Surface Modification of Titanium
and Polymide Sheet for
Adhesive Bonding
Muhammad Akram
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Surface Modification of Titanium
and Polymide Sheet for
Adhesive Bonding
Muhammad Akram
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M
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f T
ita
niu
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nd
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Surface Modification of Titanium
and Polymide Sheet for
Adhesive Bonding
Muhammad Akram
ur
fa
ce
M
od
ific
at
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n o
f T
ita
niu
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P
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behorende bij het proefschrift
Surface Modification of Titanium and Polyimide Sheet for Adhesive Bonding door Muhammad Akram
1. Plasma modificatie onder atmosferische druk heeft zich ontpopt tot een effectieve alternatieve oppervlaktebehandeling techniek voor metalen en polymeren. (dit proefschrift).
2. De duurzaamheid van lijmverbindingen is direct gerelateerd aan de oppervlakte behandeling van de grensvlakken (dit proefschrift).
3. Het falen van lijmverbindingen kan worden vertraagd door oppervlakte modificatie, maar kan niet worden vermeden in zeer vochtige omgevingen met hoge temperaturen (dit proefschrift).
4. In tegenstelling tot de lijmverbinding, kunnen verbindingen tussen mensen verbeteren na geconfronteerd te zijn met extreme omgevingsomstandigheden.
5. Vandaag de dag zou de wereld polio-vrij zijn, als de vaccinatiecampagnes niet tevens werden gebruikt om terroristen te identificeren.
6. Terrorisme kan worden verminderd door het geven van goed onderwijs en werkgelegenheid in achterstandsgebieden.
7. De vaste openingstijden van een universiteit kunnen leiden tot een reductie van de onderzoeksresultaten. Als gevolg hiervan kan de afronding van een promotieonderzoek worden vertraagd.
8. Doorwerken in het weekend en/of tot laat in de nacht maakt je geen betere wetenschapper; Het vermindert alleen de kwaliteit van het leven.
9. Zelfs een zwakke democratische regering is beter dan een sterke dictatuur die de burgerrechten negeert.
10. De vrijheid van meningsuiting impliceert niet de vrijheid om iemands gevoelens te kwetsen met je ideeën.
11. Taalbarrières veroorzaken sociale segregatie.
12. De temperatuur heeft een grotere invloed op ons geluk en onze productie (Yoshiro Tsutsui, 2013: Weather and Individual Happiness, AMS); Daarom dient de
temperatuur in universitaire werkruimtes voor iedere persoon individueel te worden geregeld.
Deze stellingen worden verdedigbaar geacht en zijn als zodanig goedgekeurd door de promotor Prof.dr.ir. L.J.Ernst
Surface Modification of Titanium and Polyimide Sheet for Adhesive Bonding by Muhammad Akram
1. Atmospheric pressure plasma modification has emerged as an effective alternative surface treatment technique for metals and polymers (this thesis).
2. The durability of adhesive bonds is directly related to the interface surface preparation (this thesis).
3. The failure of adhesive bonds can be delayed by surface modification, but cannot be avoided in very humid environments with high temperatures (this thesis)
4. In contrast to the glue connection, connections between people can improve after being confronted with extreme environmental conditions.
5. Today, the world would be polio-free, if the vaccination campaigns were not also used to identify terrorists.
6. Terrorism can be reduced by supplying good education and job opportunities in deprived areas.
7. The fixed opening hours of a university may lead to a reduction of the research results. As a result, the completion of a PhD thesis may be delayed.
8. Continuing working on weekends and/or until late at night makes you not a better scientist; It only reduces the quality of life.
9. Even a weak democratic government is better than a strong dictatorship that ignores the civil rights
10. The freedom of speech does not imply the freedom to hurting someone's feelings with your ideas.
11. Language barriers cause social segregation in society.
12. The temperature has a bigger effect on our happiness and production (Yoshiro Tsutsui, 2013: Weather and Individual Happiness, AMS); Therefore the temperature in
university working rooms should be controlled for each person individually. These propositions are considered opposable and defendable, and as such have been approved by the supervisor Prof.dr.Ir. L.J.Ernst
Proefschrift
Ter verkrijging van de graad van doctor aan de
Technische Universiteit Delft,
op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben,
voorzitter van het College voor Promoties,
in het openbaar te verdedigen op donderdag 26 februari 2015 om 12:30 uur
door
Muhammad AKRAM
Muhammad AKRAM
Dit proefschrift is goedgekeurd door de promotor:
Prof. Dr. ir. L.J.Ernst
Copromotor: Dr. Ir. K.M.B.Jansen
Samenstelling Promotiecommissie:
Rector Magnificus Voorzitter
Prof. Dr. ir. L.J. Ernst Technische Universiteit Delft, promotor
Dr.Ir. K.M.B. Jansen Technische Universiteit Delft, copromotor
Prof. Dr. S. Bhowmik Amrita University, India
Prof. Dr. Ir. H.A.Terryn Vrije Universiteit Brussel
Prof. Dr. Ir. R.Akkerman Universiteit van Twente
Prof. Dr. S.J.Picken Technische Universiteit Delft
Prof. Dr. U.Staufer Technische Universiteit Delft
ISBN/EAN : 978-94-6186-428-4
Copyright@2015 by Muhammad Akram
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the author.
