Adhesion and bonding in composites 9781000105629, 9780367450939, 9780824781491, 082478149X, 1000105628, 9781000127508, 1000127508, 9781000148831, 1000148831, 9781003066309, 1003066305

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Adhesion and Bonding in Composites

Adhesion and Bonding in Composites edited by R Y U T O K U KIYOTAKE A K I O

Y O S O M I Y A M O R I M O T O

N A K A J I M A

Y O S H I T O I K A D A TOSHIO

SUZUKI

CRC Press

& Francis Group

iton London New York CRC Press is an imprint of the Taylor & Francis Group, an informa business

First published 1990 by Marcel Dekker, Inc. Published 2019 by CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 First issued in paperback 2019 © 1990 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works ISBN 13: 978-0-367-45093-9 (pbk) ISBN 13: 978-0-8247-8149-1 (hbk) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com Library of Congress Cataloging-in-Publication Data

Adhesion and bonding in composites / Ryutoku Yosomiya p. cm. Includes bibliographical references. ISBN 0-8247-8149-X (alk. paper) 1. Composite materials. I. Yosomiya, Ryutoku TA418.9.C6A265 1989 620.1'18--dc20

[et al.].

89-36669 CIP

Preface

The remarkable p r o g r e s s of industrial technologies in recent y e a r s has made extremely high demands of materials of all t y p e s , includi n g metals, polymers, and ceramics. In the aerospace field, for example, materials with l i g h t n e s s , high mechanical s t r e n g t h , and good heat resistance are required; whereas in the field of p r o s t h e s e s , such as artificial bones and blood v e s s e l s , good biological compatibility, including resistance to thrombus, as well as desirable mechanical characteristics are required. With the increasing failure of single materials to respond to such h i g h - l e v e l requirements, expectations for composites have r i s e n . Composites are produced b y combining two or more homogenous materials such that the physically and chemically different p h a s e s that are formed will make possible the high performance r e q u i r e d . In achieving this high performance, the interaction between component materials in the i n t e r f a c e , and the properties of each c o n s t i t u e n t , are of utmost importance. This book deals with the roles played b y the component material interface in composites, with special emphasis on methods u s e d to improve the adhesion and bonding between them. Chapter 1 opens with a theoretical treatment of the interface in composites, based on previous s t u d i e s . In Chapter 2 we describe the relationship between wetting properties and adhesion, primarily of polymer matrices. In many composites, sufficient wetting of the surface of a solid phase during a s o l i d - l i q u i d combining step is the basic condition for good adhesion ( b o n d i n g ) . In composites, molecules and atoms of different materials get so close to each other as to form chemical bonds that contribute to produce a strong interaction. For this e f f e c t , control of the interfacial reaction is iii

iv

Preface

a r e q u i s i t e , and various treatments, such as coatings, surface modifications of reinforcing materials, or improvements of matrices, h a v e been u s e d . Chapters 3 through 6 are devoted to descriptions of such surface modifications of matrices or f i l l e r s . Chapter 3 deals with modifications of matrix polymers t h r o u g h chemical and physical treatments, and Chapter 4 c o v e r s modifications t h r o u g h graft polymerization. In Chapter 5 we describe modifications of inorganic f i l t e r s , and in Chapter 6 we d i s c u s s modifications of polymer matrices t h r o u g h polymer b l e n d i n g . In Chapter 7 adhesion of plastics onto metals is dealt with, and in Chapter 8, bonding of ceramics with metals. Chapter 9 deals with modification of the interface in f i b e r - r e i n f o r c e d metal composites, and in Chapter 10 we d i s c u s s the interfacial e f f e c t in c a r b o n - f i b e r reinforced composites as related to methods of carbon fiber surface treatment. Chapter 11 d i s c u s s e s methods, with examples, in the interface analysis of composites, and Chapter 12 c o v e r s dynamic analysis and methods of measuring the interfacial s t r e n g t h of f i b e r reinforced composites. We believe that the book will be of value to s c i e n t i s t s and e n g i n e e r s in the materials field and to e n g i n e e r s working with a d h e s i v e s or composites, as well as people who are now s t u d e n t s of composites. The authors e x p r e s s their sincere thanks to numerous r e s e a r c h e r s whose excellent reports provided r e f e r e n c e materials for this book. The authors are greatly indebted to D r . Maurits D e k k e r , Chairman of the Board, Marcel D e k k e r , I n c . , for his kind recommendation to publish this book, and are also deeply grateful to the editorial s t a f f . Ryutoku Yosomiya Kiyotake Morimoto Akio Nakajima Yoshito Ikada Toshio Suzuki

Contents

Preface Contributors 1.

2.

3.

iii ix

Interfacial Characteristics of Composite Materials

1

Introduction Composite Effect and Interfaces Structure and Function of Interfaces

1 1 4

Wettability and Adhesion

23

Introduction Wettability and Adhesion Force Interaction Force at Interface and Peeling Strength Surface Chemical Change of the Adhered Surface and Its A d h e s i v e n e s s

23 24 34

Surface Modification of Matrix Polymer for Adhesion

49

Introduction Complexities of A d h e s i v e S t r e n g t h Chemical Treatment of Polymers to Improve Wettability Modification b y Corona Discharge Treatment Modification b y Ultraviolet Irradiation Modification b y Plasma Treatment

49 50 50 56 61 67

42

ν

vi 4.

5.

6.

7.

8.

9.

Contents Surface Modification of Matrix Polymer by Craft Polymerization and Its Effect on Adhesion

83

Introduction Plasma Treatment Polymerization Radiation Polymerization Photo g r a f t i n g Polymerization

83 83 92 100

Modification of Inorganic Fillers for Composite Materials

109

Introduction Surface Improvement by Coupling A g e n t s Functional Groups on Inorganic Filler Surface Polymerization onto Inorganic Filler Surface Surface Modification of Inorganic Fibers

109 110 121 127 133

Surface Modification and Adhesion Improvement by the Blend Method

155

Introduction Surface Modifications b y Polymer Blend Adhesion Improvement b y Triazine Thiols Blend

155 156 168

Adhesion of Resin to Metal

181

Introduction Adhesion of Plastics to Metal Adhesion of Elastomer to Metal Modifications of Metal Surface b y Triazine Thiols

181 181 193 200

Bonding of Ceramic to Metal

209

Introduction Methods and Basis of Bonding Process Examples of Bonding Process

209 209 214

Interfacial Modifications and Bonding of FiberReinforced Metal Composite Material

235

Introduction Carbon-Fiber-Reinforced Metal Composites Silicon Carbide-Fiber-Reinforced Metal Composites Boron-Fiber-Reinforced Metal Composites Alumina (AlgO*)-Fiber-Reinforced Metal Composites Metallic-Fiber-Reinforced Metal Composites

235 237 245 249 250 251

Contents 10.

11.

12.

Index

Interfacial Effect of Carbon-Fiber-Reinforced Composite Material

vii

257

Introduction Surface Treatment of Carbon Fiber Roles of Interface in Mechanical Properties of Composites Roles of Interface in Physical Properties of Laminate CFRP

257 257 266

Interface Analyses of Composite Materials

283

Introduction Analyses of the Surface and the Interface of Matrix Analyses of the Surface and the Interface of Filler Analyses of the Interface between Filler and Matrix

283 285 296 301

Interfacial Strength of Composite Materials

311

Introduction Basic Strength of Fiber-Reinforced Composites Measurements of Interfacial S t r e n g t h

311 312 332

273

353

Contributors

RYUTOKU YOSOMIYA Department of Industrial Chemistry, Chiba Institute of Technology, Narashino-shi, Chiba, Japan

KIYOTAKE MORIMOTO Department of Composites, Tokyo Research Center, Nisshinbo I n d u s t r i e s , I n c . , Adachi-ku, Tokyo, Japan AKIO NAKAJIMA Department of Applied Chemistry, Osaka Institute of Technology, Osaka-shi, Osaka, Japan YOSHITO IΚ AD A Director of the Research Center for Medical Polymers and Biomaterials, Kyoto U n i v e r s i t y , Kyoto-shi, Kyoto, Japan TOSHIO SUZUKI Director and General Manager of the R&D Division, Nisshinbo Industries, I n c . , Chuo-ku, Tokyo, Japan

ix

Adhesion and Bonding in Composites

1

Interfacial Characteristics of Composite Materials

Ί.Ί

INTRODUCTION

Composite materials are materials of composite s t r u c t u r e comprising two or more components that differ in physical and chemical propert i e s which have been combined to provide specific characteristics for particular u s e s . The boundaries between components are referred to as solid i n t e r f a c e s . R e c e n t l y , it has become popular to consider the boundary not as a contact interface without t h i c k n e s s but as an interphase with thickn e s s e s on both components. That i s , the concept involves the e x i s t e n c e of a chemical and physical transient region or gradient in the b o u n d a r y . For example, a f i b e r - r e i n f o r c e d composite material includes t h r e e p h a s e s : the surface of the fiber side, the interface between t h e fiber and the matrix, and the i n t e r p h a s e . T h e s e p h a s e s are ref e r r e d to collectively as the i n t e r f a c e . The characteristics of the interface are dependent on the bonding at the i n t e r f a c e , the configuration, the structure around the interf a c e , and the physical and chemical properties of t h e c o n s t i t u e n t s . As a result the interface has a s t r o n g influence on the property of t h e composite material. In this chapter we describe interaction of t h e interface and structural change in t h e interface. Ί.2

COMPOSITE EFFECT AND INTERFACES

A method for the estimation of composite material performance from t h e characteristics of fillers (reinforcing material: f i b e r , powder) and matrices (polymer, metal, ceramics) and from the configuration of t h e filler is generally called a law of mixture. In the most basic form of a law of mixture, some characteristics of a composite material

1

1

Chapter

2

A=1-(f>B Figure 1.1 Relation between the properties of composites and vario u s laws of mixture. ( R e f . 1)

are r e p r e s e n t e d as a function of characteristics of constituent comp o n e n t s and their volume fractions, as shown in Figure 1.1 [ 1 ] , For a composite material (characteristics: x c ) that consists of component A ( c h a r a c t e r i s t i c s : χ ^ , volume fraction: φ^) and component Β ( c h a r a c t e r i s t i c s : x g , volume fraction: Φβ)> the basic formulas of a law of mixture are as follows: X

c

=

Φ

ΑΧΑ

+

Φ

ΒΧΒ

(1)

(parallel model, linear law of mixture, curve 1 in the f i g u r e ) and

xc

XA

XB

(2)

( s e r i e s model, curve 2 in the figure). The two c u r v e s exhibit theoretical u p p e r and lower limits, r e s p e c t i v e l y , based on a simple composite e f f e c t in general. A basic formula that generalizes (1) and (2) is

Interfacial

Characteristics

η

η

of

Composites

η

3 (3)

wherein n( —1 < η < 1) r e p r e s e n t s a structural parameter which indicates the proportion of the combination mode; that i s , the parallel mode is predominant when η i s close to 1 and the series mode i s predominant when η i s close to - 1 . In equation ( 3 ) , when η has a small absolute v a l u e , log x c =

A log χ Α + φ β log χ. Β Ά

(4)

This function i s intermediate in behavior between the parallel model and the series model ( c u r v e 3 in the f i g u r e , referred to as a logarithmic law of m i x t u r e ) . The law of mixture described above i s valid for a simple composite system with well-known s t r u c t u r e in which the rule of additivi t y holds (no interaction in the interface and no particular interfacial s t r u c t u r e ) . However, it is natural to consider that in practice, any interaction will occur in the interface due to the contact between A and B . T h e n , considering the creation of interfacial phase C, different from components A and Β , t h e following equation can be p r e sented :

Equation (5) r e p r e s e n t s a quadratic curve with a maximum (k > 0) or minimum (k < 0) d e p e n d i n g on the sign of k ( c u r v e 4 in the f i g u r e , r e f e r r e d to as a quadratic law of m i x t u r e ) . The parameter k i n v o l v e s an interaction between components A and Β and provides an e x p r e s s i o n of t h e interfacial e f f e c t . The equation s u g g e s t s that the properties of the interfacial phase must be improved to obtain an excellent composite material. On the other hand the properties of the interfacial phase are probably affected by t h e t h i c k n e s s of the phase and the intermolecular force between different molecules. To improve the properties of the interfacial p h a s e , it i s considered e f f e c t i v e to increase the t h i c k n e s s of the interfacial phase and the intermolecular force between different molecules. However, a thick phase is not always formed at the i n t e r f a c e . For example, a thick phase does not form in the case of polymer-metal d i f f e r i n g from in t h e case of polymerpolymer [ 2 ] . Even when the interfacial phase i s not t h i c k , it i s desirable to improve the interaction between different molecules, or t o form primary bonding between molecules, to improve the properties of the interfacial p h a s e . Next i s ion bonding and h y d r o g e n bonding, and van der Waals bonding i s lowest among interactions. However, some interaction

Chapter

4

1

always o c c u r s , which i s important in proyiding an i n t e r f a c e , r e g a r d l e s s of the t y p e of material i n v o l v e d . Furthermore, for interaction to be e f f e c t i v e , other f a c t o r s , s u c h as a d e c r e a s e in t h e distance between interfaces and an i n c r e a s e in t h e number of interactions, must be considered. T h e s e factors corr e s p o n d to an improvement in t h e wettability between different constituent molecules of the interface. That i s , thermodynamic wettability must be taken into account as the principal factor in t h e affinity between molecules. Wettability and adhesion a t e d i s c u s s e d in Chapter 2.

