Micro and Nanostructured Epoxy / Rubber Blends E-Book

• Herausgeber: Wiley-VCH
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 792

Veröffentlichungsjahr: 2014

Beschreibung

Epoxy resins are polymers which are extensively used as coating materials due to their outstanding mechanical properties and good handling characteristics. A disadvantage results from their high cross-link density: they are brittle and have very low resistance to crack growth and propagation. This necessitates the toughening of the epoxy matrix without impairing its good thermomechanical properties. The final properties of the polymer depend on their structure. The book focuses on the microstructural aspects in the modification of epoxy resins with low molecular weight liquid rubbers, one of the prime toughening agents commonly employed.

The book follows thoroughly the reactions of elastomer-modified epoxy resins from their liquid stage to the network formation. It gives an in-depth view into the cure reaction, phase separation and the simultaneous development of the morphology. Chapters on ageing, failure analysis and life cycle analysis round out the book.

Leseprobe

Table of Contents

Cover

Related Titles

Title Page

Copyright

Preface

List of Contributors

Chapter 1: Introduction

1.1 Epoxy Resin – Introduction

1.2 Cure Reactions

1.3 Curing Agents

1.4 Different Curing Methods

1.5 Curing of Epoxy Resins: Structure–Property Relationship

1.6 Toughening of Epoxy Resin

1.7 Rubber-Modified Epoxy Resin: Factors Influencing Toughening

1.8 Toughening Mechanisms in Elastomer-Modified Epoxy Resins

1.9 Quantitative Assessment of Toughening Mechanisms

1.10 Introduction of Chapters

References

Chapter 2: Liquid Rubbers as Toughening Agents

2.1 Introduction

2.2 Toughening of Thermoset Resins

2.3 Fracture Behavior of Rubber-Toughened Thermosets

2.4 Natural Rubbers

2.5 Liquid-Toughening Rubber in Thermoset Resins

2.6 Concluding Remarks

References

Chapter 3: Nanostructured Epoxy Composites

3.1 Introduction

3.2 Preparation Methods of the Nanostructured Epoxy Thermoset

3.3 Morphology of the Nanostructured Epoxy Thermoset

3.4 Microphase Separation Mechanism

3.5 Mechanical and Thermal Properties

3.6 Conclusions and Outlooks

References

Chapter 4: Manufacture of Epoxy Resin/Liquid Rubber Blends

4.1 Introduction

4.2 Comparison of Hardeners

4.3 Rubber-Toughened Epoxy Resins

4.4 Cure Reaction Analysis

4.5 Conclusions

References

Chapter 5: Cure and Cure Kinetics of Epoxy-Rubber Systems

5.1 Introduction

5.2 Cure Analysis

5.3 Curing Kinetics

5.4 Diffusion Factor

5.5 Differential Scanning Calorimetry

5.6 FTIR Spectroscopy

5.7 Dielectric Spectroscopy Thermal Method

5.8 Pressure–Volume–Temperature (PVT) Method

5.9 Dynamic Mechanical Analysis (DMA) and Rheological Methods

5.10 Conclusions

Acknowledgments

References

Chapter 6: Theoretical Modeling of the Curing Process

6.1 Introduction

6.2 Modeling of the Curing Kinetics

6.3 Applications of the Empirical Models

6.4 Conclusion

References

Chapter 7: Phase-Separation Mechanism in Epoxy Resin/Rubber Blends

7.1 Introduction

7.2 Thermodynamics of Phase Separation

7.3 Phase Separation in Uncured Epoxy Resin/Liquid Rubber Blends

7.4 Phase-Separation Mechanism in Cured Blends

7.5 Conclusion

References

Chapter 8: Morphology Analysis by Microscopy Techniques and Light Scattering

8.1 Introduction

8.2 Developments of Morphology Analysis in Rubber-Modified Epoxies

8.3 Different Types of Morphologies

8.4 Morphology of Toughening and Reinforcing Effects

8.5 Conclusions

Acknowledgments

References

Chapter 9: Pressure–Volume–Temperature (PVT) Analysis

9.1 Introduction

9.2 Generalities on the Behavior of the Polymers

9.3 Measurement Techniques

9.4 PvT Measures on Epoxies

References

Chapter 10: Rheology of Rubber-Toughened Structural Epoxy Resin Systems

10.1 Introduction

10.2 Epoxy Resin Chemistry

10.3 Modeling of the Cure Process

10.4 Rheological Implication of Differences in Reactivity

10.5 Rheological Studies of Cure

10.6 Toughened Epoxy Resins

10.7 Concluding Comments

Acknowledgments

References

Chapter 11: Viscoelastic Measurements and Properties of Rubber-Modified Epoxies

11.1 Introduction

11.2 Viscoelastic Behavior Below and Near Gel Point

11.3 Viscoelasticity of Cured Materials

11.4 Other Remarks

11.5 Conclusion

References

Chapter 12: Light, X-ray, and Neutron Scattering Techniques for Miscibility and Phase Behavior Studies in Polymer Blends

12.1 Introduction

12.2 Brief Theoretical Considerations of Scattering

12.3 Light Scattering Experiment

12.4 X-ray Scattering

12.5 Neutron Scattering

12.6 Conclusions and Future Outlook

Acknowledgments

References

Chapter 13: Mechanical Properties

13.1 Introduction

13.2 Morphology and Mechanical Properties of Rubber-Modified Epoxies

13.3 Fracture Toughness

13.4 Conclusion

References

Chapter 14: Thermal Properties

14.1 Specific Heat

14.2 Thermal Conductivity

14.3 Thermogravimetric Analysis of Rubber/Epoxy Systems

14.4 Kinetic Study from TGA

References

Chapter 15: Dielectric Properties of Elastomeric Modified Epoxies

15.1 Introduction

15.2 Dielectric Study in Rubber/Epoxy Systems

15.3 Summary

References

Chapter 16: Spectroscopy Analysis of Micro/Nanostructured Epoxy/Rubber Blends

16.1 Introduction

16.2 Fourier Transform Infrared (FTIR) and Raman Spectroscopy

16.3 Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM)

16.4 Other Spectroscopy

16.5 Summary

Abbreviations

References

Chapter 17: Applications

17.1 Applications of Toughened Epoxy Resins

17.2 Thermoset-Based Materials for Optical Applications Containing Azobenzene Choromophores

References

Chapter 18: Comparison of Epoxy/Rubber Blends with Other Toughening Strategies: Thermoplastic and Hyperbranched Modifiers

