water absorption and degradation in adhesive joints

184 Pages·2000·8.31 MB·English

WATER ABSORPTION AND DEGRADATION IN ADHESIVE JOINTS
A thesis submitted in conformity with the requirernents for the degree of
Master of Applied Science
Department of Mechanical and Industrial Engineering
University of Toronto
0 Copyright by Yijun Tu, 1999
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Abstract
Water Absorption and Degradation in Adhesive Joints
An MASc. Thesis
by
Yyun Tu
Department of Mechanical and Industrial Engineering
University of Toronto
1999
To assess the durability of adhesive joints, blistering as a form of degradation and
water dihion in adhesive joints were hvestigated. In addition, secondary bond effécts on the
measurement of critical strain energy release rate were examined to validate the open-faced
specimen technique, and the degradation parameter was studied in greater detail.
It was found that open-faced specimens can be used to test the hcture strength of the
prirnary-bond with the secondary bonds having an insifificant effect. The degradation parameter
concept requires M e re valuation with other adhesives over longer exposure penods.
Tt was also concluded that adhesive layers in closed joints cured at elevated temperatures have
water diffusion coefficients and equilibrium water concentrations greater than those of the buik
adhesive. However, water absorption in ciosed joints can be predicted ushg Fick's law with the
diffusion properties of the closed joints. Finally, it has been shown that blistering due to osmotic
pressure is a fom of degradation in joints exposed to high relative humidities.
Acknowledgments
1 thank my supervisor, Dr. Jan K. Spelt, for his guidance, encouragement, patience,
and kindness during this project. Thanks are extended to my wife Liming, whose love and
support made this achievement possible.
1 would like also offer rny thanks to James Wylde, AbduljaleeL Moidu, Ion Vintilescu
for helping me out on countless technical details.
Thanks are also due to my colleagues in the Materials and Process Mechanics Lab at
University of Toronto, in particuiar, Shuwen Wang, Payam Tangestanian, Marcello Papini,
Munir Ahmed, Boris Djurovic, Ivey Chiu, and Alan Wang for not only their technical assistance,
but aiso their fiendship.
1 gratefidly aclmowledge the financial support of the Natural Sciences and
Engineering Research Council of Canada, and University of Toronto. Cytec Industries Inc. kindly
provided the adhesives used in the research.
Table of Contents
Table of Contents
Abstract
Acknowledgment
Nomenciature, v
List of Tables, viii
List of Figures, uii
Chapter 1 Introduction
1.1 Background and Motivation, 1
1.2 Literature Review, 2
Water Transport in Adhesive Joints, 3
Blistering of Urganic Coatings. 4
DurabiIity and Acceierated Aging Processes. 5
1.3 Ovewew of the Thesis, 7
Chapter 2 Secondary Bond Effect on Strain Energy Release Rate of DCB
Specimens
2.1 Introduction, 9
2.2 Experimental Procedures, 10
2.3 Results and Discussion, 12
2.4 Conclusions, 15
Table of Contcnss
Chapter 3 Water Diffusion in Adhesive Joints
3.1 Introduction, 16
3.2 Matbernatics of Water Diffision into Adhesive Joints, 17
One-dimenirioml DI-ion Model, 17
Twdimensional Dz-ion Model, 19
3.3 Expenmenîal Procedures, 21
3.3.1 The RH EQ"libnum Behavior of Environment Chambers at a Given
Temperature, 21
3 -3.2 Experimentai Rocedures-Water Diffusion in Bulk Adhesives, 26
3 -3- 3 Experimental Procedures-Water Ditrusion in Saw-cut and We-cut