1.3

STRUCTURE AND FUNCTION OF INTERFACES

When a composite material (FRP) i s formed, at least one fluid component in a form of solution or melt i s mixed with other components and the mixture solidified. The s t r u c t u r e formed at the interface with a contact medium i s different to some d e g r e e from the internal s t r u c t u r e . In g e n e r a l , bonding at the interface i s described in terms of inter molecular force and surface f r e e e n e r g y , but in practice the following factors are also important as determinant factors of i n t e r facial bonding: (1) wettability, (2) chemical reaction, (3) adsorption and d i f f u s i o n , (4) residual s t r e s s l a y e r , (5) surface morphology, and (6) r o u g h n e s s e f f e c t . Factors that are important when combining inorganic materials and polymers (composite material) are d i s c u s s e d next. 1.3.1

Modification of Surface and Interfacial Interaction

In general, inorganic materials have a high surface free e n e r g y and o r g a n i c materials a low s u r f a c e e n e r g y . For favorable combination when the affinity i s low, each material requires surface treatment. (In Chapters 3 t h r o u g h 6 the surface treatment of organic and inorganic materials and their e f f e c t s are d e s c r i b e d . ) Silane and e s t e r i f i cation treatment are widely u s e d as surface treatments for inorganic materials. For example, in Table 1 . 1 [3] are shown t h e critical s u r face free e n e r g i e s of g l a s s , the surface of which i s treated with various silane treating a g e n t s (silane coupling agent t r e a t m e n t ) . From the data in the table it i s obvious that the high surface e n e r g y of glass i s changed to a low critical s u r f a c e free e n e r g y in a range near that of polymers. One reason for the application of such a treatment for reinforcing r e s i n s with glass f i b e r s (FRP) i s to d e c r e a s e t h e critical surface free e n e r g y . In addition, t h e adhesion s t r e n g t h depends to a considerable deg r e e on the pH of a silane treatment solution. Silicon wafers were treated with γ-aminopropyl triethoxysilane solutions of various pH v a l u e s and t h e n bonded with polyimide r e s i n . The influence of pH

^c (dyn/cm) 14 18 17 40—45 a 44 43 26-33a 30

Pyrex Pyrex Silica Pyrex Silica Pyrex Silica Silica

3- ( 1 , 1 - D i h y d r o p e r f l u o r o o c t o x y ) p r o p y l t r i e t h o x y s i l a n e , CF3(CF2)6CH20(CH2)3Si(0CH2CH3)3

γ -Perfluoroisopropoxypropyltrimethoxysilane, (CF3) 2CFO(CH2) 3Si(OCH3) 3

3- ( p - C h l o r o p h e n y l ) e t h y l t r i m e t h o x y silane, p-ClC6H4(CH2)2Si(OCH3)3

γ - Chlorop r o p y ltrimethoxy silane, C1(CH 2 ) 3 S i ( O C H 3 ) 3

Ethyltriethoxy silane, CH3CH2Si(OCH2CH3)3

Vinyltriethoxysilane, CH2=CHSi(OCH2CH3)3

D e p e n d i n g on catalyst u s e d . Source: R e f . 3.

a

Coupling a g e n t and s t r u c t u r e

Glass substrate

Critical S u r f a c e T e n s i o n s of Various Silane Coupling A g e n t s on Borosilicate and Table 1.1 Quartz Slides

6

Chapter

1

Figure 1.2 Peel force d i f f e r e n c e between silane- and non-silanetreated regions of a wafer plotted as a function of the pH of a γ-APS solution. The c u r v e r e p r e s e n t s extrapolation of t h e data p o i n t s . ( R e f . 4.)

in treatment solutions i s shown in Figure 1.2 [ 4 ] . Maximum adhesion s t r e n g t h i s at a pH value near 9, and adhesion s t r e n g t h d e c r e a s e s when the pH i s above or below t h i s value. It i s assumed that polyimide reacts as follows when treated at a pH of 9:

II ι ' t h - o — si— (ch 2 ) 3 — (Λ I I (a) With a pH below 9, aminosilane on the silicon surface forms intramolecular h y d r o g e n bonding during treatment as shown in ( b ) , and with a pH above 9, aminosilane forms bonds with h y d r o x y l groups on the silicon surface as shown in ( c ) ; c o n s e q u e n t l y , functional groups

Characteristics

Interfacial

CHo

I

of

Composites

7

CHo

I

NHo — 0 — S i — 0 — 3

I ο

0

Surface

(b)

0 Surface

(c)

reactive with polyimide disappear and it i s believed that this r e s u l t s in a great decrease in adhesive s t r e n g t h . Some s a y that organic material and polymer are chemically bonded b y functional groups on both ends of the silane treatment a g e n t , but Plueddeman considers t h e reaction between silanol groups and on the surface of the glass and the silane treatment agent to be r e v e r s i b l e . In that case, t h e internal s t r e s s relaxation and h y d r o g e n bonding t h r o u g h a sliding mechanism greatly affect the retention of bonding in the interface [ 5]. Recent investigations b y Ishida et al. u s i n g an ingenious FT-IR method have clarified the structure of silane-treated layers [ 6 ] . That i s , silane-treated l a y e r s do not have a simple s t r u c t u r e that permits silane coupling agent molecules to chemically bond on the g l a s s surface in the form of monomolecular l a y e r s ; r a t h e r , it forms a more complex stratified s t r u c t u r e . T h e s e multimolecular s t r u c t u r e c h a n g e s are not the result of the chemical s t r u c t u r e of the silane treatment agent o n l y , but also of the pH of the treatment solution, t h e concentration, t h e s o l v e n t , and t h e temperature, in addition to t h e surface s t r u c t u r e of the reinforcing material (inorganic material). Because the usual silane coupling agents will not react with matrices having no functional group, such as polyethylene and polyprop y l e n e , special treating a g e n t s h a v i n g molecular s t r u c t u r e s that are reactive with such matrices have been developed [ 7 ] . In Figure 1 . 3 t h e reaction mechanism of azidosilane are shown. Properties of polypropylene incorporated with t h r e e t y p e s of inorganic powder fillers treated with azidosilane are shown in Table 1 . 2 . In all cases the tensile s t r e n g t h , bending s t r e n g t h , and heat deformation temperature are improved. Aside from s u r f a c e modification of fillers (described in Chapters 5 and 10), chemical and g r a f t i n g treatments other than coupling treatments have been used to activate the s u r f a c e . R e c e n t l y , oxidation treatments such as wet HNO3 oxidation, dry air heating oxidation, and anodic oxidation of commercial carbon f i b e r s (CF) have been used b y Fitzer in a comparative investigation of various viewpoints concerni n g adhesion between t h e fiber and t h e matrix thermosetting resin in composite materials [81.

Chapter

8

1

? HeO ? ? -Si-OH + (CH50)3Si-R-S02N3 — -Si-O-Si-R-SCkNs ι ι ι Ο 0 0 ι ι ι Si—0— Si-Oi ι 0 0 I ι

6 ό

ι ι -Si-0-Si-R-S02N3 + 1 1 0 0 ι -Si-OI 0 I Δ

-CH2-CH-(CH2-CH-)n I ι CHs CH3

ι I I ι ? Ϋ ^ > -Si-0-Si-R-S02-N-C-CH3 0 ' Si 1 0 I

6 I

(CH?-CH-)n 1 CHs

Figure 1.3 Addition reaction of an azidosilane coupling agent to an inorganic filler. ( R e f . 7 . )

The relation between t h e d e g r e e of oxidation, BET surface area, and interlaminar shear s t r e n g t h (ILSS) of composite materials i s shown in Figure 1 . 4 . A maximum ILSS value is found for hightenacity CF (Sigrafil HT) ; for high-modulus CF (Sigrafil HM) the ILSS value i n c r e a s e s with i n c r e a s i n g oxidation. The behavior dep e n d s on t h e difference in surface structure of the CF. N e x t , the e f f e c t of blocking of various functional g r o u p s r e s u l t i n g from oxidation treatment on t h e ILSS of composite material i s determined. Acidic g r o u p s are blocked b y diazomethane treatment, nonacidic h y d r o x y l g r o u p s b y dimethyl sulfate treatment and carbonyl, and quinone groups by NaHB4 and s u b s e q u e n t dimethyl sulfate or diazomethane treatment, and blocked CFs are combined with epoxy resin to form composite materials. ILSS is shown in Figure 1 . 5 . For all functional g r o u p s involved in this experiment, some interaction between matrices i s f o u n d . Complete blocking of all functional groups generated r e s u l t s in ILSS v a l u e s as low as that of the original untreated f i b e r . Chemical interaction between the matrix and various functional groups r e s u l t i n g

141

Tensile modulus (103 psi)

(g/cm3)

0.92

130

0.49

180

1.225

221

0.49

780

7,230

580

2.5

4,370

None

Mica c

1. 238

261

0. 44

1,030

12,800

630

3. 0

7,190

S3046

1.245

200

0.61

620

7,700

470

10

3,960

None

1. 260

222

0. 48

700

9.500

480

4. 4

4,980

S3046

Wollastonite^

a

Source:Ref. 7. P r o f a x 6523 p o l y p r o p y l e n e , Hercules Inc. b A z i d o s i l a n e S3046, a development product of the Organics Department, Hercules I n c . c Mica Alsibronz 12, 325 m e s h , Franklin Mineral Corp. ^Wollastonite (calcium s i l i c a t e ) , g r a d e F - l , Interpace Corp. e c i a y - S u p r e x c l a y , 325 m e s h , J . M. Huber Corp.

Density

Heat deflection temperat u r e at 264 p s i ( ° F ) , D 648-72

Impact s t r e n g t h (ft—lb /in .) D256A-73 (notched izod)

Flexural modulus (103 psi)

5,800

575

B r e a k i n g elongation (%)

Flexural s t r e n g t h ( p s i ) , D790-71

4,300

None

Unfilled

Tensile s t r e n g t h ( p s i ) , D638-72

Coupling agent

Filler t y p e

1.224

157

equations (10) through (12) become combined that permit calculation of Yg. 2 . 1 shows the surface f r e e e n e r g y of various polymers obtainway [7b and 8]. W i s a thermodynamic i n d e x of a d h e s i v e n e s s .

3.5

7.8 9.1

22.1 27.6 42.3 35.6 43.0 43.7 42.4

Polytrifluoroethylene

Poly(vinylidene fluoride)

Poly ( v i n y l f l u o r i d e )

Polyethylene

Poly(vinylidene chloride)

Poly ( v i n y l chloride)

Poly(methyl m e t h a c r y l a t e )

0

0.1

1.9

0

0.2

1.3

2.1

19.4

Polytetrafluoroethylene

0.8

0.2

0.9

0

1.0

0

0

0

14.9

γ' s c

Ys

43.2

44.0

45.8

35 6

43.5

40.2

31.2

21.5

14.9

of Polymer Solids ( d y n / c m , at 20°C)

Hexafluoropropylene

, and y γ'S b

c

γY a s

ysa, ysb, y

Polymer

Table 2.1

31

38.3 (B)

43.2 (B)

43.9 (B)

39

39

40

28

44.2 (C)

44.0 (B)

25

22

18.5

16.2

Yc*

4 0 . 0 (C)

2 9 . 0 (C)

21.5 (B)

-

yc(max)*> t

36.5 33.8 42.7 42.0 20.6 24.4 29.8 42.5 38.8 33.3

Poly ( v i n y l alcohol)

Polystyrene

Poly ( e t h y l e n e terephthalate)

Nylon

Hexatriacontane

Paraffin

Polypropylene

Polyoxy methylene

P o l y ( y - m e t h y l L-glutamate) α-sheet 3-sheet 1.0 2.4

0.9

0

0

0

1.4

0.6

5.8

3.3

15.1

8.2 2. 1

1.2

0

0

0

3.1

0.5

0

-

10.7

48.0 37.8

44.6

29.8

24.4

20.6

46.5

43.8

40.6

-

52.3

50 (C) 37 (B)(C)

46.5(C)

29.8 ( B )

25.7 ( B )

20.6 (A)

46.0 (C)

43.4 ( B ) ( C )

43.0 ( B )

-

-

-

40-50 37

29

23

20-22

46

43

33

37

ysa + ygb + ysc.

^ y c , critical surface tension reported in the literature.

Letter in p a r e n t h e s e s indicates t y p e of liquid; y s =

Source: R e f . 7b and 8. * y c ( m a x ) , maximum value of y c among the v a l u e s obtained b y t h e d i f f e r e n t liquid s e r i e s A , B , and C.

26.5

Poly aery lamide

CS3

-SI

§: co §·

>>

a

g

5

&

f

Chapter

28

2

10® Jm" 2 1Q2

W

101

10°

1,

2

20

30mNm" 4 0

Steel

Js Figure 2.1 Adhesive failure e n e r g y W for two different contact p e r i o d s , plotted against t h e surface tension of the polymers used as a d h e r e n t s . Τ = 23°C; · , t = 1, 5 χ ΙΟ"2 s; o, t = 1.0 χ 10 2 s . 1, polytetrafluoroethylene; 2, polysiloxane; 3, p o l y ( v i n y l i d e n e f l u o r i d e ) ; 4, polypropylene; 5, polyethylene; 6, p o l y ( v i n y l chloride); 7, poly(methyl methacrylate); 8, p o l y s t y r e n e , 9, p o l y ( e t h y l e n e t e r e p h t h a l a t e ) ; 10, 6-polyamide; 11, 6,6-polyamide. (Ref. 9.)

T h e r e f o r e , the l a r g e r W a , the b e t t e r t h e a d h e s i v e n e s s . However, Wa estimated from t h i s table i s smaller than 100 dyn/cm and much smaller t h a n the adhesion or cohesion break e n e r g y , 10^ to 10? d y n / c m , which was measured mechanically. To examine the e f f e c t of wettability on adhesion s t r e n g t h , Zosel e t al. [9] measured t h e adhesion s t r e n g t h of polyisobutylene (PIB) o n various polymers and obtained the result shown in Figure 2 . 1 . This figure shows the s t r e n g t h measured at contact times of 1.5 χ 10~ 2 and 1 χ 10 2 s . In both c a s e s , the identical adhesion s t r e n g t h (w) i s shown when the surface free e n e r g y , y g , of the adhered material i s larger than the yg value of PIB. On the other hand, when t h e Yg value of the adhered material is lower than the yg value of PIB ( i . e . , PIB does not wet the material e n o u g h ) , the adhesion s t r e n g t h i n c r e a s e s when the yg value of the material approaches that of PIB. R e c e n t l y , it has been reported for hot melt adhesion that good wettability d o e s not necessarily contribute to adhesion s t r e n g t h .

Wettability

and

Adhesion

29

In the hot melt adhesion of a polymer-polymer or polymer-metal s y s tem, Imachi [10] examined contact a n g l e , peeling s t r e n g t h , hot melt temperature, and so o n , and obtained the result that good wettability d o e s not always bring high peeling s t r e n g t h . This may s u g g e s t that wettability i s only one of the conditions of adhesion. To examine the relationship between each component of the s u r face tension and the adhesion s t r e n g t h , Nakamae et al. deposited metallic cobalt vapor on various polymer films and measured the adhesion s t r e n g t h between a thin film of metallic cobalt and a polymer film [ 1 1 ] . The surface characteristics of the polymer film employed in this experiment are shown in table 2 . 2 . The comparison shows that the dispersion component of various polymers does not change v e r y much, but the polar component changes significantly. Such a balance between the dispersion and the polar component and the polar component itself greatly affects interaction between the polymer and t h e other adhered material. Figure 2.2 shows the adhesion s t r e n g t h and the component of s u r f a c e tension. A linear relation is o b s e r v e d between the adhesion s t r e n g t h and the polar component, but the dispersion component does not show such a relationship. Furthermore, the surface of the polye t h y l e n e terephthalate) (PET) film was treated with aqueous NaOH solution. As shown in Figure 2 . 3 , a relation between adhesion s t r e n g t h and each component of the surface tension was obtained u s i n g this treated film. In this c a s e , also, a linear relation was obs e r v e d between adhesion s t r e n g t h and the polar component. From the r e s u l t s of surface analysis by FT-IR, the adsorption int e n s i t i e s of C = 0 and OH increased proportionally to treatment time. It is estimated from this that the s t r o n g polar interaction between h i g h l y polar metallic cobalt and polar s u b s t i t u e n t s s u c h as —OH, — COOH, or CO e x i s t i n g on the PET film surface contributes greatly to the increase in adhesion s t r e n g t h . In the case of adhesion between inorganic material and polymer o r between metallic material and polymer, various surface treatments are generally undertaken to get their surface free e n e r g y values close t o g e t h e r . In this w a y , the mutual affinity i s increased and t h e atmospheric s t r e n g t h , especially the waterproof s t r e n g t h , i s increased. For example, when various surface treatments are undertaken for aluminum, y g d and ygP change as shown in Table 2.3 [12], The adhesion s t r e n g t h with e p o x y phenol resin i s superior for aluminum treated by electrolytic oxidation. However, the treated aluminum has a high adhesion s t r e n g t h to water, and the waterproof s t r e n g t h i s inferior. On the other h a n d , aluminum treated with phosphoric acid is superior in waterproof s t r e n g t h . In this system, t h e r e f o r e , t h e Ygd value of aluminum should be large in order to raise the

53 38 36 22 20

76 70 64 64 64

PET

PET - Η1 PET - Η 2 PET-H3 PET-H4

Ν aO Η - treated poly ( e t h y l e n e terephthalate)

θ r

θ a

Water

29

53 53 48 44 46

65 56 51 47 46

28 13 5 4

θ r

θ a

θ

Ethylene glycol

Contact angle

C h a r a c t e r i s t i c s of Various Polymer Films U s e d

Poly ( e t h y l e n e terephthalate)

Polymer

Table 2.2

θ 34 33 33 32 32

θ 42 42 35 31 32

Methylene iodine

47 50 52 52

44

30 30 29 29

33

Surface free energy (dyn/cm)

17 20 23 23

11

Wettability

and Adhesion 45

^ i> Q σι (μ Μ (Ν (Μ Μ

(Occurrence of a crack)

56

Penetration rate method

Liquid paraffin

20 > (Occurrence of a crack)

48

Penetration rate method

Toluene

Contact angle(°)(23°C)

R e s u l t s of Contact Angles Measured b y Tablet and Penetration Rate Methods

Calcium carbonate (Untreated)

Sample

Table 5.6

3

CD*

2. ο' §

O

Ο

ο Q θ'

ss ο α

@

X

∆

∆

O

Silica (Untreated)

Silica (Treated with TC-1 )

Silica (Treated with TC-2)

Silica (Treated with TC-3)

Talc (Untreated)

Talc (Treated with TC-1 )

∆

:

Source:

Ref.

