP is carried out using a peroxide catalyst, an accelerator, or both a catalyst and an accelerator to lower the curing temperature as well as to reduce the curing time. When a catalyst is present, resin is thermally cured at a slightly higher temperature in the absence of an accelerator, whereas in the presence of an accelerator the UPR can be cured at a low temperature. The most commonly used catalysts (initiator) are peroxides, namely methyl ethyl ketone peroxide, benzoyl peroxide, cyclohexane peroxide, acetyl acetone peroxide, etc. Among them, methylethylketone peroxide is used for low temperature (cold) curing and benzoyl peroxide is used for hot curing (temperatures $ 70 C). During the curing process, an initiator begins to decompose into highly reactive radicals that can initiate a chain reaction. Further accelerators helps to activate the initiator at lower temperature, which result in curing of UP resin at room temperature or lower temperature. Accelerators (or promoters) are always led to form metal salts, namely cobalt naphthenate, cobalt octoate, or cobalt neodecanoate. Cobalt naphthenate and dimethyl aniline are commonly used accelerators for the copolymerization of UPR copolymers with styrene. Cobalt accelerator is added at about 0.01-phr (parts per hundred resin) depending on the nature of the products required. However, a small amount of cobalt accelerator, it significantly influences the mechanical properties of resulting products.

52

CHAPTER 2 UNSATURATED POLYESTER—MACROCOMPOSITES

2.8 ADDITIVES AND INHIBITORS There are a number of additives used to improve the properties of UPRs that become an integral part of UP polymer matrices. These additives include thixotropic agents, which are used for the hand layup and/or spray layup fabrication processes. When thixotropic agents are introduced into UPR the viscosity of the resin is in turn appreciably increased and the affinity of liquid resin to flow or drain from vertical surfaces is reduced. When UPR is subjected to shear, the viscosity of the resin is reduced and it can easily be transferred into a mold. Fumed silica and some clays are commonly used thixotropic agents for UPR. Additives like pigment dispersions and color pastes can be introduced into UPR to improve its weather resistance properties. For make-up coats, finely milled pigments are blended and used with resin at a high shear rate. Several pigments can chemically interact with UPR in the paint industry and the interaction influences the speed of the gel time. Fire retardant additives such as alumina trihydrate, halogen, borate, and phosphorus compounds are incorporated into UPRs with the aim of enhancing the fire resistance properties of the resulting UP composites. UV inhibitors and stabilizers are used to avoid chalking, loss of gloss, crazing, yellowing, variation in electrical properties, embrittlement, and the disintegration of UPRs due to exposure to ultraviolet radiation. UPRs also possess insulation properties like other polymers. Conductive additives like metal, carbon particles, or conductive fibers are incorporated into UPRs in order to make them conductors and to utilize them for different electronic applications including electromagnetic interference shielding and for antistatic coatings, etc. Inhibitors have also been used in UPRs to improve their shelf life, facilitate fast curing, and to avoid the formation of any unwanted colors, odors, and/or other side effects. Hydroquinone, 4,4dihydroxybiphenyl, and catechols with 3-n-alkyl, 3-isopropy1, 3-pheny1, 4-n-alky1, 4-isopropy1, 3,5-dialkyl, and 3,6-dialkyl substitutions are used as effective inhibitors.

2.9 FABRICATION OF FIBER-REINFORCED UNSATURATED POLYESTER COMPOSITES The fabrication of glass fiber
reinforced UPR composites are carried out by (1) direct impregnation and (2) indirect impregnation methods.

2.9.1 DIRECT IMPREGNATION 2.9.1.1 Hand layup fabrication method The hand layup method is one of the most common, easy, and inexpensive open molding methods for the fabrication of UPR composites because it requires simple equipment. In a hand layup approach, first a release gel is applied onto the surface of a mold to avoid the stabbing of polymer. Fiber reinforcements are placed in a mold and UPR is mixed with a suitable curing agent/catalyst/ accelerator and poured onto the surface and then uniformly spread over the mold using a brush or roller. The curing process is carried out at room temperature or the required temperature based on the UPR and the catalyst/accelerator used, later, the mold is opened and the fabricated composite is

2.9 FABRICATION OF FIBER-REINFORCED UNSATURATED

53

taken out and used for further modifications. This process is utilized to make both large and small items such as storage tanks, tubs, and showers.

2.9.1.2 Spray layup fabrication method This method is a semi-automated version of the hand layup method. In this method, a spray gun is used to spray pressurized UPR and filler reinforcement in the form of chopped fibers into a mold for consolidation as defined above for the hand layup fabrication method. The spray layup fabrication method is utilized for the fabrication of low-load carrying parts including fairing of trucks, small boats, bath tubs, etc.

2.9.1.3 Centrifugal casting fabrication method The centrifugal casting fabrication method is one of the most progressive casting techniques and is widely used in many industries for various products. This method is related to the spray layup fabrication method except that the fibers are introduced by spear into a rotating mold to produce a pipe. The rotation offers centrifugal force for good consolidation. In this way, the rigidity of the pipe can be adjusted.

2.9.1.4 Pultrusion fabrication method Pultrusion is a continuous and automatic closed type of molding technique for the processing of composites. The pultrusion processing technique is similar to the extrusion process. The main difference is that the material is pushed through dies in the extrusion method, whereas the material is pulled through dies in the pultrusion method. This technique is used to fabricate thermoset composites with constant cross-section profiles and for the continuous production of polymer composites. In this fabrication process, distribution of fiber and alignment of resin impregnation are good result in uniform products. The reinforcement including continuous rovings or fiber mats is unrolled from basket holding rolls and passes through a UPR tank. In the UPR tank, rovings or fiber mats are dipped to get fully wetted fibers. Then, the resin saturated fibers or rovings are directed to a hot die while the preferred shape is given to the UPR-impregnated fibers or rovings with the help of dies and curing takes place in this section by applying heat and finally the hot cured composite shape is dragged using a gripper. Pultruded-UPR components have shown sufficient mechanical strength and are used for further applications. This process is a low-cost automated system with high quality products. The production rate is high as it is a continuous production process. It is a simple and straight-forward process which does not require a skilled operator. Pultrusion process techniques are used to fabricate products like solid rods, tubing, long flat sheets, channels, angled and flanged beams, and rail covers for subways as well as for high voltage work.

2.9.1.5 Filament winding fabrication method Filament winding is mainly used for the production of open structures like cylinders and/or closedend structures such as pressure vessels and tanks. This process includes winding filaments stiffly over a rotating mandrel. The filament winding technique is a very popular fabrication method used to produce composite parts including axisymmetric parts, pipes, tubes, tanks, cylinders, and spheres, etc. Fiber from continuous fiber rovings gets soaked as it is allowed through a resin bath and soaked resin as it exits from resin bath gets coiled on a mandrel, which constantly rotates on its axis of symmetry. Once the winding process is complete, the filament coil is allowed to cure.

54

CHAPTER 2 UNSATURATED POLYESTER—MACROCOMPOSITES

In distinctive filament winding instrument, the rotational speed, traverse speed of the resin bath, and other process parameters can be controlled the fabrication of polymer composites part. The filament winding fabrication method is used to manufacture composite products like storage tanks, gas cylinders, vessels, pipelines, fishing rods, missile and rocket motor cases, ducting, cement mixtures, boat masts, aircraft fuselages, and shafts for golf clubs are commonly developed with this method. Also, filament winding has expanded into nonspherical and noncylindrical engineering composite products.

2.9.1.6 Resin transfer molding Resin transfer molding (RTM) is a closed-molding fabrication process where UPR is transferred over already-placed reinforcement fibers (glass fiber, carbon fiber, aramid fiber, and natural plant fibers such as sisal, banana, nettle, hemp, and flax) which are then placed on the surface of a mold. A release gel is pasted on the surface of the mold for the easy removal of the fabricated UPR composite. In this technique, resin is infused into a fibrous preform enclosed in a closed mold. The formulated resin composition is pumped into the vented mold where it cold cures before demolding. To improve the impregnation and to reduce voids in the composites, vacuum assistance is often used and this is referred to as vacuum-assisted RTM. The RTM fabrication technique is used for the production of hollow shapes, complex structures, automotive body parts, big containers, and bathtubs, etc.

2.9.2 INDIRECT IMPREGNATION In these fabrication routes, fibers are preimpregnated or compounded with resin by sheet molding compounds (SMCs) and dough/bulk molding compounds (DMCs/BMCs).

2.9.2.1 Sheet molding compounds In this technique, the concept involves ionomer formation with carboxyl terminated polyester and it is used to increase the molecular weight of the polyester with an increase in the viscosity of the resin. The catalyzed UPR is compounded with stearate-coated calcium carbonate as a releasing agent. The resulting resin slurry is deposited onto a polyolefin release and backing films. Then the rolled-up sheet is “aged” for several days until the resin is thickened through the formation of ionomers.

2.9.2.2 Dough or bulk mold compounds DMCs and BMCs refer to dough and bulk molding compounds respectively; these are dough-like thermosets that can be processed by hot-press, compression, or injection molding methods. In contrast to SMC, DMC/BMCs have lower weight contents of random chopped strands between the lengths of 6 and 12 mm. They have a greater degree of flow than that of SMC and can be used for making more complex shaped parts. Both hot-press molding and injection molding are preferred for molding DMCs. Among these injection molding has the advantage of having cycle times of 20
50 seconds.

2.10 MECHANICAL PROPERTIES OF FIBER-REINFORCED

55

2.10 MECHANICAL PROPERTIES OF FIBER-REINFORCED UP MACROCOMPOSITES Fiber-reinforced UP composites dominate in a wide range of industrial applications. Fibrous materials, namely glass, carbon, or natural fibers are reinforced in polyester matrices and they are accordingly called fiber-reinforced polyester (FRP), carbon fiber
reinforced polyester (CFRP) and natural fiber
reinforced polyester (NFRP). Fiber-reinforced composites possess enhanced strength, stiffness, and fatigue resistance and important values for different fiber-reinforced UP composites are presented in Table 2.2. FRP composite components are used in airplanes, electronics components, automotives, rail ways, and sporting equipment. In addition to their desired mechanical properties, their resistance to corrosion and sensitivity to UV light, heat, and moisture make fiberreinforced composites suitable for different industrial, rehabilitation, and recreational applications. Glass fiber
reinforced UP composites have shown important and significant advantages over a long period of time in a broad range of applications due to their high strength, stiffness, and modulus. Zoalfakar et al. [46] developed E-glass fiber
reinforced UP composites by random orientation through hand layup technique with varying weight percentages of fiber from 15, 30, 45, to 60 wt. %. The influence of various weight percentages of glass fiber on the mechanical properties such as tensile strength, flexural strength, and impact strength was studied. An enhancement in the mechanical properties of the UP composites was observed based on the weight percentage of the glass fiber reinforced. The tensile strength values were observed to increase from 28.2 to 78.83 MPa, flexural strength from 44.65 to 119.23 MPa, and impact energy from 3.50 to 6.50 J for the fiber-reinforced UP composites. The hardness value appreciably increased from 31.5 to 47. The 60 wt.% glass fiber
reinforced UP composites achieved the optimum mechanical properties. A polyalkenyl-poly-maleic-anhydride-ester/amide polymer matrix was developed by Miskolczi et al. [47] and it was utilized as a modifier in order to improve the mechanical properties of glass fiber
reinforced UPR composites. Two different reinforcements, namely chopped glass fiber and a glass woven fabric material were utilized for the fabrication of the UP composites. An appreciable change in properties was noticed when the glass-fiber surface was modified with polyalkenyl-polymaleic-anhydride-ester/amide type additives. For example, the improvements in tensile and flexural strength noticed were 38.9% and 21.9%, respectively. Similarly, the tensile and flexural properties of the glass woven [0/90 ] fabric
reinforced composites were improved by 18.0% and 40.1%, respectively, when compared to that of untreated glass fiber
reinforced polyester composites. Cross-linked glass fiber
reinforced UPR composites were developed by Vargha et al. and their thermal and mechanical properties were studied [48]. From DMA measurement, the values of storage modulus was increased with increasing glass fiber content. Effect of glass fiber content on the storage modulus was more significant below the glass transition temperature (Tg). The temperatures detected by DMA analysis corresponding to the storage modulus of 750 MPa were above the Tg and also increased with higher glass fiber content in accordance with the real heat distortion temperature (HDT) measurements. Glass fiber reinforcement contributes to an enhanced HDT behavior in resulting UP-composite materials. A glass fiber
reinforced UPR was prepared by hand layup technique and the nature of the clay and its loading as well as mixing time on the tensile modulus, flammability, and wear resistance were evaluated by Roni Sujarwadi et al. [49] and it was found that dried clay and a higher loading

Table 2.2 Mechanical Properties of Fiber-Reinforced UP Resin Composites Type

Thickness (mm)

GFR-0 GFR-1 GFR-2 GFR-3 GFR-4 GF GV GF 1 CA4 GV 1 CA4 GF GF Clay

5 5 5 5 5

2.9 4.4 10 10

% of Filler

35 6 2 61 6 2 12% 22% 0

Max. Load (N)

Yield Strength (MPa)

Tensile Strength (MPa)

Bending Strength (MPa)

2707.5 3504.3 4827.9 5370.4 7488.9 99.4 197.2 117.4 233.4

10.29 18.63 20.32 21.08 12.54

19.76 28.25 50.82 56.53 78.83

35.33 44.65 83.08 90.55 119.23

Impact Strength (kJ/m2)

Flexural Strength (MPa)

E-Modulus (MPa)

HDT ( C)

46

95 170 135 230

127.1 63.3 154.9 88.7

2282 1864 3066 2846

47

60 . 195 10240 6 517 10290 6 334 11130 6 587

Clay (B/drying)

1

Clay (A/drying)

5

Hemp Hemp (dry) Hemp (dry) Hemp (wet) Hemp (wet) BF BF 1 0.5% NaOH BF 1 A151 WF 1 Talc WF 1 Talc RHF-1 RHF-2 RHF-3 RHF-4 RHF-5

30 30

0 10 26 10 26 90 90

560 720 1270 640 680 1100 1800

0.2 0.2 0.2 0.2 0.2

0 15 0 20 30 40 50

1700 40 32 18.22 32.53 36.26 41.06 46.40

Ref

38 14 28

5510 4200 6490 5760 8050 2800 4000 6000 28 40 82.94 89.70 100.03 109.85 118.67

48 49

50

51

52 53

GF GF 1 JF GF 1 JF (UV) KF KF KF KF 1 GF KF 1 GF KF 1 GF KF 1 GF KF 1 GF KF 1 GF POSS BWPOSS BWPOSS BUPOSS BUPOSS a

Tensile modulus. Young modulus.

b

5 10 15 515 (GF) 10 1 5 (GF) 15 1 5 (GF) 5 1 10 (GF) 10 1 10 (GF) 15 1 10 (GF) 3 3 3 3

58 52 78 32 35 40 57

78 72 102

46 28 42

3500 2800 3800 1150a 1200a 1300a 1300a

60

1300a

65

1350a

70

1400a

78

1450a

75

1400a

61 87 75 82 73

5.6 6 0.9 8.9 6 0.2 7.3 6 0.7 8.0 6 0.5 7.0 6 0.3

3960b 5267b 4600b 5009b 4460b

54

55

15

58

CHAPTER 2 UNSATURATED POLYESTER—MACROCOMPOSITES

increase the modulus. The value of the modulus increased to 11130 6 587 MPa for 5% clayloaded glass fiber
reinforced UPR composites.

2.10.1 MECHANICAL PROPERTIES OF NATURAL FIBER
REINFORCED UP MACROCOMPOSITES Composite materials are generally more expensive than other conventional materials on a weight by weight basis; however, they may be considered as more cost-effective in the long term, on the basis of their lightweight, high strength and modulus, good corrosion resistance, low coefficient thermal expansion, excellent impact behavior, damage tolerance characteristics and the ability to tailor their mechanical, electrical, and thermal properties and performances. Approximately 85% of fiber-reinforced polymer products such as boats, car and aircraft components, and furniture related products are manufactured with the use of UPR-based composites. In spite of these advantages, the widespread use of synthetic fiber-reinforced polymer composites has a tendency to weaken because of their high-initial costs, their use in nonefficient structural forms, and adverse environmental impact. In contrast, the use of renewable natural fibers as reinforcements has become major for polymer composite applications. Accordingly, developments in high-performance engineering materials from renewable and environment-friendly materials are taken up by researchers and are in progress. A substantial increase in the use of agro-based materials has been noticed in the recent past. As a result, composites made from agro-based materials such as coir fiber, coconut pith, jute sticks, ground nut and rice husk, reed, straw wood, wheat, barley, oats, rye, sisal, coir, bamboo, sugarcane, ramie, grass, reeds, banana, and papyrus have become one of the main targets of researchers. Natural fibers have many significant advantages over synthetic fibers. The main advantages of natural lignocellulose fiber reinforcement in polymer composite materials are its biodegradability, abundant availability, easy decomposability in the environment, and that it is environment-friendly. Hemp fiber
reinforced UPR composites were developed and the effects of moisture absorption on the mechanical properties were studied [50]. Hemp fiber
reinforced UP composite specimens were prepared with 0, 10, 15, 21, and 26 wt.% hemp fiber and tested at 25 C and 100 C for different durations. The tensile and flexural properties of water-immersed samples were evaluated and compared with those of dry specimens. The tensile and flexural properties of hemp fiber
reinforced UP were found to decrease with increases in the percentage of moisture uptake. The moisture-uptake-induced degradation of the UP-composite samples was significant at higher temperatures. The water absorption property of these composites at room temperature was found to follow Fickian behavior, although at 100 C it exhibited non-Fickian behavior. Banana fiber and sisal fiber-based hybrid UPR composites were prepared with 20, 30, 40, and 50 wt.% by changing the volume fraction of the two fibers. The mechanical properties increased with the increasing volume fraction of fiber. UP composites with 40% fiber content showed better performance when compared to that of other composites. The impact strength was also found to be maximal in the case of sisal-reinforced polyester composites. A high tensile strength was obtained for composites having a banana/sisal volume ratio of 3:1. As the ratio of banana was increased in the hybrid composite, the tensile strength was increased, and when the ratio of sisal was increased, the impact strength was increased. The tensile and flexural properties show a positive synergistic

2.10 MECHANICAL PROPERTIES OF FIBER-REINFORCED

59

effect, while the impact performance showed a negative hybrid effect. Keeping the volume fraction at 0.40 and the volume ratio of banana and sisal at 1:1, different layering patterns such as trilayer (banana/sisal/banana and sisal/banana/sisal) and bilayer (banana/sisal) composites were studied. The value of the tensile strength obtained was maximum in the case of the banana/sisal/banana trilayer composites. It was also observed that banana/sisal bilayer hybrid fiber
reinforced UP composites possessed better tensile and flexural properties [51]. Cellulosic fibers have been considered as cost competitive reinforcements in the polymer industry. Among the various factors, the performance of composite materials is dependent on the result of the interaction between the polymer matrix and the reinforcement. To achieve the optimum performance of the composites, sufficient interaction between the matrix resin and the cellulosic material is essential and can be achieved by the surface modification of the reinforcements. Banana fiber, obtained from the pseudo-stem of the banana plant (Musa sepientum), is a bast fiber with relatively good mechanical properties. The fiber surface was modified chemically to bring about improved interfacial interaction between the fiber and the polyester matrix. Various silanes and alkali were used to modify the fiber surface. Chemical modification was found to have a reflective effect on fiber
matrix interactions. An improved fiber
matrix interaction is evident from the enhanced tensile and flexural properties [52]. Md. Abdul Gafur et al. [52] developed woven natural fiber
reinforced UPR composites by simple cold press molding using 7.5% styrene monomer and 1.5% methyl ethyl ketone peroxide (MEKP). In addition, they used talc as an additive filler with different weight percentages (5%, 10%, and 15%) to study the effect on the different properties of the composites. It was observed that the flexural strength and modulus of the composites were increased with an increase in talc content. The thermal stabilities of the composites were also improved. Krissanti Arnis et al. [53] studied the mechanical properties of rice husk fiber
reinforced UP composites. They observed that the value of the tensile strength was increased from 18.22 to 46.40 MPa and the tensile modulus from 1.13 to 5.06 GPa with increasing fiber content weight percentages. Similar observations were also noticed in the case of the values of the flexural strength and flexural modulus when the fiber weight fraction in the composites was increased. The mechanical properties of the rice husk fiber composites were also influenced by fiber fraction and alkali treatment. Alkali modification contributed to better interfacial bonding between the rice husk fiber and the UPR matrix, which in turn resulted in an improvement in the mechanical properties.

2.10.2 MECHANICAL PROPERTIES OF NATURAL AND SYNTHETIC FIBER
REINFORCED UP MACROCOMPOSITES Jute fiber and E-glass fiber
reinforced UPR hybrid composites were developed by the simple hand layup technique at 25 C by Mubarak A. Khan et al. [54]. They noticed that 25 wt.% jute fiber
reinforced UPR composites possessed the highest mechanical properties compared to that of other compositions. The mechanical properties were increased with increases in the glass fiber in the jute fiber
reinforced UP composites. Among them, composites with a jute-to-glass ratio of 1:3 exhibited enhanced mechanical properties such as tensile strength (125%), tensile modulus (49%), bending strength (162%), and bending modulus (235%) over the untreated jute polymer composites. In order to improve the mechanical properties further, the surfaces of the jute and glass fiber were

60

CHAPTER 2 UNSATURATED POLYESTER—MACROCOMPOSITES

irradiated under UV radiation at various intensities. UV treated jute and glass fibers with a ratio of 1:3 exhibited the highest improvement in mechanical properties when compared to those of untreated hybrid composites. UV-modified jute/glass
reinforced hybrid UP composites showed the highest value for impact strength (40 kJ/m2). UP bio-macrocomposites were developed with the use of UPR as a matrix and kenaf bast (natural fiber) and glass fiber as reinforcements by S. F. Zhafer et al. [55]. In the recent past, the use of natural fibers in polymer composites has picked up due to their synergistic properties such as being lightweight, nontoxic, nonabrasive, low cost, and biodegradable. The introduction of synthetic fiber along with natural fiber into matrices could enhance the mechanical properties of the resulting natural fiber-based hybrid composites. Kenaf bast fiber (natural fiber) and glass fiber (synthetic fiber) hybridized with varying weight ratios were used as reinforcements for the preparation of UP biocomposites using the hand layup process. Data obtained from the mechanical studies inferred that the 10 wt.% glass fiber
reinforced kenaf bast-based UP composites showed enhanced mechanical properties including tensile strength and modulus when compared with those of neat and kenaf bast-based UP composites.

2.10.3 MECHANICAL PROPERTIES OF HYBRID FIBER
REINFORCED UP MACROCOMPOSITES It was reported that UPRs reinforced with bentonite-modified polyhedral oligomeric silsesquioxane (POSS) possess improved mechanical and flammability properties. It was also observed that the mechanical properties of POSS-reinforced UPR composites exhibited improvements in the values of tensile strength and impact strength of up to 44% and 59%, respectively, compared to those of neat matrix [15].

2.11 GENERAL APPLICATIONS OF FIBER-REINFORCED UP MACROCOMPOSITES Fiber-reinforced UP composites possess excellent properties and the ability to replace steel-based components due to their lower density in the number of industrial and engineering applications [56]. The anisotropic behavior of composites facilitates the alignment of the fiber reinforcement with the strain field, thus making the products stronger and lighter than those of conventional steel products. UPR-based composites are used in: • • • • • • •

composite grids/gratings and rails and ladder components; aqueous piping systems and water and fuel storage tanks and vessels; low pressure composite valves and pull tubes; modular paneling for partition walls and high-pressure accumulator bottles; flexible and floating risers, drill pipes, structural components, boxes, and housings fire water pump casings and lift pump casings; blast and fire protection.

2.11 GENERAL APPLICATIONS OF FIBER-REINFORCED

61

2.11.1 FLAME RETARDANT BEHAVIOR OF UP RESIN COMPOSITES Polymeric materials used in domestic and industrial applications require flame retardant behavior to prevent fire related accidents. Flame retardants are added to control, prevent, or minimize fire damage. Many fillers and additives are used as potential flame retardants in UPR composites. The incorporation of fillers reduces the peak heat release rate (PHRR), total heat release (THR), and the fire growth rate index (FIGRA). For example, the incorporation of ammonium polyphosphate, melamine phosphate, and alumina trihydrate as additives reduces the PHRR and THR. Further, the introduction of small amounts of reinforcements in combination with char promoting flame retardants causes total reductions in the PHRR of UPRs in the range between 60% and 70%. Fire retardant behavior in materials is considered as an important and essential parameter to ensure the safety of individuals as well as fabricated products. The use of composite materials for construction is increasing day by day because of their strength performance compared to traditional materials. Unsaturated polyester composites have been widely used in naval constructions, offshore applications, water piping, building construction, and automotive applications because of their enhanced mechanical properties and thermal stability [34,57
63].

2.11.2 ELECTRICAL PROPERTIES OF UP COMPOSITES UPR composite materials are versatile for electrical applications due to their easy fabrication, cost effectiveness, lower weight, with enhanced insulation properties. The electrical behavior of polymers is correlated to the nature and molecular design of polymers and electrical insulation is required to prevent unnecessary contact and short circuiting among electrical conductors of varying potentials [29,35]. Insulation materials support and defend materials against a wide variety of environmental hazards such as high humidity, elevated temperature, vibration, high radiation, and the presence of various gases, moisture, and other pollution. The use of glass
reinforced UP composites in the area of dielectric materials is of major importance in engineering materials. Dielectric materials can be chosen based on their dielectric constant and other properties over a varied range of temperature and dielectric field frequencies. The investigation of dielectric properties is one of the most suitable methods for studying the structural property relationship of glass-reinforced composites for engineering insulation applications. Typical electrical applications of fiber-reinforced composite components include terminals, connectors, industrial and household plug points, switches, and PCBs. Nowadays, unidirectional glass
reinforced composites have been used widely to replace porcelain in the production of high voltage insulators. There are several experimental and numerical procedures which can be used to evaluate laminates in terms of electrical properties [2].

2.11.3 AEROSPACE APPLICATIONS OF FIBER-REINFORCED UP MACROCOMPOSITES Fiber-reinforced UPR composite materials are widely used in the aircraft industry. The most commonly used fibers are glass, carbon, aramid, and natural fiber. Usually, UPR or an epoxy resin is used as a resin matrix with fiber as reinforcements. These composite components are cured by thermal treatment in the temperature range of 120 C
180 C. The first aircraft structural components were introduced during the 1950s and 1960s with the use of glass fiber
reinforced composites.

62

CHAPTER 2 UNSATURATED POLYESTER—MACROCOMPOSITES

These components include paddles and rudders, frames, randomes, fairings, helicopter canopies, rotor blades, etc. Due to their good strength and better stiffness combined with lower density, carbon fiber and boron fiber
reinforced composites were considered to replace aluminum components used in the high performance aircraft assemblies. Low density aramid fiber
reinforced composites have been used for lightweight loaded structures due to their high tensile strength. Glass fiber
reinforced composites are also considered as one of the best materials for the fabrication of weightless aircrafts and low-weight structural components. In the past few years, the use of UPRs has also increased for the fabrication of small access panels in nearly-complete aircraft surfaces, resulting in a lower weight and leading to improved performance in terms of enhanced durability and excellent corrosion resistance behavior. In recent years, fiber-reinforced UP composite materials have been used for flight control surfaces, fairings, randomes, engine cowlings, landing gear doors, fan ducts, and floor panels, etc., in aerospace applications [2,29,32].

2.11.4 CONSTRUCTION APPLICATIONS OF FIBER-REINFORCED UPR COMPOSITES Fiber-reinforced UP macrocomposites have been used in the construction industry including in grills, tanks, and long roof structures for replacing reinforced concrete and steel. UP composites are lightweight and possess good corrosion resistance, which helps extend their lifespan and contributes to their low maintenance and low repair costs, making them attractive for low stress applications. In addition, UPR composites are used for high performance structural applications. UPR composites play a significant role in replacing aluminum, steel, timber, and concrete in building construction [2,64]. Composites have been significantly adopted in construction fields in the place of conventional materials such as doors, windows, panels, furniture, structural and nonstructural gratings, roof design, bridge components, tanks, and other interior components. Many components made of UPR composites find a wide range of applications in architectural structures, shuttering, aesthetic appearance, etc.

2.11.5 MARINE APPLICATIONS OF FIBER-REINFORCED UPR COMPOSITES Since the 1940s, fiber-reinforced UPR composites have been used for commercial production in various types of boats including sail and fishing boats, dinghies, and ships; for interior and exterior parts. Fiber-reinforced UPR composites are used in the exterior of boats covered with foam or honeycomb, keels, masts, frames, poles, drums, booms, and shafting. Also, fiber-reinforced UP composites are used in the hulls, decks, and various interior components of marine products [65,66]. Fiber-reinforced composites are expected to possess an improved performance with minimal risk of failure in adverse marine conditions. The major advantage of boat components using UPR is their reduced weight, which in turn increases cruising speed, acceleration, maneuverability, and consumes less fuel. Racing ships employ advanced polymer composites more extensively than other marine structures in order to reduce the weight and to improve performance and durability. Racing 1 power boats are also constructed with hybrid UPR composites for enhanced performance as well as safety. In addition, carbon fiber
reinforced UP composites with their stealth features are important in minimizing radar reflection.

REFERENCES

63

2.11.6 SPORTING GOODS APPLICATIONS OF FIBER-REINFORCED UPR COMPOSITES Fiber-reinforced UPR composites are used in the production of sporting equipment from tennis and racket ball rackets, shafts for golf clubs, fishing rods, frames for bicycles and race bikes, snow skis, hockey and baseball sticks, sail kayaks and boats, paddles canoe hulls, surfboards, arrows, helmets, exercise components, etc. Sports shoe soles and heels are prepared using polymer composites due to their light weight, design flexibility, and vibration damping. A low weight and good strength can be achieved by the introduction of carbon fiber into UPR for cost competitive sports goods, especially in bicycle and canoe races, snow skis, tennis rackets, etc., for maintaining their lightweight without losing their stiffness [2,56]. Sports products like tennis/badminton rockets made up with fiber-reinforced polymers offer the faster damping of vibrations result in it reduces the shock-wave transferred to the player armrest while playing games. Carbon fiber
reinforced polymer composites are widely used for the production of racing bikes, bicycle frames, and fly-fishing rods and they are used for the production of golf clubs, which are preferred by popular professional golf players due to their low weight when compared to that of steel golf shafts.

2.12 SUMMARY AND CONCLUSION UPR composites are among the most economical thermoset polymers and have been used for many years in various fields including construction, marine, aerospace, sports goods, offshore applications, fire resistant components, waterlines, and building construction, etc., due to their excellent processability, low weight, good cross-linking ability, and excellent mechanical properties. In this chapter, preparation methods for UPRs, types of UPRs based on their properties and applications, different reinforcements including fibers and other materials in UPRs, the properties, applications, and fabrication techniques of UPR macrocomposites, and the most common applications in many fields including automotive, marine, transport, building, construction, aerospace, and electronic devices of UPR macrocomposites have been discussed and reported. In the upcoming years, mainly in the automotive and other industries the growing interest in the usage of natural fibers and other renewable fibers as reinforcements will be expected to improve the properties of UPR composites. New reinforcements and UPR composites are continuously being developed to be utilized in new application fields including in space rocket cryogenic fuel tanks, which must be resistant against the diffusion of gases. Also, the invention of self-reinforced and self-healed UP composites will be expected, since they offer the advantages of longevity and low maintenance apart from being lightweight and possessing durability. Self-healing UPR composites are intensively focused for their practical applicability and long life in various fields. Furthermore, intensive research will be warranted for the production of macrocomposite-reinforced hybrid composites able to be used in various fields to replace existing materials with low weight, cost effective, and excellent properties.

REFERENCES [1] S. Thomas, K. Joseph, S.K. Malhotra, K. Goda, M.S. Sreekala (Eds.), Polymer Composites, Vol. 1: Macro and Microcomposites, first ed., Wiley, 2012. [2] H.E.D.M. Saleh (Ed.), Polyester, Section 2, Polyester Composites, In Tech Open Science, 2012.

64

CHAPTER 2 UNSATURATED POLYESTER—MACROCOMPOSITES

[3] R.-M. Wang, S.-R. Zheng, Y.-P. Zheng (Eds.), Polymer Matrix Composites and Technology, Woodhead Publishing in Materials, Elsevier, 2011. [4] A.F. Avila, C.M. Paulo, D.B. Santos, C.A. Fari, Mater. Charact. 50 (2003) 281
291. [5] I. Nicoleta, H. Hickel, Dental Mater. 25 (2009) 810
819. [6] A.R. Bunsell, B. Harris, Composites 5 (1974) 157. [7] A. Mkaddem, I. Demirci, M.E. Mansori, Comp. Sci. Technol. 68 (2008) 3123
3127. [8] B.A. Bednarcyk, Comp. Part B 34 (2003) 175
197. [9] A. Tabiei, S.B. Aminjikarai, Comp. Struct. 88 (2009) 65
82. [10] H. Huang, R. Talreja, Comp. Sci. Technol. 66 (2006) 2743
2757. [11] S. Sriramula, M.K. Chryssanthopoulos, Comp. Part A 40 (2009) 1673
1684. [12] T.E. Tay, B.C. Vincent, G. Liu, Mater. Sci. Eng. B 132 (2006) 138
142. [13] P.K. Mallick (Ed.), Processing of Polymer Matrix Composites, CRC Press, 2017. [14] K. Friedrich, Z. Zhang, A.K. Schlarb, Comp. Sci. Technol. 65 (2005) 2329
2343. [15] M. Oleksy, H. Galina, Unsaturated polyester resin composites containing bentonites modified with silsesquioxanes, Ind. Eng. Chem. Res. 52 (2013) 6713
6721. [16] M.H. Mohammed, B. Dauda, Unsaturated polyester resin reinforced with chemically modified natural fibre, IOSR J. Polym. Textile Eng. 1 (2014) 31
38. [17] M. Paauw, A. Pizzi, Some filler effects on cross-linking of unsaturated polyesters, J. Appl. Polym. Sci. 50 (1993) 1287
1293. [18] K. Dinakaran, S. Devaraju, M. Alagar, Thermoset nanocomposites, in: Vikas Mittal (Ed.), Unsaturated Polyester Resin Clay Hybrid Nanocomposites, Wiley-VCH 2013. [19] M. Mandhakini, S. Devaraju, M.R. Venkatesan, M. Alagar, Linseed vinyl ester fatty amide toughened unsaturated polyester-bismaleimide composites, High Perf. Polym. 24 (2012) 237
244. [20] A. Prabu, M. Alagar, Mechanical and thermal studies of intercross-linked networks based on siliconized polyurethane-epoxy/unsaturated polyester coatings, Prog. Org. Coatings 49 (2004) 236
243. [21] K. Chozhan, M. Alagar, J. Sharmila, P. Gnanasundaram, Thermo mechanical behaviour of unsaturated polyester toughened epoxy
clay hybrid nanocomposites, J. Polym. Res. 14 (2007) 319
328. [22] K. Dinakaran, M. Alagar, Preparation and characterization of bismaleimide (N, N0 -bismaleimido-4, 40 -diphenyl methane)
unsaturated polyester modified epoxy intercrosslinked matrices, J. Appl. Polym. Sci. 85 (2002) 2853
2861. [23] K. Kanimozhi, P. Prabunathan, V. Selvaraj, M. Alagar, Vinyl silane-functionalized rice husk ashreinforced unsaturated polyester nanocomposites, RSC Adv. 4 (2014) 18157
18163. [24] K. Dinakaran, M. Alagar, Development and characterization of vinyl ester oligomer (veo) modified unsaturated polyester intercrosslinked matrices and composites, Int. J. Polym. Mater. 52 (2003) 957
966. [25] J. Sharmila, S. Premkumar, M. Alagar, Preparation and characterization of organoclay-filled, vinyl estermodified unsaturated polyester nanocomposites, High Perf. Polym. 22 (2010) 16
27. [26] K. Kanimozhi, P. Prabunathan, V. Selvaraj, M. Alagar, Thermal and mechanical properties of functionalized mullite reinforced unsaturated polyester composites, Polym. Comp. 35 (2014) 1663
1670. [27] Flexible unsaturated polyester resin, CN103396540A China Patent, 2013. [28] Preparation method of high-flexibility unsaturated polyester resin, CN107522854A, China Patent, 2017. [29] P. Bharat Dholakiya, in: H.E.D.M. Saleh (Ed.), Unsaturated Polyester Resin for Specialty Applications, Intech Open, London, 2012. [30] A.-K. KhalafAllah, L.-S. Eldein, E.-A. Elradi, H.-S. Mandour, Chemical and other properties of unsaturated polyester under various environmental conditions, Int. J. Sci. Eng. Res. 7 (2016) 1011
1019. [31] K.E. Medema, F.J.F. Van, H.B. Yokelson, Unsaturated polyester resin compositions, EP 1132429, Eur. Pat. Application, 2001.

REFERENCES

65

[32] H.V. Boenig, Unsaturated polyesters, Structure and Properties, Elsevier, Amsterdam, 1964. [33] B. Parkyn, F. Lamb, B.V. Clinton, Polyesters; Unsaturated Polyesters and Polyester Plasticizers, 2, Elsevier, New York, 1967. [34] Z. Bharat, Dholakiya, D. Kanuprasad, Pate, synthesis, characterization and glass reinforced composites of low styrene emission unsaturated polyester resin having improved fire resistance and mechanical properties, Macromol. Indian J. 3 (2007) 169
175. [35] H.T. Sunemi, Y. Fushiki, A. Nishimura, Y. Kawai, Pre polymer polyester resin compositions and electrical laminates made therefrom, Kanegafuchi Chemical Ind., CA1337915, 1996. [36] J. Kim, D. Jeong, C. Son, Y. Lee, E. Kim, I. Moom, Synthesis & application of unsaturated polyester resins based on PET waste, Korean J. Chem. Eng. 24 (2007) 1076
1083. [37] O. Toshishide, JP 2002294057, 7 p, Jpn. Kokai Tokkyo Koho, 2002. [38] W. Shoichi, N. Seiichi, K. Koichi, U. Tomoyuki, JP 11140287, 8 p, Jpn. Kokai Tokkyo Koho, 1999. [39] B. Jaljakumari, K.G.K. Warrier, K.G. Satyanarayana, C. Pavithran, J. Reinf. Plast. Comp. 7 (1998) 402
412. [40] D. Baral, P.P. De, G.B. Nando, Polym. Degrad. Stability 65 (1999) 47
51. [41] A.L.G. Saad, A.F. Younan, Polym. Degrad. Stability 50 (1995) 133
140. [42] M.R.C. Nametz, J.D. Pietro, I.N. Einhorn, Amer. Chem. Soc. 28 (1968) 204
224. [43] Tanaka, E. Iwami, Japanese Patent. JP 62201326, CA. 10822799t, 1986. [44] N. Yoshihiro, I. Yasuaki, JapanesePatent. JP 1160858, CA. 130223995g, 1997. [45] N. Kazunri, K. Haruyuki, JapanesePatent, JP 11116779, CA. 130353416e, 1997. [46] M.S. EL-Wazerya, M.I. EL-Elamya, S.H. Zoalfakar, Mechanical properties of glass fiber reinforced polyester composites, Int. J. Appl. Sci. Eng. 14 (2017) 121
131. [47] N. Cs Varga, L. Miskolczi, G. Bartha, Lipoczi, improving the mechanical properties of glass-fibrereinforced polyester composites by modification of fibre surface, Mater. Design 31 (2010) 185
193. [48] Z. Budai, Z. Sulyok, V. Vargha, Glass-fibre reinforced composite materials based on unsaturated polyester resins, J. Therm. Anal. Calorim. 109 (2012) 1533
1544. [49] O. Ujianto, D. Margetts, H. Santoso, R. Sujarwadi, Glass fiber reinforced unsaturated polyester: effect of nanoclay and mixing time on tensile modulus, flammability, and wear resistance, Solid State Phenomena 266 (2017) 110
114. [50] H.N. Dhakal, Z.Y. Zhang, M.O.W. Richardson, Effect of water absorption on the mechanical properties of hemp fibre reinforced unsaturated polyester composites, Comp. Sci. Technol. 67 (2007) 1674
1683. [51] L.A. Pothan, J. George, S. Thomas, Effect of fiber surface treatments on the fiber
matrix interaction in banana fiber reinforced polyester composites, Comp. Interfaces 9 (2002) 335
353. [52] R. Akter, R. Sultana, Md.Z. Alam, Md.R. Qadir, M.H.A. Begum, Md.A. Gafur, Fabrication and characterization of woven natural fibre reinforced unsaturated polyester resin composites, Int. J. Eng. Technol. 13 (2013) 122
128. [53] I.W. Surata, I.G.A.K. Suriadi, K. Arnis, Mechanical properties of rice husks fiber reinforced polyester composites, Int. J. Mater. Mech. Manuf. 2 (2014) 165
168. [54] A. Al-kafi, M.Z. Abedin, M.D.H. Beg, K.L. Pickering, M.A. Khan, Study on the mechanical properties of jute/glass fiber-reinforced unsaturated polyester hybrid composites: effect of surface modification by ultraviolet radiation, J. Reinforced Plast. Comp. 25 (2006) 575
588. [55] Kenaf-glass fiber reinforced unsaturated polyester hybrid composites: Tensile properties, AIP Conf. Proc. 2016, 1756, 040009 (1
5). [56] P.K. Mallick, Fiber Reinforced Composites Materials, Manufacturing, and Design, CRC Press, Taylor & Francis, 2017. [57] M.R. Ricciardi, V. Antonucci, M. Giordano, M. Zarrelli, Thermal decomposition and fire behavior of glass fiber
reinforced polyester resin composites containing phosphate-based fire-retardant additives, J. Fire Sci. 30 (2012) 318
330.

66

CHAPTER 2 UNSATURATED POLYESTER—MACROCOMPOSITES

[58] E.K. Walczak, G. Rymarz, in: N.O. Camlibel (Ed.), Flame-Retardant Unsaturated Polyester Resins: An Overview of Past and Recent Developments (Chapter 3), Intech Open, London, 2018. [59] N. Maheshwari, S. Thakur, P. Neogi, S. Neogi, UV resistance and fire retardant property enhancement of unsaturated polyester composite, Poly. Bull. 72 (2015) 1433
1447. [60] A. Tewarson, Flammability parameters of materials: ignition, combustion, and fire propagation, J. Fire Sci. 12 (1994) 329
356. [61] A. Tewarson, R.F. Pion, Flammability of plastics—I. Burning intensity, Combust. Flame 26 (1976) 85
103. [62] C.P. Fennimore, F.J. Martin, Flammability and sensitivity of materials in oxygen enriched atmospheres, Mod. Plast. 44 (1966) 141
148. [63] D.W.Van Krevelen, Properties of Polymers, Elservier/North-Holland, New York, 1975. [64] P.B. Potyrała, J.R.C. Rius, Use of fibre reinforced polymers in bridge construction. State of the art in hybrid and all-composite structures. Enginyeria de la Construccio´, Polytechnic University of Catalonia, 2011. [65] Md.M. Hossain, A.H.M.F. Elahi, S. Afrin, Md.I. Mahmud, H.M. Cho, M.A. Khan, Thermal aging of unsaturated polyester composite reinforced with E-glass nonwoven mat, AUTEX Res. J. 17 (2017) 313
318. [66] J. Graham-Jones, J. Summerscales (Eds.), Marine Applications of Advanced Fibre-Reinforced Composites, Woodhead Publishing, Elsevier, 2015.

FURTHER READING T.T. Yakovenko, I.V. Slimakovskii, O.V. Suberlyak, Electrical properties of composites based on unsaturated polyester resin, Inter. Polym. Sci. Technol. 33 (2006) 43
47.

CHAPTER

UNSATURATED POLYESTER MICROCOMPOSITES

3 Basim Abu-Jdayil1,2

1

Chemical and Petroleum Engineering Department, United Arab Emirates University, Al Ain, United Arab Emirates 2 Emirates Center for Energy and Environment Research, United Arab Emirates University Al Ain, Abu Dhabi, United Arab Emirates

3.1 INTRODUCTION Petrochemicals are the source for more than 98% of global plastic materials. These materials have many advantages over conventional building materials like their low weight, versatility, high specific mechanical properties, corrosion resistance, and relative low cost. Currently, there is huge demand for polymers and it is expected that global polymer production will reach 400 million tons in 2020 [1]. However, the extensive use of petroleum-based plastics has led to the current environmental problems where millions of tons of plastic packaging are landfilled every year, becoming significant sources of environmental pollution and harming wildlife when they are dispersed in nature. This environmental problem was the driving force for researchers to find alternatives to petroleum-based polymers. One of the solutions to this problem was to partially replace petroleum plastics with natural, biodegradable materials as fillers to form composite materials. The effective reduction of petrol-based polymer consumption will also reduce carbon dioxide emissions, which in turn would minimize global warming. In addition, the reinforcement of plastic materials with different fillers leads to improvements in the different physical, thermal, and mechanical properties of polymer composites. Natural fibers have already established a track record as a simple filler material in automobile parts. Natural fibers like sisal, banana, jute, coir, and oil palm fiber have all been proved to be good reinforcements in thermoset and thermoplastic matrices [2]. Unsaturated polyester resins (UPRs), upon curing, form highly cross-linked thermosets that possess extremely versatile properties and applications and have been popular thermosets used as polymer matrices in composites. Polyesters were initially developed for coating applications and are commonly known as “alkyd resins.” The term “alkyd” was coined by combining the first part of the word “alcohol” and the last part of the word “acid” [3]. Unsaturated polyesters (UPs) are widely produced industrially as they possess many advantages compared to other thermosetting resins including room temperature cure capabilities, reasonable price, good mechanical properties, and transparency. The curing of UP is achieved through a polymerization reaction that causes crosslinking among individual linear polymer chains. In contrast to other thermosetting resins, no byproducts are formed during the curing reaction; hence, resins can be molded, cast, and laminated at low pressures and temperatures [4]. Nonreinforced cross-linked UPRs are utilized as structural Unsaturated Polyester Resins. DOI: https://doi.org/10.1016/B978-0-12-816129-6.00003-X © 2019 Elsevier Inc. All rights reserved.

67

68

CHAPTER 3 UNSATURATED POLYESTER MICROCOMPOSITES

materials (e.g., cultured marble and solid-surface countertops), adhesives, coatings, or insulating materials in the automotive, aviation, electronics, construction, and packaging industries [5]. On the other hand, UP matrix composites have been used for many years in a broad range of technological fields such as naval construction, marine and land transportation industries, waterlines, and building construction [6]. In addition, UPR composites find use in a variety of other applications. They basically bridge the gap between conventional polymers, service polymers, and specialist engineering polymers.

3.2 INORGANIC FILLER
UNSATURATED POLYESTER RESIN COMPOSITES The reinforcement of polymer matrices using inorganic fillers is widely used, resulting in materials with excellent mechanical and thermal properties. Glass fibers are the most common of all reinforcing fibers or polymeric matrix composites [7]. Glass fiber, also called fiberglass, is a material made from extremely fine fibers of glass. Fiberglass is a lightweight, extremely strong, and robust material. Although its strength properties are somewhat lower than those of carbon fiber and it is less stiff, the material is typically far less brittle, and the raw materials are much less expensive. By trapping air within, blocks of glass fiber make good thermal insulation, with a thermal conductivity in the order of 0.05 W=m  K [8]. Fiberglass
UPR (GFUPR) composites are largely used mainly due to a combination of their low cost and good mechanical properties [9
12]. Although their strength properties are somewhat lower than those of carbon fiber and it is less stiff, the material is typically far less brittle, and the raw materials are much less expensive [13]. The flexural properties of random glass
polyester composites containing 20 and 30 wt.% of continuous fiber showed mean values of flexural strength and modulus of 84 MPa and 7 GPa and 110 MPa and 10 GPa, respectively [14]. In the work of EL-Wazery et al. [10] an E-glass fiber random oriented
reinforced polymer composite was developed by hand lay-up technique with varying fiber percentages (15%, 30%, 45%, and 60% by weight percentage). Their results showed remarkable improvement in the mechanical properties of the fabricated composites with increasing glass fiber contents. The tensile strength varied from 28.25 to 78.83 MPa, flexural strength varied from 44.65 to 119.23 MPa, and the impact energy at room temperature varied from 3.50 to 6.50 J, as a function of fiber weight fraction. The best mechanical properties were obtained with the 60 wt.% glass fiber fabricated composites. Comparing with the neat polyester resin, a significant increase in the tensile strength was reported for polyester composites composed of glass fiber [9]. Despite the fact that GFUPR composites have excellent thermal and mechanical properties, it is difficult to devise suitable disposal methods for them. Due to many environmental problems, the disposal methods for GFUPR composites and their recycling have been seriously acknowledged [12]. Glass fiber
reinforced plastics (GFRP) have been considered inherently difficult to recycle due to both the cross-linked nature of thermoset resins, which cannot be remolded, and the complex composition of the composites themselves. Presently, most of GFRP waste is landfilled leading to negative environmental impacts and supplementary added costs. With an increasing awareness of environmental matters and the subsequent desire to save resources, recycling would convert an expensive waste into a profitable reusable material. In a study by Ribeiro et al. [15] efforts were made to recycle ground GFRP waste, from pultrusion production scrap, into new and sustainable composite materials. For this purpose, GFRP waste recycles were incorporated into polyester-based

3.2 INORGANIC FILLER
UNSATURATED POLYESTER RESIN COMPOSITES

69

mortars as fine aggregates and filler replacements at different load contents and particle size distributions. The results of this study [15] revealed that GFRP waste
filled polymer mortars improved the flexural and compressive behavior compared to unmodified polyester
based mortars, thus indicating the feasibility of reusing GFRP industrial waste in concrete
polymer composite materials. GFUPR composites are used in different industrial applications such as in aviation technologies, marine technologies, in wind turbine blades, in automobile chassis and power transmissions, in hot tubs, roofing, and in various fluid containers and pressure vessels [16]. On the other hand, it is well known that the interphase of filler
polymer plays a key role in determining the mechanical interfacial properties of composites. The efficiency of the stress transfer between the fibers and the matrix is determined by molecular interaction at the interface and by the properties of the formed interphase [7]. Therefore different types of treatments were applied on glass fibers to improve the interfacial adhesion at the interface between the glass fibers and the UPR. Saline coupling agents are examples of these treatments. At interface between the glass fibers and the silane coupling agent, the hydroxyl groups of the silanes and those of the glass fiber surface can react with each other through siloxane bonding or hydrogen bonding, which lead to the adhesion process of the silane coupling agents onto glass fibers [17]. Park and Jin [7] noted that the mixture of two silane coupling agents led to increase the mechanical interfacial properties of GFUPR composites, due to the peptide bonding between C 5 O of methacryloxypropyl trimethoxy silane and 2 NH of aminopropyltriethoxy silane. The work of Kanerva et al. [16] includes an investigation of six different surface treatments for joining composites by overlamination. This work focused on the fracture toughness of accelerated aged glass fiber
UP matrix composite joints. Initial shear strength tests involved five different peel ply and tear ply surface treatments and a mechanical abrading treatment. Based on the results of the screening, a mechanical abrading treatment, a dry polyamide peel ply treatment, and a tear ply treatment with room temperature application were found to be potential treatments for the composite
composite joints [16]. In a study [18], frost retardant glass fiber
UP composites were evaluated for surface roughness. Glass fiber
reinforced composites were manufactured, using 8 ply fabric-type glass fiber mats and UP in a 50 vol.% volume fraction, using vacuum assisted resin transfer molding. The surfaces of the prepared composites were sanded using different grit sand papers. Static and dynamic contact angle measurements were performed to determine the hydrophobicity of specimens with different surface roughness. An observation of frost thickness versus time was performed to evaluate frost retardancy as a function of surface roughness. It was found that surface roughness and frost formation thickness were dependent on the grit of the sand paper used in the sanding process. These behaviors appeared to be interrelated with the optimal sanding grit size of 320 CC [18]. On the other hand, several studies in which clays were incorporated into UPR can be found in the literature [19,20]. The use of organoclays to improve certain mechanical properties of polyesters was reported. Jawahar et al. [21] demonstrated that nanocomposites with a 3% bentonite content exhibit an improvement of 85% in the wear resistance and a decrease of 35% in the friction coefficient. Ismail et al. [22] studied the sand/clay-unsaturated polyester composite. The results indicate that the compressive strength of sand/clay-polyester composite decreases with the increasing of claysand content, as well as increasing the concentration of styrene for particle size of clay between 0.5 and 1.25 mm. The total porosity and water absorption increased with increasing clay content

70

CHAPTER 3 UNSATURATED POLYESTER MICROCOMPOSITES

when using a particle size of 1.25
0.8 and 0.8
0.5 mm. For a particle size less than 0.5 mm the total porosity and water absorption decreased with an increase of clay content up to 40% after which the values increased with increasing clay/sand ratio. Infrared spectra showed the appearance of new bonds, which indicates a chemical reaction between the polyester and clay constituents. The effects of polyvinyl acetate (PVAc) and CaCO3 on the cure kinetics, mechanical properties, and surface rugosities of UPRs were studied by Lucas et al. [23]. The addition of calcium carbonate led to a decrease in the induction period of the copolymerization, without any significant action on the subsequent cure rate. The addition of both PVAc and CaCO3 led to a composite with a higher elastic modulus than that of pure resin alone, but showed a dramatic decrease in both the flexural strength and impact energy. Abu-Jdayil et al. [24], Abu-Jdayil and Al-Malah [25], and Al-Malah and Abu-Jdayil [26] formulated a polymeric thermal insulating material using different types of Jordanian clays. The results obtained indicated that the bentonite-based UPR composite has the most stable and compatible thermal, physical, and chemical properties among composites, and it proved to be a promising thermal insulating material for both domestic and industrial applications. The compressive strength, hardness, thermal conductivity coefficient, water absorption capacity, and apparent density were found to increase with increasing bentonite content in UP-based composite materials [24]. Furthermore, it was found that a composite material based on a certain type of clay (with highest content of Al2 O3 and SiO2 ) possessed superior characteristics in terms of mechanical properties as compared to calcium carbonate
containing polyesterbased composite materials [25]. Ruban et al. [27] studied the chemical resistance and mechanical properties of UPE/organoclay nanocomposites. The chemical resistance was studied under aqueous conditions. The results indicated that the mechanical properties, namely tensile and flexural characteristics, considerably increased with increasing clay content. These polymer
clay composites could be effective with positive mechanical and thermal properties against nonaqueous solvents. In general, clays can increase the mechanical properties and thermal resistance of thermosetting resins [27]. A major disadvantage of polymeric materials is their flammability and generation of toxic gases under fire. To overcome this problem, Tabatabai et al. [28], blended UPR with flue-gas desulfurization (FGD) gypsum. The thermal and mechanical results indicated that the ambient mechanical properties of polyester resin could be enhanced with up to 50% FGD gypsum content. Thermogravimetric analysis (TGA) results showed significant improvement in the mass retention proportional to the gypsum content. Under direct fire, FGD gypsum formed a protective physical barrier on the exterior surfaces of the resin. The study concluded that FGD gypsum can potentially be an effective and low-cost fire-resistant additive for polymers [28]. The main goal of a study by Doan et al. [29] was to evaluate the potential of waste stone powders as a filler in composite materials with a matrix of UP. These wastes were generated in the form of stone fragments and stone-cutting sludge. UPR composites with different stone powder fillers (50
65 wt.%) were prepared. The influence of powder type on the mechanical properties, water resistance, and thermal stability of the composites as well as their surface fracture morphologies were studied. The moduli of the composites increased by 100%, while the hardness of the composites was improved by 80% upon loading with the “waste” filler, leading to an economical material and helping to reduce waste.

