Embed Size (px)
Citation preview
Progress in Adhesion
and Adhesives
Scrivener Publishing
100 Cummings Center, Suite 541J
Beverly, MA 01915-6106
Publishers at Scrivener
Martin Scrivener ([email protected])
Phillip Carmical ([email protected])
Progress in Adhesion
and Adhesives
Edited by
K.L. Mittal
Volume 2
This edition first published 2017 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA
and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA
© 2017 Scrivener Publishing LLC
For more information about Scrivener publications please visit www.scrivenerpublishing.com.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or
transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or other-
wise, except as permitted by law. Advice on how to obtain permission to reuse material from this title
is available at http://www.wiley.com/go/permissions.
Wiley Global Headquarters
111 River Street, Hoboken, NJ 07030, USA
For details of our global editorial offices, customer services, and more information about Wiley products
visit us at www.wiley.com.
Limit of Liability/Disclaimer of Warranty
While the publisher and authors have used their best efforts in preparing this work, they make no rep-
resentations or warranties with respect to the accuracy or completeness of the contents of this work and
specifically disclaim all warranties, including without limitation any implied warranties of merchant-
ability or fitness for a particular purpose. No warranty may be created or extended by sales representa-
tives, written sales materials, or promotional statements for this work. The fact that an organization,
website, or product is referred to in this work as a citation and/or potential source of further informa-
tion does not mean that the publisher and authors endorse the information or services the organiza-
tion, website, or product may provide or recommendations it may make. This work is sold with the
understanding that the publisher is not engaged in rendering professional services. The advice and
strategies contained herein may not be suitable for your situation. You should consult with a specialist
where appropriate. Neither the publisher nor authors shall be liable for any loss of profit or any other
commercial damages, including but not limited to special, incidental, consequential, or other damages.
Further, readers should be aware that websites listed in this work may have changed or disappeared
between when this work was written and when it is read.
Library of Congress Cataloging-in-Publication Data
ISBN 978-1-119-40638-9
Cover images: K.L. Mittal
Cover design by Russell Richardson
Set in size of 10pt and Minion Pro by Exeter Premedia Services Private Ltd., Chennai, India
Printed in
10 9 8 7 6 5 4 3 2 1
v
Contents
Preface xiii
1 Surface Modification of Natural Fibers for Reinforced Polymer Composites 1
M. Masudul Hassan and Manfred H. Wagner1.1 Introduction 1
1.1.1 Natural Fibers (NFs): Sources and Classification 21.1.2 Composition of NFs 21.1.3 New Trends in the Chemistry of Cellulose 31.1.4 Action of Reducing and Oxidizing Agents 61.1.5 Drawbacks of Natural Fibers 7
1.2 Modifications of Natural Fibers 91.2.1 Physical Modifications of Natural Fibers 91.2.2 Chemical Modifications of Natural Fibers 11
1.3 Composites 161.3.1 Hybrid Composites 171.3.2 Compatibilization 171.3.3 Effect of Radiation on Fiber Composites 191.3.4 Initiative in Product Development
of NF Composites 201.4 Properties Evaluation 20
1.4.1 Lantana-Camara Fiber 201.4.2 Tea Dust-Polypropylene (TDPP) Composite 231.4.3 Water Absorption Test 271.4.4 Jute Fiber Reinforced Vinylester Composites 271.4.5 Coir Fiber Reinforced Polyester Composites 291.4.6 Effect of Alkali Treatment on Hemp, Sisal
and Kapok for Composite Reinforcement 311.4.7 DSC Analysis of Mercerized Fibers 34
vi Contents
1.4.8 XRD Analysis of Mercerized Fibers 341.4.9 SEM Analysis of Alkalized Fibers 34
1.5 Conclusions 36Acknowledgements 37References 37
2 Factors Influencing Adhesion of Submicrometer Thin Metal Films 45
A. Lahmar, A. Assaf, M.J. Durand, S. Jouanneau,
G. Thouand and B. Garnier2.1 Introduction 462.2 Experimental Details 47
2.2.1 Film Deposition 472.2.2 Measurement of the Critical Load 48
2.3 Results and Discussion 502.3.1 Scanning Electron Microscopy Observations 502.3.2 Effects of Film Thickness and Substrate Bias
on the Mean Critical Load 512.3.3 Effects of Ion Bombardment Etching of
Substrate Surface 542.3.4 Effect of Ageing Treatment after Deposition 552.3.5 Effect of Roughness of the Substrate Surface 562.3.6 Dependence of Critical Load and Thermal
Resistance on Deposition Conditions 582.3.7 Correlation Between Adhesion and
Thermal Contact Resistance 602.4 Summary 63References 63
3 Surface Treatments to Modulate Bioadhesion 67
D.G. Waugh, C. Toccaceli, A.R. Gillett, C.H. Ng,
S.D. Hodgson and J. Lawrence3.1 Introduction 67