18.1 Epoxy/Thermoplastic Blends: Development and Properties

18.2 Epoxy/Hyperbranched Polymer Blends: Development and Properties

18.3 Novel Toughening Approaches for Liquid Molding Technologies

18.4 Rubbers as Tougheners: Comparison with Thermoplastics and Hyperbranched Modifiers

18.5 Conclusions

References

Chapter 19: Reliability Testing

19.1 Introduction

19.2 Reliability Tests Used in Micro/Nanotechnologies

19.3 Behavior in Real Applications and Aging Studies of Epoxy/Rubber Blends

19.4 Conclusions

References

Chapter 20: Failure Analysis

20.1 Introduction

20.2 Methods for Failure Analysis of Epoxy/Rubber Blends

20.3 Typical Failure Modes and Failure Mechanisms of Epoxy/Rubber Blends Used in Micro and Nanotechnologies

20.4 Self Healing

20.5 Conclusions

References

Chapter 21: Life Cycle Assessment (LCA) of Epoxy-Based Materials

21.1 Introduction to Life Cycle Assessment (LCA)

21.2 Significance of Life Cycle Assessment (LCA)

21.3 Life Cycle Analysis of Epoxy Systems

21.4 Conclusion

References

Index

End User License Agreement

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Guide

Cover

Table of Contents

Preface

Chapter 1: Introduction

List of Illustrations

Figure 1.1

Figure 1.1

Scheme 1.2

Scheme 1.3

Figure 1.2

Scheme 1.4

Scheme 1.5

Scheme 1.6

Scheme 1.7

Figure 1.3

Figure 1.4

Figure 1.5

Figure 1.6

Figure 1.7

Figure 2.1

Figure 2.2

Figure 2.3

Figure 2.4

Figure 2.5

Figure 2.6

Figure 2.7

Figure 2.8

Figure 2.9

Figure 2.10

Figure 2.11

Figure 2.12

Figure 2.13

Figure 2.14

Figure 2.15

Figure 2.16

Figure 2.17

Figure 3.1

Figure 3.2

Figure 3.3

Figure 3.4

Figure 3.5

Figure 3.6

Figure 3.7

Figure 3.8

Scheme 4.1

Scheme 4.2

Scheme 4.3

Scheme 4.4

Figure 5.1

Figure 5.2

Figure 5.3

Figure 5.4

Figure 5.5

Figure 6.1

Figure 6.2

Figure 6.3

Figure 7.1

Figure 7.2

Figure 7.3

Figure 7.4

Figure 7.5

Figure 7.6

Figure 7.7

Figure 7.8

Figure 7.9

Figure 7.10

Figure 7.11

Figure 7.13

Figure 7.12

Figure 7.14

Figure 7.15

Figure 7.16

Figure 7.17

Figure 7.18

Figure 7.19

Figure 8.1

Figure 8.2

Figure 8.3

Figure 8.4

Figure 8.5

Figure 8.6

Figure 8.7

Figure 8.8

Figure 8.9

Figure 8.10

Figure 8.11

Figure 8.12

Figure 8.13

Figure 8.14

Figure 8.15

Figure 8.16

Figure 8.17

Figure 8.18

Figure 8.19

Figure 8.20

Figure 8.21

Figure 8.22

Figure 8.23

Figure 8.24

Figure 8.25

Figure 8.26

Figure 8.27

Figure 9.1

Figure 9.2

Figure 9.3

Figure 9.4

Figure 9.5

Figure 9.6

Figure 9.7

Figure 9.8

Figure 9.9

Figure 9.10

Figure 10.1

Figure 10.2

Figure 10.3

Figure 10.4

Figure 10.5

Figure 10.6

Figure 10.7

Figure 10.8

Figure 10.9

Figure 10.10

Figure 10.11

Figure 10.12

Figure 11.1

Figure 11.2

Figure 11.3

Figure 11.4

Figure 11.5

Figure 11.6

Figure 11.7

Figure 12.1

Figure 12.2

Figure 12.3

Figure 12.4

Figure 12.5

Figure 12.6

Figure 12.7

Figure 12.8

Figure 12.9

Figure 12.10

Figure 12.11

Figure 12.12

Figure 12.13

Figure 12.14

Figure 12.15

Figure 12.16

Figure 12.17

Figure 12.18

Figure 12.19

Figure 12.20

Figure 12.21

Figure 12.22

Figure 12.23

Figure 12.24

Figure 12.25

Figure 12.26

Figure 12.27

Figure 13.1

Figure 13.2

Figure 13.3

Figure 13.4

Figure 13.5

Figure 13.6

Figure 13.7

Figure 13.8

Figure 13.9

Figure 13.10

Figure 13.11

Figure 13.12

Figure 13.13

Figure 14.1

Figure 14.2

Figure 14.3

Figure 14.4

Figure 16.1

Figure 16.2

Figure 16.3

Figure 16.4

Figure 16.5

Figure 16.6

Figure 16.7

Figure 16.8

Figure 16.9

Figure 17.1

Figure 17.2

Figure 17.3

Figure 17.4

Figure 17.5

Figure 17.6

Figure 17.7

Figure 17.8

Figure 17.9

Figure 17.10

Figure 17.11

Figure 17.12

Figure 17.13

Figure 18.1

Figure 18.2

Figure 18.3

Figure 18.4

Figure 18.5

Figure 18.6

Figure 18.7

Figure 18.8

Figure 18.9

Figure 18.10

Figure 18.11

Figure 19.1

Figure 19.2

Figure 19.3

Figure 20.1

Figure 20.2

Figure 20.3

Figure 21.1

Figure 21.2

Figure 21.3

Figure 21.4

Figure 21.5

Figure 21.6

List of Tables

Table 2.1

Table 2.2

Table 2.3

Table 2.4

Table 2.5

Table 2.6

Table 3.1

Table 5.1

Table 10.1

Table 12.1

Table 12.2

Table 13.1

Table 13.2

Table 14.1

Table 14.2

Table 14.3

Table 18.1

Table 18.2

Table 18.3

Table 18.4

Table 18.5

Table 18.6

Table 19.1

Table 20.1

Table 21.1

Table 21.2

Table 21.3

Table 21.4

Table 21.5

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Edited by Sabu Thomas, Christophe Sinturel, and Raju Thomas

Micro- and Nanostructured Epoxy/Rubber Blends

The Editors

Sabu Thomas

Mahatma Gandhi University

Centre for Nanoscience and Nanotechnology

Priyadarshini Hills

Kottayam

686560 Kerala

India

Prof. Christophe Sinturel

CNRS-Université d'Orléans

Centre de Recherche sur la Matiére Divisée

1 B rue de la Férollerie

45071, Orléans Cedex 2

France

Prof. Raju Thomas

Mahatma Gandhi University

Research and Postgraduate

Department of Chemistry

Mar Thoma College

Kuttapuzha

Tiruvalla-3

Kottayam-60

689103 Kerala

India

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

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The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.

© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany

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Print ISBN: 978-3-527-33334-9

ePDF ISBN: 978-3-527-66690-4

ePub ISBN: 978-3-527-66689-8

Mobi ISBN: 978-3-527-66688-1

oBook ISBN: 978-3-527-66687-4

Preface

The book Micro and Nanostructured Epoxy/Rubber Blends is a fairly comprehensive review of the recent issues and accomplishments in the area of elastomer-modified epoxies. Recently, toughening of epoxies with elastomers and other synthetically prepared compounds has been studied widely and reported in the literature. However, to the best of our knowledge, there are no dedicated reference books on rubber toughening of epoxy resins covering the recent advancements. Recently, there are various advanced techniques to look at the microstructural issues related to the toughening aspects. This book covers almost all the issues related to the toughening of epoxies with liquid rubbers, that is, from the very initial liquid stage to the final network formation (vitrification) via the phase separation phenomenon and gelation process. The dynamics of all these phenomena have been characterized using sophisticated techniques in different length scales.

