Novel Defence Functional and Engineering Materials (NDFEM) Volume 2: Engineering Materials for Defence Applications 9789819997947, 9789819997954

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Indian Institute of Metals Series

Eswara Prasad Namburi R. J. H. Wanhill Dipak Kumar Setua   Editors

Novel Defence Functional and Engineering Materials (NDFEM) Volume 2 Engineering Materials for Defence Applications Metallurgy Materials Engineering

Indian Institute of Metals Series Series Editor U. Kamachi Mudali, Vice Chancellor, HBNI, DAE, Mumbai, Maharashtra, India Editorial Board Bikramjit Basu, Materials Research Center, IISc, Bangalore, Karnataka, India Suman K. Mishra, CGCRI, Kolkata, West Bengal, India Eswara Prasad Namburi, Ex-DMSRDE, Kanpur, Uttar Pradesh, India S. V. S. Narayana Murty, Liquid Propulsion Systems Centre, ISRO, Trivandrum, Kerala, India R. N. Singh, Mechanical Metallurgy Division, BARC, Mumbai, Maharashtra, India R. Balamuralikrishnan, DRDO, DMRL, Hyderabad, Telangana, India

About the Book Series: The study of metallurgy and materials science is vital for developing advanced materials for diverse applications. In the last decade, the progress in this field has been rapid and extensive, giving us a new array of materials, with a wide range of applications, and a variety of possibilities for design of new materials, processing and characterizing the materials. In order to make this growing volume of knowledge available, an initiative to publish a series of books in Metallurgy and Materials Science was taken during the Diamond Jubilee year of the Indian Institute of Metals (IIM) in the year 2006, and has been published in partnership with Springer since 2016. This book series publishes different categories of publications: textbooks to satisfy the requirements of students and beginners in the field, monographs on select topics by experts in the field, professional books to cater to the needs of practising engineers, and proceedings of select international conferences organized by IIM after mandatory peer review. The series publishes across all areas of materials sciences and metallurgy. An panel of eminent international and national experts serves as the advisory body in overseeing the selection of topics, important areas to be covered, and the selection of contributing authors.

Eswara Prasad Namburi · R. J. H. Wanhill · Dipak Kumar Setua Editors

Novel Defence Functional and Engineering Materials (NDFEM) Volume 2 Engineering Materials for Defence Applications

Editors Eswara Prasad Namburi Defence Materials and Stores Research and Development Establishment (DMSRDE) Defence Research and Development Organisation (DRDO) Kanpur, Uttar Pradesh, India

R. J. H. Wanhill Emeritus Principal Research Scientist Aerospace Vehicles Division Royal Netherlands Aerospace Centre Amsterdam, Flevoland, the Netherlands

Dipak Kumar Setua ACRHEM/UoH, Defence Materials and Stores Research and Development Establishment (DMSRDE) Defence Research and Development Organisation (DRDO) Kanpur, Uttar Pradesh, India

ISSN 2509-6400 ISSN 2509-6419 (electronic) Indian Institute of Metals Series ISBN 978-981-99-9794-7 ISBN 978-981-99-9795-4 (eBook) https://doi.org/10.1007/978-981-99-9795-4 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 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 Paper in this product is recyclable.

These book volumes are respectfully dedicated to all our Indian Defence Functional Materials Scientists, Technologists, and Industrials Associates, and also the Defence Personnel, who have been collaborating with the authors and editors of these volumes; and our partners and families for their continuous support, notably during 2020–2023, whose constant encouragement and kind support have been the main sources of motivation for maintaining this task until its completion. —Eswara Prasad Namburi, R. J. H. Wanhill and Dipak Kumar Setua

Series Editor’s Preface

The Indian Institute of Metals Series is an institutional partnership series focusing on metallurgy and materials science and engineering.

About the Indian Institute of Metals The Indian Institute of Metals (IIM) is a premier professional body (since 1947) representing an eminent and dynamic group of metallurgists and materials scientists and engineers from R&D institutions, academia, and industry, mostly from India. It is a registered professional institute with the primary objective of promoting and advancing the study and practice of the science and technology of metals, alloys, and novel materials. The institute is actively engaged in promoting academia–research and institute–industry interactions.

Genesis and History of the Series The study of metallurgy and materials science and engineering is vital for developing advanced materials for diverse applications. In the last decade, progress in this field has been rapid and extensive, giving us a new array of materials, with a wide range of applications and a variety of possibilities for processing and characterizing the materials. In order to make this growing volume of knowledge available, an initiative to publish a series of books in metallurgy and materials science and engineering was taken during the Diamond Jubilee year of the Indian Institute of Metals (IIM) in the year 2006. IIM entered into a partnership with Universities Press, Hyderabad, in 2006, and from 2016 the book series is under MoU with M/s Springer Nature, and as part of the IIM book series, a total of 24 books were published till 2023. The books were authored by eminent professionals in academia, industry, and R&D with outstanding background in their respective domains, thus generating unique vii

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resources of validated expertise of interest in metallurgy. The international character of the authors’ and editors has enabled the books to command national and global readership. This book series includes different categories of publications: textbooks to satisfy the requirements of undergraduates and beginners in the field, monographs on select topics by experts in the field, and proceedings of select international conferences organized by IIM, after mandatory peer review. An eminent panel of experts constitutes the advisory body in overseeing the selection of topics, important areas to be covered in the books and the selection of contributing authors.

About “Novel Defence Functional and Engineering Materials” This book on “Novel Defence Functional and Engineering Materials” with Volume 1: Defence Functional Materials, and Volume 2: Defence Engineering Materials, will be of great interest to the scientists and researchers who are involved in developing new and innovative functional polymeric materials, synthetic and special fuels and fluids, and composite systems for the development of advanced systems like hypersonic vehicles, fighter aircraft, e.g., LCA Mk II, unmanned aero vehicle systems, a variety of Defence platforms, underwater missiles and torpedoes, radar detection and deception systems, strategic applications for future MBTs, in addition to advanced futuristic technologies for structural sensors, stealth and environmental applications. Both volumes of the book cover three important categories of functional materials: viz., polymers, elastomers and textiles; high temperature ceramics; and nanomaterials, particularly the design and development of polymer matrix composites, inorganic– organic hybrids, active and responsive smart materials, magneto-rheological fluids, gels and lubricant fluids, polymeric systems and coatings for stealth and camouflage applications, with examples of a few critical products. To some extent theoretical aspects are dealt with, where appropriate, to provide core knowledge on phases and microstructures; in addition, the use of sophisticated characterization techniques, like transmission and scanning electron microscopy equipped with various analyzers; Xray diffraction; nanoindentation; atomic force microscopy; FTIR and Raman Spectroscopy; vector network and particle size analyzers; and a variety of thermal and magnetic property measurement instruments have been brought out, including a few improvised analytical techniques. Both volumes 1 and 2 contain 10 chapters each by well-known experts in the field from India and abroad, and finely edited and delivered by the editors, Dr. Eswara Prasad Namburi, Dr. R. J. H. Wanhill and Dr. Dipak Kumar Setua. They have made excellent efforts to coordinate with specialists in the respective fields of polymers, elastomers and textiles; high temperature ceramics; and nanomaterials, to provide 20 articles of high relevance. Both Volumes 1 and 2 of this Book will be a treasure for those who are interested in learning everything about functional and engineering materials of relevance to defense applications, and pursue a research career and

Series Editor’s Preface

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study. The IIM-Springer Series gratefully acknowledge the three editors and all chapter authors for their excellent efforts in covering wide range of information on the subject matter of interest to the readers. Dr. U. Kamachi Mudali FIIM, FNAE, FNASI, FASM, FNACE FAPAM, FICS, HFECSI, FIIChE FASCh. Editor-in-Chief, Series in Metallurgy and Materials Engineering Vice Chancellor Homi Bhabha National Institute (HBNI) Mumbai, India

Foreword

While there are many Materials Science textbooks, and even vade mecums, they mostly deal with the properties of specific materials such as functional, electronic, composites and nanomaterials. However, there are only a few that serve the purpose of bringing most defence-related non-metallic materials together in a single book. This book, Volume 2 of Novel Defence Functional and Engineering Materials (NDFEM), fills this particular gap. It is admirable that the authors have focused not only on new generation functional and smart materials, but also many advanced engineering and structural materials of direct relevance to Defence Systems. The authors and editors have made special efforts to include many examples of polymer hybrids and nanocomposites developed specially for Defence applications. The discussion contents that have also been integrally included in nearly all the 20 chapters of this two-volume series and the 10 book chapters of this second volume enable each and every reader to understand Defence perspectives. The fact that DRDO’s and particularly DMSRDE’s efforts in this field have been highlighted in this book is greatly appreciated.

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I have known the editors of this book: Dr. Eswara Prasad Namburi, Dr. R. J. H. Wanhill and Dr. Dipak Kumar Setua for many years and can vouch for their knowledge of the subject, and I am sure the readers of this book will be greatly benefited.

Dr. Samir V. Kamat, FNAE Chairman, DRDO and Secretary, DD R&D Defence Research and Development Organisation (DRDO) Ministry of Defence Government of India New Delhi, India

Preface

Engineering Materials pertain to those class or classes of materials that are explicitly used for engineering applications. They differ only slightly from the structural materials used for load-bearing applications. Hence, although there is a great variety of engineering materials, they are mainly confined to the sphere of Structural Materials with a few specific functional components or with some specially designed features/ attributes. The performance or efficacy of Engineering Materials depends on their structural efficiency, amenability for 360-degree solutions (meaning effectiveness right from manufacturing, assembly, systems engineering, systems maintenance, refurbishment, and recycling) and cost-effectiveness with or without multiple features. They also should be optimized for their functionality with respect to size, shape, volume, and weight. Many Engineering Materials exist: metals/MMCs, polymers/PMCs, ceramics/ CMCs, fibers/FRPs, composites, micro-materials/MEMs and nanomaterials/NMEs, and also multi-layered or multi-component structures, or even modular materials. But all materials for Defence applications need to provide advantages over normally commercially available technical materials, such that they can be considered as Defence Engineering Materials. For obvious reasons, they and their technologies are mostly subject to security and its constraints. However, in the present global scenarios some of these Defence Engineering Materials are broadly discussed if not shared or revealed in much detail. A large number of monographs are made available (such as the more than 150 DRDO Monograph Series) in the open literature. Several patents have also been published from time to time. The Indian Defence Agencies emphasize ample dissemination of their requirements in terms of QRs, and also Standards that define what could or could not be considered for Defence Applications. The Second Volume in this two-volume book series covers some of the most important and contemporary Defence Engineering Materials that are being finalized for use in various Indian Defence Systems. Although both books do not cover specific details and applications, they provide the overall background, salient scientific achievements, manufacturing and qualification methodologies, and finally their future outlook. There are chapters on: 1. Special Polymers, 2. PMCs, 3. Defence xiii

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or Technical Textiles, 4. Special Textile Materials, 5. Materials and Structures for Defence Protection, 5. Ultrahigh Temperature Ceramics, 6. Hybrid Composites, 7. Protection Systems, and 8. Camouflage and Stealth Materials. All the chapters provide more information than normally available in the open literature. They also include references to many important documents, publications, and defence standards. Each author has taken special care to include the information up to the latest developments in international research, so that these two books serve optimally as ready reference material. We, the Editors of these two special book volumes, are also sure that there are ample data not normally present in the open literature. The chapters are lucidly presented to indicate the importance and suitability for special and/or specific Engineering Application areas. A few formulations are given as examples to show the underlying depth and vastness of experimentation and/or modeling that goes into designing these materials and systems. The contents of the chapters have been finalized by comprehensive and careful reviewing by both internal Indian Defence Scientists and Indian National and International experts, and all have been coordinated and executed with finality by the editors, especially Dr. R. J. H. Wanhill. The summary and salient features of these volumes are provided by the best internationally recognized Indian experts through their expertly drafted Forewords. We are sure all readers belonging to the large confluence of scientific, engineering, technology, and systems communities especially working in Defence sector or for Defence systems will readily and easily understand the usefulness of these volumes and exploit them to derive huge benefits from this rare set of book chapters. If the initial response of a few reviewers who have had access to the books is any indication, we are sure the responses will be enthusiastic and equally appreciative. We look forward to hearing from all the readers from the wide spectrum of Indian and International Functional and Engineering Materials Research, Manufacturing, Systems Engineering and Production, and sincerely wish all the efforts painstakingly put in by nearly 60 authors over 5–6 years are justifiably rewarded.

Kanpur, India May 2023

Eswara Prasad Namburi

R. J. H. Wanhill

Dipak Kumar Setua

Acknowledgements

When I met Dr. Baldev Raj, the then Editor-in-Chief of IIM-Springer Book Series in his Director’s office at NIAS, Bengaluru soon after we published the two-volume book on Aerospace Materials and Materials Technologies, he complemented our near 3-year effort to fructify that book project. [As he liked the book contents very much, he predicted they will do well both in stands (in sales) as well as Vade Mecums (in Libraries). His predictions came true as these books did extremely well on both fronts.] He then inquired whether we can bring out similar volumes for Functional Materials, as I just started leading a premier Functional Materials Laboratory of DRDO, the DMSRDE, Kanpur. I immediately said emphatically “NO” as I or my close associates till then never worked on functional materials. But, he was persistent; and, after an year or so, I conceded and consented to do another two-volume book series—another set of Vade Mecums on Functional and Engineering Materials. Since this is a very vast subject, we agreed to cover only Defence-related materials, in which my colleagues at DMSRDE and I started having a good insight and we were greatly strengthened in our cause when Dr. Dipak Kumar Setua joined hands as he was a known international expert in the area of polymers and their derivatives. Soon, the contents took shape and all the preliminary drafts were sent for review and basic approval to Dr. R. J. H. Wanhill, by then he was a very close associate and a co-editor for three of my earlier 16 books that I edited. He liked the idea and the outline, but rendered his apologies for not being able to do the honors (partly because the contents were not in the best of the shape for any international book publications). Later, he too had to agree to Dr. Baldev Raj’s request and put in enormous efforts to bring the contents to good standards of readability and presentability. A few special efforts by both Dr. Russell and Dr. Setua improved the quality of results and further due to their undeterred insistences, a few initial drafts took totally new shape with large omittances and many new additions. Some of the authors of this book series are greatly benefited and their own contributions elsewhere too have become widely accepted. All this has happened as this project was executed to near perfection for more than five years of hardship and total dedication.

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Special mention should be made to the perseverance of all the stakeholders not to lose the sight of ultimate goal (even in the worst situations of the COVID-19 pandemic that prevailed during 2019 all the way up to 2022), i.e., to complete the book project without lowering the standards, and also to cover most of the topics that are relevant not only to Indian defence, but also to defence applications for the world over. Hence, my heart goes out to all the authors and my two co-editors and I would like to express my sincere gratitude to all these contributors; I am indebted to each one of them: Dibyendu S. Bag, Akansha Dixit, Satyen Saha, Ashok Kumar, Jitendra Kr. Bansiwal, Gobardhan Lal, D. N. Tripathi, Arun Kr. Singh, Awanish Kr. Sonkar, Richa Verma, Rajesh Kr. Tiwari, Arun Kr. Mishra, Ajitendra S. Parihar, J. N. Srivastava, P. Venkitanarayanan, Vidya S. Pandey, Saurabh Kr. Srivastava, Jitendra Yadav, Arati Kole, Mukesh Kr. Sinha, Biswa Ranjan Das, Anurag Srivastava, Priyanka Katiyar, Shraddha Mishra, Thako H. Goswami, Suresh Kumar, Ashok Kumar, Lalit M. Manocha, Himanshu B. Baskey, Ashish Dubey, Krishna Kr. Gupta, Alok Kr. Dixit, Syed M. Abbas, and T. C. Shami. Dr. Russell Wanhill’s efforts need a very special mention, especially for these book volumes. This is because in case of many book chapters, we had minimum three or four versions, and further some of these versions were differently modified by either Dr. Setua or our internal DMSRDE experts, causing utter chaos and confusion. Dr. Wanhill not only kept track of these versions and progressively worked on all the drafts, and he singlehandedly and painstakingly shaped the entire book contents. No works will suffice to thank him enough and these two volumes took final shape only due to his unparalleled efforts. I also take this opportunity to thank Dr. Setua, who went through lots of health problems, still never excused himself from this daunting task. I owe a lot to him and also learnt a lot in this process with regard to all these special defence functional and engineering materials, which were till then foreign to me. Some of my colleagues who authored many special and classified subjects took special care to work diligently not to reveal any classified information, but, at the same time, did justice to cover amply on all the important aspects of each of the materials and material systems. I thank all of them profoundly. As I said these book volumes underwent several interactions, including one to take care of stringent plagiarism limits of Springer Nature. Mr. Devroop Arya and Mrs. Bhavana Srivastava of DMSRDE as well as Mr. S. Shashinath of DMRL, DRDO, Hyderabad helped immensely with minimum 2–3 rounds of checks. Mr. Devroop, Mrs. Pratibha Singh, and Mr. Surendra Kr. Yadav of DMSRDE further helped in file maintenance as well as several rounds of formatting. A few colleagues from the various materials directorates of DMSRDE too have helped and my special thanks are due to them. I also thank several senior and yesteryear DMSRDE scientists and technologists and former directors—Prof. G. N. Mathur, Dr. K. U. Bhasker Rao, Dr. A. K. Saxena, and the present director, Dr. Mayank Dwivedi. Besides these dignitaries, my heart goes out to the colleagues and their family members whom we lost dearly due to COVID-19 pandemic, especially Dr. Mahender Prasad; Shri Avinash Pankaj of DMSRDE; and Shri Alok Mal, Director, DHR, DRDO Hqs (Formerly with DMSRDE).

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I consider it is an honor that none other than Prof. Srinivasa Ranganathan, a doyen of Materials Research in India has agreed to write the Foreword for the first of the two volumes. I am most indebted to Dr. Ranganathan. Dr. Samir V. Kamat who recently took charge as Chairman, DRDO has obliged my request and wrote the Foreword for the second volume despite his many pressing engagements within a very short time and I consider this as a special favor that he has done to me personally. I should also place on record my sincere thanks to Dr. K. Bhanu Sanakara Rao, Dr. G. Malakondaiah, Prof. V. V. Kutumbarao, Prof. J. Viplava Kumar, Prof. Rajiv Prakash, Prof. Sandeep Verma, Prof. B. S. Murty, Prof. I. Manna, Prof. D. Banerjee, Prof. Shrikant Lele, Dr. G. Sundararajan, Prof. C. Suryanarayana, Prof. K. A. Padmanabhan, Dr. Shantanu Chakrabarti, Prof. C. Ravi Ravindran, Dr. Kamachi U. Mudali, Dr. Amol A. Gokhale, Dr. G. Satheesh Reddy, Dr. Kota Harinarayana, Dr. V. K. Saraswat, and Prof. P. Rama Rao for their constant encouragement and exemplary support. Special mention should be made to the spontaneous encouragement given by Prof. Dr. Eduard Arzt and Prof. Tobias Kraus, though I could not involve them much directly in this mammoth publishing effort. The final contribution to manuscript preparations and support was provided by a few intimate dignitaries, officials, and colleagues at BEPL/NH, located at the Mind Space, HITEC City, Hyderabad, where I was on a 6-month sabbatical as Expert Advisor. I profoundly thank all of them. I consider it my honor that Springer Nature Publications has agreed to IIM Book Publication Committees’ recommendations and accepted to publish these book volumes. I place on record my profound gratitude to Mr. William Achauer, Director and Mr. Anil Chandy, Managing Director, Springer Nature Singapore Private Limited for official contracting; Mrs. Swati Maharishi, Editorial Director, Springer, New Delhi for her special, continued, and constant support; and Mr. Ashok Kumar, Senior Production Executive, Springer New Delhi for the kind follow-up. Our special thanks are due to Mrs. Ramya Somasundaram, Project Coordinator, Book Production, Springer Nature for valuable support in printing these volumes, and more importantly for bearing with us with many a number of special requests. At this juncture I should acknowledge gratefully the initiative taken by the IIM, particularly by Late Dr. Baldev Raj, without whom I would not have ventured to this project and without his invisible support I would not have completed this book project.

Eswara Prasad Namburi

Current Series Information

To increase the readership and to ensure wide dissemination among global readers, this new chapter of the series has been initiated with Springer in the year 2016. The goal is to continue publishing high-value content on metallurgy and materials science and engineering, focusing on current trends and applications. So far, eleven important books on state of the art in metallurgy and materials science and engineering have been published and three books were released during IIM-ATM 2022 at Hyderabad. Readers who are interested in writing books for the Series may contact the Series Editor-in-Chief, Dr. U. Kamachi Mudali, Former President of IIM and Vice Chancellor of Homi Bhabha National Institute (HBNI), Mumbai at ukmudali1@ gmail.com, [email protected] or the Springer Editorial Director, Ms. Swati Meherishi at [email protected].

Editorial Advisory Board (2022–2024)

Editor-in-Chief Dr. U. Kamachi Mudali FIIM, FNAE, FNASI, FASM, FNACE, FAPAM, FICS, HFECSI, FIIChE FASCh. Editor-in-Chief, Series in Metallurgy and Materials Engineering and Vice Chancellor, Homi Bhabha National Institute (HBNI), Mumbai, India

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Member Secretary Dr. Divakar Ramachandran, IGCAR, Kalpakkam

Editorial Advisory Board Prof. Bikramjit Basu, IISc, Bengaluru Dr. Suman K. Mishra, CGCRI, Kolkata Dr. Eswara Prasad Namburi, Ex-DMSRDE, Kanpur Dr. S. V. S. Narayana Murty, Liquid Propulsion Systems Centre, ISRO, Trivandrum, Kerala, India Dr. R. N. Singh, BARC, Mumbai Dr. R. Balamuralikrishnan, DMRL, Hyderabad

Contents

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Polymer Matrix Composites (PMCs) for Defence Applications . . . . . Dibyendu S. Bag, Ashok Kumar, Jitendra Kr. Bansiwal, Gobardhan Lal, D. N. Tripathi, and Eswara Prasad Namburi

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Hybrid Polymeric Composites for Defence Applications . . . . . . . . . . . Arun Kr. Singh, Richa Verma, Rajesh Kr. Tiwari, Eswara Prasad Namburi, and R. J. H. Wanhill

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Materials and Technologies for Personal Protection Systems (PPSs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Arun Kr. Mishra, Ajitendra S. Parihar, J. N. Srivastava, Rajesh Kr. Tiwari, Eswara Prasad Namburi, and P. Venkitanarayanan

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Polymeric Materials for Defence Stores in Extreme Cold Weather . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J. N. Srivastava, Vidya S. Pandey, and Eswara Prasad Namburi

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Development of Non-metallic Structural Materials for Defence Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Sourabh Srivastava, Jitendra Yadav, J. N. Srivastava, Arati Kole, and Eswara Prasad Namburi

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Defence Protective Textiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 Mukesh Kr. Sinha, Biswa Ranjan Das, Anurag Srivastava, and Eswara Prasad Namburi

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Nanofibre Web Coatings Based on Nano-Spider (NS) Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 Mukesh Kr. Sinha, Biswa Ranjan Das, and Eswara Prasad Namburi

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Superhydrophobic Textiles for Protective Clothing . . . . . . . . . . . . . . . 225 Priyanka Katiyar, Shraddha Mishra, T. H. Goswami, Anurag Srivastava, and Eswara Prasad Namburi

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Contents

High Temperature Composites for Aerospace Applications and Their (Select) Production Technologies . . . . . . . . . . . . . . . . . . . . . . 253 Suresh Kumar, Ashok Ranjan, L. M. Manocha, and Eswara Prasad Namburi

10 Functional Materials for Stealth and Camouflage Applications . . . . 287 Himanshu B. Baskey, Alok Kr. Dixit, Krishna Kr. Gupta, Ashish Dubey, S. M. Abbas, T. C. Shami, and Eswara Prasad Namburi

Editors and Contributors

About the Editors Dr. Eswara Prasad Namburi joined DRDO in 1985 after obtaining a B.Tech. and Ph.D. in Metallurgical Engineering from the Indian Institute of Technology, Banaras Hindu University. He retired in September 2022 after serving the Indian Ministry of Defence for more than three decades as an Outstanding Scientist of DRDO. Dr. Prasad has contributed immensely to the development of several defence systems, including fighter aircraft structures, engines, missiles, naval propulsion, and personal protection systems. He has been instrumental in awarding 14 defence production technologies to over 40 Indian industries. Dr. Eswara Prasad is internationally known for his research on fatigue, fracture and life extension behaviour of several aero, structural and functional materials, with over 750 wide-ranging publications, which include; 12 government white papers; 17 international edited books/ conference proceedings, Monograph on AluminiumLithium Alloys: Processing, Properties and Applications and a 2-volume Reference Work Aerospace Materials and Material Technologies. He has been Chairman, President or Member of 10+ national committees and bodies and received 15+ national and international awards and 70+ honours and recognitions. These include fellowships of six national and international science and engineering academies/institutions, notably Founder Fellow of the Indian Structural Integrity Society (FInSIS) and Fellow of the Asia Pacific Academy of Sciences (FAPAM). xxiii

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Dr. R. J. H. Wanhill is emeritus Principal Research Scientist, Aerospace Vehicles Division, Royal Netherlands Aerospace Centre (NLR), in the Netherlands. He holds two Doctorates, one from the University of Manchester (1968) and the second from the Delft University of Technology (1994). He joined the NLR in 1970, and since then has investigated fatigue and fracture of all classes of aerospace alloys. He is co-author of the book Fracture Mechanics (1984), which ran into a second edition; co-author with Simon Barter of the monograph Fatigue of Beta Processed and Beta Heattreated Titanium Alloys (2012); co-author and co-editor for the book Aluminium-Lithium Alloys: Processing, Properties and Applications (2014); co-author and coeditor for the 2-volume series Aerospace Materials and Material Technologies (2017); co-author of the monograph Corrosion and Stress Corrosion Testing of Aerospace Vehicle Structural Alloys (2018); and coauthor of the monograph Fatigue Crack growth Failure and Lifing Analyses for Metallic Aircraft Structures and Components (2019). Dr. Dipak Kumar Setua obtained his M.Sc. (Chemistry) in 1981 with institute top rank and Ph.D. (Polymer Science and Rubber Technology) in 1985, both from the Indian Institute of Technology (IIT), Kharagpur, India. Subsequently, he joined as a research scientist and spent more than three decades at the Defence Materials and Stores R&D Establishment (DMSRDE), Kanpur, India. Furthermore, Dr. Setua served DMRSRDE and DRDO as a DRDO Fellow until 2022. He spent two years (1989–1991) at the Institute of Polymer Engineering, University of Akron, Ohio, Akron, USA, as a visiting scientist and remained Director of the Advanced Centre of Research on High Energy Materials (ACRHEM), University of Hyderabad, India, during 2013–2014. Some of the of his research include short fibre-rubber composites, polymer blends and compatibility, polymer characterization, magneto-rheological elastomer, and design and development of polymer-based composites and nanocomposites and products for personal protection, aerospace systems and bio-medical applications. Dr. Setua has published more than 100 archival papers, several patents and book chapters. He has also received many prestigious honours and awards.

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Contributors S. M. Abbas Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India Dibyendu S. Bag Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India Jitendra Kr. Bansiwal Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India Himanshu B. Baskey Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India Biswa Ranjan Das Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India Alok Kr. Dixit Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India Ashish Dubey Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India T. H. Goswami Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India Krishna Kr. Gupta Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India Priyanka Katiyar Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India Arati Kole Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India Ashok Kumar Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India

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Editors and Contributors

Suresh Kumar Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India Gobardhan Lal Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India L. M. Manocha Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India Arun Kr. Mishra Defence Research and Development Establishment (DRDE), DRDO, Gwalior, Madhya Pradesh, India Shraddha Mishra Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India Eswara Prasad Namburi Former Outstanding Scientist and Ex-Director, Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India Vidya S. Pandey Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India Ajitendra S. Parihar Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India Ashok Ranjan Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India T. C. Shami Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India Arun Kr. Singh Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India Mukesh Kr. Sinha Ministry of Textiles, (On Deputation from DMSRDE, DRDO), New Delhi, India

Editors and Contributors

xxvii

Anurag Srivastava Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India J. N. Srivastava Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India Sourabh Srivastava Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India Rajesh Kr. Tiwari Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India D. N. Tripathi Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India P. Venkitanarayanan Department of Mechanical Engineering, Indian Institute of Technology, Uttar Pradesh, Kanpur, India Richa Verma Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India R. J. H. Wanhill Emeritus Principal Research Scientist, Aerospace Vehicles Division, Royal Netherlands Aerospace Centre, Amsterdam and Marknesse, Amsterdam, the Netherlands Jitendra Yadav Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India

Chapter 1

Polymer Matrix Composites (PMCs) for Defence Applications Dibyendu S. Bag, Ashok Kumar, Jitendra Kr. Bansiwal, Gobardhan Lal, D. N. Tripathi, and Eswara Prasad Namburi

Abstract Polymer matrix composites (PMCs) are consisting of reinforcing fillers embedded into polymer matrices. Nowadays, PMCs are widely used as structural materials and replacing the traditional materials. These are used in automotive, railways, missiles and marine industries, defence, aerospace and other applications. Because of the increasing demand for lightweight, corrosion and chemically resistant as well as electrically insulating and high flame retardant materials, the global composite market is growing at a Compound Annual Growth Rate (CAGR) of 4.1% from 2018 to 2023. This chapter describes the information regarding the reinforcing fillers and matrices (both thermoplastics and thermosetting) as well as the composite manufacturing processes and applications. Polymer nanocomposites are also discussed. This chapter also includes a special topic of self-healing composites including its prospects and technological demand.

1.1 Introduction Polymer matrix composites (PMCs) are multi-component systems consisting of fillers embedded in polymer matrices in order to obtain materials with improved mechanical and functional polymer properties. The filler components are usually stronger and more rigid than the polymer matrices, and individual components retain their separate identities in the composites (Seymour and Deanin 1987). The reinforcing fillers (usually high-strength fibres) improve the strength properties, whereas functional fillers impart functional properties into the composite systems. If the fillers D. S. Bag · A. Kumar · J. Kr. Bansiwal · G. Lal · D. N. Tripathi Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India E. P. Namburi (B) Former Outstanding Scientist and Ex-Director, Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 E. P. Namburi et al. (eds.), Novel Defence Functional and Engineering Materials (NDFEM) Volume 2, Indian Institute of Metals Series, https://doi.org/10.1007/978-981-99-9795-4_1

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used are in the nano-dimension regime, i.e. nanofillers, the composites are usually called polymer nanocomposites (PNCs). The fillers and polymer matrices are bonded to each other by weak intermolecular forces and/or by mechanical interlocking at their interfaces (chemical bonding is rarely involved). The properties and structural performance of PMCs usually depend upon the nature of the filler materials and matrix resin systems, and also the adhesion at their interfaces. Strong interface adhesion enables high strength and stiffness, whereas weak interfaces result in low strength and stiffness. Thus long and strong fibres with strong fibre/matrix interfaces are potentially suitable for obtaining high-strength composites. The properties of PMCs also depend on the size, shape, agglomeration and distribution of the filler materials (including fibres) in the matrix resin systems. In general, the intrinsic stiffness of bulk polymers (elastic moduli typically 2–4 GPa in the glassy state) is relatively low compared to that of most inorganic solids/ fibres (elastic moduli typically 80–250 GPa) and also high modulus organic fibres (elastic modulus > 100 GPa). Therefore, composite materials are generally fabricated by embedding such inorganic and/or organic fibres into polymer matrices to obtain materials of superior mechanical and other properties that are important for design as well as structural efficiency. Fibre-reinforced PMCs have amply demonstrated such advantages and thus have become very important materials for applications in aerospace, automobile, marine and other industries. Composites reinforced with carbon fibres are used mainly in aerospace/aircraft applications, while glass fibrereinforced composites are preferred for automobiles. PMCs are also promising structural materials for defence applications owing to certain advantages over conventional metallic materials. Some of the major advantages of PMCs include the following: • PMCs have high specific strengths and moduli, much higher than those of metallic materials. • They have light weight in comparison to metallic components. • It is possible to produce complicated shapes in composites more easily than for metallic components. • Polymer composites do not corrode, unlike many structural alloys. • Composites tend to have higher fatigue endurance limits than metals. • Composites are electrically insulating, and also give less vibration and noise. • In general, composites, particularly carbon fibre-reinforced PMCs, have low coefficients of thermal expansion, unlike metals. This is important particularly for structural components subjected to large temperature variations, e.g. spacecraft structures and satellites. It is very difficult to estimate the global production of PMCs. However, it has been reported that the global composite’s market shows many opportunities in the aerospace and defence, automotive and marine industries, construction, wind energy, pipelines and tanks, electrical and electronics and other consumer goods. The global composite material’s market is valued at around $26 billion as of 2018 and is expected to reach $38 billion by 2023, and it is forecasted to grow at a

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Compound Annual Growth Rate (CAGR) of 4.1% from 2018 to 2023. The global composite’s end-product market is expected to reach an estimated $107.4 billion by 2023. This is mainly because of the increasing demand for (i) lightweight materials in the aerospace, defence and transportation industries; (ii) corrosion and chemically resistant materials in construction, pipeline and tankage industries and (iii) electrically insulating and high flame retardant materials for the electrical and electronics industry. Nevertheless, the production of composite materials is still very small compared with other structural materials. For example, the global output of steel is about one thousand megatons, and the production of plastics also reaches hundreds of megatons. Hence there is still much scope room for composite materials to be developed and to speed up the development process (Nicolais et al. 2011; Bag et al. 2018a). In the United States, there are approximately 2000 composite manufacturing plants and material suppliers. About 65% of all composites are produced with glass fibres and polyester or vinyl ester resin, and are manufactured using the open moulding method. The remaining 35% are produced with high-volume manufacturing methods using carbon or aramid fibres. Recent trends in composites research have focussed on hightemperature capability, toughened resin systems, high-strain and hybrid fibres, fast and low-temperature curing systems, thermoplastics and self-healing composites. The present chapter first gives an overview of PMCs, covering the materials and manufacturing processes. It also briefly surveys various applications of composite materials. There is also a discussion of recent trends and priority areas of composites research, notably a review of the self-healing concept, which is being intensively investigated.

1.2 Polymer Matrix Composites (PMCs) 1.2.1 Overviews PMCs may be classified in several categories, based on the materials used and also the fabrication and production methods. Overviews are given in Figs. 1.1.and 1.2.

1.2.2 Polymer Matrices Polymer matrices used for composites bind the fibres/fillers together so as to transfer the load to the fibres, distribute the load among them and also protect fillers from the environment. They are categorized into two classes: (i) thermosetting resins, which undergo chemical reactions (e.g. cross-linking) during heating, leading to a threedimensional network structure that cannot be altered and (ii) thermoplastic resins,

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Fig. 1.1 Different types of PMCs based on the distribution of fillers and the construction of composite structures

Fig. 1.2 PMCs progressively classified according to the types of (i) fibres and (ii) resin systems (thermosets and thermoplastics)

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which are non-reactive solids (i.e. no chemical reaction occurs during processing) and reversibly soften on heating and harden on cooling. Thus, the final product is usually made by application of heat and pressure, leading to consolidation during the manufacturing of composite components. Unlike thermosetting resins, thermoplastic resins can usually be reheated and reformed into another shape, if desired. According to the class of the matrices, composites are called thermosetting composites and thermoplastic composites.

1.2.2.1

Thermosetting Resins

Thermosetting resins include epoxy resins, phenolics, amino resins, unsaturated polyesters, vinyl polyesters, polyimides and cyanate ester resins. Epoxy resins: These are usually low molecular weight and low-viscosity liquids with low shrinkage (1–5%) during cure. Epoxy composites exhibit good mechanical and thermal properties, high resistance to chemicals and corrosion, and their processing technologies are well established. They are used for pipe, tank, drum and can linings, laminated printed circuit boards, rocket motor casings and structural components for aerospace, automobile, marine and construction applications. A disadvantage is that they are more expensive than polyester resins. Phenolic resins: These have good electrical and heat insulating properties along with good mechanical properties. Mica-filled phenolic resins have very good dielectric strength (13,790–15,760 V/m) and are used in electrical appliances. Amino resins: Urea-formaldehyde and melamine-formaldehyde are used in aircraft interiors because of their low smoke and heat release properties in the event of a fire. Unsaturated polyester resins (UPR): These are widely used due to their good mechanical properties, corrosion resistance, low weight and low cost. The unsaturated sites in the polymer chains are reactive, and they cross-link with the styrene monomer via a free radical reaction. Fibre-reinforced composites of these resins are widely used to make pipes, tanks and containers and other structural components, e.g. posts, beams springs, hulls, panels, etc. Vinyl ester resins (VER): Systems contain vinyl-type unsaturation and hence their name vinyl ester resins. They have good adhesion to glass fibres; impart corrosionresistant properties to the composites; and are used for piping, tanks, coatings, for example. Thermosetting polyimides: These are a special class of resins used for hightemperature composite applications. They have high glass-transition temperatures, thermal stability and flame resistance properties that are better than those of epoxy resins. Bismaleimides (BMI) are used as prepreg matrices for largescale advanced structural composites for aerospace applications and also electrical printed circuit boards. PMR-15 resin occupies a distinguished position to be used as the matrix for advanced and high-temperature composites. Carbon fibrereinforced PMR-15 composites include aero-engines and spacecraft components

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such as duct, blades, inner cowls, swirl frames and nozzle flaps. They are also used in missiles such as in fins and bodies. Cyanate ester resins: These are also high-temperature resins with high glasstransition temperatures (up to 400 °C) and high thermal stability. They are used in the manufacture of aerospace structural composite components which meet fire protection regulations concerning flammability, smoke density and toxicity. 1.2.2.2

Thermoplastic Resins

Important examples of thermoplastics are polyamides (PA), polycarbonate (PC), polyphenylene sulphide (PPS), polyether sulphone (PES), polyetherimide (PEI) and polyether ether ketone (PEEK). Polyamides (or nylons) have moderate tensile strength (41–166 MPa), good toughness (107 J/m) and good chemical resistance properties. They are typically used for gears, speedometer and windshield wiper gears, antenna mounts, packaging and for many general-purpose applications. Polycarbonates have relatively high tensile strength (about 62 MPa) and very high impact strength (Izod test: 640–854 J/m) and are used in safety shields, helmets, cams and gears, aircraft components, electrical relay covers and computer terminals. Polyphenylene sulphide (PPS) is a temperature-resistant engineering thermoplastic. It has good resistance to abrasion, high-energy radiations and inherent flame resistance. It is particularly compatible with glass fibre reinforcement and is used in the chemical process industries such as in construction of pump housings and impellers, valves, metering devices, tower packing and as coating in tanks. Other applications include lamp sockets, coil bobbins, gears, bushings, turbines for fans, heaters or dryers and coatings for washing machines and microwave-compatible cookware. Polysulphones are popular polymers for their toughness and stability at high temperatures and are typically used for TV components, connectors, capacitor films, medical instruments and corrosion-resistant pipes. Polyetherimide (PEI) is an amorphous high-performance polymer with a high glass-transition temperature (215 °C). PEI maintains acceptable properties at high temperatures (200–300 °C) and is highly resistant to ionizing radiations, allowing its use in the nuclear industries. Relative to PEEK, PEI is cheaper, but is lower in impact strength and usable temperature. PEI is mostly used in printed circuit boards, fibre optic connectors, filter bowls, plated reflectors (in high-intensity lighting systems) and in aircraft engine components. Fibre-reinforced polyimides are used for high-temperature mechanical components including bearings, bushings, thrust washers, piston rings, gears, ball-bearing cages (retainers), valve seats, gaskets, compressor vanes and turbine blades. Polyether ether ketone (PEEK) is a high-performance semi-crystalline engineering thermoplastic having excellent chemical resistance properties and resistance to nuclear radiations. PEEK retains good mechanical properties at high

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temperatures (such as 200 °C). It has excellent electric properties and is suitable for use as an insulating material. The low flammability, smoke and toxic gas emission (low FST) properties make PEEK a very promising material for critical applications, e.g. wire and cable coverings in the aerospace and nuclear industries. It is also used as electrical connectors, valve parts and pump impellers. Recently, much interest has been paid to using PEEK as the matrix for high-performance composites. The new and high-temperature processing technologies of this novel engineering polymer must be considered and established. Some of the widely used resin systems for composite applications and their properties are given in Table 1.1. More than 95% of thermoset composites are based on epoxy and polyester resins. Currently, attention is focussed on toughened resins and high-temperature resin systems (such as polyimides, bismaleimides and cyanate esters). Also there is a demand for fast and low-temperature curing resin systems to enable greater productivity of composites. Although thermoplastic resins represent a relatively small part of the PMC industry, they are now preferred because of their toughness and recyclability. In most defence and aerospace applications, the thermosetting resins are polyesters, phenolics, epoxies and polyimides, while the favoured thermoplastic resins are PPS, PEI and PEEK. Table 1.1 Some important resin systems and their properties (Wen 2007) Resin systems

Density Glass-transition Maximum Continuous Tensile Tensile (gm/cc) temp. Tg (°C) service service strength modulus temp. (°C) temp. (°C) (MPa) (GPa)

Thermosets Epoxies

1.2–1.3 100–200

240

180

80–100 4–6

Phenolics

1.0–1.2 260

300

170

41–58

4–8

Unsaturated polyesters 1.1–1.4 70–120

200

120

60–65

0.6–1.4

Vinyl esters

1.1–1.3

210

110

30–90

3–5

Cyanate esters (CE)

1.18

250–290

400

180

70–80

3–3.49

Bismaleimides (BMI)

1.25

300

380

230

85–90

4–4.5

Polyimides (PMR-15)

1.32

320–330

300

280

55–65

2–2.5

Polyamides (PA-6)

1.13

48

130

100

50–60

2.5–3.5

Polycarbonate (PC)

1.20

143–152

140

120

55–75

2–2.4

Polyetherimide (PEI)

1.27

Thermoplastics

215

170

150

95–100 3–3.4

Polyphenylenesulphide 1.35 (PPS)

88

220

135

65–75

Polyether ether ketone (PEEK)

143

250

140

90–100 3.5–4.0

1.30

3.5–3.9

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1.2.3 Filler Materials The intrinsic stiffness of polymers (elastic moduli about 3.5 GPa in the glassy state) is relatively low, whereas most inorganic solids have high elastic moduli (typically 80–200 GPa). Moreover, inorganic solids are most often very low-cost materials and are mostly used as additives in the polymer matrices, either to improve the strength properties of the resultant polymer composites and/or to provide cost benefits for the final materials. Such additives are called fillers in PMCs. If the fillers are used to impart the high mechanical strength properties of polymers they are called reinforcing fillers. The reinforcement improves the strength and modulus of composite materials because the fibres or particles bear some or usually most of the applied load. On the other hand, fillers are called non-reinforcing fillers if their role is primarily to increase the stiffness of the polymeric materials per se, as well as providing cost benefits for the final composite materials. The filler materials generally used for making composites include carbon/graphite fibres, carbon particles, glass fibres, solid glass spheres (beads) and hollow glass spheres (balloons), boron fibres and mineral particles (such as talc, clay and silica). Other than the wide use of carbon and glass fibres, certain organic fibres like aramids (Kevlar) and nylon fibres are also used as reinforcing fillers. Also, various forms of fibres such as yarns, rovings, chopped strands, woven fabric and mats are used in advanced composites. Yarns and rovings are used in processes such as filament winding or pultrusion. Woven fabrics and mats are used for preforms in composite parts manufacturing. Nanofillers like fullerenes, carbon nanotubes (CNTs) (both single-walled nanotubes and multi-walled nanotubes), graphene, nanosilica and magnetic nanoparticles are used to achieve certain functional properties in composites. In these cases, they may be called functional fillers. Other functional fillers such as conductive fillers (polyaniline, conducting carbon, etc.), radar-absorbing fillers (such as milled carbon fibre, barium titanate powder or flakes) are used for specific applications. However, in most cases, carbon, glass, aramid and boron fibres are used in the defence sector and for various structural applications. Some of the important reinforcing fillers, their properties and applications are summarized in Table 1.2. Carbon fibres have high strength and stiffness and are used in a variety of structural and electrical applications. They are manufactured from polyacrylonitrile (PAN) polymer by heating, oxidizing and carbonizing, or else from other precursors like rayon and petroleum pitch. The PAN-based fibre is most commonly used in the advanced composite industry today. Glass fibres are the most common reinforcing fillers (available both as continuous and short fibres) for PMCs because they provide high strength at low cost. But they have demerits like the tendency to absorb moisture, poor abrasion resistance and poor adhesion to polymer matrices. In order to overcome these demerits, coupling agents, e.g. silanes are generally used for final finishing of the fibres. There are different classes of glass fibres, including A-glass, C-glass, D-glass, E-glass and

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Table 1.2 Some important reinforcing filler materials and their properties (Mortensen 2006) Fibres/class of fibres

Density (gm/ cc)

Tensile strength (GPa)

Modulus (GPa)

Application areas

Carbon fibres (CF) Standard modulus (high strength)

1.77–1.80

3.0–3.5

220–240

• Widely used for almost all parts in aircraft, satellites, antenna dishes, missiles, etc

Intermediate modulus

1.77–1.81

5.4–5.7

270–300

• Primary structural parts in high-performance aircraft

High modulus

1.77–1.80

2.8–4.5

390–450

• Space structures, control surfaces in aircraft

Ultrahigh modulus

1.80–1.82

7.0–7.5

290–310

• Primary structures in high-performance aircraft

Glass fibres (GF) E-glass

2.55

2.2–3.5

65–75

• Electric grade. General-purpose fibres for strength and high electrical resistivity • Small passenger aircraft parts, aircraft interiors, secondary parts, radomes, rocket motor casings

S-glass

2.47

4.3–4.8

85–95

• Structural grade. Reinforcement where high strength, modulus, and stability under extreme temperatures and corrosive environments are required • Highly loaded parts in small passenger aircraft

C-glass

2.56

3.31

69

• Chemical grade. Good chemical (particularly acid resistant) properties and application in corrosive acid environments • Suitable for corrosion-resistant parts and storage battery jars and other parts for industrial applications

D-glass

2.14

2.5

55

• Low dielectric constant, good electric insulation and wave penetration properties • Suitable for electrical applications and reinforcing materials for radomes (continued)

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Table 1.2 (continued) Fibres/class of fibres

Density (gm/ cc)

Tensile strength (GPa)

Modulus (GPa)

Application areas

Boron fibres

2.6

3.5

400

• Limited applications in the aerospace industries

Low modulus 1.44 (Kevler-29)

2.7–2.8

62

• Fairings, non-load bearing parts

Intermediate modulus (Kevler-49)

2.7–2.8

131

• Radomes, rocket motor casings and some structural parts • Highly loaded parts

Aramid fibres

1.44

S-glass. E-glass is the electrical grade, providing good insulation properties. Sglass contains higher percentages of alumina and silica as compared to E-glass, and is a structural grade used for structural composites: it also has about 33% higher tensile strength than E-glass. Boron fibres are manufactured by a chemical vapour deposition technique in which a fine tungsten wire or graphite filament is used as the core and boron trichloride gas as the boron source. The fibre diameters may vary from 0.1 to 0.2 mm. They have low density, high tensile strength and high modulus of elasticity. However, such fibres are difficult to weave, braid or twist because of their high stiffness. Hence, they are formed into resin-impregnated tapes for hand layup and filament winding. They have limited applications, mainly in the aerospace industry, because of their high cost. Aramid fibres (e.g. Kevlar) are obtained from aromatic polyamides. These fibres possess unique properties: high tensile strength and modulus, temperature stability, dimensional stability, flexural performance, textile processability and resistance to chemicals.

1.3 Processing and Fabrication of PMCs Processing and fabrication of polymer matrix composites are decided considering three main factors: (i) types of reinforcing agents and their size and shape, (ii) types of resin matrices and finally (iii) the types and design of end-use products. The fabrication process also depends upon the type of equipment used for the end product. Hence there are various fabrication processes for PMCs (also called fibrereinforced plastics, FRP) that are discussed in this section, see Table 1.3. The most commonly used moulding techniques for composite fabrication and their advantages and disadvantages are given in Table 1.4.

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Table 1.3 Manufacturing techniques for polymer matrix composites (PMCs): fabrication processes and resin systems Semi-finished fabrication process

Technology stage for thermoplastic resins

Technology stage for thermoset resins

Thermoplastic Prepreg

Uncommon

Widely used

Thermoplastic sheet moulding compound (SMC)/bulk moulding compound (BMC)

Uncommon

Widely used

Open moulding process

Technology stage

Technology stage

Hand lay-up

Widely used

Widely used

Robotic lay-up

Widely used

Widely used

Filament winding

Widely used

Widely used

Pultrusion

Uncommon

Widely used

Fused deposition modelling (additive R&D scale manufacturing) spry-up

Spray up widely used

Honey comb core

Uncommon

Widely used

Closed moulding process

Technology stage

Technology stage

Injection moulding

Widely used

Uncommon

Compression moulding

Uncommon

Widely used

Resin transfer moulding

R&D

Widely used

Vacuum-assisted resin infusion

R&D

Widely used

Autoclave moulding

Uncommon

Widely used

Balanced pressure fluid moulding (quickstep process)

Newcomer technology

Newcomer technology

The common feature to all composite fabrication processes is the combining of a resin and its curing agent, reinforcing fillers/fibres, and in some cases a solvent. Typically, heat and pressure are applied to provide the shape and cure the resin, leading to the finished product. In composites the role of resin is to hold the reinforcing fibres firmly together and protect them. The resin also helps to transfer the load to the fibres in the fabricated product. A curing agent, also known as hardener, reacts with the active groups of the resin and forms a network structure giving a hard plastic. The reinforcing fibres impart strength and other required properties to the composites. Sometimes a solvent may also be used in the composite fabrication process in order to dilute the resin system for better impregnation into fibres/fabrics. A non-reactive solvent reduces the viscosity of the resin system, whereas a reactive solvent becomes part of the process. Currently, several composite fabrication processes are automated. However, some are manual, which require skilled manpower during manufacture. Also, technologists are keen to use the advanced manufacturing processes to meet the demands of newly designed and complicated product parts. Since the advanced composite industry is relatively new and still developing, various other processes for fabrication are also developing to meet new high-tech requirements.