3.1.1 The Role of Wettability in Biological and Microbiological Adhesion 69
3.2 Various Surface Treatments 703.2.1 Laser Surface Treatment 703.2.2 Lithography 753.2.3 Micro/Nano Contact Printing 773.2.4 Plasma Surface Treatment 793.2.5 Radiation Grafting 813.2.6 Ion Beam and Electron Beam Processing 82
Contents vii
3.3 Prospects 853.4 Summary 89References 89
4 Hot-Melt Adhesives from Renewable Resources 101
P. Utekar, H. Gabale, A. Khandelwal and S.T. Mhaske4.1 Introduction 1014.2 Potential Renewable Base Polymers 1034.3 Lactic Acid Based Polymers as Hot-Melt Adhesives 1044.4 Soy Protein Based Polymers as Hot-Melt Adhesives 1064.5 Bio-Based Polyamides as Hot-Melt Adhesives 1074.6 Starch Based Polymers as Hot-Melt Adhesives 1094.7 Summary 111References 111
5 Relevance of Adhesion in Particulate/Fibre-Polymer Composites and Particle Coated Fibre Yarns 115
V.B. Mohan, K. Jayaraman and D. Bhattacharyya5.1 Introduction 115
5.1.1 Mechanisms of Adhesion 1185.1.2 Tests for Interfacial Adhesion in Composites 120
5.2 Theory of Interaction 1245.2.1 Adhesion Mechanism in Fibre Yarns and
Polymer Systems 1255.2.2 Surface Modification Techniques 1265.2.3 Adhesion Properties of Fibres 1305.2.4 Morphological Evaluation of Fibre Yarns
Coated with Nanoparticles 1315.2.5 Interfacial Adhesion in Particle and
Polymer Blends 1385.3 Summary 140References 142
6 Study and Analysis of Damages in Functionally Graded Adhesively Bonded Joints of Laminated FRP Composites 147
S.K. Panigrahi and Rashmi Ranjan Das6.1 Introduction 1486.2 Damage Analysis of Adhesively Bonded
Laminated Composite Joints 1496.2.1 Damage Analysis of Adhesively Bonded
Flat FRP Composite Joints 1496.2.2 Damage Analysis of Adhesively Bonded
Tubular FRP Composite Joints 151
viii Contents
6.3 Effect of Adhesive Property on Damages in Adhesively Bonded Joints 152
6.4 Effect of Functionally Graded Adhesives on Damages in Adhesively Bonded Joints 153
6.5 Conclusion 156References 156
7 Surface Modification Strategies for Fabrication of Nano-Biodevices 161
Ankur Gupta, Vinay Kumar Patel, Rishi Kant and
Shantanu Bhattacharya7.1 Introduction 1617.2 Interfacial Interactions for Proper Functioning
of Nano-biodevices 1647.3 Strategies for Surface Modification of Polymers
in Nano-biodevices 1677.3.1 Surface Modification of Polymers Through
Plasma Treatment 1687.3.2 Surface Modification of Surfaces Through
Chemical Route 1687.3.3 Surface Modification Through Silanization
of Surfaces 1697.3.4 Surface Modification of Polymers with
SAMs by Micro-contact Printing Technique 1707.3.5 Other Surface Modification Strategies 171
7.4 Benefits of Surface Modifications to Nano-Biodevices 1767.5 Summary 177References 177
8 Effects of Particulates on Contact Angles and Adhesion of a Droplet 187
Youhua Jiang, Wei Xu and Chang-Hwan Choi8.1 Introduction 1878.2 Theoretical Background of Contact Angles and
Adhesion of a Droplet 1898.3 Effects of Particulates on Static Contact Angles 191
8.3.1 Deposition of Particulates on Solid-liquid Interface 192
8.3.2 Adsorption of Particulates on Liquid-Gas Interface 194
8.3.3 Adsorption of Surfactants on Solid-Gas Interface 195
Contents ix
8.4 Effects of Particulates on Droplet Pinning 1978.4.1 Flows Within a Droplet 1998.4.2 Interactions amongst Particulates,
Solid Substrates, and Liquid-Gas Interfaces 2018.4.3 Structural Disjoining Pressure 204
8.5 Effects of Particulates on Droplet Motion 2058.5.1 Contact Line Velocity 2058.5.2 Stick-Slip Behavior 206
8.6 Summary 2108.7 Prospects 210Acknowledgements 211References 211
9 Thermal Stresses in Adhesively Bonded Joints/Patches and Their Modeling 217
M. Kemal Apalak9.1 Introduction 2179.2 Thermal Stresses 219
9.2.1 Bi-material Strips 2199.2.2 Linear Analyses 2209.2.3 Nonlinear Analyses 225
9.3 Thermal Residual Stresses 2309.3.1 Residual Stresses - Adhesive Curing 2339.3.2 Residual Stresses - Hygrothermal Ageing 246
9.4 Viscoelastic Analyses 2509.5 Fracture and Fatigue 2559.6 Summary 263References 264
10 Ways to Mitigate Thermal Stresses in Adhesively Bonded Joints/Patches 271
M. Kemal Apalak10.1 Introduction 27110.2 CFRP Strengthened Beams and Plates 27310.3 Weld-Bonded Joints, Cutting Tools 27610.4 Adhesive Joints Under Cryogenic Temperatures 27910.5 Low and High-Temperature Adhesives 28510.6 Fillers and Electrically-conductive Adhesives 289
10.6.1 Adhesive Layer with Fillers or Voids 289 10.6.2 Electrically-conductive Adhesives 292
10.7 Microelectronics, Optics and Nuclear Applications 296
x Contents
10.8 Dental Applications 30710.9 Summary 312References 314
11 Laser-Assisted Electroless Metallization of Polymer Materials 321
Piotr Rytlewski, Bartłomiej Jagodziński and
Krzysztof Moraczewski11.1 Introduction 32111.2 Application of Lasers in the Metallization of Polymer
Materials 323 11.2.1 Modification in a Gaseous Medium 324 11.2.2 Modification in Solutions 326 11.2.3 Modification of Thin Films 327 11.2.4 Modification of Composite Materials 328