3.3 NATURAL FIBER
UNSATURATED POLYESTER RESIN COMPOSITES

71

3.3 NATURAL FIBER
UNSATURATED POLYESTER RESIN COMPOSITES Natural fibers are available in abundance and are inexpensive compared to other relatively advanced man-made fibers. Lignocellulosic fibers like sisal, jute, hemp, and flax have often been used as reinforcements in polymer matrices. The use of natural cellulosic fibers to reinforcement polymers as a replacement for synthetic fillers such as glass and carbon fibers is receiving attention for making low-cost engineering materials. The advantages of natural fibers such as their low density, high specific strength, and renewability, in addition to new environmental legislation as well as consumer pressure have forced manufacturing industries to search for new materials that can substitute conventional nonrenewable reinforcing materials such as glass fiber [12]. The reinforcement of polyester with various cellulosic fibers has been widely reported. Polyester
jute [30,31], polyester
sisal [32
35], polyester
coir [36], polyester
banana
cotton [37], polyester
peach palm [38], polyester
straw [39], polyester
hemp [40], polyester
pineapple leaf [41], polyester
bagasse [42,43], and polyester
cotton
kapok [44] are examples of interesting systems. Generally, these natural fibers are low-cost fillers with low density and high specific properties. In addition, these fibers are biodegradable and nonabrasive. Natural fiber composites offer specific properties comparable to those of conventional fiber composites. However, in the development of these composites, the incompatibility of these fibers and poor resistance to moisture often reduce the potential of natural fibers in composites production [45]. Therefore a lot of research has been conducted to overcome these drawbacks. The chemical composition of natural fibers varies depending on the type of fiber. Primarily, fibers contain cellulose, hemicellulose, pectin, and lignin. They generally consist of helically wound cellulose microfibrils in an amorphous matrix of lignin and hemicellulose [46]. The properties of each constituent contribute to the overall properties of the fiber. Hemicellulose is responsible for the biodegradation, moisture absorption, and thermal degradation of fiber as it shows the least resistance, whereas lignin is thermally stable but is responsible for the UV degradation. The percentage composition of each of these components varies for different fibers [45]. On the other hand, the mechanical properties are determined mainly by the cellulose content and microfibrillar angle. The Young’s modulus of natural fibers decreases with increases in diameter. A high cellulose content and low microfibril angle are desirable properties for a fiber to be used as a reinforcement in polymer composites. The mechanical properties of natural fibers are also significantly related to the degree of polymerization of cellulose in the fiber [46]. Generally, fibers contain 60%
80% cellulose, 5%
20% lignin, and up to 20% moisture [45]. Cellulose is a natural polymer with high strength and stiffness per unit weight, and it is the building material of long fibrous cells. The selective removal of noncellulosic compounds constitutes the main objective of fiber chemical treatments [46]. The cells of flax fiber consist mostly of pure cellulose, being cemented as fascicle bundles by means of noncellulosic incrusting such as lignin, hemicellulose, pectin, protein or mineral substances, resins, tannins, dyers, and a small amount of waxes and fat. A mature flax cell wall consists of about 70%
75% cellulose, 15% hemicellulose, and pectic materials. While sisal fibers contain 67%
78% cellulose, 10%
14.2% hemicellulose, and 8%
11% lignin [46,47]. Joseph et al. [48] reported that sisal contains 85%
88% cellulose and 4%
5% lignin. These large variations in the chemical compositions of natural fibers are a result of different sources, ages, measurement methods [47].

72

CHAPTER 3 UNSATURATED POLYESTER MICROCOMPOSITES

Natural fibers used in composites fabrication exhibited variations in their diameters and lengths. Flax, hemp, and juta fibers were used in length ranges of 5
60, 5
55, and 1.5
5 mm, respectively. While the diameter range was 12
27, 25
50, and 20 µm, for flax, hemp, and juta fibers, respectively [46]. The length of sisal fibers used in different composites was between 1.0 and 30 mm and the diameter was about 100
300 µm [47,48]. Different physical and mechanical properties were determined for UPR
natural fiber composites. Among the different mechanical properties, the tensile strength received the most attention. The mechanical properties of sisal fiber composites with several different thermoset resin matrices (polyester, epoxy, phenol-formaldehyde) and a thermoplastic matrix (low density polyethylene (LDPE)) were evaluated with respect to fiber length and fiber loading [48]. Composites containing 10, 20, and 30 wt.% fiber were prepared using fiber with lengths in the range of 5
30 mm. All the composites showed a general trend of increases in properties with increasing fiber loadings. However, the optimum length of fiber required to obtain an increase in properties varied with the type of matrix. Among the polyesters, epoxy and phenol-formaldehyde composites of sisal fiber, a phenolic type resin, performed better as a matrix than epoxy and polyester resins with respect to tensile and flexural properties due to the high interfacial bonding in phenolic composites. The fiber length had no significant effect on the sisal
UPR composites in the studied range [48]. The properties of natural fiber
UPR composites have been reviewed by many researches (e.g., Refs. [46,49,50]). In general, it has been reported by many researchers that the introduction of sisal fiber into UPR matrices results in increased mechanical properties [51]. The observed increase in the mechanical properties of sisal fiber
reinforced thermoset polymer composites are ascribed to the fact that the fibers could effectively carry an applied load and distribute it within the matrix, and that the lowered brittle nature of the cured thermoset matrix led to a higher mechanical strength than that of the cured thermosets without sisal reinforcement [49]. Rajulu et al. [52] coated untreated and 2% NaOH treated bamboo fibers with epoxy, UP, and blends of the two. The chemical resistance of these fibers to acetic acid, nitric acid, hydrochloric acid, sodium hydroxide, ammonium hydroxide, sodium carbonate, benzene, toluene, carbon tetrachloride, and water was studied. The blend coated fibers showed better resistance to the chemicals mentioned. The blend coated fibers also had higher tensile strengths. This was attributed to hydrogen bonding between the UP and epoxides group. On the other hand, a dynamic mechanical analysis of banana fiber
reinforced polyester composites was carried out by Pothan et al. [2], with special reference to the effect of fiber loading, frequency, and temperature. At lower temperatures (in the glassy region), the dynamic modulus values were maximum for the neat polyester, whereas at temperatures above the glass transition temperature (Tg), the dynamic modeled values were found to be maximum for composites with a 40% fiber loading, indicating that the incorporation of banana fiber into polyester matrices induces reinforcing effects appreciably at higher temperatures. The Tg associated with the damping peak was lowered up to a fiber content of 30%. The Tg values were increased with a higher fiber content [2]. Al-Kaabi et al. [53] tested polyester composite specimens reinforced with date palm fibers (DPF) to assess their performance. The specimens were subjected to various types of mechanical and physical tests and the results showed that these fibers may yield reasonable properties and could be used for low-cost applications that require low to medium strength. Tests indicated that additional work was needed to enhance the compatibility between the fiber and the matrix. The thermal degradation and fire resistance of different natural fiber composites were studied [6].

3.3 NATURAL FIBER
UNSATURATED POLYESTER RESIN COMPOSITES

73

UP and modified acrylic resins (Modar) were used as matrix composites. Thermal degradation indicated that the Modar matrix composites were more resistant to temperature than the composites with UPR matrices. Flax fiber, due to its low lignin content, exhibited the best thermal resistance among the natural fibers studied. Water absorption of natural fiber plastic composites is a serious concern especially for potential outdoor applications. In the research of Akil et al. [54], jute fiber
reinforced UP composites were subjected to water immersion tests in order to study the effects of water absorption on their mechanical properties. Water absorption tests were conducted by immersing the composite specimens into three different environmental conditions including distilled water, sea water, and acidic solutions at room temperature for periods of up to 3 weeks. The water absorption properties of jute fiber
reinforced UP composites were found to follow so-called pseudo-Fickian behavior. The flexural and compression properties were found to decrease with increases in the percentage of water uptake. These flexural and compression behaviors were explained by the plasticization of the matrix
fiber interface and the swelling of the jute fibers [54]. In the work of Prasad and Rao [55], tensile and flexural tests were carried out on composites made from jowar natural fiber
reinforced polyester resin and these were compared with composites of sisal and bamboo developed under similar laboratory conditions. Jowar fiber has a tensile strength of 302 MPa, a modulus of 6.99 GPa, and an effective density of 922 kg=m3 . It was observed that the tensile strength of the jowar fiber composite was almost equal to that of the bamboo composite and 1.89 times that of the sisal composite, while the tensile modulus was 11% and 45% greater than those of the bamboo and sisal composites, respectively, at a 0.40 volume fraction of fiber. The flexural strength of the jowar composite was 4% and 35% greater and the flexural modulus was 1.12 times and 2.16 times greater than those of the bamboo and sisal composites, respectively [55]. Many reports in the literature have illustrated that the incorporation of fibers into polymer matrices enhances the mechanical properties of composites. Although, certain limitations are realized in natural fibers, which have hindered their applicability in advanced, strong, and hi-tech structures that require high strength and stiffness at elevated temperatures. The limitations include (1) their inferior mechanical strength compared to synthetic fibers, (2) their poor fiber/matrix adhesion due to the hydrophilic characteristic of natural fiber, and (3) their low melting temperature [49]. Most natural fibers are hydrophilic and incompatible with hydrophobic polymers. This property of fibers leads to the formation of aggregates in polymer matrices rather than a homogenous distribution. Regardless of the polarity of polymer matrices, the resulting composites will be susceptible to moisture deterioration and fungal damage during outdoor application. The moisture absorption of natural fibers may cause dimensional changes to the resulting composites and weaken the interfacial adhesion [50]. To overcome these problems, different chemical and physical treatments of fibers have been attempted. Extensive research has been carried out and reported in the literature, showing the importance of the interface and the influence of various types of surface modifications on the physical and mechanical properties of natural fiber
reinforced composites. Frequently used treatments include bleaching, acetylation, and alkali treatments. Natural fibers are chemically treated to remove lignin, pectin, waxy substances, and natural oils covering the external surface of the fiber cell wall. This reveals the fibrils and gives a rough surface topography to the fiber. Sodium hydroxide (NaOH) is the most commonly used chemical for bleaching and/or cleaning the surface of plant fibers. It also changes the fine structure of native cellulose I into cellulose II through a process known as alkalization [56]. The observed trend from the literature indicates a preference for

74

CHAPTER 3 UNSATURATED POLYESTER MICROCOMPOSITES

chemical modification (alkaline, silane, acetylation, benzoylation, acrylate and acrylonitrile grafting, maleated coupling, permanganate, peroxide, and isocyanate treatment) compared to physical modification (corona and plasma treatment) [57]. In the work of George et al. [58], a review was given on physical and chemical treatment methods that improve fiber
matrix adhesion and their characterization methods. Proposed theories to explain the interfacial bonding mechanisms of different coupling agents, which are responsible for the improvement of mechanical performance and hygrothermal stability of composites, were discussed. The effect of chemical treatments (acetylation, silane treatment, and titanate coupling) on the mechanical properties of coir and oil palm fiber
reinforced polyester composites was investigated by Hill and Abdul Khalil [59]. A small increase in the tensile strength, tensile modulus, and impact strength of the composites reinforced with modified fiber was noted. Rout et al. [60] studied the effect of alkali treatment, bleaching, and vinyl grafting on the performance of coir
polyester composites and found that among all the modifications, bleached (65 C) coir
polyester composites showed better flexural strength (61.6 MPa), whereas 2% alkali-treated coir/polyester composites showed significant improvement in tensile strength (26.80 MPa). On the other hand, Pothan et al. [61], examined the mechanical properties of various silane treated and mercerized banana fiber
reinforced polyester composites and concluded that the alkali-treated composites had higher mechanical properties due to the better packing of their cellulose chains after the dissolution of lignin, the cementing material. Sisal fibers were subjected to various chemical and physical modifications such as mercerization, heating to 100 C, permanganate treatment, benzoylation, and silanization to improve its interfacial bonding with polyester matrices [62]. Mercerized fiber
reinforced composites showed a 36% increase in tensile strength and a 53% increase in Young’s modulus while the permanganatetreated fiber
reinforced composites showed a 25% increase in flexural strength. However, the treatment has been found to cause a reduction in the impact strength. A water absorption study of these composites at different temperatures revealed that the water absorption was less for the treated fiber
reinforced composites at all temperatures compared to the untreated composites. In their trail to improve the interfacial bond between natural fibers and polyester matrices, Sreenivasan et al. [63] performed chemical surface treatments on Sansevieria cylindrica fibers (SCFs). Treatments including alkali, benzoyl peroxide, potassium permanganate, and stearic acid were carried out to modify the fiber surface. The mechanical properties of composites prepared from the chemically treated SCFs were found to be much better than those of the untreated ones. Potassium permanganate
treated S. cylindrica fiber/polyester (PSCFP) composites showed the optimum mechanical properties among the treated S. cylindrica fiber/polyester (SCFP) composites. Rajkumar et al. [64] treated sisal fibers with various reagents to improve their wetting behavior in polyester matrices. The observations demonstrated that NaOH treatment removed the hemicelluloses and other impurities to a greater extent as observed through Fourier transform infrared spectroscopy and scanning electron microscopy. In addition, it was further noted that there was a considerable increase in the surface area of the exposed cellulose, which aided in the enhancement of the wettability characteristics. Conversely, a virtual layer formed on the fiber when treated with silane and isocyanate. TGA studies revealed that there was an appreciable enhancement in thermal stability for all the treatments [64]. A work prepared by Halip et al. [65], provides a good review of the effects of different treatments on the water retention (WR) ability of natural fiber
reinforced polymer composites.

3.4 POLYESTER-BASED HYBRID COMPOSITES

75

The various types of treatments such as alkali, silane, and acetylation that were applied on natural fibers and the effect on the water behavior of the corresponding composites were analyzed.

3.4 POLYESTER-BASED HYBRID COMPOSITES Hybrid composites are formed by a combination of two or more different constituents like additives or fibers in matrices [49]. In most cases, the mechanical properties of natural fibers do not match those of glass fiber. One solution to this problem is to replace only a fraction of the glass fiber, thus making a hybrid material [12]. However, most studies on the mechanical properties of hybrid composites were based on natural/natural fiber, natural/synthetic fiber, and natural/natural or natural/synthetic/additive modified reinforced polymer composites. Hybridization is usually classified into interlaminate and intralaminate mode. Interlaminate, or simply laminate, consists of depositing layers made of different fibers, whereas in intralaminates, the different fibers are entangled within a single layer [9]. The properties of hybrid composites mainly depend on the fiber content, length of individual fibers, orientation, extent of intermingling of fibers, fiber to matrix bonding, and the arrangement of the fibers. The strength of hybrid composites is also dependent on the failure strain of individual fibers. Maximum hybrid results are obtained when fibers are highly strain compatible [66]. The hybridization of natural/glass fiber
reinforced polymer composites has been developing toward building their applications in the field of engineering and technology. Most studies clearly revealed that an increase in the mechanical properties followed an increase in the synthetic fiber percentage in the overall fiber content. However, due to the disposal issues and environmental impacts related to synthetic fibers along with the strength requirements for particular applications, a 50:50 ratio (natural/synthetic) in the overall fiber loading could be used. It offers balanced and intermediate mechanical properties compared to those of individual sisal and synthetic fiber
based composites [49]. The water absorption of glass-reinforced coir
polyester composites is less than that of coir
polyester composites [60]. The tensile, flexural, and impact properties of pineapple leaf fiber and sisal-reinforced polyester composites are improved by the incorporation of a small amount of glass fiber into these composites, showing positive hybrid effect [67]. Increase in tensile, impact, and compressive strength were seen following an increase in glass fiber ratio in a hybrid sisal fiber
glass composite [68,69]. With an even ratio of sisal/glass in the composite, balanced mechanical properties superior to those of individual sisal composites were attained. The incorporation of glass into jute fiber composites enhances the properties of the resulting hybrid composites [70]. Hybridization was successfully applied to vegetable/synthetic fiber
reinforced polyester composites in a way that the various properties responded satisfactorily to the incorporation of a third component [9]. The incorporation of both sugar palm and glass fiber into the UP matrix resulted in the enhancement of the mechanical properties of the composites. Glass/sugar palm composites were found to have an increase in tensile, flexural, and impact properties with increases in the fiber content and weight ratio of glass/sugar palm fibers [71]. Mixing natural fiber with glass fiber
reinforced polymers (GFRPs) is finding increased applications. In a study by Ramesh et al. [11], sisal
jute
glass fiber
reinforced polyester composites were developed and their mechanical

76

CHAPTER 3 UNSATURATED POLYESTER MICROCOMPOSITES

properties such as tensile strength, flexural strength, and impact strength were evaluated. The results indicated that the incorporation of sisal
jute fiber into GFRPs can improve their properties and the resulting composites can used as an alternate material for glass fiber
reinforced polymer composites. A study on reinforced polyester composites with various volume percentages of hemp, jute, and glass fibers was carried out by Raju et al. [72]. The composites thermal properties such as thermal conductivity, specific heat capacity, and thermal diffusivity were investigated. The results revealed that the thermal conductivity of the composites decreased with increases in fiber content, while quite opposite trend was observed with respect to temperature. In another type of study, Pothan et al. [73], investigated the dynamic mechanical properties over a range of temperatures of hybrid composites of glass and banana fiber in polyester matrices. All the properties were compared with those of neat polyester samples and unhybridized composites. The effects of the layering pattern of the two fibers on the ultimate viscoelastic behavior of the composites were also investigated. Composites were prepared with banana as the surface layer, glass as the surface layer, and as an intimate mixture of glass and banana. At temperatures above the Tg, the storage modulus values were found to decrease even with the addition of glass fiber for the composites with glass was the core material. The value of the storage modulus of the composites with the above-mentioned geometry was found to be different, above and below the Tg; the value above Tg being lower than that below Tg, unlike in unhybridized composite. The loss modulus curves and the damping peaks were found to be flattened by the addition of glass. The layering pattern or the geometry of the composites was found to have a profound effect on the dynamic properties of the composites. The most intimately mixed composite was found to have the highest storage modulus values of all the compositions. The storage modulus values were consistent with the results of tensile strength. The dielectric behavior of the composites was also found to be dependent on the glass fiber volume fraction as well as the layering pattern employed [73]. Shanmugam and Thiruchitrambalam [74] studied alkali-treated continuous palmyra palm leaf stalk fiber (PPLSF) and jute fibers as reinforcements in UP matrices and evaluated their static and dynamic mechanical properties. Continuous PPLSF and jute fibers were aligned unidirectionally in bilayer arrangement and the hybrid composites were fabricated using a compression molding process. A positive hybrid effect was observed for the composites due to hybridization. Increasing jute fiber loading showed a considerable increase in the tensile and flexural properties of the hybrid composites as compared to the treated PPLSF composites. The impact strength of the hybrid composites was observed to be less compared to the pure PPLSF composites. The addition of jute fibers to PPLSF and the alkali treatment of the fibers enhanced the storage and loss moduli of the hybrid composites. Composites with a higher jute loading showed maximum damping behavior. Overall, the hybridization was found to be efficient showing increased static and dynamic mechanical properties [74]. The effect of weaving patterns and random orientation on the mechanical properties of banana, kenaf, and banana/kenaf fiber
reinforced hybrid polyester composites was investigated [75]. Composites were prepared using the hand lay-up method with two different weaving patterns, namely plain and twill-type. Of the two weaving patterns, the plain-type showed improved tensile properties compared to the twill-type in all the fabricated composites. Furthermore, a maximum increase in mechanical strength was observed in the plain-woven hybrid composites compared to the randomly oriented composites. Moreover, alkali (NaOH) and sodium lauryl sulfate (SLS) treatments appeared to provide an additional improvement in mechanical strength through enhanced interfacial bonding.

3.4 POLYESTER-BASED HYBRID COMPOSITES

77

The tensile, flexural, and impact strengths of short randomly oriented banana/jute hybrid polyester composites were studied by Adhikari and Gowda [76]. Fiber volume fractions of 5%, 10%, 15%, 20%, and 25% were used while maintaining the banana/jute volume in a 1:1 ratio. The specimens were prepared in 3 and 5 mm thicknesses. The mechanical strength of the banana/jute hybrid polyester composite was found to vary with the fiber content. This study showed that the optimum fiber volume fraction for tensile and flexural strength is 15%. However, the impact strength increased with increases in the fiber volume fraction continuously up to 25%. An increase in thickness slightly increased the tensile, flexural, and impact strengths due to better interfacial adhesion between the fibers and the matrix [76]. The aim of a study performed by Romanzini et al. [77], was to investigate the effect of new, cation-exchanged, and silane-modified montmorillonite nanoclay fillers on the thermal, mechanical, and flammability characteristics of polyester/glass fiber composites in comparison with composites prepared with calcium carbonate as a filler. The coefficient of thermal expansion showed reduced values for the silane-modified samples. All the composites decomposed at lower temperatures in air and their degradation behavior differed from those exposed to nitrogen. The glass fibers acted as a physical barrier against combustion and the catalytic effect of the clay was less pronounced for the silane-modified samples. In general, consequent to the addition of clay, these composites showed improved fire performance, shorter fire duration, and slower fire growth and maintained better mechanical properties than those with calcium carbonate filler [77]. The main focus of the work carried out by Potluri et al. [78], was to predict the elastic properties of hybrid composites where a polyester matrix emended with micro-boron carbide particles was reinforced with continuous S2-glass fibers. The influence of the inclusion of boron carbide particles on the elastic properties of the S2-glass fiber
based polyester composite at different fiber volume fractions was inspected. The outcomes of this study suggested that boron carbide microparticles are one of the better reinforcement options for enhancing the elastic properties of fiber-reinforced polyester composites. Many research studies on natural/natural fiber
reinforced composites were also reported in the literature. Differences in the cellulose contents of natural fibers could help the hybridization of natural/natural fibers in composites. Combining natural fibers could fulfill the requirements of a given application and result in an environment-friendly material. These biocomposites could be used in secondary/tertiary structures and in applications requiring low strength and stiffness [49]. Idicula et al. [79] studied the dynamic and static mechanical properties of randomly oriented, intimately mixed, short, banana/sisal hybrid fiber
reinforced polyester composites. The dynamic properties such as the storage modulus, damping behavior, and static mechanical properties such as tensile, flexural, and impact properties were investigated as a function of the total fiber volume fraction and the relative volume fraction of the two fibers. Keeping the relative volume fraction of banana and sisal at 1:1, the volume fraction of the fiber was optimized. At conditions above the glass transition temperature Tg of the matrix, the storage modulus was found to increase with fiber volume fraction and maximum value was obtained at a volume fraction of 0.40. The tensile modulus and flexural strength were found to be highest at a 0.40 volume fraction, which indicates effective stress transfer between the fiber and the matrix. Keeping the total fiber volume fraction at 0.40, hybrid composites having different volume ratios of fiber and unhybridized composites were prepared and analyzed. Sisal/polyester composites showed the maximum damping behavior and the highest impact strength as compared to banana/polyester as well as hybrid composites. However, maximum stress transfer between the fiber and the matrix was obtained in composites having a

78

CHAPTER 3 UNSATURATED POLYESTER MICROCOMPOSITES

volume ratio of banana and sisal of 3:1, which showed the lowest damping behavior value and highest storage modulus value at the Tg. The tensile strength and flexural modulus were also maximal and the impact strength was minimal at this volume ratio [79]. The thermal conductivity, diffusivity, and specific heat of polyester/natural fiber (banana/sisal) composites were investigated as a function of filler concentration and for several fiber surface treatments [80]. The results showed that chemical treatment of the fibers reduced the composites thermal contact resistance. The incorporation of banana/sisal fiber induced a decrease in the effective thermal conductivity of the composites. Using NaOH and polystyrene maleic anhydride chemical treatments on the fibers allowed a significant increase of both the thermal conductivity and density values of the banana/sisal fiber composites. This showed that the chemical treatment allowed a better contact between the components (fiber/matrix) and reduced considerably the thermal contact resistance [80]. An evaluation of the effect of hybridization on the mechanical performance of short banana/ sisal hybrid fiber
reinforced polyester composites found that the tensile properties of the composites were improved by the addition of banana fibers [51]. The maximum tensile strength (58 MPa) was obtained for composites having a banana/sisal ratio of 3:1 and a total fiber content of 67 vol. %. The results were explained as being due to the smaller diameters of banana fibers compared to those of the sisal fibers and better stress transfer in unit area of banana/polyester composite. Hybrid composites were prepared with jute fabric and unshredded newspaper in a polyester resin matrix [81]. The experiment was designed to have 1:2 weights ratio jute and unshrouded newspaper with 42 (w/w)% fiber content hybrid composites and two different sequences jute/paper/ jute and paper/jute/paper of waste newspaper and jute fabric arrangement. The tensile, flexural, and interlaminar shear strength and fracture surface morphology of the composites were evaluated and compared. The test results showed that the hybrid composites had higher tensile and flexural properties than pure paper composites. Different layering sequences exhibited significant effects on the tensile, flexural, and interlaminar shear strength properties of the hybrid composites. Hybrid composites with an outer layer of jute fabric showed better tensile and flexural properties than hybrid composites with a paper outer layer. Fractured surface SEM imagery of the pure paper composite showed that the paper and the resin were arranged in separate layers and that the resin did not penetrate inside the fiber network in the paper [81]. A review work by Sanjay and Yogesha [12] gave a framework of the physical and mechanical properties of natural/glass fiber
reinforced polymers. They found that less investigations were carried out on the electrical, thermal, and dynamic properties of hybrid composites. This paper concluded that hybrid composites made of natural fiber with glass fiber exhibit better mechanical properties than natural fiber composites. Senthilkumar et al. [49], reached the same conclusion that hybridization with synthetic fibers like carbon, glass, and aramid effectively improve the mechanical properties of sisal fiber
based composites. Improvements in the mechanical properties of sisal/other natural fiber
reinforced hybrid composites were not as significant or remarkable as those for sisal/synthetic fiber
reinforced hybrid composites.

3.5 WASTE FILLERS
UNSATURATED POLYESTER COMPOSITES The low thermal conductivity of UP encourages its utilization as a matrix in the development of polymer-based thermal insulation materials. Although a lot of research has been carried out on the

3.5 WASTE FILLERS
UNSATURATED POLYESTER COMPOSITES

79

properties of natural fiber
reinforced UPR composites, little focus has been given to DPF and other wastes like scrap tires. Therefore the subsequent sections provide a comparison between different types of UPR-filler composites developed as insulating materials using waste rubber particles (WR), date palm leaf fibers (DL), date pits (DP), and red shale (RS) as filler materials. A common factor among all of these fillers is that they are all waste materials that are abundantly available in the Gulf countries and other regions of the world.

3.5.1 UNSATURATED POLYESTER RESIN In this study, the UPR and methyl ethyl ketone peroxide (MEKP) used to fabricate composites were obtained from Reichhold Norpol Company, Dubai (UAE). The polyester resin used is known commercially as Polylite 721
800E with a styrene content of 44%
46% and a viscosity of 280
330 (mPa/s. Polylite 721
800E, which is an isophthalic polyester resin, has a built-in curing agent (cobalt octoate) that gives a relatively long gel time, rapid curing combined with relatively low exothermic temperature, and short demolding time. The ratio between the catalyst and UPR used to prepare the composites was 1:100.

3.5.2 REINFORCEMENT FILLERS The generation of scrap tires is approximately 1 billion per year globally and is set to increase in the future as car and truck transportation continue to expand throughout the world. Old scrap tires have become an increasing source of waste and pollution everywhere in the world. Waste tires present not only environmental hazards but health and safety issues as well. The United States disposes of 279 million waste tires each year, while an estimated 4.8 million waste tires were generated in 2010 in the UAE [82,83]. The waste rubber used in this study, which was produced from the recycling process of scrap tires, was supplied by the Gulf Rubber Factory (GRF) located in Al Ain (UAE). The size of rubber particles used in this investigation was less than 800 µm (see Fig. 3.1). The rubber particles were used without any kind of treatment. DP and date palm wood are readily available in a number of countries. They have typically been seen as waste products from the preparation of dates and are usually discarded. In the United States, pulverized ground DP are being used on a small scale on dirt roads as a type of road basegravel. In the Middle East, it is sometimes used in animal feed. Several investigators have used DP to adsorb dyes, phenols, and heavy metals. Wastes from date palm trees in the form of date leaves, date wood, and DP are widely produced in the Arabian Gulf countries. The UAE has about 40 million date palms, more than three-quarters of which are found in Abu Dhabi, and each generating about 15 kg of waste biomass annually. In this work, date palm leaf filler was obtained from a UAE University farm located in Al Foah, Al Ain. Large pieces of date palm leaves were crushed and ground using a high energy mill to obtain fibers with various diameters and lengths. It was not easy to perform an accurate separation of fibers solely based on diameter and length. Therefore sieving of the crushed DPF gave an approximate size characterization and the mesh size was related to the diameter distribution. The DPF were screened using meshes to get fibers having sizes less than 1180 µm (see Fig. 3.2). The size of DL fibers varied between 180 and 1180 µm. The DP were obtained from the Al-Saad Date Processing Factory (UAE) and the samples were crushed,

80

CHAPTER 3 UNSATURATED POLYESTER MICROCOMPOSITES

FIGURE 3.1 (A) Waste rubber filler. (B) Micrograph of waste rubber.

FIGURE 3.2 (A) Date palm leaf fibers. (B) Micrograph of date palm leaf fibers.

ground, and then screened to ensure a size range of 100
850 µm. Fig. 3.3 shows the DP particles and their general morphology. The shale filler was provided by the Emirates Ceramic Factory (ECF) (subsidiary of Fujairah Building Industries P.S.C. (FBI)), Fujairah, UAE. It is mainly extracted from rocky sand obtained from a site located between Fujairah and Ras Al Khaimah on highway E18 in the UAE. The exact location of this site is 25 310 33.7vN 56 020 18.7vE. This shale is utilized by the main ceramic companies in the UAE—Emirates Ceramic Factory and RAK Ceramics, which is one of the largest ceramic manufacturers in the world. In addition, this shale, which is known locally as RS, is being used as a silica, alumina, and iron supplement for the adjustment of final cement product quality. In this investigation, the shale was crushed, ground, and then screened to get grain sizes less than 100 µm. Fig. 3.4 shows the shale at different stages. The shale consists mainly of silica (64.2%), alumina (17.2%), ferric oxide (8.7%), and potassium oxide (5.2%) [25].

3.5 WASTE FILLERS
UNSATURATED POLYESTER COMPOSITES

81

FIGURE 3.3 (A) Date pit (DP) particles. (B) General morphology of DP particles.

FIGURE 3.4 (A) The rocks used to produce the shale filler. (B) Shale filler used in this study.

3.5.3 COMPOSITES FABRICATION The composites were prepared using different filler concentrations which were added to the UP at room temperature. For the curing process, MEKP was added as an initiator. The maximum filler concentration was dependent of the type of filler used and its compatibility with UP to form a stable composite. The best compatibility was between the DP and UPR where the filler content reached 70 vol.%. On the other hand, it was difficult to mix more than 40 vol.% of rubber with UP. Both the DL and RS formed stable composites with 60 vol.% filler content. The composites were prepared using a high viscosity mixer and the mixture was then poured into suitable molds prepared from stainless steel. Different types of molds were fabricated to meet the requirements of the tests

82

CHAPTER 3 UNSATURATED POLYESTER MICROCOMPOSITES

FIGURE 3.5 (A) Waste rubber
UPR composites. (B) Date leaf
UPR composites. (C) DP
UPR composites. (D) RS
UPR composites.

that were performed on the prepared composites. The interior surface of the molds was coated with paraffin wax and polyvinyl acetate to prevent the samples sticking to the molds. The produced samples were then subjected to different thermal, physical, and mechanical tests according to ASTM standards. Fig. 3.5 shows UPR composite samples prepared using different fillers.

3.6 INVESTIGATION OF PHYSICAL, THERMAL, AND MECHANICAL PROPERTIES OF UNSATURATED POLYESTER RESIN COMPOSITES 3.6.1 DENSITY The density of polyester is experimentally found to be 1200 kg/m3, whereas the densities of WR, DL, DP, and RS are 800, 300, 644, and 1360 kg/m3, respectively. Fig. 3.6 shows the effect of filler loading

3.6 INVESTIGATION OF PHYSICAL, THERMAL

83

FIGURE 3.6 Effect of filler loading on the density of composites.

on the measured and calculated density of the formulated composites. The lines represent the densities calculated from the linear mixing rule and the logos represent the experimental values. For fillers with densities less than the density of neat polymer, the density of composites decreases with filler content. Only the density of the RS
UPR composites increased with shale content as the density of shale is greater than that of pure polyester. The densities of the DL
UPR and DP
UPR composites are very close, although the density of DL is clearly less than that of DP. The lowest density was reported for the 40 vol.% WR
UPR composite, which was 1053 kg/m3, while the maximum measured density was 1590 kg/m3 for the 60 vol.% RS
UPR composite. It can be seen from Fig. 3.6 that there is a clear deviation between the measured density of the composites and the theoretical density calculated from the linear mixing rule. The closest to the mixing rule is the WR
UPR composite. For the other composites, the experimental densities are slightly higher than the theoretical values. This can be attributed to the shrinkage of the composite materials on curing. The degree of shrinkage is higher as the filler content increases in the composite, which is represented by the increasing deviation of the experimental density from the theoretical values at higher filler concentrations [25]. In general, the prepared composites had low densities when compared to other construction composites and materials. The density of WR
UPR composites is lower than that of rubber-based concrete composites having densities ranging between 2220 and 1150 kg/m3 for varying rubber contents as reported by Benazzouk et al. [84] The density of UPR composites filled with 40 vol.% DL (1135 kg/m3) and DP (1149 kg/m3) is lower than that of established fibers like sisal (1450 kg/m3) and banana (1350 kg/m3) and is nearly equal to that of coir (1150 kg/m3) [55], which is an attractive parameter when manufacturing lightweight materials.

3.6.2 WATER RETENTION A WR test was performed according to ASTM D-570-98. Cylindrical specimens 30 mm long and 25 mm in diameter were used. Two types of tests were performed on the prepared composites: (1) short-term

84

CHAPTER 3 UNSATURATED POLYESTER MICROCOMPOSITES

immersion where the specimen was placed in a container of water at room temperature, rested at its edge, and entirely immersed. At the end of 24 hours the sample was removed from water, wiped free of surface moisture with a dry cloth, and weighed to the nearest 0.001 g immediately; and (2) long-term immersion where the specimen was placed in a container of water at room temperature, rested at its edge, and entirely immersed. Readings were taken at frequent intervals until the sample reached the saturation (equilibrium) condition, where no change in the sample weight was noticed. The percentage water retention (WR%) for the UP composites was calculated using the equation: WR% 5

weight of equilibrated sample 2 weight of dry sample 3 100% weight of dry sample

(3.1)

It is a measure of the percent relative increase in weight due to water imbuement or retention within the solid matrix. Fig. 3.7 shows the distilled water retention percent in a 24 hours test at room temperature for different composites as a function of filler content. Polyester resin is hydrophobic in nature so the pure resin samples had negligible water absorption [81]. In general, the WR% value of all the composites was low, and below a filler content of 30 vol.%, the WR of all the composites was comparable. For composites containing 40 vol.% filler, the WR% values were 1.95%, 1.647%, 0.62%, and 0.21% for DP, WR, DL, and RS, respectively. The water absorption increased linearly with filler loading except for in DP as it showed exponential behavior. Increasing the filler content increased the void formation in the composites which should lead to an increase in water

FIGURE 3.7 Effect of filler loading on 24 h water retention (WR) test for polyester composites.

3.6 INVESTIGATION OF PHYSICAL, THERMAL

85

absorption. In addition, Fig. 3.7 shows that the RS
UPR composites had the lowest WR, which means that the cross-linking process was highly efficient in the case of small sized RS particles, which reduced the voids needed for WR. Although the irregularity of rubber particles should have led to an increase in the voids in the composite, the WR of the WR
UPR composite was low. This can be attributed to the hydrophobicity of rubber particles, which repel water and reduced the WR of the prepared composites. On the other hand, the relatively high WR of DP composites results from the hydrophilic nature of DP due to their hydroxyl groups and to the voids induced by the poor compatibility between the natural filler and the polymer matrix. The approximate percentage of hemicellulose, lignin, and cellulose in typical DP are 17.5%, 11.0%, and 42.5% dry weight, respectively [84]. Therefore DP contain large amounts of cellulose molecules that contain a hydroxyl group which attracts water molecules through hydrogen bonding, which in turn leads to water absorption. Plant-based cellulosic fibers contain lumen; a central hollow region which acts as a capillary tube to allow water to be absorbed. Thus as the cellulosic fiber content increases in the composites, more interfacial area exists leading to an increase in water absorption [81]. Ameh et al. [85], observed similar WR behavior, which increased with increasing date seed concentrations in polyester
date seed composites. Although DL contains high amounts of cellulose, the water absorption of DL
UPR composites was very low (less than 1.1% at 60 vol.% DL content). Alrumman [86] reported lignin, cellulose, and hemicellulose contents of date palm leaves as 16.71%, 59.11%, and 16.43%, respectively. Comparing the WR of DP with date leaf fibers reveals that DL water absorption is much lesser, although DL filler has a higher cellulose content. This big difference can be attributed to the difference in shapes, as DP particles have a higher surface area (compare Figs. 3.2B and 3.3B). Relating to several studies, the absorption of natural fibers depends on the specific area of the fibers and their chemical compositions, specifically cellulose [40]. Compared to bentonite
UPR composites [24,26], RS
UPR composites have much lower WR as bentonite has the ability to retain water molecules, while shale has no ability to retain water molecules. The equilibrium water content (WReq ) is another important parameter used to characterize the ability of composite materials for long-term water absorption. This parameter can be determined experimentally if a given sample reaches equilibrium during the measurement or it can be estimated by fitting the experimental data with a convenient equation if equilibrium is not reached during the measurement. See Abu-Jdayil et al. [83] and Abu-Jdayil et al. [88] for more details about the theoretical approach to determine the equilibrium water content. Fig. 3.8 shows the water absorption results for 40 vol.% filler content composites after about 2000 hours of immersion. It can be seen that all composites reached the saturation conditions except for the DL
UPR composites. On the other hand, the results for water absorption up to saturation of the DP
UPR composites are similar to other natural fibers like hemp fiber [40], while the other investigated composites have less WR than many natural and inorganic fillers. The RS
UPR composite water absorption is at least tentimes lower than that of other composites such as hemp fiber
UPR [40], LDPE
date palm [88], and short flax/polypropylene composites [90]. The equilibrium water content (WReq ) for shale
UPR composites varies between 0.453 wt.% at 0 vol.% shale content and 1.190 wt.% at 60 vol.% shale content, which is low compared to other composites proposed for heat insulation or construction materials [88]. In addition, the water absorption of both DP
UPR and DL
UPR composites is much less than that of DPF
cement composites [87].

86

CHAPTER 3 UNSATURATED POLYESTER MICROCOMPOSITES

FIGURE 3.8 Long-term water absorption for 40 vol.% filler content composites.

3.6.3 THERMAL CONDUCTIVITY A thermal conductivity testing machine, Lasercomp FOX-200, was used to measure the thermal conductivity of the produced composites. A specific mold was fabricated according to the dimensions of the sample required by the Lasercomp heat flow instrument. The dimensions of the samples were 150 3 150 3 20 mm (see Fig. 3.5). The measurement conditions follow the standard methods reported by ASTM C1045-07. The steady state method was used in these measurements, where the thermal conductivity was determined from measurements of the temperature gradient in the composite material and the heat input. The results of the thermal conductivity measured at 25 C will be discussed here. The thermal conductivity (k) of pure polyester was measured as 0.136 W=m  K. Fig. 3.9 illustrates the measured thermal conductivity for all composites at 25 C as a function of filler content. The lowest measured thermal conductivity was 0.104 W=m  K for the WR
UPR composite at a filler content of 40 vol.%. While the highest was 0.218 W=m  K for 60 vol.% RS
UPR composites. The results for RS show that the thermal conductivity of the composites remained in the same range as the thermal conductivity of neat polyester up until a filler content of 30 vol.% (#0.15 W=m  K). Increasing the filler content beyond 30 vol.% causes a clear increase in the thermal conductivity coefficient, ranging from 0.148 to 0.218 W=m  K for 30 and 60 vol.% filler contents, respectively. This rise is expected because the shale used in this study is rich in silica and alumina, which have higher thermal conductivities compared to that of polyester. The thermal conductivity of most mineral fillers is about one order of magnitude higher than that of polymers and their incorporation considerably increases the conductivity of a composite [25]. The results illustrated in Fig. 3.9 show that the incorporation of rubber particles into polyester matrices reduces the thermal conductivity as the amount of rubber increases in the composites. This

3.6 INVESTIGATION OF PHYSICAL, THERMAL

87

FIGURE 3.9 Effect of filler loading on thermal conductivity of composites at 25 C.

is due to the low thermal conductivity of the filler (0.082 W=m K at 25 C). With the maximum filler content (40 vol.%), the thermal conductivity of the composite of rubber reaches 0.104 W=m  K at 25 C. This value is comparable with the thermal conductivity of commercial heat insulators and is less than several rubber-based composites [83]. Eiras et al. [88] found experimentally that the thermal conductivity of rubberized mortar composites varies between 0.255 and 0.443 W=m  K. Increasing the DP content, in general, caused a slight increase in the thermal conductivity coefficient of the DP
UPR composite. This may be due to the high moisture absorption capacity of DP compared to polyester resin. On the other hand, increasing the date leaf fiber content led to a slight decrease in the thermal conductivity coefficient of the DL
UPR composites. The minimum thermal conductivity of DL
UPR was reported at 30 vol.% filler content, which was 0.110 W=m  K. At a 60 vol.% filler content, the thermal conductivities of DP and date leaf composites were 0.166 and 0.128 W=m  K, respectively. The mean value of the thermal conductivity of pure date palm samples studied by Agoudjil et al. [93]was k 5 0.083 W=m  K at atmospheric pressure. Although an increase in thermal conductivity with increasing filler content has been observed, the thermal conductivities of the prepared composite materials were very promising and comparable with other proposed composites containing DPF (0.075
0.6 W=m  K) [94], hemp fibers (0.115 W=m  K) [95], and banana fibers (0.117 W=m  K) [96]. On the other hand, the thermal conductivity of composites reported here is less than the thermal conductivity of hemp, jute and glass fiber
reinforced polyester composites studied by Subba Raju et al. [72], as the thermal conductivity of their composites varied between 0.207 and 0.190 W=m  K at 30 C for filler content ranged between 18 and 36 vol.%. The thermal conductivity of banana/sisal
UPR composites was reported in the range of 0.181
0.213 W=m  K [80], which is greater than k values of investigated composites, specifically the natural fiber
reinforced composites.

88

CHAPTER 3 UNSATURATED POLYESTER MICROCOMPOSITES

3.6.4 THERMOGRAVIMETRIC ANALYSIS TGA studies were carried out using a thermal gravimetric analyzer, TGA 7 (Perkin-Elmer, USA). About 20 mg of cured composite was heated under nitrogen flow (100 mL/min) from 30 C to 950 C using a heating rate of 5 C/min. TGA was employed to evaluate the thermal stability and decomposition process of the samples. TGA provides definitive data for material and product design and aging stability information with short test times. Fig. 3.10 shows the residual mass as a function of temperature for the pure fillers and neat polyester resin. The shale powder shows the highest thermal stability with weight loss less than 6.5 wt. % up to a temperature of 950 C. The pure polyester and other fillers (DP, DL, and WR) degraded in three stages. However, the first stage, which is due to moisture evaporation, was not remarkable in the polyester and to a lesser degree in the pure rubber. The pure polyester began to degrade at 350 C, a further rapid mass decrease was observed until 500 C with about 95 wt.% mass loss during the decomposition stage. The loss of the cross-linked polyester structure can account for the main mass loss during this decomposition stage, leaving a small amount of residual char. The waste rubber started to degrade at 220 C, a further rapid mass decrease was observed until 425 C with about 56 wt.% mass loss during the decomposition stage. On the other hand, both DP and date palm leaf have approximately the same thermal behavior. The date palm leaf is a little more stable than the DP. DPs and leaves mainly consist of three components with decomposition temperatures between 200 C and 400 C, namely; hemicellulose, cellulose, and lignin. Hemicellulose decomposes between 160 C and 360 C and cellulose decomposes between 240 C and 390 C. The decomposition temperature range of lignin is wider and higher,

FIGURE 3.10 TGA of pure polyester and pure fillers.

3.6 INVESTIGATION OF PHYSICAL, THERMAL

89

FIGURE 3.11 TGA of UP-based composites.

that is, between 250 C and 700 C [97]. As shown in Fig. 3.10, the date leaf shows an initial loss of 5% at 85 C, a 50% weight loss at 325 C, and the char yield of the fiber was 23% at 950 C. The DP exhibited an initial degradation of 5% at 100 C, a 50% weight loss at 295 C, and the char yield of the fiber was 18% at 950 C. Ali and Alabdulkarem [98] found that the T50% degradation temperature of date palm tree surface fibers was 364 C, while the char yield of the same fiber was 22% at 1192 C. On the other hand, the TGA curves tend to shift to a higher decomposition temperature when shale filler is incorporated into the polyester matrix (see Fig. 3.11). Adding shale into UPs led to improved thermal behavior, with a lower maximum mass loss rate and higher char yield. These enhancements are attributed to the capability of filler to withstand high temperatures and hence it could preserve the polyester network structure from degradation.

3.6.5 MECHANICAL STRENGTH A compressive test was conducted using a universal testing machine (MTS Model MH/20 with a load capacity of 100 kN). The specimen was compressed between the upper (movable) and lower (fixed) plates of the machine. The loading continued either until the fracture of the specimen occurred or up to the point where the distance between the upper and lower plates reached a specific predetermined value and then the test was interrupted. The dimensions of the compression test specimen were 30 mm both in length and diameter. All compression tests were performed at room temperature and with an overhead speed of 1 mm/min following the ASTM D695-15 standard.

90

CHAPTER 3 UNSATURATED POLYESTER MICROCOMPOSITES

The tensile curves were obtained using the same machine as the one used for the compression test. The specimen was installed into the movable and fixed jaws of the machine. All tests ended when the specimen fractured. The dimensions of the dumbbell-shaped sample were an overall length of 100 mm, a gauge length of 20 mm, and a thickness of 4 mm. All tensile tests were performed at room temperature with a 2 mm/min overhead speed following the ASTM D638-14 standard.

3.6.5.1 Compressive strength In the compressive strength test, pure polyester samples flattened without failure at overhead displacements of up to 25 mm, whereas composite samples fractured before this value of displacement could be achieved. The impact of filler content on the compressive strength of UPR composites is presented in Fig. 3.12. It can be seen that the compression strengths of all the composites were reduced with increasing filler content, but with extents based on the nature of the filler. The WR
UPR composites exhibit the lowest compressive strengths. The compressive strength of the rubber-based composite reaches a minimum at 40 vol.%. The compressive strength of neat polyester is 103.8 MPa. On the other hand, the compressive strength of composites with a 40% rubber content is 18.7 MPa, with a reduction of 82%. Increasing the rubber content led to an increase in voids in the structure of the composites and consequently a decrease in the compressive strength. The reduction in the strength of the composites with the addition of rubber particles could also be attributed to the weak interfacial bonds between the polyester and the tire rubber particles [82]. On the other hand, as the rubber content is increased, the degree of cross-linking between the rubber particles and the polyester is reduced, which results in decreased strength. The lower modulus of rubber acts like a soft core when compared to the surrounding polyester. Under loading conditions,

FIGURE 3.12 Effect of filler loading on compressive strength of composites.

3.6 INVESTIGATION OF PHYSICAL, THERMAL

91

therefore, the rubber particles may cause essential stress concentration and decrease the overall strength of the rubber
polyester composite. On the other hand, the RS
UPR composites exhibited the highest compressive strength. Compared to the neat polymer, the compressive strength of RS
UPR matrices tends to decrease slightly with increasing filler content because of the concomitant formation of voids. However, the compression strength of composites with a 60% shale content is 94.1 MPa, with a reduction of 9.3%. Thus the decrease in compressive strength observed for increasing shale content was ascribed to the concomitant increase of matrix porosity due to air entrainment and poorer shale/matrix adhesion. In the case of date palm biomass, Fig. 3.12 shows that the composites based on DL have greater strengths than those of DP at the same filler content. For composites comprising 60 vol.% of filler, the compressive strength of the DL
UPR composite (67.8 MPa) is nearly double that of the compressive strength of the DP
UPR composite (34.3 MPa). One of the reasons can be the higher cellulose content of DL, which reaches 59.11% [87], while the cellulose content of DP is approximated to be 42.5% [84]. It is a well-known fact that the cellulose content of natural fibers is responsible for their mechanical strength. In addition, the higher affinity of DP composites for moisture absorption reduces their mechanical strength as the diffused water initiates microcracks that later lead to a reduction in the material strength. The investigation of the compressive strengths of natural fiber
reinforced composites has received little attention in the literature compared to tensile strength. However, all the prepared composites showed compressive strengths superior to those of other composites commonly used as lightweight construction materials (e.g., composites of cement, sand, and coconut and durian fiber) (2.4
3.3 MPa) [99], also outperforming stone masonry (estimated compressive strength of 20
30 MPa) [100]. John and Naidu [68] observed a marginal decrease in the compressive strength of sisal fiber/glass fiber hybrid composites over that of UPR matrices.

3.6.5.2 Tensile strength Fig. 3.13 shows the tensile strength variation of the prepared composites with the corresponding filler contents. It can be noticed that the tensile strength/failure stress and elastic modulus of UP composites decreased with increasing rubber content. Neat UP samples have been found to have a maximum/failure stress of 39.7 MPa, and an elastic modulus of 1.03 GPa, whereas filler-reinforced polyester composites have lower values. The WR
UPR composites showed the lowest values for tensile strength and elastic moduli among all investigated composites. The tensile strength of the WR
UPR composites fell from 20 MPa at 10 vol.% filler content to 4.35 MPa at 40 vol.% rubber content. For the same samples, the modulus of elasticity changes from 0.73 to 0.158 GPa. The reduced tensile strength at higher percentages of rubber particles may be due to the lower efficiency of stress transfer between the rubber particles and the polyester matrix interface because of the weak adhesion force at the interface, direct contact between particles, and possibly the existence of voids. Abu-Jdayil et al. [82] found that the lesser the size of the rubber particles, the higher the composite tensile strength at any considered rubber particle content. The two main parameters that may be responsible for the trend are the enhanced dispersion and rubber particles
polyester matrix interaction. However, the produced WR
UPR composites showed outstanding tensile strengths compared to currently used insulating materials (e.g., foam-glass, mineral fibers, and polystyrene), where their tensile strengths were less than 1.0 MPa.

92

CHAPTER 3 UNSATURATED POLYESTER MICROCOMPOSITES

FIGURE 3.13 Effect of filler loading on the tensile strength of composites.

On the other hand, the tensile strength of polyester samples reinforced with RS decreased slowly with increasing filler content. It drops linearly from 31.8 MPa at 10 vol.% filler content to 18.9 MPa at 60 vol.% filler content. In general, the RS
UPR showed the highest tensile strength at a filler content of 30 vol.% and less. The observed higher tensile strength at lower filler contents is attributed to the better dispersion of shale in the polyester resin matrix, the absence of voids or pores, and good interfacial bonding at low-filler loadings. While, the low tensile strength at high filler content can be attributed to inefficient stress transfer at the particle
matrix interface due to poor interfacial adhesion, particle-to-particle contact, and the presence of voids or pores. Considering the natural fillers, both DP and date palm leaf show good tensile strengths compared to WR
UPR composites. As can be seen in Fig. 3.13, at a low-filler content, DP
UPR shows higher tensile strength than DL
UPR composites. However, the frailer stress of DP
UPR decreases with filler content, and with an accelerated rate at filler contents higher than 30 vol.%. The tensile strength of DP
polyester composites fell from 35.1 MPa at 10 vol.% filler content, to 23.6 MPa at 30 vol.%, and then to 7.7 MPa at 60 vol.%. The corresponding elastic moduli of the previous samples were 1.05, 1.0, and 0.36 GPa, respectively. This clear reduction in tensile strength of DP
UPR composites upon increasing DP contents can be attributed to two factors, namely the poor compatibility between the natural fillers and the matrix and the agglomeration of natural fillers. Specifically, the hydroxyl groups (2OH) that are present in natural fillers make them hydrophilic, whereas polymers are hydrophobic, resulting in poor compatibility. In addition, the presence of hydrogen bonds between the particles of natural fillers leads to the agglomeration of particles and increases in the gaps between the DP particles and the polyester matrix, which can be supported by the morphology of the 30 vol.% DP
UPR composite as shown in Fig. 3.14.