25.

Slightly bad

X

∆

O O

o

O

Talc (Treated with TC-3)

:

o ∆

O

o

O

Talc (Treated with TC-2)

: Well

O

o O

X

∆

©

∆

o

∆

O

o

: Bad

O

O

o

o

∆

o

O

o

o

o o

o

X

X

X

®

X

o

o ®

X

∆

∆

O

O

®

X

Sedimentation volume

©

®

O

®

®

O

®

X

Microscope

Liquid paraffin

X

Sedimentation volume

Toluene

®

Slightly well

X

O

Calcium carbonate (Treated with TC-3)

O

∆

∆

Calcium carbonate (Treated with TC-2)

∆

X

X

X

e

Calcium carbonate (Treated with TC-1 )

Microscope

Sedimentation Volume

®

Microscope

Water

Dispersion (25°C)

D e g r e e s of D i s p e r s i o n O b s e r v e d Under t h e Microscope and b y Sedimented Powder Volume

Calcium carbonate (Untreated)

Table 5.7

Modification

of Inorganic

Fillers

121

in t h i s field, b r i n g i n g about the development of an amino silane with an unsaturated bond, carboxylic acid functional silane, cationic silane [ 1 8 , 1 9 ] , silyl peroxide [ 2 0 , 2 1 ] , and aminimide [ 2 2 ] . Table 5.5 s h o w s t h e e f f e c t s of t h e s e silane coupling a g e n t s on the properties of h i g h - d e n s i t y polyethylene-aluminum sandwich panels. Many s t u d i e s have also been conducted on the action mechanism o f titanate coupling a g e n t s , which were developed relatively r e c e n t l y , although sufficient evidence has not b e e n obtained for the bonding mechanisms or interactions between titanate coupling a g e n t s and i n organic fillers or matrix r e s i n s . Bonding between the a g e n t s and inorganic fillers is generally interpreted based on a chemical bonding t h e o r y as shown in Figure 5 . 2 , although it has not been demonstrated appropriately [ 2 3 ] . There are, h o w e v e r , many reports that propose indirect evidence which s u g g e s t s that the a g e n t s do interact somewhat with inorganic fillers [ 2 4 ] . Few titanate coupling a g e n t s h a v e nonhydrolyzable groups that can u n d e r g o a chemical reaction with t h e matrix r e s i n s . This indicates that for the most p a r t , interactions of titanate coupling a g e n t s with matrix r e s i n s result from the entanglement of molecules of t h e former with relatively long-chain h y drocarbon radicals of the latter and that the entanglement i s r e s p o n s ible for the surface modification e f f e c t . Figure 5 . 3 shows typical properties of a composite composed of a polyolefin resin and an i n o r g a n i c filler treated with a titanate coupling a g e n t . Results on the w e t t i n g and dispersion properties are given in Tables 5 . 6 and 5 . 7 , r e s p e c t i v e l y . It can be s e e n that silane coupling agents are e f f e c t i v e for increasing the rigidity, while titanate coupling agents act to improve the p r o c e s s ability and flexibility.

5.3

FUNCTIONAL GROUPS ON INORGANIC FILLER SURFACE

T h e functional groups on the surface of an inorganic filler are an important factor in i n v e s t i g a t i n g interactions between the filler and a matrix polymer at the interface b e c a u s e t h e y correspond to the e n d g r o u p s of the polymer. Such g r o u p s have not been identified i n all inorganic fillers. Functional g r o u p s on the carbon black s u r face have been studied for many y e a r s ago b y organic chemical methods, showing that t h e y exist only at the end of the graphite crystallite layer constituting the carbon black material. T h e r e are t h r e e t y p e s of o x y g e n - c o n t a i n i n g functional g r o u p s : acid, neutral, and basic. Carbon generally forms acid surface o x i d e s when heated in an o x y g e n atmosphere, and it forms basic s u r face o x i d e s when heated in an inert g a s . Both acid and basic s u r f a c e o x i d e s exist in any carbon material, although the latter always

Chapter

1 122

5

in smaller amounts. The acid functional groups include carboxyl and phenolic hydroxyl groups, while the neutral ones include carbonyl and quinone groups. However, the structures of the basic functional groups are not known in detail. Table 5 . 8 shows the distributions of oxygen-containing surface functional groups that exist on the carbon black surface [ 2 5 , 2 6 ] . It i s known that two t y p e s of groups, siloxane and silanol, exist on the surface of silicic acid. Silanol groups are left in large amounts within the filler particles formed through a condensation reaction of low-molecular-weight silicic acids. The following surface functional groups will be produced, depending on the crystal structure formed [27]:

0

• Η»^ \

Si

/

^/H V

Η \

\

/

ι ' /Si

OH

^

Si

\

I /

Si

0

/

'. |

\ Tridymite

Cristobalite

It i s accepted that the cristobalite and tridymite typed coexist on t h e amorphous silicic acid surface in a ratio of about 60% to 40%. Table 5 . 9 lists measurements made by various techniques for the silanol group in dry white carbon (Aerosil) [ 28]. The existence of hydroxyl groups has been observed on the surface of rutile- and anat a s e - t y p e titanium dioxides by the infrared-absorbing analysis method [ 2 9 , 3 0 ] . The difference between the adsorbed H O on a rutile surface and those on an anatase surface can be cfetermined by nuclear magnetic resonance absorption at 300°K [31]. The following models have been proposed for the structure of the titanium dioxide surface under oxidation and reduction conditions [32]: Η I 0 I

,

, TiIV

V i

0

/

TiIV X

\

TiIV

>

Η I 0 ^ /

^

Ti™

,τί1*1

< / | \

surface in oxidation condition

surface in reduction condition

Although hydroxyl groups also exist on an alumina surface, alumina and its hydrates assume a great number of transformed structures,

75

78

41

39

73

2. 92

1. 66

0. 71

0. 59

4. 65

FF-B-3

HAF-C

HAF-A-2

HAF-B-2

EPC

26b.

83

3. 69

FF-A-3

Ref.

90

3. 94

FF-C

Source:

%

Hydrogen >—Η

0. 86

0. 61

0. 53

0.,27

0. 67

0. 40

0. 30

meq/g

Hydroxyl >-OH

S u r f a c e Group Distributions

meq/g

5.8

Sample

Table

13

41

31

13

17

9

7

%

0. 64

0. 00

0. 22

0. 11

0. 02

0. 13

0. 03

meq/g

Quinone = > 0

10

0

13

5

1

3

1

%

0. 05

0. 12

0. 12

0.,01

0.,13

0.,11

0.,04

meq/g

Carboxylic >-C02H

1

8

7

0

3

2

1

%

0. 20

0.,18

0.,13

0. 09

0. 16

0. 13

0. 06

meq/g

Lactone >-C02

3

12

8

4

4

3

1

%

Chapter

124 Table 5. 9

5

Density and Silanol Group of Aerosil Surface Silanol groups Specific surface (m2/g)

(meq/100 g)

Packing density (OH/lOO A 2 )

Reaction at 1000°C (free H2O, Κ-Fischer titration)

180

103

3.45

Reaction with SOCl 2

178

62

2.10

Zerewitinoff method by CH3MgI or CH3L1

178

122

4.12

Reaction with SOCl 2

165

87

3.17

Titration by NaOH

145

57

2.38

Reaction with Β CI 3

145

56

2.34

Α1^+ absorption from Al(OH) 2 Cl(aq)

145

60

2.50

U 0 2 + absorption from U 0 2 ( C H 3 C 0 2 ) 2 (pH 5.4)

145

52

2.17

Methyration of CH3OH at 200— 250°C

145

55

2.29

IR spectroscopic

145

115

4.77

Reaction

Source:

Ref. 28.

resulting in different surface properties [33]. These infrared spectroscopic observations s u g g e s t that hydroxyl groups exist in all inorganic fillers and that their surfaces, which are hydrophilic in varying deg r e e s , must not be highly wettable by matrix polymers. Studies have been carried out on reactions involving the surface hydroxyl group or other oxygen-containing groups to determine their use for surface improvement. The following exchange reaction takes place when silicic acid i s immersed in an aqueous solution containing transition metal ions: M n ' + mSi(OH) ^

» M(SiO)

(n

m

~m)' +

m H-

This exchange reaction proceeds less rapidly in the order Zn(II) > Cu(II) > Ni(II) > Co(II) > Mn(II). Except for Zn, a metal ion with

Modification

of Inorganic

Fillers

125

a larger diameter and larger density of electric charge tends to undergo the ion-exchange reaction more rapidly [34]. Calcium hydroxide also reacts rapidly, as shown below. 2Si(OH) + Ca(OH) f

(SiO) 2 Ca + 2H 2 0

This reaction is actually used for surface modification. It is also possible to make a silicic acid surface undergo the following reaction in a basic solution of aluminum chloride:

HO\

Si

/

/ OH HO\

Si

/ OH χ

+3

+ [(H 2 0) 6 A£] HO \ HO \

/ Si

0

/

ki

+3CI

OH / \

0

\

Si

+ 4H +3C£

OH /

+ 4H20

\

Various chemical reactions have been attempted in order to identi f y functional groups on the surfaces of inorganic fillers. The following, for example, are important reactions for silanol group contained in silicic acid or silicates. \

\

— Si-OH + ROH

^ — Si-OR + H20

/

^

(R = CH 3 , C 2 H 5 \

— Si-OH + CH2 = CH2

-

/

\

/

^

\

/CH 3

— Si-OH + CZ - Si-CH 3

\

Si-OH + SiC^

? CEHI )

X

—Si-0-CH2-CH3 /

\

/

/

\

CH3

>- — Si-O-Si-CH 3 + HC£

CH3

\

/

CH3

CI

— Si-O-Si— Cl + HC£ / \ a

0

0

o

o 0 X 0 X

o X

C 2 H 5 OH- treated ZnO

A1203

C 2 H 5 OH- treated A12O3

CC1 4- treated A12O3

BaO'6Fe 2 O 3

C 2 H 5 OH- treated BaO-6Fe 2 O 3

Y -Fe 2 0 3

C 2 H 5 OH-treated y-Fe 2 O 3 0

0

0

o/x

x/o

o/x

x/o

o/x

o/x

o/o

o/x

x/o

x/o

o/x

x/o

o/x

x/o o/x

x/o o/x

x/o

x/o

o/o e/x

x/o

X/A

x/o

o/x

x/o

x/o

o/x

Water /CC14

o/x

o/x x/e

A/O

o/x

o/x

x/o

Butanol /water

o, good dispersion; x, bad dispersion; A, little good dispersion; 3, collected interface. Source: Ref. 35.

Q

o

0

ZnO

0

o/x

0

o/x

o/x

o

X

o/x

C 2H 50 H- treated magnetite

0

Magnetite

0

o/x

o

X

C 2 H 5 OH-treated silica gel

0

x/o

Benzene /water

A/O

X

C 2 HsOH-treated silica gel

0

Benzene

o

o

Water

Effect of Lipophilic Treatment on Various Metal Oxide Powdersa

Silica gel

Table 5.10

ISD

Oi

Cn

Chapter

Modification

of Inorganic

127

Fillers

F — Si-OH + SiFi*

• — Si-O-Si — F + HF F

\

\

— Si-OH + S02Ci /

/

\

\

— Si-OH + CH2N2

.

* — S1-O-CH3 + N 2

^ . - S i - O H + CH 3 -CH-CH 2 ^

\

\

— Si-OK + C 2 H 5 I \

— Si-OH + CH3L1 /

+ S02 + HC£

— Si-CZ

\ / * — S i - O - CH

/

^

Ο

\

CH3

CH20H

\

- S i - 0 - C 2 H 5 + KI \

- S i - O - L i + CHi* /

Of t h e s e , the esterification reaction i s used most f r e q u e n t l y . The esterification will hardly proceed beyond a certain point, and many s t u d i e s are being conducted to provide improved methods. Table 5 . 1 0 lists observations for silicic acid, alumina, zinc o x i d e , and a magnetic oxide which were immersed in ethyl alcohol, 1 - b u t y l alcohol o r carbon tetrachloride and then heated and p r e s s e d in an autoclave, which indicate that the surface of each inorganic filler (powder) b e came lipophilic b y this treatment [ 3 5 ] . 5.4 5.4.1

POLYMERIZATION ONTO INORGANIC FILLER SURFACE Polymerization by Initiator at Filler Surface

If polymerization i s induced after allowing an appropriate initiator to b e absorbed o v e r the surface of the filler, the end groups of t h e r e s u l t i n g polymer molecules are connected to the inorganic filler s u r f a c e . Some reactions have been reported that u s e s u c h initiators to p r o v i d e covalent bonding for connecting a graft polymer to an inorganic filler surface [ 3 6 ] . Powder with high fluidity can be produced b y allowing methyl methacrylate monomer to react under vacuum with α ,a'-azobis(a,Y-dimethylvaleronitrile), an initiator, adsorbed on silicic acid or titanium dioxide. When azo compounds are adsorbed to clay materials such as kaolin and bentonite, the initiator molecules separate into two g r o u p s , one

Chapter

128

CH3

CH3 +

H s N

IC — C — Ν =

/ I

Ν —

CHS

C —

I

CH3

5

NH2 θ Γ

+

\

2CI"

NHZ

Adsorbed surface I \ ]

Silica Alumina Silica t .