11.3 Modification of Polymer Composite Materials 328 11.3.1 Polyamide Composites 328
11.4 Summary 346Acknowledgement 347References 347
12 Adhesion Measurement of Coatings on Biodevices/Implants 351
Wei-Sheng Lei, Kash Mittal and Zhishui Yu12.1 Introduction 35212.2 Mechanical Test Methods of Adhesion Measurement 354
12.2.1 Cross-Cut Test 354 12.2.2 Peel Test 355 12.2.3 Scribe (Scratch) Test 356 12.2.4 Pull-Off (Tensile) Test 360 12.2.5 Single-Lap Shear Test 363 12.2.6 Blister Test 364 12.2.7 Micro- and Nano- Indentation Tests 365 12.2.8 Small-Punch Test 369 12.2.9 Micro- and Nano- Scale Tensile Testing 369 12.2.10 Four-Point Bending Test 371 12.2.11 Other Test Methods 372
12.3 Summary and Remarks 373References 374
Contents xi
13 Cyanoacrylate Adhesives 381
P. Rajesh Raja13.1 Introduction 38113.2 Synthesis and Processing 38213.3 Applications 386
13.3.1 Industrial and Consumer 386 13.3.2 Medical 390 13.3.3 Forensics 393 13.3.4 Recent Advances in Cyanoacrylate Adhesives 393
13.4 Summary 394References 394
14 Promotion of Adhesion of Green Flame Retardant Coatings onto Polyolefins by Depositing Ultra-Thin Plasma Polymer Films 399
Moustapha E. Moustapha, Jörg F. Friedrich,
Zeinab R. Farag, Simone Krüger, Gundula Hidde
and Maged M. Azzam14.1 Introduction 40014.2 Role of Adhesion in the Use of Thick
Fire-Retardant Coatings 40014.3 Strategies for Adhesion Promotion of
Flame-Retardant Coatings 40614.4 Plasma Polymerization 40914.5 Adhesion Improvement by Plasma Polymer Layers 412
14.5.1 Inorganic Flame Retardant Layers (Water Glass Layers) 412
14.5.2 Coating with Prepolymer of Melamine Resin 414 14.5.3 Curing of the Melamine Prepolymer 414
14.6 Results of Adhesion Improvement Using Adhesion-Promoting Plasma Polymers 415
14.6.1 Results of Adhesion Promotion 415 14.6.2 Locus of Adhesion Failure 418
14.7 Flame Retardancy Tests 42014.8 Thermal Behavior 42114.9 Summary 423Acknowledgement 424References 424
Preface
In 2015 we had brought out the premier volume in this series “Progress in Adhesion and Adhesives” (although we had not called it Volume 1 as we had no idea what the future plans would be) based on 13 articles published in 2014 in the journal Reviews of Adhesion and Adhesives(RAA). RAA was initiated in 2013 with the sole purpose of publishing review articles on top-ics of contemporary interest.
With the ever-increasing amount of research being published it is a Herculean task to be fully conversant with the latest research develop-ments in any field, and the arena of adhesion and adhesives is no exception. Thus topical review articles provide an alternate and a very efficient way to stay abreast of the state-of-the-art of a given subject. Moreover, anybody embarking on a new research area or an individual who just wishes to be knowledgeable about a topic are well advised to start with a good review article on topic of his/her interest.
The success of and the warm reception accorded to the premier volume provided us the impetus to bring out this sequel, designated as Volume 2. The current volume is based on 14 critical, concise, illuminating and thought-provoking review articles (published in 2016 in RAA) written by a coterie of internationally renowned subject matter experts, covering many and varied topics within the broad purview of Adhesion Science and Adhesive Technology.
The rationale for bringing out Volume 2 is the same as was applicable to its predecessor, i.e., the RAA has limited circulation so this set of books should provide broad exposure and wide dissemination of valuable infor-mation published in RAA. The chapters in this Volume are arranged in the same order as published originally in RAA. The subjects of these 14 reviews fall into the following general areas.
1. Surface modification of polymers for a variety of purposes.2. Adhesion aspects in reinforced composites3. Thin films/coatings and their adhesion measurement4. Bioadhesion and bio-implants
xiii
xiv Preface
5. Adhesives and adhesive joints6. General adhesion aspects
The topics covered include: surface modification of natural fibers for reinforced polymer composites; adhesion of submicrometer thin metals films; surface treatments to modulate bioadhesion; hot-melt adhesives from renewable resources; relevance of adhesion in particulate-polymer composites; analysis of damages in functionally graded adhesively bonded joints; surface modification strategies for fabrication of nano-biodevices; effects of particulates on contact angles and adhesion of a droplet; ther-mal stresses in adhesively bonded joints and ways to mitigate these; laser-assisted electroless metallization of polymer materials; adhesion measurement of coatings on biodevices /implants; cyanoacrylate adhe-sives; and adhesion of green flame retardant coatings onto polyolefins.