3.7 CONCLUSION

93

FIGURE 3.14 Morphology of DP
UPR composite.

Interesting behavior was observed for DL
UPR composites, where their tensile strength was approximately constant over a filler content range of 20
60 vol.% (see Fig. 3.13). The tensile strength of the 10 vol.% DL
UPR composite was 26.5 MPa, which decreased slightly to 22.8 MPa at a 60 vol.%. On the other hand, the tensile modulus of the DL
UPR composites was higher than that of the DP
UPR and WR
UPR composites and varied between 1.4 and 1.09 GPa. Compared to natural filler composites, DP and date leaf fibers show lower tensile strengths than bamboo (126.2 MPa), jowar (124 MPa), and sisal (65.5 MPa) composites [55]. In addition, the tensile moduli of DL and DP fibers are less than that of bamboo (2.75 GPa) and jowar (2.48 GPa), but comparable with sisal composites (1.9 GPa) [55].

3.7 CONCLUSION Taking into account all the investigated properties for the prepared composites, it can be concluded that the date palm leaf
based composites have the optimum behavior, with low density, thermal conductivity, and WR, accompanied with acceptable mechanical strengths. In general, the proposed natural-based composites are comparable to conventional insulation and building materials with the advantages of being safer to human beings as well as utilizing waste material. Moreover, the morphology of DP
and date leaf
polyester composites presented in Figs. 3.14 and 3.15 show that there are noticeable voids between the filler fibers and the polymer matrix. Reducing such kinds of

94

CHAPTER 3 UNSATURATED POLYESTER MICROCOMPOSITES

FIGURE 3.15 Morphology of DL
UPR composite.

gaps using different types of fiber treatments and/or hybridization methods mentioned in the previous sections will increase the compatibility between date palm materials and polymer matrices, which will eventually lead to enhancements in the mechanical behavior of composites. The utilization of such natural-based composites in different applications will have positive environmental and economic impacts.

ACKNOWLEDGMENTS The authors would like to acknowledge the financial support provided by the College of Engineering at the UAE University (Project # 31N092) and the Emirates Center for Energy and Environment Research (Project # 31R041).

REFERENCES [1] Y. Jiang, D. Maniar, A.J. Woortman, G.O. Alberda van Ekenstein, K. Loos, Enzymatic polymerization of furan-2,5-dicarboxylic acid-based furanic-aliphatic polyamides as sustainable alternatives to polyphthalamides, Biomacromolecules 16 (11) (2015) 3674
3685. [2] L.A. Pothan, Z. Oommen, S. Thomas, Dynamic mechanical analysis of banana fiber reinforced polyester composites, Comp. Sci. Technol. 63 (2) (2003) 283
293.

REFERENCES

95

[3] P.S.G. Krishnan, S.T. Kulkarni, Polyester resins, in: B.L. Deopura, R. Alagirusamy, M. Joshi, B. Gupta (Eds.), Polyesters Polyamides, Elsevier, 2008, pp. 3
40. [4] S.H. Aziz, M.P. Ansell, S.J. Clarke, S.R. Panteny, Modified polyester resins for natural fibre composites, Comp. Sci. Technol. 65 (3-4) (2005) 525
535. [5] S. He, N.D. Petkovich, K. Liu, Y. Qian, C.W. Macosko, A. Stein, Unsaturated polyester resin toughening with very low loadings of GO derivatives, Polymer. (Guildf). 110 (2017) 149
157. [6] L.B. Manfredi, E.S. Rodrı´guez, M. Wladyka-Przybylak, A. V´azquez, Thermal degradation and fire resistance of unsaturated polyester, modified acrylic resins and their composites with natural fibres, Polym. Degrad. Stability 91 (2) (2006) 255
261. [7] S.J. Park, J.S. Jin, Effect of silane coupling agent on interphase and performance of glass fibers/unsaturated polyester composites, J. Colloid. Interface. Sci. 242 (1) (2001) 174
179. [8] Textile Lerner, 2018. http://textilelearner.blogspot.com/2011/08/glass-fiber-types-of-glass-fiber_3834. html (accessed 02.12.18). [9] J.H.S. Almeida Jr., S.C. Amico, E.C. Botelho, F.D.R. Amado, Hybridization effect on the mechanical properties of curaua/glass fiber composites, Comp. Part B: Eng. 55 (2013) 492
497. [10] M.S. EL-Wazery, M.I. EL-Elamy, S.H. Zoalfakar, Mechanical properties of glass fiber reinforced polyester composites, Int. J. Appl. Sci. Eng. 14 (3) (2017) 121
131. [11] M. Ramesh, K. Palanikumar, K.H. Reddy, Mechanical property evaluation of sisal
jute
glass fiber reinforced polyester composites, Comp. Part B: Eng. 48 (2013) 1
9. [12] M.R. Sanjay, B. Yogesha, Studies on natural/glass fiber reinforced polymer hybrid composites: an evolution, Mater. Today: Proc. 4 (2) (2017) 2739
2747. [13] M.R. Sanjay, G.R. Arpitha, B. Yogesha, Study on mechanical properties of natural-glass fibre reinforced polymer hybrid composites: a review, Mater. Today: Proc. 2 (4-5) (2015) 2959
2967. [14] M. Davallo, H. Pasdar, Comparison of mechanical properties of glass-polyester composites formed by resin transfer moulding and hand lay-up technique, Int. J. Chem. Tech. Res. CODEN (USA): IJCRGG 1 (3) (2009) 470
475. [15] M.C. Ribeiro, A. Fiu´za, A.C. Meira Castro, F.G. Silva, J.P. Meixedo, M.L. Dinis, A case study undertaken recycling & reuse of glass fiber reinforced thermoset polumer wastes of composite materials industry. eSYMBIOSIS 2014, 2014. [16] M. Kanerva, E. Sarlin, M. Hoikkanen, K. Ra¨mo¨, O. Saarela, J. Vuorinen, Interface modification of glass fibre
polyester composite
composite joints using peel plies, Int. J. Adhesion Adhesives 59 (2015) 40
52. [17] T. Bayer, K.J. Eichhorn, K. Grundke, H.J. Jacobasch, FTIR spectroscopic studies of interfacial reactions between amino functionalized silicon surfaces and molten maleic anhydride copolymers, Macromol. Chem. Phys. 200 (4) (1999) 852
857. [18] J.H. Kim, D.J. Kwon, P.S. Shin, Y.M. Baek, H.S. Park, K.L. De Vries, et al., Evaluation of surface roughness and frost retardancy of a glass fiber/unsaturated polyester composite, Int. J. Heat Mass Transfer 130 (2019) 282
289. [19] H.N. Dhakal, Z.Y. Zhang, M.O.W. Richardson, Nanoindentation behaviour of layered silicate reinforced unsaturated polyester nanocomposites, Polym. Testing 25 (6) (2006) 846
852. [20] X. Kornmann, L.A. Berglund, J. Sterte, E.P. Giannelis, Nanocomposites based on montmorillonite and unsaturated polyester, Polym. Eng. Sci. 38 (8) (1998) 1351
1358. [21] P. Jawahar, R. Gnanamoorthy, M. Balasubramanian, Tribological behaviour of clay
thermoset polyester nanocomposites, Wear 261 (7-8) (2006) 835
840. [22] M.R. Ismail, M.A.M. Ali, A.A. El-Milligy, M.S. Afifi, Studies on sand/clay unsaturated polyester composite materials, J. Appl. Polym. Sci. 72 (8) (1999) 1031
1038.

96

CHAPTER 3 UNSATURATED POLYESTER MICROCOMPOSITES

[23] J.C. Lucas, J. Borrajo, R.J. Williams, Cure of unsaturated polyester resins: 2. Influence of low-profile additives and fillers on the polymerization reaction, mechanical properties and surface rugosities, Polymer. (Guildf). 34 (9) (1993) 1886
1890. [24] B. Abu-Jdayil, K. Al-Malah, R. Sawalha, Study on bentonite-unsaturated polyester composite materials, J. Reinforced Plast. Comp. 21 (17) (2002) 1597
1607. [25] B. Abu-Jdayil, K. Al-Malah, Jordanian clay-based heat insulator composites: mechanical properties, J. Reinforced Plast. Comp. 27 (14) (2008) 1559
1568. [26] K. Al-Malah, B. Abu-Jdayil, Clay-based heat insulator composites: thermal and water retention properties, Appl. Clay Sci. 37 (1-2) (2007) 90
96. [27] Y.J.V. Ruban, S.G. Mon, D.V. Roy, Chemical resistance/thermal and mechanical properties of unsaturated polyester-based nanocomposites, Appl. Nanosci. 4 (2) (2014) 233
240. [28] H. Tabatabai, M. Janbaz, A. Nabizadeh, Mechanical and thermo-gravimetric properties of unsaturated polyester resin blended with FGD gypsum, Const. Building Mater. 163 (2018) 438
445. [29] T.T.L. Doan, H.M. Brodowsky, U. Gohs, E. Ma¨der, Re-use of marble stone powders in producing unsaturated polyester composites, Adv. Eng. Mater. (2018) 1701061. [30] A.C. De Albuquerque, K. Joseph, L.H. de Carvalho, J.R.M. d’Almeida, Effect of wettability and ageing conditions on the physical and mechanical properties of uniaxially oriented jute-roving-reinforced polyester composites, Comp. Sci. Technol. 60 (6) (2000) 833
844. [31] P.J. Roe, M.P. Ansell, Jute-reinforced polyester composites, J. Mater. Sci. 20 (11) (1985) 4015
4020. [32] K.M. Nair, S.M. Diwan, S. Thomas, Tensile properties of short sisal fiber reinforced polystyrene composites, J. Appl. Polym. Sci. 60 (9) (1996) 1483
1497. [33] S.K. Pal, D. Mukhopadhyay, S.K. Sanyal, R.N. Mukherjea, Studies on process variables for natural fiber composites—effect of polyesteramide polyol as interfacial agent, J. Appl. Polym. Sci. 35 (4) (1988) 973
985. [34] G.E. Prasad, B.K. Gowda, R. Velmurugan, Comparative study of impact strength characteristics of treated and untreated sisal polyester composites, Proc. Eng. 173 (2017) 778
785. [35] S. Sangthong, T. Pongprayoon, N. Yanumet, Mechanical property improvement of unsaturated polyester composite reinforced with admicellar-treated sisal fibers, Comp. Part A: Appl. Sci. Manuf. 40 (6-7) (2009) 687
694. [36] O. Owolabi, T. Czvikovszky, I. Kovacs, Coconut-fiber-reinforced thermosetting plastics, J. Appl. Polym. Sci. 30 (5) (1985) 1827
1836. [37] K.G. Satyanarayana, K. Sukumaran, A.G. Kulkarni, S.G.K. Pillai, P.K. Rohatgi, Performance of banana fabric-polyester resin composites, Composite Structures, 2, Springer, Dordrecht, 1983, pp. 535
548. [38] M.A. De Farias, M.Z. Farina, A.P.T. Pezzin, D.A.K. Silva, Unsaturated polyester composites reinforced with fiber and powder of peach palm: mechanical characterization and water absorption profile, Mater. Sci. Eng. C 29 (2) (2009) 510
513. [39] N.M. White, M.P. Ansell, Straw-reinforced polyester composites, J. Mater. Sci. 18 (5) (1983) 1549
1556. [40] H.N. Dhakal, Z.Y. Zhang, M.O.W. Richardson, Effect of water absorption on the mechanical properties of hemp fibre reinforced unsaturated polyester composites, Comp. Sci. Technol. 67 (7-8) (2007) 1674
1683. [41] L.U. Devi, S.S. Bhagawan, S. Thomas, Mechanical properties of pineapple leaf fiber-reinforced polyester composites, J. Appl. Polym. Sci. 64 (9) (1997) 1739
1748. [42] H.M. Naguib, U.F. Kandil, A.I. Hashem, Y.M. Boghdadi, Effect of fiber loading on the mechanical and physical properties of “green” bagasse
polyester composite, J. Rad. Res. Appl. Sci. 8 (4) (2015) 544
548.

REFERENCES

97

[43] V. Vilay, M. Mariatti, R.M. Taib, M. Todo, Effect of fiber surface treatment and fiber loading on the properties of bagasse fiber
reinforced unsaturated polyester composites, Comp. Sci. Technol. 68 (3-4) (2008) 631
638. [44] L.Y. Mwaikambo, E.T. Bisanda, The performance of cotton
kapok fabric
polyester composites, Polym. Testing 18 (3) (1999) 181
198. [45] D.N. Saheb, J.P. Jog, Natural fiber polymer composites: a review, Adv. Polym. Technol. J. Polym. Processing Institute 18 (4) (1999) 351
363. [46] G. Bogoeva-Gaceva, M. Avella, M. Malinconico, A. Buzarovska, A. Grozdanov, G. Gentile, et al., Natural fiber eco-composites, Polym. Comp. 28 (1) (2007) 98
107. [47] Y. Li, Y.W. Mai, L. Ye, Sisal fibre and its composites: a review of recent developments, Comp. Sci. Technol. 60 (11) (2000) 2037
2055. [48] K. Joseph, S. Varghese, G. Kalaprasad, S. Thomas, L. Prasannakumari, P. Koshy, et al., Influence of interfacial adhesion on the mechanical properties and fracture behaviour of short sisal fibre reinforced polymer composites, Eur. Polym J. 32 (10) (1996) 1243
1250. [49] K. Senthilkumar, N. Saba, N. Rajini, M. Chandrasekar, M. Jawaid, S. Siengchin, et al., Mechanical properties evaluation of sisal fibre reinforced polymer composites: a review, Const. Build. Mater. 174 (2018) 713
729. [50] Y. Xie, C.A. Hill, Z. Xiao, H. Militz, C. Mai, Silane coupling agents used for natural fiber/polymer composites: a review, Comp. Part A: Appl. Sci. Manuf. 41 (7) (2010) 806
819. [51] M. Idicula, K. Joseph, S. Thomas, Mechanical performance of short banana/sisal hybrid fiber reinforced polyester composites, J. Reinforced Plast. Comp. 29 (1) (2010) 12
29. [52] A.V. Rajulu, L.G. Devi, G.B. Rao, R.L. Reddy, Chemical resistance and tensile properties of epoxy/ unsaturated polyester blend coated bamboo fibers, J. Reinforced Plast. Comp. 22 (11) (2003) 1029
1034. [53] K. Al-Kaabi, A. Al-Khanbashi, A. Hammami, Date palm fibers as polymeric matrix reinforcement: DPF/polyester composite properties, Polym. Comp. 26 (5) (2005) 604
613. [54] H.M. Akil, L.W. Cheng, Z.M. Ishak, A.A. Bakar, M.A. Rahman, Water absorption study on pultruded jute fibre reinforced unsaturated polyester composites, Comp. Sci. Technol. 69 (11-12) (2009) 1942
1948. [55] A.R. Prasad, K.M. Rao, Mechanical properties of natural fibre reinforced polyester composites: jowar, sisal and bamboo, Mater. Design 32 (8-9) (2011) 4658
4663. [56] L.Y. Mwaikambo, M.P. Ansell, Chemical modification of hemp, sisal, jute, and kapok fibers by alkalization, J. Appl. Polym. Sci. 84 (12) (2002) 2222
2234. [57] O. Faruk, A.K. Bledzki, H.P. Fink, M. Sain, Progress report on natural fiber reinforced composites, Macromol. Mater. Eng. 299 (1) (2014) 9
26. [58] J. George, M.S. Sreekala, S. Thomas, A review on interface modification and characterization of natural fiber reinforced plastic composites, Polym. Eng. Sci. 41 (9) (2001) 1471
1485. [59] C.A.S. Hill, H.P.S. Abdul Khalil, Effect of fiber treatments on mechanical properties of coir or oil palm fiber reinforced polyester composites, J. Appl. Polym. Sci. 78 (9) (2000) 1685
1697. [60] J. Rout, M. Misra, S.S. Tripathy, S.K. Nayak, A.K. Mohanty, The influence of fibre treatment on the performance of coir-polyester composites, Comp. Sci. Technol. 61 (9) (2001) 1303
1310. [61] L.A. Pothan, J. George, S. Thomas, Effect of fiber surface treatments on the fiber
matrix interaction in banana fiber reinforced polyester composites, Comp. Interfaces 9 (4) (2002) 335
353. [62] P.A. Sreekumar, S.P. Thomas, J. Marc Saiter, K. Joseph, G. Unnikrishnan, S. Thomas, Effect of fiber surface modification on the mechanical and water absorption characteristics of sisal/polyester composites fabricated by resin transfer molding, Comp. Part A: Appl. Sci. Manuf. 40 (11) (2009) 1777
1784.

98

CHAPTER 3 UNSATURATED POLYESTER MICROCOMPOSITES

[63] V.S. Sreenivasan, D. Ravindran, V. Manikandan, R. Narayanasamy, Influence of fibre treatments on mechanical properties of short Sansevieria cylindrica/polyester composites, Mater. Design 37 (2012) 111
121. [64] S. Rajkumar, J. Tjong, S.K. Nayak, M. Sain, Wetting behavior of soy-based resin and unsaturated polyester on surface-modified sisal fiber mat, J. Reinforced Plast. Comp. 34 (10) (2015) 807
818. [65] J.A. Halip, L.S. Hua, Z. Ashaari, P.M. Tahir, L.W. Chen, M.K.A. Uyup, Effect of treatment on water absorption behavior of natural fiber
reinforced polymer composites, Mechanical and Physical Testing of Biocomposites, Fibre-Reinforced Composites and Hybrid Composites, Woodhead Publishing, 2019, pp. 141
156. [66] M.J. John, S. Thomas, Biofibres and biocomposites, Carbohyd. Polym. 71 (3) (2008) 343
364. [67] S. Mishra, A.K. Mohanty, L.T. Drzal, M. Misra, S. Parija, S.K. Nayak, et al., Studies on mechanical performance of biofibre/glass reinforced polyester hybrid composites, Comp. Sci. Technol. 63 (10) (2003) 1377
1385. [68] K. John, S.V. Naidu, Sisal fiber/glass fiber hybrid composites: the impact and compressive properties, J. Reinforced Plast. Comp. 23 (12) (2004) 1253
1258. [69] K. John, S.V. Naidu, Tensile properties of unsaturated polyester-based sisal fiber
glass fiber hybrid composites, J. Reinforced Plast. Comp. 23 (17) (2004) 1815
1819. [70] K. Sabeel Ahmed, Investigations on Mechanical Performance Characteristics of Jute Glass Fiber Hybrid Composites (Ph.D. thesis). Anna University, Chennai, 2007. [71] S.M. Sapuan, H.Y. Lok, M.R. Ishak, S. Misri, Mechanical properties of hybrid glass/sugar palm fibre reinforced unsaturated polyester composites, Chinese J. Polym. Sci. 31 (10) (2013) 1394
1403. [72] N.S. Raju, M.I. Reddy, M.A. Kumar, K. Ramji, Study on thermo physical properties of hemp, jute and glass fiber reinforced polyester composites, Mater. Today: Proc. 5 (2) (2018) 5918
5924. [73] L.A. Pothan, C.N. George, M.J. John, S. Thomas, Dynamic mechanical and dielectric behavior of banana-glass hybrid fiber reinforced polyester composites, J. Reinforced Plast. Comp. 29 (8) (2010) 1131
1145. [74] D. Shanmugam, M. Thiruchitrambalam, Static and dynamic mechanical properties of alkali treated unidirectional continuous palmyra palm leaf stalk fiber/jute fiber reinforced hybrid polyester composites, Mater. Design 50 (2013) 533
542. [75] A. Alavudeen, N. Rajini, S. Karthikeyan, M. Thiruchitrambalam, N. Venkateshwaren, Mechanical properties of banana/kenaf fiber-reinforced hybrid polyester composites: effect of woven fabric and random orientation, Mater. Design (1980-2015) 66 (2015) 246
257. [76] R.K. Adhikari, B.K. Gowda, Exploration of mechanical properties of banana/jute hybrid polyester composite, Mater. Today: Proc. 4 (8) (2017) 7171
7176. [77] D. Romanzini, F. Cuttica, A. Frache, A.J. Zattera, S.C. Amico, Thermal and fire retardancy studies of clay-modified unsaturated polyester/glass fiber composites, Polym. Comp. 38 (12) (2017) 2743
2752. [78] R. Potluri, K. Supriya, G.V.V.N.G. Vittal, Effect of boron carbide particles inclusion on the mechanical behaviour of S2-Glass fiber based polyester composites, Mater. Today: Proc. 5 (9) (2018) 20257
20267. [79] M. Idicula, S.K. Malhotra, K. Joseph, S. Thomas, Dynamic mechanical analysis of randomly oriented intimately mixed short banana/sisal hybrid fibre reinforced polyester composites, Comp. Sci. Technol. 65 (7-8) (2005) 1077
1087. [80] M. Idicula, A. Boudenne, L. Umadevi, L. Ibos, Y. Candau, S. Thomas, Thermophysical properties of natural fibre reinforced polyester composites, Comp. Sci. Technol. 66 (15) (2006) 2719
2725. [81] S. Das, Mechanical and water swelling properties of waste paper reinforced unsaturated polyester composites, Construct. Build. Mater. 138 (2017) 469
478.

REFERENCES

99

[82] B. Abu-Jdayil, A.H.I. Mourad, A. Hussain, Investigation on the mechanical behavior of polyester-scrap tire composites, Construct. Build. Mater. 127 (2016) 896
903. [83] B. Abu-Jdayil, A.H. Mourad, A. Hussain, Thermal and physical characteristics of polyester
scrap tire composites, Construct. Build. Mater. 105 (2016) 472
479. [84] A. Benazzouk, O. Douzane, K. Mezreb, B. Laidoudi, M. Qu´eneudec, Thermal conductivity of cement composites containing rubber waste particles: Experimental study and modelling, Construct. Build. Mater. 22 (4) (2008) 573
579. [85] M.A. Al-Ghouti, J. Li, Y. Salamh, N. Al-Laqtah, G. Walker, M.N. Ahmad, Adsorption mechanisms of removing heavy metals and dyes from aqueous solution using date pits solid adsorbent, J. Hazard. Mater. 176 (1-3) (2010) 510
520. [86] A.O. Ameh, M.T. Isa, I. Sanusi, Effect of particle size and concentration on the mechanical properties of polyester/date palm seed particulate composites, Leonardo Electron. J. Pract. Technol. 26 (2015) 65
78. [87] S.A. Alrumman, Enzymatic saccharification and fermentation of cellulosic date palm wastes to glucose and lactic acid, Braz. J. Microbiol. 47 (1) (2016) 110
119. [88] B. Abu-Jdayil, A.H. Mourad, M. Hassan, Development of polymeric heat insulators based on emirati red shale filler: thermal and physical properties, Polym. Comp. 39 (S3) (2018) E1463
E1473. [89] M.A. AlMaadeed, Z. Nogellova, M. Miˇcuˇs´ık, I. Novak, I. Krupa, Mechanical, sorption and adhesive properties of composites based on low density polyethylene filled with date palm wood powder, Mater. Design 53 (2014) 29
37. [90] A. Arbelaiz, B. Fernandez, J.A. Ramos, A. Retegi, R. Llano-Ponte, I. Mondragon, Mechanical properties of short flax fibre bundle/polypropylene composites: Influence of matrix/fibre modification, fibre content, water uptake and recycling, Comp. Sci. Technol. 65 (10) (2005) 1582
1592. [91] N. Benmansour, B. Agoudjil, A. Gherabli, A. Kareche, A. Boudenne, Thermal and mechanical performance of natural mortar reinforced with date palm fibers for use as insulating materials in building, Energy Build. 81 (2014) 98
104. [92] J.N. Eiras, F. Segovia, M.V. Borrachero, J. Monzo´, M. Bonilla, J. Pay´a, Physical and mechanical properties of foamed Portland cement composite containing crumb rubber from worn tires, Mater. Design 59 (2014) 550
557. [93] B. Agoudjil, A. Benchabane, A. Boudenne, L. Ibos, M. Fois, Renewable materials to reduce building heat loss: Characterization of date palm wood, Energy Build. 43 (2-3) (2011) 491
497. [94] N. Benmansour, B. Agoudjil, A. Gherabli, A. Kareche, A. Boudenne, Thermal and mechanical performance of natural mortar reinforced with date palm fibers for use as insulating materials in building, Energy Build. 81 (2014) 98
104. [95] T. Behzad, M. Sain, Measurement and prediction of thermal conductivity for hemp fiber reinforced composites, Polym. Eng. Sci. 47 (7) (2007) 977
983. [96] S.A. Paul, A. Boudenne, L. Ibos, Y. Candau, K. Joseph, S. Thomas, Effect of fiber loading and chemical treatments on thermophysical properties of banana fiber/polypropylene commingled composite materials, Comp. Part A: Appl. Sci. Manuf. 39 (9) (2008) 1582
1588. [97] M.E. Babiker, A.R.A. Aziz, M. Heikal, S. Yusup, Pyrolysis characteristics of Phoenix dactylifera date palm seeds using thermo-gravimetric analysis (TGA), Int. J. Environ. Sci. Dev. 4 (5) (2013) 521. [98] M.E. Ali, A. Alabdulkarem, On thermal characteristics and microstructure of a new insulation material extracted from date palm trees surface fibers, Construct. Build. Mater. 138 (2017) 276
284. [99] J.C. Morel, A. Mesbah, M. Oggero, P. Walker, Building houses with local materials: means to drastically reduce the environmental impact of construction, Build. Environ. 36 (10) (2001) 1119
1126. [100] J. Khedari, B. Suttisonk, N. Pratinthong, J. Hirunlabh, New lightweight composite construction materials with low thermal conductivity, Cement Concrete Comp. 23 (1) (2001) 65
70.

100

CHAPTER 3 UNSATURATED POLYESTER MICROCOMPOSITES

FURTHER READING S. Bahadur, Y. Zheng, Mechanical and tribological behavior of polyester reinforced with short glass fibers, Wear 137 (2) (1990) 251
266. A. Benzoic, O. Douzane, K. Mezreb, B. Laidoudi, M. Qu´eneudec, Thermal conductivity of cement composites containing rubber waste particles: experimental study and modelling, Construct. Build. Mater. 22 (2008) 573
579. F. Chu, X. Yu, Y. Hou, X. Mu, L. Song, W. Hu, A facile strategy to simultaneously improve the mechanical and fire safety properties of ramie fabric-reinforced unsaturated polyester resin composites, Comp. Part A: Appl. Sci. Manuf. 115 (2018) 264
273. M.A.M. Hassan, Development of Polymeric Heat Insulators Based on Local Shale Fillers (Master thesis). UAE University, 2015. J.D. Rusmirovi´c, T. Radoman, E.S. Dˇzunuzovi´c, J.V. Dˇzunuzovi´c, J. Markovski, P. Spasojevi´c, et al., Effect of the modified silica nanofiller on the mechanical properties of unsaturated polyester resins based on recycled polyethylene terephthalate, Polym. Comp. 38 (3) (2017) 538
554.

CHAPTER

UNSATURATED POLYESTER NANOCOMPOSITES

4

Shivkumari Panda and Dibakar Behera School of Applied Sciences (Chemistry), KIIT University, Bhubaneswar, India

4.1 INTRODUCTION AND HISTORY Polymers have been the interest of numerous studies over the past four decades due to the rising interest in high performance and light weight materials design for various engineering components. 21st century scientists have developed an inspiring number of matrices that can be used for polymer-based composites such as unsaturated polyesters (UPEs), epoxies, vinyl esters, and others. Among them, UPEs represent a special rank of thermosetting polymeric materials having excellent wetting properties with reinforcement and high physical and chemical properties as well as being low cost [1]. UPE is the result of a polyesterification reaction between a dicarboxylic acid and a dihydroxy alcohol. It contains double bonds along the polymeric chain for which they are classified as vinyl types. The term UPE, a combination of reactive polymers and reactive monomers, encompasses a broad field of reinforced plastics in general and glass fiber
reinforced thermosetting resins in particular. They are used in a wide range of commercial products such as plastics, fibers, composites, and in coating applications [2]. Use of this polymer has been increasing over the past four decades due to its relatively high specific strength, corrosion resistance, design flexibility, and low density. Again, UPE is largely used commercially as fibers, plastics, composites, and is coating applications. Since 1930, UPE resin (UPR) has been used widely for various ranges of applications with major importance [3]. The wide-spread utilization of this thermoset resin is successfully related to its comparatively low cost and easy availability. Generally, the main backbones of UPE molecules are unsaturated acids, saturated acids, glycols, and cross-linking monomers. For its curing at room temperature, various initiators and promoters such as vinyl monomers like styrene, vinyl toluene, alpha methyl styrene, methyl methacrylate, and diallyl phthalate are used to create resin in its commonly used liquid form. Among these, cross-linking monomers of styrene are broadly used because of their easy availability and superior quality. However, the presence of these styrene monomers enhances the viscosity of resins. Generally, in coating formulations UPR is dissolved in a vinyl monomer (styrene) followed by cross-linking and curing in presence of initiator (methyl ethyl ketone peroxide; MEKP) to speed up the reaction. It has great utility in various

Unsaturated Polyester Resins. DOI: https://doi.org/10.1016/B978-0-12-816129-6.00004-1 © 2019 Elsevier Inc. All rights reserved.

101

102

CHAPTER 4 UNSATURATED POLYESTER NANOCOMPOSITES

structural fields for low cost housing projects as well as in marine fields. There are different types of UPE resins that can be used in various fields. These include: • • • • • • • •

General purpose UPRs (trays, boats, water tanks, and building panels for industrial applications) Flexible UPRs (decorative furniture castings and frames) Resilient UPRs (helmets, guards, and aircraft and automotive parts) Low-shrinkage UPRs Weather-resistant UPRs (gel coats and outdoor structural panels) Chemical-resistant UPRs (fume hoods, reaction, tanks, and pipes) Fire-resistant UPRs (navy boats) Marine-grade UPRs

However, the main problem of this viscous, bulky bifunctional monomer is its high reactivity, high molecular weight, low polymerization shrinkage, and cross-linked three-dimensional resin network. So, its applicability decreases due to its poor damage tolerance, low mechanical strength, and some environmental factors [4]. For this reason, various different methods with the use of blending different types of polymers, reinforcements for hybrid composites, and chemical stabilizers have been conducted to improve the mechanical and physical properties of the polymer [5]. Polymer blends show improved fracture resistance, reduced overall production costs, and enhanced handling of monomer material [6].

4.2 POLYMERIZATION The term UPE is very broad in the field of thermosetting resin and UPEs are the result of polyesterification reactions between a dicarboxylic acid and a dihydroxy alcohol as shown in Fig. 4.1.

FIGURE 4.1 Esterification reaction for preparation of isophthalic unsaturated polyester.

4.2 POLYMERIZATION

103

This is identified by the ester group. Besides this, UPEs contain double bonds along the polymeric chain for which they are classified as vinyl types. For curing UPE, vinyl monomers like styrene, vinyl toluene, alpha methyl styrene, methyl methacrylate, and diallyl phthalate are used to create resins in the commonly used liquid form. Among these cross-linking monomers, styrene is broadly used for its easy availability and superior quality. Generally, in coating formulations UPR is dissolved in a vinyl monomer (styrene) followed by cross-linking and curing by stirring in an initiator (MEKP) before its use. A schematic diagram for the cross-linking and curing reaction mechanism for UPE is given in Fig. 4.2. This polymerization reaction mechanism for curing UPE follows the free radical addition of styrene across the double bonds. Here the free radical is initiated by organic peroxide (MEKP). So no volatile byproducts are evolved.

FIGURE 4.2 Cross-linking reaction for curing of unsaturated polyester.

104

CHAPTER 4 UNSATURATED POLYESTER NANOCOMPOSITES

4.3 FABRICATION TECHNIQUES UPRs have been used in a variety of fiber reinforced fabrication procedures. Among various techniques, some important methods include hand lay-up, spray lay-up, filament winding, and compression molding. Hand lay-up has been utilized broadly due to its easy and time saving procedure. This method is used in large marine parts, sanitary ware, and storage tanks. General ideas on the mentioned types of fabrications are briefly described here.

4.3.1 HAND LAY-UP This is the easiest and oldest fabrication technique and it requires minimum infrastructural skill and no machinery. A mold consisting of a male and a female part is usually used in this process. For getting glossy structure on the product morphology, a release gel is sprayed onto the mold surface. It is also used to keep the polymer from sticking to the surface of the mold. As thermosetting resins are highly adhesive in nature, to avoid the resin sticking to the mold, a thin plastic sheet is used on both sides of the mold to integrate a suitable releasing mechanism. A reinforcement of the mat layer is placed beneath the surface of the gel coat layer. Then the liquid UPR is mixed properly in appropriate proportions with a catalyst and then with an accelerator. Then the whole mixture is poured onto the surface of the mat. The polymer is homogeneously distributed by means of a brush and a second layer of mat is placed on it. A roller is used with gentle pressure on top of the mat and polymer layer to take away the trapped air and the excess polymer present in it. This process is repeated for each layer according to requirement. Finally, it is covered by a thin plastic sheet and release gel is sprayed on the inner surface of the top mold plate that is then placed on the prepared layers at room temperature with certain applied pressure for 24 hours. Finally, the mold is opened and the cured sample is placed in an oven at high temperatures (80 C
120 C) according to the resin type. This technique has very important applications in the field of automotive instruments, aircraft parts, boat hulls, and various indoor and outdoor products.

4.3.2 SPRAY LAY-UP This technique is an expansion of the hand lay-up process. A mold hand lay-up like technique is also used here which is covered with a spray release gel for the easy removal of the prepared material from the mold. Here a reinforcement used is in the form of chopped fibers. In this process, a spray gun is used to spread out the resin as well as the reinforcement. Usually glass roving is used as the reinforcement which passes easily through the spray gun. The thermosetting matrix and reinforcement are sprayed at the same time or individually one by one. A roller is moved with gentle pressure on top of the sprayed material to take away the trapped air present. After a certain thickness of the spraying mixture is reached the product undergoes curing at room temperature. Finally, the mold is opened to remove the prepared composite for additional final processing. This technique is used for preparing low load transportation parts like small boats, bath tubs, fairing of trucks, etc.

4.4 IMPORTANCE OF UNSATURATED POLYESTER-BASED COMPOSITES

105

4.3.3 COMPRESSION MOLDING This technique is a closed molding process with the advantages of little raw material wastage, low maintenance cost, negligible residual stress, and maximum dimensional accuracy with low shrink characteristics. Here, two synchronized metal molds are used with an inactive base plate and variable upper plate where both the reinforcement and matrix mixture are placed. The entire system is placed in between a compression molder with a certain temperature, pressure, and time period according to the requirement of the resin matrix used. All three of these parameters are vital and have to be maintained properly to build the optimum required properties in the composites. After the application of pressure and temperature through a hydraulic mechanism the materials acquire the shape of the mold cavity according to its design. Insufficient applied pressure and temperature may cause poor interfacial adhesion between the fiber and the matrix. This situation may lead to the development of cracks in the resulting composite or the release of resin from the composite system through the degradation of the shape. Also, the improper application of time of these external forces of temperature and pressure causes internal defects in the composite material. So, the curing of the material may occur at room temperature or at high temperature followed by removal of the composite for further processing. This technique has high indoor and outdoor applicability from home appliances to the automobile field, electronic items, aircraft parts, and medical equipment, etc. The products have good surface morphology and flexibility.

4.3.4 FILAMENT WINDING This technique has many advantages over other fabrication methods. It is an automated process so minimum effort is required but it provides high production rates. The prepared materials have low cost, but high strength, high design flexibility, void free structures, and a good degree of uniformity. The filament winding procedure consists of fiber roving, a rotating mandrel, a winding pattern, resin impregnation, and a moving carriage. In this method, the matrix is placed in a resin bath and the fibers, in a particular orientation, are dipped completely into the bath and wound in a controlled manner around the mandrel. Fiber tension has an enormous effect on the prepared material as it affects the type, geometry, and proportion of the fiber reinforcement required on the rotating mandrel. An optimum fiber tension is necessary for this technique because a too-high fiber tension may crack the fiber surface completely or partially. After curing the layers, the final composite material is removed from the mandrel. A carriage is used to keep the roving in place and to direct them to the mandrel. This method is used to produce pressure vessels, rocket motor cases, gas cylinders, air craft parts, and some engineered materials.

4.4 IMPORTANCE OF UNSATURATED POLYESTER-BASED COMPOSITES Cured UPEs have high glass transition temperature (Tg) values. So they are too brittle and exhibit poor resistance to crack propagation for many engineering applications in the absence of reinforcements [7]. These polymers usually craze on their free surface and the crazed areas are converted into cracks, which propagate with relatively little energy absorption resulting in fracture. So, the brittleness of this class of nonbiodegradable thermosetting materials is one of their major

106

CHAPTER 4 UNSATURATED POLYESTER NANOCOMPOSITES

drawbacks. Therefore their utilization in industrial applications has been restricted to some extent. So, the modification of resin using various types of thermosetting polymers, thermoplastic polymers, and elastomers can improve the impact strength and fracture properties of these rigid polymers. The blending of UPE helps to prepare various grades of polymers which exhibit better properties than pristine UPRs. Miscible blends give better results as they have no phase separation. Again, the efficiency of a polymer blend depends on its miscibility or the compatibility of the polymer constituents. So, to get rid of such bottlenecks, sometimes reinforcing fibers, fillers, nanofillers, and additives are used. As a result, these reinforcing materials can act as strong compatibilizers, which can interact with both the immiscible ends by enhancing the compatibility between polymers. This special class of blends is called composites. Composites also have the capacity to meet a range of requirements that cannot be obtained by only blending polymers. Moreover, polymeric composites have inherent design flexibility that is used to generate complex shapes with specific properties and enhanced aesthetics. Composite materials are materials that are composed of two or more distinct phases (matrix phase and dispersed phase) having bulk properties significantly different from those of any of the constituents. Generally, in a composite material one of the phases has a volume fraction greater than 10% and the properties of one constituent are much greater ( . 5 times) than the other. Broadly speaking, composites can be considered as materials consisting of two or more chemically distinct constituents on a macroscale, having a distinct interface separating them. The primary phase, having a continuous character, is called the matrix. Matrices are usually more ductile and less hard. They hold the dispersed phase and share a load with it. The second phase (or phases) is imbedded in the matrix in a discontinuous form. This secondary phase is called the dispersed phase. The dispersed phase is usually stronger than the matrix, therefore it is also called the reinforcing phase. UPE composites may contain: •

•

•

An initiator/catalyst and an accelerator: Catalysts help to begin the polymerization process of the matrix or the blend and are used for starting the curing process. After that an accelerator is added to increase the rate of the reaction by enhancing the curing rate in a certain period of time. Thickeners: For some fabrication techniques like in-sheet molding or bulk molding, thickened resins are required. So, multivalent salts are used for this process as they interact with the
COOH group present on the polymeric chain. For example, a 5% MgO concentrated is used as a thickener for UPE. For gel coat applications some thixotropic additives are used, for example, fumed silica and precipitated silica. Fillers/nanofillers: These are generally used to reduce the cost of composites. They may also be introduced for technical reasons like lowering shrinkage during curing, for making it cheaper, and to improve the overall properties by enhancing heat resistance. Examples of biodegradable fillers include chitin, fly ash, rice husk, wood flour, and paper pulp, etc. Examples of nonbiodegradable fillers include hydroxyapatite, zinc borate, antimony trioxide. Examples of some other fillers are as follows. Raw kaolin, mica—Electrical resistance fillers MWCNTs, graphene, GNPs, graphene oxide, chitosan-functionalized graphene nanofillers, titanium dioxide, and aluminum oxide—Nanofillers.

4.6 DETAILED OVERVIEW OF NANOFILLERS

•

• •

107

Reinforcing materials: Fibers are used reinforcing materials for UPRs. Fiber-reinforced UPE composites are cost effective and have superior properties. Examples of naturally occurring organic fibers include jute, bamboo, sisal, wood, and cellulose fibers, while synthetic fibers include polyester fiber, polyamide fiber, polyvinyl alcohol fiber, and polyacrylonitrile fiber. Mold release agents: These are used in molding processes to safely release cured composites from molds used in bulk molding and sheet molding compounds. Low profile additives: These are used to reduce the shrinking process during the course of curing. They enhance the mechanical properties of composites. It consists of a styrenebutadiene rubber solution which is prepared from styrene in the presence of hydroquinone at 50 C. Examples include hydroxy-terminated polybutadiene, maleated nitrile rubber, epoxidizednatural rubber, and hydroxy-terminated natural rubber.

4.5 EMERGENCE OF UNSATURATED POLYESTER-BASED NANOCOMPOSITES Nanocomposites represent one of the most promising materials that were birthed by nanotechnology in 1984 [8]. Polymers are the most common materials (thermoplastics, thermosets, or elastomers) that are used for nanocomposite fabrication. A nanocomposite may be defined as a composite system consisting of a polymer matrix and homogeneously dispersed filler particles with at least one dimension being ,100 nm. Over the past few decades, polymer nanocomposites (PNC) have attracted much interest in both academia and industry [9] due to their excellent properties such as their large surface area/volume ratios induced by the addition of nanoadditives [10]. The reduced nanofiller size allows for the tailoring of the physical properties of composites. Most material properties can be changed and engineered dramatically through controlled size-selective synthesis and assembly of nanoscale building items. Nanoparticles and nanomaterials have unique mechanical, electronic, magnetic, thermal, optical, and chemical properties, thereby providing a wide spectrum of new possibilities in engineered nanostructures and nanocomposites for communications, biotechnology and medicine, photonics, and electronics [11]. Generally, most nanocomposites are prepared using carbonaceous materials. The most common carbon-based nanofillers are graphite (one-dimensional), graphene (two-dimensional), carbon nanotubes (three-dimensional), and fullerene (four-dimensional). These carbonaceous materials remain a fascinating material to researchers and technologists for their advanced type of reinforcing effect with improved characteristics and high applicability.

4.6 DETAILED OVERVIEW OF NANOFILLERS 1. Zero-dimensional nanofiller Zero-dimensional materials are nanoparticles in which all the dimensions are measured within the nanoscale. They have no dimensions or zero dimensions and are larger than 100 nm. The most common representation of zero-dimensional nanomaterials is nanoparticles. Characteristic properties of zero-dimensional nanomaterials include being amorphous or

108

CHAPTER 4 UNSATURATED POLYESTER NANOCOMPOSITES

crystalline, single crystalline or polycrystalline, composed of single or multichemical elements with various shapes and forms. They may exist individually or in combination with matrices that are represented as metallic, ceramic, or polymeric. Here all the dimensions of nanoparticles are measured in nanoscale. Significant growth has been seen in the field of zero-dimensional nanomaterials. Also, various types of new and modern physical and chemical techniques have been introduced for their fabrication. Presently, zero-dimensional nanomaterials such as uniform particle arrays (quantum dots), heterogeneous particles arrays, core
shell quantum dots, hollow spheres, and nanolenses have been synthesized by several researchers [12,13]. 2. One-dimensional nanofiller One-dimensional nanomaterials have one dimension of 100 nm or less. This leads to needlelike shaped nanomaterials.1D materials include nanotubes, nanorods, and nanowires. The best examples of 1D nanomaterials are montmorillonite (MMT) clays and nanographene platelets (NGPs). Here one dimension of the nanoparticle is measured in nanoscale and the other two dimensions are measured in macroscale. In the past decade, one-dimensional nanomaterials have gained increasing interest due to their importance in research and development and their wide range of potential applications. These nanomaterials have attained broad attention after the pioneering work of Iijima [14].They have a profound impact on nanoelectronics, nanodevices, nanocomposite materials, and alternative energy resources [15]. Examples of this type of nanomaterial include nanowires, nanorods, nanotubes, nanobelts, and nanoribbons that have been synthesized in laboratories [16]. 3. Two-dimensional nanofiller Two dimensional nanomaterials are not confined to the nanoscale as these nanomaterials exhibit plate-like shapes and include nanofilms, nanolayers, and nanocoatings. These are amorphous or crystalline and are made up of various chemical compositions with single-layer or multilayer structures. In recent years, the synthesis of these nanofillers has become very interesting topic in materials research, owing to their small dimensional characteristics. In a search for two-dimensional nanofillers, considerable research attention has been focused over the past few years on their development. Actually, such nanofillers with specific geometries exhibit unique shape-dependent properties and successive utilization as building blocks in the key components of nanodevices [17]. Moreover, further experiments are ongoing for the investigation of emergent applications in sensors, photocatalysts, nanocontainers, nanoreactors, and templates for 2D structures of other materials [18]. They also possess junctions (continuous islands), branched structures, nanoprisms, nanoplates, nanosheets, nanowalls, and nanodisks [19,20]. 4. Three-dimensional nanofiller Bulk nanomaterials are materials that are not confined to the nanoscale in any dimension. These materials are thus characterized by having three arbitrary dimensions above 100 nm. These materials possess a nanocrystalline structure or involve the presence of features at the nanoscale. Again, bulk nanomaterials can be composed of a multiple arrangement of nanosized crystals, most typically in different orientations. With respect to the presence of features at the nanoscale, 3D nanomaterials can contain dispersions of nanoparticles, bundles of nanowires, and nanotubes as well as multinanolayers. Generally, the performance and applications of nanomaterials depend on their sizes, shapes, dimensionality, and morphologies. Therefore it is of great interest to researchers to synthesize three-dimensional nanofillers with controlled

4.7 REQUIREMENT OF FUNCTIONALIZATION OF NANOFILLERS

109

structures and morphologies. In addition, such types of nanomaterials have wide-ranging applications in the area of catalysis, magnetic material, and electrode material for batteries. Again, they have also attracted intensive research interests because nanostructures have high surface areas and supply enough absorption sites for all involved molecules in a small space. 5. Biodegradable nanofiller Recently, nano-shaped materials have achieved a lot interest due to their exclusive properties with attractive applications. Nanoparticles possess higher surface areas, which affects their physicochemical and other reactive properties. So, an attempt must be made to set off some new alternative techniques to conventional methods for preparing organic nanoparticles. Today, biopolymers like chitosan nanoparticles (CSNPs) result in very promising materials since they show improved properties with the preservation of the material biodegradability without ecotoxicity. CSNPs have gained huge interest in various fields because of their unique organic properties. They are environment-friendly and bioactive. These CSNPs have already been used to improve the strength and washability of textiles and to confer antibacterial effects [21,22]. Several methods like microemulsion, self-assembly, and ionotropic gelation have been used to prepare CSNPs [23]. The candidature of CSNPs as a reinforcement in the composite industry proves its uniqueness by providing better properties with advanced application areas. However, sometimes the practical applications of these natural chitosan nanofillers are finite because they do not dissolve in neutral and basic aqueous media. This reduces the effective dispersion and induces nucleation when used in a polymer matrix. So, to solve such problems, the chemical modification of chitosan can be performed.

4.7 REQUIREMENT OF FUNCTIONALIZATION OF NANOFILLERS The practical applications of nanofillers remain limited due to some unsolved problems such as a lack of effective dispersion technologies as well as their nonbiodegradability. So, they have been successfully functionalized by different types of chemicals and biodegradable nanofillers to make them partially biodegradable without sacrificing their internal properties. Again, the surface modification of nanoparticles is necessary for a homogeneous distribution in polymer matrices because these particles have the tendency to agglomerate tendency. Proper dispersion is the main key to obtaining desired, and sometimes anomalous, properties. The functionalization of the surface of nanoparticles using cheap organic material is the only solution to get costeffective and highly useful nanocomposites of good compatibility with various polymer matrices. Moreover, the proper homogeneous dispersion of graphene nanoplatelets in a polymer matrix can lead to major enhancements in the properties and even provide the latest features for broad applicability. The use of graphene nanoplatelets as a polymer reinforcement has the benefit of low cost compared to carbon nanotubes. Although these expensive two-dimensional carbonaceous nanofillers have been widely used in nanocomposite preparation for a variety of applications, they have become a major source of waste disposal problems due to their poor biodegradability. The benefits of nanotechnology-based industrial and consumer goods have been broadly exposed. So, the debate of the probable effects of their extensive use in various fields is just beginning to come to light.

110

CHAPTER 4 UNSATURATED POLYESTER NANOCOMPOSITES

4.7.1 AMINE FUNCTIONALIZED CARBON NANOFILLERS (f-MWCNTS AND f-GNPS) For the development of carboxyl groups, about 1 g of both nanofillers were dispersed on the mixture of conc.H2SO4 and HNO3 (1:1(v/v)) at 80 C for a duration of 5 hours and refluxed in an ultrasonication bath at 60 C for 12 hours at a frequency of 50 kHz. Then the resulting suspension was diluted with a 1:1(v/v) water/acetone solution until there was no trace of acid in the mass of the nanofillers. The mixture was then centrifuged and filtered in a vacuum through a polytetrafluoroethylene membrane with a pore size of 0.5 μm. The carboxylated carbon nanofillers were dried at 80 C for 24 hours in a vacuum oven and then taken for amine functionalization. Amine functionalized carbon nanofillers were synthesized by the addition of 100 mL of freshly prepared thionyl chloride (SOCl2) to 500 mg of oxidized MWCNTs. Then by heating both the nanofillers at 80 C followed by refluxing for 24 hours with the extraction of SOCl2 by distillation and addition of 150 mL of triethylenetetramine (TETA) a suspension was obtained. The suspension was then refluxed at 100 C for 48 hours and cooled at room temperature. Now, it was centrifuged and dispersed in a beaker containing 300 mL of anhydrous ethanol to remove the TETA adsorbed on the surfaces of the nanofillers. The final product was filtered under vacuum and dried in an oven at 100 C for 12 hours. The resulting materials are referred to as amine-functionalized MWCNTs (f-MWCNTs) and amine-functionalized GNPs (f-GNPs).

4.7.2 SYNTHESIS OF CHITOSAN-FUNCTIONALIZED GNPS At first 2 g of GNPs were dispersed in 50 mL of a water/ethanol (1:3(V/V)) solution. Then the suspension was sonicated at 50 C at a frequency of 40 kHz for 9 hours. Before using the chitosan (CS), it was completely deacetylated through the addition of 80% deacetylated CS to a 50% NaOH solution and stirring for 8 hours at 90 C under an N2 atmosphere. The product was filtered and washed with methanol and dried repetitively three times. About 4 g of this dried CS was treated with a 0.5 M HCl solution. Finally, the GNPs
ethanol suspension was gradually added to the CS
acid solution and the mixture was stirred at 70 C for 4 hours to yield a uniform black colored solution. After reaction, the mixture was filtered through a 0.2 μm microporous poly(ether sulfone) membrane, washed with distilled water to take away the adsorbed and uncross-linked CS. Finally, the product was dried in a vacuum oven at 80 C for 48 hours.

4.7.3 SYNTHESIS OF EPOXIDIZED CHITOSAN NANOPARTICLES (EP-F-CS NPS) An epoxy group was developed on the surfaces of the CS nanoparticles through treatment with epichlorohydrin (EPI). For this, 5 mL of EPI, 10 mL of DMSO, and 10 mL of 2 mol/L NaOH solutions were mixed with 1 g of CS nanoparticles. The mixture was heated to 40 C followed by gentle agitation using a magnetic stirrer at 200 rpm for a duration of 6 hours. The mixture was then centrifuged and filtered in a vacuum through a polytetrafluoroethylene membrane of 0.45 μm pore size. Finally, the epoxidized CS nanoparticles (EP/CSNPs) were carefully washed with acetone to eliminate the unreacted products.

4.8 UNSATURATED POLYESTER-BASED NANOSIZED MATERIALS

111

4.7.4 PREPARATION OF SILANE-FUNCTIONALIZED CHITOSAN NANOPARTICLES (GLYMO-F-CSNPS) CSNPs were dissolved in 100 mL of 2% (w/w) acetic acid and kept in a magnetic stirrer for 6 hours at 80 C. Before use the solution was filtered through a sintered glass filter. Then, the mixture was stirred in a 500 mL round bottomed three-necked flask and barged with N2 for 30 minutes. After that, 0.05 mole of dibutyltin dilaurate (DBTDL) was added into the minimum volume of acetone followed by the mixing of 3-glycidyloxypropyltrimethoxysilane (GLYMO) over 15 minutes. The reaction was continued at 700 C for 24 hours under N2. The stirring of the solution was continued for 15 minutes at 30 C followed by filtration. Then, a 10% (w/w) NaOH solution was added with vigorous stirring until the functionalized polymer was fully precipitated. This solution mixture was centrifuged and scattered in a beaker containing ethanol and deionized water to take away the unreacted silane and excess acetic acid adsorbed on the surface of the CSNPs. The final-silane functionalized CSNP was dried in an oven at 100 C for 24 hours and stored in a desiccator.

4.8 FABRICATION AND CHARACTERISTICS OF UNSATURATED POLYESTER-BASED NANOSIZED MATERIALS Carbon-based polymer composites have engrossed much attention from scientists and industries because of their advanced properties with tremendous applications. This part of the chapter aims to represent the inimitable characteristics of the interaction of carbon-based nanofillers with polyester resin through van der Waals forces of attraction as well as hydrogen bonding. Nanocomposite specimens were fabricated at a low cost and evidenced high mechanical strength and good stability making them suitable for applications in the manufacturing of components for various thermal and electrical applications. The improved properties of this new fabricated multiphase composite make it a high-quality promising candidate as a thermal interface material with the capability of meeting the requirements for various modern applications.