The book starts with the state of art, new challenges, and opportunities in the area of toughening aspects of epoxies. The recent developments in the modification of epoxies and the unresolved microstructural issues in epoxy toughening are well highlighted. A comprehensive review on various functionalized liquid rubbers employed to toughen epoxy resins are mentioned. Conventional as well as new methods to fabricate different epoxy/liquid rubber blend systems are included. The different parameters of in situ cure reaction of elastomer-modified epoxies, such as the nature of reaction, reaction constants, and activation energy, which are the influencing factors of the structure-property relationship, are well discussed in this book. The reaction analysis by spectroscopy, pressure-volume-temperature (PVT), dielectric spectroscopy, and differential scanning calorimetry (dynamic/isothermal) methods are comprehended. An in-depth study on thermodynamics of phase separation behavior, demixing phenomenon, and in situ phase growth of epoxy-rubber blends are included. The details on the analysis of morphological characteristics of phase-separated elastomers using optical microscopy (OM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and scanning tunneling microscopy (STM) are discussed in the book. In fact, the morphological parameters affect the toughening mechanisms in rubber-modified epoxies. Therefore, the morphology and toughening mechanisms have been correlated. The chemorheological aspects and physical transformations during cure of elastomer-incorporated epoxies are provided along with a comprehensive coverage of viscoelastic behaviors, which is a signature of morphology of rubber-modified epoxies. The miscibility aspects and characteristics of phase separation behavior are analyzed and discussed using light scattering, X-ray scattering, and neutron scattering studies.

Morphology characteristics and mechanical properties including fracture toughness of rubber-modified epoxies are well documented in this book. Thermogravimetric analysis (TGA) of rubber/epoxy systems and its application to analyze the reaction kinetics are discussed. Dielectric properties and different dielectric parameters of elastomer-modified epoxies are covered in the book. Various spectroscopy techniques to follow the chemical structures and types of interactions in modified blends are examined using Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), ultraviolet spectroscopy (UV), and electron spin resonance spectroscopy (ESR). A broad discussion on the applications of rubber-modified epoxies including aerospace, industrial, and other fields are incorporated in the book.

Toward the final part of the book, the effects of different environmental parameters on the properties of elastomer-modified epoxies and aging are explained. The failure of modified epoxies have been explained based on morphological criteria and are analyzed by the use of acoustic emission spectroscopy. Finally, the life cycle analyses of rubber-modified epoxy systems are well described.

Raju Thomas, PhD

Christophe Sinturel, PhD

Sabu Thomas, PhD

List of Contributors

Ibrahim Abdullah

Universiti Kebangsaan Malaysia (UKM), Polymer Research Center (PORCE)

Faculty of Science and Technology

School of Chemical Sciences and Food Technology

Bangi, 43600 Selangor

Malaysia

Ishak Ahmad

Universiti Kebangsaan Malaysia (UKM), Polymer Research Center (PORCE)

Faculty of Science and Technology

School of Chemical Sciences and Food Technology

Bangi, 43600 Selangor

Malaysia

Sahrim Bin Hj Ahmad

Universiti Kebangsaan Malaysia (UKM), Polymer Research Center (PORCE)

Faculty of Science and Technology

School of Chemical Sciences and Food Technology

Bangi, 43600 Selangor

Malaysia

Mimi Azlina Abu Bakar

Department of Mechanical Engineering

Universiti Teknologi MARA

Shah Alam

Selangor

Malaysia

Titu Bjenescu

C.F.C., Consulting Department

13, Chemin de Riant-Coin

La Conversion, CH-1093

Switzerland

Marius Bâzu

National Institute for Microtechnologies

Head of the Reliability laboratory

126A, Erou Iancu Nicolae Street

Bucharest

Romania

Nicolas Boyard

Université de Nantes

Laboratoire de Thermocinétique de Nantes

UMR 6607 CNRS

La Chantrerie

rue Christian Pauc

BP 50609

Nantes Cedex 3

F-44306

France

Gianluca Cicala

University of Catania

Department of Industrial Engineering

Engineering Faculty-DIIM Edificio 10

Viale Andrea Doria 6

Catania

Italy

Didier Delaunay

Université de Nantes

Laboratoire de Thermocinétique de Nantes

UMR 6607 CNRS

La Chantrerie

rue Christian Pauc

BP 50609

Nantes Cedex 3

F-44306

France

Nidamarthy Vasantha Kumar Dutt

Indian Institute of Chemical Technology CSIR-IICT

Chemical Engineering Division

Uppal Road

Hyderabad

500007 Andhra Pradesh

India

Bejoy Francis

Mahatma Gandhi University

St Berchmans College

Research and Postgraduate

Department of Chemistry

Changanassery

686101 Kerala

India

María José Galante

Institute of Materials Science and Technology (INTEMA)

University of Mar Del Plata

J.B Justo 4302

Mar del Plata

Argentina

Swayampakula Kalyani

Indian Institute of Chemical Technology (CSIR-IICT)

Chemical Engineering Division

Membrane Separation Group

Uppal Road, Hyderabad

Andhra Pradesh

India

Hanieh Kargarzadeh

Universiti Kebangsaan Malaysia (UKM), Polymer Research Center (PORCE)

Faculty of Science and Technology

School of Chemical Sciences and Food Technology

Bangi

43600 Selangor

Malaysia

Shinu Koshy

Mahatma Gandhi University

School of Chemical Sciences

Priyadarshini Hills P.O.

Kottayam

686560 Kerala

India

Vattikuti Lakshmana Rao

Vikram Sarabhai Space Centre

Polymers and Special Chemicals Division

Thiruvananthapuram

695022 Kerala

India

Yuan Meng

Zhejiang University

Department of Polymer Science and Engineering

MOE Key Laboratory of Macromolecular Synthesis and Functionalization

Polymer Building, Room 421

Hangzhou

China

Antonela B. Orofino

Institute of Materials Science and Technology (INTEMA)

University of Mar Del Plata

J.B Justo 4302

Mar del Plata

Argentina

Patricia A. Oyanguren

Institute of Materials Science and Technology (INTEMA)

University of Mar Del Plata

J.B Justo 4302

Mar del Plata

Argentina

Jyotishkumar Parameswaranpillai

Cochin University of Science and Technology

Department of Polymer Science and Rubber Technology

Cochin

682022 Kerala

India

Richard A. Pethrick

University of Strathclyde

WestChem

Department of Pure and Applied Chemistry

Cathedral Street

Glasgow

G1

UK

Qi Qin

Wuhan University of Technology

School of Chemical Engineering

Xiongchu Avenue

Wuhan, Hubei 430070

China

Chikkakuntappa Ranganathaiah

University of Mysore

Department of Studies in Physics

Manasagangotri

Mysore

006 Karnataka

India

Yerrapragada Venkata Lakshmi Ravi Kumar

Indian Institute of Chemical Technology (CSIR-IICT)