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Table 1.4 Comparison of the commonly used composite moulding processes (Agarwal et al. 2006; Hull and Clyne 1996) Moulding process

Advantages

Disadvantages

Cycle time

Prepreg process

Better resin/fibre control

Labour intensive for large complex parts

5–10 h

Preforming process

Good mouldability with complicated shapes and the elimination of trimming operation

• Cost-effective only for • 45–75 s for large complicated compression process • 4–5 min for vacuum shape parts • Large scrap generated forming process when fibre mats used

Reaction transfer moulding (RTM) process

• Inside and outside finish possible with thickness control • More complex parts possible with vacuum-assisted RTM

• Low-viscosity resin necessary • Possibility of void formation without vacuum

• 8–10 min for large parts • 3–4 min with vacuum-assisted RTM

Compression moulding

Favoured method for mass production with high fibre volumes

Expensive set-up cost for low production

1–2 min, which again may vary with thickness of the parts

Sheet moulding compound (SMC)

Cost-effective for production volume 10 to 80 tonne/year

Minimum weight savings potential (more wastage)

1–2 min

Reaction injection moulding (RIM)

Low cost tooling Difficult to control the where prototypes can process be made with soft tools

Bulk moulding compound (BMC)

Effective for low-cost base materials

• Low fibre content 30–60 s • Randomly oriented fibres • Low structural quality • Poor surface finish

Extrusion compression moulding

• Fully automated • Variety of polymers and fibres can be used with fibre volume up to 60% by weight

Not suitable for surface finish parts

Injection moulding Fully automated, fast Expensive set-up cost cycle, variety of for low production thermoplastic polymers and short fibres/fillers can be used with filler volumes up to 40–60% by weight

1–2 min

3–6 min

30–60 s

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1.3.1 Types and Stages of PMC Manufacturing Processes Overviews of the manufacturing processes are widely available in the literature (Lukaszewicz et al. 2012; Krishna Mohan et al. 2019; StrongBrent 2008; Advani and Hsiao 2012; Fernlund et al. 2018). The basic composite fabrication processes are associated with the resin formulation and/or prepregs which are used in the moulding process (open or closed moulding) for fabrication of composite parts either in batches or continuously. (1) Formulation: This is the process where a resin, curing agent and any other necessary components are mixed together in a required proportion. This process may involve adding the components manually into a small mixing vessel. In the case of larger volume processes, the components are pumped into a mixing vessel. The potential hazards involved in the formulation are associated with skin, eye and respiratory contact with the ingredients: precautions must be taken during mixing the ingredients. (2) Prepregging: This process refers mainly to thermosetting resins and curing agent mixtures whereby these are impregnated with the reinforcing fibres. These impregnated reinforcements are commonly called prepregs, and are available mainly in three forms: woven fabrics, roving, and unidirectional tape (Lukaszewicz et al. 2012; Krishna Mohan et al. 2019). These thermosetting prepregs are partially cured resin systems in which resin fluidity is arrested. Once the resin mixture has been impregnated with the fibres, the prepreg must be stored in a refrigerator or freezer until used in the manufacturing process. This low-temperature storage prevents the chemical reaction (curing) from occurring prematurely. Fabrics and tapes are provided as continuous rolls in widths up to 2 metres and lengths up to several metres. The fabric or tape thickness constitutes one ply in the construction of a multiply layup. Impregnated roving is wound onto cores or bobbins and is used for filament winding. Prepreg materials are used widely in making advanced composites particularly for aircraft and aerospace, and missile applications. Prepregs have several advantages compared to traditional hand layups of fibre bundles or filament winding, but also a few disadvantages. The advantages are: • It is easier to achieve maximum strength properties as compared to hand layups, since hand layups typically have excess resin, which reduces the overall properties and increases brittleness. • Prepregged components have greater uniformity and repeatability. • The products have less waste. Prepregs will bleed excess resin during the curing process but all of the excesses of hand layups—cups of resin, messy rollers, drips—are no longer a problem. • The products also take less curing time. After the heat-curing cycle is completed, the part is ready for service. It is not required to wait the standard 48 h to allow a full cure as in a typical hand layup.

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The disadvantages are: • Prepregs are costly and the cost of the products is more in comparison to other processes. • Prepregs have storage problems and limited shelf life. Such prepregs are stored at low temperatures in order to avoid cross-linking. N.B: The above discussion of prepregs concerns thermosetting resins. Prepreg processability and conformability using thermoplastic matrices has been growing across many industries because of the key advantages of thermoplastics over thermoset counterparts. Thermoplastics are inherently less brittle than thermosets, resulting in composites with higher impact resistance properties. However, continuous fibre-reinforced thermoplastic prepregs are less technically mature than thermoset systems. On the other hand, most thermoplastic prepregs do not accrue low-temperature storage time before processing, because the resin is fully polymerized before part consolidation and can be stored at room temperature. Such prepregs undergo physical changes (melting and solidifying) rather than chemical reactions during consolidation. Moreover, the processing times for fibre-reinforced thermoplastics can in principle be much shorter than those of thermosets. (3) Moulding process: This may be open or closed moulding. • Open moulding processes are those where the part being manufactured is exposed to the atmosphere. This process is typically used for fabrication of large components for boats, marine ships, industrial boilers, storage tanks, etc. The fabricator typically uses a manual process. The liquid resin mixtures may be applied by several techniques such as hand layup, pultrusion or a spray process onto a reinforcing material, followed by curing to obtain the final composite component. • Closed moulding processes are those in which part of the manufacture takes place in a closed vessel or chamber. Actually, this process is performed in a mould consisting of two sections that open and close in each moulding cycle and is typically used for fabrication of medium-to-small-sized components. The liquid resin mixture or prepreg material may be manually put into the vessel/chamber for the curing step. In the case of liquid resin mixtures, these may be pumped into the vessel/chamber for the curing step. Closed moulding/forming processes are of three types based on their counterparts in conventional plastic moulding. These are injection moulding, compression moulding and transfer moulding (StrongBrent 2008; Advani and Hsiao 2012; Fernlund et al. 2018). The tooling cost of closed moulding is more than twice the cost of a comparable open mould owing to the more complex equipment required in this process. On the other hand, the advantages of a closed moulding process are: (i) good finish on all part surfaces, (ii) higher production rates, (iii) closer control over tolerances and (iv) enabling fabrication of more complex three-dimensional shapes.

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(4) Batch or continuous processes: Batch (or sequential) processes involve manufacture of a single part at a time, in sequence. This type of process is usually required where the part being made is small and complex in shape, when the curing phase is critical, or where a small number of parts are involved. On the other hand, continuous processes are typically automated to some degree and are used to produce larger numbers of identical parts relatively quickly. These continuous processes are typified by pumping the resin mixture into the mould, followed by closed curing. (5) Advanced fabrication: autoclave and vacuum bagging process: In this process, a charge material (prepregs or resins and fabrics) is placed in a mould that is contained in a plastic bag (Fernlund et al. 2018). This assembly is placed inside an autoclave, a vacuum is applied to the bag to remove entrapped air and volatiles, and then heat and pressure are applied to cure the resin and obtain the fabricated part. Usually an inert atmosphere (nitrogen or carbon dioxide)e is provided inside the autoclave to avoid any oxidation or unwanted degradation of the materials. This process is most suitable for large and intricate high-performance parts/components such as aircraft wings, flaps, and missile canisters. However, this process requires a large initial capital investment, since a large autoclave is needed.

1.3.2 Challenges of PMC Manufacturing Processes The manufacture of composites requires (i) the preparation of a well-defined assembly of fibres according to the desired shape of a required component, (ii) impregnating this fibre assembly with the resin system and (iii) in the case of thermosets, curing the resin by applying heat and pressure, keeping the shape of the component intact. The major challenges are • Efficient handling of the fibres and resins along with the impregnation of resins into fibres, and the application of uniform heat and pressure during curing to produce the final component. • Stiff fibres do not flow. Hence continuous fibres are (i) stacked in layers and are successively impregnated with the resin, or (ii) formed into a preform, and then inserted in a mould for component fabrication. Alternatively, the resin can be injected into the mould under pressure to impregnate the fibres. Either way, these process techniques must be closely controlled. • Composites requiring high-volume percentages of fibres are difficult to achieve. • The curing stage, involving geometry-dependent pressure and temperature distributions for the components, must be precise to achieve the required properties in the composites.

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1.4 Thermoset Composites: Types and Overall Properties Thermoset composites (TSCs) use thermosetting resins as the matrices, which hold the fibres firmly in place in structural composites (Jouyandesh et al. 2019). The widely used thermosetting resins in composites are listed in Table 1.1. The most common thermosetting resin used today is a polyester resin, followed by vinyl ester and epoxy resins. Resins like bismaleimide, polyimide and cyanate ester resins are used in special and high-tech applications. Note that silicone and polyurethane resins are also used. Thermosetting resins are popular because they remain in a low-viscosity liquid state before curing. This low viscosity allows (i) convenient impregnation of resins between reinforcing fibres (such as glass, carbon or aramid fibres), (ii) easy fabrication, (iii) easy to remove air during manufacturing and (iv) the ability to rapidly manufacture products using a vacuum or positive pressure pump when employing the closed mould manufacturing process. Besides the relative ease of manufacturing of thermosetting composites, the resins are typically low-cost materials. Furthermore, thermosetting resins can exhibit excellent properties which include • Resistance to solvents, corrosive substances and high temperatures. • Excellent adhesion and finishing (polishing, painting, etc.). • The composites have very good fatigue strength and tailored elasticity. In the manufacturing of thermoset composites, the uncured resin molecules are cross-linked (cured) via a catalytic chemical reaction, which is most often exothermic. Curing creates extremely strong molecular bonds, changing the resin from a liquid to a solid. Once cured, a thermosetting resin, and hence a thermoset composite, cannot be reversed or reformed. This means that the recycling of thermoset composites is extremely difficult. Although the thermoset resin itself is not recyclable, there are a few new companies who have successfully removed the resins through pyrolization and are able to reclaim the reinforcing fibres.

1.4.1 Market for Thermoset Composites The rapid growth of composites’ use is observed, especially in marine, transportation, construction and aerospace applications. Moreover, the market for thermoset composites was valued at US$ 41.98 billion in 2016, and is projected to reach US$57.98 billion by 2021.The most widely used thermoset resins are epoxies. The thermoset epoxy resin composites may be tailored by varying the curing systems and reinforcements. There are two main types of epoxy composites, i.e. glass fibre and carbon fibre-reinforced composites. Thermoset epoxy composites are used widely in the automotive, transportation, construction, aerospace and defence industries, and also for sporting goods and in the electronics industry. The wide-ranging use of epoxy composites is beneficial for

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market growth, in particular, the growing automotive and transportation industries in the Asia-Pacific and North America regions. The global epoxy composite market in 2016 was US$ 21.04 billion and is forecasted to reach US$ 39.19 billion in 2023 (Market Research and Future Report (MRFR) 2019).

1.5 Thermoplastic Composites: Types and Overall Properties Thermoplastic composites (TPCs) are now being replacing thermoset composites (TSCs) and complex metallic parts in aerospace applications. The initial drivers for usage of thermoplastic composites are their relatively good impact resistance (durability) and inherent flame resistance properties. Moreover, thermoplastic composites offer a new perspective because of their recyclability via a novel grinding process and/or melt processing, which are not possible for TSCs. TPCs enable shorter production times, but require new fabrication techniques. High-performance thermoplastic composites are considered as the future generation of structural materials for aviation and defence industries owing to their better damage tolerance, chemical and thermal properties. Besides aerospace and defence applications, fibre (glass, carbon and natural fibres) reinforced thermoplastic composites have increasingly found uses in the automotive and renewable energy sectors. This is because of their properties like light weight, construction potential, integral design and relatively good impact properties (Stewart 2011).

1.5.1 Market for Thermoplastic Composites In addition to the thermoplastic composites based on commodity-orientated thermoplastic resins (e.g. polypropylene, polyamide or nylon), this class of composites includes high-performance thermoplastic resins such as polyether ether ketone (PEEK), polyphenylene sulphide (PPS) and polyetherimide (PEI), as mentioned in Sect. 1.2.2.2. PEEK-based TPCs using carbon fibres and glass fibres are considered to be two of the most promising candidates for various structural applications (Stewart 2011). However, the necessary high-temperature processing of PEEK is a major hindrance to its use for composite fabrication and technological advancement. On the other hand, PEEK prepregs are becoming commercially available and the structural applications of PEEK composites are increasing. For example, particulate reinforced PEEK composites are fabricated by means of compression moulding and/or injection moulding and are being widely studied (Wang et al. 1997; Goyal et al. 2005; Kuo et al. 2005).

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Nanoparticle-filled PEEK composites are reported to be promising highperformance nanocomposites with significant improvements in the tribological characteristics, resulting in considerably decreased frictional coefficients and wear rates. Moreover, nano-sized silica or alumina particles (15–30 nm) used to fill PEEK composites have demonstrated an improvement of elastic modulus and tensile strength by 20–50% (Goyal et al. 2005; Kuo et al. 2005). The inclusion of the ceramic nanofillers can also substantially improve the composite thermal stability (Goyal et al. 2005), and nano-sized silica particles could improve the dimensional stability and storage modulus of the resulting PEEK nanocomposites (Kuo et al. 2005). Some further remarks on nano-particle composites are made in Sect. 1.6. The thermoplastic composites’ market is especially based on thermoplastic resins (e.g. polypropylene, polyamide, polyetheretherketone and hybrid polymer combinations) and is experiencing significant growth because of the demand in various applications such as consumer goods and sports, electrical and electronics, automobiles, railways, marines, aerospace and defence. Moreover, the market for aerospace and defence applications shows the highest CAGR owing to growing Government investments and increasing global demand for both commercial and military aircraft. The thermoplastic composites’ market is projected to grow from USD 28.0 billion in 2019 to USD 36.0 billion by 2024, at a CAGR of 5.2% (https://www.marketsan dmarkets.com/PressReleases/thermoplastic-composite.asp).

1.5.2 Thermoplastic Composites (TPCs) Versus Thermosetting Composites (TSCs) Thermoplastic composites (TPCs) offer a unique opportunity to replace metals such as steel and aluminium alloys because of their light weight, excellent formability, corrosion resistance and strength properties. They provide sufficiently strong components even when being around 50% lighter than steel as well as 30% lighter than aluminium (although some of these strength advantages have to be reduced owing to relatively low resistance to impact damages). The attractive combinations of properties ensure TPCs are in high demand, since they allow designers to create lighter aircraft, more energy-efficient cars, and stronger oil and gas pipes. TPCs offer the potential for shorter processing times compared to TSCs, but there are disadvantages that have prevented them from largely replacing TSCs in the aerospace industry: • High processing temperatures, which increase the costs of prepregs and complicate the use of conventional processing equipment. • Low tack (adhesion) and quality variations of the prepregs, requiring expensive manual handling operations. • Thermoforming of continuous fibre-reinforced thermoplastics has proven to be much more difficult than first anticipated due to the tendency of the fibres to wrinkle and buckle if not maintained under tension.

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• Superior toughness and damage tolerance have largely been negated by the development of much improved thermoset resins. • Solvent and fluid resistance properties remain major barriers to the use of amorphous thermoplastic composites.

1.6 Polymer Nanocomposites Polymer nanocomposites, already mentioned in Sect. 1.5.1, are composites obtained by the combination of polymer matrices and nanofillers (nanometre-scale dimensions: less than 100 nm). These fillers are usually one-dimensional nanotubes and nanofibres (such as carbon nanotubes), two-dimensional layered minerals like clay (silicates) (such as montmorillonite, saponite and hectorite) and graphene or three-dimensional spherical particles (such as fullerenes, nano-CaCO3 ). In contrast to conventional composites, i.e. the so-called micro-composites where the reinforcement is on the order of microns, significant improvements of polymer properties can be achieved with a small amount of nanofillers in polymer nanocomposites. This is because of the large surface-area-to-volume-ratio of nanofillers when compared to the micro- and macro-fillers in conventional composites. Hence, over the past decade, the advancement of composite research has progressed to polymer nanocomposites (Mai and Yu 2006; Kumar et al. 2016; Katiyar et al. 2017; Ahmad et al. 2019a, b; Dey et al. 2020). Properties like mechanical properties (elastic strength, modulus, stiffness, fracture toughness), impact resistance, scratch resistance, barrier resistance and flame retardance, as well as optical, electrical and magnetic properties, are superior in polymer nanocomposites as compared to conventional composites. A general schematic of the formation of nanocomposites is shown in Fig. 1.3. Exfoliated organo-clay (5.3 wt%) filler showed an increase in strength by 42% and an increase in modulus by 84% in nylon-6 nanocomposites compared to the unfilled composite. This is also significantly higher than when using unmodified clay particles as filler (Kojima et al. 1993; Okada and Usuki 1995). Surface-treated montmorillonite (with 12-amino lauric acid) containing nylon-6 nanocomposite was observed to have large increase in tensile strength (55%), modulus (91%) and heat distortion temperature (134%). However, there was no substantial change in impact strength (Ponnamma et al. 2017). Similarly, various other nanofillers have been incorporated into various polymer matrices when following different strategies/techniques to obtain nanocomposites. This subject of polymer nanocomposites has taken a major advance over the last three decades, and its discussion is beyond the scope of the present chapter. Interested readers may consult the relevant books (Mai and Yu 2006; Okada and Usuki 1995; Ponnamma et al. 2017).

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Fig. 1.3 Schematic representation of formation of conventional composites and nanocomposites (two nanoforms, intercalated and exfoliated, are shown for the dispersion of a nanoclay silicate in the polymer matrices)

1.7 Applications of PMCs PMCs are widely used in various structural and other high-performance engineering applications (Chung 2019; Arhant and Davies 2019; Mittal et al. 2018). These applications include the aerospace and defence, transportation (automobiles, railways and marine) sectors, civil infrastructures and personnel/military protection systems such as bulletproof jackets and ballistic helmets. Composites reinforced with carbon fibres are used preferably in aerospace applications while glass fibre-reinforced composites are used for automobiles. Table 1.5 lists numerous examples of PMC applications along with their manufacturing technologies in several sectors. It is practically impossible to provide a complete list of applications of polymer composites. However, some details of applications are given in the following lists:

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Table 1.5 Some PMC applications and manufacturing process technologies Products and components

Materials used for composites

Processing technology

Missiles and space Missile canisters fairings and Carbon fibres and epoxy resins; fins carbon fibres and polyimides

Filament winding and also autoclave or compression moulding

Rocket cases

Filament winding

Glass/Kevlar fibres; polyimides, epoxy or cyanate resins

Satellites: Carbon/Kevlar fibres, epoxy Antennas, antenna support resins of different grades structures, antenna reflectors, solar array wing

Compression moulding

Boron fibres, polyimide and/or Space shuttle: Main frame and ribs-trusses modified epoxy resins struts, frame stabilizing braces, nose landing gear and drag-brace struts

Compression moulding/ autoclave process

Aircraft Stabilator, horizontal and vertical tail skins

Boron fibres Epoxy, polyimide and/or cyanate ester resins

Autoclave/compression moulding and/or filament winding

Flap, leading edge, airbrakes, Carbon/glass fibres, PEEK and/or Autoclave/compression wing skin, control surfaces, epoxy or polyimides moulding and/or filament front fuselage winding Automobiles and trucks Bumpers Dashboards

Glass fibres/natural fibres, polypropylene, ABS (acrylonitrile-butadiene-styrene)

Thermoforming or compression moulding

Personnel protection systems Ballistic helmet

Kevlar/glass fibres; Epoxy/ Compression moulding phenolic resins and their prepregs

Bullet proof jacket

Kevlar fabrics and epoxy or UHMWPE fibres

Compression moulding

• Aerospace and Defence: PMCs have wide applications in aerospace and defence sectors, including structures for transport and military aircraft and gliders; aircraft internal fittings, partitions and floors, galley units and trolleys; space and satellite components, and launch vehicles; ground support equipment and components; personnel armour and ballistic items; and transit containers: see Table 1.5 and also the additional Sects. 1.7.1 and 1.7.2. • Automobiles: Body panels, leaf springs, drive shafts, bumpers, doors, racing car bodies, luggage racks, tyres, etc. Car bumpers are generally made of glass fibrereinforced polypropylene (PP); dashboards of glass fibre-reinforced acrylonitrilebutadiene-styrene (ABS), see Table 1.5.

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• Railways: Railway track and signal components, traffic signs, seating window masks and partitions, train berths, windows, flooring, etc. Glass fibre-reinforced ABS and polypropylene (PP) are used for berths, and glass fibre-reinforced polyvinyl chloride (PVC) is used in flooring • Marine/maritime: Boat hulls, canoes and boats, yachts, kayaks, surf and sailboards, lifeboats and rescue vessels, boat accessories, window masks, cruise liners. • Sports goods: Golf clubs, skis, fishing rods and tennis rackets. • Electricals and electronics: Panels, circuit boards, housings, switch gears and boxes, cabinets, insulators and connectors, booms, distribution posts and pylons, transformer elements, ladders and cableways. • Biomedical applications: Medical implants, orthopaedic devices, X-ray tables, other medical items, assemblies and equipment components. • Building and construction: Permanent and temporary formwork and shuttering, partitions, cladding, polymer concrete, structural and decorative building elements, cabins and housing, bridge elements and sections, quay facings, signposts and street fencing, walkways and staging. • Engineering and industrial applications: PMCs are used in water control engineering, sewage treatment plants, water and industrial storage tanks, energy harvesting systems like wind energy blades, other assemblies and fittings, sundry enclosures, pallets, safety helmets, trays, bins, profiles, pipes, staging, pump components, partitions, staging and walkways, scrubbers and weirs. PMCs are widely used in the chemical and oil industries, especially for chemical- and corrosion-resistant components like chemical storage vessels, linings, pipes, ducts, chimneys, grid flooring, pressure vessels, processing tanks, fume hoods, cooling tower components and other assemblies and enclosures. • Agriculture Sector: PMCs are used in the production of feed troughs, containers and enclosures, fencing, equipment components, partitions, staging, flooring, silos and tanks.

1.7.1 Aircraft Applications Aircraft structural materials must have outstanding engineering properties and also, at the same time, enable the manufacturing of lightweight and durable structures (i.e. the airframe). Aluminium alloys have predominated in aircraft industries since introduction of the Boeing 247 (1933) and Douglas DC-2 (1934), but carbon fibrereinforced polymer composites increasingly provide competition, Fig. 1.4. There are two more points to note with reference to Fig. 1.4: (i) tactical aircraft tend to use more composites than transport aircraft; (ii) military aircraft with special qualities (VTOL, stealth) have higher percentages of composites than their more conventional contemporaries. (N.B: The Northrop Grumman B2 has a high-cost composite airframe mainly because of the radar-absorbing properties. This aircraft

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Fig. 1.4 Growth of consumption of CFRP composites in aircraft since 1970

is a notable example of using composites for both their structural and functional properties.) The introduction of the Boeing 787 (2011) and Airbus A350 (2015) represents a potential ‘game changer’ for civil transport aircrafts. The airframes are about 50% and 52% composites, respectively (Fig. 1.5). However, some authorities doubt whether in future similar aircraft size category will have such high percentages of composite structures, since they are more expensive than comparable aluminium alloy structures. There are also other disadvantages, including much less damage tolerance (Wanhill 2017). Nevertheless, PMCs based on high-performance thermoplastics like polyether ether ketone (PEEK), polyphenylene sulphide (PPS), polybenzimidazole (PBI) and their blend matrices are widely used in secondary aircraft structures such as radomes, access panels for fuel tanks, pylon fairings, door handles and gear covers.

Fig. 1.5 Airframe material distributions and percentages for the Boeing 787: the Boeing Company

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1.7.2 Personnel Protection Systems PMCs are widely used in personnel protection systems, especially as armour materials for protection of soldiers such as bulletproof jackets, ballistic helmets, anti-mine boots, blast protection suits and anti-riot shields. The PMCs used in these kinds of defence applications are described in more detail in Chaps. 1, 2 and 3 of Vol. 1 and Chaps. 2, 3, 4 and 10 of Vol. 2 of this two-volume book.

1.8 Special Topic: Self-healing PMCs When fibre-reinforced polymer composites are subjected to impact damage and repeated cycles of thermal and mechanical stresses they are prone to microcracking and delamination which lead to mechanical degradation and structural failures (Gamstedt and Talreja 1999). The detection of damage and its management are necessary for optimum service use (Wegst et al. 2015). The best way to manage PMC damage is to develop self-healing structures that have both structural and functional properties. This self-healing concept partially eliminates the damage and offers some extension of the service life for PMC components. Different design approaches have been reported in the literature to develop self-healing polymeric materials and composite structures (Zhang and Rong 2011). These are reviewed in this section. Table 1.6 Different healing approaches and test methods to determine the healing efficiency for self-healing polymers and composites Matrices and materials

Healing approach/ mechanism

Stimulus

Healing efficiency (%)

Test methods

References

Mechanical

93

Flexural strength

Pang and Bond (2005a)

Microencapsulation: (extrinsic)

Mechanical

80–93

Fracture toughness, Tensile strength

Sananda et al. (2006)

Microvascular network: (extrinsic)

Mechanical

60–70

Fracture toughness

Williams et al. (2007)

Carbon fibres (extrinsic)

Electrical

77

Impact strength

Murphy et al. (2008)

Molecular diffusion (intrinsic)

Mechanical

100

Fracture toughness

Lin et al. (1990)

Photo-induced healing (intrinsic)

Photon

26

Flexure strength

Blaiszik et al. (2010)

Thermosets and/ Hollow fibres or their (extrinsic) composites

Thermoplastics

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1.8.1 Design Methodologies for Self-healing Composites The design methodologies for self-healing polymeric materials and composite structures depend upon the polymeric matrix materials, see Table 1.6 (Arhant and Davies 2019; Mittal et al. 2018; Wanhill 2017; Gamstedt and Talreja 1999; Wegst et al. 2015; Zhang and Rong 2011; Pang and Bond 2005a; Sananda et al. 2006; Williams et al. 2007; Murphy et al. 2008; Lin et al. 1990; Chung et al. 2004). There are two types of design approaches: intrinsic and extrinsic methods. In the extrinsic method of self-healing, the healing precursors/agents are incorporated separately into the matrices and composite structures, whereas in the intrinsic method, no additional healing agents are needed, the inherent functionality of polymer matrices allows automatic healing. The healing efficiencies listed in Table 1.6 should be regarded with caution. For example, recovery of more than 90% of the pristine resin fracture toughness properties is reported in epoxy composites embedded with microcapsule containing healing agent (Blaiszik et al. 2010; Smith 2012). However, limited investigation to report PMC fracture toughness recovery exceeds 80% (Blaiszik et al. 2010). This insufficient healing result appears to be generally true for PMCs which is has been attributed to insufficient amounts of self-healing agents to fill cracks and delaminations, and thermal loss of the healing reaction to the fibres (Smith 2012). An example of measuring and evaluating the healing efficiency is given in Sect. 8.4.

1.8.2 Extrinsic Approaches for Self-healing Composites Cracks and damage in polymers and composites can be repaired (‘healed’) by more or less conventional extrinsic methods like (i) ‘hot plate’ welding in thermoplastics, where the polymers are heated up to the glass-transition temperature (T g ) to enable molecular inter-diffusion towards the damage sites and (ii) resin injection to restrict the delamination of composites. Also, reinforcing patches can be used to restore the strengths of laminates containing fibre breakages. However, these conventional extrinsic strategies have not been ideal solutions, because they required regular structural monitoring and then manual intervention for repairing any detected damage. Alternative so-called extrinsic healing strategies have been invented to overcome these concerns and also to develop self-healing polymer composites (White et al. 2001; Pang and Bond 2005b; Trask et al. 2007). In these strategies, the extrinsic healing agents are embedded into the polymer matrix/ composites by including them in microcapsules, hollow fibres or vascular networks, which act as reservoirs. These reservoirs subsequently release the healing agents in locations where damage occurs. The released healing agents combine with catalysts embedded into the composites during fabrication to polymerize and thus repair the damage.

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Liquid Healing Agent-Filled Microcapsules

This concept using liquid healing agent-filled microcapsules dates from 2001 (White et al. 2001). Dicyclopentadiene (DCPD) was chosen as the healing agent inside microcapsules based on poly (urea-formaldehyde). The filled microcapsules and Grubbs’ catalyst were embedded into an epoxy-based composite during fabrication. The basic idea and molecular chemistry involved are illustrated in Figs. 1.6 and 1.7, respectively. Thus when (i) a crack develops in the composite, the local microcapsules break and release the liquid healing agent, which by capillary action fills the crack; (ii) the catalyst located near or at the crack causes the healing agent to polymerize within the crack and heals it. Using this concept a healing efficiency of more than 70% was achieved (White et al. 2001). However, as mentioned in Sect. 5.8.1, healing efficiency values should be treated with caution. The healing process and efficiency depend on many factors, such as (a) the robustness of the microcapsules, which have to withstand incorporation into the resin; (b) the necessary fracture of the microcapsules and release of the healing agent into the cracked location via capillary action and (c) the healing agent must interact with the embedded catalyst sources to initiate polymerization and/or cross-linking, thereby resulting in solidification that repairs the crack. Fig. 1.6 Schematic representation of self-healing via the micro-encapsulation method (White et al. 2001)

1 Polymer Matrix Composites (PMCs) for Defence Applications Ph

PCy3 Ru

H

27

Cl

*

Cl PCy3

Grubb's Catalyst

Dicyclopentadiene Cross-linked polymer Fig. 1.7 The chemistry of healing (in micro-encapsulation self-healing method) based on ring opening metathesis polymerization (ROMP) of dicyclopentadiene (DCPD) using Grubbs’ catalyst (White et al. 2001)

1.8.2.2

Liquid Healing Agent-Filled Hollow Fibre Embedment

Self-healing composites have also been demonstrated using healing agents stored in embedded hollow glass fibres (Pang and Bond 2005b; Trask et al. 2007; Bleay et al. 2001). Figure 1.8 gives schematics of two approaches: (i) both the healing agent (resin system) and hardener incorporated into hollow fibres and (ii) the healing agent incorporated into hollow fibres and the hardener added to microcapsules. The critical issues of hollow fibre embedment are: (i) fibres must be broken to release the healing precursor; (ii) a low-viscosity healing agent (resin) must be used to facilitate fibre infiltration and (iii) the use of hollow glass fibres in carbon fibrereinforced composites may lead to problems with respect to the coefficient of thermal expansion.

Fig. 1.8 Resin- and hardener-filled hollow glass fibres/microcapsules embedment for fabricating self-healing composites

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There are two main limitations to these concepts of using healing agents in microcapsules. Firstly, the local amounts of healing agent are necessarily small, being contained in microcapsules. Secondly, and more importantly, in this approach healing is achieved only once at each crack location. To overcome this limitation, a multiple healing approach of using a microvascular network has been adopted. Also a multiple healing process can be designed to be intrinsic to polymers via a reversible covalent network and supramolecular network formations, see Sect. 1.8.3.

1.8.2.3

Liquid Healing Agent-Filled Microvascular Network Embedment

The microvascular self-healing approach is analogous to the biological vascular system of many plants and animals, and has been explored to heal larger areas of epoxy composite materials (Toohey et al. 2007). In this approach, the healing precursor is incorporated into capillaries or hollow channels that are interconnected one dimensionally, two dimensionally or three dimensionally. Any damage triggers the self-healing (Wu et al. 2003; Huang et al. 2009; Hansen et al. 2009). Such millimetre-scale networks are inserted into PMCs during their fabrication, and they enable multiple-cycle self-healing, providing a continuous flow of healing agent to the damage sites.

1.8.3 Intrinsic Approaches for Self-healing Composites To achieve inherent self-healing functionality, the healing moieties are introduced into the basic chemical structures of polymer materials. Different thermo-reversible chemistries, non-covalent (secondary) and ionic interactions in polymers have been augmented to enable self-healing. The self-healing may be triggered by external sources such as heat, pressure and light energy. Such intrinsic approaches offer multiple healing processes, which are unobtainable from the extrinsic microcapsule and hollow fibre approaches. Three examples are given here:

1.8.3.1

Thermo-Reversible Network Formation Based on Diels–Alder (DA) and Retro-DA Reactions

Diels–Alder (DA) reactions are self-contained reactions and offer a dynamic chemistry without use of a catalyst. Such a reaction has been used for thermo-reversible network formation in the intrinsic healing process (Chen et al. 2002, 2003). Thermodynamically, the DA reactions are exothermic. On the other hand, the retro-Diels– Alder (r-DA) reaction generally proceeds at elevated temperature and is endothermic in nature. Several self-healing polymers based on this strategy are listed in the literature (Pramanik et al. 2013, 2014).

1 Polymer Matrix Composites (PMCs) for Defence Applications Fig. 1.9 Thermally reversible highly cross-linked furan-maleimide-based polymer network (Chen et al. 2002)

O

O O

O

+

O O

O

O

N

O

29

O O

O

O

O

O

O

3M

O O

O

4F

O

O N

O

N

O

O

O

O

O

O

O

O

O

O

O

O

O O

O

O

O

N

O O

90-120oC to Room Temperature

120oC

O

O

3M4F

O

O N

O

The functional reversibility of linear polymer main-chain and side-chain pendant groups in forming network polymers offers the bulk material dynamic chemistry needed to design thermally re-mendable polymer networks and composites. Bismaleimide and tetra-furan derivatives have been used to create a thermally activated self-healing carbon fibre PMC using a DA reaction. The tris-maleimide (3 M)- and tetra-furan (4F)-based polymer (3M4F) formed a cross-linked structure via a DA reaction. When heated to 120 °C, it is de-polymerized via the r-DA reaction, resulting in the initial units, see Fig. 1.9 (Chen et al. 2003). It was shown that fractured polymer materials heated to 120 °C and then tested showed 83% recovery of the original strength.

1.8.3.2

Ionomer-Based Network Formations

Thermo-reversible physical interactions based on ionomers lead to intrinsic healing in polymers. Ionomers are polymers having 15–25 mol% ionic groups. These polymers have pendant ionic (e.g. carboxylic acid) groups that are either moderately or completely neutralized with metal or quaternary ammonium ions. Owing to the reversible nature of ionic bonds, this concept is used to design self-healing polymeric

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Fig. 1.10 Effect of a high-velocity impact on ionomeric chains: a before impact and b after impact (Sun et al. 2006; Kalista et al. 2007). The process is reversible owing to the heat supplied by the impact

systems. The mechanism of healing is shown in Fig. 1.10, whereby an impact causes damage, but also the heat to repair it. Puncture reversal after ballistic impacts in thermoplastic poly(ethylene-comethacrylate) copolymers has been reported (Sun et al. 2006; Kalista et al. 2007; Bag et al. 2018b). See also Sect. 8.5 for the commercial products: Surlyn® and Nucrel® . Molecular re-arrangement of ionomers after a high-velocity projectile impact heals the punctured system. The factors influencing the healing process depend not only on heat but also the plasticizing effect of different acid groups, the ionic clusters and the order-to-disorder transition phenomenon. This type of self-healing mechanism is utilized in polymers used in food packaging, membrane separation, roofing materials, automobile parts, golf ball covers and coatings.

1.8.3.3

Supra-Molecular Interaction-Based Network Formations

Supra-molecular polymers are emerging materials that exhibit the reversibility and responsiveness of non-covalent interactions. Based on this strategy, a self-healing rubber has been prepared using vegetable oil esters (Cordier et al. 2008). A thermoreversible rubber with cross-linking of maleated ethylene/propylene copolymers has been reported to exhibit self-healing based on hydrogen bonding and ionic interactions (Wang et al. 2012). This material can be remoulded several times at 80 °C. True reversible (physical) cross-links were confirmed by Fourier-transform infrared (FTIR) spectroscopy.

1.8.4 Evaluation of the Healing Efficiencies of PMCs The terms ‘healing ability’ and ‘healing efficiency’ are used to express the restoration/ recovery of a property assessed by the self-healing process. In general, the healing

1 Polymer Matrix Composites (PMCs) for Defence Applications

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Fig. 1.11 Load–displacement curves for virgin and healed specimen tests using tapered double cantilever beams (TDCBs) (White et al. 2001)

efficiency (η) is quantified as the ratio of a property of a healed material to that of the property of the pristine (undamaged) material. As an example, fracture tests were performed using a tapered double cantilever beam (TDCB) specimen to determine the crack-healing efficiency of epoxy composite materials, Fig. 1.11. Inter-laminar fracture toughness testing was carried out according to ASTM 5528-01. The load–displacement curve of a self-healing composite demonstrates recovery of about 75% of the virgin fracture load. Thus the healing efficiency is determined according to the following relations: ηG = virgin

healed PIC virgin

PIC

vigin

=

K IC

virgin

K IC

(1.1)

healed where PIC and PIC are the critical loads at fracture for the healed and pristine virgin healed and K IC are the mode I fracture toughnesses (virgin) specimens, and the K IC of the healed and pristine specimens, respectively. The healing efficiency (%) in terms of the tensile strengths of materials can be evaluated by the following relation:

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ηG = virgin

where σChealed and σC specimens.

σChealed virgin

σC

× 100%

(1.2)

are the tensile strengths of the healed and pristine (virgin)

1.8.5 Applications and Prospects for Self-healing Polymers and Composites Developing composite materials with self-healing capabilities may significantly increase the service lives, and can also improve the safety aspects, as mentioned earlier. Thus it is highly desirable to introduce such novel materials into aerostructures, automobiles, and other components and personal items. Limited products of self-healing materials are currently available in the market in the form of paints, coatings and films, elastomers and tyres, and composites. However, a number of prototypes have great promise for commercial applications. Some examples, listing the manufacturers, are given here: • NextGen Aeronautics have developed thermally reversible products called ‘Mendomer-400’ and ‘Mendomer-401’. These are enriched with a small amount of magnetic particles, yielding highly cross-linked polymer composites. The mendomer–magnetic particle composites are capable of self-healing using a magnetic field. In addition, they exhibit a shape-memory effect. These self-healing and morphing capabilities are being integrated into air-launched missile control surfaces. • Cornerstone Research Group Inc. (CRG) has developed smart composite structures called reflexive composites, capable of sensing damages and self-healing during operation. The key components of reflexive composites are Veriflex® EH shape-memory polymers (fully cross-linked epoxy-based thermosets) that can restore up to 85% of the original mechanical performance within 3 min. • NanoComposix in association with SensorMetrix commercialized cross-linked thermally repairable polymers that can potentially heal thermally- and mechanically induced microcracks. • Self-healing conductive polymer composites, consisting of insulating semicrystalline polymers and electrically conductive carbon fillers, are available on the market for many years as resettable (or self-healing) fuses such as Polyfuse® (Littlefuse), PolySwitch® (Tyco Electronics) and Multifuse® (Bourns). • DuPont markets Surlyn® and Nucrel® , which are ionomers capable of self-healing ballistic damages. These materials are used as interlayers for laminated bulletproof glass, and ballistic protection self-sealing cabin and fuel line systems of helicopters.

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• Nissan Motor Co. Ltd. commercialized a clear paint named ‘Scratch Shield’ in 2005. This paint contains a highly elastic resin and attenuates scratches in the exterior paints on automobiles, providing both aesthetic improvement and functional protection. The damage ‘repair’ takes place overnight or maybe somewhat longer, depending upon the environmental temperature and depth of scratch. Other companies using a similar concept are Hyundai and Toyota Motors. • Chinese Hongchang Chemical Co. Ltd. has developed a commercial anti-scratch and self-healing wood coating for use as furniture varnish. • High Impact Technology has developed a commercial polyurethane (PU) coating called Battle Jacket™. This can be used for self-healing bullet holes, and is suitable for resealing helicopter fuel tanks. • Self-healing rubbers based on supramolecular chemistry have been demonstrated. The Reverlink™ compound marketed by Arkem comprises 10 grades of supramolecular elastomers featuring self-healing characteristics. Such materials are composed of more than 60% fatty acid oligomers derived from vegetable oils. The industrial applications of these supramolecular elastomers are being explored for sealing joints, impact protection, shock-absorbing layers, industrial gloves, conveyor belts, anticorrosion coatings (for metals) and additives for adhesives, paints, varnishes, pastes and sealants. • Self-sealing liners are used in helicopter fuel tanks.

1.9 Technological Implications Many more self-healing materials and technologies have been developed, and have been recently reviewed (Kumar et al. 2020). Together with several advanced technologies these have paved new paths for both civilian and military applications. However, the field is not yet mature and there are many possibilities for further developments. The demands for self-healing materials and technologies need to be properly addressed in aerospace, defence and other industrial sectors. Composite materials having both self-healing and morphing functionalities have great potential for adaptive aerostructures in defence applications. For example, the US Defence Department has estimated that the global market for unmanned air vehicles (UAVs) will be worth US$ 55 billion by 2020.

1.10 Summary and Concluding Remarks Polymer matrix composites (PMCs) are widely used as structural materials for high-performance applications such as in aerospace, ground transportation and civil infrastructures. Composites reinforced with carbon fibres are used mainly in aerospace/aircraft applications, while glass fibre-reinforced composites are preferred

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for automobiles. PMCs are also promising structural materials for various defence applications owing to certain advantages over conventional metallic materials. Recent trends in composite research are as follows: • Attention to higher temperature resins, such as polyimides, bismaleimides and cyanate ester resins, to enable continuous service at 200–300 °C, especially for jet engine casings. • Development of fast and low-temperature curing resin systems to enable greater productivity and the associated economic advantages of efficient processing. • Toughened thermosetting resins and thermoplastic resin systems for better impact properties. • Development of high-strain carbon fibres and hybridization of reinforcing fabrics for better mechanical properties. • Thermoplastic composites are favoured for future-generation composite materials with high impact properties, recyclability and out-of-autoclave processing. • Self-healing composites have attracted special attention, since they offer the prospects of longer service lives and improved safety aspects. Thermoplastic composites (TPCs) are increasingly favoured for replacing thermoset composites (TSCs) and metallic complex parts in aerospace applications. This is not only due to their low density, excellent mechanical properties and processability, but also with respect to the relatively new perspective of recycling TPC materials. Currently, over 2000 aircraft are dumped in ‘graveyards’ and it is estimated that over 5000 commercial airliners may be withdrawn from service in the next 20 years (Keivanpour et al. 2013). Thus the increase of disposal costs and the End-Of-Life (EoL) regulations push the aircraft industries towards more efficient solutions for recycling (Mezei and Boros 2016). In contrast to TSCs, the TPCs have better prospects for recyclability (Rush 2007; Li and Englund 2017). Acknowledgements The authors gratefully acknowledge the funding from DRDO for conducting most of the cited studies at DMSRDE. They particularly are grateful to Prof. GN Mathur, Dr. KU Bhasker Rao and Dr. AK Saxena for direct involvement as well as association, and to Drs. G Satheesh Reddy and Samir V. Kamat, Former and Present Chairmen, DRDO for their exemplary encouragement for the past 5 years during which this mammoth effort is made. They are indebted to Dr. R.J.H. Wanhill without whose expert review and extensive corrections this book chapter would not have taken this present shape.

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Chapter 2

Hybrid Polymeric Composites for Defence Applications Arun Kr. Singh, Richa Verma, Rajesh Kr. Tiwari, Eswara Prasad Namburi, and R. J. H. Wanhill

Abstract Composite materials have become an integral and essential class of materials for modern technological developments. The concept of the hybridization technique in polymeric composite development is finding many new and unexpected prospects in providing material solutions for high-end structural applications such as Defence and the aerospace sector. Hybridization has been used not only to improve the performance of composite structures but also to reduce the weight and overall cost for various applications. Several natural fibres like jute, hemp, and kenaf, which are abundantly available, have been found to be suitable substitutes for synthetic fibres in engineering applications. The lower cost and ease in the availability of some of the readily available synthetic fibres are the primary factors for their widespread applications. Considerable enhancements in the mechanical properties have been observed for hybrid composites consisting of different synthetic fibres, with or without additional natural fibres. Keywords Design philosophy · Hybrid composite · Natural fibre · rule of mixture · Synthetic fibre

A. Kr. Singh · R. Verma · R. Kr. Tiwari Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India E. P. Namburi (B) Former Outstanding Scientist and Ex-Director, Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India e-mail: [email protected] R. J. H. Wanhill Emeritus Principal Research Scientist, Aerospace Vehicles Division, Royal Netherlands Aerospace Centre, Amsterdam and Marknesse, Amsterdam, the Netherlands © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 E. P. Namburi et al. (eds.), Novel Defence Functional and Engineering Materials (NDFEM) Volume 2, Indian Institute of Metals Series, https://doi.org/10.1007/978-981-99-9795-4_2

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2.1 Introduction Our scientific understanding has increased tremendously over the last two centuries, as may be seen from technological progress and the availability of products. Newer technologies have been developed to synthesize special materials like plastics, polymers, ceramics, and metal alloys. With the invention of polymers and plastics, it became possible to consider replacing traditional materials like metals, which have the disadvantages of relatively high densities and—in some cases, notably irons and steels—susceptibility to corrosion. Recently, scientists and engineers have begun to consider mixing different materials to develop a new class of hybrid composite materials that have improved properties. Nowadays, composite materials are continuously expanding the horizon of application areas in all branches of engineering. The primary constituents of any composite are combined in such a fashion as to improve their intrinsic worth, while minimizing any negative aspects. Composite materials are commonly classified into two discrete levels: • The matrix constituent. The major composite classes include Organic Matrix Composites (OMCs), Metal Matrix Composites (MMCs), and Ceramic Matrix Composites (CMCs). The term organic matrix composite is generally assumed to apply to Polymer Matrix Composites (PMCs). • The reinforcement form—fibre-reinforced composites, laminar composites, and particulate composites. PMCs are the most commonly used materials: they are also known as FRPs— fibre-reinforced polymers (or plastics). These materials use polymer-based resins as the matrix, and a variety of fibres, e.g. glass, carbon, and aramid fibres, as the reinforcements. In general, the mechanical properties of polymers are inadequate for many structural purposes. In particular, polymer strengths and stiffnesses are low compared to those of metals and ceramics. These difficulties can be overcome by reinforcing other materials with polymers in the form of fibres. A distinct advantage is that PMCs do not require high pressures and temperatures for processing. For this reason, among others, PMCs have experienced rapid development over the last decades and have become popular for both low- and high-end structural applications. Composite materials are increasingly being used for structural engineering materials and non-structural materials in industrial applications all over the world, including India. This fact shows that composite materials have become essential to modern technological developments. Typical applications for PMCs include aerospace, marine, and automotive components, sporting goods, electrical components, and household appliances. Among these many applications, there has been much research on structural composites for blast and ballistic protection systems, based on high-strength reinforcement such as ultra-high molecular weight polyethylene (UHMWPE), p-aramid, glass, and carbon fibre fabrics. The development of new materials for any high-end structural applications such as ballistic protection systems can be divided into two basic areas: (i) wholly new

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Fig. 2.1 Contribution of major industrial countries to the development of hybrid composites (Mochane et al. 2019)

materials from advances in chemistry, polymer science, metallurgical science, and related sources of innovation and (ii) use of commercially available materials in new combinations and using available commercial production facilities. In view of the potential cost benefits, option (ii) could sometimes—or even often—be preferable. In this context, there is much scope for developing hybrid PMCs. Figure 2.1 shows that many countries are following this approach, with China being much the most active, followed by India and the USA. This chapter discusses hybrid composites for ballistic armour applications, including their classifications, candidate materials, fabrication processes, hybrid composite characterization, and structure behaviour. The chapter also presents a broad prospective of hybrid composites as advanced material solutions for Defence applications.

2.2 Hybrid Composite Materials Hybrid composites may be defined in terms of combinations of two or more natural and/or synthetic fibres. As discussed by Yamamoto et al. (1989), such combinations are re-energized to form strong chemical bonding, based on their compatibility. There are many types, including inter-ply, intra-ply and fabric hybrids, and short-cut fibre hybrids (Thwe and Liao 2002). The performance of a hybrid composite depends on the types of fibres and matrices and their properties; fibre orientations in the matrix; and the processing parameters involved in moulding composite laminates (Tamil et al. 2018). Optimum choices for hybridization improve not only the performance of composite structures and laminates but also reduce their weight. Figure 2.2 presents a broad classification of hybrid structural composites.

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Fibre reinforced composite

Thermoplastic matrix composite

Thermoset matrix composite

Particulate composite

Random Orientation composite

Random Orientation composite

Structural composite

Single layer composite

Multi layer composite

Continuous Discontinuous fibre composite fibre composite Unidirectional fibre composite Bidirectional fibre composite

Random orientation

composite Preferred orientation

composite

Sandwich composite Laminate composite Hybrid laminate

composite

Fig. 2.2 Classification of hybrid composite materials (Summerscales and Short 1978)

Mixing or hybridizing different types of reinforcing fibres within a structure can be accomplished in two ways: (i) inter-ply hybridization, where layers (laminas) of different fibre types are laminated together and (ii) intra-ply hybridization, where different fibre types are mixed within the individual laminas. In inter-ply hybridization each layer of material is stacked according to the required properties. An example of this type of hybridization is the use of high- and low-modulus fibres to tailor the radial strain gradient in very high-pressure storage containers. Hybrid composites have been reviewed by several authors (Short and Summerscales 1979, 1980; Hardaker and Richardson 1980; Swolfs et al. 2014; Kulkarni et al. 1976; Kulkarni and Rosen 1976) and can be divided into 6 groups, as shown in Fig. 2.3.

2.3 Design Philosophy for Hybrid Composites The design philosophy of hybrid composites is well illustrated by an example: a combination of glass and natural fibres (in a resin matrix). Glass fibres are common reinforcements in composites owing to their excellent mechanical properties and durability. The main drawbacks of glass fibres are their (relatively) high density, poor machinability, and recyclability, and they are not eco-friendly. On the other hand, natural fibres such as kenaf and flax have properties approaching those of synthetic fibres and are sustainable and environmental friendly. These natural fibres are, however, hydrophilic, which on their own makes them inadequate for structural applications. By introducing both types of fibres (synthetic and natural) into a hybrid composite, the disadvantages of each constituent can be compensated and diluted.

2 Hybrid Polymeric Composites for Defence Applications

(a)

(d)

Fibre Type A

a. b. c. d. e. f.

(b)

(e)

Fibre Type B

43

(c)

(f )

Fibre Type C

fibre-by-fibre mixtures (also known as "intimate" hybrids [10, 11]) tow-by-tow mixtures (also known as "discrete" or "zebra" hybrids [10, 11]) layer-by-layer mixtures skin-core-skin structures (also known as sandwich structures) internal ribs external ribs

Fig. 2.3 Different types of hybrid composite (Short and Summerscales 1980)

The behaviour of hybrid composites may be considered as a weighted sum of the properties of the individual components. The hybridization concept provides choices to design engineers in the spectrum of matrix and reinforcement materials available to achieve the desired properties for a specific application. As a consequence, a balance in cost and performance can be achieved through proper material design (Thwe and Liao 2003). The properties of a hybrid composite mainly depend upon the fibre content, lengths of individual fibres, their orientation, the extent of fibre intermingling and arrangement, and the fibre-to-matrix bonding. The strengths of hybrid composites depend on the failure strains of individual fibres. Optimum (maximum) strengths are obtained when the fibres are highly straincompatible (Sreekala et al. 2002). However, there is a synergistic effect as well: an apparent extra improvement in the properties of a composite containing two or more types of fibres (Jones 1994). Selection of the types of compatible fibres and the levels of their properties are of prime importance. This selection is determined by the

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Efficient hybrid composite processing

Synergism

Polymer matrix modification

Fibre treatment /modification

High performance hybrid composite

Fig. 2.4 Hybridization design philosophy (Jones 1994)

purpose of hybridization, requirements imposed on the material or the construction being designed, as shown in Fig. 2.4. In more detail, the successful use of hybrid composites is determined by the chemical and mechanical properties and physical stability of the fibres/matrix system.

2.4 Candidate Materials For Defence applications, some of the current material solutions involve highstrength materials, e.g. for use in ballistic protection structures. Sometimes these can be defeated by more advanced threats. Technological advancements are continuing in order to counter these newer threats: examples are the combining of highstrength ceramics with composite backing materials using synthetic fibres (aramid, UHMWPE, glass, carbon, and boron fibres). However, such advancements bring other issues with them, specifically with respect to the composite backings. Besides the requirements of lightweight and high-impact strength, the costs and durability, including service life extension, also play an important role to enhance their use in various Defence applications. Before being considered for these applications it is necessary to have an extensive database on candidate materials and their compatibility and suitability for the design and development of hybrid composites.