This book consolidating plentiful information on a number of top-ics of current interest should be valuable and useful to materials science, nanotechnology, polymers, bonding, biomedical, composites researchers in academia, government research labs and R&D personnel in a host of industries. Yours truly sincerely is sanguine that Volume 2 will receive the same warm welcome as its forerunner by the materials science community in general and the adhesionists in particular.
Now is the pleasant task of thanking those who were instrumental in shaping this book. First I am thankful to the authors of review articles for their enthusiastic support for bringing out Volume 2 as they felt that this was a very useful medium for bringing the information to a wider audi-ence. Also, I should thank Martin Scrivener (publisher) for conceiving the idea of these books and for his steadfast interest in and support for this book project.
Kash Mittal P.O. Box 1280 Hopewell Jct., NY 12533 E-mail: [email protected] April 2017
*Corresponding author: [email protected]
1
Surface Modification of Natural Fibers for Reinforced Polymer Composites
M. Masudul Hassan1* and Manfred H. Wagner2
1Department of Chemistry, M C College, National University, Sylhet-3100, Bangladesh 2Berlin Institute of Technology (TU Berlin), Institute of Materials Science and Technology,
Polymer Engineering/Polymer Physics, D-10623 Berlin, Germany
AbstractRecent advances in engineering, natural fibers development and composites science offer significant
opportunities for new, improved materials which can be biodegradable and recyclable and can also
be obtained from sustainable resources at the same time. The combination of bio-fibers like betel
nut, banana, coir, jute, rice straw, tea dust and various grasses with polymer matrices from both non-
renewable (petroleum based) and renewable resources to produce composite materials that are com-
petitive with synthetic composites such as glass fiber reinforced polypropylene or epoxide has been
getting increased attention over the last decades. This article provides a general overview of natural
fibers and bio-composites as well as the research on and application of these materials. A special
emphasis is placed on surface modification of natural fibers to attain desired composite properties.
The roles of compatibilizers and radiation on the natural fiber-polymer composites are also included.
A discussion about chemical nature, processing, testing and properties of natural fiber reinforced
polymer composites completes this article.
Keywords: Natural fiber, surface modification, compatibilizer, radiation, hybrid composite,
mechanical properties
1.1 Introduction
The demand for natural fiber reinforced polymer composites is growing rapidly due to
their high mechanical properties, significant processing advantages, low cost and low
K.L. Mittal (ed.) Progress in Adhesion and Adhesives, Volume 2 (1–44) © 2017 Scrivener Publishing LLC
1
2 Progress in Adhesion and Adhesives, Volume 2
density. Natural fibers are renewable resources in many countries of the world; they are
cheaper, pose no health hazards and finally provide a solution to environmental pollution
by finding new uses over expensive materials and non-renewable resources. Furthermore,
natural fiber reinforced polymer composites form a new class of materials which seem to
have great potential in the future as a substitute for scarce wood and wood based materials
in societal applications [1].
Lignocellulosic natural fibers like jute, sisal, coir, and pineapple have been used as rein-
forcements in polymer matrices. Natural fibers of vegetable origin include bast, leaves, and
wood fibers. They may differ considerably in their physical appearance but they have, how-
ever, many similarities that identify them as one family. The characteristics of the fibers
depend on the individual constituents and the fibrillar structure. The fiber is composed
of numerous elongated fusiform fiber cells. The fiber cells are linked together by means of
middle lamellae, which consist of hemicellulose, lignin and pectin. Growing environmen-
tal awareness has spurred the researchers worldwide to develop and utilize materials that
are compatible with the environment. In this process natural fibers have become suitable
alternatives to traditional synthetic or man-made fibers and have the potential to be used in
cheaper, more sustainable and more environmentally-friendly composite materials [2–3].
1.1.1 Natural Fibers (NFs): Sources and Classification
Natural organic fibers can be derived from either animal or plant sources. The majority of
useful natural textile fibers are plant derived, with the exception of wool and silk. All plant
fibers are composed of cellulose, whereas fibers of animal origin consist of proteins. Natural
fibers, in general, can be classified based on their origin, and the plant-based fibers can
be further categorized based on part of the plant they are recovered from. An overview of
natural fibers and some photographs of NFs are presented in Figures 1.1 and 1.2, respec-
tively [4–5].
Plant fibers are a renewable resource and have the ability to be recycled. The plant
fibers leave little residue if they are burned for disposal, returning less carbon dioxide
(CO2) to the atmosphere than is removed during the plant’s growth.
Chemically the lignocellulosic fibers comprise cellulose, hemicellulose, lignin, pectin
and small amounts of waxes and fat. Several important sources of ligno cellulosic materials
are listed [6] in Table 1.1, Dinwoodie [7] summarizes the polymeric state, molecular deriva-
tives and function of cellulose, hemicellulose, lignin and extractives (see Table 1.2).
