Epoxidised Natural Rubber: Properties & Applications 9811988358, 9789811988356

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Siti Salina Sarkawi Fatimah Rubaizah Mohd Rasdi Veronica Charlotte   Editors

Epoxidised Natural Rubber Properties & Applications

Epoxidised Natural Rubber

Siti Salina Sarkawi · Fatimah Rubaizah Mohd Rasdi · Veronica Charlotte Editors

Epoxidised Natural Rubber Properties & Applications

Editors Siti Salina Sarkawi Malaysian Rubber Board Sungai Buloh, Selangor, Malaysia

Fatimah Rubaizah Mohd Rasdi Malaysian Rubber Board Sungai Buloh, Selangor, Malaysia

Veronica Charlotte Malaysian Rubber Board Kuala Lumpur, Malaysia

ISBN 978-981-19-8835-6 ISBN 978-981-19-8836-3 (eBook) https://doi.org/10.1007/978-981-19-8836-3 © Malaysian Rubber Board 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Foreword

The Malaysian Rubber Board (MRB) was established in 1998 through a merger among Rubber Research Institute of Malaysia (RRIM), Malaysian Rubber Research and Development Board (MRRDB) and Malaysian Rubber Exchange and Licensing Board (MRELB). As a government statutory body, MRB primarily assists with development and modernisation of the Malaysian rubber industry. This involves all aspects from cultivation of rubber trees, to the extraction and processing of raw rubber, followed by the manufacture and marketing of both rubber and rubber products. MRB has carried out extensive research and development on epoxidised natural rubber (ENR) since the 1980s. The initial commercial production of ENR was undertaken by Kumpulan Guthrie in 1990, but the technology was subsequently returned to MRB for further commercialisation and development. As a result of this, ENR was successively trademarked as Ekoprena® with an improved processing technology developed by MRB. This new technology was transferred to Felda Rubber Industries (FRISB) in 2010, currently known as Felda Global Ventures (FGV) Rubber, which produced two commercial grades, Ekoprena 25® and Ekoprena 50® . The production of Ekoprena was enlisted as a strategic project under the National Key Economic Area (2010–2020), aimed at enhancing Malaysia’s economic growth. Through this national-level initiative, MRB conducted a series of research collaborations to evaluate the performance of Ekoprena® in a range of rubber products, specifically tyres, engineering and industrial rubber goods as well as latex-based products. Among the collaborators involved as part of this research were Prasarana Malaysia Berhad, Felda Global Ventures (FGV), Felda Transport Services Sdn Bhd, Nadicorp Holdings Sdn Bhd, PROTON, Kulitkraft, Jebco, Doshin Rubber, FT Hose and Hose, Hebe Rubber, MyKPK, M-Xell Chemicals and Balai Seni Negara. Outcomes of these collaborations include important contributions towards the assessment of market potential for Ekoprena® in an effort to increase its market uptake in the rubber products industry. The uptake of Ekoprena® is expected to have an impact on increasing natural rubber production and subsequently improving the income of Malaysian rubber smallholders. Resultantly, an increase in both local consumption and contribution of the rubber industry to national exports is projected.

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This book encompasses a compilation of the most recent developments and data accumulated during the project implementation. It provides a better understanding on the properties of ENR or its commercial trademark Ekoprena® , leading to a range of unique applications in various products such as paint and sound dampening products. Authored by experts in their respective fields, the chapters of this book comprise an introduction along with a technical overview of Ekoprena® , delineating its properties and applications as a readable source material for Ekoprena® and ENR users across the board. Dato’ Dr. Zairossani Mohd Nor Director General Malaysian Rubber Board Kuala Lumpur, Malaysia

Preface

Modified natural rubbers, obtained via either physical or chemical routes, have been exhaustively investigated to allow for optimisation of its properties. Epoxidised natural rubber (ENR) is by far a successful development in the domain of natural rubber modification. Malaysian Rubber Board (MRB) has improved the ENR processing and production technology, trademarked as Ekoprena® . While the epoxidation level of ENR can be customised according to specific requirements, two commercially available grades are the ENR 25 and ENR 50, consisting 25% and 50% epoxidation levels, respectively. The distinctive properties of ENR at various epoxidation levels allow for a wide range of diversity in its applications. The ENR 25 exhibits enhanced dampening and hysteresis, resulting in wet grip property improvement appropriate for tyre tread compounds. The ENR 50 further reveals good oil resistance and air impermeability, paving potential applications in oil-resistant hoses, seals, connectors and tubes as well as bladders, inner tubes and inner liners. Moreover, the ENR material has demonstrated promising prospects in latex-based products including paint, adhesives and foam. A collation of research findings and remarkable developments vis-à-vis ENR and its applications executed by MRB in recent years is documented in this book. It represents the very first all-encompassing publication on ENR, ranging from its chemistry to processing technology, rheology of raw ENR, compounding and properties of ENR, as well as ENR applications in tyres, technical rubber goods, footwear and latex-related products, among others. Highlighted in this book are unexplored novel and exclusive applications of ENR, in addition to new products manufactured with ENR. Into the bargain, the perspective of environmental sustainability and life cycle analysis of ENR processing as well as economic and market trends of this specialty rubber are emphasised in the concluding chapter. The target readership and potential audience anticipated for this book are consumers of ENR or Ekoprena® in the manufacture of respective products as well as investors who are enthusiastic about producing ENR-based products. The practicality of the outlined contents is intended for and applicable to scientists, engineers,

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product designers, technicians and students dealing with this specialty rubber and its diverse applications. Siti Salina Sarkawi Malaysian Rubber Board Sungai Buloh, Selangor, Malaysia Fatimah Rubaizah Mohd Rasdi Malaysian Rubber Board Sungai Buloh, Selangor, Malaysia Veronica Charlotte Malaysian Rubber Board Kuala Lumpur, Malaysia

Acknowledgement

The editors wish to thank all contributing authors, who have made this book project a reality. The authors are MRB research officers who were directly involved in the research and development work on Ekoprena® under the NKEA programme on Commercialising Ekoprena® and Pureprena® . The invaluable cooperation from subject matter experts within MRB in reviewing each chapter is gratefully acknowledged. We also would like to accord special thanks to officers from the Publication and Library Unit of MRB, namely Muhammad Ghazally Rosli, Nadhyrah Rosenita Nordin, Mohd Norafendy Mohd Nordin, Azlina Pawan and Mazwanie Mahdzir, for their excellent assistance in editing and proofreading the chapters. We appreciate the unwavering assistance from Christoph Baumann since the initial idea of this book was discussed and throughout the publishing journey. The authors would also like to thank the Director General of MRB for permission to publish this book.

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Contents

Epoxidised Natural Rubber and Its Chemistry . . . . . . . . . . . . . . . . . . . . . . . Fatimah Rubaizah Mohd Rasdi and Yusniwati Mohamed

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The Processing Technology of Epoxidised Natural Rubber . . . . . . . . . . . . Manroshan Singh Jaswan Singh, Nurul Hayati Yusof, and Fatimah Rubaizah Mohd Rasdi

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The Properties of Materials from Raw Epoxidised Natural Rubber . . . . . Nurul Hayati Yusof, Manroshan Singh Jaswan Singh, Yusniwati Mohamed Yusof, Fauzi Mohd Som, and Fatimah Rubaizah Mohd Rasdi

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General Compounding and Properties of Epoxidised Natural Rubber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Siti Salina Sarkawi, Ahmad Kifli Che Aziz, and Nik Intan Nik Ismail Epoxidised Natural Rubber in Tyre Applications . . . . . . . . . . . . . . . . . . . . . Siti Salina Sarkawi, Roland Ngeow, Ahmad Kifli Che Aziz, Rohaidah Abdul Rahim, Rassimi Abdul Ghani, Teku Zakwan Zaeimoedin, and Nurul Hayati Yusof

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Epoxidised Natural Rubber in Technical Rubber Goods . . . . . . . . . . . . . . . 141 Nik Intan Nik Ismail, Shamsul Kamaruddin, Mohamad Asri Ahmad, and Mahmud Iskandar Seth A Rahim Epoxidised Natural Rubber in Footwear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 Hani Afiffa Mohd Hanif, Mohamad Asri Ahmad, and Kok Chong Yong Epoxidised Natural Rubber in Latex Related Products . . . . . . . . . . . . . . . . 191 Asrul Mustafa, Dazylah Darji, Rohani Abu Bakar, Ruslimie Che Ali, Muhammad Syaarani Danya, Shamsul Kamaruddin, Kok Lang Mok, Norhanifah Mohd Yazid, Fatimah Rubaizah Mohd Rasdi, and Roslim Ramli

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Contents

Epoxidised Natural Rubber in Other Applications . . . . . . . . . . . . . . . . . . . . 251 Suhawati Ibrahim, Nurul Hayati Yusof, Nik Intan Nik Ismail, Dayang Habibah Abang Ismawi Hassim, Siti Salina Sarkawi, and Fatimah Rubaizah Mohd Rasdi Environmental Sustainability and Life Cycle Assessment for Epoxidised Natural Rubber (ENR) Processing . . . . . . . . . . . . . . . . . . . . 271 Zameri Mohamed, Fatimah Rubaizah Mohd Rasdi, and Pretibaa Subhramaniyun Economic and Market Trends of Specialty Rubber . . . . . . . . . . . . . . . . . . . 283 Norlee Ramli and Shafizal Yusof

About the Editors

Dr Siti Salina Sarkawi is a senior research officer attached to the Innovation and Elastomer Technology Unit under the Technology and Engineering Division of the Malaysian Rubber Board (MRB). She joined MRB in 2000 and holds a Bachelor of Chemical Engineering from Vanderbilt University, Tennessee, USA, as well as a MSc. in Polymer Material Science from UMIST, UK. She obtained her PhD from Twente University, the Netherlands, in 2013 and published a thesis entitled ‘Nano-Reinforcement of Tire Rubber: Silica Technology for Natural Rubber’. She has been involved with various aspects of material sciences and rubber technology, particularly in areas of mixing and compounding, rubber formulation, compound design, rubber analysis and processing technologies. She is actively involved in consultancy works on evaluation of new materials and additives for rubber and compound development, providing technical advisory services to the industry and related government agencies as well as lecturing on topics related to rubber and tyre technology. Her research interests and expertise include silica technology, green tyres, epoxidised natural rubber, deproteinised natural rubber, bio-oils, bio-fillers, nano-fillers and graphene.

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About the Editors

Dr Fatimah Rubaizah Mohd Rasdi is currently heading the Advancement and Innovation of Specialty Materials Unit under the Technology and Engineering Division in Malaysian Rubber Board. She holds a Bachelor of Chemical Engineering (Hons) from Universiti Teknologi Malaysia (UTM) and obtained her Ph.D. from Newcastle University, UK, in 2014. Her thesis was entitled ‘Continuous Screening using Mesoscale Oscillatory Baffled Reactor’. She later graduated with a Masters of Business Administration in 2020 from UNISEL, Malaysia. Her research interests and expertise include production and processing of specialty rubber materials, especially epoxidised natural rubber and deprotenised natural rubber. She is also actively involved in standards development at both the local and international levels. She has presented papers at national and international conferences on topics related to process intensification and specialty rubbers coupled with publications in various journals. Dr Veronica Charlotte is a senior research officer attached to the Publication and Library Unit under the Support Services Division of the Malaysian Rubber Board since 2005. Prior to joining MRB, she earned a B.Eng (Hons) in Biochemical Engineering in 2004 from University of Wales Swansea in the UK. She gained an insider perspective of the publishing process in her role as an editor of MRB publications, besides firsthand experience working with authors, reviewers and fellow editors within the discipline. Drawing on this, she pursued a doctorate and graduated with a PhD in Applied Linguistics from the University of Nottingham Malaysia in 2019. Her thesis was entitled ‘Publication of Rubber Research Articles: Investigating a Discourse Event from a Macro Genre Perspective’ and focused on the specific context where the authors themselves are located. The research findings corroborated evolution of a research article from cradle to grave and involvement of gatekeepers within the publishing process before the ultimate readership can be reached. She has since published both technical and conference papers as well as feature articles in various volumes, in addition to a book chapter in the M.A.K Haliday Library Functional Linguistics Series on Specialized Discourses and Their Readerships, edited by David Banks and Emilia Di Martino.

About the Editors

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Her expertise and passion are dedicated to science and scientific publications in particular, traversing journal research articles, various periodicals, monographs and books.

List of Figures

Epoxidised Natural Rubber and Its Chemistry Fig. 1 Fig. 2 Fig. 3 Fig. 4 Fig. 5 Fig. 6 Fig. 7

Two modification routes for natural rubber [5] . . . . . . . . . . . . . . . . a Epoxy structure and b unsaturated double bond (C = C) of epoxidised natural rubber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Epoxidised natural rubber structure produced from an in-situ epoxidation process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The mechanism of alkene epoxidation with peracids [6] . . . . . . . . Secondary ring opening of epoxide groups (Scheme 1) . . . . . . . . . Ring opening of adjacent epoxide groups to yield five-membered cyclic ether (Scheme 2) . . . . . . . . . . . . . . . . . . . . . . Crosslinking of ENR via ring-opened epoxide groups . . . . . . . . . .

2 4 6 6 7 7 7

The Processing Technology of Epoxidised Natural Rubber Fig. 1

Fig. 2 Fig. 3 Fig. 4

Fig. 5 Fig. 6 Fig. 7

Flowchart on ENR production which resembles that of commercially available ENR grade in Malaysia known as Ekoprena® [1] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Schematic diagram depicting the counter-current of a steam coagulator for ENR latex coagulation during its processing [3] . . . Effect on Mooney viscosity from different maturation and alkaline soaking process conditions [1] . . . . . . . . . . . . . . . . . . . Colloidal properties of LATZ preserved concentrated latex as represented by a particle size distribution b particle morphology c zeta potential and d flow behaviour . . . . . . . . . . . . . Epoxidation reaction time period for different grades of ENR when the reaction temperature is maintained at 60 °C . . . . . . . . . . Adsorption of non-ionic surfactant on the surface of rubber particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Changes in the zeta potential with an increase in surfactant concentration in the latex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

13 14 14

17 18 18 19

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Fig. 8 Fig. 9

Fig. 10

Fig. 11 Fig. 12 Fig. 13 Fig. 14 Fig. 15 Fig. 16

List of Figures

Epoxidised latex with various surfactant levels. The latex remained stable above 2 phr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Changes in pH with increasing ammonia solution for latex with a fixed surfactant and b fixed hydrogen peroxide concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of ammonia addition with the presence of excess hydrogen peroxide in the epoxidised latex with a no foaming observed after reaction b foam increased to 5 times of the initial volume after a few minutes and c stable foam with large bubbles observed after a few hours due to drainage of the liquid component in the foam . . . . . . . . . . . . . . . . . . . . . . . . . Various forms of ENR produced from the neutralised ENR latex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Guideline for DRC and concentration of hydrogen peroxide for the ENR reaction using field latex as a starting material . . . . . . Refractive index and density of ENR with the extent of epoxidation. Extracted from [20] . . . . . . . . . . . . . . . . . . . . . . . . . The ultrafiltration system for concentration of latexes. Extracted from [21] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Viscous form of LENR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Processing stages in LENR production . . . . . . . . . . . . . . . . . . . . . . .

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The Properties of Materials from Raw Epoxidised Natural Rubber Fig. 1 Fig. 2 Fig. 3 Fig. 4 Fig. 5

Fig. 6 Fig. 7 Fig. 8

Fig. 9

Fig. 10 Fig. 11

ENR packed as a single 33.33 kg bale and b 36 bales in a 1.2 tonne pallet [4] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vinylic proton from NR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Proton from epoxy group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 H-NMR spectrum of raw NR using 400 MHz with tetramethylsilane (TMS) as the reference for δ = 0 ppm . . . . 1 H-NMR spectra of raw ENR 25 and ENR 50 using 400 MHz with tetramethylsilane (TMS) as the reference for δ = 0 ppm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 C NMR spectrum of raw NR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 C NMR spectra of raw ENR 25 and ENR 50 . . . . . . . . . . . . . . . . Epoxidation level for ENR 25 measured with low-field NMR spectrometer Nanalysis 60 Pro (1 H frequency of 60 MHz) is 26% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Secondary ring-opening of epoxide groups with a Scheme 1: trans-diol products and b Scheme 2: five membered cyclic ether products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Furan and diol ring-opening peaks in NMR spectrum . . . . . . . . . . . Particle size distribution of ENR latex after a preparation and b concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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List of Figures

Fig. 12

Fig. 13 Fig. 14 Fig. 15 Fig. 16 Fig. 17

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Fig. 20 Fig. 21 Fig. 22 Fig. 23 Fig. 24

Fig. 25

Fig. 26

Fig. 27 Fig. 28 Fig. 29 Fig. 30 Fig. 31

SEM images at 10,000 × magnification showing morphology of ENR rubber particles after a preparation and b concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zeta potential of ENR latex after preparation and concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of shear rate and volume fraction on the shear stress of ENR latex after a preparation and b concentration . . . . . . . . . . . Effect of volume fraction on the a yield stress and b plastic viscosity of ENR latexes fitted to the Bingham model . . . . . . . . . . Effect of shear rate on the viscosity of ENR latex after a preparation and b concentration at various volume fractions . . . . . Effect of volume fraction (φ) on the a relative viscosity and b maximum packing fraction of ENR latex after preparation and concentration. Extracted from [17] . . . . . . . . . . . . . . . . . . . . . . Changes in the moduli (G’ and G”) as a function of strain (%) at 1 Hz and 25 °C of ENR latex after a preparation and b concentration. Extracted from [17] . . . . . . . . . . . . . . . . . . . . . . . . . . Changes in the moduli (G’ and G”) as a function of frequency at 25 °C of ENR latex after concentration at a φ = 0.584 (57 wt%), b φ = 0.505 (49 wt%), c φ = 0.495 (48 wt%) and d φ = 0.485. Extracted from [17] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of concentrated ENR latex volume fraction on the a moduli and b tan δ at 1 Hz and 25 °C . . . . . . . . . . . . . . . . . . . . . . . . Interaction between particles as observed at various tan δ values a tan δ < 1, b tan δ = 1 and c tan δ > 1 . . . . . . . . . . . . . . . . . Layer thickness, Δ of the adsorbed surfactant as a function of ENR latex volume fraction (φ) . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical structures of a ENR and b LENR . . . . . . . . . . . . . . . . . . Effect of surfactant concentrations (0 to 3.0 wt%) on the stability of the ENR 50 latex during reaction (destabilised condition is represented by the “x” symbol) . . . . . . . A scheme showing LENR 50 reaction mechanism produced from ENR 50 at pH 8 using sodium nitrite (NaNO2 ) and hydrogen peroxide (H2 O2 ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of degrading agent concentrations (sodium nitrite and hydrogen peroxide at a 1:1 ratio) on M n and M w of LENR 50 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of degrading agent concentrations on gel content of LENR 50 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FTIR spectra of ENR 50 and LENR 50 . . . . . . . . . . . . . . . . . . . . . . 1 H-NMR spectra of ENR 50 and LENR 50 . . . . . . . . . . . . . . . . . . . Expanded 1 H-NMR spectra of ENR 50 and LENR 50 from 9 to 10 ppm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Expanded 1 H-NMR spectra of ENR 50 and LENR 50 indicating presence of diol groups at 3–4 ppm . . . . . . . . . . . . . . . . .

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Fig. 32 Fig. 33 Fig. 34 Fig. 35 Fig. 36 Fig. 37 Fig. 38 Fig. 39 Fig. 40 Fig. 41 Fig. 42

List of Figures

Expanded 1 H-NMR spectrum of LENR 50 indicating furan and diol chemical shifts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 C NMR spectrum of LENR 50 . . . . . . . . . . . . . . . . . . . . . . . . . . . . Particle size distribution of ENR 50 and LENR 50 latexes . . . . . . . FESEM images of a ENR 50 latex (x10K) and b LENR 50 latex (x15K) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermograms of ENR 50 and LENR 50 . . . . . . . . . . . . . . . . . . . . . a TG curves and b DTG curves for ENR 50 (yellow line) and LENR 50 (orange line) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Maximum degradation temperature of LENRs with different epoxidation level percentages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Contact angle of ENR 50 and LENR 50 . . . . . . . . . . . . . . . . . . . . . . Effect of number average molecular weight, M n on the viscosities of LENR 50 batches . . . . . . . . . . . . . . . . . . . . . . . Molecular weights of LENR 50 during storage . . . . . . . . . . . . . . . . Epoxidation level of LENR 50 upon storage . . . . . . . . . . . . . . . . . .