4.8.1 PREPARATION AND PROPERTIES OF UPE/GRAPHITE NANOCOMPOSITE Graphite is a new class of two-dimensional extraordinary carbonaceous matter which has always remained a fascinating material to researchers and technologists for its broad applications as lubricants in industries and high temperature gaskets [24]. In many aspects it is considered as the most stable and purest form of carbon and a great component of carbon nanotubes owing to its better inplane stiffness with superior thermal, mechanical, electrical, and transport properties. Graphite flake is an advanced type of thermodynamically stable reinforcing filler material with improved characteristics and high applicability. It is a good conductor of heat and electricity due to the presence of delocalized electrons, which are free to move throughout the sheet. Here carbon atoms are chemically bonded together as layers in the form of regular hexagons by strong covalent bonds and van der Waals forces of attraction. The nanoscale shape of this nanomaterial, shows a great enhancement in interfacial area as well as improved properties even taken in very low mass addition. [25
28]. Graphite has a 3D bulk limit for multilayer graphene, with the number of layers tending

112

CHAPTER 4 UNSATURATED POLYESTER NANOCOMPOSITES

to infinity, and is an excellent heat conductor [29]. Again, graphite-reinforced polymer composites with high transport properties in the form of thermal and electrical conductivity and superior processability have drawn a lot of attention [30
32]. So for current reinforcing candidates of the graphite family, graphite flake has excellent thermal conductivity compared to other carbon nanofillers because of its large specific surface area [33]. Graphite/UPE composites were prepared by solvent swelling by dispersing varying contents of graphite flakes in acetone. They were mixed thoroughly by sonication for about 1 hour at a temperature of 60 C. Then the suspension was added to UPR and stirred in a magnetic stirrer to evaporate the solvent. Again, this UPE/graphite mixture was degassed under vacuum at 50 C. After that the mixture was left to cool. Then, 5% (w/w) cobalt naphthanate accelerator and then 3% (w/w) MEKP catalyst was mixed with the mixture and stirred for 3 hours at 100 C. When the catalyst was completely dissolved, the whole mixture was poured into a mold and subjected to hot-press (5 t) and the curing was carried out at 120 C in a convention oven for 2 hours. The mechanical properties of the UPE/graphite nanocomposites containing varying contents of graphite were studied. Graphite loading had a tremendous effect on the tensile strength of the nanocomposites. Strong interfacial interaction between the nanofiller and the polymer matrix due to the good dispersion of the nanofiller enhanced the homogeneity as well as mechanical properties of the composites. The tensile strength of the nanocomposites improved as the graphite loading increased and the utmost value (45 MPa) was attained at a 2 wt.% graphite content. At that particular loading, the distribution and dispersion started to progress as the nanofiller concentration became greater, acting as a support to resist the maximum part of the load through effective interfacial stress transfer. With further increases in nanofiller loading, the tensile strength decreased due to the aggregation of the nanofiller at high concentrations. When the amount of graphite flakes reaches an optimum content (3 wt.%) and the distance between two graphite flakes is so small that they may be apt to stack together easily due to Van der Waals forces, it decreases the tensile strength. This leads to an inhomogeneous dispersion in the polymer matrix and decreases the required properties. Actually, proper distribution of graphite reduces the chain flexibility of the blend by improving the cross-linking density and enhancing the stiffness. At a 2 wt.% of graphite flakes nanofiller, the lowest value of fracture strain was observed as the brittleness of the nanocomposite gradually decreased with the addition of this nanofiller. Further addition of nanofiller increased the value as the brittleness increased. One of the most attractive properties of graphite is its potential to increase electrical conductivity at low concentrations. This is capable of dramatically enhancing the conductivity of polymer composites when they are introduced into an insulating polymer. Sometimes conductive composites are produced through the introduction of a much lesser quantity of conducting material which can create an effective conductive pathway by making successful filler contact. Using a threshold concentration for electrical conductivity, the graphite dispersion in matrices can be measured. Electrical properties are associated with two parameters, namely electrical conductivity and percolation threshold. The relation of electrical conductivity with resistivity is given in Eq. (4.1). Electrical conductivityðσÞ 5 1=resistivityðρÞ

(4.1)

At a certain volume fraction of filler loading, there is a sudden increase in electrical conductivity of composites known as the percolation threshold (Pc). Polymers filled with graphite nanofiller have outstanding electrical properties with a slightly higher percolation threshold. This higher value

4.8 UNSATURATED POLYESTER-BASED NANOSIZED MATERIALS

113

is due to the proper orientation and better dispersion of the nanofiller. Here the electrical conductivity of UPE/graphite nanocomposites with varying contents of filler is presented in Fig. 4.3. It can be observed that for the nanocomposites the conductivity increases in a continuous trend up to a 2 wt.% of filler loading. This clearly proves the percolation theory. This result is due to the better homogeneous dispersion of the nanofiller in the UPE matrix. The percolation theory is based on the distribution of nanoparticles from the conducting network in the polymer matrix. Here, at a 1 wt.% of filler loading a conductive part is formed by the contact of the graphite flakes. At this filler concentration the conductivity is lower than that of a 2 wt.% nanofiller content due to the smaller number of additives. Again, increasing the filler content up to 2 wt. %, enhances the conducting particles by increasing the number of conducting networks. So, this high electrical conductivity is the result of the high aspect ratio of the graphite nanofiller and its homogenous distribution. At 3 wt.% of nanofiller content the conductivity decreases due to the

FIGURE 4.3 Electrical conductivity of (A) UPE/graphite (1 wt.%), (B) UPE/graphite (2 wt.%), and (C)UPE/graphite (3 wt.%) nanocomposites.

114

CHAPTER 4 UNSATURATED POLYESTER NANOCOMPOSITES

aggregation of fillers which hinders the conduction process. Here various factors such as concentration and aggregation of fillers, processing technique of composites, and distribution of fillers in the matrix affect the electrical conductivity and percolation threshold of the composites. Again, some geometric factors like filler aspect ratio are the most important properties to determine the percolation threshold. Curliness and randomly oriented nanofillers in the polymer matrix also help to increase the magnitude of the conductivity compared to straight nanofillers. This may be due to the compactly interconnected graphite network constructed within the polymer. Thermal conductivity, one of the transport properties of a material, is determined by the vibration of phonons and the thermal motion of electrons. In this case, transport properties are coefficients of the ratio between flux and the driving force with direction gradient. Here thermal conductivity flux 5 heat flux (q). Mathematically thermal conductivity is represented in Eq. (4.2). Directional driving force 5 dT/dx (temperature gradient) Mathematically thermal conductivity


5 K 5 q=
dT=dx 5 Q=A =ðΔt=x0 Þ

(4.2)

where q 5 Q/A,q is the rate of heat flow (Q) across a given cross-section (A) and X0 is the distance. This is one of the theories to explain the fact behind the thermal conductivity of the composite. Fig. 4.4 shows the thermal conductivity of graphite/UPE nanocomposites with varying contents of nanofiller at three different temperatures. Actually, the thermal conductivity increases with an increase in temperature. This is expected as graphite possesses high thermal conductivity and the introduction of this also enhanced the conductivity of the UPE polymer. Much less graphite in the polymeric matrices enhanced the thermal conductivity of the nanocomposites as 2D-graphite is a good conductor of phonons. These nanocomposites have tremendous applicability in power electronics, thermal pastes, and miniaturized electronic devices [22]. From the figure we can observe that the thermal conductivity increases by 20% for 1 wt.% of nanofiller at 30 C, whereas at a 2 wt.% filler loading the conductivity value increases to about 30%. Here the enhancement may be due to the increase in the filler volume. As the filler has a high thermal conductivity, the high aspect ratio of graphite produced a better heat conductive network in the UPE matrix. This network allowed for the efficient conduction of phonons over a long distance. Another factor that induced the transport of phonons for increasing thermal conductivity is the uniform dispersion of graphite in the UPE matrix. At a 3 wt.% filler content the thermal conductivity of the nanocomposite decreased due to the clustering of the nanofiller as well as an increase in the viscosity of the matrix
filler mixture, which caused very weak cross-linking between the reinforced nanofiller and the matrix. Huge content of nanofillers creates an interfacial layer and increases the coupling effect of fillers with resin, which dampens the vibrational scattering of phonons at the interface. This process diminishes the heat flow and the efficiency of fillers as thermal conductors in resin matrices [23]. So, basically thermal conductivity depends on the uniform distribution of nanofiller in matrices and the surface area of the nanomaterial, which decrease the mean free path of heat carriers and enhance thermal conductivity. The water transport properties can be seen in Fig. 4.5 which depicts the data of water concentration versus time for the UPE/graphite nanocomposites with different concentrations of nanofiller. The specimens remained at 120 C under vacuum for 24 hours for complete dryness and then in a distilled water chamber at a constant temperature of 25 C. Fig. 4.6 shows the sorption values at equillibrium (Ceq) of different types of nanocomposites for varying filler loadings.

4.8 UNSATURATED POLYESTER-BASED NANOSIZED MATERIALS

115

FIGURE 4.4 Thermal conductivity of different types of nanocomposites.

FIGURE 4.5 The water concentration in relation to time (Ct) as a function of time (h) for different types of nanocomposites.

116

CHAPTER 4 UNSATURATED POLYESTER NANOCOMPOSITES

FIGURE 4.6 The sorption values at equillibrium for different types of nanocomposites.

At a 1 wt.%. graphite concentration, the sorption value of Ceq is slightly higher as a small amount of nanofiller has no effect on the diffusion coefficient. At higher water activity (a 5 1), the 2 wt.% graphite-filled UPE/graphite nanocomposite showed the lowest Ceq and water absorption values. This reduction of water transport activity may be due to the improved filler matrix interaction which prevents the swelling of the composite and the easy penetration of water molecules. Again, uniformly distributed graphite may create a synergistic effect with the UPE matrix and make the nanocomposite suitable for various structural applications such as aeronautical and wind turbine material. At a filler content of 3 wt.% the water uptake increases due to the deterioration of the filler
matrix bonding. Furthermore, the distortion of the polymer network occurs due to the relaxation mechanism which leads to the movement of a polymeric chain segment and allows the penetration of water. So, the addition of a 2 wt.% graphite nanofiller reduced the moisture absorption and enhanced the durability of the nanocomposites. These UPE/graphite nanomaterials showed enhanced storage moduli with increases in nanofiller content as represented in Fig. 4.7. As the temperature increased, the nanocomposites showed a sudden drop in storage moduli as well as in Tg related to the transition of the materials from a glassy state to a rubbery state. At a higher graphite content of up to 2 wt.% a higher storage modulus was observed above the Tg or in the rubbery region. The 2 wt.% graphite/UPE nanocomposite showed the highest Tg of 145 C and about 25% and 30% higher storage modulus than the 1 and 3 wt.% nanocomposites, respectively. The improvement in this property for the nanocomposites may be due to the better reinforcement of the nanofiller and the restricted movement of the polymer chains caused by the chain confinement effect of the graphite flakes. The nanocomposites at 2 wt.% also showed higher Tg, which suggested the improved cross-linking density and greater crystallinity of the nanocomposites. This tendency enhances their capacity to take in a vibration and scatter it all through the material without any failure. Moreover, at 2 wt.% nanofiller content the values of the storage modulus and Tg attained the utmost value with a diminished loss modulus (tanδ). Reduction in loss modulus are due to the better curing conditions of nanofiller with the matrix and enhanced degree of cross-linking that lead to an improved degree of particle distribution and good adhesion. That causes reduced availability of free space in the nanocomposite and hinder the segmental motion of polymeric chains under loading. Again, at the highest graphite concentration

4.8 UNSATURATED POLYESTER-BASED NANOSIZED MATERIALS

117

FIGURE 4.7 Dynamic mechanical spectra of different types of nanocomposites.

of 3 wt.% the availability of vacant space increases, leading to a slightly decreased cross-linking density and increased polymeric chain flexibility due to the clustering of the nanofiller. This tendency may cause the free movement of each polymer molecules. So, here the storage modulus value decreases and the damping coefficient value increases. The thermal stability of the nanophase composite system was determined by TGA. Fig. 4.8 depicts that an increase in nanofiller loading increases the thermal stability of UPR. The TGA results indicate only one phase of thermal decomposition platform, indicating a one-step process. The weight loss below 150 C was due to the loss of water molecules and beyond 150 C was related to the loss of bonded water molecules present in the polymeric chain of the nanocomposite. For the graphite nanofiller
reinforced nanocomposites, the thermal stability increases may be due to the increase in cross-link density and improved polymer filler interaction. Again, this increased thermal stability may be due to the better cross-linking and compatibility between the matrix and the nanofiller which creates a convoluted path in the composites to restrict the diffusion of volatile decomposition products. The presence of these nanofillers in the optimum concentration of 2 wt.% results in a compactly packed structure without any cracks and voids and induces thermal resistance. In addition, the Tmax shows higher value by confirming higher thermal stability at this particular concentration. But the incorporation of 3 wt.% nanofiller into the blend matrix showed an approximately 10% reduced thermal decomposition temperature compared to its corresponding 2 wt.% graphite-filled nanocomposite due to the aggregation of fillers and poor surface characteristics resulting in thermal instability.

4.8.2 PREPARATION AND PROPERTIES OF MWCNTs AND GNPs REINFORCED HYBRID UPE NANOCOMPOSITES Of late, the majority of the scientific community has shown much interest in incorporating carbonbased nanofillers such as carbon black, expanded graphite, and carbon nanotubes (CNTs) into the preparation of polymeric nanocomposites. Currently, graphene nanoplatelets (GNPs), one of the

118

CHAPTER 4 UNSATURATED POLYESTER NANOCOMPOSITES

FIGURE 4.8 Thermogravimetric analysis of different types of nanocomposites.

stiffest known honeycomb-type carbon materials, have gained the extensive attention of the modern scenario owing to its inherently high mechanical strength with good electrical and thermal conductivity. So, GNPs are regarded as a promising carbon nanofiller for nanocomposites due to their good balance of properties and cost. Again, CNTs (1D carbon nanofiller) have been used as a prominent nanofiller material with significant mechanical, chemical, electrical, and thermal properties. CNTs and GNPs share many similar properties with only structural differences. Attempts have been made to integrate these two carbon-based materials in order to utilize the merits of both. So, both CNTs and GNPs are used in various novel approaches to design high-performance hybrid nanocomposites with better applicability. Generally, CNTs tangle and aggregate easily due to their size and GNPs undergo restacking because of their thermodynamically unstable large specific surface area, strong Van der Waals bonds, and π 2 π stacking forces between layers. This leads to comparatively poor interactions between nanofillers and polymer matrices. So, to enhance their dispersion and compatibility with matrices various surface functionalization methods have been developed. Again, to explore their extraordinary potential, good dispersion of the CNTs and GNPs are compulsory and may lead to the formation of efficient three-dimensional networks. Here attempts have been made to achieve stable and finely dispersed GNPs and MWCNTs in UPE matrix by amine functionalization. Again, the preparation of MWCNTs
GNPs/UPE hybrid nanocomposite is an alternative method to obtain cost effective nanocomposites with balanced properties. So, for getting a satisfactory efficiency, a homogeneous distribution of nanofillers in the UPE matrix is necessary. Long-sized MWCNTs can make a bridge between the multilayer GNPs and stop their reagglomeration. On the other hand, exfoliated GNPs inhibit the reaggregation of CNTs and improve their dispersion. Here the main outcome of the present study was to investigate the improvement of properties of UPE nanocomposites after the addition of 1D MWCNTs and 2D GNPs as single nanofillers and as a hybrid nanofiller. The nanofillers were functionalized with triethylenetetramine (TETA) to get amine functionalized carbon nanofillers to facilitate the dispersion process.

4.8 UNSATURATED POLYESTER-BASED NANOSIZED MATERIALS

119

For preparing single and hybrid carbon nanofiller
reinforced nanocomposites, initially the nanofillers were added to ethanol and stirred in a high intensity ultrasonicator for 24 hours at ambient temperature conditions with mechanical stirring. The suspensions were mixed with the UPE by sonication at 80 C for 2 hours. Then 2% (w/w) catalyst (MEKP) was added by stoichiometry followed by further stirring for 1 hour to get a homogeneous stable suspension. Now 0.5% (w/w) cobalt naphthanate accelerator was added to the solution in parts by weight and stirred for 30 minutes. Before starting the fabrication, a gel coat with 2% (w/w) DCP was evenly brushed into a 2-plate mold and cured for 2 hours at room temperature. After drying at room temperature, the nanocomposites were further dried in a vacuum oven at 120 C for 12 hours with a hot press to remove the entrapped solvent. All the mechanical characteristics were enhanced with the introduction of both the MWCNTs and GNPs in the UPE matrix and the optimum values were achieved at 1:1 wt.% of nanofiller loading. After that particular concentration the mechanical properties diminished. The findings clearly revealed that the addition of these carbonaceous nanofillers as the reinforcement made the bionanocomposite capable of resisting the optimum amount of stress by allowing effective interfacial load transfer. Better reaction conditions with uniform nanofiller dispersion enhances all the mechanical properties at this particular filler concentration. The remarkable decrease of properties at a higher concentration of nanofiller is the result of irregular dispersion due to the nucleation of the nanofiller. The viscoelastic properties with respect to temperature for the fabricated hybrid nanocomposite with varying contents of the raw carbon-based nanomaterials showed better reinforcement of the nanofiller and the restricted movement of the polymer molecules. This may cause a chain confinement effect in both the nanofillers. It has been observed that at a 1:1 wt.% of MWCNTs/GNPs content, the nanocomposite showed the highest Tg of 120 C and storage modulus value of 5025 MPa. The reasons behind this are the improved cross-link density and greater crystallinity of the nanocomposite. After this particular concentration, the value further decreased with the addition of more nanofiller because of the lack of uniform distribution in the polymer matrix phase. Irregular nanofiller distribution may create the possibility of filler agglomeration and reduce the overall properties. The thermal properties of materials are associated with the release of adsorbed species, heat change in chemical reactions, and decomposition. So, the introduction of filler has a trivial effect on these properties. Only the introduction of a nanofiller has an enormous effect on thermal degradation on the basis of filler
matrix interaction. For 1:1 wt% loading of both MWCNT and GNP nanofiller
reinforced hybrid nanocomposites, degradation occurred in a high temperature range with enhanced thermal stability compared to that of single nanofiller
filled nanocomposite. This increased thermal stability may be due to the better cross-linking and compatibility between the blend and the nanofiller. The presence of these nanofillers in the optimum concentration of 1:1 wt.% results in a compactly packed structure without any cracks and voids and induces thermal resistance. Also, the thermal conductivity value increases at 1:1 wt.% nanofiller loading because of the better dispersion of the nanofiller, which eases the movement of phonons from the nanofillers to the matrix system. At this particular concentration of nanofiller, the highest thermal conductivity is observed especially due to the homogenous dispersion of GNPs, which makes it easier to create heat conductive paths. But thermal conductivity decreases with increases in the concentration of nanofiller due to the aggregation of the nanofiller which results in weak cross-linking between the reinforced nanofiller and the matrix.

120

CHAPTER 4 UNSATURATED POLYESTER NANOCOMPOSITES

4.9 HIGH PERFORMANCE NANOCOMPOSITES FROM VARIOUS UPE-BASED BLENDS: A NEW TREND Thermosetting UPEs are broadly utilized in the fiber-reinforced plastic (FRP) industry and in various decorative coatings. The extensive use of these resins is due to their low cost, simplicity in processing, ease of combination with reinforcements, fast curing at room temperature without any volatile byproducts, exceptional dimensional stability, and the wide variety of grades available. FRP is used in a range of engineering applications due to its high strength-to-weight ratio, excellent chemical resistance, ease of fabrication, and versatility of product design. General purpose (GP) grade UPE is a blend of styrene with the condensation product of a diol like 1, 2-propylene glycol along with a mixture of dibasic acids like maleic acid (MA) and phthalic acid in the form of an anhydride. When the cross-linking is initiated, with the help of a catalyst and an accelerator, styrene forms polystyrene chains which cross-link the polyester chains at the sites of unsaturation. Cured UPEs have high Tg values. So, they are too brittle and have poor resistance to crack propagation, making them unsuitable for many engineering applications in the absence of reinforcements. Brittleness of this class of nonbiodegradable thermosetting materials is one of their major drawbacks. Therefore their utilization in industrial applications has been restricted to some extent. So, the modification of resin, using various types of thermosetting polymers, thermoplastic polymers, and elastomers can improve the impact strength and fracture properties of these rigid polymers. The blending of UPE helps to prepare various grades of polymers which exhibit better properties than pristine UPR. Miscible blends give better results as they have no phase separation. Again, the efficacy of a polymer blend depends on their miscibility or the compatibility of the two polymers. So, to get rid of such bottlenecks, sometimes reinforcing fibers, fillers, nanofillers, and additives are used. As a result, these reinforcing filler or fiber materials can act as strong compatibilizers by enhancing the compatibility between polymers. Polymer blending is the synergistic mixing of structurally different homopolymers or copolymers to enhance some vital material qualities while maintaining a fraction of their original properties. Here, structurally dissimilar polymers are combined through the activity of secondary bond powers and no covalent bonding exists between them. This is a well-known and financially effective method employed in the design of novel materials. Again, this methodology is normally less expensive and less time consuming than the development of a new monomer for polymer synthesis. A major benefit of polymer blends is the broad variety of material properties that can be obtained by varying the blend composition. Scientist and researchers have concentrated on biodegradable or partially biodegradable blends as they offer a potential solution to contamination and waste transfer related issues created from plastics transfer. The idea of mixing biodegradable and traditional polymers to improve their biodegradability has drawn wide attention and commercial usage in the current situation. Polymer-based blends combine a number of significant properties from each of the mix constituents. The utilization of polymer mixes and composites represents an extremely versatile procedure for plotting inventive materials that satisfy certain prerequisites like: • • •

Lesser costs without giving up the original properties; Capability to modify properties without the development of any entirely new polymers; Proper utilization of organic or inorganic waste materials.

4.10 APPLICATIONS

121

Polymer blends are classified based on their miscibility, morphology, methods of preparation, and utility. Among these, miscibility is the most significant criteria and, on this basis, blends can be simply divided into four categories. • • • •

Miscible (homogeneous) polymer blend: A polymer blend that has a particular phase structure and exhibits a single glass transition temperature. Immiscible (heterogeneous) polymer blend: A polymer blend made of two polymers having two different phases with two glass transition temperatures. Compatible polymer blend: Compatible implies complete miscibility or macroscopic uniformity across the entire composition range. Incompatible polymer blend: This is a system in which any region of incomplete miscibility observed is regarded as incompatible.

The blending of polymers is sometimes hindered owing to their low compatibility. This is based on the concept that most polymers are immiscible and contradictory. Incompatibility between polymer pairs results in poor phase morphology and is the reason behind the weakened mechanical properties of the majority of polymer blends. Currently, much attention has been given to the utilization of nano-shaped compatibilizers due to their minimum size, small stacking portion, and large aspect ratios. There are different strategies for deciding the coordination between two polymers utilized for mixing. Among these, well known strategies are solubility parameter calculations and others depend on glass transition temperatures. Another modern trend of preparing UPE blend-based materials is bionanocomposites. In recent years, bionanocomposites have raised great research interests because of the increasing demand for environment-friendly products. Here, biopolymers or synthetic polymers are formulated with fillers of nanometric dimensions. As a new candidate in the nanoworld, biodegradable polymeric nanocomposites are impressive as they can be resulted from petroleum as well as renewable sources. The reinforcement of bionanoparticles in synthetic resins also yields bionanocomposites. Several cellulosic nanoparticles have been used as reinforcements in synthetic polymers to impart some desirable properties like decreasing shrinkage and creep resistance. Finally, one can say that (1) bionanocomposites can be produced from both biopolymers and nonrenewable nanomaterials (carbon nanotubes and nanoclay), and (2) bionanocomposites can result from organic nanoparticles and blends of petroleum-derived polymers with bio-based thermosetting polymers. Here, ecofriendly nanoparticles are used as novel reinforcing biodegradable nanofillers in the preparation of nanocomposites. They are cost effective and exhibit distinctly improved properties. Another interesting thing related to this, is the proper utilization of organic polymer waste through biodegradation and bioconversion. These wastes are inexpensive, biodegradable and exhibit no health hazards.

4.10 APPLICATIONS This discussion has described the far-reaching applications of UPE that have been realized over the past decade. UPEs have drawn attention in multiple field applications due to their properties like

122

CHAPTER 4 UNSATURATED POLYESTER NANOCOMPOSITES

low cost, ease of processing, good heat distortion characteristics, low coefficient of liner expansion, resistance to stress crack formation, low flammability, and the highly cross-linked nature of the 3D polymer network which improves the resistance capacity to softening and deformation at high temperatures. Actually, nanostructured materials have gained much attention due to their ability to achieve properties superior to those of the original polymers. So, it has been thoroughly discussed here that carbon nanofillers have the potential to modify UPE matrices whether they are used individually or as hybrid nanofillers. Again, nanofillers are functionalized to achieve better dispersion as well as interfacial adhesion to UPE matrices and to enhance the properties. Carbon nanofillers positively improve the properties including the minimum formation of microcracks with improved wetting properties particularly when they are in hybrid form. Studies have discovered that these nanocomposites have a great deal of applications in various engineering and nanoelectronics fields. However, a variety of other aspects such as the effect of various chemicals on the thermal stability and corrosion resistance of the nanocomposites need to be investigated so that they will become emerging future materials.

4.11 CONCLUSION This chapter emphasizes the use of a variety of nonbiodegradable and biodegradable nanomaterials as highly promising reinforcing fillers to tune the mechanical, thermal, and electrical properties of UPE and the blend-based matrices in which they are embedded. The resulting nanocomposites have received much attention because of their prospective to gain properties superior to those of traditional engineering materials. Studies have discovered major improvements in some significant properties endowed with reduced corrosion and swelling properties that have a great deal of applications in the engineering and nanoelectronics fields. Furthermore, this work presents valuable and unique methods for introducing biodegradable chitosan onto the surface of graphene nanoplatelets as a nanofiller, functionalization of CSNPs with epichlorohydrin and 3-glycidyloxypropyltrimethoxysilane. All these novel reinforcing bionanofillers can be successfully introduced into UPE blend
based matrices and can make the most excellent nanocomposites that may contribute to the requirement of the modern trend. All nanocomposites are efficient due to their better co-ordination of nanofiller with the blend. Here, the nanocomposites possess better cross-linking density and crystallinity than the pristine matrix which enhances their potential for application. Thus this nanocomposite could be a good candidate for a variety of structural applications with the ability of advanced functioning.

ACKNOWLEDGMENT The authors of this chapter are thankful to KIIT University for their support and help. The assistance provided by NIT, Rourkela and IIT, and Kharagpur during the completion of the experimental works are greatly acknowledged.

REFERENCES

123

REFERENCES [1] B. Dholakiya, Unsaturated Polyester for Specialty Applications, Intech, New York, 2012 (Chapter 7). [2] I. Goodman, J.A. Rhys, Saturated Polymers, Iliffe, London, 1965. [3] P.W. Morgan, Condensation Polymers: By Interfacial and Solution Methods, Inerscience, Publishers, New York, 1965. [4] A. Frodet, P. Arland, Unsaturated Polyesters, Comprehensive Polymer Science, Pargamon Press, New York, 1989 (Chapter 19). [5] H.F. Mark, N.G. Gaylord, N.M. Bikales, Encyclopedia of Polymer Science and Technology, John Wiley and Sons, New York, 1970. [6] A.B. Cherian, L.A. Varghese, E.T. Thachil, Epoxy modified, unsaturated polyester hybrid networks, Eur. Polym. J. 43 (4) (2007) 1460
1469. [7] K. Dinakaran, M. Alagar, Development and characterization of vinyl ester oligomer (veo) modified unsaturated polyester inter crosslinked matrices and composites, Polym. Plast. Tech. Eng. 52 (11) (2003) 957
966. [8] P. Gatenholm, J. Kubat, A. Mathiasson, Biodegradable natural composites. I. Processing and properties, J. Appl. Polym. Sci. 45 (9) (1992) 1667
1677. [9] I.K. Varma, B.S. Rao, M.S. Choudhary, V. Choudhary, D.S. Varma, Effect of Styrene on vinyl ester resin properties I, Die Angew. Makromol. Chem. 130 (1) (1985) 191
199. [10] R. Roy, S. Komarneni, Nanophase and nanocomposite materials, Mater. Res. Soc. (1984) 241. [11] Y.W. Mai, Z.Z. Yu, Polymer Nanocomposites, Woodhead Publishing Ltd, 2006. [12] J. Wang, M. Lin, Y. Yan, Z. Wang, P.C. Ho, K.P. Loh, CdSe/AsS core-shell quantum dots: preparation and two-photon fluorescence, J. Am. Chem. Soc. 131 (2009) 11300. Available from: https://doi.org/ 10.1021/ja904675a. [13] U.K. Gautam, S.R.C. Vivekchand, A. Govindaraj, G.U. Kulkarni, N.R. Selvi, C.N.R. Rao, Generation of onions and nanotubes of GaS and GaSe through laser and thermally induced exfoliation, J. Am. Chem. Soc. 127 (11) (2005) 3658
3659. [14] S. Iijima, Helical microtubes of graphitic carbon, Nature 354 (1991) 56
58. [15] S.V.N.T. Kuchibhatla, A.S. Karakoti, D. Bera, S. Seal, One dimensional nanostructured materials, Prog. Mater. Sci. 52 (2007) 699. [16] L.M. Cao, X.Y. Zhang, H. Tian, Z. Zhang, C.X. Gao, W.K. Wang, Boron notride nanotube nanojunctions, Nanotechnology 15 (2004) 139. [17] B.H. Hong, J.Y. Lee, C.W. Lee, J.C. Kim, S.C. Bae, K.S. Kim, J. Am. Chem. Soc. 123 (43) (2001) 10748
10749. [18] D. Pradhan, K.T. Leung, Vertical growth of two-dimensional zinc oxide nanostructures on ITO-coated glass: effects of deposition temperature and deposition time, J. Phys. Chem. C 112 (5) (2008) 1357. [19] J.N. Tiwari, F.M. Pan, R.N. Tiwari, S.K. Nandi, Facile synthesis of continuous Pt island networks and their electrochemical properties for methanol electro oxidation, Chem. Commun. 48 (2008) 6516
6518. [20] B.B. Nayak, D. Behera, B.K. Mishra, Polymer composites with functionalized nanoparticles, J. Am. Ceram. Soc. 93 (2010) 3080
3083. [21] S.H. Lim, S.M. Hudson, Review of chitosan and its derivatives as antimicrobial agents and their uses as textile chemicals, J. Macro. Sci. Polym. Rev. 43 (2) (2003) 223
269. [22] K.F. El-tahlawy, M.A. El-bendary, A.G. Elhendawy, S.M. Hudson, The antimicrobial activity of cotton fabrics treated with different crosslinking agents and chitosan, Carbohyd. Polym 60 (4) (2005) 421
430. [23] S.A. Agnihotri, N.N. Mallikurjana, T.M. Aminabhavi, Recent advances on chitosan- based micro- and nanoparticles in drug delivery, J. Controlled Release 100 (1) (2004) 5
28.

124

CHAPTER 4 UNSATURATED POLYESTER NANOCOMPOSITES

[24] M. Naffakh, A.M. Diez-Pascual, C. Marco, G. Ellis, M.A. Gomez-Fatou, Opportunities and challenges in the use of inorganic fullerene-like nanoparticles to produce advanced polymer nanocomposites, Prog. Polym. Sci. 38 (8) (2013) 1163
1231. [25] H.J. Salavagione, G. Martinez, G. Ellis, Recent advances in the covalent modification of graphene with polymers, Macromol. Rapid Commun 32 (22) (2011) 1771
1789. [26] A.M. Diez-Pascual, M. Naffakh, Mechanical and thermal behaviour of isotactic polypropylene reinforced with inorganic fullerene-like WS2 nanoparticles: effect of filler loading and temperature, Mater. Chem. Phys. 141 (2) (2013) 979
989. [27] S. Ghose, D.C. Working, J.W. Connell, J.G. Smith, K.A. Watson, D.M. Delozier, et al., Thermal conductivity of Ultem (TM)/ carbon nanofiller blends, High Perform Polym. J 18 (6) (2006) 961
977. [28] M. Moniruzzaman, K.I. Winey, Polymer nanocomposites containing carbon nanotubes, Macromolecules 39 (16) (2006) 5194
5205. [29] K.M.F. Shahil, A.A. Balandin, Graphene-multilayer graphene nanocomposites as highly efficient thermal interface materials, Nano Lett. 12 (2) (2012) 861
867. [30] Q. Liu, X.B. He, S.B. Ren, T.T. Liu, Q.P. Kang, X.H. Qu, Fabrication and thermal conductivity of copper matrix composites reinforced with Mo2C or TiC coated graphite fibers, Mater. Res. Bull. 48 (11) (2013) 4811
4817. [31] J. Barcena, J. Maudes, J. Coleto, J.L. Baldonedo, J.M.G.D. Salazar, Microstructural study of vapour grown carbon nanofibre/copper composites, Compos. Sci. Technol. 68 (6) (2008) 1384
1391. [32] P.M. Adams, H.A. Katzman, G.S. Rellick, G.W. Stupian, Characterization of high thermal conductivity carbon fibers and a self reinforced graphite panel, Carbon 36 (3) (1998) 233
245. [33] C.Y. Wen, G.W. Huang, Application of a thermally conductive pyrolytic graphite sheet to thermal management of a PEM fuel cell, J. Power Sources 178 (1) (2008) 132
140.

CHAPTER

MICROMECHANICS OF SHORT-FIBER AND PARTICULATE COMPOSITES

5

´ Eduardo A.W. de Menezes1, Leandro Friedrich1, Angelica Colpo1 and Sandro C. Amico1,2 1

Post-Graduation Program in Mechanical Engineering, UFRGS, Porto Alegre, Brazil 2Post-Graduation Program in Mining, Metallurgical and Materials Engineering, UFRGS, Porto Alegre, Brazil

5.1 INTRODUCTION Micromechanics refers to the study of heterogeneous materials considering the interaction of the constituents in detail, allowing designers to tailor effective properties and to represent anisotropic composite materials as an equivalent homogeneous material by estimating the average responses [1]. A statistically representative volume element (RVE) is considered for this purpose, allowing for acquisition of the exact solution, in many cases related to periodic structures [2]. Unlike isotropic materials, the experimental evaluation of a unidirectional composite lamina is quite costly and time-consuming due to the higher number of independent elastic constants and the five strength parameters, which are a function of the constituents’ volume fraction, packing geometry, and processing quality, among others, justifying the need to develop analytical models to easily predict composite behavior [3]. Before applying continuum mechanics relations, where solids are considered homogeneous at macroscale with stresses and strains uniformly satisfying constitutive laws, one must consider that, at microscale, many materials show inhomogeneities such as cracks, voids, or inclusions. To circumvent this problem, a sufficiently large RVE (which is discussed later), able to statistically represent the overall material, is defined in order to represent the microstructure. By applying prescribed tensile loads or displacements at the RVE, the resulting stresses and strains can be correlated with the whole structure at the macroscale by means of the averaging principle [2]. This process is illustrated in Fig. 5.1. Early research in the micromechanical field date back to the 19th century, with the works of Maxwell and Rayleigh on electrical conductivity in heterogeneous media, while the early work on solid mechanics was carried out by Voigt (parallel model) and Reuss (series model) [4] at the beginning of 20th century. Nevertheless, Eshelby [5,6] was the first to propose solving the problem of ellipsoidal particles embedded in infinite media for the elastic field, and many subsequent approaches, namely variational, effective medium, self-consistent, dilute, and Mori
Tanaka models [2], are greatly based on the solutions of Eshelby.

Unsaturated Polyester Resins. DOI: https://doi.org/10.1016/B978-0-12-816129-6.00005-3 © 2019 Elsevier Inc. All rights reserved.

125

126

CHAPTER 5 MICROMECHANICS OF SHORT-FIBER

FIGURE 5.1 Illustration of the overall process, from microscale to macroscale, for the analysis of heterogeneous materials. Adapted from H. Yin, Y. Zhao, Introduction to the Micromechanics of Composite Materials, CRC Press, Boca Raton, FL, 2016.

In recent years, many different models have been derived aiming at more accurately predicting composite behavior based on micromechanical theories, avoiding some of the assumptions from Eshelby’s works or incorporating correction factors, such as empirical coefficients, stress concentration factors, crowding factors, or interface characteristics. This chapter focuses on providing the basic micromechanics concepts such as RVE, averaging, and eigenstrain, and to report Eshelby’s findings, along with various micromechanical models developed from them or apart, designed to predict the mechanical properties such as engineering constants and tensile strength of short-fiber composite materials with random and aligned fiber distributions, along with particulate composites of different particle geometries, highlighting the particular assumptions and hypotheses used.

5.2 INHOMOGENEITIES Many natural materials are heterogeneous when analyzed microscopically, that is, formed by different constituents or phases, even if they appear to be macroscopically homogeneous. These materials may be distinguishable on a small length scale and may have different mechanical properties and orientations. Cracks, voids, particulates, layers, fibers, laminates, grains, and distortions in the crystalline lattice, generally called defects, are actually inhomogeneities. Composites in general, polycrystalline, porous, and cellular materials are typical examples of materials that have inhomogeneities.

5.2 INHOMOGENEITIES

127

Defects can be characterized in two ways [4,7], namely (1) where the materials themselves are the source of disturbance, called eigenstrain or eigenstress, for example, displacements and inclusions; and (2) those that only under external action induce perturbation in the uniform field, for example, particles, voids, and cracks. This classification helps in understanding the formal equivalence between inhomogeneous and homogeneous materials with the presence of eigenstrain and eigenstress. Micromechanics mainly focuses on the behavior of heterogeneities and estimates of their effects (homogenization) on the global properties and material performance. Composite materials may greatly benefit from it because there are many influential microstructural aspects such as geometry, distribution, and interaction between phases, therefore requiring complex multiscale analysis. According to Bohm [8], in these approaches, stress and strain fields in inhomogeneous materials are divided into contributions corresponding to different length scales. These length scales are assumed sufficiently different so that for each scale variation: 1. Fluctuations of the stress and strain fields at smaller length scales (microfields, “fast variables”) influence the macroscopic behavior at larger length scales only via their volume averages. 2. Gradients of stress and strain fields, as well as compositional gradients, at larger length scales (macrofields, “slow variables”) are not significant at smaller length scales, where these fields are assumed to be locally constant and are described in terms of uniform “applied” or “far field” stresses and strains. In this context, an important task of micromechanics is to link mechanical relations between different length scales. To understand how this is done, some typical defects are discussed here, presenting classical approaches and analytical solutions for linear elastic media [4,9].

5.2.1 INCLUSIONS Fig. 5.2A illustrates a homogeneous material in the domain D containing a subdomain Ω of distinct mechanical properties. This subdomain is called inhomogeneity and may represent the defects mentioned before (e.g., voids, particulates, layers, fibers, laminates, and grains). However, when the

FIGURE 5.2 Illustration of domain with an (A) inhomogeneity and (B) an inclusion.

128

CHAPTER 5 MICROMECHANICS OF SHORT-FIBER

subdomain has the same mechanical properties as D and is endowed with a nonmechanical strain, that is, not caused by external excitation, this is known as eigenstrain and is called inclusion. The eigenstrain represents a set of strains that are not caused by external loads, existing in their absence and generated by different physical sources such as thermal expansion, plastic and residual deformation, phase transformation, and others [4]. Bodies free of any other external forces in the presence of inclusions restricted on their surface, generate internal stresses called eigenstress. Eigenstress is a result of an incompatibility in eigenstrain. The phenomenon can be understood as self-equilibrated internal stress. This concept of eigenstrain was first introduced by Eshelby [5] and later used by Mura [10] to describe local inelastic deformations. To understand the influence of eigenstrain on material behavior, one may consider a homogeneous elastic solid for which the general stress
strain relation is described by: σij 5 Cijkl εkl

(5.1)

where σij 5 σji is the Cauchy stress, εkl 5 εlk the strains, and Cijkl 5 Cklij the constitutive elastic matrix. In the presence of eigenstrain, Eq. (5.1) can be rewritten as: σij 5 Cijkl ðεkl 2 εtkl Þ

where

εtkl

denotes the eigenstrain and the term

εkl 2 εtkl

(5.2)

represents the elastic deformation.

5.2.2 ESHELBY MODEL The closed form depicted in Eq. (5.2) cannot be used to represent stress and strain distributions when inclusions with arbitrary geometries and eigenstrain fields are present. This motivated the studies of Eshelby (1916
81), who developed one of the most important analytical solutions in micromechanics [9,11]. Eshelby [5,6] developed a model for an elastic and homogeneous ellipsoidal inclusion defined by its main axes (a) contained in an infinite linear elastic matrix (Fig. 5.3). Eshelby related the contracted form (total deformation) and the transformed form (perturbation in the deformation field) by means of a fourth-order tensor, called the Eshelby tensor (S) as in: εij 5 Sijkl εtkl 5 const in Ω

(5.3)

The tensor depends on the elastic constants and the inclusion geometry. S is symmetric in the first and second pair of indices but, in general, is not symmetric to change in these pairs: Sijkl 5 Sjikl 5 Sijlk ;

Sijkl 6¼ Sklij

(5.4)

The stresses inside the inclusion can be found using Eq. (5.2). If the deformations are constant then the stresses are: σij 5 Cijrs ðSrskl 2 Irskl Þεtkl 5 const in Ω

(5.5)

where I is the identity tensor given by: 1 Irskl 5 ðδrk δsl 1 δrl δsk Þ 2

(5.6)

5.2 INHOMOGENEITIES

129

FIGURE 5.3 Ellipsoidal inclusion in an unbounded domain. Adapted from D. Gross, T. Seelig, Fracture Mechanics With Introduction to Micromechanics, Springer, New York, 2006.

This relation is valid only within the domain Ω. Outside the inclusion, stresses and deformations are not constant and decrease asymptotically as they move away from the inclusion. For a spherical inclusion (a1 5 a2 5 a3 5 a) in an isotropic material, for example, there is no dependence on the geometry or the main axes, reducing the Eshelby solution to:
 1 1 Sijkl 5 α δij δkl 1 β Iijkl 2 δij δkl 3 3

(5.7)

where α and β are scalar parameters given by: α5

11ν 3K 2ð4 2 5νÞ 6ðK 1 2μÞ 5 ; β5 5 3ð1 2 νÞ 3K 1 4μ 15ð1 2 νÞ 5ð3K 1 4μÞ

(5.8)

Mura [12], Li and Wang [13], Torquato [14], and Markenscoff and Gupta [15] present some examples of closed expressions for the case of isotropic materials and extensions of the Eshelby solution for problems of heterogeneity.

5.2.3 EQUIVALENT EIGENSTRAIN The Eshelby results allowed for the development of several models to estimate homogenized elastic tensor through the equivalent eigenstrain method. The equivalent inclusion method is a homogenization process that uses linear elasticity theory and Green’s functions [7] to establish the equivalence between an eigenstrain field (or eigenstress) and inhomogeneity that can be replaced by an eigenstrain field with an equivalent mechanical effect [13], basically, making the material homogeneous through an eigenstrain and thus using the results of Eshelby.

130

CHAPTER 5 MICROMECHANICS OF SHORT-FIBER

Consider a domain given by volume V of inhomogeneous behavior described by an elastic tensor Cijkl ðxÞ and with displacements ubi applied in the boundary @V, as shown in Fig. 5.4A. If the volume forces are negligible, the boundary value problem is governed by [4]: σij;j 5 0; σij 5 Cijkl ðxÞεkl ; ui j@V 5 ubi

(5.9)

0 Now consider the same volume V of a homogeneous material with elastic constants Cijkl (Fig. 5.4B). The fields in this problem, indicated by subscript 0, can be described as: 0 σ0ij;j 5 0; σ0ij 5 Cijkl ε0kl ; u0i j@V 5 ubi

(5.10)

The next step is to represent the equivalence of the inhomogeneities through eigenstrain (Fig. 5.4C). To describe the behavior of heterogeneous material without the inhomogeneities, the difference between the fields are defined as: B

B

ui 5 ui 2 u0i ; εij 5 εij 2 ε0ij σ~ ij 5 σij 2 σ0ij

Reorganizing gives:

(5.11)

0 σ~ ij 5 Cijkl ðxÞεkl 2 Cijkl ðεkl 2 ε~ kl Þ

    0 021 0 σ~ ij 5 Cijkl ε~ kl 1 Cijkl Crstq ðxÞ 2 Crstq εtq

(5.12)

(5.13)

Thus the differences between stress fields when the displacements in the boundary are neglected are: 0 σ~ ij 5 0; σ~ ij 5 Cijkl ð~εkl 2 εtkl Þ; u~i j@V 5 0

(5.14) 0 (Cijkl )

(εtkl ðxÞ).

Eq. (5.14) describes a value problem in a homogeneous material with eigenstrain To know the equivalent eigenstrains that represent the heterogeneity of the material, the term in Eq. (5.14) may be isolated as:   021 0 εtij 5 2 Cijkl Ckltq ðxÞ 2 Ckltq εtq

(5.15)

The equivalent eigenstrain method turns a complex problem (Fig. 5.4A) into a simplified problem (Fig. 5.4D). The distributed eigenstrain still partially depends on the deformation field of the

FIGURE 5.4 Representation of: (A) heterogeneous material; (B) homogeneous comparison material; (C) equivalent eigenstrain; and (D) homogenized original problem [4].

5.2 INHOMOGENEITIES

131

original problem through the difference of the elastic properties of the materials, that is, 0 Cijkl 2 Cijkl . To obtain the relationship between equivalent eigenstrain and the Eshelby tensor, first consider an ellipsoidal inhomogeneity of domain Ω inserted into an elastic matrix with a constant deformation field applied in the boundary (ε0 ) as shown in Fig. 5.5A. The elastic tensor of inhomogeneity and matrix is given by CI and Cm , respectively. Now, simplify the problem comparing the same domain and boundary conditions with a homogeneous material with an eigenstrain (Fig. 5.5B). Using the equivalent eigenstrain method to determine εtkl ðxÞ, it can be isolated in Eq. (5.11) and then substituted into Eq. (5.15), considering C 0 5 Cm ,   εt ðxÞ 5 2 Cm21 :ðCI 2 Cm Þ: ε~ ðxÞ 1 ε0

(5.16)

If the deformation field at the boundary is constant, the difference in deformations (i.e., fluctuations) ε~ is just the value of the eigenstrain, which can be determined by the Eshelby tensor, ε~ 5 S:εt 5 const

(5.17)

Replacing Eq. (5.16) with Eq. (5.17) and solving for εt :

 21 εt 5 2 S1ðCI 2Cm Þ21 :Cm :ε0 in Ω

(5.18)

Inside the inhomogeneity, the total deformation in terms of the external applied deformation field ε0 is  21 ε 5 11S:Cm 21 :ðCI 2Cm Þ :ε0 5 const or 0 ε 5 ΛN I :ε 5 const

(5.19)

where ΛN I is a fourth-order tensor known as influence tensor or concentrator tensor. If the strain field is constant, the stress also is and, inside the inhomogeneity, σ 5 CI :ε. Substituting ε from Eq. (5.19) and considering ε0 5 σ0 =Cm , one finds that

FIGURE 5.5 Representation of: (A) ellipsoidal inhomogeneity; and (B) homogeneous material with eigenstrain.

132

CHAPTER 5 MICROMECHANICS OF SHORT-FIBER

21 0 σ 5 CI :ΛN I :Cm :σ or N 21 0 σ 5 BN with BN I :σ I 5 CI :ΛI :Cm

(5.20)

where BN I is also an influence tensor. That is, the Eshelby tensor introduces the geometric and mechanical properties of the inhomogeneity in the relationship between deformations and stresses applied at the boundary and inside the inhomogeneity. The described procedure is called the equivalent eigenstrain method, which is analogous to the original process created by Eshelby. In the case of inhomogeneities as cavities or cracks, a domain Ω of negligible stiffness CI 5 0 is assumed in the relations for the inhomogeneities (Eqs. 5.19 and 5.20). Some special cases are presented in Refs. [16
18].

5.3 ELASTIC PROPERTIES When it is necessary to determine the behavior of an unknown material, one usually chooses to perform mechanical tests such as tensile, compression, or bending. From these, material properties like Young’s modulus, Poisson’s coefficient, rupture and yield stresses, and others are obtained. In the case of a material such as steel, the properties hardly change for similar specimens because of its homogeneity at almost all length scales. However, when a heterogeneous material on a large (visual) length scale is tested, that will represent the behavior of a particular specimen only. This occurs for a variety of materials and is not restricted to the visual scale since even macroscopically homogeneous materials may contain heterogeneities at the microscopic level. To overcome that difficulty, different homogenization techniques were proposed to evaluate effective (mean) elastic properties by finding a statistically homogeneous volume at the appropriate scale that is characterized by no change in the mechanical material properties.

5.3.1 REPRESENTATIVE VOLUME ELEMENT The macroscopic properties of a heterogeneous material can be determined through micromechanical models that consider a particular volume of the solid to represent the effective material behavior. In an elastically homogeneous material, this volume is called the RVE [19
21]. In a continuum medium, the choice of RVE must take into account three characteristic parameters (Fig. 5.6), namely the characteristic length of the heterogeneous medium (L), the characteristic length of the RVE (l), and the length of the heterogeneities (d). For the homogenization process to be valid, these parameters must satisfy certain conditions [22], namely: 1. l ,, L: the RVE size should be small in comparison to the structure to ensure that the homogenized equivalent material defined a continuum medium, so that traditional tools as differentiation and integration could be applied, allowing the determination of stress and deformation fields. 2. l .. d: the size of the heterogeneities should be small compared to the size of the RVE. This condition guarantees that the RVE is statistically representative of the particles at the microscopic level, thus ensuring the reliability of the properties obtained by the homogenization process.

5.3 ELASTIC PROPERTIES

133

FIGURE 5.6 Illustration of a representative volume element.

5.3.2 AVERAGING If one considers two very distinct scales, a material point of coordinates x at the macroscopic level is related to the volume V at the microscopic level, where tensions and deformations prevail as microfloating fields, that is, these microfields influence the behavior of the larger scale only through volumetric averages. On the other hand, at the macroscale, the stress and strain field gradients are not significant for the smaller scale because locally these fields may be considered constant and can be described in terms of stresses or strains uniformly applied at the boundary, as shown in Eq. (5.21): εðxÞ 5 hεi 1 ε0 ðxÞ; σðxÞ 5 hσi 1 σ0 ðxÞ

(5.21)

where εðxÞ and σðxÞ: microfields hεiΩ and hσiΩ : macrofields ε0 ðxÞ and σ0 ðxÞ: fluctuating microfields

The macrostresses and macrostrains are defined for a coordinate point x as the volumetric averages in a solid of volume V,  1 σij 5 V

ð

 1 σij ðxÞdV; εij 5 V V

ð

εij ðxÞdV

(5.22)

V

Considering a traction or displacement applied at the surface of the solid, the mean stress and strain fields in V are found through the divergence theorem,  1 σij 5 V

ð

 1 Ti xj dA; εij 5 2V @V

ð

@V

ðui nj 1 uj ni ÞdA

(5.23)

134

CHAPTER 5 MICROMECHANICS OF SHORT-FIBER

5.3.3 GENERAL RELATIONS FOR ESTIMATING THE EFFECTIVE MECHANICAL PROPERTIES The equivalent or effective mechanical properties of composites are determined by homogenization techniques [21], which take into account the geometric and mechanical characteristics of all the constituents and their distributions. In this sense, analytical approaches have been developed over the years. However, it is necessary to understand the initial hypotheses and conditions for the proposed models to be valid. At a microscopic level, one can say that the law of elasticity for any coordinate point x can be written in the form: σij 5 Cijkl ðxÞεkl ðxÞ

(5.24)

While the elasticity tensor that relates macrostresses and macrostrains is represented by:  σij 5 Cijkl  hεkl i

(5.25)

However, C  can be considered a material property only if the equality between the mean strain energy density hU i in the V also represents the amount of macroscopic energy, that is, the following equality must be satisfied:   hU i 5 1=2εij Cijkl εkl 5 1=2 εij Cijkl  hεkl i

(5.26)

Combining Eqs. (5.25) and (5.26) yields: 

  σij εij 5 σij εij

(5.27)

Eq. (5.27) is known as Hill’s theorem [23]. This condition implies that fluctuations in the microscopic field do not generate work, that is:    σ~ ij ðxÞ 5 σij ðxÞ 2 σij ; ε~ ij ðxÞ 5 εij ðxÞ 2 εij ; σ~ ij ε~ ij 5 0

(5.28)

In summary, Hill’s theorem approximates energetically the homogeneous equivalent medium to the heterogeneous microstructure material, and when the equality depicted in Eq. (5.27) is satisfied, it represents an RVE. In that case, the microfields of stresses and deformations considered in the boundary are given by their volumetric means. Clearly, for Hill’s condition to be satisfied within a macroscopic body under a nonuniform external load, the sample should be large enough to have small field fluctuations relative to its size. For any perfectly bound heterogeneous body in the absence of body forces, two important loading states satisfy Hill’s condition [4,13], namely: 1. Linear displacements: ui 5 ε0ij xj on @V where ε0ij 5 const, and according to Eq. (5.23) with Ð @V xi nj dA 5 Vδij :  εij 5 ε0ij

(5.29)

2. Uniform tractions: Ti 5 σ0ij nj on @V where σ0ij 5 const, and according to Eq. (5.23) one gets:  σij 5 σ0ij

(5.30)

Eqs. (5.29) and (5.30) are also related to the average strain and the average stress theorems, respectively. Using the same idea of equivalent inclusion (Eqs. 5.19 and 5.20) within a domain

5.3 ELASTIC PROPERTIES

135

V of the body, the stress and strain fields are linearly dependent on the load applied on the surface. Thus 1: εij ðxÞ 5 Λijkl ðxÞε0kl

for

ui 5 ε0ij xj

on

@V

(5.31)

2: σij ðxÞ 5 Bijkl ðxÞσ0kl

for

Ti 5 σ0ij nj

on @V

(5.32)

where Λ and B are the already mentioned influence or concentrator tensors and depend on the microstructure in every domain V, so: hΛi 5 1; hBi 5 1

(5.33)

Eqs. (5.24) and (5.25) can be rewritten and again equalized to satisfy Hill’s theorem:  C  :hεi 5 hC:εi; C21 :hσi 5 hεi 5 C21 :σ

(5.34)

For each boundary condition (1) and (2), one has: C ðuÞ 5 hC:Λi  21 CðtÞ 5 C21 :B

(5.35) (5.36)

An alternative representation is also obtained by inserting Eqs. (5.31) and (5.32) into the mean strain energy density in Eq. (5.26):   21 C ðuÞ 5 ΛT :C:Λ ; CðtÞ 5 BT :C21 :B

(5.37)

From C ðuÞ and C ðtÞ , it is clear that the effective material properties depend on the type of prescribed boundary condition applied to the contour and that they cannot be separately considered as real mechanical properties. However, the difference between them can be interpreted as the quality of the observed domain, that is, whether the volume is representative or not. For example, consider a biphasic material, with a continuous phase (matrix) and a dispersed phase (an inhomogeneity other than cavities or cracks), with the volume fraction of the inhomogeneity VI . In this case, there are two boundary conditions for C: CðuÞ 5 Cm 1 VI ðCI 2 Cm ÞΛI  21 C ðtÞ 5 Cm21 1VI ðCI21 2Cm21 Þ:BI

(5.38) (5.39)

It is still necessary to determine the concentration tensor and then the effective tensor. Bearing in mind that the concentration tensor is a function of the Eshelby tensor related through the equivalent inclusion method and that carries information on the mechanical properties of the inhomogeneity and its shape.