1.1.2 Composition of NFs
Natural plant fibers are composed of cellulose fibers, made of helically wound cellulose
micro-fibrils, bound together by an amorphous lignin matrix. Lignin keeps the water in the
fibers acts as a protection against biological attack and as a stiffener to give stem its resist-
ance against gravity forces and wind. Hemicellulose found in the natural fibers is believed
to be a compatibilizer between cellulose and lignin. The cell wall in a fiber is not a homo-
geneous membrane [8–9]. Each fiber has a complex, layered structure consisting of a thin
Surface Modification of Natural Fibers for Reinforced Polymer Composites 3
primary wall which is the first layer deposited during cell growth encircling a secondary
wall. The secondary wall is made up of three layers and the thick middle layer determines the
mechanical properties of the fiber. The middle layer consists of a series of helically wound
cellular micro-fibrils formed from long chain cellulose molecules. The angle between the
fiber axis and the micro-fibrils is called the microfibrillar angle. The characteristic value of
microfibrillar angle varies from one fiber to another. These micro-fibrils typically have a
diameter of 10–30 nm and are made up of 30–100 cellulose molecules in an extended chain
conformation and provide mechanical strength to the fiber. Study on jute cellulose, hemi-
cellulose and lignin [10–11] suggests that these consist of units as shown in Figures 1.3–1.5.
1.1.3 New Trends in the Chemistry of Cellulose
The chemistry of cellulose now under development will make possible the use of cellulose,
the most important and widespread polymer, for manufacturing a great variety of materi-
als with new structures and endowed with valuable properties quite different from those of
ordinary cellulose products. The transformation of natural cellulose containing one type of
reactive groups (primary and secondary alcohol groups) into high molecular weight com-
pounds which, depending on processing conditions, will contain almost any of the known
reactive functional groups.
Cellulose reacts as a trihydric alcohol with one primary and two secondary alco-
hol groups per glucose unit. The relative reactivity of the hydroxyl groups of both low
Wood fibers
Bast Leaf Seed/fruit Grass
Examples:
Jute, hempExamples:
Cotton, coir
Examples: Soft and
hardwoods
Straw
fibers
Examples: Bamboo
fiber, switch grass
Examples: Sisal,
pineapple leaf
fiber
Reinforcing natural fibers/fillers
Examples: Corn,
wheat, rice straw
Non-wood biofibers
Figure 1.1 Overview of natural fibers.
4 Progress in Adhesion and Adhesives, Volume 2
Hemp
Coir
Jute
Lantana-Camara Tea
Seaweed
Abaca
Sisal
Figure 1.2 Photographs of some natural fibers.
Surface Modification of Natural Fibers for Reinforced Polymer Composites 5
Table 1.1 Chemical compositions of various lignocellulosic materials.
Lignocellulose
source Cellulose (%) Hemicellulose (%) Lignin (%)
Other
constituents (%)
Hardwood 43–47 25–35 16–24 2–8
Softwood 40–44 25–29 25–31 1–5
Coir 32–43 10–20 43–49 4.5
Cotton 95 2 0.9 0.4
Hemp 70.2 22.4 5 5.7
Henequen 77.6 4.8 13.1 3.6
Jute 71.5 13.6 13.1 1.8
Kenaf 36.0 21.5 17.8 2.2
Ramie 76.2 16.7 0.7 6.4
Sisal 73.1 14.2 11.0 1.7
Table 1.2 Cellulosic component, polymeric state, derivatives and function.
Component Polymeric state Derivatives Function
Cellulose Crystalline highly oriented large
molecule
Glucose “Fiber”
Hemicelluloses
small molecules
Semi-crystalline mannose, xilose Galactose “Matrix”
Lignin Amorphous large 3-D molecule Phenyl propane “Matrix”
Extractives Some polymeric; Other
nonpolymeric polyphenols
Terpenes
CH2OH
CH2OH
H
H OH
H
HH
H HOHOH
OHOH H
H H
OO
H
O O
CH2OH
H OH
OH
OH H
H
OH
CH2OH
H
HH
H H
X
OH
OH
O O
Figure 1.3 Structure of cellulose.
6 Progress in Adhesion and Adhesives, Volume 2
COOH
CH2OH
H
H OH
H
HH
H HOH
OH
OCH3
OH H
H H
OO
H
O O
CH2OH
H OH
OH
OH H
H
OH
CH2OH
H
HH
H H
n
OH
OH
O O
Figure 1.4 Structure of hemicellulose.
CH3O
CH3O
CH3O CH3OCH3O
CH3O
CH3O
O
OCH3
OH
OH
OHOHHO
HO
HO
OH OH
OCH3
OCH3
CH3O
OCH3
OH OH
OH
OH
OH
OHOH
OCH3
OCH3OH
OH
OH
OH
OH
OH OH
HO
HO
O
O
OO
OO
O
O O
OO
Figure 1.5 Structure of lignin.
molecular mass carbohydrates and cellulose has been studied [12]. In the former, the
2- and 6-hydroxyl groups are usually the most reactive. With cellulose, certain data
indicate a preferential reactivity of the 2-hydroxyl and others of the 6-hydroxyl group.
The manifold reactions of cellulose may be conveniently divided into two main kinds:
those involving the hydroxyl groups and those involving or causing a degradation of
the chain molecules. The former includes the following reactions: (1) Esterification:
nitration, acetylation and xanthation. (2) Etherification: alkylation and benzylation.