5.4A HaO .1 1 \

= N

-ON=N

ο N^

Ο

Interlayer >

o

>

Homo polymer

A®

Silica Alumina Silica Figure 5 . 4 Schematic representation of polymerization of monomers initiated by free radicals attached to bentonite. (Ref. 36, 37.)

e x i s t i n g between the layers in the form of a dication and the other on the surface in the form of a monocation, in a ratio of about 80:20 i n the case of bentonite. As shown in Figure 5.4, polymerization is initiated mainly between the layers if radicals formed by thermal decomposition coexist with such monomers as methyl methacrylate, s t y rene, vinyl acetate, chloroprene, and acrylonitrile [37]. For example, polymerization of styrene occurs at a glass surface treated with Lewis acids, including the tetrachloride and titanium tetrachloride. The following complex is likely to be formed in this reaction [38] : χ —Si—OH + TiCl. /

4

\ Si—OH *TiCl. / 4

Modification

of Inorganic

Fillers

—Si—OH-TiCl. + CH = / 4 Δ.

129

CH ι

-Si—CH - CH + CH = ·" Δ \ Ζ

— S i - C H 0 - CH + TiCl.OH / Δ ι 4

CH ,

— — S i - [ C H - CH — ] / Δ ι Π

Vinyl polymers are produced on a silicic acid surface through similar reactions. The mechanism is as follows:

/°\

V

-Si-O-SiI

- S i - O-Si-

I

°,H

-Si-

«

-siι

I

soa2

si-

9

Si-

?

—

(1)

I

θ θ

N2C£

o

-Si-

I

I

° Si-

-

(o

-Si-

τ

-Si-O-SiI

NH2

o

•

°|H

°

I

O-Li

-Si—

N02

W θ N2C£

Φ

H2

-Si-

I

I

(3)

θ θ N2SR

-

9

-Si-

>

-si-

Φ"

+ N2

(4)

Mft

9

-Si-

(5)

Chapter

1 130

5

A p h e n y l group connected to the silicic acid surface b y covalent b o n d i n g i s formed b y reactions (1) and ( 2 ) . Polymerization i s initiated as a result of thermal decomposition of diazonium salts or such d e r i v a t i v e s as thioether, which can g i v e radicals. Grafting of s t y r e n e , acrylic acid, or vinlypyridine can be performed b y t h e s e reactions. T w e n t y to forty percent of the polymer molecules will be g r a f t e d to t h e s u r f a c e of silicic acid. There are many excellent s t u d i e s on graft reactions at carbon black s u r f a c e , i n c l u d i n g t h o s e b y Ohkita et al. [ 3 9 - 4 3 ] . Some typical graft reactions are shown below.

CBll

J - o· +

CBll

>

CH - CH 2

A

C00H + CH 2 = CH

CBll

Jr COO + CH 3 - CH

(8)

X CB|1

>"C00 + CH3—CH I X CB I

5.4.2

J — C

[ CH 2 -CH-}-CH 2 1 n-2 X X

|| O

CH I

(9)

- 0 -—f-CH- C H 2 — ] — CH - CH3 I n-l I X X

Copolymerization with Monomer Adsorbed on Inorganic Filler Surface

An acrylic a c i d / s t y r e n e copolymer i s formed with s t r o n g bonding to t h e s u r f a c e of titanium dioxide if acrylic acid or methacrylic acid adsorbed on titanium dioxide i s d i s p e r s e d in s t y r e n e monomers and polymerization i s initiated b y a benzoyl peroxide initiator, as follows [ 3 6 ] .

Modification

131

Fillers

of Inorganic

St

If sodium montmorillonite is u s e d , the compounds can be copolymerized naturally with butadiene, cis-2-buten, trans-2-butene, or propionamide without using initiators [44a]. Most of the resulting polymer molecules are connected to the surface and cannot be extracted [ 44]. Polymerization of acrylic acid, methyl acrylate, acrylonitrile, 4-vinyl pyridine, and styrene between the layers of montmorillonite can begin without the existence of an initiator. In this reaction, hydrogen bonding i s produced between hydrogen of the monomer and o x y g e n on the crystal surface or between oxygen of the former and a hydroxyl group on the latter, leading to a regular arrangement of the resulting molecules. It is reported that for montmorillonite, monomers penetrate the gaps between the lattice layers, forming polymer there, in three different ways, as shown in Figure 5.5 [45]. When hydroxyl groups on an inorganic filler surface are allowed to react with vinyl isocyanate, vinyl-substituted titanium dioxide or silicic acid will be formed, which can then undergo a copolymer!zation reaction with vinyl monomers such as styrene. It is possible to u s e such reactions to connect polymer molecules directly to an inorganic filler. For example, polyethylene oxide /isocyanate compounds are easily added to silica through the following reactions [ 46] : i - B u ( 0 - C H 2 - C H 2 ) x 0 H + OCNRNCO t-Bu(0-CH

Δ

—

(10)

-CH 0 ) OCONHRNCO Δ

X

f - B u ( 0 - C H - C H JJ OCONHRNCO + SiOH 2 22xχ

(11)

i—Bu(0 *CH *CH ) OCONHR—NHCOO—Si Δ

Δ

X

Polymerization can also be initiated at an inorganic filler surface b y irradiation of h i g h - e n e r g y light such as γ - r a y s . Such reactions have been studied for vinyl chloride, methyl methacrylate, and acrylonitrile on the surface of carbon black or magnesium oxide, and styrene and calcium carbonate on magnesium oxide, as well as styrene and methyl methacrylate on zinc oxide [36,47]. The mechanism of the γ-radiation-induced polymerization of methyl methacrylate on a glass powder surface is interpreted as follows:

Chapter

1 32

7}

9.3A

/ . Silicate lattice / / / / / / / i/

Ο

5

Ο

4-

Q

7.7k

A. Silicate lattice / /

/ / / / / / / / / / / /

9.3A

A Silicate l a t t i c e / V

j _

/ / / / / / / /

A / A t

TYPE 2

TYPE 1

, Silicate lattice / „

/ / / / / > / / / A TYPE 3 Figure 5.5 complexes.

Schematic representation of monomer-montmorillonite (Ref. 45.)

- Al—OH

Y

"ray

» - A l · + OH·

CH,

I

I

3

-Al· + CH = C 2 I I CO

I

OCH-

(12) CH„

I

I

3

~ -A1--CH.-C· 2 I I CO

I

OCH-

(13)

of Inorganic

Modification

CH q I I 3 —Al —CH —C· +nCH = 2 2 I I CO

I

OCH 3

I

Fillers

CH q I 3 =C I CO

133 CH q I I 3 — -Al-CH - C 2 I I CO

OCH 3

OCH 3

CH q I 3 -[-CH-C-] 2 I CO

I

I

°'CH3 (14)

5.5

SURFACE MODIFICATION OF INORGANIC FIBERS

In this section we deal with surface modification and adhesion of inorganic fibers, including glass fiber and carbon fiber, which are widely used for fiber-reinforced composite materials. 5.5.1

Surface Modification by Grafting and Adhesion Properties of Glass Fibers

Silanol and siloxane groups are likely to be the major functional groups on a glass fiber surface since glass consists mainly of S1O2. It is widely known that when heated, silanol groups decompose into siloxane groups, releasing water in the process, whereas siloxane groups formed at moderate temperatures will rehydrate to form silanol groups in the presence of water. Figure 5.6 shows the results of a study by Davydov et al. [48] , who investigated the effects of heat treatment on silanol concentration at a silica surface. Silanol groups existing at a glass surface are connected to water through hydrogen bonding. It has been reported that water molecules adsorbed to silanol groups in a ratio of 1:1 will remain strongly adsorbed even after thorough degassing [49]. Furthermore, the conditions of glass surfaces differ depending on the environment, particularly moisture. There generally exist one or more layers of free water, each having a thickness equal to the molecular diameter, which i s weakly adsorbed on the surface. The effects of such adsorbed water on the surface should be taken into account in considering the reactions of silanol and siloxane groups at a glass fiber surface. The reactivity of these functional groups is not discussed here because it was outlined earlier in the chapter and because Boehm's detailed report [50] is available. The adhesion between the glass fiber and matrix is the most important factor in relation to the properties of a glass fiber/polymer composite. Incomplete adhesion will cause deterioration of the mechanical, electrical, and moisture-resistant properties of the composite.

Chapter

1 134

5

3.0 α

ο I 2.5 -1

0.7 0.6 0.5 0.4 0.3 0.2

0.1 0.0 0

2 0 0 4 0 0 6 0 0 8 0 0 1000 1200

Figure 5 . 6 C u r v e s 1, total water loss due to thermal treatment of h y d r o x y l a t e silica g e l s VI(a) and V I I ( b ) . ο and measured v a l u e s . C u r v e s 2, content of surface structural water for silica g e l s VI I I I ( / - S i - O H + D.O and A - S i - O D + H O , e x c h a n g e at 150°; ( Δ - S i 2

I

I

2

I - O H + D n O , e x c h a n g e at 20°C) and VII ( A - S i - O H + D „ 0 ) , Δ

j

Δ

I de-

p e n d i n g on the temperature of treatment. Curves 3, content of inn e r structural water of silica gels VI and VII (obtained b y d i f f e r e n c e of ordinates corresponding to c u r v e s 1 and 2 ) . ( R e f . 48.)

Laird et al. [51] have reported that in fiber-reinforced polymer, the d i f f u s i o n rate of water along the interface between the glass fiber and polymer i s about 450 times greater than that through t h e resin l a y e r s , indicating the complete adhesion of glass fiber and polymer i s h i g h l y important. In general, the critical surface t e n s i o n , y c , at a chemically clean g l a s s surface i s greater than 80 d y n / c m , while it d e c r e a s e s to 20 to 40 dyn/cm when the s u r f a c e i s stained with organic material. Moreo v e r , it can d e c r e a s e to 28 and 46 d y n / c m , r e s p e c t i v e l y , in an atmosphere at relative humidity 95% and 1%, e v e n if there i s no organi c contamination (Table 5.11) [ 5 2 ] . Wetting of glass fiber by liquid polymer t e n d s to be incomplete and voids are o f t e n formed since the liquid polymers used for composites usually have a surface tension

Modification

of Inorganic

135

Fillers

Table 5.11 Spreading Behavior on Glass of Pure Liquids Having Negative and Positive Spreading Coefficients on Bulk Water a

Surfac Surfacee tension, tension ,

Contact angle θ ( d e g ) , or spreading behavior of drop on glass equilibrated at:

^LV ^LV (dy (dyn n /cm) /cm )

95% RH

65-53% RH

Methylene iodine

50.8

36

31

13

Τ etr abromoeth ane

47.5

36

9

9

1 -Methylnaphthalene

38.7

7

-

5

Li CH,

Bu —^ CH2— CH=C — C H 2 C H 2 - C H 2 0 C-C= CH

(3)

It was difficult to living-polymerize polar vinyl monomers and to introduce ended double bonds capable of being polymerized quantitatively. Recently, the following new polymerization technique has been developed. The synthesis of macromers having an introduction ratio of functional groups of about 100% has been realized [24,25]. CHO3 ι CH 2 =C-C0-CH. 3 ii 0

HF2 CH 3 . CHo

= /OSi(CH3)3 x

?H% CH3-C — C H COCH3 11 0

0CH,

2

C

, , u COCH3 11 0

/ OSi(CH 3 ) 3 CH2-C = C^ \ °"CH3

(4>

The reactivity of the ended double bond is often a problem for applications of the foregoing macromers. The data reported previously indicate that the reactivity of macromers is as high as that of the corresponding low-molecular-weight models [ 2 6 - 2 9 ] . Figure 6.5 shows a composition curve of copolymers of poly (methyl methacrylate) with stearyl methacrylate [20]. In general, it is thought that the macromer exhibits almost the same reactivity as that of the corresponding low-molecular-weight monomer, at least in the copolymerization reaction. The above-mentioned graft copolymers have interfacial activity. When blended with other polymeric solids, the graft copolymers are deposited and adsorbed to the surface of the solids. As one utilization of the phenomenon noted above, the surface of polymeric base materials can be modified and rendered a function by using a tailored graft copolymer. Figure 6. 6 shows the degree of surface modification of p o l y m e t h y l methacrylate) by measurement of the contact angle [29]. The surface

Adhesion

Improvement

by the Blend

165

Method

100

*

80 /

60

40

cy 20

20

J_

40

SMA in

JL

60

feed

80

J

100

(mol*)

Figure 6.5 Copolymer composition data for copolymerization of SMA with · , MM A, 9 - 2 2 wt % c o n v e r s i o n s ; o, PMMA macromonomer (Mn = 1650), 39 wt % conversion; o, PMMA_macromonomer (Mn = 4080), 61 wt % conversion; macromonomer (Mn = 4080), 35 wt % conversion. ( R e f . 20.)

of this poly(methyl methacrylate) was modified u s i n g a graft copolymer of which the backbone of which i s composed of poly(fluoroalkyl acrylate) (which has a water-repelling function) and the branch parts of poly(methyl methacrylate) macromer. That i s , a small amount of t h e graft copolymer was added to poly (methyl methacrylate), dissolved i n b e n z e n e , and formed into a film. The contact angles between water and the film s u r f a c e , which i s in contact the air and the g l a s s , were measured. The r e s u l t s h a v e revealed that the air-side surface of the poly(methyl methacrylate) film i s modified nearly to the water repellency of fluorocarbons by the addition of 0.2% graft copolymer. On the other h a n d , random copolymers with the same composition as t h e block copolymers have relatively l e s s e f f e c t on the surface modification [ 3 0 , 3 1 ] . Such surface modifications can also be achieved by u s i n g graft copolymers that contain hydrophobic segments as the branch. For example, small amounts of graft copolymers containing poly (dimethyl

0.4 0.2

^ *

Contact wi th Contact air

Λ

GF-2 glass

with

ο Ο

-0.2

-0.4 0.01

j

L 0.1

-j

L 1

Macromonomer

10

100

(wt*)

Figure 6.6 Surface modification of poly(methyl methacrylate) b y poly(fluoroalkyl acrylate)-methyl methacrylate macromonomer. (Ref. 29.)

0.4-

(/) Ο Ο 0 -0.1 -0.2 _i 0.01

L 0.1

1.0 Siloxane content (wt%)

166

1 0 15 in the blend

Adhesion

Improvement

by the Blend

Method

167

0.4V

ίΑ Δ

S

0

X4 -0.1

A

A—A °Z=

"Π

•

-0.2L

10

Siloxane content (wt%) in the blend Figure 6.8 Contact angle of water droplet at 20°C on the air-side s u r f a c e of various silicone polymer-PMMA blend films after treatment with n-hexane. (For an explanation of t h e symbols, and for contact angle values before n - h e x a n e treatment, see Figure 6 . 7 . ) ( R e f . 31.)

siloxane) macromer as the branch were added to poly(methyl methacrylate) and formed into a film on a glass plate. Figure 6.7 s h o w s values for contact angles of both t h e air-cont a c t side and the glass-contact side film surface [ 3 1 ] . The measurement r e s u l t s reveal that the surface of poly(methacrylate) i s rendere d as hydrophobic as poly (dimethyl siloxane) for a graft polymer addition amount of about 1%. The durability of such surface modificat i o n s was examined by surface treatment with n - h e x a n e , which is a good solvent for poly(dimethyl s i l o x a n e ) . No reductions in the s u r f a c e hydrophobic property of the film were found.

Figure 6 . 7 Contact angle of water droplet at 20°C f o r various silicone polymer-PMMA binary blends ( A , air-side s u r f a c e ; G, g l a s s s i d e s u r f a c e ) . The symbols r e p r e s e n t the following silicone polymers: o , GM-211; · , GM-213; • , GM-411; • , GM-413; • , PDMS; v, PMTS; a , MTS-45; A MTS-25. ( R e f . 31.)