1. Introduction……….………..1
1.1. Objectives………...3
1.2. Scientific importance of this research work……….………....4
1.3. Approach and methodology………..…..…………...6
1.3.1. Selection of Materials .………..……….7
1.3.2. Analysis of base materials………..……….….7
1.3.3. Surface modification……….….8
1.3.4. Analysis after modification……….………..…..8
1.3.5. Characterization of high performance polyimide adhesive………..8
1.3.6. Joining of polyimide & titanium sheet by high performance adhesive...8
1.3.7. Studies of mechanical properties of joints……….9
1.3.8. Evaluation of performance and durability of the improved adhesive Bond strength at elevated temperatures and moisture levels………….………...9
1.3.9. Correlation of failure modes and bond strength with moisture saturation level and temperatures……….……….…….10
1.3.10. Analysis of results and conclusions……….…..10
1.4. Structure of the thesis………..…11
1.5. Application in industry………..………..12
1.6. References………..………..………...…….13
2. Literature review……….……….……….………...16
2.1. Introduction to adhesion……….18
2.1.1 Definition of adhesive and adhesive bonding………....18
2.1.2 Fundamentals of adhesive bonding………..………19
2.1.3 Classification of adhesives………..….20
2.1.4 Process of adhesive bonding………..……21
2.1.5 Advantages of adhesive bonding………..….21
2.1.6 Limitation of adhesively bonded joints………..…23
2.1.7 Requirements of a good adhesive bond……….25
2.1.8 Theories of adhesion………...26
2.2. Introduction to titanium and its alloys……….34
2.2.1 Characteristics of titanium……….………….…..35
2.2.2 Production and fabrication of titanium………...38
2.2.3 Grades of titanium……….….38 2.2.4 Applications of titanium……….….…39 2.2.5 Precautions………43 2.3. Introduction to polymers………..…...44 2.3.1 Homopolymers………..….44 2.3.2 Co polymers………..45
2.3.3 High performance polymers……….46
2.3.4 Polyimide……….46
2.3.5 Applications of polyimide………...48
2.4. Introduction to surface treatment (Preparation)………49
2.4.1. Surface treatments of titanium……….……….….49
3.2 Classification of adhesives………..61
3.2.1. Natural adhesives………..…....61
3.2.2. Synthetic adhesives………....62
3.2.3. Classification by chemical composition……….…..62
3.2.4. Aromatic polymer adhesives……….…..63
3.2.5. Polyimide adhesives……….….….64
3.3 Experimental……….………..67
3.2.6. Thermal gravimetric analysis (TGA)………...67
3.2.7. Differential scanning calorimetry (DSC)……….….67
3.2.8. Fourier transform infrared spectroscopy (FTIR)……….…………69
3.4 Results and discussion………..……….………..70
3.5 Conclusions………..………...74
3.6 References……….75
4. Surface modification of titanium for adhesive bonding with high temperature Polyimide adhesive ………..….76
4.1 Introduction……….……….………..….…77
4.2 Experimental………..………..……….……..80
4.2.1. Materials……….………...80
4.2.2. Mechanical treatment……….……….……….81
4.2.3. Atmospheric ressure plasma treatment……….………..….….…81
4.2.4. Contact angle measurement & surface energy estimation…….………....…….82
4.2.5. Surface roughness measurements……….…………..….…..83
4.2.6. Adhesive joint preparation and tensile lap shear testing……….………….……..83
4.2.7. Microscopic studies of substrate surfaces and fractography….…..…..……..83
4.2.8. X-Ray Photo Electron Spectroscopy (XPS)……….………..…………..84
4.3 Results……….………..………….86
4.3.1. Contact Angle on surface modified titanium………..…………...86
4.3.2. Surface energy of titanium samples………..………..90
4.3.3. Surface roughness measurements………..……….….91
4.3.4. Scanning electron microscope results……….…….93
4.3.5. XPS analysis of titanium samples……….…..96
4.3.6. Lap shear tensile properties of adhesive bonded joints………..………….…103
4.4 Discussion………..………105
4.5 Conclusions………..……….………112
4.6 References………...……….113
5. Surface modification of polyimide sheet for adhesive bonding ………..……..116
5.1 Introduction……….……….………..………..………….117
5.2 Experimental...…...119
5.2.1. Materials………..………..119
5.2.2. Atmospheric pressure plasma treatment………....120
5.2.3. Contact angle measurement and surface energy estimation………..….121
5.2.4. Adhesive joint preparation and tensile lap shear testing………..…………122
5.2.5. SEM analysis of substrate surfaces and fractrography………..…………122
5.3.2. Surface energy of polyimide sheet (Meldin 7000)………..126
5.3.3. Lap shear tensile properties of adhesively bonded joints……….…..127
5.3.4. Scanning electron microscopic analysis of polyimide (Meldin 7000).…….….128
5.4 Discussion……….……….130
5.5 Conclusions……….………..…….132
5.6 References……….…………..…..133
6. Durability of titanium and polyimide sheet adhesive bonds ……….136
6.1. Introduction………..……….……….………138
6.2. Experimental………141
6.2.1. Test samples preparation for Q5000 testing………...141
6.2.2. Test sample preparation for conditioning and lap shear testing………….….142
6.2.3. Test procedures for Q5000 moisture sorption test……….……….143
6.2.4. Test procedure for moisture and dry conditioning of titanium lap shear samples……….………..………..……….…..144
6.2.5. Test procedures for lap shear testing………...………145
6.3. Results……….………146
6.4. Discussion ………..………….……156
6.5. Conclusions……….……....…..157
6.6. References……….………...…..158
7. Conclusions and recommendations……….………..…….161
7.1 Introduction……….……….……….…..….….162
7.2 Selection of high temperature adhesive………….……….……….………..162
7.3. Conclusion concerning surface treatment of titanium for adhesive bonding…..……..163
7.4. Conclusions concerning surface treatment of polyimide sheet for adhesive bonding with titanium……….……….……….……….…164
7.5. Conclusions concerning pre-conditioning of adhesively bonded titanium samples at high temperature and moisture conditions…………..…….………....165
7.6. Final Conclusions………..……….………..166
Summary……….………...………..……168
Samenvatting………..………..………..…….171
Symbols and abbreviations…...………..………...174
Publications/Conferences…………..………..…………..……….175
Acknowledgement...…....177
1
CHAPTER – I
2
Titanium is one of the most effective materials for structural application of spacecraft and aviation. Titanium alloys are widely used in solid rocket booster cases, guidance control for pressure vessel and other different applications demanding lightweight and reliability. The aerospace industry is also a larger market for titanium products and adhesive bonding is advantageous in terms of its fabrication.
There are mainly two main characteristics because of which titanium is widely used in aviation and in the aircraft industry, one is the corrosion resistance and an other one is the highest strength to weight ratio of any metal. In its unalloyed state titanium is as strong as steel but 45% lighter than steel. Titanium has a density of 4.56g/cm3.For comparison, the density of mild steel is 7.85g/cm3. Commercial (99.2% pure) grades of titanium have an ultimate tensile strength of about 434MPa, equal to that of some steel alloys, but these are 45% lighter (Donachie 1988). Titanium is 60% heavier than aluminum, but more than twice as strong as the most commonly used 6061-T6 aluminum alloy.
Due to their high tensile strength to density ratio (Barksdale 1968), high corrosion resistance (Britannica 2006), and its ability to withstand moderately high temperatures without creeping, titanium alloys are used in aircraft, armor plating, naval ships, spacecraft, and missiles (Lide 2005; Britannica 2006). For these applications, titanium alloyed with aluminum, vanadium, and other elements is used for a variety of components including critical structural parts, firewalls, landing gear, exhaust ducts (helicopters), and hydraulic systems. In fact, about two thirds of all titanium metal produced is used in aircraft engines and frames (Emsley 2001).
