WATER ABSORPTION AND DEGRADATION IN ADHESIVE …

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 d i h i o n 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 r evaluation 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, 1 7

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. 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 D W i o n 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 D z f i i o n 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


4.2 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 ~ g due 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


5.2 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


5.3.1 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


Chapter 6 Conclusions and Future Work


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

e l i n e a r thermal expansion rate of adherend

A'( A + 6 ) p i n Chapter 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

k s m o t i c pressure

p-density of adhesive in dry state

û-angle of spherical cap fonned by blister outer surface

-phare angle, a r c 4 $ ( 2 1 J

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 (xpypz) at time r

Cs-quilibrium water concentration in percentage of the mass of hosting adhesive

D-water d i fkion 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

G m n t i c a l strain energy release rate of adhesive joints

G ~ r i t i c a l strain energy release rate of adhesive joints at pure mode 1

~ ~ O - ~ o n t r o l value of Gc measured fiom undegraded adhesive joints

Nomenclanne

h-half thickness of cast adhesive layer or haif thickness of bondline in closed joints

h-adherend thickness

H-thickness of adhesive layer on open-faced specimens

I-haif length of adhesive sandwich in 2D diffusion model

L-iength of adhesive sandwich in 1D diaision model

M-mass of adhesive in dry state

M-mass of absorbed water at equilibrium in adhesive

Mrmass of absorbed water at t h e r in adhesive

n, m-natural numbers

N-number of data points used in statistical assessrnent

p-pressure exerted in blisters by the liquid

p,--critical pressure for blisters to expand

r-base radius of blisters

R-in Chapter 3 and Section 4.4.1 gas constant, 8.3 14 J/(K-mol); in 4.4.2, radius of blister outer

sudàce in the plane perpendicular to adhesive surface and through the center of the blister

RH-relat ive humidity

S .D.-standard deviation

t-time of water absorption or aging

T-temperature

v-volume of adhesive

w-half width of adhesive sandwich

x, y, z-spatial coordinates of adhesive layer

vii

List of Tables (page nurnber given at end)

Table 2- 1 Gc values of specimens of different bond types at phase angles of O0 and 4S0, 12

Table 2-2 Gc averages of difKerent factor groups in secondary and tertiary bond tests, 13

Table 3- 1 Relationship between NaOH concentration and RH fiom 24OC to 85'C, 23

Table 3-2 Test schedule for RH equilibration of environment chamber and probe response to

step RH changes, 24

Table 3-3 Time required to re-establish equilibrium RH (combined effects of actuai chamber

equili bration and probe response), 25

Table 3-4 Probe response time for step changes in RH, 25

Table 3-5 Properties of saw-cut sandwiches used in water absorption experiments at 85OC, 100%

RH and 65'C, 100% RH, 30

Table 3-6 Properties of knïfe-cut sandwiches used in water absorption experiments at 85'C,

100% RH and 65OC, 100% RH, 3 1

Table 3-7 Properties of uncut adhesive sandwiches, 34

Table 3-8 Equilibrium water concentrations of Cybond 4523GB (Batch B-6404) up to 1 L days of

aging at 6S°C, 8S°C and 100% RH, 85% RH, 60% RH (N is the number of data

points collected fiom each specimen and used in specimen average or number of

specimens in grand average), 36

. . . vlll

List of Tables

Table 3-9 Diffusion coefficient of Cybond 4523GB (Batch B-6404) up to 1 1 days of aging at

65OC, 8S°C (IV is the number of data points collected nom each specimen and used Ui

specimen average or number of specimens in grand average. SD. is reported in

percentage of the average), 37

Table 3-1 0 Equilibrium water concentrations of Cybond 1 126 (Batch B-LX-6748) up to 39 days

of aging at 6S°C and 100% RH, 85% RH (N is the number of data points collected

from each specimen and used in specimen average or number of specimew in grand

average), 3 7

Table 3- 1 1 Diffusion coefficient of Cybond 1 126 (Batch B-LX-6748) up to 39 days of aging at

65OC (N the is number of data points collected fkom each specimen and used in

specimen average or number of specimens in grand average. S.D. is reported in

percentage of the average), 38

Table 3- 12 Comparison of equilibrium water concentrations and ditrusion coefficients of

Cybond 4523GB at 35OC, 65OC and 8S°C between present and previous work, 39

Table 3- 13 Comparison of equilibrium water concentrations and d i h i o n coefficients of

Cybond 1 126 at 65OC between present and previous work, 40

Table 3-14 Specific humidity of moist air at 65OC and 85°C; LOO%, 85%, and 60% RH, 42

Table 3- 1 5 Water absorption data for uncut Cybond 4523GB specimens at 65OC and 85OC with

100% RH. MJMs and 1 D prediction were calculated using &ta fiom fiesh, cast

wafers, 57

Table 3-16 Effective diffusion coefficient (DE) of saw-cut and uncut Cybond 4523GB specimens

with bondline thickness 2h, 59

Table 3-17 Absorption data of Me-cut Cybond 4523GB specimens at 85OC with 100% RH.

MAMs and ID prediction were caiculated using data fiom fÎesh, cast wders (The

boadline thiclcness is designated by the 3rd and 4& digits of the specimen codes: 10-

1.12 mm. 05-4-56 mm, 02-0.24 mm; nominai width is designated by 5* and firn

digits: 85-8.5 mm, 65-6.5 mm, 45-4.5mm), 60

Table 3- 18 Absorption data of knife-cut Cybond 4523GB specimens 65OC with 100% RH.

MJMs and 1 D prediction were calculated using data fiom fiesh, cast wafers (The

bondline thickness is designated by the 3rd and 4Lh digits of specimen codes: 1 O-

1.12 mm, 05-4.56 mm, 02-0.24 mm; nominal width is designated by 5& and 6&

digits: 85-8.5 mm, 65-4.5 mm, 454.5mm), 63

Table 3- 19 & of knife-cut specimens with bondline thickness 2h and specimen width 2w

(length was 1 19- 163 mm) aged at 85'C with 100% RH. Bold face numbers are

averages of &/D for two replicates (fiesh, cast wafer D= 12.3 x 1 o - ' ~ m2/s), 66

Table 3-20 & of knife-cut specimens with bondline thickness 2h and specimen width 2w

(length was 148- 163 mm) aged at 65OC with 100% RH,. Bold face nurnbers are

averages of &/D for two replicates (fiesh. cast wafer B 6 . 9 ~ 1 o - ' ~ m2/s), 67

Table 3-21 T m e to reach 95% of fÎesh, cast wafer saturation at 100% RH for adhesive layers

absorbing water via edges and as a totally exposed wafer, 69

List of Tables

Table 3-22 Estimateci residd thermal stress in adhesive layer of joints ( c d at tSO°C, aged at

6S°C and 8S°C) with various bondline thicimess. The mechanical and thermal

properties of adherend are those of AAI 100-0 (Ee70 GPa, m . 3 , ~ 2 . 2 ~ 1 O-'

PC) and the &ta of the adhesive ( E A = ~ MPa (the value of rubber because it is

leathery at the aging temperatures with Cs), ~ ~ 4 . 5 , a ~ 4 . 5 ~ lo%) are best

estimates on epoxies at the aging temperatures. -0.1 mm, 88

Table 3-23 Cs values of Cybond 4523GB aged at 6S°C and 85*C, 100% RH with different

situations. N is the number of specirnens. S.D. was calcuiated using the number of

data points collected fiom each specimen in 1 specimen tests or using the number of

specimens in multi-specimen tests, 9 1

Table 4-1 Survey on btistering of an open-faced Cybond 4523GB specimen with 0.4 mm thick

adhesive layer aged at 8S°C, 100% RH, 102

Table 4-2 Survey on blisterhg of an open-faced Cybond 1 126 specimen witb 0.4 mm thick

adhesive layer aged at 6S°C, 100% RH, 102

Table 4-3 Chemical compositions of blister liquids fiom Cybond 1 126 and Cybond 452368

specimens, 1 13

Table 4-4 Chemical compositions of adhesives Cybond 1 126 and Cybond 4523GB, 1 13

Table 4-5 Concentration of ionic species in water containhg Cybond 1 126 and Cybond

4523GB, 1 15

Table 4-6 Table 4-6 Heights of blisters induced by swelling of adhesive above air bubbles of

difTeremt sizes on Cybond 4523GB specimens aged at 85OC, 100% RH, assuming that

the adhesive layer has reached its equilibrium water content (4.88%), 125

Table 5-1 Testing schedule for Cybond 4523GB specimens, DP was caldated based on the

diffusion properties measured fiom fiesh, cast wafen (at 6S°C, ~ 6 . 9 ~ IO*') m2/s,

Cs=2.20% with 85% RH and Cfl.78% with 60% RH; at W°C, ~=12.3x10-" m2/s,

C~=2.90% with 85% RH and Cs=l. 15% with 60% RH), 13 5

Table 5-2 Testing scheduie for Cybond 1126 specimens aged at 6S°C, DP was calculated based

on the diffusion properties measured fiom cast wafers ( D= 1 0 . 2 ~ 1 O-') m2/s, C~3 .62%

with 85% RH), 136

Table 5-3 DP and average Gc of Cybond 452368 specimens degraded at 6S°C and 85OC and

tested at dry state and (v.60° (Gc of undegraded Cybond 4523GB specimens was 394

Um2), 142

Table 5-4 DP and average Gc of Cybond 1126 specimens degraded at 65OC and tested at dry

state and p 6 0 ° (The Gc value of undegraded Cybond 1 126 specimens was 2233

~/m'), 143

Table A-1 Low relative humidity (30% and 60%) water uptake of Cybond 1 126 wafers exposed

at 65OC after 24 hour 65OC drying, A3

List of Figures

List of Figures (page number given at end)

Fig. 3- 1 Two-dimensional diaision model, 1 9

Fig, 3-2 An environment chamber during adhesive absorption testing, 22

Fig. 3-3 A saw-cut or knife-cut adhesive sandwich, 29

Fig. 3-4 An uncut adhesive sandwich showing end clamps and Teflon spacers in place, 33

Fig. 3-5 A water absorption plot of Cybond 4523GB, 35

Fig. 3-6 Relationship between specific humidity and equilibrium water concentration of Cybond

4523GB at 6S°C and 85OC. A - ~ ~ O C , 60% RH; 8, G-6S°C, 85% RH; C, H-6S°C,

100% RH; %85OC, 60% RH; E-8S°C, 85% RH; F, I-8S°C, 100% RH, 42

Fig. 3-7 Mass gain per unit area of AA5454-0 aluminum control plates for saw-cut specimens at

100% RH, 44

Fig. 3-8 Mass gain per unit area of AAl100-0 aluminum control sheets for knife-cut specimens

at 100% RH, 45

Fig. 3-9 Mass gain of Teflon spacers for uncut specimen absorption testing (100% RH), 45

Fig. 3- 10 Water absorption of saw-cut Cybond 4523GB sandwiches (2k 1.12 mm, 2w=8.5 mm,

aged at 85OC, 100% RH). A--# 8581; B4# 8582; C, D-ID and 2D predictions using

data from fiesh, cast wafers; - - - - - - - - - uncerlainty envelope of 1 D prediction based on

fl S.D. for D (S.D.=23%), 47

xiii

List of Figures

Fig. 3- 1 1 Water absorption of saw-cut Cybond 4523GB sandwiches (2k1.12 mm, 2 ~ 6 . 5 mm,

aged at 85'C, 100% RH). A 4 856 1 ; B-# 8562; C, D-1 D and 2D predictions using

data fiom hsh, cast wafers; - ---- - - - - uncertainty enveiope of 1 D pmdiction based on

S S . D - for D (S.D.=23%), 48

Fig. 3- 12 Water absorption of saw-cut Cybond 4523GB sandwiches (2h=1.12 mm, 2w=8.5 mm,

aged at 65OC, 100% RH). A-# 658 1 ; BfC 6582; C, D-1 D and 2D predictions using

data nom fresh, cast wafers; ---- - - - - - uncertainty envetope of 1 D prediçtion based on

k3S.D. for D (S.D.4 7%), 48

Fig. 3- 13 Water absorption of saw-cut Cybond 4523GB sandwiches (2h4.12 mm, 2 ~ 6 . 5 mm,

aged at 65OC, 100% RH). A 4 6561; B-# 6562; C, D-1D and 2D predictions using

data fiom fresh, cast wafers; - - - - - - - - - uncertainty envelope of ID prediction based on

k3S.D. for D (S.D.=17%), 49

Fig. 3-14 Peeled adhesive surf'ace of sample 8582 (aged at 8S°C, 100% RH for 78 days). 49

Fig. 3-15 Peeled adhesive surface of a fiesh joint, 50

Fig. 3- 16 Peeled aluminurn surface of sample 8582 (aged at 8S°C, 100% RH for 78 days), 50

Fig. 3- 17 Peeled aluminum surface of a h s h joint, 5 1

Fig. 3- 18 Width effect on water absorption of saw-cut Cybond 4523GB specimens a 8S°C,

100% RH (2k1-12 mm), 55

Fig. 3- 19 Width effect on water absorption of saw-cut Cybond 452368 specimens at 65'C,

100% RH(2h=1.12 mm), 55

Li of Figures

Fig. 3-20 Water uptake of uncut Cybond 4525GB specimens at 85OC, 100% RH. A 4 85 1, B-

# 852, C-average of # 851 and 852 1D prediction using &ta fiom h s h , cast wafers,

- - - - - - uncertainty envelope of ID prediction based on G S D . for D (S.D.= 1 3%), 58

Fig. 3-2 1 Water uptake of uncut Cybond 4525GB specimens at 65"C, 100% RH. A 4 65 1, B-

# 652, C-average of # 65 1 and 652 ID prediction using data fiom k s h , cast wafers,

- - - - - - uncertaintty envelope of 1D prediction based on +3S.D. for D (S.D.=27%), 59

Fig. 3-12 Bondline thickness effect on M& of knïfe-cut Cybond 4523GB specimens at 8S°C

with 100% RH (2w=8.5 mm),- 1 D prediction using data fkom fiesh, cast wafers,

- - - - - - - uncertaintty envelope of 1 D prediction based on +t S.D. for D (S.D.4 3%), 70

Fig. 3-23 Bondline thickness effect on M m s of Imife-cut Cybond 5423GB specimens at 85OC

with 100% RH ( 2 ~ 6 . 5 mm), - 1 D prediction using data fiom fiesh, cast wafers,

- - A - - - - uncertainty envelope of 1 D prediction based on fl S J). for D (S .De= 13%), 70

Fig. 3-24 Bondline thichess effect on M M s of knife-cut Cybond 5423GB specimens at 8S°C

with 100% RH ( 2 ~ 4 . 5 mm),- ID prediction using data fiom fiesh, cast wafers,

- - - . 0 - uncertainty envelope of 1 D prediction based on cl S .D. for D (S.D.=13%), 7 1

Fig. 3-25 Bondline thickness effect on M/Ms of knife-cut Cybond 5423GB specimens at 65OC

with 100% RH (2w=8.5 mm), -1D prediction using data fiom k s h , cast wafers,

. . . . . . . uncertainty envelope of 1D prediction based on k3S.D. for D (S.D.=27%), 71

Fig. 3-26 Bondline thickness effect on M/Ms of Me-cut Cybond 5423GB specimens at 6S°C

with 100% RH (2~4.5 mm), - 1 D prediction using data fiom fiesh, cast wafers,

--..-.. uncertainty envelope of ID prediction based on k3S.D. for D (S.D.=27%), 72

List of Figures

Fig. 3-27 Bondline thickness effect on M m s of Me-cut Cybond 5423GB specimens at 6S°C

with 100% RH (21~4.5 mm), - 1D prediction using data fiom eesh, cast wafers,

.--.--.- uncertainty envelope of 1 D prediction based on +7S .D. for D (S.D.=27%), 72

Fig. 3-28 Width effect on M M s o f f e-cut Cybond 4523GB specimens at 8S°C, 100% RH

(2h4 .12 mm), - 1D prediction of the two narrowest specirnens using &ta fkom

fiesh, cast wafers, - - - - - - uncertainty envelope of 1 D prediction based on k3S.D. for D

(S.D.=13%), 73

Fig. 3-29 Width effect on M m s of knife-cut Cybond 4523GB specimens at 8S°C, 100% RH

(2k0.56 mm), - ID prediction of the two narrowest specimens using &ta from

fies4 cast wafers, - - - - - - uncertainty envelope of 1 D prediction based on &3S-D. for D

(S.D.=13%), 74

Fig. 3-30 Width effect on M m s of knife-cut Cybond 4523GB specimens at 8S°C with 100% RH

(2k0.24 mm), - 1D prediction of the two narrowest specimens using daîa fiom

fresh, cast wafers, - - - - - - uncertainty envelope of 1D prediction based on k3 S.D. for D

(S.DI=13%), 74

Fig. 3-3 1 Width effect on M m s of knife-cut Cybond 4523GB specimens at 65°C with 100% RH

(2k1.12 mm), - 1D prediction of the two narrowest specimens ushg data &orn

fiesh, cast wafers, - - - - - - uncertainty envelope of ID prediction based on k3S.D. for D

(S.D.=27%), 75

Fig. 3-3 2 W idth effect on M/Ms of knife-cut Cybond 4523GB specimens at 6S°C with 1 00%

RH (2k0.56 mm), - 1D prediction of the two narrowest specimens using &ta h m

fiesh, cast wafers, - - - - - - - uncertainty envelope of 1 D prediction based on &3S.D. for D

(S.D.=27%), 75

List of Figures

Fig. 3-33 Width effect on MJMs of knife-cut Cybond 4523GB specimens at 65OC wiîb 100% RH

(2k0.24 mm), - 1D prediction of the two narrowest specimens using data fiom

fiesh, cast wafers, - - - - - - - uncertainty envelope of 1 D prediction based on k3 S.D. for D

(S.D,=27%), 76

Fig. 3-34 Bondline thickness effect on M i î i of Me-cut Cybond 4523GB specimens at 8S°C

with 100% RH (2w=8.5 mm), 76

Fig. 3-35 Bondline thickness effect on MJ2L of Me-cut Cybond 4523GB specimens at 85OC

with 100% RH ( 2 ~ 6 . 5 mm), 77

Fig. 3-3 6 Bondline thickness effect on M42L of knife-cut Cybond 452368 specimens at 8S°C

with 100% RH ( 2 ~ 4 . 5 mm), 77

Fig. 3-37 Bondline thickness effect on Mi2L of laufetut Cybond 4523GB specimens at 6S°C

with 100% RH ( 2 ~ 8 . 5 mm), 78

Fig. 3-38 Bondline thickness effect on MJ2L of We-cut Cybond 4523GB specimens at 6S°C

with 100% RH ( 2 ~ 4 . 5 mm), 78

Fig. 3-39 Bondline thickness effect on Mi2L of knife-cut Cybond 4523GB specimens at 6S°C

with 100% RH (2w4.5 mm), 79

Fig. 3-40 Width effect on Mt/2L of hifesut Cybond 4523GB specimens at 8S°C, 100% RH

(2h=1.12 mm ), 80

Fig. 3-41 Width eEect on MJ2.L of knife-cut Cybond 4523GB specimens at 8S°C, 100% RH

(2k0.56 mm ), 80

Fig. 3-42 Width effect on MJ2L of knife-cut Cybond 4523GB specimens at 8S°C, 100% RH

(2k0.24 mm), 81

List of Figtacs

Fig. 3-43 Width effect on M a of knife-cut Cybond 4523GB specimens at 6S°C, 100% RH

(2k-1.12 mm), 81

Fig. 3-44 Width effect on MJîL of biifezut Cybond 4523GB specimens at 6S°C, 100% RH

(2k0.56 mm), 82

Fig. 345 Width effect on MJ2L of knife-cut Cybond 4523GB specimens at 6S°C, 100% RH

(2k0.24 mm), 82

Fig. 3-46 Hypothesis of interfacial diffusion in a closed jo& 83

Fig. 3-47 ReIationship between bondline thickness and water uptake rate for knife-cut specimens

(85OC), 86

Fig. 3-48 ReIationship between bondline thickness and water uptake rate for knife-cut specimens

(65OC), 86

Fig. 3-49 Recorded water absorption of saw-cut specimens 858 1 and 8582 (2k1.12 mm,

2 ~ 8 . 5 mm, aged at 85OC, 100% RH) and Fickian prediction using diffusion

properties measured fiom closed joints (D&i 1 . 0 ~ 1 0'13 m2/s, CS=5 .4%), 92

Fig. 3-50 Recorded water absorption of saw-cut specimens 8561 and 8562 (2h1.12 mm,

2 ~ 6 . 5 mm, aged at 85'C, 100% RH) and Fickian prediction using d i h i o n

properties measured from ail the closed joints in saw-cut group (&-6 1 . 0 ~ 1 o - ' ~ m2/s,

Cs=5.4%), 92

Fig. 3-5 1 Recorded water absorption of saw-cut specimens 658 1 and 6582 (2h= 1.1 2 mm,

2 ~ 8 . 5 mm, aged at 6S°C, 100% RH) and Fickian prediction using diffusion

properties measured fiom al1 the closed joints in saw-cut group(D& 5. l x 1 O-" m2/s,

cs=5. 1 %), 93

List of Figures

Fig. 3-52 Recorded water absorption of saw-cut specimens 656 1 and 6562 (2h= 1.12 mm,

2 ~ 6 . 5 mm, aged at 6S°C, 100% RH) and Fickian prediction using diffusion

properties measured nom closed joints (k-1 5. 1 x 1 m2/s, Cs=5. 1 %), 93

Fig. 3-53 Recorded water absorption of uncut specimens 85 1 and 852 (2kO-56 mm, 2w=7S

mm, aged at 8S°C, 100% RH) and Fickian prediction using diffusion properties

measured fiom al1 the closed joints in uncut group (D&2.0~10"~ m2/s, Cs=5.9%), 94

