Surface Preparation Techniques for Adhesive Bonding (Materials Science and Process Technology Series)

by

Raymond

F. Wegman

Adhesion Ledgewood,

Reprint

r;:;:::l

NOVES

~

Westwood,

Associates New Jersey

Edition

PUBLICATIONS New Jersey,

U.S.A.

Copyright @ 1989 by Raymond F. Wegman No part of this book may be reproduced any form or by any means, electronic or including photocopying, recording or by tion storage and retrieval system, without in writing from the Publisher. Library of Congress Catalog Card Number: ISBN: O-8155-1198-1 Printed in the United States

or utilized in mechanical, any informapermission 88-34528

Published in the United States of America by Noyes Publications Fairview Avenue, Westwood, New Jersey 07675 10987654

Library

of Congress Cataloging-in-Publication

Wegman, Raymond F. Surface preparation

techniques

Data

for adhesive bonding.

Includes bibliographies and index. 1. Adhesives. 2. Surface preparation. I. Title. TP966.W44 1989 668.3 88-34526 ISBN O-8155-1198-1

To all those engineers and scientists who have dedicated their careers to the advancement of the field of materials and processing for adhesive bonding; and to my wife, Rose, whose encouragement, indulgence, and sacrifice made this book possible.

Preface

The purpose of this handbook is to provide information on processing adherends prior to adhesive bonding. Where sufficient data was available the processes are given in the form of process specifications. Further, where available, data is given to enable potential users a basis for the selection of the process most suitable for their particular application and facility. It is known that many of the chemicals used in these processes are hazardous due to their strong oxidizing properties, or may be toxic or hazardous to one’s health. It is the user’s responsibility to assure that proper safety, handling and disposal procedures are implemented and monitored when any of these methods is employed. Raymond

vii

F. Wegman

ACKNOWLEDGMENTS My grateful thanks to Dr. David W. Levi for his help and encouragement; Dr. John D. Venables and his staff at the Materials and Surface Science Department at Martin Marietta Laboratories, for supplying the numerous micrographs and drawings of treated surfaces.

NOTICE To the best of the Publisher’s knowledge the information contained in this publication is accurate; however, the Publisher assumes no liability for errors or any consequences arising from the use of the information contained herein. Final determination of the suitability of any information, procedure, or product for use contemplated by any user, and the manner of that use, is the sole responsibility of the user. Mention of trade names or commercial products does not constitute endorsement or recommendation for use by the Publisher. The book is intended for information only. It is known that many of the chemicals used in the processes described are hazardous, due to their strong oxidizing properties, or may be toxic or hazardous to one’s health. It is the user’s responsibility to assure that proper safety, handling and disposal procedures are implemented and monitored when any of these methods is employed. Expert advice should be obtained at all times when implementation is being considered.

.. .

VIII

Contents

Preface ............................................................................

vii

Acknowledgments ...........................................................

viii

Notice ..............................................................................

viii

1. Introduction .............................................................

1

Adhesion ..............................................................................

1

Specific Adhesion ....................................................

2

Mechanical Adhesion ...............................................

3

Bonding ................................................................................

5

References ...........................................................................

7

2. Aluminum and Aluminum Alloys ...........................

9

Introduction ..........................................................................

9

FPL Etch ..............................................................................

10

Phosphoric Acid Anodize Process ......................................

14

P2 Etch Process ..................................................................

17

Chromic Acid Anodize Process ...........................................

19

Preparation of Aluminum Alloys by the Optimized FPL Etch (Sulfo-Chrom Etch) ..........................................

22

Apparatus ................................................................

22

Materials ..................................................................

22

Processing ...............................................................

23

Restrictions ..............................................................

24

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ix

x

Contents Preparation of Aluminum Alloys by the Phosphoric Acid Anodize Process (PAA) ............................................

24

Apparatus ................................................................

24

Appearance .............................................................

25

Materials ..................................................................

26

Processing ...............................................................

26

Restrictions ..............................................................

27

Anodic Coating Inspection .......................................

27

Preparation of Aluminum Alloys by the P2 Etch (SulfoFerric Etch) ...............................................................

27

Apparatus ................................................................

27

Materials ..................................................................

28

Processing ...............................................................

29

Restrictions ..............................................................