60 60 61 62 62 63 63 64 65 66 66

General Compounding and Properties of Epoxidised Natural Rubber Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5 Fig. 6

Fig. 7 Fig. 8

Effect of filler loading on tensile strength of a carbon blackand b silica-filled ENR vulcanisates at various epoxidation levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The hybrid reinforcement of ENR with silica consists of these combinations: a interaction between the silanol group of silica with the epoxide group in ENR; b coupling of silica with ENR through ring-opening of ENR; and c silane coupling bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of filler loading for a carbon black- b silica-filled ENR vulcanisates on modulus M100 and c carbon black- d silica-filled ENR vulcanisates on modulus M300 at various epoxidation levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of filler loading on elongation at break of a carbon black- and b silica-filled ENR vulcanisates at various epoxidation levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of filler loading on hardness of a carbon black- and b silica-filled ENR vulcanisates at various epoxidation levels . . . . . . Effect of filler loading on compression set (22 h at 70 °C) of a carbon black- and b silica-filled ENR vulcanisates at various epoxidation levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of filler loading on resilience of a carbon black- and b silica-filled ENR vulcanisates at various epoxidation levels . . . . . . Effect of filler loading on abrasion resistance of a carbon black- and b silica-filled ENR vulcanisates at various epoxidation levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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List of Figures

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Fig. 12 Fig. 13

Fig. 14 Fig. 15 Fig. 16 Fig. 17 Fig. 18 Fig. 19 Fig. 20 Fig. 21

Fig. 22

Effect of filler loading on tear strength of a carbon blackand b silica-filled ENR vulcanisates at various epoxidation levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of filler loading on air permeability constant of a carbon black- and b silica-filled ENR vulcanisates at various epoxidation levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparative ageing of a carbon black- b silica-filled ENR vulcanisates on tensile strength and elongation at break and c carbon black- d silica-filled ENR vulcanisates on modulus M100 and M300 at respective epoxidation levels under accelerated ageing conditions for 7 days at 70 °C . . . . . . . . . Summary of the effect of filler loadings on physical properties of the ENR vulcanisates . . . . . . . . . . . . . . . . . . . . . . . . . . The a storage modulus (E’) and b tan delta (δ) curves of NR, ENR 25 and ENR 50 vulcanisates filled with 50 phr carbon black . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The effect of epoxidation on the properties of ENR . . . . . . . . . . . . Percentage volume change of rubber in IRM 901 (70 h at 100 °C) and IRM 903 (70 h at 100 °C) . . . . . . . . . . . . . . . . . . . . . A linear relationship of solubility parameter and epoxidation level [22] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Relationship between wet grip, epoxidation level and Tg of ENR vulcanisates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Coefficient of friction for ENR 25 silica tread compound from the Plint rubber friction test . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of car stopping distance between silica-filled ENR and a control tread on dry and wet asphalt [24] . . . . . . . . . . . Interaction of graphene with ENR and silica . . . . . . . . . . . . . . . . . . Hybrid reinforcement of ENR with nanofillers; ENR/silica and ENR/silane/silica bondings; and the physical interactions of ENR with silica and graphene [33] . . . . . . . . . . . . . . . . . . . . . . . . The effects of ENR as a compatabiliser on filler-filler interaction in GO/CB filled NR [38] . . . . . . . . . . . . . . . . . . . . . . . . .

xxi

79

80

81 82

83 86 88 89 91 92 93 94

95 96

Epoxidised Natural Rubber in Tyre Applications Fig. 1

Fig. 2 Fig. 3 Fig. 4

Prediction of tread properties from the tan δ curve as a function of temperature for NR and ENR compounds. Note: The curve is from DMA testing at temperatures between −100 °C and +100 °C . . . . . . . . . . . . . . . . . . . . . . . . . . . . STAR3 plot of rolling resistance and wet grip ratings of ENR compounds compared with NR and other blends . . . . . . . . . . . . . . . Cross-sectional view of tread components for a complex extrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PCR tyres 195/55R15 85 V with silica-filled ENR tread . . . . . . . .

102 103 106 107

xxii

Fig. 5

Fig. 6 Fig. 7 Fig. 8 Fig. 9

Fig. 10 Fig. 11 Fig. 12 Fig. 13

Fig. 14 Fig. 15

Fig. 16 Fig. 17 Fig. 18 Fig. 19

Fig. 20 Fig. 21

Fig. 22

Fig. 23 Fig. 24

List of Figures

Comparison of test car stopping distance between silica-filled ENR and control tread tyres on both dry and wet asphalt surfaces [28, 32] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Steady-state cornering test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Slalom test on a wet asphalt surface . . . . . . . . . . . . . . . . . . . . . . . . . Subjective ride and handling rating of the ENR 25–silica PCR tyre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of wear rates of PCR tyres size 195/55R15 between ENR 25 filled with silica, ENR 25 filled with dual fillers (silica/CB) and control sSBR/BR/NR filled with carbon black/silica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Endurance and on-the-road testing of ENR 25–silica PCR tyres for 60,000 km travelling distance . . . . . . . . . . . . . . . . . . . . . . . Summary of ENR 25–silica tyre performance . . . . . . . . . . . . . . . . . Tread pattern used for ENR retreads . . . . . . . . . . . . . . . . . . . . . . . . . Projected tread life of ENR retreads from an on-road trial using city buses. The trials were carried out on different routes and over different durations using the same ENR/BR retread compound. Ten buses were used on each route . . . . . . . . . . Comparison of different routes on the wear of ENR and control-retreaded tyres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of left and right sides of a ENR and b control-retreaded tyres tested during road trials on different bus routes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Treadwear rating of ENR blend retreads during the on-road trial using city feeder buses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Average wear performance of ENR blend retreads during the on-road trial using city feeder buses . . . . . . . . . . . . . . . . Temperature hysteresis profile of ENR blends . . . . . . . . . . . . . . . . . ENR retread trial for trailers of a tankers and b cargoes. The trial was in collaboration with Felda Transport Services Sdn Bhd as the truck operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Axle/wheel locations on the rigid four-axle trucks for the trial . . . . Wear rating of tanker (T) and cargo (C) trailers at third axle (low severity condition) and fourth axle (high severity condition) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Treadwear (mm/1,000 km) of tube and tubeless ENR blend (ENR/NR/BR) motorcycle tyres. 70/90-17 and 80/90-17 motorcycle tyres were fitted on the front and rear wheels, respectively . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of tyre wear for tubeless ENR and commercial motorcycle tyres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of rolling resistance for ENR motorcycle tyres with commercial motorcycle tyres . . . . . . . . . . . . . . . . . . . . . . . . . .

109 110 111 111

112 113 114 115

116 116

117 117 118 120

121 121

122

124 125 126

List of Figures

Fig. 25 Fig. 26 Fig. 27 Fig. 28

FTIR spectra for a precipitated silica (VN3) b raw ENR 25 and c ENR 25–silica masterbatch . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermogravimetric (TGA) curves of raw ENR 25 (line) and ENR–silica VN3 masterbatch (short-dashed) . . . . . . . . . . . . . . Mooney viscosity for ENR-silica masterbatches with silica concentrations from 20 to 60 phr . . . . . . . . . . . . . . . . . . . . . . . . . . . FESEM images of ENR–silica masterbatch (magnification ×500) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xxiii

130 131 133 134

Epoxidised Natural Rubber in Technical Rubber Goods Fig. 1 Fig. 2 Fig. 3 Fig. 4 Fig. 5 Fig. 6 Fig. 7 Fig. 8 Fig. 9 Fig. 10

Fig. 11 Fig. 12 Fig. 13 Fig. 14 Fig. 15 Fig. 16 Fig. 17 Fig. 18

Percentage volume swelling of rubbers in hydrocarbon oils tested for 70 h at 100 °C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Percentage volume swelling of rubbers in solvents tested for 70 h at 23 °C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dynamic mechanical curves of ENR 50, NBR and ENR 50/NBR blends as a function of temperature . . . . . . . . . . . . . . . . . . Volume swelling of ENR 50/NBR blends with various ratios, tested for 70 h at 100 °C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fuel resistant seal of ENR 50/NBR blend . . . . . . . . . . . . . . . . . . . . Dynamic mechanical curves of CR, ENR 50 and CR/ENR 50 blends as a function of temperature . . . . . . . . . . . . . . . . . . . . . . . Volume swelling of CR/ENR 50 blends at various ratios tested for 70 h at 100 °C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Air permeability coefficient (Q) at 25 °C, (m2 . Pa) of CR/ENR 50 blends at various ratios . . . . . . . . . . . . . . . . . . . . . . . Abrasion resistance of CR/ENR 50 blends at various ratios . . . . . . ' The dependence of a storage modulus (G ); and b tan δ for NR, ENR 25 and ENR 50 as a function of temperature at 170 Hz [10] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ' The dependence of storage modulus (G ) for NR, ENR 25 and ENR 50 as a function of frequency [10] . . . . . . . . . . . . . . . . . . The sound absorption coefficient of dry rubber foams for NR, ENR 25 and ENR 50 compared with commercial foams [24] . . . . The sound absorption coefficient of latex foams for NR, ENR 25 and ENR 50 compared with commercial foams [24] . . . . Static stiffness of engine mounts using a variation of compounds and carbon black loadings . . . . . . . . . . . . . . . . . . . . . Dynamic stiffness of engine mounts filled with 26 phr carbon black tested at a variation of frequencies . . . . . . . . . . . . . . . . . . . . . Dynamic stiffness of engine mounts filled with 36 phr carbon black tested at a variation of frequencies . . . . . . . . . . . . . . . . . . . . . Tan δ of engine mounts filled with different carbon black loadings tested at 15 Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ENR 50 and DPNR rubber bushes . . . . . . . . . . . . . . . . . . . . . . . . . .

143 143 146 148 149 151 153 154 155

156 157 158 159 162 163 163 164 165

xxiv

Fig. 19 Fig. 20 Fig. 21 Fig. 22

List of Figures

Graph of static test for a ENR 50 and b DPNR . . . . . . . . . . . . . . . . Graphs of a dynamic stiffness and b loss angle for ENR 50 and DPNR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Green rubber sound damper [34] . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of product density . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

166 167 168 170

Epoxidised Natural Rubber in Footwear Fig. 1 Fig. 2 Fig. 3 Fig. 4 Fig. 5 Fig. 6

Typical sole construction of a safety shoe and b military boot . . . . Typical footwear construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification of conductive, dissipative, and insulative materials and footwear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon black-filled antistatic soles with various amounts of whitening agent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Volume resistivity of ENR blends with a non-black antistatic agent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Prototype of safety footwear with ENR-based antistatic soles . . . .

175 175 180 182 185 186

Epoxidised Natural Rubber in Latex Related Products Fig. 1 Fig. 2 Fig. 3 Fig. 4 Fig. 5 Fig. 6

Fig. 7 Fig. 8 Fig. 9 Fig. 10

Fig. 11 Fig. 12 Fig. 13 Fig. 14

Malaysia’s export values for latex-based products from 2010 to 2020 [2] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Removal of surfactant in ENR latex particles by ultrafiltration membrane technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 H NMR of surfactant and ENR . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tensile strength of neat and compounded ENR 25 and ENR 50 latex castings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flow chart of the dipping process . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of a total solids content b coagulant concentration c dwelling time and d former temperature on thickness of dipped ENR film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tensile strength and elongation at break of ENR, NR, NBR and PC gloves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Volume of changes in oil at different swelling times of ENR, NR, NBR and PC gloves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Permeation performance levels detailed in BS EN 16523-1:2015 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a Viscosity as a function of shear rate at 25 °C for LATZ and ENR latex; b shear stress as a function shear rate at 25 °C for LATZ and ENR latex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FoamScan® analyzer configuration . . . . . . . . . . . . . . . . . . . . . . . . . . Foam volume versus time for a LATZ latex and b ENR latex . . . . Foamability and stability study of latex: a LATZ latex and b ENR latex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ENR latex foam fabrication process . . . . . . . . . . . . . . . . . . . . . . . . .

192 193 194 194 195

197 198 198 199

202 203 204 205 206

List of Figures

Fig. 15 Fig. 16

Fig. 17 Fig. 18 Fig. 19 Fig. 20 Fig. 21 Fig. 22 Fig. 23 Fig. 24 Fig. 25 Fig. 26 Fig. 27 Fig. 28 Fig. 29 Fig. 30 Fig. 31 Fig. 32 Fig. 33 Fig. 34 Fig. 35 Fig. 36 Fig. 37

Fig. 38

Fig. 39

Gelling time of LATZ and ENR latex foam. Green point = gelling time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a Sound absorption panel absorbs unwanted sound to improve the quality of performance and b Sound insulation panel mitigates noise propagation from one room to another . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FESEM image of an open-cell structure of ENR latex foam at 40X magnification, scale bar = 100 μm . . . . . . . . . . . . . . . . . . . Schematic diagram of impedance tube method for measuring SAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Schematic diagram of impedance tube method for measuring STL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sound absorption coefficient (SAC) of GRSI, PU and ENR latex foam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sound transmission loss (STL) of GRSI, PU and ENR latex foam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Configuration of vibration-damping test . . . . . . . . . . . . . . . . . . . . . . Adhesive classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Malaysia’s exports and imports of polymer and rubber-based adhesives (sourced by the International Trade Centre (ITC)) . . . . . Application of wallpaper adhesive on a concrete wall b wooden wall and c drywall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methods of wallpaper adhesive applications a notched trowel b roller c paste machine and d brush . . . . . . . . . . . . . . . . . . . ENR latex multicolour adhesive . . . . . . . . . . . . . . . . . . . . . . . . . . . . Use of ENR latex multicolour adhesives in art and craft . . . . . . . . . Applications of adhesive as a painting medium and craft on a canvas b wood and c collage . . . . . . . . . . . . . . . . . . . . . . . . . . . Application of adhesive for various papers a A4 paper b newspaper and c craft paper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ENR latex paint in wall murals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ENR latex paint for office, laboratory and house interiors . . . . . . . Images of an elephant paddock painted with ENR latex paint . . . . ENR latex paint on road kerbs and as parking lines . . . . . . . . . . . . . A corporate building painted with ENR latex paint . . . . . . . . . . . . . Example of packaging prototype . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison paintings made with different types of painting media on cotton canvas: a acrylic painting medium b ENR-based paint and c water colour . . . . . . . . . . . . . . . . . . . . . . . . Appearance of paper sheet applied with three different painting media after 15 min of water immersion: a acrylic painting medium b ENR-based paint and c water colour . . . . . . . . The appearance of paper sheets after exposure to UV light for 7 days a acrylic painting medium b ENR-based paint and c water colour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xxv

208

209 209 209 210 211 211 212 214 215 218 218 219 221 221 225 230 231 232 233 233 234

234

235

235

xxvi

List of Figures

Epoxidised Natural Rubber in Other Applications Fig. 1

Fig. 2 Fig. 3

Fig. 4 Fig. 5 Fig. 6

Fig. 7

Fig. 8

Fig. 9 Fig. 10

Fig. 11

Fig. 12

Fig. 13

SEM images of the blended SPEEK/ENR 50 membranes with a 0 wt% b 0.5 wt% c 1.0 wt% d 1.5 wt% e 2.0 wt% and f 2.5 wt% ENR 50 loadings. Reprinted from [12], Copyright 2020, with permission from Elsevier . . . . . . . . . . . . . . . Bond strength of NR adhesives comprising various Mn of LENR 50s as a tackifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparative bond strength of NR-based adhesives formulated with LENRs and commercial hydrocarbon tackifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bond strengths of NR/LENR-based adhesive compared with commercial carpet adhesives . . . . . . . . . . . . . . . . . . . . . . . . . . . Micrograph of plywood substrate obtained using an optical microscope (10× magnification) . . . . . . . . . . . . . . . . . . . . . . . . . . . . Schematic illustration of the ENR and TPU interaction. Reprinted with permission from Springer: [49], Copyright 2012 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of ENR/TPU blends with EPDM/PP TPV on the degree of swelling and compression set properties. Reprinted with permission from Springer: [50], Copyright 2012 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A crosslinking reaction between the ENR and the PVC. Reprinted with permission from Springer: [55], Copyright 1991 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A crosslinking reaction between ENR, PVC and XNBR [54] . . . . a Morphology of the ENR microfibrils observed at 2000x magnification and b Detailed morphology structure of the ENR microfibrils 5000x magnification. Reprinted (adapted) with permission from American Chemical Society [59]. Copyright 2015 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photograph of 1 kg spool of a PLA/NR blend and b PLA/ENR 25 blend filaments prototype (diameter = 1.75 mm ± 0.05) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of blend ratio on the a tensile strength and b the elongation at break (EB) of the PLA/NR and PLA/ENR 25 blends ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3D printing products from PLA/ENR 25 blend filaments . . . . . . .

254 256

256 257 257

262

262

263 263

264

265

265 266

Environmental Sustainability and Life Cycle Assessment for Epoxidised Natural Rubber (ENR) Processing Fig. 1 Fig. 2

ENR 25 process flow representing the system boundary for this case study [6] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterisation to produce 1 kg of ENR 25 from gate to gate (Gtog) based on impact categories . . . . . . . . . . . . . . . . . . . .

275 277

List of Figures

Fig. 3 Fig. 4

Damage assessment to produce 1 kg of ENR 25 from gate to gate (Gtog) based on damage categories . . . . . . . . . . . . . . . . . . . Weighting to produce 1 kg of ENR 25 from gate to gate (Gtog) based on impact categories . . . . . . . . . . . . . . . . . . . . . . . . . .

xxvii

278 280

Economic and Market Trends of Specialty Rubber Fig. 1 Fig. 2 Fig. 3 Fig. 4

Fig. 5 Fig. 6 Fig. 7 Fig. 8 Fig. 9 Fig. 10 Fig. 11 Fig. 12 Fig. 13 Fig. 14

Major events that heightened uncertainty causing significant ramifications globally [2] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Asia as the leading driver of global growth [4] . . . . . . . . . . . . . . . . Steady economic growth path prior to 2020, enabled by continued access to market-based financing in Malaysia [5] . . . Trend of population growth with respect to age structure, as well as the poor, middle class and urban shares of the population . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Range of smallholders age, a 2012 survey by the Malaysian Rubber Board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . National debt as a percentage of GDP [13, 14] . . . . . . . . . . . . . . . . National debt status in 2007|Pre-global financial crisis . . . . . . . . . . National debt status in 2020|COVID-19 [4] . . . . . . . . . . . . . . . . . . . Number of regional trade agreements in force [16] [Information as of June 2020] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Global NR demand [7, 18] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Potential market for Ekoprena® (Based on the market for SSBR and NdBR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a Shift in economic weight to Asia and b Share of the world GDP forecast [30] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Economic activity forecast to tilt towards Asia (Source Oxford Economics) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Forecast of tyre market growth rate in Asia from 2020–2025 (% CAGR) [31] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

285 286 286

289 290 291 291 292 293 295 297 298 299 299

List of Tables

Epoxidised Natural Rubber and Its Chemistry Table 1 Table 2 Table 3

The extension of chemical modification onto natural rubber [6] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Common epoxidising agents for diene elastomers [12] . . . . . . . . Epoxidation via latex routes for peracetic acid and performic acid systems [14] . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 5 5

The Processing Technology of Epoxidised Natural Rubber Table 1

Properties of centrifuged HA and LATZ preserved natural rubber latex concentrates [13] . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

16

The Properties of Materials from Raw Epoxidised Natural Rubber Table 1 Table 2 Table 3 Table 4 Table 5 Table 6 Table 7 Table 8

Specification of TSR grades under the SMR scheme [2] . . . . . . . Importance of each test under the SMR scheme for raw NR [2] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Typical raw rubber properties of ENR 25 and 50 by manufacturers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Specified values of raw ENR rubber produced from both field and concentrated latex . . . . . . . . . . . . . . . . . . . . . . Typical properties of ENR latex after preparation and concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties of LENR prepared from ENR 50 latex after storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of pH on the properties of ENR 50 latex and LENR 50 . . . Absorption peak area ratio for ENR 50 and LENR 50 . . . . . . . . .

32 33 33 34 41 53 54 57

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xxx

List of Tables

General Compounding and Properties of Epoxidised Natural Rubber Table 1 Table 2 Table 3 Table 4 Table 5 Table 6 Table 7 Table 8 Table 9 Table 10 Table 11 Table 12 Table 13 Table 14 Table 15 Table 16 Table 17

Proposed ENR formulations using a semi-EV vulcanisation system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mixing steps for the carbon black-filled ENR masterbatch . . . . . Mixing steps for the silica-filled ENR masterbatch (1st stage–mixing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mixing steps for the silica-filled ENR masterbatch (2nd stage–remilling) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Physical properties of carbon black-filled ENR vulcanisates (30 phr N234) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Physical properties of silica-filled ENR vulcanisates (30 phr Zeosil 1165MP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The effect of increasing carbon black and silica loadings on ENR vulcanisate properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of carbon black-α and silica-β filled ENR vulcanisates (45 phr) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of Tg between NR, ENR 25 and ENR 50 vulcanisates (50 phr carbon black) . . . . . . . . . . . . . . . . . . . . . . . . . Effect of epoxidation levels on physical properties of carbon black-filled ENR vulcanisates (60 phr) . . . . . . . . . . . . . Effect of epoxidation levels on physical properties of silica-filled ENR vulcanisates (60 phr) . . . . . . . . . . . . . . . . . . . Physical Properties of black-filled (50 phr N330 Black) ENR vulcanisatesa compared to NR, CR, NBR and IIR . . . . . . . . Solubility parameter of ENR and other rubbers [22] . . . . . . . . . . Relative air permeability* of ENR and other rubbers . . . . . . . . . . Relative wet grip of ENR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Silica-filled ENR PCR tyrea performance [24] . . . . . . . . . . . . . . . Physical properties of the ENR vulcanisate . . . . . . . . . . . . . . . . . .

71 71 72 72 73 73 82 83 83 84 84 87 90 90 91 92 94

Epoxidised Natural Rubber in Tyre Applications Table 1 Table 2 Table 3

Table 4 Table 5 Table 6 Table 7 Table 8

Rubber used in tyres and its related properties . . . . . . . . . . . . . . . Tyre performance indicator from the tan delta versus temperature curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of tangential and intermeshing rotor types on the compound viscosity and filler dispersion of silica-filled ENR 25 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mooney viscosity of ENR 25 filled with 75 phr silica, mixed using a 270L tangential rotor internal mixer . . . . . . . . . . . Physical properties of silica-filled ENR 25 tread . . . . . . . . . . . . . . Results of safety and regulation tests for silica-filled ENR tyres [28, 32] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Silica-filled ENR PCR tyrea performance [32] . . . . . . . . . . . . . . . Fuel consumption of silica-filled ENR tyre treadsa [28, 32] . . . . .