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2.4.1 Reinforcement Materials High-strength synthetic polymeric materials are mostly used as the reinforcement fibres for composites to be developed for blast and ballistic (protective armour) applications. Recently, over the past few years, natural fibres are being considered for use in such applications. Figure 2.5 presents commonly used synthetic as well as natural reinforcement fibres that are currently being considered, alone or in combination, as candidate reinforcement fibres for composite armours. In fact, one of the most recently researched topics for the development of composite armours is that of using natural fibres as replacements for synthetic fibres. This is because of their advantages such as biodegradability, renewability, and abundant availability as compared with synthetic fibres. Combinations of both types of fibres in organic matrices also offer new opportunities to produce multifunctional materials and structures for advanced applications. Table 2.1 lists the main engineering properties for natural fibres being considered for advanced composites. However, the literature on these fibres has reported limitations such as low thermal stability, high flammability, high moisture absorption, and variations in mechanical properties. These limitations have encouraged the development of hybridization techniques to combine natural fibre reinforcements with synthetic fibre reinforcements. The available choices of these synthetic fibres are shown in Table 2.2. Reinforcement fibres

Natural fibres

Flax Hemp Kenaf Jute

Synthetic fibres

Organic fibres

Inorganic fibres

Polyethylene Polyester Aramid

Glass Carbon Silicon carbide Boron

Fig. 2.5 Candidate fibre reinforcements (Ahmad et al. 2015)

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Table 2.1 Natural fibre reinforcements (Pickering et al. 2016; Gurunathan et al. 2015) Reinforcement fibres

Density g/cm3

Tensile strength (MPa)

Young’s modulus (GPa)

Elongation (%)

Flax

1.4

800–1500

60–80

1.2–1.6

Jute

1.3–1.48

393–800

13–26.5

1.16–1.8

Hemp

1.48

550–900

70–80

1.6–4.0

Kenaf

1.4

284–930

21–60

1.6–2.0

Table 2.2 Synthetic fibre reinforcements (Bilisik 2017) Reinforcement fibre

Density g/cm3

Tensile strength (MPa)

Young’s modulus (GPa)

Elongation (%)

Glass

2.44–2.55

220–480

65–95

1.8–3.2

Carbon

1.77–1.82

280–750

220–450

1.4–1.8

Polyethylene

0.97

280–300

100–120

2.5–3.8

Aramid

1.44–1.48

230–280

120–170

2.0–2.5

Silicon carbide

3.16–3.20

Boron

2.38–2.54

360–440 360–400

380–400

2.4.2 Matrix Materials Selection of the matrix in any composite is based on the requirements for the shape, surface appearance, environmental resistance, and overall durability of the composite product. Depending on the required processing techniques, there are two types of matrix/resins, thermoplastics and thermosets, which are commonly used for fibrereinforced composites (Rodriguez-Castellanos and Rodrigue 2016). Thermoplastic Resins Thermoplastics resins are much used as composite matrices owing to their lightness, low cost, and ease of processing. Table 2.3 lists a number of the most popular ones. A basic property requirement in any resin is its interfacial adhesion with the reinforcements. The interfacial adhesion can be enhanced by chemical, physical and biological treatments, and this has been the subject of many investigations for improving the interfacial adhesion between the matrix and natural fibres, thereby enhancing the overall properties of the resulting composite materials (Park and Seo 2011). The chemical treatments can include alkalis, alone or in combination with other treatments, such as acetylation and salinization. Furthermore, coupling agents or compatibilizers are also utilized to improve interfacial adhesion, which in turn further improves the properties of the resulting composite products (Couture et al. 2016; Stelescu et al. 2017; Guo et al. 2017; Maadeed et al. 2014; Zolfaghari et al. 2013; Mohammed et al. 1885).

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Table 2.3 List of thermoplastic resins (Fei et al. 2017) Resin

Advantages

Disadvantages

Polypropylene

• Highly resistant to corrosion • Excellent insulator • Highly malleable

• UV degradation • Chain degradation • Poor bonding properties

Polyethylene

• Chemical resistance • Wear resistance • Sharply defined melting point

• Low stiffness and creep resistance at high temperatures • Poor dimensional stability

Nylon

• Excellent mechanical properties • Good toughness • Outstanding fatigue resistance

• High shrinkage • Lack of stability • Antistatic properties

Polycarbonate

• Good UV resistance • Good impact resistance • High refractive index

• Poor abrasion resistance • Susceptible to scratches • Toxic nature

Polyetherimide

• Melt range rather than a defined melting point • Good impact resistance

• Toughness and impact resistance • Isotropic dimensional stability

Polyetherether ketone • Good mechanical properties • Performance at elevated temperatures • Good peeling resistance • Low toxicity

• Hydrophobic nature • Very high cost • High processing temperatures

Thermoset Resins Thermoset resins form three-dimensional cross-link networks via curing. Table 2.4 lists examples, which include: epoxy resins, phenolic resins, polyurethanes, acrylics, alkyds, furans, polyimides, vinyl esters, and unsaturated polyesters. Polyester and epoxy thermosets have been the most studied for hybrid composites (Atiqah et al. 2014). Recent investigations of thermoset-based hybrid composites are described in references (Guo et al. 2017; Dhakal et al. 2015; Sapiai et al. 2015; Kumar et al. 2015; Saba et al. 2016).

2.5 Composite and Hybrid Composite Processing Routes The subject of hybrid composite processing is extremely broad. It must be emphasized that unlike metal processing, the final properties of composite parts not only depend upon the individual material properties of fibres and matrices but are also governed by the processing route adopted to fabricate these composite. This section describes some of the important fabrication routes of hybrid composites and also discusses the major factors influencing the end product properties.

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Table 2.4 List of thermoset resins (Park and Seo 2011) Resin

Advantages

Disadvantages

Polyester

• Low cost • Easy to process • Good chemical and moisture resistance • Fast cure time • Room temperature cure

• Flammable • Toxic smoke upon combustion • Average mechanical properties

Vinyl ester

• • • • • •

• Flammable • Smoke released upon combustion • Mechanical properties are not as good as epoxies

Epoxy

• Excellent mechanical properties • Expensive • Good chemical and heat • Requires high processing resistance temperatures to achieve good properties • Good adhesive properties • Moisture resistant • Variety of compositions available

Low cost Ease of processing Low viscosity Room temperature cure Moisture resistant Good mechanical properties

Unsaturated polyester • Ease of handling • Relatively low cost • Wide range of fabrication processes

• Low modulus • Long odour • Shrinkage

Phenolic

• • • •

• Low mechanical properties • Not suitable for RTM process • Voids due to the release of water vapour during cure

Cyanate esters

• Excellent strength and toughness • High cost • Very low moisture absorption • Superior electrical properties

Low cost Flame-resistant Low smoke Good for ablative and rocket nozzles

2.5.1 Prepreg Moulding This process uses continuous filament reinforcements with pre-determined quantities of matrix material per unit area or length. In the case of thermosets, the prepregs are already semi-cured. When heated and subjected to pressure, the matrix materials soften and begin to flow. For thermoplastics, this process is reversible, but for thermosets further heating causes the matrices to transform to the fully cross-linked irreversible state and softening stops.

2.5.2 Autoclave Moulding Autoclave moulding is illustrated in Fig. 2.6. Although expensive, it is a widely used process for aerospace products. It provides good consolidating pressure to the part

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Fig. 2.6 Autoclave moulding (http://www.gurit.com)

during the curing (or cross-linking) of the matrix and also minimizes porosity due to the entrapment of reaction products. High compaction pressures are used to achieve maximum possible density and virtually 0% porosity.

2.5.3 Filament Winding This is a very important processing technique, frequently used in realizing engineering products like rocket motor cases, pressure vessels, tanks, containers, and canisters. This process makes use of continuous fibre tows (or rovings) impregnated in coordination with resin or pre-impregnated tow pregs, see Fig. 2.7. After wetting them with a liquid matrix in a resin bath, the tows from multiple spools of fibres are drawn out and collimated to form a tape of unidirectional (UD) rovings before being deposited on the surface of a rotating mandrel. Alternatively, UD or fabric prepreg tapes can be used.

2.5.4 Hand Lay-Up Process This is one of the simplest processes of making composite parts, using dry reinforcements and catalyzed liquid resin. This process is sometimes referred to as brush bucket technology, as it requires practically no investment but for the tooling. Dry reinforcement in different forms can be used, e.g. chopped strand mats, woven fabrics, woven rovings and chopped fibres. Rollers are used to compact and consolidate the lay-up. The volume fractions of fibres and matrices are controlled via measured

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Fig. 2.7 Filament winding (http://www.gurit.com)

quantities of constituents, and the homogeneity of the mixed constituents depends on the skill of the operator. Although this process can give good results, it is not suitable for high-performance structures (Fig. 2.8).

Fig. 2.8 Hand lay-up process (http://www.gurit.com)

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Fig. 2.9 Spray lay-up process (http://www.gurit.com)

2.5.5 Spray Lay-Up Process In the spray lay-up process, continuous fibres are fed into a chopper, and catalyzed resin is introduced from a separate storage chamber, see Fig. 2.9. The chopped fibres and liquid resin are blended in a mixing chamber and then deposited on the tool surface by a jet of compressed air.

2.5.6 Resin Transfer Moulding (RTM) This is a high-quality and popular industrial process owing to its short cycle time and suitability to high-volume production. This process is schematically illustrated in Fig. 2.10. Like many other processes, RTM requires matched die moulds, which when assembled form a cavity representing the geometry of the product. A variant is vacuum-assisted resin transfer moulding (VARTM). In this process, the resin is injected from one side of the mould, while the other side is connected to the inlet side of a vacuum pump. This accelerates the complete wetting of the dry reinforcement preform and hence reduces the process cycle time.

2.6 Mechanical Property Evaluation of Hybrid Composites Various techniques are used to study the micro- and macroscale properties of composite structures. For microscale studies, these techniques are Scanning electron microscope (SEM), Transmission electron microscope (TEM), Field ion microscope

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Fig. 2.10 Resin transfer moulding (RTM) (http://www.gurit.com)

(FIM), Scanning tunnelling microscope (STM), Scanning probe microscopy (SPM), Atomic force microscope (AFM), and X-ray diffraction topography (XRT). Similarly, for optical properties we have Fourier transform infrared spectroscopy (FTIR), Thermoluminescence (TL), and Photoluminescence (PL) techniques, whereas X-ray diffraction (XRD), Small-angle X-ray scattering (SAXS), Energy-dispersive X-ray/ spectroscopy (EDX, EDS) are used for studies requiring X-radiography. Besides these microscale techniques, mechanical testing is done to determine the major engineering properties. These include tensile, compressive, torsional, creep, fatigue, toughness, and hardness testing. The mechanical characteristics of hybrid polymer composite depend on several factors: (i) type and dispersion and distribution of the reinforcements in the chosen polymer matrix; (ii) interfacial adhesion between the reinforcements and the matrices. With respect to the reinforcements, the list includes their aspect ratios (high for long fibres), the fibre dimensions and orientations, surface modifications to improve fibre/matrix adhesion, mechanical properties, and the loading modes for the finished products. Numerous studies have reported the effective development and deployment of hybrid polymeric composites made from both thermoset and thermoplastic resins with natural and synthetic fibre reinforcements (Akil et al. 2014; Kureemun et al. 2018; Ramana and Ramprasad 2017; Safri et al. 2017). There are two main methods of characterization: analytical and experimental evaluations.

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2.6.1 Analytical Evaluations The rule of mixtures (ROM) is often used to predict the mechanical behaviour of hybrid composite materials. There are several ROM models reported in the literature. Essabir et al. (2016) discuss the various models of Voigt, Reuss, Hirsch, and Tsai– Pagano. For non-hybrid composites, the general ROM equations are used to predict the composite moduli, see Eq. 2.1: E c = E f V f + E m Vm

(2.1)

V f + Vm = 1or Vm = 1 − V f

(2.2)

E c = E f V f + E m (1 − V f )

(2.3)

Hence,

where E f and V f are the Young’s modulus and volume fraction of the reinforcement, and E m and V m are the Young’s modulus and volume fraction of the matrix. (The subscripts c, f , and m represent the composite, the reinforcing fibres, and the matrix, respectively.) The volume fractions of the fibre reinforcements and polymer matrix are also described by Eqs. (2.2) and (2.3). For hybrid composites the ROM equations have to be modified. The Young’s modulus (E hc ) of hybrid composites has been derived (Venkateshwaran et al. 2012) as a sum of two systems (polymer composite reinforced with fibre type 1 and polymer composite reinforced with fibre type 2), as shown in Eq. 2.4, and by assuming that the strain of the hybrid equals that of each system. E hc = E c1 Vc1 + E c2 Vc2

(2.4)

where, E c1 and E c2 are the elastic moduli for composites 1 and 2; and V c1 and V c2 represent the volume fractions of systems 1 and 2, respectively, as given by Eqs. 2.5, 2.6, and 2.7: Vc1 =

Vf1 Vt

(2.5)

Vc2 =

Vf2 Vt

(2.6)

+ Vf2

(2.7)

Vt = V

f1

where V t is the total reinforcement volume fraction. Similarly, the tensile strength (σ) of the hybrid composite can be derived through the application of an equilibrium force (F hc ) on the hybrid cross-sectional area (A), as given in Eqs. 2.8, 2.9, and 2.10:

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Fhc = F f 1 + F f 2 + Fm and F = σ · A

(2.8)

σhc Ahc = σ f 1 A f 1 + σ f 2 A f 2 + σm Am

(2.9)

Then,

Hence, σhc Ahc = σ f 1 V f 1 + σ f 2 V f 2 + σm (1 − V

f1

− V f 2)

(2.10)

The prediction model(s) of mechanical properties offer several advantages, including cost reductions (less experimental testing) and minimizing the time needed for the design of new products for certain applications.

2.6.2 Experimental Evaluations Experimental evaluations of hybrid composites characteristics are done to measure and understand the effects of different fibres and their volume fractions, and the matrix properties in hybrid composites Table 2.5 lists the types of tests. Also, some of the major testing results for hybrid composites are summarized in Table 2.6. Table 2.5 Experimental evaluation tests employed for hybrid composites Testing

Test standard

Test machine (m/c)

Density

ASTM D792

–

Hardness testing

–

Universal hardness testing m/c

Tensile testing

ASTM D3090

Universal testing m/c

Compressive strength testing

ASTM D 695-M

Universal testing m/c

Flexural testing

ASTM D 790M-86

Universal testing m/c

Inter-laminar shear strength testing

ASTM D2344-84

Universal testing m/c

Fracture toughness testing

ASTM D 5045

Universal testing m/c

Drop weight impact testing

ASTM D 5628

Impact testing m/c

High strain rate testing

–

Split Hopkinson pressure bar

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Table 2.6 Some research work on hybrid composites Hybrid composite

Reinforcement fibres

Details

Features

Refs.

Synthetic/ natural fibres

Glass/Jute fibres

• Hybrid had higher mechanical properties than non-hybrid jute-reinforced composite • Reduction of degradation of jute fibres

• Balanced performance and cost • Reduced moisture absorption • Improved mechanical properties

Akil et al. (2014)

Carbon/Flax fibres

• Hybrid had higher mechanical properties than non-hybrid woven flax-reinforced composites

• Improved mechanical properties

Kureemun et al. (2018)

Carbon/Jute fibres

• Hybrid had higher mechanical properties than non-hybrid jute-reinforced composites

• Improved mechanical properties • Hybrid can replace carbon reinforced composite

Ramana and Ramprasad (2017)

E-glass/Kevlar-29 fibres

• Hybrid had higher • Placement Muhi et al. mechanical position of (2009) properties than Kevlar important non-hybrid E-glass for impact and performance Kevlar-29-reinforced composites

Synthetic/ synthetic fibres

Carbon/UHMWPE • Ballistic fibres performance of circular CFRP and UHMWPE fibre composite plates were measured Kevlar/UHMWPE fibres

• Improved ballistic performance

Karthikeyan et al. (2013)

´ • Projectile impact test • Improved Cwik et al. using a 7.94 mm ballistic limit, up (2017) diameter steel ball to 20%, compared to similar laminates reinforced with UD fibres (continued)

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Table 2.6 (continued) Hybrid composite

Reinforcement fibres

Details

Features

Refs.

Natural/ natural fibres

Kenaf/Jute fibres

• Hybrids absorbed less water than non-hybrid kenaf or jute-reinforced composites • Hybrids had higher mechanical properties than non-hybrid kenaf or jute-reinforced composites

• Low cost • Minimal improvement of mechanical and water resistance properties

Maslinda et al. (2017)

Kenaf/Hemp fibres • Hybrid composites absorbed less water than non-hybrid kenaf or hemp-reinforced composites • Hybrid composites had higher mechanical properties then non-hybrid kenaf or hemp-reinforced composites

• Low cost • Minimal improvement of mechanical and water resistance properties

Maslinda et al. (2017)

2.7 Prospective for Hybrid Composites for Defence Applications This section briefly describes the overall and specific composite requirements for Indian Defence, although it should be noted that composite materials have many applications in other sectors. The increasing trend of composites usage in India and elsewhere shows that these materials are now integral and essential parts of modern technological developments. In the event of modern military confrontations, the combat personnel, their support equipment, vehicles, and other military hardware require personal and ballistic armour systems for protection against a variety of enemy threats: bullets, projectiles, mines, rockets and missiles, see Fig. 2.11. In giving this protection armour is required that is as light as possible while providing maximum protection against ballistic threats. The lightness of composite materials and their high strength (especially in combination with other materials like ceramics) are considered to be the main criteria for using them as components of armours. Besides the protection of personnel, there is a need to counter high-velocity armour piercing projectiles to protect vehicles and installations and also aircraft. On the other hand, and necessarily in contrast, fibre-reinforced composite materials are used for fabrication of

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Fig. 2.11 Various threats for defence platforms

rockets and missiles, i.e. they are essential contributors to active countermeasure technologies. Composite materials industries in India gained international perceptible participation in the early 1970s, when MIS Fibre Glass Pilkington Limited set up their factory in Mumbai (then Bombay). Since that time remarkable progress has been made in the area of high-strength fibre/fabric development. However, widespread use of these materials is strongly linked to costs in a developing country like India. Thus if we are able to bring down their costs, their use for various applications will greatly increase. The concept of hybridization in composite material technologies is considered to be one of the proven ways to fulfil the composite requirements for Defence applications, at relatively low costs. Numerous industrial research and scientific institutions have already recognized the great importance and potential of these technologies, not only for Defence applications, and are giving priorities to all aspects of their development. In addition, and in order to provide cost-effective composite material solutions, work is being done in many R&D institutes to determine the potential uses of natural and abundantly available fibres. Some examples have been given in this chapter.

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2.8 Conclusions In this chapter, the concept of recent hybridization techniques for polymeric composites has been surveyed, and their basic aspects have been described. In addition, newer reasonable-cost material solutions for high-end structural applications in Defence are discussed. Emphasis is placed on the fact that hybridization provides a unique opportunity to employ reasonably high percentages of natural fibres in combination with synthetic fibres. This approach enables reducing the overall costs of composite products for these high-end applications.

References Ahmad, F., Choi, H.S., Park, M.K.: A review: natural fiber composites selection in view of mechanical, light weight, and economic properties. Macromol. Mater. Eng. 300, 10–24 (2015) Akil, H.M., Santulli, C., Sarasini, F., Tirillo, J., Valente, T.: Environmental effects on the mechanical behaviour of pultruded jute/glass fibre-reinforced polyester hybrid composites. Compos. Sci. Technol. 94, 62–70 (2014) Atiqah, A., Maleque, M.A., Jawaid, M., Iqbal, M.: Development of kenaf-glass reinforced unsaturated polyester hybrid composite for structural applications. Compos. B Eng. 56, 68–73 (2014) Bilisik, K.: Two-dimensional (2D) fabrics and three-dimensional (3D) preforms for ballistic and stabbing protection: a review. Text. Res. J. 87(18), 2275–2304 (2017) Couture, A., Lebrun, G., Laperriere, L.: Mechanical properties of polylactic acid (PLA) composites reinforced with unidirectional flax and flax-paper layers. Compos. Struct. 154, 286–295 (2016) ´ Cwik, T.K., et al.: Design and ballistic performance of hybrid composite laminates. Appl. Compos. Mater. 24, 717–733 (2017) Dhakal, H.N., Sarasini, F., Santulli, C., Tirillo, J., Zhang, Z., Arumugam, V.: Effect of basalt fibre hybridisation on post-impact mechanical behaviour of hemp fibre reinforced composites. Compos. A Appl. Sci. Manuf. 75, 54–67 (2015) Essabir, H., Bensalah, M.O., Rodrigue, D., Bouhfid, R., Qaiss, A.: Structural, mechanical and thermal properties of bio-based hybrid composites from waste coir residues. Fibers Shell Part. Mech. Mater. 93, 134–144 (2016) Fei, M.-E., Xie, T., Liu, W., Chen, H., Qiu, R.: Surface grafting of bamboo fibers with 1,2-epoxy4-vinylcyclohexane for reinforcing unsaturated polyester. 24, 5505–5514 (2017) Guo, G., Chen, J.C., Gong, G.: Injection molding of polypropylene hybrid composites reinforced with carbon fiber and wood fiber. Polym. Compos. 39, 3329–3335 (2017) Gurunathan, T., Mohanty, S., Nayak, S.K.: A review of the recent developments in biocomposites based on natural fibres and their application perspectives. Compos. A Appl. Sci. Manuf. 77, 1–25 (2015) Hardaker, K.M., Richardson, M.: Trends in hybrid composite technology. Polym.-Plast. Technol. Eng. 15(2), 169–182 (1980) http://www.gurit.com Jones, F.R.: Handbook of Polymer Composites, Longman Scientific and Technical (1994) Karthikeyan, K., Russell, B.P., Fleck, N.A., Wadley, H.N.G., Deshpande, V.S.: The effect of shear strength on the ballistic response of laminated composite plates. Euro. J. Mech. A/Solids 42, 35–53 (2013)

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Chapter 3

Materials and Technologies for Personal Protection Systems (PPSs) Arun Kr. Mishra, Ajitendra S. Parihar, J. N. Srivastava, Rajesh Kr. Tiwari, Eswara Prasad Namburi, and P. Venkitanarayanan

Abstract High-strength and high-modulus fibres have revolutionized the design of personnel protection systems (PPSs), e.g. a bulletproof jacket (BPJ) and ballistic products like helmets and blast protective shoes. A BPJ is a lightweight personal protective gear that is worn as armour on the upper part of a wearer’s body: it helps to absorb the impact energy of bullets fired from small arms as well as to disperse the kinetic energy of fragments formed by disintegration of the bullets inside the jacket. The configuration of a complete BPJ with all its constitutive components and the fabrication technologies involved are discussed in this chapter. The advantages of using fibre-reinforced polymer composites (FRPs) in ballistic items include their high specific energy absorption capacity, lightweight, corrosion resistance, ease of fabrication and essentially the generation of less back face signature (BFS), as a result of trauma formed after the firing, compared to metals, alloys or ceramics. A detailed discussion is given of the efficacy of ballistic or combat helmets with respect to their design, selection of materials and fabrication technologies. To achieve the best ballistic resistant properties, the helmets are fabricated with hybrid fibres, e.g. composites of Kevlar and ultrahigh molecular weight polyethylene (UHMWPE). Anti-mine infantry boots (BAMI) are also discussed.

A. Kr. Mishra Defence Research and Development Establishment (DRDE), DRDO, Gwalior, Madhya Pradesh, India A. S. Parihar · J. N. Srivastava · R. Kr. Tiwari Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India E. P. Namburi (B) Former Outstanding Scientist and Ex-Director, Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India e-mail: [email protected] P. Venkitanarayanan Department of Mechanical Engineering, Indian Institute of Technology, Uttar Pradesh, Kanpur, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 E. P. Namburi et al. (eds.), Novel Defence Functional and Engineering Materials (NDFEM) Volume 2, Indian Institute of Metals Series, https://doi.org/10.1007/978-981-99-9795-4_3

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3.1 Bulletproof Jackets (BPJs) 3.1.1 Introduction Armours have always played a significant role in the protection of warriors/soldiers (The et al. 2009). In the modern era, armour is necessary for counter-insurgency operations against terrorist attacks and in low-intensity conflicts (LICs). The present advanced and functional materials used to construct body armours are meant to provide better protection against possible threats and also reduce the weight for ease of use. Body armour or BPJ is an integral part of the personnel protection system against fragmented ordnance and small arms and is therefore indispensable. Both soft and hard types of bulletproof body armours were introduced after the development of cannons and guns (http://www.nij.gov/topics/technology/bodyarmour/pages/impacteffectiveness.aspx). These weapons hurl high-speed projectiles with enough energy to penetrate thin layers of metals. The thickness of armour materials like the DRDO-developed Jackal steel can be increased to meet any perceived threat level, but that would be too cumbersome and heavy for carrying out normal combat operations. Reliable and wearable bullet-resistant armour was not developed until the 1960s. Even with modern body armours, which are highly sophisticated designs exploiting ceramic plates and polymeric fibres, the weight for inhibiting everincreasing destructive capability presents a huge challenge (Cavallaro and Ranges 2011). For example, body armour presently constitutes almost 30% of a soldier’s operational load, and is the single largest weight item. It must be noted that besides this personal protection system (PPS), today’s soldiers are extensively loaded with weapons, ammunition, communication equipment and food for survival in the battlefield. The basic principle of PPS development is to increase the efficiency of soldiers and give them advantages in surviving engagements and returning to base unscathed. The fabrics used in body armour restrict the movement of impacting bullets by stopping and deforming them into disc shapes, thereby spreading the energy over a larger area. Some layers may be penetrated, but as the bullets deform the energy is absorbed by a larger surface area, mostly within the armour itself. Any residual energy is absorbed by the wearer and can cause trauma. In fact, even without penetration, modern small arms contain enough energy to cause blunt force trauma at the impact point. Therefore, BPJ specifications will include limits to the impact energy delivered to the body as well as resistance to penetration.

3.1.2 Components of BPJs High-strength and high-modulus fibres have revolutionized the design of lightweight BPJs. High-performance fibres (Russell et al. 2013) used in ballistic products are characterized by low density, high strength and high energy absorption capacity.

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Examples of such materials are glass (E&S glass), aramid (Kevlar, Twaron), highperformance polyethylene (HPPE) fibres (Dyneema, Spectra) and PBO (Zylon). The ballistic performance of a material depends upon its ability to absorb localized energy very rapidly and dissipate it over a wider area. In fibre-based armour, the tenacity, elongation to rupture and the sonic velocity of the fibre are related to its specific modulus. The specific energy of absorption is related to the specific breaking strength and the strain to rupture. A BPJ or bullet-resistant vest (BRV) is an example of personal protective gear. A BPJ consists of soft armour panels (SAPs) and hard armour panels (HAPs) enveloped in a jacket cover, whereas a BRV only has SAPs. Essentially, any complete BPJ consists of three parts: the jacket outer cover, SAPs and HAPs. SAPs are made from advanced woven fibres sewn into vests and other soft clothing, or are based on non-woven pre-impregnated fabric. They are capable of protecting the user from threats like a submachine carbine (SMC), small-calibre handguns, shotgun projectiles and small fragments from explosives such as hand grenades. At present, the SAPs are being made of polymer composites comprising high-performance fabrics of Kevlar, Ultra-High Molecular Weight Polyethylene fibres (UHMWPE), and glass and carbon fibres. These fabrics consist of one or more layers of long yarns held in place by a secondary non-structural stitching thread. The main yarns can be any of the structural fibres in any combination. The stitching process allows a variety of fibre orientations to be combined into one fabric.

3.1.2.1

Jacket Cover

A BPJ jacket cover/carrier or harness is simply an outer jacket made up of nylon or other fabric to accommodate SAPs and HAPs. These are located in outside and inside pockets, which are of different sizes. The fabric used for the jacket is coated with a water-repellent polymer like polyurethane, polychloroprene or polyvinyl chloride. On the front of the jacket cover, in addition to a big pocket for the front HAP, some small pockets are provided to keep two rifle magazines, two hand grenades, one hand-held wireless set and two light machine gun (LMG) magazines: all of which allowing a soldier to assume a frontal prone position. Velcro fasteners are provided to close the pockets. Figure 3.1 shows a camouflaged jacket cover containing HAPs and SAPs, while a schematic view of the complete BPJ is given in Fig. 3.2.

3.1.2.2

SAPs

An SAP is an inner part of the BPJ that comes into contact with the wearer’s body (Miller Daniel Jeffrey 2011). The SAP has quilted ballistic fabric layers stitched or semi-stitched to protect against 9 mm SMC bullets. The SAP also has a trauma attenuation pad (foam) to absorb the energy of bullets and to provide comfort to the wearer. The following are the different types of SAPs used in the construction of a typical BPJ:

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Fig. 3.1 BPJ jacket cover with HAP (outside) and SAP (inside) developed at DMSRDE

Fig. 3.2 Schematic of the locations of the HAPs and SAPs in a BPJ

(i) Front (along with sides) SAP provides protection to vital organs (heart, lungs, liver) from bullets with kinetic energy up to 700 J (velocity up to 400 m/s). (ii) Back SAP provides the same protection as the front SAP when bullets come from behind. (iii) Groin SAP provides protection to the groin area.

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(iv) Front collar SAP provides protection to the throat. (v) Back Collar SAP provides protection to the neck. 3.1.2.3

HAPs

HAPs are armour plates which are rigid (unlike SAPs) and are placed inside the outer side pockets of the BPJ jacket cover. These hard armour plates are made of either metals, layered polymeric matrix composites (PMCs) of ballistic fabric or hybrid ceramic tile and PMC laminates. HAPs are capable of defeating (stopping) bullets of rifles like AK-47s and Self Loading Rifles (SLRs). However, HAPs are always tested with SAP backing as per the National Institute of Justice (NIJ) or other international standards. These standards specify that the trauma (bulge behind the jacket) due to the bullet should be below 44 mm. Figure 3.3 shows a HAP with its two components: ceramic front (orange colour) and polymer backing (green colour). The following are the different types of HAPs: (i) Front HAP provides protection to the vital organs of human body from rifle bullets with kinetic energy up to 2000 J and velocity up to 850 m/s. (ii) Two side HAPs provide protection from rifle bullets coming from either side. (iii) Groin HAP provides protection to the groin area from rifle bullets. (iv) Back HAP provides protection to the back from rifle bullets. (v) Throat HAP is a panel providing protection to the throat from rifle bullets. Fig. 3.3 Hard Armour Panel (HAP): ceramic tile (orange) + PMC backing (green)

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Table 3.1 GSQR 1438 threats, component areas and numbers of shots to withstand Component

Area (cm2 )

Ammunition

No. of shots

Front and back SAPs Groin SAP Neck/collar SAP Total Area SAPs

3800 500 500 4800

9 mm SMC

12 3 0 15

Front HAP Back HAP Groin HAP Two side HAPs Throat HAP Total area HAPs

1000 1000 375 750 270 3395

AK-47 (MSC & HSC), SLR AK-47 (MSC), SLR AK-47 (MSC), SLR AK-47 (MSC), SLR AK-47 (MSC), SLR

8 8 3* 3* 1# 23

* BFS measured for first shot only, steel core

#

No BFS measurement, MSC—mild steel core, HSC—hard

3.1.3 Threat Perceptions for BPJs Advances in global weapons technology have caused major changes in the threat levels imposed on armed forces, paramilitary and security personnel. These changes are due to the emergence of new advanced materials used in munitions. In lowintensity conflict (LIC) operations the greatest challenge is to give forces full protection against all possible lethal munitions: security forces face different types of challenges from traditional warfare to cross-border terrorism and rescue operations from natural disasters. Furthermore, the threats are not only for defence personnel but also for all types of mechanized support like infantry combat vehicles (ICVs), main battle tanks (MBTs), helicopters and aircraft. Some idea of the problems is indicated by the Indian Army General Staff Qualitative Requirements, GSQR1438, which specify the threats to BPJ components for medium-sized BPJs weighing less than 10.4 kg and having a Back Face Signature (BFS)* below 25 mm, see Table 3.1.

3.1.4 HAP Functions The following list gives the functions of BPJ HAPs, (i) After the bullet is fired, Fig. 3.4a, and received by the facing ceramic tile, Fig. 3.4b, the BPJ starts to blunt the bullet, as shown in Fig. 3.4c. (ii) The composite laminate absorbs the bullet energy by de-lamination, de-bonding between fibre and matrix, fibre pull-out and fracture, Fig. 3.4d. (iii) Accordingly, the bullet changes to a ‘mushroom’ shape, Fig. 3.4e (YohannesRegassa and RatnamUppala 2014). (iv) HAPs can arrest bullets of velocity up to 850 m/s (i.e. kinetic energy between 1000 and 2700 J), see Fig. 3.4f.

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Fig. 3.4 HAP bullet-stopping and mushrooming Fig. 3.5 Shapes of deformed bullets extracted from the tested BPJ HAPs

Each successive layer of a HAP continues to absorb energy until the bullet is arrested. An actual view of the deformed (mushroom-shaped) and arrested bullets extracted from HAPs is given in Fig. 3.5.

3.1.5 Manufacture of HAPs 3.1.5.1

Polymeric HAPs

HAPs made only from polymers involve fabrication of laminates whose design is optimized based on a specific combination of area density, shape and size. The fabrication process requires the use of compression moulding in a hydraulic press and subsequent curing of the resin matrix in an autoclave. Modern technology also employs sophisticated electronic process controllers. The steps for manufacturing polymeric (PMC) HAPs are as follows:

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Step 1 Cutting the required number of layers of fabric (PMC prepreg) from the roll as per the template (size) of the HAP; Step 2 Thorough visual inspection in order to ensure no cuts or damage in the fabric; Step 3 Stack all the layers of fabric and place in a mould, which has been heated to the curing (thermosets) or softening (thermoplastics) temperature range, about 150 °C; Step 4 Insert thermocouples in the core of the prepreg and continue heating the mould to maintain its temperature up to curing; Step 5 Close the mould and apply a small pressure until the core temperature reaches the vaporization temperature of any volatile matter present in the prepreg; Step 6 Apply final pressure, as optimized, when the core temperature exceeds the vaporization temperature of the volatiles (usually 60–70 °C); Step 7 Continue heating the mould to maintain the curing temperature of the matrix until the core temperature reaches the curing temperature; Step 8 Stop heating, and start cooling mould till the core temperature is well below 60 °C, all the while maintaining the final pressure mentioned in Step 6; Step 9 Reduce pressure to zero and open the mould to take out the component for air cooling to room temperature; Step 10 Finish the edges to remove extra unpressed ‘flash’, if any, using either hand-cutting tools or an automated water jet cutting machine for mass production. The processing cycle, enumerated in the above is shown in Fig. 3.6, and the shape of a PMC-based HAP is shown in Fig. 3.7.

Fig. 3.6 Processing cycle for PMC-based HAPs

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Fig. 3.7 HAP (PMC)

3.1.5.2

Ceramic-Faced Hybrid HAPs

Hybrid ceramic-PMC panels consisting of an integrated ceramic front face and composite backing (Torki 2012) require the selected materials to work in tandem for achieving the necessary performance characteristics during durability testing, stopping the bullet, managing the bullet’s energy and momentum and finally preventing trauma to the wearer. The manufacturing of hybrid ceramic-faced HAPs (Fisher 2011) involves processes such as pressureless sintering, hot pressing and reaction bonding. Typical ceramic tiles (St. Asenov et al. 2013) for BPJ HAPs are shown in Fig. 3.8.

3.1.5.3

Joining

Joining of the ceramic with the backing PMC is a crucial step in the manufacturing of BPJs. A number of factors decide the joining techniques, including (i) types of materials to be joined; (ii) desired component functions for example, strength; (iii) operational temperature and environment (coefficient of thermal expansion CTE, Refractive Index, UV, RH); (iv) applied mechanical stresses (static and dynamic); (v) strain rate dependence; (vi) mechanical impedance; (vii) required level of joint air tightness; and (viii) component design and cost. The step-by-step process for

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Fig. 3.8 Ceramic tiles for HAPs

joining/fabricating hybrid HAPs after fabrication of the ceramic tiles and PMCs is as follows: Step 1 Visually inspect the reaction bonded ceramic (e.g. boron carbide)front panel (tile) for any cracks and other defects; Step 2 Join the ceramic front panel to the PMC backing using a suitable elastomeric adhesive and under the application of heat and vacuum; Step 3 Finish the component edges to remove any extra materials; Step 4 Wrap the above hybrid composite panel inside the waterproof sealing fabric, usually polyurethane (PU)-coated Nylon. This fabric should be 20 mm oversize along the edges to enable sealing the edge contours, preferably using a hand-held machine; Step 5 Remove the surplus waterproof material. 3.1.5.4

Structural Integration

After joining the ceramic with the composite backing, the complete BPJ system is required to qualify for durability and structural integrity. Durability of the ceramic facing during the service life is generally achieved by the use of materials like polymeric foam, felts, honeycombs, and other impact-absorbing materials. For good performance of the armour during ballistic events the complete armour must be wrapped to confine ceramic fragments emanating from the impact point.

3.1.6 Ballistic Testing and Evaluation Body armour standards for ballistic testing and evaluation vary due to differences in threat scenarios in particular environments. As a result, the armour testing methods

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also vary significantly from country to country or even from region to region. Law enforcement statistics show that many injuries and killings happen due to the mishandling of weapons. Hence, protection from one’s own weapon becomes a necessity. While many standards exist in the literature, a few of them have acquired a status as ‘models’. The US National Institute of Justice (NIJ) Ballistic and Stab documents are the broadly accepted standards (Ballistic Resistance of Body Armour 2008). In addition to the NIJ, the UK Home Office Scientific Development Branch (HOSDB) standards are also used by a number of countries and organizations. These ‘model’ standards are adapted, as necessary, by other countries including India. The NIJ Standard 0101.06 has specific performance standards for bulletproof jackets and vests used by law enforcement agencies and is widely employed as the ‘model’ standard. The US and NATO military armour designs are also tested against ARMY MILSTD-662F and STANAG 2920 standards for V50 (the velocity at which the bullet is expected to perforate 50% of the armour). These two standards address different operational demands of fragment protection, and the standard parameters in ballistic testing of bulletproof jackets and vests involve: (i) No Perforation from intended bullets; (ii) Restricted back face signature (BFS), popularly known as ‘TRAUMA’ (44, 25 mm or less; as per user’s requirement); (iii) Armour ballistic strength (V50 Test: for a given bullet type) (Ballistic Resistance of Body Armour 2008); (iv) No Perforation from fragments. Ballistic testing by destructive tests of HAPs and SAPs is the only test method to qualify a BPJ. Most ballistic testing routines are based on standards to establish a guaranteed minimum ballistic resistance. A schematic of a HAP testing set-up is given in Fig. 3.9, and the post-testing trauma measurement procedure for a HAP + SAP assembly is shown in Fig. 3.10.

3.2 Ballistic Helmets 3.2.1 Introduction Steel helmets were in use up to the 1980s, before the development of fibre-reinforced composite materials. In 1981, the USA introduced the first of its Personal Armour Systems for Ground Troops (PASGT). The PASGT design was optimized in terms of area of protection and weight in order to cover all important parts of the head along with the neck. PASGT helmets made of glass fibre-reinforced phenolic/epoxy resin were used till 1985 and Kevlar-based helmets until 2006. After the development of gel-spinning technology to draw UHMWPE fibres, ballistic helmets have been fabricated with hybrid composites of Kevlar and UHMWPE. The advantages of using fibre-reinforced composite materials include

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Fig. 3.9 Schematic of HAP testing set-up. Source NIJ 0101 06 (Ballistic Resistance of Body Armour 2008)

Fig. 3.10 HAP + SAP testing and trauma measurement

(i) high specific energy absorption during impact loading per unit weight; (ii) lightweight; (iii) corrosion resistance; (iv) less BFS (trauma) (Freitas et al. 2014)— extremely important; and (v) ease of fabrication. The compression moulding technology has been well established for the fabrication of prepreg-based fabric, and modern machining processes accomplish automatic cutting of the helmet shell. This makes efficient use of time in finishing/cutting helmet shell. Various advanced lightweight ballistic helmets have been developed, and are in use in various countries. A few examples are enhanced combat helmets, advanced combat helmets, lightweight helmets and enhanced performance combat helmets. In the present chapter, the ballistic or combat helmets are discussed with respect to the design and materials and fabrication technologies to protect against low- and

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medium-velocity bullets and shrapnel (Wallace 2012). A typical helmet is shown in Fig. 3.11, and details are given in Fig. 3.12.

Fig. 3.11 Ballistic Kevlar helmet with camouflage IR-absorbing paint. Source DMSRDE helmet

Fig. 3.12 Components of a ballistic helmet: a Three point chin strap, b Trauma reduction foam (TRF), c Helmet shell, d Crown foam with pad, e Rivets to attach harness assembly. Source DMSRDE helmet

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Of the different components of the ballistic helmet, the helmet shell is the most important part for ballistic performance. The base materials selected as per design criteria consider the following parameters: (i) (ii) (iii) (iv) (v) (vi)

Threat level: type of bullet against which protection is required; Design pattern; Area of protection/size of the helmet; Weight: in terms of area density in kg/m2 ; BFS level; Relevant mechanical properties of a flat panel representing the ballistic helmet material (interlaminar shear strength, tensile strength, toughness, ductility index); (vii) V50 values of the materials; (viii) Ergonomics, e.g. fitting on the head; (ix) Cost.

3.2.2 Design Criteria for the Helmet Shell The helmet must be designed for maximum protection with minimum possible weight. The types of materials used and the design in terms of fit are very important. Statistical data show that head injuries constitute 21% of all injuries, whereas the area of head is about 10% of the total body area. Helmets are placed in two size categories on the basis of cephalometry (anthropometrics) data (Kulkarni et al. 2013), as follows: (i) Medium size: head circumference 48–52 cm; (ii) Large size: head circumference 52–57 cm. The surface area of a medium-sized helmet shell is about 1312 cm2 . On this basis, the design pattern is selected for the overall shape of the helmet. The USA has developed the PASGT system for ballistic helmets and bullet-resistant vests. The ballistic performance of a ballistic helmet is judged not only by the ability to stop the specified bullets but also by minimizing the amount of blunt trauma (BFS) behind the shell (Cannon March 2001): the trauma size should not be more than 13 mm.

3.2.3 Threat Levels for Ballistic Helmets The threat levels for ballistic helmets are less stringent as compared to those for BPJs: the USA Law Enforcement Agency has specified the various threat levels for ballistic helmets as II, II-A, III, III-A and IV; ballistic helmets are designed for a maximum level of III-A, while BPJs are designed for level IV. Ballistic helmets are designed for a maximum impact energy level of 790 J: this covers 9 mm SMC bullets. However, level IV goes to an impact energy level of 4000

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J to counterarmour-piercing projectiles/bullets. The reason for the ballistic helmet’s lower design impact energy level of 790 joules is that bullets transfer all their energy to helmets very rapidly, within a few microseconds. The crucial capability of the helmet material is how fast it disseminates the impact energy to adjacent areas, such that the shock does not damage the human head (Li et al. 2015; Magnus Aare and SveinKleiven 2007). In this respect, it has been found experimentally that if the energy of ammunition is higher than 790 J, the weight of the helmet will be more than 2.0 kg. With such a heavy load the amount of thrust generated by the stopped bullet will be more than sufficient to break the neck. This is why all ballistic or combat helmets are designed to the III-A limit of coping with a 9 mm bullet. In general, the threat levels against a ballistic helmet for a 0.38'' or 9 mm bullet fired from three different weapons are as follows: (a) (b) (c) (d)

0.38'' ball by revolver: V = 183 ± 9 m/s 9 × 19 mm ball by pistol: V = 350 ± 9 m/s 9 × 19 mm ball by SMC: V = 397 ± 15 m/s 9 × 19 mm ball by MP-5: V = 430 ± 15 m/s.

The design of a ballistic helmet should be such that all the above-specified bullets will be stopped by the helmet when fired from 10 m distance, and the trauma (BFS) should not be more than 13 mm. If the BFS is more than 13 mm there can be internal head injuries that can cause death, as found by animal studies. In India, the threat level and protection efficiency of ballistic helmets has been defined by the Indian Army in their respective GSQRs. A good design of helmet dissipates the impact energy of the bullet by delamination and fibril formation. Examples are given in Figs. 3.13 and 3.14.

Fig. 3.13 Delamination in a helmet after stopping a bullet. Source DMSRDE helmet

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Fig. 3.14 Formation of fibrils after bullet impact. Source DMSRDE helmet

3.2.4 Materials for Different Components of the Ballistic Helmet The performance requirements for the major components of ballistic helmets are as follows: • • • •

Helmet shell: The main ballistic part of the helmet. Trauma reduction foam: Reduction of the BFS after bullet arrest in helmet shell. Harness assembly provides cushioning to the head. Three-point chin strap system: Adjustment of proper fitting during combat operation. • Camouflage UV-absorbing paint: Protection of the helmet shell from degradation and water absorption (also camouflaging). • Rivets: Fixing the harness assembly to the helmet shell. • Edge beading: Rubberized beading glued on the helmet shell edges to protect the edges from moisture ingress and external damage that may cause delamination in the shell.

3.2.5 Energy Absorption Mechanism in a Composite-Based Ballistic Helmet During the past 10 years, ballistic helmets are made from fibre-reinforced plastic (FRP) composites owing to their better energy absorption characteristics during bullet impact. FRP composites have layered fibre/matrix structures in which the fibres are the reinforcement and the matrix is the load transfer medium. The various steps representing the energy absorption mechanisms by the matrix and the reinforcement

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are depicted in Fig. 3.15 and also described here (Sheikh et al. 2009; Chang and Chang 1987):

Fig. 3.15 Energy absorption mechanisms in FRP armour laminates

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Table 3.2 Energy absorption and related properties of FRPs in ballistic helmets Mechanism of energy absorption

Material index/constitutive parameter

Compression of helmet just below impact

Specific compressive strength of fibres

Compression in the region surrounding the impacted zone

Specific compressive strength of composite laminate

Shear plugging phenomenon

Bonding between fibres and matrix

Stretching and tensile failure of primary yarns

Specific tensile strength of fibres under tensile load

Stretching and tensile failure of secondary yarns

Specific tensile strength of fibres under tensile and shear load

Conical deformation on back face of target

Interlaminar shear strength (ILSS) and flexural strength of laminate

Delamination

ILSS: Mode II (in-plane shear) dynamic strain energy release rate (Cheng 2004)

Fibre pull-out

Bonding between fibres and matrix

Matrix cracking

Toughness of matrix: energy absorbed in matrix cracking per unit volume

Friction between bullet and helmet composite material during impact event

Abrasion resistance of fibres, thermal conductivity of laminate

• • • • • • • • • •

Compression of material directly below the projectile; Compression in the region surrounding the impact zone; Shear; plugging. Stretching and tensile failure of primary yarns; Stretching and tensile failure of secondary yarns; Conical deformation on back face of target; Delamination; Fibre pull-out (separation of fibres from the matrix); Matrix cracking; Friction between projectile and FRP.

All these energy absorption mechanisms play significant roles in stopping bullets. Table 3.2 lists the material properties associated with each of the energy absorption mechanisms.

3.2.6 Fabrication Methodologies Compression moulding is the best-suited method for fabricating ballistic helmets. A suitable mould is designed and fabricated using P-10 and EN 24 steels with surface quality getting the desired finish of the helmet shell. The moulding is done in a hydraulic press, and the three important parameters that need to be optimized are the pressure, temperature and curing time.

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Temp. (°C)

Degassing Point 2 P≤220 kg/cm 140

80

P ≤40 kg/cm

2

50

Room Temp.

12

22

50

60

Time (Min)

Fig. 3.16 Compression moulding cycle for Kevlar/epoxy ballistic helmet

3.2.6.1

Fabrication of Thermoset Prepreg Ballistic Helmet

Thermoset (generally epoxy) prepregs are commercially available, especially those of Kevlar-reinforced epoxy and Kevlar-reinforced phenolics. The prepregs are cut in flower shapes and arranged in stacks. The thickness of one layer of the prepreg is generally 0.3 mm, and the total thickness of the helmet is 8.0 mm. Hence, about 27–28 layers are required. Epoxy-based systems are cured at 140 °C for 45 min and post-cured at 150 °C for 2 h. Figure 3.16 shows a typical compression moulding cycle.

3.2.6.2

Fabrication of Thermoplastic Prepreg Ballistic Helmet

The fabrication of thermoplastic (e.g. UHMWPE, Dyneema/Spectra) ballistic helmets is different from thermoset FRP helmets in terms of cycle temperature, degassing and optimum pressure. For thermoplastics, curing (resin cross-linking) is not needed, and therefore no degassing is required during the compression moulding operation. The flower-shaped prepreg has to be held in an oven at 60–70 °C for moisture removal. The processing temperature is in the range of 122–126 °C and the pressure is 140–200 bar for Dyneema-based prepregs. For Spectra-based prepregs, the processing temperature is also122–126 °C, but the pressure range is 130–180 bar.

3.2.7 Hygroscopic Study of Helmet Shell Materials One of the most important properties that needs to be checked is the hygroscopic nature of the composite helmet shell fabricated by the compression moulding process.

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If water is later absorbed by the helmet shell, its ballistic performance will be reduced: water absorption increases the BFS value. Hence, the FRP materials (e.g. Kevlar, Dyneema and Spectra) are tested for water absorption in dry and wet conditions using the drop weight impact test at room temperature. The other main parameters (total energy absorbed, maximum load and the energy at maximum load) are also evaluated from this type of test. The drop weight tests are done with 10 × 10 cm2 flat laminates in dry and wet conditions, whereby ‘wet’ specimens are dipped in water for 30 min, removed and tested. Some of the results are given in Figs. 3.17, 3.18 and 3.19. These results show that Dyneema and Spectra have almost negligible water absorption, and so their impact properties are almost the same in dry and wet conditions. However, Kevlar is hygroscopic and the impact performance is less under wet conditions [N.B: Kevlar impregnated with an advanced resin system shows less susceptibility to water absorption. Hence, advanced Kevlar prepregs are also strong candidate materials for ballistic helmets].

Fig. 3.17 Total energy absorption under dry and wet conditions by composite laminates of Dyneema, Spectra and Kevlar

Fig. 3.18 Maximum load under dry and wet conditions for composite laminates of Dyneema, Spectra and Kevlar

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Fig. 3.19 Energy absorption at maximum load under dry and wet conditions for composite laminates of Dyneema, Spectra and Kevlar

3.2.8 Ballistic Evaluation of Helmet Shells The optimization in design, material configuration and processing parameters needs to be validated by a final ballistic evaluation test of the moulded helmet shell against a 9 × 19 mm bullet. Two main parameters are evaluated: (i) perforation/non-perforation and (ii) the BFS in mm. The testing is done with a specially designed helmet-shaped fixture filled with the clay (either Roma Plastilina or Indian Clay). When the bullet is arrested by the helmet shell there is a transient deformation inside the helmet. This transient deformation in the helmet shell creates a permanent dent on the clay. The indentation depth is called the trauma/BFS behind armour blunt trauma.

3.2.9 Scope of Future Upgrading and Improvement of Ballistic Helmets Current ballistic helmets provide protection against 9 × 19 mm ball ammunition fired from a pistol, SMC or MP-5. As per US NIJ level III, the AK-47 bullet with mild steel core is a common threat all over the world. There are BPJs that provide protection against AK-47 mild steel core ammunition and even to a higher threat level like hard steel core bullets. However, the current design and materials used in ballistic helmets do not protect against AK-47 bullets, since they would be too heavy. Thus, there is an urgent need of modifications, e.g. to the front parts of ballistic helmets, to provide protection against both 9 mm bullets and also AK-47 bullets. This extra protection can be accomplished with the introduction of an extra attachment generally called a PATKA. It is an add-on part which can be made of steel or ceramic and integrated into the front part of the helmet. Fibre-reinforced composites are not suitable for PATKAs because they would be too thick to fit within the ballistic helmet. Steel is also ruled out because it would be too heavy. The remaining option is ceramic materials, which should be designed as detachable components that can be

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replaced once they are cracked or broken after stopping one or two bullets. Probable candidates for PATKAs are boron carbide, silicon carbide, boron–silicon carbide and boron carbide toughened by carbon nanotubes (CNTs). To further improve the ballistic performance, the design of PATKAs should be such that they can be added on to any ballistic helmet to improve its ballistic protection from 9 mm ammunition to 7.62 × 39 mm ammunition (AK-47). For this purpose, there is a need to work on the design and optimization of dynamic fracture toughness for lightweight ceramics. The other candidate material is a shear thickening fluid (STF). The ballistic performance (BFS) of Kevlar impregnated with silica nanoparticles is significantly enhanced (Joselin and Jacob Wilson 2014). The STF behaviour changes from a fluid to a thick gel/solid in a very short time when the applied stress is above a threshold value (Decker et al. 2005). N.B: STF coating of Kevlar also reduces the BFS (Park and Yoon 2012).