Chemical Engineering Division

Membrane Separation Group

Uppal Road

Hyderabad

500007 Andhra Pradesh

India

Elham Mostafa Sadek El Akiaby

Petrochemical Department

Egyptian Petroleum Research Institute

Nasr City, Cairo

Egypt

Luciana Sáiz

Institute of Materials Science and Technology (INTEMA)

University of Mar Del Plata

J.B Justo 4302

Mar del Plata

Argentina

Christophe Sinturel

CNRS-Université d'Orléans

Centre de Recherche sur la Matiére Divisée

B rue de la Férollerie

Orléans Cedex 2

France

Vincent Sobotka

Université de Nantes

Laboratoire de Thermocinétique de Nantes

UMR 6607 CNRS

La Chantrerie

rue Christian Pauc

BP 50609

Nantes Cedex 3

F-44306

France

Mark D. Soucek

The University of Akron

Department of Polymer Engineering

S. Forge Street

Akron

44325-0301 Ohio

USA

Raju Thomas

Mahatma Gandhi University

Research and Postgraduate Department of Chemistry

Mar Thoma College

Kuttapuzha

Tiruvalla-3

Kottayam-60

689103 Kerala

India

Sabu Thomas

Mahatma Gandhi University

Centre for Nanoscience and Nanotechnology

Priyadarshini Hills

Kottayam

686560 Kerala

India

Humberto Vázquez-Torres

Universidad Autónoma Metropolitana

Unidad Iztapalapa

Departamento de Física

San Rafael Atlixco 186

Col Vicentina

C.P 09340 México D.F.

Mexico

Dhanya Vijayan

Cochin University of Science and Technology

Department of Polymer Science and Rubber Technology

Cochin

682022 Kerala

India

Poornima Vijayan P.

Mahatma Gandhi University

School of Chemical Sciences

Kottayam

Kerala

India

Xiaojiang Wang

The University of Akron

Department of Polymer Engineering

S. Forge Street

Akron

44325-0301 Ohio

USA

Ying Yi

Wuhan University of Technology

School of Chemical Engineering

Xiongchu Avenue

Wuhan

Hubei 430070

China

Yingfeng Yu

Fudan University

Department of Macromolecular Science

State Key Laboratory of Molecular Engineering of Polymers

No 220, Handan Road

Shanghai

China

Aiqing Zhang

South-Central University for Nationalities

Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission and Ministry of Education

College of Chemistry and Materials

Minzu RD, Hongshan

Wuhan

430074 Hubei Province

China

Daohong Zhang

South-Central University for Nationalities

Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission and Ministry of Education

College of Chemistry and Materials

Minzu RD, Hongshan

Wuhan

430074 Hubei Province

China

Junheng Zhang

South-Central University for Nationalities

Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission and Ministry of Education

College of Chemistry and Materials

Minzu RD, Hongshan

Wuhan

430074 Hubei Province

China

Xinghong Zhang

Zhejiang University

Department of Polymer Science and Engineering

MOE Key Laboratory of Macromolecular Synthesis and Functionalization

Polymer Building, Room 421

Hangzhou

China

1Introduction

Raju Thomas, Christophe Sinturel, Sabu Thomas and Elham Mostafa Sadek El Akiaby

1.1 Epoxy Resin – Introduction

Epoxy resin belongs to the principal polymer under the term thermosetting resins, which covers a wide range of cross-linking polymers including unsaturated polyester resins, phenol-formaldehyde resins, and amino resins. Thermosetting polymers form an infusible and insoluble mass on heating, due to the formation of a covalently cross-linked and thermally stable network structure. They are generally amorphous and possess various desirable properties such as high tensile strength and modulus, easy processing, good thermal and chemical resistance, and dimensional stability. The term epoxy resin is applied to both prepolymers and to cured resins; the former is characterized by a three-membered ring known as the epoxy, epoxide, oxirane, or ethoxyline group. The epoxy group is shown in Figure 1.1.

Figure 1.1 Epoxy group.

1.2 Cure Reactions

In the cured resin all reactive groups may have reacted, so that although they no longer contain epoxy groups, the cured resins are still called epoxy resins. Commercial epoxy resins contain aliphatic, cycloaliphatic, or aromatic backbones. The capability of the epoxy ring to react with a variety of substrates imparts versatility to the resins. Treatment with curing agents gives insoluble and intractable thermoset polymers. In order to facilitate processing and modify cured resin properties, other constituents may be included in the composition: fillers, solvents, diluents, plasticizers, and accelerators.

Epoxy resins are highly reactive, presumably due to the strained three-membered ring structures, and react with many nucleophilic and electrophilic reagents. Therefore, a wide variety of organic compounds having active hydrogen atoms can be used as curatives. Those include amines (both aliphatic/aromatic and primary/secondary), phenols, carboxylic acids, thiols, anhydrides, and so on. The general reactions of the epoxy resin with these compounds are represented in Scheme 1.1.

Scheme 1.1 Reactions between epoxy and different curing agents.

1.3 Curing Agents

Optimum performance properties are attained by cross-linking an epoxy resin with a curing agent or hardener so as to form a three-dimensional insoluble and infusible network. The choice of curing agents depends on the application and on the handling characteristics such as viscosity, pot life, and gel time; use of properties such as mechanical, chemical, thermal, electrical, and environmental limitations and cost. Curing agents are either catalytic or co-reactive.

1.3.1 Catalytic Cure

Catalytic curing agents function as initiators for epoxy resin homopolymerization. Catalytic cures are initiated by Lewis acids and bases such as boron trihalides and tertiary amines, respectively. Catalytic curing agents can be used for homopolymerization, as supplemental curing agents with polyamines, or as accelerators for anhydrides-cured systems. Catalytic curing agents have high-temperature resistance capacity and long pot lives. However, they have long cure cycles at high temperature. The materials are used as adhesives and for electrical encapsulation purposes.

1.3.2 Co-reactive Cure

On the other hand, the co-reactive curing agents act as a comonomer in the polymerization process. Among this are a wide variety of compounds such as amines (both primary and secondary), mercaptans, isocyanates, carboxylic acids, acid anhydrides, and so on.

1.3.2.1 Primary and Secondary Amines

These are the most widely employed curing agents in epoxy resin chemistry. As a result of the reaction between the epoxy group and the primary amine, a secondary alcohol and amine are generated. The reaction is depicted in Scheme 1.2. The secondary amine, in turn, reacts with the epoxy group to give a tertiary amine and two secondary hydroxyl groups [1].

Scheme 1.2 Reactions between epoxy and primary amine.

In general, hydroxyl groups accelerate the rate of curing of amines, among which polyfunctional alcohols are the best accelerators. A mechanism has been proposed [1] in which the hydrogen atom of the hydroxyl group partially protonates the oxygen atom on the epoxy group, rendering the methylene group more susceptible to attack by the nucleophilic amine. The reaction is represented in Scheme 1.3.