100 101

105 105 106 107 108 108

List of Tables

Table 9 Table 10 Table 11 Table 12 Table 13 Table 14 Table 15 Table 16 Table 17 Table 18 Table 19 Table 20 Table 21 Table 22 Table 23 Table 24 Table 25 Table 26

Data value for steady-state cornering on a dry asphalt surface [28, 32] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Data value for the wet slalom test [28, 32] . . . . . . . . . . . . . . . . . . ENR-retreaded tyre performance . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of on-road performance with laboratory abrasion data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Physical properties of ENR blend vulcanisates . . . . . . . . . . . . . . . ENR-retreaded tyre performance (eco-labelling) . . . . . . . . . . . . . ENR and commercial retreads performancea . . . . . . . . . . . . . . . . . Treadwear ratings for tanker and cargo trailers . . . . . . . . . . . . . . . ENR 25-based motorcycle tyre tread vulcanisate properties . . . . ENR-based motorcycle tyre performance . . . . . . . . . . . . . . . . . . . Properties of 10 wt% silica dispersion . . . . . . . . . . . . . . . . . . . . . . Observations on ENR–silica masterbatch coagulation with acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties of ENR 25 latex (starting material) . . . . . . . . . . . . . . . . Properties of ENR 25–silica masterbatch . . . . . . . . . . . . . . . . . . . . Properties of ENR–silica masterbatch and thermogravimetric analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . Onset and glass transition temperatures for ENR 25 and ENR 25–silica masterbatches . . . . . . . . . . . . . . . . . . . . . . . . . Formulation for ENR–silica wet masterbatch compound . . . . . . . Mechanical properties of ENR–silica masterbatch vulcanisates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xxxi

110 111 115 118 119 120 122 122 123 124 128 129 129 129 131 132 135 136

Epoxidised Natural Rubber in Technical Rubber Goods Table 1 Table 2 Table 3 Table 4 Table 5 Table 6 Table 7 Table 8 Table 9 Table 10 Table 11 Table 12 Table 13 Table 14

Formulation of ENR fuel resistant seal . . . . . . . . . . . . . . . . . . . . . Cure characteristics of the ENR fuel resistant seal . . . . . . . . . . . . Dynamic glass transition temperature of ENR 50/NBR blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical properties of ENR 50/NBR at various blend ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Formulation of ENR hose cover . . . . . . . . . . . . . . . . . . . . . . . . . . . Rheological properties of CR/ENR 50 blends . . . . . . . . . . . . . . . . Dynamic glass transition temperature of CR/ENR 50 blends . . . . Mechanical properties of CR/ENR 50 blends . . . . . . . . . . . . . . . . Engine mounting compound formulation . . . . . . . . . . . . . . . . . . . Physical properties of ENR blends (26 phr carbon black) . . . . . . Physical properties of ENR blends (36 phr carbon black) . . . . . . Formulation for each rubber bio-composite . . . . . . . . . . . . . . . . . . Static stiffness of each rubber compound . . . . . . . . . . . . . . . . . . . . Formulation of ENR-based rubber sound damper . . . . . . . . . . . . .

144 145 146 146 150 150 151 152 160 161 161 165 166 169

xxxii

Table 15 Table 16

List of Tables

A typical two stage mixing cycle in an internal mixer and kneader for sound damping product manufacturing . . . . . . . . ENR-based sound damper product performance . . . . . . . . . . . . . .

169 170

Epoxidised Natural Rubber in Footwear Table 1 Table 2 Table 3 Table 4 Table 5 Table 6 Table 7 Table 8 Table 9

Typical formulation of ENR-based marching boot soles . . . . . . . ENR compound properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Typical formulation of ENR-based safety boot soles . . . . . . . . . . Comparison of physical properties for developed ENR safety boot soles with commercial safety boots . . . . . . . . . . . . . . . Typical compounding recipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Physical properties of the ENR-based composite for antistatic footwear application . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of volume resistivity between ENR and other rubbers with 15 phr of conductive filler . . . . . . . . . . . . . . . . . . . . . Examples of conductive ENR blends . . . . . . . . . . . . . . . . . . . . . . . Comparison between synthetic and ENR-based soles . . . . . . . . . .

176 177 178 178 181 182 183 184 187

Epoxidised Natural Rubber in Latex Related Products Table 1

Table 2 Table 3 Table 4 Table 5 Table 6 Table 7 Table 8 Table 9 Table 10 Table 11 Table 12 Table 13 Table 14 Table 15

Comparison of ENR latex properties before and after the concentration process using an ultrafiltration membrane system . . . . . . . . . . . . . . . . . . . . . . . . Latex properties of ENRLC and HA [11] . . . . . . . . . . . . . . . . . . . Permeation performance levels for different types of gloves according to BS EN 16523-1:2015 . . . . . . . . . . . . . . . . . . . . . . . . . Physiochemical properties of LATZ latex and ENR Latex . . . . . . Compounding formulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gelling formulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Damping ratio (ζ) of ENR latex foam fabricated at different density levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . List of ingredients for the ENR latex wallpaper adhesive . . . . . . . Technical and application data for the ENR latex wallpaper adhesive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . List of ingredients for ENR 25 latex multicolour adhesive . . . . . . Technical and application data for the ENR latex multicolour adhesive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . List of ingredients and amounts for ENR paper adhesive formulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Technical and application data for the ENR latex paper adhesive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Peel adhesion for ENR, NR and commercial paper adhesives on various papers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Relationship between drying time and peel of adhesion of ENR latex, NR latex and commercial paper adhesives . . . . . . .

193 195 199 201 207 207 213 216 217 219 220 222 223 224 224

List of Tables

Table 16 Table 17 Table 18 Table 19 Table 20

Physical properties of ENR, NR and commercial paper adhesives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The physical and performance properties of ENR latex paint . . . A comparative analysis of ENR latex paint and commercial paints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ingredients and preparation compositions of ENR-based paint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Colour fading percentages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xxxiii

224 228 229 234 236

Epoxidised Natural Rubber in Other Applications Table 1

Table 2

Water uptake and average particle sizes of the dispersed ENR 50 in a polymer matrix of pristine SPEEK, Nafion117 and blended SPEEK/ENR 50 (SPE50) membranes [12] . . . . . . . . Properties of TPENR (DV) in comparison to other thermoplastic and NBR vulcanisates [44] . . . . . . . . . . . . . . . . . . .

254 261

Environmental Sustainability and Life Cycle Assessment for Epoxidised Natural Rubber (ENR) Processing Table 1 Table 2

Table 3 Table 4 Table 5

Characteristics of raw effluent from ENR processing [3] . . . . . . . Regulatory standards for watercourse discharge of effluent from SMR processing (or other conventional rubber grades) [4] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effluent treatment systems for ENR rubber processing . . . . . . . . Average LCI table based on 1 pallet (1200 kg) of ENR 25 production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Weighting values to produce 1 kg of ENR 25 from gate to gate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

273

273 274 276 279

Epoxidised Natural Rubber and Its Chemistry Fatimah Rubaizah Mohd Rasdi and Yusniwati Mohamed

Abstract The main motivation for modification of natural rubber is to enhance its properties and expand its application in various rubber product sectors. There are two routes of modification which are viable with natural rubber, namely the physical and chemical. The latter involves reactions onto the unsaturated structure of natural rubber that result in changes to its structure. Amongst the chemically modified natural rubbers are hydrogenated and epoxidised natural rubbers. However, only epoxidised natural rubber (ENR) is of interest in the current publication. Various methods of epoxidation are available, and these include either a solution or latex environment but epoxidation via peracids method appears to be preferred due to better reaction rates and product purity. The epoxidation level in natural rubber can also be increased to 90%. However, 5 to 75% epoxidation level is a favoured range as it results in minimal secondary ring-opened structures such as trans-diol and five-membered cyclic ether. Keywords Epoxidised natural rubber · Chemical modification · Epoxidation level · Performic acid · Peracetic acid

1 Introduction The introduction of rubber to Malaysia by British colonists in 1877 was primarily due to the ideal climate and soil conditions for plantation. Since its introduction, production increased dramatically with a huge surge in demand for rubber from around the world [1]. By the 1930s, Malaysia was amongst the countries responsible for producing three-quarters of the world’s rubber demand [2]. Nevertheless, in the 1990’s, synthetic rubber (SR) had been introduced commercially as a material to substitute natural rubber (NR) [3]. This was due to shortage of natural rubber from the previous world wars. As synthetic rubber is derived generally from petrochemical feedstock, it is able to provide equivalent or better properties than natural rubber in its various applications, specifically rubber products. Some of the drawbacks of natural F. R. Mohd Rasdi (B) · Y. Mohamed Malaysian Rubber Board, Sungai Buloh, Selangor, Malaysia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. S. Sarkawi et al. (eds.), Epoxidised Natural Rubber, https://doi.org/10.1007/978-981-19-8836-3_1

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F. R. Mohd Rasdi and Y. Mohamed

rubber are its sensitivity towards heat, oxygen and ozone [4] due to the unsaturated double bond structure of natural rubber, which consists of cis-1,4-polyisoprene. To maintain its position as the material of choice in a variety of product applications, natural rubber was subjected to modification processes in an effort to overcome its drawbacks. Through these modification approaches, natural rubber was reintroduced to adapt to specific application needs especially in niche applications, i.e. oil resistance application. Moreover, through this approach, dependency on synthetic rubber is minimised while aligning with increased awareness for sustainable materials considering natural rubber is produced from the Hevea brasiliensis rubber tree species. Thus, natural rubber is undeniably a more sustainable material compared with synthetic rubber in the rubber products market sector. There are two routes to modify natural rubber, namely via chemical and physical methods (Fig. 1). Physical modification only involves incorporation of compound ingredients or blending approaches which do not result in any changes to the natural rubber structure. The chemical modification route, however, involves reactions on the unsaturated structure of natural rubber. This can be performed through grafting using different polymers, attachment of pendant functional groups or through intramolecular changes [5]. The classes of such transformation of the polymer chain are listed in Table 1 [6]. As the chemical modification route is classified as a transformation process of polymer chains, it does involve increment in reagent costs considering the modified reaction. It is estimated that approximately 10% of price increment is applicable for each mole percentage modification [6]. It is important that any modification should bring significant improvements to ensure commercial viability of the material. Amongst the examples of chemically modified natural rubbers are

Fig. 1 Two modification routes for natural rubber [5]

Epoxidised Natural Rubber and Its Chemistry

3

Table 1 The extension of chemical modification onto natural rubber [6] Type of reaction

Extent, mol %

Effects on properties

Surface

1 40 35

Layer thickness, Δ (nm)

Fig. 22 Layer thickness, Δ of the adsorbed surfactant as a function of ENR latex volume fraction (φ)

30 25 20 15 10 5 0 0.450

0.475

0.500

0.525

0.550

0.575

0.600

Latex volume fraction (ɸ)

where [n] = 2.5. The layer thickness (Δ) values calculated using Eq. 8φ from 0.4975 up to 0.584 are shown in Fig. 22. At the highest φ of 0.584, the Δ was ~ 20 nm, and it increased with reducing φ, reaching a value of ~ 35 nm at φ of 0.4975. Since tan δ = 1 (G’ = G”) at φ = 0.4975, the layer thickness when the adsorbed polymer layers start to interact with each other is ~ 35 nm, which is a considerably thick layer. Very high stability is therefore conferred to the rubber particles which hinder film formation in the manufacture of products such as gloves [26, 27]. Work is currently in progress to epoxidise natural rubber with a thinner adsorbed polymer layer for application in dipped glove manufacturing.

5 Properties of Liquid Epoxidised Natural Rubber (LENR) Liquid epoxidised natural rubber (LENR) is a sticky, viscous and soft form of ENR. The appearance varies depending on the molecular weight (MW). The shorter the polymer chain, i.e. molecular weight, the softer and stickier LENR is. LENR is prepared by degradation of ENR in the latex stage, i.e. water phase in the presence of chemicals such as redox degrading agents [28, 29], distinguished from the dissolution of ENR in organic solvents. This route is considered greener compared with the solvent route as it is environmentally friendly, and the redox degrading agents are

52

N. H. Yusof et al. O

O

*

*

(a) ENR

X2

Y2

*

*

(b) LENR

X1

Y1

Fig. 23 Chemical structures of a ENR and b LENR

water-soluble chemicals. To expand ENR application in niche sectors, LENR was synthesised from ENR using the degradation method. In general, ENR and LENR have similar structures, as shown in Fig. 23a and b, respectively. X2 and Y2 (ENR) represent the repeating units for both epoxy (C–O–C) and isoprene (C = C) units before degradation respectively, whereas X1 and Y1 (LENR) represent the repeating units obtained after degradation. Numbers 2 and 1 denote epoxy and C = C before and after degradation, in which LENR (X1,Y1) has a shorter repeating unit compared to ENR (X2,Y 2) due to chain scission by degradation. In this sub-chapter, ENR 50 latex is used as a starting material to produce LENR 50.

5.1 Conditions for Preparation of LENR 5.1.1

ENR 50 Latex Storage Time

Table 6 shows the properties of LENR 50 prepared using ENR 50 latex stored at different periods before degradation was performed. The number average molecular weight, M n and weight average molecular weight, M w for ENR at Day 0 were 14.3 × 104 g/mol and 34.5 × 104 g/mol, respectively, with high gel content, i.e. 40 wt%. The epoxidation level was calculated as 49 mol %, which reflected the efficiency of the epoxidation of ENR targeted at 50 mol %. The molecular weight and gel content were then significantly decreased after degradation of ENR took place, i.e. LENR 1, 3 and 6. However, the epoxidation level showed a slight increase which may be due to the formation of C–O–C resulting from incomplete chain scission of C = C during degradation. When ENR latex is stored up to 150 days, some changes in the molecular weight, epoxidation level and gel content are observed in the resulting LENRs. This implies that ENR latex can be stored for up to 150 days before degradation takes place considering the properties of LENR 50 that remained fairly consistent throughout the storage period of latex.

5.1.2

Surfactant Concentrations on ENR 50 Latex Stability

The effect of surfactant concentration on ENR 50 latex stability throughout the reaction is shown in Fig. 24. The destabilisation period was represented by the “x” symbol. ENR latex destabilised at early stages of the reaction at lower surfactant

The Properties of Materials from Raw Epoxidised Natural Rubber

53

Table 6 Properties of LENR prepared from ENR 50 latex after storage Storage of ENR 50 latex (Days)

Prepared LENR 50

Number average molecular weight, Mn × 104 , g/mol

Weight average molecular weight, Mw × 10 4 , g/mol

Epoxidation level, mol %

Gel content, w/w %

0

ENR 50

14.3

34.5

49.0

40.0

1

LENR 1

1.40

5.04

52.1

0.59

60

LENR 3

1.39

4.24

54.7

0.73

150

LENR 6

1.57

4.65

54.5

0.75

Fig. 24 Effect of surfactant concentrations (0 to 3.0 wt%) on the stability of the ENR 50 latex during reaction (destabilised condition is represented by the “x” symbol)

concentrations but the stability improved as the surfactant concentration increased. Presence of the surfactant hinders the latex particles from coalescing and ensures optimal penetration of chemicals during degradation, over a period of 8 h. Figure 24 shows that latex stability was sustained throughout the reaction with surfactant concentrations between 2–3 wt% at 65–70 °C for 8 h.

5.1.3

pH of ENR 50 Latex

Table 7 shows the effect of pH on the properties of ENR 50 latex and LENR 50 . In this study, ENR latex was the control for every pH condition studied, i.e. 6, 8 and 11 to prepare LENR. Epoxidation level of ENR latex varied minimally from batch to batch. It was observed that increasing the pH of ENR latex affected the properties of the resulting LENR. At pH 8, changes to the epoxidation level of LENR was the lowest, whereas changes to the Mn and Mw were the highest. On the other hand, at

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N. H. Yusof et al.

Table 7 Effect of pH on the properties of ENR 50 latex and LENR 50 Mn x 104, g/mol pH 6 8 11

ENR

LENR

52.5 51.6 52.2

62.1 56.8 60.9

Changes (%) 18 10 17

ENR

LENR

9.153 11.115 7.208

1.206 1.239 1.872

Mw x 104, g/mol Changes (%) 87 89 74

ENR

LENR

29.951 48.209 17.031

3.353 4.534 5.610

Changes (%) 89 91 67

pH 6 and 11, epoxidation level of LENR as well as changes to the Mn and Mw were vice versa. LENR is prepared in two steps using peroxynitrite produced by the reaction pathways between sodium nitrite (NaNO2 ) and hydrogen peroxide (H2 O2 ). In the first pathway, alkene from ENR 50 is further epoxidised by the first mole of peroxynitrite to form more epoxide groups in ENR 50. This reaction is then followed by the second pathway, whereby the second mole of peroxynitrite oxidises H2 O2 with the second mole of peroxynitrite to produce nitrite ion, water and oxygen. The nitrite ion which is highly nucleophilic will then attack the epoxy ring of ENR 50 through chain scissions to reduce the molecular weight and form LENR 50. Fig. 25 shows LENR 50 reaction mechanism produced from ENR 50 at pH 8 using NaNO2 and H2 O2 [30–32]. pH 8 was selected for the reaction as it produced LENR 50 with the lowest changes in epoxidation level as well as highest changes in Mn and Mw . At highly basic conditions, pH 11, H2 O2 tends to decompose more into water and oxygen. In the state of reduced H2 O2 , the second reaction step could not take place effectively, leading to lower chain scissions which resulted in higher changes to the epoxidation level (17% increase) but lower changes in molecular weight (67%). Similar changes in epoxidation level and molecular weights at pH 11 were observed at pH 6, however the changes in pH 6 were attributed to H2 O2 oxidising the alkene to epoxy strongly [33]. Compared to pH 6 and pH 11, the opposite was observed for pH 8 where the changes in epoxidation level were lower with highest change in molecular weights, due to more stable H2 O2 towards the second step of the reaction.

5.1.4

Degrading Agent Concentrations

Figure 26 shows the effect of degrading agent concentrations on the molecular weight of LENR 50. The M n and M w were substantially decreased as the degrading agent concentrations increased. The decrease implies that the chemicals are readily utilised to scission the ENR polymer chain. Figure 27 shows the effect of degrading agent concentrations on the gel content of LENRs. ENR 50 as a control contained the highest gel content, i.e. 52 w/w%

The Properties of Materials from Raw Epoxidised Natural Rubber

55

Fig. 25 A scheme showing LENR 50 reaction mechanism produced from ENR 50 at pH 8 using sodium nitrite (NaNO2 ) and hydrogen peroxide (H2 O2 )

Fig. 26 Effect of degrading agent concentrations (sodium nitrite and hydrogen peroxide at a 1:1 ratio) on M n and M w of LENR 50

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Fig. 27 Effect of degrading agent concentrations on gel content of LENR 50

which then decreased remarkably until a value of less than 1 w/w% when higher amounts of degrading agent concentrations were used. The increased concentration of chemicals shortens the polymer chains resulting in lower gel content as a result of reduced cross-linking junctions and entanglements in the polymer.

5.2 Chemical Properties of LENR 5.2.1

Characterisation of Functional Groups Using Fourier Transform Infra-Red (FTIR) Spectroscopy

Figure 28 shows the FTIR spectra ranging from 4000 to 400 cm−1 for ENR 50 and LENR 50. The absorption peaks of ENR appeared at 3464, 2962, 1663, 1460, 1370, 879 and 840 cm−1 which were identified as OH (hydroxyl) stretching, CH2 stretching, C = C stretching, CH2 stretching, CH3 deformation, C-O-C (epoxy ring) and CH olefin wagging, respectively. In addition, the absorption peaks at 1119, 1069 and 1022 cm−1 were identified as C-O (ester) stretching. In the case of LENR, the absorption peaks ascended at 3459, 1083–1025 and 875 cm−1 and were identified as OH stretching, C-O stretching and C-O-C, respectively. The increase of peak intensity at 3459 cm−1 suggests the formation of OH groups associated with side reactions during the chain scissions of ENR. The formation of chain scissions was supported by the appearance of absorption peaks at 1774 and 1716 cm−1 , which were identified as C = O (carbonyl) groups of aldehyde and ketone, respectively. In addition, the broad region at 1083–1025 cm−1 and the peak at 875 cm−1 of LENR indicate the increase of C-O and C-O-C, respectively. A schematic mechanism was proposed indicating the redox reagents to be attracted only to C = C and C–O–C bonds for chain scission [34, 35]. Moreover, CH2 was not significantly affected by the degradation of ENR to produce LENR [36, 37]. Hence, this peak was used as a reference for quantifying the absorption peak area ratio of

The Properties of Materials from Raw Epoxidised Natural Rubber

57

Fig. 28 FTIR spectra of ENR 50 and LENR 50

functional groups, for instance OH, C = O, C = C, C-O and C–O–C before (ENR) and after (LENR) degradation. The absorption peak area ratios for ENR 50 and LENR 50 are presented in Table 8. From the table, the peak area ratios of OH, C−O and C–O–C of LENR were higher than ENR, reflecting on the similar observations of peak intensity in Fig. 28. The peak area ratio of both OH and C = O groups of LENR increased twofolds, while the peak area ratio of C = C of LENR decreased compared with ENR Table 8 Absorption peak area ratio for ENR 50 and LENR 50

Peak assignment

Absorption peak area ratio AFG /A2962 ENR

LENR

CH2 (reference peak)

1

1

OH

1.16

2.71

C=O

0.10 (1734 cm−1 )

0.05 (1774 cm−1 ) 0.23 (1716 cm−1 )

C=C

0.07

0.01

C = O attached to amine

0.40

0.33

C-O

3.46

5.56

C–O–C

0.82

1.36

AFG = Peak area of Functional Group; A2962 = Peak area of CH 2

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indicating the formation of C = O as functional groups from chain scission of C = C. Furthermore, the peak area ratio of C−O−C of LENR was higher than ENR, probably due to the peroxynitrite ion reaction on the alkene of ENR. Hence, the FTIR spectroscopy and absorption peak area ratios confirmed that degradation occurred, in which more polar groups, i.e. OH (hydroxyl), C = O (carbonyl), C-O, C−O−C groups were formed. According to previous work, a small amount of hydroxyl and carbonyl groups belonged to the functional ends of LENR. These findings corresponded with the degradation of liquid natural rubber (LNR), where carbonyl and hydroxyl groups formed after degradation in the base medium [29, 32, 38].