Polyimide constitutes an important class of materials because of many desirable characteristics. Thermosetting polyimides are known for thermal stability, good chemical resistance, radiation resistance, wear resistance, good adhesion properties and excellent mechanical properties. Thermosetting polyimides exhibit very low creep and high tensile strength. These properties are maintained during continuous use to temperatures of 280°C and for short excursions, as high as 482°C. Polyimides are also inherently resistant to flame combustion and do not usually need to be mixed with flame-retardants. Typical polyimide parts are not affected by commonly used solvents
3
and oils (e.g including hydrocarbons, esters, ethers, alcohols and freons). They also resist weak acids but are not recommended for use in environments that contain alkalis or inorganic acids.
Polyimide has a density of 1.43 g/cm3 and its tensile strength is in the range of 75-90MPa. Polyimide has an elongation at break of 4-8% and they have a glass transition temperature above 400°C. Since polyimides are thermally stable at high temperature they are a popular choice for structural parts in aerospace applications where metal replacement is required with lightweight materials. Polyimide adhesives are used for joining metals and high temperature polymers because their coefficient of thermal expansion is comparable to that of metals.
1.1 Objective
The work of particular relevance in this research project has been on the enhancement of initial adhesion and of bond durability of high temperature resistant polymer such as polyimide (PI) to titanium. Considered are surfaces, modified through mechanical treatment (e.g. grit blasting) and electrochemical treatment like with atmospheric-pressure plasma using different gases. These surface treatments result into a change in wetting characteristics of the substrate surface, followed by joining modified substrate surfaces with adhesive and curing the adhesive bond, using ultra high temperature resistant adhesive such as polyimide (PI).
The work has been designed to improve adhesion characteristics while keeping in view its application to aerospace industry. Therefore, challenges will be taken for retention of adhesion and durability of adhesive bonding of high performance polymer to titanium under moist environmental conditions and elevated temperatures. Characterization of the adhesive coating layer (e.g. moisture saturation level and coefficient of diffusion) will be discussed. Subjection of the adhesively joined modified samples to some of the aerospace environments, especially under high temperature, cryogenic atmosphere and humid as well as chemical atmospheres is performed. Surface modification of polyimide sheet is performed using atmospheric pressure plasma and its effect on the adhesive bond strength is analyzed. Characterization of the modified polyimide and titanium surfaces will be performed to
4
establish the relationship between adhesive bond stability and surface profiles of titanium and polyimide.
1.2 Scientific importance of the research work
The use of titanium for structural application of spacecraft has already shown its potential during the early Mercury and Apollo program. Titanium alloys are still widely used in solid rocket booster cases, guidance control pressure vessels and other different applications demanding lightweight and reliability (Keohan 1998). In this context, it is also important that the aerospace industry is a larger market for titanium products primarily because of light weight material, high temperature resistance and corrosion resistance (BOEING). Recently, the AIRBUS Company is giving special attention to surface modification of titanium and its alloys, which could enhance the adhesive bond durability. In general, surface modification of titanium is carried out by chromic acid anodization. However, the alternative surface preparation technique of titanium is emphasized and that would be of interest for future research. Therefore, in the present investigation, the surface modification of titanium is carried out by atmospheric pressure plasma ion implantation and apparently, it could be of interest to AIRBUS in terms of adhesive bond durability.
It is also noted that in search of long time behavior and efficient service performance in the context of future generations of aerospace, there is an increasing need of metal-polymer composite. Therefore, work of particular relevance to this project has been on the improvement of high performance polymer-titanium composite through high performance adhesive bonding, by performing various surface modifications on metal and on polymers substrates. Based on these considerations, high temperature resistant polymeric sheet such as polyimide (Meldin) sheet, which also has excellent cryogenic properties has been selected to be joined to titanium sheet by employing ultra high temperature resistant polyimide adhesive.
However, in general, these polymers exhibit insufficient adhesive bond strength due to their reasonably low surface energy. Thus, it is necessary to modify the surface of the polymer. Surface modification essentially incorporates various polar functional groups on the polymer surface as well as increases the surface roughness. This will
5
help to enhance its surface energy and ultimately leads to the improved adhesive bond strength.
Several surface modification methods are employed to modify the polymer surfaces, such as chemical treatments, thermal treatments, mechanical treatments, and electrical treatments. An atmospheric pressure plasma treatment method will be used in the present investigation, it results into a better uniformity in surface modification of the polymers (Liston 1993; Yao 1993; Akovali 1995; Dutta 1996; Bhowmik. S 1998; Banik 1999; Bonin 2000). Moreover, it is a dry treatment method, which is better suited for industrial applications. It is now well established fact that the plasma treatment creates physical and chemical changes (such as cross linking, formation of free radicals and oxygen functionalization in the form of polar groups on the polymer surface). These changes result in an improvement of the wetting and therefore of the adhesion characteristics (Suzuki 1986; Dorn 1990; Liston 1993; Alexander 1997; Kettle 1997; Bonin 1998; Bag 1999; Bhowmik. S 2001; Chattopadhyay 2001). Further, it is emphasized that different gases could be used for generating different gas plasmas. Herewith the plasma could play a dominant role in the surface modification of polymers and titanium alloys.
The most important problem in polymer-metal adhesive joining is that the joint will be exposed to various environments. Therefore, not only the adhesion in terms of joint strength but also the durability of the joint is of major concern. It has been established that various environments can attack the polymer-adhesive interface as well as the metal-adhesive interface. This could be due to migration of foreign particles at the bond line and consequently, premature failure could take place at the interface, resulting into a significantly low joint strength.
In this regard, a surface of high temperature resistant polyimide (Meldin 7001) sheet and a titanium alloy (Ti6Al4V) will be modified by using atmospheric pressure plasma at various process parameters. It is expected that due to plasma ionization, the chemical and micro structural features of the titanium surface could change significantly. This could result into a substantial improvement in terms of durability of the adhesive bond of polyimide to titanium under aerospace environments. The adhesively bonded samples with this modified titanium interface will be subjected to various moisture and temperature levels in a humidity chamber. Also dry aging of
6
adhesively bonded modified titanium interface samples will be performed at elevated temperature.
In view of the above facts, surface modified polyimide sheet and titanium will be characterized by contact angle and surface energy measurement to identify the best wettability of the polymer interface as well as the metal surface. Physicochemical changes of the polymer and titanium surfaces will be characterized by various studies such as: surface roughness profiler, optical microscope, scanning electron microscope SEM (EDS), atomic force mass spectroscopy (AFM), X-ray photoelectron spectroscopy (XPS) analysis.