Fig- 3-54 Recorded water absorption of uncut specimens 65 1 and 652 ( 2 H . 5 6 mm, 2w=7.5

mm, aged at 6S°C, 100% RH) and Fickian prediction using diffusion properties

measured fiom al1 the closed joints in uncut group (-30.6~ 1 0-13 m2/s, Cs=4.5%), 94

Fig. 3-5 5 Recorded water absorption of knife-cut specimens 85 1085 1 and 85 10852 ( 2 k 1 .O mm,

2 ~ 4 3 . 5 mm, aged at 8S°C, 100% RH) and Fickian prediction using diffusion

properties measured fiom al1 the closed joints in Me-cut group ( ~ ~ 5 4 . 4 ~ 10-l~ m2/s,

Cs=5.5%), 95

Fig. 3-56 Recorded water absorption of Me-cut specimens 850245 1 and 8502452 (2k0.24

mm, 2 w 4 . 5 mm, aged at 8S°C, 100% RH) and Ficlcian prediction using diffusion

properties measured fiom closed joints in knife-cut group (&-54.4~ 10'" m2h,

Cs=5.S%). 95

Fig. 3-57 Recorded water absorption of knife-cut specimens 6510851 and 6510852 (2h=1.0 mm,

2~43 .5 mm, aged at 6S°C, 100% RH) and Fickian prediction using diffusion

properties measured nom closed joints (&=l2.8x 1 0-l3 m2/s, C H . 1%), 96

List of Figures

Fig. 3-58 Recorded water absorption of Me-cut specimens 650245 1 and 6502452 (2H.24

mm, 2 w 4 . 5 mm, aged at 65OC, 100% RH) and Fickian prediction using dinusion

properties measured h m closed joints in We-cut group (&12.8~10-*~ m2/s,

C&.1%), 96

Fig. 4- 1 A b i i s t e ~ g area on a Cybond 1 126 open-faced specimen aged for 120 days at 6S°C

with 100% RH, 100

Fig- 4-2 A cross section of a blister on a Cybond 1 126 open-faced specimen aged for 120 days at

6S°C with 100% RH. The specimen was cut using a table saw, 100

Fig. 4-3 Delamination at the edge of a Cybond 1 126 open-faced specimen aged for 120 &YS at

6S°C with 100% RH, 101

Fig. 4-4 Height and base diameter of a blister, 10 1

Fig. 4-5 A purposely contaminated AA 606 1 -T65 1 plate before bonding, 1 03

Fig. 4-6 The appearance of a blister on Cybond 4523GB closed joint specimen 85 1085 1 (aged at

85°C with 100% RH for 1 10 days), 105

Fig. 4-7 A blistered spot of Cybond 4523GB specimen 85 1085 1 on the adhesive after removal of

the adherend (aged at 8S°C with 100% RH for 1 10 days, the adherend thickness was 0.1

mm), 105

Fig. 4-8 A blistered spot of Cybond 4523GB specimen 85 1085 1 on the adherend (aged at 85OC

with 100% RH for 1 10 days, the adherend thickness was 0.1 mm), 106

Fig. 4-9 A blistered spot of Cybond 4523GB spechen 8565 lon adhesive after removal of

adherend (aged at 85OC, 100% RH for 78 days, the adherend thickness was 1 mm), 107

Fig. 4- 10 A blistered spot of Cybond 4523GB specimen 8565 Ion adherend side (aged at 8S°C,

100% R H for 78 days, the adherend thickness was 1 mm), 107

List of Figures

Fig. 4-1 1 A blistered spot on an open-faced specimen. Beneath the air is the adherend, 108

Fig- 4-12 Water cor~centration through depth of non-blistered Cybond 4523GB adhesive Iayer

(0.4 mm) on open-faced specimens aged at 8S°C, 100% RH (D= 12.3 m2/s), 109

Fig. 4-1 3 Morphology of duminurn surface undemeath a blister on a Cybond 1 126 specimen

(B65 100 16 1) aged for 105 days at 6S°C, 100% RH. The light particles are adhesive

residue, Il 1

Fig. 4- 14 Morphology of aluminum surface near the blister shown in Fig. 4- 1 3. The light

particles are adhesive residue, 1 1 1

Fig. 4- 15 Morphoiogy of adherend underneath a blister on a Cybond 1 126 specimen

(B65 1 OONCT), 1 16

Fig. 4- 16 Morphology of adherend undemeath a blister on a Cybond 4523GB specimen

(A65 1 OONCT), 1 16

Fig. 4- 17 Morphology of adherend underneath a blister on a Cybond 1 126 specimen

(B65100161), 117

Fig. 4- 18 Morphology of adherend undemeath a blister on a Cybond 4523GB specimen

(A851001 12), 117

Fig. 4-19 Morphology of adherend undemeath a blister on a Cybond 4523GB specimen

(A65 1 OONCT), 1 18

Fig. 4-20 Blistering process in open-faced specimen and closed joint, 122

Fig. 4-2 1 A blister due to swelling of adhesive surrounding an entrapped air bubble, 124

Fig. 5- 1 One-dimensional diffusion mode1 in an open-faced specimen, 128

Fig. 5-2 Pre-cracking a DCB specimen, 132

List of F i m m

Fig. 5-3 DCB sample clamped on load jig showing crack path associated with loading such that

Fl>F2, 133

Fig. 5-4 R-cuve of an undegraded Cybond 4523GB DCB specimen (#AS, (IF604,138

Fig. 5-5 R-cuve of an undegraded Cybond 1 126 DCB specimen (#Bol, ~ 6 0 9 , 138

Fig. 5-6 R-curve of a degraded Cybond 4523GB DCB specimen (#A85085280-3, WO",, 140

Fig. 5-7 R-curve of a degraded Cybond 4523GB DCB specimen (#A65085216-2, -04, 140

Fig. 5-8 R-curve of a degraded Cybond 1 126 DCB specimen (#B65085250-2, @O?, 141

Fig. 5-9 The relationship between G ~ / G ~ O and DP of Cybond 4523GB specimens degraded at

6S°C and 85OC, and tested at dry state at p60° (The GC* of the present work was 394

~ / r n ~ , and the GCO of ref. [3] was 405 ~ / r n ~ . P-present work, 65'C; x-present work,

85°C; A-ref [3], 6S°C), 144

Fig. 5- 10 The relationship between Gc and DP of Cybond 1 126 specimens degraded at 6S°C and

tested at dry state at 6 0 " and ( ~ F O O (x-present work, p60°, ~cO=2230 ilm'; A-

ref. [3], y O O , ~~O=595 j/m2), 145

Fig. A- 1 Water uptake of a Cybond 1 126 wafer at 6S°C, 30% RH, Al

Fig. A-2 Water uptake of a Cybond 1 126 wafer at 6S°C, 60% RH, A2

Chaptcr 1 Introduction

Chapter 1 Introduction

1.1 Background and Motivation

It is believed that water is among the most hostile agents in the degradatioa of

adhesive joints, but there is d l no effective way to predict the durability of adhesive joints in

wet or humid environments.

Great effort has been made to correlate the strength loss with water content and time

in adhesive joints. Becaw an imdesirably long time is needed for a joint to be aged to yield a

noticeable strength loss, a nurnber of aging processes have been proposed to accelerate adhesive

joint degradatioa. Basic techniques of accelerated aging include increasing the relative humidity,

elevating the temperature, and reducing the diffusion path length by directly exposing the

adhesive layer to rnoisture or drilling holes at the center of joints. The open-faced specimens used

in the present research incorporated dl of the three principles. Because open-faced specimens

require a secondary bond to form closed joints for fhcture testing, secondary bond effects, (Le.,

the effect of increased bondline thickness and the effect of additional curing cycle), must be

understood to ensure that the open-faced specimen technique generates undistorted data.

The transport and distribution of water in an adhesive layer are cntical to predicting

joint degradation. It is believed that Fick's law is a fairly accurate description of water -ion

Chaptcr 1 Introduction

into a buk adhesive, but interfacial difihion and thermal or swelling stresses may affect the

absorption rate and sanirateci water concentration in adhesive joints.

During the present study, blisterhg was observed on the open-faced specimens at

100% relative hurnidity. This phmornenon has also k e n observed by other researchers".

Blistering results in delamination of the adhesive Iayer fiom the adherend. In fact, blistering also

appeared in closed joints, leading to the speculation that blistering could be a part of, ifnot the

main, mechanisrn of joint degradation.

Motivated by the above questions and observations, the present research examined: a)

some issues of open-faced specimen bonding technique and the effects of the secondary bond; b)

water absorption in both buik adhesive and closed joints; c) blistering as a degradation process in

both open-faced specimens and closed joints; and d) the validity of the degradation parameter

@P) proposed by James Wylde and spel+, which assesses the cumulative effect of the adhesive

water content over time on the fracture resistance of adhesive joints.

1.2 Literature Review

The literature review wilI focus on three aspects which correspond to the objectives of

this study: water transport inside adhesive joints, blistering of organic coatings (adhesive layer),

and studies of adhesive joint durability using accelerated aging processes.

Water Transport in Adhesive Joints

According to comyn4, in the case of metai adherends, water may enter joints by: a)

diffiision through the adhesive; b) transport dong the intefice; c) c a p i k y action through cracks

and crazes in the adhesive.

As early as 1987, Leidheiscr and ~unke* found that a water layer of many monolayers

thick can build up at the coating/substrate interface when an organic-coated substrate is exposed

to water or high relative humidities-

Drain et aL6 reported that the wicking of water dong the adhesive/adherend intefiace

is dominant in the water absorption of certain adhesive joints. As weil, Nakamura et al? found

that the diffllsion coefficient of water is larger at the interface than in the adhesives, and that

moisture absorption is determined largely by difbion of water at the interface.

Using the relationship between Young's modulus and water concentration in an

adhesive joint, Wylde and spelt' found more water in the joints tàan the amount predicted by

Fickian d ' ï i o n , and concluded that interfaciai diffiision was possible.

Some research has revealed that an interface zone exists in the adhesive layer that has

different mechanical properties than those of the bulk adhesive. peretz8 suggested that the curing

process md curing conditions between the adherends in an adhesive joint causes the adhesive to

become non-homogenous. ln other words, thin adhesive boundary layers which show different

mechanical properties, such as Young's moduius and Poisson ratio, are formed at interfaces.

Safavi-Ardebili et al.' found an interface zone of irregular thickness (nominally between 2 and 6

pn) in a aluminum-epoxy system using s c d g electron microscopy, ion itchhg, energy-

dispersive x-ray analysis, acoustic microscopy, and nano-indentation. The authors reported that

the interface region had, on average, an effective elastic modulus ( ~ / ( l - v ~ ) ) that was 13% greater

than that of the bulk resin, and that the interface region was aiso approximately 4% harder than

the bulk adhesive.

~illeheden", on the other hanci, found that there was no experimental evidence

supporthg the existence of a boundary layer, and concluded that, within the accuracy of the

deformation measuring system, the adhesive matend in an adhesive joint can be regarded as

king homogeneous.

Kim, Giliat and ~ r o u t m a n ~ ~ ~ ~ examined the efféct of stress on the ditfuion

properties of graphite-epoxy composites and showed that the diffusion coefficient increased and,

in some cases the equilibrium water content went up slightly under tensile stress.

The literature review shows that many studies support the possibility of interfaciai

m i o n and stress effects on diniision in polymers , although the &ta remain relatively sparse.

Blkfering of Organic Cdings

~unke' ' reviewed the blistering of organic coatings. He suggested that osmosis is the

most important mechanism of blistering at the coating-substrate interface.

Lefebvre et al.I2 Uivestigated the sharp &op in adhesion which seemed to occur

whenever an epoxy a d h e ~ g to an inorganic substrate was equilibrated with air whose relative

humidity exceeded a cntical value characteristic of the epoxy. They showed that osmotic ceils

that form around artincially introduced water-soluble impurities can cause a loss of adhesion at a

predictable critical relative humidity.

Sargent and ~shbee*' investigated the swelling of adhesive layer in closed joints

usiag a photoelastic technique. The auîhors found that a significant part of the s w e h g resdted

fiom osmosis and that disc-shaped cracks formed in the adhesive.

The initiation of osmotic c e h in adhesive joints requires Yree water" (as opposed to

"bound water" that exists as separateci molecules in the adhesive). Naim et d . I 3 detected fke

water in adhesive joints using dielectric spectroscopy. ~ i f s h i n ' ~ also rrported the presence of

water in a fieezable state in polymers, i.e. the existence of 6ree water.

The Merature shows that blisterhg studies have focused on the interface between

metals and organic coatings, such as paints. But the mechanisms of b l i s t e ~ g in adhesive joints

and the role it piays in joint degradation remah unexplored.

Durabifiîy and Accelerated Aging Rocesses

Having entered a joint, water may cause strength loss by one or a combination of the

following mechanisms4: a) altering the properties of the adhesive in a reveaible mamer (e.g.,

plastickation), or an irreversible manner (e.g., hydrolysis, cracking, or crazing); b) attaclchg the

adhesive/adherend interface either by displacing the adhesive or by hydrating the metal or the

metal oxide surface of the adherend; c) inducing sweiiïng stresses in adhesive joints.

Nakamura et al.' also pmvided a good summary of mechanisms that have been

proposed to affect joint strength under environmental attack. In the same paper, the authors

correlated the water concentration with the decay of the wet shear strength of adhesion by

introducing a "breakage parameter". This was a surface failure parameter such as that related to

hydrogen bond breakage in the case of interfacial failure or an aging parameter related to buik

adhesive degradation in the case of cohesive failure. To some extent, this theory revealed the

relationship between durability and water concentration, but it failed to consider the cumulative

effect of t h e , a crucial factor in any aging process.

As mentioned before, hain et alO6 claimed that interfacial diffusion contributed to

joint degradation, but no quantitative correlation between water concentration and strength loss

was established.

Several other authon, notably Brewis et al." and Bowditch et al? found a critical

water concentration below which water has no efXect on the strength of an adhesive joint.

~arker" attempted to comlate water ingress to strength change whde studying the

effects of curing temperature on durability, It was noticed that the general trends for normalized

strength (Le. strength divided by initial strength) vs. normalized water concentration (Le. water

concentration divided by equilibrium water concentration) were similar for different adhesives. It

was concluded that there may exist a relationship between strength loss and water content.

Wylde and s p e d proposed a degradation parameter (DP), an integral of water

concentration at a given point inside an adhesive joint with respect to time, to characterize the

degree of degradation for a given adhesive system at a given temperature. The DP concept

established a quantitative relationship between watcr aging and strength loss of adhesive joints,

taking into account the spatial distribution of water and its cumulative effect over tirne.

Harris and ~ a ~ " concluded that when the mode of joint failure was cohesive (in the

adhesive), changes in joint properties due to the ingress of moisture depend mainly on the

distribution of moisture in the adhesive layer, and elevated temperature can be used to increase

the rate of diffusion and hence accelerate the rate at which the changes occur.

Accelerated exposure was demonstrated by Bowditch et al. l6 who proposed a

specimen in which a filler made of adherend material was distributed throughout an adhesive

resin. These specimens were exposed to environmental attack and tested for changes in the

elastic modulus. The critical water content below which joins were unaEected was not observed,

Cbapter 1 Introduction

and it was concluded that atîack of adhesion interfaces by water was reversible and that at the any

given equilibrium water content the potential of water for degradation of adhesion interfaces was

associated with the partitioning of water beniveen the adherend surface and adhesive.

A modified single-lapshear (SLS) specimen was proposed by Anowsmith and

add dis on'^ in which holes were driiied in an SLS specimen to increase the surface area for water

ingress and decrease the average diffùsion path length.

Chang et a l o introduced a iayered materiai in which a film was cast on one

adherend and exposed to environmental attack. The specimen was then tested for adhesive

disbondhg in a tensiie test.

Similarly, Giunta et al?' employed an "open-faced" notched coating adhesion (NCA)

test to develop a technique for detecting the strength loss in adhesive pefiormance without

having to age specimens for long periods of tirne. The K A specimens may allow aging to occur

more quickly than in the traditional closed adhesive bond geometry. Due to the "open-faced"

feature, the water diffusion path of an NCA specimen is the thickness of the adhesive layer,

which greatly reduces the total time to mach equiiibrium.

The DP concept proposed by Wylde and spelt? is one of the most promising

correlations beîween the degradation of adhesive joints and water aging. Yet, it lacks sufficient

experimental resuits to properly validate the concept.

1.3 Ovewiew of the Thesis

Chapter 2 assesses the effects of the secondary bond on fiacture energy measured

using DCB specimens. Both the bondline thichess effect and the curing cycle effect are

investigated. In addition, some observations regarding adhesive selection for secondary bonds are

discussed As weii, the shortcomings of difEerenî secondary bond adhesives such as Hysol9309

and Cyhnd 4523GB in Cybond 4523GB primary bonded joints are reporteci.

Chapter 3 discwses water diffiision in cast adhesive wafers and closed joints.

Diffusion coefficients and equilibrium water concentrations for bulk Cybond 4523GB and

Cybond 1 126 are presented. The sections on closed joint diaision present the data for three

gmups of Cybond 452368 sandwiches: saw-eut specimens, M e - c u t specimens, and uncut

specimens. Basic mathematics of diaision in adhesive joints are introduced in this chapter. In

addition, possible expianations (i.e., interfacial m i o n and thermal stress effect) to water

transport and Fickian mode1 for water absorption in closed adhesive joints are presented.

Chapter 4 examines the blistering phenornenon in open-faced specimens and closed

joints. Using several sets of experiments inspired by accidental observations during the agbg of

open-faced specimens, this chapter investigates the mechanisms for two categories of blistering;

namely, blistering due to osmotic pressure and blistering due to adhesive swelling. It presents

some implications of blisterhg for the prediction of joint strength loss due to degradation in

water.

Chapter 5 discusses the validation of the Degradation Parameter OP) and some new

experimentai techniques. The experiments focus on the correlation between joint hcture

strength loss and DP. Coaunents on DP of open-faced specimens are also presented.

Chapter 6 presents general conclusions of the present work. Some suggestions for

fbture work are proposed.

Chape 2 Secondas, Bond Effect on Strain Enagy Release Rate of DCB Spccimeos

Chapter 2 Secondary Bond Effect on Strain Energy Release

Rate of DCB Specimens

2.1 Introduction

To accelerate the degradation of adhesive joints, open-faced specimens were prepared

for agiog. Aged specimens were then closed using secondary adhesive bonds. Generally,

compared with a single bond adhesive joint, a secondary-bonded joint has a iarger bondiine

thickness (primary bondline thickness plus seconciary bondline thickness), one more curing cycle,

and a potential for crack propagation in the secondary bondline or at the interface between the

primary and secondary bonds. The open-faced technique as an accelerated aghg method is valid

oniy if the secondary bond has a negligible effect on the stmin energy release rate Gc.

Wylde and spelt3 found that a secondary bond had a negligible effect on Gc for the

Cybond 4523GB-aiuminum system. But the authon observed that for the Cybond 1 126-

aiuminum system, the presence of a secondary bond induced a small but statisticdy signifiant

deviation arnong the data, and concluded that the bondline thïckness could not account for the

deviation. However, Wylde and spel9 did not completely analyze the effects of dinerent factors

of secondary-bonded joints on Gc.

To justa the use of the secondary bond technique as a part of accelerated aging, the

present research examined secondary bond effects using double cantilever beam W B )

specimens.

Chapter 2 Secondary Bond Wcct on Strain Energy Release Rate of DCB Specimens

2.2 Experimental Procedures

In this study, Cybond 4523GB, a 150°C cured one-part adhesive, was used as the

primary adhesive system. Hyso19309EA, a room temperature cured two-part adhesive, and

Cybond 4523GB were chosen as adhesives for secondary bonds. The following specimen groups

were prepared to address the possible ef5ects of three factors: bondline thickness, extra curing

cycles, and a crack tip damage zone which is not inside the primary adhesive:

standard, closed single bond specimens ( A U , A4-2) of Cybond 4523GB with a

0.4 mm bondline thickness as a control;

a standard, closed single bond specimen (A5) of Cybond 4523GB with a 0.5 mm

bond line thickness as a control;

a secondary bond specimens (AA5-1, AA5-2) of 0.4 mm tbick Cybond 4523GB as

the primary adhesive with 0.1 mm thick Cybond 4523GB as the secondary bond;

secondary bond specimen (AC5) of 0.4 mm thick Cybond 4523GB as the primary

adhesive and 0.1 mm thick Hysol9309EA as the secondary bond;

tertiary bond specimens (AAC6-1, AAC6-2) of 0.4 mm thick Cybond 4523GB,

another 0.1 mm thick Cybond 4523GB as the secondai-y bond, and O. lmm thick Hysol9309EA

as the tertiary bond.

The tertiary bond plate was prepared to check if there were a large nurnber of air

bubbles and voids at the secondary bond which could weaken the secondary bond sigdicantly.

The result showed that the using of a constant clamphg pressure yielded a uniform secondary

bond with a few number of air bubbles and voids. Then the plate was closed using Hysol

9309EA to form a joint with a thickness of approximately 0.6 mm and two 1 50°C curing cycles.