Scheme 1.3 Mechanism of the reaction between epoxy and amine.

Amine-cured products, in general, have good elevated temperature performance, chemical resistance, long pot life, and low moisture absorption. Low compatibility with epoxy resins, toxicity, and long cure cycles are certain disadvantages. Amine curators are highly applicable for high-performance composites and coatings, filament winding, and for electrical encapsulation purposes. A few examples of amine curatives are listed in Figure 1.2a–e.

Figure 1.2 Structure of amine curatives. (a) 3,3′-Diaminodiphenylsulfone, (b) 4,4′-diaminodiphenylsulfone, (c) 4,4′-methylenedianiline, (d) 4,4′-methylenebis(2,6-diethylaniline), and (e) 4,4′-methylenebis (3-chloro-2,6-diethylaniline).

1.3.2.2 Mercaptans

The epoxy-mercaptan reaction is faster than the epoxy-amine reaction, especially at low temperatures; the reaction is accelerated by primary and secondary amines (Scheme 1.4).

Scheme 1.4 Reaction between epoxy and thiol.

1.3.2.3 Isocyanates

Epoxy groups react with isocyanates or with hydroxyl groups to produce oxazolidone structures or a urethane linkage, respectively, which are depicted in Scheme 1.5. The main advantages are fast curing at low temperature, good flexibility, and solvent resistance. Moisture sensitivity and toxicity limit its application to power coatings and maintenance coatings.

Scheme 1.5 Reactions between epoxy and isocyanate.

1.3.2.4 Carboxylic Acids

Carboxylic acids react with epoxy groups to form β-hydroxy propyl ester, which, in turn, reacts with another carboxylic acid to yield a diester. The hydroxyl ester can also undergo polymerization by the reaction of the secondary hydroxyl group with the epoxy. The possible reactions are summarized in Scheme 1.6.

Scheme 1.6 Reactions between epoxy and carboxylic acid.

1.3.2.5 Acid Anhydrides

Acid anhydrides react slowly with epoxy resins even at 200 °C [2, 3]. Both esterification and etherification reactions occur during the reaction. Secondary alcohols from the epoxy backbone react with the anhydride to form a half ester, which reacts with an epoxy group to give the diester. Esterification also occurs as a competing reaction between the secondary alcohol and epoxy group to form β-hydroxy ether. Basic catalysts favor esterification.

Other mechanisms are also reported [4–6]. Among this is a theory [7] based on the initiation by reaction of the tertiary amine with the epoxy group, giving rise to a zwitter ion that contains a quaternary nitrogen atom and an alkoxide anion. The anion reacts with the anhydride group to obtain a quaternary salt, the anion of which reacts first with the epoxy group and then with the anhydride. Consequently, a diester is formed and the carboxyl anion is regenerated to reinitiate the cycle. The reactions are represented in Scheme 1.7.

Scheme 1.7 Reactions between epoxy and anhydride the presence of tertiary amine accelerator.

Some of the examples of anhydride curatives are listed along with their structures (Figure 1.3). Good mechanical and electrical properties; low shrinkage; and viscosity, long pot life, and the slight color of anhydride curators supersede certain disadvantages such as long cure cycles at high temperature and brittle nature.

Figure 1.3 Structure of anhydride curatives.

1.4 Different Curing Methods

Epoxy resin systems are cured by various methods. These include chemical curing (under ambient or increased temperature as with conventional thermal curing), microwave curing, and radiation curing (electron-beam (EB) and ultraviolet (UV) curing) [8]. The mechanism of curing methods differs in these methods. Thermal curing takes place through a step polymerization mechanism, which follows throughout the curing process, whereas radiation curing leads to chain polymerization involving initiation, propagation, and termination steps. The mechanical characteristics of the cured matrix differ in both cases.

1.4.1 Thermal Curing

In a study by Chekanov et al. [9], epoxy resin based on diglycidyl ether of bisphenol F (DGEBF) was cured using an aliphatic amine both frontally and in a batch-cure schedule. Both calorimetric and viscoelastic studies were performed to determine the glass transition temperatures (Tg), storage modulus (E′), and tan delta (tan δ) of cured samples. Tensile properties of both types of cured materials were almost similar. A certain research group [10] have in situ analyzed the dielectric properties and infrared (IR) spectroscopy with a view to comparing the reaction mechanism of thermally cured and microwave-cured epoxy resins. Gu et al. [11] have investigated the cure reaction of an epoxy system by thermal mode using differential scanning calorimeter (DSC) technology (both dynamic and isothermal) and reported that the curing involved two consecutive reactions. The heat of the cure reaction and the overall kinetic parameters were determined. The reaction kinetics of two epoxy systems comprising stoichiometric amounts of DGEBA (diglycidyl ether of bisphenol-A)/DDS (diamino diphenyl sulfone) and DGEBA/m-PDA (m-phenylene diamine) were cured using microwave and thermal energy [12]. In the case of the DGEBA/m-PDA system, the rate constants of the primary amine-epoxy reaction were equal to that of the secondary amine-epoxy reaction and the etherification reaction was negligible for both modes of cure. On the other hand, for the DGEBA/DDS system, the reaction constant for the primary amine-epoxy reaction was greater than that of the secondary amine-epoxy reaction. The etherification reaction was negligible only at low cure temperatures for both cure methods. A time-temperature-transformation (TTT) diagram was also computed. At higher isothermal cure temperatures, the vitrification time was shorter for microwave curing.

The influence of a hyperbranched polymer on the thermal and photocuring of DGEBA epoxy resin has been investigated [13]. During thermal curing, the addition of a low amount of water slowed down the reaction, whereas further addition of water accelerated the process. During isothermal photocuring, water decreased the rate of the reaction at low temperatures, whereas it accelerated at high temperatures. Moreover, the addition of water decreased the glass transition temperature in both curing techniques. Thermal curing reactions of two multifunctional epoxy resins using the acetyl esterified calixarene (CA) derivatives as curing agents were analyzed by Xu et al. [14]. The glass transition and decomposition temperatures (Td) were measured by DSC and thermogravimetric analysis (TGA), respectively. They were able to generate cured resins with excellent Tg using the CA derivatives and also concluded that the Tg of the cured resins was strongly affected by the degree of esterification of CA derivatives.

1.4.2 Microwave Curing

Microwave curing can reduce the time of the epoxy resin substantially. The cure times can be reduced to the range of minutes from hours through the use of microwave curing. Nightingale and Day [15] and Zainol et al. [16] have significantly reduced the cure time of carbon/epoxy composites and two bismaleimide resins, respectively, by using microwave curing. As the microwave energy is largely concentrated on the sample with greater efficiency, curing is highly economic for commercial manufacturing with shorter cure cycles [17–19]. In a study, Boey and Yap [20] examined the effect of microwave curing on a DGEBA epoxy resin with three different amine hardeners, namely, DDS, diamino diphenyl methane (DDM), and m-PDA; all led to faster curing compared to conventional thermal curing. The vitrification time for DGEBA epoxy resin with DDS and m-PDA hardeners was much shorter for microwave curing [12].