3.3 Anti-Mine Infantry Boots (BAMI) 3.3.1 Introduction Anti-personnel (AP) mines are used to restrict /stop the movement of enemy armed forces. The mines are buried in the ground and are activated by a load/force. The blast of AP mines can cause fatal injuries to soldiers and especially injure the legs, which may need amputation. Similarly, there are Anti-Tank (AT) mines and Improvised Explosive Devices (IEDs) that can be activated when a vehicle/MBT passes over them. These can damage the vehicle/tank and injure the personnel inside. Although soldiers engaged in low-intensity conflicts wear BPJs and ballistic helmets to protect vital organs, these are no defence against AP mines. BAMI can provide adequate protection from the blast overpressure of an AP mine as well as from the accompanying shrapnel and splinters. Armed forces of most countries prepare a minefield plan during mine deployments so that they can restrict entering the mined areas to their own personnel. However, some unorganized forces, e.g. terrorists, do not make systematic layouts of minefields, and the unidentified mines blast accidentally and cause many deaths of people and animals living in nearby areas. It is also important, of course, to safely remove mines at the end of hostilities. The BAMI boots discussed in this section have chrome-tanned leather uppers which protect feet from the debris of AP blasts and also adverse climate conditions. The boot soles have two types of configuration: heel (rear) and ball (front) sides: (1) The heels are made of FRP deflectors. A fraction of the total blast energy delivered to these FRP deflectors (in the form of kinetic energy) will be dissipated by plastic deformation of the deflectors, and the rest will be deflected towards the ground. Ceramic honeycomb material of different shapes is embedded between the FRP deflectors as a shock-absorbing material: much energy is absorbed in crushing/pulverizing of the ceramic honeycomb material inserts. Hence, a very

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small fraction of the energy generated by the AP mine blast will reach to user’s foot. (2) The ball (front) configuration has ceramic honeycomb material and woollen felt, which is lighter in weight and comfortable to wear. Both the heel and ball protective configurations are embedded inside a conventional polyurethane rubber oversole, which is flexible and wear-resistant.

3.3.2 Anti-Personnel Mines There are mainly two types of AP mines: (i) fragmentation type and (ii) blast type. Fragmentation mines are designed to kill and maximize damage to the enemy by spreading high-speed fragments of the blasted explosive. On the other hand, blast mines are intended to temporarily incapacitate the infiltrating enemy: wounded soldiers create more problems for transportation, medical treatment and additional security. The total weight of a fragmentation mine ranges from 500 to 5000 g, and the explosive content varies from 75 to 900 g. Blast mines are much lighter, 75–630 g, and the explosive content ranges from 28 to 300 g. AP mines, see Fig. 3.20, are generally placed about 20 mm below the ground When the pressure plate is stepped on, a weight of 24 kg is sufficient to depress the firing pin so that it hits a highly sensitive detonator, which then ignites the main explosive charge. Detonation of the explosive produces a high-pressure blast wave travelling at a supersonic speed. This blast wave produces an impulsive force that depends upon the explosive charge and distance from explosive. Figure 3.21 explains the pressure behaviour of an explosion conducted in a free air blast: t A is the time of arrival of the detonation front at a particular point. As the wave propagates, it loses energy and the pressure amplitude decreases, see Fig. 3.22.

3.3.3 Features of DMSRDE-Developed Anti-Mine Boots 3.3.3.1

Boot Anti-Mine MK-I

Boot Anti-mine MK-I provides protection against the blast of a non-metallic AP mine during de-mining operations. The configuration of the boot is shown in Fig. 3.23. The salient features include (i) boot weight of 5.0 kg per pair; (ii) an attenuated peak overpressure of 45 kg/cm2 is achieved, which is safe; and (iii) a soldier can safely encounter an accidental blast, and these boots are suitable for any terrain.

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Fig. 3.20 Schematic diagram of an Anti-personnel mine

Fig. 3.21 Pressure–time history of an air blast in free air (Baker 1973)

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Fig. 3.22 Blast wave propagation with decreasing amplitude over distance (Baker 1973)

Fig. 3.23 Boot Anti-mine MK-I for de-mining operations involving non-metallic AP mines. Source DMSRDE

3.3.3.2

BAMI

BAMI boots, see Fig. 3.24, have been developed to protect against the blast of AP non-metallic mines during assault operations. The salient features include (i) the boot has a combat/ammunition design and has been indigenously developed; (ii) weight less than 3.0 kg/pair; (iii) suitable for assault operations in flat areas, desert and mountain regions; (iv) performance adjudged superior in comparison to boots of foreign origin; (v) peak pressure of 45,000 kg/cm2 for a 35 g HE mine is attenuated to less than 160 kg/cm2 , which is safe against amputation level injury; and (vi) the developed boots meet the requirements of GSQR 1059 for assault operations.

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Fig. 3.24 Shape of BAMI boots. Source DMSRDE

3.3.4 Materials for BAMI Ceramic honeycombs are routinely used to mitigate the blast overpressure and fragmentation from high explosive devices. Other ballistic and blast protection materials include textiles, FRPs, ceramic composites and metals (limited use in vehicle protection owing to high densities. The list of materials and their characteristics are given in Table 3.3, followed by important details. Important details concerning the BAMI materials: Table 3.3 Materials for BAMI and their features Component

Material

Feature

Upper

Chrome tanned leather

Heat resistant, free from ageing; strong enough to protect from blast debris

Heel side inserts

Ceramic honeycomb (cordierite: mullite)

Blast pressure attenuating and lightweight material; dissipates energy of blast by irreversible process of crushing; high compressive strength

FRP deflectors Deflect blast wave; dissipate shock by widening its angle; absorb shock energy by initiation of cracks

Ball side insert

Sole

Half-cut composite plate

Absorbs shock energy by cracking

Ceramic honeycomb

Blast pressure attenuating and lightweight material; dissipates energy of blast by irreversible process of crushing; high compressive strength

Woollen/ Kevlar felts

Weight-saving materials for dissipating pressure

Polyurethane

Flexible and wear-resistant, with adhesive properties to encapsulate the heel and ball side inserts

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(1) Honeycomb-structured materials have two specialities. These are (i) a porous structure with high air volume, since air is the best attenuator and mitigates blast waves/pressure; (ii) high stiffness with low weight, which is important for attenuation of overpressure. (2) The boot sole has a hybrid ceramic honeycomb and FRP deflector structure with multiple interfaces. The ceramic honeycomb materials crush to attenuate the blast wave overpressure. Multiple interfaces also attenuate the blast wave owing to mismatch of impedance. In more detail, when the blast wave impinges on one of the layers of hybrid structure it is partially (i) absorbed, (ii) transmitted, and (iii) reflected. The transmitted part goes to the next hybrid layer to be further absorbed/transmitted/reflected and so on. The reflected portions of the blast wave become attenuated by absorption in the previous layer(s).

3.3.5 Fabrication Process of BAMI Boots BAMI boots are manufactured according to the shape and design as per GSQR requirements: (i) all FRP deflectors are manufactured by hand lay-up; (ii) ceramic honeycomb inserts are made by an extrusion/sintering process; (iii) leather uppers are prepared by stitching; and (iv) moulding of the boots with leather uppers and hybrid composite/ceramic honeycomb inserts is done in an automatic injection-moulding machine.

3.3.6 Characteristic Features of BAMI Boots 3.3.6.1

Sole Stiffness and Elongation at Break

Because a high-pressure pulse (wave) is induced in an extremely short time, it is necessary for the material to have (i) sufficient rigidity to withstand the loading and (ii) sufficient capacity to cater for the deflection that is likely to be imposed. Such characteristics are rare in materials, since stiffer materials are generally less flexible, while flexible materials are less stiff. This means that, for the sole material, there are seemingly contradictory requirements of adequate stiffness and good tensile elongation. These requirements are met by polyurethane, which has been found to be the best sole material.

3.3.6.2

Capacity to Attenuate High Pressure

Sandwich structures of ceramic honeycomb inserts and FRP deflectors work as shockabsorbing materials. This configuration absorbs/deflects a pressure wave travelling

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towards the foot. As mentioned earlier, a blast pressure of 45,000 kg/cm2 is attenuated to below 160 kg/cm2 on the foot.

3.3.6.3

Heat Retention/Dissipation Capacity

BAMI boots have chrome-tanned leather uppers to retain heat in winter and dissipate it in summer, thereby protecting the user from adverse environmental conditions.

3.3.6.4

Stability and Comfort

BAMI boots for assault operations obviously give more protection than the MK-1 used for de-mining operations.

3.3.6.5

Lightweight

The weight of a pair of BAMI boots is about 3 kg. This is much less than the 5 kg weight of a pair of MK-1 boots.

3.3.7 BAMI Boot Testing and Characterization Various BAMIs with different weights have been tested against simulations of the blast of AP mines containing 35 g HE. In these tests the boots were placed above a pellet of chemical explosive (CE) buried below 2 cm of soil, see Fig. 3.25. Pressure sensors were located inside the boots just above the CE pellet position. A weight of about 35 kg was placed on the sensor/boot combinations. The sensor cables are connected to control room instrumentation. The mines were activated by electric detonation. The transmitted pressures through the boots were recorded by the sensors. Table 3.4 shows all the results. The highest recorded transmitted pressure was 95 kg/ cm2 , but it is clear that the boot weight is not the determining factor: lighter boots, especially the C versions gave much better attenuation of the blast pressure.

3.4 Technological Accomplishments The Indian Army has been unable to procure BPJs meeting the stringent GSQR 1438 requirements from external sources. However, the DMSRDE, Kanpur, has successfully designed, developed, manufactured and successfully qualified all three sizes of BPJs (small, medium and large) for GSQR 1438, including the conditioning

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Fig. 3.25 Schematic diagram of blast trial set-up for determination of transmitted pressure from a simulated AP mine blast

Table 3.4 Test results for transmitted pressure versus weights of various BAMI boots

Boot sample no Weight (kg) Transmitted pressure (kg/cm2 ) A-1412

1.402

70

A-1399

1.380

65

A-1410

1.403

60

A-1387

1.380

65

A-1374

1.368

60

A-1349

1.340

50

B-1361

1.355

85

B-1369

1.362

95

C-1356

1.348

45

C-1396

1.378

50

protocols for SAPs and HAPs against AK-47 bullets (mild and hard steel cores) and 7.62 SLR bullets. Ballistic helmets made from FRPs delaminate from the impact of high-energy projectiles like rifle bullets. This delamination causes unacceptable BFS/trauma even when bullets are arrested in the helmet shell. To overcome this, the DMSRDE has developed a ballistic helmet incorporating Kevlar FRPs. This helmet can arrest as many as 10 projectiles while keeping the BFS/trauma below an ‘acceptable’ level of 13 mm. N.B: Bullets stopped between the 4th and 5th layers of the helmet shell. Protection from AP mine blasts is extremely important for any soldier. Antimine boots are the most important personnel protective equipment (PPE) against

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anti-personnel mine blasts. The DMSRDE has made considerable R&D efforts to develop the anti-mine boots designated as BAMI: these are suitable for infantry assault operations when these are necessary. Among the essential requirements as per GSQR-1059 for these boots are (i) reduction in blast pressure from a 35 gm explosive charge to safe limits; (ii) facilitation of all assault operations; and (iii) protection of soldiers’ legs from the blast of Anti-Personnel Non-metallic Mines (APNM-M14).

3.5 Conclusions The BPJ, Ballistic Helmet and BAMI boots and their components like SAPs, HAPs, helmet harnesses, and ceramic inserts have been discussed in this chapter, together with fabrication technologies for these personnel protective systems. The weight of personal protection systems should be as low as possible without compromising the ballistic performance. Extensive research has been carried out worldwide for the last 15 years for using advanced high-strength fibres like Kevlar, Dyneema and Spectra for FRPs in ballistic protection systems. Dyneema HB 80/50, Kevlar KM2 and a hybrid of Dyneema HB 80/50 and Kevlar KM2 have been found to best meet the requirements of PPEs. However, there is contemporaneous research into further developments of armaments, ammunition and mines that are causing an increasing threat perception for BPJs, ballistic helmets and BAMI. Therefore, research into improving these protective systems must be continued. Acknowledgements The authors are grateful to Dr. VK Saraswat, Member (S&T), NITI Aayog, GoI, Dr. S Christopher, Dr. G Satheesh Reddy, Dr. Samir, V. Kamat, Dr. SC Sati Maj. Gen. SK Kulkarni and Lt. Gen. JK Sharma and several others at the Indian Infantry and DRDO Corporate CCs and DGs, as also Dr. Manjeet Singh, DS and Director, TBRL, Chandigarh and his colleagues for providing a very rare opportunity to these authors, who have been tasked to develop and qualify these vital PPSs for Indian Army. Special scrutiny of this book chapter contents by the Technical Committee of DMSRDE too is gratefully acknowledged.

References Aare, M., Kleiven, S.: Evaluation of head response to ballistic helmet impacts using finite element method. Int. J. Impact Eng. 34(3), 596–608 (2007). https://doi.org/10.1016/j.ijimpeng.2005. 08.001 Baker, W.E.: Explosions in the Air. University of Texas Pr., Ausint (1973) Ballistic Resistance of Body Armour.: NIJ Standard-0101.06 (2008) Cannon, L.B.: Behind Armour Blunt Trauma—An emerging problem. J. R. Army Med. Corps 147(1), 87–96 (2001). https://doi.org/10.1136/Jramc-147-01-09 Cavallaro, P.V.: Ranges, Soft Body Armour: an overview of materials, manufacturing, testing, and ballistic impact dynamics. Engineering, and Analysis Department (2011)

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Chang, F.K., Chang, K.Y.: A progressive damage model for laminated composites containing stress concentrations. J. Compos. Mater. 21(9), 834–855 (1987) Cheng, W., Itoh, S., Kuo, A.Y.: Calculation of dynamic strain energy release rates due to shock wave loading. Mater. Sci. Forum 465–466, 415–420 (2004). https://doi.org/10.4028/www.Sci entific.net/MSF465-466.415 Decker, M.J., Egres, R.G., Watzel, E.D., Wagner, N.J.: Low velocity ballistic properties of shear thickening fluid (STF)—Fabric composites. In: Proceedings of 22nd International Symposium on Ballistics, pp. 14–18. Van Couver BC (2005) Fisher, J.: Validation of a Simple Go/No-Go Damage Detection System for Personal Ceramic Body Armour Using Pressure Sensitive Film. Iowa State University, Ames (2011) Freitas, C.J., Mathis, J.T., Scott, N., Bigger, R.P., Mackiewicz, J.: Dynamic response due to behind helmet blunt trauma measured with human head surrogate. Int. J. Med. Sci. 11(5), 409–425 (2014). https://doi.org/10.7150/ijms.8079 http://www.nij.gov/topics/technology/body-armour/pages/impacteffectiveness.aspx Joselin, R., Wilson, W.J.: Investigation on impact strength properties of kevlar fabric using different shear thickening fluid composition. Defence Sci. J. 64(3), 236–243 (2014). https://doi.org/10. 14429/dsj.64.7322 Kulkarni, S.G., Gao, X.L., Horner, S.E., Zheng, J.Q., David, N.V.: Ballistic helmets: their design, materials and performance against traumatic brain injury. J. Compos. Struct. 101, 313–331 (2013). ISSN 0263-8223 Li, Y.Q., Li, X.G., Gao, X.L.: Modeling of advanced combat helmet under ballistic impact. J. Appl. Mech. 82(11), 9 (2015). https://doi.org/10.1115/1.4031095 Miller, D.J.: Design and Analysis of an Innovative Semi-Flexible Hybrid Personal-Body-Armour System. Graduate Theses and Dissertations. University of South Florida (2011). http://schola rcommons.usf.edu/etd/3247 Park, J.L., Yoon, B.I., Paik, J.G., Kang, T.J.: Ballistic Performance of p-aramid fabrics impregnated with shear thickening fluid, Part-I effect of laminating sequence. Text. Res. J. 82(6), 527–541 (2012). https://doi.org/10.1177/0040517511420753 Regassa, Y., Likeleh, G., Uppala, R.: Modeling and simulation of bullet resistant composite body armour. Int. J. Res. Stud. Sci. Eng. Technol. [IJRSSET] 1(3), 39–44 (2014). ISSN 2349-4751 (Print) & ISSN 2349-476X (Online) ©IJRSSET 39 Russell, B.P., Kandan, K., Deshpande, V.S., Fleck, N.A.: The high strain rate response of UHMWPE: from fibre to laminate. Department of Engineering, Cambridge University, Trumpington Street, Cambridge, CB2 1PZ, UK (2013) Sheikh, A.H., Bull, P.H., Keplar, J.A.: Behaviour of multiple composite plates subjected to ballistic impact. J. Compos. Sci. Technol. (Elsevier) 69(6), 704 (2009). https://doi.org/10.10.16/J.Com pscitech.2008.03.022 St. Asenov, L.L., Toncheva, K.: Promising ceramic materials for ballistic protection. J. Chem. Technol. Metall. 48(2), 190–195 (2013) The BJA/PERF Body Armour Survey (2009) Protecting Nation’s Law Enforcement Officers. Phase II Final Report to BJA, USA Torki, A.M.: Dynamic mechanical properties of hybrid nanocomposite materials. Doctoral Dissertation University of Belgrade, Faculty of Technology and Metallurgy, Belgrade (2012) Wallace, D.: Combat helmets and blast traumatic brain injury. J. Military Veterans Health (Austral. Military Med. Assoc.) 20(1) (2012). e-ISSN No.: 1839-2733

Chapter 4

Polymeric Materials for Defence Stores in Extreme Cold Weather J. N. Srivastava, Vidya S. Pandey, and Eswara Prasad Namburi

Abstract Armed forces’ general stores fulfil the requirements of troops in both peace and war situations with respect to the soldiers’ operational ease, food, shelter and climatic challenges. Different terrains in nation have a vast range of temperature, humidity, wind and geographical conditions that offer challenges in designing the stores. Selection of the necessary functional materials needs much attention since these materials should be able to meet not only the functional requirements but also have cost-effectiveness and longevity. Most of the stores currently being developed are polymers or polymeric composite materials because they are lightweight with good strength (there are exceptions, where metals are indispensable). In addition, extreme cold weather (ECW) offers challenges to thermoplastic polymers owing to their ductile-to-brittle transition at sub-zero temperatures and consequent loss in toughness and impact strength. This problem can however be overcome by modifying the polymers with special additives. Different types of polymers commonly used in the development of the stores are discussed with respect to their source, processing methods and structure–property relationships. Stringent trial standards that are necessary for defence requirements, product design and testing are also highlighted.

J. N. Srivastava · V. S. Pandey Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India E. P. Namburi (B) Former Outstanding Scientist and Ex-Director, Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 E. P. Namburi et al. (eds.), Novel Defence Functional and Engineering Materials (NDFEM) Volume 2, Indian Institute of Metals Series, https://doi.org/10.1007/978-981-99-9795-4_4

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4.1 Introduction: High- and Low-Temperature Effects Armed forces’ stores have specific qualitative requirements which differ from those of common utility stores. Specifications of materials for defence store items are more stringent and product-specific owing to their use in harsh service and climatic conditions. Also, these materials need to be suppliable in bulk; cost-effective; and lightweight and durable, especially if they are to be used for field operations by military personnel. Most of the stores used during such operations are prone to accidental falls (from a certain height) and hence require a certain degree of impact strength in all climatic conditions. These stores are also subjected to different temperatures depending on their usage as well as terrain conditions. Typical applications are water bottles, food containers, boots, gloves, snow goggles, ballistic goggles, tents and shelters. The temperature range to which defence stores are generally exposed is between −50 and +50 °C, owing to the vast range of climatic conditions in different terrains. Items like water bottles and food containers may be exposed to boiling water temperature (100 °C at low altitude) in order to sterilise them and also to store hot foodstuffs. The strength of thermoplastic polymers above normal ambient temperatures is therefore important. For amorphous polymers, the glass transition temperature (T g ) determines the upper temperature limit (about 5–10 °C less than the T g ) at which the material retains its mechanical strength. This temperature limit is called the heat deflection temperature (HDT), see Fig. 4.1. On the other hand, for low-temperature applications, the service temperatures should be above the ductile-to-brittle transition temperature of the polymers in order

Fig. 4.1 Thermal transitions of a polymer: Tm is the melting temperature

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to retain toughness and impact strength. Under extreme cold weather (ECW) conditions, the temperature may drop to −50 °C or below in snowbound and glacial regions. Many polymers have good toughness and strength at room temperature but not at such low temperatures; so selection of the proper polymeric materials and their desired modifications to cope with ECW becomes important. The effect of low temperature on mechanical behaviour and service life largely depends upon (i) the glass transition temperature, T g , (ii) the characteristic ratio, C ∞ , which is defined as the actual end-to-end expansion of a polymer chain compared to the theoretical chain length, and (iii) the crystalline phase melting temperature (T m ) (Jang et al. 1984). In practice, the T g occurs over a range of temperatures. This region is a critical factor in the design of polymeric stores. Also, as may be inferred from Fig. 4.1, the effects of increasing temperature on the mechanical properties (in this case the modulus) have to be considered, albeit these effects are less drastic. Therefore, in the present chapter, the emphasis is on the low-temperature behaviour of polymers suitable for defence stores.

4.2 Low-Temperature Behaviour of Polymers Extensive studies have been carried out by many researchers on the structure– property relationships of thermoplastic polymers (Vincent 1960; Legrand 1969; Norman Brown and Ward 1983) with respect to changes in temperature. Some key factors responsible for changes in the mechanical behaviour of these polymers are discussed in the following subsections.

4.2.1 The Ductile-to-Brittle Transition Vincent (1960) has done seminal work on the study of ductile and brittle failures of thermoplastic polymers. His observations point to the fact that the chances of brittle failure of polymers can be reduced by the following factors: • • • • • •

Increasing the molecular weight; Reducing the crystallinity; Adding plasticisers; Blending rubbery polymers; Reducing cross-linking; Removing residual strain.

The addition of plasticisers, blending of rubbery polymers and reduction of crosslinking reduce the yield strength. Also, the moduli of low molecular weight polymers decrease faster with increasing temperature. This latter effect is illustrated schematically in Fig. 4.2, as part of a general diagram concerning the viscoelastic behaviour of polymers with respect to temperature.

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Fig. 4.2 Viscoelastic behaviour of polymers with respect to temperature

Polymeric materials generally undergo appreciable plastic deformation (yielding and ductility) before breaking. A polymer’s brittle failure occurs below the ductileto-brittle transition temperature, T g , whereas above this temperature yielding takes place. The value of T g is mainly dictated by the morphology and chain structure of the polymer, while the ductile-to-brittle behaviour is controlled by the characteristic ratio (C ∞ ) (Flory and Fisk 1966) and entanglement density (ν). The intrinsic ductility increases as C ∞ decreases. Polymers attain their maximum intrinsic ductility when C ∞ reaches its lowest value: for example, C ∞ = 2 for tetrahedral skeletal bonds in a freely rotating chain. Two commonly known polymers with very low characteristic ratios are polycarbonate (C ∞ = 2.4) and polysulphone (C ∞ = 2.2) which are considered as the toughest polymers. Entanglement is understood as physical cross-links and increases toughness by increasing the resistance to (i) void formation and growth and (ii) crack propagation. High entanglement and a low characteristic ratio give high toughness. On the other hand, polymers having low entanglement combined with a large characteristic ratio tend to be brittle. In summary, the toughness of a polymer depends on many factors, which can be broadly categorised as intrinsic and extrinsic factors (Griffith 1921). The main intrinsic factors are molecular structure, cohesive energy, average molecular weight, molecular weight distribution and morphology or degree of crystallinity. The extrinsic factors related to toughness are strain rate, stress state, temperature, specimen geometry and notch or crack size and shape.

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4.2.2 Glass Transition Temperature (Tg ) Tg is one of the most important thermophysical properties of amorphous polymers. An appropriate description (but not definition) for the glass transition temperature could be as the melting point of amorphous polymeric materials. However, melting is observed at a sharp and fixed temperature, whereas the glass transition is mostly in a temperature range. This difference is because melting is a true first-order phase transition, whereas the glass transition is a pseudo-second-order transition (Brittleductile transition temperature 2017). Many theories have been proposed to explain the glass transition in polymers, and these are mainly divided into two categories: kinetic and equilibrium theories: (1) In the kinetic theory, glass transition is considered as a dynamic process. Glassification is achieved when chain segment movements which are kinetic units get ‘frozen’. When heated from a very low-temperature polymers are much below the brittle temperature, and the first solid-state transition occurs when localised bond motions take place as shown in Fig. 4.3, i.e. stretching and bending of bonds begin and vibration of side chains may start. (2) Gibbs and Di Marzio proposed and developed the first equilibrium theory of the glass transition in which they estimated the changes in entropy with increasing temperature and extended the postulation that when a thermodynamic secondorder transition is reached the conformational entropy becomes zero (Gibbs and Dimarzio 1958). All conformations are essentially ‘frozen’ below this temperature.

Fig. 4.3 Different types of bond movements in a polymer chain (Krevelen and Nijenhuis 2009)

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It is generally accepted that polymeric materials below their glass transition are brittle. However, this definition is too broad and in practice, there are many exceptions. Thermal transitions or relaxations in polymers commonly termed as T α , T β and T γ can help in understanding the structure–property relationships in polymers with respect to temperature. The T γ transition refers to stretching and bending movements of localised bonds and side chains, and these are the first movements after increasing the temperature and free volume of the polymer from its frozen state. T β is reached on further increasing the temperature and free volume, whereby movement of whole side chains and 4–8 backbone atoms starts. T α is reached at a temperature when large-scale movement of large chain segments starts. Further study on polymer structure–property relationships has shown that: (1) For polymers such as polystyrene or polyphenyl methacrylate, which have large bulky side groups, only the large-scale movement occurs. In this case, the transition from brittle-to-ductile behaviour coincides with the α-transition: i.e. T α = T g. (2) For the majority of polymers the brittle-to-ductile transition occurs at a much lower temperature (T b ), than the glass transition temperature, i.e. T b < T g . However, for most amorphous polymers glassification coincides with the βtransition, i.e. T b = T β = T g . (3) Similarly, for very tough polymers such as polycarbonate and polysulphone, and many semi-crystalline polymers, the brittle-to-ductile transition temperature corresponds with the γ-transition, and therefore T b = T γ . Researchers have proposed a number of relationships to relate the β-transition temperature to the glass transition temperature. The brittle-to-ductile transition temperature is a function of the intrinsic chain stiffness or flexibility as found by Wu (1992) and he proposed the following relationship: {

} Tβ /{Tα } = Tb /Tg = 0.135 + 0.082C∞

(4.1)

which is applicable only to polymers with T b < T g , or for polymers without bulky side groups (i.e. polymers with C ∞ ≤ 10.5). With the T b and T g data from several sources, for both amorphous and semi-crystalline polymers a similar relationship has also been established (Brittle-ductile transition temperature 2017): Tb /Tg = 0.208 + 0.069C∞

(4.2)

The ratio T b /T g as a function of the chain stiffness (C ∞ ) is shown in Fig. 4.4. The first secondary transition below T g , the so-called β-relaxation, is of practical importance. This became evident after Struik’s (1978) finding that polymers are brittle below T b and undergo creep and ductile fracture between T b and T g . The β-relaxation is characteristic of each individual polymer since it is connected with the start of free movements of special short sections of the polymer chain. Some empirical relationships may be given for a closer approximation of T b (Krevelen and Nijenhuis 2009), as follows:

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Fig. 4.4 Relation between characteristic ratio and brittle (T b ) and glass transition (T g ) temperatures (Brittle-ductile transition temperature 2017)

For semi-crystalline polymers, the following relation was proposed: Tb = 0.8Tg − 40 = 0.5Tm −25

(4.3)

For non-crystallisable glassy polymers, another empirical equation was proposed: Tb + Tg = 635 K

(4.4)

Several authors have proposed correlations between the chemical structure and the glass transition temperature of polymers. It was assumed that the structural groups in the repeating unit provide weighted additive contributions to the T g . Correlations for the T g in the general form are given below (Strobl 2007): Tg

∑

si =

∑

si Tgi

(4.5)

where si is a weight factor attributed to a given structural group and T gi is the specific contribution to T g of that structural group. Different assumptions for si were proposed in the literature. Barton and Lee (1968) suggested si to be equal to the weight or mole fraction of the relevant group in relation to the structural unit. Weyland et al. (1970) put si equal to Z i , the number of backbone atoms of the contributing group.

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4.2.3 Characteristic Ratio and Freely Jointed Chain Kuhn and Kuhn (1948) proposed that a polymer chain has N K links of length lK without any restrictions on the successive bond angles. A linear chain of polymer was modelled as a random coil in space having freely jointed chain units. The rootmean-square distance Rrms between the ends is given by: Rrms = NK 1/2 lK . Also, the 2 = 6R 2g . gyration radius, Rg = (N K /6)1/2 lK . These relationships give Rrms The radius of gyration, Rg , is estimated as the average value of the first moment of all segments of the chain with respect to the centre of mass of the molecule. For a straight rod-like polymer with a fully extended chain, the so-called Kuhn length 2 /Rmax and is related to the stiffness of a polymer chain. is Rmax = N K l K = Rrms A polyethylene chain, assuming it to be a freely jointed chain, has a Kuhn length of approximately 1.54 Å. In the real polymer chain, there are two main effects that restrict the rotation of bonds around the chain backbone. The first restriction is the excluded volume effect, which is defined as the exclusion of space for a polymer segment, i.e. two polymer segments cannot occupy the same position in space. The second restriction is the hindering of internal rotation of the backbone, which also has an influence on expansion of the chain. Both restrictions are taken into consideration to quantify the chain expansion, and a characteristic ratio C ∞ (N ν → ∞) as a measure of the expansion is thus introduced. This gives an idea about the real polymer chain end-to-end distance compared to an ideal chain with bond length lν (Kuhn 1934): ) ( C∞ = R02 / Nν lν2

(4.6)

In Eq. (4.6), R0 is the root-mean-square end-to-end distance of a coiled polymer chain, N ν is the number of units in the chain backbone and lν is the root-mean-square length of a unit.

4.2.4 Couchman–Karasz (C–K) Equation The Couchman–Karasz (C–K) equation is often used for predicting the glass transition temperatures of random copolymers and amorphous mixtures of polymers (Couchman and Karasz 1978). The Eq. (4.7) is based on the entropy (S) continuity at the T g : Sglass (T , p) = Sliq (T, p)

(4.7)

In the case of blends and copolymers, the specific entropy of the mixture at T g is calculated as a weight-fraction-weighted sum of pure-component entropies, and the change of entropy due to mixing,

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( ) ∑[ ( )] ( ) ωi · Si Tg,mix + ΔSmixing Tg,mix Stot Tg,mix =

(4.8)

i

where the glass transition temperature of the mixture is T g,mix in the glass or liquid rubber state; the mass fraction is ωi of component i; and the change of entropy due to mixing of the polymer components is ΔS mix . For heating the individual components and the mixture, the corresponding changes in specific entropy are ( ) ( ) ] [ Si,glass Tg,mix = Si,glass Tg,i + Cpi,glass ln Tg,mix /Tg,i

(4.9)

( ) ( ) ] [ Si,liq Tg,mix = Si,liq Tg,i + Cpi,liq ln Tg,mix /Tg,i

(4.10)

where ΔC pi is the change of heat capacity when the transition from glass to the rubber (liquid) state takes place. This is the C–K equation that is applied to miscible multicomponent mixtures and correlates the dependence of the glass transition temperature on composition via entropy continuity. For a two-component mixture, Eqs. (4.9) and (4.10) can be rearranged as ) ( ) ( InTgm = ω1 ΔC p1 InTg1 + ω2 ΔC p2 InTg2 / ω1 ΔC p1 + ω2 ΔC p2

(4.11)

In Eq. (4.11), components 1 and 2 are denoted by the subscripts 1 and 2. The other symbols are as denoted earlier in Eqs. (4.9) and (4.10). The C–K equation is found to be valid for estimating the glass transition temperature of many random copolymers. However, this equation should not be applied to polymer blends and random copolymers with significant specific intermolecular interactions, since then the equation may lead to large deviations from the true T g of the mixture. An interesting example is a blend of polystyrene (PS) and poly (2,6-dimethylp-phenylene oxide) (PPO), which have similar structural groups and no specific interactions (Gaur and Wunderlich 1981). The C–K predicted values for such a blend are in excellent agreement with the experimental values as shown in Table 4.1. Table 4.1 Experimental and predicted values of glass transition temperatures of PS–PPO blends (Gaur and Wunderlich 1981)

Glass Transition Temperature of PS–PPO Blends Weight fraction PS

Experimental T g (K)

Predicted Tga (K)

0.0

489

489

0.2

461

459.6

0.4

434

434.8

0.5

425

423.3

0.6

413

412.3

0.8

396

391.7

1.0

379

379

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The glass transition temperatures have been calculated with the software 3PsT g from Triton Road, and calculated with experimental discontinuous increment in specific grades of PS: 294.7 J/g K, PPO:265.5 J/g K.

4.3 Characterisation and Testing of Polymers for Product Development Polymeric materials can be characterised by numerous methods depending on the properties and behaviour of the material to be investigated. The Defence store materials discussed in this chapter require a few important tests that validate their applicability in extremely cold weather. The visco-elastic behaviour of polymers (among many other parameters) depends mainly on external factors such as stress, temperature and time. Some common mechanical and visco-elastic behaviour studies are briefly mentioned here: • Creep: Maintain a fixed load and monitor the change in length with respect to time. • Stress Relaxation: Extend the polymer to a fixed length and then monitor the force required to keep a constant length with respect to time. • Stress–Strain: Sample length is increased at a fixed rate and the change in load is monitored with respect to sample length. • Dynamic Mechanical Analysis: The polymer is extended to a certain amount and then oscillates. Monitor the load with respect to deformation at a given temperature. Some frequently used test methods are described in Sects. 4.3.1–4.3.4.

4.3.1 Impact Testing Polymeric materials for structural applications sometimes undergo impact loads or rapid stress loading. There are some test methods that can assess the ability to withstand such loads. The most common methods are the Charpy and Izod impact tests: the Charpy test is generally used for metals, whereas the Izod test is very popular for polymeric materials (Joseph Gordon 2003). The Izod pendulum test of impact toughness is described in ASTM Standard D 256. The average of four or five readings is taken as the impact strength of the material. Comparisons of materials can be done by using a standard notched-beam test specimen, since the total impact energy depends on notch shape, length and also size of the specimen (Joseph Gordon 2003). A test specimen clamped at both ends of the beam is struck at the centre by a pendulum weight, see Fig. 4.5, and the energy required to break the specimen is measured. The loss in energy of the pendulum

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Fig. 4.5 Izod pendulum impact tester (ASTM 2010)

weight before and after the impact equates to the energy (E F ) required to break the specimen: E F = m g (h S − h E )

(4.12)

where hS is the height of the pendulum hammer at the starting position; hE is the height at the end position; m is the mass of the pendulum hammer; and g is the gravitational acceleration. The energy absorbed by the specimen can be reported either as the value of the energy per unit thickness at the notch in the specimen (J/m) or the energy per unit area (J/m2 ) at the notch of the specimen. The standard ASTM test specimen has the dimensions 63.5 × 12.7 × 3.2 (in mm). The fracture of a polymeric material sample in an impact test has two main components of the total absorbed energy: (i) the energy for plastic deformation at the tip of the notch and (ii) the energy required to overcome the molecular cohesive energy before fracture. Polymeric materials above the brittle transition temperature (T b ), generally undergo appreciable plastic deformation before fracture takes place. Under such ductile failures, the polymers absorb high-impact energy much more than the energy required for fracture. The Izod test is rapid and so can also be performed on accurately cooled specimens at sub-zero temperatures. At lower temperatures the mobility of polymer chains decreases, and brittle fracture occurs below the ductile–brittle transition temperature. There is a sharp decrease in impact energy when the polymer passes through the ductile–brittle transition zone (see Fig. 4.6) and eventually attains the lowest value at a temperature below the brittle transition temperature (T b ).

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Fig. 4.6 Dependence of impact energy of polymers on temperature

Although the Izod test is apparently simple, the impact energy depends on a few intrinsic and extrinsic factors which should be taken into consideration while selecting a material. Some important intrinsic factors responsible for a polymer’s toughness are molecular weight, molecular weight distribution, morphology, crystallinity, cohesive energy, etc. Extrinsic factors mainly depend on the design of the test method and ambient conditions such as size and shape of specimen and notch, weight and shape of the pendulum hammer, impact speed and temperature.

4.3.2 Dynamical Mechanical Analysis (DMA) DMA is used to measure the glass transition temperature and modulus for studying the viscoelastic behaviour of polymers. In this method oscillatory deformations are applied to the material and the mechanical response is measured at different temperatures. The data enable the solidus and liquidus phase behaviour of polymers to be modelled mechanically with combinations of springs and dashpots (Ferry 1980). The main principle of DMA is that a sample subjected to a sinusoidal oscillating stress responds with a similar frequency of sinusoidal oscillation provided the material remains within its elastic limits. In the case of a viscous material, the response will be an out-of-phase strain wave. The phase difference between the applied and measured response frequency varies between the limits of 90° for a Newtonian liquid and 0° that for a perfectly elastic solid. The measured phase angle for a viscoelastic material will therefore be between 0° and 90° (Ferry 1991).

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4.3.3 Melt Flow Index (MFI) Test Melt Flow Index (MFI) is a measure of the degree of ease of melt flow and is a test to investigate the melt rheology of thermoplastics. For polymer processing, the values of MFI correlate with the polymer grades that should be chosen for different processes. The units are in g/10 min as per Melt Flow Index-ASTM D1238, ISO 1133 (ASTM 2010). The main information obtained from this test is (Shenoy and Saini 1996): • Selection of suitable materials for a particular processing technique. • Indirect evidence of some changes, e.g. chain scission, in the molecular structure of polymer chains.

4.3.4 Tensile Test: Standard Testing Methods Structural applications require well-determined tensile properties of the materials, including tensile elongation to fracture and fracture strength. An example of tensile testing under increasing stress (force measurement) is shown in Fig. 4.7. The displacement rate influences the test result for polymeric materials and is thus kept in the range of 0.2–20 inches per minute (Odian 1991). In the ISO system, the analogous test that measures tensile properties is ISO 527, whereas the ASTM has the Standard ASTM D638 (ASTM D638-14). Both tests are reliable, and their results do not vary significantly, such that both are extensively used in the process of material selection.

Fig. 4.7 Tensile test instrument and test specimen (Odian 1991)

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Some other test methods specific to polymer testing are as follows: • ASTM D638, ISO 527 (tensile test of plastics); • ASTM D412, JIS K-6301 (tensile test of vulcanised rubbers); • ASTM D 882-95A (tensile test of thin plastic sheet).

4.4 Polymeric Materials for Defence Stores Polymeric materials for the development of various Defence stores are selected according to their application, qualitative requirements and service conditions. Some frequently used materials are described in Sects. 4.4.1–4.4.7.

4.4.1 Polycarbonates (PCs) These are a group of thermoplastic polymers with carbonate groups in their structures. Polycarbonates are used in engineering as well as in commodity items. They are strong, tough and optically transparent and find many applications, since they can be easily worked, moulded and thermoformed. PCs have high impact resistance but the scratch resistance is not so good: therefore silicon-modified PCs with improved scratch resistance are used in some applications. A hard silicon-based coating is mostly applied for optical applications of polycarbonates. Optical properties of some grades of PC are comparable to those of Poly Methyl Methacrylate (PMMA), but polycarbonates are much stronger and tougher, and have a broader range of service temperature in both high and low extremes. The Tg of PCs is about 147 °C (Yannas and Lunn 1971). They soften gradually above this temperature and develop some flow above 155 °C. For processing, the mould assembly, inlet, outlet, etc., must be kept at high temperatures, generally above 80 °C to make strain- and stress-free products. Low molecular weight grades are easier to mould than higher grades, but their strength is correspondingly lower. The tougher grades of PC are difficult to process because the higher molecular weight leads to high melt viscosity. PCs can be processed and formed at room temperature using sheet metal techniques because these enable large plastic deformations without breaking or even cracking: heating may not be necessary even for small radius sharpangle bends. This makes PCs useful in prototyping applications where electrically non-conductive or transparent parts are needed. A special grade of silicon-modified polycarbonate is used for bulk manufacturing of flex water bottles for the armed forces. Food-grade polycarbonate is the most suited for water bottles owing to its unique combination of the following properties: • Excellent impact strength among polymeric materials. • Widest available service temperature range, −40 to +100 °C. • 100% UV-resistant so there is good service life.

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• Excellent dimensional stability. • Good optical clarity and excellent transparency for visual observation of turbidity in natural source water. • Excellent low-temperature ductility. • Easy to manufacture. • High strength-to-weight ratio. • Low thermal conductivity. The material grade PC LEXAN EXL series (http://www.sabic.com) is selected for water bottles because LEXAN polycarbonate (PC) EXL is a siloxane copolymer resin and an injection moulding (IM) grade with FDA and EU food contact compliance. This resin offers extreme low temperature (−40 °C) ductility in combination with medium flow characteristics and excellent processability, which provides shorter IM cycle times compared to normal grades of PC. Some typical properties are MFI 10– 11 g/10 min (ASTM D1238); heat deflection temperature (HDT) 120 °C (ASTM D648); tensile yield strength 56–58 MPa (ASTM D638); and Izod impact strength 824 J/m(23 °C) and 712 J/m (−30 °C). N.B: LEXAN EXL resin is available in transparent and opaque colours and is an excellent candidate for a broad range of food contact applications (http://www.fda. gov/Food).

4.4.1.1

Processing

PCs can be processed in various shapes by injection moulding and blow moulding techniques. For hollow shapes, conventional extrusion blow moulding or injection stretch blow moulding can be used. More specifically, for the selection and design of a manufacturing process, the unique properties of PCs must be understood. Some properties and process parameters playing important roles are: Properties: • Thermal diffusivity: Polycarbonates have high thermal diffusivity (Zhang et al. 2002), which means that there is a tendency to lose heat relatively faster from the melt to the mould, barrel and nozzle while processing than most other plastics. This faster heat transfer can lead to ‘delamination’ during moulding. The thermal diffusivity of the chosen polycarbonate grade is very important for the final moulding process. Also, it may be difficult to control the polycarbonate temperature: accurate temperature control is essential. • Viscosity: Because the melt viscosity of polycarbonate resins is relatively high it is advisable to keep the applied pressure constant and use a high initial screw velocity (see under process parameters). These values may, however, be less than those required for processing materials like polypropylene. • Drying characteristics: Polycarbonates tend to rapidly adsorb moisture from the ambient environment during transport to the process area and also while kept waiting before moulding. Hence, their drying before processing is very important.

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Undried PCs not only create difficulty in processing, causing splay, but also result in reduced dimensional accuracy of the product. PC resins must be dried with a desiccant to less than 0.02% moisture for optimal performance of the products. Process parameters: • Screw design: Polycarbonate product moulding is difficult on a machine with a screw designed for processing a general-purpose olefin. This is because these screws give too-rapid compression. Screws with moderate feed lengths of about 7 turns and long compression sections of about 8–10 turns may be more suited for PC processing (www.plasticsmachining.com). Special-grade PC water bottles developed for the armed forces require very specific processing parameters: screw L/D = 27; and length of feed, compression and melting sections in the combined ratios of 5:3:2, respectively. It has also been found that either preheating the material to 120 °C or keeping the temperature in the feeding section at 170 °C greatly reduces wear of the processing equipment. • Smooth contact surfaces: The screw should be chromium-plated to create a smooth wear-resistant surface and minimise contact of the resin with the screw base metal since PCs tend to adhere to iron alloys and rough (pitted) metal surfaces owing to wear degradation. Other areas, including all internal surfaces, endcaps and nozzle internal flow paths should also be chromium-plated. • Machine purging: Thorough purging of the moulding machine is required after manufacturing polycarbonate products. Without proper purging, remanent polycarbonate materials may bond adjacent steel parts. This can even lead to valve breakage when the screw is turned, owing to the valve adhering to the endcap. Also, if the screw is not given sufficient time to warm up before it is turned, the glue-like melted material can sometimes chip off some of the plating from the screw. N.B: More details on processes and process parameters, including equipment illustrations, are given later in this chapter, in Sect. 4.5.3.

4.4.2 Polyurethanes (PUs) The polyurethane structure consists of carbamate (urethane) links between organic units. Polyurethanes are generally formed by reacting a di- or poly-isocyanate with a polyol, and both thermosetting and thermoplastic forms can be synthesised. Nonisocyanate-based polyurethanes were also recently developed because of concerns about isocyanate health hazards and toxicity (Phillips and Parker 1964). Polyurethane foams have many applications like microcellular foam seals and gaskets, high-resilience foam seatings, and rigid foam insulation panels. Elastomeric wheels and tyres for applications such as skateboard wheels, roller coasters, escalators, shopping carts and elevators are also commonly found. They are also extensively used as surface coatings, high-performance adhesives and surface sealants.

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109

Thermoplastic Polyurethane Rubbers

The reactions of polyols, diisocyanates, and glycols produce block copolymers in which hard blocks with glass transition temperatures well above normal ambient temperature are separated by soft rubbery blocks. In a typical process of manufacturing thermoplastic polyurethane elastomers, a prepolymer is first produced by reacting a polyol, such as a linear polyester with terminal hydroxyl groups or a hydroxyl-terminated polyether, and having molecular weights in the range of 800– 2500, with an excess of diisocyanate (usually of the MDI type) to give a mixture of isocyanate-terminated polyol prepolymer and unreacted diisocyanate. This mixture is then reacted with a chain extender such as 1,4-butanediol to give a polymer with long polyurethane segments whose block lengths depend on the extent of excess isocyanate and the corresponding stoichiometric glycol. The overall reaction is shown in Fig. 4.8 (Dunnols 1979). Specifically, the polyurethane segments show high intersegment attraction, such as hydrogen bonding, and may be able to crystallise and form hard segments. In such polymers, hard segments with Tg well above normal ambient temperature are separated by polyol soft segments, which in the mass are rubbery in nature. Hard and soft segments are arranged alternately along the polymer chain.

Fig. 4.8 Polyurethane chain configuration (Dunnols 1979)

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Polyurethane Properties and Advantages for Product Development

Polyurethanes are much used in the development of Defence stores like snow goggle and all-terrain goggle frames, food container insulation and gaskets. Polyurethanes perform well for troop requirements and are equally good in desert summer and extremely cold conditions (down to −50 °C in glaciated regions). The following list specifies a number of properties favouring the use of polyurethanes (Oertal 1985): • High load bearing capacity: Specific grades of polyurethanes have high load capacity in both compression and tension. Under a heavy load, polyurethanes will change shape and then recover almost completely when the load is removed. • Hardness: Polyurethanes can be manufactured with hardnesses ranging from 20 SHORE A to 85 SHORE D by manipulating the prepolymer’s molecular structure. • Abrasion and impact resistance: High wear resistance at low temperatures. • Tear resistance and flexibility: High tear resistance and very good performance under high flex fatigue. • Oil, grease and water resistance: Polyurethane properties remain stable with very low swelling in water, oil or grease. They last for many years in seawater applications. • Harsh environments: Polyurethanes are highly resistant to temperature extremes and harsh environmental conditions. Many grades do not undergo material degradation. • Wide range of resilience: A resilience range of 0–40% is required for shockabsorbing elastomer applications, and 40–65% resilience for high-frequency vibration applications, where quick recovery is needed. Also, the toughness of polyurethanes is generally enhanced by high resilience. • Strong bonding properties: Polyurethanes can be easily bonded to a range of materials, including plastics, metals, and wood during the manufacturing process. This property makes polyurethanes very useful for wheels, rollers and inserts.

4.4.3 Ethylene Propylene Diene Monomer (EPDM) Rubbers EPDM rubbers are closely related to a copolymer of ethylene and propylene, i.e. ethylene propylene rubber, and are a terpolymer of ethylene, propylene and a diene component (Ravishankar 2012). EPDMs are outstandingly resistant to weather, ozone and heat. They also have good resistance to ketones, ordinary diluted acids and alkalis. Polymerisation of ethylene and propylene is done to produce EPDM rubbers in which about 3–8% of a diene monomer is added for fast vulcanisation with sulphur as it provides the desired cross-linking sites (Green and Wittcoff 2003). N.B: EPDM rubbers differ significantly from the diene hydrocarbon rubbers in that their level of unsaturation is much lower. This makes them more heat, oxygen and ozone-resistant.

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EPDM rubbers have good ozone, oxygen and weather resistance and are also heat-resistant at moderate temperatures. Their physical and chemical properties make them to be extensively used in automobile industry for different components like car bumpers, heater hoses, body and chassis parts, etc. Other applications include wire and cable insulation, appliance parts, hoses, gaskets and seals, and coated fabrics. For defence stores, EPDMs are used in gaskets, frames and sealing rings of equipment used in cold weather conditions.

4.4.4 Linear Low-Density Polyethylene (LLDPE) LLDPE is a substantially linear polyethylene with a significant number of short branches of pendant alkyl groups. The linearity contributes towards strength, whereas the branches impart toughness. LLDPE is commonly made by copolymerisation of ethylene with longer-chain higher alpha-olefins such as butene, hexene or octene. The absence of long-chain branching distinguishes LLDPE structurally from conventional low-density polyethylene (LDPE), and it is made at lower temperatures and pressures. The copolymerisation process produces an LLDPE polymer that has a narrower molecular weight distribution and significantly different rheological properties than LDPE, owing to its more linear structure (Market Study: Polyethylene LLDPE 2014). Also, LLDPE is less shear-sensitive because of its shorter chain branching and narrower molecular weight distribution. This means that LLDPE remains more viscous during a shearing process such as extrusion, and it is more difficult to process as compared to LDPE of equivalent melt flow index. On the other hand, LLDPE does not strain harden as much as LDPE when deformed, since it has lower viscosity at all strain rates. LLDPE’s rheological properties may be summarised as ‘stiff in shear’ and ‘soft in tension’. LLDPE is now replacing the conventional LDPE in many applications because of the combination of product performance characteristics and favourable production economics. For many products, LLDPE processing can use existing equipment for processing LDPE (blow moulding, injection moulding, rotational moulding, etc.) Also, LLDPE films are increasingly used in food packaging as ice bags and retail merchandise bags, and as industrial liners and garment bags. Good flex properties and environmental stress-crack resistance combined with good low-temperature impact strength and low warpage make LLDPE suitable for injection-moulded parts for household wares, closures and lids. Extruded pipe and tubing made from LLDPE exhibit good stress-crack resistance and good bursting strength (Furumiya et al. 1985).