Chapter 2 Secondary Bond Enect on Strain Energy Relcasc Rate of DCB Specimcns

The secondary bond and tertiary bond specimens were made of open-fâced plates

which were prepared ushg the procedure described by Wylde and spelr'. n e primary surfàces

were roughened using 220 grit sandpaper and rinsed using acetone. The plates were then dried at

room temperature for about 20 min. The thiclcness of the secondary and tertiary bonds was

controiled by glass beads in the adhesive for Cybond 4523GB. For Hysol9309EA, the bondline

thickness control was indirectly achieved using a constant clamping pressure. Each ofthe groups

was cut fiom one plate which yielded 3 DCB specimws- The adhesives Cybond 4523GB and

Hysol9309EA used in this study were fiom one batch.

The role of crack propagation monitoring in these tests was to measure crack length

and confïrm that cracking oçcurred at the interface between the aluminum and the primary

adhesive or inside the primary adhesive hndline. A combined effective mamcation of lOOx

was achieved using the CCD video camera (mode1 XC-73, the Leica microscope (Wld MB),

and a 30 cm monitor. If the crack began to propagate near the secondary bond interface (the

interface between the primary bond and the secondary bond), that data point was discarded- To

illustrate the lower fracture energy that corresponding to cracking at the secondary interface, the

resdts of AM-1 at both O0 and 48', and AAS-2 at 48' are also shown in Table 2-1.

The hcture tests were conducted as reported by Wylde and spelt3. Phase angles of 0'

and 48', were controiled by adjusting the load jig describeci by Feralund and ~pelt?. The primary

adherend (adherend bonded directly to primary adhesive) was located in this load jig so that it

carried the greater load, which tended to drive the crack to the primary bond-aluminum interface.

Chapter 2 Secondary Bond Effect on S u a h Energy Rclease Rate of DCB Specimau

2.3 Resuits and Discussion

The results of secondary and tertiary bond effects on DCB specirnens of Cybond

4523GB joints are show in Table 2-1.

Table 2-1 Oc values of specimens of different bond types at phase angles of O0 and 4 8 O

I I 1 total specimen bond type bondlùie I I I thiclaens

A41

A 4 2

AS

AM-1

AM-2

AC5

AAC6- 1

AAC6-2

Gc ( ~ / m ~ ) 228 single

(mm) 0.4

bond single bond singie bond secondary bond secondary bond secondary bond tertiary bond tertiary bond

204

*results of secondary bond interface cracking

SD- (~lm')

7.5

0.4

0.5

0.5

0.5

0.5

0.6

0.6 208

It was reportcd by Ferniund and spelt? that the Gc value at a phase angle of 0' was

2 13 Um2 (S.D.=18.6 J/m2), and approximately 340 J/m2 (S.D.= 55 ~ lrn~) for a phase angle of Mo.

As shown in Table 2-1, the results for both single bond and secondary bond specimens agree well

with these values. Table 2-2 shows the data of Table 2-1 grouped according to various factors

I I J

N

17

3.9

5.3

8 I NIA

Gc (~1x.n~) 36 1

16 NIA

Sm. (Um2) 18.8

N

14

Chaoter 2 Secondarv Bond Eaéct on S e Enersr Release Rate of DCB Smximens

(bondline features which potentiaily have an effect on the hcture strength). The fact that Gc is

relatively constant M e r illustrates the insensitivity of the h c t u r e energy to these factors.

When crack propagation occurs at the interfaces (AM- 1 at both 0' and 4g0, AAS-2,

AAC6- 1 and AAC6-2 at 4S0 only, see Table 2-1) ktween two adhesive layers, the measured Gc

value is below the average of the rest spechens and may be an indication of poor bonding at the

secondary interface. Hence, this lower Gc shodd not be taken as the hcture strength of the

primary bond-

Table 2-2 Gc averages of different factor groups in secondary and tertiary bond tests

factor

single bond

secondary bond

tertiary bond

single high temp. curing cycle two high temp. curing cycles 0.4 mm bondline thickness 0.5 mm bondline thickness 0.6 mm bondline thickness grand average

/ Gc average

specimens average

4- 1, A4-2,

2.8 2 AAC6-1, NIA AAC6-2

17.6 3 Acll,A4-2, 347 AS, AC5

8 -3 3 AAS-2, AAC6- NIA 1, AAC6-2

6.4 2 A4-1, A4-2 346

13.3 3 A5, AAS-2, 349 AC5

2.8 2 AAC6-1, NIA AAC6-2

14.2 7 A4-1, A4-2, 347 AS, AM-2, ACS, AAC6- 1, AAC6-2

S.D. (~/rn')

17.3

NIA

NIA

15.2

NIA

22.0

14.1

NIA

15.2

Chapter 2 Sccondary Boad Effiect on SPain Energy Rclease Rate of DCB Spccixncns

Conceming the effects of bond line thicicness (see Table 2-2), the experimentai data

revealed that the Gc value was essentidy constant with respect to the bondline thicimess ranging

Fom 0.4-0.6 mm. The standard deviation was less than 7% for 0.4 mm ( A 4 1, A4-2), 0.5 mm

(A5, AC5, U S - 2 at 0' only), and 0.6 mm (AAC6-1, AAC6-2 at 0' only). 'Ihis result is

consistent with Femlimd and ~ ~ e l t ' s ~ ~ observation that for Cybond 4523GB, the fiactute energy

was a weak fiinction of the bondline thickness. Similarly, Wyide and spelt3 came to the same

conclusion regarding the Cybond 4523GB adhesive system.

The anaiysis of extra curing time in Table 2-2 shows that one more curing cycle at

1 50°C for 1 .S hours (at 0' ody) did not contribute any signincant di&rence cornparrd witb

specimens (A4-1, A4-2, A5, AC5) with a single curing cycle. The standard deviation of these

two groups was less than 4% (ody 0° &ta are available). This phenornenon could be explained

by the cross-link process. M e r the adhesive is completely cured, the cross-link network inside

the bulk adhesive wiii be M y established, Therefore, as long as the adhesive does not reach

temperatures higher than that endured during the previous curing cycle, subsequent cycles will

not change the fiacture strength of the adhesive.

Another inference based on the results is that the bond between the primary and

secondary adhesives was the main obstacle to implementing the secondary bond technique,

necessitating carefui secondary adhesive selection and rigorous pretreatment of the primary

adhesive surface. The Gc value should be used as the main criterion in the selection of the

seconciaq adhesive. In cases where Gc values are not available, assuming that Gc has a positive

correlation with lap shear strength, which is usually given on the technical data sheet, lap shear

strength can be used as a substitute for Gc in comparing candidate adhesives. The present work

has demonstrated that the selection of a secondary bond adhesive (Hysol9309EA in this case)

Chapter 2 Secondary Bond Effect on Strain Energy Release Rate of DCB Specimais

based on lap shear strength was satisfactory. An additional consideration for secondary adhesive

selection would be the Young's modulus because the secondary adhesive Young's modulus may

affect the stress state aromd the crack tip.

In high mode ratio (greater mode il) tests, the primary-bond adhesive itself would be a

good selection to form the secondary bond because the Young's moâulus does not change over

the primary and secondary bonds and the high mode ratio ensures that the crack will propagate in

the primary bond h aged specimens ttiat are tested dry, assuming that aging does not alter the

Young's modulus signifïcantly, because the aged primary bond will have a lower Gc, the

primary-bond adhesive will form a stronger bond whether it is tested under mode 1 (pure

opening) condition or at a higher mode ratio.

2.4 Conclusions

Conceming the secondary bond effect on the strain energy release rate (Gc) of DCB

specimens ushg Cybond 4523GB as primary bond, within the accuracy of hcture testing, the

present work has corne to the following conclusions:

1. Within the range of 0.4-0.6 mm, bondline thickness does not have a significant

effect on Gc.

2. An extra curing cycle which is identical to the first one (cured at 150°C for 1.5

hours) does not change Gc.

3. The primaxy adhesive is a good selection for the secondary bonding of aged

specimens at al1 mode ratios and also of fiesh specimens which will be tested at a hi& mode

ratio.

Chapter 3 Water DBùsïon in A8iesive Joints


3.1 Introduction

It is well known that the loss of hcture strength in adhesive joints is often

attributable to the presence of water. It is hypothesized that determinhg factors are the spatiai

distribution of water and its cumulative effect over tirne. The water distribution is govemed by

the exposure t h e , water concentration in the environment, and the diB.sion properties of

adhesives at a given temperature. Assumuig that water diffusion obeys Fick's laws, we c m

readily describe water diffusion behavior using the diffusion coetticient (D, m2/s) and the

saturated or equilibrium water concentration (Cs, %14. A literature review indicated that water

diffusion properties of adhesives have not k e n widely studied. It was necessary to mesure the

diffusion properties, D and Cs, for adhesives Cybond 4523GB and Cybond 1 126, which were

used to form the primary bonds in the adhesive joints in the present research.

In a closed adhesive joint, water diffusion certainly occurs over the bondline

thickness, which is governed by Fick's law, but it is not well understood whether the possible

thermal stress due to the temperature difference between curing and aging, and the formation of

interfaces between the adhesive layer and the adherends affect the water diffusion behavior.

These two questions are crucial for describing adhesive joint degradation by water because

thermal stress would alter the diaision properties, and interfacial diffusion would change the

diffusion path and the distribution of water inside a joint.

Chapf er 3 Water Diffiision in Adhesive Joints

Previous work4 has shown that, for certain adhesives, both the diffusion coefficient

and the equiiibrium water concentration are sensitive to stress. The literahue review also

suggested that an interfacial zone, having mechanical properties difZerent fiom those of the buik

adhesive, rnay form during curing, which raises the possibility that the interfacial zone may have

a different water diffusion properties than the buik adhesive. Moreover, it may be that water

enters at an interface through micro-channels resuitiag fiom poor bonding or specimen cutting,

and perhaps osmosis due to residual salt species fiom pretreatment-

3.2 Mathematics of Water Dinusion into Adhesive Joints

One-dimensional Dirusion Model

One-dimensional Fickian diffusion has been described in many referen~es~~~. Due to

the importance of the mode1 to the present research, it is briefly reviewed below.

in Cartesian coordinates, Fick's law is

where D is the d i f i i o n coefficient of water into the adhesive and C is the water concentration of

point (x,y,z) at t h e t. In the present study, water concentration C is defined as the ratio of mass

of absorbed water at tirne t to the initial mass of adhesive, i.e., M,IMo.

if diffusion is restricted to one dimension, Say, the z-direction, such as the case

presented by a thin cast adhesive wafer absorbing water in a moist environment where diffusion

into the edges of the wafer can be ignored, Eqn. (3-1) reduces to

Chape 3 Wata Diffuaon in Adhesive Joints

If the thin wafer is considemi as a semi-innnite film (the ongin of z abscissa is at the

center of the w a k thickness), the solution for the case of an adhesive wafer with a half-thickness

h is

where Cs is the equilibriwn (saturated) water content of the adhesive, i.e., Ms/Mo, Ms is the m a s

of water absorbed at equilibrium.

This gives the concentration of water at point z within the wafer d e r t h e t. It may be

integrated to give the total uptake by the wafer at various times. This is u s d y expressed as the

fractional water uptake M/Ms,

Over short times (up to about 0.6 MJMs), the Gractional uptake is approximated by the following

expression:

This equation provides an approach for detemiining the diffusion coefficient D, while

Ms can be found by taking the average value of mass uptake after saturation (the plateau portion

of the M/MS- & curve).

Shen and springe? have shown that Eqn. (3-4) can be closely approximated by the

simpler expression given below as Eqn (3-6). This approximation can be used for calculating D

using al1 data acquired before saturation.

Chapter 3 Water Difftsion in Adhesive Joints

Two-dimensional D~jjfmion Model

Ifthere is no water transport occuning at interfaces, and specimens have a large

length to width ratio so that the diffusion at both ends can be ignore4 the water uptake M, could

be predicted by Eqn. (3-4) assumùlg one-dimensional diffusion.

On the other band, a two-dimensional water absorption model can be developed to

predict water uptake in rectanguiar adhesive sandwiches. Refeming to Fig. 3-1, h is the half-

thickness of the adhesive layer, 1 and w are the half-length and half-width of the sandwich,

respectively. Water is diffiising into the adhesive layer in both the x and y directions, while the z

direction is shielded by adherends.

Fig. 3-1 Two-dimensional diffusion model

Applying Eqn. (3-1) to the model,

Chapter 3 Water m i o n ia Adhesive Joints

Since it is assumed that no water transportation occurs at the interfaces, Le., the water

concentration wiii be d o r m in the z direction, and that C=Cs at the boundaries. The water

concentration may then be modified so that zero boundary conditions are obtained, i.e.,

T(x, y s o=c(xt Y* 0-Cs- (3-8)

The boundary conditions are then given as

(1) RIs y ,t)=O,

(3 &L JJ .r)=O,

(3 Gxs IV ,[)=O,

(4) axs -w ,t)=O,

with the initial condition

( 5 ) 5(xs y $)=O-

Using the technique of separation of variables, Eqn. (3-7) can be solved as

where

and

O bviously,

Chapter 3 Water Diffiision in Adhesive Joints

where v is the volume of the adhesive layer, and p is the initiai dry density of the adhesive.

Therefore,

Combining with Eqn. (3-13), htegrating Eqn. (3-9) over the length (21) and width (2w) yields

6 4 4 .Y2 m

Mt = M~ --xz 1 expl-(A + p) Dr].

rniO (2n + 1)'(2m + 1)-

In case of 1D diffusion, one dimension, say w in Eqn. (2-14), is considered to be W t e . This

rnakes AD? infinitesimal because D and t are relatively srnall.

Eqn. (3-1 4) reduces to Eqn. (3-4), the 1D case.


3.3.1 The R H Equilibrium Behavior of Environment Chambers at a Given

Temperature

During the measurement of water diffusion properties and open-faced specimen

aging, the environment chambers were periodically opened for specimen weighing and

inspection. It was assumed that the periodic openhg of the environment chambers would not

significantly affect the relative humidity (RH). To prove the validity of this assumption, a

monitoring experirnent was carried out.

The monitoring tests were conducted in environmental chambers that were

constmcted fiom ordinary food containers (Fig. 3-2)- The supporthg grating for adhesive

samples was made of plastic rods with a diameter of 4 mm. The ends of the rods were fked

Chamer 3 Water Diffiision in Adhesive Joints

through the holes in the walls of the container, and sealed ushg a silicone sealant. To measure

RH, a probe was put into the chamber through a hole (diameter 3 / 4 3 and the gap between the

probe and the hole was sealed with a foam disk and tape (see Fig. 3-2). Otherwise, the probe hoie

on the cover was closed using a rubber plug, The chamber cover was sealed to the wdIs with

tape during testing.

Relative humidity in each chamber was controiled by an NaOH solution. The

relationsiiip between NaOH concentration and RH is shown in Table 3- 1. Ln the range f?om room

temperature to 8S°C, the relationship is almost independent of temperature".

< to multirneter foam disk ,

\

\ NaOH solution I

View A-A

probe

Note: Silicone sealant in View A-A, and seaiing tape around the foam disk and cover edges are not shown

f ig. 3-2 An environment chamber during adhesive absorption testing

Three chambers of different RH (93%, 53%, and 6%) were prepared 24 hours before

monitoring to ensure that the chambers would reach equilibrium. Ushg an Omega HX-94C RH-

Chapter 3 Water Dïfiùsion in Adhesive Joints

temperature probe, the chamber RH was recorded at merent time intervals, more fiequently at

the beginning of m o n i t o ~ g .

Table 3-1 Relationship between NaOH concentration and RH from 24'C to 8S°C

Two processes were contributing to the equilibration of the recorded RH: the actuai

equilibrating behavior of the charnbers and the probe response to step changes in RH. Four tests

were camied out at 24OC: two assessed the combined effects of chamber equilibration and probe

response step RH changes, one 6om 53% RH to 6% RH in the chamber with a initial probe

reading 93% RH and the other fkom 53% RH to 93% RH in the charnber with a initial probe

reading 7% RH; the others assessed the probe response to step changes, one fiom 6% RH to 53%

RH and the other fiom 53% RH to 93% RH.

In experiments 1 and 2 (Table 3-2), both 6% RH and 93% RH chambers had been

exposed to room environment (53%RH, 24'C) for 2 hours before they were close for testing, and

the probe was put into the chamber upon closing. In experiment 1, the probe had been placed

NaOH concentration (% w-t.)

O

7

10

17

30

RH (%)

1 O0

93

91

85

71

Chapter 3 Water DiffUsion in Adhesive Joints

into the 93% chamber for approximately 2 hours before it went into the 6% chamber; therefore,

the initial reading for this process was 93%. in experiment 2, the probe was directiy moved fiom

the 6% chamber to the 93% chamber, setting the initial reading at 7%.

Ln experiments 3 and 4 (Table 3-2), the probe had reached a stable reading (6% or

53%) before it was exposed to the new RH (53% or 93%, respectively). The results (Table 3-4)

showed îhat it took approximately 30-60 min to reach a stable reading when the probe was

placed into an environment chamber with a significantiy different RH fÎom its initial state-

Table 3-2 Test schedule for RH equilibration of environment chamber and probe

response to step RH changes

I experhent I initial RH I fuial RH

category

The results of experiments 1 and 2 are Iisted in Table 3-3. Both cases showed that, 1

hour after closing, the recorded RH difference was within 10% of the equilibriurn RH; after 2

hours. the measured RH was within 2% of the equilibrium value. It should be noticed that these

two tests represented an extreme situation; the normal open pend was less than 30 s for

specimen weighing and Iess than 5 min for inspection.

These results were used to guide the conduct of the water absorption experiments.

probe 1 chamber No.

combined effects of chamber equilibration and probe response the probe response to RH step changes

probe 1 chamber

1

2

93%

7%

3

4

53%

53%

6%

93%

6%

93%

6%

53%

53%

93%

53%

93%

53%

93%

Chanter 3 Water Diffiision in Adhesive Joints

Table 3-3 l ime requited to r~s tab l i sh equilibrium RH (combined effecfs of actuaC

cham ber equilibration and probe response)

1 probe initial reading: 93% 1 probe initial readîng: 7% l

Table 3 4 Probe response time for step changes in RH

expected equilibrium chamber RH: 6% Tirne (min) 1 RH (%)

expected equilibriurn chamber RH: 93% Tirne (min)

probe initiai reading: 6%

equilibrium chamber RH: 53%

RH (Yo)

probe initiai reading: 53%

equilibrium chamber RH: 93%

Time (min) Time (min) RH(%) RH (%)

53 O 6 1 O

Chabter 3 Water DZfkîon in Adhesive Joints

3-3-2 Eiperimental Procedures- Water Diffusion in Bulk Adhesives

To measure the d i h i o n properties (diffiision coefficient and equilibrïum water

concentration) of Cybond 4523GB and Cybond 1 126, wafers of these adhesives were cast using

milled, half-inch-thick, AA6O6 1 -T6 plates coated with TFE mold release agent (Miller-

Stephenson- 122N/C02). The wafer thichess was controlled by 0.375 mm thick Teflon sheet

spacers, one ply for the 0.4 mm thick wafers and two plies for 0.8 mm thick wafers. The wafers

were cured under the sarne conditions as the open-faced specimens descncbed in Chapter 5-

Before king put into environment chambers (Fig. 3-2), the wafers were cut into specimens of

approximately 50mmxSOmm using a scalpel, and the thicknesses were measured ushg a digital

caliper at 20 difEerent locations on each group (2-3 pieces) of wafers. The readings were averaged

to obtain a nominal thickness for each group for the calculation of the average diffiision

coefficient.

The environment chambers were placed in ovens at pre-set temperatures. The

combinations of temperature and RH to which the specimens were exposed were identicat to the

conditions of the adhesive joint degradation chamben in this research; namely, 8S°C at 4 relative

humidities: 100% RH, 85% RH, 60% RH, and 30% RH; and 65OC at the same 4 relative

hwnidity levels. The aging temperature were selected below or around the glass transition

temperatures (Tg) of the adhesives. For Cybond 1 126, Tg was measured at 6 lfO.S°C using

differential scanning calorimetry (The Lab of Inorganic Molecules, Polymersy and Materials,

Department of Chemistry, University of Toronto). And the Tg of Cybond 1523GB is reportedt3 as

1 1 4e°C. Therefore, the Cybond 1 126 specimens were tested only at 6S°C (Although 6S°C is

slightly higher than T, it was employed in the absorption and DP verification in Chapter 5 to

duplicate the experimental settïngs of ref. [3 1).

Chapter 3 Water Difkion in Adhesive Joints

Since the adhesive surfaces of the open-faced specimens were sanded with 220 grit

sandpaper before degradation to remove residues of mold release agent, it was of interest to

explore the effect of sanding on the diffusion properties. Sanded wafers were rinsed using

acetone after sanding, while the unsanded wafers were cleaned with a Kimwipe tissue using

acetone.

The solubility of the mold release agent with acetone was checked by observing the

contact angle of a water droplet on coated and clean aluminum surfaces. The droplets had a

contact angle about 45' on the surface sprayed with mold release agent, while the contact angle

on the same surface cleaned with acetone was 0°, suggesting that the mold release agent had been

cornpletely removed-

The wafers were dried in a vacuum oven at 30-50°C for 24 hours before being put

into environment chambers. The dried mass of the specimens was taken as the initial mass (Mo).

During the sorption tests, the specimens were periodically taken out for weighing. Before

weighmg, specimens were put on a Kimwipe tissue in room air and dried for about half an hou.

Specimen mass was measured using a Sartorius GMBH electronic analytical balance (Type

1 7 12) with an accuracy of H.0 1 mg.

3.3.3 Experimental Procedures- Water Diffusion in Saw-cut and Knife-cut

Sandwiches

The objective of these experiments was to check whetber the absorption behavior of

the closed aluminum-Cybond 4523GB joints differed fiom that of cast wafers. With the

assurnption of no interfacial diffusion and no thermal stress effect, the predicted water uptake

Chapter 3 Watcr DBùsion in Adhesive Joints

using the Fickian mode1 was compared with the measured data; if the p d c t e d water uptake was

iess than the measurements, then water may have entered the joints via the intedaces or thermal

stress may have changed the diEusion properties.