(3) Replacement of –OH by –NH2 and halogen. (4) Replacement of –H in –OH by Na.
(5) Oxidation of –CH2OH to –COOH. (6) Oxidation of secondary –OH groups to alde-
hyde and carboxyl and (7) Formation of addition compounds with acids, bases, and
salts. The various possible types of oxidized groups formed in the cellulose molecule are
shown in Figure 1.6.
1.1.4 Action of Reducing and Oxidizing Agents
Reducing agents have no effect on cellulose while oxidizing agents readily convert it to
oxycellulose. For chemical treatment of fibrous materials, various oxidizing agents are
Surface Modification of Natural Fibers for Reinforced Polymer Composites 7
widely used: chlorinated lime, sodium hypochlorite, hydrogen peroxide, sodium chlorite,
sodium and potassium chromates, and such acids that are capable of oxidizing, such as,
for instance, nitric acid. These reagents may cause intense oxidation of cellulose functional
groups and breakage of chains as a result of glucosidic linkage rupture. The oxidizing agents
first act on the functional groups located on the cellulose fiber surface and then progres-
sively penetrate into the depth of the fiber. There are oxidizing agents which mainly affect
the primary alcohol group at the 6th carbon atom, while other oxidizing agents principally
react with the secondary alcohol groups at the 2nd and 3rd carbon atoms, breaking the
pyran ring. Figure 1.7 represents the oxidation process [13].
1.1.5 Drawbacks of Natural Fibers
Most natural fibers are hygroscopic in nature, i.e., they take in or give out moisture to their
surrounding atmosphere. When NFs neither absorb nor give out moisture to the air around
them they are said to be in equilibrium with that particular atmosphere. The amount of
moisture held by NFs can be expressed in two ways: by moisture content, or moisture
regain. The equilibrium moisture held by NFs when exposed to atmospheres of differ-
ent relative humidities shows appreciable hysteresis according to whether absorption from
low humidities or desorption from high humidities is concerned [14–16]. In general, the
physico-mechanical behavior of NFs depends on the shape and size of cellulose molecule,
fibrillar arrangement, various bonds, and interaction of non-cellulosic components of the
fiber. The individual fiber filaments of an NF are composed of a number of ultimate cells
cemented together by an isotropic, non-cellulosic intercellular substance (hemicellulose,
OH
H
OHH
O
O
OH H
1
23
4
5
CH2OHNO2 Oxidation leads to –COOH
Random oxd. leads to –CHO, >CO, and –COOH
Periodate oxidation leads to
dialdehyde formation
6
Figure 1.6 Possible types of oxidized groups in cellulose.
8 Progress in Adhesion and Adhesives, Volume 2
lignin and pectin) which forms a layer of middle lamella in between the fiber cell walls. The
walls of the fiber cells are thick and lignified and except for the original cracks, these are
relatively smooth in size. The ultimate fiber cells are elongated in the direction of the stem
axis with pointed or tapering ends and appear more or less polygonal with well-defined
angles in a cross section.
The residual oil is the major contaminant in the NF products and creates greater prob-
lems in addition to the natural and inherent defects such as falling off of fiber from fiber
products. Another drawback of an NF, which is responsible for its limited use, is that of
discoloration due to the development of yellow to brown color after sufficient exposure
to light. Moreover, there is a major drawback associated with the application of NFs for
reinforcement of resin matrices. Due to presence of hydroxyl and other polar groups in
various constituents of an NF, the moisture uptake is high (approx. 12.5% at 65% rela-
tive humidity & 20 °C) by dry fiber. All this leads to (i) poor wettability with resin, and
(ii) weak interfacial bonding between NF and the relatively more hydrophobic matrices.
Environmental performance of such NF composites is generally poor due to delami-
nation under humid conditions. Thus, it is essential to pretreat the surface of the NF,
Figure 1.7 Effect of oxidizing agents on cellulose.
O
H O
H OH
HIO4Pb(OCOCH3)4
CH2OH
N2O4
OHHH
O
H
O
H O
H OH
HC=O
OHHH
O
H
O
H O
H OH
COOH
OHHH
O
H
O
H O
O
COH HOC
O
H
O
H
O
H O
O
CH2OH
HCIO2
CH2OH
CH
H
O
HC O
H
Surface Modification of Natural Fibers for Reinforced Polymer Composites 9
so that its moisture absorption is reduced and the wettability by the resin is improved.
Hence cellulosic fibers have some inherent drawbacks which can be briefly enumerated
as follows:
a. poor solubility in common solvents, which makes improvements in fibers and
yarns through spinning processes almost impossible;
b. poor crease resistance, which makes garments made from cellulosic fibers crumple
easily during wear;
c. lack of thermoplasticity, which is a requirement for heat setting and shaping of gar-
ments; and
d. poor dimensional stability which results in distortion of the garment during laun-
dering and ironing. These drawbacks, and the fact that cellulose has encountered
stiff competition from synthetic fibers, have directed attention toward improving
the properties of cellulose.