High performance polyimide adhesive is characterized by TGA, DSC and FTIR analysis. Lap shear testing under static loading will be carried out to identify the best adhesive joint. The coefficient of moisture diffusion and the moisture saturation level in polyimide adhesive layers in the adhesive joints will be determined. Durability studies of the best joint will be carried out, at various moisture saturation levels and exposed at different temperatures for specified times. Parallel experiments are run to monitor the changes in the properties of the polymer and adhesive itself when subjected to these environments. Finally, under these moisture and temperature conditions, the durability of the joints shall be correlated to the mode of failure of the joints under static loading.
1.3 Approach and Methodology
The aim of this research project is to optimize adhesive bond strength of titanium and polyimide with high performance adhesive, by using different surface treatments. Then, the investigation of stability of these adhesively bonded metal polymer composites with improved bond strength is performed under various environmental conditions. Finally, we suggest a metal polymer adhesive joint, whose bond strength is comparable to the bond strength of joints produced after conventional surface treatment, and which are suitable for the use in aerospace environment applications. It will involve following steps in the process.
7 1.3.1 Selection of Materials
Ti6Al4V-Grade 5 is the most manufactured titanium alloy. With a very high strength to weight ratio, and excellent resistance to elevated temperatures, this titanium alloy is widely used in aerospace industry and subsea applications.
During the past three decades since the commercialization of polyimide, an impressive variety of polyimides has been synthesized for both commercial and scientific interest. In this research project Polyimide sheet of the Meldin 7000 series from Saint Gobain is used.
Meldin 7000 polyimide exhibits extremely high stability at elevated temperatures and is ideal for electrical and thermal insulating applications.
As a high temperature resistant polyimide adhesive we selected “124-41” high temperature and thermally conductive PI adhesive from Creative Materials, Inc MA, USA. This polyimide adhesive has a glass transition temperature higher than 250°C and is thermally stable above a temperature of 380°C.
1.3.2 Analysis of base materials
Investigation of base materials (before modification) is performed by a sequence of experimental tests listed below
a. Surface characterization of titanium alloy and polyimide (Meldin 7000) sheet, by measuring the contact angle and by estimating the surface energy b. Measuring the surface roughness of the unmodified titanium alloy and
polyimide sheet
c. Optical microscope studies
d. SEM (EDS) studies of titanium and polyimide sheets e. DSC , TGA , and FTIR analysis of polyimide adhesive
f. AFM analysis of the untreated polyimide sheet g. XPS analysis of the untreated titanium surface
8 1.3.3 Surface Modification
a. Surface modifications of titanium have been performed with mechanical treatment and with atmospheric pressure plasma.
b. Surface modifications of polyimide sheet have been performed by atmospheric pressure plasma.
1.3.4 Analysis after modification
a. Surface characterization by measuring the contact angle and by estimating the surface energy
b. Surface roughness measurement of modified titanium surface with a Wyko surface roughness profiler
c. Investigation of chemical changes on the titanium surface by XPS studies d. Investigation of surface morphology of modified titanium and of
polyimide sheet with SEM (EDS) studies
e. Surface roughness analysis of modified polyimide sheet with AFM studies
1.3.5 Characterization of high performance polyimide adhesive
a. DSC analysis of polyimide adhesive b. TGA analysis of the polyimide adhesives
c. FTIR analysis of the polyimide adhesive for understanding the curing kinetics
1.3.6 Joining of polyimide (Meldin 7000)sheet with titanium alloy by high performance adhesive
Three different kinds of samples will be prepared by joining base specimens as well as plasma modified specimens. The titanium alloy and polyimide Meldin 7000 sheet samples are prepared and joined with high performance polyimide adhesive according to ASTM standards (D-1002).
a. polyimide sheet– polyimide adhesive – titanium sheet b. titanium sheet – polyimide adhesive – titanium sheet
9
c. polyimide sheet – polyimide adhesive – polyimide sheet
1.3.7 Studies of mechanical properties of joints
For lap shear testing, samples mentioned at 1.3.6 are categorized into groups mentioned below and lap shear testing is performed according to ASTM standards.
a. Static lap shear (SLS) testing of adhesively bonded basic titanium substrate b. Static lap shear (SLS) testing of adhesively bonded basic PI substrate c. Static lap shear (SLS) testing of titanium substrates joined after mechanical
treatment
d. Static lap shear testing (SLS) of titanium substrates joined after mechanical and atmospheric pressure plasma treatment
e. Static lap shear testing (SLS) of adhesively bonded polyimide substrate after atmospheric pressure plasma treatment
f. Static lap shear (SLS) testing of modified titanium and modified polyimide joined together with high performance adhesive.
1.3.8 Evaluation of performance and durability of the improved adhesive bond strength at elevated temperatures and moisture levels
a. Sample preparation for moisture sorption analysis.
b. Determination of the diffusion coefficient (D) and the moisture saturation level (MSat) for high performance polyimide adhesive.
c. Calculation of the time required to reach the moisture saturation level, for a polyimide adhesive layer in substrate joints.
d. Pre-conditioning of the lap shear test specimen, in a humidity chamber at various elevated temperatures and relative humidity levels.
e. Dry aging of the specimens prepared for the lap shear testing.
f. Static lap shear testing of preconditioned samples and dry aged samples with a tensile testing machine.
10 1.3.9 Correlation of failure modes and bond strength with moisture saturation
level and temperatures
a. Sample preparation for moisture sorption analysis
b. Determination of the time required to reach 25%, 50% and 70% moisture saturation in the adhesive layer.
c. Preconditioning of titanium lap shear samples in the moisture oven at elevated temperature and relative humidity level for a given number of days for the above mentioned moisture saturation levels.
d. Lap shear testing of preconditioned samples with 25%, 50% and 75% moisture saturation levels
e. Correlation between moisture saturation level and adhesive bond strength
1.3.10 Analysis of results and conclusions
The analysis of the experimental results is presented in the discussion section of each chapter. Based on the results and arguments made in the discussion section of each chapter, a conclusion is drawn in the last part. Manuscripts are prepared from each chapter and communicated to international peer reviewed journals for publication. Further recommendations are made for industry and scientific researchers about metal and polymer surface optimization.
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1.4 Structure of the thesis
The dissertation on the work carried out has been organized into seven chapters with contents as outlined below.
After describing the context of the work objectives of the research project in
Chapter 1, the necessary background literature review is presented in Chapter 2. It
begins with an introduction to adhesion and different theories of adhesion. A critical review of literature about selection of titanium alloy based upon its application in industry is carried out. Other composites, frequently used in automotive and aerospace industries, are also discussed. A literature review on polyimides for the selection of the most suitable sheet is performed. A literature review for the selection of an appropriate high performance polyimide adhesive is also performed. There is also a discussion about the different methods of surface modification and the methods of characterization of the modified metal and polymer surfaces, by surface energy estimation, measurement of the surface roughness, SEM, XPS and AFM analysis. It also deals with limitations and drawback of conventional surface treatment methods and identifies the needs for searching some alternative techniques. Chapter 2 ends up with some of the problems identification during surface treatment and adhesive bonding of titanium alloys and polyimide sheet, while using them for industrial application. It defines parameters and the work line of the research project.