Two groups of sandwich specimens of Cybond 4523GB were fabricated at different

times during this experiment. The adherends of the £irst group (8 specimens) were made of

AA5454-0 sheet of 1 mm thick, and the specimens were cut from 2 bonded plates of

approximately 60 mmx2 10 mm using a diamond saw (Stmers Accutom-2). The results of these

samples showed a significantly higher water absorption rate and saturated water content (see

Section 3 -4.3). These results, however, could be used as the evidence of a higher D and Cs in

sandwich diffusion ody if there was no possibility that interfaces and bondlines of the specimens

had been damaged during saw-cutting. To avoid saw-cutting operation a second group of 36

specimens was prepared, this tirne using 0.1 mm thick AA 1 100-0 as adherends and cutting them

fkom 2 bonded sheets for specimens of the same thickness bondiine (Total 6 bonded sheets were

fabricated. The size of the sheets was also approximately 60 mmx2 10 mm) ushg a sharp knife.

The adherends of the sarnples in these NO groups (saw-cut and knife-cut) were

pretreated and bonded using the procedures described in Chapter 5. The thin adherend group was

clamped between a pair of milled hdf-inch-thick aluminum plates during bonding and curing.

Different adhesive batches were used for each group. The diffusion properties of these two

batches were slightly dif5erent (see Section 3.4.1).

It was speculated that interfacial diffusion may occur in a very thin region near the

adherend surface and that different adhesive thicknesses would be helpfiil revealing the pattern of

diffusion in the interfacial zone. The bondline thickness in the group with 0.1 mm adherends

(knife-cut) was controlled by two plies of nominally 0.5 mm thick Teflon spacers for 1.12 mm

Chapter 3 Wacer DiBision in Adhesive Joints

bondline thickness, one ply of nominally 0.5 mm thick Teflon spacers for 0.56 mm bondline

thickness, and 0.2 mm thick aluminum spacers for the 0.24 bondline thickness. The thick

adherend (1 mm, saw-cut) group had only one bondline thickness: 1.12 mm.

Knife cutting was performed ushg a scdpel to cut through the adherends to the

adhesive layer on both sides and then breaking the specimen by hand. This was uitended to

minimize any damage to the bondline due to vibration during saw-cutting. Both saw-cut and

knife-cut specimens were carefdly filed in the lengthwise direction (Fig. 3-3) in an attempt to

remove any damaged zone near the edges.

adherends adhesive layer -

l

I L (filing direction) !

Fig. 3-3 A saw-cut or k n i f ~ u t adhesive sandwich

After filing, the specimens were cleaned using a Kimwipe tissue soaked with acetorie.

The specimens were then dried in room air over 2 hour before weighing. Because these

specirnens were closed joints, and the room air had a relatively low temperature and relative

humidity (approximately 24OC, 55% RH), it was assumed that they could not absorb enough

water to violate the assumption that they were essentially dry pnor to sorption testing.

Chapter 3 Water D i m o n in Adhesive Joints

The haif-width of a sample was the diffiision path for the joint in 1D diffiision. In

order to Vary the time to saturation, the saw-cut sandwiches were divided h t o two sub-groups of

different widths, 6.5 mm and 8.5 mm; while the knife-cut group were made ïnto three different

widths, 4.5 mm, 6.5 mm, and 8.5 mm. The lengths varied, but the ratios of length to width were

al1 greater than 13, which ensured that the one-dimensional diffusion mode1 was reasonabiy

accurate. For details of the sample features, refer to Tables 3-5 and 3-6.

Table 3-5 Properties of saw-cut sandwiches used in water absorption experiments at

8S°C, 100% RH and 6S°C, 100% RH

Note: The specimens were numbered in such a way that the f h t two digits stand for temperature. the third digits for

nominal widths (8-8-5 mm, M . 5 mm), and the last digit for the replicate number. The thickness of the

adherends was 1 mm and adhesive layer thickness was 1 -1 2 mm.

The initial mass of the adhesive inside the specimens was detennined by subtracting

the mass of the adherends fiom the total initial mass of the specimens. The adherend mass was

calculated using the adherend volume and density. The densities were rneasured as 2.665 &rn3

and 2.625 &rn3 for AA5454-0 (the 1 mm thick sheet) and AA 1 100-0 (the 0.1 mm thick sheet),

respectively.

Chapter 3 Water D i o n in Adhesive Joints

Table 3-6 Properlies of knife-cut sandwiches used in water absorption experiments at

85"C, 100% RH and W°C, 100% RH

adherend adhesive specimen 2wxL totai adberend adhesive ,-(g) - (g)-. (mrnxmm) m s s (g) (g) mas (g)

0.5921 1.71 72 6510851 8.27x155.5 2.9749 1 0.7494 2.2255

Note: The specimens were numbered in such a way that the k t nvo digits stand for temperature, the third and forth

digits for adhesive layer thicknesses (10-1 1.2 mm, 01-0.56 mm, 0 2 4 . 2 4 mm), the f i f i and sixth digits

for nominal widths, and the last digit for the replicate number. The adherend thickness was 0.1 mm-

The specimens were put into chambers of 100% R H at 8S°C and 6S°C with 2

replicates at each specimen size and temperature. The mass of absorbed water was measured

using a Sartorius GMBH electronic analytical balance (Type 17 12, accuracy M.0 1 mg) at 6- 10


day intervais at the beghhg , extending to intervals of about 2 weeks later on, and reaching

about one month for the last few data points.

Because of the possible mass change of the alilminwn surfaces due to oxidation and

hydration, control pieces of aluminum sheets (cut fiom the same sheet as the adherends) were

pretreated, aged, and weighed at the same tirne as the bonded specirnens. The mass changes of

the control pieces were scaled to the adherends according to the exposed surface area.

The initial malis of each specimen and the alruniniun sheet correction was deductecl

fiom the total m a s to obtain the water uptake (Mt). The water uptake was then converted into the

fractional water content (Mt/Ms) and compared witii the fiactionai water content predicted using

one-dimensional and two-dimensionai Fickian diffusion models with d i f i i o n properties fkom

fiesh, cast wafers-

3.3.4 Erperimental Procedures- Water Drfsihn in Sandwiches with (Incut

Edges

Although knife-cutting and filing were relatively gentle operations, they could still

potentially damage the joints creating micro-cracks at the bondline which might then provide

channels for water ingress. To M e r avoid possible damage due to cutting and filing, four

adhesive sandwiches with uncut edges were fabricated.

The uncut sandwiches were aiso made of 1 mm thick AA5454-0 adherends as with

the saw-cut groups, but the adhesive batch is the same as that used in knife-cut specirnens. The

adherend surface pretreatment was the same as for the open-faced specirnens (Chapter 5). M e r

pretreatment(Tef1on spacers were also cleaned using acetone), fou. sets of plates and spacers

Chapter 3 Water D i h i o n in Adhesive Joints

were grouped and weighed to record the mas. The adhesive mass was then obtained by

subtracting the aluminum and Teflon mass fiom the total sandwich mass.

ï h e sandwich structure (Fig. 3-4) was unique in that the adherends were different in

size. The larger adherend (bottom plate) served as the base of a sandwich, the smaiier adherend

(top plate) deterrnined the sandwich width, while two Teflon spacers at the ends contr011ed the

Iength and thickness of the adhesive layer. Table 3-7 shows the properties of the specimens.

-, clamp, both ends /

View A

View B-B

Top View

\ adhesive top plate / bottom plate \ / Teflon spacer. both ends

i

i ! i

t i

1

top plate

I

Fig. 3 4 An uncut adhesive sandwich showing end clamps and Teflon spacers in place

Chavter 3 Water Diffusion in Adhesive Joints

During bonding, 2-3 times the amount of adhesive needed to form the bondline was

spread approxirnately at the center of the bottom plate. Then, two spacers and the top plate were

put into position, and two 2-inch paper clamps were applied to the ends over the spacers.

The clamped specimens were left at room temperature for 2-3 hour, during which

time the adhesive spew was periodically removed using a small spatula. Before curing, the

bottom plate surface was cleaned using a Kimwipe tissue soaked with acetone to remove residuai

adhesive due to spatula operation. Depending on the position of the clamps, adhesive continued

oozing out of the bondline or contracted during curing due to decreasing viscosity and other

thermal effects. This produced a variable bondline width, but the variance was relatively small

( 1 -3 % of the bondline width).

Table 3-7 Properties of uncut ad hesive sandwiches

1 spacer 1 thickness(mrn) / 0.5 ( 0.5 1 0.5 1 0.5 I

l ~ Pr"peq

I outer surface (cm2) 1 57.88 1 58.29 1 58.59 1 58.52 1

#65 1

135

7.58

1 .O

149.32

20.02

1 .O

135

top plate

bottom

plate

1

total m a s (g)

adherend and spacer m a s ( g )

adhesive mass (g)

length (mm)

width (mm)

thickness(mm)

length (mm)

width (mm)

thic kness(mm)

(mm)

$652

135

7.44

1 .O

149.29

20.00

1 .O

134

adhesive

layer

Teflon

7.34

0.56

1s

5

L 1

#85 1

135

7.58

1 .O

149.26

19.79

1 -0

136

2w (mm)

2h(mm)

len* (mm)

wvidth (mm)

17.54047

16.77800

0.76247

17.4435

16.64992

0.79358

#852

135

7.34

1 .O

149.33

19.92

1 .O

132

7.58

0.56

12

5

1 7.58

0.56

12

5

17.4437 1

16.6 1595

0.82776

17,45238

16.68063

0.76575

7.44

0.56

12

5

Chapter 3 Water DiffLsion in Adhesive Joints

The absorption tests were again conducted at 100% RH at two temperatures, 65OC and

8S°C, with two replicates in each case. The water uptake measurements and data processing

procedures were the same as for the saw-cut and hife-cut groups.

3.4 Results and Discussion

3.4.1 Water Diffusiort Promies of Bulk Adhesives

Plots of fiactional water uptake (the ratio of the mass of absorbed water at

-

time

the mass of absorbed water at equilibrium (MJMs)) versus root time (& ) are helpfui in the

- de teda t ion of dinusion properties. A typical MJMs - Jt plot is s h o w in Fig. 3-5. The plateau

portion of the plot corresponds to the saturated state, and the ascending portion can be considered

linear up to about 0.6 (MMs).

O 200 400 600 800 root time (s '")

Fig. 3-5 A water absorption plot of Cybond 4523GB

After the mass of the absorption specimen reached a relatively constant value, the

average mass increase was taken as the equilibrium water content Ms. The percentage Ms over

Mo (initial adhesive mas) was obtained as the equilibrium water concentration Cs. The diffusion

Chapter 3 Water DiBision in Adhesive Joints

coefficient was calcdated using the exponentid approximation of Eqn. (3-6). Each pair of M M s

- fi data which was below the plateau yielded a D value and the average of these D values

measured fiom one specimen was taken as the D of the specimen. In the case of CS, the plateau

values were averaged for each specimen.

The average values of the Cs and D at both 6S°C and 85OC of Cybond 4523GB batch

B-6404 (used in knife-cut and uncut groups) are shown in Tables 3-8 and 3-9. Those of the

Cybond 4523GB batch LX4945 (used m the saw-cut group, up to 27 days of aging, only I

specimen was tested) were: 5 12.3 x 1 0 - l ~ m2/s (N=4, s . D . = ~ . ~ x IO-'' m2/s, 23%), and Cs

=4.13% ( N 4 , S.D.=0.23%) at 8S°C and 100% RH; D= 10.0~10- '~ m2/s (N=4, S.D.4.7 ~10 - l3

m2/s. 17%), and Cs =3.66% (N4, S.D.=O.OS%) at 65OC and 100% RH.

Table 3-8 Equilibrium water concentrations of Cybond 4523GB (Batch 8-6404) up to 11

days of aging at 6S°C, 8S°C and 100% RH, 85% RH, 60% RH (N is the number of

data points collected f'rom each specimen and used in specimen average or

number of specimens in grand average)

m

specimm 1 Cs 1 N 1 S.D. 1 suface 1 RH 1 specimen (%) (96) condition (96)

A-8-1 4.89 6 0-16 sanded A-6- 1

A-8-3 4.90 3 0-02 unsanded

grand averagel 468 1 3 1 0.02 1 N/A 1 85 1 Ad-3

A-84 2.90 3 0-06 sanded A-6-4

A-8-6 1.15 6 0.04 unsanded 60 F=- grand average grand average 1.13 2 0.03 NIA 1-

' (%) 1 1 zi Iconditiom 3.86 10 sanded

1 1 1

3.79 1 3 1 0.06 1 unsanded

1

2.20 1 3 1 0.01 1 sanded

0.83 10 0.01 sanded

0.78 1 3 1 0.02 1 sanded I

0.73 3 0.01 unsanded


Table 3-9 Diffusion coefficient of Cybond 452368 (Batch B-64û4) up to 11 days of aging

at 65OC, 85OC (N is the number of data points collected from each specimen and

used in specimen average or number of specimens in grand average. S.D. is

reported in percentage of the average)

RH (%) (m2/sx I O-' condition

60

, 1 4.2 2% 1 unsanded

specimen

A-8- 1

9.1 2 18% sanded

7.9 3 6.7% sanded

6-8 3 11.6% unsmded

6.9 5 27% N/A

D N S-D, surface RH specimen (m2/sx IO-") condition (96)

12.3 3 19% sanded A-6- 1

A-8-3

A-8-5

A-8-6

Tables 3-1 0 and 3-1 1 shows the d i h i o n data at 65OC for Cybond 1 126 batch B-LX-

10.1 3 20% unsanded A-6-4

14.0 1 N/A sanded 60 A-6-5

13.6 4 6% unsanded A-6-6

grand average

6748. Cybond 1 126 was tested o d y at 65'C, because 85OC was beyond the glas transition

temperature (Td of the adhesive. See Appendix A for the experiments at 60% and 30% relative

humidity.

123 5 13% N/A grand average

Table 3-10 Equilibrium water concentrations of Cybond 1126 (Batch 6-LX-6748) up to 39

days of aging at 6S°C and 100% RH, 85% RH (N is the number of data points

collected from each specimen and used in specimen average or number of

specimens in grand average)

1 RH (%) i specimen 1 Cs (%) 1 N 1 S.D. (%) 1 surface condition 1 I l B- 1 I 10.3 / 5 1 0.50 1 unsanded 1

I 1

1 85 1 B-3 1 3.62 1 5 1 0.22 1 unsanded 1 grand average

t I

9.16 I 2 I 1.60 N/A

Table 3-1 1 Diffusion cmfficient of Cybond 1126 (Batch 8-LX-6748) up to 39 days of

aging at 6S°C (N is the num ber of data points collected from each specimen and

used in specimen avenge or number of specimens in grand avenge. S.D. is

reported in percentage of the average)

I 1 100 1 B-2 1 6.12 1 4 1 24% 1 unsanded

B- 1

I 1 grand average 1 10.2 1 2 / 56% 1 NIA

S.D. 1 surface condition RH (%)

It was assumed that the diffusion coefficient was independent of the relative humidity

at a given therefore the averagevalues of D in Tables 3-9 were taken across the

two levels of relative humidity at both 85OC and 65°C. The 85% RH measurements for both

adhesives at both temperatures were conducted to obtain only Cs.

The results shown in Tables 3-8 to 3-1 1 indicate that the standard deviation (S.D.) of

Cs was relatively small (up to 7.7% of the average for Cybond 4523GB, and 17% for Cybond

1 126)), while S.D. of D was up to 20% w i t b a specimen and 27% arnong the specimens for

Cybond 4523GB, and up to 24% within a specimen and 56% among the specimens for Cybcmd

1 126. This confkms that D is sensitive to experimental variability, which is consistent with

Althof s experience2'. In the case of Cybond 1 126, the initial water content might have had a

significant effect on its diffusion coefficient measurement (refer to Appendix A for further

discussion on the eEect of initial water content in Cybond 1126). For Cybond 4523GB, the

overall (batch-to-batch plus specimen-to-specimen) standard deviation in the D measurement

was 12% of the average at 85OC and 22% at 6S°C.

Surface condition in Tables 3-8 to 3-1 1 refers to the sanding with 220 grit sandpaper

before absorption testing. Although light sanding changed the morphology and increased the

specimen 1 D (m2/sx l 0-14) 1 N

142 17 24% unsanded

Chapter 3 Water D W o n in Adhesive Joints

surface area of the specimens, the experimental results showed that this did not significantly

affect the d i h i o n coefficient. As expected, sanding the surface of the bulk adhesive did not

affect the equilibrium water content.

It can be seen fiom Table 3- 12 that the diffusion coefficients of Cybond 4523GB are

roughly consistent with the measurements of cet [3], but the equilibrium water concentrations

are comparable with those in ref. pl only at 6S°C, 100% RH. Naim et al.13 reported a Cs of 3.5%

for Cybond 4523GB immersed in 70°C water? which is close to the present result at 65OC. 100%

RH (3.69%).

Both the diffusion coefficient and equilibrium water concentrations of Cybond 1 126

are significantly different fiom the results of ref. [3] (Table 3-13). Batch-to-batch variance and

inconsistent mixing processes may be partly responsible for the dserences, but a thorough

Table 3-1 2 Comparison of equilibrium water concentrations and diffusion coefficients of

Cybond 452308 at 3S°C, 6S°C and 85% between present and pievious work

I I Ref. [3] I present work

temperature (OC) 35

RH (%)

L

Cs (%)

100 1 3.5

Cs (%) D (m2/sx 1 0-13)

5.1

D (m2/sx I O-13)

N/A NIA

Chapter 3 W a t a DiBision in Adhcsivc Joints

Table 3-1 3 Cornparison of equilibrium water concentrations and diffusion coefficients of

Cybond 11 26 at 66% between present and previous work

Ref. 131 1 present work

It has been reported that D is independent of relative humidity. For most polymenc

materials, the relationship between D and T is governed by the Arrhenius ~ ~ u a t i o n ~ ~

RH (%)

100

85

60

30

where Do is a base value of the diffusion coefficient; EA is the surface activation energy of the

material; and R is the gas constant (8.3 14 J/(K-mol)). Although a typicai structural adhesive is a

mixture of resin and minera1 mer, the present work showed that Arrhenius Equation was valid to

within reasonable error for Cybond 4523GB. Based on the present data, the coefficient Do and

-8 2 the siaface activation energy EA of adhesive Cybond 4523GB were estimated to be 2.2~10 m /s

and 2% 1 o4 Jlmol, respectively. The Do and EA of Cybond 1126 cannot be caiculated because

only 65OC measurements were conducted on this adhesive.

The results showed that the equilibrium water concentration varied significantly with

changes in relative humidity at a given temperature. But the data also revealed that the ratio of

equilibrium water concentrations at different Ievels of relative humidity was ahost constant,

regardless of the temperature. In the case of Cybond 4523GB, the ratios Cs of 100% RH, 85%

RH and 60% RH were approximately 4.5:2.7:1 at both 85OC and 6S°C. Although the cornparison

Cs (%)

18.8

NIA

2.8

0.37

D (m21sx 1 0-14)

0.289

Cs (%) -

9.16

3.62

N/A

NIA

D (m2/sxl~-'4)

10.2

Chapter 3 Water Difîùsion in Adhesive Joints

between different temperatures is not available for Cybond 1 126, it is noted that the ratio of Cs at

100% RH and 85% RH at 65OC was about 5:2.

On the other hand, the equilibrium water concentration was not a strong function of

temperature at a given relative humidity. This conclusion is consistent with the results of Wylde

and spelt3 and other r e ~ e a r c h e r s ~ ~ ~ ~ . In the case of Cybond 4523GB, the ratios of Cs at 85OC and

6S°C are 1.32: 1 (1 00% RH), 1.3 1 : 1 (85% RH), and 1.45: 1 (60% RH). Again, it shouid be noted

that these tbree ratios are roughly the same.

The equilibrium water concentration is strongly dependent on the nature of the

adhesive. For organic buik adhesives, the equilibrium water content varies over a wide range as

temperature and relative humidity change. The data of Bowditch et a1F9 yielded a value of 2.8 for

the ratio of Cs at 100% RH and 60% RH at a given temperature (not specified) for Jeffamine

Dl000 and Jeffamine D230 filled epoxide resin (Araldite GY250) adhesive. Gledhiii et ai.% 30

data can be translated into a value of 1.9 for the ratio of Cs at 100% RH aud 55% RH (20°C) for

another epoxy adhesive. It seems that each adhesive has its own relationship between Cs and RH

at a given temperature.

It is of interest to correlate Cs and the amount of water available in the air. The

amount of water in moist air can be characterized by the specific humidity (ratio of the mass of

the water vapor to the mass of the dry air in the mixture). The specific humidities at the

experirnental settings adopted in the present research are listed in Table 3-14.


Table 3 4 4 Specific humidity of rnoist air at 6S"C and 85OC; 100%. 86%, and 60% RH"

I experimental setting specifk humidity

temperature (OC) 1 RH (%) 1 (g of watedg of air)

0.2 O -4 0.6 0.8 specific humidity (g of water/g of air)

o Present research x Ref. [3]

Fig -3-6 Relationship between specific humidity and equili brium water concentration of

Cybond 452308 at 6S°C and 85OC. A4S°C, 60°h RH; B, G-ûS0C, 86% RH; C, H-

6S°C, 100% RH; D-8S°C, 60% RH; E a ° C , 85% RH; F, I-8S°C, l W% RH

CbaDter 3 Water DiBision in Adhesive Joints

Figure 3-6 shows the relationship between the specific humidity and the equilibrïum

water concentration of Cybond 4523GB. It noteworthy that water avaiiability is not the only

factor that determines the equilibrium water concentrations in this adhesive; temperature seems

to play a significant role, and the adhesive absorbed water more efficiently at 6S°C than it did at

85°C. ~ l t h o f ~ observed that several adhesives demoastrated the same trend with 20°C, 95% RH

(specific humidity 0.014) and 70°C, 95% RH (specific humidity 0.265).