In a study by Yusoff et al. [21], a comparison has been studied between microwave heating and conventional thermal heating in fabricating carbon/epoxy composites by employing two types of epoxy resin systems using the resin transfer molding (RTM) technique. The curing of the two composites, namely, LY5052-HY5052-carbon and the DGEBA-HY917-DY073-carbon composite systems, were studied at 100 and 120 °C. Better temperature control and faster rate of polymerization were attained with microwave heating for both epoxy systems. Both conventional and microwave heating yielded almost similar glass transition temperatures (120 °C for DGEBA systems and 130 °C for LY/HY5052 systems). In addition, microwave-cured composites had higher void contents than conventionally cured composites. Besides, C-scan traces showed that all composites, regardless of the methods of curing, had minimal defects.

Another interesting study explains the curing of an epoxy resin system using the microwave heating method [22], which also has been cured using the conventional oven method. The cured resins have been compared using a number of techniques. The mechanism of the reaction was found to be slightly different in the two cases. The epoxy-amine reaction occurs to a greater extent than the epoxy-hydroxyl reaction in the microwave-cured resin. This change in the reaction path did not change the dielectric properties for the thermally cured and microwave-cured materials. Owing to the difference in the network structure of the samples cured by the different methods, broadening of the glass transition for microwave-cured epoxy resins was observed.

In their work on low-temperature curing of epoxies with microwaves, Hubbard et al. [23] established that the variable frequency microwave (VFM) technology can cure epoxy materials in a wide variety of applications at much faster times (<10%) than that of convection or IR heating. Advantages of curing an epoxy at a low temperature with VFM are reduced shrinkage, reduced stress, and a more uniform film. As the entire adhesive is heated at the same instant with microwave energy, the network formation is uniform throughout the bulk. With convection heating, the outside of the film is heated sooner and to a higher degree than the material on the inside.

A comparative work on the mechanical properties of epoxy–anhydride resins, where the systems were cured by thermal and microwave heating, was reported by Tanrattanakul and SaeTiaw [24]. Two anhydride hardening agents and three types of accelerators were employed. Thermal curing was performed at 150 °C for 20 and 14 min for resins containing 1% and 4% accelerator, respectively. Microwave curing was carried out at a low power (207 or 276 W) for 10, 14, and 20 min. All mechanical characteristics including tensile, impact, and flexural properties, and viscoelastic properties were investigated. Equivalent or better mechanical properties were obtained by microwave curing, in comparison with those obtained by thermal curing. Microwave curing also provided a shorter cure time and an equivalent degree of conversion. The glass transition temperatures (tan δ) of the thermally and microwave-cured resins were comparable, and their activation energies were in the range of 327–521 kJ mol−1.

Chang et al. [25] developed two nanocomposites to study the dielectric properties. They developed multiwalled carbon nanotube (MWCNT)/epoxy (EP) composites using microwave curing (m-MWCNT/EP) as well as by thermal curing (t-MWCNT/EP). Results show that the two types of composites have greatly different dielectric properties. With the same content of MWCNTs, the m-MWCNT/EP composites show a much higher dielectric constant and lower dielectric loss than the t-MWCNT/EP composites because of their different structures. Further study showed that it is possible to control the dispersion and spatial distribution of carbon nanotubes using a different curing technique to obtain high-performance composites with unexpected dielectric properties, especially those with a very high dielectric constant and a low dielectric loss.

1.4.3 Radiation Curing

1.4.3.1 Electron Beam Curing

EB curing of composites is an innovative processing method, begun in the late 1970s, that uses high-energy electrons from an accelerator to initiate polymerization and cross-linking of a matrix resin. Reduced cure time, low-temperature cure, greater design flexibility, and unlimited material shelf life are some of the advantages of this method compared to the conventional thermal curing. As a low-cost and nuisance-free technology, the method has been widely used in recent years [26–28]. The effect of the EB on the fiber and its sizing has been studied by Saunders and Singh [29–31] based on epoxy acrylate composites.

EB curing of filament-wound epoxy acrylates and bismaleimide resin composites has been studied by Béziers since the 1970s [32]. The curing of acrylates, being a vinyl monomer, normally proceeds through a free radical mechanism that is sensitive to oxygen. This leads to high shrinkage and high moisture absorption and ultimately to substandard mechanical properties. The cationic curing of epoxy resin by using appropriate onium salts as initiators [33, 34] were analyzed by Crivello et al. In fact, a variety of epoxy resin matrixes have been cured using EB considering its excellent properties and processing characteristics [35, 37, 38]. The majority of research studies focus on the development of resin systems and optimization and not on the fundamental theory of EB curing. However, a few studies are related to the mechanism of EB curing and its dependence on materials parameters [39–42]. In this context, the mechanism proposed by Hult and Sundell [43], Ledwith [44], and Crivello et al. [36] on EB-induced cationic polymerization in the presence of onium salt photoinitiators is recommendable, in spite of the lack of dependable analysis and validation for the basic reaction process.

In a study by Sui et al. [45], the effects of different initiators and diluents on Shell Epon 828 radiation reaction were discussed, and these were also studied by heat treatment. The experimental results proved that cationic photoinitiators can initiate EB radiation curing of epoxy resin, while conventional heat-cured systems are not always suitable for the EB curing. The diluents generally decreased the curing degree of the resin, which was not prominent for reactive diluents. The proposed mechanism was the cationic ring-opening polymerization process followed by the production of protonic acid that can initiate a polyreaction. The characteristics of the cured system consisted of many lamellar structures.

Alessi et al. [46] studied the hydrothermal aging of epoxy thermoplastic blends, used as matrices for carbon fiber composites, cured by EB. Irradiation has been carried out at mild temperatures. Radiation-cured epoxy-based matrices have been subjected to a thermal and moisture absorption aging treatment. The vitrification phenomenon specifically characterizes the aging response of the material. Very complex chemical and physical phenomena occur during aging, causing further curing and degradation reactions and plasticization. The mechanical fracture behavior was affected by curing and degradation reactions and has been discussed on the basis of the different curing degree and the developed morphology. A balanced effect between the embrittlement and toughness, which are due to post-curing reactions and separation phase phenomena, respectively, was for the system that showed the best mechanical behavior after aging.

1.4.3.2 Gamma Ray Irradiation

Surface properties of polymeric materials, such as films, fibers, powders, and molded objects [47], can be altered by a new technique using irradiation-induced grafting. This can induce chemical reactions at any temperature without any catalyst in the solid, liquid, and gas phase, and is a safe method against environmental pollution. This technique has been applied to carbon fibers [48]. Epoxy resin and chloroepoxy propane reacted with carbon fibers by a co-irradiation grafting method and acrylic acid was graft-polymerized onto the fiber surface via a pre-irradiation grafting method. The roughness, amount of oxygen-containing functional groups, and surface energy were all found to increase significantly after irradiation grafting. The irradiation grafting, the mechanism of which was proposed by radical reactions, improved marginally the tensile strength of carbon fibers. Moreover, it was reported that the interlaminar shear strength of treated carbon fiber/epoxy was enhanced by at least 17.5%, compared with that of untreated carbon fiber/epoxy. This has been suggested as a method to modify the physicochemical properties of carbon fibers and to improve the interfacial adhesion of composites.