30

Preparation of Aluminum Alloys by the Chromic Acid Anodize Method (CAA) .............................................

30

Apparatus ................................................................

30

Appearance .............................................................

31

Materials ..................................................................

32

Processing ...............................................................

33

Restrictions ..............................................................

34

Specific Controls ......................................................

34

References ...........................................................................

35

3. Titanium and Titanium Alloys ................................

38

Introduction ..........................................................................

38

Preparation of Titanium by the Chromic Acid Anodize Process .....................................................................

51

Apparatus ................................................................

51

Materials ..................................................................

52

Processing ...............................................................

52

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Contents

xi

Restrictions ..............................................................

53

Preparation of Titanium by the Turco 5578 Process ..........

53

Apparatus ................................................................

53

Materials ..................................................................

54

Processing ...............................................................

54

Restrictions ..............................................................

55

Preparations of Titanium by the Liquid Honel Pasa-Jell 107 Process ..............................................................

55

Apparatus ................................................................

55

Materials ..................................................................

55

Processing ...............................................................

56

Restrictions ..............................................................

57

Preparation of Titanium by the Dry Hone/Pass-Jell 107 Process .....................................................................

57

Apparatus ................................................................

57

Materials ..................................................................

58

Processing ...............................................................

58

Restrictions ..............................................................

59

Preparation of Titanium by the Alkaline Peroxide Process .....................................................................

59

Apparatus ................................................................

59

Materials ..................................................................

60

Processing ...............................................................

60

Restrictions ..............................................................

61

Preparation of Titanium by the Stabilized Phosphate Fluoride Process .......................................................

61

Apparatus ................................................................

61

Materials ..................................................................

61

Processing ...............................................................

62

Restrictions ..............................................................

63

References ...........................................................................

63

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xii

Contents

4. Steel and Stainless Steel ........................................

66

Introduction ..........................................................................

66

Carbon and Alloy Steels ......................................................

66

Bonding to Oily Steel ...........................................................

68

Preparation for Bonding to Oily Steel .......................

68

Conversion Coating of Steel ................................................

70

Stainless Steel .....................................................................

70

Specialty Stainless Steels ...................................................

72

Preparation of Steel by the Phosphoric Acid-Alcohol Process .....................................................................

72

Apparatus ................................................................

72

Materials ..................................................................

73

Processing ...............................................................

73

Variations .................................................................

73

Restrictions ..............................................................

73

Preparation of Steel by the Nitric-Phosphoric Acid Process .....................................................................

74

Apparatus ................................................................

74

Materials ..................................................................

74

Processing ...............................................................

74

Preparation of Stainless Steel by the Sulfuric AcidSodium Dichromate Process ....................................

74

Apparatus ................................................................

74

Materials ..................................................................

75

Processing ...............................................................

75

Restrictions ..............................................................

76

Preparation of Stainless Steel by the Sulfuric AcidSodium Bisulfate Process ........................................

76

Apparatus ................................................................

76

Materials ..................................................................

77

Processing ...............................................................

77

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Contents

xiii

References ...........................................................................

78

5. Copper and Copper Alloys .....................................

80

Introduction ..........................................................................

80

Preparation of Copper and Copper Alloys by the Ferric Chloride-Nitric Acid Process .....................................

81

Apparatus ................................................................

81

Materials ..................................................................

81

Processing ...............................................................

82

Restrictions ..............................................................

82

Preparation of Copper and Copper Alloys by the Ferric Sulfate-Sulfuric Acid/Sodium Dichromate Sulfuric Acid Process .............................................................

83

Apparatus ................................................................

83

Material ....................................................................

83

Processing ...............................................................

84

Preparation of Copper and Copper Alloys by the Black Oxide Process ..........................................................

85

Apparatus ................................................................

85

Materials ..................................................................

85

Processing ...............................................................

86

Restrictions ..............................................................

86

References ...........................................................................

87

6. Magnesium ..............................................................

88

Introduction ..........................................................................

88

Preparation of Magnesium and Magnesium Alloys; by the HAE Alkaline Anodic Process-Ml L-M-45202 Type 1, Class A, Grade 2 .........................................

89

Apparatus ................................................................

89

Appearance .............................................................

91

Materials ..................................................................

92

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xiv

Contents Processing ...............................................................