3.4.2 Results and Discussion-Mass Gain of Adherend Surface and Teflon

Spacers

The measured mass increase of a sandwich specimen in a 100% RH environmental

chamber was more than that due to water alone. The results of the present research revded that,

at 65'C, the oxidation and hydration of the adherend (aluminum) surface had a significant effect

on the m a s increase: in the case of saw-cut specimens, the mass increase due to aiuminum

surface reactions was up to about 6% of the absorbed water in the adhesive; in the case of knife-

cut specimens, the mass increase due to aluminum surface reactions was up to about 75% of the

absorbed water for 0.2 mm thick bondline specimens. The water absorption of the Teflon spacers

in the uncut specimens contributed little to the mass increase.

The results of the aluminum surface and Teflon spacer mass gain are shown in Figs.

3-7 to 3-9. It was observed that aluminum specimens at 65OC had gained more mass per unit area

than that at 8S°C. This remains unexplained since the metal came fiom the same sheet and was

pretreated identically. Part of this discrepancy may simply be due to the variability inhemnt in

corrosion measurements.


Fig. 3-7 Mass gain per unit area of AA5454-0 alurninum control plates for sawtut

specimens at 100°h RH

The regression equations for the mass gain per unit area of the control plates for saw-

cut specimen absorption testing was obtained for predicting the adherend mass increase of the

uncut specimens. These two groups adopted the same aluminum (AA5454-0, 1 mm tbick) sheet

as adherends. In the case of 65OC,

dM = -2 x 10''' t ' + 2 x 1 O-'' t - 5 x 1 O-' t ' + 0.0006r - 0.02 1 9 ( r s 1 320 hours) (3-1 6)

dM = 0.295 ( t > 1320 hours)

In the case of 85'C,

( d . = 0.04 (1 > 307 hours)

where dM is the mass gain per unit surface area in mglcm2 and t is time in hours.


Fig. 3-8 Mass gain per unit area of AA1100-0 aluminum control sheets for k n i f ~ u t

specimens at 100% RH

Fig. 3-9 Mass gain of Teflon spacers for uncut specimen absorption testing (100% RH)

Chapter 3 Wanr m o n in Adhesive Joints

The mass of a pair of Tenon spacers in an uncut specimen was about 0.1468 g (1 2

mmx5 mmx0.5 mm). In the 65OC case, a larger water absorption situation, the mass gain of

Teflon sheet was about 0.22%, making the mass of water absorbed by the two spacers 0.00032 g.

This value is approximately 0.8% of the adhesive saturation water content, suggesting that water

absorption by Teflon spacers had a negligible effect on the water content of the bondine

adhesive. Moreover, ody about one quarter of the spacer surface was exposed to the moist

environment (Fig. 3-4).

3.4.3 Results and Discussion- Water Diffusion in Clused Joints

Saw-cut specimens

The results of al1 the three testhg groups, saw-cut (Figs. 3-10 to 3-13), uncut (Table

3- 1 S ) , and knife-cut (Tables 3- 1 7 and 3- la), demonstrated that water d i f i e d into these joints

faster than into the bulk adhesive and that Cs was Iarger than expected. With a significantly

larger initial slope than that corresponding to the bulk adhesive ciinusion coefficient, the plots of

MJMs versus root time varied significantly fiom the Fickian diffüsions. The one-dimensional

and two-dimensional predictions were calculated using diffusion properties from cast wafers (see

Section 3.4.1) assuming that there was no interfacial diffusion or any thermal and swelling stress

effects existing in these sandwiches. Because the ratios of length to width of the samples were

relatively large (over 20), the two models yielded almost the same results.

As mentioned before, one possible explanation for the accelerated water uptake and

higher equilibrium water concentration seen in Figs. 3- 10 to 3- 13 was that the process of saw-

cutting and filing could have induced micro-cracks which provided chaanels for water ingress.

This possibility was addressed by the testing of hife-cut and uncut samples. Moreover, aged

Chapter 3 Water Dïfhion in Adhesive Joints

saw-cut samples were dyed with an acetone solution of liquid papa for 20 days after the

absorption test. They were then dried at a 6S0C for 48 hours. The dried specimens were then

etched using an NaOH solution of approximately 20% concentration by weight till the adherends

remained only about 0.1 mm thick. The specimens were then removed, and the remaining

aluminum was peeled off and examined using a microscope and the SEM.

The images of both the adhesive and alumuium (adherend) surfaces were compared

with those of a newly fabricated (fie&) specimens, The results showed that, although the surface

of the aged adhesive were rougher and more adhesive residue was on the aluminum surface, but

no evidence of systematic micro-cracks or white liquid paper particles was observed (Figs. 3-14

to 3- 17). The SEM sites were chosen near the edges of the samples, where any damage should

have k e n more evident. It was concluded that the roughened surface resulted fiom aging.

Fig. 3-1 0 Water absorption of sawcut Cybond 4523GB sandwiches (2h1.12 mm, 2 ~ 8 . 5

mm, aged at 8S°C, 100% RH). A,-4 8581; &r 8582; C, D-10 and 2D pmdictions

using data from fresh, cast wafers; - - - - - uncertainty envelope of 1 D prediction

based on 13s.D. for D (S.D.=23%)

Chapter 3 Water Diffirsion in Adhesive Joints

Fig. 3-11 Water absorption of sawcut Cybond 452308 sandwiches (2hr1.12 mm, 2 ~ 6 . 5

mm, aged at 8S°C, 100% RH). A 4 8561; B-# 8562; C, D-1D and 20 predictions


based on k3S.D. for D (S.0.=23%)

Fig. 3-12 Water absorption of saw-cut Cybond 4523GB sandwiches (2k1.12 mm, 2 ~ 8 . 5

mm, aged at 6S°C, 100% RH). A 4 6581 ; B-ir 6582; C, D-ID and 20 predictions


based on k3S.D. for D (S.D.=llOh)

Chaoter 3 Water Diffirsion in Adhesive Joints

Fig. 3-1 3 Water absorption of saw-cut Cybond 452368 sandwiches (2h4.12 mm, Zm.5

mm, aged at 6S°C, 100% RH). AX 6561 ; B-lr 6562; C, 0-1 D and 20 predictions

using data from fresh, cast wafers; - - - - - uncertainty envelope of 1 O prediction

based on k3S.O. for O (S.D.=t 7%)

Fig. 314 Peeled adhesive surface of sarnple 8682 (aged at 85*C, 100% RH for 78 d a p )

49

Chapter 3 Water m o n in Adhesive Joints

Fig. 3-15 Peeled adhesive surface of a fresh joint

Fig. 3-16 Peeled aluminum surface of sample 8582 (aged at 8S°C, 100% RH for 78 days)


Fig 3-17 Peeled aluminum surface of a fresh joint

In searching for possible explanations for the unexpectedly hi& Cs observed with

saw-cut specimens, the foiiowing rasons were considered:

1) Because the amount of adhesive was obtained by subtracting the calcdated

aiuminum mass from the total sample mass (Section 3.3), an overestimated adherend volume

ancilor aluminum density could have yieided a larger adherend mass, thereby reducing the

adhesive mass.

2) Cs of the cast adhesive wafer was underestirnated.

3) Bonding and curing had developed an interfacial zone which had a higher Cs than

that of the bulk adhesive.

4) Biistering might have collected fiee water (as opposed to the bund water in the

adhesive).

Chapta 3 Water Difkîon m Adhesive Joints

5) Long-tenn aging, combined with residual stresses resulting h m the curing of the

joints, might have induced micrrrcracks or other form of structurai change in the adhesive which

provided more space for water absorption.

Conceming the first possibiiity, it is hard to believe that such a consistent error (50%)

could have k e n made. The possible density rneasurement error can also be d e d out fiom the

following reasoning: A 50% error increase in Cs needs a 33% underestimate of the adhesive

mass, which requires the aliiminrim density be measured about 1 1% larger than the true value

(refer to Table 3-4). The magnitude of S.D. for this measurement was only 0.43%.

Regarding the second possibility, Cs of the adhesive batch (batch LX-6945,4.13%)

used for the saw-cut samples was less than that of batch B-6404 (4.88%). If the latter value was

the true value of Cs for batch LX-6945, the error wouid be approxirnately 18%, whereas the

observed difference between the Cs of wafers and that of the sandwiches was about 50%-

The interfacial zone @ossibility three) has a small thickness (nominally 2-6 pm in the

alurninum-epoxy system of Safavi-Ardebili et al.'). Assuming that the extra water gained by the

samples of the present research was ai l accommodated by two interfacial zones, the 50% extra

water would be equivalent to a 18 pm thick water film on both sides of the joints, This must be

far greater than the capacity of the interfacial zone.

Blistering (possibility four) was observed only in sampie 8565 1 of the 8 samples that

were examined (Section 4.2)' and it absorbed more water than 85652 (Fig. 3- 1 1 ), so, blisterhg

alone did not account for the gain of extra water in generai.

The tiAh possibility seems to be most plausible for the higher Cs in the closed joints,

especially, when it is examined together with the results fiom the lmife-cut and uncut groups,

which demonstrated the same phenornenon, although to a lesser degree. ifthe adhesive had

Chmter 3 Water Difiüsion in Adhesive Joints

developed micro-cracks or other forms of permanent stnictural change, then a repeat absorption

test on the adhesive îtseif(i.e., a wafer) should yield the same higher Cs value seen with the

sandwiches. To check this possibility, saw-cut sandwich specimens (aged at LOO% RH. 6S°C and

85OC for 78 days) were etched using an NaOH solution approximately 20% concentration by

weight till the adherends were about O- 1 mm thick, and then the adherends were peeled off.

These aged adhesive wafers were then drïed and repeat absorption testing was carried out at

100% RH and 65OC or 8S°C, depending on the original sandwich coaditiom. M m o of the repeat

absorption specimens aged at 6S°C was measured at 7 and 45 days of aging, and that of the 85OC

aged specimens was measured at 1 1 and 45 days of aging to obtain the apparent Cs. The resuits

(averages of the two measurements) of the repeat absorption tests at 65OC and 8S°C were,

respectively, 6.1 % (S.D.=0.09%, N=2) and 7.5% (S.D.=2.1%, N=2); i.e., MJMo was higher than

Cs of fiesh, cast wafers aged for 4 weeks under the same condition (3.66% and 4.13%,

respectively, see Section 3.4. i for statistics) and higher that the overall apparent Cs of the

initially closed joints that were etched: 5.1% (2 specimens, S.D.=0.35%, N=8) at 6S°C, and 5.4%

(4 specimens, S.D.=1.3%, N=16) at 8S°C.

Two fiesh, cast Cybond 4523GB (batch B-6404) wafers were employed to M e r

explore the effect of long-term aging. MJMi was measured after 60 and 76 days of aging: at

6S°C it was 4.4% (1 specimen, S.D.4.4%, N=2), and at 8S°C it was 6.3% (1 specimen,

S.D.=0.34%, N=2), suggesting that long-term aging increases Cs for the adhesive. It is unknown

whether M a of the specimens aged for 76 days had reached the final Cs value.

It seems that because of residual thermal stress or the stress due to adhesive swelling

in closed joints aged at 85OC reached a higher M m o (approxirnately 5.4%) within two weeks

(Figs. 3- 10 and 3-1 1, the same phenornenon was observed with the knife-cut specimens aged at

Chapter 3 Water Dithsion in Adhesive Joints

8S°C, see Table 3-17), during which k s h cast wafers were s t i i i showing a relatively stable lower

Cs. These observations show that, dthough long-tem aging had certain effects on the higher

apparent Cs, the closed joint itself seemed to elevate Cs at an earlier stage of absorption.

The consistency of the saw-cut absorption data are illustrated in Figs 3- 1 8 and 3- 19

which compare the sandwich water uptake per unit length of bondline edges (width of the

specimen was ignored, i.e., bondlhe edge length was taken as twice the specimen length) of 8.5

mm and 6.5 width specimens at 8S°C and 65OC. It can be seen (ïig. 3- 1 8) that at 85°C the

amount of water absorbed per unit length of the sandwich bondiine was the same for the

specimens of dBerent width up to roughly the point 0.3 g/m of absorption, after which the

narrower specimens reached a plateau and the wider specimens continued to pick up water. This

phenornenon was expected as the diffiision process in the closed joints can be depicted by two

diffusion fionts, which are qualitatively defïned as the boundaries of the dry and wet parts of

adhesive, moving towards the center of the joint. Once the two fronts met, the specimen was

about to reach saturation. The same water absorption route of specimens of different widths

shown in Fig. 3-19 suggests that at a lower temperature (6S°C) it took longer for the specimens to

reach saturation, therefore, the absorption behavior of 8.5 mm and 6.5 mm wide specimens

wodd split off fiom each other later than at 8S°C. This is consistent with the data of Table 3-1 2,

showing that D is greater at 8S°C than at 6j°C.

It has been noted (Figs. 3- 10 to 3-1 3) that the apparent Cs of the saw-cut specimens

did not seem to change with the width of the specimens at a given temperature, with the ratio

between the apparent Cs of closed joint to the Cs of fiesh, cast wden king 1.3 1 at 85'C and

1.39 at 65OC. The effective Fickian diffusion coefficients (9) for the saw-cut group are

presented in Table 3-16 of the next subsection.

Fig. 3-18 Width effect on water absorption of sawcut Cybond 4S23GB specimens at

8S°C, 100% RH (21i31.12 mm)

Fig. 3-19 Width effect on water absorption of u w c u t Cybond 462368 rpecimens at

65OC, 100% RH (21p1.12 mm)

Chabter 3 Watet Dïfbion in Adhesive Joints

Uncut Specimens

To address the possibility that saw cutting damaged the edges of the adhesive joints,

thereby ailowing faster water absorption, the foliowing data were obtained with specimens

without cut edges.

Table 3-1 5 represents the water absorption results of 4 mcut specimens. Because

there was no aluminum control plate employed in this test to account for changes in aluminum

mass. the results are compiled in two ways: d e d u h g the possible aluminum surface mass gain

using the data of the control plates used with the saw-cut specimens (the aluminum plates used in

these two groups were the same), and without aluminum surface mass correction. Table 3-1 5

gives only 1 D predictions because the 1 D and 2D models yield virtually the same value for large

lengtldwidth ratio specimens.

The results show that the uncut specimens also absorbed water at a faster rate than the

Fickian prediction, and as with the saw-cut group, reached a higher Cs. Thus, the saw-cutting and

filing alone cannot explain the accelerated diffiision rate and higher Cs as compared with cast

wafers aged for a shorter tirne.

Figures 3-20 and 3-2 1 compare the measured water uptake with the 1D Fickian

prediction using diffusion propexties fiom cast wafers (see Tables 3-8 and 3-9) and assuming that

no interfacial diffusion or thermal stress effect exists- MJMs reported in both figures was with the

aluminum surface mass change correction. The ID prediction envelope shows that enors in D

alone could not account for the discrepancy between the measwed water uptake in the closed

joints and Fickian prediction. As expected, the 8S°C tests reached Cs earlier than at 6S°C (D was

higher at 85OC).


Table 3-15 Water absorption data for uncut Cybond 452308 spedmens at 65% and 8S0C

with 1 OOOh RH. M,/& and 1 D prediction were calculated using data from fresh, cast

wafers.

(85'C I M , / M ~ (surface mas change 1 1 1 1 1 I

1 O 1 0.38 1 0.65 1 1.18 1 1.14 132 1 1.23

85 1 total initial specimen mass (g)

1 00%

RH)

17.4435

corrected) MJMs (without sinface mass

852

change correction) M/Ms 1 D Fickian prediction

(85°C

100%

RH)

(65'C I M , / M ~ (surface mass change 1 O 1 0-54 1 0.80 1 0.93 1 1.04 1 1 1 1-37

17.45899

O

total initial specimen mass (g)

651

O

M/Ms (sinface m a s change corrected) M/Ms (without d a c e mass change correction) MJMs 1 D Fickian prediction

17.47082

0.40

17.45238

total initial specimen masç (g)

1 00%

0-17

O

O

O

RH)

(65°C [ML& (surface mass change I 1

1 O 1 0.43 1 0.62 1 0.89 1 1.02 1 1.34 1 1.41

17.491 63

0.7 1

17.46466

17.54047

corrected) MjMs (without sirrface m a s

652

1 1 029 1 0.46 1 0.59

0.3 1

0.33

0.17

change correction) M M s 1 D Fickian prediction

17.48998 17.4%83

1.24

17,47685

17.55672

O

total initial specimen mass (g)

100%

TabIe 3- 16 lists the effective d i h i o n coefficient (DE) and its ratio to the actual

diffusion coefficient o f cast wafers for both saw-cut and uncut specimens. Assiiming that the

water absorption in the closed joints obeyed Fick's law, & was calculated fining the M/MS-&

initial (up to 0.6 MJMs or higher if the data maintains a linear relationship between MJMs

and &) slopes into Fickian 1 D mode1 (Eqn. (3-5)). This assumes that & is a material constant

17.4936:

0-78

0.60

0.65

0.29

O

RH)

1 1.20 ( 1.38

0.90

17.49684

17.5679

OS7

17.4437 1

corrected) MJMs (without surface mass

1.29

1.13

1-19

0.47

0.12

change correction) M,/hfs 1 D Fickian predictioa

17.4952

17.57788

0.97

O

1 .O8

1.15

0.60

0.22 1

O

17.50349

17.58388

133

17-48 178 17.45794

0.46

1 t.SO264

1.3 1

1.37

0.79

0.35

17,46758

0-13

1.28

1.35

0.9 1

17.58946'

1.54

17.489 1

0.78

1

17.5961

0.44

022

1.74

1 ïSOîO5

1 -25

1.98

0.6 1

1 7.504

0.36

0.75

1.49

0.46

1.91 1.97

0.63 0.77

Chauter 3 Water Dïflbion in Adhesive Joints

regardless of the dimensions of specimens. In the present work, it was observed that f i was

relatively constant at a given temperature and was higher at 85OC than at 65OC.

O 500 1000 1500 2000 2500 3000 3500

mot time (sin)

Fig. 3-20 Water uptake of uncut Cybond 452508 specimens at 85'C. 100% RH. A Y 851,

B-# 852, C-average of # 851 and 852 10 prediction using data frorn fresh, cast

wafers, - - - - uncertainty envelope of I D prediction based on k3S.D. for D (S.D.=13@h)

The effective d i h i o n coefficients reported in Table 3-16 were calculated using

diffusion data measured f?om closed joints. ï h e measurement of & was not afTected by long-

terrn aging since the sandwiches reached approximately 0.6MJMs (the longest time used to

calculate DE), within 3 weeks at both 6S°C and 85°C.

The overall apparent Cs of uncut specimens was estimated to be 5.9% (1.2 1 times of

the bulk adhesive value, based on 2 specimem, S.D.=0.44%, N=8) at 8S°C and 4.5% (1.22 times

of the bulk adhesive value, based on 2 specimens, SD.=0.65%, N=6) at 6S°C assuming that

when MJMs shown in Table 3-15 was over 1, it had reached the plateau value.

Chapter 3 Water Diffirsioa in Adfiesive Joints

O 500 1000 1500 2000 2500 3000 3500

root time (sin)

Fig. 3-21 Water uptake of uncut Cybond 452SGB specimens at 65OC. 100% RH. H 651,

H 652, C-avenge of # 651 and 652 1 D prediction using data from fresh, cast

wafers, - - - - uncertainty envelope of 1 D prediction based on 13s.D. for D (S.D.=27%)

Table 3-1 6 Effective diffusion coefftcient (DE) of saw-cut and uncut Cybond 4523GB

specimens with bondline thickness 2h

k?*"P

saw-cut

2k1.12 mm

uncut 2 k 0 . 5 6 mm

temperature and fresh, cast wafer d i h i o n coefficient

Chapter 3 Water Dü3Mon in Adhesive Joints

Kn~ye-cut Specimens

Both the saw-cut and uncut specirnens showed that water diffused in closed joints at

an accelerated rate cornpared with fksh cast wafers and reached a higher apparent Cs. The knife-

cut specimens, in three bondiine thicknesses and three widths, were employed to m e r study the

water d i f i i o n patterns in closed joints.

The resuits of knife-cut specimens absorbing water at 8S°C and 65OC with 100% RH

are listed in Tables 3-17 and 3-1 8. As expected, al1 these specimens absorbed water at a faster

rate and, again, the water content of the adhesive reached a level higher than the Cs of the fksh,

cast adhesive wafers. The water uptake reported in these two tables was calculated deducting the

mass gain due to alurninum surface reactions using the control plate technique (see Fig. 3-8).

Table 3-17 Absorption data of k n b u t Cybond 452308 specimens at 85OC with 100%

RH. M/Ms and 1D prediction were calculated using data from fresh, cast wafers

weeks. (The bondline thickness is designated by the 3rd and 4th digits of the

specimen codes: 10-1 -1 2 mm, 054 .55 mm, 024 .24 mm; nominal width is

designated by 5th and 6th digits: 8 5 4 . 5 mm, 65-6.5 mm, W . S m m )

time (hours)

root time (si")

specimen 1 M&

1 prediction

Chapter 3 Water D=on in Adhesive Joints .