The curing of certain systems, composed of DGEBA and DGEBF as epoxy resins and triarylsulfoniumhexafluoroantimonate (TASHFA), and triarylsulfonium hexafluorophosphate (TASHFP) as initiators, was investigated using EB and γ-ray irradiation [49]. The chemical and mechanical characteristics of irradiated epoxy resins were compared after curing up to 50 kGy in N2 and air atmosphere. The effect of oxygen on the radiation curing of epoxy resin was established. The chemical structures of cured epoxy were characterized by Fourier transform near-infrared (FTNIR). The gel fraction and the stress at yield of epoxy resins irradiated by E-beam and γ-ray in N2 atmosphere were also compared with those of epoxy resins irradiated by E-beam and γ-ray in air. The degree of curing of epoxy resins irradiated in N2 regardless of E-beam and γ-ray was higher than that of epoxy resins irradiated in air. The gel fraction of two epoxy resins increased markedly with the increase in dose, and the values of the gel fraction of DGEBF samples were higher than that of DGEBA. The gel fraction of DGEBF irradiated by γ-ray was higher than that of the epoxy irradiated by E-beam at the same dosage. Therefore, the thermal stability and stress at yield of epoxy resins irradiated in N2 were higher than that of epoxy resin irradiated in air.

1.5 Curing of Epoxy Resins: Structure–Property Relationship

During curing, epoxy resin is converted into a hard infusible material as a result of cross-linking reactions, which ultimately lead to a network structure. Curing agents or hardeners are widely known to promote curing of epoxy resins. Curing can occur by either homopolymerization initiated by a catalytic curing agent or by a polyaddition/copolymerization reaction with a multifunctional curing agent. Initially, linear growth of the chain occurs followed by branching and finally leads to cross-linking. This irreversible reaction leads to an increase in molecular weight, which results in the physical transformation of the system from a viscous state to an elastic gel. The temperature at which this transformation occurs is known as gel point, which is critical in polymer processing as the polymer will not flow beyond this limit, and hence this temperature determines the processability of the system. Gelation mainly depends on the functionality of components, stoichiometry, and reactivity of the system, and does not inhibit the curing reaction. As curing proceeds further, the viscosity of the system increases because of the increase in molecular weight, and the reaction becomes diffusion controlled and eventually quenched as the material vitrifies [50–52]. This is again another physical transformation and occurs during the curing reaction called vitrification of the growing chain or network. During this conversion, a substantial increase in cross-link density, glass transition temperature (Tg), and ultimate properties takes place. The change in the state of the system from viscous liquid to elastic gel and finally to glassy solid begins to occur as the Tg of the developing network becomes equal to the curing temperature. At the vitrification point, the reaction shifts from chemically controlled to a diffusion-controlled state. The net changes during the curing of the thermoset can be expressed in terms of a TTT diagram. The S-shaped gelation curve and the vitrification curve divide the time-temperature plot into four distinct states of the thermosetting cure process: liquid, gelled rubber, ungelled glass, and gelled glass. Tg0 is the glass transition temperature of the unreacted resin mixture, Tg, the glass transition temperature of the fully cured resin, gel Tg is the glass transition temperature of the resin at its gel point. The temperature region below the glass transition of the unreacted resin (Tg0) represents the solid state and hence the reaction is believed to occur very slowly. This temperature region defines the storage temperature for unreacted resins. The reaction initiates above Tg0 and continues till the rise in Tg becomes equal to the cure temperature at which vitrification will commence. Thereafter, the reaction becomes diffusion controlled and is eventually quenched when vitrification is complete. In a stage between gel Tg and Tg, gelation precedes vitrification, and a cross-linked rubbery network forms and grows until its Tg coincides with the cure temperature. The reaction will be quenched at this stage. Tg is the minimum cure temperature required for the completion of cure. Above Tg, the thermoset will remain in the rubbery state, unless other oxidative cross-linking or chain scission occurs. Gelation and vitrification determine the handling, processing, and development of the ultimate properties of the cured resin.

1.6 Toughening of Epoxy Resin

Epoxy resins are characterized by their outstanding properties such as high thermal and corrosion resistance, good thermomechanical properties, and adhesion to various substrates. In spite of these desirable properties, the major disadvantages are their low toughness and poor crack resistance and brittle nature at room temperature. Hence, epoxy materials are to be toughened for many end-use applications. In a broader sense, toughness refers to the measure of the materials' resistance to failure, without much compromise in their enviable thermomechanical properties.

1.6.1 Different Toughening Agents

The most important method for toughening epoxy resins is the incorporation of a dispersed toughener phase in the epoxy matrix. Various important toughener phases are mentioned here, the details of which were described in the preceding sections. The various toughener phases include liquid rubbers, rigid particulates, core–shell particles, and thermoplastics.

1.6.1.1 Liquid Elastomers for Toughening Epoxy Matrices

Reactive liquid rubbers that are initially soluble in the base epoxy resin [53–55] as well as rubbers that are initially immiscible [56, 57] in the resin are used as toughening agents. In the former case, phase separation of rubber takes place during cure, whereas in the latter case the rubber phase will be in the precipitated state. Nevertheless, this type of rubbers generates heterogeneity and hence a very low content is enough for toughening. The former type of rubbers is considered as the best modifiers. On the other hand, rubbers with different functionalities, which permit to form covalent interactions at the epoxy interface, normally serve as the best tougheners. When low-modulus rubber particles are dispersed into the brittle resin matrix, the force that induces and propagates cracks is dissipated by the rubbery phase and prevents the catastrophic failure of the matrices. Significant increase in peel strength, impact properties, and fracture toughness has been achieved without much loss in tensile and thermal properties.

Major liquid rubbers used as tougheners are carboxyl-terminated butadiene-co-acrylonitrile (CTBN) [58–66], hydroxyl-terminated polybutadiene (HTPB) liquid rubbers [67–73], epoxy-terminated liquid rubbers [74–78], amine-terminated butadiene acrylonitrile (ATBN) rubbers [78–82], acrylic elastomers [83–87], and other synthetic elastomers [88–93]. The details of liquid elastomer modifiers are explained in the preceding chapter.

1.6.1.2 Rigid Crystalline Polymers

Epoxies can be toughened using rigid crystalline polymers, even though it is difficult to visualize how such rigid crystalline domains could provide an efficient toughening mechanism. One such possible mechanism could operate by phase transformation, which is well known for ceramic materials [94]. In zirconia-containing ceramics [95–98], the metastable tetragonal phase of zirconia is incorporated into the ceramic, and under the influence of the stress field around the crack tip, this phase transforms to the stable monoclinic phase. Compressive stresses are set up on one of the phases as the monoclinic phase is less dense than the tetragonal phase. This superposes on the tensile stress field ahead of the crack tip producing shear deformations, which result in the increase in critical fracture energy.