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4.4.5 High-Density Polyethylene (HDPE) HDPE is widely acknowledged for its high strength-to-density ratio. The density of HDPE ranges from 0.93 to 0.97 g/cm3 . Compared to LDPE, HDPE has stronger intermolecular forces due to lesser branching, resulting in higher tensile strength. The difference in strength is proportionately much higher than the difference in density, which gives HDPE a higher specific strength, i.e. it is suitable for lightweight applications. The lack of branching in HDPE is ensured by appropriate reaction conditions and the choice of catalysts, namely Ziegler–Natta catalysts (Renfrew and Morgan 1960). Some common applications of HDPE are food storage containers, piping for water supply and sewage, recyclable plastic bottles, wood–plastic composites, folding chairs and tables. HDPE is resistant to corrosive substances and therefore finds wide application as protective liners in steel pipes and plastic packaging bags. Specific grades of HDPE may be used for ballistic plates, far-IR lenses and 3D printer filaments. Hollow shapes like water bottles and jugs can be manufactured by blow moulding in a fast and efficient manner, and this application amounts to about 8 million tons (1/3 worldwide production) of HDP use. Furthermore, there is a very wide range of applications of HDPE owing to its superior properties compared to conventional packaging materials like glass, metal and cardboard, and also the health and environmental issues of Bisphenol A in PVC and PC (NIIR Board of Consultant and Engineers 2006).

4.4.6 Nylons Nylons are thermoplastics and the name is generic for aliphatic or semi-aromatic polyamides. Nylons can be easily melt-processed into fibres, films or shapes. Nylon was first produced using diamines in 1935, by DuPont’s research facility, and it is the first commercially successful synthetic thermoplastic polymer. Wartime (World War II) usage of nylon increased the production, since it was used by military in manufacturing parachutes and their cords (Kohan 1973). Different property variations in nylon can be achieved by mixing it with a wide variety of additives. Mixtures of diamines (−NH2 ) and diacids (−COOH) of different proportions can be polymerised together to synthesise copolymers of nylons.

4.4.6.1

Nylon Properties

These aliphatic polyamides are linear polymers, which are specifically employed in certain applications where the adjacent polar –CONH– (amide) groups give rise to high inter-chain attraction in the crystalline zones and thus, excellent combination of high toughness with adequate flexibility, especially above their glass transition

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temperatures. Melting point of Nylons is high (around 200 °C) due to high inter-chain attraction in the crystalline zones formed by amide chain segments. Mechanical properties such as high toughness, impact strength, and flexibility of nylons are affected significantly by the amount of crystallisation, ambient temperature and humidity Rest of the properties of various grades of nylons are almost similar. Influence of moisture can be understood by the example, that Young’s moduli for nylon-6,6 and nylon-6 decrease by about 40% with the absorption of 2% moisture (Nelson 1976). If provided with a highly crystalline surface layer, nylon can offer good abrasion resistance. Surface crystallinity can be enhanced by annealing in a temperature range of 150–200 °C in a non-oxidising fluid. The properties of nylons strongly depend on the degree of crystallisation and the size of morphological spherulites. Both these factors are generally influenced by the processing conditions. The effect of humidity on the properties of the polymer will also be less in case of higher degree of crystallinity that leads to less water absorption.

4.4.6.2

Nylon Applications

The most important application of nylons is as fibres, which account for nearly 90% of the world production of all nylons. The remaining 10% is used for certain plastic applications. Nylons are not used as general-purpose polymers, like polyethylene and polystyrene, since they are more than twice the cost. Nylons have steadily increasing applications as plastic materials for speciality purposes where the combination of rigidity, toughness, abrasion resistance, gasoline resistance and reasonable heat resistance is important. The largest plastic applications of nylons have been in mechanical engineering, most commonly for gears, bearings, cams, valve seats and bushes. Zippers made of nylon last longer than traditional ones of fabric or metal. Nylon moving parts have the advantage that they may often be operated without lubrication, and they may often be moulded in one piece. PU-coated nylon fabrics are frequently used in the development of defence stores outer covers owing to their properties like low-temperature flexibility and ductility as well as abrasion resistance, waterproofing and water repellence. Also, nylons have excellent tear strength when the areal density is in the range of 300–350 GSM or g/ m2 .

4.4.7 Bakelite Polyoxybenzylmethylenglycolanhydride is popularly known as Bakelite and is recognised as an early plastic. It is a phenol formaldehyde thermosetting resin synthesised by the condensation reaction of phenol with formaldehyde. The resin was first developed by Leo Baekeland, a Belgian-American chemist, in 1907. The urea-formaldehyde and phenol-formaldehyde resins are thermoset polymers having

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Table 4.2 Properties of urea-formaldehyde and melamine-formaldehyde (Brydson 1982) Property

Urea-formaldehyde (α-Cellulose filled)

Melamine-formaldehyde (Cellulose filled)

Specific gravity

1.5–1.6

1.5–1.55

Tensile strength 103 lbf/in2 MPa

7.5–11.5 52–80

8–12 55–83

Impact strength (ft-lbf)

0.20–0.35

0.15–0.24

Dielectric strength (90 °C) V/0.001 in

120–200

160–240

Volume resistivity (Ω m)

1013 –1015

109 –1010

Water absorption (mg) 24 h at 20 °C 30 min at 100 °C

50–130 180–460

10–50 40–110

wide applications. In fact, phenolic resins are recognised as the first commercially produced polymers that are purely synthetic (Brydson 1982). Under the requisite heat and pressure conditions, different shapes and forms are produced by using the precursor (A-stage resin) which is a low-molecular-weight prepolymer. Generally, processing is done by compression moulding and often fillers are also added mainly to improve physical properties and to reduce resin content. Because the cross-linked polymer from the phenol-formaldehyde reaction is insoluble and infusible, for commercial applications it is necessary to first produce a tractable and fusible low-molecular-weight prepolymer that can be transformed into the cross-linked polymer whenever required. Bakelite was initially used for its electrical non-conductivity and heat-resistant properties in telephone casings, electrical insulators, radio and highly diverse products, e.g. kitchenware, pipe stems, jewellery, children’s toys and firearms. In recognition of its significance as the world”s first synthetic plastic, Bakelite was designated a National Historic Chemical Landmark by the American Chemical Society on November 9, 1993 (Gould 1959). Some useful properties of two commonly used Bakelite-type formaldehydes are given in Table 4.2. Synthesising Bakelite is a multi-stage catalytic process in which phenol and formaldehyde are heated in the presence of a catalyst such as zinc chloride, hydrochloric acid or the base ammonia. This firstly results in Bakelite A, which is a liquid condensation product soluble in acetone, alcohol or additional phenol. On further heating, the product becomes partially soluble and can be further softened by heat. Prolonged further heating, however, results in a sticky gum-like insoluble residue.

4.4.7.1

Compression Moulding of Bakelite

The moulding process for Bakelite is flexible with respect to the feed material, which has a direct impact on casting speed. Both resin powder, as well as partially cured and

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preformed slugs, can be used as the starting material, and there is a high casting yield. The moulding process can also be made faster by removing the casting without any significant cooling (Whitehouse et al. 1967). Another property of Bakelite processing favouring faster production is that after moulding the surface finish of Bakelite is very smooth and does not require any additional process step for finishing. Because of the smooth finish and also its resistance to heat, solvents and scratches, Bakelite also finds good engineering applications.

4.4.7.2

Applications of Bakelite

Because of its distinct properties, Bakelite is used in a wide range of applications. Its effective and wide application dates back to World War II when it was used in many products like electrical systems, machine parts, telephones and radio sets. Today it has wider applications ranging from adhesives, moulding compounds, varnishes and binding agents in the automobile industry as well as household commodities. Since phenolic resins are thermosetting and heat resistant, they also have potential applications like ablative heat shields and high-temperature composite materials for spacecraft and warheads. For defence stores development, Bakelite is widely used for insulated handles, knobs and grips. A typical application is a handle made from aluminium sheets sandwiched between Bakelite. This handle is inserted into the mould cavity in which phenol formaldehyde (Bakelite) powder is poured. The upper mould closes and the die is compressed at around 4–6 bar pressure at 145–155 °C. The curing time is 2– 3 min. Then the mould is opened and extra material on the edges (flash) is removed to obtain the finished Bakelite handle. An example is given, later in Fig. 4.11d.

4.5 Material Selection, Product Development and Manufacturing Processes The materials are selected based on the service requirements of particular stores, including loads, environments and temperatures. The material selections are followed by product designs based on conventional design tools as well as modelling and simulation inputs. Then special properties like the rheology and melt flow are considered to decide on the most suitable processing technique for product development.

4.5.1 Material Selection For defence stores, the materials are selected according to the troop requirements. One of the most important factors, also for the design, is that the product must be

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Table 4.3 User quality and trial requirements and corresponding desired material properties Quality requirement/trial directive

Desired material properties

All-terrain test

−50 °C to +50 °C service temperature; negligible water absorption (ambient RH up to 90%)

Drop test

Good impact strength

Lifting and hanging

Good tensile strength

Top loading

Good compressive and flexural strengths

Wind load

Good flexural and tensile strengths

Hand grip

Smooth surface finish; insert mouldability

Food grade

FDA-compliant and overall migration test qualification

Insulation

Low thermal conductivity; should withstand temperature extremes

Visibility

Amorphous or semi-crystalline polymers with good transmittance of optical light (>85%)

Lightweight

High strength; lower density

Cushioning and sealing

Elastomeric; low T g ; partially cross-linked

lightweight. The most suitable polymer for a particular application is then selected on the basis of its functional, chemical, mechanical and physical properties. Various grades of a particular polymer are then finally considered to design the product. Table 4.3 shows some of the user quality and trial requirements and the desired material properties. Material properties identified on the basis of user requirements set the criteria for material selection. Table 4.4 gives some widely used polymers and their properties.

4.5.2 Product Development Material selection for defence stores is obviously an important, even critical, step in product development. Depending on the qualitative requirement of a particular store, first, the broader category of the material is identified, e.g. whether polymers, metals, ceramics, metal oxides, textiles or elastomers. If polymers are to be used as construction materials for some or all of the components of the stores, then selection of the type and grade of a particular polymer becomes specific to the requirement in order to develop the best product with the minimum input of time, money and energy. A brief description of some typical grades of polymers selected for different applications and the qualitative requirements of the defence general stores is given in the following three examples, Sects. 4.5.2.1–4.5.2.3.

−63 −65 to −35 −65 to +80

−110 −70 −40 to + 90 30 450 J/m 35 0.916 0.33 0.11

T g (°C)

Brittleness temperature T b * (°C)

Service temperature (°C)

Tensile strength (MPa)

Izod impact strength

Flexural strength (MPa)

Density (g/cc)

Thermal conductivity (W/m K) (Whitehouse et al. 1967)

Thermal diffusivity (mm2 /s) (https://thermt est.com/materials-database) 0.19

0.293

1.2–1.55

1–115

1330 J/m

1–70

−40 to + 110

–

−40

Thermoset

TPU 115

327

PTFE

0.09

0.24–0.28

1.084

117

116 J/m

87

*

3–17

−50 to + 150

−45

−54

122

EPDM

0.124

0.25

2.2

NB

0.1

0.20

0.9–2.0

NB

160 J/m NB

27

−150 to + −73 to 220 +204

−196 to − −240 150

47–50

215–223

Nylon6

Where T b /T g = 0.135 + 0.082 C ∞ (Souheng 1992) C ∞ : Characteristic Ratio (Flory and Fisk 1966) T b : Brittleness Temperature T g : Glass Transition temperature

0.14

0.58

0.96

NB

NB

0.5

51

120–160

Melting point (T m ) (°C)

PU

LLDPE

Parameters

0.23

0.46–0.52

0.93–0.97

31.7

69 J/m

25–28

−40 to + 83

−70

−110

130

HDPE

Table 4.4 Properties of polymers (Brittle-ductile transition temperature 2017) used in the development of defence general stores

220

Thermoset

Bakelite

0.144

0.19–0.22

1.2–1.22

1.2

600–900 J/ m

50–65

−40 to + 130

0.18

0.2

1.3

95

9–11 J/m2

50

−40 to + 140

−80 to −60 –

147

260

PC

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(a) Water bottle with insulation

(b) Water bottle main body

Fig. 4.9 Flex water bottle

4.5.2.1

Flex Water Bottle

A flex water bottle is shown in Fig. 4.9. Such bottles are used for storage and carriage of potable water without appreciable change in the original water temperature for up to 6 h. They have been designed and developed for 1 L capacity using various polymeric materials. Brief descriptions of the qualitative requirements and the polymers selected for the application are given in Table 4.5.

4.5.2.2

All-Terrain and Snow Goggles

All-terrain and snow goggles are illustrated in Fig. 4.10. These provide eye protection from UV light, glare, dust, dirt, fogging and wind-chill in extremely cold weather, and have been designed using suitable polymeric materials. Brief descriptions of the qualitative requirements and the polymers selected for these applications are given in Table 4.6.

4.5.2.3

Food Containers and Carriers

Different food containers and carriers for storage and transport of food for individuals as well as for a group have been designed and developed using strong food-grade polymers that can withstand a wide range of service temperatures. Some of these products are shown in Fig. 4.11. Brief descriptions of the qualitative requirements and the polymers selected for these applications are given in Table 4.7.

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Table 4.5 Water bottle requirements and selection User quality requirements

Polymer requirements

Selected polymers and properties

1. must withstand temperature Low-temperature ductility and PC-Lexan EXL grades (http:// range −40 to It +50 °C high-temperature dimensional www.sabic.com/en/products/ stability specialities/lexan-copolymer): • Widest available service temperature range, −40 to +120 °C • Excellent dimensional stability • 100% UV-resistant 2. Lightweight

High strength to density ratio (SDR)

SDR 45–48 MPa/(g/cc)

3. Stored water should not develop any kind of odour

Anti-scratch and microbial resistance

Good scratch and microbial resistance due to siloxane modification

4. No cracks during use

Low T b

−60 °C

5. Water quality should be visible from outside

Excellent transparency

Good optical clarity and excellent transparency (minimum 80%)

6. Bottle must withstand sudden impact or shocks during normal drops

High-impact strength

• Excellent impact strength (Izod Impact Strength (600–850 J/m) • Excellent low-temperature ductility

7. Bottle should not be so rigid Good flexural Strength that it causes discomfort when carried (worn)

90 MPa

8. Stored water should be at Low thermal conductivity the original temperature during operation or exercise

0.19–0.22 W/m K for PC 0.015 W/m K effective thermal conductivity for insulation cover (special grade polyurethane-coated nylon with poly fibre insulation for low temperature)

9. Food grade

Minimal migration/leaching

Much below IS 9845 (1998) specification and FDA-compliant

10. Mass producible

Economical manufacturing

Easy to manufacture due to conducive melt flow and thermal properties

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(a)

(b)

(c)

Fig. 4.10 a All-terrain goggles. b Snow goggles. c Goggle mask

4.5.3 Manufacturing Processes The process for bulk production depends on the design, size and quantity of the stores to be manufactured. The most commonly used manufacturing processes include injection, blow, extrusion and compression moulding. However, a particular product may need a specific set of processes in order to manufacture it in a cost-effective and timely manner. As an example, the process for the production of water bottles as per armed force requirement of quality and quantity is described here. The PC water bottle can be produced either by Extrusion Blow Moulding (EBM) or Injection Stretch Blow Moulding (ISBM). Both processes have their advantages and limitations, but the IBSM process is recommended for manufacturing these bottles because the end product has excellent visual and dimensional quality, especially for the threads and neck portion; no pinch-off scrap; excellent thickness control; and very few surface defects.

4.5.3.1

Injection Stretch Blow Moulding Process (ISBM)

As the name suggests, ISBM is a combination of the two familiar processes of injection moulding and blow moulding. These processes can be carried out on a singlestage machine or a double-stage machine (Harper 2006). These will be reviewed before further discussion of the production of water bottles.

Single-Stage Machine (Single-Heat Approach) The preform (parison) is directly transferred to be blown into the finished article in the blowing section, see Fig. 4.12. This ensures the optimum use of residual heat from the initial injection moulding process. In the single-stage process, there are two versions of the ISBM machine, see Figs. 4.13 and 4.14. The three-station machine (Fig. 4.13) uses the same heat and conditioning step + blowing on the same station, whereas in the four-station machine (Fig. 4.14) conditioning and blowing are done on different stations. Four-station machines are generally preferred for processing somewhat bulkier and larger articles, especially if they are thicker. On the other hand,

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(a) Food container

(c) Carrier meat chest

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(b) Internal components of food container

(d) Aluminium alloy mess tin with insulating Bakelite handles

Fig. 4.11 Food containers and carriers

for smaller articles, the three-station machine is more suitable owing to a faster rate of production.

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Table 4.6 All-terrain and snow goggle requirements and selection User quality requirements

Polymer requirements

Lenses

1. Approx. 100% UV opaque and good service life

Selected polymers and properties Optical grade PC-Lexan (http://www.sabic.com/en/pro ducts/specialities/lexan-cop olymer )/Makrolon (http:// www.plastics.covestro.com/)

UV (A & B)-resistant and UV-stable

2. Clear and wide field of view Excellent transparency Anti-scratch resistance

100% UV (A & B)-resistant. Major portion of UV is absorbed by stabiliser and much less by the main matrix material Good optical clarity (excellent transparency, minimum 80–87%) Anti-scratch coating

3. Must withstand temperature Low-temperature ductility and • Widest available service from −40 to +50 °C high temperature dimensional temperature range −40 to + stability 100 °C • Excellent dimensional stability 4. Lightweight

High strength to density ratio (SDR)

SDR 40–48 MPa/(g/cc)

5. No cracks during assembly or disassembly with frame

Low-temperature ductility and • Excellent low-temperature low brittleness temperature ductility • Brittleness temperature − 60 °C

6. Must withstand sudden impact or shocks during normal drops

High impact strength

• Excellent impact strength (Izod Impact Strength (600–950 J/m)

7. Fog should not condense much on external surface

Hydrophobic surface properties

Anti-fog coating on the surface

8. Optical grade

Ophthalmic requirement

Qualifies for ANSI Z 87.1

9. Mass producible

Economical manufacturing

Easy to make by injection moulding

Frame/mask

Polyurethane rubber (http:// www.tpu.covestro.com/en/pro ducts/Desmopan)/polyur ethane thermoplastic elastomers (TPUs)

1. Must withstand temperature from −40 to +50 °C

• Widest available service temperature range −50 to + 100 °C • Excellent dimensional stability and surface finish • 100% UV resistant for good service life (continued)

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Table 4.6 (continued) User quality requirements

Polymer requirements

Selected polymers and properties

2. Lightweight

Good Strength-to-Density Ratio (SDR)

SDR 25 MPa/(g/cc)

3. No cracks during assembly or disassembly with frame

Low brittleness temperature

−60 °C

4. Flexible enough to avoid discomfort during wearing

Soft surface Good flexural strength

Shore A Hardness 70 and excellent low-temperature flexibility

5. Must withstand sudden impact or shocks during normal drops

High-impact resilience

Impact resilience up to 63%

6. Minimum heat flow to surroundings

Low thermal conductivity

0.293 W/m K

7. Mass producible

Economic manufacturing process

Easy to manufacture by extrusion and injection moulding (very good thermal and flow properties)

Double-Stage Machine (Dual-Heat Approach) After the preform is made by injection moulding it is transferred to the companion machine for blow moulding, see Fig. 4.15. This may include a storage period, which has the disadvantage that residual heat from the injection moulding process is not available.

ISBM Process Choice for Defence Water Bottles A single-stage ISBM process is preferred for the defence water bottles. The main reason is to avoid any contamination and hence deterioration of the food grade quality of the material during the transfer and reheating of the preform. A description of the processing steps follows: • PC ISBM injection moulding of the preform with the finished neck that includes threads at one end. • Maintain the temperature of the preform (parison) above the glass transition temperature T g before high-pressure air is blown into the preform in a metallic mould. • During the blowing process, the preform is simultaneously stretched using a core rod, see Fig. 4.12. This stretching significantly increases the strength of the finished product. • The finished bottle is cooled inside the mould and then ejected.

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Table 4.7 Food container and carrier requirements and selection User qualitative requirements

Polymer requirements

Main body

Selected polymers and properties HDPE/LLDPE

1. Must withstand temperature Low-temperature ductility and • Good service temperature range from −40 to +50 °C High-Temperature range −40 to +90 °C • Excellent dimensional dimensional stability stability 2. Lightweight

Good strength to density ratio

SDR 25 MPa/(g/cc)

3. Stored foodstuff should not accumulate or develop any kind of odour

Good microbial resistance Anti-scratch resistance

Good microbial and scratch resistance

4. Should not develop many scratches and any cracks during service life

Good scratch resistance and low brittleness temperature

Shore hardness D 68 Brittleness temperature − 70 °C

5. Must withstand sudden impact or shocks during normal drops from 1.5 m

High-impact strength

• Excellent impact strength • Good low-temperature ductility

6. The stored foodstuff should Low thermal conductivity and be at the original provision of foam insulation temperature for up to 6–8 h filling

Twin-walled design of containers filled with PU Foam (thermal conductivity 0.019 W/m K)

7. Food grade

Minimal migration/leaching

Much below IS 9845 (1998) specification and FDA-compliant

8. Mass producible

Economical manufacturing

Easy to manufacture by injection moulding (small containers) and roto-moulding (large containers)

Insulating handle

Bakelite: phenol formaldehyde

1. Must withstand temperature High-temperature dimensional • Service temperature range range from −40 to +150 °C stability −40 to +150 °C • Excellent dimensional stability 2. Lightweight

Low density

Density 1.3 g/cc

3. Acts as insulation to the handle of the hot case

Low thermal conductivity

0.2 W/m K

4.5.3.2

ISBM Process Sequence

The ISBM process is very fast and efficient for manufacturing hollow thin-walled products. However, the processing requires careful monitoring of many process parameters in order to ensure smooth and efficient bulk production. The exact process sequence is described in detail as follows:

4 Polymeric Materials for Defence Stores in Extreme Cold Weather

Fig. 4.12 Single-stage injection stretch blow molding ISBM

Fig. 4.13 Three-station ISBM machine

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Fig. 4.14 Four-station ISBM machine

Fig. 4.15 Double-stage ISBM

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Fig. 4.16 Three sections of the screw

• Material preparation: Material preparation and drying parameters have a particularly important influence on the quality of the finished product, as well as lowering the rejection rates. Moisture can cause hydrostatic degradation of the polymer during processing and shows up as streaks or small bubbles on the surface. The PC should have a residual moisture content ≤0.02%. It is dried at approximately 120 °C for about 3–4 h for high-speed dryers and 2–3 h for dry air dryers before moulding, to ensure optimum processing, appearance and mechanical properties. Lower temperatures do not ensure a sufficient level of drying, and at higher temperatures, there is a risk of granules sticking together and being unsuitable for processing. • Preform production by injection moulding: This step is based on the total weight, the thicknesses at various profiles and other design parameters. The main ones are: • Machine and temperature: Universal three-section screws, see Fig. 4.16, and barrier screws are used (http://www.tpu.covestro.com/en/products/Desmopan). A progressive temperature setting is used from the hopper to the nozzle in order to obtain a melt temperature between 300 and 330 °C and to prevent material flow and dripping. Maintaining an optimal mould temperature is essential for achieving a stressfree end product and also for good release. The mould and core temperatures for three-station and four-station machines are, respectively, 80–100 and 40–60 °C. The cycle time and cooling times will be 50–60 s. The overall cycle will take 60–70 s. • Injection speed and pressure: High injection speeds are needed to shorten injection times and avoid premature freezing of the melt. Injection pressures are kept high enough (maintained around 70–105 MPa) to fill the mould and as low as possible to obtain parts without flash marks. • Screw speed: Fairly high screw speeds are used in order to finish plasticising before the end of the cooling time. However, material degradation occurs when high screw speeds are combined with too-high back pressures due to overheating of the plastic. • Process parameters: Typical parameters applied for processing are as below:

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Melt temperature

295–315 °C

Rear—Zone 1 temperature

215–295 °C

Middle—Zone 2 temperature

280–305 °C

Front—Zone 3 temperature

295–315 °C

Nozzle temperature

290–315 °C

Mould temperature

70–95 °C

Screw speed

40–70 rpm

• Preform conditioning: After injection moulding the preform is conditioned (preheated) in a hotpot to the temperature needed for blowing. The maximum preform temperature is selected in such a way that the material is sufficiently softened (above its T g ) for the blowing process. At the same time, too high a temperature would give excessive drag-down. Also, in the hotpot, the preform is heated to a different level in the axial direction. This allows control of the wall thickness variations during blowing. N.B: The transfer of heat between the preform and the hotpot wall is influenced by the gap between them. • Blowing the preform into the final bottle shape: Inside the blow mould, the preform is stretched to its definitive length in the axial direction by a blowing mandrel. At the same time, it is pre-blown in the radial direction at a pressure of 1.5 bars. The bottle receives the definitive shape in the second stage. For the second stage, the blowing pressure is kept near 10 bar. The mould temperature will be between 60 and 80 °C (approximately 75 °C). There should be sufficient ventilation to ensure that no optical defects appear.

4.5.4 Defence Store Manufacturing Defects The end product, whatever it is, should be free from the following defects: • Flash: This is excessive and unwanted material along the mould parting line or between mould components. This generally happens due to incorrect melt temperatures. • Sinks and voids: Sinks are depressions on the component surface where the material shrinks away from the mould surface. Voids are sections in the centre of the component where the material shrinks away from itself, leaving a small cavity. Both sinks and voids are attributed to incorrect melt temperatures. • Burning: This is the result of gases and volatile compounds becoming trapped, compressed and heated in the moulds. This is generally caused by excessively high melt temperatures along with high clamp pressure, which reduces vent depth and prevents these volatile compounds from escaping. • Flow lines: Flow lines result from poor adhesion of the material to the mould surface. A low mould temperature is the main contributing factor to flow lines.

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• Poor surface finish: This is generally the result of poor or non-uniform adhesion of the polymer material to the mould surface. Various contributing factors include incorrect melt temperatures, incorrect screw speeds, incorrect mould temperatures and poor mould surface finish. • Incorrect dimensions: Incorrect dimensions are encountered typically as a result of polymer shrinkage due to incorrect mould temperatures, which cause incorrect cavity pressures. • Splay, bubbles and blisters: Splay appears as surface lines parallel to the flow. Bubbles and blisters appear due to volatiles in the resin. These are generally caused by incorrect handling of the resin, including dehydration; timely removal of old material; and excessively high screw rotations that may introduce air into the nozzle and barrel.

4.6 Concluding Remarks There are numerous applications of polymers for defence stores. Depending on the usage of the product, the material is selected so that it can withstand the service conditions under different temperatures and in different climates and terrains. Some applications presented in this chapter make it clear how important it is to select the appropriate polymeric materials, especially for extremely low-temperature applications. For example, an insulated water bottle, which may have very good strength, while, in service under high-temperature desert conditions, may be prone to fracture and failure in glaciated regions. For applications where the rubbery nature of a material is required (e.g. goggle frames, gaskets, washers, footwear and gloves) the polymer glass transition temperature, T g , and degree of cross-linking are the most important parameters. On the other hand, in the case of materials requiring high strength and stiffness (e.g. tent frames and poles), the most suitable materials are polymeric composites consisting of a low-temperature polymeric matrix and reinforcing high-strength fibres. Some applications such as water bottles, ballistic shields and snow goggle lenses require some degree of flexibility combined with high toughness and good ductility at low temperatures. The use of polymers at low temperatures is a challenge because they tend to be brittle at sub-zero temperatures, and it is desired to have almost the same mechanical properties at room temperature and very low temperatures. There are very few ‘pure’ polymers that can maintain their impact strength and toughness at low temperature. However, modification of these polymers by blending, copolymerisation, plasticiser additions and thermal treatment may help in achieving sufficient toughness for the development of defence stores. Such modified materials can provide solutions to this problem, but they themselves have problems like phase segregation and anisotropy that must be carefully addressed.

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Acknowledgements The authors are grateful to the DRDO, India, for providing the financial and infrastructural support to carry out the defence stores product development work. The authors thankfully acknowledge the contributions of several officers and staff involved in stores development, for their untiring efforts to successfully complete the assignments from conceptual design to bulk production of defence stores, some of which are described in this chapter. The authors are indebted to Dr. R.J.H. Wanhill for many useful modifications and corrections, which have vastly improved the technical value of the manuscript and also, much more significantly its readability.

References Astm, D.: ASTM D 256-04: standard test methods for determining the Izod pendulum impact resistance of plastics (2010) ASTM D638-14: standard test method for testing tensile properties of plastics ASTM D1238-04: standard test method for melt flow rates of thermoplastics by extrusion plastometer Barton, J.M., Lee, W.A.: Polym. Lond. 9, 602 (1968) Brittle-ductile transition temperature.: Polymer properties database (2017). http://Polymerdatabase. com/polymerphysics. Accessed 21 July 2017 Brydson, J.A.: Plastics Materials. Butterworth Scientific, London (1982) Brown, N., Ward, I.M.: The influence of morphology and molecular weight on ductile-brittle transitions in linear polyethylene. J. Mater. Sci. 18, 1405–1420 (1983) Couchman, P.R., Karasz, F.E.: A classical thermodynamic discussion of the effect of composition on glass-transition temperatures. Macromolecules 11(1), 117–119 (1978) Dunnols, J.: Basic Urethane Foam Manufacturing Technology, Technomics, Westport, CT (1979) Ferry, J.D.: Viscoelastic Properties of Polymers, 3rd edn. Wiley, New York (1980) Ferry, J.D.: Some reflections on the early development of polymer dynamics: viscoelasticity, dielectric dispersion and self-diffusion. Macromolecules 24(19), 5237–5245 (1991) Flory, P.J., Fisk, S.: Effect of volume exclusion on the dimensions of polymer chains. J. Chem. Phys. 44, 2243–2248 (1966) Furumiya, A., Akana, Y., Ushida, Y., Masuda, T., Nakajima, A.: Relationship between molecular characteristics and physical properties of linear low density polyethylenes. Pure Appl. Chem. 57(6), 823–832 (1985) Gaur, U., Wunderlich, B.: Heat capacity and other thermodynamic properties of linear macromolecules. IV. Polypropylene. J. Phys. Chem. Ref. Data 10, 1051–1064 (1981) Gibbs, J.H., Dimarzio, E.A.: Nature of glass transition and the glassy state. J. Chem. Phys. 28, 373–384 (1958) Gould, D.F.: Phenolic Resins. Van Nostrand Reinhold, New York (1959) Green, M.M., Wittcoff, H.A.: Organic Chemistry Principles and Industrial Practice. Wiley, New York (2003) Griffith, A.A.: The phenomenon of rupture and flow in solids. Philos. Trans. Roy. Soc. A 221(582), 163–198 (1921) Gordon, M.J.: Jr. Industrial Design of Plastics Products, p 199. Wiley, New York, ISBN 978-0-47123151-7 (2003) Harper, C.A.: Handbook of plastic processes, Ch 5. In: Norman, Lee, C. (eds.) Blow Moulding. Wiley-Interscience, Hoboken, NJ (2006) Jang, B.Z., Uhlmann, D.R., Vander Sande, J.B.: Ductile-Brittle transition in polymers. J. App. Polym. Sci. 29(11), 3409–3420 (1984) Kohan, M.I.: Nylon Plastics, p. 201. Wiley, New York (1973) Kuhn, W.: The Shape of fibrous molecules in solution. Kolloid Z. 68(1), 2–15 (1934)

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Kuhn, W., Kuhn, H.: Rigidity of chain molecules and its determination from viscosity and flow birefringence in dilute solutions. J. Colloid Sci. 3(1), 11–32 (1948) Legrand, D.G.: Crazing, yielding, and fracture of polymers. I. Ductile brittle transition in polycarbonate. J. App. Polym. Sci. 13(10), 2129–2147 (1969) Lexan Copolymer. http://www.sabic.com/en/products/specialities/lexan-copolymer. Accessed 03 Jan. 2018 Market Study: Polyethylene LLDPE.: 2nd edn. Ceresana Market Research (2014) Materials thermal properties database. https://thermtest.com/materials-database. Accessed 21 Sept. 2017 Nelson, W.E.: Nylon Plastics Technology. Published for the Plastic & Rubber Institute by NewnesButterworths, London (1976) NIIR Board of Consultant & Engineers.: Plastic Films, HDPE and Thermoset Plastics. Asia Pacific Business Press, New Delhi (2006) Odian, G.: Principles of Polymerization, 3rd edn. J. Wiley, New York (1991) Oertal, G.: Polyurethane Handbook. Hanser Publishers, Munich (1985) Phillips, L.N., Parker, D.B.V.: Polyurethanes: chemistry, Technology and Properties. Iliffe Books, London (1964) Polycarbonate-plastic machining & fabricating. www.plasticsmachining.com. Accessed on 30 August 2017 Ravishankar, P.S.: Treatise on EPDM. Rubber Chem. Technol. 85(3), 327–349 (2012) Renfrew, A., Morgan, P.: Polythene—The Technology and Uses of Ethylene Polymers, 5th edn., p. 255. Iliffe, London, UK (1960) SABIC Lexan Copolymer. http://www.sabic.com. Accessed 12 May 2017 Science & Research (Food). http://www.fda.gov/Food. Accessed 03 June 2017 Shenoy, A.V., Saini, D.R.: Thermoplastic Melt Rheology and Processing. Marcel Dekker Inc., New York (1996) Thermoplastic Polyurethanes. http://www.tpu.covestro.com/en/products/Desmopan. Accessed on 21 Sept. 2017 Strobl, G.: The Physics of Polymers. Springer, Heidelberg (2007) Struik, L.C.E.: Orientation effects and cooling stresses in amorphous polymers. Polym. Eng. Sci. 18(10), 799–811 (1978) Van Krevelen, W., Te Nijenhuis, K.: Properties of Polymers. Elsevier, Oxford UK (2009) Vincent, P.I.: The tough-brittle transitions in thermoplastics. Polymer 1, 425–444 (1960) Weyland, H.G., Hoftyzer, P.J., Van Krevelen, D.W.: Prediction of glass transition temperature of polymers. Polymer 11, 79–87 (1970) Whitehouse, A.A.K., Pritchett, E.G.K., Barnett, G.: Phenolic Resins. Iliffe, London (1967) World of Polycarbonates. http://www.plastics.covestro.com/ Accessed on 12 Nov. 2017 Wu, S.: Secondary relaxation, brittle-ductile transition temperature and chain structure. J. Appl. Poly. Sci. (45), 619–624 (1992) Yannas, I.V., Lunn, A.C.: Infrared spectroscopic evidence for polycarbonate chain backbone motion below Tg. Polym. Lett. 9, 611–615 (1971) Zhang, X., et al.: Measurements of the thermal conductivity and thermal diffusivity of polymer melts with the short hot wire method. Int. J. Thermophys. 23(4), 1077–1090 (2002)

Chapter 5

Development of Non-metallic Structural Materials for Defence Systems Sourabh Srivastava, Jitendra Yadav, J. N. Srivastava, Arati Kole, and Eswara Prasad Namburi

Abstract The present chapter discusses the pilot plant scale synthesis of ceramic precursor polymeric materials, Polydimethylsilane (PDMS) and Polycarbosilane (PCS), as well as organic polymers, Polyetheretherketone (PEEK), and Polyurethane (PU) for the development of production technologies of strategic structural materials such as SiC and other modular capacities in several Defence applications. A detailed ' description of PEEK by reacting 4,4 -difluorobenzophenone, hydroquinone, K2 CO3 , and using diphenylsulphone (DPS) as a solvent is provided. Similarly, the reaction process is illustrated for PU by the Prepolymer route. Several key characterisations of PCS, PDMS, PEEK, and PU are also described.

5.1 Introduction Structural materials must have adequate sustainability under different types of mechanical stresses, physical conditions, and chemical environments for equipment, systems, and structures. With the advent of science and technology, these materials were initially predominantly metals and alloys. However, to some extent, they are gradually being replaced by advanced ceramics and polymers owing to these materials’ outstanding properties. To set the background for this chapter, Fig. 5.1 shows a general classification of structural materials.

S. Srivastava · J. Yadav · J. N. Srivastava · A. Kole Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India E. P. Namburi (B) Former Outstanding Scientist and Ex-Director, Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 E. P. Namburi et al. (eds.), Novel Defence Functional and Engineering Materials (NDFEM) Volume 2, Indian Institute of Metals Series, https://doi.org/10.1007/978-981-99-9795-4_5

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Fig. 5.1 Classification of structural materials

5.1.1 Pilot Plants and DMSRDE Use Pilot plants are vital for several reasons: (i) to obtain experimental data either for new processes or new compounds; (ii) to provide valuable data for the design of a full-sized commercial plant; (iii) to reduce the risk associated with full-sized plants; and (iv) they are substantially less expensive to build than full-scale plants. The fundamental aspect of scaling up or down of physical and chemical processes is the principle of similarity. Dimensional analysis and differential equations are the two techniques available to derive the similarity criteria between laboratory scale and pilot scale processes for a physical system. Process Development is the identification, via literature and other sources, of a proper synthesis process and selection of raw materials. Scientific data and experience derived from laboratory scale investigations help to obtain optimized process parameters and kinetics data. Scale-up activity represents the synthesis of the knowledge accumulated in the various phases of process development, from the design of laboratory experiments until the operation of pilot plants and eventually establishing industrial plants. At DMSRDE we have concentrated on Pilot plant scale synthesis of (i) Polydimethylsilane (PDMS) precursor for Polycarbosilane; (ii) Polycarbosilane (PCS) for development of SiCf and SiC–SiCf high temperature composites; (iii) Polyetheretherketone (PEEK) for the Light Combat Aircraft (LCA) door panels and fuel delivery components and (iv) Polyurethane (PU) Adhesive for the LCA windshield.

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5.1.2 Supervisory Control and Data Acquisition (SCADA) Supervisory Control and Data Acquisition (SCADA) is a collection/network of equipment that provides an operator at a remote location with enough information to determine the status of a particular piece of equipment or entire substation, enabling actions to be taken regarding the equipment or network. The basic units of SCADA are shown in Fig. 5.2. These consist of different blocks, namely Human-machine Interfaces (HMIs), Supervisory System, Remote Terminal Units (RTUs), Programmable Logic Controller (PLC), Communication Interfaces, and SCADA Programming. The main units of SCADA systems are discussed below: (1) Human-machine Interfaces (HMIs): HMI systems facilitate graphical presentation of data for the operating personnel. An HMI is an input-output device that is used by linking to the SCADA system’s software programs and databases for providing system management information, including the scheduled maintenance procedures, detailed schematics, logistic information, and trending and diagnostic data for a specific sensor or machine. (2) Supervisory System: The supervisory system is used as a server for communicating between the SCADA system equipment, such as RTUs, PLCs, sensors, and the HMI software used in control room workstations. In smaller SCADA systems the Master or Supervisory station comprises a single PC. In larger SCADA systems the supervisory system comprises distributed software applications, disaster recovery sites, and multiple servers. (3) Remote Terminal Units (RTUs): Physical objects in the SCADA systems are interfaced with the microprocessor-controlled electronic devices called RTUs.

Fig. 5.2 Basic units of SCADA systems

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Fig. 5.3 Architecture of a PLC

These units transmit telemetry data to the supervisory system and receive messages from the master system for controlling the network. (4) Programmable Logic Controllers: PLCs are connected to the sensors for collecting the sensor output signals in order to convert them into digital data. PLCs are used instead of RTUs because of PLCs advantages like flexibility, configuration, versatility, and affordability compared to RTUs.The architecture of a PLC is shown in Fig. 5.3. (5) Communication Infrastructure: SCADA systems generally use a combination of radio and direct wired connections. But large systems like power stations and railways frequently employ Synchronous Digital Hierarchy/Synchronous Optical Network (SDH/SONET). There are some compact protocols used in SCADA systems—a few communication protocols, which are standardized and recognized by SCADA vendors—and these send information only when the supervisory station polls the RTUs. (6) SCADA Programming: SCADA programming in a master system or HMI is required for creating maps and diagrams to give important situational information in case of an event or process failure. Standard interfaces are used for programming most commercial SCADA systems: programming can be done using a derived programming language or C language. SCADA is popular in several sectors, e.g. process industries, oil and gas, electric power generation, distribution and utilities, water and waste control, agriculture/irrigation, manufacturing, transportation systems, and other related applications. There are different types of SCADA systems that can be considered, since SCADA architectures span four generations:

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First Generation: Monolithic or Early SCADA systems, Second Generation: Distributed SCADA systems, Third Generation: Networked SCADA systems. Fourth Generation: ‘Internet of things’ technology SCADA systems.

5.2 Silicon Carbide In recent years, several important ceramics-based advances have been achieved, such as the discovery of a variety of functional properties associated with Polymer Derived Ceramics (PDCs), ceramic matrix, and metal-matrix composites. This non-metallic material is of great interest because of its high temperature strength retention as well as thermal stress resistance, coupled with its low density. As a consequence, different types of Si-based ceramic materials such as SiC, Si3 N4 , and Si2 N2 O are already available commercially.

5.2.1 Silicon Carbide Properties and Types Silicon carbide (SiC) has excellent mechanical properties such as Young’s modulus (380–430 GPa) and flexural strength (500–550 MPa) compared to alumina and boron carbide. Silicon carbide exists in about 250 crystalline forms. Alpha silicon carbide (α-SiC) is the most commonly encountered polymorph; it is formed at temperatures higher than 1700 °C and has a hexagonal crystal structure (similar to Wurtzite). The beta modification (β-SiC), with a zinc blended crystal structure (similar to diamond), is formed at temperatures below 1700 °C, as shown in Fig. 5.4. Table 5.1 illustrates the structures and properties of various types of silicon carbide crystals (Kipping 1924; Burkhard 1949, 1951; Miller and Michl 1989).

5.2.2 PDC-Derived Silicon Carbide Interest in polysilanes was aroused in 1975 with the discovery by Yajima and Hayashi (Yajima et al. 1975) that the permethyl polymer (Me2 Si)n could be transformed into silicon carbide by heating to high temperatures. Since then the pyrolysis of various organosilicon polymers has been developed as a major method to produce ceramic fibres. The schematic diagram for the synthesis of a silicon carbide–silicon carbide fibre (SiC–SiCf ) composite starting from PDMS is given in Fig. 5.5. Polycarbosilane (PCS), synthesized from polydimethylsilane (PDMS), is the most suitable precursor for generating SiC fibres (see Sect. 5.2.3). SiC fibres are well known for their excellent properties, such as high tensile strength, high stiffness, excellent heat, oxidation and corrosion resistance, and intermiscibilities/

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Fig. 5.4 α-SiC Wurtzite structure and β-SiC structure (like zinc blende crystal structure)

Table 5.1 Properties of major silicon carbide polytypes Properties

Structure-3C (β)

Structure-4H

Structure-6H (α)

Crystal structure

Zinc blende (cubic)

Hexagonal

Hexagonal

Bandgap (eV)

2.36

3.23

3.05

Density

(g/cm3 )

3.21

3.21

3.21

Lattice constants (Å)

4.3596

3.0730; 10.053

3.0730; 15.11

Bulk modulus (GPa)

250

220

220

Thermal conductivity (w/cm K)

3.6

3.7

4.9

Fig. 5.5 Flow diagram for SiC–SiCf composite formation

compatibilities with resins, metals, and ceramics. Therefore, they find certain applications in aerospace and high temperature applications (Yajima et al. 1978, 1977a, 1979; Fritz 1967). Ceramic composites (SiCm –SiCf ) are produced via the pyrolysis method using resin and fibres of polycarbosilane (Yajima et al. 1977b; Hasegawa et al. 1980). These composites find applications as structural materials in convergent and divergent flaps, flame holders, and nozzles of the exhaust systems of high performance aerospace engines.

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5.2.3 Synthesis Technologies for SiC Fibres SiC fibres have been developed in the following stages: (1) Mark I fibres: Yajima et al. (1975) first produced a PCS by cleavage of the dodecamethylcyclohexasilane ring and then fractionating the same to yield the “high” molecular weight of ~1500. These fibres were brittle and had only limited handling strength and hence, poor spinnability. (2) Mark II Fibres: Yajima et al. (1978) added between 1 and 4% of dichlorodiphenylsilane to overcome the above Mark I Fibre limitations. It was believed that the introduction of phenyl groups, which were more stable to temperature than methyl groups, would result in partial suppression of the dehydrogenation condensation reaction. In this way, it was hoped to make the molecular structure more linear or in-chain. This resulted in a 30% improvement in the spinnability of the fibres, i.e. there was a 30% reduction in the diameters for the same strength. It was also noted that the mechanical characteristics of the final SiC fibres were superior to those of the Mark I fibres. (3) Mark III Fibres: As part of an investigation into the process of thermal decomposition of organometallic polymers, Yajima et al. (1977a) synthesized PCS using polyborodiphenylsiloxane as a catalyst, which was easily produced by dissolving boric acid and diphenyldichlorosiloxane in anhydrous n-butyl ether. The solution was then refluxed in a nitrogen atmosphere for 18 hours, and then heated for 1 hour at reduced pressure at 300 °C. This gave polyborodiphenylsiloxane in a 92% yield. The spinnability of this Mark III polymer was greatly improved compared with that of the Mark I: so much so, that when cured it could be handled and subjected to tensile testing. The tensile strength of Mark III fibres was in the range of 20–59 MPa. The work carried out on these polycarbosilane systems led to the successful development of the first SiC-based ceramic fibres. These are known commercially as Nicalon.

5.2.4 Directly Produced Silicon Carbide This technology is based on rice husk, which is commonly available in large quantities in rice-growing countries. Rice husk was first used by Lee and Cutler (1975) as a starting material for the production of SiC in a high temperature furnace. Both silica and carbon are present in rice husk, making the formation of SiC easy using high temperature pyrolysis, since this route appears to be promising. In the Rice Husk (RH) route, both silica and carbon Krishnarao and co-workers (Krishnarao et al. 1998) also carried out extensive work on the preparation of SiC from rice husk at 1400–1900 °C in an inert atmosphere to produce a mixture of whiskers and particulates. Singh et al. (2002) produced ultrafine SiC, in powder form, in an Arc Thermal Plasma Reactor. More generally, a number of processes are available for the synthesis of silicon carbide with various types of raw materials, including rice husk. The two main

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processes of SiC synthesis are (i) the High Temperature Tubular Furnace Process, and (ii) the Arc Thermal Plasma Process. At the DMSRDE method (ii) has been used on a pilot scale.

5.3 Polyetheretherketone (PEEK) and Polyurethanes (PUs) 5.3.1 PEEK Polyetheretherketone (PEEK) is a thermoplastic used in structural applications. It is a colourless organic polymer originally introduced by Victrex PLC, then Imperial Chemical Industries (ICI) in the early 1980s. Paulo et al. (2003) described the work on machinability studies for structural applications. Proton exchange membranes for fuel cell applications was reported by Li et al. (2005) by direct synthesis of SPEEKK (sulphonatedpolyetheretherketone ketone). Conceiçãoet al. (2008) synthesized modified PEEK derivatives and characterized them for applications in membranes and composites science. PEEK has been explored as dental implants where Najeeb et al. (2016) observed the application to be less stress shielding compared to titanium, owing to a close match of the mechanical properties of bone and PEEK. During the studies carried out by Panayotov et al. (2016), they mentioned PEEK-based materials as an important group of biomaterials used for bone and cartilage replacement. Also, there is the possibility of its use in 3D printing processes. At DMSRDE the PEEK synthesis process for 2 kg batches was established, please see Sect. 5.4.4.

5.3.2 PUs Polyurethanes (PUs) are formed by reacting a polyol (an alcohol with two or more than two reactive hydroxyl groups per molecule) with a diisocyanate(aromatic/ aliphatic) in the presence of suitable catalysts and additives. Professor Dr. Otto Bayer and his co-workers discovered Polyurethane in 1937 (Akindoyo et al. 2016). Petrovi´c and Ferguson (1991) reported that in the mid-1950s PUs were commonly available in the form of elastomers, coatings, rigid forms, and adhesives. They also reported the thermal degradation behaviour and related properties (Petrovi´c et al. 1994). PUs possess good physical and mechanical properties as well as biocompatibility, and are therefore used in many medical appliances as in surgical drapes, wound dressing, and hospital bedding. Zhou et al. (2014) prepared a (polyethylene oxide)-modified medical polyethylene to improve the hemocompatibility. Synthetic biodegradable polymers for orthopedic devices are also prepared from PUs (Middleton et al. 2000).

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Fig. 5.6 Polyurethane synthesis method overviews

Other relevant studies are: (i) structural engineering of PU coatings for high performance applications (Chattopadhyay and Raju 2007), (ii) a review of polyurethane and polyurethane composites recycling and recovery (Zia et al. 2007), (iii) thermal stability and flame retardance (Chattopadhyay and Dean 2009), and (iv) a comprehensive review on polyurethane types, synthesis and applications (Akindoyo et al. 2016). There are two ways of synthesizing PUs: (i) the prepolymer method and (ii) the direct method, see Fig. 5.6. As is clear from the preceding list of studies (Akindoyo et al. 2016; Petrovi´c and Ferguson 1991; Petrovi´c et al. 1994; Zhou et al. 2014; Middleton et al. 2000; Chattopadhyay and Raju 2007; Zia et al. 2007; Chattopadhyay and Dean 2009), PUs have a wide range of applications. A summary, with unavoidable repetitions, is given here: (1) Automotive: body fasteners, suspension system parts, seals and gaskets, belts, battery covers, hoses, electronics covers, adhesives, and bumper stops. (2) Adhesives and sealants: Anti-mine boots, Shoes; aerospace, marine; magnetic media binders. (3) Coatings: floors, roofs, hovercraft blades, wire and cable, fabric; aerospace, and marine. (4) Engineering Components: gears, sprockets, crane bumpers, and military rockets. (5) Mining and slurry transportation: lined pipe, water valves, pump impellers, hopper car linings, conveyor belts, cyclones, and spray coatings. (6) Fabric laminates and simulated leather: shoes, upholstery, clothing, luggage, and business card holders. (7) Medical and dental: Drug test media, medical tubing, catheters, blood bags, bandages, dental water extractors, orthodontic clamps, and packaging. (8) Food: hopper cars, grain buckets, and grain chute linings.

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5.4 Pilot, Laboratory, and Bench Scale Synthesis In this main section of the chapter, we discuss synthesis methods for PDMS (5.4.1), PCS (5.4.2), Silica (5.4.3), PEEK (5.4.4), and PUs (5.4.5).