Table 3-1 7 continued

specïmen

8505851

2.07997

O

O

2,0553

mass (g)

M/Ms

M/bfslD prediction mas (g)

specimen

8505852

2.1 1092

0.50

021

2.0846

2.14863

1.09

0-40

2.1 1906

1 2,13803

0.93

033

2.1 1 O7 1

0.48 M/Ms lCf/MS ID prediction m a s (g)

1.04

specimen

0-90 O

O

133471

138

0.49

1.35587

1.25

0.48

2.54937

2.15707

123

0.48

2.13005

122

0.2 1

1.33942

0.48

1.35885

1 -20

0.60

1.35403

1.18

0.60

2.53286

2.15109

1-13

0.60

2,125 18

M m s

1 .O8

0.75

2,48324

1 .O6

0.76

1.640 18

1 .O5

0.75

1.60864

1 -04

0.76

1.05689

1.15

0.74

1-03958

1.12

0.74

1.13

O 1 0.56

0.33

1.35076

0.97

specimen

8510651

specimen

8510652

specimen

8505651

0-60

1.35687

1 22

0.67

1.35505

1.21

0.67

2.53508

2-1 5îSl

1.1 5

0.67

2. 12587

0.40

1.35587

1.17

0-33

1.34797

0.95

0-33

253324

M/MS

MMs ID prediction mass (g)

hf/Ms

M&lD prediction m a s (g)

M/Ms

bf/Ms 1D prediction

850285 1

specimen

8502852

1.1 1

0.82

2.48669

1.10

0-83

1.641 62

1 .O8

0.82

1.60785

1.14

0.40

I.35294

1.14

0.40

2.54416

' M m ID prediction mass (g)

M/Ms M& ID prediction m a s @ )

O

1.3227

O

O

2.43567

O

O

2,38975

O

O

1.5895

O

O

0.67

1.35765

1-1 1

0.73

1.3523

1-10

0.73

2.52054

214257

0.99

0-73

2.1 1 572

0.2 1

1.3362

0.52

031

2-49355

1-21

0.52

2,49334

1.18

0.53

1.64639

1.19

0.52

1.43

0.83

1.35862

135 ,

0.83

2.54893

215803

1.24

0.83

2.13084

0.94

0.87

2.4708

0.92

0.88

1 -63399

0.92

0.87

1.60319

0.98

1.27

0.62

2-50053

1.26

0.64

1.64904

1.24

0.63

1.61404

0.65

0.27

2.44646

0.65

0.28

1.63047

0.65

0.27

m a ~ ~ ( g )

1 -26

0.94

2.49657

1.21

0.95

1.64554

1 1.17

0.95

1.6139

1-23

0.73

1.35466

specimen

8505652

1.61695

1 .O9

0.43

2,48481

1 .O8

0.44

1 -641 08

1 .O8

0-43

1.58946 1.56012

1.15

0.95

1 .O6061

1.33

0.94

1 .O4288

1 -29

0.94

0.83

1.36285

1.609621

1 .O2

0.83

1 .OS724

1.16

0-8 1

1 .O3975

1.12

0.82

0.9 1

0.88

1 .OS473

1 .O3

0.86

1.03722

0.99

0.98

M/Ms

M/Ms ID prediction m a s (g)

O

O

1 .O3323

specimen

8502652

1-22

0.63

1 .OS92

1.37

0.64

0.27

1 .O4594

0.63

0.6 1

1 -04306

1.30

0.62

0.27

1.03002

0.66

0.27

prediction mass (g)

M/Ms M& 1D prediction

1 .O6

0.43

1.05584

1.1 1

1 .O 17 12

O

O

1-16

0.53

I.05776

1.20

0.43

1.03 872

1 .O8

0.43

0.5 1

1.04 197

1 -25

0.74


Table 3-17 continued

specimen

8510451

specimen

85 10452

specimen

850545 1

specimen

8505452

specimen

850245 1

- (g) ' 1.49096 1-5323 1 1 S 5 5 1 1,55654 1 .556 13 1.54436 1,54403 1.53833

M/M, O 0.75 1-17 1.19 1.18 0.97 0.96 0.85

M&lD O 0.40 0.63 0.76 0.84 0.94 0.97 0.99 prediction mas (g) 1-63654 1.68633 1.7064 1.70654 1-70525 ' 1.69459 1.6936 1.68347

Mm. O 0.83 1.16 1.16 t -14 0.96 0.94 0.94

M W s ID O 0.4 1 0.64 0.74 0.86 0.94 0.97 0.99 prediction m a s (g) 1.08458 1.1 1232 1.12332 1.12226 1.12339 1.1 1772 1.1 1852 1.1 1383

M/Ms O 0.86 1.19 1.16 1.19 1-01 1 .O3 0-89

M&ID O 0.40 0.63 0.73 0.85 0-94 0.97 0.99 prediction mass(g) 1.12413 1.15138 1.16373 1,16376 1.16462 1.15702 1.15749 1.15377

M&w O 0.81 1.16 1.16 1.19 0.96 0.97 0.86

M m s ID O 1 0.40 0.62 0.72 0.84 0.93 0.97 0.98 prediction 1 mas (g) 1 0.73909 0.75 178 0.75692 0.757 16

prediction m a s (g) 0.67979 0.69127 0.69555 0.69587

M/MS 1D O 0.42 0.65 0.7 1 prediction

For the specimens aged at 8S°C, 100% RH, after the water content reached (1.1 -

1 .2)Cs, it was seen in al1 cases that the water content feu back to the level of the fiesh bulk

adhesive (Table 3-1 7, Figs. 3-22 to 3-24) before nsing again. It was likely that this water content

drop was due to a lower RH (about 95%) resulting fiom a leak in the chamber. By the time of the

last data points (9583200s, 2660 hours), blisters were observed on 13 of 18 of the specimens (see

Section 4-2), therefore it seemed that the final water content increase was due to k water

accumulated inside the blisters. not b o n d water distributed over the adhesive.


Noting that the apparent Cs was not afFected by the specimen size at a given

temperature (Tables 3-1 7 and 3-1 8, Figs. 3-22 to 3-27) as with saw-cut specimens, the overall

apparent Cs of knife-eut specimens aged at 8S°C was estimated at approximately 5.5% (1.13

times of the value of fiesh cast wafers, based on 18 specimens, S.D.=0.42, N=84) by averaging

the plateau MAMs values which were not affected by possible leaking or b l i s t e~g ; and that of

the specimens aged at 6S°C to be 4.1% (1.10 times the fksh bulk adhesive value, based on1 8

specimem, S.D.= 0.29%, IV=64). It is interesting to observe that Me-cut joints aged at both

85°C and 65'C had an apparent Cs approximately 10% larger than the respective fiesh, cast wafer

Cs. The same pattern c m also be seen with saw-cut specimens (page 54) and uncut specimens

(page 58), although the ratios between the apparent Cs of closed joints and the Cs of k s h , cast

wafers were higher.

Table 318 Absorption data of knifeeut Cybond 452308 specimens 6S°C with 100% RH.

MjMs and 1 D prediction were calculated using data from fresh, cast wafers (The

bondline thickness b designated by the 3* and digits of specimen codes: 1û-

1.12 mm, 05-0.56 mm, 0 2 4 . 2 4 mm; nominal width is designated by E and 6*

digits: 85-8.5 mm, 6 M . S mm, 4 M . 5 m m )

1

root time (s IR) O

mass (g) 2.97487

Chapter 3 Water DiBision in Adbesive Joints

Table 3-18 continued

specimen

6505851

specimen

6505852

specimen

6502851

specimen

6502852

specimen

6510651

specimen

6510652

specimen

6505651

specimw

6505652

specimen

6502651

maSS (g)

Mms MMs ID prediction m a s (g)

M m s

M/Ms ID prediction mass (g)

M/Ms

M/Ms 1 D prediction mass(g)

ML&

MMs ID prediction mass (g)

M m s

M/MslD prediction mass (g)

M/Ms

M/bfslD prediction mas (g)

M m s

M/MS ID prediction m a s (g)

M/Ms

.IM/MS ID prediction m a s (g)

MMs

M/MS ID prediction

2.0923 1

O

O

2.05962

O

O

1.22988

O

O

1,19409

2.1 0403

0.19

0.16

2-07 164

0.20

0.16

1.23504

0-15

0.16

1.19933

2-1 1302

035

0.25

2.08207

0.40

025

1.24069

0.40

0.25

1.20319

O

O

2.28854

O

O

229247

O

O

1 S356 1

O

O

1.54044

O

O

0.32

025

2.32395

0.52

0-33

2.32789

0.52

0.33

1 S557

0.50

0.33

1.56087

0.51

0.33

0.16

0.16

2.30937

0.30

0.21

2.31295

0.29

0.21

1 -547 16

0.28

0.21

1.55246

0.29

0.21

2.1 171 3

0.44

030

2.08499

0.46

030

1.24 174

0.44

0.30

1.20475

0.40

0.31

2-33] 77

0.64

0.40

3,33434

0.61

0.40

1 S6O28

0.63

0.40

1.56402

0.59

0.40

0.93375

O

O

0.943 1 1

0.48

0.39

2.1 2614

0.62

0.36

2.09082

0.58

037

124353

0.53

0.36

1.20765

0.55

0.37

234094

0.78

0.48

2.34466

0.77

0.48

1 56539

0.77

0.49

1 S693 1

0.74

0.49

0.93847

0.21

0.20

0.94607

0.68

O47

2.13751

0.75

0.45

2.10 1 62

0.70

0.45

135429

0.86

0.45

1.21266

OS8

0-46

2.35673

0.97

0.60

2.35877

0.94

0.60

1.5727

0.87

0.60

1 S799

0.94

0.60

0.941 8

0-39

0.95208

0.87

0.58

2.14537

0.83

0-51

2.1 1 099

0.81

0.52

1.2585

0.87

0.51

1.22104

0.32

0.83

0.52

2.3668

1.08

0.67

2.36861

1.05

0.67

1 -57923

0.97

0.67

1.58547

1.00

0.67

l

0.95553

0.89

0.65

2.15645

1.02

0.56

2.1 1946

0.94

0.57

1 -26459

1.10

0.56

1.22546

0.98

0.57

2.37546

1.19

0.73

2,37806

1.17

0.73

1,58387

1.05

0.73

1.59243

1.16

0.73

0.95862

1.01

0.71

2.16295

1-12

0.66

2.123 14

0.99

0.68

1.26665

2-1 645

1.14

0.76

2- 1 27 1 6

1.07

0-77

1.26728

1.00

0.68

2.3766

1.20

0.83

2.37734

1.14

0.83

1.58483

1-05

1.02

0-77

2.37537

1.17

0-91

2.37643

1.12

0.91

1 -58524

1.05

0.9596

1.00

0.82

4

0.67

1.2232

0.9597

0.98

0.89

1.15

0-76

1.228

0.83

1 .S9296

1.14

0.83

0.91

1.59237

1.11

0.91

Cha~ter 3 Water D i i o n in Adhesive Joints

Table 3-1 8 continued

As an indicator of the water absorption rate of the closed joints, effective diffusion

coefficients (&) were again calculated assuming 1D Fickian diffusion fiom the edges. & was

calculated ushg Eqn. (3-9, the linear approximation, if there were M M e 0 . 6 data (e-g.

specimens 85 1 085 1,85 1 O85 1, etc., and al1 the 65'C aged specimens), or Eqn. (3-6), the

specimen

6502652

specimen

6510451

0.96349

0-93

spechen

6510452

specimen

6505451

specimen

6505452

specimen

6502151

specimen

6502452

mass (g)

' M . M ~

M/MS 1 D prediction mass (g)

0,94877

0.37

032

1.66245

0.96807

1.05 1.13

0.89

1.6857

1.08

0-99

1.65702

M M s

M m l D prediction rnass (g)

M/Ms

M M s 1D prediction ~~

mass (g)

M/MS

M/Ms lD prediction mass (g)

M/MS

M m s ID

0,94094

O

O

1.62779

0.96774~0.96819

1.08

0.87

0.59

1.1 1634

0.83

0.58

1.12188

0.77

0.57

0.65282

0.85

0.58

0.95075

0.50

0.39

1.66952

M m s

MfMs ID prediction mass (g)

0.94559

0.20

0.20

1.64964

0.81

1.69024

1.18

0.97

1.66084

0.65 1 0.71

0.87

0.58

1.64084

O

O

1.09362

O

O

1.10048

O

O

0.64265

O

O

1.68752

1.16

0.88

1.6589

O

O

1.59989

0.95431

0.75

0.47

1.6743 1 1 -69385

1.28

0-92

1.66367

1.08

0.99

1.12854

1.03

0-99

1.13547

1.02

0.99

0.66054

0.97

0.99

0.95854

0.80

0.58

1.68238

0-98

0-69

1.6447

0.43

0.31

1.10488

0-40

0.30

1.1 1185

0.40

0.29

0,64846

0.47

0.30 prediction mass (g)

MMs

MihfslD ~rediction

0.6532

0.99

0.99

1-10

0-82

1.653 17

1.17

0.98

1.13165

1.16

0.97

1.1365

1.08

0.96

0.661 S8

1.07

0.97

0-73

0.49

1.1 1281

0.69

0.48

1.1 1895

0.66

0.47

0.65164

0.74

0.48

0.63938

0.30

0.30

0.45

0-30

1 -62022

0.63529

O

O

0.65383

1.09

0.97

125

0.93

1.13347

0,64278

0.60

0.48

0.72

0.48

1.63442

1-17

0.89

1.12951

0-96

0.70

1.1 191 1

0.64795

0.87

0.81

1-09

0.83

1.12499

0.65 128

1.00

0.88

0.64372

0.68

0.58

0.93

0.68

1-12518

0.90

0.67

0,65357

0.92

0.68

0.65485

1.27

0.92

0.6451 1

0.82

0.69

1-06

0.81

1.13019

0.99

0.80

0.65695

1-02

0.81

1.15

0.87

1,13383

1.05

0.86

0.66071

1.19

0.87

1.27

0.92

1.13854

1.19

0-91

0.66225

1.25

0.92

C b t e r 3 Water D i i o n in Adhesive Joints

exponential approximation, if MJMsc0.6 data were not available (e.g. specimens 85 1045 1,

85 1045 1, etc.). Again, the Ms h m the sandwiches was used to calculate the & values.

It can be observed fiom Tables 3-19 and 3-20 that a higher temperature yielded a

higher DE, consistent with the results of the saw-cut and uncut specimens.

In ref. [4], Comyn discussed Althof s data which indicated that the diffusion

coefficients measured fiom sandwiches were higher than those measured corn cast films.

Cracking and corrosion at sandwich edges were cited as the factors which had induced the

difference between the diffusion coefficients. in the present work, the difference between

predicted and measured water uptake was evident fiom the beginning of absorption, which

cannot be explained using the concept of opening of sandwich edges. Furthemore, mass changes

due to corrosion of the adherends have been accounted for using alurninum control sheet. Thus,

another mechanism should be investigated to explain the differences between the d i h i o n

coefficients measured with sandwiches and wafers in the present work.

Table 3-1 9 DE of knife-cut specimens with bondline thickness 2h and specimen width 2w

(length was 119-163 mm) aged at 8S°C, 100% RH. Bold face numbers are avenges

of DEJD for two replicates (fresh, cast wafer 0=12.3~10-'~ m21s)


Table 3-20 DE of knifeeut specimens with bondline thickness 2h and specimen width 2w

(length w u 14-163 mm) aged at 6S°C with 400% RH. Bold face numbem are

average. of &IO for two replicates (fresh, ast mfer M.9~10-13 m2/s)

Iudging fiom Table 3-1 9, it seems that & may be a fhction of 2w for specimens

with 2h fiom 0.24 mm to 1.12 mm. A t-test showed that the means of & for the specirnens aged

at 85OC with 2w=8.5 mm, and 2k1.12 mm and the those aged at 85OC with 2 ~ 4 . 5 mm, and

2h= 1.12 mm (the 1" and 3rd rows in the ln column of Table 3- 19) are the sarne, should be

rejected at 90% confidence level. A test in the the 3rd column between the ln and 3rd rows

suggested the nul1 hypothesis should also be rejected at 90% confidence level. However, these

cases are among the largest differences of ail the possible cornparisons with respect to different

widths and bondline thicknesses in these two tables, and the sample sizes in the tests were so few

that the t-test results should not be conclusive. On the hand, a t-test in the znd c01um.n between

row 2 and row 3 showed that the nul1 hypothesis c m be rejected at ody 50% confidence level.

Although no systematic error was identified to be responsible for it, there is no theoretical

explmation for this width effect. Table 3-20, in fact, shows that at 6S°C, 9 was independent of

specimen dimensions. Given it is a constant at a specific temperature, we cm hnd 9 to be

5 4 . 4 ~ 1 0-13 m2/s (average of al1 specimens in Table 3-1 9, S.D.= 1 0 . 5 ~ l0-l3 m2/s, N=18) at 8S°C

and 12.8~ 1 o - ' ~ rn% (average of al1 specimens in Table 3-20, SD.= 2 . 2 ~ 1 o - ' ~ m2/s, N48) at

65OC.

Again, because aii the specimens at both 6S°C and 8S°C reached 0 . 6 M M within 4

weeks, it should be noted that effect of long-tem aging was not likely to be involved in the 9

measurement of knife-cut specimens.

The ID Fickian prediction of M M s does not change witti the bondIine thickness, and

the recorded data show the same result (Tables 3-1 7 and 3-1 8, Figs. 3-22 to 3-27), although the

knife-cut specimens absorbed water at a higher rate than cast wafers.

If there was interfacial diffusion during the absorption, and if interfaciai diffusion and

d i f i i o n fiom the edges are independent of each other, interfacial diffusion wodd be the same

regardless of the bondline thickness. Therefore, it may be expected that &/D would be larger for

thinner bondlines, since interface d i h i o n would represent a larger hc t ion of the water uptake.

However, as shown in Figs. 3-22 to 3-27 and Tables 3-19 and 3-20, at a given width, the

specimens with different bondline thickness had roughly the same &/D for the specimens aged

at both 85OC and 65OC. This phenornenon may suggest that there was no interfacial diffusion and

the sandwiching increased the diffiision coefficient of the bullc adhesive, or, on the other hand,

that interfacial diffusion was not a separate process (see Section 3.4.4); water absorbed fiom the

interfacial zone might diaise into the bondline in the direction perpendicular to the interface

diffusion, changing water concentrations of both the interfacial zone and the rest of adhesive.

It is of interest to compare the measured tirne for the adhesive in the sandwiches to

reach saturation with two extreme scenarios: first, water penetrates the adhesive solely via the

edges by Fickian d i e i o n ; second, the adhesive layer is exposed on al1 faces to the moist


environment (wafer diffusion). If interfacial diffusion has a significantly higher effective D than

that of the bulk adhesive, the interfacial d i fkion Font will move weil ahead of the edge

diffusion front and, could perhaps provide suEcient water so that the adhesive absorbed water as

a wafer. Table 3-21 shows the t h e to mach 95% of fiesh, cast wafer saturation for both edge

diffusion and wafer diffusion, It can be seen that the measured saturation tirne for the Me-cut

specimens fell behveen the two extreme cases, although closer to the wafer diffusion model.

Table 3-21 Tirne to mach 95% of fresh, cast wafer saturation at i00% RH for adhesive

layers absorbing water via edges and as a totally exposed wafer

Figures 3-22 to 3-21 show that at W°C, 100% RH the measured MJMs was well

above the upper limit of the 3S.D. envelope, confimiing the conclusion drawn fiom saw-cut and

uncut specimens that the discrepancy between the recorded data and the Fickian prediction using

fiesh, cast wafer data was not due to errors in the difhsion coefficient of the bulk adhesive. In

Figs. 3-25 to 3-27 (6S°C, 100% RH), the 3S.D. upper limits meet the measured data curve, but

the general trend is still above the envelope.

measured time

agedat 6S°C

1600

1700

1650

1100

(mm)

8.5

6.5

(mm) 1.12

0.56

0.24

1.12

measured time

aged at 8S°C

360

380

360

0.56

O -24

1.12

calculated tirne for calculated tirne for

edge

5650

5670

5650

3380

edge

8000

8750

8650

wafer

91

24

4.5

89.5

3350

wafer

138

35.5

6.7

134 290

35.5

6.6

136

4820

23.5 ' 295 1350

1600

720

4820

5 160

2380

3350

1570

4.4

85.5

300

230

Chaptcr 3 Wata Diffiision in Adbesive Joints

Fig.