1.6.1.3 Hygrothermal Toughening Agents

Functional oligomers for toughening epoxy resins exhibiting low water absorption characteristics and suitable for high-temperature applications are reported by many researchers. Several amine-terminated silicone oligomers were used by Takahashi et al. [99] to toughen epoxy resins for its application as encapsulants for semiconductor-integrated circuit devices. Siloxane oligomers offer advantages such as lowering Tg values for the dispersed rubbery particles and good thermal stability. Moreover, siloxane oligomers exhibit good weatherability, oxidative stability, high flexibility, and moisture resistance [100]. Further, siloxane-modified epoxy leads to the formation of a very hydrophobic and chemically bound surface coating [101] and improves the wear properties [102].

1.6.1.4 Core–Shell Particles

Core–shell rubber particles are a type of preformed thermoplastic particles with a glassy shell that can be designed to adhere better with resin and a rubbery core to improve toughness. Reactive core–shell-type hyperbranched block copolymers with onion-like molecular architecture were used as flexibilizers and toughening agents for thermosets. The system was reported to achieve good miscibility, low viscosity, and better interfacial adhesion [103]. Preformed particles will not undergo phase separation and will remain in the shape in which they were added to the neat resin or composite. Consequently, these particles may be developed before the resin formulation and then added to the thermosetting resin or developed in situ (during resin formulation) before the resin formulation is catalyzed or cured. An important characteristic of core–shell particles as tougheners for epoxies is the role of the particle/epoxy interface [104]. A discrete interface is generated between the particles and the matrix epoxy by the shell polymer of core–shell particles. However conflicting, there are many studies regarding the effect of shell composition on toughening of epoxies.

1.6.1.5 Nanoparticles for Epoxy Toughening

Nanoparticulate composites are observed to be impact tougheners for epoxy matrices. They showed superior material properties than microscale fillers [105] for toughening. In a noticeable work, Wetzel and coworkers [106] have modified epoxy matrix with various amounts of micro- and nanoscale particles of CaSiO3 (4–15 μm) and alumina (Al2O3) (13 nm) for reinforcement purposes. The influence of these particles on the impact energy, flexural strength, and dynamic mechanical thermal properties were investigated. The analyses showed that the stiffness, impact energy, and fracture strain of the epoxy matrix were improved by a low-level addition (1–2 vol%) of alumina particles. Similarly, only 2 vol% of CaSiO3 was enough to improve the flexural modulus and wear resistance of the matrix epoxy.

Many other particles such as nanostructured TiO2 [107], nano-ZnO [108] and nano-SiO2 [109], and nanoclay [110] were dispersed in epoxy resin to improve both the mechanical and the thermal properties because of their large specific surface area.

It was found that reinforcement with nanoparticles improved the fracture toughness at room temperature, but decreased the fracture toughness at the cryogenic temperature in spite of their toughening effect.

1.6.1.6 Thermoplastic Modification of Epoxy Resin

Thermoplastics are one of the best toughening agents for thermosetting polymers, particularly for epoxies, as they will not reduce the glass transition of the cured network. Detailed studies have been conducted on mechanical properties, fracture behavior, phase separation, and morphology of polysulfone (PSF)-modified epoxies, and perhaps this is believed to be the most widely studied thermoplastics employed for toughening of epoxy resin [111]. Hedrick et al. [112] employed phenolic hydroxyl and amine-terminated PSF oligomers to toughen epoxies and observed an increase in fracture toughness by 100% without any loss in thermal properties. Other researchers [113] have also noticed fracture toughness and fracture energy increase in PSF-modified epoxy resins. Cure kinetics have been analyzed in detail by different analytical techniques [114–116] on PES (poly(ether sulfone)) and PSF-modified epoxy resins cured with DDM hardener and an autocatalytic model was used to explain kinetics. They have also studied rheological changes of PES-modified epoxies and explained the earlier phase separation of PES as due to its higher viscosity compared to that of the resin [117, 118].

Functionally terminated polyether ether ketone (PEEK) is a versatile modifier for epoxy. It is characterized by toughness, stiffness, thermo-oxidative stability, chemical and solvent resistance, electrical performance, flame retardant nature, and retention of physical properties at high temperature [119]. Fracture and thermal properties of DDM-cured phenolphthalein PEEK, and polyetherketone cardo (PEK-C)/epoxy blends showed property reduction with the addition of a modifier, whereas a series of amine-terminated oligomers based on tert-butyl hydroquinone (TBHQ) and methyl hydroquinone (MeHQ) exhibited toughness improvement [120]. This was caused by the final morphology, that is, the homogeneous nature in the former case [121] changed to a phase-separated one in the latter case [122–124].

1.6.1.7 Block Copolymers as Modifiers for Epoxy Resin

Block-copolymer-modified epoxy resins have generated significant interest because it was demonstrated that the combination could lead to nanostructured thermosets. Amphiphilic block copolymers self-assemble to form a variety of well-defined ordered and disordered microstructure morphologies such as spherical, cylindrical, lamellar, and gyroid phase [125, 126]. Studies show that introduction of block copolymers in epoxy resin can greatly improve mechanical and physical properties including tensile, flexural, impact, wear resistance, thermal resistance, and performance [127, 128]. The fracture toughness, modulus, and other thermomechanical properties of the nanostructured blends prepared using block copolymers are generally higher than that of pristine epoxy resin [129–132]. In addition, with the use of block copolymers as the toughening agents, the size of the resulting features can be controlled and are typically on the order of tens of nanometers.

1.7 Rubber-Modified Epoxy Resin: Factors Influencing Toughening

Liquid rubbers are widely used as potential modifiers for epoxy toughening. Commonly employed liquid rubbers are copolymers of butadiene and acrylonitrile. Rubbers with different functionalities, which permit to form covalent interactions at the epoxy interface, normally serve as the best tougheners. When low-modulus rubber particles are dispersed into the brittle resin matrix, the force that induces and propagates cracks is dissipated by the rubbery phase and prevents the catastrophic failure of the matrices. Significant increase in peel strength, impact properties, and fracture toughness has been achieved without much loss in tensile and thermal properties.

1.7.1 Concentration Effects

Increasing the concentration of the rubber content reduces the tensile strength and the modulus of the brittle matrix. This is due to the plasticization of the matrix by the incorporated rubber. In the case of polyamide (PA)-ethylene-propylene-diene monomer (EPDM) blend, the relationship between Izod impact strength and rubber content is linear up to 30% of rubber and shows a decrease on further inclusion of rubber [133, 134].

1.7.2 Particle Size and Distribution of Rubber

The graph of toughness versus rubber particle size gives a maximum toughness for an optimum particle size [135, 136]. The optimum particle size depends on the entanglement density of the matrix and the nature of rubber [137]. This decreases