Chapter 3 deals with characterization and evaluation of high performance
polyimide adhesive. DSC and TGA are performed to establish the thermal stability and determination of the glass transition temperature of high performance polyimide adhesive. FTIR analysis is performed to understand the cure kinetics, crystallization, and cross-linking reactions during adhesive bonding.
Chapter 4 deals with surface modification of titanium with different surface
modification techniques and its effects on the adhesive bond strength. This chapter concludes with the surface modification technique that is most suitable to obtain the optimized adhesive bond strength of polyimide adhesive and titanium alloy substrate. It also involves investigation of the modified titanium surface with XPS analysis and establishes a relationship between the lap shear strength, the surface energy and the surface characteristics of the titanium alloy surface.
12 Chapter 5 of the thesis deals with surface modification of polyimide sheet with
atmospheric pressure plasma and draws a conclusion about the relationship between the surface energy and the polyimide sheet lap shear joint bond strength when bonded using high performance polyimide adhesive.
In Chapter 6, the durability of the improved adhesive bond strength of titanium samples at elevated temperature and moisture levels is investigated. Samples subjected to different relative humidity, temperature levels are subsequently subjected to mechanical testing. A relationship between relative humidity and temperature with the adhesive bond strength is established at the end of this chapter.
Chapter 7 concludes with the outcome of this research work and defines the
direction of future research work in the field of adhesive bonding of metals and polymers for high temperature applications.
1.5 Application in industry
Adhesive bonding has many advantages over other ways of joining materials, but its use is often restricted by temperature and environmental limitations (Liston 1993; Critchlow and Brewis 1995; Higgins 2000; Chattopadhyay 2001). This project will help to remove some of these limitations.
Adhesive bonding of dissimilar materials is playing an increasingly important role in the manufacture of aerospace structures, satellite, rockets, and vehicles for road and rail transport. The results of this project will be helpful and will provide a guideline for selection of adhesives coatings (Chapter 3) & surface modification techniques (Chapter 4, Chapter 5) for optimized adhesive bond strength in different environmental conditions for aerospace, automotive, and offshore marine engineering (Chapter 6). In these areas the problems are analogous to those in aerospace applications, although far less acute. Thus, in general, the manufacturing industry will certainly benefit from the advances made in this project. More broadly, the project aims to elucidate the fundamental mechanisms of adhesion in the bonds as described and so will benefit to scientists working in the field of adhesion and adhesive bonding of metals and polymers.
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1.6 References
Akovali, G., Rzaev, Z. M. O. and Mamedov, D. H (1995). "Plasma Surface Modification of Propylene Based Polymers by Silicon and Tin Containing Compounds." Journal of Applied Polymer Science, 58: 645-651.
Alexander, M. R., Jones, F. R. and Short, R. D. (1997). "Radio Frequency Hexamethyldisiloxane deposition: A Comparison of Plasma and Deposit Chemistry." Plasma and Polymers 2(4): 277-300.
Bag, D. S., Kumar, V. P. and Maiti, S (1999). "Chemical Modification of LDPE Film." Journal of Applied Polymer Science, 71: 1041-1048.
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Bhowmik. S, G. P. K., Ray. S and Barthwal. S. K , (1998). "Surface Modification of High Density Polyethylene and Polypropylene by DC Glow Discharge and Adhesive Bonding to Steel." Journal of Adhesion Science and Technology 12(11): 1181-1204.
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Chattopadhyay, S., Ghosh, R. N., Chaki, T. K. and Bhowmik, A. K. (2001). "Surface Analysis and Printability Studies on Electron Beam-Irradiated Thermoplastic Elastomeric Films from LDPE and EVA Blends." Journal of Adhesion Science and Technology 15(3): 303-320.
Critchlow, G. W. and D. M. Brewis (1995). "Review of surface pretreatments for titanium alloys." International Journal of Adhesion and Adhesives 15(3): 161-172.
Donachie, M. J., Jr. (1988). TITANIUM: A Technical Guide. Metals Park, OH, USA.
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Yao, Y., Liu, X. and Zhu, Y., (1993). "Surface Modification of High Density Polyethylene by Plasma Treatment." Journal of Adhesion Science and Technology
16
CHAPTER – II
17
Adhesive bonding, as a joining technology, is already applied in ancient times using natural materials. The first known application of adhesive is the use of bitumen [a natural substance that contains hydrocarbons found on the surface of the earth in tar or asphalt pits] about 36,000 years ago. Various adhesive materials of animal or vegetable origin were used in ancient Egypt 3,300 years ago. With the development of synthetic polymeric materials, higher loaded joints in more demanding applications became possible. Nowadays, in our daily life, many products are used without even noticing the bonded joints in for example food packaging, consumer goods, furniture, sporting goods, electronic equipment, cars, trains, aircraft, buildings, etc. Adhesive bonding is the most suitable method of joining both for metallic and non-metallic structures where strength, stiffness and fatigue life must be maximized at a minimum weight (Mittal 2003, Packham.D.E; 2005).
Polymeric adhesives may be used to join a large variety of possible material combinations including metal-metal, metal-plastic, metal-composite, composite-composite, plastic-plastic, metal-ceramic systems. Wetting and adhesion are also studied in some detail in the present chapter. Since the successful surface pretreatments of the adherents for the short- and long-term durability and performance of the adhesive joints mostly depend on these factors. Wetting of the adherents by the adhesive is critical to the formation of secondary bonds according to the adsorption theory. It has been theoretically verified that for complete wetting (i.e. for a contact angle θ, equal to be zero), the surface tension of the adhesive must be lower than the surface energy of the adherend. Therefore, the primary objective of a surface pretreatment is to increase the surface energy of the adherend as much as possible. The influence of surface pretreatment and aging conditions on the short and long-term strength of adhesive bonds should be taken into account for durability design. Some form of substrate pretreatment is always necessary to achieve a satisfactory level of long-term bond strength. In order to improve the performance of adhesive bonds, the adherends surfaces (i.e. metallic or non-metallic) are generally pretreated using the (a) physical, (b) mechanical, (c) chemical, (d) photochemical, (e) thermal, or (e) plasma method.