5.3.4 ANALYTICAL APPROXIMATIONS Over the years, analytical approximation models, namely the self-consistent method [24], Mori
Tanaka [25], differential scheme [11], dilute distribution and the limits of Hashin and Shtrikman [26], Voigt [27], and Reuss [28], have been created based on certain hypotheses. However, the basic difference between these models is how they consider the interactions between inclusions, which influence behavior, for instance, when the content of inclusions exceeds a certain limit.

136

CHAPTER 5 MICROMECHANICS OF SHORT-FIBER

FIGURE 5.7 Mori
Tanaka homogeneization scheme.

In the case of diluted distribution, for instance, the heterogeneities are distributed throughout the homogeneous matrix, but the interaction among them is neglected as well as the interaction with the boundary of the RVE. Hence, each heterogeneity is defined as being located in an infinite domain subject to uniform strain or stress fields. This solution is valid only for very low volume fractions of heterogeneities (VI ,, 1) [7,24]. On the contrary, in the self-consistent method, focus is not only placed on the effect between heterogeneities, at certain distances, but also on the contact between them [4,11,26]. In this case, no material plays the role of matrix and, therefore, the interaction between inclusions is considered implicitly [29]. Mori and Tanaka [25] are among the many researchers in the mesomechanics field who, in the 1970s, developed mathematical models for heterogeneous materials by applying the concepts of microscopic midfields to analyze the macroscopic properties of materials. Differently from the dilute model (or diluted estimate), the hypothesis of interaction between defects is considered, and its application extended to cases containing a greater volume fraction of defects. This model assumes that distant from the heterogeneity, the field of matrix deformations can be approximated by the constant field hεim or hσim . In the case of an ellipsoidal inhomogeneity with CI 6¼ 0, as depicted in Fig. 5.7, the deformation within the domain Ω is constant ðε 5 hεiI Þ, and the concentration tensor is given by Eq. (5.19), now called ΛIðMTÞ by convention. And, from Eq. (5.37), one gets: hεi 5 Vm hεim 1 VI hεiI  21  21 hεiI 5 ΛIðMTÞ :hεi ΛIðMTÞ 5 VI 11Vm ΛN21 5 11Vm Sm :Cm21 :ðCI 2Cm Þ I

(5.40)

Finally, the effective elastic tensor of Mori
Tanaka for the case of prescribed displacements in the boundary (Eq. 5.38) is given by: ðuÞ CðMTÞ 5 Cm 1 VI ðCI 2 Cm Þ:ΛIðMTÞ

(5.41)

If one considers the case of an isotropic spherical heterogeneity where CI 6¼ 0 within an isotropic matrix, the elasticity effective tensor is simplified. Thus the effective bulk (K) and the effective shear moduli (G) are respectively given by:  KðMTÞ 5 Km 1 VI

ðKI 2 Km ÞKm ðGI 2 Gm ÞGm ; GðMTÞ 5 Gm 1 VI Km 1 αð1 2 VI ÞðKI 2 Km Þ Gm 1 βð1 2 VI ÞðGI 2 Gm Þ

where α and β are scalar parameters as shown in Eq. (5.8).

(5.42)

5.4 MICROMECHANICS OF SHORT-FIBER COMPOSITES

137

The Mori
Tanaka estimation is the most used homogenization model for heterogeneous materials, especially when the volume content of equivalent inclusions is moderate. Variant forms of this method and new approaches can be found in Refs. [30
33].

5.4 MICROMECHANICS OF SHORT-FIBER COMPOSITES Short fiber
reinforced composites are widely used in medicine, automotive, aerospace, and naval applications [34]. Their advantages against unreinforced polymers include higher stiffness, strength, creep resistance, and better ageing and weathering properties. In many cases, especially for brittle matrices, toughness and dimensional stability are also improved [35]. However, for tough matrices, toughness may even be reduced [36]. And, in comparison to long fiber composites, they present low cost and manufacturing advantages, but with poorer mechanical properties. Manufacturing processes have a limited capability to align short fibers [37]. One of the best results of this is the high performance-discontinuous fiber (HiPerDiF) method, where about 66% of fibers are aligned within 6 3 degrees [38]. This usually translates into short-fibers that are randomly distributed. This type of orientation and anisotropic behavior, make finite element modeling generally laborious and time-consuming due to the problematic construction of RVEs, especially for 3D problems [39]. In order to accurately predict the mechanical properties of a composite, a good strategy is the use of micromechanical models based on the constituents’ individual properties, usually available in closed-form solutions. These models involve a series of assumptions and simplifications, the usual ones being [40]: 1. 2. 3. 4. 5.

Fibers are considered to be isotropic and straight with uniform properties along the length. Matrix and fiber are perfectly bonded and interface properties are neglected. Fillers are uniformly distributed in a periodic pattern, far enough from each other. Materials are linear elastic. Loads are applied at infinity.

Additionally, the necessary individual properties of the constituents may be hard to find, especially for fibers. This set of hypotheses can make strength predictions significantly deviant from experimental data. Despite that, numerical results may reasonably agree [41,42], and can be used to avoid exhaustive experimental tests in early design stages, allowing quick extrapolations for different fiber contents. In this context, this section aims to provide classical and newer micromechanical models available in closed-form solutions to predict the mechanical properties of composite materials reinforced with short fibers, covering both unidirectional orientation (1D) and random distribution.

5.4.1 UNIDIRECTIONAL ARRANGEMENT Under an applied stress, stress is assumed to be transferred from matrix to fibers through a shear mechanism, where stress builds up from the fiber ends, reaching a maximum value at the center of the fiber [43]. In order to effectively reinforce a composite with positive effects in stiffness and

138

CHAPTER 5 MICROMECHANICS OF SHORT-FIBER

strength, the fiber must be of a critical length (lc), as expressed in Eq. (5.43), with a value of about 1 mm for most carbon and glass fibers, considering an aspect ratio range between 20 and 150 [44]. lc 5

σT1;f φf 2τ if

(5.43)

where φf is the fiber diameter, σT1;f is the fiber tensile strength, and τ if is the interfacial shear strength. Fig. 5.8 illustrates stress the distribution for different values of fiber length lf relative to lc, and this has a marked effect on the tensile strength (σTl ) of composites. If lf ,lc, then the load transfer to the fiber is not maximized, and failure is dominated by matrix strength, as given by Eq. (5.44). If lf $ lc, the fiber ultimate stress is reached, and tensile strength is given by Eq. (5.45) [43]. ! lf τ if Vf 1 ðσm Þε;f Vm ; forlf , lc 5 φf
 1 0
 φf σT1;f ult A 1 ðσm Þε;f Vm ; forlf . lc σT1 5 @1 2 Vf σT1;f ult 4lf τ i σT1

(5.44)

(5.45)

where Vf and Vm are the fiber and matrix volume fractions, respectively, and ðσm Þε;f is the stress supported by the matrix at the fiber failure, which, for an isostrain condition, is given by: ðσm Þε;f 5

σm Em E1f

(5.46)

In order to predict the engineering constants, a semiempirical model based on the fiber geometry was proposed by Halpin
Tsai [45]. The parameter ζ is dependent on fiber geometry (length

FIGURE 5.8 Stress distribution along fiber length, considering (A) the length below its critical value, (B) equal to critical value, and (C) above its critical value [44].

5.4 MICROMECHANICS OF SHORT-FIBER COMPOSITES

139

and cross section), where ζ 5 0 gives the lower bound, and ζ 5 N gives the upper bound. The values indicated in Eqs. (5.47
5.50) were estimated under a conservative viewpoint for circular fibers [45].
 ! E1;f

 Em 2 1 1 1 ζη1 Vf lf ; η1 5
 E1 5 Em ; ζ 52 E1;f 1 2 η1 Vf φf 1ζ

(5.47)

Em


 E1;f

 Em 2 1 1 1 ζη2 Vf E2 5 Em ; η2 5
 ; ζ 52 E 1;f 1 2 η2 Vf 1ζ

(5.48)

Em


 G12;f

 21 Gm 1 1 ζη3 Vf
 G12 5 Gm ; η3 5 ; ζ 51 G12;f 1 2 η3 Vf 12

(5.49)

Gm

ν 12 5 ν 12;f Vf 1 ν m Vm ; ν 12 5 ν 12

E2 E1

(5.50)

Alternatively, a model for predicting G12 was proposed by De and White [35], based on the average between the upper (1) and lower (
) bounds as presented by Halpin
Tsai. Gð122Þ

5 Gm 1

1 G12;f 2 Gm

0

!

Vf 1

Vm 2Gm

G12 5

;

Gð121Þ

5 G12;f 1 @

1 Vm

1 Gm 2 G12;f

1

Vf 2G12;f

A

Gð121Þ 1 Gð122Þ 2

(5.51)

(5.52)

Another way to predict the Young’s modulus of short aligned fibers is to use the Cox shear-lag model [46]. Perfect bonding, no longitudinal gaps between fibers (see Fig. 5.9A), and the equal lateral contraction of fibers and matrix are assumed, so that shear stresses are transferred to the fiber during axial strain [36]. The shear-lag parameter is given by: 0

11=2 8πG m
A η5@ E1;f πφ2f ln 2R φ

(5.53)

f

where 2R is the spacing between fibers. For hexagonal arrangement (see Fig. 5.9B), R is defined by: R 5 φf

!1=2 π pffiffiffi 2Vf 3

(5.54)

The shear-lag parameter is inserted to correct the rule of mixtures (ROM), that is,
1 ηl tanh 2f @ AVf 1 Em Vm E1 5 E1f 1 2 ηlf 0

2

(5.55)

140

CHAPTER 5 MICROMECHANICS OF SHORT-FIBER

FIGURE 5.9 (A) Short-fiber composite showing fiber distribution and the absence of gaps. (B) Hexagonal arrangement applied to evaluate fiber spacing [36].

5.4.2 RANDOM ARRANGEMENT For composites with randomly oriented fibers, the reinforcement volume content is more limited, and decreases at an exponential rate as the fiber aspect ratio increases [47]. In some processes, a significant fiber length reduction is expected, and the fiber conformation also changes, violating the assumption of straight fibers. The mean fiber orientation angle (θmean) can be obtained through a probabilistic density function (g(θ)), as depicted in Eq. (5.56) [36], where minimum and maximum fiber angles (θmin and θmax, respectively) are between 0 and π/2, and q1 and q2 are shape parameters. θmean 5

ð θmax θmin

ðsinθÞ2q1 21 ðcosθÞ2q2 21 θ gðθÞdθ; gðθÞ 5 Ð θmax 2q1 21 ðcosθÞ2q2 21 dθ θmin ðsinθÞ

(5.56)

Several models have been proposed for the prediction of the composite engineering constants, most of them based on empirical or semi-empirical approaches for isotropic materials, with a defined application range. These models may be classified in 2D (composites with one dimension lower than the others, as plates and laminae), with all fibers lying in the same plane, and in 3D (three dimensions in the same order), rarely used in practice due to difficulties in controlling fiber orientations in 3D [48]. The first approach consists of balancing the longitudinal and transversal Young’s moduli evaluated in the previous section to derive E and G, with ν computed from them assuming an isotropic behavior [49]. E2D 5

3 5 1 4 E1 1 E2 ; E3D 5 E1 1 E2 8 8 5 5

(5.57)

5.4 MICROMECHANICS OF SHORT-FIBER COMPOSITES

1 1 E2D G2D 5 E1 1 E2 ; ν 2D 5 21 8 4 2G2D

141

(5.58)

Christensen and Waals [50] applied the strength of materials relations to estimate the Young’s modulus of a short fiber
reinforced polymer based on the average value considering all possible fiber orientations. Due to the complexity of the derived expressions, the authors simplified the results for composites with Vf below 20% in order to fit both experimental and theoretical predictions: E2D 5

    Vf Vf E1;f 1 1 1 Vf Em ; E3D 5 E1;f 1 1 1 ð1 1 ν m ÞVf Em 3 6

(5.59)

Aiming to derive a model to accurately predict the behavior of polymers reinforced by short glass fibers, Manera [51] focused on fiber volume fractions for usual manufacture methods and parts (10%
40%) and typical polymeric matrix properties (2 GPa , Em , 4 GPa and ν  0.4), yielding: E2D 5



 16 8 2 3 1 E1;f 1 2Em Vf 1 Em ; G2D 5 E1;f 1 Em Vf 1 Em 45 9 15 4 3

(5.60)

Alternatively, Cox [46] and Pan [48] elaborated models based on probabilistic density functions to account for fiber orientation angles. By expanding g (θ) in π-periodic functions to obtain the stiffness matrix, and assuming the composite as isotropic, neglecting matrix contribution, Cox model [46] can be written as: Vf Vf ; E3D 5 E1;f 3 6 Vf Vf G2D 5 E1;f ; G3D 5 E1;f 8 15 1 1 ν 2D 5 ; ν 3D 5 3 4 E2D 5 E1;f

(5.61) (5.62) (5.63)

Considering again the assumption of isotropic materials, g(θ) in Eq. (5.56) is independent of the direction, that is, it becomes a constant (g0). By subjecting g(θ) to normalization conditions and making θ 5 π/2 one gets [48]: gðθÞsinðθÞ

ð π=2

2π=2

dθ 5 1 ‘

g0 5

1 π

(5.64)

ROM is now modified to incorporate g0:

   Vf Vf E2D 5 E1;f Vf gðθÞ 1 1 2 Vf gðθÞ Em 5 E1;f 1 12 Em π π

   Vf Vf 1 12 νm ν 2D 5 ν 12;f Vf gðθÞ 1 1 2 Vf gðθÞ ν m 5 ν 12;f π π

(5.65) (5.66)

In the case of 3D, the fiber orientation relative to the 1-3 plane must also be taken into account, yielding [48]: E3D 5 E1; f



 Vf Vf Vf Vf 1 12 Em ; ν 3D 5 ν 12; f 1 12 νm 2π 2π 2π 2π

(5.67)

142

CHAPTER 5 MICROMECHANICS OF SHORT-FIBER

Table 5.1 Results From Micromechanical Models and Experiments for Two Random Composites [52] Curaua/Polyester

E-glass/Polyester

Method

E (MPa)

G (MPa)

ν

E (MPa)

G (MPa)

ν

Experimental Wolff Christensen Manera Cox Pan

8360 8662 7734 8575 3600 6314

2590 3140
3216 1350 2321

0.330 0.379
0.333 0.333 0.360

17640 17890 15160 16730 10770 13080

6140 6301
6273 4038 4814

0.350 0.420
0.333 0.333 0.358

and the shear modulus can be directly obtained through: E2D E3D ; G3D 5 2ð1 1 ν 2D Þ 2ð1 1 ν 3D Þ

G2D 5

(5.68)

Menezes et al. [52] applied these models for random composites to glass (Vf 5 38%) or curaua (Vf 5 30%) fiber/polyester composites and compared the results with experimental data, as shown in Table 5.1. The micromechanical equations were evaluated using dedicated software MECHGcomp [53]. The models were found to reproduce, with relatively good accuracy, the experimental data, except for the Cox model, which neglects the matrix contribution for the composite stiffness. Regarding tensile strength, there is no fully satisfactory theory for its prediction [54]. An empirical model was proposed by Matthews and Rawlings [54], where the factor η1 accounts for the strengthening efficiency of short fibers, and the factor η2 for the orientation efficiency (0.375 for random fibers in 2D and 0.2 in 3D). The tensile strength in the case of short fibers can be computed through Eqs. (5.44
5.45), and for long fibers through ROM (Eq. (5.69)). 

σT1



5 σT1;f Vf 1 ðσm Þε;f Vm  T σ 2 ðσm Þε;f Vm T T σ 5 η1 η2 σ1;f Vf ; η1 5  1T short fibers σ1 long fibers 2 ðσm Þε;f Vm long fibers

(5.69) (5.70)

5.5 MICROMECHANICS OF PARTICULATE COMPOSITES High costs and technical difficulties in manufacturing fiber composites limit their use in many applications, making particulate composites a viable alternative for a wide number of applications [55]. Particles are typically added to improve Young’s modulus and strength [56], but they can also increase fracture toughness when the filler (usually a ceramic material) acts toward reducing thermal stresses [57]. The particles may be polymers, rubbers, ceramics, or metallic materials [58]. Due to the randomness of particle distribution, in a scale much larger than that of the particle size and spacing, these composites behave as quasi-homogeneous and quasi-isotropic [58]. This

5.5 MICROMECHANICS OF PARTICULATE COMPOSITES

143

key assumption for particles is considered in conjunction with the assumptions included in the previous section. In view of the important role played by the type, shape, and spatial arrangement of the reinforcing phase on the mechanical behavior of composites, this section discusses the models separately based on particle geometry.

5.5.1 SPHERICAL PARTICLES Considering a cubic unit cell with a spherical particle inside and applying the ROM: σT Aunit

cell

5 σTp Ap 1 σTm Am

(5.71)

where A is the equivalent area and the subscripts p and m denote the particle and the matrix, respectively. By expressing the strength as function of Young’s modulus and substituting the areas [55]: E5

Vp0:67 Em
1 2 Vp0:33 1 2

Em Ep


  1 1 2 Vp0:67 Em

(5.72)

This approach can also be extended for G (see Eq. 5.77), and ν is obtained from the isotropic assumption (Eq. 5.78) [55]: G5

Vp0:67 Gm
1 2 Vp0:33 1 2 ν5

Gm Gp


  1 1 2 Vp0:67 Gm

E 21 2G

(5.73)

(5.74)

Other models applied to particulate composites aim to estimate the bulk modulus (K), shear modulus (G), and the remaining constants from the isotropic assumption. Based on the averaging principle and dilute theory applied to spherical particles (shown earlier in this chapter), these properties are obtained by [59]:



 3Km 1 4Gm 3Kp 2 3Km K 5 Km 1 1 Vp 3Km 3Kp 1 4Gm  

 15ð1 2 ν m Þ Gp 2 Gm G 5 Gm 1 1 Vp 2Gp ð4 2 5ν m Þ 1 Gm ð7 2 5ν m Þ

(5.75) (5.76)

For the nondilute case, the bulk modulus of an isotropic media reinforced by isotropic spherical particles, idealized as grains (rigid inclusion in a nonrigid matrix), may be obtained by submitting it to hydrostatic compression. The equilibrium equation and the radial displacement ur could be written as [9]: @σr 2 @2 ur 2 @ur 2 1 ðσr 2 σθ Þ 5 0; 1 2 2 ur 5 0 r r @r r @r @r

(5.77)

where σr and σθ are radial and hoop stresses, respectively, r is the particle radius, and ur is the radial displacement.

144

CHAPTER 5 MICROMECHANICS OF SHORT-FIBER

By solving the differential equation and applying the boundary conditions, one can derive the composite bulk modulus [60]: 2 K 5 Km 4




11

4Gm 3Km



η1 Vp

1 2 η1 Vp

3 5

(5.78)

From the superposition principle, any uniform state of stress may be derived from a uniform tension system, yielding the same dilatation value for all three directions simultaneously [60]. An analogous procedure can be applied to compute the shear modulus under tensile stress: 2 G 5 Gm 4

where η1 5

Kp Km 2 1 Kp 4Gm Km 1 3Km

11




7 2 5ν m 8 2 10ν m



1 2 η2 Vp

η2 Vp

3 5


 G ð8 2 10ν m Þ Gmp 2 1
 ; η2 5 G ð8 2 10ν m Þ Gmp 1 ð7 2 5ν m Þ

(5.79)

(5.80)

Eq. (5.78) shows inappropriate singular points, and Eq. (5.79) may result in values out of Hashin
Shtrikman bounds for shear modulus [26] (see Eq. (5.90)). In order to solve these issues, Christensen [9] adopted an additional assumption when solving Eq. (5.77), proposing a three-phase model, as depicted in Fig. 5.10, where the outer cylinder has an infinite radius so that the imposed strain is sufficiently far away from the filler and the matrix, producing a pure shear state. Under equal average strain, all three phases store the same strain energy, and Eshelby’s formulation for imposed displacement can be applied to evaluate the stored energy. Equivalents of the bulk modulus and shear modulus are shown in Eq. (5.81), for a step-by-step solution proposed by Christensen [9].

FIGURE 5.10 Three-phase model.

5.5 MICROMECHANICS OF PARTICULATE COMPOSITES

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 0   2 η5 6 η25 2 η4 η6 Vp Kp 2 Km A K 5 Km 1 h i ; G 5 Gm @ ð1 2 Vp ÞðKp 2 Km Þ η4 11 4 K 1 G m

3

145

(5.81)

m

where

   Gp    Gp  η 1 5 49 2 50ν p ν m 2 1 1 35 ν p 2 2ν m 1 35 2ν p 2 ν m ; Gm Gm



 Gp Gp η 2 5 5ν p 28 17 14 ; Gm Gm Gp η35 ð8 2 10ν m Þ 1 ð7 2 5ν m Þ G





m  

 10 7 5 Gp Gp Gp η4 5 8 2 1 ð4 2 5ν m Þη 1 Vp3 2 2 63 2 1 η 2 1 2η 1 η 3 Vp3 1 252 2 1 η 2 Vp3 Gm Gm Gm

   Gp 2 50 2 1 7 2 12ν m 1 8ν 2m η 2 Vp 1 4ð7 2 10ν m Þη 2 η3 Gm





 

 10 7 5 Gp Gp Gp η5 5 2 2 2 1 ð1 2 5ν m Þη 1 Vp3 1 2 63 2 1 η 2 1 2η 1 η 3 Vp3 2 252 2 1 η 2 Vp3 Gm Gm Gm

 Gp 3 1 75 2 1 ð3 2 ν m Þη 2 ν m Vp 1 ð15ν m 2 7Þη 2 η3 2 Gm





 

 10 7 5 Gp Gp Gp η6 5 4 2 1 ð5ν m 2 7Þη 1 Vp3 2 2 63 2 1 η 2 1 2η 1 η 3 Vp3 1 252 2 1 η 2 Vp3 Gm Gm Gm

   Gp 1 25 2 1 ν 2m 2 7 η 2 Vp 2 ð7 1 5ν m Þη 2 η3 Gm

(5.82)

Since these models rely on isotropic linear elasticity, the Young’s modulus and Poisson’s ratio can be evaluated, respectively, through: E5

9GK 3K 2 2G ; ν5 3K 1 G 6K 1 2G

(5.83)

The model presented by Einstein [61] for evaluating the viscosity of particles immersed in fluid systems was assumed to apply to the shear modulus of spherical particles in nonrigid matrices, yielding good results for low concentrations only due to the assumption that particles are distant from each other [62].   G 5 Gm 1 1 2:5Vp

(5.84)

In order to avoid this limitation, a semiempirical model was proposed by Mooney [63] to fit experimental data for volume fractions of up to 40%, taking into account the space-crowding effect of the suspended spheres on one another, represented by F in Eq. (5.85). The most probable value for F was shown to be around 1.35, considering densely packed spheres in a facecentered array (it could be less for polydispersity) being the upper bound (simple cubic array) F 5 1.91.

2:5Vp G 5 Gm exp 1 2 FVp



(5.85)

146

CHAPTER 5 MICROMECHANICS OF SHORT-FIBER

Another correction to Einstein’s model was proposed by Guth [64]. Based on experimental observations of carbon particles immersed in a fluid medium, he proposed a generalization of Eq. (5.84) by inserting an empirical term to account for sphere interactions. In order to address the spheres “chaining” effect for higher volumes, which caused rapid growth in stiffness, a quadratic dependence on Vp was introduced. Although the measured property was viscosity, the author extended the formulation for E and G.

 E 5 Em 1 1 2:5 Vp 1 14:1 Vp2 ; G 5 Gm 1 1 2:5 Vp 1 14:1 Vp2

(5.86)

Aiming to create a micromechanical model to accurately predict the Young’s modulus of concrete, Counto [65] elaborated on an empirical model that accurately fits a wide range of experimental data [62], thus surpassing the limitations of previous models. 2

3- 1

612Vp1=2 7 1 7 E56 4 Em 1 12V 1=2  5 p E 1E m p 1=2

(5.87)

Vp

Regarding tensile strength, Nicolais model [66] assumes that there is no adhesion between inclusions and matrix, therefore the inclusions cannot carry any of the load, thus:
 σT 5 ðσm Þult 1 2 1:21 Vp2=3

(5.88)

Angrizani et al [42] compared the predictions of the above models with experimental data obtained for unsaturated polyester reinforced by calcium carbonate for three different filler mass fractions. Micromechanics calculations were performed by dedicated software MECH-Gcomp [53], and good correlation between analytical and experimental results were found, as illustrated in Fig. 5.11.

5.5.2 ELLIPSOIDAL PARTICLES Aiming to develop a micromechanical model independent of phase geometry assumptions, the Hashin
Shtrikman model [26] applied variational principles to bound the strain energy, and consequently the engineering constants. The principles of minimum potential and complementary energy were applied to extremize the functionals of strain energy, yielding upper (1) and lower (
) bounds for bulk and shear moduli under isotropic elasticity conditions: K ð2Þ 5 Km 1 Gð2Þ 5 Gm 1

1 Kp 2 Km

Vp ; K ð1Þ 5 Kp 1 1 3Km3V1m4Gm

Vp 1 Gp 2 Gm

1

6ðKm 1 2Gm ÞVm 5ð3Km 1 4Gm ÞGm

; Gð1Þ 5 Gp 1

Vm 3Vp 3Kp 1 4Gp

1 Km 2 Kp

1

1 Gm 2 Gp

Vm 6ðK 1 2G ÞV 1 5 3Kp 1 4Gp Gp ð p pÞ p

(5.89)

(5.90)

where the Young’s modulus and Poisson’s ratio can be computed through Eq. (5.83). These bounds were found to be in good agreement with experimental data. Also, for low differences in particle and matrix stiffnesses, the upper and lower bounds yielded close results, reaching infinity for a rigid phase, and zero for voids or cavities [26].

5.5 MICROMECHANICS OF PARTICULATE COMPOSITES

147

FIGURE 5.11 Comparison of experimental and analytical results of Young’s modulus (A) and shear modulus (B) for unsaturated polyester reinforced by calcium carbonate.

The Guth model [64] was also generalized for ellipsoidal particles by inserting the particle aspect ratio (p) into Eq. (5.86):

 E 5 Em 1 1 0:67pVp 1 1:62p2 Vp2 ; G 5 Gm 1 1 0:67pVp 1 1:62p2 Vp2

(5.91)

If p 5 1 (for spherical particles), Eq. (5.91) is equivalent to Eq. (5.86). The Mooney model [63] was also generalized for ellipsoidal particles by Brodnyan [67]: G 5 Gm exp

 2:5Vp 1 0:4075ðp21Þ1:508 Vp 1 2 FVp

(5.92)

where F follows Mooney’s proposal, and 1 , p , 15. For p 5 1, Eq. (5.92) becomes Eq. (5.85).

148

CHAPTER 5 MICROMECHANICS OF SHORT-FIBER

Another important model is the generalization of the Eshelby theory performed by Chow [68], where an additional strain tensor was inserted to account for the interaction between surrounding fillers:


0 E 5 Em @1 1

Kp Km


 1 G 2 1 η1 1 2 Gmp 2 1 η3 A 2η2 η3 1 η1 η4

(5.93)

and the parameters are (assuming p $ 1):





 Gp Gp Kp 2 1 Vm η7 ; η2 5 1 1 2 1 Vm η8 ; η3 5 1 1 2 1 Vm η5 ; Gm Km

Gm      Kp 4πη9 4πη9 2 2 2π 2 η11 η10 ; η6 5 2 4 η11 2 π η10 ; 2 1 Vm η6 ; η5 5 η4 5 1 1 Km 3 3

   4π 4π 2 3η11 η9 2 4 η11 2 2π η10 ; η7 5 2 2 3  12p  2



   4π 2 3η11 p 4π 3 1 η ; 1 4π 2 η ; η 5 η8 5 2 η 9 11 10 9 2 12 3

8π 1 2 ν m p i 1 1 2 2ν m 2π p h  2 12 ; η11 5 2 cosh21 p η10 5 3 p p 21 8π 1 2 ν m ðp2 21Þ2 η1 5 1 1

(5.94)

It is important to highlight that if p 5 1, then the Chow model coincides with the Kerner model for spherical particles. Since both particles and matrices are assumed to be isotropic, their bulk moduli K can be found using: Km 5

Em Ep  ; Kp 5  3ð1 2 2ν m Þ 3 1 2 2ν p

(5.95)

5.5.3 CUBIC PARTICLES Regarding composites reinforced by cubic fillers, two essentially different models were elaborated by Paul [69] and Ishai [70]. The first was based on the assumption of a cubic filler in a cubic matrix, submitted to uniform normal stress at the boundaries: 2

3   1 1 Ep =Em 2 1 Vp2=3 5 E 5 Em 4  
2=3 1 1 Ep =Em 2 1 Vp 2 Vp

(5.96)

whereas Ishai solved the problem by considering uniform displacement, achieving results extremely similar to the Hashin
Shtrickman lower bound: 0

E 5 Em @1 1

1

Vp 1=3 Ep =Em Ep =Em 2 1 Vp

A

(5.97)

Regarding tensile strength and considering again a cubic particle embedded in a cubic matrix, Nielsen [71] proposed a model considering no adhesion between filler and matrix, considering a stress concentration factor Kt, with a value that lies within 0
1 (a 0.5 value was suggested by Nielsen).
 σT1 5 0:5 ðσm Þult 1 2 Vp2=3

(5.98)

REFERENCES

149

5.6 CONCLUSION Fundamental concepts required for understanding micromechanics and the derivation of micro
macro relations along with their limitations were briefly reviewed in this chapter. The process of defining an RVE capable of representing a heterogeneous medium was discussed in detail, along with the required steps to compute eigenstresses and eigenstrains to extend RVE properties to macroscale through an averaging principle. After introducing the fundamental findings of Eshelby [5], Hill [23], Hashin
Shtrickman [26], and Mori
Tanaka [25], various closed-form solutions for predicting the mechanical properties of composites available in the literature were presented, aiming to encompass a wide range of applications. The models may considerably deviate from one another due to their different assumptions and simplifications, usually with restrictions regarding filler geometry or constituents’ contents and mechanical properties. Micromechanical models were separately presented based on filler geometry, covering random and aligned short-fiber composites and particulate composites reinforced by spherical, ellipsoidal, or cubic fillers. It has been shown that micromechanical relations are able to predict engineering constants of composite materials based on fiber and matrix properties and volume fractions with reasonable accuracy. Through the application of closed-form analytical solutions one can quickly estimate the properties of composite materials and also extrapolate results for different reinforcement contents, thus avoiding experimental tests in the early design stages, thereby saving time and money.

ACKNOWLEDGMENT The authors gratefully acknowledge the Coordination for the Improvement of Higher Education Personnel (CAPES) and the National Council for Scientific and Technological Development (CNPq).

REFERENCES [1] E.J. Barbero, Introduction to Composite Materials Design, second ed, CRC Press, Boca Raton, FL, 2011. [2] H. Yin, Y. Zhao, Introduction to the Micromechanics of Composite Materials, CRC Press, Boca Raton, FL, 2016. [3] A.K. Kaw, Mechanics of Composite Materials, second ed, CRC Press, Boca Raton, FL, 2006. [4] D. Gross, T. Seelig, Fracture Mechanics With Introduction to Micromechanics, Springer, New York, 2006. [5] J. Eshelby, The determination of the elastic field of an ellipsoidal inclusion and related problems, Proc. R. Soc. Lond. A. Math. Phys. Sci. 241 (1957) 376
396. [6] J. Eshelsby, The elastic field outside an ellipsoidal inclusion, Proc. R. Soc. Lond. A. Math. Phys. Sci. 252 (1959) 561
569. [7] G. Dvorak, Micromechanics of Composite Materials, Springer, New York, 2012. [8] H.J. Bohm, A short introduction to continuum micromechanics, Mechanics of Microstructured Materials, Springer Verlag, Vienna, 2004, pp. 1
40. [9] R.M. Christensen, Mechanics of Composite Materials, Dover, Mineola, 2005.

150

CHAPTER 5 MICROMECHANICS OF SHORT-FIBER

[10] T. Mura, Micromechanics of Defects in Solids, Springer, Dordrecht, 1987. [11] S. Nemat-Nasser, M. Horri, Micromechanics: Overall Properties of Heterogeneous Materials, Elsevier, Amsterdam, 1999. [12] T. Mura, Inclusion problem, Appl. Mech. Rev. 41 (1988) 15
20. [13] S. Li, G. Wang, Introduction to Micromechanics and Nanomechanics, World Scientific, Singapore, 2008. [14] S. Torquato, Random Heterogeneous Materials: Microstructure and Macroscopic Properties, Springer, New York, 2002. [15] X. Markenscoff, A. Gupta, Collected Works of J.D. Eshelby: The Mechanics of Defects and Inhomogeneities, Springer, Dordrecht, 2006. [16] A. Gurson, Continuum theory of ductile rupture by void nucleation and growth: Part I- yield criteria, J. Eng. Mater. Technol. 99 (1977) 2
15. [17] M. Kachanov, B. Shafiro, I. Tsukrov, Handbook of Elasticity Solutions, Kluwer Academic Publishers, Dordrecht, 2003. [18] T. Mura, Micromechanics of Defects in Solids, Martinus Nijhoff Publishers, Dordrecht, 1982. [19] W. Drugan, J. Willis, A micromechanics-based nonlocal constitutive equation and estimates of representative volume element size for elastic composites, J. Mech. Phys. Solids. 44 (1996) 497
524. [20] C. Huet, Application of variational concepts to size effects in elastic heterogeneous bodies, J. Mech. Phys. Solids. 38 (1990) 813
841. [21] T. Kanit, S. Forest, I. Galliet, V. Mounoury, D. Jeulin, Determination of the size of the representative volume element for random composites: statistical and numerical approach, J. Mech. Phys. Solids. 40 (2003) 3647
3679. [22] L. Domieux, D. Kondo, J. Ulm, Microporomechanics, John Wiley & Sons, Ltd, Chichester, 2006. [23] R. Hill, A self-consistent mechanics of composite materials, J. Mech. Phys. Solids. 13 (1965) 213
222. [24] B. Budiansky, On the elastic moduli of some heterogeneous materials, J. Mech. Phys. Solids. 13 (1965) 223
227. [25] T. Mori, K. Tanaka, Average stress in matrix and average elastic energy of materials with misfitting inclusions, Acta Metallurgica 21 (1973) 571
574. [26] Z. Hashin, S. Shtrikman, A varational approach to the theory of the elastic behavior of multiphase materials, J. Mech. Phys. Solids. 11 (1963) 127
140. ¨ ber die Berechnung zwischen den beiden Elastizita¨tsconstanten isotroper Ko¨per, [27] W. Voigt, U Wiedemanns Annalen der Physik und Chemie 38 (1889) 573
578. [28] A. Reuss, Berechnung der Fliessgrenze von Mischkristallen auf Grund der Plastizita¨tsbedingung fu¨r Einkristalle, Zeitschrift fu¨r Angewandte Mathematik und Mechanik 9 (1929) 49
58. [29] T. Zohdi, P. Wriggers, A model for simulating the deterioration of structural-scale material responses of microheterogeneous solids, Comp. Methods Appl. Mech. Eng. 190 (2001) 2803
2823. [30] Y. Benveniste, A new approach to the application of Mori
Tanaka’s Theory in composite materials, Mech. Mater. 6 (1987) 147
157. [31] T. Chen, G.J. Dvorak, Y. Benveniste, Mori-Tanaka estimates of the overall elastic moduli of certain composite materials, ASME J. Appl. Mech. 59 (1992) 539
546. [32] V. Buryachenko, Micromechanics of Heterogeneous Materials, Springer, New York, 2007. [33] A. Zaoui, Continuum micromechanics: survey, J. Eng. Mech. 128 (2002) 808
816. [34] C. Chen, C. Zhang, C. Liu, Y. Miao, S.C. Wong, Y. Li, Rate-dependent tensile failure behavior of short fiber reinforced PEEK, Comp. Part B: Eng. 136 (2018) 187
196. [35] S.K. De, J.R. White, Short Fibre-Polymer Composites, Woodhead Publishing, Cambridge, 1996. [36] S.Y. Fu, B. Lauke, Y.W. Mai, Science and Engineering of Short Fibre Reinforced Polymer Composites, Woodhead Publishing, Cambridge, 2009. [37] J. Henry, S. Pimenta, Modelling hybrid effects on the stiffness of aligned discontinuous composites with hybrid fibre-types, Comp. Sci. Technol. 152 (2017) 275
289.

REFERENCES

151

[38] H. Yu, K.D. Potter, M.R. Wisnom, A novel manufacturing method for aligned discontinuous fibre composites (High Performance-Discontinuous Fibre method), Comp. Part A 65 (2014) 175
185. [39] L. Nazarenko, H. Stolarski, H. Altenbach, Thermo-elastic properties of random composites with unidirectional anisotropic short-fibers and interphases, Europ. J. Mech. A Solids 70 (2018) 249
266. ¨ chsner, M. O ¨ chsner, The Finite Element Analysis Program MSC Marc/Mentat, second ed., [40] A. O Springer, Cham, 2018. [41] S. Banerjee, B.V. Sankar, Mechanical properties of hybrid composites using finite element method based micromechanics, Comp. Part B 58 (2014) 318
327. [42] C.C. Angrizani, E.A.W. Menezes, F.F. Luz, A.A. Muller, S.C. Amico, Uso de software acadeˆmico MECH-Gcomp para ensino de compo´sitos particulados, Revista de ensino de engenharia 36 (1) (2017) 13
23. [43] G. Eckold, Design and Manufacture of Composite Structures, Woodhead Publishing, Cambridge, 1994. [44] W.D. Callister, Materials Science and Engineering, seventh ed, John Wiley & Sons, York, 2007. [45] J.C. Halpin, J.L. Kardos, The Halpin-Tsai equations: a review, Polym. Eng. Sci. 16 (5) (1976) 344
352. [46] H.L. Cox, The elasticity and strength of paper and other fibrous materials, Br. J. Appl. Phys. 3 (1952) 72
79. [47] R. Turkovich, L. Erwin, Fiber fracture in reinforced thermoplastic processing, Polym. Eng. Sci. 23 (13) (1983) 743
749. [48] N. Pan, The elastic constants of randomly oriented fiber composites: A new approach to prediction, Sci. Eng. Comp. Mater. 5 (2) (1996) 63
72. [49] E.G. Wolff, Introduction to the Dimensional Stability of Composite Materials, Destech Publications, Lancaster, 2004. [50] R.M. Christensen, F.M. Waals, Effective stiffness of randomly oriented fibre composites, J. Comp. Mater. 6 (1972) 518
532. [51] M. Manera, Elastic properties of randomly oriented short fiber-glass composites, J. Comp. Mater. 11 (1977) 235
247. [52] E.A.W. Menezes, C.C. Angrizani, R. Smidt, S.C. Amico, Propriedades mecaˆnicas de compo´sitos com fibras curtas aleato´rias com auxı´lio de software online, 13 Congresso Brasileiro de Polı´meros, CBPol 2015, Natal, Brasil, 2015. [53] S.C. Amico, E.A.W. Menezes, MECH-Gcomp, UFRGS [Online]. Available: www.ufrgs.br/mechg. (accessed 20.06.18.). [54] F.L. Matthews, R.D. Rawlings, Composite Materials: Engineering and Science, Woodhead Publishing, Cambridge, 1994. [55] S.K. Mital, P.L.N. Murthy, R.K. Goldberg, Micromechanics for particulate reinforced composites, NASA Tech. Memorandum 107276 (1996). [56] S.J. Park, M.K. Seo, Interface Science and Composites, Academic Press, 2011. [57] I.M. Low, Advances in Ceramic Matrix Composites, Woodhead Publishing, 2013. [58] I.M. Daniel, O. Ishai, Engineering Mechanics of Composite Materials, second ed, Oxford University Press, New York, 2006. [59] R. Pal, Electromagnetic, Mechanical, and Transport Properties of Composite Materials, CRC Press, Boca Ranton, FL, 2015. [60] E.H. Kerner, The elastic and thermo-elastic properties of composite media, Proc. Phys. Soc. 68 (1956) 808
813. [61] A. Einstein, Investigation on Theory of Brownian Motion, Dover, English translation, New York, 1956. [62] S. Ahmed, F.R. Jones, A review of particulate reinforcement theories for polymer composites, J. Mater. Sci. 25 (1990) 4933
4942. [63] M. Mooney, The viscosity of a concentrated suspension of spherical particles, J. Colloid Sci. 6 (2) (1951) 162
170.

152

CHAPTER 5 MICROMECHANICS OF SHORT-FIBER

[64] E. Guth, Theory of filler reinforcement, J. Appl. Phys. 16 (20) (1945) 20
25. [65] U.J. Counto, The effect of the elastic modulus of the aggregate on the elastic modulus, creep and creep recovery of concrete, Mag. Concrete Res. 16 (48) (1964) 129
138. [66] L. Nicolais, M. Narkis, Stress-strain behavior of styrene-acrylonitrile/glass bead composites in the glassy region, Polym. Eng. Sci. 11 (3) (1971) 194
199. [67] J.G. Brodnyan, The concentration dependence of the newtonian viscosity of prolate ellipsoids, Trans. Soc. Rheol. 3 (1959) 61
68. [68] T.S. Chow, Effect of particle shape at finite concentration on the elastic moduli of filled polymers, J. Polym. Sci. Polym. Phys. Ed. 16 (1978) 959
965. [69] B. Paul, Prediction of elastic constants of multiphase materials, Trans. Metallurgical Soc. AIME 218 (1960) 36
41. [70] O. Ishai, L.J. Cohen, Elastic properties of filled and porous epoxy composites, Int. J. Mech. Sci. 9 (1967) 539
546. [71] L.E. Nielsen, Simple theory of stress-strain properties of filled polymers, J. Appl. Polym. Sci. 10 (1966) 97
103.

CHAPTER

BLENDS, INTERPENETRATING POLYMER NETWORKS, AND GELS OF UNSATURATED POLYESTER RESIN POLYMERS WITH OTHER POLYMERS

6

Pragnesh N. Dave1 and Ekta Khosla2 1

Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar, India 2Department of Chemistry, Hans Raj Mahila Maha Vidyalaya, Jalandhar, India

6.1 INTRODUCTION HISTORY The fact that some natural oils besides alkyd resins can be dried by certain additives and employed as coatings was realized long ago. This results from a polymerization reaction of the unsaturated moieties in ester molecules (Fig. 6.1). Carleton Ellis’ original patents with regard to polyester resins egressesed in the 1930s. While profit-making production started in 1941 by now reinforced with glass fiber for radar domes, also known to as radomes. Unsaturated polyester resins (UPRs) consist of two polymers, that is, a short-chain polyester bearing, polymerizable double bonds and a vinyl monomer. The curing reaction involves the copolymerization of the vinyl monomer with the double bonds of the polyester. During the process of curing, a threedimensional network is formed. UPR is yielded in this process, belonging to the thermoset class. UPRs have interesting applications in compression molding (sheet molding compounds), injection molding (bulk molding compounds), filament winding, resin transfer molding (RTM), pultrusion, and the hand lay-up process [1]. 85% of fiber-reinforced polymer (FRP) products such as aircraft parts, motor covers, belt guards, water-cooling towers, boats, architectural parts, chairs in ducts and other process equipment in chemical plants and paper mills, water pipes, and chemical containers are manufactured using polyesters, and they are used in offshore applications, construction, and the paint industry [2,3]. The determination of the gel time and curing time is a very significant step in the processing of UPRs while manufacturing composite products. The curing reaction should be accomplished in a governable manner to attain a high-quality product [4]. On account of the enormous amount of applications where flammability does not matter, such as underground mine bolts, pipes, and in many marine applications, the percentage of flame-retardant resins relative to all UPRs is less than Unsaturated Polyester Resins. DOI: https://doi.org/10.1016/B978-0-12-816129-6.00006-5 © 2019 Elsevier Inc. All rights reserved.

153

154

CHAPTER 6 BLENDS, INTERPENETRATING POLYMER NETWORKS

HO−CH 2 CH 2−OH Ethylene glycol (Structure)

Propy lene glycol (Structure)

Neopentyl glycol (Structure)

Trimethylol propane (Structure)

Glycerol (Structure) FIGURE 6.1 Diols and triols used for unsaturated polyester resins.

2% (v) [5]. UPR, like other polymer materials, has a restriction in fire retardancy. UPR liberates volatile vapors that trigger the ignition of fire when it is thermally degraded [6]. The addition of halogen can improve the fire retardancy of composites, nonetheless the use of a halogen additive may create environmental problems due to halogen radical and halo acid production which is harmful to human health when burnt [7]. The pie chart in Fig. 6.2 shows the world consumption of UPRs (https://ihsmarkit.com/products/ unsaturated-polyester-resins-chemical-economics-andbook.html, accessed on August 1, 2018). Fig. 6.2 shows the worldwide consumption of UPRs, which are thermosetting polymers, broadly used in the preparation of polymer composites [8
10]. The enhanced use of these materials is due to their comparatively low cost, good compatibility, ease of processing with a variety of fillers, and large selection of diverse types of monomers. These polymers are distinguished for the excellent balance between their mechanical, electrical, and chemical properties as compared to other engineering polymers. Some limitations of unsaturated polyesters (UPs) are their lower mechanical and thermal properties, which restrict their usage for some applications. To overcome these imperfections various modified fillers have been added [8
10].

6.1 INTRODUCTION

155

FIGURE 6.2 Worldwide consumption of unsaturated polyester resins.

Conventional fillers serve the purpose of improving the mechanical properties and decreasing manufacturing costs, but their use is restricted due to the phase separation and agglomeration of filler particles, inching toward a drastic worsening of the material properties [11]. The incorporation of clay into UPRs can result in the expansion of their mechanical, thermal, barrier and chemical properties, wear resistance, and flame retardancy [12
16]. This can be achieved with less filler content than what is used in most conventional composites. Important patents filed in this research area have been typified in Table 6.1. The high degree of cross-linking in UPR makes these composites more fragile and they possess lesser impact strength, which, unfortunately, restrains their use in high-performance applications. Hence, improving the toughness of UPRs has been the foremost foundation of the work of scientists. For the modification of UPR—improved toughness, the main method includes the introduction of elastomeric, nanosized fillers for modifying the chemical structure and forming interpenetrating polymer networks (IPNs) [17
20]. To improve the impact strength, IPNs of commercial UP and polyurethanes (PUR) have been used. These materials are employed in reaction injection molding (RIM) technology [21,22]. UPs represent the main class of thermosetting molding resins. They are products of the condensation of saturated and unsaturated dicarboxylic acids or anhydrides with alcohols. Propylene glycol is the most commonly used alcohol and phthalic and maleic anhydride are the most common saturated and unsaturated anhydrides being used. Condensation products (reactive resin) form very durable structures and coatings when cross-linked with vinyl reactive monomers (e.g., styrene).

156

CHAPTER 6 BLENDS, INTERPENETRATING POLYMER NETWORKS

Table 6.1 Some Industrially Important Interpenetrating Network Patents US Patent Number

Assignee

Vinyl chloride resin composition Latex paints Composite resin particles and the preparation thereof Copolyetherester elastomeric compositions Interpenetrating polymer network (IPN) of epoxy resin (ER), polyallyl polymer, and anhydride Maleic anhydride
ER prepolymer, (vinyl or isopropenyl) phenyl glycidyl ether and anhydride Semi-IPN for tougher and more microcracking resistant high temperature polymers

5132359 5124393 5115020 5112915 5110867

Mitsubishi Rayon Company, Ltd. Union Oil Company of California Nippon Paint Co., Ltd. General Electric Company Akzo NV

5106924

Westinghouse Electric Corp.

5098961

Polyurethane
poly(vinyl chloride) interpenetrating network Process for the preparation of stable interpenetrating polymer blends, comprising a poly(vinyl aromatic) polymer phase and a poly(alkylene) phase Porous membrane formed from IPN having a hydrophilic surface Novel damping compositions Microporous waterproof and moisture vapor permeable structures, including the processes of manufacture and useful articles thereof Epoxy
polyimide blend for low temperature cure, highperformance resin systems and composites Use of reactive hot melt adhesive for packaging applications Thermoplastic resin composition Embedded lens retroreflective sheeting with flexible, dimensionally stable coating Processible polyimide blends Vinyl chloride resin composition Impact-resistant resin composition Copolyetherester elastomeric compositions IPN of an aliphatic polyol(allyl carbonate) and ER IPN of blocked urethane prepolymer, polyol, ER, and anhydride Electron beam irradiated release film Thermoplastic polyester resin composition

5091455

The United States of America as represented by the Administrator of the National Aeronautics and Space Administration W. R. Grace & Co.-Conn.

Patent Title

5084513

Shell Internationale Research Maatschappij B.V.

5079272

Millipore Corporation

5066708 5066683

Rohm and Haas Company Tetratec Co., Aluminum Company of America operation

5021519

Aluminum Company of America

5018337

4996101 4994522 4994523 4992506 4957981 4923934

National Starch and Chemical Investment Holding Corporation Mitsubishi Rayon Co., Ltd. Minnesota Mining and Manufacturing Company Lockheed Corporation Mitsubishi Rayon Company Limited Mitsubishi Rayon Company Limited General Electric Company Akzo N.V. Todd A. Werner (Inventor)

4921882 4918132

Hercules Incorporated Mitsubishi Rayon Company Limited

5011887 5008142

6.2 CLASSIFICATION OF POLYESTER RESINS

157

Table 6.1 Some Industrially Important Interpenetrating Network Patents Continued

Patent Title Process of preparation for a new memory thermoplastic composition from polycaprolactone and polyurethane, products obtained by this process, and its use particularly in orthopedics Resin blends exhibiting improved impact properties Matrix
matrix polyblend adhesives and method of bonding incompatible polymers Impact modified poly(alkenyl aromatic) resin compositions Interpenetrated polymer films Enhanced melt extrusion of thermoplastics containing silicone IPNs Thermosetting composition for an IPN system Interpenetrating polymeric network comprising polytetrafluoroethylene and polysiloxane Thermosetting cyanate resin and the use thereof for the production of composite materials and IPNs

US Patent Number

Assignee

4912174

Laboratoires D’Hygiene et de Dietetique (L.H.D.)

4902737 4886689

General Electric Company Ausimont, U.S.A., Inc.

4882383

General Electric Co.

4845150 4831071

Foster-Miller Inc. ICI Americas Inc.

4766183 4764560

Essex Specialty Products, Inc. General Electric Company

4754001

Bayer Aktiengesellschaft

The properties of cross-linked resin can be altered by varying the type and amount of acids and glycols. UPRs are used in the production of sanitary-ware, tanks, gratings, fiber-reinforced plastics and fiber-filled plastic articles, pipes, and high-performance components for the marine and transportation industries such as in closure panels, body panels, fenders, boat hulls/decks, and other large glass fiber
reinforced parts. UPRs also find use in coatings and adhesives.

6.2 CLASSIFICATION OF POLYESTER RESINS Polyesters are broadly classified into unsaturated and saturated polymers. These are two broad divisions split up as: 1. Saturated: a. Fibers and films: These were based on the reaction of terephthalic acid with ethylene glycol and are linear, high molecular weight polymers which do not experience any cross-linking reactions. b. Plasticizers: These are completely saturated polyesters and are usually referred to as polymeric plasticizers. c. Polyester/polyurethanes: These are polyesters having high hydroxyl contents that react with various isocyanates to form polyurethane. These polyesters find extensive use as foams, elastomers, surface coatings, and adhesives.

158

CHAPTER 6 BLENDS, INTERPENETRATING POLYMER NETWORKS

2. Unsaturated: a. Laminating and casting resins: These are based on dibasic acids and dihydric alcohols. The polyester unit formed must be capable of copolymerizing with a vinyl-type monomer, thereby giving a vinyl
polyester copolymer or cured polyester having a thermoset structure. b. Alkyds: In general, these are of the same types as those described in (a), although the glyptal (surface coatings)-types are modified with oils or fatty acids. This term was also used to describe a group of thermosetting molding materials based on the reaction of a dihydric alcohol with an unsaturated acid such as maleic acid in place of the conventionally used phthalic acid. Vinyl-type monomers are also essential to speedy cross-linking and curing and these are used as molding powders for compression and transfer molding techniques. On the basis of their structure, polyester resins may be classified into these groups: (1) ortho-resins, (2) iso-resins, (3) bisphenol-A fumarates, (4) chlorendics, and (5) vinyl ester (VE) resins. 1. Ortho-resins: These are based on maleic anhydride (MA), phthalic anhydride (PA) or fumaric acid, and glycols. They are also recognized as general-purpose resins. Characteristics: PA is comparatively low in cost and provides a rigid link in the backbone. Limitations: It reduces the thermal resistance of laminates, while limited chemical resistance and processability are other problems associated with these resins. Owing to the presence of the pendant methyl group, the resulting resins are less crystalline and more attuned with wideranging reactive diluents (styrene) compared to those obtained using ethylene glycol, diethylene glycol (DEG), and triethylene glycol and they yield products with inferior electrical properties. 2. Iso-resins: These are prepared using MA/fumaric acid, isophthalic acid, and glycol. Characteristics: These resins are higher in price than ortho-resins and also have substantially higher viscosities; therefore, a higher proportion of reactive diluents (styrene) are needed. Isophthalic resins are of a higher caliber as they have better thermal and chemical resistance and mechanical properties. Limitations: The presence of higher quantities of styrene imparts improved water and alkali resistance to cured resins. 3. Bisphenol-A fumarate resins: Characteristics: The introduction of bisphenol-A into the backbone bestows a higher degree of inflexibility and stiffness with improved thermal performance. By reacting ethoxy-based bisphenol-A with fumaric acid, they are synthesized.

n

6.2 CLASSIFICATION OF POLYESTER RESINS

159

4. Chlorendic resins: Characteristics: To boost flame retardancy, chlorine/bromine-containing anhydrides or phenols are exploited in the preparation of UPRs. For example, the reaction of chlorendic anhydride/chlorendic acid with maleic acid/fumaric acid and glycol yields resin with better flame retardancy than general-purpose UPR. Other monomers used, include tetrachloro- or tetrabromophthalic anhydride. The bromine content must be at least 12% to make a self-extinguishing polyester [23].