93

Preparation of Magnesium and Magnesium Alloys by the Dow 17 Anodic Process-Mil-M-45202 Type 1, Class C .....................................................................

94

Apparatus ................................................................

94

Appearance .............................................................

95

Materials ..................................................................

95

Processing ...............................................................

96

Restrictions ..............................................................

97

Preparation of Magnesium and Magnesium Alloys by the Chromic Acid Treatment Processes ..................

97

Apparatus ................................................................

97

Materials ..................................................................

98

Processing ...............................................................

98

References ...........................................................................

99

7. Other Metals ............................................................ 100 Introduction ..........................................................................

100

Beryllium ..............................................................................

100

Processing of Beryllium by the Sodium Hydroxide Process ............................................................ 101 Processing of Beryllium by the BERYLCOAT ”D” Process ............................................................ 101 Cadmium (Cadmium Plating) ..............................................

103

Abrasive Scouring .................................................... 103 Chromium (Chromium Plate) ...............................................

104

Abrasive Cleaning of Chromium ............................... 104 Hydrochloric Acid Treatment of Chromium ............... 104 Gold ......................................................................................

104

Abrasive Cleaning of Gold ....................................... 104 Abrasive Scouring of Gold and Gold Plate ............... 104

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Contents

xv

Nickel and Nickel Alloys ......................................................

104

Abrasive Cleaning of Nickel ..................................... 105 Nitric Acid Etch ........................................................ 105 Sulfuric-Nitric Acid Pickle ......................................... 105 Nickel Plated Parts ..............................................................

105

Abrasive Scouring .................................................... 105 Platinum ...............................................................................

105

Abrasive Cleaning .................................................... 105 Abrasive Scouring .................................................... 106 Silver ....................................................................................

106

Uranium ...............................................................................

106

Abrasive Cleaning .................................................... 106 Acetic Acid-Hydrochloric Acid Processing ................ 106 Nitric Acid Processing .............................................. 107 Zinc ......................................................................................

107

Abrasion .................................................................. 107 Acid Etch ................................................................. 107 Sulfuric Acid/Dichromate Etch .................................. 108 Rare Metals, .........................................................................

108

Metal Matrix Composites .....................................................

108

References ..........................................................................

108

8. Plastics .................................................................... 109 Introduction ..........................................................................

109

Organic Matrix Composites .................................................

109

Peel Ply ................................................................... 111 Abrasion .................................................................. 111 Thermoset Materials ............................................................

112

Thermoplastic Materials ......................................................

113

Acetal Copolymer (Celcon) ..................................................

113

Chemical Etch .......................................................... 113 This page has been reformatted by Knovel to provide easier navigation.

xvi

Contents Acetal Homopolymer (Delrin) ..............................................

114

Chemical Etch .......................................................... 114 “Satinizing” U.S. Patent 3,325,426 (Du Pent) ........... 114 Acrylonitrile-ButadieneStyrene ............................................

116

Abrasive Treatment .................................................. 116 Chemical Etch .......................................................... 116 Cellulosics ............................................................................

116

Ethylene-Vinyl Acetate ........................................................

116

Fluorinated Ethylene-Propylene (Teflon, FEP) ...................

117

Sodium Naphthalene Complex Process (ASTM D2093) .............................................................. 117 Nylon ....................................................................................

117

Abrasive Treatment .................................................. 117 Chemical Etch .......................................................... 118 Abrasive Treatment .................................................. 118 Polyaryl Sulfone ...................................................................

118

Solvent Wash ........................................................... 118 Abrasive Treatment .................................................. 119 Chemical Etch .......................................................... 119 Polycarbonate (Lexan, Merlon and Tuffak) .........................

119

Flame Treatment ..................................................... 119 Oven Dry ................................................................. 119 Abrasive Treatment .................................................. 120 Polyethylene ........................................................................

120

Chemical Treatment ................................................. 120 Flame Treatment ..................................................... 120 Newer Methods of Treating Low Density Polyethylene ..................................................... 121 Polymethylmethacrylate (Plexiglas or Lucite) .....................

122

Abrasive Treatment .................................................. 122

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Contents Polymethylpentane ..............................................................

xvii 122

Abrasive Treatment .................................................. 122 Chemical Treatment (Dichromate) ........................... 122 Chemical Treatment (Permanganate) ...................... 123 Polyphenylene Sulfide (Ryton) ............................................