Chapter

1 168

5

In the c a s e of systems to which random copolymer or homopolymer was added, t h e reduction in contact angle was remarkable. Figu r e 6 . 8 shows the t e s t r e s u l t s . It i s generally understood that s u c h d i f f e r e n c e s in durability are caused b y t h e functions of the poly(methyl methacrylate) backbone constituent, which dissolved into t h e b a s e poly (methyl methacrylate) as an anchor. Only recently have attempts to modify solid s u r f a c e s b y utilizing t h e surface activities of block and graft copolymers been made. T h e r e are few applications in t h e composite material f i e l d s . Typicall y , the substitutional u s e of a polar group containing block and g r a f t copolymers for silane coupling a g e n t s , improvement in t h e i n terfacial adhesion of matrix r e s i n s by addition of t h e s e block and g r a f t copolymers, t h e n deposition of the copolymers to the resin s u r f a c e , and s o o n , are s u g g e s t e d .

6.3

ADHESION IMPROVEMENT BY TRIAZINE THIOLS BLEND

6.3.1

Reactions of Triazine Thiols

Many s t u d i e s on c r o s s - l i n k i n g adhesion between different materials b y u s i n g triazine thiols have recently been made [ 3 2 ] . With regard t o obtaining h i g h - q u a l i t y composite materials, blending of triazine thiols with filler-matrix s y s t e m s is of wide interest as one method o f enhancing interaction between the filler and t h e matrix. Table 6 . 3 l i s t s typical triazine thiols (the abbreviations are designated in p a r e n t h e s e s . ) The various reactions of triazine thiols previously r e p o r t e d are illustrated below. TSS +

I χ halogenated

MgO

+ MgX2

TSS

rubber MgO

"T" S τ S

(5)

In c o e x i s t e n c e with metal o x i d e s such as MgO, triazine thiols f u n c tion as a c r o s s - l i n k i n g agent for halogen-containing polymers [ 3 3 ] .

TSS + — f CH9-CH9 A— +R00R

^

PE

>

S

J

(6) +

2R H

°

Adhesion

Improvement

by the Blend

Method

169

In coexistence with peroxides, triazine thiols function as a crosslinking agent for polyethylene, butyl rubber, and so on.

s - ch2-ch(oh)*cn2K > Τ S ~ ch2'CH(0H)-CH2R

TSS + CH2-CH-CH2-R V

TSS + -OCOCH=CHCOO-

>

-OCO· CH'CH, 2 •COOI s Τ s -OCO· CH ·CH,; COO-

TSS + MO(M)

>

TSS-M-SST +H20

(7)

(8)

(9)

Reaction formulas (7) and (8) express the functions of triazine thiols as a curing or cross-linking agent for epoxy compounds and unsaturated polyesters. Formula (9) expresses the reaction of triazine thiols with metal oxides [ 3 4 , 3 5 ] . Triazine thiols have a high d e g r e e of reactivity with many polymeric, inorganic, and metallic materials which implies that triazine thiols are useful for the surface modification of composite materials.

Table 6.3

Properties of Triazine Thiols

R N ^ N HS

SM Μ

-R

PK a

F(TT)

-SH

H(Na)

6.5

AF(AN)

-NHC6H5

H(Na)

5.5

DB(DBN)

-N(C4H9)2

H(Na)

4.1

DA

= -N(CH2CH CH2)2

Η

4.2

OL

-NHC18H35

Na

Source:

Ref. 32.

Chapter

1 170 6.3.2

5

Modification of Polymers b y Triazine Thiols for Adhesion

The bonding of polymers to other polymers i s dependent on t h e solubility parameters and reactivities of the polymers u s e d , whether or not bonding i s possible. From the viewpoint of solubility parameters and reactivity, polymers are grouped as follows: (1) the solubility parameters and reactivities are both different; (2) either the solubili t y parameters or the reactivities are different; or (3) the solubility parameters and reactivities are about the same. In case 1, where the polymers are bonded with most difficulty, typically in bonding of poly (vinyl chloride) (PVC) to an ethylenepropylene-diene terpolymer (EPDM), PVC is added with an appropriate plasticizer to adjust the apparent solubility parameter to close to that of EPDM, and also with triazine thiols, so that a strong adhesion between PVC and EPDM can be obtained. Table 6.4 illustrates t h e addition e f f e c t s of the triazine thiols used for adhesion of PVC to various rubbery polymers [ 36]. The added chloranil functions as an accelerator of the reaction between the thiol groups of the triazine thiols and the EPDM molecules [37]. In bonding of PVC to polyethylene (PE), a PVC sheet containing a triazine thiol and MgO was overlaid on a polyethylene sheet containing a triazine thiol and a peroxide, and then h o t - p r e s s e d , so that the PVC-PE sheets were strongly bonded, which is expressed b y reaction formula ( 1 0 ) . Figure 6.9 shows the test results [38,39].

PVC MgO

DA

PVC

PVC-

SyN^S Ν Ν \

Ν I

ch2-ch=ch2

δ Η

PE Peroxide DA

γ - Ν γ SH V

N

PVC

PVC SyNyS Ν Ν V

/ \ CH2 ch2 I I PE-CH CH - PE ch/

CHo CH? I I PE — CH CH — PE \N2X

(10)

Generally, fluororubbers are difficultly bonded polymeric materials. However, fluororubbers containing a triazine thiol and a tetraonium salt in an appropriate combination can be bonded sufficiently well to

Adhesion

Improvement

Table 6.4

Compound15

by the Blend

Method

171

Vulcanizing Adhesion of PVC to Rubber a Triazine trithiol in PVC compound 0 (phr)

1

—

3 -

3 2

—

3 -

3 3

-

3 -

3 4

—

3 -

3 5

-

3

6

-

3

7

—

8

—

3 3 -

3 sp value of PVC;

Rubber and additives Rubber

sp Value

Chloranil

EPDM EPDM EPDM EPDM

7.9

IR IR IR IR

7.9

BR BR BR BR

8.4

SBR SBR SBR SBR

8.6

CIR CIR

8.0

CO CO

9.4

CR CR

9.4

—

NBR NBR NBR NBR

9.6

—

—

-

1 1 —

-

1 1 -

1 1 — -

1 1 —

-

—

-

9.6; press conditions:

1 -

1 1

Peel strength (kN/m) 0.3 2.0 0.3 3.0 0.5 1.2 0.8 2.4 0.3 4.8 0.6 5.7 0.6 4.2 0.8 6.6 0.8 3.4 0.6 9.6 1.2 10.4 4.0 7.2 4.7 14.5

170°C χ 30 min.

^Rubber compound: 1. EPDM 100 parts, SRF black 50 phr, oil 30 phr, MBTS 2 phr, sulfur 0.5 phr, ZnO 5 phr, St 1 phr, chloranil 0 or 1 phr. 2. IR 100 parts, SRF black 50 phr, MBTS 2 phr, sulfur 0.5 p h r , ZnO 5 phr, St 1 phr, chloranil 0 or 1 phr. 3. BR 100 parts, SRF black 50 phr, MBTS 2 phr, sulfur 0.5 phr, ZnO 5 phr, St 1 phr, chloranil 0 or 1 phr.

Chapter

1 172 Table 6 . 4 4.

5

(Continued)

SBR 100 p a r t s , SRF black 50 p h r , MBTS 2 p h r , sulfur 0 . 5 p h r , ZnO 5 p h r , St 1 p h r , chloranil 0 or 1 p h r . CIR 100 p a r t s , SRF black 50 p h r , 6 - d i b u t y l a m i n o - l , 3 , 5 - t r i a z i n e 2,4-dithiol 3 p h r , MgO 5 p h r , St 1 p h r . CO 100 p a r t s , SRF black 50 p h r , triazine trithiol 2 p h r , MgO 3 p h r , CaC03 5 p h r . CR 100 p a r t s , SRF black 50 p h r , triazine trithiol 2 p h r , MgO 5 p h r , ZnO 5 p h r , St 1 p h r . NBR 100 p a r t s , SRF black 50 p h r , oil 10 p h r , MBTS 2 p h r , s u l f u r 0 . 5 p h r , ZnO 5 p h r , St 1 p h r .

5. 6. 7. 8. C

PVC compound: PVC 100 p a r t s , DOP 50 p h r , triazine trithiol 0 or 3 p h r , MgO 5 p h r , Sta. 1 p h r . Source: R e f . 36.

N(CH2CH=CH>2

φ Φ

RP-101 1t

(L 0

Ο

0.5

DCP / DAAT Figure 6 . 9

C r o s s - l i n k i n g adhesion between PVC and PE.

(Ref.

36.)

Adhesion

Improvement

by the Blend

Method

173

Carbon number of alkyl chain Figure 6.10 Influence of carbon number of (C n H2n + 1)4 NBr on the peel strength of adherends. o, cross-linking adherends of FR/ NBR; · , cocross-linking adherends of FR/CHR. Adhesion conditions: 160°C, 30 min under 5 MPa and postcure 180°C for 20 min. (Ref. 41.)

polar rubbers such as butadiene-nitrile copolymeric rubbers (NBR), epichlorohydrin rubbers, chlorinated rubbers, chloroprene rubbers ( C R ) , and the like. Here the triazine thiol functions as a crosslinking agent, and the tetraonium salt acts as a catalyst for activati n g the thiol groups [ 4 0 , 4 1 ] . Also, the structure of the ammonium salt exerts a strong influence on the peeling resistance of the bonded sheets. Figure 6.10 shows the e f f e c t s of alkyl groups of tetraalkyl ammonium bromide with different carbon numbers on the adhesion of a fluororubber to NBR, a chlorohydrin-ethyleneoxide copolymeric rubber (CHR). The results support the suggestion that the basicity and migration capability of the ammonium salt are related to the reaction between the fluororubber and polymeric rubbers at the interf a c e . As shown in Table 6.5, PVC-fiber materials with high bonding

Chapter

1 174

5

Table 6.5 Effect of Fabrics 8 on Peel Strength in Adhesion of PVC^ Sheets to the Fabrics Peel strength 0 ( k g / 3 cm)

Denier

Fabrics Nylon 6,6

840

2.1 (0.2)

Nylon 6

420 110

3.5 ( 0 . 4 ) 3.1 ( 0 . 4 )

1000 100 75

1.2 ( 0 . 1 ) 2.0 ( 0 . 3 ) 1.3 ( 0 . 2 )

Polyester

a

Epoxy treatment: Epoxy G-100 5 g , methanol 100 ml, 20°C χ 5 min, heat treatment for 30 min at 165°C. b PVC plastisol: Zeon 121 100 p a r t s , DOP 60 phr, DB 3 p h r , MgO 5 phr. Adhesion condition: 180°C χ 20 min. Q (—) Blank values in parentheses. Source: Ref. 45.

Γ

SH N^N

in in

niif

'/Cu-Zn Plate/, / / / / /

/ / / / ' '

EPDM Compound

Figure 6.11

N

S-^ff^S

hs-^^SH Trtaine Trithiol (TT)

I

Μ

/ Cu^Cu/Cu 7 C u / C u / V κ Cu-Zn Plate treated by τ τ

/ y

EPDM Vulcanization

X M^M

Ν

w Λ

Schematic diagram of cross-linking model.

(Ref.

36.)

Solvso MeOH

0.3

0.3

TT

TT —Na

^ High-olefin oil ( b p , 1 5 0 - 2 40°C). Source: Ref. 42.

°Polyethylene glycol (MW = 300).

kpeel s t r e n g t h a f t e r heat treatment.

Peel s t r e n g t h before heat treatment.

60

130

15

60 d

MeOH

0.3

TT

2.0

5

80

H2O

0.3

TT — Na

2.6

5

80

H2O

0.3

DB—Na

4.0

5

80

H2O

0.3

AN—Na

4.1

15

PEG

2.5 3.2

15 15

3.4

4.1

130

1.0

TT

15

3.0

PS ah ahaa

130

0.3

TT

PEG

0.03

TT

15

Time (min)

C

Temperature (°C) 130

Solvent

C

Concentration (%)

8.0

8.9

6.5

2.1

7.0

8.9

8.8

9.4

7.2

PSbhb

Pee Peell s t r e n g t h (kN/m)

PEGC

Triazin Triazinee thio thioll

Τ reatment conditions

E f f e c t s of Triazine Thiols and Solvents on the Peel S t r e n g t h in the Vulcanizing Adhesion of Table 6 . 6 EPDM to Brass Plates Treated with Triazine Thiol Solutions

cn

i a

3 a S CD

ct> to

c

ι Ό s «> 3 a> 3

8" CO δ'

Chapter

1 176

5

Table 6.7 Cross-Linked Adhesion of Metal and Rubber Containing Triazine Thiols Compound (phr)

°C x min

Peel strength (kN/m)

Cu/CHR

DB 1, AF :L, MgO 5, FEF 40

160 X 40

10.6

Cu/CHC

F 0.5, AF 1, C a C 0 3 10, FEF 40

170 X 30

12.6

Cu/SBR

F 3, MBTS 3, S 0.5, St 1, ZnO 5

160 X 20

17.0

Cu—Ni/SBR

F 3, MBTS 3, S 0.5, St 1, ZnO 5

160 X 20

8.5

Cu—Sn/SBR

F 3, MBTS 3, S 0.5, St 1, ZnO 5

160 X 30

12.7

Cu/NBR

F 3, MBTS 3, S 0 . 5 , St 1, ZnO 5

170 X 30

12.0

Cu/EDPM

F 3, MBTS 3, S 0 . 5 , St 1, ZnO 5

180 X 30

10.6

Cu/CR

F 3, MBTS 3, S 0.5, St 1, ZnO 5

170 X 20

7.6

Cu/PE

DCP 3

160 X 30

9.0

Cu/PVC

F 3, PEG 5, MgO 3, RP 101 DOP 30, FEF 10

180 X 20

9.8

Metal/rubber

Source:

Ref. 32.

s t r e n g t h s were obtained by heat-pressing fibers of fabrics treated with an epoxy compound against PVC containing a triazine thiol [ 45]. 6.3.3

Adhesion of Polymers to Metals Utilizing Triazine Thiols

Triazine thiols are able to react with metals and metal oxides and form specific surface films as mentioned above [formula ( 9 ) ] . When a metal material was dipped into a triazine thiol solution, an organic film dozens to several hundred of micrometers thick can be formed. This film has characteristics such as high resistance to heat and water, and inclusion of functional groups such as thiol, disulfide,

Adhesion

Improvement

by the Blend

Method

177

mercaptide g r o u p s , and s o o n , which are active for the polymer [ 4 2 ] , A c c o r d i n g l y , when a polymeric sheet i s overlaid on the organic film and h e a t e d , t h e polymer molecules are d i f f u s e d into the film layer and react with the active functional g r o u p s to form a s t r o n g adhesion l a y e r . Figure 6.11 shows a model of t h e adhesion mechanism [ 4 4 ] . Table 6 . 6 lists the e f f e c t s of treatment condition on the adhesion b e t w e e n a triazine thiol-treated b r a s s board and EPDM. The peeling s t r e n g t h i s considerably enhanced b y t h e heat treatment. Such improvements in adhesion between a b r a s s plate and a polymer can be obtained without triazine thiol treatment of t h e b r a s s plate simply b y adding a triazine thiol to a polymer and bonding the polymer to an untreated brass plate [ 4 3 , 4 4 ] . Table 6.7 shows the test r e s u l t s . T h e s e results support the mechanism through which the triazine thiol molecules contained in the polymer are d i f f u s e d to the interface between t h e polymer and the b r a s s s h e e t , and react with t h e b r a s s surface to form such a film.