18 Almost all pretreatment methods bring some degree of change in surface roughness, but mechanical surface pretreatment i.e. grit-blasting is usually considered as one of the most effective methods to control the desired level of surface roughness and joint strength. The overall effect of mechanical surface treatment is not limited to the removal of contamination or to an increase in surface area. This also relates to changes in the surface chemistry of adherends. Suitable surface pretreatment increases the bond strength by altering the substrate surface in a number of ways. Some of these steps are (a) increasing the surface tension by producing a surface free from contaminants (i.e. surface contamination may cause insufficient wetting by the adhesive in the liquid state for the creation of a durable bond), (b) by removal of the weak cohesion layer or the pollution present at the surface, and (c) increasing the surface roughness by changing the surface chemistry and hence producing a macro/microscopically rough surface.
Introduction to adhesion
2.1
An adhesive is a material that is applied to the surfaces of articles to join them permanently by an adhesive bonding process. Adhesives join parts together by transmitting stresses from one part to another in a way that distributes the stresses more uniformly than is the case with mechanical fasteners. Adhesive bonding often provides structures that are at least as strong as conventional assemblies at lower cost and weight. Many adhesives readily join dissimilar materials, including plastics. Adhesives can be classified in a number of ways, e.g., by chemical structure or functionality, and are categorized as natural or synthetic. Theories of adhesion include mechanical interlocking, electrostatic, diffusion, and adsorption/surface reaction theories.
2.1.1 Definition of adhesive and adhesive bonding
An adhesive is a substance capable of forming bonds to each of the two or more parts comprising the final object (ASTM 2012). Adhesive bonding is used as a joining technique next to other joining techniques like welding, soldering, riveting, screwing, bolting, etc. An adhesive is defined as “a polymeric substance with visco-elastic behavior, capable of holding adherends together by surface attachment to produce a joint of high shear strength.” A feature of adhesives is the relatively small quantities
19 required relative to the weight of the final object. An entirely satisfactory definition for adhesion has not been found. The following definition has been proposed by (Wu S 1982) “Adhesion refers to the state in which two dissimilar bodies are held together by intimate interfacial contact such that mechanical force or work can be transferred across the interface." The interfacial forces holding the two phases together may arise from van der Waals forces, chemical bonding, or electrostatic attraction. The mechanical strength of the system is determined not only by the interfacial forces, but also by the mechanical properties of the interfacial zone and the two bulk phases. In mechanical fastening, the strength of the structure is limited to that of the areas in contact with the fasteners (Joining techniques 1976). It is common to obtain adhesive bonds that are stronger than the strength of the adherends.
2.1.2 Fundamentals of adhesive bonding
The words adhesion and cohesion are important terminology in adhesive bonding. Adhesion is the state in which interface forces hold two surfaces together and cohesion is the state in which chemical and physical forces hold the constituents of a mass of material (the adhesive) together.
There are two principal types of adhesive bonding: structural and nonstructural. Structural adhesive bonding is bonding for applications in which the adherends (the objects being bonded) may experience large stresses up to their yield point. Structural adhesive bonds must be capable of transmitting stress without losing of integrity within design limits (Mittal K.L 2003). Bonds must also be durable throughout the useful service life of a part, which may be years. In addition to possessing significant resistance to aging, a structural bond is defined as having a shear strength larger than 7MPa. Nonstructural adhesives are not required to support substantial loads, but merely hold lightweight materials in place. This type of adhesive is sometimes called a “holding adhesive.” Pressure-sensitive tapes and packaging adhesives are examples of nonstructural adhesives. Smooth surfaces are an inherent advantage of adhesively joined structures and products.
One of the important features of adhesive bonding is that exposed surfaces are not defaced and contours are not disturbed, such as happens with mechanical fastening systems. This feature is important in function and appearance. Aerospace structures, including helicopter rotor blades, require smooth exteriors to minimize
20 drag and to keep low temperatures. Lighter weight materials, rather than are used with conventional fastening, can often be used with adhesive bonding because the uniform stress distribution in the joint permits full utilization of the strength and rigidity of the adherends (Ebnesajjad 2011). Adhesive bonding provides much larger areas for stress transfer throughout the part, thereby decreasing stress concentrations in small areas.
Many adhesives readily join dissimilar materials, including plastics metals and composites, if proper surface treatments are used. Adhesives can be used to join metals, plastics, ceramics, cork, rubber, and combinations of materials. Adhesives can also be formulated to be conductive.
Where temperature variations are encountered in the service of an item containing dissimilar materials, adhesives perform another useful function. Flexible adhesives are able to accommodate differences in the thermal expansion coefficients of the adherends, and therefore prevent damage that might occur if stiff fastening systems were used.
Sealing is another important function of adhesive joining. The continuous bond seals out liquids or gases that do not attack the adhesive or sealant. Adhesives/sealants are often used in place of solid or cellular gaskets. Mechanical damping can be imparted to a structure using adhesives formulated for that purpose. A related characteristic, fatigue resistance, can be improved by the ability of such adhesives to withstand cyclic strains and shock loads without cracking. In a properly designed joint, the adherends generally fail in fatigue before the adhesive fails. Thin or fragile parts can also be adhesively bonded. Adhesive joints do not usually impose heavy loads on the adherends, as in riveting, or localized heating, as in welding. The adherends are also relatively free from heat-induced distortion (Petrie 2007).
2.1.3 Classification of adhesives
Adhesives can be classified in a number of ways, like based on structure or functionality. Polymeric materials, which fall within the classifications of thermoplastics, thermosetting resins, elastomers, and natural adhesives, may serve the adhesive functions. Adhesives are categorized into two classes: natural and synthetic. The natural group includes animal glue, casein-and protein-based adhesives, and natural rubber adhesives. The synthetic group has been further divided into two subcategories: industrial and special compounds. Industrial compounds include
21 acrylics, epoxies, silicones etc. An example of the specialty group is pressure-sensitive adhesives.
2.1.4 Process of adhesive bonding
Polymeric adhesives may be used for joining the non-metallic third materials in the adhesive bonded joints. The adhesives, generally a macromolecular or polymeric material are used. Other materials, which are used, include for instance ceramic materials for very high temperature applications. The close contact between atoms in an adhesively bonded joint is created by using the organic intermediate layer between the adherend surfaces. If the atoms of the organic molecules are capable to come near the surface of the substrate at inter-atomic distance, a chemical or physical bond will be formed.
In practice with a metal bonded joint, the adhesive material is not adhering directly to the metal, but to the always-present oxide layer. This means that the strength of a bonded joint not only depends on the strength of the adhesive material but also on the strength of the oxide layer or other intermediate layers.