5.2.2

Structural Characterisation Using Nuclear Magnetic Resonance (NMR) Spectroscopy

Figure 29 shows the 1 H-NMR spectra of ENR 50 and LENR 50. The signals that appeared at δ = 1.6 (Fig. 29a), 2.0 (Fig. 29b) and 5.1 ppm (Fig. 29c) were assigned to methyl proton (−CH3 ), methylene proton (−CH2 −) and methine proton (−CH=) of cis-1,4 -isoprene units, respectively, whereas the signals which appeared at δ = 1.2 (Fig. 29d) and 2.6 ppm (Fig. 29e) were assigned to methyl proton (−CH3 ) and methine proton (−CH=) of the epoxy group. This implies that the epoxidation level of LENR was retained after degradation of ENR. (d) (a)

(b) (c)

ENR 50

(a)

(e)

(c)

(b) (b) LENR 50

Fig. 29

1 H-NMR

spectra of ENR 50 and LENR 50

(d)

(e)

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Fig. 30 Expanded 1 H-NMR spectra of ENR 50 and LENR 50 from 9 to 10 ppm

Figure 30 shows the expanded 1 H-NMR spectra of ENR and LENR from 9 to 10 ppm (as depicted in Fig. 29). Two new proton signals appeared at the chemical shift from 9 to 10 ppm in the 1 H-NMR spectrum of LENR. These proton signals at δ = 9.3–9.4 and 9.8 ppm belonged to the proton of aldehyde (O = C−H) functional end group in the LENR structure [36, 39]. Figure 31 shows the expanded 1 H-NMR spectra of ENR 50 and LENR 50 at the chemical shift from 3 to 4 ppm (as depicted in Fig. 29). The small proton signals from 3.2 to 3.4 ppm were assigned to the proton of the hydroxyl group (Fig. 31f) and the proton linked to the carbon of the secondary alcohol (Fig. 31g). These assignments suggest the formation of hydroxyl groups, in particular, diol in ENR and LENR compared with the hydroxyl functional end groups. The formation of the diol group may have occured due to a secondary epoxide ring-opening reaction during epoxidation and degradation to form ENR and LENR, respectively [29, 32, 40]. There was also a broad signal that appeared at δ = 3.6–3.8 ppm as shown in Fig. 32 (as depicted in Fig. 29). The signal indicated a furan ring which may form due to the interaction between two adjacent epoxy groups during the opening reaction [33, 40]. It can be deduced that the formation of aldehyde (carbonyl) as a functional end group, and that of diol and furan as other functional groups occurred during the degradation of ENR to LENR. These other functional groups are considered as epoxy derivatives, calculated from NMR measurements based on the amount of diol and furan to the overall functional groups in the structure. These epoxy derivatives increased significantly after LENR formation [28, 29, 41]. Apart from this, there was also a significant proton signal at δ = 3.58 ppm that appeared between the furan

ENR 50

f g

f g

LENR 50

Fig. 31 Expanded 1 H-NMR spectra of ENR 50 and LENR 50 indicating presence of diol groups at 3–4 ppm

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Fig. 32 Expanded 1 H-NMR spectrum of LENR 50 indicating furan and diol chemical shifts

and diol signals which was attributed to the non-ionic surfactant, Teric used as the stabiliser for the preparation of ENR from NR latex. Figure 33 shows the 13 C NMR spectrum of LENR 50 which shows the chemical shifts at (a) δ = 22.28 ppm, (b) 134.67 ppm, (c) 124.57 ppm and (d) 32.96 ppm assigned to (−C(CH3 ) = CH−), (−C(CH3 ) = CH−), (−C(CH3 ) = CH−) and (−CH2 C(CH3 ) = CHCH2 −), respectively. Signals at (e) δ = 15.96 ppm, (f) 60.77 ppm, (g) 77.07 ppm and (h) 25.60 ppm were assigned to (-C(CH3 )OCH-), (-C(CH3 )OCH-), (-C(CH3 )OCH−) and (−CH2 C(CH3 )OCHCH2 −) groups of epoxidised cis-1,4-isoprene units, respectively.

Fig. 33

13 C

NMR spectrum of LENR 50

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5.3 Morphology of LENR The particle size distribution of both ENR 50 and LENR 50 latexes measured using laser diffraction technique is shown in Fig. 34. ENR latex showed a similar particle distribution as that of natural rubber latex, namely a bimodal particle distribution [42]. There were two peak modes of ENR latex particle size observed around 0.3 microns and 1 micron, respectively. The LENR latex particles showed a narrower distribution, with the single peak shifted to a slightly larger size region indicating larger sized LENR particle compared with that ENR latex [43]. To visualise both these latex particles, field emission scanning electron microscope (FESEM) was used. FESEM images for (a) ENR 50 and (b) LENR 50 latexes at 15,000 × magnification are illustrated in Fig. 35. The ENR latex had a relatively larger population of smaller spherical size particles compared with that of LENR latex particles. On the other hand, LENR particles were larger in size and slightly elongated. ENR latex particles are composed of compact and tightly coiled long chains which may lead to a smaller particle size [42, 44]. In the case of LENR latex, the particles were much larger probably due to the presence of more linear and shorter rubber chains. As a result, the chains are less entangled with a higher degree of motion within the particles, resulting in an increase in particle hydrodynamic volume.

Fig. 34 Particle size distribution of ENR 50 and LENR 50 latexes

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Fig. 35 FESEM images of a ENR 50 latex (x10K) and b LENR 50 latex (x15K)

5.4 Thermal Properties of LENR 5.4.1

Glass Transition Temperature Using Differential Scanning Calorimeter (DSC)

Figure 36 shows the DSC thermograms of ENR 50 and LENR 50. Single glass transition temperatures (Tg) were observed for both latexes indicating the homogeneity of the rubbers. The Tg of LENR at -20 °C was higher compared to the Tg of ENR at -27 °C by 7o C. Furthermore, the abrupt slope of LENR was observed in contrast with ENR may be due to higher interactions of polar groups [38, 45] present on the backbone of LENR. Moreover, the decrease of C = C could also suppress chain flexibility in LENR, leading to a substantial increase in its glass transition temperature.

Fig. 36 Thermograms of ENR 50 and LENR 50

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Fig. 37 a TG curves and b DTG curves for ENR 50 (yellow line) and LENR 50 (orange line)

5.4.2

Decomposition Using Thermogravimetric (TG) Analysis

Figure 37 show the TG and DTG curves of ENR 50 and LENR 50. The TG curves revealed a single-step degradation for both rubbers, showing degradation of the major polymers. Compared with ENR 50, the TG curve of LENR 50 was shifted to the right as temperature increased, which reflects the increase in maximum decomposition temperature of LENR 50, i.e. 432 °C compared with ENR 50, i.e. 382 °C. This suggests that the increase in polar groups improves the thermal stability of LENR. Figure 38 shows the effect of degrading agent concentrations on the maximum degradation temperature of NR, ENRs and LENRs. The maximum degradation temperatures (0 phr) increased for ENR25 and ENR50 when compared to NR which indicate higher thermal stability for ENR as the epoxidation level increased. Interestingly, the degradation temperatures also increased for LENR 25 and LENR 50 compared to ENR (15 phr and 20 phr). This may be due to the formation of diol, carbonyl and furan groups that contributes to better thermal stability.

Fig. 38 Maximum degradation temperature of LENRs with different epoxidation level percentages

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5.5 Wettability Using Contact Angle Analysis Figure 39 shows the wettability of ENR 50 and LENR 50 measured using contact angle analysis. The measurement was conducted by observing the response of a water droplet on the sample surface. The contact angle of LENR was lower than ENR, indicating its higher surface energy and superior hydrophilicity. This may be attributed to the presence of polar groups on the structure of LENR. Though so, the values for both ENR and LENR decreased over time, reflecting the increase in hydrophilicity and surface energy with time. Fig. 39 Contact angle of ENR 50 and LENR 50

5.6 Viscosity Measurement Using Brookfield Viscometer The viscosity measurement was performed at 120 °C to measure the flow characteristics of the polymers. Since LENR is intended for adhesive application, thus, viscosity is one of the important parameters to be determined. The relationship of number average molecular weight, M n , and viscosity for five batches of LENR 50 are shown in Fig. 40. The viscosity measured was in the range of 11,000 to 16,000 cP for M n ranging from 6500 to 7500 g/mol which further indicates consistent values. Overall, the viscosity of LENR was averaged to be at 13,000 ± 2,000 cP.

The Properties of Materials from Raw Epoxidised Natural Rubber

65

20000 18000

Viscosity, cP

Viscosity, cP

16000 14000 12000 10000 8000 6000 5000

6000

7000

8000

9000

Number average molecular weight, Mn

Fig. 40 Effect of number average molecular weight, M n on the viscosities of LENR 50 batches

5.7 Storage Stability of LENR Figure 41 shows the number average molecular weight, M n , and weight average molecular weight, M w upon storage. The LENR was kept at ambient temperature for nearly two years and collected for molecular weight measurements at various time intervals. From the graph, the M n and M w for the 1st day of storage were 1.186 × 104 g/mol and 4.126 × 104 g/mol, respectively. Both molecular weights fluctuated at the early stages of storage. However, a consistent trend was observed as the storage time increased. The average fluctuation was calculated to be within ± 3000 g/mol for both M w and M n , and thus indicating no substantial changes in molecular weights of LENR upon storage. This implies that LENR was stable for 758 days ( 2 years of storage) [41, 46]. The changes in LENR 50 epoxidation levels during storage are shown in Fig. 42. The epoxidation level was calculated based on the amount of epoxy groups in the rubber from the NMR spectrum. The epoxidation level of LENR 50 on the 1st day of storage was 55.6 mol% and it showed a slight decrease with increasing storage time up until the 360th day. This indicated a less active opening of the epoxy ring, confirming a more controllable and stable LENR 50 [41, 46].

6 Conclusions The characterisation and properties evaluation of ENR rubber, ENR concentrated latex and liquid ENR were extensively characterised to understand the materials and their potentials for various applications, especially for niche markets. This indirectly expands the NR application segment by providing an alternative and sustainable material from a renewable source with properties comparable to its synthetic counterparts. The main specifications of ENR rubber are epoxidation level, ring-opening

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Fig. 41 Molecular weights of LENR 50 during storage

Fig. 42 Epoxidation level of LENR 50 upon storage

level, glass transition temperature, Mooney viscosity, ash content, nitrogen content and Lovibond colour index. The 1 H-NMR spectrometry is the preferred method to determine the epoxidation level of raw ENR and LENR. Concentrated ENR latex, however, showed a unimodal narrow particle size distribution with lower zeta potential isoelectric point. The condition of ENR latex as a starting material is crucial before the degradation reaction for LENR production. A suitable level of epoxidation and pH of latex determine the desired properties of LENR. The molecular weight and gel content of LENR produced are dependent on the observance of these conditions.

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References 1. ISO International Standard (2000) Rubber, raw natural — guidelines for the specification of technically specified rubber (TSR). ISO 2020:1–10 2. Malaysian Rubber Board (2018) SMR bulletin No 7 revised edition 2018. Malaysian Rubber Board: 1–145 3. Gelling IR (1991) Epoxidised natural rubber. J Nat Rubb Res 6(2):271–297 4. Mohd Nor Z (2007) EKOPRENA the “Green” material. MRB Rubb Technol Dev: 6–7 5. Burfield DR, Kooi Ling L, Kia Sang L, Soon N (1984) Analysis of epoxidized natural rubber, a comparative study of DSC, NMR, elemental analysis and direct titration methods. Polym (Guildf) 25(7):995–998 6. Davey J, Loadman M (1984) A chemical demonstration of the randomness of epoxidation of natural rubber. Br Polym J 16(3):134–138 7. Gnecco S, Pooley A, Krause M (1996) Epoxidation of low-molecular-weight Euphorbia lactiflua natural rubber with “in situ” formed performic acid. Polym Bull 37:609–615 8. ISO International Standard (2019) ASTM D3677 – 10 standard test methods for rubber— identification by infrared spectrophotometry 9. Heping Y, Zongqiang Z, Guang L, Qifang W (2008) Processing characteristics and thermal stabilities of gel and sol of epoxidized natural rubber. Eur Polym J 44(2):453–464 10. Mohd Rasdi FR, Mohamed Yusof Y, Mohd Som F (2006) Evaluation of epoxidized natural rubber from latex concentrate. Young Res Conf Appl Sci 11. Takayuki S, Warunee K, Seiichi K (2007) Characterization of epoxidized natural rubber by 2D NMR spectroscopy. Polym (Guildf). 48 (3):750–757 12. Hamzah R, Abu Bakar M, Khairuddean M, Mohammed IA, Adnan R (2012) A structural study of epoxidized natural rubber (ENR 50) and its cyclic dithiocarbamate derivative using NMR spectroscopy technique. Molecules 17:10974–10993 13. Tengfei L, Xuhui Z, Zhenghai T, Baochun G (2015) Renewable conjugated acids as curatives for high performance rubber/silica composites. Green Chem 17(6):3301–3305 14. ISO (2014) ISO 124 Latex, Rubber – Determination of Total Solids Content 15. ISO (2005) ISO 126 Natural Rubber Latex Concentrate – Determination of Dry Rubber Content 16. ISO (2011) ISO 125 Natural Rubber Latex Concentrate – Determination of Alkalinity 17. Singh M, Mohd RF (2019) Colloidal properties of epoxidized natural rubber latex prepared via membrane separation technology. J Rubb Res 22(4):153–167 18. Blackley D (1997) Polymer latices science and technology. Fundam Principles 1(2) Chapman & Hall 19. Hunter R (1981) Zeta potential in colloid science: principles and applications. Academic Press, London 20. Singh M, Esquena J, Solans C, Booten K, Tadros T (2014) Influence of hydrophobically modified inulin (INUTEC NRA) on the stability of vulcanized natural rubber latex. Colloids Surf A 451:90–100 21. Kronberg B (2001) Chapter 22: Measuring adsorption. In: Holmberg K (ed) Handbook of applied surface and colloid chemistry. Wiley, England 22. Bingham E (1922) Fluidity and plasticity. McGraw-Hill, New York 23. Quemada D (1998) Rheological modeling of complex fluids I. The concept of effective volume fraction revisited. Eur Phys J AP 1:119–27 24. Tadros T (2010) Rheology of dispersions: principles and applications. Wiley, Weinheim 25. Nestor J, Obiols-Rabasa M, Esquena J, Solans C, Levecke B, Booten K et al (2008) Viscoelastic properties of polystyrene and poly(methyl methacrylate) dispersions sterically stabilized by hydrophobically modified inulin (polyfructose) polymeric surfactant. J Colloid Interface Sci 319:152–159 26. Darji D, Md Said M (2010) Vulcanisation and coagulant dipping of epoxidised natural rubber. Pertanika J Sci Technol 18(2):421–425

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27. Che Ali R, Mohd Yazid N, Mohd Rasdi FR, Mustafa A (2015) Effect of pre-vulcanization time on the latex particles, surface morphology and strength of epoxidised natural rubber films. In: CAMS-2015: 3rd International Conference on Chemical, Agricultural and Medical Sciences: 64–69 28. Yusof NH, Darji D, Nesan KVB, Mohd Rasdi FR (2018) Preparation of liquid epoxidized natural rubber in latex stage by chemical degradation. In: AIP Conf Proc 1985(1):040006 29. Yusof NH, Darji D, Mohd Rasdi FR, Nesan KVB (2021) Preparation and characterisation of liquid epoxidised natural rubber in latex stage by chemical degradation. J Rubb Res 24(1):93– 106 30. Bach R, Glukhovtsev M, G C (1998) High-level computational study of the stereoelectronic effects of substituents on alkene epoxidations with peroxyformic acid. J Am Chem Soc 120(38):9902–9910 31. Lobachev V, Rudakov E (2006) The chemistry of peroxynitrite. Reaction mechanisms and kinetics. Russ Chem Rev 75(5):375 32. Ibrahim S, Daik R, Abdullah I (2014) Functionalization of liquid natural rubber via oxidative degradation of natural rubber. Polym (Basel) 6(12):2928–2941 33. Bac N, Terlemezyan L, Mihailov M (1993) Epoxidation of natural rubber in latex in the presence of a reducing agent. J Appl Polym Sci 50(5):845–849 34. Klinklai W, Kawahara S, Mizumo T, Yoshizawa M, Sakdapipanich J, Isono Y et al (2003) Depolymerization and ionic conductivity of enzymatically deproteinized natural rubber having epoxy group. Eur Polym J 39(8):1707–1712 35. Rooshenass P, Yahya R, Gan S (2018) Preparation of liquid epoxidized natural rubber by oxidative degradations using periodic acid, potassium permanganate and UV-irradiation. J Polym Environ 26(4):1378–1392 36. Rooshenass P, Yahya R, Gan S (2016) Comparison of three different degradation methods to produce liquid epoxidized natural rubber. Rubb Chem Technol 89(1):177–198 37. Yongyue L, Changjin Y, Bangqian C, Kui X, Zhong J, P Z, et al (2013) Thermal degradation of epoxidized natural rubber in presence of neodymium stearate. J Rare Earths 31(5):526–30 38. Khalyavina A, Häußler L, Lederer A (2012) Effect of the degree of branching on the glass transition temperature of polyesters. Polym (Guildf) 53(5):1049–1053 39. Thitithammawong A, Srangkhum S, Rungvichaniwat A (2011) Hydroxytelechelic natural rubber from natural rubber and epoxidised natural rubber. J Rubb Res 14(4):230–440 40. Davey J, Loadma M (1984) Chemical demonstration of the randomness of epoxidation of natural rubber. Br Polym J 16(3):134–138 41. Darji D, Yusof NH, Mohd Rasdi FR (2018) Shelf life of liquid epoxidized natural rubber (LENR). In: AIP Conf Proc 42. Sriring M, Nimpaiboon A, Kumarn S, Sirisinha C, Sakdapipanich J, Toki S (2018) Viscoelastic and mechanical properties of large-and small-particle natural rubber before and after vulcanization. Polym Test 70:127–134 43. Tangpakdee J, Mizokoshi M, Endo A, Tanaka Y (1998) Novel method for preparation of low molecular weight natural rubber latex. Rubb Chem Technol 71(4):795–802 44. Tarachiwin L, Sakdapipanich J, Tanaka Y (2005) Relationship between particle size and molecular weight of rubber from Hevea brasiliensis. Rubb Chem Technol 78(4):694–704 45. Agapov A, Wang Y, Kunal K, Robertson C, Sokolov A (2012) Effect of polar interactions on polymer dynamics. Macromol 45(20):8430–8437 46. Darji D, Yusof NH, Mohd Rasdi FR (2021) Characterization of liquid epoxidized natural rubber (LENR) upon storage. J Rubb Res 24(3): 435-446

General Compounding and Properties of Epoxidised Natural Rubber Siti Salina Sarkawi, Ahmad Kifli Che Aziz, and Nik Intan Nik Ismail

Abstract Epoxidation of natural rubber results in an orderly increase for both polarity and glass transition temperature of the Epoxidised Natural Rubber (ENR), which alters the physical properties of ENR vulcanisates. Basic compounding and physical properties of the ENR vulcanisate, particularly ENR 25 and ENR 50 are delineated here. The reinforcing effect of carbon black and silica in ENR compounding are compared and described further. The effect of epoxidation level on the physical properties of the ENR vulcanisates are also explained. The epoxidation level greatly influences resilience, tear strength and air permeability of the ENR vulcanisate. On the other hand, loading of fillers (carbon black or silica) has an enhanced effect on tensile strength and abrasion resistance than the epoxidation level, whilst hardness is less affected by the epoxidation level. Moreover, the advantages of ENR with special emphasis on its oil resistance characteristic and low gas permeability are reviewed. The oil resistance of ENR 50 falls between that of polychloroprene (CR) and medium grade nitrile rubber (NBR-34% acrylonitrile content). Similar improvements are observed for gas permeability, where ENR 50 has good impermeability, similar to butyl rubber. Keywords Epoxidised natural rubber · Oil resistance · Air permeability · Resilience · ENR 25 · ENR 50

1 Introduction Chemical modification of natural rubber (NR) via the epoxidation process in latex yields a speciality rubber termed epoxidised natural rubber (ENR) [1, 2]. Depending on the degree of epoxidation ENR has improved and unique properties compared with NR [3–5] in terms of oil resistance, damping and gas permeability. An increase in the epoxidation level of ENR increases the glass transition temperature of the rubber systematically [6, 7]. This contributes to changes in physical properties of the S. S. Sarkawi (B) · A. K. Che Aziz · N. I. Nik Ismail Malaysian Rubber Board, Sungai Buloh, Selangor, Malaysia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. S. Sarkawi et al. (eds.), Epoxidised Natural Rubber, https://doi.org/10.1007/978-981-19-8836-3_4

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ENR vulcanisates. An extent of epoxidation also enhances the polarity of ENR. The presence of polar epoxide groups in the ENR allows for compatibility with highly polar fillers such as silica [1, 2, 8]. This chapter discusses compounding and mixing considerations for the ENR compound. Basic physical properties of ENR vulcanisates particularly ENR 25 and ENR 50 are presented. The effect of epoxidation level on the physical and dynamic properties of the ENR vulcanisates are described. The reinforcing effect of carbon black and silica in ENR compounding are compared. Additionally, the properties of ENR vulcanisates in comparison with synthetic rubbers such as chloroprene rubber (CR), nitrile rubber (NBR) and butyl rubber (IIR) are also included.