5.4.1 PDMS Synthesis Synthesis of PDMS (from DMDS) follows a Wurtz-type coupling method where DMDS is reacted with an alkali metal (sodium) under continuous stirring and in the presence of a phase transfer catalyst and in an inert atmosphere, see Fig. 5.7. After completion, the reaction mass is quenched with organic alcohol and subsequently washed with water. This is followed by separation of the organic layer, which contains the PDMS recovery of the PDMS from the organic layer involves filtration followed by drying. PDMS has been synthesized using a laboratory scale set-up at the level of 100 gm/ batch level, and several experiments were done at this level to optimize the process parameters and study the reaction kinetics. Then PDMS was successively produced with a 1 kg/batch bench scale set-up to generate engineering data for establishing the Pilot plant, followed by upgrading to a 10 kg/batch PLC-based PDMS Pilot plant. A block diagram of the process is shown in Fig. 5.8, and a process flow diagram for the Pilot plant is shown in Fig. 5.9. This diagram gives a schematic of a jacketed limpet coiled reactor, sodium melting tank, shell and tube heat exchanger, centrifuge, and vacuum tray drier with overall nitrogen blanketing facilities. The Pilot Plant has been fabricated, erected, and commissioned and PDMS has been successfully produced. The major components of the Pilot Plant are the Reactor, Sodium Melting Tank, DMDS Feed Tank, Solvent Feed Tank, Condenser, Condensate Receiver, Wash Water Tank, Centrifuge, Vacuum Tray Drier, and Programmable Logic Controller (PLC) unit. Some of the details of the Reactor system are given in Table 5.2. The following points give more details of the process: (1) The jacketed limpet coiled Reactor (MOC-Hastelloy C-276) is equipped with a 45° pitched blade turbine coupled with an anchor stirrer for mixing the reactants:

Fig. 5.7 Chemical reaction during PDMS synthesis

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Fig. 5.8 Block diagram of PDMS synthesis

Fig. 5.9 Process flow diagram for the PDMS Pilot plant

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Table 5.2 Specification of PDMS reactor and condenser PDMS reactor (i)

Working volume

:

120 L

(ii)

Reactor volume

:

165 L

(iii)

MOC

:

Hastelloy C-276 clad with SS-316

(iv)

Working temperature

:

300 °C

(v)

Working pressure

:

Vacuum to 15 kgf/cm2

(vi)

Top dish head

:

Standard dished head

(vii)

Working L/D

:

1.15/1

(viii)

Agitator (stirrer) type

:

45° pitched (6-bladed) coupled with anchor

(ix)

Agitator motor

:

3 hp

Shell and tube heat exchanger (Condenser) (i)

Type of condenser

:

Fixed tube Sheet type

(ii)

Heat Transfer area

:

2 m2

(iii)

Tube MOC

:

Hastelloy C-276

(iv)

Tube quantity

:

22

(v)

Pass

:

Shell Pass –1, Tube Pass –1

(vi)

Tube side fluid

:

Organic vapours

(vii)

Shell side fluid

:

Coolant

(2)

(3)

(4)

(5)

(6)

there are also baffles for further enhanced mass transfer. Continuous stirring is vital for controlling the heterogeneous phase’s highly exothermic reaction. The sodium is melted in toluene in the Melting Tank for better control of the heterogeneous phase reaction and easy handling of the large amount of hazardous sodium. The molten sodium is charged into the Reactor and kept under continuous stirring in the presence of an inert atmosphere. The polar solvent and phase transfer catalyst are added at low temperature. Then the temperature is increased to 100 °C and the other reactant (DMDS) is fed into the Reactor in a laminar flow at the rate of 0.15 l/min. After completion of the reaction, the reaction mass is quenched with isopropanol and drained into the Extraction Unit where it is washed with water to separate the organic and inorganic layers. The PDMS- and toluene-containing organic layer is charged into the Centrifuge. The purified PDMS is retained on the inner surface of the basket, and is recovered and kept inside the Vacuum Tray Dryer. A SCADA system using an HMI and PLC was employed for data acquisition and monitoring and controlling the polymerization process. The PLC greatly facilitates control of the vigorous exothermic reaction.

The optimized process parameters at the Pilot scale are listed in Table 5.3. Thermogravimetric analysis (TGA) of the PDMS (Fig. 5.10) showed that a 4.6% weight loss occurred at about 200 °C. This weight loss may be attributed to the

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Table 5.3 Optimized Pilot plant process parameters for PDMS Pilot plant scale process parameters of polydimethylsilane Batch time

:

5h

Temperature

:

110 °C

Sodium to DMDS ratio

:

1: 0.5 mol/mole

Sodium to Toluene Ratio

:

1: 5 gm/ml

Toluene to Dioxane Ratio

:

10: 1 ml/ml

Sodium to catalyst ratio

:

1: 0.001gm/ml

DMDS addition rate

:

0.15 l/min

Agitator RPM

:

140

vaporization of trapped solvent in the polymer and decomposition of low molecular weight polymers and other trapped impurities. At about 343 °C a weight loss of 69.6% weight was observed. This may be attributed to the evolution of hydrocarbon gases owing to the cleavage of Si–CH3 groups. The weight loss continued up to 450 °C, with about 20% residue left. This sequence of temperatures and weight loss values agrees very well with the literature. 120

6 343.06°C 4.653% (0.05835mg)

100

Weight (%)

80

69.60% (0.8728mg)

60

40

2

Deriv. Weight (%/min)

4

514.65°C 4.785% (0.06000mg)

100.61°C

0 20

0

0

200

400

600

Temperature (°C)

Fig. 5.10 TGA characterization of PDMS

800

-2 1000 Universal V4.7A TA Instruments

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5.4.2 PCS Synthesis At the laboratory level polycarbosilane (PCS) is synthesized by thermal rearrangement, see Fig. 5.11, in which polyborodiphenylsiloxane (PBSO) is used as the catalyst at 1 wt% of PDMS. The block diagram of PCS synthesis is shown in Fig. 5.12. The laboratory level synthesis was carried out at the 0.5 kg/batch level in a four-necked flask with reflux condenser. The thermal rearrangement reaction was continued for 100 h, and the temperature gradually rose to 400 °C. The product was dissolved in toluene and the solution was filtered to remove insoluble by-products and unreacted reactants. The optimized process parameters at the laboratory scale are listed in Table 5.4. The data obtained from laboratory scale synthesis and experience helped in determining the specifications for Pilot plant synthesis of PCS at the 10 kg/batch level. The PCS Pilot plant consists of a limpet coiled jacketed Reactor (MOC-Monel-400), shell and tube heat exchanger, catch pot, and receiver tank together with associated piping and instruments. Because of the endothermic as well as exothermic nature of the reaction, a PLC-operated SCADA system with HMI facility helps to operate the plant smoothly and in a hazard-free manner. The cooling at elevated temperatures is achieved by a combination of air and a liquid cooling medium. The temperature profile as optimized at Pilot plant level is shown in Fig. 5.13. The reaction time was reduced from 100 h at the laboratory scale to 55 h at the Pilot plant scale. The PCS Fig. 5.11 Chemical reaction of PCS synthesis

Fig. 5.12 Block diagram of PCS synthesis

Table 5.4 Optimized laboratory scale process parameters for PCS

Catalyst

:

1% PBSO

Reaction time

:

40 h

Atmosphere

:

argon

Temperature

:

400 °C

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400

Temperature 0C

350 300 250 200 150 100 50 0

0

5

10

15

20

25

30

35

40

45

50

55

Time (Hours) Fig. 5.13 Heating profile at Pilot scale for PCS synthesis

Pilot Plant flow diagram is shown in Fig. 5.14, and specifications of the Pilot plant equipment are given in Table 5.5. The thermal behaviour of the PCS was studied by using TGA. The TGA curves are shown in Fig. 5.15. In the first stage, the TGA weight loss curve shows a slight weight loss between 100 and 270 °C. This weight loss is due to evaporation of the reaction product H2 and some low molecular weight polycarbosilane (Akindoyo et al. 2016; Chen et al. 2008). In the second stage, the TGA curve falls off rapidly from 270 to 450 °C, corresponding to a large exothermic peak in the DTA curve. The weight loss in this region is 24.4 wt%, and it could be due to evaporation of gaseous products such as CH4 and H2 because of dehydrogenation and dehydrocarbonation condensation of polycarbosilane molecules. In the third stage, the TGA weight loss curve continues to fall from 450 to 640 °C, and the corresponding weight loss is 6.7 wt%. This weight loss could be due to decomposition of organic side groups such as Si–H, Si–CH3 , and C–H in Si–CH2 –Si (Shukla et al. 2004). Above 640 °C there is hardly any weight loss and the TGA plot shows crystallization of the amorphous phase. The high molecular weight leads to low weight loss during the cross-linking and pyrolysis, and thereby increases the ceramic yield.

5.4.3 SiC Synthesis from Rice Husk by the Arc Plasma Method The DMSRDE has indigenously developed the Arc Thermal Plasma Process for direct synthesis of SiC from rice husk, using a pot type 50 kW extended Arc Plasma Reactor with graphite electrodes. A schematic diagram of the Reactor is shown in Fig. 5.16. The possible reactions of the process can be written as:

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Fig. 5.14 Flow diagram for PCS Pilot plant: R = Reactor; C = Condenser; CP = Catch Pot; T = solvent recovery tank; M = Agitator coupling Table 5.5 Specification of PCS Pilot plant and equipment Specification of reactor (i)

Reactor volume

:

65 L

(ii)

Working volume

:

50 L

(iii)

MOC

:

Monel-400

(iv)

Working temperature

:

Up to 450 °C

(v)

Working pressure

:

Atmospheric

(vi)

Limpet Coil

:

25 NB SS-304 coil with 100 mm pitch

Specification of shell and tube heat exchanger (Condenser) (i)

Type of condenser

:

Fixed tube sheet

(ii)

Heat transfer area

:

1 m2

(iii)

Shell/tube pass

:

1/2

(iv)

Shell side fluid

:

Organic vapours

(v)

Tube side fluid

:

Coolant

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Fig. 5.15 TGA characterization of PCS

C(s) + SiO2 (s) = SiO(g) + CO(g)

(5.1)

SiO2 (s) + CO (g) = SiO(g) + CO2 (g)

(5.2)

C(s) + CO2 (g) = 2CO(g)

(5.3)

2C(s) + SiO(g) = SiC(s) + C(g)

(5.4)

Resulting in the overall reaction as: SiO2 (amorphous) + 3C (amorphous) → SiC + 2CO

(5.5)

In this process, raw rice husk samples were directly added to the reactor for plasma treatment. The product obtained consisted of SiC and unreacted carbon and silica. The removal of excess carbon and silica was performed by burning the pyrolysed product

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Fig. 5.16 Schematic diagram of the arc plasma reactor: 1. Mild steel casing 2. Graphite crucible 3. Plasma 4. Bubble alumina 5. Graphite base, 6. Alumina bush 7. Bottom electrode (graphite) 8. Water outlet 9. Copper connector 10. Water inlet 11. Magnesialining 12. Exhaust 13. Graphite bush 14. Top electrode (graphite) 15. Water inlet 16. Copper connector 17. Water outlet 18. Electrical insulation 19. Alumina bush 20. Rack and pinion arrangement 21. Charge

at 700 °C in an oxidizing atmosphere to remove carbon, followed by treatment with concentrated HF to remove silica. A graphite crucible was used as the hearth of the Reactor and was connected to a graphite electrode. This whole assembly constitutes the anode. The top graphite electrode, the cathode, had an axial hole to pass plasma-forming gas, and was attached

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Fig. 5.17 Block diagram for arc thermal plasma process

to a rack and pinion mechanism for vertical movement. Easily ionisable argon gas was used to form the plasma, thereby considerably extending the arc volume. The Reactor was connected to a 50 kW DC power source. The arc was initiated by shorting the electrodes. Experiments were carried out in batch operations, and the process conditions such as power and time were varied. The block diagram of this Arc Thermal Plasma Process route is shown in Fig. 5.17, and the process flow chart is given in Fig. 5.18. The pure SiC product consisted of micron-scale particles, Fig. 5.19a. Heat-treatment at 1800 °C in a high temperature tubular furnace resulted in whisker formation, Fig. 5.19b.

5.4.4 Bench Scale Synthesis of PEEK '

PEEK is synthesized by reacting 4,4 -difluorobenzophenone, hydroquinone, K2 CO3 and diphenylsulphone (DPS) as a solvent: the reaction process is illustrated in Fig. 5.20. The product is crude PEEK containing a large amount of DPS, with some potas' sium fluoride and unreacted 4,4 -difluorobenzophenone, hydroquinone, and K2 CO3 . The DPS and other impurities can be removed as follows: (i) a fine powder of ' crude PEEK was treated with methanol at ambient temperature to remove 4,4 difluorobenzophenone; (ii) treatment with water and acetone to help separate KF, hydroquinone, and K2 CO3 very easily from the desired product; and (iii) removal of DPS. However, removal of DPS is a challenging task, see Sect. 5.4.4.1, because it is present in solid solution in the crude PEEK. Also, we note here that DPS must be removed because its presence in PEEK drastically affects its mechanical properties, which are important in Defence applications.

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Fig. 5.18 Process flowchart for preparation of the final product, fine silicon carbide

Fig. 5.19 SEM micrographs: a Pure SiC after arc thermal plasma treatment of rice husk; b whisker formation owing to further treatment at 1800 °C in a high temperature tubular furnace

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Fig. 5.20 PEEK synthesis reaction steps

5.4.4.1

Purification Technique(s) for PEEK

Widely-used techniques for purification/separation in industries are: (i) distillation; (ii) absorption; (iii) adsorption; and (iv) solid–liquid extraction. Of these, solid–liquid extraction is the only technique suitable for PEEK purification. This was evaluated at the bench scale. Figure 5.21 shows the purification set-up for Peek synthesized in 2 kg batches. Based on extensive research the optimized process parameters for purifying 2 kg/batches of crude PEEK were determined and are summarized in Table 5.6.

5.4.4.2

Thermogravimetric Analysis (TGA) of PEEK

The Thermal decomposition of PEEK studied by TGA was a two-step process, with the first step of thermal decomposition centred at 480 °C and attributed to the random chain scission of the ether and ketone bonds. The second decomposition step occurred above 660 °C, see Fig. 5.22, and is attributed to the cracking and dehydrogenation of the cross-linked residue produced in the first stage of decomposition. This second step results in a thermally stable carbonaceous char (Lodhe et al. 2015; Banerjee et al. 2015) (Fig. 5.23).

5.4.5 PUs Synthesis Polyurethanes are synthesized by the prepolymer method, see Fig. 5.24, in which reacting difunctionalPolyol, i.e., Poly Tetra Methylene Ether Glycol (PTMEG) is ' reacted with difunctional 4,4 -dicyclohexylmethane diisocyanate (H12 MDI) to get the prepolymer. This prepolymer is subsequently mixed with either 1,4 Butane diol

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Fig. 5.21 Purification set-up for PEEK

Table 5.6 Optimized process parameters for purification of PEEK

Solvent

Acetone

Crude PEEK size

500 μm

Thimble mesh size

270 μm

Pure PEEK size

150 mesh

Purification time

8h

Purity of PEEK

99 ± 0.5%

Recovery of DPS

98%

or an amine, which are the chain extenders, catalysts, and additives, to get the desired PU. The synthesis reaction can be shown as in Fig. 5.25, together with the PU polymer structure. We note here that PUs are unique materials that offer the elasticity of rubber combined with the toughness and durability of metals. Polyurethanes have outstanding resistance to oxygen, ozone, sunlight, and general weather conditions that make their usage very attractive for aerospace applications. Also, polyurethane adhesives are excellent because (i) they can interact with the substrate through polar interactions (e.g. hydrogen bonding) and (ii) via molecular composition changes, whereby the adhesive stiffness, elasticity, and cross-linking

5 Development of Non-metallic Structural Materials for Defence Systems

Fig. 5.22 Block diagram for synthesis and purification of PEEK

Fig. 5.23 TGA characterization of PEEK

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Fig. 5.25 PU synthesis methods

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can be tailored to suit specific needs. PU adhesives are also highly transparent, and together with compatibility with polycarbonate (PC) and polymethylmethacrylate (PMMA) make them suitable for interlayers for windshields. The block diagram for PU adhesive synthesis is shown in Fig. 5.26. At DMSRDE PUs have been synthesized on a laboratory scale to obtain the required d properties for use in Anti–mine Blast Protective suits for the military (Army). Based on the optimized process parameters obtained on the laboratory scale, a Pilot plant for the production of transparent PU Adhesives was planned at the level of 25 kg/batch. The specifications for the Pilot plant were finalized, and the plant was erected and commissioned. More than 1000 kg of the required PU has been produced successfully in this plant. The specifications for the PU Reactor are listed in Table 5.7. The optimised process parameters for the 25 kg/batch Pilot Scale synthesis are given in Table 5.8. The various mechanical properties of PUs, namely the Melt Flow Index (MFI), tensile strength, and % elongation have been studied as functions of the block ratios

Fig. 5.26 Block diagram for synthesis of PU adhesives

Table 5.7 Specifications for the PU reactor Description

Specifications

Reactor total/working volume

32 lit/25 lit

Reactor MOC

SS-316 shell, 5 mm thickness

Jacket MOC

M.S. shell, 5 mm thickness

H/D ratio

1:1

Reactor working pressure

100 mm Hg to 3 kg/cm2

Jacket pressure

10 kg/cm2

Working temperature Max. Vessel

125 °C

Working temperature Max. Jacket

180 °C steam

Stirrer type

Anchor coupled with pitched blade

Motor HP/RPM

1.0 HP/50–500

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Polyol: Isocyanate: diol (mole ratios)

1:4.25:3.25

Temperature

120 °C

Time

1.5 h

Catalyst

0.05% of BDO

Additives

0.1% of prepolymer

of the reactants, see Fig. 5.27. This Figure shows that the MFI and % elongation of PEEK decreases with increasing block ratio, whereas the tensile strength increases with increasing block ratio. Table 5.9 summarizes the optimized combination of these properties.

Fig. 5.27 Studies of MFI, strength, and elongation properties of PEEK

Table 5.9 Optimized MFI, strength, and elongation properties of PU

Melt flow index (MFI)

:

6.45 gm/10 min

Tensile strength

:

14.18 MPa

% elongation

:

523

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5.5 Applications for Defence Systems PDMS is used as a precursor for Polycarbosilane (PCS), and PCS is employed for the production of SiC fibres which leads to the formation of ceramic matrix composites. These will be used for high temperature applications, e.g. as structural materials in the exhaust system of the Kaveri Gas Turbine Engine and the nose cone of the Light Combat Aircraft (LCA). SiC fibres made from PCS have good properties such as high temperature strength, light weight, and weather resistivity. SiC matrix composites are used as composite armour plates in bulletproof vests for armed forces, owing to its good properties mentioned above. These properties make it suitable for frontline battle operations. SiC matrix composites are also expected to overcome the shortcomings of currently used carbon–carbon friction material in aircraft landing gear brakes, since they resist fading and temperature increases. SiC also finds applications as a semiconductor material in strategic areas. SiC in powder form is a potential precursor material for making armour and turbine blades in the LCA engine, as well as for the inner linings of high temperature reactors. Other, more innovative, uses of SiC include its use in graphene production. PEEK is an engineering thermoplastic that possesses excellent chemical resistance, light weight, high strength, high temperature resistance, and good resistance to burning. Owing to these excellent properties PEEK is very useful for aerospace applications. The applications of PEEK include insulating tubes in fuel delivery systems, and secondary structural panels (door and base panels) for the LCA. PEEK is also used in the Kaveri engine seal assembly and in the Main battle Tank (MBT) hydropneumatic suspension unit (HSU). PUs have outstanding resistance to oxygen, ozone, sunlight and general weather conditions, and good strength/weight ratios. These properties also make them useful for aerospace applications: (i) PUs remain flexible during continuous use at temperatures up to 80 °C and down to −20 °C; (ii) the transparency is above 80%, making them useful as interlayer adhesives for windshields fabricated by joining polycarbonate and polymethylmethacrylate sheets. PUs also have very high impact strength, making them useful especially for fabricating Anti–mine Personnel Blast Protection Suits; and also they are ideal materials for loaded wheels, heavy-duty couplings, metal-forming pads, shock pads, expansion joints, and machine mounts.

5.6 General Summary and Remarks The present chapter discusses five non-metallic materials: (i) PDMS, (ii) PCS, (iii) SiC, (iv) PEEK, and (v) PU, for their potential defence structural applications. Owing to the importance of these materials for the Defence sector, there has been a requirement to develop Pilot plant production of these materials. Hence the various steps for a detailed engineering study to establish Pilot plants were carried out as follows: (a) project scheduling; (b) process calculations including materials and energy balances;

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(c) laboratory scale synthesis; (d) block flow diagrams; (e) process flow diagrams; (f) process and instrumentation drawings; (g) specifications and drawings for all equipment (process, mechanical, structural, piping, electrical and instrument); (h) control philosophy and automation; (i) safety systems; and (j) regulatory steps. The details of Supervisory Control And Data Acquisition (SCADA) have also been discussed. The process parameters were optimized on a laboratory scale, and further data were obtained at the Pilot plant level. (SCADA) have also been discussed. The production processes were carried out at Pilot plants using PLC-based remote control for safe and smooth operation. Much attention was given to the selection of proper raw materials and structural materials for the Reactors, together with the heating/ cooling modes in each case. One of the key factors in the development of these non-metallic materials was that the solvent used should be non-toxic and non-hazardous. Adequate design must account for the flammability and explosiveness of the raw materials, their byproducts, and the final product recovery method. These aspects have been discussed thoroughly. The future trends and features influencing next-generation Pilot plants are: (i) outsourcing; (ii) automation; (iii) fugitive emissions; (iv) multiple trains; (v) online analytical capabilities; (v) safety and control system interaction; (vi) wireless technology; (vii) instrument availability; (viii) instrument multi-functionality; and (ix) unit size. Acknowledgements The authors acknowledge the support and vital inputs by Shri S. K. Singh, Senior Scientist, IMMT (CSIR), Bhuwaneshwar, India; and also the critical review by Dr. R. J. H. Wanhill.

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Chapter 6

Defence Protective Textiles Mukesh Kr. Sinha, Biswa Ranjan Das, Anurag Srivastava, and Eswara Prasad Namburi

Abstract Material scientists and particularly textile technologists across the globe have conveyed the fundamental aspects and state of the art of defence protective textiles (DPTs) which are of interest to strategic sectors. An overview of DPTs on various aspects of manufacturing including processing methods and fabrication technologies of different products has been discussed. In addition, the present chapter also deals with research initiatives of the Defence Materials and Stores Research and Development Establishment (DMSRDE), DRDO, Kanpur in the development of specialty DPTs with emphasis on challenges and recent trends. Activated carbon spheres (ACS) have been found to be one of the most promising materials for the development of lighter weight and durable DPTs particularly for chemical, biological, nuclear, and radiological (CBRN) protection. Keywords Technical textiles · Advanced textile materials · Defence products · Manufacturing and qualification

6.1 Introduction Technical textiles comprised of fibres, yarns, and fabrics are capable of performing high-performance technical and functional properties besides meeting the desirable aspects of design, aesthetic, decorative, surface appearance, and texture. In short, M. Kr. Sinha Ministry of Textiles (On Deputation from DMSRDE, DRDO), New Delhi, India B. R. Das · A. Srivastava Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India E. P. Namburi (B) Former Outstanding Scientist and Ex-Director, Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 E. P. Namburi et al. (eds.), Novel Defence Functional and Engineering Materials (NDFEM) Volume 2, Indian Institute of Metals Series, https://doi.org/10.1007/978-981-99-9795-4_6

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technical textiles are defined as industrial textiles covering both functional and highperformance textiles. Specialized functional fabrics which are covered under the umbrella of technical textile include agrotech (agro textiles), buildtech (construction textiles) clothtech (clothing textiles), geotech (geo textiles), hometech (home textiles), indutech (industrial textiles), meditech (medical textiles), mobiltech (automotive textiles), packtech (packaging textiles), protech (protective textiles), and sportech (sports textiles) (Getu and Sahu 2014; Vaishnav and Sharma 2015; Das and Nizam 2014). The old concept of Defence protective textiles (DPTs), which are Protech, emphasized on functional properties of fabrics rather than aesthetics has been changed and now DPTs should contain both functional as well as aesthetics (surface properties) so that the soldiers should look smart and intelligent with enhanced fighting capability (Das and Nizam 2014). Modern versions of DPTs are meant to fulfil the stringent army requirement to protect the troop during combat operations, emergency operations, and critical survival situations to protect themselves and minimize casualties during war scenarios. Therefore, the range of textile materials used in Defence has a highly diversified spectrum, viz., protective clothing for extreme cold weather (ECW); chemical, biological, nuclear, and radiological (CBRN) protective clothing (specific performance for neutralizing deadly warfare agents); bulletproof soft armour composite panels; camouflage fabrics; inflatable boats and protective covers like tents, shelters, and habitats, and textile-based medical category items (Kovacevic et al. 2012; Young et al. 2000; Boopathi et al. 2008). Using commercially available functional fibres enables product development for a specific application such as moisture management, conductive, hydrophilic, antibacterial, flame retardant, antistatic, smart, and intelligent fibres, and fabrics. The advantages of using such functional fabrics could be tailored into the development of various DPTs suitable for applications, e.g., sensors, electromagnetic interference shielding (EMI) clothing, and lightweight cold weather protective clothing with a high degree of comfort management (Matteo and Alessandro 2014). Commercially available high-performance and functional fibres like Silicon Carbide (Nicalon), Carbon, p-aramid (Kevlar), Poly P-Phenylenebenzobisoxazole (PBO, Zylon), Ultrahigh molecular weight polyethylene (UHMWPE, Spectra, and Dyneema) have found increasing importance in defence products (Kumar 1991; Prasad et al. 2016). These high-performance fibres have specific physical properties unique to these fibres, e.g., higher tensile strength, chemical, and superior thermal resistance compared to conventional grades of polymeric fibres. Polymeric high-performance fibres have tenacity in the range of 15–50 g/d and tensile modulus of 150–400 g/d. Multinational companies like DuPont, Honeywell, Dutch State Mines (DSM), and Niclon are having global trade monopolies for the manufacture of such high-performance fibres (Kumar 1991). Defence and Aerospace sectors are primary consumers of these fibres for making nuclear, biological, and chemical (NBC) clothing, aerostat, inflatable tents, and shelters for desert and cold regions, and ballistic items, e.g., bullet proof jacket (BPJ), ballistic helmets. In the area of nano-textiles, research and development initiatives focus more on the development of specialized nano-finishes and coating,

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functional dyes, and auxiliaries, incorporating nano-sized additives into fibres for creating functional composite fibres and nanofibre webs coated on non-woven fabrics (Huang et al. 2003). In today’s war scenario, various sophisticated armaments, e.g., missiles, tanks, and aircrafts, are used. Thus the soldiers need military functional DPTs based uniform which plays an important role in deciding the fate of war and the number of causalities inclusive of themselves and civilians. DPTs, therefore, have significance in the context of several major challenges to be addressed in order to keep pace with technological and product upgradation. They are (a) weight of fabric (g/m2 ), (b) functional performance, (c) comfort and durability, (d) aesthetic and design, (e) surface characteristics, and (f) to produce fabrics with functionalities such as hydrophilic, anti-microbial, wash-n–wear finishes, soft finishers and chemical warfare and biological protection. By using specialized nano-finishes and a choice of high-performance functional textiles based on chitosan, sliver particles, chlorosulphonated polyethylene, fluoroelastomer, etc., the mission can be succeeded. This book chapter is an attempt to give insight on technological development which helped to meet various challenges of DPTs to obtain optimized lighter fabrics meeting functional requirements. The description has been dedicated to the continual effort made on technological research to overcome the above impediments, data utilization, use of functional and high-performance fibres, nanofibre web coatings, smart colourants (chameleon fibres for auto camouflaging), bi-component fibres, enzymatic surface modification (eco textiles), semi-permeable fabrics (clothing engineering/comfort and microclimates), tri-layered breathable membrane sandwiched fabric, micro-denier fibres, 3D weaving technology, friction spinning technology, film laminated fabric and adhesion technology. Several natural fibres melt blown fibre, spinning (non-woven) technology, multifunctional fibres/fabrics, functional auxiliaries, and finishing agents and phase change materials (PCMs) are occasionally being discussed for the purpose.

6.2 Classification of DPTs DPTs are developed to meet the specific requirements of different war field scenarios such as CBRN protective clothing, cold weather clothing (blankets, mattresses, sleeping bags, gloves boots, etc.), life jackets, tents, tarpaulins, canvas, load-carrying fabric (ropes, harness, and rucksack), blast and NBC protective tent and shelter, combat operational suits, camouflaged fabrics, medical items and garments (wound healing and dressings textiles, respiratory masks and decontamination kits, causality evacuation bag, insect repellent clothing), awning and canopies (Adanur and Tiwary 1997). Thus, the classification of DPTs is highly complicated and all kinds of protection syndromes could not be covered under a single umbrella. However, an attempt has been made to classify the DPTs based on our practical research experiences enumerated in Fig. 6.1.

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Fig. 6.1 Classification of DPTs

Individual protective equipment (IPE) for CBRN protection provides protection to soldiers against weapons of mass destruction (WMD). Current trend WMD has been switched to CBRN from the old NBC concept to meet more stringent warfare requirements. Therefore, new generation DPTs can be divided into either (a) permeable items, e.g., breathable suit, Casualty bag (full), Casualty bag (half) and Facelet mask, and (b) impermeable items—Decontamination suit, Overboot, Gloves and Haversack to provide protection against chemical warfare agents, biological and viral agents, soft beta nuclear radiation and radiological dust in fall-out operations in war field. The recent trend also focuses on the inclusion of biological and chemical sensors in the CBRN suits for threat detection (Kovacevic et al. 2012; Young et al. 2000; Boopathi et al. 2008; Karkalic et al. 2015). Heavy textiles include tents, protective covers (vehicles/aircraft), tarpaulins, and canvas shelters for casualties in army hospitals, inflatable radome for both cold and hot weather (Van et al. 1998; Rubeziene et al. 2008; Surdu et al. 2015; Ali et al. 2016). Further, the DPTs composed of heavy textiles are classified into two major segments—(1) functional shelters and tents and (2) strategic shelters or protective covers. (1) Functional Shelter and Tents: These are mainly used for providing shelters for soldiers engaged in mortar position controller, combat operational tents, tent desert, and tent arctic large. (2) Strategic Shelter or Protective Covers: These types of shelter are created for covering war equipment and animations, e.g., shelter for missiles coverings of Pent House and night shelters, tank and artillery vehicle covers. High altitude clothing gears (HACGs) for protection against extremely low temperature (ECW) from −40 to −50 °C vary from harnesses to jackets and trousers to cover each and every part of the human body, as illustrated in Fig. 6.2. The primary functional requirements for these items are to meet UV protection, wind, and waterproofing with adequate water vapour transmission (Bivainyte et al. 2012; Kasturiya et al. 1999; Purane and Panigrahi 2007). In the war field, the soldiers are faced with a wide range of threats. Conventionally army combat uniforms consist of shirts made from cotton cellulose poplin fabric and trousers made from drill cotton fabric disruptively printed in order to provide

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Fig. 6.2 Classification of HACG

camouflage protection in the visible to NIR region. However, the trend has been changed and in the current scenario, a soldier is to be equipped with clothing that provides him simultaneous protection from all potential threats to enhance his capabilities. Soldier in the new millennium will, therefore, be considered as a ‘Solider of a complete system’ capable of generation of electricity, multifunctional protective performance, and lower weight. The classification suggests combining most of the functional properties in one umbrella and inserting and equipping the fabric with small conductive chips for electronic data processing, ammunition charging, sensors, wireless networking, and communications, and inserting tubings for cooling within the garment for low physiological stresses (enhanced comfort). The lightweight clothing will assist the soldiers to remain efficient even in extreme climates and harsh conditions. Therefore, in the coming era all functional properties of clothing will be combined into one multifunctional grid type of weave fabric structure to evolve a smart and intelligent protective clothing (SIPC) (Rubeziene et al. 2008; Dolez and Vu-Khanh 2009), and naturally, the type of clothing will be changed from DPTs to SIPC.

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6.3 Manufacturing Process of DPTs 6.3.1 CBRN Agents Amongst the WMDs, Chemical warfare agents (CWAs) are one of the most brutal agents which are created by mankind (Ganesan et al. 2010; Lal and Tripathi 2009; Khalil 2015). Exposures to CWAs are anticipated to kill humans or any living organism. Typical CWAs are listed in Table 6.1. All these CWAs mainly attack and affect the eyes and skin of human beings, except the chocking agents which act through the respiratory system. Therefore, skin protection from these CWAs is foremost important in the warfare zone (Ganesan et al. 2010). A biological warfare (BW) agent involves the use of living organisms which could also be used as a WMD. Classical biological warfare agents include bacteria, viruses, fungi, protozoa, and toxins from organic matter. In the present time, nuclear weapons are derived from both nuclear fusion and fission types of bombs. However, destruction due to nuclear fusion, e.g., a hydrogen bomb cannot be protected by the use of protective clothing. A nuclear fission results in the emission of harmful radiation in the form of alpha, beta, and gamma rays at the site of detonation and also in the vicinity as a fall-out radiation (Lal and Tripathi 2009). The fission type of nuclear attack can be protected by shielding using protective fabrics containing heavy metals, e.g., lead (Pb), lead oxides, and others. Table 6.1 Types of chemical warfare agents (CWAs) (Ganesan et al. 2010) Name of CWAs

Chemical structure

Types of CWAs

Physical state at 20 °C

Phosgene CG

COCl2

Choking agent

Gas

Diphosgene DP

C2 Cl4 O2

Choking agent

Liquid

Tabun GA

C5 H11 N2 O2 P

Nerve agent

Liquid

Sarin GB

C4 H10 FO2 P

Nerve agent

Liquid

Soman GD

C7 H16 FO2 P

Nerve agent

Liquid

Nitrogen mustard HN-1

C6 H13 Cl2 N

Blistering agent

Liquid

Sulphur mustard HD

C4 H8 Cl2 S

Blistering agent

Liquid

Arsine SA

AsH3

Blood agent

Gas

Cyanogen chloride CK

CNCl

Blood agent

Gas

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Table 6.2 Leading manufacturers of CBRN protective suit Sr. No

Suit brand

Company

Country

1

Demron

Radiation Shield Technologies, Coral Gables

USA

2

Saratoga

Blucher, Erkrath

Germany

3

Safeguard TM

Karcher, Schwaikheim

Germany

4

Remploy

Frontline, Birkenhead, Merseyside

UK

5

Protective suit type II

Supergum, Tel-Aviv

Israel

6

L-1 chemical protective suit

Slavyanskaya, Slavyansk-on-Kuban, ul. Victory

Russia

6.3.2 CBRN Protective Garments 6.3.2.1

International Scenario

Various global players manufacturing the CBRN suits are listed in Table 6.2. However, only Demron is a radiation-jamming fabric used by Radiation Shield Technologies, USA to develop the item. Demron shields effectively the high-energy alpha (α) and beta (β) and also reduces the low-energy gamma (γ) radiations (Karkalic et al. 2015; Lal and Tripathi 2009). It can be coupled with different protective chemical and biological agents as well. The fabric provides radiation protection by Pb shielding, although the made-up item is lighter weight, soft, and flexible. However, CBRN suit manufacturing technologies with details of material compositions and fabrication details are closely guarded secrets.

6.3.2.2

Manufacturing Technique of CBRN Suit

CBRN suit devotement is a challenging and on-going research activity that needs continuous improvement in terms of heat stress/comfort and weight reduction. Previously most of the chemical protective adsorbent layers were developed with film/ membrane/charcoal/foam and active carbon spheres (ACSs). The earlier developed CBRN suits displayed many limitations like poor adhesion of ACS with the substrate fabric, low crushing strength of ACS, poor peeling strength of lamination, and low durability due to frequent carbon shedding. In addition, the composite layer was quite stiff and hence inadequate to meet the desired comfort level besides, they are nondurable, heavyweight, have poor adhesion/lamination with fabric, stiff (discomfort on wear), low air permeability, and poor porosity for necessary water vapour transmission. The state-of-the-art technology for ACS has been developed by Blucher, GmbH/Germany (Saratoga™) and it offers the highest degree of mustard vapour adsorption with minimum stress development under static and dynamic conditions (Karkalic et al. 2015; Khalil 2015). Presently, most of the reported CBRN suits are based on activated carbons in the form of spherical shape and are coated and affixed

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Fig. 6.3 A schematic flow diagram of CBRN suit manufacturing technology

on the fabrics. A schematic manufacturing process of the CBRN suit is presented in Fig. 6.3. Body protection against CWAs can be envisaged through, (a) the disintegration of the chemicals by the adsorbing materials, (b) by using semi-permeable material that allows passage of only air but not CWAs, and (c) the use of suitable carbonaceous materials as effective adsorbent of CWAs. Out of all these propositions, the last one is the most commonly used and acceptable worldwide. The role of carbonaceous adsorbents is to adsorb the CWAs in their surface pores and the method of preparation of ACS involves the use of a variety of polymer resins, e.g., phenolic types, polyacrylonitrile (PAN) or polystyrene (PS) sulphonate resins which are subjected to first drying at 100–110 °C, followed by their stepwise carbonization under an inert atmosphere at 900–1100 °C, and subsequent heating at the rate of 3–5 °C/min. Partially converted carbon, thus obtained, is subjected to activation by carbonization at ~950 °C underflow of carbon dioxide in order to obtain high-strength micro-porous activated carbon spheres (Chen et al. 2009a). The micro-porosity is generated on the surface of ACS due to the release/decomposition of H2 , CO, and CO2 . The flow diagram of the manufacturing process of ACS is shown in Fig. 6.4. Characteristic properties of ACS used for the development of DPTs are given in Table 6.3. In order to impart higher ACS strength, the metallic compound is also impregnated in the carbonaceous resin as a doping material. Figure 6.5a gives an SEM photograph

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Acticated carbon

Activated carbon fibres

Activated carbon powder or granules

Polymeric base material Phenolic Resin

Carbonization PAN

Viscose

Activation Carbonization

Carbonization

Activation Activation Fig. 6.4 General flow diagram of the ACS manufacturing process

Table 6.3 Properties of ACS (Sinha et al. 2017) BET surface Total pore area (m2 /g)6.1 volume (cc/ g)6.1

Micropore volume (cc/ g)6.1

CTC adsorption (wt %)2

Bulk density (g/ cc)3

Crushing strength (kgf/ bead)4

Diameter (mm)5

~1050–1100

0.39–0.65

40–70

0.55–0.64

1.5–2.5

200–500

0.35–0.56

1

Nitrogen adsorption isotherm at 196 °C, obtained by Thermoforming surface area analyzer, 2 ASTM D3467, 3 Graduated cylindrical column, 4 IS: 1060 Pt 1 and 5 Foot-dial guage, under pressure 200 g/cm2

of the shape of an ACS, Fig. 6.5b the presence of Al particles used for doping, and Fig. 6.5c the nature of the surface pores responsible for CWAs adsorption. The two most important properties govern the performance of CBRN suits, e.g., strength and porosity. Strength influences the adhesion and susceptibility to crushing of the ACS thereby the extent of shedding, while the porosity affects the mustard gas adsorption capacity. Also, these properties are contrary in nature. An increase in porosity concomitantly reduces the strength of the ACS. Hence, a surface area of 1050–1100 m2 /g and crushing strength of ~1.5–2.0 kgf/bead are the optimum values of ACS for the manufacture of the CBRN suit. Adsorbent materials are fixed on the substrate fabric and subsequently laminated by non-woven fabrics to prepare laminated composite fabrics via a continuous coating and lamination process (Boopathi

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Fig. 6.5 SEM images of ACS: a spherical shape, b elemental composition form EDX analysis, and c surface porosity (Chen et al. 2009b)

et al. 2008; Lal and Tripathi 2009). A schematic flow set-up of the continuous coating and lamination process is shown in Fig. 6.6.

Fig. 6.6 Continuous coating and lamination process for ACS laminated fabric

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The whole manufacturing process of manufacturing ACS laminated fabric is divided into three zones, (a) Coating, (b) fixation, and (c) lamination. The base fabric includes polyester, nylon, cotton, viscose, polyester/viscose, and polyester/ cotton blends knitted or non-woven of ~100–120 g/m2 . A polyurethane-based adhesive (40–50 g/m2 ) is printed on the substrate fabric in a hexagonal close-packed (HCP) pattern. ACS in the weight range of 200–210 g/m2 is sprayed on the base fabric through a mesh (size 0.3–0.5 mm) attached to a hopper (as shown in Fig. 6.6). The optimization of the coating with varied add-on content of ACS is carried out with respect to air permeability. ACS content 190 g/m2 of ACS, better air permeability (comfort) is found. An ACS add-on of ~210 g/m2 has been found to fulfil the necessity of comfort level (air permeability (~50–65 cc/cm2 /sec at 10 mm water head), very good chemical protection, and acceptable weight of the coated textile when compared with the international standards (Defence Materials Stores Research and Development Establishment-technical report 2015a, 2016). Fixation of ACS on the substrate is carried out by passing the fabric through a series of drying chambers maintained at 160–170 °C, under the stretched condition to minimize any widthwise shrinkage through suitable guide rolls, then passed through heated rolls for curing at 160– 170 °C at a residence time period of 40–60 s. After curing, the fabric is ready for lamination. Spun-bonded polyester non-woven fabric (35–40 g/m2 ) is impregnated with low-density polyethylene (LDPE) dots aggregated on the fabric surface which is responsible for adhering to ACS-coated fabric during the lamination process. A microscopic examination of polyester non-woven fabric confirms the presence of LDPE dots (Fig. 6.7). A schematic view of the lamination process is shown in Fig. 6.8. The ACS-adhered woven fabric and non-woven fabric are mounted on roll shafts in such a manner that, when unwounded, the LDPE-coated side of the nonwoven fabric faces the side of the ACS-adhered woven fabric. The two fabrics then pass together on heated rolls maintained at 140–150 °C for effective lamination. Subsequently, through a number of guide rolls the fabric is wound on a delivery roll. Normally, the linear speed of coating and that of lamination is maintained at 5–6 m/min when ACS was physically and uniformly deposited and laminated with the non-woven fabric. The chemical protection level of the optimized samples is found to meet the protection requirement of mustard gas penetration time, even after washing for six wash cycles (Defence Materials Stores Research and Development Establishment-technical report 2015a, b). The outer layer of the CBRN suit is polyester fabric printed in a camouflage pattern having multifunctional properties, e.g., flame retardancy, water and oil repellency, antistatic, air permeability, and adequate mechanical strength (tensile and tear). To meet these functional properties, it is necessary to choose types of fibre, select fabric construction, and additives for functional finishes. Recent trends exist for the use of specialty performance fibres like nomex, keramel, linzing FR and its blend, and vectron as woven in different forms, e.g., twill, rib stop or plain weave, or knitted structures. As per international standards, the outer shell consists of ~90– 110 g/m2 nylon/cotton blended twill for woodland camouflage or tri-blend (cotton/

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Fig. 6.7 Microscopic image of polyester non-woven fabric impregnated with LDPE adhesive dots

Spun Bonded Non-woven (LDPE powder) ACS Adhered Polyester Base fabric Thermal Bonding via Hot Roll Melt Process ACS laminated NBC adsorbent fabric Fig. 6.8 A schematic flow diagram of the Lamination process

nylon/kevlar) twill for desert camouflage fabrics in addition to water-repellent treatment. The outer shell fabric is normally abrasive resistant and tear strength of ~5–6 kgf with rib stop weave junction which prevents the spreading of tearing is highly preferred (Fig. 6.9). The representative image of CBRN fabric displaying its various constituent’s layers is shown in Fig. 6.10. The manufactured layer is stitched in different sizes from large to small-sized suits and subsequently seam locked with IR sealing to avoid penetration of CWA inside during the use of the suit (Boopathi et al. 2008; Defence Materials Stores Research and Development Establishment-technical report 2016). Apart from the CBRN suit, other clothing items like Overboot and Gloves are also used for the protection of soldiers’ feet and hand respectively in contaminated

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Fig. 6.9 SEM image of outer shell fabric (MacMillin 2014)

Fig. 6.10 Different layers of CBRN fabric (https://www.blauer.com/chembio_xrt/)

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environment. Bromobutyl isobutylene isoprene (BIIR) rubber compound with antistatic and flame retardant additives are used in the development of these items for at least 24 h break-through time (BTT) in mustard vapour. Under the same category, belt, haversack, personal decontamination kit bag, face mask, colour detector paper, and first aid medical items are carried by individual during CBRN emergency. A multilayer polymeric film which is sandwiched between specially coated and finished polyester/nylon fabric is used as auxiliary components, e.g., decontamination suit which is manufactured by a multilayer polymeric film laminated with polyester fabric. This suit is used for the evacuation of army personal from war front contaminated with CWAs. Because these are not wholly textile-based items, a detailed discussion of their fabrication is avoided in this chapter.

6.4 Heavy Textiles Heavy textiles, in the form of tents and shelters, provide protection to soliders from rough to normal as well as in wet climatic conditions and therefore, should be waterproof and structurally stable for outdoor applications. Heavy textiles used to be produced by coating with paraffinic hydrocarbon wax on cotton fabric (~80–110 g/ m2 ) have limitations like their durability, poor breathability, and failure in abnormal climatic conditions. The main functional requirements of tent fabric are sufficient water repellency, waterproofness, and breathability. However, as the waterproofness of the tent is increased, the breathability deteriorates due to the blocking of the fabric pores by the use of the excess coating. Modern shelters and tents fabrics, besides being waterproof and flame retardant, are also ultraviolet (UV) resistant. However, as per the scenario, additional requirement arises, e.g., the Arctic tent should also be windproof in addition to becoming waterproof and breathable (Van et al. 1998; Surdu et al. 2015). In the case of tents required for ammunition storage, the fabric should be antistatic to dissipate any charge generation during storage. Apart from these, DPTs for tents are required to be printed with camouflage paints for matching/ concealment with background terrains (either snow bond desert or green belt areas). Nowadays shelter tents are also made of blast resistant and CBRN protective. Two main factors need to be considered while designing the tent fabric structures, (a) selection weaving patterns of fibres, and (b) introduction of desirable functionality. Textile fabrics used for the fabrication of tents are heavy textiles and weight and density of tent fabrics normally range between 350 and 650 g/m2 , depending upon the end user requirements. Figure 6.11 shows a set of optical micrographs of different compact weave patterns/designs of these fabrics. Compact woven structure offers a lightweight, waterproof, breathable, UV-resistant tent (Saravanan 2007; Das et al. 2010). The particular fabric is a durable, tightly compact, dense woven structure and possesses small pores’ sizes (~30–40 μm) for trapping of functional coated material (fluorocarbon) or PTFE which facilitates a higher degree of adhesion. This fabric also shows a flat surface, good abrasion resistance, and excellent breathability, and does not get stiffened and the surface feel or texture remains soft and smooth.

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Fig. 6.11 Optical microscopy images of various types of compact weaved fabric structures (magnification level 50X) (Schacher et al. 2012)

Materials and design of tent fabric observed significant changes with the introduction of man-made nylon, polyester, and acrylic fabrics in the search for a more comfortable shelter with enhanced barrier layer properties from rains, mild dew, cold, hot, and humidity. These lead to the current development of fabrics encompassing both waterproof and permeability to water and air vapour (Sen 2001; MacMillin 2014; Singha 2012). Tightly woven structures coated with micro-porous polymeric membrane to control air permeability and hydrophilicity are introduced. Modern shelter fabrics are produced by coating or lamination of closely woven fabrics with (1) hydrophilic membranes, (2) micro-porous membranes/films, and (3) hydrophobic coated textiles (Surdu et al. 2015; Schacher et al. 2012; Frances and Cooper 2007).

6.4.1 Hydrophilic Membranes A hydrophilic membrane consists of functional groups such as −O−, −OH−, −CO− or −NH2 which are present in main chains of the polymers like Polyvinyl alcohol (PVA), Polyacrylamide (PA), and Polyethylene oxide (PEO). These polymers are water sensitive and when exposed to or remain in contact with say rainwater for an extended period, either they swell or dissolve allowing rapid diffusion of hot air vapour from inside the tent generated by the metabolic activities of human being.

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Fig. 6.12 SEM images of, a micro-filament fabric, b PTFE micro-porous film, and c fabric surface coated with micro-porous PTFE film (Fung 2002)

6.4.1.1

Micro-porous Membranes and Films

These are made of polyurethane, fluorocarbon, polytetrafluoroethylenes, acrylics, polyamino acids membranes, or films having pore size ~0.1–50 mm and are laminated onto the weaved fabric. Figure 6.12 shows a decrease in the inter-yarn gaps due to the coating of micro-porous film, which suffices water vapour and air transportation but obstructs penetration of water/rain droplets. In case of micro-porous membranes of PTFE, where each of the carbon atoms in the main polymeric chains are fully surrounded/bonded to fluorine (F) atoms, high carbon-fluorine (C-F) bond strength also render chemical inertness, UV and photo degradation resistances.

6.4.2 Hydrophobic Textiles Commercially available methods for the development of such technical grades fabrics involve, (a) tightly woven fabric structure of either PET or Nylon, as shown in Fig. 6.12a, b woven structure of cotton or other synthetic polymers coated with water-repellent polymers, e.g., PTFE having coating density 20–80 g/m2 . A suitable coating technique is used to layer the fabric by spreading through specialized textile auxiliaries, and putting on one or more layers of coating for the creation of waterproofness. The coatings’ compositions are formulated in a suitable solvent in the form of dough and applied as a thick paste onto the fabric and dried subsequently, cured for adhesion and fixation of the coating film. Commonly used PVC/ PU-coated polyester fabrics for tent application are manufactured by this method. Another commercially practised coating method consists of spreading of a melted polyethylene polymer (PE) over the fabric substrate and chilled down to prepare PEcoated sheets which are highly durable. Apart from waterproofing, tent fabrics also need some additional functional properties, e.g., flame retardancy, anti-microbial, and mothproof which are achieved by mixing functional ingredients into the dough. A waterproof/protective tarpaulin and tent have to perform multi-purpose tasks and therefore, their base fabrics need to be multifunctional in nature (Singha 2012). The

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coating formulation with different textile grades of polymers, e.g., Polyvinyl chloride (PVC), Polyurethane (PU), acrylic, Polytetrafluoroethylene (PTFE), Polychloroprene rubber (e.g., Neoprene–DuPont CR) and Chlorosulphonated Polyethylene rubber (e.g., Hypalon DuPont) are extensively used for making these multi-purpose tents. Furthermore, it is also possible that the coating formulations consist of reinforcing fillers to improve the mechanical and specialized additives, e.g., UV radiation absorbers, coloured pigments, heat stabilizers, and antioxidants for improvement of the service life of the products.

6.5 High Altitude Clothing Gear (HACG) Cold weather clothing gears include protective clothing to protect the human body from exposure at very low temperature at −50 to −60 °C which could cause various infectious diseases and loss of life. These gears, named as extreme cold weather protective clothing system (ECWPCS), have five important factors for the design of their fabrics, (a) thickness—thicker the fabric greater is the insulation, (b) dryness— in wet conditions the fabrics no longer provide adequate insulation, (c) wind proofness—wind reduces insulation, so fabric should be windproof, (d) coverage—whole body part should be covered by same amount of insulation, and (e) flexibility— multiple layers may be easily added or removed to avoid being either too hot or too cold (Kasturiya et al. 1999; Das et al. 2013). Breathable and waterproof clothing system transfers moisture vapour which depends upon, (a) absolute moisture comfort gradient, (b) temperature gradient across the waterproof breathable protective layers (top and bottom), (c) humidity factor on the clothing microclimate, and (d) interface between the water vapour and the fabric layers. While designing a breathable ECWPCS, the basic consideration that needs to be practised is (i) waterproofness, durability/flexibility of coating/lamination, comfort, thermal insulation capacity, aesthetic design, water vapour transmission rate (WVTR), efficiency against wind chill, strength properties (e.g., tear, tensile and peeling strength), flexing and abrasion resistance, launderability (washing fastness), seam sealing, the adhesion strength of coating with the fabric, and resistance to UV and insecticides. Amongst so many factors to combine together into one protective gear evidently is a challenging task. However, two of the most important properties are clothing comfort and thermal insulation (Giesbrecht 2003).