In contrast to other joining methods such as riveting and bolting, bonding has no adverse effect on the material characteristics of the surfaces to be bonded e.g.: drilling of holes in the joined parts, damaging them, and creating stress concentrations. There is also no adverse effect of high temperatures on mechanical properties or distortion by local heating as is the case with most welding or soldering processes.
In the manufacturing environment, bonding technology permits characteristic material properties to be utilized to the utmost.
2.1.5 Advantages of adhesive bonding
An important advantage of adhesive bonding is the improvement of the properties of the structure. The already mentioned design for improved fatigue and damage tolerance properties can result in fail–safe characteristics of a structure. Special structural design concepts are possible with specific characteristics. Sandwich panels, built-up laminates, and large doublers around cutouts use the adhesively bonded joining of materials over large surface areas.
22 The adhesively bonded joint should be designed to be mainly loaded in shear, due to the relatively lower tensile strength of the interfaces. However, by a careful joint design the efficiency of the joint (ratio of the strength of the joint to the undisturbed substrate) can be larger than one.
Adhesively bonded structures show a more uniform distribution of stresses in the bonded areas, without loss of cross-section or stress concentrations due to fastener holes. Stress concentrations may be present in bonded joints due to eccentricity in the joint. As bonding processes can be performed with relatively low-heat, geometric distortions do not occur and there is no effect on substrate properties. For these reasons, adhesive bonding is suitable for component assembly, with special adhesives with good gap filling properties and long open times for larger components. The aforementioned structural advantages can lead to lower weight designs with the same performance. These weight savings often simultaneously leading to cost savings are especially beneficial in aerospace applications, but become more and more of interest in other industrial sectors.
Another advantage of the adhesively bonded structure is the smooth surface. There is no deformation due to mechanical fastening or heat distortion like welding. For aerospace structures with high requirements for aerodynamic smoothness, this is an advantage. In other application areas like automotive and shipbuilding, this means less rework to get the smooth aesthetic outlook needed. The bonded joints are air and watertight due to their continuous nature. This means that the joint can act as a pressure barrier like in the pressure cabin of aircrafts, watertight joints in ships, and fuel tight joints in fuel tanks of aircraft.
Adhesively bonded joints create the possibility to new structural design solutions. The adhesively bonded joint in structural applications has not only a load carrying, but also a shape defining function. Sandwich structures, where high strength materials are used as face sheets with a relative weak material such as foam, used as a core, show an excellent bending stiffness. By laminating, the composite materials can be created with improved, often tailored characteristics compared to the properties of the individual layers.
Below is a summary of advantages or specific aspects of bonding technology:
23
Joins thin or thick materials of any shape
Joins similar or dis-similar materials (Petrie E.M. 1996)
Minimizes or prevents electrochemical (galvanic) corrosion between dissimilar materials
Resists fatigue and cyclic loads
Provides joints with smooth contours
Seals joints against a variety of environments
Insulates against heat transfer and electrical conductance (in some cases adhesives are designed to provide such conductance)
The heat required to set the joint is usually too low to reduce the strength of the metal parts
Provides an attractive strength/weight ratio
Quicker and/or cheaper to form than mechanical fastening
Joining of surfaces without direct contact
Smooth air and water tight joints
Joining of heat-sensitive materials
Weight-saving
Bonding of materials over large areas
Uniform distribution of stresses in bonded areas
High vibration damping and shock absorption
Easy integration into automated production processes
Joining of different materials
Prevention of crevice corrosion
2.1.6 Limitation of adhesively bonded joints
An important disadvantage in the design of bonded structures is the change in properties of the polymeric adhesive materials with temperatures. Especially in aircrafts, operating in a very wide range of temperatures of -60°C at cruising altitude to a heated structure of +90°C on the ground in tropical desert areas, the design has to consider these changes in conditions having an effect on adhesive properties. Special adhesives have been developed for even higher temperature applications, such as engine surrounding structures or aerodynamic heating effects by supersonic flight. Furthermore, moisture saturation by environmental exposure has an additional effect
24 on adhesive properties. Clearly the designer has to take these effects on properties into account in his design and design verification (De Lollis N.J. 1970, Rabilloud.G; 2000).
The bond does not permit visual examination of the bond area (unless the adherends are transparent) (Sharpe L.H. 1966).
Careful surface preparation is required to obtain durable bonds, often with corrosive chemicals.
Long cure times may be needed, particularly where high cure temperatures are not used.
Holding fixtures, presses, ovens, and autoclaves, not usually required for other fastening methods, are necessities for adhesive bonding.
Upper service temperatures are limited to approximately 177°C in most cases, but special adhesives, usually more expensive, are available for limited use up to 371°C.
Rigid process control, and emphasis on cleanliness, is required for most adhesives.
The useful life of the adhesive joint depends on the environment to which it is exposed.
Natural or vegetable-origin adhesives are subject to attack by bacteria, mold, rodents, or vermin.
Exposure to solvents used in cleaning or solvent cementing may present health problems.
Adhesively bonded joints are difficult to disassemble. For joints, where due to manufacturing, maintenance or reparability reasons, disassembly of
components during the life time of a product is desirable, mechanically fastened joints are preferred.
Adhesive bonded joints have to be designed as shear loaded joints, as the interface tensile strength is often relatively low.
Relatively complex manufacturing processes necessary to guarantee reliable, durable adhesive bonded joints.
25 2.1.7 Requirements of a good adhesive bond
The basic requirements for a good adhesive bond are (Petrie 2007).
a. Proper choice of adhesive b. Good joint design
c. Cleanliness of surfaces
d. Wetting of surfaces that are to be bonded together
e. Proper adhesive bonding process (solidification and cure)
a. Proper choice of adhesive
There are numerous adhesives available for bonding materials. Selection of the adhesive type and form depends on the nature of adherends, performance requirements of the end use, and the adhesive bonding process.
b. Good joint design
Imparting strength to a joint by design is possible. A carefully designed joint can yield a stronger bond by combining the advantages of the mechanical design with adhesive bond strength to meet the end use requirements of the bonded part.
c. Cleanliness
To obtain a good adhesive bond, starting with a clean adherend surface is essential. Foreign materials such as dirt, oil, moisture, and weak oxide layers must be removed; otherwise, the adhesive would be bonded to weak boundary layers rather than to the substrate. Various surface treatments exist that remove or strengthen the weak boundary layers. Such treatments typically involve physical or chemical processes, or a combination of both (Ebnesajjad 2006).
d. Wetting
Wetting is the displacement of air (or other gases) present on the surface of adherends by a liquid phase. The result of good wetting is a greater contact area between the adherends and the adhesive over which the forces of adhesion may act (Satas .D 2001).