2 Mixing and Compounding of ENR Two commercial grades of ENR are available, namely ENR 25 and ENR 50, which correspond to 25 mol % and 50 mol %, respectively. Semi-efficient vulcanisation (semi-EV) and efficient vulcanisation (EV) cure systems are both recommended for the sulphur vulcanisation of ENR. Although the conventional sulphur system (high sulphur/low accelerator) is an alternative, it is less favoured due to its poor ageing characteristics [7]. An EV system is particularly useful if good compression set values are required. Special attention should be given when mixing the ENR with fillers as presence of the filler increases the Mooney viscosity and the chances of scorch taking place. As ENR is a polar rubber, it is highly compatible with polar fillers such as silica. ENR 25 and ENR 50 can be reinforced with a silica filler in the absence of a coupling agent. However, for the ideal combination to optimise processability and improve vulcanisate properties, reinforcing the ENR with silica and a small amount of coupling agent is required. The recommended amount of silane coupling agent used for silica reinforced ENR is about 2–4 wt% relative to the amount of silica used [9, 10]. This leads to a reduction of 60–80% in the amount of coupling agent used as compared with its usage in regular NR compounds. The suggested basic compounding formulations for carbon black and silica-filled ENR 25 using a semi-EV vulcanisation system is shown in Table 1. The processing behaviour of ENR is different from that of NR due to its chemical nature. In particular, pH can have a large effect on its scorching characteristics. Hence, a base compound such as calcium stearate is required to control processing safety [3]. The recommended amount of calcium stearate is 2 phr for ENR 25 and 5 phr for ENR 50 [2]. The use of calcium stearate improves the air-ageing properties of ENR compared with NR [2]. If the mill-sticking problem is encountered, an additional level of stearic acid and a partial sulphur crosslinking can be incorporated to improve the cohesive strength of the ENR [11]. Recommended mixing cycle for carbon black and silica masterbatches in a Banbury internal mixer (BR1600) are given in Tables 2 and 3, respectively. Normally, silica compounds mixing requires several stages where the remill stage (2nd slow

General Compounding and Properties of Epoxidised Natural Rubber Table 1 Proposed ENR formulations using a semi-EV vulcanisation system

Ingredients

71

Parts per hundred rubber (phr) Carbon black

Silica

ENR

100

100

Calcium stearate

2e

2e

Carbon black

As required

−

Silica

−

As required

−

4 wt% relative to silica content

Processing oilb

5

5

Zinc oxide

3

3

Stearic acid

2

2

Antioxidant

2

2

Sulphur

1.5

1.5

CBSc

1.5

1.5

DPGd

–

2

Coupling

agenta

a Bis-(triethoxysilylpropyl)

tetrasulphide (TESPT) distillate aromatic extract c N-cyclohexyl-2-benzothiazyl sulfenamide d Diphenyl guanidine e Amount of calcium stearate is increased to 5 phr if ENR 50 is used b Treated

stage: Table 4) is carried out to increase silanisation efficiency and reduce the viscosity of the compound [10, 12]. Sulphur and accelerators should be added to the above masterbatch either in the final slow stage or earlier, in the mill. In the latter case, the mixing time should be kept to a minimum to avoid the possibility of the compound sticking to the rolls. Table 2 Mixing steps for the carbon black-filled ENR masterbatch

Time (min)

Step

0

Load rubber + calcium stearate

1

Load half of black and oil

1½

Load remainder of black

2¼

Load powder ingredients (excluding sulphur and accelerators)

3

Sweep

4

Dump

72 Table 3 Mixing steps for the silica-filled ENR masterbatch (1st stage–mixing)

S. S. Sarkawi et al. Time (min)

Step

0

Load rubber + powder (including calcium stearate)

1

Load half of silica, coupling agent and oil

2

Load remainder of silica, coupling agent and oil

3

Sweep and maintain temperature at 140 °C for another 3 minutes

Note: Dump after 3 min above 140 °C

Table 4 Mixing steps for the silica-filled ENR masterbatch (2nd stage–remilling)

Time (min)

Step

0

Load masterbatch

1½

Sweep

3

Dump

3 Basic Physical Vulcanisate Properties of ENR General physical properties of ENR 25 and ENR 50 reinforced with 30 phr carbon black N234 and precipitated silica Zeosil 1165MP are summarised in Tables 5 and 6, respectively. Generally, both ENR 25 and ENR 50 vulcanisates filled with carbon black have slightly lower tensile strength than the NR vulcanisate. A slight reduction is observed for the elongation at break and modulus M300 of ENR 25 compared with NR. However, ENR 50 vulcanisate has a lower elongation at break but higher M300 compared to both NR and ENR 25 vulcanisates. The hardness and compression set of ENR 25 and ENR 50 are almost comparable. Abrasion resistance of ENR 25 and ENR 50 are similar and inferior to that of the NR. Reduction in resilience was observed for ENR 25 compared with NR, and is greater for ENR 50. Tear strength of ENR 25 and ENR 50 is inferior to NR, whilst air permeability of the ENR 25 and ENR 50 vulcanisates is superior to NR. These behaviours are also applicable to ENR 25 and ENR 50 vulcanisates filled with silica.

4 Comparison of Reinforcing Effect of Carbon Black and Silica in ENR Compounds Reinforcing fillers are used in rubber compounding to improve resultant properties. Two of the most common reinforcing fillers are carbon black and silica. The amount of filler loading greatly influences the physical properties of rubber vulcanisates [13]. The effect of carbon black (N234) and silica (Zeosil 1165MP) loading on the physical properties of ENR vulcanisates are shown in Figs. 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10. The compound formulations are shown in Table 1.

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Table 5 Physical properties of carbon black-filled ENR vulcanisates (30 phr N234) Properties

NR

ENR 25

ENR 50

Tensile strength, MPa

34.5

26.7

24.6

Elongation at break, %

712

670

600

Modulus at 100%, MPa

1.5

1.6

2.1

Modulus at 300%, MPa

6.5

5.9

7.4

Hardness, IRHD

58

52

49

Resilience, %

76

62

46

Abrasion vol. loss, mm3

148

213

211

Abrasion resistance index

125

87

88

Compression set, 22 h at 70 °C, %

22

28

27

Tear strength (Crescent), kN/m

103

27

18

8.13

3.58

0.72

Air permeability coefficient (Q) at 25 °C,

×10–17

,m2 ·Pa

Table 6 Physical properties of silica-filled ENR vulcanisates (30 phr Zeosil 1165MP) Properties

NR

ENR 25

ENR 50

Tensile strength, MPa

32.8

28.3

20.7

Elongation at break, %

600

570

430

Modulus at 100%, MPa

2.0

1.7

2.3

Modulus at 300%, MPa

10.0

8.7

11.1

Hardness, IRHD

55

52

57

Resilience, %

84

70

43

Abrasion vol. loss, mm3

148

194

215

Abrasion resistance index

125

95

86

Compression set, 22 h at 70 °C, %

21

26

23

Tear strength (Crescent), kN/m

99

67

17

10.44

3.47

1.08

Air permeability coefficient (Q) at 25 °C,

×10–17

,m2 ·Pa

Generally, an increase in filler loading increases the tensile strength to an optimum before it decreases [14]. Both carbon black and silica-filled NR have an optimum tensile strength at 30 phr filler loading (Fig. 1). However, for carbon black-filled ENR vulcanisates, the optimum tensile strength is shown at a higher loading of 45 phr. Silica-filled ENR vulcanisates behave differently where the ENR 10 follows the optimum of NR, whilst the optimum for ENR 25 and ENR 50 shifted to a lower loading of 15 phr. The effect of filler loading on the tensile strength of silica-filled ENR relates to the increase in crosslink density of silica-filled ENR due to the strong chemical interactions between ENR and silica in the presence of coupling agents [9]. The mechanism of a hybrid reinforcing effect of ENR and silica with a silane system consists of a combination of coupling and interactions between ENR and silica as illustrated in Fig. 2 [15].

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Fig. 1 Effect of filler loading on tensile strength of a carbon black- and b silica-filled ENR vulcanisates at various epoxidation levels

In Fig. 3, both moduli M100 and M300 of the ENR vulcanisates show the increase with carbon black and silica loading as expected. In carbon black-filled ENR 25 vulcanisate, the moduli M100 and M300 are almost comparable to the NR vulcanisate (Fig. 3a, c). However, the ENR 50 vulcanisate exhibits a higher moduli than the ENR 25. For silica-filled vulcanisates, ENR and NR display different behaviours. In silica-filled ENR vulcanisates, a continuous increase in M100 and M300 was observed with increasing silica loading (Fig. 3b, d). However, in NR filled with silica, the M100 started to level off, whereas the M300 decreased above the optimum of 45 phr silica. The increase in modulus of silica-filled ENR vulcanisate is due to restriction of the ENR chain motion at higher silica loadings as a result of the increase in crosslink density. Higher crosslink density of the silica-filled ENR is mainly due to more physical and chemical interactions between the ENR, silica and coupling agent compared with the NR [8]. Furthermore, no M300 is obtained for silica-filled ENR 50 at above 60 phr as the elongation at break is below 300%. This reflected a moderate influence of epoxidation level on the moduli of the ENR vulcanisates. The elongation at break is expected to decrease with increasing filler loading [14]. The elongation at break for carbon black-filled ENR and NR vulcanisates are constant and only start to decrease at the optimum loading of carbon black of 30 phr (Fig. 4). Both ENR 10 and ENR 25 show comparable elongation at break with NR at various carbon black loadings. The elongation at break of ENR 50 is inferior to both ENR 25 and NR. In silica-filled ENR vulcanisates, the elongation at break reduces with increasing silica loading (Fig. 4b). Compared with carbon black, the reduction in elongation at break is more pronounced in silica-filled ENR, and the effect of epoxidation can be observed. A decrease in the elongation at break for silica-filled ENR 50 vulcanisates occurs above 60 phr silica, where the elongation at break drops

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Fig. 2 The hybrid reinforcement of ENR with silica consists of these combinations: a interaction between the silanol group of silica with the epoxide group in ENR; b coupling of silica with ENR through ring-opening of ENR; and c silane coupling bonding

below 300%. The greater reduction in elongation at break for ENR-silica vulcanisates relates to the higher modulus at high silica loadings. The increase in crosslink density from the ENR-silica-silane interactions in silica-filled ENR (Fig. 2b) resulted in ENR chains becoming increasingly restricted during the deformation and in turn, decreasing elongation [9, 16]. Hardness of the ENR vulcanisate increases with increasing filler loading for both carbon black and silica as shown in Fig. 5. The compression set of the ENR vulcanisate increases with increasing filler loading for both silica and carbon black (Fig. 6). As rubber is a visoelastic material, a part of its energy is elastically stored and partly dissipated as heat during deformation. Resilience however is related to the tan δ and Tg of the rubber. In highly resilient rubber, most of the energy is returned and only a small portion is lost as heat. Conversely in rubber with low resilience, most of the impact energy is absorbed and dissipated as heat. In Fig. 7 both carbon black and silica loading cause a significant decrease in the resilience of the ENR vulcanisates.

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Fig. 3 Effect of filler loading for a carbon black- b silica-filled ENR vulcanisates on modulus M100 and c carbon black- d silica-filled ENR vulcanisates on modulus M300 at various epoxidation levels

The resilience of ENR 10 is almost comparable to NR. Addition of fillers reduces the elasticity of the rubber and increases heat loss. For ENR 25, silica has a lesser effect in reducing resilience compared with carbon black. This is mainly due to chemical bonding between the ENR, silica and coupling agent compared with the physical interaction between ENR and carbon black. The increase in both carbon black and silica loading in the ENR vulcanisate improves the abrasion resistance up to an optimum of 45 phr before it starts to deteriorate (Fig. 8).

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Fig. 4 Effect of filler loading on elongation at break of a carbon black- and b silica-filled ENR vulcanisates at various epoxidation levels

Fig. 5 Effect of filler loading on hardness of a carbon black- and b silica-filled ENR vulcanisates at various epoxidation levels

Tear strength of the ENR vulcanisate increases with increasing filler loading, then decreases after reaching an optimum level. The optimum loading of tear strength for carbon black-filled ENR is 60 phr (Fig. 9a). However, for silica-filled ENR, the optimum loading varies according to ENR grades where for ENR 25 it is at 30 phr and a plateau trend was observed for ENR 50 (Fig. 9b).

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Fig. 6 Effect of filler loading on compression set (22 h at 70 °C) of a carbon black- and b silica-filled ENR vulcanisates at various epoxidation levels

Fig. 7 Effect of filler loading on resilience of a carbon black- and b silica-filled ENR vulcanisates at various epoxidation levels

The increase in both carbon black and silica loading reduces the air permeability constant of the NR and ENR 10 vulcanisates (Fig. 10). However, no effects were observed on the air permeability constant at both filler loadings for ENR 25 and ENR 50. The ENR 25 vulcanisate shows a reduction of about half in air permeability from the NR vulcanisate with either carbon black or silica fillers. A good ageing resistance of ENR vulcanisates is obtained with the semi-EV vulcanisation system as shown in Fig. 11. The tensile strength and elongation at break retentions are comparable for both ENR and NR vulcanisates with carbon

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Fig. 8 Effect of filler loading on abrasion resistance of a carbon black- and b silica-filled ENR vulcanisates at various epoxidation levels

Fig. 9 Effect of filler loading on tear strength of a carbon black- and b silica-filled ENR vulcanisates at various epoxidation levels

black and silica reinforcements (Fig. 11a, b). This was also applicable for M100 and M300 retention after ageing (Fig. 11c, d). Hence, using either a semi-EV or EV cure system for ENR vulcanisates is recommended to obtain optimal ageing resistance. It is a known fact that the conventional high sulphur formulation for ENR causes ageing inferior resistance to that of NR with a reduction in tensile strength and elongation at break as well as an increase in the modulus [2, 5].

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Fig. 10 Effect of filler loading on air permeability constant of a carbon black- and b silica-filled ENR vulcanisates at various epoxidation levels

A summary of the effect of increasing filler loadings on the properties of ENR vulcanisates is given in Table 7. The tensile strength, tear strength and abrasion resistance increase to an optimum level before declining with the increase in filler loadings. However, tear strength of the silica-filled ENR 50 is not affected by filler loadings. The modulus, hardness and compression set increase with the increase in filler loadings. Conversely, the elongation at break and resilience decrease with increasing filler loadings. Nonetheless, the filler loadings do not influence air permeability for ENR 25 and ENR 50. A schematic representation of the relationship, which generally applies to carbon black and silica reinforcements is presented in Fig. 12. Table 8 shows the typical comparative properties of NR, ENR 25 and ENR 50 vulcanisates at the optimum carbon black loading of 45 phr. In contrast, the optimum for silica loadings varies with the properties.

5 Effect of Epoxidation Level on Properties of ENR Vulcanisates ENR rubber has unique properties depending on the degree of epoxidation. It is well established that the glass transition temperature (Tg) of ENR varies linearly with the degree of epoxidation [5]. The Tg of ENR increases by 0.93 °C for every one mole per cent of epoxidation [6, 7]. Raw ENR 25 has a Tg of −45 °C and ENR 50 has a Tg of −22 °C [17]. The effect of epoxidation level on the Tg of ENR vulcanisates can be seen from the temperature hysteresis profile measured using a Dynamic Mechanical Analyser

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Fig. 11 Comparative ageing of a carbon black- b silica-filled ENR vulcanisates on tensile strength and elongation at break and c carbon black- d silica-filled ENR vulcanisates on modulus M100 and M300 at respective epoxidation levels under accelerated ageing conditions for 7 days at 70 °C

(DMA) as shown in Fig. 13. The ENR 25, ENR 50 and NR vulcanisates are reinforced with 50 phr carbon black. The onset or a significant drop in the storage modulus (E’) is assigned as DMA Tg [18], and peak temperature of the tan delta (δ) curve is commonly referred to as Tg of the vulcanisate. In Fig. 13, the shift in the tan δ peak and onset of E’ from NR to ENR 25 and ENR 50 are clearly seen, indicating the rise of the vulcanisates Tgs with increasing epoxidation. Difference in the Tg from the onset of E’ with the peak of tan δ is about 10 °C. Tg comparisons for NR, ENR 25 and ENR 50 vulcanisates are shown in Table 9, where upon vulcanisation, the Tg increases by 10–25 °C. The changes in Tg with epoxidation level is also reflected in the properties of ENR vulcanisates. The effect of epoxidation levels on physical properties of carbon

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Table 7 The effect of increasing carbon black and silica loadings on ENR vulcanisate properties Properties

Carbon black

Silica

Tensile strength

Increase to an optimum before decreasing

Increase to an optimum before decreasing

Elongation at break

Decreases

Decreases

Modulus at 100%

Increases

Increases

Modulus at 300%

Increases

Increases

Hardness

Increases

Increases

Compression set

Increases

Increases

Resilience

Decreases

Decreases

Abrasion resistance

Increase to an optimum before decreasing

Increase to an optimum before decreasing

Tear strength

Increase to an optimum before decreasing

Increase to an optimum before decreasing**

Air permeability coefficient (Q)

Constant (ENR 25 and ENR 50) Decreases (ENR 10)

Constant (ENR 25 and ENR 50) Decreases (ENR 10)

**

For ENR 10 and ENR 25, but no effect on ENR 50

Fig. 12 Summary of the effect of filler loadings on physical properties of the ENR vulcanisates

black- and silica-filled ENR vulcanisates are shown in Tables 10 and 11, respectively. Comparison is made between NR, ENR 10, ENR 25 and ENR 50, representing the increase in the epoxidation levels up to 50 mol %. The vulcanisates are reinforced with 60 phr filler of either carbon black (N234) or silica (Zeosil 1165MP), following the formulation in Table 1. Generally, the tensile strength of the ENR vulcanisate is slightly lower than that of NR. For carbon black-filled vulcanisates, the tensile strength is less affected by

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Table 8 Comparison of carbon black-α and silica-β filled ENR vulcanisates (45 phr) Properties

NR

ENR 25

ENR 50

Black Silica Black Silica Black Silica Tensile strength, MPa

32

31

28

24

26

19

Elongation at break, %

646

514

587

402

547

322

Modulus at 100%, MPa

2.1

2.8

2.4

3.1

3.1

3.8

Modulus at 300%, MPa

10.1

15.1

10.8

16.2

12.2

17.6

Hardness, IRHD

64

60

67

64

55

65

Compression set, 22 h at 70 °C, %

24

22

28

25

26

25

Abrasion vol. loss,

mm3

148

146

180

160

193

212

Abrasion resistance index

125

127

102

116

96

87

Resilience, %

69

79

43

61

41

34

Tear strength (Crescent), kN/m

117

112

70

64

30

20

Air permeability coefficient (Q) at 25 °C ×10–17, 8.6 ,m2 ·Pa

9.1

3.2

2.8

0.5

0.9

α Carbon β Silica

black N234 zeosil 1165MP

Fig. 13 The a storage modulus (E’) and b tan delta (δ) curves of NR, ENR 25 and ENR 50 vulcanisates filled with 50 phr carbon black Table 9 Comparison of Tg between NR, ENR 25 and ENR 50 vulcanisates (50 phr carbon black)

Types of rubber

Raw (uncured) Tg, °C

Onset storage modulus (E’), °C

Peak tan delta, °C

NR

−70

−56

−46

ENR 25

−45

−32

−22

ENR 50

−22

−10

0

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Table 10 Effect of epoxidation levels on physical properties of carbon black-filled ENR vulcanisates (60 phr) Properties

NR

ENR 10

ENR 25

ENR 50

Tensile strength, MPa

28.0

25.6

25.7

24.4

Elongation at break, %

550

570

570

490

Modulus at 100%, MPa

3.1

3.0

2.8

3.9

Modulus at 300%, MPa

14.1

13.2

12.6

15.4

Hardness, IRHD

77

72

66

72

Compression set, 22 h at 70 °C, %

24

28

28

30

Abrasion vol. loss,

mm3

155

171

182

204

Abrasion resistance index

119

108

102

91

Resilience, %

53

42

40

23

Tear strength (Crescent), kN/m

125

118

105

46

Air permeability coefficient (Q) at 25 °C, ×10–17 ,m2 ·Pa

8.36

5.14

2.52

0.84

Table 11 Effect of epoxidation levels on physical properties of silica-filled ENR vulcanisates (60 phr) Properties

NR

ENR 10

ENR 25

ENR 50

Tensile strength, MPa

28.4

28.9

21.5

17.9

Elongation at break, %

500

570

380

270

Modulus at 100%, MPa

2.8

2.2

3.3

4.8

Modulus at 300%, MPa

14.7

12.2

15.9

–

Hardness, IRHD

63

59

64

66

Compression set, 22 h at 70 °C, %

18

26

28

25

Abrasion vol. loss, mm3

148

180

171

208

Abrasion resistance index

125

102

108

89

Resilience, %

72

71

58

28

Tear strength (Crescent), kN/m

119

112

48

21

Air permeability coefficient (Q) at 25 °C, ×10–17 ,m2 ·Pa

9.07

5.98

3.29

0.89

the epoxidation level showing only a slight difference between ENR 10, ENR 25 and ENR 50. This is consistent with previous work reported by Baker et al. where tensile strength for carbon black-filled ENR 25 is almost comparable with ENR 50 [2, 6]. The effect of epoxidation levels on ENR tensile strength was clearly observed in silica-filled vulcanisates (Fig. 1b). The tensile strength of silica-filled ENR 50 is inferior to that of NR. This complies with the work by Kaewsakul et al. where an increase in epoxide content influences the tensile strength of silica-filled ENR, especially at epoxidation levels higher than 29% [9]. These findings can be explained by the increase in crosslink density in silica-filled ENR resulting from strong chemical interactions between the ENR and silica in the presence of a coupling agent [19].