-2h=l*l2 mn -2h=l.l2 mn + 2h=0.56 mn + 2H.56 nxn * 2W.24 mn - 2h=0.24 nxn

-- - - - - -

3-22 Bondline thickness effect on Ad& of knife-cut Cybond 452368 specimens at

8S°C, 100% RH ( 2 ~ 8 . 5 mm),- 1 D prediction using data from fresh, cast wafers,

- - - - uncertainty envelope of tD prediction based on 3S.D. for D (S.D.=13*k)

Fig. 3-23 Bondline thickness effect on Mms of k n i f ~ u t Cybond 4523GB specimens rt

85OC, 100°h RH (2-6.5 mm),- 10 prediction using data from fresh, cast waferr,

uncertainty envelope of I D prediction based on k3S.D. for D (S.D.=13%)

Chapter 3 Water DZfkion in Adhesive Joints

O 1 O00 2000 3000 4000

root time (P)

Fig. 3-24 Bondline thickness effect on MJMS of knife-cut Cybond 4523GB specimens at

8S°C, 100% RH ( 2 ~ 4 . 5 mm),- 1D prediction using data from fresh, cast wafers,

- - - - uncertsinty envelope of 1 D prediction based on k3S.D. for D (S.D.=13%)

O 1000 2000 3000 4000

root time (P)

Fig. 3-25 Bondline thickness effect on Mms of knife-cut Cybond 4523GB specimens at

6S°C, 100% RH ( 2 ~ 8 . 5 mm),- 1 D prediction using data from fresh, cast wafers,

- - - - uncertainty envelope of 1 D pmdiction based on k3S.D. for D (S.D.=27%)


O 1000 2000 3000 4000

root time (sIR)

Fig. 3-26 Bondline thickness effect on Mms of knifeuut Cybond 452308 specimens at

6S°C, 100% RH ( 2 ~ 6 . 5 mm),- I D prediction using data from fmsh, cast wafers,

- - - - uncertainty envelope of I D prediction based on 23S.D. for D (S.D.=2T0h)

Fig. 3-27 Bondline thickness effect on A#& of knife-cut Cybond 4523GB specimens at

6S°C, 100% RH ( 2 ~ d . 5 mm),- 1 D prediction using data from fres h, cast wafers,

- - - - uncertainty envelope of 1D prediction based on k3S.D. for O (S.D.=ZI%)

Chapter 3 Water DBùsion in Adhesive Joints

Figures 3-28 to 3-33 show the width effect on M m . The 3S.D. envelope sbown in

the figures are based on the average dimensions of the nanuwest specimens. It also c m be seen

that the measured M/Ms was above the upper limits of the 3S.D. envelopes.

The independence of & with respect to specimen dimensions teporteci in Tables 3-19

and 3-20 can be observed by examining the initial dopes of the specimens shown in Figs. 3-22 to

3-3 3. Before equilibriurn, d l of the absorption data for specimens with the same width

approximately lie on the same curve (Figs. 3-22 to 3-27), which suggests that samples of the

same width but dBerent bondline thickness have the same & in the same absorption

environment. For the specimens with different width (Figs. 3-28 to 3-33), the combination of a

lower MJMs and a Iarger width aiso yieided a constant &, as expected with Eqn. (3-9, Le.,

MJMs is inversely proportional to the width of the specimens untill approximately 0.6MJMs.

Fig. 3-28 Width effect on Mms of knifacut Cybond 452308 specimens at 8S°C, 100% RH

(2h4.12 mm), - 1 D prediction of the two narrowest specimens usina data from

fresh, cast wafers, - - - - uncertainty envelope of 1 O prediction based on k3S.D. for D

(S.D.=13%)

Chapter 3 Water Diatsion in Adhesive Joints

O 1000 2000 3000 4000

root time (SI")

Fig. 3-29 Width effect on Md& of knife-cut Cybond 452308 specimens at 8S°C, 100% RH

(2h-0.56 mm),-1D prediction of the two narrowest specimens using data from

fresh, cast wafers, - - - unceitrinty envelope of 1 D prediction based on k3S.D. for O

O 1000 2000 3000 4000

root time ( s I R )

Fig. 3-30 Width effect on Mms of knife-cut Cybond 452308 specimens at 8S°C, 100% RH

(2h0.24 mm),-1 D prediction of the two nanowest specimens using data frorn

fresh, cast wafers, - - - uncertainty envelope of 1D prediction based on k3S.D. for D

Chapta 3 Wara Diflbïon in Adhesive Joints

O 1000 2000 3000 4000

root time (sr")

Fig. 3-31 Width affect on M m of knife-cut Cybond 452368 specimens at 6S°C, 100%

RH (2rnl.12 m m ) , 10 prediction of the two narrowest specimens using data

from fresh, cast wafers, - - - uncertainty envelope of I D prediction based on k3S.D.

for 0 (S.D.=27%)

O 1000 2000 3000 4000

rwt time (s'I2)

8s Fig. 3--idth eflect on Mms of knifscut Cybond 452308 specimens at 6S°C, 100% RH

(2k0.56 mm), -1 D prediction of the two narrowest specimens using data from

fresh, cast wafers, - - - uncertainty envelope of I D pmdiction based on k3S.D. for D

(S.D.=27%)

Chapter 3 Water Dinusion in Adhesive Joints

O 1000 2000 3000 4000

root time (s '")

Fig. 3-33 Width effect on M& of knifmut Cybond 452308 specimens at 65OC. 100% RH

(2h0.24 mm), -1 D prediction of the two nanowest specimens using data from

fresh, cast wafers, - - - uncertainty envelope of 10 prediction based on 23S.D. for D

(S.D.=27%)

root time

Fig. 3-34 Bondline thickness effect on M/ZL of knife-cut Cybond 452368 specimens at

85'C with 100% RH (2-8.5 mm)

Chapter 3 Wata Diffiision in Adhesive Joints

root time (da)

Fig. 3-35 Bondline thickness effed on M/2L of knife-cut Cybond 452308 specimens at

8S°C with 100% RH (2ud.5 mm)

O 500 1000 1500 2000 2500 3000 3500

root time (SI")

Fig. 3-36 Bondline thickness effect on M/2L of knifwut Cybond 452368 specimens at

8S°C with 100% RH ( 2 ~ 4 . 5 mm)

Chapta 3 Water DiûÙsion in Adhesive Joints

O 500 1000 1500 2000 2500 3000 3500 4000

rwt time (sIR)

Fig. 3-37 Bondline thickness effect on Md21 of knife-cut Cybond 452368 specimens at

6S°C with 100% RH (2-8.5 mm)

O 500 1000 1500 2000 2500 3000 3500 4000

mot time (slR)

Fig. 3-38 Bondline thickness effect on M42L of knifwut Cybond 462368 specimens at

6S°C with 100% RH (2-6.5 mm)


Fig. 3-39 Bondline thickness effect on kli(2L of knife-cut Cybond 4523GB specimens at

65OC with 100% RH ( 2 ~ 4 . 5 mm)

Figures 3-34 to 3-39 show M m at a given specimen width and Figs. 3-40 to 3-45

show M a L at a given bondline thickness, where 2L is the total length of the bondline edges,

ignoring the width of the specimens. These graphs show the consistency of the data fiom knife-

cut specimens. As expected, Figs. 3-34 to 3-39 show that the water uptake of specimens was

proportional to bondhe thickness. In Figs. 3-40 to 3-45, it has been shown that at the beginning

of absorption, specimens with different widths absorbed approximately the same amount of water

at a given t h e . After a cenain tirne, the narrowest specimen split off the common curve, and

later, the second narrowest specimen (Fig. 3-43 presents the clearest picture). It would be

reasonable to assume that the two "split-ofl" points comesponded to the tunes when the diffusion

fronts fiom both edges of the two narrower specimens met at the center.

Chaptcr 3 Watcr Difiùsion m Adhesive Joints

F ig. 3-40 Width effect on Md2L of knifeuut Cybond 452308 specimens at 8S°C, 100% RH

(2k1.12 mm)

raot Ume (s lR)

Fig. 3 4 1 Width effect on M/?L of knife-cut Cybond 452308 specimens at 8S°C, 100% RH

(2h=0.56 mm)

Chapter 3 Water Difiùsion in Adhesive Joints

mot time (dR)

Fig. 3 4 2 Width effect on M/u of knifa-cut Cybond 452308 specimens at 85OC, 100% RH

(2h=0.24 mm)

O 500 1000 1500 2000 2500 3000 3500 4000

mot time

Fig. 3-43 Width effect on M/2L of knifacut Cybond 452308 specimens at 6S°C, 100% RH

(2h=1.12 mm)

Chabter 3 Water DiBision in Adhesive Joints

O 500 1000 1500 2000 2500 3000 3500 4000

root time (SI")

Fig. 3-44 Width effect on M/2L of knifecut Cybond 452368 specimens at 6S°C, f00% RH

(2M0.56 mm)

O 500 1000 1500 2000 2500 3000 3500 4000

root time (SI")

Fig. 345 Specirnen width effect on M/ZL of knife-cut Cybond 4S23GB specimens at 6S°C,

100% RH (2 î~0 .24 mm)


3.4.4 Models of Water D~jjfusion in Ciosed Joints

Interfacial D~fjfusion Hypothesis

The present experimental observations and previous workg have led to a hypothesis

that an interfacial zone, fonned during bonding and cwing, may have possessed a higher

d i f i i o n coefficient for water. In this model, water enters a joint via both the interfacial zone and

the central bulk adhesive. The water brought into the joint via the interfaciai zone is not retained

at the interface, but difbses to the central dry part of the sandwich in 2D diffusion.

In the 2D interfacial diffusion model (Fig. 3-46), both the edge diaision and

interfacial diffusion are govemed by Fick's law, but diaision coefficient in the interfaces would

be about I to 2 orders of magnitude greater than that of the bulk adhesive. The diffusion fiom the

interface would also obey Fick's law, with a D equal to the bulk value. The process is

complicated by the variations in time and location of the amount of water at the interface.

interfacial d i rnion adherend

edge

diaision

- edge

< interfacial zone adherend

Fig. 346 Hypothesis of interfacial diffusion in a closed joint


In a similar situation, ~ishe?' proposed a mode1 to describe the seIf-diffiision of silver

dong grain boundaries and through the interiors of crystals. Fisher used the analogy of heat

transfer dong a copper foi1 imbedded in cork,

Foilowing Fisher's modeling, the water concentration C in the interfkcial zone varies

according to the equation:

where Dr is the diffusion coefficient in the interfacial zone; ais the thickness of the interfacial

zone; and x and y are parallel and perpendicular to the interface, respectively.

The concentration in the buik adhesive obeys Eqn. (3-7):

The combined Equations (3-1 8) and (3-7) can be solved numerically provided that the

ratio of D'/D and 6 are known. This was beyond the scope of the present work.

Figures 3-34 to 3-39 reveal the relationship between bondline thickness and water

uptalce rate expresxd as MJ(2L -& ). If interfacial diffusion existed in the closed joints, M,

should have two components: a contribution fiom interfacial diffision and a contribution fiom

edge diffiision (bulk diffiision). If the initial slope of each curve in Figs. 3-34 to 3-39 is plotted

versus 2h, the interfacial contribution should be reflected in a nonzero y intercept. Figures 3-47

and 3-48 show that M / ( ~ L . & ) (average of two replicates at each case up to 0.6MJMs) was

proportional to the bondline thickness, but the interfaciai component is not evident, Le., each line

has approximately a zero intercept.


This resuit may indicate that there was, in fact, no interfacial diffusion, but, on the

other hand, it may also suggest that the front of interfacial diffusion was not far ahead of that due

to bulk d i h i o n (edge diffusion and diffiision fiom the interface). This may support the

following interfacial d i h i o n process: interfacial d i h i o n provided sunicient water to make the

bondline 2D diffiision, but the interfacial diffusion development was in part detennined by water

availability. If the bondline adhesive absorbed water fiom the interface, then the interfacial

diffusion couid fidly develop only when the bondline adhesive had reached its equilibrium water

concentration. This process would result in the interfacial diffusion fiont movùlg almost at the

same rate as (or slightly leading) the edge diffiision fiont. This explanation seems to be an

answer to the zero y intercept in Figs. 3-47 and 3-48 while maintaining the interfacial diffiision

hypothesis, but this process should have produced different & m n g the specimens with

different bondline thicknesses, which was not observed in the experiments. This contradiction

suggests that interfacial diffuçion may not exist in the closed joints.

Another piece of evidence that is contrary to interfacial diffusion is that the time for

the closed joints to reach equilibrium was approximately the same regardless of the bondline

thickness at a given specimen width (Tables 3-17 and 3-18). Ifthe bulk adhesive had absorbed

water fiom the interface and interfacial diffusion was not a fùnction of bondline thickness, then

the thinner specimens should have reached equilibrium faster (i.e., a higher &). The nearly

constant time to equilibrium and the constant & with respect to bondine tfiickness suggest that

the closed joints absorbed water solely îÎom edge diffusion in a 1D Fickian way, alôeit at a faster

rate than predicted using D fkom cast wafiers.


Fig. 347 Relationship between bondline thickness and water uptake rate for knife-cut

specimens (8S°C)

O 0.2 0.4 0.6 0.8 1 1.2

2h (mm)

Fig. 3-48 Relationship between bondline thickness and water uptake rate for knife-cut

specimens (6S°C)

Cbabter 3 Water D W i o n in Adhesive Joints

Io addition, Figs. 3-47 and 3-48 appear to show thaf for the wune bondline thickness,

the larger the specimen width, the p a t e r the water uptake rate of the specùnen. Although this

observation appears to be consistent with the values of & shown in Tables 3-19 and 3-20, it is

seen that the differences in & are not significant (see page 67 for details).

Residual Stress ffypothesis

The anaiysis in Section 3.4.3 has s h o w that, although an interfacial cWùsion mode1

can reasonably interpret some of the phenomena observed in water dinùsion in closed joints, it is

inconsistent with the bondline-thickness-independence revealed in the foilowing ways: the

relationship between the & and 2h, the zero interfacial d i h i o n component shown in

MJ(2L - 4 ) - 2 h plots, and the nearly constant time to equilibrïum for specimens with varied 2h.

It has been noted that water absorption is sensitive to stress in many adhesives4. it is

also known that the difference in thermal expansion rates of adhesives and adherends induces

residud stress in joints during curing and cooling. An adhesive under tende stress may have a

higher diffusion coefficient and equïiibrium water concentration4. The investigation of water

absorption behavior of adhesive Cybond 4523GB under stress was, however, beyond the scope of

the present work.

As shown in ref. [37], the thermal stress ain the adhesive layer of a sandwich subject

to temperature difference AT is as follows:

Chapter 3 Wata DiBision m Adhcsïve Joints

where E is Young's moddus in MPa, vis Poisson ratio, a is the hear thermal expansion rate in

IPC, hA is the halftbickness of the adhesive layer and hu is the thichiess of adherend in m. It

should be noted that stress in the plane parailel to the plate surface is equal in b t h x and y

directions, and that the stress perpendicuiar to the plate surface is zero. Obviously, a i s

independent of the length and width of a joint.

Table 3-22 ptesents residual t h e d stresses estimated using Eqn. (3- 19) for

specimens aged at 85OC and 6S°C. Although mis a h c t i o n of hh it can be seen nom Table 3-22

that hA has essentiaily no effect on the residual thermal stress with the &ta adopted in the present

reseach so that it did not have a significant effect on the diffiision coefficient of the adhesive

(Tables 3-19 and 3-20). It is emphasized that the swelling of the adhesive are ignored, thereby

overestimating the tende residual stress.

Table 3-22 Estimated residual thermal stress in adhesive Iayer of joints (cured at lSO°C,

aged at 6S°C and 8S°C) with various bondline thickness. The mechanial and

thermal properties of adherend are those of Ml11 0010 (Efl7O GPa, ~ 0 . 3 ,

a p 2 . 2 ~ 1 0 ? ~ ) and the data of the adhesive (EAr7 MPa (the value of rubber

are best estimates on epoxies at the aging temperaturesu". hd.l mm

1 aghg temperature (OC) 1 65 1 85 I I

AT (OC) 85 65

Chapcer 3 Wata DiBision in Adhesive Joints

Residual themial stress seems to reasonably explain the water absorption phenornena

in closed joints observed in the present work. Although the 85OC aged spechens should have

lower residual stress thui 6S°C aged specimens, the experiments have shown that the 8S°C aged

specimens had a higher &; however, this rnay be simply due to a temperature effect.

As estimated in Section 4.4.2, the linear expansion rate of Cybond 4523GB due to

swelling is approximately 2% at 85OC with a saturated water content. This is far larger than the

thermai expansion and so the net effect of swelling is likeiy to produce compressive stress in the

joints. The effect of sweiiing on water absorption in closed joints is not clear at this point. It

should be noted that the analysis of Section 4.4.2 may have considerably overestimated the

swelling by ignoring voids inside the adhesive. A~so, the swelling of the adhesive will depend on

the distribution of water in the joints; before saturation, the adhesive would sweii more on the

edges than at the center of the joints, and at the beginning of absorption, the effect of sweliing

wodd be less signincant compared to the effect of residual thermal stress.

Another factor that might have wntributed to the higher & and Cs of closed joints

was that curing may have produced anisotropy in the adhesive Iayers; i.e., the presence of the

adherends created a lower D for diffusion perpendicular to the interface. But the measurements

of Safavi-Ardebili et al9. shows that the thickness of an interfacial zone which might have

different diffusion properties than the bdk adhesive was only 2-6 pm, suggesting that anisotropy

would be negligible in the present bondline thickness range (0.24-1.12 mm).

Fickian Characterizrrtion of W~ict D t ~ u s w n in Closed Joints

It has been shown that Cybond 4523GB closed joints had a higher Cs and D than cast

adhesive wafers. However, & and the apparent Cs of the sandwiches seemed to be independent

Chapter 3 Water m o n in Adhesive Joints

of dimensions of the specimens. & values for sawcuf uncut, and knife-cut specimens have ken

gathered in Tables 3-1 6,3- 1 9, and 3-20; apparent Cs values of these spechens at 100Y0 RH are

compiled in Table 3-23. The onset of the absorption plateau was identified when M&Ms excceded

the fiesh, cast wafer Cs. It seems that Cs was elevated by two factors: the dwation of aging and

the formation of closed joints. Comparing the fint and the third rows of Table 3-23, it is

observed that the longer aging time yielded a higher Cs. Examining the second and the fifth rows,

the same trend cm k seen. However, these observations are not conciusive because the tests in

rows two and three had ody one specimens. Assuming the populations involved are norrnally

distributed, a t-test (the standard deviations are unknown and unequal) is employed to check the

mean ciifference between row one and row six at 65°C and 85°C' and between row one and row

seven (they were fiom the same batch of adhesive, and each group had minimum 3 specimens) at

65OC and 8S°C. The t-test shows that the nul1 hypotheses (that the means of wafers and closed

joints in each respective cornparison are the same) can be rejected at least 90%, 98%, 95%, 99%

levels of confidence. Again, the sample sizes of cast wafers and uncut specimens in these tests

may be too few to be conclusive. Given that Cs of closed joints was higher than the value of

fiesh, cast wafers, based on the data of the present research, it is difncult to infer whether the

Uicrease was due to time effect or formation of closed joints.

The & values were less aEected by aging time since the data used to calculate DE lay

in the early stages of aging (up to 0.6 MJMs), which corresponded to approximately 3 weeks for

most closed joints.

Figures 3-49 to 3-58 show that, regardiess of the! specimens dimensions, using & and

apparent Cs measured fiom closed joints, water dithision behavior in closed joints at both 65OC

and 8S°C can be fairly accurately characterized by Fick's law. The apparent Cs is shown in Table

Chapter 3 Water DiBision in Adheske Joints

3-23, and & was the average fiom Tables 3- l6,3-l9, and 3-20 for each respective group and

temperature. These predictioas were calculated using Eqn. (3-6), the expmentid approximation

of fiactional water uptake in ID Fickian diffiision. AU the m u t and uncut specirnens are

shown in Figs. 3 4 9 to 3-54. The Me-cut specimens shown in Figs. 3-55 to 3-58 were chosen to

represent the largest specimen size ( 2 ~ 8 . 5 mm, 2k1.12 mm) and the smallest specimen size

(2r-4.5 mm, 2k0.24 mm). No size effects c m be observed in these graphs.

Table 3-23 Cg values of CybOnd 452308 aged at 65% and 8S°C, 100% RH with diHemnt

situations. N is number of specimens. S.D. was calculated using the number of

data points collected from each specimen in 1 specimen tests or using the number

of specimens in multi-specimen tests

specimen 1 durationof 1 65°C I 85°C

and its d i f i son pah

fiesh, cast &ers (uncut and knife-cut batch)? 2 M . 8 8 mm fiesh, cast wafers (saw-cut batch) 2 M . 8 8 mm long-term aged cast wafers (uncut and knife-cut batch) 2 H . 8 8 mm repeat absorption of wafers fkom saw-cut sandwiches 2k1.12 mm saw-cut sandwiches 2 ~ 4 . 5 and 8.5 mm uncut sandwiches 2 ~ 7 . 5 mm knife-cut

agini3

I l &lys

27 days

76 days

45 days after 78 days of a@% as sandwiches 78 days

1 15 days

141 days at sandwiches 2u4.S, 6.5, and 8.5 mm

63d, 77* &y

omet of plateau

at 3"day

3"&y

not recorded

not recorded

37"', 66* &y 63" day

49", 6SoC and 153 days at 85°C

1 sLh, 22"6 &y

onset of plateau

at 3"day

2dday

not recorded

not recorded

Cs (%) Cs (%)

4.88

4.13

6.3

7.5

S.D. (%)

S-D. (96)

0.02

0.23

0-34

2.1

N

13*, 25& &y 34& day

1 5 ~ ,

N

3

L

1

1

1.3

0.44

0-42

3

1

1

1

4

2

18

3.69

3.66

4.4

6.1

5.1

4.5

4.1

5.4

5.9

5.5

4

2

18

0.24

0.05

0.40

0.09

0.35

0.65

0.29

Chaprer 3 Water Dïfhsïon in Adhesive Joints

Fig. 3-49 Recorded water absorption of saw-cut specimens 8581 and 8582 (2ht1.12 mm,

2-8.5 mm, aged at 8G°C, 100°h RH) and Fiekian prediction using DE and CS

measured from al1 elosed joints in the saw-cut group ( ~ ~ 6 1 . 0 ~ 1 0 - ' ~ rn21s, C&.4%)

4 prediction

Fig. 3-50 Recorded water absorption of saw-cut specimens 8561 and 8562 (2k1.12 mm,

2 ~ 6 . 5 mm, aged at 8S°C, 100% RH) and Fickian prediction using & and CS

measund from al1 closed joints in the sawcut group ( & = 6 l . 0 x w 3 m21s, Cp5.4%)

Chapter 3 Water DiBision in Adbesive Joints

+# 65851 * # 65852 + prediciton

-

O 500 1000 1500 2000 2500 3000

mot time


2-8.5 mm, aged at 6b°C. 100% RH) and Fickian prediction using 4 and CS

measund from al1 closed joints in the sawcut group (D~=IS.I~IO"' m21s, Cs=S.l%)

O 500 1000 1500 2000 2500 3000

mot time ( s r R )

-+- prediction


2-6.5 mm, aged at 6S°C, 100% RH) and Fickian prediction using 4 and CS

measund from closed joints in sawtut group (Dp15.1 XIO"' m21s. Cs=S.l%)


+# 851 * # 852 + prediction

1000 2000 3000

mot time (P)

Fig. 3-53 Recorded water absorption of uncut specimens 851 and 852 (2hd.56 mm,

2 ~ 7 . 5 mm, aged at 8S°C, 100% RH) and Fickian prediction using OE and Cs

measured from closed joints in uncut group (D&~.OXIO"~ m2/s, cs=s.9%)

O 1000 2000 3000 4000

rwt time ( s lR )

Fig. 3-54 Recorded water absorption of uncut specimens 651 and 652 (2k0.56 mm,

2 ~ 7 . 5 mm, aged at 65OC. 100% RH) and Fickian prediction using DE and Ca

measured from al1 closed joints in uncut group (&=30.6~10"~ m21s, Cs=4.5%)

Ch2I~ter 3 Water Diffusion in Adhesive Joints

O 1 O00 2000 3000 4000

root t h e

Fig. 3-55 Recorded water absorption of knifeuut specimens 8510851 and 8510852

(2m1.12 mm, 2 ~ 8 . 5 mm, aged at 8S°C, 100% RH) and Fickian prediction using 4

and Cs measured from al1 closed joints in knife-cut group (D&~AX~O-" m21s.