REFERENCES 1.

D . J . Meier, Theory of Block Copolymers: I. Domain Formation in A—Β Block Copolymers, J. Polym. S c i . , C26, 81 (1969).

2.

E. Helfand and Z. R. Wasserman, Block Copolymer T h e o r y : 4. Narrow Interphase Approximation, Macromolecules, 9, 879 (1976).

3.

L. Leibler, Theory of Microphase Separation in Block Copolymers, Macromolecules, 13, 1602 (1980).

4.

Τ . Ono, H. Minamiguchi, T . S o e n , and H. Kawai, Domain Structure and Viscoelastic Properties of Graft Copolymer, Kolloid. Z. Z. Polym., 250, 394 (1972).

5.

H. Hasegawa and T . Hashimoto, Morphology of Block Polymers Near a Free S u r f a c e , Macromolecules, 18, 589 (1985).

6.

T . Hashimoto, Y. Tsukahara, K. Tachi, and H. Kawai, Domain Boundary Mixing and Mixing in Domain" E f f e c t s on Microdomain Morphology and Linear Dynamic Mechanical R e s p o n s e , Macromolecules, 16, 648 (1983).

7.

Y. Tsukahara, N . Nakamura, T . Hashimoto, and H. Kawai, Structure and Properties of Tapered Block Polymers of S t y r e n e and I s o p r e n e , Polym. J . , 12, 455 (1980).

8.

A . G. Kanellopoulos and M. J . Owen, The Adsorption of Polydimethylsiloxane Polyether ABA Block Copolymers at the

1 178

Chapter Water/Air and Water /Silicone Fluid Interface, J. Colloid face Sci., 35, 120 (1971).

9.

5

Inter-

M. Kawaguchi and A. Takahashi, Ellipsometric Study of the Adsorption of Comb-Branched Polystyrene onto a Metal Surface, J. Polym. Sci. Polym. Phys. Ed., 18, 943 (1980).

10.

M. J. Owen and T. C. Kendrick, Surface Activity of Poly sty rene-Polysiloxane-Polystyrene ABA Block Copolymers, Macromolecules, 3, 458 (1970).

11.

D. H. Napper, Flocculation Studies of Sterically Stabilized Dispersions, J. Colloid Interface Sci.t 32, 106 (1970).

12.

J. V. Daw kins and G. Taylor, Nonaqueous Polymethylmethacrylate Dispersion, Radical Dispersion Polymerization in the Presence of AB Block Copolymers of Polystyrene and Polydimethylsiloxane, Polymer, 20, 599 (1979).

13.

J. V. Daw kins and G. Taylor, Micelle Formation by AB Block Copolymers of Polystyrene and Polydimethylsiloxane in n-Alkans, Makromol. Chem., 180, 1737 (1979).

14.

D. G. LeGrand and G. L. Gaines, J r . , Surface Activity of Block Copolymers of Dimethylsiloxane and Bisphenol-A Carbonate in Polycarbonate, Polym. Prepr. Am. Chem. Soc. Div. Polym. Chem., 11, 442 (1970).

15.

M. J . Owen and J. Thompson, Siloxane Modification of Polyamides, Br. Polym. J., 4, 297 (1972).

16a.

G. L. Gains, J r . , and G. W. Bender, Surface Concentration of a Styrene-Dimethylsiloxane Block Copolymer in Mixtures with Polystyrene, Macromolecules, 5, 82 (1972).

16b.

G. L. Gains, J r . , and G. W. Bender, Surface Studies on Multicomponent Polymer Systems by X-Ray Photoelectron Spectros c o p y , Macromolecules, 12, 1011 (1979).

17.

A. Takahashi, H. Wakabayashi, K. Honda and T . Kato, Wettability and Composition of Styrene-Tetrahydrofuran Block Copolymers, Kobunshi Ronbunshu, 35, 269 (1978).

18a.

Y. Yamashita, Surface Properties of Styrene-Tetrahydrofuran Block Copolymers, J. Macromol. Sci. Chem. Ed., A13, 401 (1979).

18b.

A. Takahashi and Y. Yamashita, Morphology, Crystallization, and Surface Properties of Styrene-Tetrahydrofuran Block Polymers, Adv. Chem., 142, 267 (1975).

Adhesion

Improvement

by the Blend

Method

179

19.

Κ. K. Roy, D. Pramanick, and S. R. Palit, Application of Dye Techniques in the Study of Chain-Transfer Properties of Thiols, Makromol Chem,, 153, 71 (1972).

20.

K. Ito, N. Usami, and Y. Yamashita, Syntheses of Methyl Methacrylate-Stearyl Methacrylate Graft Copolymers and Characterization by Inverse Gas Chromatography, Macromolecules, 13, 216 (1980).

21.

Κ. E. J. Barrett, Dispersion Polymerization in Organic Media, John Wiley & Sons L t d . , Chichester, West S u s s e x , England, 1975.

22.

B. W. Jackson, U . S . Pat. 3,689,593 (1972).

23.

G. O. Shulz and R. Milkovich, Graft Polymers with Macromonomers: I. Synthesis from Methacrylate-Terminated Polystyrene, J. Appl. Polym. Sci., 27, 4773 (1982).

24.

O. W. Webster, W. R. Hertler, D. Y. Sogah, W. B. Farnham, and Τ. V. RajanBabu, Group-Transfer Polymerization: I. A New Concept for Addition Polymerization with Organosilicon Initiator, J. Am. Chem. Soc., 105, 5706 (1983).

25.

D. Y. Sogah and O. W. Webster, Telechelic Polymers by Group Transfer Polymerization, J. Polym. Sci. Polym. Lett. Ed., 21, 927 (1983).

26.

Y. Yamashita, Synthesis of Anphiphilic Graftcopolymers from Polystyrene Macromonomer, Polym. J., 14, 255 (1982).

27.

M. Maeda, Y. Nitadori, and T. Tsuruta, Synthesis of New Monomers Having a Primary Amino Group by Lithium Alkylamide Catalyzed Addition Reaction of N-Alkylethylenediamines with 1,4-Divinylbenzene, Makromol. Chem., 181, 2251 (1980).

28.

A. Revillon and T. Hamaide, Macromer Copolymerization Reactivity Ratio Determined by GPC Analysis, Polym. Bull., 6, 235 (1982).

29.

Y. Yamashita, Y. Tsukahara, K. Ito, M. Okada, and Y. Tajima, Synthesis and Application of Fluorine Containing Graftcopolymers, Polym. Bull., 5, 335 (1981).

30.

Y. Yamashita and Y. Tsukahara, Modification of Polymer by Tailoerd Graft Copolymers, Polym. Sci. Technol., 21, 131 (1983).

31.

Y. Kawakami, R. A. Murthy, and Y. Yamashita, Surface Active Properties of Silicone Containing Polymers, Polym. Bull., 10, 368 (1983).

32.

K. Mori and Y. Nakamura, Stabilization of Interface of Different Materials, Surf. Jpn., 23, 709 (1985).

Chapter

1 180

5

33.

Κ. Mori and Y. Nakamura, Crosslinking of Halogen-Containing Rubbers with Triazine Dithiols, Rubber Chem. Technol., 57, 34 (1984).

34.

K. Mori and Y. Nakamura, Action of Triazine Thiols and Their Metal Salts to Peroxide, J. Appl. Polym. Sci., 26, 691 (1983).

35.

K. Mori, Functionality of Metal Surface, Denki Kagaku, (1986).

36.

K. Mori and Y. Nakamura, Crosslinking Adhesion Between Different Materials, J. Adhes. Soc. Jpn., 19, 382 (1983).

37.

K. Mori and Y. Nakamura, Adhesion of Rubbers to Polyvinyl chloride During Vulcanization, Plast. Rubber Process. Appl., 3, 17 (1983).

38.

Y. Nakamura, K. Mori, and H. Nishina, Improvement of Adhesion Properties and Compatibility Between Polyethylene and Polyvinylchloride by Crosslinking, J. Adhes. Soc. Jpn., 21, 95 (1985).

39.

Y. Nakamura, K. Mori, Y. Yoshida, and K. Tamura, Improvement of Adhesion and Compatibility between Polyethylene and Polyvinylchloride by Crosslinking, Kobunshi Ronbunshu, 41, 531 (1984).

40.

Y. Nakamura, K. Mori, and K. Wada, Crosslinking Adhesion of Fluoro Rubber to Nithile of Epichlorohydrin Rubber, J. Soc. Rubber Ind. Jpn., 57, 561 (1984).

41.

Y. Nakamura, K. Mori, and K. Wada, Improvement of Some Ρ Properties of Fluorine-Containing Crosslinked Blend Rubber by Crosslinking, Kobunshi Ronbunshu, 41, 539 (1984).

42.

K. Mori, Y. Nakamura, M. Shida, and I. Nishiwaki, Vulcanizing Adhesion of EPDM on Surface Treated Brass Plate by Triazine Thiols, J. Soc. Rubber Ind. Jpn., 57, 376 (1984).

43.

Y. Nakamura, M. Saito, K. Mori, and Y. Asabe, Coupling Effects of Trithiocyanuric Acid for Vulcanizing Adhesion of Styrene-Butadiene Rubber on Copper Plate, Kobunshi Ronbunshu, 37, 389 (1980).

44.

Y. Nakamura, K. Mori, and K. Tamura, Coupling Effect of 6-Dibutylamino-1,3,5-triazine-2,4-dithiol for Adhesion of Polyethylene on Copper Plate, J. Adhes. Soc. Jpn., 17, 308 (1981).

45.

K. Mori and Y. Nakamura, Adhesion of Soft PVC Sheets to Nylon Fabrics, Kobunshi Ronbunshu, 35, 375 (1978).

46.

Y. Tsukahara, K. Kohno, H. Inoue, and Y. Yamashita, Surface Modification of Polymer Solids by Graft Copolymers, Chem. Soc. Jpn., 6, 1070 (1985).

54, 96

7

Adhesion of Resin to Metal

7.1

INTRODUCTION

The surfaces of solids generally adsorb C 0 2 , 02» N 2 , and H 2 0 from the atmosphere and are frequently covered with molecular films of fats and oils. When such a metal is mechanically polished or cleaned with an organic solvent for degreasing purposes, wetting of the adh e s i v e , or adhesion, can be improved. Although the contact angle between the metal surface and water just after cleaning is approximately 70 to 90°, showing lipophilic characteristics, an oxidized film i s produced on it in an atmosphere that is hydrophilic in nature [ 1 ] . For instance, an oxide layer such as F e 2 0 3 and F e 3 0 4 i s produced on the surface of iron. Such a layer spontaneously developes r u s t , which prevents adhesion because it is brittle and contaminated. The rusty surface of metal is therefore mechanically polished or pickled to form continuous, active, tough oxide film by further surface treatment. Recently, quantitative measurement of the adhesion properties of polymer films to metals has also progressed [ 2 , 3 ] . 7.2 7.2.1

ADHESION OF PLASTIC TO METAL Metal Surface and Adhesion

Levine et al. [4a,4b] investigated the contact angle, roughness of s u r f a c e , and adhesive strength under shear by binding epoxy resin t o various surface-treated steels. The results, shown in Table 7.1, reveal that the contact angle is always reduced, and consequently the adhesive strength is increased, by treating the steel with a degreasing solvent. By treating the steel with chromic acid of pH 0.1 or less or 50% HNO3 for 10 s , the contact angle i s reduced to 30 to 38° and the surface roughness becomes 10 to 20 ym. The adhesive s t r e n g t h , however, ranges from 104 to 130% of the base strength of steel treated with trichloroethylene. 181

Chapter

1 182

5

Table 7.1 Relationship Between Surface Treatment, Contact Angle and Tensile Shear in an Epoxy Adhesive System a Contact angle (deg)

Surface treatment

Tensile Shear (psi)

Surface roughness (Min.)

Effect of Solvent Treatment As received* 5

77

500

10-15

Standard solvent cleaned (Trichloraethylene)

42

1940

10- 15

Toluene

59

1795

10-15

Heptane

51

1800

10-15

Methyl ethyl ketone

47

1805

10-15

Ethyl acetate

43

1945

10-15

35 34 34

2140 2210 2190

10-15 10-15 10-15

Additional Solvents

Methyl chloroform + treatment in ultrasonic vibrator 5 min 13 min 20 min

Effect of Mechanical Treatment 0 Sisal buffing (30 s)

42

2000

5

Polishing (30 s + Sisal 1 min)

42

1980

5

Polishing (1 min)

41

2100

10

B u f f i n g ( 1 min)

44

2000

1 0 - 15

Grit blasting

36

2425

8 0 - 100

Effect of Chemical T r e a t m e n t Chromic acid:

pH 0 . 6 - 0 . 8

42

1960

10- 20

Chromic acid:

pH < 0 . 1

38

2170

10- 20

38 37 35

2008 2128 2215

10- 20 10- 20 10-20

Hydrochronic acid diluted to 50% with water 1.0 min 3.5 min 7.5 min

Adhesion Table 7.1

of Resin

to Metal

183

(Continued) Contact angle (deg)

Surface treatment

Tensile shear (psi)

Surface roughness (Uin.)

Effect of Chemical Treatment^ Nitric acid diluted to 50% with water 1 s 5 s 10s

38 35 35

2320 2500 2545

10-20 10-20 10-20

Sulfuric acid sodium dichromate etch 0.5 min 2.0 min 5.0 min

34 34 34

2060 2020 2100

25-30 40-50 60-70

Hydrofloric acid 1.0 min 2.5 min

29 29

2170 2240

10-15 10-15

36

2320

10-20

Alkaline etch pH 12.6, 180°F, 10 min

Adhesive; Epoxy resin (Epon 828)/methane diamine/Versamide 115 = 100:22.5:20 b A d h e r e n d ; Steel (SAE 1010), 10 ^ 15 yin surface finished, stored i n trichloroethylene for 4 - 5 h at room t e m p . , hand wiped and rinsed i n clean trichloraethylene, dried with a hot air gun. Q Standard solvent-cleaned followed by mechanical treatment and again solvent cleaning. ^Standard solvent-cleaned followed by chemical treatment rinsing with water and drying with hot-air gun. Source:Ref. 4a.