Therefore, the limited use of natural fiber composites is also connected with some
other major disadvantages still associated with these materials. The fibers generally show
low ability to adhere to common non-polar matrix materials for efficient stress trans-
fer. Furthermore, the fibers inherent hydrophilic nature makes them susceptible to water
uptake in moist conditions. Natural fiber composites tend to swell considerably with water
uptake and as a consequence mechanical properties, such as stiffness and strength, are
negatively influenced. However, the natural fiber is not inert. The fiber-matrix adhesion
may be improved and the fiber swelling reduced by means of chemical, enzymatic or
mechanical modifications.
1.2 Modifications of Natural Fibers
To achieve some improvements, the physical and chemical structures of cellulose must be
altered.
1.2.1 Physical Modifications of Natural Fibers
The physical structure of cellulose can be altered either by swelling or by regeneration.
Cellulose can be swollen in a suitable swelling agent and then partially deswollen by removal
of the swelling agent. There is practically no change in the chemical structure of the cellu-
lose, whether fiber, crumb, or film, but there are considerable changes in the physical form
resulting in an enhancement of strength, luster, and reactivity.
1.2.1.1 Plasma Treatment
Plasma has been extensively used as a physical method for the modification of polymers
[17–20]. The plasma treatment of natural fibers affects the surface only within a few tens
of nm and thus does not affect the bulk properties of fibers [21]. It was observed that the
10 Progress in Adhesion and Adhesives, Volume 2
plasma treatment can induce dramatic changes in the surface morphology of natural plant
fibers [22]. More specifically, some tiny grains, cracks and longitudinal grooves appeared on
the surfaces of the plasma-treated flax fibers, indicating that plasma treatment causes deg-
radation and increases the surface roughness of the flax fibers. Jute fibers were treated with
oxygen plasma in different plasma reactors with different plasma powers. It was reported
that all treatments increased the tensile strength and flexural strength of the resulting jute
fiber-unsaturated polyester composites [23].
1.2.1.2 Physical Activation Processes on Cellulose
1.2.1.2.1 Nonionizing (Low Energy) Radiation
Low-energy, radiation-induced grafting involves the use of ultraviolet or visible light sup-
plied by a suitable source. The energy is used to cause excitation of the sensitizer, causing
generation of radical species which may then attack the substrate. It is shown that ultra-
violet radiation can be used to initiate grafting. Since this type of radiation is not of suf-
ficiently high energy to break C-C or C-H bonds, a photosensitizer must be added to the
system [24]. Sodium 2,7- anthraquinonedisulfonate and 2-methylanthraquinone are used
as sesitizers to graft acrylamide, styrene and other monomers onto cellulose (Cellophane)
and cellulose acetate films. Approximately 0.5% of the sensitizer (based on monomer) is
used [25].
1.2.1.2.2 Ionizing (High Energy) Radiation
With all types of high-energy radiations such as gamma rays, X-rays, alpha particles, and
protons, primary event consists of the formation of ions resulting from the scission of C-C
or C-H bonds belonging to the cellulose, the monomer, or the solvent. The ions are rapidly
converted into free radicals, and in nearly every known case of radiation polymerization
or radiation grafting, a radical mechanism, rather than an ionic mechanism, accounts for
the initiation and growth steps [26–27]. When polymeric materials are subjected to irra-
diation by ionizing radiation such as -rays from Cobalt-60 (6oCo) or high-energy electron
beams generated from electron accelerators, active sites, usually free radicals, are formed
in the polymeric materials. When these active sites are brought into contact with reactive
monomers, either simultaneously during irradiation (direct or simultaneous method) or
after irradiation (post irradiation method), the active sites initiate polymerization of the
reactive monomers to form chemically different polymer chains (graft chains) bonded to
the polymeric materials (polymer substrates). In the presence of monomer, the possible
product from the irradiation of cellulose, which will lead to the formation of graft copoly-
mers, can be represented as Figure 1.8.
It can be proposed that the localization of the absorbed energy in the cellulose initiates
photochemcial reactions, thereby leading to free radical formation. The chain scission by
the photon of light is the primary reaction resulting in free radical formation. In the case of
cellulose, graft reaction takes place at the main backbone. The formation of free radicals by
chain scission is shown in Figure 1.9 and additional modes of radical formation are shown
in Figure 1.10.
Surface Modification of Natural Fibers for Reinforced Polymer Composites 11
1.2.2 Chemical Modifications of Natural Fibers
1.2.2.1 Fundamental or Basic Aspects
Intimate molecular contact at the fiber-matrix interface is necessary to obtain strong interfacial
intereaction. Without an intimate molecular contact, the interfacial adhesion will be very weak,
and accordingly the applied stress that can be transmitted from one phase to the other through
the interface will be very low. Natural fibers are amenable to modifications as they bear hydroxyl
groups from cellulose and lignin. The hydroxyl groups may be involved in hydrogen bond-
ing within the cellulose molecules thereby reducing the activity towards the matrix. Chemical
modifications may activate these groups or can introduce new moieties that can effectively
interact with the matrix. In order to improve the fiber-matrix adhesion a pre-treatment of the
fiber surface or the incorporation of a surface modifier during processing is required.
Several processes have been developed to modify polymers and fiber surfaces including
chemical treatments, radiation treatment, plasma treatments, surface grafting, etc. which
are shown in Table 1.3 [28–59]. These cause physical and chemical changes in the surface
layer without affecting the bulk properties [60]. The chemical structure of cellulose can be
altered in several ways:
1. By substitution of the cellulose hydroxyl, the cellulose molecules are altered through
introducing side groups, usually by an etherification or an esterification reaction.