123

Abrasive Treatment .................................................. 123 Polypropylene ......................................................................

123

Chemical Treatment ................................................. 123 Flame Treatment ..................................................... 124 New Methods for Treating Polypropylene ................ 124 Polystyrene ..........................................................................

125

Abrasive Treatment .................................................. 125 Chemical Treatment ................................................. 125 Polysulfone ..........................................................................

125

Chemical Treatment ................................................. 125 Solvent Wash ........................................................... 125 Abrasive Treatment .................................................. 126 References ...........................................................................

126

9. Rubbers ................................................................... 127 Introduction ..........................................................................

127

Neoprene .............................................................................

128

Abrasive Treatment .................................................. 128 Cyclization ............................................................... 129 Chlorination ............................................................. 130 Ethylene-Propylene-Diene Terpolymer (EPDM) .................

130

Abrasive Treatment .................................................. 130 Cyclization ............................................................... 131 Silicone Rubber ...................................................................

132

Solvent Cleaning ...................................................... 132 Soap-and-Water ...................................................... 132 This page has been reformatted by Knovel to provide easier navigation.

xviii

Contents Priming .................................................................... 132 Butyl Rubber ........................................................................

132

Abrasive Treatment, ................................................. 133 Cyclization ............................................................... 133 Chlorination ............................................................. 134 Priming .................................................................... 134 Chlorobutyl Rubber ..............................................................

134

Abrasive Treatment .................................................. 134 Cyclization ............................................................... 135 Chlorination ............................................................. 136 Priming .................................................................... 136 Chlorosulfonated Polyethylene ............................................

136

Abrasive Treatment .................................................. 136 Priming .................................................................... 137 Nitrile Rubber .......................................................................

137

Abrasive Treatment .................................................. 137 Cyclization ............................................................... 137 Chlorination ............................................................. 138 Priming .................................................................... 139 Polyurethane Elastomers ....................................................

139

Abrasive Treatment .................................................. 139 Priming .................................................................... 139 Synthetic Natural Rubber ....................................................

140

Abrasive Treatment .................................................. 140 Cyclization ............................................................... 140 Chlorination ............................................................. 141 Styrene-Butadiene Rubber ..................................................

141

Abrasive Treatment .................................................. 141 Cyclization ............................................................... 142 Chlorination ............................................................. 143 Priming .................................................................... 143 This page has been reformatted by Knovel to provide easier navigation.

Contents

xix

Polybutadiene ......................................................................

143

Abrasive Treatment .................................................. 143 Cyclization ............................................................... 144 Chlorination ............................................................. 144 Solvent Wipe ............................................................ 145 Fluorosilicone Elastomers ...................................................

145

Solvent Clean .......................................................... 145 Epichlorohydrin Rubbers .....................................................

145

Solvent Cleaning ...................................................... 145 Polysulfide Rubber ...............................................................

146

Abrasive Treatment .................................................. 146 Chlorination ............................................................. 146 Polypropylene Oxide Rubber ..............................................

146

Solvent Cleaning ...................................................... 146 Fluorocarbon Elastomers ....................................................

146

Chemical Etch .......................................................... 147 Polyacrylate or Polyacrylic Rubber ......................................

147

Abrasive Treatment .................................................. 147

lndex ............................................................................... 148

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1

Introduction

ADHESION Adhesion is a surface phenomenon, i.e., adhesion is controlled by the condition of the surface of the adherend. The ASTM’ defines adhesion as “the state in which two surfaces are held together by interfacial forces which consist of valence forces or interlocking actions or both.” Adhesion between surfaces which are held together by valence forces is called specific adhesion; this is the same type of force which gives rise to cohesion. Cohesion is defined as the state in which particles of a single substance are held together by primary or secondary forces. As used in the field, cohesion is defined as the state in which the particles of the adhesive (or the adherend) are held together. Adhesion parts together sion.

between surfaces in which the adhesive holds the by interlocking action is known as mechanical adhe-

Both specific adhesion and mechanical adhesion are important to the understanding of how adhesion is affected by surface preparation. Allen* in his discussion of the fundamentals of adhesion concluded by stating ‘I.. . an adhesive bond achieves its strength from the combination of a variety of sources; (mechanisms) . . . For these mechanisms, the relative importance and the proper way which they should be combined will vary from one example to another, but none should be excluded without very careful consideration and exploration.” 1