Sandwiches, 27
3 -3- 4 Experimentai Rocedures-Water Diffusion in Sandwiches with Uncut Edges, 32
3.4 Results and Discussion, 35
3 -4.Z Water DitEsion Ptoperties of Bulk Adhesives, 35
3 A.2 Results and Discussio~)-Mass Gain of Adherend Surface and Teflon Spacers, 43
3.4.3 Results and Discussion--Water DWion in Closed Joints, 46
Sm-eut specimens. 46
Uncut Specimens, 56
ffige-cur Specimens, 60
3.4.4 Models of Water diaision in Closed JO*, 83
interfocial Dzfiion Hypothesk, 83
Residwl Stress Hypathesis, 87
Table of Contents
Fich-an Characterizarion of Water Drmion in Chsed Joints. 89
3.5 Conclusions, 97
Chapter 4 Blistering in Adhesive Joints
4.1 Introduction, 98
4.2 Experimental Observations, 99
4.2.1 Blistering on Open-faced DP specimens, 99
4.2.2 Blisterhg on Purposely Contambted Specimens, 102
4.2.3 Blistering in Closed Joints, 104
4.3 Analysis and Discussion, 107
4.3.1 Water Uptake due to Fickian Dinùsion in Blisters, 107
4.3 -2 Corrosion and Blistering, 1 10
4.3.3 Chernical and Morphological Analysis of Blisters, 1 12
4.4 Mechanisms of Blistering in Adhesh Joints, 118
4.4.1 B k t e ~dgue to Osmosis, 1 18
4.4.2 Blisterhg due to Swelling of Adhesive Surrounding Air Bubbles, 123
4.5 Conclusions, 126
Chapter 5 Experimental Investigation of the Degradation Parameter
5.1 Introduction, 127
5.2 Experimental Proceàures, 128
5 -2.1 Experimental Proceàures Adopted fiom Ref. [3], 128
5 -2.2 Specimen Fabrication, 130
Table of Contents
5.2.3 Fracture Tests, 13 1
5.2.4 Testing Scheduies, 133
5.3 Results and Discussion, 137
5.3.1 Control Values of Gc for Joints of Cybond 4523GB and Cybond 1 126, 137
5 -3.2 Results of Degraded Cybond 4523GB and Cybond 1 126 Specimens, 13 8
5.4 Conclusions, 146
Chapter 6 Conclusions and Future Work
6.1 Conclusions, 148
6.2 Future Work, 149
References, 150
Appendix A, Al
Nomenclature
a-in Chapter 4, hear sweihg expansion rate; in Chapter 5, Fz/Fi
aA-linear thermal expansion rate of adhesive
elineart hermal expansion rate of adherend
A'( A + 6)
p i n C hapter 4, hctional swelling expansion; in Chapter 5, - 1
u3
6-thickoess of interfacial zone in adhesive joint
+Poisson ratio
v4-Poisson ratio of adhesive
-Poisson ratio of adherend
ksmotic pressure
p-density of adhesive in dry state
û-angle of spherical cap fonned by blister outer surface
J
-phare angle, a r c 4 $ ( 2 1
a-crack length
A-distance between Ioading point and the clamp closer to the Ioading pins of Ioad jig
-stance between the two end restraints on load jig
c-concentration of solution
C,C (x,y,zt*water concentration of point (xpypza)t time r
Cs-quilibrium water concentration in percentage of the mass of hosting adhesive
D-water difkion coefficient in adhesive
Do-base value of diffusion coefficient in Arrhenius Equation
D '-water diff'usion coefficient in interfacial zone of adhesive joint
a-ffective water diffusion coefficient in closed joints in Fickian diffiision mode1
DP4egradation parameter
e-height of blisters
E-Young 's rnodulus
EA-in Section 3.4.1, surface activation energy of adhesive surface; in Section 3.4.4, Young's
moduius of adhesive
&-Young3 modulus of adheread
FI-force applied to the upper adherend of DCB joint clarnped in a ioad jig
F d o r c e applied to the lower adherend of DCB joint clamped in a load jig
G-applied strain energy release rate
Gmntical strain energy release rate of adhesive joints
G ~ r i t i c asltr ain energy release rate of adhesive joints at pure mode 1
~ ~ O - ~ o n tvraolule of Gc measured fiom undegraded adhesive joints