5. VE resins: Characteristics: Bisacryloxy or bismethacryloxy derivatives of epoxy resins (ERs) carry unsaturated sites only in the terminal position and are developed by the reaction of acrylic acid or methacrylic acid with ER (e.g., diglycidyl ether of bisphenol-A, epoxy of the phenol-novolac type, or epoxy based on tetrabromobisphenol-A). These resins were first marketed under the trade name of Epocryl in 1965 by Shell Chemical Company. In 1966, Dow Chemical Company, under the trade name of Derakane resins, introduced a similar series of resins for molding purposes. The viscosities of neat resins are high; hence, reactive diluents (e.g., styrene) are added to obtain low viscosity solutions (100
500 poise). Notable advances in VE resin formulations are low-styrene-emission resins, hybrid grades that balance performance and economy, automotive grades with high tensile strength and heat deflection temperatures, and materials for corrosion resistance [24]. The effect of presence of electrolyte on a VE resin and its greatly filled quartz composites have just been reported on [25].

160

CHAPTER 6 BLENDS, INTERPENETRATING POLYMER NETWORKS

6.2.1 SYNTHESIS AND CHARACTERIZATION OF INTERPENETRATING POLYMER NETWORKS An IPN can be defined as a mixture of two cross-linked polymers when at least one of them is synthesized and/or cross-linked with another. If another polymer that is capable of cross-linking separately is added to a UPR, the physical properties can be enhanced dramatically. The components that make up an IPN are thermodynamically incompatible and a transition region of two phases is formed in such a system. The whole complex of IPN properties is determined by the accessibility and characteristics of this area. Other special types of such systems are also addressed as hybrid systems. The properties of an adhesive derived from the mixture of oligomers are similar to UPR and a prepolymer with end isocyanate groups (or macrodiisocyanate) based on polydiethylene glycol adipate of molecular weight 800 and Toluene diisocyanate (TDI) (a mixture of the 2,4- and 2,6-isomers in a ratio of 65:35), with the surfactant Alkyl triazole glycoside (ATG) added. The adhesive is cured by the effect of a redox system, for which MEKP(O) and cyanoacrylate (CN) are commonly used.

n

6.2 CLASSIFICATION OF POLYESTER RESINS

161

a. Polyurethanes: Polyurethanes are like UPR, compounds that simultaneously form a crosslinkable polyurethane which are added to poly glycols and diisocyanates. The rate of reaction of one component might be estimated to be reduced by the dilution effects of the other components. On the other hand, during free radical polymerization, the reaction may become diffusion controlled and a Trommsdorff effect (a self-acceleration of the by and large rate of the polymerization) emerges. When the polymerizing in mass becomes more viscous as the concentration of polymer increases, the mutual deactivation of the rising radicals is stalled, while the other basic reaction rates such as initiation and propagation remain constant. For a UPR
polyurethane system, the rate of the curing process is increased considerably in comparison to pure homopolymers. Validating reactions between the polyurethane isocyanate groups and the terminal UP carboxyl groups were suggested to possibly lead to the formation of amines (Eq. 6.1). R 2 N 5 C 5 O 1 R0 COOH-R 2 NHCO 2 O 2 CO 2 R0 -R 2 NHCO 2 R0 1 CO2

b.

c.

d.

e.

(6.1)

These amines may act as promoters of the curing process. Moisture, which does not influence the curing reaction of UPR, would also lead to the formation of amines by the reaction of the isocyanate groups with water. A tricomponent IPN system consisting of castor oil-based polyurethane components, acrylonitrile, and a UPR (the main component) was synthesized with the purpose of strengthening the unsaturated UPR. By incorporating urethane and acrylonitrile structures, the tensile strength of the matrix (UPR) decreased and the flexural and impact strengths were increased. Epoxides: Mixtures of UPR systems and ERs also form IPNs. Since a single glass transition temperature (Tg) for each IPN is observed, it is suggested that both materials are compatible. On the other hand, an interlock between the two rising networks was suggested because in the course of curing, a retarded viscosity increase was observed. A network interlock is indicated by a lower total exothermic reaction during simultaneous polymerization in comparison to the reaction of homopolymers. In bismaleimide-modified UP
ERs, the reaction of the UP with the ER could be established by IR spectral studies. The absorption of bismaleimide into the ER enhanced both the mechanical strength and thermal behavior of the ER. Vinyl ester resins: UPs modified with 30% of VE oligomer are tougheners for the UP matrix. The introduction of VE oligomer and bismaleimide into UPR improves its thermomechanical properties. Phenolic resins: An IPN consisting of a UPR and a resol-type phenolic resin showed improved heat resistance but also suppressed smoke, toxic gas, and heat release during combustion in comparison to a pure UPR. Organic
inorganic hybrids:

Organic
inorganic polymer hybrid materials can be synthesized using a UP and silica gel. First the UP is prepared and to this polyester the silica gel precursor is added, that is, tetramethoxysilane,

162

CHAPTER 6 BLENDS, INTERPENETRATING POLYMER NETWORKS

methyltrimethoxysilane, or phenyltrimethoxysilane. Gelling of the alkoxysilanes was achieved at 60 C using HCl catalyst in UPR. It was established by nuclear magnetic resonance spectroscopy that the polyester hydrolyze during the acid treatment. Finally, the IPN was formed by the photopolymerization of the UPR. It is assumed that between the phenyltrimethoxysilane and the aromatic groups in the UPR pinteractions arise.

6.3 POLYURETHANE HYBRID NETWORKS The mechanical properties of UPR can be significantly enhanced by incorporating a polyurethane linkage into the polymer network. The mechanical properties also can be altered by the same techniques used in segmented polyurethanes. The basic concept is to use soft segments and hard segments. A polyester is prepared with an excess of diols and diluted with styrene as usual. Additional diols as chain extenders are mixed into the resin solution. 4,4-Diphenylmethane diisocyanate dissolved in styrene is used to form hybrid linkages. Desirable peroxides are added to begin the radical curing. The curing begins with a reaction between the isocyanates and the hydroxyl groups, thus forming the polyurethane linkage. Then the cross-linking reaction takes place. The mechanical properties of the hybrid networks were usually improved by the assimilation of a chain extender at room temperature. Hexanediol increased the flexibility of the polymer chains, ensuing in a higher deformation and impact resistance of the hybrid networks. Hybrid networks with ethylene glycol as the chain extender are stiffer. The synthesis and characterization of IPNs was carried out by blending two thermosets and this method is extensively used [26
29]. This has been shown to be a promising way to extend the range of properties of thermosets and, hence the applications of polymer products. A string of studies on IPNs have discovered enhanced mechanical properties [30]. Epoxy/poly vinyl acetate IPNs are branded for toughness [31]. Epoxy/polydimethylsiloxane IPNs show potential toughening and better impact and thermal strength [32]. Epoxy/acrylate IPNs are described to display improved elongation at break, toughness, modulus, and tensile strength [33
35]. Likewise, IPNs of polyurethane
polystyrene, polyacrylates and polybenzoxazine, polymethacrylate, and epoxy
amine networks are reported to have higher tensile strength and elongation at break, improved thermal and surface free energy, and exhibit gas barrier properties [36
38]. The usage of polymer blends to make new materials for specialty applications is becoming widely employed for a range of applications. IPNs belong to a category of polymer blends with special characteristics such as impact strength modification and pH sensitive hydrogels. In semi-IPNs only one of the constituents is cross-linked. Polyurethane
polyester IPNs have been synthesized to improve the flexibility of the resulting polymer network. In recent years, the blending of two thermosets through IPNs has been extensively studied [39
47]. Several studies on IPNs have revealed improved mechanical properties [48,49]. Lin et al. [50,51] investigated the chemorheology and kinetics of epoxy/UP IPNs.

6.4 CURRENT RESEARCH ON UNSATURATED POLYESTER

163

The enhanced cracking energy-absorbing capability of epoxy/acrylic IPNs has also been reported in the literature [48]. The entanglement of the two interlocked networks exhibited an improved toughness on the mechanical properties of epoxy/UP IPNs [52]. The physical and mechanical properties of PUR elastomers are related to the phase separation of hard from soft blocks, which depends on the solubility parameter of the blocks, crystallinity of the phases, temperature, and the sample’s thermal record [53]. Hence, the degree of phase separation, structural and dynamical heterogeneity, and morphology of the resulting phases (size, shape, orientation, and domain associations) have been extensively investigated. Their dependence on factors such as chemical composition, degree of crystallinity, molecular masses and the ratio of segments, the introduction and structure of the side chains or functional groups, the preparation method, and sample storage has been the subject of numerous studies. PUR can be used as an underwater acoustic absorption material and/or damping material [54
58]. The ability to absorb mechanical energy and convert it into heat is its uppermost around the Tg, where the rate (frequency) of mechanical action is equal to the rate (frequency) of coordinated molecular chain segment motion [59]. Therefore the damping capacity is connected to motional heterogeneity, which can be adjusted by varying the composition of soft and hard segments and their mutual ratio and the introduction of functional groups and side chains [54,60
65]. Damping capacity can be determined from dynamic mechanical properties. In two damping methods that are interesting from an engineering point of view, the values for the loss modulus and the loss factor tan(δ) are important. An extensional damping requires a high E value, while a constrained layer damping requires a high tan value [59]. With the aim of being efficient in the wide temperature and frequency ranges encountered in real damping applications, damping materials should exhibit a high tan value ( . 0.3) above the temperature range of no less than 60 C
80 C. PUR frequently shows an inadequately broad tan(δ) peak. Besides, the usage of PUR in structural materials can be hindered by its meager thermal stability and mechanical properties. So as to widen the temperature range with adequately high damping peaks and to obtain materials with adequate properties, polyurethanes can be combined with polymers possessing high modulus and strength through the preparation of IPNs [66,67]. Changes in the physical and mechanical properties of networks compared to the pure components depend on the degree of phase separation and morphology. The degree of phase separation in IPNs depend on numerous factors such as the miscibility, mutual ratio, crystallinity, and the Tg of the components, the degree of cross-linking of individual networks and the interconnection of networks, the method and conditions (temperature, pressure) of preparation, and the rate of forming [68]. The effect of blending on the damping ability of PUR IPNs with various polymer components such as ER [27
30,67
69] VE resin [67,69
72], UPR [73], acrylates [67,69,74
77], poly(vinyl chloride) (PVC) [67
69,78], and polystyrene [66,67,69,79] has been extensively prepared and characterized.

6.4 CURRENT RESEARCH ON UNSATURATED POLYESTER UPR is the class of superior and applied polymers, which find use in a number of diverse applications [80
82]. UPRs were selected primarily for making FRPs using any molding technique because of their ease of handling and fabrication and low cost as compared to ER.

164

CHAPTER 6 BLENDS, INTERPENETRATING POLYMER NETWORKS

They are primarily used in compression molding (sheet molding compounds), injection molding (bulk molding compounds), pultrusion, resin transfer molding, filament winding, and the hand lay-up process [83]. About 85% of FRP products (like boats, car and aircraft components, and chairs) are manufactured using polyesters [84]. Different kinds of polyesters have been synthesized over the past few decades from various types of diols and diacid chlorides. Thermally stable products made of polyesters derived from isophthalic and terephthalic acids with bisphenol-A have successfully been commercialized [85]. These polyesters were normally hard to process because of their inadequate solubility in organic solvents and their melting temperature or high Tg relating to their rigid structures [86]. Therefore the growth of polyesters for use at high temperatures with improved solubility was an important goal. To improve the solubility and processability of polymers without the extreme loss of their high thermal stability, polar and flexible groups were introduced into the backbone of the polymers [87
91]. The insertion of bulky side chain affects solubility because this strategy produces a severance of chains and the lowering of the chain packing by way of molecular mobility leading to the enhancement of solubility concurrently [92
95]. (99) It has been identified that a large number of polymers containing heterocyclic rings in their main chain were resistant to high temperatures (98). New polyesters containing rigid segments such as pyridine rings that possess high Tg values and enhanced solubility in organic solvents were synthesized by researchers (99; 100). The challenge in synthesis of UPRs riveted on the enhancement of chemical inertness, barrier properties, low friction coefficient and low surface tension, solvent and high temperature resistant that in essence are transferred to other polymeric materials by blending or copolymerization. Some augmentation such as excellent resistance to corrosion, water and atmospheric agents, formulations for resins and foams, and several others were also present in patent literature [87]. Nowadays, the macroscopic properties of polymers and other complex materials are mainly interpreted by understanding the underlying microscopic phenomena. The temperature dependence of the polymer affects average relaxation time and the molecular mobility decide the properties at a particular temperature. Toward this end, an energy landscape model based on the nature of structural evolution in a super cooled liquid approaching the glassy state was developed. According to this scheme, relaxation behavior was considered as strong and fragile, depending on the rate with which the associated properties were modified as the temperature passes through the glass transition region [82]. The continuing search for polymers with improved or unusual properties guided a considerable level of interest in the behavior of so-called rigid-rod polymers. Such materials were of curiosity owing to their potential to form fibers of particularly high strength [80
82]. This rigid-rod polymer had achieved commercial success in a variety of applications, particularly those relying on its exclusive combination of high strength and low density [87]. Their lack of processability was a major drawback in the commercial exploitation of the many rigid-rod systems. However, the usage of UPR resin was used to overcome this drawback [90]. Polyester resins also called unsaturated copolyesters, are based on a polyester backbone in which both a saturated acid and an unsaturated acid are condensed with a dihydric alcohol [82]. An inspection of the scientific literature reveals that few unsaturated copolyesters based on the interaction of unsaturated diols and saturated acids were synthesized and studied [86]. Aromatic polyesters and copolyesters containing phenylindane units with Tg between 235 C and 253 C were

6.6 POLYMER GELATION AND VITRIFICATION

165

documented in the literature [87]. In the contemporary era, attempts have been made toward the syntheses of polymers containing chromophoric groups, for instance, the aromatic azo groups which can form a part of the main chain [89]. Therefore polymers that possess the azo group have potential use in a variety of applications [93
95]. The aromatic azo group, because of the existence of cis-trans isomerism and further effects on the photochromic properties of polymers, is of special interest. New UPs and copolyesters based on some dibenzylidenecycloalkanones and containing meta- and para-azo groups in the main chain were investigated relating to the synthesis and characterization of them. Through the interfacial polycondensation of various monomers, new interesting classes of linear UPs polyesters based on dibenzylidenecycloalkanones were synthesized [96]. The effects of a cycloalkanone ring in the polymer backbone on the properties of polymers were also studied.

6.5 POLYMER BLENDS IPNs are formed by two cross-linked polymers, so they both constitute a network, whereas semiIPNs are blends, in which only one of the constituents is cross-linked. Very little work has been reported in the area of producing IPNs with oligomers; the production of IPNs for practical applications has been described, but only a few papers in this respect have appeared in the literature. The industrial fabrication of most polymers involves the formation of oligomer as byproducts. This is not only a waste of resources but also a source of pollutants in landfills and water deposits. Also, for these IPN-like materials, the microstructure is essential, so that characterization can direct a better understanding of the synthesis
properties relationships.

6.6 POLYMER GELATION AND VITRIFICATION Since the beginning of this century, the polymer industry has been producing thermosetting resins (viscous liquids with the ability to harden permanently) owing to their excellent chemical, thermal, and mechanical properties along with their easy, controllable, inexpensive, and fast molding and production. More specifically, these thermosets are highly elastic (very ductile), strong (adequate stiffness but still quite tough), dimensionally stable, and resistant to heat and corrosion agents; which are all important requirements regarding finished products and their fabrication, processing, and use in several fields, for example, automobile and marine transportation (e.g., protective coatings, hulls, and auto bodywork compounds) and civil infrastructure construction (e.g., covers, bathroom components and fixtures, pipes, tanks, and fittings). These properties are the result of a highly cross-linked network composed by polymer chains and an additional monomer (solvent), which is highly reactive. In order to obtain this highly stable and strong network, the formation of covalent bonds is necessary between the polymer chains and the solvent monomers, which is only possible if they comprise functional groups that can react with each other such as the alkene groups (due to their carbon
carbon double bonds).

166

CHAPTER 6 BLENDS, INTERPENETRATING POLYMER NETWORKS

FIGURE 6.3 Schematic illustration of a curing reaction and its steps, from unlinked chains to gel formation.

To accomplish this reaction between the functional groups, it is also necessary for an ion or a radical to be present, which are obtained by heating, heating and compression, or light irradiation of chemical compounds called initiators. Ions or radicals work like a trigger by breaking some double bonds, which generate other reactive free ions or radicals. These would be able to “attack” the other double bonds along the chains and, as a result, a copolymerization occurs between the initial polymer and the solvent, producing the final resin. This whole process is also known as a curing reaction and it is not as simple as it seems. It comprises two solidification phases—gelation and vitrification—and two more subreactions—polymer and solvent homopolymerization—which are worthy of consideration (Fig. 6.3). Gelation consists of a liquid (sol phase) to rubber (gel phase) transition controlled by the kinetics of the reaction, in which the resin molecular weight and viscosity increases considerably. After gelation, chains begin to lose their mobility (due to the increase of cross-linked network density) and a diffusion-controlled rubber
glass transition occurs, known as vitrification. This is a significant stage as it determines the rate and degree of the reaction conversion and enables some modifications to the structure and the properties of the final resin. Concerning the subreactions, it is also important to note that besides the copolymerization between the polymer and the solvent, covalent bonds are also created within the polymer chains or between the solvent monomers as shown in Fig. 6.4 These homopolymerization reactions have dissimilar kinetics and affect, in different ways, the macro- and micro-structure of the final cross-linked network. UPs have been extensively used in the biomedical and environmental fields. Due to their inherent biodegradability (ester linkages), biocompatibility, and cross-linking ability (carbon
carbon double bonds), they had developed into the most appropriate and expectant candidate for the production of resins which require not only superb physicochemical properties but also good biological properties. Classically, UPs comprise glycols and saturated and unsaturated acids as monomers. The types and amounts of monomers used define the composition and the properties of UPs: glycols and saturated acids are accountable for strength and thermochemical resistance in UPs; and unsaturated acids allow for the cross-linking of UPs in curing reactions due to their double bonds.

REFERENCES

167

FIGURE 6.4 Schematic illustration of inter (a.1) and intra (a.2) polymer homopolymerization and solvent monomer homopolymerization.

6.7 CONCLUSION AND FUTURE DIRECTIONS The prospects of UPRs in the global automotive composites market look to be full of opportunities in various applications including body panels, closure panels, grille opening reinforcement, heat shields, fenders, headlamp reflectors, pickup boxes, and others. The production of UPRs in the worldwide automotive composites market is predicted to grow at a compound annual growth rate (CAGR) of 5.3% from 2016 to 2021. The major agents of change for market growth are the rising demand for lightweight resources and the performance profit of reinforced composites over competitor materials. Properties such as high tensile strength, lightweight, ease of processability, and good corrosion resistance and surface tension make UPR composites ideal candidates for developing lightweight and fuel-efficient vehicles.

REFERENCES [1] J.L. Vilas, J.M. Laza, M.T. Garay, M. Rodriguez, L.M. Leon, Unsaturated polyester resins cure: kinetic, rheologic, and mechanical dynamical analysis. II. The glass transition in the mechanical dynamical spectrum of polyester networks, J. Polym. Sci. Part B: Polym. Phys. 39 (2001) 146
152. [2] L.U. Devi, S.S. Bhagawan, S. Thomas, Mechanical properties of pineapple leaf fiberreinforced polyester composites, J. Appl. Polym. Sci. 64 (1997) 1739
1748. [3] C.M.C. Pereira, M. Herrero, F.M. Labajos, A.T. Marques, V. Rives, Preparation and properties of new flame retardant unsaturated polyester nanocomposites based on layered double hydroxides, Polym. Degrad. Stabil. 94 (2009) 939
946. [4] B. Salar, in: Fibre Reinforced Polyester Composites. H. El-D. M. Saleh (Ed.), 2012, pp. 135
166. [5] http://www.cfa-hq.org/press-release22.htm (accessed 5.09.02.).

168

CHAPTER 6 BLENDS, INTERPENETRATING POLYMER NETWORKS

[6] W.W. Isao, S. Shinichi, Environmental release and behavior of brominated flame retardant, Environ. Int. 29 (2003) 665
682. [7] C. Gerard, G. Fontaine, S. Bourbigot, New trends in reaction and resistance to fire of fire
retardant epoxies, J. Mater. 3 (2010) 4476
4499. [8] R.R. Sengupta, M. Bhattacharya, S. Bandyopadhyay, A.K. Bhowmick, A review on the mechanical and electrical properties of graphite and modified graphite reinforced polymer composites, Prog. Polym. Sci. 36 (2011) 638
670. [9] C. Varga, N. Miskolczi, L. Bartha, G. Lipo´czi, Improving the mechanical properties of glass-fibrereinforced poly-ester composites by modification of fibre surface, Mater. Design 31 (2010) 185
193. [10] S.S. Ray, M. Okamoto, Polymer/layered silicate nano-composites: a review from preparation to processing, Prog. Polym. Sci. 28 (2003) 1539
1641. [11] K.A. Carrado, L. Xu, In situ synthesis of polymer clay nanocomposites from silicate gels, Chem. Mater. 10, 1440
1445. [12] R.K. Bharadwaj, A.R. Mehrabi, C. Hamilton, C. Trujillo, M. Murga, R. Fan, et al., Structure
property relationships in cross-linked polyester
clay nanocomposites, Polymer 43 (2002) 3699
3705. [13] X. Kornmann, L.A. Berglund, J. Sterte, E.P. Giannelis, Nanocomposites based on montmorillonite and unsaturated polyester, Polym. Eng. Sci. 38 (1998) 1351
1358. [14] M. Chieruzzi, A. Miliozzi, J.M. Kenny, Effects of the nano-particles on the thermal expansion and mechanical properties of unsaturated polyester/clay nanocomposites, Composites,. Part A 45 (2013) 44
48. [15] P. Jawahar, R. Gnanamoorthy, M. Balasubramanian, Tribologicalbahaviour of the clay-thermoset nanocomposites, Wear 261 (2006) 835
840. [16] P. Kiliaris, C.D. Papaspyrides, Polymer/layered silicate (clay) nanocomposites: an overview of flame retardancy, Prog. Polym. Sci. 35 (2010) 902
958. [17] M. Lubi, C. Ravi, K.A. Ahammed, Modification of unsaturated polyester resin using elastomers, J. Elastomers Plast. 32 (2000) 60
72. [18] M. Zhang, R.P. Singh, Mechanical reinforcement of unsaturated polyester by Al2O3 nanoparticles, Mater. Lett. 58 (2004) 408
412. [19] B. Gawdzik, T. Matynia, E. Chmielewska, Modification of unsaturated polyester resin with bismaleimide, J. Appl. Polym. Sci. 82 (2001) 2003
2007. [20] D. Tang, J. Zhang, D. Zhou, Influence of BaTiO3 on damping and dielectric properties of filled polyurethane/unsaturated polyester resin interpenetrating polymer networks, J. Mater. Sci. 40 (2005) 3339
3345. [21] L. Valette, C. Hsu, Polyurethane and unsaturated polyester hybrid networks: 2. Influence of hard domains on mechanical properties, Polymer 40 (1999) 2059
2070. [22] X. Ramis, A. Cadenato, J. Morancho, Polyurethane unsaturated polyester interpenetrating polymer networks: thermal and dynamic mechanical thermal behaviour, Polymer 42 (2001) 9469
9479. [23] G. Gunduz, S. Ozturk, Flame retardance and mechanical properties of 1, 3 propylene glycol based unsaturated polyester, Polym. Plast. Technol. Eng. 33 (1994) 245. [24] P. Mapleston, Vinyl ester. Performance upgrades fit many markets, Mod. Plast 70 (40) (1993). [25] D. Sagi-Mana, M. Narkis, A. Siegmann, R. Joseph, H. Dodiuk, The effect of marine environment on a vinyl ester resin and its highly filled particulate quartz composites, J. Appl. Polym. Sci 69 (1998) 2229
2234. [26] D. Klempner, L.H. Sperling, L.A. Utracki (Eds.), Interpenetrating Polymer Networks, American Chemical Society, Washington, DC, 1994. CONF-910812-ISBN 0-8412-2528-1; TRN: 95:002683-0002. [27] X.W. He, J.M. Widmaier, J.E. Herz, G.C. Meyer, in: D. Klempner, K.C. Frisch (Eds.), Advances in Interpenetrating Polymer Networks, 4, Technomic, Lancaster, PA, 1994, p. 321. [28] V. Abetz, G.C. Meyer, A. Mathis, C. Picot, J.M. Widmaier, in: S.C. Kim, L.H. Sperling (Eds.), IPNs Around the World: Science and Engineering, Wiley, Chichester, 1997, p. 203.

REFERENCES

169

[29] J.M. Widmaier, G.C. Meyer, in: D. Klempner, K.C. Frisch (Eds.), Advances in Interpenetrating Polymer Networks, 1, Technomic, Lancaster, PA, 1989, p. 155. [30] L. Rosu, C.N. Cascava, D. Ciobanu, D. Rosu, D.E. Ion, C. Morosanu, et al., Effect of UV radiation on the semi-interpenetrating polymer networks based on polyurethane and epoxy maleate of bisphenol A, J. Photochem. Photobiol. A: Chem. 169 (2005) 177
185. [31] A. Lapprand, C. Arribas, C. Salom, R. Masegosa, M. Prolongo, Epoxy resin modified with poly (vinyl acetate), J. Mater. Process Tech. 143
144 (2003) 827
831. [32] T.V.T. Velan, M.I. Bilal, Aliphatic amine cured PDMS-epoxy interpenetrating network system for high performance engineering applications-development and characterization, Bull. Mater. Sci. 23 (5) (2000) 425
429. [33] B. Das, D. Chakraborty, A. Hajra, Epoxy resin/poly(ethyl acrylate)—interpenetrating polymer networks: engineering properties and morphology, Eur. Polym. J. 30 (11) (1994) 1269
1276. [34] D. Chakrabarty, Interpenetrating polymer networks: engineering properties and morphology, Polym. Gels Networks 6 (1998) 191
204. [35] B. Das, D. Chakraborty, Epoxy-poly (2-ethylhexyl acrylate) interpenetrating polymer networks morphology and mechanical and thermal properties, Polym. Gels Networks 3 (2) (1995) 197
208. [36] C. Hui, C. Jiann-Ming, Synthesis and properties of polyurethane-polystyrene-grafted interpenetrating polymer networks using a microgel process, J. Appl. Polym. Sci. 50 (1993) 495
501. [37] L. Hua, S. Yi-Che, W. Chih-Feng, H. Chih-Feng, S. Yuung-Ching, C. Feng-Chih, Thermal properties and surface energy characteristics of interpenetrating polyacrylate and polybenzoxazine networks, Polymer 49 (2008) 4852
4860. [38] V. Simic, S. Boileau, L. Bouteiller, L. Gallez, P. Merlin, Gas barrier and adhesion of interpenetrating polymer networks based on poly(diurethanebismethacrylate) and different epoxy-amine networks, Eur. Polym. J. 38 (2002) 2449
2458. [39] L.H. Sperling, Interpenetrating Polymers and Related Materials, Plenum Press, New York, 1981. [40] S. Nishi, T. Kotoka, Complex-forming poly (oxyethylene):poly(acrylic acid) interpenetrating polymer networks. 1. Preparation, structure, and viscoelastic properties, Macromolecules 18 (8) (1985) 1519
1525. [41] D.L. Siegfried, D.A. Thomas, L.H. Sperling, Thermoplastic interpenetrating polymer networks of a triblock copolymer elastomer and anlonomeric plastic. II. Mechanical behavior, Polym. Eng. Sci. 21 (1) (1981) 39
46. [42] D.J. Hourston, R.J. Satguunathan, Latex interpenetrating polymer networks based on acrylic polymers. I. Predicted and observed compatibilities, Appl. Polym. Sci. 29 (10) (1984) 2969
2980. [43] M.C.O. Chang, D.A. Thomas, L.H. Sperling, Characterization of the area under loss modulus and tan δ
temperature curves: acrylic polymers and their sequential interpenetrating polymer networks, J. Appl. Polym. Sci. 34 (1) (1987) 409
422. [44] R.B. Fox, J.L. Bitner, J.H. Hinkley, W. Carter, Dynamic mechanical properties of some polyurethaneacrylic copolymer interpenetrating polymer networks, Polym. Eng. Sci. 25 (3) (1985) 157
163. [45] H. Chen, J.M. Chen, Synthesis and properties of polyurethane, polystyrene-grafted interpenetrating polymer networks using a microgel process, J. Appl. Polym. Sci. 50 (3) (1993) 495
501. [46] D.J. Hourston, Y.J. Zia, Semi- and fully interpenetrating polymer networks based on polyurethane
polyacrylate systems. II. Polyurethane
poly (methyl acrylate) semi-1-interpenetrating polymer networks, Appl. Polym. Sci. 28 (1983) 3745
3758. [47] S.C. Kim, L.H. Sperling, IPNs Around the World: Science and Engineering, John Wiley & Sons, New York, 1997. [48] M.S. Lin, S.T. Lee, Mechanical behaviours of fully and semi-interpenetrating polymer networks based on epoxy and acrylics, Polymer 38 (1) (1997) 53
58. [49] M. Akay, J.G. Cracknell, Epoxy resin
polyethersulphone blends, J. Appl. Polym. Sci. 52 (5) (1994) 663
688.

170

CHAPTER 6 BLENDS, INTERPENETRATING POLYMER NETWORKS

[50] M.S. Lin, R.J. Chang, Chemorheology on simultaneous IPN formation of epoxy resin and unsaturated polyester, J. Appl. Polym. Sci. 46 (1992) 815
827. [51] M.S. Lin, R.J. Chang, T. Yang, Y.F. Shih, Kinetic study on simultaneous interpenetrating polymer network formation of epoxy resin and unsaturated polyester, J. Appl. Polym. Sci. 55 (1995) 1607
1617. [52] M.S. Lin, C.C. Liu, C.T. Lee, Toughened interpenetrating polymer network materials based on unsaturated polyester and epoxy, J. Appl. Polym. Sci. 72 (1999) 585
592. [53] C. Hepburn, Polyurethane Elastomers, Elsevier, London, UK, 1992. [54] K.H. Yoon, S.T. Yoon, O.O. Park, Damping properties and transmission loss of polyurethane. I. Effect of soft and hard segment compositions, J. Appl. Polym. Sci. 75 (2000) 604
611. [55] K.H. Yoon, J.G. Kim, D.S. Bang, Damping properties and transmission loss of polyurethane. II. PU layer and copolymer effect, Fibers Polym. 4 (2) (2003) 49
53. [56] P.H. Mott, C.M. Roland, R.D. Corsaro, Acoustic and dynamic mechanical properties of polyurethane rubber, J. Acoustical Soc. Am. 111 (2002) 1782
1790. [57] C.H. Zhang, Z. Hu, G. Gao, et al., Damping behavior and acoustic performance of polyurethane/lead zirconate titanate ceramic composites, Mater. Design 46 (2013) 503
510. [58] W. Sun, X. Yan, X. Zhu, Dynamic mechanical and underwater acoustic properties of the polyurethane/ epoxy resin blend elastomers filled with macroporous poly(vinyl acetate co-triallyl isocyanurate) resin beads, J. Appl. Polym. Sci. 122 (2011) 2359
2367. [59] L.H. Sperling, Basic viscoelastic definitions and concepts, in: R. Corsaro, et al. (Eds.), Sound and Vibration Damping with Polymers, ACS Symposium Series, American Chemical Society, Washington, DC, 1990, pp. 5
22. Chapter-1. [60] W.P. Chen, S. Schlick, Study of phase separation in polyurethanes using paramagnetic labels: effect of segment molecular weight and temperature, Polymer 31 (1990) 308
314. [61] W.P. Chen, D.J. Kenney, K.C. Frisch, et al., Study of phase separation in polyurethane using paramagnetic labels effect of soft segment molecular weight, structure, and thermal history, J. Polym. Sci. Part B: Polym. Phys. 29 (1991) 1513
1524. [62] W.P. Chen, K.C. Frisch, D.J. Kenney, et al., Effect of soft segment molecular weight and 3- methyl side group on microstructural separation in polyurethane elastomers, J. Macromol. Sci. Part A: Pure Appl. Chem. 29 (1992) 567
587. ˇ [63] J. E`ulin, M. Andreis, I. Smit, et al., Motional heterogeneity and phase separation of functionalized polyester polyurethanes, Europ. Polym. J. 40 (2004) 1857
1866. [64] J. Culin, I. Smit, Z. Veksli, A. Anzlovar, M. Zigon, et al., Phase morphology of functionalized polyester polyurethanes. Effect of functional group concentration, Polym. Int. 55 (3) (2006) 285
291. [65] J. Culin, S. Frka, M. Andreis, A. Anzlovar, M. Zigon, et al., Motional heterogeneity of segmented polyurethane
polymethacrylate mixtures: an influence of functional groups concentration, Polymer 43 (14) (2002) 3891
3899. [66] D.J. Hourston, F.U. Schafer, Polyurethane/polystyrene one-shot interpenetrating polymer networks with good damping ability: transition broadening through crosslinking, internetwork grafting and compatibilization, Polym. Adv. Technol. 7 (4) (1996) 273
280. [67] L.H. Sperling, Interpenetrating polymer networks, Encyclopedia of Polymer Science and Technology, John Wiley & Sons, 2004. [68] Y.C. Chern, S.M. Tseng, K.H. Hsieh, Damping properties of interpenetrating polymer networks of polyurethane-modified epoxy and polyurethanes, J. Appl. Polym. Sci. 74 (2) (1999) 328
335. [69] H. Janik, Progress in the studies of the supermolecular structure of segmented polyurethanes, Polimery 55 (6) (2010) 421
430. [70] C.H. Chen, W.J. Chen, M.H. Chen, Y.M. Li, Simultaneous full-interpenetrating polymer networks of blocked polyurethane and vinyl ester. II. Static and dynamic mechanical properties, J. Appl. Polym. Sci. 71 (12) (1999) 1977
1985.

REFERENCES

171

[71] C.L. Qin, D.Y. Zhao, X.D. Bai, Vibration damping properties of gradient polyurethane/vinyl ester resin interpenetrating polymer network, Mater. Chem. Phys. 97 (2006) 517
524. [72] C.L. Qin, W.M. Cai, D.Y. Tang, Damping properties and morphology of polyurethane/vinyl ester resin interpenetrating polymer network, Mater. Chem. Phys. 85 (2004) 402
409. [73] D.Y. Tang, J.S. Zhang, D.R. Zhou, L.C. Zhao, Influence of BaTiO3 on damping and dielectric properties of filled polyurethane/unsaturated polyester resin interpenetrating polymer networks, J. Mater. Sci. 40 (2005) 3339
3345. [74] S. Desai, I.M. Thakore, A. Brennan, S. Devi, Thermomechanical properties and morphology of interpenetrating polymer networks of polyurethane
poly(methyl methacrylate), J. Appl. Polym. Sci. 83 (2002) 1576
1585. [75] J. Culin, I. Smit, M. Andreis, Motional heterogeneity and phase separation of semi-interpenetrating networks and mixtures based on functionalised polyurethane and polymethacrylate prepolymers, Polymer 46 (2005) 89
99. [76] J. Culin, M. Andreis, Z. Veksli, ESR-spin labelling study of semi-interpenetrating networks and polymer mixtures based on functionalized polyurethanes and polymethacrylates, Eur. Polym. J. 41 (2005) 1874
1882. [77] J. Culin, M. Andreis, I. Smit, Z. Veksli, A. Anzlovar, M. Zigon, Motional heterogeneity and phase separation of functionalized polyester polyurethanes, Eur. Polym. J. 40 (2004) 1857
1866. [78] S.N. Jaisankar, R.M. Sankar, K.S. Meera, A.B. Mandal, Thermoplastic interpenetrating polymer networks based on polyvinyl chloride and polyurethane ionomers for damping application, Soft Mater. 11 (2013) 55
60. [79] N.V. Babkina, Yu. S. Lipatov, T.T. Alekseeva, Damping properties of composites based on interpenetrating polymer networks formed in the presence of compatibilizing additives, Mech. Comp. Mater. 42 (2006) 385
392. [80] M. Modesti, L. Zanella, A. Lorenzetti, R. Bertani, M. Gleria, Thermally stable hybrid foams based on cyclophosphazenes and polyurethanes, Polym. Degrad. Stab. 87 (2) (2005) 287
292. [81] M. Tiitu, A. Talo, O. Fors´en, O. Ikkala, Aminic epoxy resin hardeners as reactive solvents for conjugated polymers: polyaniline base/epoxy composites for anticorrosion coatings, Polymer 46 (18) (2005) 6855
6861. [82] N.T. Qazvini, N. Mohammadi, Dynamic mechanical analysis of segmental relaxation in unsaturated polyester resin networks: effect of styrene content, Polymer 46 (21) (2005) 9088
9096. [83] G.L. Grunewald, V.H. Dahanukar, K.R. Criscione, Effects of a 3-alkyl-,4-hydroxy- and/or 8-aromaticsubstituent on the phenylethanolamine N-methyltransferase inhibitor potency and α2-adrenoceptor affinity of 2,3,4,5-tetrahydro-1H-2-benzazepines, J. Med. Chem. 9 (8) (2001) 1957
1965. [84] R. Shah, S. Devi, Preconcentration and separation of palladium(II) and platinum(IV) on a dithizone anchored poly(vinylpyridine)-based chelating resin, Anal. Chim. Acta 341 (2
3) (1997) 217
224. [85] G. Cavallo, V. Venditto, L. Annunziata, S. Pragliola, P. Longo, G. Guerra, Thermal crosslinking of ethene copolymers containing 1,2-cyclopropane units, Polymer 46 (9) (2005) 2847
2853. [86] B. Tamami, M. Kolahdoozan, Synthesis of thiiranes from oxiranes in water using polymeric cosolvents, Tetrahedron Lett. 45 (7) (2004) 1535
1537. [87] M. Messori, M. Toselli, F. Pilati, C. Tonelli, Unsaturated polyester resins modified with poly (ε-caprolactone)
perfluoropolyethers block copolymers, Polymer 42 (25) (2001) 09877
09885. [88] H. Ito, Injection Molded Semicrystalline Polymers: Structure Development and Modeling, 2nd ed., Encyclopedia of Materials: Science and Technology, 2001, pp. 4082
4087. [89] Y. He, S. Shuai, A. Cao, K. Kasuya, Y. Doi, Y. Inoue, Enzymatic biodegradation of synthetic atactic poly(R, S 2 3-hydroxybutyrate) enhanced by an amorphous non biodegradable polymer, Polym. Degrad. Stab. 73 (2) (2001) 193
199.

172

CHAPTER 6 BLENDS, INTERPENETRATING POLYMER NETWORKS

[90] Y.J. Huang, W.C. Jiang, Effects of chemical composition and structure of unsaturated polyester resins on the miscibility, cured sample morphology and mechanical properties for styrene/unsaturated polyester/ low-profile additive ternary systems. 1: miscibility and cured sample morphology, Polymer 39 (25) (1998) 6631
6641. [91] M. Wesseling, F. Kuppler, R. Bodmeier, Tackiness of acrylic and cellulosic polymer films used in the coating of solid dosage forms, Eur. J. Pharm. Biopharm. 47 (1) (1999) 73
78. [92] J.K. Fink, Montanuniversita¨t Leoben Leoben, Austria in “Reactive Polymers Fundamentals and Applications, a Concise Guide to Industrial Polymers”, William Andrew Publishing, Norwich, NY, 2005. [93] W.D. Cook, M.D. Zipper, A.C.H. Chung, PVAc phase precipitation in vinyl ester and polyester resins, Polymer 39 (22) (1998) 5431
5439. [94] Q. Li, Y. Lin, F. Wang, Study on a comb-like polymer electrolyte based on the backbone of ethylene
maleic anhydride copolymer, Solid State Ionics 109 (1
2) (1998) 145
150. [95] P.G. Klein, B.W. Evans, I.M. Ward, Solid-state carbon-13 n.m.r. studies of molecular motion in an aromatic copolyester, Polymer 39 (15) (1998) 3349
3354. [96] J.M. Go´mez de Salazar, M.I. Barrena, G. Morales, L. Matesanz, N. Merino, Compression strength and wear resistance of ceramic foams
polymer composites, Mater. Lett. 60 (13
14) (2006) 1687
1692.

CHAPTER

ROLE OF NANOFILLERS IN BLENDS, INTERPENETRATING POLYMER NETWORKS, AND GELS OF UNSATURATED POLYESTER RESIN POLYMERS

7

Shalini Chaturvedi1 and Pragnesh N. Dave2 1

2

Samarpan Science and Commerce College, Gandhinagar, India Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar, India

7.1 INTRODUCTION Polymer nanocomposites have long existed and been used as additives in polymers such as carbon black, pyrogenic silica, and diatomite. Nevertheless, their characterizations and effects on various properties induced by the nanometric scale of fillers were not fully understood at these times. The real starting point corresponds to the understanding of the action of these fillers in the 1990s and much research then started on various fillers. The demand for continual improvement in the performances of thermoplastic and thermoset polymer materials has led to the emergence of new technologies. Nanofiller lists increased within years. Nanofillers are also used in matrix in interaction with traditional fillers. Nanofillers can significantly improve or adjust the different properties of materials into which they are incorporated such as the optical, electrical, mechanical, thermal, and/or fireretardant properties, sometimes in synergy with conventional fillers. Nowadays, the development of polymer nanocomposites is one of the most active areas of development in nanomaterials. Presence of nanoparticle in polymer strengthening electrical conduction and barrier properties of it against temperature, gases, and liquids as well as the possible improvement of fire behavior. Polymers are generally known to be good insulating materials due to their stable physical and chemical properties [1 3]. Both the mechanical and electrical properties of polymers, however, can be further improved or modified with the addition of inorganic fillers as demonstrated by increases in the mechanical strength of composites and changes in their electrical conductivity. It is well-known that the properties of composites can also change with the dispersion state, geometric shape, and surface quality of the filler particles as well as their particle size. For example, the effect of carbon black dispersion in polymer blends on the electrical conduction properties of composites and the dependence of electrical properties on the shape and distribution of the filler particles were reported [4 6]. Owing to the recent commercial availability of nanoparticles, the outlook for composite materials with new or modified physical properties has become even Unsaturated Polyester Resins. DOI: https://doi.org/10.1016/B978-0-12-816129-6.00007-7 © 2019 Elsevier Inc. All rights reserved.

173

174

CHAPTER 7 ROLE OF NANOFILLERS IN BLENDS, INTERPENETRATING

brighter. Nanoscale fillers are different from bulk materials and conventional micron-sized fillers due to their small size and corresponding large surface area. It is expected that the addition of nanoparticles into polymers would lead to an unprecedented ability to control the electrical properties of filled polymers [7 10]. Furthermore, nanocomposite materials with length scales smaller than 1 Am are becoming necessary as the size of modern electronic devices reach submicron scales. Therefore understanding how to control the resistivity and permittivity of nanofilled polymer nanocomposites is an essential step toward the controlled engineering of nanocomposite materials in future electrical applications [11 14]. Nanofillers are stimulating increasing interest, including on a commercial scale, for the reinforcement of rubbers [1]. Their primary particles, with at least one dimension of one or a few nanometers, can be individually dispersed in a rubber matrix and a great reinforcement effect has been already demonstrated. Among nanofillers, clays [1 3] and carbon nanotubes (CNTs) [4 8] are the most investigated ones. In most reports in the literature, nanofillers are used in neat polymer matrices, in the absence of any nanostructured fillers. However, an increasing interest is evident for hybrid filler systems, which are based on both a nanofiller and a nanostructured filler. As a matter of fact, the findings from these studies could pave the way for large scale applications of rubber nanocomposites.

7.2 NANOPARTICLES Nanofillers are generally categorized into three geometries, namely equiaxial (particles), rod (fiberlike), and sheet. A significant advantage of small-sized particles is their high surface-to-volume ratio which is varied by the particle geometry. The properties of polymer nanocomposites can be change by varying the type of nanoscale fillers used in the system. Many types of nanofillers are commercially available today. Clay or montmorillonite is one of the most studied polymer nanocomposites. Other fillers include carbon, aluminum oxide, and silica. The following subsections describe examples of fillers [15,16].

7.3 FILLERS Traditionally, fillers were considered as additives, which, due to their unfavorable geometrical features, surface area, or surface chemical composition, could only moderately change the characteristics of a polymer. Their major contribution was in lowering the cost of materials by replacing the more expensive polymers, while another possible economic advantage was their faster molding cycles as a result of increased thermal conductivity. Depending on the type of filler, other polymer properties could be affected; for example, melt viscosity could be significantly increased through the incorporation of fibrous materials. On the other hand, mold shrinkage and thermal expansion would be reduced, which are common effects of most inorganic fillers [17,18].

7.3.1 CLASSIFICATION AND FUNCTION OF NANOFILLERS The term filler is very broad and encompasses a wide range of materials, including a variety of solid particulate materials (inorganic, organic). There is significant diversity in the chemical

7.3 FILLERS

175

structures, forms, shapes, sizes, and inherent properties of the various compounds that are used as fillers (Fig. 7.1). They are usually rigid materials, immiscible with the matrix in both molten and solid states. Fillers may be classified as inorganic or organic substances and further subdivided according to chemical family as shown in Table 7.1 [18 21]. Different types of nanofillers have been reported within a few years (nanoclays, nanooxides, CNTs, polyhedral oligomeric silsesquixanes (POSS), etc.) as well as matrices in which they are used and interactions with traditional fillers. Nowadays, the development of polymer nanocomposites is one of the most active areas in the development of nanomaterials. The properties imparted by nanoparticles are various and focus particularly on strengthening electrical conduction and

FIGURE 7.1 Schematic illustration of polymer/layered nanofiller composite. A.M. Gumel, M.S.M. Annuar, Nanocomposites of polyhydroxyalkanoates (PHAs), RSC Green Chem. 2015(30):98 118.

Table 7.1 Chemical Types of Fillers for Polymers Chemical Type

Examples

Inorganic oxides Hydroxides Salts Silicates Metals Carbon graphite Natural polymers Synthetic polymers

Glass, MgO, SiO2, Sb2O3 Al(OOH)3, Mg(OH)2 CaCO3, BaSO4, CaSO4, phosphates Talc, mica, kaolin, wollastonite, montmorillonite, nanoclay, feldspar, asbestos Boron, steel Carbon fibers, graphite fibers and flakes, carbon nanotubes, carbon black Cellulose fibers, wood flour and fibers, flax, cotton, sisal, starch Polyamide, polyester, aramid, polyvinyl alcohol fibers

176

CHAPTER 7 ROLE OF NANOFILLERS IN BLENDS, INTERPENETRATING

barrier properties against temperature, gases, and liquids as well as the possible improvement of fire behavior. As a method consisting of reinforcing polymer chains at the molecular scale in the same way as the fibers at the macroscopic scale. Nanocomposites [22,23] represent the new generation of two-phased materials, associating a basic matrix to nanofillers inserted between polymer chains. Nanofillers can significantly improve or adjust the different properties of materials into which they are incorporated such as the optical, electrical, mechanical, thermal properties and/or fire-retardant properties, sometimes in synergy with conventional fillers. The properties of composite materials can be significantly impacted by the mixture ratio between the organic matrix and the nanofillers.

7.3.2 ROLE OF NANOFILLERS Polymer nanocomposites have been widely used in many fields. By introducing nanoparticles as fillers, researchers are able to get reinforced materials and new materials with novel properties such as stronger mechanics, enhanced optical properties, and improved conductivity. Though experimental techniques have rapidly advanced to enable better control of materials at the atomic level, there is still a lack of fundamental understanding of the dynamics and structure properties relations of polymer nanocomposites. In this thesis, computer simulations are used to study the molecular structures and connections between the micro and macro properties of a variety of nanocomposites. The goal is to understand the role of nanofillers in complex nanocomposite systems and to assist in nanocomposite design. Nanoplatelet fillers such as clays have shown superior effects on the properties of polymer gels. Molecular dynamic simulation was used to study nanoplatelet-filled composite gel systems, in which short-range attraction exists between the polymer and nanoplatelet fillers. It is shown that the polymers and nanoplatelet fillers formed organic inorganic networks with nanoplatelets acting as cross-link junctions, and the network eventually percolated the system as the fillers reached a critical concentration. Stress auto-correlation and step-strain tests were applied to investigate the mechanical properties; the results show that the simulated composites changed from fluid-like to solid-like. The mechanical changes were consistent with the percolation transition and the gelation mechanism was therefore believed to be similar to that of pure polymer physical gels. It was observed that the platelets aggregated into a local intercalation structure, which significantly differs from typical spherical fillers. This unique intercalation structure was examined by radial distribution function and ordering parameters. The effects of intercalation on the properties of the platelet composites were discussed in comparison to spherical fillers. Nanofillers have been widely used in polymer blends to improve the interfacial compatibility of otherwise immiscible polymers. In the second system, the interfacial behavior of binary polymer blends with different types of fillers is investigated. The interfacial tension and shear resistance were studied as a function of filler polymer interaction, filler concentration, and species of filler. Filler polymer interaction was found to be the key factor in improving the interfacial compatibility. The results show that nanofillers reduce both interfacial tension and interfacial slip under strong filler polymer interaction. The effects of nanofillers, however, differ significantly from each other according to their shapes. The structure of nanofillers at the interface and their effects on interfacial behaviors were analyzed. The self-assembly of polymers into a columnar structure, while subject to a thin film environment, provides an economic route to fabricate polymer solar cells (PSC) with high conversion efficiency. In this work, two immiscible polymers segregate into to a percolating columnar

7.3 FILLERS

177

structure when confined to a thin film. By adding nanofillers with specific functionalities, the segregation of nanofillers to the polymer polymer interface can be templated. This process is proven to be surface tension driven and is a result that is particular for thin film geometries, where the thickness is under a critical value. The results provide a theoretical basis for the column structure formed in a self-assembling PSC system, and can help to select polymer candidates that optimize PSC efficiency. These studies serve as theoretical guidelines for engineering novel nanocomposites and could lead to the design of materials with new and improved properties.

7.3.3 IN BLENDING Polymer blending has been used as a successful method to develop new polymeric materials with improved or new synergy of many specific properties for diverse applications. Silica nanoparticles (hydrophilic and hydrophobic), layered silicate, surface-modified nanosilicates (organoclays), single- and multiwalled CNTs, and graphene are the most commonly used nanofiller in the production of polymer nanocomposites and nanomaterial-containing polymer blends. X-ray diffraction is the easiest and most widely used technique to probe nanostructures of layered silicate filled nanocomposites. During the past two decades, a combination of polymer blending and nanoparticle incorporation has proven to be an effective strategy with a great tailoring potential to develop a variety of new polymer composites for a wide range of prescribed applications. It was demonstrated that high-throughput experimentation can be used as a suitable tool to study the phase behavior of blends. Cocontinuous polymer blends are composed of two or more immiscible or partially miscible polymers coexisting within the same volume in multiple interpenetrating networks (IPNs). They can be created by either the melt compounding of immiscible polymers or the phase separation of partially miscible polymer pairs via spinodal decomposition. Polymer blends with cocontinuous structures have significantly improved mechanical properties and they have applications in conductive plastics, porous membranes for filtration, and tissue scaffolds for drug delivery devices. As the cocontinuous morphology is in a nonequilibrium state, thermodynamic instability causes the morphology to coarsen during postmixing processing, which is a major drawback for applications. In order to control and optimize the phase morphology, nanofillers have been localized and jammed at the interface as an effective method to suppress coarsening and to stabilize the cocontinuous structure during annealing. However, the mechanisms involved in the stabilization of morphology by interfacial nanofillers are not yet fully understood. This thesis seeks to systemically study the structure processing properties relationships of nanofiller-stabilized cocontinuous polymer blends by providing insight into these three questions: (1) how do thermodynamic factors determine nanofiller localization and their morphology stabilization ability? (2) How do kinetic factors affect nanofiller migration during melt compounding and coarsening suppression during annealing? (3) How is the morphology dynamics of nanofiller-stabilized polymer blends connected to their rheology response during annealing? Concerning the thermodynamic factors, this thesis approaches the problem by incorporating nanofillers with different surface properties into cocontinuous polymer blends. The different hydrophobicities of silica nanoparticles and the different polarities of graphene nanoplates determine the different localizations of these nanofillers in the polymer blends. Nanofiller localization in one polymer phase or at the interface is explained by the system’s tendency to minimize its free energy. The wetting coefficients, which are derived from the Young’s equation and calculated

178

CHAPTER 7 ROLE OF NANOFILLERS IN BLENDS, INTERPENETRATING

based on the surface energies of the nanofillers and two polymer components, have been applied to predict the localizations of the nanofillers in the polymer blends. Concerning kinetic factors, different processing parameters during melt compounding were systemically investigated to study their effect on the migration and localization of nanofillers and their corresponding morphology stabilization ability during annealing. The proper sequence of addition of components is crucially significant to achieve interfacial localization; nanofillers were generally premixed with the thermodynamically less favorable phase, and then melt compounded with the thermodynamically more favorable phase to enable nanofillers to migrate from the premixed phase to the interface. The effect of different melt compounding times is also systemically studied in cocontinuous polymer blends stabilized by grapheme nanoplates; it was found that blends with short melt compounding times had more nanofillers jammed at the interface and more effective coarsening suppression ability during annealing. In order to correlate the morphology dynamics with rheology, rheology time sweeps were combined with morphology information from confocal microscopy, scanning electron microscopy, and transmission electron microscopy. It was found that morphology coarsening results in the shrinkage of the interfacial area and the jamming of interfacial nanofillers. The nanofillers jammed at the interface contributed to the stabilization of the cocontinuous morphology and the formation of a 3D nanofiller network. The nanofiller network gave rise to an increase of storage modulus during annealing and to the typical gel-like behavior in rheology frequency sweeps.