6.5.1 Inner Layer The layer exists next to the skin as the innermost layers and should have high vapour permeability to transfer or wick away the liquid sweat from the body surface. It is the breathable layer for cold weather clothing. The fabrics used for the purpose are weaved with absorptive hydrophilic yarns (mostly of natural fibres) which show

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Fig. 6.13 Effect of a hydrophilic comfort finish, b improved moisture management by double-faced finish (Benjamin and Schmitt 2008)

higher degree of water vapour transmission from inside to outside of the body. A relatively tight woven compact fabric structure weaved from natural hydrophilic fibre reveals higher efficiency of transport of water vapour as compared to identical fabric made of non-absorptive hydrophobic fibres (Giesbrecht 2003). Natural fibre provides further benefits by rapidly absorbing the liquid water droplets produced by the skin due to heat stress and holding them inside the clothing as sweat condensate. Polypropylene—Polartec® fabrics and polyester micro-fibre are also used for the construction of the inner layer. Furthermore, technological advancement of the inner layer for moisture management finds usage of super-hydrophilic multifunctional fabrics developed by Reliance industries Ltd., India, and Bayer, Germany. The mechanism of moisture management for the generation of comfort clothing is shown in Fig. 6.13.

6.5.2 Middle Layer Middle layer materials can entrap more air, so offers higher total insulation values (TIVs) and is usually a combination of multiple layers instead of only one. Polartec® , Windstopper® , and PowerStrech® materials, fleece, pile, wool, down, polyamides, polypropylene, and coarse polyester fibres are used in the form of filling. PES fleece, piled fabric, and non-woven (needle-punch polyester) battings of different crosssectional fibres and bird’s feathers are generally used as an insulating layer. At present, needle-punch PET non-woven fabric (400–815 g/m2 ) of different cross sections and hollow polyester are emerging textiles for cold weathering fabrics. The TIVs for various materials are PET (round)—0.66 to 0.93, PET (trilobal)—0.69 to 0.95, PET (hollow)—0.64 to 0.81, Acrylic—0.46 to 0.60, and Wool—0.0.56 to 0.70. These results indicate that PET with trilobal cross section, as depicted in Fig. 6.14, is the best choice for use as insulting materials compared to wool and acrylic (Benjamin and Schmitt 2008).

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Fig. 6.14 Micrograph of fibre encapsulated webbing (Siddiqui and Sun 2014)

6.5.3 Outer Layer The outer layer should be waterproof and also breathable. Commonly PU-coated nylon fabrics are used for this purpose so that cold air and water can’t reach the inner layer of the fabric. In case of cold weather properties of yarn play an important role. Construction of fabric is made such that the inter-yarn pore spaces are kept as little as possible to provide higher protection against penetration of wind and rain droplets. At the same time, the outer surface should also composed of nonabsorptive (hydrophobic yarns) to diminish wetting by rain, sleet, or snow. A specialized weaving by micro-denier fibres results in breathable waterproof and windproof for cold weather textiles (Mukhopadhyay 2002). Ventile woven fabric was first manufactured by Shirley Institute, UK for an outer breathable layer. This fabric was woven from Egyptian cotton having fine long staple twisted mercerized yarns. This fabric works on the principle of waterproofness derived from the swelling of fibres. Once the fibres get swollen, the inter-yarn distance and fabric pore sizes are drastically reduced thus limiting the passage of water. At the same time, wetting due to rain or immersion in water leads to swelling of cotton yarn and the pore size reduces to 3– 4 μm which was found to provide protection to the wearer in severe rainy conditions up to 20 min. For manufacturing of waterproof fabrics for prolonged heavy rain, both the concept of yarn swelling as well as a judicious selection of water-repellent treatment needs to be coupled. Lower degree of water repellency will allow absorption of water on the surface, which will swell cotton yarn and tighten the inter-yarn pores leading to restriction of further penetration of water inside the protective fabrics. Ventile yarns are dense, have higher ends and pick per inch (high cover), and have plain weave with warp threads run in pair off which is known as Oxford construction (Özçelik et al. 2007; Sakthivel and Ramachandran 2012; Chang et al. 2017).

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Fig. 6.15 SEM image of hydrophilic PVA layer partly removed from PTFE surface (Holms 2000)

On a similar concept, inter-yarn spaces and the hydrophilic (cotton) nature of the fibres allows an adequate high moisture vapour transmission rate, otherwise, the fabric will get damp which reduces the insulation of the clothing. Normally, PET or nylon fabric is coated with breathable PU material or laminated with PTFE membrane or hydrophilic breathable coatings of PVA. A combination of very fine micro-porous and hydrophilic films is ideal and one of the latest developments of fabric for efficient outer layer HACG. However, there is a need for a judicious balance between hydrophilicity and hydrophobicity, and essential to laminate fabrics with copolymers having both properties. Hydrophilicity due to functional groups on polymer chains, as discussed earlier, also facilitates the establishment of proper adhesion and peel strength of coating with the fabric substrate (Das and Nizam 2014). A bi-component micro-porous film is highly useful for HACG applications, as shown in Fig. 6.15 (Surdu et al. 2015). The advantages of such laminated structure are a higher degree of breathability, enhanced waterproofness, lighter weight, and service life of up to 10 years. The enhanced breathability is controlled by micro-porosity (0.1–50 μm) of the film acting as a semi-permeable membrane and allows the transfer of hot air molecules (generated by human metabolism). However, at the same time, it does not allow the water molecules to penetrate inside the body.

6.5.4 New Trends in HACG HACG needs continual improvement in comfort and insulation level. Advancement of fibre technology has introduced high thermal insulating materials, e.g., microdenier fibre, bi-component fibre (sea-islands), hollow fibre encapsulated technology, and melt blown and needle-punch non-woven fabrics. A micro-fibre is defined as a

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Fig. 6.16 SEM images: a multifilament micro-denier yarn, and b cross-sectional fabric made with multifilament micro-denier yarn (Purane and Panigrahi 2007)

fibre (including staple fibres and filaments) of linear density of approximately 0.3– 1.0 dtex. Fibres finer than 0.3 dtex are referred to as super-micro-fibres. Fabrics woven from micro-fibres are breathable and inherently soft feel and good drape (Das et al. 2013; Mukhopadhyay 2002). Therefore, fabrics made with these fibres are suitable for the fabrication of the middle layer of high-altitude clothing due to good thermal insulation (TIV = 0.91). Melt spinning, wet spinning, and dry spinning can be employed to manufacture micro-fibres. Representative images of micro-denier multifilament yarn and fabric made with such yarns are shown in Fig. 6.16. Hills, Inc., West Florida has been engaged for decades in the development of fibre spinning equipment and technology for bi-component fibres. These bi-component fibres are extruded with two different polymers in the form of sheath/core, side by side, segmented pie, islands-in-the-sea, and mixed fibres. These specially oriented spun fibres can have various fibre cross section from hollow, trilobal, and round to cross shape and are commercially available in all forms like staple, filament, and microfilament. Sheath-core type of fibre is suitable for making conductive fibres, where the sheath polymer is PAN or PET or Nylon and the core consists of conducting polymers like polythiophene and metal oxides (homo or mixed). The spun filaments have a denier range from 2 to 5 dpf (12–20 μm). Submicron filaments called as ‘islands-in-the-sea’ are useful for making high-altitude protective fabrics. Where the island polymer is generally polyester (20 wt%) and the sea is normally a watersoluble polymer, e.g., PVA (80 wt%). Firstly the fabric made of ‘islands-in-the-sea’ filaments is converted into knitted or woven and subsequently the sea polymer is dissolved to leave small submicron island filaments (hole type of structure) on the surface of the fabric which act as an efficient cold air trapper. The advantage of hollow fibres over normal grade fibres is realized in terms of their large surface area to volume ratio (approx. 100 times). One of the inventions of scientists dealing with cotton by Kurabo (Japan), where a hollow cotton product composed of cotton and PVA fibres is developed called SPINAIR (Jerzy et al. 2014).

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PVA fibres are introduced into the cotton yarn during the yarn spinning process and subsequently dissolved and removed from the fabric in the finishing process, thereby leaving a final yarn only of cotton fibres. The structure of yarn and fabrics made out of these fibres is loose and porous. The particular yarn has been developed to improve the thermal insulation of cotton fabrics. The air trapped inside the yarn between the cotton fibres increases the thermal resistance of the fabrics. Research Centre, USA has developed temperature-adaptable hollow fibres which are beneficial for heavy textiles (Jerzy et al. 2014; Khoddami et al. 2009; Trask et al. 2007). Reliance, India has developed hollow polyester fibres, which again is highly beneficial for DPTs. In the global context, hollow polyester fibres developed by Teijin Fibres Ltd show cross section with eight lobes that appear like octopus suitable for HACG due to their high insulation and efficient moisture management. In case of self-healing of composite structures, hollow glass fibres are used (Fig. 6.17). Primaloft has developed cold weather fabrics by encapsulation of down-barrier webbing material into fibres. Such functional webbing absorbs moisture, heat is conducted away from the body much faster due to dried conditions (fibres absorb approx. 100–250% weight in water). There is another category of advanced fibre, different from hollow fibre, and is called ‘specialized functional fibre’. The advantages of using such fibre in fabrics are the scope of elimination of one of the textile processes (finishing) and the durability of the functional properties increased more than 20–30 times (Kumar 1991). Specialty and functional fibres types and applications are tabulated in Table 6.4. All these functional fibres and the technology required for their manufacture are proprietary and very expensive.

Fig. 6.17 SEM image of hollow glass fibres (Gurudatt et al. 1997)

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Table 6.4 Functional and high-performance specialty fibre for DPTs applications Fibres/brand

DPTs applications

Meta-aramid

Bulletproof jackets, body armour, helmets and NBC outer fabrics

Para-aramid

Ballistic fabrics and flame resistance clothing

Modacrylic fibres

Fire resistance blankets, curtains, carpets, sofa covers

Super absorbent fibre (acrylic)

Feminine hygiene products and inner layer of DPTs—NBC and HACG

Ultra-high molecular weight polyethylene (UHHMPE)

Bulletproof vests, ballistic body armour, and base fabric for aerostat

Carbon fibre

CBRN adsorbent materials

Fire retardant (FR) viscose

CBRN outer and inner layers

Flame retardant (FR) polyester

Fire fighter suits and CBRN outer layer

High tenacity/super high tenacity nylon

Ropes

High tenacity/super high tenacity polyester

Heavy textiles and CBRN fabrics

High Tenacity/Super high tenacity polypropylene

UV protective textiles

High tenacity/super high tenacity viscose

CBRN

Polytetrafluoroethylene (PTFE)

Heavy textiles and barrier layer for CBRN

PBI (Polybenzimidazole)

HACG and CBRN fabrics

PBO

High tensile strength and high flame resistance properties this bulletproof vests, body armour

Anti-microbial/anti-fungal/anti-bacterial fibres

CBRN, Heavy textiles, inner layer HACG, combat dessert wear, protective clothing for doctors, adverse climatic conditions

Phenolic fibre

Automotive and electrical components

Conductive fibres

Electronics manufacturing garments, clean room garments, military garments

Multifunctional fibres

CBRN—outer layer

6.6 Research Initiatives at DMSRDE for Development of Functional DPTs DMSRDE has played an important part in the development of several high-tech and value added fibres and fabrics for DPTs applications. Besides, the laboratory has also continuously focused on bulk production of the entire spectrum of DPTs like NBC suits (Mark I to Mark V) (Singh et al. 2000; Chatterjee et al. 1998), ECW clothing (Products and technologies under material cluster 2014; https://www.drdo.gov. in/drdo/labs1/DMSRDE/English/indexnew.jsp?pg=products.jsp), and heavy textiles (tent, shelters for Army and Defence Equipment). The research focuses more on the aspects of fabric designs, development, and manufacturing of functional fibres,

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yarns, and fabrics used in DPTs. Some representative photographs of the indigenously developed DPTs are shown in Fig. 6.18. Besides material and technology development, all these DPTs govern by stringent functional norms and requirements which are specified as joint service qualitative requirements (JSQRs). Some of the important DPTs functional features and JSQR quality control norms with adopted technology by DMSRDE are specified in Table 6.5. Examples of interesting findings on case study pertaining to the development of the NBC Suit at DMSRDE are reported in the following sections. Case Study-I: In the development of NBC suits (Fig. 6.18a), varieties of polymeric adhesive were tested like Polyvinyl Acetate (PVAc), Polystyrene, Polyvinyl Chloride (PVC), Polyurethanes, Phthalic Anhydride, Polyols, Polytetrafluoroethylene and Polyvinylidene Chloride. Acrylate was found to be the most suitable adhesive. It was observed that in other cases except for the acrylate adhesive, poor rub-off resistance, cracking of the coating layer during flexing, low air permeability and moisture vapour transmission, low washing fastness during laundering, and stiffening of the adsorbent layer. Case Study-II In the same invention (Fig. 6.18a), rigorous experimental trials were conducted with different types of substrate fabrics such as polyester, nylon, cotton, viscose, polyester/viscose, and polyester/cotton blends. The coating of ACS on polyester/ viscose blended fabric turned out to be more efficient than others. In the case of polyester and nylon fabric, there was shrinkage of fabric during coating which resulted in the formation of streaks marks. There was also the occurrence of thermal damage on cotton and viscose fabrics during the coating process. Hence, the blend of polyester/viscose fabric was selected to have negligible shrinkage and thermal damage. Amongst the various blend proportions of polyester and viscose (20:90, 30:70, 40:60, 67:33, 80:20), the 67:33 blend ratio was found to be optimum in terms of fabric strength and air and water vapour transmission properties. Case Study-III Similarly for various NBC items, the non-woven fabric is an integral part. Various non-woven fabrics of polyester and polypropylene with gsm ranging from 10 to 100 were used. It was seen that polypropylene non-woven led to shrinkage and resulted in patches, bubbles formation, and non-uniform mass distribution across the width. Lower gsm (40), a crease mark was observed on the adsorbent layer with poor peel strength. The optimized gsm was 35–40 based on the requirements.

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Fig. 6.18 Representative photographs of, a NBC suit, b decontamination suit, c life jacket, d glacier clothing gear, e glacier tents, and f glacier poncho (https://www.drdo.gov.in/drdo/labs1/ DMSRDE/English/indexnew.jsp?pg=products.jsp; https://www.drdo.gov.in/drdo/labs1/DMSRDE/ English/indexnew.jsp?pg=achieve.jsp)

6.7 Innovative Materials Phase change materials (PCMs) are known as thermos-regulating materials. The absorbed/stored heat in a PCM is released into the environment through a reverse cooling process (Dakuri and Hayavadana 2014; Wang et al. 2008). During the process, the material temperature decreases continuously and the release of heat

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Table 6.5 Salient features of DMSRDE developed DPT products (Sinha et al. 2017; Products and technologies under material cluster 2014; https://www.drdo.gov.in/drdo/labs1/DMSRDE/English/ indexnew.jsp?pg=products.jsp; https://www.drdo.gov.in/drdo/labs1/DMSRDE/English/indexnew. jsp?pg=achieve.jsp) Sr No

DPTs items

Functional features

Adopted technologies

1

CBRN suit

Wearing time Launderability BTT Protection time Overall Suit weight Self-life

≥45 days Activated carbon sphere 6 field trials ≥24 h ≤2.75 kg 10 years

2

Life jacket

Weight Buoyancy

1 kg 18.5 kg (min.)

Polyethylene-based foam technology

3

HACG

Protection Waterproof Windproof UV protective

–40 °C

Acrylic pile thermal insulating material and breathable fabric technology

4

Heavy textiles

Tent glacier (for 10–12 men) Length Width Height Wall height Weight

4.2 m 4.3 m 2.35 m 1.55 m 290 kgs

Double-layer principles. Waterproof and insulating materials, aluminized fabric

energy is utilized in textiles as a latent heat storage material and they open the scope to develop thermo-regulated textiles and clothings (Shim et al. 2001). Normally, the PCMs materials are based on paraffin with melting point ranges from 20 to 40 °C {heat absorbing endothermic process}, and their solidification from the melt occurs at 30–10 °C {heat releasing exothermic process}. Besides the paraffin, hydrated inorganic salts, polyhydric alcohol-water solution, polyethylene glycol, polytetramethylene glycol, aliphatic polyester, etc. can also be used as PCMs for textiles. The heat transfers during the melting or crystallization processes without temperature change make PCMs an interesting source of heat storage material. Nowadays PCM encapsulated fibre material is developed where fiber itself can accommodate and regulate temperature change by a two-component heat transfer mechanism (Shin et al. 2005). These PCM-based textiles are useful for making tent, shelter, and inner layers of HACG DPTs fabrics. The technique of fabrication of smart fibre by encapsulating PCM into the fiber of different cross sections and shapes like round, square, and triangular; trilobal is important for the extent of heat released during the cooling process for HACG.

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6.8 Test Methods for Characterization of DPTs A list of standard test methods pertaining to the characterization of DPTs is illustrated in Table 6.6 (Haase 2005; Debnath 2010; Zeinab et al. 2006).

6.9 General Summary and Remarks The chapter provides an in-depth discussion on DPT technology, classification, and overview of the National and International scenario and the technology gap that exist in the use of functional and high-performance fibres for high-performance DPTs. Practical product-oriented research works have been illustrated with suitable examples on DMSRDE’s research initiatives on DPTs as per stringent army’s functional norms (JSQRs). Micro denier encapsulated fibres and PCMs are the best options for HACG items. DMSRDE has established the ACS-coated fabric technology in order to develop CBRN suits for protection from chemical/biological warfare agents, while offering the highest degree of comfort at any climatic conditions varying from hot and humid, desert to plain land. These also offer protection against chemical warfare agents in liquid, vapour, and aerosol forms. Concept of moisture management in clothing signifies a higher degree of comfort even in high humid conditions and further works are needed to establish weaving and processing parameters for such DPTs products. Such efforts will open the path for replacing multi-layered defence clothing with a single-layered multifunctional structured fabric and elimination of the tedious fabric finishing and coating operations. Besides, till date, life estimation of fabric is a challenging task mainly due to the involvement of various types of materials having different photodegradation factors but their effect is interrelated and cumulative. Therefore, the design of DPTs comprising of a heterogeneous material composition where each individual component of DPTs will have a different rate of photodegradation behaviour will significantly affect the functional properties, e.g., photoprotective dyes used to protect the fabric degradation either by absorbing or reflecting UV light stipulated challenges are countered by ways to find out the correlation between material degradation and exposure time. Worldwide DPTs are manufactured either by using high-performance fibre or functional fibres. The potential application includes technical textiles (military textiles) with better functional properties and lightweight fabrics such as singlelayered CBRN clothing, lightweight tent and shelter fabrics, and camouflaging fabrics for radar attenuation/dissipation. The research activity on functional fibres will also create the fundamental understanding for developing futuristic defence protective clothing.

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Table 6.6 Standard methods pertaining to DPTs Properties

Test methods

Tear strength

ISO 9073-4 (Textiles—Test methods for nonwovens—Part 4: Determination of tear resistance)

Tensile strength and elongation at break

ISO 13934-1 (Textiles—Tensile properties of fabrics—Part 2: determination of maximum force using the grab method)

Air permeability

ISO 9237 (Textiles—determination of the permeability of fabrics to air)

Waterproofness

EN 20811 (Textile fabrics—Determination of resistance to water penetration—Hydrostatic pressure test)

Burning behaviour

ISO 15025 (Protective clothing—Protection against heat and flame—Method of test for limited flame spread) and BS Handbook, 1974, p 4/169

UV degradation

AATCC—test method 111 Weather Resistance of Textiles: Exposure to Daylight and weather

Heat stress (Sweating guarded-hotplate)

ISO 11092 Physiological effects—Measurement of thermal and water vapour resistance under steady-state conditions (sweating guarded-hotplate test)

Static charge measurement

DIN EN 1149-5 Protective clothing—Electrostatic properties—Part 5: Material performance and design requirements; German version EN 1149-5:2008

Conditioning

ASTMD 1776-04 Standard practice for conditioning and testing textiles

Carbon shedding

ISO 7854: 1995, Clause-3, method ‘A’—De-Mattia method

Breathability (comfort)/water vapour transmission rate (WVTR)

ASTME 96, procedure B (upright cup method) Standard Test Methods for Water Vapour Transmission of Materials

Pilling/abrasion

ASTMD 4157-02 Abrasion Resistance of Textile fabrics (Oscillatory Cylinder Method)

Thermal insulation of clothing using a heated manikin

ASTMF1291-05 Measuring the Thermal Insulation of Clothing Using a Heated Manikin

Evaporative resistance of clothing using a sweating manikin (dynamic conditions)

ISO 15831:2004 Clothing—Physiological effects—Measurement of thermal insulation by means of a thermal manikin

Bacterial filtration efficiency (BFE)

ASTMF2101-14 Evaluating the bacterial filtration efficiency (BFE) of medical face mask materials, using a biological aerosol of staphylococcus aureus

Anti-microbial testing of outer fabrics

AATCC 100-2012 antibacterial finishes on textile materials

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Chapter 7

Nanofibre Web Coatings Based on Nano-Spider (NS) Technology Mukesh Kr. Sinha, Biswa Ranjan Das, and Eswara Prasad Namburi

Abstract This chapter covers various aspects of nanofibre web coatings: fundamental theories, principles and operational process of Nanospider (NS) technology, experimental results, characterization methods and commercial applications. The concept of integration of nanofibre webs in defence protective textiles (DPTs), and research at the Defence Materials & Stores Research & Development Establishment (DMSRDE), DRDO, Kanpur, on nanofibre coated web technology are highlighted. This research has revealed that NS technology is promising for the development of lighter weight functional fabrics with enhanced comfort, particularly for military clothing and equipment.

7.1 Introduction Electrospinning is an established process to produce polymeric nanofibres by electrically charging a suspended droplet of polymer melt or solution ejected by a spinneret. It is a simple experimental set-up that provides the opportunity for control over the thickness and composition of the nanofibres, together with the generation of porosity for enhancing the surface area. The diameters of the polymer fibres vary from micrometres (10–100 μm) to nanometres (10–1000 nm), thereby resulting in very large surface areas, particularly for nanofibre webs.

M. Kr. Sinha Ministry of Textiles (On Deputation from DMSRDE, DRDO), New Delhi, India B. R. Das Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India E. P. Namburi (B) Former Outstanding Scientist and Ex-Director, Defence Materials and Stores Research and Development Establishment (DMSRDE), Defence Research and Development Organisation (DRDO), Kanpur, Uttar Pradesh, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 E. P. Namburi et al. (eds.), Novel Defence Functional and Engineering Materials (NDFEM) Volume 2, Indian Institute of Metals Series, https://doi.org/10.1007/978-981-99-9795-4_7

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The principle of electrospinning is to use a high electric field to draw polymer solution or melt from an orifice to a collector. Figure 7.1 shows a representative set-up. Electrospun nanofibres ranging from 10 to 1000 nm have been produced by applying an electrical potential (≤80 kV) to polymeric solutions of several materials (Reneker and Chun 1996; Huang et al. 2003; Inai et al. 2005). A solution is held at the tip of a capillary tube by surface tension, and an electrical potential is applied to provide a charge to the polymer solution. Mutual charge repulsion in the polymer solution causes a force directly opposing the surface tension: initially, an increase in electrical potential leads to elongation of the hemispherical surface of the solution at the tip of the capillary tube, forming a conical shape known as the Taylor cone (Yarin et al. 2001). Increasing the electric potential to overcome the surface tension results in emission of a charged jet of solution from the Taylor cone. The charged jet undergoes instabilities and gradually thins down in air primarily owing to elongation and solvent evaporation (Reneker et al. 2000; Frenot and Chronakis 2003). The jet eventually forms randomly oriented nanofibres on a stationary metallic collector (Doshi and Reneker 1995). Other techniques for the production of polymer nanofibres include melt blowing, solvent casting and multi-component processes involving extrusion techniques (Das 2010). Although these methods have significantly higher productivity than electrospinning, the latter gives more control over the formation of finer fibres. Hence, electrospinning is favoured for the nanostructuring of materials as fine fibres. The latest trends in electrospinning include co-axial spinning, which produces fibres with different core and shell morphology (Zhang et al. 2004). Various other

Fig. 7.1 Representative image of a conventional electrospinning set-up (Das 2010)

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modifications are also available, e.g. (i) a multi-layering technique in which different polymer solutions are deposited layer by layer, and (ii) multi-jet electrospinning of different polymers to form composite structures or hybrid nanofibres (Murugan and Ramakrishna 2007). Very recently, the concept of reactive electrospinning has emerged, whereby cross-linking of polymeric fibres is done during spinning. This technique avoids an additional step of post-spinning cross-linking for water based solvent-polymer systems (Kim et al. 2005). Several researchers have tried to improve the production capability of conventional electrospinning by using multiple needle electrospinning. However, scaling-up of this method is subject to several issues, of which the main ones are (a) polymer clogging at the spinneret nozzle, thereby limiting the production throughput, and (b) alteration of the electric field by the presence of nearby needles, which degrades the web and fibre uniformity (Huanga et al. 2006). To avoid these problems a “needle-free” electrospinning method has been developed. The latest development in “needle-free” electrospinning is Elmarco’s ‘Nanospider (NS) technology’, which provides significant scaling-up benefits in terms of production rate and product quality as compared to conventional electrospinning (Huanga et al. 2006). This NS technology is well recognized by industry leaders as providing excellent web and fibre uniformity, high productivity, and the ability to consistently meet key performance characteristics (Das 2010). In this technology, droplet formation and jet initiation is a self-organized process, whereby numerous jets are formed simultaneously from droplets generated on the surface of a wire. Table 7.1 gives a comparative analysis between conventional electrospinning (CS) and NS electrospinning. It is seen that among other differences the NS technology has ample scope for bulk quantity production. Table 7.1 Comparisons between conventional and NS electrospinning (Stanislav 2011) Production variables

Conventional (CS) set-up

Nanospider (NS) machine

1. Mechanism

Needle forces polymer Polymer is in a bath. An even downwards distribution is maintained on the Drips and fibres deposited in web electrode via rotation

2. Hydrostatic pressure

Production variable—required to be kept level across all needles

None

3. Taylor cone separation Defined mechanically by needle distances

Nature self-optimizes distance between Taylor cones

4. Polymer concentration Often 10% of solution

Often 20% or more solution

5. Fibre diameters

80, 100, 150, 200, 250 and higher 80, 100, 150, 200, 250 and Standard deviation likely to vary higher over fibre length Standard deviation ± 30%

6. Throughput

~0.05–0.26 g/m2 /hour

0.09–0.24 g/m2 /min

7. Production (m/min)

No bulk scale machine

Up to 60 m/min

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7.2 Description of NS Technology 7.2.1 Principles Electrospinning is a highly complex process in which electrostatic forces play a vital role in forming the nanofibres. In the case of NS, the polymer solution is kept in a rotating spinning-bath and subjected to a very high voltage (~40–80 kV). When the solution is forced to emerge and be exposed to the high voltage, it undergoes several physical changes before becoming shaped into a nanofibre web, namely (i) droplet generation, (ii) Taylor cone shape, (iii) jet initiation, (iv) a whipping instability region, and (v) solidification into nanofibres (Persano et al. 2013; Stanislav 2011; Mirjalili and Zohoori 2016; Eichhorn and Sampson 2010; Balamurugan et al. 2011; Heidi et al. 2006): (1) Droplet generation: In the first stage, as soon as the polymer solution is captured by the spinning electrode (SE), it forms spherical droplets at the SE surface, see Fig. 7.2 (Petrik 2009). The droplet formation is controlled by the solution conductivity, viscoelasticity and surface tension. (2) Taylor cone shape: When the droplets are subjected to a high enough voltage their shape begins to change from spherical to conical, see Fig. 7.3 (Andrady 2008). (3) Jet initiation: After the Taylor cones form they elongate into unstable fibreshaped jets owing to high electrostatic forces. (4) Whipping instability region: The unstable jets undergo bending and undulating movements during their passage towards the collecting electrode. Figure 7.4 is an image of the whipping region of a jet. A dramatic increase of the jet surface areas occurs, whereby the surface charge density is greatly lowered and the effect of the high voltage is suppressed.

Fig. 7.2 Droplets generated over the surface of the spinning electrode (SE) (Petrik 2009)

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Fig. 7.3 Droplet deformation into a conical (Taylor cone) shape (Andrady 2008)

Fig. 7.4 SEM image of web patterns formed in the whipping instability region (Shin et al. 2001)

(5) Solidification into nanofibres: The rate of evaporation of the solvent is important during jet whipping, such that the solvent is completely evaporated before the fibre-shaped jet arrives at the collecting electrode. For a high vapour pressure solvent, the viscosity of the jet may reach levels too high to allow further deformation early in the whipping instability stage, thereby resulting in thicker nanofibres. Solvent volatility is therefore a key consideration in controlling fibre diameters. The vapour pressures of different solvents used in the NS technique are listed in Table 7.2. A judicious selection of solvent and optimised process parameters enables extremely fine nanofibres to be prepared.

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Table 7.2 Vapour pressures of different solvents used for NS electrospinning (Aussawasathien 2005) Polymer (abbreviation)

Solvent types

Vapour pressure (kPa at 20 °C)

Nylon6,6, PA-6.6

Formic acid

4.600

Polyurethanes (PU)

Dimethyl formamide (DMF)

0.380

Polybenzimidazole (PBI)

Dimethyl accetamide

0.178

Polycarboate (PC)

DMF & Tetrahydrofuran (1:1) 0.380 & 19.30

Polyacrylonitrile (PAN)

DMF

0.380

Polyvinyl alcohol (PVA)

Distilled water

2.30

Polylactic acid (PLA)

Methylene chloride & DMF

47.4 & 0.380

Polymethacrylate (PMMA)

DMF & toluene (1:9)

0.380 & 9.90

Polyethylene oxide (PEO)

Distilled water

2.30

Polyethylene terephtalate (P Dichlormethane & ET) trifluoracetic

47.4 & 11.0

Polystyrene (PS)

Tetrahydrofuran

19.30

Polymethacrylate (PMMA)

Tetrahydrofuran, acetone & chloroform

19.30, 24.0 & 21.0

Polyamide (PA)

Dimethylacetamide

0.178

Collagen

Hexafluoro-2-propanol

16.0

Polyvinylidene difluoride (PVDF)

DMF & dimethylacetamide (1/1)

0.380 & 0.178

Polyether imide (PEI)

Hexafluoro-2-propanol

16.0

7.2.2 Operation of a Nanospider (NS) Machine Figure 7.5 shows a schematic of nanoweb formation and an actual coating machine. The process is based on the potential difference between the positively charged spinning electrode (SE) and the negatively charged collecting electrode (CE). The SE is immersed in the polymer solution in the spinning head. The high potential difference between the electrodes leads to extrusion of polymer solution from the SE in the form of a conical nanofibre web, which is deposited on a moving fabric, e.g. a polypropylene (PP) spin-bonded nonwoven fabric. The SE rotation provides a constant source of specified solution quantity to be extruded. Several types of rotating electrodes have been developed for industrial machines, see Fig. 7.6. However, the drum roller type (R.H. example in Fig. 8.6) is the most used. Tables 7.3 and 7.4 present the material and operational parameters that are important for determining the morphology and functional properties of electrospun fibres (Persano et al. 2013; Sinha et al. 2013a, 2014a). These parameters are discussed in some detail in Sect. 7.3.

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Fig. 7.5 The NS technique: a schematic of nanoweb formation and b a nanoweb coating machine (Sinha et al. 2014a)

Fig. 7.6 Various types of spinning electrodes (SEs) (Stanislav 2011) Table 7.3 NS technology process and material parameters (Huanga et al. 2006) Polymer properties

Solution properties

Other variables

Molecular weight

Viscosity

Rotation of SE (rpm) Speed of CE (m/min)

Molecular weight distribution

Concentration

Throughput rate

Glass-transition temperature

Surface tension

Electric field strength (kV)

Solubility

Electrical conductivity

Relative humidity and Temperature Distance between SE & CE Geometry of electrodes

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Parameters

Range

Voltage (kV)

–CH3 > –CF2 > –CF2 H> –CF3 (Nishino et al. 1999). The regularly aligned –CF3 groups lead to a calculated water contact angle (WCA) of about 120°, together with the lowest surface energy of 6.7 mJ/m2 .

Surface

Fig. 8.2 Schematic representation of a solid surface. Circles denote atoms, straight lines represent chemical bonds of electrons, and curved lines show unsatisfied bonds of the top layer of atoms at the surface

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Fig. 8.3 Effect of surface tension and surface free energy on wettability

8.2.2 Wettability A liquid droplet placed on a solid surface generally starts to spread up to some extent. The spreading depends on the intermolecular forces interacting between the two phases, and these forces are known as the adhesive force (intermolecular attraction between the solid surface and liquid molecules) and the cohesive forces (intermolecular attractions between the liquid molecules). If the adhesion between the solid and liquid molecules is greater than cohesion among the liquid molecules themselves, then the liquid has a tendency to spread over the solid surface. This phenomenon is called ‘the wetting process’. Alternatively, if the intermolecular cohesive forces between the liquid molecules are stronger than the adhesive force between the solid and the liquid molecules, the liquid will remain as a droplet on the solid surface and does not spread. The wettability phenomenon is well explained in Fig. 8.3: if the surface tension (γ L ) of the liquid is less than the solid surface energy (γ S ), the liquid wets the surface. Liquids with surface tension higher than the solid surface energy are repelled by the surface. If the surface energy is sufficiently low the droplet tends to take a circular shape.

8.2.3 Contact Angle Consider a liquid drop in equilibrium on a solid surface, Fig. 8.4. The angle θ is determined by a balance between three interfacial forces, namely the interfacial tensions between the solid and liquid surface (γ SL ), between solid and vapour (γ SV ) and between liquid and vapour (γ LV ). The angle θ is referred to as the ‘contact angle’ or ‘wetting angle’, and is a quantitative measure of the wetting of a solid by a liquid. If the angle θ is less than 90°, the liquid is said to wet the solid. If it is greater than 90°, it is said to be non-wetting. Specifically, hydrophilic (water-attracting) materials are substances which show a water contact angle (WCA) lower than 90°, while hydrophobic (water-repelling) materials have a WCA greater than 90°. The contact angle is determined from Eq. (8.1): γSV = γSL + γLV cosθ

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Fig. 8.4 Force equilibrium for a liquid drop

γSV

γLV

Vapour

θ

Liquid

γSL

Solid

γSV − γSL = γLV cosθ cosθ =

γSV − γSL γLV

(8.1)

This is Young’s equation (Young 1805), which describes the balance of forces acting on a liquid droplet spreading on a completely smooth and homogeneous surface. However, it is difficult to achieve perfectly flat surfaces, such that roughness must be considered for practical situations. Wenzel (1936) proposed a model by ' modifying Young’s equation to describe the contact angle (θ ) of a rough surface, as follows: cosθ ' =

R f (γ SV − γSL ) = R f cosθ γLV

(8.2)

where the parameter (Rf ) indicates the roughness factor of the surface and may be mathematically defined as Rf =

actual area of rough surface geometrically projected area

Since R f is always >1, the surface roughness enhances the hydrophilicity of hydrophilic surfaces and the hydrophobicity of hydrophobic surfaces. Therefore, in the Wenzel regime, see Fig. 8.5, the roughness affects both hydrophilicity and hydrophobicity.

8.2.4 Contact Angle Hysteresis For any given solid/liquid interaction there exist ranges of contact angles. These angles can be static or dynamic contact angles. When the three phase boundaries (liquid/solid/vapour) are in a static condition, the angles produced are called ‘static contact angles’. However, it should be borne in mind that the static angle created by a single drop of liquid (say 10 μl) may not necessarily be the same when several steps are used to create similar but smaller droplets (say 2 μl) from the larger one.

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Fig. 8.5 Effect of roughness on contact angle. Reprinted with permission from reference (Cassie and Baxter 1944)

The recent history of interactions will influence the values of static contact angles, and these can be different from each other for each droplet. On the other hand, the situation will be very different when liquid is added or removed from an already existing static liquid drop on a solid surface: (i) addition of liquid to an existing droplet can expand its size and also increase the contact angle; but (ii) removing liquid can reduce the size and also the contact angle. When a drop expands the angle created is called the ‘advanced contact angle’, and when it contracts the new angle is called the ‘receded contact angle’. The advanced angle approaches a maximum value, whereas the receded angle approaches a minimum value. However, both lie within the range of contact angles, see Fig. 8.6. If there is a continuous change of droplet size, either by adding or removing liquid, the three phase boundaries (liquid/solid/vapour) are in motion. Hence the angles produced are called ‘dynamic contact angles’, which obviously may be either ‘advancing contact angles’ (θ adv ) or ‘receding contact angles’ (θ rec ). The difference between the advancing and receding contact angle values is called the ‘contact angle hysteresis’ (θ H ). θ H has been used to describe surface properties such as heterogeneity, roughness and mobility. The surface domains restrict the motion of the contact line for

Fig. 8.6 Schematic illustration of advancing (θ adv ) and receding (θ rec ) contact angles

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heterogeneous surfaces. In the case of chemical heterogeneity, these domains represent areas with different contact angles from the surrounding surface. For example, when wetting with water the hydrophobic domains on the surface pin the motion of the contact line as the liquid advances, thus increasing the contact angle. When water recedes any hydrophilic domains on the surface hold back the draining motion of the contact line, thereby decreasing the contact angle. This means that for the WCA the advancing angles characterize hydrophobic domains and receding angles characterize hydrophilic domains.

8.2.5 Factors Influencing the Hydrophobic Property The importance of contact angle measurement in the textile and fibre industry is well known. Cotton is usually wetted by water and considered as a hygroscopic or hydrophilic fibre. However, synthetic fibres possess definite contact angles against water: for example, nylon gives a contact angle of about 40°. Most of the polymers like polyethylene, polypropylene and PTFE are highly hydrophobic in nature since they show high contact angle behaviour with water, i.e. 96°, 102° and 109° respectively.

8.2.6 Mechanisms of Superhydrophobicity and Oleophobicity Several models have been proposed to explain the mechanisms of superhydrophobicity and oleophobicity. For example, the Wenzel model is only valid when liquid droplets fully penetrate into roughness grooves; see Fig. 8.7a. This concept was later refined by Cassie and Baxter in 1944, resulting in the Cassie–Baxter state (Werner et al. 2005). It was proposed that air is entrapped in the grooves and liquid droplets will penetrate slightly into the grooves, forming a heterogeneous surface regime, ' Fig. 8.7b. Cassie proposed an equation to describe the contact angle (θ ) for a heterogeneous surface composed of solid and air and introduced the wetted area fraction ' of solid ( f SL ) to calculate the contact angle (θ ). The wetted area fraction ( f SL ) is defined as the area fraction of solid that is in contact with the liquid. The value of ( f SL ) is obtained by dividing the liquid/solid contact area by the projected area. Similarly, the area fraction of liquid ( f LV ) can be calculated by dividing the liquid/vapour contact area with the projected area. Thus ' the WCA (θ ) can be expressed according to Eq. 8.3, assuming a WCA with air of 180°. cosθ ' = f SL cosθ + f LV cos180◦

(8.3)

Equation 8.3 can be rearranged by putting the f LV value in terms of f SL , i.e. f LV = 1 − f SL . This results in Eq. 8.4:

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Fig. 8.7 Schematic representation of theories of superhydrophobicity: a Wenzel state, water droplet completely wetting the solid surface; b intermediate state between Wenzel and Cassie–Baxter states, with water droplet moving inside the air-containing grooves; c Cassie–Baxter state, water droplet partially resting on solid surface and with air entrapped within the grooves

cosθ ' = f SL cosθ + (1 − f SL )cos180◦ = f SL (cosθ + 1) − 1

(8.4)

In Eq. (8.4) the fraction (1 − f SL ) describes the area in contact with air and is applicable only to the case where liquid just touches the top of the surface, Fig. 8.7c. The parameter f SL ranges from 0 to 1. At f SL = 0, the liquid droplet does not touch the surface at all and at f SL = 1, the surface is completely wetted and behaves like a smooth surface. Most importantly, the small contact area between the liquid droplet and solid surface in the full Cassie–Baxter regime allows the droplet to slide easily over the surface. Bico et al. (1999) proposed a more complex equation to account for the partial penetration of liquid into surface grooves. Since the grooves are filled with air, the contact angle with the surface will always be increased relative to the behaviour observed on a smooth substrate having an identical chemical composition, owing to the extremely hydrophobic nature of air. It is therefore obvious that the surface topography has a profound effect on surface wettability, and in fact, the roughness amplifies the hydrophobicity. Therefore, Eq. 8.4 has been further modified to account for the actual surface roughness (Rf ), see Eq. 8.5: cosθ ' = R f f SL cosθ + f SL − 1

(8.5)

It has been observed that there is a critical value of f SL below which the Cassie regime exists and above which the Wenzel regime exists; and this critical value of f SL represents a thermodynamically more stable state (Bico et al. 2001; Callies and

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Quéré 2005; Lafuma and Quéré 2003). The transition from the Cassie to Wenzel regime occurs at a certain critical contact angle (θ c ) defined by Eq. 8.6: cosθc =

f SL − 1 R f − f SL

(8.6)

Rf and f SL were already been defined in Eqs. 8.2 and 8.3. Hence at contact angles larger than θ c , there are air pockets present under the droplet, which, in turn, exists in the Cassie regime. Lafuma and co-workers also pointed out that the Wenzel regime was the equilibrium state of the Cassie regime (Bhushan and Jung 2007). From the foregoing Equations and discussion, we see that to obtain superhydrophobic surfaces it is necessary to design rough surfaces whereby the contact area of liquid and air is increased. This can be done either by reducing the surface free energy or fabricating rough surface structures or both. Another important characteristic of a solid–liquid interface is the contact angle hysteresis (θ H ). Low contact angle hysteresis results in very low water roll-off angles. Figure 8.8 illustrates the roll-off angle α, which is defined as the angle to which a surface should be tilted for water droplets to roll-off: in fact, a very low water contact angle hysteresis is required to do this (Jung and Bhushan 2008; Nosonovsky and Bhushan 2007a). Low roll-off angles are important in fluid dynamics and selfcleaning surfaces. (It is understood that some slippage occurs during roll-off.) The difference between the cosines of the advancing and receding angles is related to ' ' and θrec are defined as in the roughness of a nominally smooth surface, and the θadv Eq. 8.7: ' ' cosθadv − cosθrec = R f (1 − f LA )(cosθadv − cosθrec ) + Hr

(8.7)

where H r is the outcome of surface roughness, which is equal to the total perimeter of the asperity per unit area. It may be observed from Eqs. (8.5) and (8.7) that the increase in f LA → 1 results in increasing the contact angle (cos θ → −1, θ → π ) and decreasing the contact angle hysteresis (cosθ adv − cosθ rec → 0). In the limiting case of a very small solid–liquid fractional contact area under the droplet, when the contact angle is large, i.e. (i) (cosθ ≈ −1 + (θ − π)2 /2 and sinθ ≈ π − θ ) and the contact angle hysteresis is small, i.e. (ii) (θ adv ≈ θ ≈ θ rec ), Eqs. (8.5) and (8.7) are reduced to / ) ( θ ' − π = 2(1 − f LA ) R f cosθ + 1 (8.8) cosθadv − cosθrec ' θadv − θ 'rec = (1 − f LA )R f sinθ (√ ) cosθ − cosθ adv rec = 1 − f LA R f / ( ) 2 R f cosθ + 1

(8.9)

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Fig. 8.8 Liquid droplet in contact with tilted rough surface profile (the tilt angle is α)

α

θadv

θrec

For a homogeneous interface f LA = 0, whereas for a heterogeneous composite interface, f LA is not zero. It is observed from Eqs. 8.7–8.9 that for a homogeneous interface, the increase in roughness (high Rf ) enhances the contact angle hysteresis (high values of θ adv − θ rec ). Whereas for a composite interface, f LA tends to unity ( f LA → 1) and thus provides both a high contact angle and small contact angle hysteresis (Nosonovsky and Bhushan 2007a, b). This means that a composite interface is desirable for superhydrophobicity. Further, if the θ H difference is higher between the Wenzel and Cassie-Baxter states, then there might be a transition from the Wenzel to Cassie-Baxter states that makes the wetting metastable (Wang et al. 2009; Lafuma and Quéré 2003). Moreover, both the Cassie–Baxter and Wenzel states are extreme states, and when external pressure is applied on the Cassie–Baxter surface a transition will occur from the Cassie–Baxter state to the Wenzel state, all the while passing through the various intermediate states shown in Fig. 8.7b. In fact, external pressure is the dominant factor in causing a transition from the Cassie–Baxter state to the Wenzel state. This has the consequence that highly stable superhydrophobic surfaces exist only when the Cassie–Baxter state cannot be pressure-transformed into the Wenzel state. These stable superhydrophobic surfaces are termed robust surfaces. Since these robust surfaces are highly dependent on the surface morphology in the Cassie–Baxter state, we may also refer to robust Cassie–Baxter states.

8.2.7 Essential Parameters for Robust Cassie–Baxter States The purposeful surface treatment of substrates at micro- and nanoscale levels increases their roughness and robustness. The important parameter that enhances the robustness of the surface is the presence of re-entrant topographies. Re-entrant topographies can be introduced in random or regular patterns. Re-entrant structures with random patterns can be prepared in the form of nanoparticles, nano-wires and nano-pores using simple cost effective methods such as chemical synthesis, electrospinning and candle soot depositions (Deng et al. 2012; Leng et al. 2009; Steele et al. 2008). However, it is difficult to obtain and control uniform surface morphologies on large areas, and this shows the limitation of random patterns. On the other

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hand, re-entrant structures with regular patterns can be prepared in the form of Tshaped micro-pillars (Liu and Kim 2014), nanonails (Ahuja et al. 2008), microhoodoos (Tuteja et al. 2007), inverse trapezoids (Im et al. 2010; Choi et al. 2013) and textile fibres (Pan et al. 2012; Choi et al. 2009). Chemical etching and microinjection compression moulding (μ-ICM) are other methods used to develop regular re-entrant topographical structures. Superoleophobic surfaces can be created by incorporating re-entrant surface texture along with modified surface chemistry: this means surfaces that support a robust composite (solid–liquid–air) interface and have contact angles greater than 150° with various low-surface-tension liquids. Because the surface tensions of oils and organic liquids are much lower than that of water, see Table 8.1, most textile surfaces can be made oleophobic for a solid–air–oil interface by lowering the surface energy. Table 8.1 Surface tension of various liquids (Li et al. 2017)

Liquid

Surface tension (mN/m) at 25 °C

n-Hexane

18.4

n-Octane

21.6

n-Decane

23.8

n-Dodecane

25.4

Amyl alcohol

25.8

n-Hexadecane

27.5

Octanol

27.5

Dodecanol

29.4

Methyl myristate

29.4

Diethyl glutarate

32.3

Cyclohexane

33.4

Diethyl adipate

36.0

Chlorobenzene

36.0

Ethyl benzoate

37.2

Dimethyl malonate

37.4

Benzyl chloride

40.0

Monoacetin

41.8

Diethylene glycol

45.2

Glycerol

63.4

Water

72.3

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8.3 Materials and Process Techniques Surface morphology, surface chemistry and re-entrant topography of surface structures are deciding factors in determining the superhydrophobic and oleophobic properties of textile surfaces. Five typical wetting states that have been exhibited by nature on animal and plant superhydrophobic surfaces are illustrated in Fig. 8.9 (Li et al. 2017; Wang and Jiang 2007; Feng et al. 2002). Figure 8.9a–e differentiate the various superhydrophobic states, i.e. Wenzel state (e.g. rose petal), totally air-supporting superhydrophobic surfaces in the Cassie-Baxter state (e.g. butterfly wings), the metastable state between the Wenzel and Cassie-Baxter states (e.g. waterstrider), surfaces in the micro/nano-structured two-tier “lotus” state, and a partially wetting “gecko” state. The unique property of lotus leaves has been attributed to the combination of a low-surface-energy waxy layer and a rough structure with nano protrusions on microbumps on lotus leaves as depicted in Fig. 8.10a–d. Nature also provides several examples of anisotropic super wettability. Butterfly wings (Fig. 8.9g) exhibit anisotropic rolling/pinning superhydrophobic states (Zheng et al. 2007). In addition to these low-adhesion superhydrophobic examples, some organisms also provide examples of high-adhesion superhydrophobic properties, such as gecko feet (Fig. 8.9j) (Autumn et al. 2000, 2002; Lee and Lee 2007). The hierarchical surface architecture illustrated by lotus leaves, Fig. 8.10, aims to reduce the interfacial adhesion of water droplets since air is entrapped in the grooves of the roughened surface. This reduces the liquid-to-solid contact area and thus makes the lotus leaf surface less prone to adhesion by water and dust particles (Singh et al. 2012). As already discussed in previous sections, the presence of nano- or microhierarchical topographies, re-entrant structures on the surface and application of lower surface energy materials on the substrate surface mainly decide the superhydrophobicity of any surface. Textiles generally possess inherently hierarchical surface structures, and these vary according to the fibre chemistry, yarn properties and fabric structures. However, the surface chemistry of textile fabrics needs to be

Fig. 8.9 Five typical examples of anti-wetting surfaces. a Wenzel state, b Cassie state, c WenzelCassie state, d “lotus” state, e “gecko” state; and several common phenomena for anti-wetting in nature, i.e. f rose petal, g butterfly wings, h waterstrider, i lotus leaf, j gecko. Reprinted with permission from reference (Wang and Jiang 2007)

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Fig. 8.10 a Lotus leaves in nature: self-cleaning behaviour, b microstructures as observed by SEM, c protrusions, d the wax tubules on lotus leaves. Reprinted from Reference (Koch et al. 2009) with permission

altered either by chemical modification of the surface or by attaching low surface energy materials to the surface in order to achieve superhydrophobicity. Many techniques have been used to develop super hydrophobic surfaces on textile fabrics. Figure 8.11 illustrates various physical and chemical methods for preparing robust superhydrophobic textile surfaces. These include the sol-gel technique, layer-by-layer deposition, electrospinning, plasma processing, and polymer grafting for roughening the surface, followed by surface modification with low surface energy materials such as fluorinated compounds, long chain alkyl compounds, non-fluorinated polymers, and silicone compounds.

8.3.1 Roughening of Textile Surfaces Generally, textile surfaces can be roughened either by attaching inorganic and organic materials or their hybrids of micro- or nanoparticles, using a variety of techniques that include low temperature sol-gel processing, hydrothermal processing, complex coating, and layer-by-layer methods. Inorganic nanoparticles, such as, ZnO, SiO2 , TiO2 , have low affinity for textile fibres, hence it is very important to make these particles compatible with the textile substrates via chemical modification, thereby enhancing the stability of the superhydrophobic coating and its robustness during fabrication of superhydrophobic textiles.