2. By reacting cellulose with bi- or polyfunctional compounds, which results in the
production of cross-links or resinification products in the cellulose, thereby stabi-
lizing its structure.
3. By combining synthetic polymers with cellulose to produce materials with
improved properties. This process is known as grafting, usually done by
CH2OH
H H OO
H
H
H
H
H
IrradiationH
H
OO
HOH OH
OH OH
CH2OH
H
Hor or or
or
H
OO
HOH
OH
CH2OH
H OO
H
H
HOH
OH
CHOH
CH2OH
H
H
H
OO
HOH
OHH
CH2OH
H
H
H
O
HOH
OHH
Figure 1.8 Possible free radicals formation by irradiation of cellulose.
12 Progress in Adhesion and Adhesives, Volume 2
modifying the cellulose molecules through creation of synthetic polymers that
confer certain desirable properties on the cellulose without destroying its intrin-
sic properties.
Much research has been done on grafting polymeric molecules onto cellulose to pro-
duce materials with new properties intermediate between those of cellulose and those of
synthetics.
CH2OH
CH2OH
H
H
H
H
O
HOH
OH
O
H
H
HOH
H
H
OH
OO
CH2OH
Irradiation
CH2OH
H
H
H
H
O
HOH
OH
Irradiation Irradiation
CH2OH
H
H
H
H
O
HOH
OH
H
H
O
+
HOH
H
H
OH
OO
CH2OH
CH2OH
H
H
+
+
HOH
H
H
OH
OO
O
O
CH2OH
H
H
HOH
H
H
OH
O
OOO
H
H
H OH
OH H
H
Figure 1.9 Free radicals formation by chain scission of cellulose.
Surface Modification of Natural Fibers for Reinforced Polymer Composites 13
CH2OH
H
H , , ,
H
H
O
OH
OH
Dehydrogenation: Free radical formation by hydrogen abstraction
Free radical formation by bond cleavage
Irradiation
Irradiation
Irra
dia
tio
nIr
rad
iati
on
O
CH2OH
H
H
H
O
HOH
OH
OH
CHOH
H
H
H
H
O
HOH
OH
O
,
, ,
CH2OH CH2OH
HH
O
HOH
OH
O
H
H
H
H
O
HOH
OH
O
CH2OH
CH2OH
CH2OH
H
H
H
O
HOH
H
OH
O
Free radical formation by dehydroxymethylation
Dehydroxylation: Free radical formation by hydroxyl abstraction
H+
H
H
O
HOH
H
OH
O
H
H
H
O
HOH
H
OH
HO
O
CH2OH
H
H
H
OHH
OH
O
CH2OH
H
H
H
O
HOH
HOH
CH2
H
H
H OH
O
HOH
HO
Figure 1.10 Additional modes of free radicals generation.
14 Progress in Adhesion and Adhesives, Volume 2
1.2.2.2 Grafting Reactions on Cellulose in an NF
Grafting of vinyl monomers with different functional groups (–OH, -Cl, -C N, etc.) onto
cellulose is a typical free radical polymerization reaction [9, 61] which involves three
distinct aspects, namely, initiation, propagation, and termination. Initiation consists of
two steps. The first step is to produce free radicals on the cellulose backbone from the
initiator. This is generally achieved by abstraction of a hydrogen atom from the cellulose
molecule.
The second step entails the addition of a monomer molecule to the cellulose free radical,
resulting in the formation of a covalent bond between the monomer and the cellulose and
in the creation of a free radical on the newly formed branch. Thus, a chain is followed by
many subsequent additions of monomer molecules to the initiated chain, thereby propa-
gating the chain. Termination occurs by combination, where the radicals of two growing
polymer chains are coupled Figure 1.11.
Or by disproportionation where a hydrogen atom is abstracted by one chain from the
other Figure 1.12. Termination may also occur by reaction with impurities, initiator, or
activated monomer, or by a chain transfer process.
1.2.2.3 Mercerization of Fibers
Alkali treatment of natural fibers, also called mercerization [62] is the usual method to
produce high quality fibers. Alkali treatment increases surface roughness, resulting in bet-
ter mechanical interlocking and the amount of cellulose exposed on the fiber surface. In the
alkali treatment, the following reaction takes place: Addition of aqueous sodium hydroxide
(NaOH) to natural fiber promotes the ionization of the hydroxyl group to the alkoxide.
Fiber – OH + NaOH Fiber – O – Na +H2O
Table 1.3 Various surface treatment methods for natural fibers.
Natural fiber Treatment method (s) Reference (s)
Jute Chemical, Radiation and Plasma [28–40]
Rice straw Chemical and Radiation [26–27]
Banana Chemical [41–43, 89]
Betel nut Chemical [44–45, 82]
Tea dust Chemical [46]
Sisal Chemical [47–53, 64, 113]
Seaweed Chemical and Radiation [26, 45, 54, 81]
Flax Chemical and Plasma [55–57, 22, 68, 98]
Hemp Chemical [58]
Lantana-Camara Chemical [59, 124]