7.3.4 INTERPENETRATING POLYMER NETWORKS AND UNSATURATED POLYMER RESINS Unsaturated polyesters have been widely used as resin components for composites in the building industry and the electrical industry, as glass fiber reinforced composites. They are extremely popular because of their low manufacturing costs, easy processing, and low densities. However, typical unsaturated polyester resins (UPRs) have some drawbacks such as polymerization shrinkage, inherent brittleness, and low resistance to crack propagation due to their high degree of cross-linking [24 27]. Therefore in recent years, chemical modification by the reactive blending of UPR and other thermosets to form IPNs is a promising way to extend the range of properties of those thermosets and hence increase the number of applications for polymer products in industrial demand [28]. IPNs obtained by the blending of unsaturated polyesters and epoxy resin have been extensively studied [29 31]. Unsaturated polyester bismaleimide modified epoxy matrix systems [22], vinyl ester oligomer modified unsaturated polyester with varying percentages of bismaleimide [32], unsaturated polyester polyurethane prepolymers [33 35], and unsaturated polyester epoxidized novolac resins have been developed. Chemical bonding between elastomer and UPR using methacrylate end-capped nitrile rubber, epoxy-terminated nitrile rubber, or isocyanate end-capped polybutadiene [36] and IPNs based on polyethylene glycol diacrylate and epoxy [37] have been also studied. IPNs are, ideally, compositions of two or more chemically distinct polymer networks held together exclusively by their permanent mutual entanglements [38]. They can be formed by one of two methods as sequential or simultaneous IPNs. A sequential IPN is where one network is formed and then swollen with a second cross-linking system, which is subsequently polymerized. The second type is the simultaneous IPN, in which the two network components are polymerized together [39].

REFERENCES

179

7.4 CONCLUSION Nanoscale dimensions can increase significantly the physical interactions and physicochemical and chemical interfaces in materials. The morphologies obtained for nanocomposites and the ability to modify the interfaces are essential to maximize their properties. Nanofiller particles in polymer matrices play a key role, mainly in mechanical properties. The interfacial strength between a filler and a polymer is a very important factor because a lack of adhesion between two phases will result in early failure. Other physical properties such as optical, magnetic, electronic, thermal, wear resistance, barrier to diffusion, water resistance, and/or flame retardancy can be strongly affected by nanoparticle dispersion in polymer matrices. Regarding biocomposites, nanoscale-organized composites with perfect dispersions provide better substrate conditions for cellular interactions, particularly in the cell adhesion and proliferation state, when compared to conventional composites. Nanofillers are gaining increasing interest, including on a commercial scale, for the reinforcement of rubbers. Nanofillers are used in neat polymer matrices. However, an increasing interest is evident for hybrid filler systems based on both a nanofiller and a nanostructured filler.

REFERENCES [1] D. Ratna, N. Manoj, R. Varley, R. Singh Raman, G.P. Simon, Clay reinforced epoxy nanocomposites, Polym. Int. 52 (9) (2003) 1403 1407. [2] B. Gua, D. Jia, C. Cai, Effects of organo-montmorillonite dispersion on the thermal stability of epoxy layered silicate nanocomposites, Eur. Polym. J. 40 (8) (2004) 1743 1748. [3] J.Y. Lee, H.K. Lee, Characterisation of organo-bentonite used for polymer nanocomposites, Mater. Chem. Phys. 85 (2) (2004) 410 415. [4] M.A. Treece, J.P. Oberhauser, Processing of polypropylene-clay nanocomposites: single-screw extrusion with inline supercritical carbon dioxide feed versus twin-screw extrusion, J. Appl. Polym. Sci 103 (2007) 884 892. [5] M. Zanetti, S. Valesella, M.P. Luda, L. Costa, in: C.A. Wilkie, G.L. Nelson (Eds.), Fire and Polymers IV. Materials and Concepts for Hazard Prevention, 922, ACS Symposium Series, 2006. [6] M. Roy, J.K. Nelson, L.S. Schadler, C. Zou, J.C. Fothergill, The influence of physical and chemical linkage on the properties of nanocomposites, in: Annual Report Conference on Electrical Insulation and Dielectric Phenomena, CEIDP, 2005, pp. 183 186. [7] X.-L. Xie, Q.-X. Liu, R.K.-Y. Li, X.-P. Zhou, Q.-X. Zhang, Z.Z. Yu, Processability and mechanical properties of PVC/CaCO3 nanocomposites prepared by in situ polymerization, Polymer 45 (19) (2004) 6665 6673. [8] T. Tanaka, Dielectric nanocomposites with insulating properties, IEEE Trans. Dielectr. Electr. Insul. 12 (5) (2005) 914 928. [9] Y. Cao, P.C. Irwin, K. Younsi, The future of nanodielectrics in the electrical power industry, IEEE Trans. Dielectr. Electr. Insul. 11 (5) (2004) 797 807. [10] T. Imai, F. Sawa, T. Ozaki, T. Shimizu, S. Kuge, M. Kozako, et al., Effects of epoxy/filler interface on properties of nano or micro composites, IEEJ Trans. Fund. Mater. 126 (2) (2006) 84 91. [11] M. Roy, J.K. Nelson, R.K. MacCrone, L.S. Schadler, Polymer nanocomposite dielectrics the role of the interface, IEEE Trans. Dielectr. Electr. Insul. 12 (4) (2005) 629 643. [12] S. Singha, M.J. Thomas, Polymer composite/nanocomposite processing and its effect on the electrical properties, IEEE Conf. Electr. Insul. Dielectr. Phenomena (CEIDP) (2006) 557 560.

180

CHAPTER 7 ROLE OF NANOFILLERS IN BLENDS, INTERPENETRATING

[13] C. Zou, J.C. Fothergill, S.W. Rowe, A water shell model for the dielectric properties of hydrated silicafilled epoxy nano-composites, IEEE Int. Conf. Solid Dielectr. (ICSD) (2007) 389 392. [14] G.J. Papakonstantopoulos, M. Doxastakis, P.F. Nealey, J.L. Barrat, J.J. de Pablo, Calculation of local mechanical properties of filled polymers, Phys. Rev. E 75 (2007) 1 13. [15] A.P. Harsha, U.S. Tewari, Tribo performance of polyaryletherketone composites, Polym. Testing 21 (2002) 697 709. [16] P.M. Ajayan, L.S. Schadler, P.V. Braun, Nanocomposite Science and Technology, Wiley-VCH, Weinheim, 2003, pp. 122 131. [17] W.Y. Kim, N.V. Greidanus, C.P. Duncan, B.A. Masri, D.S. Garbuz, Porous tantalum uncementedacetabular shells in revision total hip replacement: two to four year clinical and radiographic results, Hip Int. 18 (2008) 17 22. [18] A. Siegmeth, C.P. Duncan, B.A. Masri, W.Y. Kim, D.S. Garbuz, Modular tantalum augments for acetabular defects in revision hip arthroplasty, Clin. Orthop. Relat. Res. 467 (2009) 199 205. [19] P.E. Purdue, P. Koulouvaris, B.J. Nestor, T.P. Sculco, The central role of wear debris in periprostheticosteolysis, Hss J. 2 (2006) 102 113. [20] R. Tsukamoto, P.A. Williams, H. Shoji, K. Hirakawa, K. Yamamoto, M. Tsukamoto, et al., Wear of sequentially enhanced 9-Mrad polyethylene in 10 million cycle knee simulation study, J. Biomed. Mater. Res. B Appl. Biomater. 86 (2008) 119 124. [21] A. Thabet, Y.A. Mubarak, M. Abdrabo, Dielectric properties response of industrial applications by nanocomposite materials, in: 2nd International Conference on Energy Engineering (ICEE-2010), High Institute of Energy, South Valley Universty, Aswan, Egypt, December 27 29, 2010. [22] M. Biron, Thermosets and Composites, Technical Information for Plastics Users, Elsevier Ltd, 2004. [23] J. Gloaguen, J. Lefevre, Nanocomposites polymers/silicates en feuillets, Techniques de l’ing´enieurAM 5205 19 (2007). [24] D.R. Paul, S. Newman (Eds.), Polymer Blends, 2, Academic Press, New York, 1978. [25] G. Qipeng, Z. Haifeng, Z. Sixun, M. Yongli, Z. Wei, J. Mater. Sci. 34 (1999) 123. [26] A.P. Mouritz, Z. Mathys, Compos. Struct. 47 (1999) 643. [27] G.T. Egglestone, D.M. Turley, Fire Matter. 18 (1995) 255. [28] G. Lubin (Ed.), Handbook of Composites, Van Nostrand, New York, 1982. [29] M.-S. Liu, C.H.-C.H. Liu, Ch-T. Lee, J. Appl. Polym. Sci. 72 (1999) 585. [30] Z.G. Shaker, R.M. Browne, H.A. Stretz, P.E. Cassidy, M.T. Blanda, J. Appl. Polym. Sci. 84 (2003) 2283. [31] S.J. Park, W.B. Park, J.R. Lee, Polym. J. 31 (1999) 28. [32] K. Dinakaran, M. Alagar, J. Appl. Polym. Sci. 85 (2002) 2502. [33] V. Ludovic, Ch-P. Hsu, Polymer 40 (1999) 2059. [34] M.X. Xu, W.G. Liu, Y.L. Guan, Z.P. Bi, K. DeYao, Polym. Int. 38 (1995) 205. [35] C.A. Benny, T. Abraham Beena, T.E. Thomas, J. Appl. Polym. Sci. 100 (2006) 449. [36] L. Suspene, Y. Show Yang, J.-P. Pascault, Rubber toughened plastics, in: C. Keith Riew, A.J. Kinloch (Eds.), Advances in Chemistry., 1993, American Chemical Society, Washington, DC, 1993, p. 168. [37] M.S. Lin, K.T. Jeng, K.Y. Huang, Y.F. Shin, J. Polym. Sci. Polym. Chem. Ed. 31 (1993) 3317. [38] L.H. Sperling, V. Mishr, Polym. Adv. Technol. 7 (1995) 197. [39] K. Dean, W.D. Cook, M.D. Zipper, P. Burchill, Polymer 42 (2001) 1345.

FURTHER READING K. Dinakaran, M. Alagar, J. Appl. Polym. Sci. 85 (2002) 2853.

CHAPTER

UNSATURATED POLYESTER RESINS: BLENDS, INTERPENETRATING POLYMER NETWORKS, COMPOSITES, AND NANOCOMPOSITES

8

Hom Nath Dhakal1 and Sikiru Oluwarotimi Ismail2 1

School of Mechanical and Design Engineering, Advanced Materials and Manufacturing (AMM) Research Group, University of Portsmouth, Portsmouth, United Kingdom 2Manufacturing, Materials, Biomedical and Civil Division, School of Engineering and Technology, Hutton Building, University of Hertfordshire, Hertfordshire, United Kingdom

8.1 INTRODUCTION 8.1.1 POLYMERIC MATRIX COMPOSITE Composites are classified according to the matrix used: polymeric matrix composite (PMC), ceramic matrix composite (CMC), or metallic matrix composite (MMC). Within composites the matrix serves three major functions, namely it supports and transfers stresses to the fibers, which carry most of the load, it protects the fibers against physical and environment damage, and it reduces the propagation of cracks in the composite by virtue of the ductility and toughness of the plastic matrix. “Poly” comes from the Greek word for “many” and “mer” comes from the Greek word for “parts.” Therefore a polymer is a long chain of molecules made from a series of repeating basic units called mer. The most commonly used matrix materials for composites are polymeric. In general, the mechanical properties of polymers are inadequate for many structural applications. Precisely, their strength and stiffness are significantly low compared to metals and ceramics. Consequently, there are considerable benefits to be gained by reinforcing polymers with suitable fibers and fillers.

8.1.2 IMPORTANT POLYMER MATRICES The classification of polymers can be carried out based on their configurations, thermophysical properties, and the polymerization reactions that occur on these polymers.

Unsaturated Polyester Resins. DOI: https://doi.org/10.1016/B978-0-12-816129-6.00008-9 © 2019 Elsevier Inc. All rights reserved.

181

182

CHAPTER 8 UNSATURATED POLYESTER RESINS

8.1.3 THERMOSETS The matrix phase plays a crucial role in the performance of polymer composites. Both thermosets and thermoplastics are attractive as matrix materials for composites and nanocomposites. The formulation of thermoset composites is complex because of the large number of components involved such as base resins, curing agents, catalysts, and hardeners. Thermoset polymers have cross-linked structures that are formed from chemical bonds. They are less sensitive to heat because when heat is applied, liquid resin becomes rigid through a process known as vitrification. The resulting crosslinking structures prevent the polymer from flowing and melting again, thus providing thermal stability to the polymer. Some examples of thermoset polymers are epoxy, vinyl esters, phenol formaldehyde, urethane, and unsaturated polyesters (UPs) [1]. It has been estimated that over three-quarters of all matrices of PMCs are thermosetting polymers. Thermosets are resins which readily cross-link during curing. Curing involves the application of heat and pressure or the addition of a catalyst, also known as a curing agent or hardener. Importantly, the properties of thermosets depend mainly on their cross-link density and the length of cross-linkage. Bonds are mainly primary (covalent type) giving stiffer, stronger, mainly 3D isotropic properties, and less flow in the solid state even at elevated temperatures. Because of their cross-links, reheating cannot reshape thermosets; thermosets just degrade on reheating, and in some cases may burn, but do not soften sufficiently for reshaping [2]. An increase in cross-link density gives higher strength, stiffness, chemical resistance, and glass transition temperature (Tg), but lower strain at break and toughness. Thus optimal curing is preferred. Volume contraction (shrinkage), about 4% 8%, takes place on curing. Thermosets may be used in higher temperatures (maximum service temperature of 450 C). They are more resistant to chemical attack than most thermoplastics. The stiffness is increased as the weak Van der Waals bonding between polymer chains is replaced by stronger cross-links.

8.1.3.1 Unsaturated polyesters UPs provide the foundation for developing cross-linked polyesters. Step polymerization is the first reaction used to form UPs. The polyester monomer necessary for cross-linking polyesters is formed from antifreeze (ethylene glycol), which has hydroxyl groups on either end. The antifreeze acts as an organic base and because organic acid (vinyl diacyl chloride) is involved, it becomes an acidbased reaction. Condensation polymerization results from this reaction as hydrogen chloride (HCl) is released. These hydroxyl groups continue to react to form polyester, which acts as a raw material for cross-linking [3]. Unsaturated polyester resins (UPRs) are the backbone of the composites industry, representing approximately 75% of the total resins used. They contain unsaturated material, maleic anhydride, or fumaric acid, as part of the dicarboxylic acid component, as represented in Fig. 8.1. The finished polymer is dissolved in a reactive monomer such as styrene to yield a low viscosity liquid. The resin is cured when the monomer reacts with the unsaturated sites on the polymer, thus converting it to a solid thermoset structure. Depending upon the basic building blocks, UPs can be divided into class structures, namely orthophthalic (ortho), isophthalic (iso), dicyclopentadiene, and bisphenol-A fumarate resins [4]. The condensation reaction produces a long chain polymer and water, as further shown in Fig. 8.2. UPs are extremely versatile in properties and applications. They are popular thermosets used in polymer matrix composites. The reinforcement of polyester matrices with cellulosic fibers has been

8.1 INTRODUCTION

183

FIGURE 8.1 The condensation reaction of dicarboxylic acid and dialcohol.

FIGURE 8.2 Condensation reaction producing a long chain polymer and water.

Table 8.1 Mechanical Properties of Unsaturated Polyester Properties Tensile strength Elongation at break Flexural strength Flexural modulus Volume shrinkage Heat Dist. Temp Water absorption—24 h

Pure Resin 47 2.2 90 3700 9 63 10

Unit

Test Method 2

N/mm % N/mm2 N/mm2 %  C Mg

BS BS BS BS BS BS BS

2782: 2782: 2782: 2782: 2782: 2782: 2782:

Part Part Part Part Part Part Part

3: Method 3: Method 3: Method 3: Method 6: Method 1: Method 4: Method

320C: 1976 320C: 1976 335A: 1978 335A: 1978 644A: 1986 121A: 1991 430A: 1983

Properties data adapted from Reichhold, UK. MSDS date sheet.

widely reported. Dhakal et al. [5] reported that due to the low viscosity of UP, it wets properly in nonwoven hemp fiber/polyester composites, and can be cured at room temperature with the use of the catalyst, methyl ethyl ketone peroxide (MEKP). Many works have been widely reported on the use of polyester as a matrix material for natural fiber reinforced composites. Polyester sisal [6], polyester coir [7], polyester banana cotton [8], polyester straw [9], and polyester cotton kapok [10] are some examples of promising composite systems. UPs are widely produced in industrial scale as they possess many advantages compared to other thermosetting resins including room temperature cure capability, good mechanical properties, low viscosity, and transparency [11]. The mechanical and physical properties of UP are presented in both Tables 8.1 and 8.2.

184

CHAPTER 8 UNSATURATED POLYESTER RESINS

Table 8.2 Physical Data in Liquid State at 25 C Properties

Value

Units

Test Method

Viscosity: ICI Cone and Plate Brookfield SP2 at 12 rpm Density Acid value Monomers Monomer content Flash point Geltime: 2% Butanox M50 Stability at 20 C from date of manufacture

190 220 900 1100 1.10 ,25 Styrene 42 46 32 13 17 6

cps cps g/cm3 mg KOH/g

BS BS BS BS BS

%  C mins months

2782: Part 2782: Part 3900: Part 2782: Part 2782: Part

7: Method 730C: 1994 7: Method 730C: 1992 A12: 1975 4: Method 432B: 1976 3: Method 335A: 1978

BS 3900: Part A9: 1986 BS 2782: Part 8: Method 835B: 1980

Properties data adapted from Reichhold, UK. MSDS date sheet.

FIGURE 8.3 Free-radical production from the chain reaction of peroxide.

8.1.3.2 Unsaturated polyester with catalyst UP could be cross-linked (cured) using a room temperature curing catalyst, MEKP, in a 50 wt.% solution with phthalate UN 2563. The catalyst mixes with the resin in a ratio of 10 cm3 to 1 kg resin based on the specification of the manufacturer. The resin is cured by free-radical polymerization (production from the chain reaction of peroxide), as depicted in Fig. 8.3. This process was successfully used or demonstrated by De Rosa et al. [12] and Dhakal et al. [13].

8.1.3.3 Unsaturated polyester composites Several works have been reported on UP composites with different reinforcements (fibers) and fillers. Monti et al. [14] investigated both experimentally and numerically the delay effect of carbon nanofiber on the cure kinetics and chemorheology of UPRs. Vilay et al. [15] reported the improved mechanical properties (higher tensile and flexural properties at higher fiber contents), storage modulus, and water absorption resistance of bagasse fiber reinforced UP composites treated with sodium hydroxide and acrylic acid when compared to untreated fiber based composites. These results were similarly reported regarding the chemical (maleic anhydride, styrene, acrylic acid, and acetic anhydride) treatment of Alfa fiber/UP composites as investigated by Bessadok et al. [16].

8.2 POLYMER NANOCOMPOSITES

185

Dhakal et al. [5] investigated and reported that both the tensile and flexural (mechanical) properties of nonwoven hemp fiber reinforced UP composites decreased with an increase in the percentage of moisture uptake. Similar results were reported for the flexural and compression properties of jute fiber reinforced UP composites by Md Akil et al. [17]. Jiang et al. [18] reported an improvement of the mechanical properties (impact resistance, interfacial shear, and tensile strengths) as well as antihydrothermal ageing responses of carbon fiber/UP composites. Similarly, Sawpan et al. [19] obtained an improvement in the interfacial shear strength of hemp fiber reinforced polylactide and UP composites. In their another work, they found that the flexural strength of these composites decreased with fiber contents, unlike the flexural modulus that increased with fiber contents. Although, the fiber matrix adhesion was later enhanced as both the flexural strength and modulus were increased with alkali and silane fiber treatments [20]. In addition, Wu et al. [21] improved the interfacial bond and impact toughness of carbon fiber/UP composites with a vinyl ester-carbon nanotube sizing agent. There were increases in surface roughness, improvements in the amount of polar functional groups and the wettability of the carbon fibers after coating treatment as well as the inter-laminar shear strength, interfacial adhesion, surface energy, chemical bonding, mechanical interlocking, and impact toughness as similarly reported by Jiang et al. [22]. Also, both the mechanical and fire safety properties of ramie fabric reinforced UPR composites have been enhanced by coating the surface of the ramie fabric with a phosphorus- and nitrogen-containing silane coupling agent. The experimental work conducted by Negawo et al. [23] on the mechanical, structural, morphological, and dynamic mechanical properties of alkali treated Ensete stem fiber reinforced UP composites revealed increases in both the flexural strength and Young’s modulus of the composites as well as better interfacial interaction and afore-mentioned properties when compared with the untreated counterparts. Dhakal et al. [13] reported that jute/UP composites withstood higher loads when subjected to a low-velocity impact test at 30 C.

8.2 POLYMER NANOCOMPOSITES The term nanocomposite describes a two-phase material, where one of the phases is dispersed in the other on a nanometer (1029 m) scale [24]. Layered silicate reinforced nanocomposites have been developed to demonstrate effective improvements in both mechanical and thermal properties. These properties were enhanced by the proper dispersion of clay [25 34] precisely making use of size effects. Polymer nanocomposites (PNCs) are polymers (thermoplastics, thermosets, or elastomers) that have been reinforced with small quantities (less than 5% by weight) of nanosized particles having high aspect ratios greater than 300 (l=d . 300, where l and d are fiber length and diameter). It is important to mention that the surface area becomes key when particles are between 10 and 20 nm. The most studied nanocomposites are hybrids based on layered inorganic compounds such as nanoclays. Historically, polyamide nanocomposites were the first nanocomposite materials to be developed, mainly because of the barrier properties imparted by nanoclays to the resin. The first nanocomposite on the market was a polyamide 6, developed by Toyota Central Research and Development Laboratories about a decade ago. The new material developed by Toyota showed

186

CHAPTER 8 UNSATURATED POLYESTER RESINS

high improvements in mechanical and barrier properties and resistance as compared to the pristine matrix and this at a low clay content (4 wt.%) [35]. These attractive characteristics already suggest a variety of possible industrial applications for PNCs. These applications include, but are not limited to: • • • • •

Automotive (gas tanks, bumpers, and interior and exterior panels) Aerospace (flame retardant panels and high-performance components) Construction (building sections and structural panels) Electrical and electronics (electrical components and printed circuit boards) Food packaging (containers and wrapping films).

Polymers have been successfully reinforced with various particulate fillers in order to improve their strength and stiffness, as well as to enhance their fire retardance and barrier properties. The addition of particulate fillers often results in brittleness and opacity. Also, in these reinforced composites, the dispersion on a nanometer scale between the polymer and the fillers and additives is always challenging in terms of obtaining a homogenous mix. If a homogenous dispersion on the nanometer scale is achieved, the mechanical, thermal, and barrier properties can be significantly improved.

8.2.1 LAYERED SILICATE The structure of a typical smectite clay nanolayer is represented in Fig. 8.4 [37]. It shows the oxygen framework (solid circles) where each nanolayer consists of two tetrahedral sheets containing mainly silicon (Si) and occasionally aluminum (Al). It also contains a central octahedral sheet occupied by magnesium (Mg), Al, and others. Mn1.xH2O represents the hydrated inorganic exchange cation (normally, sodium and calcium) that occupies the gallery space between the nanolayers. Each silicate nanolayer has a lateral dimension of 200 2000 nm and a thickness of about 1 nm. These nanolayers stack up and form tactoids (disordered crystalline droplets) which are typically of 0.1 1 μm thick. The most important factor in the success of polymer reinforcement is the aspect ratio of the clay particles. Clays with a platy structure and a thickness of less than 1 nm are optimal. The length and width of these clays are in the micron range, with aspect ratios of between 30:1 and 1500:1. Moreover, montmorillonite (MMT) is a natural clay that is used widely in polymers. It is a type of smectite that can absorb water and has a layer structure of aluminum octahedron sandwiched between layers of silicon tetrahedron. Each layered sheet is slightly less than 1 nm thin with surface dimensions extending to about 1000 nm or 1 μm with an aspect ratio about 1000:1 and a surface area close to 750 m2/g. Consequently, MMT clay is hydrophilic; therefore, it is inherently not compatible with most polymers and must be chemically modified to make its surface more hydrophobic. The most widely used surface treatments are ammonium cations that can replace existing cations available on the surface of the clay. The treatments work on the clay to minimize the attractive forces between agglomerated platelets.

8.2.2 NANOCOMPOSITES STRUCTURE Polymer layered silicate composites are ideally divided into three general types: conventional composites, delaminated nanocomposites, and intercalated nanocomposites. In addition, in the

8.3 NANOCOMPOSITE PREPARATION

187

FIGURE 8.4 Structure of layered silicate [30,36].

structures of conventional composites, layered silicate acts as a conventional filler. Intercalated nanocomposites consist of a regular insertion of polymer between the silicate layers and exfoliated polymer clay nanocomposite, where 1 nm thick layers are dispersed, forming a monolithic structure at the microscale [38].

8.3 NANOCOMPOSITE PREPARATION There are mainly three different methods to synthesize polymer layered silicate nanocomposites, namely in situ polymerization, exfoliation adsorption, and melt intercalation, as subsequently and briefly discussed. The ability to remove inorganic cations initially intercalated between silicate layers (commonly called galleries) is extremely important for the synthesis of PNCs.

8.3.1 IN SITU POLYMERIZATION In this method, layered organo-silicate is swollen within liquid monomer (or monomer solution); this allows for the monomer molecules to diffuse into silicate layers. Polymerization is initiated

188

CHAPTER 8 UNSATURATED POLYESTER RESINS

FIGURE 8.5 Basic steps of melt intercalation method.

either by heat, radiation, or organic initiator or fixed in the galleries before the swelling step by the monomer.

8.3.2 EXFOLIATION ADSORPTION In exfoliation adsorption, layered silicate is exfoliated using a solvent in which the polymer (or prepolymer in the case of insoluble ones such as polyimide) is soluble. This technique has been widely used with water-soluble polymers such as poly(vinyl alcohol) and poly(ethylene oxide), to mention but a few. Also, the solvent is evaporated.

8.3.3 MELT INTERCALATION Melt intercalation follows basic steps, as illustrated in Fig. 8.5. Layered silicate is mixed with a polymer in molten state; if the layer surfaces are sufficiently compatible with the chosen polymer, the polymer can enter the galleries. The strategy in this process is to blend a molten thermoplastic with an organo-silicate in order to optimize polymer/layered silicate interactions. The process is sufficiently rapid to take place in a conventional mixing extruder [38].

8.4 UNSATURATED POLYESTER NANOCOMPOSITES The suitability of UPs for the preparation of electrically conductive thermosetting nanocomposites at low nanotube concentrations has been investigated by Battisti et al. [39]. The work carried out by Builes et al. [40] reduced the brittleness of UP as well as with increased flexural modulus of UP nanocomposites modified with sisal microfibrillated cellulose (MFC) and PEO-b-PPO-b-PEO block copolymer, as both dispersing and nanostructuring agents for sisal MFC and UP. The cured UP/ clay nanocomposite exhibited an increase in its thermal (heat resistance) and mechanical (hardness) properties. However, there was an increase in the softening temperature melt and solution viscosity of the UP as well as a decrease in flammability due to the presence of the clay [41]. The flame retardance properties of UPR nanocomposites were improved by introducing phosphorus, nitrogen, and silicon-coexisting elements into boron nitride nanosheets, as reported by Wang et al. [42]. The addition of nanoclay particles reduced the coefficient of the thermal expansion of UPR and a higher Young’s modulus was observed when larger amounts of MMT clays were used, as observed after both bending and tensile tests [43]. Dhakal et al. [29] experimentally showed a significant correlation between the nanomechanical properties and decree of clay dispersion, known as

8.5 IMPORTANT MECHANISMS FOR PROPERTY IMPROVEMENT

189

interlayer d-spacing of clay particles, in layered silicate reinforced UP nanocomposites. They reported that the mechanical properties such as hardness and elastic modulus increased with the addition of a certain weight percentage of nanoclay, which made these reinforced nanocomposites perform better than their unreinforced counterparts. Chakradhar et al. [27] introduced UP into epoxy resin to enhance the impact and tensile strengths of epoxy/UP/clay nanocomposite systems. They obtained an optimized result at 3 wt.% clay content, when compared with the neat blend (0 wt.% clay), as similarly reported by Ahmed et al. [25], as well as Oleksy and Galina [32]. Moreover, the thermal stability, electrical conductivity, and mechanical properties (tensile strength, elongation at break, and hardness) of UP/Egyptian bentonite nanocomposites were improved with an organoclay content of 7 wt.% as an optimum value [31]. With respect to the conductivity of PNCs, Shah et al. [44] highlighted that PNC can be an effective method toward tackling the practical problems against the breakthrough of solid polymer electrolytes as applicable to lithium ion batteries/dye-sensitized solar cell. Both the low-velocity impact toughness and creep-stain responses of vinyl ester matrix nanocomposites based on layered silicate have been enhanced by Alateyah et al. [45] with the addition of clay or the presence of layered silicate.

8.5 IMPORTANT MECHANISMS FOR PROPERTY IMPROVEMENT The need for the improvement of the various properties of nanocomposites is based on the fact that the interfacial interaction between organically modified layered silicate (OMLS) and the matrix are important. Layered silicate has a layer thickness in the order of 1 nm and a high aspect ratio. A small weight percentage of OMLS, when dispersed throughout a matrix, provides a higher surface area for polymer filler interfacial interaction than that of conventional composites. However, in polymers with low polarity, the improvement is not significant since there is a lesser compatibility between the clay and polymer. The interaction between layered silicate and PNCs has three different structural types, as subsequently discussed.

8.5.1 INTERCALATED NANOCOMPOSITES In this phase, the insertion of a polymer into layered silicate occurs in a regular fashion, crystallographically, despite the clay to polymer ratio (Fig. 8.6). The properties of this phase resemble those of ceramic materials.

FIGURE 8.6 Structural types of nanocomposites [46].

190

CHAPTER 8 UNSATURATED POLYESTER RESINS

8.5.2 FLOCCULATED NANOCOMPOSITES This phase is the same as the previous one. However, the silicate layers are occasionally flocculated due to hydroxylated edge edge interaction between the layers of silicate (Fig. 8.6).

8.5.3 EXFOLIATED NANOCOMPOSITES Most individual clay layers are separated in continuous polymer matrices by an average distance which depends on the concentration of silicate layers. In the exfoliation stage, the clay loading is often much lower than in the intercalated phase. However, this structure is difficult to achieve as clay is naturally hydrophilic and polymers are hydrophobic, leading to difficulties in exfoliation in polymer matrices due to incompatibility. In order to improve the interaction between layered silicate and polymer, the surface of layered silicate is normally treated to make it more hydrophobic, thus facilitating the phase of exfoliation. The treatment of the surface can be performed by the ionexchange reaction of Na1, Ca12 [47], or K1 [48] with cationic surfactants including the four kinds of alkylammonium cations, namely primary, secondary, tertiary, and quaternary [47]. Also, this modification can lead to an increase in the d-spacing between the layers of silicate, due to the presence of an alkylammonium chain intercalated in the gallery to produce organoclay.

8.6 STRUCTURAL CHARACTERIZATION OF NANOCOMPOSITES There are two different and effective methods that are commonly used to characterize nanostructures, namely X-ray diffraction (XRD) and transmission electron microscopy (TEM).

8.6.1 WIDE ANGLE X-RAY DIFFRACTION XRD is used in nanocomposite research to determine the degree of exfoliation of organoclays in polymer matrices. The gap between the silicate layers of a given polymer increases as it delaminates when organoclay disperses through it. This gap separation is represented as a change in the peak form positioning when compared to the X-Ray diagram of a pure organoclay, as depicted in Fig. 8.7. An equation known as Bragg’s Law is normally used to determine the degree of exfoliation in a polymer matrix. The equation is represented thus: nλ 5 2dsinθ

(8.1)

where n 5 integer indicates the peak number (1), λ 5 the wavelength of the X-rays, d 5 spacing between the crystallographic planes, and θ 5 the incidence angle of the X-ray beam. The intercalation usually increases the interlayer spacing; thus, leading to a shift of the diffraction peak toward lower angle values (angle and layer spacing values as related through the Bragg’s

8.7 PROPERTIES OF POLYMER CLAY NANOCOMPOSITES

191

3.0 nm

Intensity

2.2 nm Delaminated (c) 1.5 nm 1.0 nm (b)

Intercalated

1.1 nm

Immiscible 1.0

3.0

5.0

7.0

(a) 9.0

2θ (deg)

FIGURE 8.7 Typical XRD patterns from polymer/layered silicate composites: (A) PE 1 organoclay - no formation of a nanocomposite, (B) PS 1 organoclay - intercalated nanocomposite, and (C) siloxane 1 organoclay delaminated nanocomposite [36].

relation: nλ 5 2dsinθ, where λ is the wavelength of X-ray radiation, d is the spacing between diffractional lattice planes, and θ is the measured diffraction (glancing) angle).

8.6.2 TRANSMISSION ELECTRON MICROSCOPY TEM micrographs, as shown in Fig. 8.8, are used to characterize structures for intercalated and exfoliated nanocomposites. Importantly, TEM reveals intercalation better than XRD when the spacing between layers becomes large ( . 8 nm).

8.7 PROPERTIES OF POLYMER CLAY NANOCOMPOSITES Several properties of polymer matrices have been improved significantly by the addition of layered silicate into polymers. The main properties which have been enhanced are, but not limited to: Mechanical properties: Tensile and flexural strengths and moduli, wear and fatigue. Thermal properties: Electrical and thermal properties, dimensional stability (lowering of coefficient of thermal expansion).

192

CHAPTER 8 UNSATURATED POLYESTER RESINS

FIGURE 8.8 (A) Intercalated nanocomposite, (B) exfoliated nanocomposite, and (C) unintercalated TEM micrographs of polymer layered silicate nanocomposites, showing filler distribution [30].

Physical properties: Density, magnetic, optic, and flame retardancy. Several reports have suggested that the incorporation of layered silicate into polymer matrices decreases the impact properties of layered silicate reinforced nanocomposites. A reported work on Nylon-6 based nanocomposites [49] showed some reduction in impact strength from 20.6 to 18.1 J/ m2 when 4.7 wt.% of nanoclay was added.

8.7.1 TENSILE PROPERTIES The tensile modulus, which is an indication of stiffness, has shown to be remarkably improved when nanocomposites are formed with a layered silicate. N6CNs prepared through the intercalative ring opening polymerization of e-caprolactam, leading to the formation of exfoliated nanocomposites, exhibited increases of 90% with the addition of only 4 wt.% of exfoliated clay. The main reason attributed to the great improvement of modulus in the case of N6CNs, was the presence of strong interaction between the matrix and the silicate layers via the formation of hydrogen bonds [50].

8.7.2 FLEXURAL PROPERTIES Table 8.3 shows the increases in tensile and flexural properties with the introduction of clay on spent PA-12. As little as 5 wt.% clay loading could improve the tensile and flexural properties significantly.

8.7.3 THERMAL PROPERTIES The thermal stability of polymeric materials is usually studied by thermogravimetric analysis (TGA). Generally, the incorporation of silicate into polymer matrices enhances the thermal stability

8.8 HYBRID COMPOSITES

193

Table 8.3 The Effect of the Addition of Layered Silicate Into a Polymer Matrix on the Mechanical Properties [51] Samples

Tensile Strength (MPa)

Tensile Modulus (GPa)

Spent Spent Spent Spent

40.45 ( 6 3.63) 44.64 ( 6 4.12) 48.64 ( 6 1.51) 52.46 ( 6 3.5)

0.656 0.712 0.758 0.732

PA-12 PA-12 (1 wt.%) PA-12 (3 wt.%) PA-12 (5 wt.%)

( 6 0.04) ( 6 0.026) ( 6 0.031) ( 6 0.025)

Flexural Strength (MPa)

Flexural Modulus (GPa)

73.85 ( 6 1.81) 81.58 ( 6 2.12) 92.98 ( 6 2.49) 90.53 ( 6 4.18)

2.229 2.443 3.022 2.853

( 6 0.094) ( 6 0.091) ( 6 0.121) ( 6 0.202)

by acting as a superior insulator and mass transport barrier to the volatile products generated during decomposition. A reported work by Blumstein [52] has revealed the improved thermal stability of a polymer layered silicate (PLS) nanocomposite that combined poly(methyl methacrylate) (PMMA) and MMT. It was shown that PMMA intercalated between the galleries of MMT clay resisted thermal degradation under conditions that would otherwise completely degrade pure PMMA. These PMMA nanocomposites were prepared by the free-radical polymerization of MMA intercalated in the clay. TGA data revealed that both linear PMMA and cross-linked PMMA intercalated into MMT layers with a 40 C 50 C higher decomposition temperature. It was conclusively stated that the stability of the PMMA nanocomposite was not only due its different structure, but also to the restricted thermal motion of the PMMA in the gallery. Similarly, Brostow et al. [26] studied the dynamic friction and wear rate of nanocomposites of PMMA and Brazilian MMT clays as function of the concentration of clay. It was reported that an increase in the quantity of the Brazilian clay caused an increase in the dynamic friction of the nanocomposites. This phenomenon was attributed to the sticky nature of the clay. In addition, at 1 wt.% clay concentration, resistance to abrasion was provided by the clay. However, a significant increase in wear was observed as the clay concentration was increased. Resultantly, increases in brittleness and clay agglomeration were observed. Furthermore, a PMMA-modified MMT (PMMA/Mt-CTA) nanocomposite exhibited better thermal stability and affinity in terms of the removal of pesticides from aquatic solutions, which qualified it as an innovative water treatment technique as well as an appropriate material for packaging applications when compared with pristine PMMA, as studied and reported by Youssef et al. [53]. The mechanical (tensile, flexural, and nanoindentation: elastic modulus and hardness), water repellence, and thermal properties of vinyl ester matrix nanocomposites based on layered silicate were enhanced [28]. This improvement was attributed to the presence of layered silicate.

8.8 HYBRID COMPOSITES The importance of hybridization technique cannot be underestimated in composite technology as improved and desirable synergetic properties of the combined constituents are achieved in hybrid composites that may be difficult and unrealistic or unfeasible in each of the fibers or fillers present in nonhybrid composites.

194

CHAPTER 8 UNSATURATED POLYESTER RESINS

8.8.1 NANOCLAY/UNSATURATED POLYESTER RESIN/NATURAL FIBER HYBRIDS The effects of natural fiber reinforcement on the mechanical, physical, and thermal properties of nanoclay/polymer hybrid composites have not yet been reported. However, there are some promising reports that indicate the use of hybrid composites made up of both natural and glass fibers. The effect of glass fiber reinforcement on the mechanical and thermal properties of polymer clay nanocomposites has been widely reported. Therefore it is expected that the introduction of natural fibers into the nanoclay/polyester nanostructure would enhance the mechanical and thermal properties. Significantly, the hydrophilic nature of both natural fiber and layered silicates suggests a good compatibility between the two materials. Moreover, the properties of natural fiber reinforced composites have been improved by Pavithran et al. [54] using sisal glass hybrid laminates subjected to impact loading, while Rozman et al. [55] improved both the flexural and tensile moduli of polypropylene oil palm empty fruit bunch glass fiber hybrid composites. Precisely, Haq et al. [56] improved the toughness of hybrid bio-based UP/soybean oil reinforced composites with the incorporation of nanoclay and natural hemp fibers, but at detriment to the stiffness and hygro-thermal properties, as similarly reported by Haq et al. [57] in their investigation on thermophysical and characterization of another bio-based UP/layered silicate nanocomposite. Also, Atiqah et al. [58] developed a kenaf glass reinforced UP hybrid composite for structural applications. They reported that the treated kenaf reinforced hybrid composite showed better fiber matrix interfacial bonding, resulting in higher values of flexural, tensile, and impact strengths when compared to other untreated combinations. Dhakal et al. [59] reported that both sodium hydroxide surface treatment and glass fiber hybridization of hemp fiber/ UP composites improved the thermal stability and wetting responses.

8.9 CONCLUSION Based on this extensive study, three common structural types of nanocomposites are intercalated, flocculated, and exfoliated. The introduction of nanoclay into UP nanocomposites improved the properties of the concerned nanocomposites up until a certain threshold value (wt.%); either below or above this level the optimized performances of the composite systems are compromised. Among these nanoclays, the most commonly used are MMT and bentonite due to their chemical compatibility with UP toward achieving better interfacial bonding. Importantly, improvements in properties such as thermal stability (fire resistance), electrical conductivity, and several mechanical properties (tensile, flexural, interfacial/laminar shear strengths, hardness, and elongation at break, among others) are commonly reported for both UP composites and nanocomposites, depending on the manufacturing processes, the volume fraction as well as the types of fibers, fillers, and matrices used. In addition, TEM and XRD are two commonly and effectively used techniques for the analysis of the morphology and characterization of UP composites, nanocomposites, and hybrid composite systems, in addition to TGA, differential scanning calorimetry, and scanning electron microscopy. Conclusively, the design and development of improved, hybrid, UP/nanoclay, natural fiber reinforced composites and nanocomposites still remain important areas of research toward the advancement of innovative composite development and materials engineering, at large.

REFERENCES

195

REFERENCES [1] L.H. Van Vlack, Materials for Engineering: Concept and Application, Addison-Wesley Publishing Company, Michigan, 1982. University of Michigan, Michigan. [2] T. Richardson, A Design Guide, Industrial Press Inc, New York, 1987. [3] T. Miller, Introduction to composites, Composites Institute, fourth ed., Society of the Plastics Industry, New York, 1998. [4] M.M. Schwarz, Composite Materials Handbook, second ed., McGraw Hill, Inc, New York, 1992. [5] H.N. Dhakal, Z.Y. Zhang, M.O.W. Richardson, Effect of water absorption on the mechanical properties of hemp fibre reinforced unsaturated polyester composites, Comp. Sci. Technol. 67 (7 8) (2007) 1674 1683. [6] S.K. Pal, D. Mukhopadhyay, S.K. Sanyal, R.N. Mukherjea, Studies on process variables for natural fibre composites-effect of polyesteramide polyol as interfacial agent, J. Appl. Polym. Sci. 35 (4) (1988) 973 985. [7] O. Owolabi, T. Czvikovszky, I. Kovacs, Coconut-fibre-reinforced thermosetting plastics, J. Appl. Polym. Sci. 30 (1985) 1827 1836. [8] K.G. Satyanarayana, K. Sukumaran, A.G. Kulkarni, S.G.K. Pillai, P.K. Rohatgi, Performance of banana fabric-polyester resin composites, in: I.H. Marshall (Ed.), Composite Structures 2, Springer, Dordrecht, 1983, pp. 535 548. [9] N.M. White, M.P. Ansell, Straw reinforced polyester composites, J. Mater. Sci. 18 (5) (1983) 1549 1556. [10] L.Y. Mwaikambo, E.T.N. Bisanda, The performance of cotton kapok fabric polyester composites, Polym. Testing 18 (3) (1999) 181 198. [11] S.H. Aziz, M.P. Ansell, S.J. Clarke, S.R. Panteny, Modified polyester resins for natural fibre composites, Comp. Sci. Technol. 65 (3 4) (2005) 525 535. [12] I.M. De Rosa, H.N. Dhakal, C. Santulli, F. Sarasini, Z.Y. Zhang, Post-impact static and cyclic flexural characterisation of hemp fibre reinforced laminates, Comp. Part B: Eng. 43 (2012) 1382 1396. [13] H.N. Dhakal, V. Arumugam, A. Aswinraj, C. Santulli, Z.Y. Zhang, A. Lopez-Arraiza, Influence of temperature and impact velocity on the impact response of jute/UP composites, Polym. Testing 35 (2014) 10 19. [14] M. Monti, D. Puglia, M. Natali, L. Torre, J.E. Kenny, Effect of carbon nanofibers on the cure kinetics of unsaturated polyester resin: thermal and chemorheological modelling, Comp. Sci. Technol. 71 (2011) 1507 1516. [15] V. Vilay, M. Mariatti, R.M. Taib, M. Todo, Effect of fiber surface treatment and fiber loading on the properties of bagasse fiber reinforced unsaturated polyester composites, Comp. Sci. Technol. 68 (2008) 631 638. [16] A. Bessadok, S. Roudesli, S. Marais, N. Follain, L. Lebrun, Alfa fibres for unsaturated polyester composites reinforcement: effects of chemical treatments on mechanical and permeation properties, Comp. Part A: Appl. Sci. Manuf. 40 (2009) 184 195. [17] H. Md Akil, L.W. Cheng, Z.A. Mohd Ishak, A. Abu Bakar, M.A. Abd Rahman, Water absorption study on pultruded jute fibre reinforced unsaturated polyester composites, Comp. Sci. Technol. 69 (2009) 1942 1948. [18] D. Jiang, L. Xing, L. Liu, S. Sun, Q. Zhang, Z. Wu, et al., Enhanced mechanical properties and antihydrothermal ageing behaviours of unsaturated polyester composites by carbon fibres interfaced with POSS, Comp. Sci. Technol. 117 (2015) 168 175. [19] M.A. Sawpan, K.L. Pickering, A. Fernyhough, Effect of fibre treatments on interfacial shear strength of hemp fibre reinforced polylactide and unsaturated polyester composites, Comp. Part A: Appl. Sci. Manuf. 42 (2011) 1189 1196.

196

CHAPTER 8 UNSATURATED POLYESTER RESINS

[20] M.A. Sawpan, K.L. Pickering, A. Fernyhough, Flexural properties of hemp fibre reinforced polylactide and unsaturated polyester composites, Comp. Part A: Appl. Sci. Manuf. 43 (2012) 519 526. [21] Z. Wu, H. Cui, L. Chen, D. Jiang, L. Weng, Y. Ma, et al., Interfacially reinforced unsaturated polyester carbon fiber composites with a vinyl ester-carbon nanotubes sizing agent, Comp. Sci. Technol. 164 (2018) 195 203. [22] D. Jiang, L. Liu, J. Long, L. Xing, Y. Huang, Z. Wu, et al., Reinforced unsaturated polyester composites by chemically grafting amino-POSS onto carbon fibers with active double spiral structural spiralphosphodicholor, Comp. Sci. Technol. 100 (2014) 158 165. [23] T.A. Negawo, Y. Polat, F.N. Buyuknalcaci, A. Kilic, N. Saba, N. Jawaid, Mechanical, morphological, structural and dynamic mechanical properties of alkali treated Ensete stem fibers reinforced unsaturated polyester composites, Comp. Struct. 207 (2019) 589 597. [24] M. Motomatsu, T. Takahashi, H.Y. Nie, W.H. Mizutani, H. Tokumoto, Microstructure study of acrylic polymer-silica nanocomposite surface by scanning force microscopy, Polymer 38 (1) (1997) 177 182. [25] B.J. Ahmed, J.H. Tomma, F.S. Matty, Synthesis, characterizations and study of mechanical properties for blended nanocomposites biased on new epoxy resin, J. Natl. Sci. Res. 5 (18) (2015) 19 29. [26] W. Brostow, M. Dutta, J. Ricardo de Souza, P. Rusek, A. Marcos de Medeiros, E.N. Ito, Nanocomposites of poly (methyl methacrylate) (PMMA) and montmorillonite (MMT) Brazilian clay: a tribological study, Express Polym. Lett. 4 (9) (2010) 570 575. [27] K.V.P. Chakradhar, K.V. Subbaiah, M.A.M. Kumar, G.R. Reddy, Blended epoxy/polyester polymer nanocomposites: effect of “nano” on mechanical properties, Polym. Plast. Technol. Eng. 51 (2012) 92 96. [28] A.I. Alateyah, H.N. Dhakal, Z.Y. Zhang, Water absorption behavior, mechanical and thermal properties of vinyl ester matrix nanocomposites based on layered silicate, Polym. Plast. Technol. Eng. 53 (4) (2014) 327 343. [29] H.N. Dhakal, Z.Y. Zhang, M.O.W. Richardson, Nano-indentation behaviour of layered silicate reinforced unsaturated polyester nanocomposites, Polym. Testing 25 (2006) 846 852. [30] V. Mittal, Polymer layered silicate nanocomposites: a review, Materials 2 (3) (2009) 992 1057. [31] M.A. Motawie, N.M. Ahmed, S.M. ElMesallamy, E.M. Sadek, N.G. Kandile, Unsaturated polyesters/layered silicate nanocomposites: synthesis and characterization, IOSR J. Appl. Chem. 7 (10) (2014) 34 43. [32] M. Oleksy, H. Galina, Unsaturated polyester resin composites containing bentonites modified with silsesquioxanes, Ind. Eng. Chem. Res. 52 (2013) 6713 6721. [33] E. Ruckenstein, Y.N. Yuan, Nanocomposites of rigid polyamide dispersed in flexible vinyl polymer, Polymer 38 (1997) 3855 3860. [34] B.K.G. Theng, The Chemistry of Clay-Organic Reactions, Wiley, New York, NY, 1974. [35] A. Usuki, M. Kawasumi, Y. Kojima, A. Okada, T. Krauchi, O. Kamigaito, Swelling behaviour of montmorrillonite cation exchanged for ω-amino acid by ε-caprolactam, J. Mater. Res. 6 (1993) 1185 1189. [36] M. Alexandre, P. Dubois, Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials, Mater. Sci. Eng. R: Rep. 28 (1-2) (2000) 1 63. [37] E.P. Giannelis, R. Krishnamoorti, E. Manias, Polymer-silica nanocomposites: model system for confined polymers and polymer brushes, Adv. Polym. Sci. 138 (1999) 108 147. [38] X. Kornmann, H. Lindberg, L.A. Berglund, Synthesis of epoxy-clay nanocomposites. Influence of the nature of the curing agent on structure, Polymer 42 (2001) 4493 4499. [39] A. Battisti, A.A. Skordos, I.K. Partridge, Percolation threshold of carbon nanotubes filled unsaturated polyesters, Comp. Sci. Technol. 70 (2010) 633 637. [40] D.H. Builes, J. Labidi, A. Eceiza, I. Mondragon, A. Tercjak, Unsaturated polyester nanocomposites modified with fibrillated cellulose and PEO-b-PPO-b-PEO block copolymer, Comp. Sci. Technol. 89 (2013) 120 126.

REFERENCES

197

[41] M. Keˆdzierski, P. Penczek, Unsaturated polyester/montmorillonite nanocomposites prepared by in situ intercalative copolyaddition, Polimery 49 (11 12) (2004) 801 805. [42] D. Wang, X. Mu, W. Cai, L. Song, C. Ma, Y. Hu, Constructing phosphorus, nitrogen, silicon-cocontained boron nitride nanosheets to reinforce flame retardant properties of unsaturated polyester resin, Comp. Part A: Appl. Sci. Manuf. 109 (2018) 546 554. [43] M. Chieruzzi, A. Miliozzi, J.M. Kenny, Effects of the nanoparticles on the thermal expansion and mechanical properties of unsaturated polyester/clay nanocomposites, Comp. Part A: Appl. Sci. Manuf. 45 (2013) 44 48. [44] M.S.A.S. Shah, P. Basak, S.V. Manorama, Polymer nanocomposites as solid electrolytes: evaluating ionpolymer and polymer-nanoparticle interactions in PEG-PU/PAN semi-IPNs and Titania Systems, J. Phys. Chem. C 114 (2010) 14281 14289. [45] A.I. Alateyah, H.N. Dhakal, Z.Y. Zhang, B. Aldousiri, Low-velocity impact and creep-strain behaviour of vinyl ester matrix nanocomposites based on layered silicate, Int. J. Polym. Sci. 2014 (2014) 1 10. [46] S.S. Ray, K. Okamoto, M. Okamoto, Structure property relationship in biodegradable poly (butylene succinate)/layered silicate nanocomposites, Macromolecules 36 (7) (2003) 2355 2367. [47] T.D. Fornes, P.J. Yoon, D.L. Hunter, H. Keskkula, D.R. Paul, Effect of organoclay structure on nylon 6 nanocomposite morphology and properties, Polymer 43 (22) (2002) 5915 5933. [48] D. Yebassa, S. Balakrishnan, E. Feresenbet, D. Raghavan, P.R. Start, S.D. Hudson, Chemically functionalized clay vinyl ester nanocomposites: effect of processing parameters, J. Polym. Sci. Part A: Polym. Chem. 42 (6) (2004) 1310 1321. [49] L.M. Liu, Z.N. Qi, X.G. Zhu, Studies on nylon 6/clay nanocomposites by melt-intercalation process, J. Appl. Polym. Sci. 71 (1999) 1133 1138. [50] A. Okada, M. Kawasumi, A. Usuki, Y. Kojima, T. Kaurauchi, O. Kamigaito, Nylon 6-clay hybrid, Mater. Res. Soc. Symp. Proc. 171 (1990) 45 50. [51] B. Aldousiri, H.N. Dhakal, S. Onuh, Z.Y. Zhang, N. Bennett, M.O.W. Richardson, Effect of layered silicate reinforcement on the structure and mechanical properties of spent polyamide-12 nanocomposites, Comp. Part B: Eng. 43 (3) (2012) 1363 1367. [52] A. Blumstein, Polymerization of adsorbed monolayers. II. Thermal degradation of the inserted polymers, J. Polym. Sci. Part A: General Paper 3 (7) (1965) 2665 2672. [53] A.M. Youssef, F.M. Malhat, A.A. Abdel Hakim, I. Dekany, Synthesis and utilization of poly (methyl methacrylate) nanocomposites based on modified montmorillonite, Arab. J. Chem. 10 (2017) 631 642. [54] C. Pavithran, P.S. Mukherjee, M. Brahmakumar, A.D. Damodaran, Impact properties of sisal-glass hybrid laminates, J. Mater. Sci. 26 (2) (1991) 455 459. [55] H.D. Rozman, G.S. Tay, R.N. Kumar, A. Abusamah, H. Ismail, Z.A.M. Ishak, Polypropylene-oil palm empty fruit bunch-glass fibre hybrid composites: a preliminary study on the flexural and tensile properties, Eur. Polym. J. 37 (6) (2001) 1283 1291. [56] M. Haq, R. Burguen˜o, A.K. Mohanty, M. Misra, Bio-based unsaturated polyester/layered silicate nanocomposites: characterization and thermo-physical properties, Comp. Sci. Technol. 68 (2008) 3344 3351. [57] M. Haq, R. Burguen˜o, A.K. Mohanty, M. Misra, Bio-based unsaturated polyester/layered silicate nanocomposites: characterization and thermo-physical properties, Comp. Part A: Appl. Sci. Manuf. 40 (2009) 540 547. [58] A. Atiqah, M.A. Maleque, M. Jawaid, M. Iqbal, Development of kenaf-glass reinforced unsaturated polyester hybrid composite for structural applications, Comp. Part B: Eng. 56 (2014) 68 73. [59] H.N. Dhakal, Z.Y. Zhang, N. Bennett, Influence of fibre treatment and glass fibre hybridisation on thermal degradation and surface energy characteristics of hemp/unsaturated polyester composites, Comp. Part B: Eng. 43 (2012) 2757 2761.