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Sol-Gel Technique

Superhydrophobic Surface

Layer by Layer Surface Roughening

Electrospinning Plasma Grafting Fluorinated compounds

Lowering the surface energy

Long chain alkyl compounds Non-fluorinated polymers Silicone Compounds

Fig. 8.11 Different processing techniques for designing superhydrophobic textile surfaces

8.3.1.1

Sol-Gel Technique

The sol-gel technique is the most common process for preparing sols, gels and nanoparticles. In this process a colloidal heterogeneous suspension (a sol) is formed by the hydrolysis and polymerization reaction of the precursor: on complete polymerization and loss of solvent, this leads to the liquid sol changing into the solid gel phase. This method consists of three basic steps. First, nano-particulate sols are prepared by acid or base catalyzed hydrolysis of metal alkoxide precursors in aqueous media or other water-miscible solvents like alcohol (e.g. the Stober method). Organic solvents are preferred owing to their high sol stability, excellent adherence on textile fabrics and easy evaporation at low temperature. Secondly, these nanosols are applied on the textile substrates using a simple pad-dry-cure or dip-coating method. In this step, the metal oxide or other nanoparticles are condensed to form a lyogel layer on the

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substrate surface. Thirdly, the solvent is evaporated resulting in a xero gel layer with a rough structure (Mahltig et al. 2005). The surface roughness of the textiles fabricated via the sol-gel method has many advantages over other techniques owing to the following reasons: i. Silica and other metal oxide nanoparticles of size ranging from 1 to 100 nm can be easily synthesized. Transparent and well-adhered coating of oxide layers can be produced on textiles. ii. Sol-gel methods are low temperature synthesis methods suitable for textile application. iii. Two or more nanoparticles (hybrid) can be synthesized simultaneously. iv. Extremely homogeneous composite nanosols can be produced in this process. v. Ultra high purity nanosols are made during the process. vi. The oxide layers thus formed impart enhanced mechanical properties to the fabrics, e.g. wear and abrasion resistance. vii. Other functional additives such as inorganic particles and polymers may also be attached with these oxide layers. viii. Conventional coating systems such as pad-dry-cure and simple dip-coating methods are employed to perform nano-particulate coating of textile surfaces under normal temperature and pressure conditions. Final properties, such as density, porosity, roughness, critical thickness and mechanical properties can be controlled by tailoring the condensation and drying parameters of the sol-gel method. The nanosols obtained by acid hydrolysis result in dense layers with weakly cross-linked products in the condensation step. On the other hand, the nanosols obtained by alkali catalysis form nano-particle layers with larger pores. Some hydrophobic additives with long alkyl silane chains, alkyl groups (R) or perfluoro-alkyl groups are introduced in the inorganic oxide nanolayers via chemical modifications in order to enhance the surface roughness and water-repellency. Solgel methods have been utilized in many cases to develop superhydrophobic textiles for various practical applications, e.g. rain clothes and tents and oil repellent fabrics (Hikita et al. 2005; Zhu et al. 2011). Some examples from the literature on cotton textiles are listed here: (1) Transparent and durable super hydrophobic silica-coating films on cotton substrates at low temperatures have been successfully prepared (Daoud and Xin 2004; Daoud et al. 2004). The coatings were produced via co-hydrolysis and poly-condensation of a hexadecyl trimethoxy silane (HDTMS), tetra ethoxy orthosilcate (TEOS), and 3-glycidyloxy propyl trimethoxy silane (GPTMS) mixtures. Hydrophobic and self-assembling properties of hexadecyl groups together with the grainy structure of the surface convert the cotton surface from superhydrophilic to superhydrophobic with a water contact angle (WCA) of 141°. The hydrophobic properties of the coating were maintained after repeated washing and were attributed to the linking ability of GPTMS, which promotes a high level of adhesion at the interface.

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(2) The biomimetic procedure for fabrication of superhydrophobic cotton textiles by generating in situ silica particles having amine groups on their surfaces, which were covalently bonded to the cotton fibres has also been reported (Hoefnagels et al. 2007). The amine groups were then utilized to react with mono-epoxyfunctionalized polydimethyl siloxane to produce a superhydrophobic surface. Alternatively, introduction of perfluoro-alkyl chains to the silica particle surface also makes the already superhydrophobic textile highly oleophobic. This was demonstrated by a 140° static contact angle and 24° roll-off angle for 15 μL sunflower oil droplets. (3) It was reported that the sol-gel coating of TiO2 followed by reaction with stearic acid and 1H, 1H, 2H, 2H-perfluorodecyl trichloro silane on cotton fabrics makes the surface superhydrophobic (Xue et al. 2008). It was found that the superhydrophobicity of the fabrics relied on the roughness caused by the sol-gel coating rather than the concentration of TiO2 . The appropriate particle size of silica sol was prepared via alkaline hydrolysis of tetraethoxy silane in a mixture of ethanol and water, together with perfluorooctylated quaternary ammonium silane as a coupling agent. The silica sol was applied to cotton fabrics by the conventional paddry-cure process to generate superhydrophobicity on the cotton surface (Yu et al. 2007). The fabrics treated with both silica sol and perfluorooctylated quaternary ammonium silane coupling agent showed high hydrophobicity and oleophobicity. (4) Superhydrophobicity can also be imparted on to cotton fabric via a combined treatment of silica nanoparticles and cost effective commercial water-repellent agent (Bae et al. 2009). Cotton fabrics treated with silica nanoparticles of average diameter 378 nm and water contact angles (WCA) above 130° show excellent hydrophobicity even with a very low concentration (0.1 wt%) of water-repellent agent: this compares with a lack of hydrophobicity on cotton treated only with neat water-repellent agent. (5) Stable superhydrophobic surfaces with water contact angles over 170º and sliding angles below 7° can be produced by simply coating with a particulate silica sol solution of co-hydrolyzed TEOS/fluorinated alkyl silane dispersed in ammonia solution (NH3 · H2 O) (Wang et al. 2007a). Various substrates which include textile fabrics (e.g. polyester, wool and cotton) and electrospun nanofibre mats can be made superhydrophobic employing this method. It should be noted that the above-mentioned sol-gel derived superhydrophobic coatings were mostly applied to cotton textiles. It is well recognized that there are many reactive hydroxyl groups on cotton fibres, such that covalent bonding between coating materials and cotton substrates favour robust sol-gel coatings on cotton-based textiles.

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Layer-By-Layer (LBL) Deposition

The layer-by-layer (LBL) technique is a versatile method for fabricating a thin layer film, and is based on the concept of self-assembled nanolayers. The LBL process enables modifying the multi-composite molecular assemblies with controlled molecular structures and also maintains a high degree of control over the thickness. The electrostatic self-assembly (ESA) technique has attracted the most attention because of its simplicity and efficiency. In the LBL method, polyelectrolytes with opposite charges are alternately deposited on the fabric surface with intermediate wash steps, and the adsorption cycles can be repeated to increase the deposition thickness (Zhang et al. 2007). Electrostatic self-assembly by the LBL process is schematically represented in Fig. 8.12. A flexible negatively-charged textile material is initially immersed in a polycationic solution (1) followed by washing in solution (2) to remove excess molecules. The substrate containing the cationic layer is subsequently immersed in the anionic polyelectrolyte solution (3). Further washing in solution (4) can remove the molecules that are not effectively attached to the substrate. The LBL technique can be used to apply thin nanocomposite layers and also to impart a wide range of functionalities on fabric surfaces. Some examples are listed in the following paragraphs: (1) Highly hydrophilic cotton fabrics were rendered superhydrophobic via multilayer electrostatic LBL assembly of polyelectrolyte/silica nanoparticles on cotton fibres, followed by a fluoro alkyl silane treatment. The surface morphology and hydrophobicity of the silica nanoparticle coated fibres could be tailored by controlling the multilayer numbers. In the cases of 1 or 3 layers, with a static contact angle larger than 150°, the fabrics showed sticky properties with high contact angle hysteresis (>45°). For cotton fabrics assembled with 5 layers or more, a slippery superhydrophobicity with a contact angle hysteresis lower than 10° was achieved. The buoyancy of the superhydrophobic fabric was examined by using a miniature boat made with the fabric. Moreover, the superhydrophobic cotton fabric showed reasonable durability in withstanding at least 30 machine-wash cycles (Zhao et al. 2010). (2) Coating thin polymeric layers using the electrostatic self-assembly LBL method can produce durable anti-wetting polyester fabrics (Joung and Buie 2015). Silica nanoparticles were uniformly dispersed on the polymer layers, then deposited on the fabric by an electrophoretic deposition technique, and finally stabilized by a heat treatment. The modified fabric showed a high static contact angle and low contact angle hysteresis, and also retained its original colour, flexibility, and air permeability. The hydrophobicity of the coating layer was maintained over 500 h during skin friction resistance testing. Novel designing and implementing those novel designs for superhydrophobic textile surfaces using nano/micro-scale inorganic materials can be accomplished by several methods and methodologies (Kolen’ko et al. 2004; Sayilkan et al. 2007; Asilturk et al. 2006; Xu and Cai 2008; Uyama et al. 1998; Abidi 2009; Stephen and Hoon 2007; Park et al. 2010; Ma et al. 2005; Sas et al. 2012; Tendero et al. 2006;

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Fig. 8.12 Schematic representation of electrostatic self-assembly (ESA) by the LBL process. A flexible textile material, for example bearing a –ve charge, is initially immersed in the polycationic solution (1) followed by washing in solution (2) to remove excess molecules. The substrate containing the cationic layer is subsequently immersed in the polyanionic solution (3). The molecules not effectively attached can be removed in the washing solution (4)

Song and Rojas 2013; Leroux et al. 2008; Kang and Sarmadi 2004). A few important and latest ones are covered in the previous book Chaps. 6 and 7 on advanced textiles, especially electrospinning. The others include Hydrothermal syntheses, Grafting and Plasma technologies. Some of these effectively employ to reduce the surface energy by different mechanisms of surface modifications and the following subsections cover these aspects in detail.

8.3.2 Lowering of Surface Energy by Chemical Modification Surface roughening by itself is not sufficient enough to achieve a superhydrophobic surface. Roughening should also be accompanied by coating or by treatment with low surface energy chemicals. The commonly used reactive low-surface-energy chemicals are long alkyl chain fatty acids, long alkyl chain thiols, alkyl or fluorinated organic silanes, perfluorinated alkyl agents, polydimethyl siloxane, and their combinations. Commercially available products such as water-repellent agents and a poly (acrylate-g-siloxane) textile finishing agent can also be used to impart superhydrophobicity.

8.3.2.1

Surface Modification by Fluorinated Compounds

The most commonly used chemicals to make the surface hydrophobic are fluoroalkyl silanes owing to their extremely low-surface-free energy and the easy reaction of the silane groups with the hydroxyl groups on coatings. Large numbers of superhydrophobic surfaces utilizing fluoroalkyl silane surface modification have been reported. A simple dip-coating process was developed for delivering a conformal

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coating of fluorodecyl polyhedral oligomeric silsesquioxane molecules on commercial fabrics (Choi et al. 2009). The coated fabrics exhibit reversible deformationdependent tuneable wettability and also have the capacity to switch their surface wetting properties between super-repellent and super-wetting with respect to a wide range of polar and non-polar liquids.

8.3.2.2

Surface Modification by Alkyl Molecules

For surface modification by alkyl molecules, normal commercial cloths have been modified with suitable gold micro/nano-structures (Wang et al. 2007b). The asprepared product was then immersed in an ethanol solution of n-dodecanethiol to make the surface hydrophobic. Superhydrophobic cloths with the highest water contact angles close to 180° can be made by this treatment.

8.3.2.3

Surface Modification by Non-fluorinated Polymers

In some cases, non-fluorinated polymers were also used for surface modification. Thus a thin coating of non-fluorinated hydrophobic polymeric film was deposited on the poly(ethylene terephthalate) fabric covered with epoxidized silica nanoparticles (Ramaratnam et al. 2007). The hydrophobic polymeric film contained 29 wt% of styrene and 1.4 wt% of reactive maleic anhydride groups, and coating with this polymeric film generated an ultra-hydrophobic textile surface. It is important to mention here that the coating will permanently anchor on to the textile fibre boundaries because of the chemical attachment of nanoparticles and polymers to the surface.

8.3.2.4

Surface Modification by Silicon Compounds

Coating of transparent, stable and nanoscale polymethyl siloxane onto cellulosebased materials by a simple chemical vapour deposition (CVD) process, followed by hydrolyzation and polymerization, can result in a superhydrophobic textile surface (Shenghai et al. 2007).

8.3.3 Evaluation Techniques 8.3.3.1

Evaluation of Superhydrophobicity by Goniometry

The most widely accepted technique for measuring contact angles is the sessile drop method. In this method, the image of a droplet placed on a surface is analysed. This optical method for determining the contact angle is known as goniometry. A typical modern goniometer consists of a light source, sample stage, lens and a CCD camera.

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Fig. 8.13 Schematic representation of contact angle goniometry

A motorized dosing system dispenses a certain amount of probe liquid at a defined rate onto the solid surface to form a sessile drop. Cameras and software are used to capture and analyse the drop shape. Figure 8.13 is a schematic representation of a modern contact angle goniometer. This instrument is also equipped with additional capabilities which include a high temperature environmental chamber, a pressure/ vacuum chamber, a tilting base and a fully automatic drop dispensing setup for measuring the advancing and receding contact angles.

8.3.3.2

Shape Analysis and Baseline Detection for Determining Static Contact Angle

During a contact angle measurement, the drop image is initially subjected to a grey level analysis that determines the contour line around the phase boundary in the drop image. Subsequently, this drop contour is determined mathematically. The contact angle is obtained from the angle between the drop contour function and the sample surface and the projection of the contact angle in the drop image is known as the baseline. The mathematical description of the baseline depends on its shape: i.e. a straight line equation for a flat surface, and a circular function for curved substrates.

8.3.3.3

Models for Contour Analysis

Drop shape analysis can be done by various methods and the following sub-section will provide a glimpse of these methods. (1) Circle method: In this method, it is assumed that the drop shape is in the form of a circular arc, and the height and width of the rectangle enclosing the arc are determined. This method is applicable for very small contact angles and drop volumes. The limitation of this method is that only few a pixels at the point

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of inflection and at both sides are used instead of the whole contour. Thus the measurement is more susceptible to interference inaccuracies in these areas. (2) Conic section method: In this method, it is assumed that the drop shape takes the form of an ellipse and that it fits the general conic section equation. This method is also known as the tangent 1 method. The contact angle is determined as the angle between the baseline and the tangent to the conic section curve at the three phase contact point. (3) Polynomial method: This method is also known as the tangent 2 method and it evaluates only the phase contact region. There is no specific requirement for the contour shape since the polynomial adapts itself to any curve at the three phase contact point. (4) Young-Laplace method: The Young-Laplace fit is particularly suitable for symmetrical drop shapes. It determines the characteristic drop shape under the influence of gravity via a complicated calculation method. The Young-Laplace method is well known for its use in determining the surface tension from the shape of a pendant drop. 8.3.3.4

Selection of an Appropriate Model for Contour Analysis

Several factors influence the selection of a model for contour analysis of any drop shape. This section describes different parameters to guide the selection: (a) Contact angle: The contour of a drop with small contact angle and small drop volume can be well described by the arc shape. For contact angles ≤20°, the circle method is most suitable. On the other hand, when the contact angle lies between 20° to 100°, the contour assumes an elliptical shape, and can be well described by the conic section method. The polynomial method and YoungLaplace fit can be used throughout the whole measuring range of contact angles above 10°. (b) Drop Volume: It is well known that the drop contour changes from a circular to elliptical shape as the volume of the drop increases, owing to its own weight. This can be better understood by using diiodomethane as a test liquid, since it has a low surface tension and high density. For such types of liquids, the polynomial method or Young-Laplace fit should be used for drop volumes greater than 3 μl. (c) Deposition of the drop: For contour analysis not only is the drop size important but also the drop deposition method should be taken into account. For example, in the case of advancing or receding contact angles, the drop deposition volume changes continuously and the needle tip also is positioned within the drop. Therefore the methods that are insensitive to contact between the drop and needle should be used, namely the tangent 2 method, conic section or polynomial methods. When measuring the dynamic contact angles it is necessary to avoid distortion of the drop shape by the needle. This means that large volume drops are recommended to be used. (d) Symmetry of the drop: The heterogeneity of the sample surface will deform the drop shape. This means that the contact angles at the right- and left-hand sides

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of the drop will differ. The same problem occurs when measuring the tilt angle. In this case, the drop is deformed by the tilt. The circle method and YoungLaplace fit cannot be used in such cases, since these methods are applicable only to symmetrical drops. However, the tangent 2 method can be used for asymmetrical drops because it can detect the differences between the right- and left-hand contact angles.

8.4 Select Scientific Studies at DMSRDE The DMSRDE research group is involved in developing superhydrophobic and oleophobic textile surfaces for various Defence and civilian applications. In this section, two cases will be discussed in which an inherently flame retardant polyester surface and a cotton surface were made superhydrophobic and highly oleophobic. The work includes the methodology for development of the materials and their characterization. Evaluations of their superhydrophobic and oleophobic properties are also included.

8.4.1 Study #1 (a) Superhydrophobic textile surfaces have been prepared by in situ coating of freshly prepared silica nanosol on cotton substrates and inherently flame retardant polyester substrates, followed by surface modification with poly(dimethyl siloxane) (PDMS), and hexadecyltrimethoxysilane (HDTMS) respectively. The precursor tetraethoxyortho silicate (TEOS) concentration was varied from 0.025 to 0.075 molar concentrations. The concentration of PDMS and HDTMS was varied from 7 to 14% (w/w) concentration and 2–8% (w/w) respectively. (b) The prepared textile surfaces were characterized by Scanning Electron Microscopy (SEM) for surface characteristics and morphology. The SEM micrographs depicted a very fine, uniform, well-adhered and crack-free coating of nanoparticles, for example, Fig. 8.14, which is for the polyester surfaces. (c) The as-prepared cotton surfaces showed a static water contact angle ~149° with 0.03 mol TEOS and 8% PDMS concentration, see Fig. 8.15. The developed polyester surfaces demonstrated a static water contact angle (WCA) ~ 150° and roll-off angle 150°, b sliding angle η2 > … ηN , where ηN = 377 Ω), whereby the impedance values are correlated with the effective complex permittivity and permeability of each layer. The prime goal of this multilayered configuration is to reduce the overall reflection over a certain frequency region of interest. Thus the objective function for minimizing the reflection coefficient can be expressed as: F(η1 , d1 . . . η N , d N ) = − max[RC]

(10.3)

√ where ηi = μN /εN for i = 1, 2,..N and γi = j2π/λ0 ηi for i = 1 to N + 1, representing the intrinsic impedances and propagation constants for each layer; and ‘μN ’ and ‘εN ’ represent the complex permittivity and permeability of the individual layers. The characteristic impedances (Z i ) and reflection coefficient (RC) in dB can be expressed as (Pozar 1997): [ Z i = ηi−1

1 + ⎡i−1 exp(2γi−1 di−1 ) 1 − ⎡i−1 exp(2γi−1 di−1 )

] (10.4)

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⎡i =

Z i − ηi for i = 2, 3, . . . N + 1 Z i + ηi RC (d B) = 20 log10 |⎡ N +1 |

(10.5) (10.6)

Equations 10.4–10.6 enable estimation of the reflection coefficient of the multilayer absorber. Numerical optimization is used to reduce the reflection value as far as possible.

10.2.4 Concise Description of the Microwave Absorption Mechanism When any microwave/radar signal impinges on the surface of an absorber, the incident wave undergoes reflection, absorption and transmission. The reflection from any object can be greatly reduced using: (i) absorption, (ii) scattering, and (iii) cancellation. The absorption of energy depends primarily on the complex permittivity and permeability of the absorber: the loss mechanisms for a dielectric absorber are dielectric losses and conduction losses; for magnetic absorbers, the magnetic loss tangent, hysteresis, electron spin resonance and domain wall resonance are the important parameters. The scattering is mainly attributed to the fillers, which resemble thin wire/dipole structures. Cancellation helps in summing up the incident and reflecting signal with a phase difference of ±180°, thus making the net amplitude of the resultant signal zero. Quarter wavelength destructive interference helps in microwave attenuation, and this effect becomes more pronounced for multilayer absorber. The multilayer absorber configuration gives more attenuation of the incident energy as well as enhancing the absorption bandwidth. One of the critical factors for the design of microwave absorber is impedance matching. This means that the absorber’s impedance should be similar to the free space impedance in order to avoid any abrupt discontinuity at the air-material interface.

10.2.5 Functional Materials Based Microwave Absorbers: Dielectric Absorbers Functional materials based on dielectric composites possess nonmagnetic behaviour, ' and their properties can be specified in terms of the complex permittivity (ε* = ε − '' j ε ), with the real part primarily specifying the energy storage capacity, and the imaginary part or the loss tangent providing the absorption capability. For efficient microwave absorption, a higher dielectric loss tangent in the specified frequency band is usually required.

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In the early years, lossy dielectric filler materials such as graphite, carbon black, amorphous carbon, and carbon fibres were used for various applications such as pyramidal foams and radar absorbing coatings and composites. However, in recent years various advanced composites based on nanoparticles of Ag, Ni, Fe, Co, ZnO, conductive polymers (PANI, PPy), different forms of carbons (CNTs, graphene, reduced graphene oxide, graphene nanosheets) are often being used (Zhu et al. 2014; Bhattacharya et al. 2014; Lv et al. 2015). In this subsection, we discuss only carbon materials. Extensive work has been carried out on dielectric absorbers that achieve high dielectric loss using carbon materials: carbon black, carbon nanotubes, graphite flakes, expanded graphite, graphene, and chopped and milled carbon fibres (Oh et al. 2004; Ling et al. 2009). Some details are given here: Carbon black as the potential filler material was used in 1936, whereby a resonant microwave absorber was synthesized using carbon black and titanium dioxide (Saville 2005). During WWII carbon black based absorber paint (HARP) was developed at the Massachusetts Institute of Technology (MIT). Various articles are reported in the literature showing the potential use of this filler material. Oh et al. (2004) carried out a detailed study including the electromagnetic characterization of carbon black powder embedded into a glass fabric reinforced plastic (GFRP) matrix in the X band frequency range (8–12 GHz), as shown in Fig. 10.4. Figure 10.4 shows that the use of carbon black as the filler constituent gives approximately 10 dB absorption bandwidth in the X band region. To enhance the absorption bandwidth, carbon black was mixed with silicon carbide powder, see Fig. 10.5a, b, by Liu et al. (2011). The diagram in Fig. 10.5 shows indeed that the absorption bandwidth has been increased, and this effect increases with increasing SiC content.

Fig. 10.4 Comparison of the measured and simulated reflection loss of carbon black based composites. Courtesy Oh et al. (2004), Musal and Hahn (1989)

294 Fig. 10.5 a Morphologies of SiC particulate. b CB particulate. c Reflection loss studies of Carbon black-SiC composites. Courtesy Liu et al. (2011)

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Carbon nanotubes (CNTs) are another potential filler material. CNTs possess excellent electrical and mechanical properties owing to their nanoscale sizes [length 1– 100 nm]. Also, CNTs are lightweight, have high conductivity and large aspect ratios, all of which are favourable for efficient microwave absorbers. Figure 10.6 shows the performance of a typical CNT-based absorber where a GFRP laminate is used as the matrix (Lee et al. 2006). It is seen that multilayer CNT-based microwave absorbers provide quite high values of absorption in the X band frequency region. Graphite flake is another prominent filler material. It was originally investigated during WWII. Fan et al. (2009) reported on the microwave absorbing capability of graphite flakes dispersed in an ethanol solution of phenol-formaldehyde cement (PFC). On solidification, the resultant absorber composite consisted of various weight percentages of graphite flakes (20–35%) embedded in the matrix. The absorption properties are presented in Fig. 10.7, which shows that 25% weight percentage of filler material gives quite high absorption when using a 2 mm thick sample.

Fig. 10.6 a Enlarged SEM photograph of MWCNT. b Reflection loss performances of MWCNTGFRP laminates. Courtesy Lee et al. (2006)

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Fig. 10.7 a SEM micrographs of graphite flakes. b Measured reflection loss properties of graphite flakes composites. Courtesy Fan et al. (2009)

Expanded graphite: Gogoi et al. (2014) used expanded graphite in Novolac phenolic resin. The SEM images and the absorber performances for a sample are shown in Fig. 10.8: the composite with 8% of filler fraction gives a minimum 10 dB absorption bandwidth in the entire X band frequency region. Graphene Much more recently, graphene has created significant attention as a potential candidate for microwave absorbers. Graphene consists of two-dimensional (2D) sheets possessing extraordinary electrical, thermal and mechanical properties, and with high specific surface area (Stankovich et al. 2006). Furthermore, graphene’s

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Fig. 10.8 a SEM micrographs of expanded graphite flakes. b Reflection loss properties of Expanded graphite flakes composites. Courtesy Gogoi et al. (2014)

unusual electron band structure offers the potential of new electron conduction phenomena that could provide improved impedance matching characteristics. This electron band structure also introduces strong dipole polarization relaxation and other loss mechanisms, which are all in favour of electromagnetic wave absorption. A graphene based composite thus appears attractive for dielectric microwave absorbers, also because of the possibility of making thinner and lightweight structures. Recent work by Cao et al. (2015) shows the microwave absorption performance of a graphene based composite with various weight percentages (3,7,11%) of graphene in a silica xerogel, see Fig. 10.9. For a composite thickness of only 2.4 mm a 10 dB absorption bandwidth was achieved over the entire X band.

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Fig. 10.9 Microwave absorption properties of ultrathin graphene composites: a the RL of the 3, 7 and 11 wt% composites at 413 K with a thickness of 2.1 mm versus frequency, b the RL of the 3, 7 and 11 wt% composites at 12.4 GHz with a thickness of 2.1 mm. Courtesy Cao et al. (2015)

Carbon fibres The use of carbon fibres as the potential dielectric filler material was recently shown by Rosa et al. (2009). They were able to design broad band microwave absorbers using the carbon fibres shown in Fig. 10.10. It can be observed from this figure that a carbon fibre based composite (thickness 4.5 mm (Rosa et al. Aug. 2009)) gives a minimum 10 dB absorption in the X band frequency range; and a minimum 7 dB reflection loss in the entire 6–18 GHz range. We note here that Baskey et al.

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Fig. 10.9 (continued)

(2014) have investigated 2 mm thick microwave absorbers, based on carbon black and carbon fibre fillers, using an epoxy as the matrix material.

Fig. 10.10 a SEM image of carbon fibres. b Comparison of the measured and calculated reflection coefficient of a composite made from these carbon fibres. Courtesy Rosa et al. (2009)

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10.2.6 Functional Materials Based Microwave Absorbers: Magnetic Absorbers Magnetic absorbers possess both dielectric and magnetic properties, and hence their electromagnetic characteristics may be specified in terms of the complex permittivity ' '' ' '' ' (ε* = ε − j ε ), and the complex permeability (μ* = μ − jμ ) with the real part μ '' indicating the magnetic strength and the imaginary part μ representing the magnetic loss. Magnetic composites usually also exhibit dielectric behaviour, since it is difficult to synthesize pure magnetic materials having zero dielectric signature. Thus magnetic absorbers are specified as magneto-dielectrics possessing both complex permittivity and complex permeability. The electromagnetic characterization of magnetodielectric composites is therefore more complicated compared with pure dielectric composites. However, from the design point of view, magneto-dielectric composites provide more flexibility because there are more parameters which can be chosen to achieve optimum performance. In fact, for design, the magneto-dielectric composites might sometimes be advantageous owing to the presence of both dielectric loss and magnetic loss. Most magnetic magneto-dielectric composites for the design of microwave absorbers are based on various forms of ferrites. These are discussed here. Ni–Zn ferrites Musal and Han (1989) investigated the reflection loss properties of Ni-Zn based ferrite absorbers working in the 100–1000 MHz [VHF/UHF] frequency region as shown below in Fig. 10.11. It can be observed from this graph that the absorber shows a minimum 10 dB reflection loss over the full frequency region with a relatively moderate thickness of 5.5 mm. Further, the performance is better at lower frequencies, mainly due to high permeability values at these lower frequencies. Mn–Zn ferrites Kim et al. (1996) studied the absorption performances of MnZn ferrites using different filler ratios in the host matrix. They observed that the absorber frequency range can be tuned by varying the filler volume ratio, as shown in Fig. 10.12. Nanocomposite ferrites: Recently Tyagi et al. (2011) reported about hard and soft ferrite nanocomposites using nickel-and zinc-substituted strontium hexaferrite for designing magnetic absorber composites with a thickness of 2.5 mm. The proposed absorber showed high values of dielectric and magnetic loss tangent combined with high values of dielectric constant and permeability. Figure 10.13 gives an example of their reflection loss results.

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Fig. 10.11 Reflection loss properties of Ni–Zn ferrite. Courtesy Musal and Hahn (1989)

10.2.7 Artificial Materials (FSS-Based) Microwave Absorbers Artificial materials can be defined as having structures consisting of unit cells with electrical sizes smaller than the operating wavelength. These unit cells may be considered as analogous to atomic or molecular cells in natural materials. Electrical properties such as permittivity and permeability values achievable with artificial materials cannot usually be realized using natural materials. For microwave absorbers, these artificial materials are mostly realized using frequency-selective surface (FSS) based structures. FSS-based structures are sometimes also referred to as metamaterial structures, as a qualification that they possess exceptional properties (negative permittivity and permeability) in specified frequency bands. These materials have received significant attention in recent years for the design of microwave absorbers. They control the path of the incident electromagnetic radiation such that the amount of reflection, transmission and absorption can be tailored for the required frequency band and band width. FSS-based structures consist of periodic perforations on a metallic sheet backed by a substrate. They are basically resonance structures with a specific unit cell geometry that is designed in one or two dimensions. The unit cell dimensions and geometry determine the overall behaviour of the FSS-based structure. These structures have been used for various applications since their inception during WWII. In the early days, FSS structures were primarily used for defence, especially to reduce the radar cross-sections of stealth ships. However, the concept has now been explored for various astronomy applications, e.g., Cassegrainian sub-reflectors for parabolic-dish antennas and phase arrays (Liu et al. 2016). In recent years, FSS-based structures have also been used to design frequency-selective windows (Lee 1971) and waveguide

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Fig. 10.12 Reflection loss properties of Mn–Zn ferrite at a various matching thickness, b various frequencies with respect to volume fraction of materials. Courtesy Kim et al. (1996)

filters (Seager and Vardaxoglou 2001); electromagnetic bandgap surfaces (EBGs) (Moustafa and Jecko 2008); and various types of antennas. Lastly, they have also been used for terrestrial and airborne radomes, and to provide electromagnetic shielding in certain frequency ranges. FSS-based structures possess certain filtering characteristics that are primarily due to the cumulative effect of the shape, size, periodicity, and EM properties of the substrate. They may be broadly classified into four types of filters: Low pass, High pass, Band pass and Band stop. In electrical engineering terms, an FSS-based structure behaves like a typical RLC circuit with either series or parallel resonance, whereby the resistance (R) and inductance (L) are present due to the metallic part, and the capacitance (C) is due to the gaps between various metallic layers. For FSS-based structures to be used as electromagnetic absorbers, various types of unit cell designs can be used in conjunction with a substrate grounded by a metallic sheet which behaves as a perfect electrical conductor (PEC). The absorption is mainly

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Fig. 10.13 Reflection loss properties of strontium hexaferrite based magnetic absorber. Courtesy Tyagi et al. (2011)

due to the loss provided by the unit cell (which is generally resistive in nature) and the substrate. Usually, the absorber performance is a narrow band in nature, but newer designs enable broad band absorbance performances. To do this, lossy dielectric/ magnetic substrates are used in combination with unit cell FSS-based structures: these are called hybrid absorbers. Traditionally, dielectric based and magnetic material based absorbers have been used for electromagnetic absorbers. However, FSS-based structures are lighter and thinner, and as mentioned earlier, can be tailored for the required absorption bandwidth.

10.2.8 Theoretical Design of FSS-Based Structures When electromagnetic energy is incident on any periodic structure, currents are induced in its elements, and these currents reradiate EM waves from the structure. Various methods are used for the analysis of FSSs, e.g. the mutual impedance method; method of moments (MoM); finite element method (FEM); finite-difference timedomain (FDTD) method; and the equivalent circuit (EC) method. Amongst these, the EC method is a simple and powerful technique because it models FSSs as energy storage inductive and capacitive parameters that can be determined from the shapes of the elements. The basic rule for designing FSSs is to make the unit cell size equal to the halfwavelength (λ/2). However, the final dimensions need to be precisely fine tuned to meet the desired frequency response. A straightforward explanation will be presented

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here, with the aid of Fig. 10.14. Thus a square loop structure can be separated into vertical and horizontal conducting strips, which can be modelled as inductive and capacitive components. In order to design FSSs for electromagnetic absorbers it is essential that they should couple to the maximum amount of incident electromagnetic energy. A problem is that it is often difficult to select the unit cell parameters. Some important points are listed here: 1. Proper unit cell geometry of FSS, i.e. it should be a circular/hexagonal loop for maximum coupling of the magnetic field, and an electric dipole type for coupling of the incident electric field. 2. The dimension of the unit cell should be about λ/2 for an FSS-based structure, but λ/4 for metamaterial structures (the wavelength corresponds to the central frequency). 3. The structure should be designed in such a way that the effect of inductance and capacitance contribute to a resonance mechanism. 4. The surface current and electric field distribution should be analysed for the proposed structure: this gives insight concerning the coupling mechanism. 5. Finally, the power loss density and reflection loss with a metal backing should be analyzed.

Fig. 10.14 Conducting patch elements and their equivalent circuit representations

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10.2.9 Examples of FSS-Based Microwave Absorbers Single band absorbers: Landy et al. (2008) proposed a metamaterial structure based on an electric ring resonator as shown in Fig. 10.15. This consists of two conductive layers (a) and (b) separated by an Fr-4 dielectric spacer (c) having a thickness of 0.6 mm. The rear layer (b) has a cut wire structure. The (simulated) reflection (green), transmission (red) and absorbance (blue) performances are shown in (d). These results show that absorption is almost 100% at 11.65 GHz. The full width at half maximum (FWHM) was 4%. From these results, it was concluded that near-perfect absorption is possible in narrow bandwidth. Baskey et al. (2015) reported a flexible ultrathin FSS-based absorber as shown in Fig. 10.16. This consists of circular rings with split-separated arms on four sides. The split arms behave as inductors, whereas the gap between them introduces the

Fig. 10.15 a Electric resonator and b cut wire components of unit element; c unit element with dielectric spacer and direction of wave propagation; d simulated pattern for reflectance, absorbance and transmission. Courtesy Landy et al. (2008)

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E

k

H

(a)

(b) Fig. 10.16 a Unit cell of FSS and b Fabricated FSS film. Courtesy Baskey et al. (2015)

capacitors. This inductor and capacitor configuration is responsible for coupling of the incident electromagnetic energy. The FSS-based structure is designed using flexible polyimide thin sheet (thickness 135 μm, εr = 3.5, tan δ = 0.00025) having copper as the conducting lines. The back surface of the structure is metallic in order to ensure that the EM energy does not get transmitted from the rear side of the structure. The proposed structure has been simulated under the periodic boundary condition along the ‘x’ and ‘y’ directions and with the incident wave propagation along the ‘z’ direction. The simulated absorptivity values under transverse electric (TE) and transverse magnetic (TM) polarization for various incidence angles are shown in Fig. 10.17. It can be observed from Fig. 10.17 that the maximum absorptivity is 99.5% at 11.20 GHz, whereas the Full Width at Half Maximum (FWHM) bandwidth is 300 MHz. It can also be observed that the EM absorption of the structure remains constant up to an incidence angle of 40°, after which there is a small shift in the absorption frequency. In order to understand the absorption mechanism of the FSS absorber, the surface current at the designated absorption frequency is plotted as

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Fig. 10.17 Simulations of the absorptivity under. a TE polarization. b TM polarization at various incidence angles. Courtesy Baskey et al. (2015)

shown in Fig. 10.18. It can be observed from this figure that the power loss is a maximum at the interface between the split arms of the ring. Multi-band and band enhanced microwave absorbers: Baskey et al. (2015) recently reported a multi-band metamaterial-inspired structure using hexagonal closed rings (HCR) and the octa-star strips (OSS) based topology, designed in a periodic pattern as shown in Fig. 10.19. The structure was designed using copper (conductivity of

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Fig. 10.18 a Top and b bottom surface current plots; c power loss density at 6.35 GHz. Courtesy Baskey et al. (2015)

5.8 × 107 s/m; thickness 0.035 mm), with the rear layer of the structure terminated by copper as the ground plane. The copper patterns are separated by a dielectric FR-4 substrate with thickness (t) of 1.0 mm, relative permittivity (εr ) of 4.3 and loss tangent of 0.025. It was observed that during the incidence of the plane wave, hexagonal closed rings (HCRs) and octa-star strips (OSS) induce the inductance, while the dielectric substrate on which the structure is designed produces the capacitance. These two effects combine to create the absorption at 4.10, 6.15 GHz, 10.05 GHz and 15.52 GHz, respectively, with absorption values of 98.72, 99.20, 99.21 and 99.90%, as shown in Fig. 10.19b.

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Fig. 10.19 a The front view of the unit cell together with the direction of the incident plane wave; b simulated absorbance of the structure. Courtesy Baskey et al. (2015)

From the above studies, it was observed that the operation of ordinary metamaterials is confined within a narrow absorption bandwidth, since the dispersive resonances are exploited to control the permittivity and permeability of the structure. The narrow absorption bandwidths are a major problem for applications like stealth technology, which requires broader absorption bandwidths. An enhanced bandwidth absorber based on double resonance has been proposed, and this has several crucial advantages. The proposed metamaterial absorber consists of two conductive layers with a single substrate between them (Lee and Lim 2011). The top layer has a dual electric resonator (DER), as shown in Fig. 10.20a. The bottom layer has a ground plate without patterning as shown in Fig. 10.20b, and the complete fabricated structure is shown in Fig. 10.20d. The DER consists of a three-finger interdigital capacitor pattern to manipulate the electrical response and increase the coupling losses so that the transmitted wave can be reduced. The DER is designed using a hexagonal shape that is six-fold rotationally-symmetric about the propagation axis, making this absorber polarization-insensitive. As a result, and as shown in Fig. 10.20d, the proposed absorber has double resonance with two distinct absorption peaks at 9.75 and 10.3 GHz, with 98% absorption; and an FWHM of 11% at 10 GHz. This is remarkable, since the substrate was only 0.6 mm thick. Broad band metamaterial absorbers: Various techniques and structures have been adopted to improve the absorption bandwidth of metamaterial based absorbers. These techniques include the use of multilayer absorber structures, incorporating multiple resonant elements of the same shape in a single unit cell by scaling their geometric dimensions and using lumped elements and multi-resonant structural design. Figure 10.21a shows a gradient metamaterial structure using the lumped resistor configuration, reported by Yang and Liu (2014) for a substrate thickness of 1.6 mm. Figure 10.21b shows the absorber performance: broad band absorption complying to a minimum of 90% absorption for both TE and TM polarization was obtained over the range of 15–18 GHz.

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Fig. 10.20 a–d Schematic of the unit cell, together with the direction of the incident plane wave; e the measured absorbance of the structure. Courtesy Lee and Lim (2011)

Xiong et al. (2013) investigated the broad band performance of the multilayer structure shown in Fig. 10.22a and based on Teflon and Rogers substrates. Figure 10.22b shows that using this multilayered configuration a minimum of 90% absorption was achieved over the entire 8–18 GHz frequency region. Hybrid microwave absorbers As may be seen from some of the previous examples, artificial materials structures and dielectric/magnetic filler based composites are

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Fig. 10.21 a The schematic of unit cell structure, b measured absorbance of the structure. Courtesy Yang and Liu (2014)

Fig. 10.22 a The schematic of unit cell structure and b measured absorbance of the structure. Courtesy Xiong et al. (2013)

unable to produce broad band absorption performances at relatively lower sample thicknesses (1–2 mm). The use of functional materials as microwave absorbers, particularly in stealth technology, requires broad band absorber performances at minimal thicknesses in order to save weight. In this context, a hybrid absorber configuration is one of the most favoured solutions. In fact, hybrid absorbers, virtually by definition, employ both artificial materials and dielectric/magnetic filler based composites. This gives two degrees of freedom for tailoring enhanced absorption bandwidths or broad band performances. Zou et al. (2008) studied a hybrid absorber combination using a split ring resonator + thin wire metamaterial (MM) structure combined with a carbonyl-iron powder (CIP) based magnetic sheet, as shown in Fig. 10.23a–c. The metamaterial structure was designed using a 0.35 mm thick FR-4 substrate and a 2.5 mm thick CIP sheet. It can be observed from Fig. 10.23d that combinations of these materials show

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Fig. 10.23 a Schematic design of metamaterial (MM), b fabricated metamaterial structure. c CIPC sheet. d RL performances for combination of CIPC and metamaterial. Courtesy Zou et al. (2008)

remarkable improvements in the absorption level as well as improvement in the 5 dB absorption bandwidth. Li et al. (2017) described a hybrid absorber configuration where a resistive FSS was sandwiched between a glass fibre reinforced composite (GFRC) as shown in Fig. 10.24a. Figure 10.24b shows the performances of a 5 mm hybrid absorber configuration: a minimum of 90% absorption was achieved over the entire 8– 17 GHz frequency range. Recently, Baskey and Akhtar (2017) showed using a hybrid absorber configuration with a flexible FSS sheet combined with a dielectric sheet (Fig. 10.25a), gives a minimum of 10 dB absorption in the entire X band frequency region (Fig. 10.25b): this is most remarkable, since the hybrid total thickness is only 1.335 mm.

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Fig. 10.24 a The schematic design of hybrid configuration. b Comparison between the simulated and measured performances. Courtesy Zou et al. (2008), Li et al. (2017)

10.3 Functional Materials for Camouflage Applications Functional materials in the forms of sheets and coatings are often used for concealing strategic targets from interrogating sensors working in the thermal or visual bands of the electromagnetic spectrum. Several investigators have studied camouflaged textile-based materials intended for both the visible and near infrared bands (380– 1200 nm) (Rubeziene et al. 2008). The Intermat group (www.intermatstealth.com) has reported low emissivity paint that reduces thermal radiation emitted from an object. Hellwig and Weber (2012) described a tarpaulin made by coating infrared (IR) based material onto one side of a woven/knitted glass fabric, with the other side coated with a material having reflectivity values in the visual and infrared region. Karlsson reported on thermal camouflage based materials working in the 3–5 and 8–12 μm frequency bands (Karlsson 1985). These proposed material based systems consist of woven fabric for which one side of the fabric is a metalized layer of polyethylene and the other side is coated with low emissivity materials. Besides camouflaging in the thermal and visible frequency regions, functional materials based on various dielectric and magnetic fillers are employed for the design of radar-interactive textile products. A summary is given in Table 10.1. Camouflage and its associated systems are very important for strategic objects, whether stationary or moving. Textile-based materials have a prominent role, since they provide camouflage products for visible, NIR, thermal, and radar regions of the electromagnetic spectrum.

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Fig. 10.25 a Schematic design of the hybrid configuration. b Comparison between the simulated measured results. Courtesy Baskey and Akhtar (2017)

10.3.1 Camouflage in the Visible Region The most plentiful, reliable, and easily-used enemy sensors are in the visual region (400–800 nm) of the electromagnetic spectrum. Therefore camouflaging in the visible region is extremely important. In this region, surveillance and detection of objects is primarily done by eye, aided or unaided, or by optical instruments or electro-optical sensors. Each object has its own visual characteristics that make it stand out from the background. The visual signatures include shape, size, colour, optical textures, patterns, shadows, and reflectivity of the object and background. Camouflaging against visual

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Table 10.1 Electromagnetic spectrum regions and required camouflage properties Radiation spectral range

Wavelength/ frequency

Requirements for camouflage properties

UV

200–400 nm

Matching of optical/UV reflectance properties of snow and ice

Visible spectrum

400–800 nm

Matching of colour, reflectance, optical texture, patterns, outline altering, shadow elimination and appearance of the background

NIR

750–1500 nm

Reflectance matching with background

Far IR/ thermal

3–5 and 8–14 μm

Hot/cold spot suppression, minimization of thermal contrast, follow diurnal variation of background, suppression of prominent features

Radar

2–18 GHz

Reduction in RCS for avoiding radar detection

sensors employs various techniques, including hiding, blending-in, and deception. Usually, two or more techniques are used to obtain the required camouflage efficacy in this region, and two or more methods are simultaneously considered. Hiding technique Objects are physically hidden by the deployment of natural materials such as natural or cut vegetation, and artificial materials/systems such as nets, screens of natural materials or smoke screens. So-called obscurant materials, like smoke screens, create a temporary camouflaging medium between a target and the detection system. Besides smoke, these can be in various forms, such as artificial aerosol-based materials consisting of dry and wet particles dispersed or suspended in air. The interaction of radiation with particles of the obscurant lowers the signal strength below the sensor threshold, or blocks the radiation used by threat target acquisition and weapons systems. The blocking takes different forms, such as absorption, scattering, emission and transmission. The effectiveness of obscurant materials also depends on concentration, particle size, shape, orientation, and refractive index. Blending-in technique Here the object is made to blend in with the background by using optical principles to produce an illusory effect, whereby a target appears to be an integral part of the background when interrogated by visual sensors. To blend the object into the background, the colour and reflectivity of the external outer surface should be made similar to that of the background. Deception technique In this technique a deliberate effort is made to mislead by various means, such as manipulation, distortion or falsification of evidence, to induce the antagonists to react in a manner prejudicial to their interest or to the benefit of the deceiver. Deception equipment can be categorized as dummy and decoy devices. Dummy devices involve making an artificial object similar to the real one in terms of shape, geometry and colour. Decoy devices are artificial objects that are matched with the actual objects. These systems are deployed with various characteristics and features of tactical equipment such as physical dimensions, emissions, and similar deployment locations.

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10.3.2 Camouflage in the Near Infrared (NIR) Region Near Infrared (NIR) sensors work on the principle of variation in reflectance of the target(s) and the background. Low Light Level Television, LIDAR and Image Intensifiers are frequently used sensors working in this region (Ramana Rao 1999; Rubeziene et al. 2008). To reduce detection by these sensing equipments it is essential that the tactical equipment/target should be painted/coated with materials having NIR reflectance values similar to those of the backgrounds; or covered with printed/coated fabrics with dyes having NIR reflectance values representative for the terrain. Each colour in the camouflage pattern has to have a specified reflectance value. To do this it is recommended to use (i) special dyes and pigments, incorporating strongly IR absorbing pigments into printing paste form, and (ii) special and minimizing IR reflectance coatings/layers. NIR reflectance values for certain desired shades are as follows: • • • •

Olive Green (Shade 220 of IS: 5): 45 ± 10%, Light Green (Shade 278 of IS: 5): 55 ± 10%, Dark Brown (Shade 412 of IS: 5): 20 ± 5%. Sand Colour (Shade 388 of IS: 5): 40 ± 5%.

10.3.3 Camouflage in the Thermal Region Every object generates heat and has thermal signatures, typically with thermal radiation wavelengths from 0.75 μm to 1 mm, which falls between the visible and microwave regions of the electromagnetic spectrum. In general, the emitted thermal radiant intensity of any target depends on its intrinsic temperature and surface finish, and it increases with an increase in temperature. More specifically, the emission peak intensity of any object shifts towards lower wavelengths as the object’s temperature increases. N.B: Every (test) object has a set of unique signatures that contain spatial, spectral and temporal details by which it can be identified and distinguished from other objects or backgrounds (Plesa et al. 2006). Thermal signatures can be obtained from thermal imaging of test objects. The signatures depend on various parameters such as (i) the shape, size, and temperature of the object; (ii) the background temperature, surface emissivity, earthshine, sunshine, skyshine; and (iii) the state of the object, i.e. stationary or moving, time of day, seasonal variations; and (iv) the operating band of the detecting radiometer. It is often the case that strategic objects are structures made of metals and coated with dark camouflage-colour paints. This means that the structural materials have high specific heat values and large heat storing capacity. As a result, when these materials are exposed to the sun, they absorb heat quickly because of the high absorptivity of the colours, and retain heat throughout the exposure. The rate and degree of heating depend upon the material properties. Thus, different objects exhibit different thermal behaviour/apparent temperature profiles with time.

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Background and vegetation have their own diurnal thermal profile, which is different from the object’s temporal thermal profile. This makes it possible to detect the object (target) by thermal sensors. The constituents of the atmosphere (e.g. water vapour, carbon dioxide, carbon monoxide and aerosols, dust, dirt, carbon, sea salt, water droplets (haze or fog), smoke, and artificial aerosols) have a significant effect on the propagation of radiation to the thermal radiation sensing system. Three major phenomena are observed due to the presence of these atmospheric constituents: (i) reduction in radiant intensity value recorded by the sensor; (ii) path radiance scattered into the field of view (FOV) that reduces the target contrast and image fidelity owing to turbulence; and (iii) small angle scattering (Hudson 2006). The objective of thermal camouflage is to control the surface temperature of the object in such a way that its thermal matches that of the background. The materials/ system for thermal camouflaging should be lightweight and should have the thermal characteristics of the background and also (i) a minimum solar loading effect; (ii) the desired insulation for suppression of hot spots; (iii) be easily applicable for all weather conditions; and (iv) be durable and insensitive to dust or damage by bushes etc. There are three main options: Low emissivity paints/anti-thermal paints The thermal radiations emitted by a target depends on temperature and emissivity. Therefore paints with various emissivity values representative for the target reduce the (apparent) temperature difference. Furthermore, using low emissivity paints the radiation patterns from the target can be made to resemble those of the background. This reduces the thermal signature. An example is given in Fig. 10.26. Figure 10.26a is an image where only the left half of a building has been painted, and this part of the building shows thermal blending with the environment. Figure 10.26b shows a test object coated with low emissive paint, and it is seen that the object blends in with the environment. Textile-based thermal camouflage systems Textile-based camouflage products such as multispectral camouflage nets (MSCN), multispectral personnel camouflage equipment (MSPCE), ponchos, thermal camouflage sheet and mobile camouflage system (MCS) are used to reduce the detection range of thermal cameras. These equipments are made of multilayer systems comprising various types of coated fabrics and laminates: namely, a radar-absorbent material coated layer, a high reflective material coated/laminated aluminized layer, an insulating foam layer, and a coated outer layer with the desired emissivity/reflectivity values according to the terrain. These multilayer fabric systems reduce the thermal contrast by way of thermal insulation (www.baesystems.com). Camouflage in the microwave region Camouflaging for ground-based objects is accomplished using textile-based products. Radar detection of any object is based on the amount of the return signal from the object when it is being interrogated by the sensor. If the return signal is weak, the range of detection of the object reduces, resulting in improved survivability of the object. Textile-based nets e.g. radar scattering camouflage nets (RSCNs) are often employed. An RSCN consists of a specially designed fabric with the inclusion of metallic elements. These metallic

Fig. 10.26 Thermal image of a building painted with low emissivity paint on left half portion and b combat vehicle painted with low emissivity paint in striped pattern

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Fig. 10.27 Deployment of a radar scattering camouflage net (RSCN) on a missile launcher vehicle: a missile launcher. b RSCN deployed on the launcher

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elements in the fabric act as scattering centres for radar waves. A typical image of an RSCN deployed over a missile launcher vehicle is shown in Fig. 10.27. Another camouflage system based on textile-based materials is the multispectral camouflage net (MSCN) which, as its name implies, has camouflage properties operating for the radar, thermal and also visual ranges. This type of system is very useful, since it can be employed for the protection of various strategic targets. MSCNs are also lightweight and suitable for all-weather conditions.

10.4 Conclusions It is evident that functional materials have significant roles in the design and realization of microwave absorbers. Functional materials that are primarily dielectric and/or magnetic in nature and artificial materials (FSS/metamaterials) have been concisely described. The applicability of these materials for the design of microwave absorbers in stealth technology has been briefly discussed. In addition, a distinct but related application, namely camouflage by using speciality materials and systems, has been discussed. Various camouflage techniques employed for visual, near infrared and thermal regions are mentioned. This includes the role of technical textile-based products such as multispectral camouflage nets and multispectral camouflage equipment and their applications. Acknowledgements The authors are grateful to their colleagues, associates and advisors of the stealth and camouflage programmes of DMSRDE, especially Dr. S Christopher and Dr. G Satheesh Reddy, the Former Chairmen of DRDO and Dr. Samir V. Kamat, the present Chairman, DRDO and Secretary, DD R&D, MoD, GoI for their constant encouragement and kind support of many a year. They all also gratefully wish to thank Professor GN Mathur for his vision for this specialization at DMSRDE, DRDO, Kanpur way back in the late 1990s when India was making its first strides into Stealth and Camouflage Technologies.

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