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In addition, at higher epoxidation levels, the self-association of epoxide groups may occur, leading to an increase in the degree of crosslinking [9]. However, this observation was not seen by Baker et al. since their work on silica-filled ENR did not involve coupling agents [2, 6]. Resilience, tear strength and air permeability are all strongly affected by the epoxidation level of ENR. Resilience decreases with an increase in epoxidation content of ENR as shown in Fig. 7, due to the increment in Tg, consistent with previously reported work [4]. A marked effect of higher epoxidation levels on resilience is clearly seen for silica-filled ENR 50. The ENR 50 vulcanisate shows a reduction of more than half in resilience with either carbon black or silica fillers compared with the NR vulcanisate. The tear strength (crescent) decreases with the rise in epoxidation levels of ENR. The reduction in tear strength of the carbon black-filled ENR with epoxidation is also shown by Baker et al. [2, 6]. The effect is more pronounced in silica-filled vulcanisates for both ENR 25 and ENR 50 as shown in Fig. 9b. The silica-filled ENR 50 vulcanisate shows an 80% reduction in tear strength compared with that of NR. Air permeability is greatly influenced by the epoxidation level as shown in Fig. 10. This relates to the reduction in free volume clusters in ENR with the presence of an epoxide group. The ENR 25 vulcanisate shows a reduction in air permeability by half from that of NR vulcanisate, whereas the ENR 50 exhibits less than half of the air permeability of ENR 25. Epoxidation levels have a moderate influence on modulus, compression set and abrasion resistance of ENR vulcanisates (Figs. 3, 6 and 8). For moduli M100 and M300, the effect of epoxidation levels can be observed at silica loadings higher than 30 phr. The abrasion resistance of ENR is inferior to NR and at high epoxidation levels in the case of ENR 50, a marked reduction was observed. This is due to the high Tg of ENR 50 compared with the NR vulcanisate. There is a close relationship between abrasion resistance and Tg [20]. In addition, epoxidation levels have lesser influence on the hardness of ENR vulcanisates as clearly observed in Fig. 5. In summary, the effects of epoxidation on the properties of ENR are schematically illustrated in Fig. 14. Note that the effect of epoxidation levels are more pronounced in resilience, tear strength and air permeability of the ENR vulcanisates. The effects of epoxidation on oil resistance and wet grip will be discussed further in the next section. Tensile strength, abrasion resistance, modulus and compression are minimally influenced by ENR epoxidation levels, whilst hardness is not affected at all.

6 Comparison of ENR with Synthetic Rubbers 6.1 Basic Properties of ENR and Synthetic Rubbers Epoxidised natural rubber (ENR) is a modified natural rubber with properties closer to synthetic rubbers than natural rubber. ENR contains polyisoprene as its main

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Fig. 14 The effect of epoxidation on the properties of ENR

chain and the epoxy structure randomly distributed in the rubber backbone, which increases resistance to swelling by hydrocarbon oils and solvents. ENR also has low air permeability, better damping and good bonding whilst retaining its superior strength advantages over that of synthetic rubbers [2, 4]. In this section, the physical properties of ENR vulcanisates are compared with synthetic rubber to emphasise its strength. Table 12 compares the physical properties of carbon black-filled (50 phr) ENR vulcanisates with synthetic rubbers, namely chloroprene rubber (CR), nitrile butadiene rubber (NBR) and isobutylene-isoprene rubber (IIR) vulcanisates at similar filler loadings. Results show considerable differences between the rubbers in terms of density, hardness, tensile strength, tear strength and moduli. The hardness of ENR 25 is comparable to CR and NBR, whereas ENR 50 is comparable to IIR. Compared to NR, results show that the density of ENR increases with the increase in epoxidation level, which agrees with the previous finding [6]. The ranking for density of ENR compared with synthetic rubber in an increasing trend is as follows: NR < IIR < ENR25 < NBR < ENR50 < CR As expected, the basic strength of ENR is inferior to NR. It was observed that the tensile strength of ENR is 22.4 MPa, lower than NR’s 28.6 MPa. The lower tensile strength of ENR is associated with a decrease in crystallinity as the epoxide levels increase [2]. The tensile strength of both ENR 25 and ENR 50 is comparable to NBR and superior to both CR and IIR. The tear strength of ENR 25 is superior to

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Table 12 Physical Properties of black-filled (50 phr N330 Black) ENR vulcanisatesa compared to NR, CR, NBR and IIR Physical properties Density,

Types of rubber

g/cm3

SMR L

ENR 25

ENR 50

CR

NBR

IIR

1.125

1.157

1.194

1.346

1.173

1.130

Hardness, IRHD

65

62

67

60

61

66

Tensile strength, MPa

28.6

22.2

22.4

18.0

23.5

15.9

Elongation at break, %

566

499

442

465

493

602

Modulus at 100%, MPa

2.8

2.7

3.2

1.9

2.7

1.9

Modulus at 300%, MPa

13.1

13.2

14.2

11.0

12.0

6.3

Resilience-Dunlop, %

62

53

32

54

44

31

Tear strength–Crescent, kN/m

139

86

46

36

60

38

Compression set, 1d at 23 °C, %

10

12

15

5

11

10

Compression set, 1d at 70 °C, %

26

50

51

10

27

20

a Using

conventional vulcanisation system

all non-crystallising rubbers evaluated (NBR, CR and IIR). Although ENR 50 has a lower tear strength value than NBR, it is higher than that of IIR and CR. A noticeable reduction in resilience properties was observed with increasing epoxidation level due to the changes in its glass transition temperature (Tg). Gelling reported that as damping increases, the rebound resilience decreases [6]. Resilience of ENR 25 is similar to CR, whereas ENR 50 is comparable to IIR. The ranking for resilience of ENR compared with synthetic rubber in a decreasing trend is as follows: NR > CR > ENR25 > NBR > ENR50 > IIR In Table 12, the compression set of both ENR 25 and ENR 50 vulcanisates are inferior to NR and other synthetic rubbers. This is due to the conventional vulcanisation system used. Peroxide and an efficient vulcanisation (EV) system are recommended to improve the compression set of ENR. CR has an excellent compression set when compared with ENR, NR and other synthetic rubbers.

6.2 Oil Swelling Resistance of ENR Rubber’s resistance to hydrocarbon oils is normally measured by determining their volume swell in standard oils. Standard oils such as IRM 901 (aromatic content 3%) and IRM 903 (aromatic content 14%) are low polar hydrocarbon and high polar hydrocarbon oils, respectively. Both oils are commonly used to measure oil resistance as recommended in ISO 1817 [21]. This test method attempts to simulate service conditions through accelerated controlled testing but may not directly correlate with

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actual performance since service conditions vary too widely. The changes in the volume of samples are calculated before and after oil immersion. Hence, low changes represent high oil resistance of the vulcanisates. Figure 15 compares the oil-resistant properties of ENR with those of NR, CR, low grade of NBR (18% acrylonitrile content) and a medium grade of NBR (34% acrylonitrile content). Compared with NR, the results showed that modification of at least 25% degrees of epoxidation has significantly improved oil resistance, particularly for the standard oil IRM 901. It is shown that swelling values of ENR 25 in IRM 901 is comparable with the low grade of NBR. Meanwhile, ENR 50 is a highly polar rubber due to more oxirane groups on its backbone, and it has therefore been proven to show oil resistance comparable to medium grade NBR when tested in IRM 901 oil. As both ENR 50 and medium grade NBR are polar rubbers and the IRM 901 oil is a low polar hydrocarbon, ENR and NBR have smaller changes in volume and mass due to different polarities during immersion at 100 °C for 70 h. Moreover, the high polar hydrocarbon oil IRM 903, known as an aggressive standard oil, is also used for swelling measurements. Compared with NR and ENR 25, the volume change of ENR 50 is significantly lesser, indicating better oil resistance. Amongst the oil-resistant synthetic rubbers, such as the low grade of NBR and CR, ENR 50 retains the lowest volume change in IRM 903. Hence, it is proven that ENR 50 has better oil resistance over the low grade of NBR followed by CR. Nevertheless, the oil resistance of ENR 50 is slightly inferior to the medium grade of NBR, as shown by a marginal difference in the volume change between the two rubbers. This could be because NBR possesses excellent resistance to hydrocarbon liquids due to the presence of acrylonitrile groups. Overall, the oil resistance of ENR 50 in the high polar hydrocarbon oil lies between those of CR and the medium grade of NBR. This result aligns with a study carried out

Fig. 15 Percentage volume change of rubber in IRM 901 (70 h at 100 °C) and IRM 903 (70 h at 100 °C)

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Fig. 16 A linear relationship of solubility parameter and epoxidation level [22]

by Baker et al. [2] and Gelling [6]. According to the study, oil resistance of ENR 50 is close to NBR medium (32%) acrylonitrile content and superior to polychloroprene rubber (CR) even in highly swelling oils such as ASTM No. 3, equivalent to IRM 903. Also, higher levels of epoxidation might further improve the oil resistance. However, such grades of ENR would become more expensive and possess lower strength due to its limitation to undergo strain crystallisation when reinforced with carbon black [6]. Hence, ENR 50 is an ideal choice for making an oil-resistant rubber product if a balanced property condition between strength and oil resistance is required. The oil resistance of rubber can also be estimated based on solubility parameters [22].

6.3 Solubility Parameter of ENR A linear correlation is observed with the solubility parameter where it increases with an increase in the level of epoxidation as shown in Fig. 16 [22]. As the mol % of epoxide groups in the rubber increases, the movement of chain mobility decreases. This is due to the fact that epoxide groups restrict or hinder the polymer chain rotation. Therefore, the ENR 50 with high Tg tends to have a high solubility parameter. A comparison of solubility parameters of different rubbers is shown in (Table 13) and Fig. 16, respectively. ENR is suggested for application as an adhesive. Knowledge in solubility parameters will enable the correct choice of solvents for the adhesive.

6.4 Air Impermeability Table 14 compares the permeation constants of various rubbers with NR. At a constant temperature of 23 °C, the relative air permeability ranges from the highest being NR,

90 Table 13 Solubility parameter of ENR and other rubbers [22]

S. S. Sarkawi et al. Rubber

Solubility parameter, (MPa1/2 )

NR

16.8

ENR 25

17.4

ENR 50

18.1

CR

17.6

NBR

18–20.5a

a NBR

Table 14 Relative air permeability* of ENR and other rubbers

depending on ACN content

Rubber

Rating

NR

100

ENR 25

29

ENR 50

7

Butyl rubber (BIIR)

6

NBR

12

* NR is taken as the reference and other rubbers are rated accordingly at 25 °C (m2 ·Pa)

followed by ENR 25, NBR, ENR 50 and IIR. It is seen that the air permeability of ENR 50 is comparable with that of IIR. This trend is also in line with previous studies [2–7]. The IIR is primarily known for excellent air permeability due to its highly efficient intermolecular packing. In this case, the epoxidation level of ENR progressively improves its air permeability due to the presence of the epoxide group. Hence, the free volume clusters in ENR are smaller than those in the NR. This is attributed to the lower diffusion coefficient of oxygen molecules due to local hindrance in the motion of oxygen molecules by epoxy functional groups on the ENR compared with NR [23]. The low permeability of ENR 50 which is as good as those of the NBR and IIR is an advantage, particularly in applications such as bladders and inner tubes. Moreover, higher tensile strength and elongation at break of the ENR 50 offers an added advantage compared with synthetic rubbers. However, for applications in tyre inner liners, a blend of ENR 50 with NR is recommended as the blend possesses better bonds than ENR 50 alone. However, ENR 50 is not readily compatible with NR thus resulting in weak bonding to an NR casing. The correct blending ratios for optimum performance needs to be determined, depending on the specific formulations used.

6.5 Wet Grip Wet grip is an important property for tyres largely, although it also applies to shoe soles and floor coverings. This property is related to the loss tangent (tan δ) at a high

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frequency. At a range from 0 °C to ambient temperature, tan δ correlates with skid behaviour such as grip and traction of a tyre on either wet or dry roads [20]. Tan δ value at 0 °C is commonly used as an indication of the wet grip of a tyre [13, 20]. The increase in epoxidation level of ENR is followed by the increase in its Tg and consequently, the increase in damping and hysteresis. Tg influences the position of tan δ-temperature profiles. In Fig. 13, tan δ peak of the ENR vulcanisate shifted to higher temperatures with the increase in epoxidation levels. This is followed by the increase in tan δ value at 0 °C for ENR 25 and ENR 50. Wet grip also increases with the increase in epoxidation level of ENR. A comparative wet grip rating for typical passenger car tyre tread formulations based on NR, ENR 25 and ENR 50 is shown in Table 15. The linear relationship between the wet grip rating, epoxidation level and Tg of ENR vulcanisates is illustrated in Fig. 17. Although ENR 50 has a higher wet grip than ENR 25, it is not recommended for use in ordinary tyre treads due to its relatively higher rolling resistance. ENR 25 is the suitable option for tyre tread applications owing to its good balance of wet grip and rolling resistance. Coefficient of friction can be used to indicate wet grip. Higher average of coefficient of friction indicates better grip and traction. The coefficient of friction for the ENR 25-silica vulcanisate measured using Plint rubber friction tester is shown in Fig. 18. The average coefficient of friction for ENR 25-silica vulcanisate is higher at 1.4528 compared with NR/BR-carbon black at 1.0059. Table 15 Relative wet grip of ENR

Wet grip ratinga

NR

ENR 25

ENR 50

Carbon black-filled

100

152

353

Silica-filled

67

178

378

a Wet

grip rating based on tan delta at 0 °C from DMA testing at 10 Hz. Carbon black-filled NR is taken as the reference and the other compound rated accordingly

Fig. 17 Relationship between wet grip, epoxidation level and Tg of ENR vulcanisates

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Fig. 18 Coefficient of friction for ENR 25 silica tread compound from the Plint rubber friction test

The performance test to evaluate wet grip of a silica-filled ENR-based tread tyre against a control-based tread tyre of size 195/55R15 85 V was conducted according to UNECE R117 requirements on wet adhesion at the IDIADA Technology Centre, Spain. The control tyre was an sSBR/BR/NR tread filled with carbon black/silica. The tread pattern and construction of the control tyre were similar to the silica-filled ENR tread tyre. Results showed that the ENR PCR tyre with silica-filled ENR tread compound achieved excellent performance on wet grip (A rating) as shown in Table 16. The silica-filled ENR tread gives a shorter braking distance compared with the control tread especially on wet surface conditions of asphalt as shown in Fig. 19. The braking performance test conducted at various speeds shows that silica-filled ENR tread exhibits better grip and stopping distances than the sSBR/BR/NR filled with carbon black/silica control tread. On a wet surface, the silica-filled ENR tyre stops at 8.1 m shorter than the control tyre at a test speed of 80 km/h. Table 16 Silica-filled ENR PCR tyrea performance [24]

Sample

Wet grip index

Wet grip rating

ENR 25/Silica

1.57

A

Control

1.27

C

a 195/55R15

85 V

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Fig. 19 Comparison of car stopping distance between silica-filled ENR and a control tread on dry and wet asphalt [24]

7 Reinforcement of ENR with Nanofillers The discovery of new nanoscale materials such as organoclay, nano-silica, carbon nanotubes, graphite oxide and graphene materials receives considerable attention in the reinforcement of filled rubber [25–27]. This is mainly due to their outstanding properties, such as larger surface area, excellent thermal, mechanical, electrical, barrier and optical properties [28, 29]. For example, Young’s modulus and tensile strength rapidly increase with the addition of carbon nanotubes (CNTs) in the ENR matrix due to the chemical interactions of polar functional groups in ENR with CNT surfaces [30]. Another advantage of nanofillers is the use of dual filler reinforcements in the rubber. A dual filler deals with a combination of two different types of nanofillers or nanofillers with conventional fillers, such as carbon black and silica. More interestingly, these hybrid fillers have a synergistic reinforcing effect on rubber properties whilst acting as benefit retainers to the individual filler [31]. Carbon nanotube (CNT)/carbon black (CB) dual filler showed the least loss of conductivity in ENR during the external strain. This arises from the polar chemical interactions between ENR and the functional groups on the filler surfaces [32]. The study concluded that the ENR and CNT/CB composite is beneficial in sensor applications, particularly in health monitoring, motion detectors and other related products. The hybrid reinforcement of ENR with nano-silica and graphene is achieved at low graphene filler loadings as shown in Table 17. The addition of graphene reduces the filler-filler interaction and increases the bound rubber content of the ENR-silica compound (Fig. 20) [33]. The synergistic effect as a result of hybrid reinforcement of the ENR with silica/silane and graphene system is illustrated in Fig. 21. The abrasion resistance of the ENR-silica compound improved with the addition of graphene. Further improvement in the dynamic properties especially tan δ at 60 °C for low rolling resistance is obtained with the ENR-silica-graphene vulcanisate [34].

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Table 17 Physical properties of the ENR vulcanisate NR/BR-CB

ENR-silica

ENR-silica/Graphene

NR

70

–

–

BR

30

–

–

ENR 25

–

100

100

Silica

–

55

55

TESPT

–

2.2

2.2

Carbon black

53

–

Graphene

–

–

3

Tensile strength, MPa

22

25

26

Elongation at break, %

650

450

440

M100, MPa

1.5

2.7

3.3

M300, MPa

7.0

14.7

16.2

Hardness, IRHD

66

67

67

Abrasion resistance index (ARI)

140

94

108

Ingredientsa

Properties

a Other

ingredients in the compounds in phr: zinc oxide 3, stearic acid 3, calcium sterate 2, 6PPD 1, TMQ 1, TDAE oil 8, sulphur 0.7, TBBS 1.5 and TBzTD 0.25

Fig. 20 Interaction of graphene with ENR and silica

On the other hand, ENR can improve the dispersion of nanofiller particles in the rubber matrix. It was reported that the incorporation of an optimum concentration of ENR in NBR-silica composites gave technological properties comparable to those containing a coupling agent and NBR-CB ISAF [35]. ENR is also used as a compatibiliser, as an alternative to a coupling agent, particularly to improve the dispersion of nanofillers such as organoclay and nano-silica in NR/NBR [36]. This shows that silica reinforced NR can be substantially improved by adding ENR as a compatibiliser, but it is somewhat less effective compared with a silane coupling agent [37].

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Fig. 21 Hybrid reinforcement of ENR with nanofillers; ENR/silica and ENR/silane/silica bondings; and the physical interactions of ENR with silica and graphene [33]

Studies have also shown that graphene’s effect is more pronounced in the modified natural rubber and silica system due to the higher interactions of graphene with silica and ENR [33]. Nik Ismail et al. reported that graphene oxide (GO) sheets can be integrated into a natural rubber by reducing filler-filler interaction and improving rubber-filler interaction using ENR as a reinforcement modifier (Fig. 22). The results suggest that the presence of a polar functionality epoxide group significantly enhances the reinforcing efficiency of GO in the NR compounds. This synergistic effect resulted from ENR as a reinforcement modifier for hybrid reinforcement of a GO/CB filled NR composite [38]. In addition, another study reported that by adding ENR 50 to a natural rubber/organoclay nanocomposite, an optimal clay dispersion was achieved [39]. This was reflected in the stiffness of the nanocomposites obtained from both dynamic mechanical thermal analysis (DMTA) and tensile tests. The tensile and tear strengths of the ENR 50 containing nanocomposites were also superior to the ENR 25 compatibilised and uncompatibilised compounds [39].

8 Conclusion Epoxidised natural rubber, both ENR 25 and ENR 50, can be compounded and mixed similar to regular NR. Both semi-efficient and efficient vulcanisation sulphur cure systems are recommended for ENR. As ENR is a polar rubber, it is highly compatible with polar fillers such as silica. ENR 25 and ENR 50 can be reinforced with a silica filler with or without minimum usage of coupling agents. The recommended amount

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Fig. 22 The effects of ENR as a compatabiliser on filler-filler interaction in GO/CB filled NR [38]

of silane coupling agent for silica reinforced ENR is about 2–4 wt% relative to the amount of silica used. An increase in the epoxidation level of ENR resulted in an orderly increase in its polarity and glass transition temperature which alters the physical properties of the ENR vulcanisates. The epoxidation level greatly influenced resilience, tear strength, air permeability, solubility parameter, oil resistance and wet grip of the ENR vulcanisates. However, tensile strength, abrasion resistance, modulus and compression set are moderately affected by the epoxidation level, whilst hardness was the least affected. The reinforcing effects of carbon black and silica in the ENR vulcanisates were compared. The tensile strength, tear strength and abrasion resistance increased to an optimum level before decreasing with the continued increase in filler loading. However, filler loadings have no effect on the tear strength of ENR 50. The modulus, hardness and compression set increased with an increment in filler loading, whilst elongation at break and resilience decreased with increasing filler loadings. Conversely, filler loading does not influence air permeability. The advantages of ENR, with special emphasis on its oil-resistant character and low gas permeability, are disclosed. The oil resistance of ENR 50 falls between that of polychloroprene (CR) and medium grade nitrile rubber (NBR-34% acrylonitrile content). Similar improvements were observed concerning gas permeability where ENR 50 has good impermeability, similar to butyl rubber. In summary, ENR 25 has comparable behaviours to NR in terms of its strength and other physical properties. ENR 50 however, is totally a different elastomer compared with NR, especially its unique properties, resistance and physical properties.