198

CHAPTER 8 UNSATURATED POLYESTER RESINS

FURTHER READING F. Chu, X. Yu, Y. Hou, X. Mu, L. Song, W. Hu, A facile strategy to simultaneously improve the mechanical and fire safety properties of ramie fabric-reinforced unsaturated polyester resin composites, Comp. Part A: Appl. Sci. Manuf. 115 (2018) 264 273. S. Pavlidou, C.D.P. Papaspyrides, A review on polymer layered silicate nanocomposites, Prog. Polym. Sci. 33 (12) (2008) 1119 1198. A. Usuki, M. Kawasumi, Y. Kojima, A. Okada, T. Krauchi, O. Kamigaito, Swelling behaviour of montmorrillonite cation exchanged for ω-amino acid by ε-caprolactam, Polym. Sci. Part A: Polym. Chem. 31 (1993) 1755 1758. A. Usuki, M. Kawasumi, Y. Kojima, A. Okada, T. Krauchi, O. Kamigaito, Swelling behaviour of montmorrillonite cation exchanged for ω-amino acid by ε-caprolactam, J. Mater. Res. 8 (1993) 1174 1178.

CHAPTER

AGING BEHAVIOR AND MODELING STUDIES OF UNSATURATED POLYESTER RESIN AND UNSATURATED POLYESTER RESIN-BASED BLENDS

9

Emmanuel Richaud and Jacques Verdu ´ Paris, France PIMM, UMR 8006, ENSAM
CNRS
CNAM, HESAM Universite,

9.1 INTRODUCTION Unsaturated polyester resins (UPRs) are usually obtained from a prepolymer being a condensation product of unsaturated anhydride (or diacid) and a diol. This highly viscous liquid is dissolved in a reactive low viscosity solvent, commonly styrene (or sometimes methyl methacrylate). The curing is initiated by peroxide and metallic salt catalyst of the peroxide decomposition often called “accelerator”. It induces the copolymerization of double bonds hold by both the prepolymer and the solvent to give a network. The wide use of such materials in applications ranging from tanks, tubes, and vessels to shipbuilding or outdoor swimming pools makes it necessary to well understand their long-term stability in the presence of external factors that cause degradation (temperature, oxygen, water, chemicals, radiations, etc.). The present chapter is, hence, aimed at: • •

•

presenting the main mechanisms involved in network degradation; proposing kinetic schemes and providing the corresponding equations, thereby allowing for the prediction of changes in network structure (at a molecular scale) and the influence of the cited causes of degradation; using the available structure
property relationships or establishing empirically new ones to determine the effect of the observed changes at microscopic scale on network properties, especially thermomechanical properties. There are basically two kinds of aging mechanisms.

1. Physical aging basically corresponds to changes in composition (water absorption, plasticizer loss) and in chain conformation, especially when interchain distances are modified, but without modification of the polymer chemical structure. The main kinds of physical aging are structural relaxation (with a decrease of free volume) and physical aging by solvent ingress or plasticizer Unsaturated Polyester Resins. DOI: https://doi.org/10.1016/B978-0-12-816129-6.00009-0 © 2019 Elsevier Inc. All rights reserved.

199

200

CHAPTER 9 AGING BEHAVIOR AND MODELING STUDIES

loss. In this chapter, focus will be placed on humid aging, which induces plasticization and differential swelling, thereby generating strains and thus being a possible cause of matrix or fiber
matrix interfacial damage. The latter will favor water penetration into the material and thus the eventual propagation of chemical aging deep sample layers. 2. Chemical aging is due to reactions with external agents such as water, oxygen, solar UV, or ionizing radiations. Since polyesters are often used in contact with water, emphasis will be put on polyester composite interactions with water. In this chapter, particular attention will be paid to the phenomenon of osmotic cracking, in which degradation at the microscopic scale leads to the formation of macroscopic cracks.

9.2 CHANGES IN MECHANICAL PROPERTIES OF NETWORKS 9.2.1 NETWORK PLASTICIZATION BY SOLVENTS The penetration of water or other small molecules into the polymer provokes a strong glass transition temperature (Tg) decrease illustrated, for example, in the case of UPR and its composites [1]. From a theoretical point of view, let us recall that the free volume theory allows for the change of Tg to be predicted in the case of polymer blends and polymer
solvent mixtures by the simplified relationship seen in Eq. (9.1). 1 12φ φ 5 1 Tg Tgp Tgw

(9.1)

In common “dry” polyester networks, the Tg is in the order of 380 K and the TgW is in the order of 120 K. The above relationship can be rearranged as: 1 1 B 1 0:0057φ Tg TgðdryÞ

(9.2)

Thus Tg is expected to decrease of about 8 K per per percent of absorbed water. In common polyesters, water absorption does not exceed 1
2%. The effects of plasticization are thus limited, but can be critical when the material is submitted to high static loads. Glassy modulus (Eglassy) depends on the density of cohesive energy: KB11. CED where K is the bulk modulus and CED the Cohesive Energy Density [2]. In the case of plasticized networks, the interchain distance increases, so it is expected that the elastic glassy modulus decreases. The glassy modulus (Erub) depends on the elastically active chain concentration. It decreases with plasticizer content [3]: 1=3

Erub 5 3 3 φP 3 ½EAC 3 RT

(9.3)

Plastification by water, therefore, results in a decrease in mechanical properties such as glassy and rubbery elastic modulus [1,4]. Identically to Tg changes, depletions of the glassy and rubbery moduli are expected to be weak since water absorption is low. Moreover, in the absence of any other chemical damages, these changes would be characterized by: • •

an equilibrium state, a reversible aspect.

9.2 CHANGES IN MECHANICAL PROPERTIES OF NETWORKS

201

According to Apolinario et al. [5], the elastic modulus actually plateaus in the case of an immersion at 30 C. The drop of elastic modulus is about 5% (for a 1% water uptake) and samples recover their initial properties after drying. However, this seems to not always be the case [1], meaning that chemical damage can overlap either in the matrix or at the matrix
fiber interface.

9.2.2 OSMOTIC CRACKING Osmotic cracking has, in particular, been observed for water diffusion. Its mechanism can be summarized as follows: • •

•

water diffuses into the polymer and fills a “void” (preexisting cavity) or forms a cluster (see Section 9.3.2); short soluble molecules (e.g., curing initiator byproducts, unreacted styrene, or short fragments generated from hydrolytic degradation) dissolve in this water phase and lower its chemical potential; this latter step provokes further diffusion of water through the polymer (which behaves as a permeable membrane) so as to equilibrate its chemical potential. The difference in chemical potential between dissolved and external water results in an osmotic pressure given by van’t Hoff’s law: P 5 RTΣci

(9.4)

ci being the concentration in soluble molecules in the “cavity.” At a certain stage, the osmotic pressure exceeds the polymer stress at break, which induces an osmotic cracking. It results in a well recognizable disk crack with radial lines [6] (Fig. 9.1).

FIGURE 9.1 Microscopic observations of disk cracks for osmotic cracking. Reprinted with the permission of Elsevier.

202

CHAPTER 9 AGING BEHAVIOR AND MODELING STUDIES

A rise in temperature induces an increase in water solubility in polymers and the redissolution of the water phase in polyesters and, thus, possible self-healing. Conversely, a decrease in temperature induces a supersaturation of water in polymers and the initiation of new disk cracks [7]. Last, the initiation time for disk cracks would obey the Arrhenius law [6].

9.2.3 CHANGES INDUCED BY CHEMICAL AGING Jefferson et al. [1], compared the changes in Tg and elastic modulus (at glassy state) in virgin polyester materials (matrices with carbon nanofibers and/or glass fibers), those polyester materials after water immersion (weight gain about 0.1%
1%), and unsaturated polyester (UP) materials immerged in water and then dried. Their results, illustrated in Fig. 9.2, show that water penetration is not totally reversible. Comparable results were obtained for UP immersion in ethanol (Table 9.1) [8]; where mass uptake curves displayed a maximum followed by a decrease ascribed to the leaching of small molecules, and the ultimate stress decreased with ethanol aging. However, one sees that a σR decrease cannot be explained only by ethanol ingress. Actually, the decrease for σR is observed to be lower for instances of higher ethanol uptake. These (irreversible) changes are associated to the chemical degradation described in Section 9.3. E’glassy Unaged Immerged + dried Immerged

Storage modulus

E’rubbery

3.6 GPa 60 MPa 3.1 GPa 50 MPa 2.9 GPa 65 MPa

Temperature

FIGURE 9.2 Effect of water immersion on modulus [8].

Table 9.1 Changes for Unsaturated Polyesters Resin (UPR) Composites in Presence of Ethanol Mass Uptake (%)

Ultimate Stress (MPa)

T ( C)

7 Days

30 Days

0 Days

30 Days

90 115

3.2 2.5

1 0.3

393

166 142

9.2 CHANGES IN MECHANICAL PROPERTIES OF NETWORKS

203

The elastic modulus at glassy state actually depends on cohesive energy more than on cross-link density. In the case of hydrolysis, moderately polar ester groups are converted into highly polar ones (carboxylic acids and alcohols). However, there are no studies evidencing an increase in glassy moduli with hydrolysis (see, e.g., Fig. 9.2 where a decrease is observed). At temperatures above the Tg, thermoset networks are in the rubbery state. Elastic behavior is given by the Flory approach, according to which the Young’s modulus is proportional to the concentration of elastically active chains, n0 [3]: E5

3ρRT 5 3n0 RT MC

(9.5)

where ρ is the density, MC the average molar mass between cross-links, R the gas constant, and T the absolute temperature. Eq. (9.5) is a theoretical equation valid for ideal networks. The analysis of a wide series of UPR networks has shown that the modulus
cross-link density is rather:


E 5 3ρRT 3

 1 2 1:5 MC

(9.6)

illustrating the effect of dangling chains [9]. It is clear that chemical changes leading to either chain scission or cross-linking will modify the concentration of elastically active chains and later the rubbery modulus. For degraded networks having undergone chain scission (s) and/or cross-linking (x), it can be proposed that: n 5 n0 2 ψ 3 s 1 φ 3 x

(9.7)

with ψ 5 1 for tetrafunctional networks and ψ 5 3 for trifunctional networks.UPR are possibly tetrafunctional networks but there are degradation studies based on these relationships. The glass transition of a fully cured network (TgN) is given by the DiMarzio’s equation [10]: TgN 5

Tgl 1 2 KDM Fn0

(9.8)

where KDM is the DiMarzio’s constant; n0 is the cross-link density (mol/kg); Tgl is the glass transition of a “virtual” linear polymer (n0 5 0); F is the flex parameter (kg/mol) related to the molar mass per rotatable bond. The calculation of the parameters of DiMarzio’s law is illustrated in the case of epoxies [11]. In the case of UPR, an attempt to describe glass transition of networks was done [12], but it seems that it is only possible to predict Tg values with F, Tgl, and n0 coming from structural considerations if KDM is an adjustable parameter. The most obvious reason is the nonideality of polyesters and, in particular, the possible presence of cross-link nodes where: •

the cross-link bridge is made of several homopolymerized styrene (S) units, which can be considered as trifunctional: E

HC CH

E

Sj E

HC CH

E

204

•

CHAPTER 9 AGING BEHAVIOR AND MODELING STUDIES

the cross-link bridge is made of homopolymerized fumarates or maleates, which can be considered as hexafunctional: S E

HC CH

E

E

HC CH

E

S

One can see here that UPRs are further from ideal networks than epoxies, for example. However, the determination of KDM for virgin networks can allow for the estimation of the residual cross-link density after a chemical aging and later, the concentration in chain scissions and crosslinks. Tg changes induced either by chain scission or cross-linking can be related to changes in yielding properties since it was proposed in thermosetting networks that [13]:   σY 5 C Tg
T

and



Tg
T εY 5 500

(9.9)

 (9.10)

Considering ultimate properties, it seems that both ultimate stress and strain decrease following the initial tensile curve of networks as a “rupture envelope” at least in the case of hydrolyzed networks (Fig. 9.3).

FIGURE 9.3 Decrease of ultimate stress and strain for various polyester networks having undergone various levels of hydrolysis [14]. Reprinted with the permission of Elsevier.

9.3 MECHANISMS OF PHYSICAL AGING BY SOLVENT INGRESS

205

9.2.4 INTERFACIAL DAMAGES IN COMPOSITES In UPR
glass fiber composites, it is known that the interface is a domain where water is expected to be more soluble than in the matrix [15] as well as being subject to a faster diffusion. The consequences of aging are then worst in the case of composites in which water promotes: • • •

matrix
fiber decohesion [16] evidenced by a decrease in the interfacial shear strength [17] composite delamination [17] the hydrolysis of coupling agents [18]

9.2.5 CONCLUSION The failure of UPR and UPR composites originate from physical aging (i.e., diffusion of small molecules in the network without changes in its architecture) and/or chemical aging (i.e., irreversible mechanisms inducing chain scissions and/or cross-linking). The main mechanisms associated with these degradation modes will be presented in Section 9.4, Mechanisms of Chemical Degradation. Some of these changes can be described using polymer physics laws from values of water uptake or concentration in cross-link. If these latter can be described by kinetic law, it may permit the changes in engineering properties (and later the lifetime) to be predicted.

9.3 MECHANISMS OF PHYSICAL AGING BY SOLVENT INGRESS 9.3.1 COMPATIBILITY WITH ORGANIC SOLVENTS Sorption isotherm for organic solvents into polymers is described by the Flory
Huggins equation:


ln

 P 5 lnð1 2 φp Þ 1 φp 1 χφ2p P0

(9.11)

in which P/P0 is the water activity in the external medium around the polymer; φp is the polymer volume fraction in the polymer
water mixture; and χ is the polymer
solvent interaction expressing the affinity of polymers for certain chemicals (“like seeks like”). It can be calculated by: χ5

2 Vm   δpolymer 2δsolvent RT

(9.12)

δ being the solubility parameter with δUP B 22 MPa1/2 [19]. Low χ values correspond to high polymer
penetrant compatibility. It is, hence, not surprising that aromatic halogenated solvents such as chlorobenzene display a high affinity with UPR [20,21], whereas apolar solvents (cyclohexane) or highly polar solvents (water) are poorly soluble in polyesters. Extensive tables reporting the compatibility of different kinds of UPR with several sorts of fluids and chemicals can be found in Ref. [22].

206

CHAPTER 9 AGING BEHAVIOR AND MODELING STUDIES

The diffusion of solvents into UPR has been scarcely investigated in existing literature. The transport phenomena of penetrants in polymers can be described by [23]: M ðt Þ 5 k 3 tn MN

(9.13)

M(t) and MN being the mass uptake at any time and at the theoretical equilibrium, respectively, and n describing the mechanism of diffusion. Schematically: •

•

n 5 0.5 means that the diffusion obeys Fick’s law as often observed in polymer networks when the polymer chains have a high mobility allowing an easy penetration of the solvent (with the diffusion rate, Rdiff, being slower than the polymer relaxation rate, Rrelax). n 5 1 means that the diffusion is either anomalous, that is, the diffusion and relaxation rates are almost of the same order or otherwise it means that the diffusion rate is much higher than the relaxation rate of a given polymer so that a sharp diffusion profile is observed.

The “diffusion” aspects will be recalled in more detail in the case of water (see Section 9.3.2). According to Deslandes, the diffusivity of chemicals in UPR is first driven by their viscosity since diffusivity increases with reciprocal viscosity [24]. The resistance to several chemicals seems to be improved by the addition of large quantities (about 40%) of microscopic fillers such as ATH, mica, or calcium carbonate [25] or small quantities (typically 1%
5%) of nanoclays [26]. On the contrary, the presence of microvoids increases both the equilibrium concentration and the sorption rate [24].

9.3.2 SORPTION AND DIFFUSION OF WATER Water is a highly polar solvent, therefore, its solubility in polyester is expected to be very low along with the subsequent damage. However, water is able to react with the ester groups of UPR so that the physical aging of water (reversible) is often accompanied by irreversible damages [1] occurring either in the matrix or at the fiber
matrix interface. Humid aging is, hence, by far one of the most important mechanisms for UPR degradation. Beside the chemical interaction (described in Section 9.4.1), two key parameters describe the polymer
water interaction: • •

the polymer affinity with water (the hydrophilicity or solubility), and the rate of water penetration into the polymer matrix (the diffusivity).

9.3.2.1 Solubility In composite matrices such as UPs, Henry’s law is often used to describe the sorption of water in a matrix. It can be written as: CS 5 σ 3 a

(9.14)

where Cs is the equilibrium concentration of water in the polymer, σ is the solubility coefficient, and a is the activity of water (i.e., the ratio of its external pressure over the saturation pressure at the considered temperature). σ is in the order of 0.36
0.42 cm3/mmol [27] corresponding to about 1% of water uptake after an immersion aging. The general shape of curves for water ingress is

9.3 MECHANISMS OF PHYSICAL AGING BY SOLVENT INGRESS

207

Relative mass uptake

Level of solubility

Time

FIGURE 9.4 Theoretical shape of kinetic curves for water uptake and possible subcases. ➀ Water uptake without chemical aging or leaching. ➁ Water uptake followed by hydrolysis. ➂ Water uptake accompanied by leaching of unreacted chemicals. ➃ Water uptake followed by hydrolysis and extraction of hydrolysis byproducts.

depicted in Fig. 9.4 [28] (In some cases, the difference between type ➃ and ➂ curves is unclear, particularly since relatively high quantities of polymerization initiator and catalyst are used and can be extracted by surrounding water). According to Fig. 9.4, determining the true equilibrium mass uptake is not always easy since other mechanisms can overlap, in particular the effects of hydrolysis (particularly very strong mass loss at long immersion durations) on mass uptake curves is observed only at elevated temperatures [29]. Despite this issue, the orders of magnitude of equilibrium water uptake are reported in Tables 9.2 and 9.3 together with available experimental details [27,30
32]. It is often proposed to describe water absorption in polymer by [33]: H5

X wm 3 M 5 ni Hi 1800

(9.15)

where M is the molar mass of the repetitive unit; ni is the number of groups able to bind with Hi molecules of water, and wm is the equilibrium water absorption (expressed in wt.%). In the case of polyesters, the hydrophilicity in networks is expressed as [31]: k 5 k1 3 ½ester 1 k2 3 ½ether 1 k3 3 ½chain ends

(9.16)

Here, the term [chain ends] accounts for acid or alcohol groups loacted at the extremity of prepolymers. More precisely, it seems that the contribution to hydrophily of those acids and alcohols is higher than ethers and esters. The effect of diacid or diol on the hydrophilicity of prepolymers has been illustrated for example in [34]. Maleates have been shown to increase the hydrophilicity either in prepolymers [34] (Fig. 9.5) or in cross-linked networks [32].

Table 9.2 Effect of Diol, Styrene Content, Prepolymer, and Temperature on the Water Solubility of Unsaturated Polyester Resin (UPR) (NB: All isotherms display the shape ➀, Fig. 9.4) Prepolymer M (50%) 1 M (50%) 1 M (50%) 1 M (46%) 1 M (46%) 1 M (46%) 1 M (46%) 1 M (46%) 1 M (46%) 1 M (46%) 1 M (46%) 1 M (46%) 1 M (70%) 1 M (70%) 1 M (70%) 1 M (46%) 1 M (46%) 1 M (46%) 1 M (46%) 1 M (46%) 1 M (46%) 1 M (46%) 1 M (46%) 1 M (46%) 1 M (46%) 1 M (46%) 1 M (46%) 1 M1 I M1 I M1 I

I I I I I I I I I I I I I I I I I I I I I I I I I I I

(50%) (50%) (50%) (54%) (54%) (54%) (54%) (54%) (54%) (54%) (54%) (54%) (30%) (30%) (30%) (54%) (54%) (54%) (54%) (54%) (54%) (54%) (54%) (54%) (54%) (54%) (54%)

Diol

Styrene (%)

Catalyst

PG PG PG PG PG PG PG PG PG PG PG PG PG PG PG NPG NPG NPG DEG DEG DEG DPG DPG DPG PG PG PG PG PG PG

38 38 38 44 44 44 42 42 42 40 40 40 45 45 45 45 45 45 38 38 38 42 42 42 43 43 43 45 45 45

MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP

(1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1

Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct

(0.2%) (0.2%) (0.2%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%)

T ( C)

HR (%)

WH2O (%)

25 25 25 30 40 50 30 40 50 30 40 50 30 40 50 30 40 50 30 40 50 30 40 50 30 40 50 30 50 70

100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 95 95 95

0.65 0.56 0.85 0.72 0.89 0.91 0.84 1.02 1 1.12 1.36 1.44 1.12 1.36 1.44 0.47 0.56 0.75 1.06 1.29 1.58 0.74 0.92 1.14 0.97 0.92 1.16 0.85 0.83 1.08

[27] [27] [30] [31] [31] [31] [31] [31] [31] [31] [31] [31] [31] [31] [31] [31] [31] [31] [31] [31] [31] [31] [31] [31] [31] [31] [31] [32] [32] [32]

M1 M1 M1 M1 M1 M M M M M M

I I I I I

PG PG 1 PG 1 PG 1 PG 1 PG PG PG NPG NPG NPG

NPG NPG NPG NPAG

45 38 38 38 38 40 40 40 40 40 40

MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP

(1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1

Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct Co Oct

(0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%)

90 30 50 70 90 30 50 70 30 50 70

M, maleate; I, isophthalate; PG, propylene glycol; NPG, neopentyl glycol; DPG, dipropylene glycol; DEG, diethylene glycol.

95 95 95 95 95 95 95 95 95 95 95

1.25 0.94 1.08 1.1 1.32 3 3.1 2.8 1.7 1.7 1.8

[32] [32] [32] [32] [32] [32] [32] [32] [32] [32] [32]

Table 9.3 Effect of Postcuring on Shape of Sorption Isotherm (Fig. 9.4) and Water Solubility in Unsaturated Polyester Resin (UPR) [28] Prepolymer

Catalyst

T ( C)

HR (%)

Isotherm

WH2O (% w/w)

O (60%) 1 O (60%) 1 O (60%) 1 O (60%) 1 I I I I I O (60%) 1 O (60%) 1 O O I I I I I

MEKP (1%) 1 CN (0.3%) 1 DMA (0.05%) AC MEKP (1%) 1 CN (0.3%) 1 DMA (0.05%) PC BPO (1%) 1 DMA (0.3%) AC BPO (1%) 1 DMA (0.3%) PC BPO (2%) 1 DMA (0.3%) AC BPO (2%) 1 DMA (0.3%) PC MEKP (0.5%) 1 CN (0.3%) 1 DMA (0.05%) AC MEKP (0.5%) 1 CN (0.3%) 1 DMA (0.05%) PC MEKP (4%) 1 CN (0.3%) 1 DMA (0.1%) AC MEKP (1%) 1 CN (0.3%) 1 DMA (0.05%) AC MEKP (1%) 1 CN (0.3%) 1 DMA (0.05%) PC BPO (1%) 1 DMA (0.3%) AC BPO (1%) 1 DMA (0.3%) PC MEKP (4%) 1 CN (0.3%) 1 DMA (0.1%) PC MEKP (0.5) 1 CN (0.3%) 1 DMA (0.05%) PC MEKP (0.5) 1 CN (0.3%) 1 DMA (0.05%) AC BPO (2%) 1 DMA (0.3%) AC BPO (2%) 1 DMA (0.3%) PC

30 30 30 30 30 30 30 30 30 65 65 65 65 65 65 65 65 65

75 75 75 75 75 75 75 75 75 75 75 75 75 75 75 75 75 75

➀ ➀ ➂ ➃ ➀ ➀ ➀ ➀ ➀ ➁ ➁ ➃ ➃ ➀ ➀ ➃ ➃ ➃

0.8 0.8 0.8 0.7 0.6 0.5 0.4 0.5 0.55 1 1.3 0.6 1 1 1 0.75 0.7 0.7

M (40%) M (40%) M (40%) M (40%)

M (40%) M (40%)

MEKP, methyl ethyl ketone peroxide; CN, cobalt naphthanate; DMA, N,N-dimethylaniline; BPO, benzoyl peroxide; AC, as cast; PC, post cured.

9.3 MECHANISMS OF PHYSICAL AGING BY SOLVENT INGRESS

211

FIGURE 9.5 Hydrophilicity of prepolymer as a function of diacid or diol.

According to Table 9.2, increasing the styrene (S) content induces a decrease in maximal water uptake consistently with the fact that S units are almost apolar and unable to interact with water molecules. It was even proposed that: wmax 5

wmax prepol ð 1 2 sÞ

(9.17)

Where s is the styrene weight fraction. It is, however, difficult to correlate the polyester hydrophilicity with prepolymer one: it was for example observed that prepolymers are almost 3-times more hydrophilic than corresponding polymerized network [34]. Two reasons are proposed: first, copolymerization with styrene induces a certain hindrance of the ester groups decreasing their affinity to water, and secondly, there is no obvious link between water affinity of prepolymer being at rubbery state and networks being usually in glassy state. According to Table 9.3, there is a small but positive effect of postcuring on the affinity of polyesters to water (i.e., water uptake increases with post curing). Two reasons might be envisaged: •

•

post curing generates groups with a higher affinity to water (e.g., it can be imagined that the ester groups held by maleate are less hydrophilic than the succinate groups obtained after polymerization of the double bonds). the disappearance of unreacted groups (e.g., styrene) is higher in incompletely cured UPRs, which lowers the maximal level of mass uptake and permits a shift from strong type ➃ behavior (Fig. 9.4) to more moderate behavior, that is, type ➃ curves with a lower rate of mass depletion or even type ➂ curves. From Eq. (9.13), the effect of temperature is expected to obey Arrhenius law [35]:


2E CS 5 C0 3 exp RT



(9.18)

where E 5 ES 1 EP, ES and EP being the activation energies, respectively, for solubility coefficient and for water vapor pressure. The fact that the equilibrium concentration increases with temperature for any given kind of UP network [31,32] means that ES 1 EP . 0, that is, ES . 243 kJ/mol [35].

212

CHAPTER 9 AGING BEHAVIOR AND MODELING STUDIES

FIGURE 9.6 Sorption isotherm of water in Unsaturated Polyester Resin (UPR) together with the estimation of mean cluster size [30].

Henry’s law means that the sorption isotherm displays a linear shape. However, experimental data sometimes display a positive curvature (Fig. 9.6). If the latter cannot be fitted by Eq. (9.10) with a fixed χ value (or rather that χ decreases with water affinity), it means that water first dissolves randomly in the polymer matrix (at low water activity) and then forms clusters, that is, aggregates of dimers or trimers or more water molecules (at higher water activity). The presence of clusters can be evidenced either: • •

experimentally using dielectric spectroscopy [36] or FTIR [37], or theoretically from Zimm
Lundberg analysis allowing for the assessment of the MCS (mean cluster size) [38,39] from the clustering function: fZL 5

    G11 @a1 =φ1 5 2 1 2 φ1 3 21 ν1 @a1 T;P

(9.19)

where G11 is a cluster integral for water and ν 1 and ϕ1 are the partial molecular volume and volume fraction of water, respectively. When fZL is below 21 value, no clustering occurs. MCS is thus given by: MCS 5 1 1 φ1 3


 G11 ν1

(9.20)

MCS can also be estimated from χ values used for simulating the sorption isotherm: MCS 5

1

  @χ 1 2 2χφ1 1 1 2 φ1 : @lnφ 

1

(9.21) T;P

Fillers usually reduce the permeation of water (and many other chemicals) [40]. It seems, however, that there is an optimum effect. In the case of highly filled UP composites, the water uptake is even shown to clearly increase with filler content [41]. This can be explained by the

9.3 MECHANISMS OF PHYSICAL AGING BY SOLVENT INGRESS

213

competition of two phenomena. First, fillers increase tortuosity [42] and maybe decrease the hydrophilicity of matrices, second, however, their presence favors the existence of an interface where water is preferentially absorbed and diffuses faster [15]. Natural fibers promote strong water absorption [43]. The case of interpenetrating polymer networks (IPNs) was illustrated in studies dealing with vinyl ester networks (BisGMA cured with styrene) mixed with epoxy systems. Interestingly, it seems that the water uptake for 50:50 networks made of epoxy/diamine and polyester is closer to values observed for epoxy/diamine. It is hence suggested that blending only affects the preexponential factor (see Eq. 9.18) rather than the activation energy [44].

9.3.2.2 Diffusivity Several models describe the rate of water diffusion in polymers: 1. Case I diffusion is observed when the characteristic time for diffusion is shorter than the typical time of polymer motions. In this case, the rate of water uptake, M(t)/MN, increases linearly with time. 2. Fick’s law is observed when the characteristic time for diffusion is higher than the typical time of polymer motions. In this case, the rate of water uptake, M(t)/MN, increases linearly with the square root of the time. Another key characteristic of Fickian diffusion is the existence of a plateau at a long exposure time (contrarily to Langmuir diffusion). Relative mass uptake can be simulated from the analytical resolution of Fick’s law [45]:
 X 
 N
M ðtÞ 8 1 D 3 ð2n11Þ2 3 π2 3 t 3 exp 2 512 2 3 2 MN π 4e2 n50 ð2n11Þ

(9.22)

If the relative mass uptake linearly increases with the square root of time, the diffusion obeys Fick’s law and D can be calculated from the slope. This equation actually admits an approximate solution for low levels of water sorption (M/MN ,0.6): MðtÞ 4 5 3 MN e

rffiffiffiffiffiffiffiffiffiffiffi D3t π

(9.23)

3. The “Langmuir” law is observed when the water diffusing in a given polymer is either “free” or “bound,” that is, if it displays some “strong” interactions with some sites of the polymer structure (e.g., water-unreacted oxiranes in the case of epoxy/diamine matrices) [46]. One of its main experimental features is the existence of a double sorption plateau. There is, however, no evidence of “Langmuir” diffusion in polyesters. Experimentally, the linearity of mass uptake versus square root of the time is sometimes an approximation. However, most authors have assumed the diffusion obeys Fick’s law so as to assess the diffusivity values given in Table 9.4. Despite the experimental scattering [31], it seems that D obeys Arrhenius law:


2 ED D 5 D0 3 exp RT



For the materials given in Table 9.3, ED takes a value close to 25
30 kJ/mol.

(9.24)

214

CHAPTER 9 AGING BEHAVIOR AND MODELING STUDIES

Table 9.4 Experimental Values of Water Diffusivities in Unsaturated Polyester Resin (UPR) [31] Prepolymer M M M M M M M M M M M M M M M M M M M M M M M M

(46%) 1 (46%) 1 (46%) 1 (46%) 1 (46%) 1 (46%) 1 (46%) 1 (46%) 1 (46%) 1 (70%) 1 (70%) 1 (70%) 1 (46%) 1 (46%) 1 (46%) 1 (46%) 1 (46%) 1 (46%) 1 (46%) 1 (46%) 1 (46%) 1 (46%) 1 (46%) 1 (46%) 1

I I I I I I I I I I I I I I I I I I I I I I I I

(54%) (54%) (54%) (54%) (54%) (54%) (54%) (54%) (54%) (30%) (30%) (30%) (54%) (54%) (54%) (54%) (54%) (54%) (54%) (54%) (54%) (54%) (54%) (54%)

Diol

Styrene (%)

Catalyst

PG PG PG PG PG PG PG PG PG PG PG PG NPG NPG NPG DEG DEG DEG DPG DPG DPG PG PG PG

44 44 44 42 42 42 40 40 40 45 45 45 45 45 45 38 38 38 42 42 42 43 43 43

MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP MEKP

(1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1 (1.5%) 1

Co Co Co Co Co Co Co Co Co Co Co Co Co Co Co Co Co Co Co Co Co Co Co Co

Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct Oct

(0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%) (0.5%)

T ( C)

D (mm2/s1)

30 40 50 30 40 50 30 40 50 30 40 50 30 40 50 30 40 50 30 40 50 30 40 50

6.9 3 1029 12 3 1029 (14
19) 3 1029 5.6 3 1029 8.6 3 1029 10 3 1029 2.1 3 1029 2.7 3 1029 3.8 3 1029 10 3 1029 13 3 1029 22 3 1029 10 3 1029 13 3 1029 18 3 1029 6.8 3 1029 11 3 10211 21.5 3 1029 11.7 3 1029 16.5 3 1029 23 3 1029 12.5 3 1029 16 3 1029 24.5 3 1029

Fick’s law is based on the assumption that diffusivity does not change with the penetrant concentration. However, a concentration-dependent diffusivity was observed [30] as depicted in Fig. 9.7 showing unambiguously the existence of a non-Fickian process; since the plot of relative mass uptake does not intersect a 0 for t 5 0, and the water diffusivity depends on its concentration which was mathematically modeled as: D 5 D0 3 expðγ 3 cÞ

(9.25)

where γ is a plastification coefficient describing the fact that the sorption of the penetrant increases free volume in the matrix which facilitates its diffusion. A complete investigation by Marais et al. [30] shows that D is multiplied by about 3 when water activity increases from 0.2 to 1 (Fig. 9.7). In conclusion, water diffusion in UPR seems to be Fickian at first, but the finest investigations (featuring tests under several water activities) suggest a more complex mechanism for water diffusion. This remains to be linked with the structure of UPR.

9.4 MECHANISMS OF CHEMICAL DEGRADATION

M(t)/M∞

215

a=1 D = 1.42 × 10–8 cm2/s a = 0.2 D = 0.39 × 10–8 cm2/s

t1/2

FIGURE 9.7 Plot of relative mass versus square root of the time for UP at 25 C.

Table 9.5 Several Aging Modes and Their Consequences at Macromolecular Level Hydrolysis Chain scission Cross-linking

1 0

Thermal Oxidation

Radio Oxidation

Photolysis and Photooxidation

1 Post polymerization

1 1 Post polymerization, irradiation under vacuum or of thick materials

9.4 MECHANISMS OF CHEMICAL DEGRADATION 9.4.1 HYDROLYSIS As previously discussed, it was observed that: •

•

the interaction of UPRs with water leads first to a weight increase corresponding to water absorption, but later a mass decrease is observed corresponding to the leaching of small molecules even in fully cured matrices [17]. UPRs display a strong drop in their mechanical properties in the presence of gaseous ethanol at 90 C or 115 C [8], whereas ethanol is only poorly soluble in UP.

The main reason is the existence of chemical damage (chain scission reactions—Table 9.5) overlapping with the physical damage (plastification of the network by the absorbed fluid). The rate of chain scissions formation is, in principle, given by: ds 5 k 3 ðE0 2 sÞ 3 w dt

(9.26)

where k is the rate constant of hydrolysis, E0 the initial concentration of hydrolysable groups (esters), w the water concentration, and s the concentration of chain scissions.

216

CHAPTER 9 AGING BEHAVIOR AND MODELING STUDIES

Eq. (9.25) can be integrated as: s 5 E0 3 ½1 2 expð 2 k 3 w 3 tÞ

(9.27)

sBk0 3 E0 3 w 3 t

(9.28)

And simplified as: Hence, it seems possible, in principle, to predict the rate of degradation from relatively simple data: water affinity, concentration of esters (depending on the structure of prepolymer and styrene content), and rate constant of prepolymer hydrolysis. This simplified approach only gives a rough estimation of degradation since there are many complications including either chemical: (1) copolymer effect, (2) autocatalytic effects, (3) chain end effects, or physical factors: (4) changes in hydrophilicity with aging conversion degree, (5) effects linked to water diffusion. 1. The copolymer effect: the hydrolysis rate constant in UP made of phtalate and maleate/fumarate is observed to differ from values observed in corresponding pure phthalate or maleate/fumarate UPs due to ester
ester interactions or hindrance by styrene [34]. 2. Autocatalytic effects: Hydrolysis generates a carboxylic acid which can act as a catalyst of further hydrolysis events. The differential system becomes [47]: ds 5 k0 3 ðE0 2 sÞ 3 w 1 kcat 3 ðE0 2 sÞ 3 s 3 w dt

(9.29)

Some examples of the autocatalytic hydrolysis of polyester-based polymers can be found in Refs. [46,48], but the case of UPR has not been investigated. 3. The chain ends are possibly more reactive than constitutive units located in the middle of chains [14] (Scheme 9.1). CH3

O

CH2 CH O C

CH3

+ H2O

O

CH2 CH OH

C O

C HO C O HO

H O

SCHEME 9.1 Self-decomposition of chain ends in UPR.

4. Changes in water affinity induced by hydrolysis: esters (being moderately polar) are converted into much more hydrophilic species (carboxylic acid, alcohol). It is, hence, possible that water equilibrium uptake is expressed as [14]: w 5 w0 1 a 3 s

(9.30)

where s expresses the number of hydrolysis events. The systematic study of hydrolysis (or even aging) induced changes in water affinity remains, however, scarcely studied. 5. Water diffusion effects: water is submitted to competition between the reactions of hydrolysis and diffusion from the surface layers to deeper layers. This results in the existence of a degraded layer, the thickness of which can be approximated by [14]: z2 5

Dw k½w

(9.31)

9.4 MECHANISMS OF CHEMICAL DEGRADATION

217

Table 9.6 Values of Thickness of Degraded Layers for Various Aging Conditions Aging Mode

Experimental Conditions

Hydrolysis

100 C 2 100%HR

Kinetic Parameters Dw 5 (6 6 1) 3 10211 m2/s k0 5 (4
30) 3 1027 s21

Photoaging Thermal aging



60 C
SEPAP 12
24 160 C

EDw 5 25
30 kJ/mol (Table 9.4) EH 5 65 kJ/mol [47] EDO2 5 20
45 kJ/mol (Table 9.7) EOX 5 50 kJ/mol [58]

zdegraded (µm) 450
1200

140 600

Since water solubility hardly depends on temperature, the thickness of this degraded layer depends on temperature: Ez 5 1/2ðED
Ek Þ

(9.32)

It is well documented that ED , Ek so the thickness of the degraded layer measured at elevated temperature (during an accelerated aging test) is quite different from the value observed in “service” conditions. Such diffusion-reaction coupling can be predicted using numerical models [49,50]. Some experimental values (for hydrolytic aging together with other aging modes) are given in Table 9.6. In terms of structure
properties relationships, existing data show that maleates and fumarates are more unstable than isophthalates and orthophthalates because of higher hydrolysis rate constants and higher hydrophilicity. Generally, isophthalates induce a higher stability than orthophthalates [51,52]. Neopentyl glycol induces a higher stability than propylene glycol [52,53] and ethylene glycol
based polyesters are also quite unstable [34]. As hydrolysis at moderate temperatures (typically below UPR
glass transition) is an aging mode leading to polymer failure, there is an increasing interest in using hydrolysis as a recycling mode for UPR networks by regenerating feedstock (in particular the diol). Subcritical water (with the use of strong alkali: NaOH or KOH) can potentially allow for the recovery of a great part of glycol after a treatment at 230 C for 1 hour [54]. There are, however, some complexities linked to high temperature hydrolysis in the presence of additives (KOH, phenol) in UPR where secondary reactions (typically above 200 C) can generate a wide variety of chemicals [55].

9.4.2 THERMAL AGING Thermal degradation is a radical in chain mechanism. At relatively high temperatures ( . 200 C), the thermolysis of the polymer backbone is by far the main source of radicals, whereas at “moderate” temperatures (,150 C, i.e., in UPR service conditions), the thermal decomposition of hydroperoxides becomes kinetically prominent. In the case of thermal degradation at high temperatures, a kinetic analysis of nonisothermal degradation curves suggested that degradation first occurs on styrene groups [56] in good agreement with the analysis of volatiles [57]. The case of thermal aging of UPRs at moderate temperatures (at which the main source of radical generation is hydroperoxides) is in part different. In the case of thermal aging of polyester

218

CHAPTER 9 AGING BEHAVIOR AND MODELING STUDIES

monitored by FTIR, Arrieta et al. [58] observed that the main chemical change was the formation of an anhydride (Fig. 9.8). Their interpretation was that anhydrides are formed by the oxidation of methylene in the α-position of the ester group. They confirmed their reasoning from analysis of a thermally oxidized prepolymer (where anhydrides were also observed) and from the absence of acetophenone groups characteristic of polystyrene oxidation [59]. The most plausible mechanism is based on the radical attack of C
H in the α-position of the ester groups. The main consequences of this mechanism are the formation of anhydride and some chain scissions (Scheme 9.2).

FIGURE 9.8 FTIR spectra of a UPR (A) and its prepolymer (B) thermally oxidized at 160 C [58]. Reprinted with the permission of Elsevier.

O C O CH CH2

O

CH3

O

CH3

OH

C O C

OOH

C O C

HO CH2 O C FTIR: 3465 cm–1

1. X° 2. O2 3. P-H O

O

O

C O C CH3

CH2

O

O° °OH

C O C CH3

CH2

CH2

FTIR: 3465 cm–1

CH3 O

O

C O C CH2 O

O

C O° H3C C CH2 O C OH FTIR: 1690 and 3300 cm–1

SCHEME 9.2 Mechanism of radical oxidation of prepolymer in unsaturated polyester resin (UPR).

9.4 MECHANISMS OF CHEMICAL DEGRADATION

219

Another consequence of thermal aging at high temperatures is the high yield of volatiles resulting from advanced chain scission processes (Table 9.4). According to Arrieta et al. [58], they can occur even under an inert atmosphere (i.e., from the direct thermolysis of the polymer), but are strongly accelerated by oxygen (Fig. 9.9). Shih et al. [57] studied the high temperature thermal degradation of epoxy
polyester IPNs. According to their results: •

•

Modulated thermogravimetric analysis (TGA) curves can permit the extraction of kinetic parameters of apparent kinetic models for nonisothermal degradation. It seems that epoxy allows for the thermal stability of UPR and IPNs to be improved, which later display kinetic parameters in between those of UPR and epoxies. The authors, however, stated that the degradation of UPR and epoxy in IPNs do not interfere. Pyrolysis tests followed by GC/MS for thermal degradation at relatively high temperature analysis reveal that almost the same degradation products are evolved from IPNs and epoxies (mainly bisphenol-A, isopropenyl phenol, and phenol), whereas the products characterizing the thermal degradation of polyesters (in particular styrene monomers, dimers, and even trimers) are absent. Those two results can be considered as somewhat difficult to reconcile. The durability of IPNs thus appears to be an open issue.

Identically to hydrolytic aging, a degraded layer is generated [60] due to a diffusion-reaction coupling as schematized in Fig. 9.10.

FIGURE 9.9 Kinetic curves for mass loss for a UP network thermally aged at 160 C [58]. Reprinted with the permission of Elsevier.

220

CHAPTER 9 AGING BEHAVIOR AND MODELING STUDIES

FIGURE 9.10 Degradation profile of oxidized materials (NB: x 5 0 and x 5 L correspond to edges in contact with atmospheric air).

Its thickness can be approximated by [61,62]: TOL2 5

DO2 3 ½O2  rOX

(9.33)

where rOX is the oxidation rate on the surface of polymer, DO2 and [O2] are the oxygen diffusivity and concentration in a given polymer matrix, respectively. It is hence crucial to determine the oxygen transport properties in UP matrices. Some values as reported by Pauly [63] are presented in Table 9.7.

9.4.3 RADIOLYTIC AGING The effect of irradiation on UPs was described by Wilski [64] in the case of high-dose irradiation under an inert atmosphere, and lower dose irradiation under air. The results unambiguously show competition between two phenomena. The first one occurs in the absence of oxygen and results in an increase (at least in the earliest exposure doses, i.e., lower than 1 MGy) of thermomechanical properties (deflection temperature being linked to Tg, flexural and impact strengths). The second one is favored by the presence of oxygen and induces the depletion of the mentioned properties (Fig. 9.11). The most reasonable explanation is common with other polymers and is based on the existence of a degraded layer as developed in Section 9.4.2. ➀

Under nitrogen (or in the bulk of thick materials where aging mechanisms are anaerobic), irradiation generates radicals reacting by coupling (cross-linking): Polymer 1 hν-P 1 1=2H2 P 1 P0 H-PH 1 P0 P 1 P -PH 1 double bond ðdismutationÞ P 1 P -P 2 P ðcross-linkingÞ

This supplementary radiation induced cross-linking improves the mechanical properties as observed by Charlesby et al. [65] in the case of the radio-curing of UPR.

Table 9.7 Oxygen Permeation Parameters for Unsaturated Polyester Resin (UPR) Main Components (%) UP resin Limestone Short GF UP resin Limestone China clay Up resin

25 52.7 15 26 26 38.3 100

Up resin ATH 1 Zn Borate

32 68

Sample Thickness (cm) 0.2

0.18

0.02 0.02

T ( C)

1015 3 P

108 3 D

ED (kJ/mol)

107.S

ES (kJ mol21)

Refs.

23 40 60 23 40 60 25 100 25 100

1.04 2.24 5.21 3.08 7 17 0.66 2.34 0.29 2.36

0.57 1.26 2.77 1.03 2.8 8 3.6 19.5 2.3 13.3

35

1.82 1.78 1.88 3 2.5 2.13 0.18 0.12 0.13 0.18

0

[63]

21

[63]



[58]



[58]

45

20.8 21.6

Polyester resins were made from terephthalic acid, fumaric acid, and butanediol 1,4. D is expressed in cm2 /s, S in cm23/Pa, P 5 D 3 SNB: For data from Ref. [58], it was assumed that EP 5 ED and ES 5 0.

222

CHAPTER 9 AGING BEHAVIOR AND MODELING STUDIES

Properties High dose rate (absence of air) Low dose rate (presence of air)

Dose

FIGURE 9.11 Properties of unsaturated polyester during irradiation [64].

➁ In the presence of oxygen (or in the oxidized layer of bulk materials), alkyl radicals react with oxygen and lead to peroxyl radicals and later hydroperoxides; the decomposition of which generating several products among which are chain scissions: P 1 O2 -POO POO 1 PH-POOH 1 P POOH-PO 1 HO PO -P 5 O 1 chain scissions

In the case of the irradiation of thick UPR materials, both phenomena can simultaneously occur; their relative proportion depending on the depth in the irradiated polymer. Another reason for the heterogeneity in irradiated polymers is the attenuation of irradiation in the thickness: I 5 expð2 μ 3 zÞ I0

(9.34)

where I0 is the initial intensity of γ-ray, I is the intensity of the ray transmitted from a shielding material of thickness x, and μ is the linear attenuation coefficient. It was observed that the use of lead monoxide particles (with a weight fraction of about 5%) made possible the attenuation of a coefficient comparable to concrete for Cs-137 or Ir-192 sources, but that even highly filled (more than 30%) UPRs could not efficiently shield Co-60 radiations [66].

9.4.4 PHOTOAGING Polyesters are reported to undergo photolytic processes such as the conversion of fumarates into maleates, the dimerization and decomposition of maleates (Scheme 9.3) [67], and the photocleavage of esters (Scheme 9.4) [68]: Photoaging also induces an in-chain radical oxidation mechanism, but in which radicals mainly come from some side reactions of radicals (such as those created in Scheme 9.4) and the decomposition of peroxides and hydroperoxides. The relative rate of photolytic and photooxidation processes depends on the temperature, nature, and intensity of the spectral source, on the molar absorptivity of compounds (and subsequently the

9.4 MECHANISMS OF CHEMICAL DEGRADATION

223

O O

hν

O O

T

O

O

O

O FTIR: 1646 cm–1

FTIR: 1641 cm–1

O O O

O O

O

O O

O

O

O O

O

O

FTIR: 1735 and 1788 cm–1

FTIR: 1754 cm–1

SCHEME 9.3 Mechanisms of photoisomerization and photodimerization of fumarate units.

O

O

O

C O CH CH2

C O°

C OH

FTIR: 1700-1680 cm–1

CH3

O

O

O

HC CH C O CH CH2

HC CH C O°

HC CH C OH

FTIR: 1715-1680 cm–1

CH3

SCHEME 9.4 Mechanism of photocleavage of polyesters.

thickness of material) [69], and on the presence of oxygen. For example, according to [70], phthalates and fumarate/maleate groups are responsible for absorption at long wavelength ( . 300 nm), whereas the photooxidation at short wavelength might involve S units as well [71]. In summary: •

radicals are created either from the photolysis of an impurity or of hydroperoxides and ketones: X-P POOH-PO 1 HO -2P 1 carbonyls

•

they propagate to regenerate hydroperoxides: P 1 O2 -POO POO 1 PH-POOH 1 P

224

•

CHAPTER 9 AGING BEHAVIOR AND MODELING STUDIES

several termination mechanisms can coexist depending on the concentration of oxygen [72]: P 1 P -inactive products P 1 POO -inactive products POO 1 POO -inactive products

In UPRs, several reactive sites are suspected to be involved. The appearance of yellowing products absorbing at 295
400 nm (similarly to polystyrene) led Michaille et al. [67] to propose a photooxidation mechanism in which styrene units are the main oxidation site (Scheme 9.5). O

O

O C HC CH C O H C

O

O

O C HC CH C O HOO C CH2

CH2

O

O

O C HC CH C O O

O

O

C

FTIR: 1690 cm–1 UV: 300-450 nm

CH2°

O C HC CH C O HO° °O C CH2

O

O

O C HC CH C O HO C CH2

SCHEME 9.5 Mechanism of radical oxidation of styrene in unsaturated polyester resin (UPR).

Polyester prepolymer can also undergo radical attack depending on the concentration of styrene as a cross-linking agent [67]. In other words, the tertiary C
H group of S units would be more oxidizable than poly(propylene glycol maleate) or poly(propylene glycol isophthalate), but at a low styrene concentration, these later become the predominant sites of propagation. A part of this “duality” can be explained from the relative kinetics of propagation: P1 OO 1 P1 H-P1 OOH 1 P1  P1 OO 1 P2 H-P1 OOH 1 P2  P2 OO 1 P1 H-P2 OOH 1 P1  P2 OO 1 P2 H-P2 OOH 1 P2 

r11 r12 r21 r22

Here 1 would correspond to styrene units and 2 to prepolymer units. The relative rates of propagation would be: r11 k11 ½P1 H 5 r12 k12 ½P2 H

(9.35)

9.4 MECHANISMS OF CHEMICAL DEGRADATION

225

There are some relationships linking structure and sensitivity to oxidation according to which [73]: logkij ð30 CÞ 5 16:2
0:0477 3 BDEj   Eij 5 0:55 3 BDEj
262

(9.36) (9.37)

if i is a secondary peroxyl radical and j the broken C
H. While, logkij ð30 CÞ 5 15:2
0:0477 3 BDEj   Eij 5 0:55 3 BDEj
262

(9.38) (9.39)

if i is a tertiary peroxyl radical. One can easily verify that: • •

the competition between each propagation reaction depends on the concentration of the reactive sites (propylene glycol or styrene), and using reasonable values of BDE (375 for S C
H, 395 kJ/mol for propylene glycol ones), propagation rate constants can cross, that is, that k312 . k321 at moderate temperatures and k321 . k312 at higher temperatures.

In other words, it can be concluded that the oxidative degradation of UPR must be described by a cooxidation model [74], which requires a complex stage of kinetic parameters estimation. Those equations linking reactivity (the rate constant) with structure (the bond dissociation energy) also permit for the confirmation of why neopentyl glycol permits a decrease in the oxidation effects compared to propylene glycol [70] because of the absence of tertiary (weak) C
H bonds. One of the most undesired consequences of photoaging and photooxidation is the yellowing of the material. Several processes might be responsible including: 1. Photolytic processes (i.e., occurring without oxygen) can induce a strong change in the aspect of UP materials, featuring gloss, and color changes. Michaille et al. [70] pointed out that the photolysis of phthalate is identical to that observed in the photolysis of polybutylene terephthalate (PBT) [75] (Scheme 9.6). O C O° O C O CH2

° H2C

O °

° C O CH2 O C ° °O CH2

SCHEME 9.6 Mechanism of photodissociation.

226

CHAPTER 9 AGING BEHAVIOR AND MODELING STUDIES

Then radicals recombine to give several kinds of benzophenones responsible for yellowing: O

O

C O

C

The work of Geuskens also revealed the role of polyenes formed during the photolysis of polystyrene absorbing at 280
450 nm [71]: CH2

C CH

CH

n