Chapter

1 184

5

Upon dipping aluminum into water, β — A^Og'Si^O (bayerite) and α—ΑΙ2Ο3Ή2Ο (boehmite) are produced at temperatures, respectively, of below 160°F (N71°C) and above 160°F. It is said that a stable amorphous oxide layer, Ύ—Al 2 03, i s transformed into extremely adh e s i v e β — Al2C>3-3H20 by treating the surface with a mixture of chromic acid and sulfuric acid or by anodizing in phosphoric acid and chromic acid solutions. The microscopically rough structure of the dense oxide layer formed by these treating methods develops a high mechanical bonding strength with resin, including epoxy resin. In addition, it improves the wetness of the polymer material by removing impurities such as hydrocarbon adsorbed on the metal surface. The OH group on the oxide surface is chemically bonded to the polymer material. It i s believed, for example, that an extra epoxy group in the epoxy resin causes the following reaction in the presence of a proper catalyst [5] : AZ-0 A£-0H

CH.-CH-R /

Ψ

0H-CH2-+CH-R

A£-0-CH-R

1

CH2-OH A£-0~

HO-CH-R

1

+

CH 2

A£-0-CH2-CH-R

1

OH

Moisture in the atmosphere acts on the oxide layer to produce flaky aluminum hydroxide (AI2O3· 2 H 2 0 ) , which l e s s e n s adhesion to t h e Al substrate, as shown in Figure 7.1. It has been found that nitrile trismethylene phosphate (NTMP) is effective as an inhibitor of that reaction. NTMP i s adsorbed on the surface of the oxide from t h e aqueous solution and a Ρ—Ο—Al bond is formed by substituting a hydroxyl group as shown in Figure 7.2 [ 6 ] . Mechanical bonding of the oxide layer with polymer material, chemical bonding on the interface, and stability of the oxide layer are required to increase the adhesive strength and durability of the polymer material bound to the metal surface. The surface of copper is generally covered with an oxide film, CuO, in the atmosphere and the sublayer i s the diffusion layer of C u 2 0 . Since C u 2 0 contributes to adhesion, it i s necessary to form an active film of C u 2 0 by any proper surface treatment method. Vaziranfs surface-treating method [7] is as follows. Copper is etched i n an acidic bath of concentrated phosphoric acid/concentrated nitric acid/water (75:10:15) for 30 s , or low-copper alloy is dipped into 2 0% NaOH aqueous solution at 200°F (%93°C) for 5 min. The low-

of Resin

Adhesion

185

to Metal

Figure 7.1 Schematic drawing of the mechanism deduced from crack propagation during wedge t e s t i n g . In a humid environment, the original oxide is converted to a hydroxide that adheres poorly to t h e aluminum substrate. The crack propagation rate is faster here than in a dry atmosphere, where the crack propagates directly through the adhesive. (Ref. 6.) copper alloy is treated in an aqueous solution containing 1% each of permanganate and NaOH at 200°F for 5 min accompanied by washing with water between each treatment. The result is copper covered with a continuous, mechanically tough, thin (500 to 1000 A) oxide film without corrosion. Adhesives that attack copper reduce the durability of adhesion. For instance, a certain adhesive of silicon-RTV

Η

Η

0

Η

0

0.

0

Γ » ϊ - ΑΙ - 0 - ΑΙ - 0 - Al

+

—

? „0Η2 0 - Ρ - CH2 - Ν 0-

"CH 2

Aluminum oxide surface

0 0

/

* P

CH

/

/

x

0

Ν

\

q-

CH0 0

/

\

ΡN

^0 Q-

0 P

x

6 0H0

1 1 1 I 0 - A l - 0 - A 1 - 0 - A 1 - 0 — Al Figure 7.2 Model for the adsorption of NTMP onto aluminum oxide s u r f a c e s . The deprotonated NTMP molecule replaces adsorbed h y droxyl ions, resulting in Ρ—Ο—Al bonding. (Ref. 6.)

Chapter

1 186

5

silicon-RTV evidences acetic acid at curing, and ethylene-acrylate copolymer produces a salt with copper under certain conditions. To prevent the copper from corroding, after acidic bath etching as described above, electrolytic oxidation i s carried out in an electrolyte composed of 3% each sodium chromate, sodium citrate, and sodium carbonate at a current density of 10 A / f t 2 for 1 min using a stainl e s s steel anode and a cathode of the copper being treated. B y this method, uniform, corrosion-resistant chromium oxide film as thin as approximately 25 A containing as little as 1.0 y g Cr/cm 2 as determined b y x - r a y spectrometry and atomic-absorption spectroscopy i s formed. This oxide film is useful for improving adhesive strength and durability. Copper scarcely adheres to polyethylene in general. According t o Bright et al. [ 8 ] , this is because copper inhibits the oxidation of polymer. The thick, black-matted oxide film formed by oxidizing copper improves adhesion. The oxide can be produced from a solution of sodium chlorite, sodium triphosphate, or sodium hydroxide. Using coulometry, Baker et al. [9a] analyzed the oxide on the surface of copper before and after hot-melt coating with polyethylene (PE) to examine the adhesion to polyethylene of oxidation-treated copper. The results indicate that cupric oxide, CuO, is decreased, and cuprous oxide, CU2O, i s increased, in the oxide film adhering to polyethylene. It i s believed that cupric oxide oxidizes polyethylene to improve adhes i v e n e s s as shown in the following formula: 2CuO + PE

C u 2 0 + PE{0)

(1)

Meanwhile, in work by Evans et al. [9b] it was estimated from t h e analytical results of CuO and C u 2 0 on the surface of copper s h e e t s with and without hot-melt coating of polyethylene carried out at 200°C for 20 min under optimal conditions for copper-polyethylene adhesion that C u 2 0 is produced according to the following reaction rather than the interaction of CuO with polymer: CuO + Cu

Cu20

4Cu + 0 2

2Cu 2 0

(2)

Peel strength is greatly enhanced by a presputtering treatment of the Teflon prior to the deposition of Cu. Without such a treatment, the Cu-Teflon showed a peel strength of l e s s than lg/mm, and the Cu film can easily be removed by Scotch tape. The peel strength increases by more than 20 times at 10 sec of presputtering, and reaches 50g/mm after 30 sec of sputtering. The peel strength remains nearly constant at longer sputtering times. It i s shown that, for a finite chemical bondi n g , an appreciable contribution to the peel strength i s possible from t h e morphology changes observed [ 1 0 ] .

of Resin to Metal

Adhesion 7.2.2

187

Roughness of Metal Surface and Adhesion

Although the relationship between the roughness of a metal surface and t h e adhesion attainable is given in numerous literature references, it i s not necessarily reproducible. This is because adhesive failure is not reproducible: that i s , it does not always take place on the interf a c e , although local cohesive failure frequently takes place there. Mechanical surface treatment to roughen the surface is therefore carried out to enlarge the specific surface area, to form a cleaned and highly active surface, and to control rapid brittle fracture in the shear or fracture test. When the adhesive penetrates voids in the rough surface, the penetrating depth can be expressed by the following equation, based on the theory of capillarity: u - ιν h

"

c°s θ

Lv

Q

(3)

where YLv is the surface tension of the liquid adhesive, θ the contact angle representing wetness of adhesive, ρ the density of the adhesive, and R the radius of the capillary tube. If we represent the factor of adhesive area increase due to roughn e s s by γ , that of loss due to void by g , and that of s t r e s s concentration by S , the apparent adhesion work, Wa, is expressed by Y /gS, Assuming that liquid is moved along the distance dx on the rough s u r f a c e , the interfacial area between solid and liquid and that between gas and liquid are increased or decreased in proportion to γ d x . We thus have the equation Y ( Y

sv-

Y

sL> =

Y

Lvcos

θ

'

(4)

where γ is the coefficient of roughness, a correction term for surface roughness, and 9f is the contact angle on a rough surface. Assuming that the surface of a solid does not adsorb the vapor of a liquid, as mentioned in the preceding section, we have Y

sv

=

Y

sL

+ Y

Lv

008

6

(5)

From equations (4) and ( 5 ) , the equation expressing the relationship between the surface roughness and the contact angle i s cos θτ When the metal surface is roughened, it becomes easy to wet because the apparent constant angle θτ between the liquid and the metal surface becomes smaller than the real contact angle, Θ. Since the surface roughness where the most effective adhesion is realized

Table 7.2

Surface Preparation V e r s u s Lap Shear S t r e n g t h 8

Group treatment

X (psi)

s (psi)

c (%v)

1

Vapor d e g r e a s e , grit blast 90-mesh g r i t , alkaline clean, Na2Cr20y— H2SO4, distilled water

3091

105

3.5

2

Vapor d e g r e a s e , grit blast 90-mesh g r i t , alkaline clean, ^ 2 0 ^ 0 7 — H2SO4, tap water

2929

215

7.3

3

Vapor d e g r e a s e , alkaline clean, Na2Cr 2 0 7 —H2SO4, distilled water

2800

307

10.96

4

Vapor d e g r e a s e , alkaline clean, ^ 2 0 ^ 0 7 — H 2 S O 4 , tap water

2826

115

4.1

5

Vapor d e g r e a s e , alkaline water, chromic—Η 2 SO4, deionized water

2874

163

5.6

6

Vapor d e g r e a s e , tap water

2756

363

1.3

7

Unsealed anodized

1935

209

10.8

8

Vapor d e g r e a s e , grit blast 90-mesh grit

1751

138

7.9

9

Vapor d e g r e a s e , wet and d r y s a n d , 100 + 240-mesh grit Ν 2 blown

1758

160

9.1

10

Vapor d e g r e a s e , wet and d r y s a n d , wipe off with sandpaper

1726

60

3.4

11

Solvent wipe, wet and d r y s a n d , wipe off with sandpaper (done rapidly)

1540

68

4

12

Solvent wipe, sand (not wet and d r y ) , 120 grit

1329

135

1.0

13

Solvent wipe, wet and d r y s a n d , 240 grit only

1345

205

15.2

14

Vapor d e g r e a s e , aluminum wool

1478

-

-

15

Vapor d e g r e a s e , 15% NaOH

1671

-

-

16

Vapor d e g r e a s e

837

72

17

Solvent wipe ( b e n z e n e )

353

-

18

As received

444

232

Na2Cr20 7 —H2SO4,

8.5 -

52.2

Resin employed in Epon 934 Shell Chemical Company; cured 16 h at 75°F p l u s 1 h at 180°F. Fillet on overlap left i n t a c t , a d h e s i v e on s i d e s of specimens removed. Source: R e f . 11.

Adhesion

of Resin

to Metal

189

generally depends on the combination of adherend and adhesive, the adhesive conditions must be examined closely. According to Chessin et al. [11] , the adhesive strength under shear in the adhesion system aluminum/Epon 934 is increased by increasing grit size from 90 mesh to 24 mesh in the order chromic acid mixture treatment > grit blasting = grit blasting + chromic and mixture treatment > mechanical abrasion + chromic and mixture treatment. The results are shown in Table 7.2. Rogers reported that the adhesive strength under shear in an aluminum/modified epoxy resin system i s increased more by treatment with sharp grains such as quartz powders [12] than by treatment with dull grains such as glass beads. Few reports on the quantitat i v e relationship between surface roughness and adhesive strength have been published. Malpass et al. [13,14] reported on the relationship between oxide film thickness, peel strength under shear, and the void by measuring the diameter of hexagonal-prism pores (120 A in H2SO4 bath and 330

cφ ο L. Φ α.

if)

Peel strength (30min)

JL

10

14

18

2 Ο Q.

22

VOLTAGE (V) Figure 7.3 Variation in peel strength and porosity with anodizing voltage for films formed in phosphoric acid at approximately constant current density. Anodizing times: · , 60 min; o, 30 min. (Ref. 14.)

1 190

Chapter

POLISH THRU \μ DIAMOND DUST

\ ΐΟμ INCHES

Η

0.002 INCH

POLISH THRU \μ DIAMOND DUST • CHROMATE ETCH

] 1 \0 μ INCHES

H

0.002 INCH

LAPPED 2X

J Ι0μ INCHES

Η

0.002 INCH

LAPPED 2λ 4 CHROMATE ETCH

J \0μ INCHES

H

0 0 0 2 INCH

SANDBLASTED WITH 4 0 - 5 0 MESH GRID

]

H

0.002 INCH

250

μ INCHES

Figure 7.4 Talysurf profilometer t r a c e s for 304 stainless steel s u r faces. (Ref. 15.)

A in H3PO4 bath) regularly formed on anodized aluminum film. The r e s u l t s are shown in Figure 7 . 3 . J e n n i n g et al. [15] published diagrams of the surface r o u g h n e s s (measured u s i n g a r o u g h n e s s meter) of aluminum and stainless steel abraded chemically or mechanically (Figure 7 . 4 ) and of the relations h i p between adhesive s t r e n g t h under tension and temperature characteristics (Figure 7 . 5 ) . 7.2.3

A d h e s i v e n e s s of Metallic Fiber to Matrix in Metallic Fiber-Reinforced Plastic

Few reports on t h e a d h e s i v e n e s s of metallic fiber to matrix h a v e been p u b l i s h e d , although t h e r e are many reports on t h e u s e of metallic fiber to reinforce plastic [ 1 6 - 1 8 ] . Surface treatment of metallic f i b e r - r e i n f o r c e d plastics includes (1) cleaning the metal s u r f a c e , (2) r o u g h e n i n g the metal s u r f a c e , (3) applying oxide film, (4) applying chemical conversion c o a t i n g s , and (5) coating with another metal. T h e e f f e c t of s u r f a c e treatment of metallic fiber on the a d h e s i v e n e s s t o e p o x y resin of s t e e l wire treated b y various methods according to McGarry et al. [16] i s shown in Figure 7 . 6 . As the figure s h o w s , treatment b y phosphoric acid i s extremely e f f e c t i v e . The mechanical properties of laminated s h e e t s are shown in Table 7 . 3 . The table r e v e a l s , h o w e v e r , that t h e e f f e c t of coating with

5

12

Ο

—ι

ι

10

20

ι

ι

ι

40

60

80

TEST TEMPERATURE (°C) Figure 7.5 Variation of joint strength with temperature for Epon 815-Versamid 140 (60/40) adhesive and 6061 T6 Al adherends. Dashed lines indicate bulk polymer data of Ishai. (Ref. 15.)