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References 1. Rubber Research Institute of Malaysia (1983) Epoxidised natural rubber. Raziq Prinkraf Sdn Bhd, 1–10 2. Baker CSL, Gelling IR (1987) Epoxidized natural rubber. In: Whelan A, Lee KS (eds) Developments in rubber technology, vol 4. Elsevier Applied Science Publishers Ltd, 87–118 3. Baker CSL, Gelling IR, Newell R (1985) Epoxidized natural rubber. Rubb Chem Technol 58:67–85 4. Baker CSL, Gelling IR, Samsuri A (1986) Epoxidised natural rubber. J Nat Rubb Res 1(2):135– 144 5. Gelling IR (1987) Epoxidized natural rubber. Nat Rubb Technol 18(2):21–29 6. Gelling IR (1991) Epoxidised natural rubber. J Nat Rubb Res 6(3):184–205 7. Gelling IR (1991) Epoxidised natural rubber. Prog Rubb Plast Tech 7(4):271–297. (MRPRA Reprint 1389) 8. Sarkawi SS, Kaewsakul W, Sahakaro K, Dierkes WK, Noordermeer JWM (2015) A review on reinforcement of natural rubber by silica fillers for use in low-rolling resistance tyres. J Rubb Res 18(4):203–233 9. Kaewsakul W, Sahakaro K, Dierkes WK, Noordermeer JWM (2014) Cooperative effects of epoxide functional groups on natural rubber and silane coupling agents on reinforcement efficiency of silica. Rubb Chem Technol 87:291–310 10. Martin PJ, Brown P, Chapman AV, Cook S (2015) Silica-reinforced epoxidised natural rubber tire treads–performance and durability. Rubb Chem Technol 88(3):390–411. https://doi.org/ 10.5254/rct.15.85940 11. Amu A, Dulngali S (1989) Easy processing epoxidised natural rubber. J Nat Rubb Res 4(2):119– 132 12. Noordermeer JWM, Dierkes WK (2009) Chapter 3—Silica-filled rubber compounds in rubber technologist’s handbook, vol 2. Shawbury, UK, Smithers-Rapra Technology, 59–95 13. Rodgers B, Waddell W (2005) Chapter 9—The Science of Rubber Compounding. In Erman B, Mark JE, Roland CM (eds) Science and technology of rubber, 3rd edn. Elsevier, Academic press, 401–454 14. Gerspacher M, Wesley W, Fillers A (2001) Carbon black. In: Baranwal KC, Stephens HL (eds) Basic elastomer technology. Rubber division. Am Chem Soc, Akron, 57–81 15. Sarkawi SS, Che Aziz AK, Abdul Rahim R, Abdul Ghani R, Kamaruddin AN (2016) Properties of epoxidized natural rubber tread compound: the hybrid reinforcing effect of silica and silane system. Polym Polym Compos 24(9):775–781 16. Rader CP (2001) Vulcanization of rubber. In: Baranwal KC, Stephens HL (eds) Basic elastomer technology. Akron, Rubber Division, Am Chem Soc, pp 165–190 17. Malaysian Rubber Board (2022) Monograf ENR. ISBN 978-983-2088-72-1 18. ASTM D7028-07 Standard Test Method for Glass Transition Temperature (DMA Tg) of Polymer Matrix Composites by Dynamic Mechanical Analysis (DMA) 19. Kaewsakul W, Sahakaro K, Dierkes WK, Noordermeer JWM (2014) Verification of Interactions between silica and epoxidised squalene as a model for epoxidised natural rubber. J Rubb Res 17(3):129–142 20. Nordsiek KH (1985) The integral rubber concept—an approach to an ideal tire tread rubber, vol 38. Kautschuk Gummi Kunststoffe. Jahrgang Nr 3/85 21. ISO 1817-Rubber, vulcanized or thermoplastic—Determination of the effect of liquids 22. Gelling IR, Tinker AJ, Abdul RH (1991) Solubility parameter of epoxidised natural rubber. J Nat Rubb Res 6(1):20–29 23. Yoo JI, Jiang Y, Kim JK (2019) Prediction of gas permeability by molecular simulation. Elastomers Compos 54(3):175–181 24. Che Aziz AK, Sarkawi SS, Abdul Ghani R, Kamaruddin A, Abdul Rahim R, Yen Wan N, Mohd Rasdi FR (2018) Silica reinforced ENR tires. Tire Technol Int 90–93 25. Jibin KP, Prajitha V, Thomas S (2021) Silica-graphene oxide reinforced rubber composites. Mater Today Proc 34(2):502–505

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26. Bokobza L (2017) Mechanical and electrical properties of elastomer nanocomposites based on different carbon nanomaterials. J Carbon Res 3(2):10 27. Liu YB, Li L, Wang Q (2010) Reinforcement of natural rubber with carbon black/nanoclay hybrid filler. Plast Rubb Comp 39(8):370–376 28. Kuilla T, Bhadra S, Yao D, Kim NH, Bose S, Lee JH (2010) Recent advances in graphene based polymer composites. Prog Polym Sci 35:1350–1375 29. Singh V, Joung D, Zhai L, Das S, Khondaker SI, Seal S (2011) Graphene based materials: past, present and future. Prog Mater Sci 56:1178–1271 30. Nakaramontri Y, Nakason C, Kummerloewe C, Vennemann N (2015) Effects of in-situ functionalization of carbon nanotubes with bis (triethoxysilylpropyl) tetrasulfide (TESPT) and 3-amino-propyltriethoxysilane (APTES) on properties of epoxidized natural rubber–carbon nanotube composites. Polym Eng Sci 55(11):2500–2510 31. Nik Ismail NI, Ansarifar A, Song M (2013) Effect of hybrid reinforcement based on precipitated silica and montmorillonite nanofillers on the mechanical properties of a silicone rubber. Polym Eng Sci 54(8):1909–1921 32. Krainoi A, Johns J, Kalkornsurapranee E, Nakaramontri Y Chapter carbon nanotubes reinforced natural rubber composites. http://dx.doi.org/https://doi.org/10.5772/intechopen. 95913 33. Sarkawi SS, Abdul Aziz A, Che Aziz AK, Abd Rahim R, Nik Ismail NI (2017) Properties of graphene nano-filler reinforced epoxidized natural rubber composites. J Polym Sci Technol 2(1):36–44 34. Sarkawi SS, Abd Aziz A, Abd Rahim R, Che Aziz AK (2016) Novel dual filler: a study of dual-nanofiller reinforcement of epoxidized natural rubber. Tire Technol Int: 98–100 35. George KM, Varkey JK, Thomas KT, Mathew NM (2002) Epoxidized natural rubber as a reinforcement modifier for silica-filled nitrile rubber. J Appl Polym Sci 85(2):292–306 36. Ahmad ZR, Mamauod SNL (2017) Physical and mechanical properties of ENR compatibilized NR/NBR blends reinforced nanoclay and nanosilica. Macromol Symp 371:27–34 37. Sengloyluan K, Sahakaro K, Dierkes WK, Noordermeer JWM (2014) Silica-reinforced tire tread compounds compatibilized by using epoxidised natural rubber. Eur Polym J 51:69–79 38. Nik Ismail NI, Sarkawi SS, Mohd Rasdi FR, Abdul Aziz A (2017) Ekoprena as a reinforcement modifier for graphene oxide/carbon black filled natural rubber. Proc Int Rubb Conf 1(1):704– 713 39. Teh PL, Mohd Ishak ZA, Hashim AS, Karger-Kocsis J, Ishiaku US (2004) Effects of epoxidized natural rubber as a compatibilizer in melt compounded natural rubber–organoclay nanocomposites. Eur Polym J 40(11):2513–2521

Epoxidised Natural Rubber in Tyre Applications Siti Salina Sarkawi, Roland Ngeow, Ahmad Kifli Che Aziz, Rohaidah Abdul Rahim, Rassimi Abdul Ghani, Teku Zakwan Zaeimoedin, and Nurul Hayati Yusof

Abstract Epoxidised natural rubber (ENR), a specialty polymer derived from modification of natural rubber, has unique physical and dynamic properties. Wet grip, rolling resistance and noise are among the special properties of ENR which are desirable for tyre applications. Hence, ENR and its blends have been largely investigated for usage in tyres including passenger cars, trucks, buses and motorcycle tyres. The wet grip and rolling resistance ratings of ENR compounds compared with NR and other blends are summarised in the STAR3 plot. The plot also shows the effect of epoxidation level and type of filler used on the wet grip and rolling resistance ratings. Prototypes of passenger car tyres, truck/bus tyres and motorcycle tyres have been prepared and subjected to various laboratory and road trials. Results of the ENR car tyre show that the silica-filled ENR 25 tread compounds exhibit superior wet grip, improved rolling noise and good rolling resistance. The road testing using a sedan car for tyre size 195/55R15 85 V demonstrated that the ENR 25-silica car tyres gave good mileage with a projected tread life of 90,000–100,000 km. Large-scale road trials with ENR retreads for truck and bus tyres were also conducted. ENR blend retreads show acceptable wear with the benefit of wet grip for safety of the vehicle. In addition, properties of ENR–silica wet masterbatch are highlighted for new potential tyre applications. Keywords Epoxidised natural rubber · ENR 25 · Tyre · Wet grip · Rolling resistance · Rolling noise

1 Introduction Tyre is a major application for both natural and synthetic rubbers. About 75% of the total consumption of natural rubber is for tyres [1, 2]. Styrene-butadiene rubber (SBR) is the most consumed synthetic rubber category, and about 65% of SBR usage is for tyres, mainly passenger car tyres [3]. Earlier research has shown that ENR has S. S. Sarkawi (B) · R. Ngeow · A. K. Che Aziz · R. Abdul Rahim · R. Abdul Ghani · T. Z. Zaeimoedin · N. H. Yusof Malaysian Rubber Board, Sungai Buloh, Selangor, Malaysia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. S. Sarkawi et al. (eds.), Epoxidised Natural Rubber, https://doi.org/10.1007/978-981-19-8836-3_5

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comparable properties to its synthetic counterpart [4]. In that respect, ENR has been largely investigated for application in tyres and its components. The development of tyre compounds has been mostly stimulated from rules and regulations such as tyre labelling and restriction of chemicals, as well as developments in technology and environmental concerns. EU tyre labelling specifies three main tyre properties, namely fuel consumption, wet grip and rolling noise. This chapter highlights the special properties of ENR that validates its application in tyres, especially wet grip, rolling resistance and noise. The usage of ENR in passenger car tyres, truck and bus tyres as well as motorcycle tyres is delineated in this chapter. Prototypes of ENR passenger car tyres, truck/bus tyres and motorcycle tyres have been prepared and subjected to various laboratory and road trials. Largescale road trials with the ENR retreads for truck and bus tyres were conducted in collaboration with third parties. Results of the evaluation from these trials are discussed here accordingly. In addition, the preparation and properties of ENR–silica wet masterbatch are included for new applications in tyres.

2 Tyre Properties Tyre is a complex product, and the selection of material for its components is very important. The common rubber used in tyre compounds and its related tyre properties are presented in Table 1. Selection of compounds for tyre components will depend on achieving the best possible compromise between physical properties, ease of processing and manufacturing as well as the final tyre properties. Three most important properties of tyre, which are often called the magic triangle of tyres, are wet grip, rolling resistance and treadwear [5, 6]. Improvement in any one of these properties conventionally leads to a trade-off in the other two properties. Improvement in wet grip usually results in an increase in the rolling resistance for general-purpose elastomers [7], which has remained uncontested for many years. Table 1 Rubber used in tyres and its related properties Tyre property

NRa

IRb

SBRc

BRd

IIRe

EPDM/EPMf

Road handling

G

G

VG

G

E

G

Wear resistance

G

G

VG

E

G

VG

Tear resistance

E

VG

P

P

G

P

Low heat build-up

VG

E

P

P

P

P

Permeability

P

P

P

P

E

P

Ageing resistance

P

P

G

G

VG

E

Ozone resistance

P

P

P

P

VG

E

E = Excellent VG = Very Good G = Good P = Poor a Natural rubber; b Polyisoprene rubber; c Styrene-butadiene rubber; d Butadiene rubber; e Butyl rubber; f Ethylene Propylene Diene rubber

Epoxidised Natural Rubber in Tyre Applications Table 2 Tyre performance indicator from the tan delta versus temperature curve

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Temperature zone (°C)

Tyre performance indicator

−60 to −20

Abrasion resistance

−20

Low-temperature properties

+20

Wet traction

+20 to +60

Rolling resistance

+80 to +100

Heat build-up

The development of modified elastomers such as solution styrene-butadiene rubber (sSBR), oil-extended emulsion styrene-butadiene rubber (OESBR) and epoxidised natural rubber (ENR) has shown that these elastomers have the best of both properties: a high wet grip and a low rolling resistance without sacrificing wear potential [6, 8–11]. The development of highly dispersible silica [4, 12, 13] and silica–silane systems [14–16] has also enabled the improvement in two of these three properties. As rubber is a viscoelastic material, its behaviour can be characterised by the loss tangent (tan δ) against temperature curve [17, 18], normally measured using dynamic mechanical analysis (DMA). During the DMA test, a viscoelastic material undergoes sinusoidal shear deformation, where storage modulus (G' ), the stored energy representing the elastic characteristic of the material, and loss modulus (G'' ), the energy dissipated as heat, representing the viscous characteristic of the material, are measured. From these data, the ratio of loss tangent, or tan δ, is defined as: tan δ =

G '' G'

(1)

It is generally accepted that wet grip and rolling resistance mechanisms have different temperature–frequency characteristics and hysteresis–temperature profiles [19]. The important properties of tread rubber can be predicted from the tan δ as a function of temperature [2, 20] as listed in Table 2. It has been shown that there is a correlation between the glass transition temperature and abrasion resistance [20]. At a temperature range from 0 °C to ambient temperature, the tan δ correlates with skid behaviour such as grip and traction of the tyre on either wet or dry roads [20]. At a temperature range between 30° and 70 °C, also the running temperature of the tyre gives a correlation with its rolling resistance. Hence, the tan δ (G'' /G' ) value at 0 °C is commonly used as an indicator for wet grip, and the tan δ value at 60 °C is an indicator for rolling resistance [21]. The schematic representation of tyre performances from the tan δ curve as a function of temperature for both NR and the ENR 25 compounds is illustrated in Fig. 1. The comparison study which involved ENR 25 filled with various loadings of silica and carbon black by Baker et al. [22] shows that silica-filled ENR compounds have both high wet grip and low rolling resistance, which have the potential for tyre tread compounds. The measurements of the both wet grip and rolling resistance ratings were taken using a two-wheel Schallamach trailer and Heenan–Froude tyre test rig, respectively. The results showed that the silica-filled ENR 25 compound behaved

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Fig. 1 Prediction of tread properties from the tan δ curve as a function of temperature for NR and ENR compounds. Note The curve is from DMA testing at temperatures between −100 °C and + 100 °C

differently compared with general-purpose elastomers as reported by Morton and Krol [7, 22]. Tyre companies were previously interested but avoided uptake of these technologies owing to low fuel prices, while environmental concerns were not a priority [23]. Extensive current research on ENR compounds and their blends has been carried out for tyre applications [24]. Comparison of carbon black and silica-reinforced NR, ENR 25 and ENR 50 as well as ENR blends on the wet grip and rolling resistance ratings is summarised in Fig. 2 (STAR3 plot) [25]. The wet grip rating was calculated from the tan δ value at 0 °C, whereas the rolling resistance was calculated from the tan δ value at 60 °C, from the DMA temperature sweep testing at 10 Hz. NR filled with 55 phr carbon black was taken as a reference, and other rubber compounds were rated accordingly. A higher wet grip rating implies higher tan δ at 0 °C than the reference compound, which indicates better wet traction. As for a higher rolling resistance rating, tan δ at 60 °C was lower than the reference compound, and this indicated lower rolling resistance which consequently resulted in better fuel economy. A tyre tread compounder would ideally aim to achieve the top right quadrant in Fig. 2; however, no particular combination of elastomer and filler allows for this optimal result. The Morton and Krol [7] line (dashed–dotted line) is included in the STAR3 plot (Fig. 2) for comparison. It is worth noting that in the plots published by Morton

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Fig. 2 STAR3 plot of rolling resistance and wet grip ratings of ENR compounds compared with NR and other blends

and Krol [7] and Baker [22, 26], a higher wet grip rating for a particular tyre tread compound indicates better wet traction. However, a lower rating for rolling resistance in their plot [7, 22, 23] indicates lower fuel consumption, which is an inverse to the ratings shown in the STAR3 plot. A higher rolling resistance rating in the STAR3 plot (Fig. 2) points towards lower fuel consumption for the compound. The carbon black-filled NR, silica-filled NR compound and gum NR (unfilled NR) crossed the Morton and Krol [7] line in the STAR3 plot (Fig. 2). This line shows that with general-purpose elastomers like NR, improved wet grip rating usually resulted in decreased rolling resistance rating [7]. The gum NR compound also exhibits the highest rolling resistance rating, while the gum ENR 25 compound exhibits among the highest wet grip rating. STAR3 plot (Fig. 2) consists of two distinguished lines, namely the carbon black line as a solid line and silica line as a dashed line. A silane coupling agent was used for all the silica-filled compounds prepared in this study. Improvement in wet grip and rolling resistance ratings for the silica–silane ENR compound compared with the carbon black and carbon black/silica compounds is seen in the STAR3 plot (Fig. 2), where the silica line is shifted from the carbon black line. Most of the dual filler systems with blends of ENR 25 and other elastomers fall around the carbon

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black line region. The results show that silica-filled ENR vulcanisates successfully extended the magic triangle of a tyre wet grip and rolling resistance properties. ENR 25 filled with 50 phr silica has the best combination of wet grip and rolling resistance ratings compared with NR filled with 55 phr carbon black (the reference compound) making it suitable for tyre tread applications. ENR 25 compound has also exhibited a better rolling resistance rating compared with the ENR 50 compound, which exhibited the highest wet grip rating measured in this study. ENR 50 can be considered for tyre applications, where high wet traction is required. The effect of epoxidation can be visibly observed from the STAR3 plot (Fig. 2). Both carbon black-filled ENR 50 and silica-filled ENR 50 compounds exhibit high wet grip ratings. A range of wet grip ratings can be distinguished between ENR 50, ENR 25 and NR, indicating the influence of the level of epoxidation. As the epoxidation level of ENR increased, the wet grip performance improved. The range of wet grip ratings for ENR 25 and its blends is located in the middle of the STAR3 plot and is higher than that of NR compounds. For comparison, the ENR 25 gum is shown to have a wet grip rating comparable to filled ENR 50. The performance of silica-filled ENR 25 tyre compounds on a passenger car radial tyre (195/5515 85 V) was conducted according to UNECE R117 requirement on wet adhesion, rolling resistance and rolling noise emission at the IDIADA Proving Ground in Spain. The ENR tyres with silica-filled ENR tread compound achieved excellent performance on its wet grip (A rating) based on EU tyre label [27], with a marginal improvement on noise level and rolling resistance [28]. For tyre compounders, achieving the top right quadrant in the STAR3 plot (Fig. 2) is challenging in practice. A balance of wet grip and rolling resistance ratings needs to be achieved in addition to the third most important property of a tyre tread which is its wear resistance [5], as discussed in the other sections of this chapter.

3 Passenger Car Tyre Tread The ENR Passenger Car Radial (PCR) tyre was designed with its tread compound based on silica-reinforced ENR 25 to provide balance in the overall car tyre performance, especially wet grip, rolling resistance and rolling noise. The prototype of ENR PCR tyres with a tread liner based on the silica-reinforced ENR 25 compound was prepared, and its performance was tested with various tyre tests.

3.1 ENR PCR Tyre Manufacturing PCR with the tread compound using the ENR 25 with silica filler can be manufactured similar to the conventional PCR tyre that uses a silica-filled compound. However, it may require stringent processing control as the viscosity of tread compounds filled with a high loading of silica can be higher and continue to increase rapidly upon

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storage. In the case of PCR tread based on the ENR 25 formulation containing a silica filler loading higher than 70 phr, it is highly recommended to utilise an intermesh rotor internal mixer with three stages of mixing when mixing the tread compound. With an intermesh rotor internal mixer, better processing characteristics and higher filler dispersion can be achieved compared with utilisation of a tangential rotor internal mixer [29]. Table 3 presents a comparison between the intermeshing rotor and tangential rotor internal mixer on processing characteristics and filler dispersion of ENR 25 filled with medium and high loadings of silica. Nonetheless, the tangential rotor internal mixer can still be used to mix the compound although it may require more mixing stages. For the production of tread profile, the extrusion process should preferably be carried out within 24 h after the mixing process to minimise the effect of increasing storage viscosity, especially for extrusion with a cold feed extruder. On the other hand, a remilling process on an open mill or an internal mixer can be introduced to reduce viscosity, if the compound has been stored for over 72 h. The hot feed extrusion process can also be employed for production of the tread profile for highly viscous compounds. Table 4 presents the Mooney viscosity of the ENR 25 filled with 75 phr silica mixed using a 270 L tangential rotor internal mixer after four stages of mixing and the effect of storage on viscosity of the compound. ENR with higher epoxidation levels tends to have weaker tack and adhesion properties towards non-polar polymers such as NR or SBR [30]. Therefore, remaining components in the tread profile containing other than the ENR 25 rubber compound in its base, wings and cushion are highly recommended to be extruded together with the ENR 25 tread using a complex extruder. This method will improve both tack and adhesion between the tread profiles with other tyre components while providing additional dimensional stability. Silica-filled ENR tread was also found to exhibit excellent characteristics of tyre wet grip [28]. Therefore, the compound serves as a Table 3 Effect of tangential and intermeshing rotor types on the compound viscosity and filler dispersion of silica-filled ENR 25 Parameter

75 phr silica

55 phr silica

Tangential

Intermesh

Tangential

Intermesh

Compound viscosity, ML [1 + 4] at 100 °C

54.3

50.1

77.6

63.2

Filler dispersion, %

50.1

86.2

48.2

93.5

Table 4 Mooney viscosity of ENR 25 filled with 75 phr silica, mixed using a 270L tangential rotor internal mixer Process After four stages of mixing, ML [2 + 4] at 100 °C After 72 h of mixing, ML [2 + 4] at 100 °C After remilling on open mills, ML [2 + 4] at 100 °C

Batch 1

Batch 2

86

85

>200

>200

63

62

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Fig. 3 Cross-sectional view of tread components for a complex extrusion

Table 5 Physical properties of silica-filled ENR 25 tread

Properties

Batch 1

Batch 2

Tensile strength, MPa

18.56

16.39

Elongation at break, %

343

300

Modulus M100, MPa

3.76

3.83

Modulus M300, MPa

16.79

16.36

Hardness, IRHD

67

67

cap tread component that remains in contact with the road surface for complex or multiplex extrusion processes as shown in Fig. 3. During tyre building, other tread components such as the base and cushion will promote better adhesion between the tread profile and tyre carcass. On the other hand, a sufficient stitching process needs to be applied on the tread, especially on the tread joint area during the tyre building process. This will improve tyre performance as the PCR tyre is subjected to high-speed tests and driving. The curing temperature and time for the tyre should be appropriately controlled to optimise its final properties. As PCR tyres are commonly vulcanised at higher curing temperatures, precaution should be taken as the silica-filled ENR tread compound can have a higher cure reversion, thus affecting its final performance. The physical properties can be maintained within normal conventional based tyre tread specifications if proper temperature and time control are observed during tyre vulcanisation. The addition of an anti-reversion agent assists to minimise the cure reversion of the silica-filled ENR tread compound at a higher temperature. A study has shown that addition of an anti-reversion agent will reduce the cure reversion behaviour of silica-filled ENR 25 tread compounds [31]. Table 5 shows the physical properties of a PCR tread compound based on ENR 25 filled with 75 phr silica and 2 phr of silane coupling agent. The physical appearance of a PCR tyre with its tread based on ENR 25 is depicted in Fig. 4.