2

Surface Preparation Techniques for Adhesive Bonding

Specific Adhesion According to the theory relating to specific adhesion, this type of adhesion involves the establishment of some kind of attraction between the atoms and the molecules which make up the adhesive and the adherends. These attractions may involve primary bonding forces, which tend to be quite strong; hydrogen bonding, which yields intermediate strength; and the weaker secondary (Van der Waals) forces. Primary bonding may be covalent or ionic in nature. Covalent bonding involves the sharing of electron pairs between adjacent atoms. Ionic or electrostatic forces are the type of primary bonds that are found in ionic crystals. Another type of primary bond is the metallic bond which is similar to the covalent bond except that it involves the valence electrons in the metal. This type of bonding is discussed by Verink: Wegman and Levi4 and Salomon.’ Secondary bonding involves dipole-dipole interactions, induced dipole interactions and dispersion forces. Secondary bonding be comes important in adhesion when nonpolar or chemically inert surfaces are involved. Hydrogen bonding may be considered a special case of dipole interaction, since hydrogen bonds result from the sharing of a proton by two electron-negative atoms. However, hydrogen bond strengths are of the same order as a weak primary bond. Another type of bond, discussed by DeLolli$ is described as a chemisorbed bond. This type of bonding is proposed as the reason why adhesive promoters, such as primers and coupling agents, are successful in overcoming potentially weak boundary layers and result in good durable bonds. In dealing with specific adhesion, good contact must be obtained between the adhesive and the surface of the adherend. To obtain this molecular contact with a solid adherend requires the wetting of the solid surface by the liquid adhesive. During the bonding process even the solid adhesive must go through a liquid phase. To understand the conditions of adequate wetting one must consider the role of surface energetics in adhesion. This, simply stated, requires that in order for a liquid to wet and spread on a solid surface the critical surface tension of the solid or solids must be greater than the surface tension of the liquid. In the case of polar solids such as metals and metal oxides this requirement is easily met, because the surface energies of the solid, provided that

Introduction

3

the surfaces are clean, are greater than 500 dyne/cm while the surface energies of the liquid adhesives are less than 500 dyne/cm.4 However, even if good wetting and good contact between the adhesive and adherend are obtained, it may be difficult to obtain good durable bonds if there is a weak boundary layer on the adherend. Therefore, it is necessary to carefully select the proper surface preparation technique for the particular adherend, in order to make sure that such a weak boundary layer does not occur, which would make the bond practically useless. Surface energetits is further discussed by Kaelble.’ Mechanical Adhesion Mechanical adhesion results from an interlocking action between the surface structure of the adherend and the adhesive or primer. Bickerman* proposed that adhesion was due to the inherent roughness of all surfaces. He accepted the fact that molecular forces of attraction caused an adhesive to wet and spread on the surface. Once this was achieved, however, Bickerman felt that mechanical coupling between the adhesive and the inherently rough adherend was more than enough to account for bond strength. Surface roughness can also account for a number of negative factors such as trapped gas bubbles, as described by DeBruyne,g and imperfect molecular fit, as described by Eley.‘O Many arguments have been presented against Bickerman’s theory and inherent surface roughness should be considered to be a contributing factor only, rather than a basic factor in the theory of adhesion. In 1977, Chen et al.‘l proposed that the surface of Forest Products Laboratory (FPL) etched aluminum consisted of fine finger-like structures of aluminum oxide which were determined to be about 50 Angstroms thick and approximately 400 Angstroms in height. Each of the fine fingers protruded from the tripoint of the oxide structure. This finding was made possible by advanced electron microscopy using such techniques as the scanning transmission microscope. Similar findings were made when the surface of phosphoric acid anodized aluminum was studied. Further discussions and stereomicrographs of treated surfaces are presented in Chapter 2 of this book. In 1980, Ditcheck et al. I2 described the morphology position of titanium adherends prepared for adhesive In this work they discussed macro-rough and micro-rough

aluminum

and combonding. surfaces.