3.600

3.400 w o. f ο ζ LU α: I L >U ω LU Q

3 monocrystal) proved to require a temperature above 1750°C, and coloring, c r a c k i n g , and s i n t e r i n g of the ceramic surface were noticeable only above 1900°C [ 1 3 ] . It i s believed that t h e role of added Mn i n v o l v e s MnO formed according to t h e reaction Mn + H 2 0

^ MnO + H 2

reacting with AI2O3 in the base plate or S i 0 2 included therein as an impurity, forming aluminate or silicate and combining with Mo grains on t h e ceramic s u r f a c e [ 1 4 ] . In other w o r d s , t h e Mo—Mn p r o c e s s i s said to be u s e f u l when including acidic oxides s u c h as t h o s e in t h e b a s e plate or S i 0 2 included t h e r e i n , the M0O3 addition p r o c e s s b e i n g e f f e c t i v e when including basic o x i d e s s u c h as CaO and MgO [15]. As mentioned a b o v e , when u s i n g an Mo—Mn s y s t e m , t h e formation of MnO followed b y reaction thereof with t h e base plate or i n cluded S i 0 2 i s e x p e c t e d , although control of the atmosphere i s d i f f i c u l t . For t h i s p u r p o s e , MnO —S1O2—AI2O3 [ 1 6 - 1 8 ] , M0O3—Mn02~ Ti02—S1O2 [ 1 9 ] , and S 1 O 2 — M n O — A I 2 O 3 — C r 2 0 3 — F e 2 0 3 — C u 2 ° s y s t e m s , for example, were u s e d ; other s y s t e m s , including CaO— A l 2 0 3 [18,21], CaO—A1203—Si02 [18], MgO—Al203—Si02 [13,22], MgO —CaO — S i 0 2 [18], and M g O — A 1 2 0 3 — S i 0 2 — A 1 2 0 3 [ 2 3 ] , have also been employed. Moreover, systems such as N a 2 0 — C a O — S i 0 2 and kaolin ( S i 0 2 — A l 2 0 3 — H 2 0 ) C 23 ] h a v e been added to improve t h e sinterability of A l 2 0 3 and MgO. 8.3.2

Bonding by the Active Metal Process

In t h e active metal p r o c e s s , v e r y active metals such as Ti and Zr, t o g e t h e r with Ni, C u , and A g , which form alloys that p r e s e n t a relat i v e l y low melting point with the active metals, are i n s e r t e d between ceramics and metals that are to be bonded so as to produce a eutectic

Bonding

of Ceramic

to Metal

215

Table 8.2 Composition and Melting Points of Various Solders Composition (wt %)

Melting point (°C)

Cu—28 Ti

880

Ni— 71.5 Ti

955

F e - 3 2 Ti

1085

C o — 1 8 . 6 Ti

1135

Cu— 47 Zr

885

Ni—83 Zr

960

F e - 8 4 Zr

934

M o - 6 9 Zr

1520

Ni-Ti-Zr

900

Source:

R e f . 24.

s t r u c t u r e , and are bonded in vacuum or inert g a s e s in a single heat treatment. As shown in Table 8 . 2 , t h e r e are various solder compos i t i o n s , with a wide range of bonding temperatures [ 2 4 ] . For e x ample, the contact angle of an Ti—Ni system with ceramics i s saturated within 5 to 10 min, as shown in Figure 8 . 3 . Moreover, analytical r e s u l t s through ΕΡΜΑ of the d i f f u s i o n aspect of r e s p e c t i v e elements in the bonded zone of sapphire (α—AI2O3) to titanium alloy are shown in Figure 8 . 4 [ 2 5 ] . The bonding mechanism in this case i s believed to be based on the fact that the Ti in Ti—Ni solder s e lectively accumulates on the sapphire i n t e r f a c e , part of which i s o x i d i z e d , and the resulting titan o x i d e s (T1O2, TiO, T1O3) react with s a p p h i r e ; or Ti ions d i f f u s e into sapphire c r y s t a l s to form on the bonded interface an intermediate layer consisting primarily of Ti— Ο — A l - b a s e d solid solutions or compounds, which constitutes a solid and vaccum-tight bonded b o d y . Based on the s u r f a c e e n e r g y of various elements and the solubili t y of Ti t h e r e i n , Nicholas et al. have shown that in addition to S n , I n , which has low surface e n e r g y and i s easily saturatable with T i , i s a v e r y favorable alloying element, a small amount of which will improve wettability remarkably, as shown in Figure 8 . 5 , and that A l , A u , and Ag are moderately favorable elements, Ga and Ni b e i n g non favorable or e v e n deteriorating elements [ 2 6 ] .

1 216

Chapter

Ti-Ni alloy on Forsterite Ceramic (in Vac.) Si on High Alumina Ceramic (1450eC in H2) Si on High Alumina Ceramic (1450eC in He) Ti-Ni alloy on High Alumina Ceramic (1050eC in Vac.) Ti-Ni alloy on Ti Plate (1050eC in Vac.)

20

30

40

TEMPERATURE

50

60

(min)

Figure 8.3 (Ref. 1.)

C h a n g e i n c o n t a c t a n g l e of metal-ceramic with time.

Figure 8.4 25.)

Diffusion a s p e c t s of elements in the sealed part.

(Ref.

5

Bonding

of Ceramic

217

to Metal

150 σ> Φ Ό

/

Λ / ^

P-Q '

tf

\

\

LU

δ ιοο ζ< ΙΟ
c φ L.

φ "

Rlg08 I Dwlght

c - C

Η Ο ® "ΜΗ*

BOVINE ALBUMIN

Et-S-C(0)-CFs

> @ λ

N-C-CFs

(1974) M.MIUard & Masrl (1974)

PLASMA INITIALED PAA GRAFTS ON PP

BcCls

-COaH

-COt")tBo^

HaO MELTING PE ON Al

ψ

Brt

C=C

A. Bradley I M. Czuha. Jr. (1975)

->

Br Brtggs et al

C- C

(1977) OH MMA / HYDROXYPROPYL MA

I

rv.

(CFsCO)eO

-Cte

-CHi

J. Hammond et al (1978)

EPOXY / ESTER PRIMER

CORONA TREATED PE

COe Na

AaNOs

>

Br Br

Brs

C=C

- COt" Afl*

->

I I -c-c

Spell I Chrlstleson (1978)

CORONA TREATED LDPE

NoOH

COeH

- CO»" Na*

Brtggs I Kendallson Polymer (1979)

Ο

Brt

CHe - C

F

F

F

F

_>

Br

ο »

I -C-C kr

NH - NHff C=Ο

>

C

NK^F F F

Figure 11.5 (Ref. 9.)

XPS studies using polymer surface derivatization.

Interface

Analyses

of Composite

Materials

Reaction

Product

C6F5CHO

-NHa

-COzH

-COzH

-CO2CH2CF3

(CF3C0)20

-C00C0CF3

•O2CCF3

-CO2H

'c = Ο f hcl·-

Figure 11.6 (Ref. 10.)

N=CH-CeF5

CF3CH20H CeHnNCNCeHn

OH

-C02H

293

8

1) KOH(ROH) 2) CeFsCHzBr

-CO2CH2C6F5

CeFsNHNHg

X=NNHCeF5

Hg(CF3C02)2 CI3CH2OH

-C-Hg(CF3C02) -C-OCH2CCI3

NqQH(ROH)

•C02"Na'

Relations of PE-Ar with various t a g g e d r e a g e n t s .

The surface of Teflon film (FEP) i s chemically treated with, for example, an ammonium solution of sodium, to provide adhesive prop e r t i e s . Figure 11.7 p r e s e n t s structural c h a n g e s in the film s u r f a c e , treated as described above, examined by two methods, ESCA and the contact angle method [ 1 1 ] . The s p e c t r a of F^g and C^g ( — C F 2 — ) are clearly shown in blank FEP ( 1 ) , but when treated with Na/NH3 (2) t h e y disappear

CH·

α·

FEP CONTROL

17jOOOcp·

17J500cpt

k17.500cp·

Na/NHs

3

-^222·

iWOOcpi

Na/NHs ABRADED

Na/NHs 96 HR AT 200*C

5

Fie

Na/NHs 100 HOURS IN FADEDMETER

do =52 * = 16

MJSOOcpt

lOjOOOcpft

laooocp·

^^floocpt

aI2JBOOCM

f / \

\

\

. WOOcp·

28t500cpS

6

Na/NHs 16 DAYS IN BOILING WATER

ISjSOOcpt -2J0OOcpt

9/SOOcp· 7 7

8 °

Na/NHs CLOROX

IOOOOCP·

..«JDOOcp·

l HjDOOcp»

GOLD CRYSTALLIZED FEP

9» =54 Br =0

12J500cpt

32£00cp«

Od =38 9r =0

300

290

290

540

636 530

700 690 690

tV

BINDING ENERGY

Figure 11.7 ESCA spectra and water contact angles from Teflon FEP before and after various surface treatments. (Ref. 11.) 294

Interface

Analyses

300

of Composite

290

Materials

295

280

ΑΓ PLASMA TREATED

« ' ' « ' »

300

I

290

I I 1 1

280

N2 PLASMA TREATED

i I I I I I I I I I I

300

290

w

eV

280

Figure 11.8 ESCA C - l s peaks of Teflon samples treated with Ar and Ν2 plasma. (Ref. 12.)

296

Chapter 10

almost completely and then appear in the spectrum of a hydrocarbon having carbonyl and carboxyl groups. Because this change does not proceed at depths above 100 to 200 A, its high sensitivity was made clear by ESCA and the reason for its good adhesiveness due to the Na/NH3 treatment was also made clear. The treatment of Teflon with Ar or Ν2 plasma, as shown in Figure 11.8, also causes a decrease in the peak of C^g (—CF^—)> accompanied by the appearance of new components due to C—N, 6 — Ο , and so on [12]. This s u g g e s t s that in Ar plasma treatment, radicals are formed first and then oxygen atoms are captured by the etching, and that in Ν2 plasma treatment, C — Ν bonds are formed by direct reaction.

11.3

ANALYSES OF T H E SURFACE AND T H E I N T E R F A C E OF FILLER

There are some hydroxyl groups on the surface of the filler of the glass fiber or oxide group [13]. In the case of carbon fiber, various functional groups are formed by various oxidation treatments [14]. It is well known that these functional groups are v e r y useful in interactions with surface-treating agents such as silane coupling agents or with matrix resins. The functional groups formed by oxidation treatment of carbon fiber, if detected by ESCA as shown in Figure 11.9 [15], are known to be mainly hydroxyl in the case of weak oxidation, and carboxyl in the case of strong oxidation: The results coincide with our results with ordinary chemical analysis [14]. To analyze these surface functional groups by separating with higher sensitivity, a method using the chemical modification reaction, which includes F in the reactant, i s designed as shown in Figure 11.10 [16]. Two or three other examples [17] were examined in the same way by ESCA (XPS); the relation between the O/C ratio of the surface functional groups for various oxidation treatments and the shear strength of the composite material when manufactured are reported in Table 11.3 [ 1 ] , In another example the functional groups on the carbon black surface were measured by FT-IR and the carboxyl (—COOH) groups were quantitatively analyzed [18]. As described previously, there are silanol groups (Si—OH) on the surface of silica (S1O2) and glass fiber. In one study these groups were analyzed quantitatively using ESCA by separating the peak of the O^g spectrum. As shown in Figure 11.11 curve fitting resulting in a value of 532. 4 eV for the chemical shift of O^g and 533. 3 eV for Si—OH makes it clear that the Si—OH groups decrease to about half at 500°C by heat treatment [19]. Because composite materials use these fillers after treatment of silane coupling agents, it is important to study the interaction

Interface C1S

Analyses

of Composite

297

Materials

difference spectra

290

285

BINDING ENERGY

Figure 11.9 tion. (Ref.

"COOH

280 (eV)

C - l s digital difference spectra for CF surface oxida15.)

(CF3C0)20

-CO2COCF3

-COH

-CO2CCF3

(a) >c = 0

C6HF4NH-NH2—^

> c =

N,NHC6HF4

CF3CH2OH

_ C02CH2-CF3

(b)

- COOH

>

(c) Figure 11.10 Relations of carbon fiber surface with various tagged reagents. (ReL 16.)

Chapter

298

11

Table 1 1 . 3 Relation Between Functional Groups and Various Surface Treatments of Carbon Fiber b y XPS OlS O/C (%)

Surface treatment None

4.3

-C—0*H II Ο

—C—OH Shear s t r e n g t h II (MPa) of Ο* e p o x y composite

—

—

34.6

Air oxidation (400°C , 1 h)

19.1

0.42

0.58

41.5

Chromic acid (15 min)

21.6

0.40

0.60

38.7

HN03 (3 h)

27.8

0.46

0.54

37.4

Hypochlorous acid (25 h)

22.3

0.37

0.63

46.2

Source:

Ref.

1.

between the filler and the coupling agent. There are many examples of analyses that u s e FT-IR. Figure 11.12 shows an analysis of silica treated with a diaminosilane coupling agent [ 2 0 ] . In this case N-2-aminoethyl-3-aminopropyltrimethoxysilane (AAPS) is adsorbed b y interaction similar to a covalent bond. This i s caused by the s t r o n g h y d r o g e n bond between a silanol group and an amino group on the s u r f a c e of silica; the adsorption model i s shown in Figure 11.13 [20]. Glass fiber treated with silane coupling agents was analyzed u s i n g ESCA; the s t r u c t u r e around Si was analyzed u s i n g the Augur parameter obtained from the peaks of Si^g (Eg 1845 eV) and Si(KLL) A u g u r (Ej{ 1605 e V ) , and from the e n e r g y d i f f e r e n c e between the two. T h e s e analyses have revealed (Figure 11.14) that silane molecules combine t h r o u g h treatment with silane-coupling a g e n t s , and f u r t h e r , t h e y react with the adjacent silanol group (Si—OH) to form s t r o n g b o n d s [ 2 1 ] . Similar r e s u l t s obtained by analysis u s i n g the Raman spectrum s u g g e s t that the action of the silane-coupling a g e n t s i s as follows [22]: C H 2 = C H Si ( 0 - C 2 H 5 ) 3 + 3H 2 0 + 3 C o H_0H ΔΌ

— CH 2 = CH S i ( O H ) 3 (i)

Interface

Analyses

of Composite

Materials

299

SiCfH

(c)

(f)

Figure 11.11 The O - l s spectral c u r v e - f i t t i n g results for Cab-O-Sil silica at (a) - 1 2 0 ° C , ( b ) 30°C, (c) 100°C, (d) 300°C, (e) 500°C, and ( f ) 700°C. (Ref. 19.)

Si—0 stretching of the AAPS siloxane groups

1100

1

1

3500

I

I

3100

I 2700

I

I 2300

»

| 1900

I

' 1500

'

« 1100

I

I

c m -i

Figure 11.12 FT-IR absorbance s p e c t r a of diaminosilane coupling agent and Cab-O-Sil s y s t e m . A , Cab-O-Sil-treated 1% AAPS in toluene solution at 130°C; B , heat-cleaned Cab-O-Sil; C, difference spectrum of A - B . (Ref. 20.)

A. DIAMINOSILANE ON SILICA

SURFACE

B. TRIAMINOSILANE ON SILICA

C : CH2

SURFACE

Θ : NH or NH2

Figure 11.13 Proposed s t r u c t u r e of diaminosilane (AAPS) and triaminosilane adsorbed on silica s u r f a c e s . (Ref. 20.)

Analyses

Interface

of Composite

Materials

301

NHzCsHeSi(0C2HS)3 χ \ C - C - C - C - C - C -

Silane Treated Carbon Fibers

OH > Si -

Untreated Glass Fiber Ε

OH ι o - Si -

O -

t

OH ι Si -

NH2C3H6S1(0C2H5)2 ι OH Ο OH

Silane Treated -

'si

Ο -

NH2C3H6SL o r I - Si

0

^ -

0 I 0 - Si

Figure 11.14 f i b e r s . (Ref.

- 0

Possible coupling of silane to carbon and glass 21.)

I ύ -Si*· OH + CH2 = CH Si (OH) 3· Glass

11.4

Silane Treated Sized Glass Fiber Ε

-Si^—O-Si-CH = CH2+ 2 H2O Dry

j

Glass (2) ANALYSES OF THE INTERFACE BETWEEN FILLER AND MATRIX

It i s possible to analyze and examine spectroscopic ally a broken-out section of composite material or a peeled adhesive s u r f a c e . 11.4.1

Analysis Examples of Adhesion Interface

Figure 11.15 shows an ESCA projection of a Pd-PET adhesion interface of electrostatic printing material, made by s p a t t e r i n g Pd thin film (

1/2

Ζ

Plastic regions growing at fiber e n d s .

Figure 12.12 where

Q =

G ε m

r^ cos α

η ( r m - r ρ cosh η A

„ , 2 2 Ε (r — τ» cos α) a a f r^ cos α

2G (r

m

-

r )r t i 2

2oG r cos m a

2

ε =

Ε r (r a i m

Vrm

"

r

2

ι

2

r„ cos a ) ί