3.2 ENR Tyre Performance Regarding the aspect of safety, the PCR tyre based on silica-filled ENR tread does not compromise the standards or regulations required by most internal tyre regulatory bodies. The safety data for common test standards have shown that the tyre complies with the United Nations Economic Commission for Europe Regulation No.

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Fig. 4 PCR tyres 195/55R15 85 V with silica-filled ENR tread

30 (UNECE R30) and Federal Motor Vehicle Safety Standards No. 139 (FMVSS 139) regulations [28, 32]. Table 6 shows results of the standard regulation testing conducted on the ENR/silica PCR tyre size 195/55R15 85 V. High-speed performance is a major concern for PCR tyres, whereby UNECE R30 specifies the test speed according to speed rating of the tyre. Additionally, few samples of the tyre tested for extreme endurance did not show any defect or failure upon completion of the test procedure. Table 7 presents the performance of silica-filled ENR-based tread tyres in terms of wet grip, rolling resistance and noise level compared with the control tyre tread size of 195/55R15 85 V. The tread of the control tyre is based on sSBR/BR/NR tread filled with carbon black/silica with a similar tread pattern and construction as the silicafilled ENR tread tyre. The ENR PCR tyre with silica-filled ENR tread compound achieves excellent performance on wet grip (A rating) with a marginal improvement on noise level and rolling resistance (Table 7). The tyre testings are conducted according to the UNECE R117 requirements on wet adhesion, rolling resistance and rolling noise emission at the IDIADA Technology Center in Spain. It is worthy to note Table 6 Results of safety and regulation tests for silica-filled ENR tyres [28, 32] Tests/standards

Result

MS 149 Tyre marking, bead unseating, plunger energy, endurance and high speed

Pass

FMVSS 139 Tyre marking, bead unseating, plunger energy, endurance and high speed

Pass

UNECE R30 Tyre marking, high speed

Pass

Extreme endurance (in-house) Duration: 120 h Speed: 100 km/h Load: 130% of load index

Pass

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Table 7 Silica-filled ENR PCR tyrea performance [32] Test

ENR 25/silica

Rating

Control

Rating

Wet grip index

1.57

A

1.27

C

Rolling resistance coefficient (Cr)

9.44

E

10.4

E

Rolling noise, dB(A)

70

2 waves

71

2 waves

a

195/55R15 85 V

that changing the tread compound to silica-filled ENR gave the benefit of 1 dB(A) reduction in rolling noise without changing both tread pattern and tyre construction. The rolling resistance of silica-filled ENR 25 tread achieved around 9% lower than the control tread. The difference in rolling resistance results is almost close to the previous study which achieved around 13% difference in rolling resistance when compared with silica-filled ENR 25 PCR tread and silica-filled sSBR/BR tread [23]. As rolling resistance for PCR tyres based on silica-filled ENR tread is lower than the control tyre of SBR/BR/NR filled with carbon black/silica tread, it also has the ability to reduce energy consumption of the vehicle. Our study has shown that the use of silica-filled ENR tread tyres can lower the fuel consumption of a car [28, 32]. Table 8 presents the fuel consumption of a 1.5L petrol engine-powered test car at variable speeds upon driving on an asphalt test track when comparing the silica-filled ENR tyre with a control tyre comprising carbon black/silica-filled sSBR/BR/NR tread. On average, the fuel consumption of the tyre with silica-filled ENR 25 tread is 1.8% lower than the control tyre tread. More obvious differences in fuel consumption are observed when the test car was driven at a high speed, which increases up to 2.65% in fuel consumption. The results of low fuel consumption of the test car fitted with the ENR 25 tyre tread were contributed by the low rolling resistance coefficient value of the tyre (Table 8). Hence, it can be seen that reduction of 9% in rolling resistance of the tyre using silica-filled ENR 25 tread gives about 1.8% decrease in fuel consumption. This finding is similar to the study by US NHTSA [33] and Michelin [34] on passenger cars where a 10% reduction in tyre rolling resistance led to a 1.1–2.0% decrease in fuel consumption. This relates to the estimation that 4.2% Table 8 Fuel consumption of silica-filled ENR tyre treadsa [28, 32]

Test speed, km/h Fuel consumption, L/100 km

Difference, %

Control Silica-filled ENR 50

4.16

4.08

−1.92

70

4.85

4.81

−0.82

80

5.36

5.28

−1.49

90

6.03

5.90

−2.16

120

8.31

8.09

−2.65

Average difference a

−1.79

1.5 L petrol engine powered test car, tested on track

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of fuel energy in a car is used to overcome rolling resistance. Since only 12.6% of energy in the fuel is transmitted to the wheels, tyre rolling resistance consumes about one third (1/3) or 33% of the total usable energy [23]. The lower fuel consumption of ENR tyres will also lead to a reduction of greenhouse gases and preserve the hydrocarbon-based energy. Braking performance test conducted at various conditions, speeds and weight of the test car has shown that the silica-filled ENR tread exhibits better grip and stopping distances than the sSBR/BR/NR filled with carbon black/silica control tread. The silica-filled ENR tread can produce a shorter stopping braking distance compared with the control tread at all surface conditions of asphalt as shown in Fig. 5. With the application of an anti-lock brake system (ABS), the stopping distance for silica-filled ENR tyre was 5.9 m shorter than the control tyre at a test speed of 120 km/h on a dry surface. On a wet surface, a more significant result was observed, whereby the silica-filled ENR tyre stopped at 8.1 m shorter than the control tyre at a test speed of 80 km/h [28, 32]. On the other hand, a previous study has also shown that the silica-filled ENR tyre stopped 3.6 m shorter on wet Bridport pebble and 3.2 m shorter on wet tarmac when tested using a locked trailer wheel at a speed of 40 km/h when compared with a commercial tyre with silica-filled sSBR/BR tread [28]. The braking distance trend results were in agreement with the wet grip results in Fig. 5 and confirm that the silica-filled ENR tyre has a good grip on both wet and dry surfaces.

Fig. 5 Comparison of test car stopping distance between silica-filled ENR and control tread tyres on both dry and wet asphalt surfaces [28, 32]

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Fig. 6 Steady-state cornering test

Table 9 Data value for steady-state cornering on a dry asphalt surface [28, 32]

Speed, km/h

G-force Silica-filled ENR

Control

80

0.748

0.707

90

0.816

0.792

100

0.899

0.896

Ride and handling tests determined the comfort feel and control ability of the vehicle towards high speed and cornering during driving. The relationship between lateral grip and responsiveness of steering input during cornering is important characteristics of tyre handling. Figure 6 and Table 9 present the method for a steady-state cornering test and the test results. When a car is subjected to high-speed cornering in a steady state, it produces a centrifugal force opposite to the bending direction. The interface between the tyre and road generates a centripetal force or lateral friction force to retain the car on its travel path. The friction force data will translate into G-force or coefficient of lateral friction which is an indication of the lateral grip of the four tyres [35]. The silica-filled ENR tyre produced better results compared with the control tyre of sSBR/BR/NR filled with carbon black/silica in the objective steady-state cornering test. Silica-filled ENR tyres produce a higher maximum coefficient of lateral friction than control tyres during cornering at test speeds of 80, 90 and 100 km/h. With the fitment of silica-filled ENR tyres, the test car can be safely driven without tyre lateral slippage during the high-speed cornering test on a dry asphalt surface. This indicated that the tyre also produced a high lateral friction during cornering. A similar result was obtained in the wet slalom test, whereby silica-filled ENR treads showed a higher G-force value and the ability to be driven at higher speeds on a snaky path. Figure 7 and Table 10 show the wet slalom test and its results, respectively. The car fitted with silica-filled ENR tread can be driven at a higher speed passing through the 18 m gap of every cone on the wet asphalt surface without

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Fig. 7 Slalom test on a wet asphalt surface

Table 10 Data value for the wet slalom test [28, 32]

Parameter

Silica-filled ENR

Control

Maximum driving speed, km/h

71.96

68.35

G-force

0.959

0.893

Control

Fig. 8 Subjective ride and handling rating of the ENR 25–silica PCR tyre

Ride Comfort

Roll/Yaw

Interior Noise 7.2 7 6.8 6.6 6.4 6.2

ENR

Steering

Handling & Tracking

loss of control, neither exceeding the road width nor hitting any cone. The results have been influenced by the high grip and traction characteristic of silica-filled ENR in a lateral and longitudinal direction. In a subjective assessment, silica-filled ENR tyre showed a better rating in terms of handling and tracking, besides good rollover resistance. Figure 8 illustrates the subjective ride and handling rating evaluated by trained personnel. The high grip ability of ENR tyres provides better ratings on some ride and handling characteristics, although the tyre rated slightly low in steering ability and ride comfort as compared with the control tyre. The high lateral grip of the tread requires more steering effort for the driver to make a turning. Still, the tyre with silica-filled ENR tread showed a better overall rating than the tyre with the control tread.

3.3 ENR Tyre Wear Performance Wear is one of the major concerns in tyre performance. Besides affecting operational costs, it also impacts the environment by influencing disposable issues once tyres are worn. There are several laboratory abrasion test methods such as DIN, Akron and PICO to determine wear performance of tyre treads. However, some findings have

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shown that the ENR 25 tread filled with silica has variable laboratory test results and may not reflect the actual tyre wear performance [9, 36]. Therefore, actual tyre wear performance can only be determined by conducting a road trial on the tyre. One of the most common parameters to determine actual tyre wear during a road trial is by using treadwear data. Treadwear rate indicates treadwear or tread depth lost in millimetres at a specific travelling distance. The value is most important for the determination of tread depth lost for every 1,000 km of vehicle movement. A lower wear rate indicates better wear performance of the tyre. Based on a previous study, ENR/silica tends to have inferior properties on wear performance which could be improved by blending with BR [37]. However, this study did not include a coupling agent in the rubber formulation. In the case of silica-filled ENR tread tyres with an addition of small amounts of silane coupling agent, the tyre obtained a slightly lower wear rate when compared with the control tyre, sSBR/BR/NR filled with carbon black/silica. Figure 9 presents the tyre treads’ wear rate of the test tyre, size 195/55R15. The wear performance of silica-filled ENR tread is equivalent to sSBR/BR/NR filled with carbon black/silica. The wear rate is significantly lower for ENR 25 filled with dual filler of silica/carbon black tyre indicating better mileage for the ENR system. This might be due to the hybrid reinforcing effect of carbon black and silica in providing wear resistance and strength towards the rubber. The endurance and on-the-road testing of ENR/silica tyres was conducted using a sedan car (Proton Persona 1.6A) for 60,000 km (Fig. 10). At the end of the testing, the ENR 25/silica tyre was still in good condition and the projected tread life of the tyre was about 90,000–100,000 km, which indicates a longer mileage obtained when using the ENR tread compound.

Fig. 9 Comparison of wear rates of PCR tyres size 195/55R15 between ENR 25 filled with silica, ENR 25 filled with dual fillers (silica/CB) and control sSBR/BR/NR filled with carbon black/silica

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Fig. 10 Endurance and on-the-road testing of ENR 25–silica PCR tyres for 60,000 km travelling distance

A summary of tyre performance of the ENR 25–silica PCR tyres is shown in Fig. 11. Improvement in the tyre performance with the ENR tread compound makes it an attractive choice for tyre application.

4 ENR Retreads for Commercial Vehicle Tyres ENR retreads are the precured treads based on blends of ENR 25 designed for commercial vehicle tyres with an improvement in tyre performance, especially wet grip for safety as well as mileage and durability. The ENR retreads have been subjected to road tests involving bus and truck tyres. A road test generally relates to the severity of wear, where it is defined as the absolute loss in tread height per unit distance of a reference tyre tread, usually expressed as mm/1000 km [38]. Tyre wear is influenced by various factors which are categorised as follows: (a) (b) (c)

tyre- and vehicle-independent factors including road surface and weather conditions [39–41]; vehicle driving factors include steering forces, driving and braking forces, driving speed and tyre load [38]; and tyre factors such as tyre construction, tread pattern and tread compound [20, 42].

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Fig. 11 Summary of ENR 25–silica tyre performance

4.1 ENR Bus Tyres The ENR precured retread was manufactured in collaboration with a commercial tyre retreader. The ENR blend compounds were produced in an Intermix® 270 L internal mixer. The extrusion of tread liner was conducted using a hot feed extruder followed by vulcanisation of the liner at 160 °C for 20 min on a heated press. The retreading process was carried out according to MS224 specifications and requirements [43]. The ENR precured tread liners with an urban tread pattern (Fig. 12) were retreaded on a tyre size 275/70R22.5. The ENR-retreaded tyre performance in terms of rolling resistance, wet grip and rolling noise is shown in Table 11. The tests were conducted at the IDIADA proving ground according to UNECE R117. Figure 13 illustrates results of the large-scale road trial using ENR retreads in city buses on different routes, with ten buses on each route. Different routes give different wear results, indicating the major influence of tyre independent factors which include road surface and driving factors. The average projected tread life of the ENR/BR (40/60) blend is about 39,000 km. It is interesting to note that ENR-retreaded tyres exhibit lower wear (%) compared to control tyres either in city traffic, intercity or highway routes as shown in Fig. 14. The intercity route is a combination of highway and city traffic. Different routes gave different results of wear, where both highway and intercity resulted in higher wear than city traffic for the control and ENR-retreaded tyres. This is due to the higher speed applied while driving on the highway. It was established that wear is greatly

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Fig. 12 Tread pattern used for ENR retreads

Table 11 ENR-retreaded tyre performance

Performance rating

ENR-retreaded Tyrea

Rolling resistance coefficient (Cr)

6.96

Fuel efficiency class

D

Wet grip index

1.17

Wet grip class

B

External rolling noise dB(A)

73

Class/black wave

2 waves

a

275/70R22.5 C3 tyre

dependent on speed [23]. For ENR-retreaded tyres, the city route shows the lowest wear. On the intercity route, a significant difference in wear is observed between the right and left tyres for both ENR and control tyres (Fig. 15). It has been thus established that the right tyre wears differently to the left tyres, which depends on the number of turns taken in either direction [23], and hence, rotating the position of a tyre is recommended on regular basis. On the intercity route, the tyres are subjected to more cornering during driving in a combination of both highway and city traffic. It is known that cornering and load influence wear of the tyre [23]. As buses in Malaysia are driven on the right side and the number of passenger seats are positioned towards the right side of the bus, the load on the right is higher than the left tyres. Hence, it is expected that the right tyres exhibit higher wear than left tyres as seen with control tyres. However, it is different for ENR tyres where the left tyres wear greater than the right tyres. It may relate to the effect of filler–polymer and filler–filler interaction on the hysteresis, which exhibited different wear performance for the tyres used in this study.

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Fig. 13 Projected tread life of ENR retreads from an on-road trial using city buses. The trials were carried out on different routes and over different durations using the same ENR/BR retread compound. Ten buses were used on each route

Fig. 14 Comparison of different routes on the wear of ENR and control-retreaded tyres

For city traffic routes, a difference in wear between the left and right tyres is observed albeit to a lesser degree. For highway driving, wear difference between the left and right is lesser for both ENR and control tyres. It has also been established that the speed of the vehicle greatly influences tyre wear [23], and the tread compound effect contribution is lesser at a higher speed.

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Fig. 15 Comparison of left and right sides of a ENR and b control-retreaded tyres tested during road trials on different bus routes

In another large-scale evaluation of ENR retreads in the Klang Valley, wear performance of retreaded tyres was evaluated using 87 city feeder buses over a 6–12 month duration. These buses travel daily, connecting commuter rail stations on 25 different routes. One-way travelling distance covered by these buses varied from 4 to 14 km. On average, the travelling distance of these buses per month is between 3,000 and 5,000 km. Several ENR blends, as well as NR blends, were evaluated in this road trial. The treadwear rating and wear rate of ENR blends are shown in Figs. 16 and 17. The NR/BR (70/30) blend was taken as a reference compound and noted as 100. The other blends were rated according to the reference compound. The ENR/NR/BR (20/20/60) blend outperformed the reference compound by 6% and SBR/BR (40/60) by 15%. The other ENR blends, namely ENR/BR (40/60), ENR/NR/BR (30/40/30) and ENR/NR/BR (20/30/50), showed inferior wear to the reference compound.

Fig. 16 Treadwear rating of ENR blend retreads during the on-road trial using city feeder buses

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Fig. 17 Average wear performance of ENR blend retreads during the on-road trial using city feeder buses

Table 12 Comparison of on-road performance with laboratory abrasion data

Compound blend

DIN abrasion resistance Index

Wear performance, km/mm

Treadwear rating

NR/BR (70/30)

119

2544

100

ENR/NR/BR (20/20/60)

181

3030

106

SBR/BR (40/60)

249

2652

91

NR/BR (40/60)

250

2590

88

ENR/BR (40/60)

152

2247

78

ENR/NR/BR (30/40/30)

159

2061

72

ENR/NR/BR (20/30/50)

159

1718

57

Treadwear is a complex phenomenon influenced by many dependent and independent tyre factors. Hence, it is challenging to correlate results from the road testing to simple laboratory abrasion tests, and quite often, these contradict one another. It is reported that ENR 25–silica displayed poor wear performance compared with ENR 25 black [19]. It has been established that addition of BR increases abrasion resistance but with a slight loss in wet grip of ENR 25 for the ENR/BR blend [37]. A comparison of laboratory results (DIN abrasion resistance index) with road data (wear performance, treadwear rating) of ENR blends is given in Table 12.

Epoxidised Natural Rubber in Tyre Applications Table 13 Physical properties of ENR blend vulcanisates

119

Properties

NR/BR

ENR/NR/BR

ENR/BR

Tensile strength, MPa

20

22

20

Elongation at break, %

680

480

460

Modulus at 100%, MPa

1.9

2.2

1.9

Modulus at 300%, MPa

6.6

13.0

11.0

Hardness, IRHD

67

65

72

Resilience, %

57

53

52

DIN abrasion vol. loss, mm3

76

77

98

DIN abrasion resistance index

249

181

152

The blend ratios: NR/BR 40/60; ENR/NR/BR 20/20/60; ENR/BR 40/60

The ENR/NR/BR (20/20/60) blend exhibited a higher laboratory abrasion resistance index than the control NR/BR (70/30) compound and displayed better wear performance during on-road testing. Although the laboratory abrasion resistance index for the SBR/BR (40/60) and NR/BR (40/60) blends is almost double of the control (NR/BR 70/30), the on-road testing displays inferior wear performance and treadwear rating. It has been reported that NR is the optimum elastomer at intermediate severity, while SBR is the preferred elastomer at very low severity, and BR is the preference for high severity applications [44]. The SBR/BR (55/45) compound displayed the worst performance compared with the NR/BR (80/20) compound when used in the fourth axle (high severity), although it exhibits the highest laboratory abrasion index [23]. Other ENR blends have exhibited a better laboratory resistance index than the control, and the actual tyre on-road trial resulted in poor wear performance. Physical properties of the selected ENR blend vulcanisates are shown in Table 13. The ENR blends possessed comparable tensile properties compared with the NR/BR blend. The temperature hysteresis profile in Fig. 18 confirms that all ENR blends exhibit a higher tan delta at 0 °C than that of NR/BR or SBR/BR blends. This indicates a promising wet grip property with ENR retreads which is vital for the safety of truck and bus tyres. The results are further confirmed by the high wet grip index (B rating based on EU tyre label) from wet adhesion testing conducted on the retreaded ENR tyre size 275/70R22.5 (Table 11). Based on the performance results listed in Table 14, the ENR/BR-retreaded tyre has successfully passed requirements for the certification of eco-labelling of retreaded tyres for C3 tyres under the SIRIM ECO 56.

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Fig. 18 Temperature hysteresis profile of ENR blends

Table 14 ENR-retreaded tyre performance (eco-labelling) Performance rating

ENR-retreaded tyrea

Eco-labelling (SIRIM ECO56) requirement

Wet grip index

1.23

>0.64

Wet grip class

B

Rolling resistance coefficient (Cr)

7.37

Fuel efficiency class

E

External rolling noise dB(A)

71

Class/black wave

1 wave