26
e. Adhesive bonding process
Successful bonding of parts requires an appropriate process. The adhesive must not only be applied to the surfaces of the adherends; the bond should also be subjected to the proper temperature, pressure and hold time. The liquid or film adhesive, once applied, must be capable of being converted into a solid in one of three ways. The method by which solidification occurs depends on the choice of adhesive. The ways in which liquid adhesives are converted to solids are (Petrie 2007).
Chemical reaction by any combination of heat, pressure and curing agents
Cooling from a molten liquid
Drying as a result of solvent evaporation
2.1.8 Theories of adhesion
The definition of the word “adhesion” depends on whether the viewpoint is macroscopic or microscopic. However, it is important to realize an intimate contact between the adherend and the adhesive is necessary for the adhesion forces to be operative. The various theories of adhesion essentially differ in qualifying the nature of these inherent adhesion forces. It is difficult to fully ascribe, adhesive bonding to an individual mechanism. A combination of different mechanisms is most probably responsible for bonding. The extent of the role of each mechanism may vary for different adhesive bonding systems. An understanding of these theories is helpful for further research work in adhesive bonding.
The bonding of an adhesive to an object or a surface is the sum of a number of mechanical, physical, and chemical forces that may overlap and influence one another. As it is not possible to separate these forces from one another, we distinguish between mechanical interlocking (caused by the mechanical anchoring of the adhesive in the pores and the uneven parts of the surface), electrostatic forces (regarding to the difference in electro-negativities of adhering materials), by the adhesion mechanisms. It deals with intermolecular physical and chemical bonding forces that occur at the interfaces of heterogeneous systems. This adhesion mechanism is explained in the case of the intermolecular forces by the adsorption theory and in the case of chemical interactions by the chemical sorption theory. The processes that play a role in the
27 bonding of similar types of thermoplastic high-polymer materials, e.g. homogeneous systems, can be partly explained with the diffusion theory.
These theories are briefly addressed here to highlight the essential concepts, on which they are based.
Mechanical Interlocking
This theory postulates that the adhesion is achieved because of flowing of an adhesive into a rough surface and the resulting ‘interlocking.’ Thus, this mechanical anchoring between the adhesive and the adherend prevents the removal of adhesive from the substrate. However, it is important to realize the degree of roughness that is being considered and the spreading of the adhesive that is achieved. It is meaningful to remember that the increase in roughness also results in availability of more area for intimate contact (Hopkins.D.G; 1925). The various surface treatments themselves have been divided on the basis of roughness produced (i.e. pore size on the surface of the adherend) (Venables J.D 2002) as:
Group I: Surface treatments that produce no micro-roughness (pore size< 0.1mm) or Macro-roughness (pore size> 0.1mm).
Group II: Surface treatments that result in a large degree of macro-roughness. Group III: Surface treatments that result in a large degree of micro-roughness due
to a porous oxide layer, with little or no macro-roughness produced.
Fig.2.1: Mechanical interlocking theory
This theory explains a few examples of adhesion such as rubber bonding to textiles and paper adhesion but not on molecular scale. Certain pretreatments result in micro- or nano-roughness on the adherend surface, which can improve the bond strength and the durability by providing mechanical interlocking. Beyond mechanical interlocking, the enhancement of the adhesive joint strength due to the roughing of the
28 adherend surface may also result from other factors such as formation of a larger surface, improved kinetics of wetting and increased plastic deformation of the adhesive. In the case that the surface roughness leads to stress concentrations at the interface, the effect may even result in a lower joint strength.
Adhesives frequently form stronger bonds to porous abraded surfaces than they do to smooth surfaces. However, this theory is not universally applicable, for good adhesion also occurs between smooth surfaces (DeMejo. L.P 1999). There is data available in literature, which suggests an increased joint strength and bond durability because of increased surface roughness. There are also contrary observations indicating that increased roughness can lower the joint strength (Allen 1993).
Diffusion Theory
This theory proposed by Voyutskii and Vakula states that the polymer-polymer adhesion results from inter-diffusion of polymer molecules across the interface. The diffusion theory is primarily applicable when both the adhesive and the adherend are polymers with relatively long-chain molecules capable of movement. The nature of materials and bonding conditions will influence whether and to what extent diffusion occurs. The diffuse interfacial (interface) layer typically has a thickness in the range of 10–1,000Å (1–100 nm). Solvent cementing or heat welding of thermoplastics occurs due to diffusion of molecules (Petrie 2007).
The theory was proposed to account for the experimental results dealing with adhesion between dissimilar polymers when satisfactory explanations could not be reached by applying other existing theories. The theory accounted for effects of contact time, influence of time and temperature on bonding rate, and the influences of polymer molecular weight and polymer structure. Additionally, these models take into account the motion of the entire chain across the interface. While diffusion applies well for cases of self-adhesion or autohesion, its sole use to provide a satisfactory explanation for polymer-polymer adhesion is questionable. High molecular weight thermoplastic polymers that often display a very high melt viscosity may not diffuse easily within the time frame of most bonding operations (Pizzi.A; 1999, Ebnesajjad 2008).
29 Fig.2.2: Inter-diffusion of Polymers (Diffusion Theory)
Electrostatic Theory
This theory proposes that adhesion takes place due to electrostatic effects between the adhesive and the adherend. It is based on the argument that the electrostatic forces arising from the junction potentials between the contacting adhesive and the substrate will contribute significantly towards the forces required for rupturing the bonds. This theory was proposed by Derjaguin who assumed the formation of an electrical double layer due to electron transfer during contact. An electron transfer is supposed to take place between the adhesive and the adherend as a result of unlike electronic band structures (Deraguin.B.V 1967). Electrostatic forces in the form of an electrical double layer are thus formed at the adhesive–adherend interface. These forces account for the resistance to separation.
While this concept may be useful to explain some specific instances of adhesion, substantial doubts have been cast regarding its overall utility. These include improved adhesion strengths with lowering of temperature for a large variety of systems (lower temperatures will favor smaller charge densities and hence poorer electrostatic forces) and negligible changes in adhesion performance with gross variations in the electronic character of the adhesives (Possart .W 1988). The contribution of the electronic mechanism in nonmetallic systems to adhesion has been calculated and found to be small when compared with that of chemical bonding (Roberts 1977).
Adsorption theory
The adsorption theory is the most generally accepted model; it was introduced by Sharpe and Schonhorn (Schonhorn 1972). The adsorption theory states that the materials will adhere because of the inter-atomic and intermolecular forces that are established between the atoms and molecules in the surfaces of the adhesive and the substrate after their intimate contact. This theory is the most important mechanism in