4

Surface Preparation Techniques for Adhesive Bonding

From their determination of the morphology of various surfaces they predicted the order of bondability of the surfaces and the reliability of the resultant bonds. Their predictions were confirmed by Brown13 using the wedge test and by Wagman and Levi,14 using stress durability testing. This work is described in more detail in Chapter 3. Some excellent micrographs of these surfaces were preresolution scanning electron sented by Venables15 using extended microscopy. This work shows that mechanical adhesion on a microscopic level does play a major role in adhesion and the durability of the resultant bonds. Therefore, as in the case of specific adhesion, it is equally important that proper surface preparation be used when one considers mechanical adhesion. Whether one subscribes to the specific adhesion theory, the mechanical adhesion theory, or a combination of the two, surface preparation is a major item that must be considered. At a National Research Council Workshop held in Washington, DC6 it was suggested that any study of the interface should begin with the realization that the traditional picture of the two-dimensional nature of the interface is outdated. It was further suggested that the interface is a three-dimensional “interphase.” This interphase was described as “extending from some point in the adherend where the local properties begin to change from the bulk properties, through the interface, and into the adhesive to where the local properties approach the bulk properties.” The interphase can extend from a few to a few hundred nanometers. On the adherend side it includes morphological alterations in the adherend near the surface, as well as oxides, whether deliberately constructed or native to the adherend surface. The oxide layer can vary in porosity and microstructure. Absorbed gases may be present on the oxide surface giving rise to conditions unfavorable to a good durable bond. A polymeric adhesive may be affected by the presence of the adherend surface. The polymeric network may be altered in the region close to the adherend surface, giving rise to a material which is different in composition and mechanical properties. This is caused by diffusion of certain adhesive components into the oxide surface leaving a layer of adhesive which contains a smaller amount of these components than does the bulk. Acceptance of the interphase as a working model of the actual joint interface leads to the conclusion that the logical beginning of a strong, durable adhesive bonded joint is the selection of the proper surface treatment.

Introduction

5

BONDING Adhesive bonding is widely accepted as the method for joining material, distributing stress, and producing structures that could not otherwise be made. The successfu I performance of these structures depends upon the ability to control all of the independent steps of the production line. Wegman and Levi17 describe the various considerations involved in setting up a satisfactory adhesive bonding shop. A summary of these steps includes:

( 1)

Receiving materials. These materials include adherends, adhesives, and the chemicals and solvents required for cleaning.

(2)

Knowledge of the fabrication processes for the parts to be bonded. The process by which the individual parts were produced may affect the type of cleaning process which will be required. The use of cutting oils, release agents, or the type of binder used in the manufacture of a casting mold can have an effect upon the selection of a cleaning process and the success of subsequent bonding steps. Mating of parts to be bonded. Some cleaning processes such as acid etching will remove material, while processes such as anodizing and other coating processes will add material. Therefore, in some designs it is important to know how much material was removed or added to allow for a satisfactory glue line. Machined parts should be mated prior to application of adhesive to insure there are no misfits which could cause voids, surface bu Iges or stress concentration points. Surface preparation. This is often the most critical, complex, and frequently abused step in the bonding sequence.

(3)

(4)

It

is important

preparation

(a)

to

remember

these

factors

regarding

:

Know the adherend material, tion and how it was prepared.

its surface

condi-

surface

6

Surface

Preparation

Techniques

for Adhesive

Bonding

(b)

Use good safety cleaning agents.

(c)

For wiping parts, only clean white cotton cloths with no sizing or finish or paper towels with little or no organic binders should be used. Never use synthetic materials.

(4

Hot solvent washing not be used to clean secondary bonding materials have been safe.

(e)

Train, qualify and requalify ate the cleaning facility.

0) (cl)

Check the cleaning

0-d

and health

practices

when

using

or vapor degreasing should composite materials or for preparations, unless these tested and proven to be personnel

who

oper-

to

detail

line regularly.

Clean1 iness, vigilance and attention are of primary importance on the job. Don’t rely on faith. rials, personnel and results at all times.

Know your process, matequality control tests and

Another important but often neglected step in processing for adhesive bonding is quality control. Control of the temperature of the solution, control of the drying between immersions and control of the type and flow of rinse waters are all very important. One of the most critical steps requiring control is the rinsing of adconcluded that “the rinsing operation herends. McNamara, et al.” during FPL pretreatment of aluminum surfaces for bonding is crucial to the success of the bonding operation.” Immersion rinse solutions with a pH