- ff 850245 1 + ff 8502452 + prediction

Fig. 3-56 Recorded water absorption of knife-cut specimens 8502451 and 8602462

(21r0.24 mm, 2w4.5 mm, aged at 8S°C, 100% RH) and Fickian prediction using 4

and CS measured from al1 cfosed joints in knife-cut group(~+~4.4x1 O"' m21s,

Cs=5.S%)

Chanter 3 Water D i o n in Adhesive Joints


(2hr1.12 mm. 2wr8.5 mm, aged at 6b°C, 100% RH) and Fickian prediction using &

and Cs measured from aII closed joints in knïfe-cut group (&=12.8x10*~~ m'ls,

Cs=4.1 %)

- -- # 650245 1

++ # 6502452 + prediction


(2hr0.24 mm, 2w4.S mm, aged at 6S°C, 100% RH) and Fickirn prediction using 4

and Cs measured from al1 clowd joints in knifceut group ( ~ ~ f 1 2 . 8 ~ 1 0 " ~ m21s,

cs=4.1 %)

Chapter 3 Wata Diffusion in Adhesive Joints

3.5 Conclusions

Based on the present experimental study and anaiyses, the following conclusions

regarding water difhision in closed Cybond 4523GB joints can be drawn:

1. & and Cs of closed joints are higher than those of cast wafers, and are

independent of the dimensions of the joints.

2. Water absorption behavior of closed joints can be characterized by Fickian law

with the diffusion properties rneasured f b m closed joints.

3. Interfacial diffiision was not observed in aluminum-Cybond 623GB joints.

Chaptcr 4 Blistcring in Adhesivc Joints


4.1 Introduction

Blistering has been observed at the interface between the adhesive and the a l d u m

s u b ~ t r a t e ' ~ - ~ ~ . In accelerated aging of open-faced specimens, b l i s t e ~ g induced delamination is a

hindrance to the experimental investigation of degradation due to bound watet in adhesive, and,

more importantly, this kind of delamination may be a mode of fdure in adhesive joints in certain

circumstances. Blisterhg as a form of corrosion undemeath organic coatings has been rigorously

studied", and cracking due to osmosis in adhesives has been i n ~ e s t i ~ a t e d ~ ~ .

There were two types of blisters observed on open-firced specimens. The f k t was

blisters due to sweiling of adhesive surtounding air bubbles in the bondline and they did not

affect the bondhg interface1'. The second type is blisters full of water, which generally c a w d

delamination at the interface. In the present work, after blistering had been observed on the open-

faced plates which were being aged for degradation parameter @P) verification (Chapter 5) , a

systematic effort was made to investigate the mechanism and its role in joint degradation. The

focus of this study was: a) the causes of bliste~g; b) the fkequency of blistering on open-faced

specimens of Cybond 4523GB-aluminum and Cybond 1 126-aluminum; c) the pattern of blister

growth; and d) blistering in the closed joints.

Chapter 4 Blist9nng in Adhesive Joints

4.2 Erperimentd Observations

4.2. I Blisterhg on Open-faced DP specimens

The blistering first observed on open-faced DP specimens had the following features:

1. Bhtering initiated d e r appmximately 3 weeks of aging at 6S°C with 100% RH

and 85OC with 100% RH.

2. Blisters were not distributed evenly over the adhesive surface (Fig. 4-1).

3. Delamination occurred undenitath most of the blisters (Fig. 4-2). No air bubbles

entrapped inside the adhesive layer were observed. At the edges of the plates, delamination was

also observed (Fig. 4-3).

4. The number and size of the blisters grew over tirne, with heights up to 3 mm and

diameters ranging fiom 1 to 8 mm (Tables 4-1 and 4-2).

5. Among the diffierent RH levels (30%, 60%, 85%, and 100%), ody 100% RH

induced blistering, and almost al1 the çamples at 100% RH blistered at both 65°C and 85OC for

both Cybond 1126 and Cybond 4523GB adhesive bond.

6. The blisters were fidl of liquid water. A pH test indicated that the water was

neutral-

In order to monitor the development of blisters, two blistering specimens (Cybond

1 126-AA606 1 -T6S 1 and Cybond 4523GB-AA606 1 -T6S l), were randomly chosen to be

investigated. On each plate. a severely blistered area was designated as the investigation zone.

The area of each investigation zone was approximately 8 cm2. The nurnber of blisters in each

investigation zone was counted periodically, and the height of a blister and base diameters (Fig.

4-4) of two other blisters were measured at the same tirne. It should be noted that the base

diameters were only estimated using a caliper.

f ig. 4-1 A blistering area on r Cybond 1126 open-faced specimen aged for 120 days at

65OC with 100% RH

adhesive layer, 0.4 mm thick

Fig. 4-2 A cross section of a blister on a Cybond 1 t 26 open-faced specimen aged for 120

days at 6S°C wl(h 100% RH. The spocinrn was cut using a tabk u w

Ch* 4 Blisterhg in Adhesive Joints .-

Fig. 4 3 ûeîamination at the edge of 8 Cybond 11 26 openIfrced speccinwn rged for 120

days at 6S°C with 100% RH

base diameter

adhesi~ laver height

Fig. 4 4 Height and base dirmetnr of r blkdrr

Tables 4-1 and 4-2 are the results of the blisterhg survey on the rnonitored specimens.

Blisters increased in height until they cracked.


Table 4-1 Suwey on blistering of an open-faced Cybond 452368 spedmen with 0.4 mm

thick adhesive Iayer aged at 8S°C, 100°h RH

1 thne of aging 1 height of 1 number o f blisters in 1 base diameter of 1 base diameter of 1 (&YS)

43

Table 4-2 Survey on blistering of an open-faced Cybond 1126 specimen with 0.4 mm

97

thick adhesive Iayer aged at 6S°C, 100% RH

blister # 1 (mm) 0.54

1 time ofaging 1 height of 1 number of blisters in 1 base diameter of 1 base diameter of ]

0.29

the investigationzone 15

Note: The monitored blisters coiiapsed afkr the survey. Exact time of collapse is unknown.

17

( d a ~ s ) 1 O5

4.2.2 Blistering on Purposely Contamirtated Specimens

blister #1 (mm) -

I

pressure re kased

In order to understand the role of residual etching solution and other surface

contamination in blistering, three open-faced specimens were made with Cybond 1 126 AA6O6 1 -

T65 1 and three with Cybond 4523GB-AA606 1-T65 1. AI1 the surfaces of the six specimens were

blister #2 (mm) -

blister Xl (mm) 1 -44

the investigation zone 22

blisters #1 (mm) -

blisters #2(mm) -

Cbapter 4 Blisterhg in Adhesive Joints

pretreated with a newly-made etching solution (ASTM D265 1-79), and special attention was paid

to the rinsing process to make sure that no solution would remain on the surfaces. Among the

three plates of each adhesive system, two were purposely contamioated with diluted etching

solution (about one tenth of its original strength) and mold release agent (MS-122NK02, Miller-

Stephenson Chernical Company, Inc.) separately in a 4x4 matrix before bonding (Fig. 4-5). The

contaminated areas were created using a foam plug of 5 mm diameter soaked with diluted

etching solution or mold release agent These plates were then dried in rwm air for about one

hou.

270 mm

plate etching solution mold release agent

e o a o O 0 0 0 contaminated contaminated

5-7 mm 5 0 a 0 t

O 0 0 0 area area

a o a e 0 0 0 0

Fig. 46 A purposely contaminated AA6061-TB51 plate before bonding

Of the three plates of each adhesive system, one of the purposely-contaminated plates

(specimen A65 100CT for Cybond 4523GB and B65 lOOCT for Cybwd 1 126) and the clean

control plate (specimen A65 100NCT for Cybond 4523GB and B65 100NCT for Cybond 1126)

were placed to 100% RH chamber at 65OC for aging. The second ariificially-contaminated plate

(specimen A65085CT for Cybond 4523GB and B65085CT for Cybond 1 126) went into 85% RH

chamber at 65'C. After three weeks of aging, blisters appeared at the etching solution

Chapta 4 Bîistering in Adhesive Joints

contamination spots in the 4x4 rnatrix pattern on plates of both adhesive systems aged at 100%

RH, but no blisters were ever observed on mold reiease agent contamination spots nor at 85%

RH. It is noteworthy that on the c l a n control plates aged at 100% RH, there dso appeared some

relatively srnail (1-2 mm maximum diameter afier about one month of aging) blisters, but no

blisters developed on the control plates aged under 85% RH.

4.2.3 Blistering in Closed Joints

Closed joints of two different adherend thickness (0.1 mm for knife-cut specimens

and 1 mm for saw-cut specimens, see Section 3.3) were fabricated to study the behavior of water

diaision in closed joints. Unexpectedly, many of these specimens also blistered and provided

valuable information on the mechanisms of bIistering in closed adhesive joints and its role in

degradation-

Blisters on knife-cut Cybond 4523GB specimens were visible on adherends of both

sides afier 1 10 days of aging (Fig. 4-6). On the specimens of 0.24 mm bondine thickness, the

same blister codd often be seen on either adherend if the diameter was over 3 mm. Of the 18

specimens aged at 85OC with 100% RH, 13 developed blisters. The number of blisters on each

specimens ranged fiom 2-5 with a maximum diameter 5.5-6 mm. Al1 the Cybond 4523GB

specimens aged at 6S°C with 100% RH had not blistered aAer 240 days of aging. Six Cybond

1 126 closed joint specimens were fabncated and aged at 6S°C mth 100% RH; no b i i s t e ~ g was

observed on these specimens after 90 days of aging.

Some of the blistered thin adherend Cybond 4523GB specimens were peeled offto

examine the blisters. Figures 4-7 and 4-8 show the morpho1ogy of a blistered spot on the

adhesive and adherend sides, respectively.

Chmer 4 Büsmhn in Adhesive Joints

Fig. 4-6 The appernnce of r blkdbr on Cybond 152308 CM joint specimen 8510851

(aged at 8S°C w#h 100% RH for H O days)

Fig. 4-7 A blistemd #pot of Cybond 452368 spacimen 8510851 on the rdheshre rfbr

rernoval of the rdherend ( r a d rt 8S°C w)th 1- RH for 110 d m , the rdhonnd

thickness was 0.1 mm)

Chapter 4 Blisterhg in Adhesive Joints

Fie. 4 8 A blistered spot of Cybond 452368 apedmen 8S108St on the rdharend (and at

8S°C with 100% RH for 110 dap, the adharend thkkner m s 0.1 mm)

Figs. 4-8 shows that there was considerable amount of adhesive residue at the center

of the blister, suggesting that the crack propagation was cohesive (in the adhesive layer) at the

beginning of the blister development, and near the blister edge, crack propagated at the interface.

Arnong the 8 one-mni-thick-adherend Cybond 4523GB closed joint specimens, after

78 days of aging, only No. 8565 1, which was aged at 85°C with 100% RH, developed blisters.

Due to the larger thickness (1 mm) of the adherend, the blisters grown at the interfàce of the saw-

cut specimen could not be seen on the outer surfàce of the adherend. The adherends of these

specimens were etched using NaOH solution tiil approximately 0.1 mm remaiwd and then peeled

off to examine effect of the long-tenn aging on adhesive (see Saw-cut Specimens, Section 3.4.3).

Unexpectedly, a blister was observeci in a thick adheread close joint (#8565 1). Figures 4-9 and 4-

10 present the pictures of a blistered spot of sample 8565 1 on the adhesive and adherend sides,

respectively. Again, adhesive residue can be seen on the adberend (Fig. 4-10).

Cha~ter 4 Blisterinn in Adhesive Joints

Fig. 4-9 A blisterad spot of Cybond 4S23GB spocimen 85651 on adheshre aRer remaval of

adherend (aged at 8S°C, 100% RH for 78 dam, #e rdhemnd aikkneu was 1 mm)

Fig. 4-1 O A blisbred spot of Cybond 452308 8pechii.n 85651 on adherend side (aged rt

8S°C, 100% RH for 78 day., the adhorend Ihkkii.8~ wps 1 mm)

Chapter 4 Blistaing in Adhesive Joints

4.3 Analysis and Discussion

4.3.1 Water Uptake dire !O F i c k h D~Yfusiort in BIWers

If water diffllsion in an adhesive layer is governed by Fick's law, it is possible to

describe the process of water diftiising into an air void (blister). At a blistered spot of an open-

faced specimen, if water concentrations at points within the adhesive iayer are known, then F, the

mass flux of water, can be calculated using Fick's first law:

where D is the diffusion coenicient, z is the depth in the adhesive layer fÎom the open surface

(Fig. 4- 1 l), and C(r. f) is the hctional water concentration in the adhesive of depth z at time t.

The water concentration in the adhesive layer changes during diaision. Water

collection undemeath the adhesive layer needs an air void as shown Fig. 4-1 1. Ifwater c m only

penetrate the adhesive fiom one side, and no water gain or lose occurs at the opposite side (as in

a non-blistered area), then the gradient of hctional water concentration over the thickness will

change with time according to Eqn. (M)'?~. This is plotted in Fig. 4-12 for Cybond 4523GB at

U°C, 100% RH.

Hz0

! 21- I

Fig. 4-1 1 A blistered spot on an open-faced specimen. Beneath the air is the adhemnd

Chapter 4 Biistering in Adhesive Joints

++ l hour + 2 hours 4 12 hours ++ 24 hours + 36 hours + 48 h o u

Fig. 4-1 2 Water concentration through depth of non-blistemd Cybond 4523GB adhesive

Iayer (0.4 mm) on open-hced specimens aged at 8s°C, 100% RH (P12 .3 XIO-" m21s)

It c m be seen fiom Fig. 4-12 that the water concentration gradient is large at shon

times (e. g., 1 hour of dinusion) near the surface contacting the moisnire. After approximately 2

hours of diffuison, water reaches the interface between adhesive and air disc. ifthe water at the

interface remains in the adhesive (e.g., at the non-blistered area), then C(0.4 mm, t ) will increase

as depicted in Fig. 4- 12. At the adhesive-air interface of the void, the water concentration on the

adhesive side will be affected by two processes: first, at least a part of water at the interface will

enter the blister air void, decreasing the water concentration on the adhesive side of the interface;

second, the collected water will elevate the relative humidity in the blister air void, setting the

lower limit of interface adhesive water concentration. The interaction of the two processes wilt

ultimately stop the water transport through the adhesive layer. This suggests that blisters would

Chapter 4 BLnmng in Adhesive Joints

not fil1 completely with iiquid water. Since this is contrary to the experimental obsewations,

diffusion alone could not be the only driving force for the blisters to fiU and grow with liquid

water. Osmosis, provides a logical explmation for the water transport through the adhesive layer

fiom the environment chambers into the blisters.

4.3.2 Corrosion and Blisterhg

Osmotic pressure is generated by a difference in concentrations of ionic species in an

aqueous system. During blisterhg observed in the present experiments, the adhesive could act as

a membrane which is permeable to water molecules.

As presented in Section 42, sait ions (Na', S O ~ ~ - , etc.) in the sodium

dichromate/sulfuric acid etchiug solution could have produced an osmotic pressure to induce

blistering, but on the specimens which were rigorously rinsed using distilled water aller etching,

blisters were also observed. This led to the speculation t h t metal corrosion underneath the

adhesive layer produced some ions. To examine this possibility, the aluminum surfaces of the

blistered spots and nearby non-blistered spots were examined using SEM. The adhesive layer

over the targeted spots was removed using a scalpel. Figures 4-1 3 and 4- 14 show these sites for a

Cybond 1 126 specimen. The cornparison of Cybond 4523GB specimens could not be analyzed in

this way because the adhesive over the non-blistered area could not be removed without

damaging the surface.

Fig. 4-1 3 Morphology of aluminum surface undemeath a blister on a Cybond 1126

specimen (865100161) aged for 105 days at 6G°C, 1ûûW RH. The light parücles are

adhesive residue

Fig. 4-14 Morphology of aluminum surhce near the blister show in Fig. 443. The light

particles are adhesive msidue

Chapta 4 Blisterhg in Adhesive Joints

Figures 4-13 and 4-14 show that there was no significant ciifference between the

aiuminum surfaces of blistered and non-blistered sites.

Other pieces of evidence that are contrary to alurriinum corrosion are: a) A common

form of corrosion of organic-coated aliuninum is anodic undermining, which manifests itseifas

threadlike filaments33, not round-shaped blisters. b) The chemical composition analysis of blister

water showed that aluminum species were a minor component in the solution (Table 4-3). c)

Corrosion of aluminum would have produced a low pH value in the blister ~ i ~ u i d ~ ~ , but a pH test

using testing paper @Hydraion paper, Micro Essentiai Labonitory Inc.) indicated that the blister

Iiquids of both Cybond 1 126 and Cybond 4523GB specimens were neutral.

4.3.3 Chernical and Morplr ological Anak'ysis of Blisters

Ethe water accumulateci in the blisters was due to osmotic pressure, then the ionic

species which were responsible for the osmosis should be detected in the liquid. To check the

chemical composition of blister water, approximately 0.3 ml of liquid fiom blisters on Cybond

1 126 specimen B85 100 16 1 (aged at 6S°C, 100% RH for 170 days) and approximately 0.5 ml of

liquid fiom Cybond 4523GB blisters on specimen A85100086 (aged at 85'C, 100% RH for 119

days) were collected using 3 ml syringes. These blister liquids were analyzed using neutron

activation analysis w, University of Toronto Slowpoke facility). The resuks of NAA for both

Cybond 1 126 blister iiquid and Cybond 4523GB blister liquid are shown in Table 4-3.

Chapter 4 Blisterhg in Adhcsive Joints

Table 4-3 Chemical compositions of blister liquids from Cybond 1126 and Cybond

4523GB specimens

Table 4-3 shows that barium and chlorine were detected in significant arnounts, but

these ions did not corne fiom residuai etching solution nor the aliuninum, Ieading to the

speculation that they might be Ieached from the adhesive layer. NAA was employed again to

analyze the chernical compositions of the two adhesives, Cybond 4523GB and Cybond 1 126.

The results for cast adhesive wafers are listed in Table 4-4. The mass of the analyzed samples

was 1 10 mg for Cybond 1 126 and 3 10 mg for Cybond 4523GB. The two adhesives both

contained a relatively large quantity of magnesium and chlorine, and Cybond 1 126 contained a

large amount of barium, which was consistent with the chernical composition of the blister

liquids. Manganese also appeared in the adhesives, but it was not detected in the liquids, showing

that manganese codd not be leached out fkom the adhesives.

species

Ba

M g

Na

concentration of Cybond 1 126 blister liquid (ppm)

20.1

240.3

21.1

concentration of Cybond 4523GB blister liquid (ppm)

- 354

61

Chapter 4 Blisterkg in Adhesive Joints

Table 44 Chemical compositions of adhesives Cybond i l 26 and Cybond 4523GB

1 species 1 concentration in Cybond 1 126 1 concentration in Cybond 4523GB 1

The following expriment was performed to M e r confïrxn that ionic species cm be

leached out fiom the adhesives into water. Scraps of 0.8 mm thick Cybond 4523GB wafers and

0.4 mm thick Cybond 1126 wafers (0.95658 g of Cybond 1126 and 2.44561 g of Cybond

4523GB) were soaked into 20 ml distiIied water at 6j°C for 4 weeks. T'en, the liquids were

analyzed using inductively coupted plasma atomic emission spectroscopy (ICP-AES) for metal

species and ion chromatography (IC) for non-metal species, at the Analytical Laboratory for

Environmental Science Research and Training (ANALEST), University of Toronto.

Approximately the same amotmt of pure distilled water (fiom the same source as the soaking

water) served as the blank during liquid chernical composition analysis. Table 4-5 shows the

concentrations of Al, Mg, and Cl ions, confirming the possibility that ionic species in the

adhesive can be leached into water, thcreby causing frnther water absorption by osmosis. This

process of leaching and osmotic water transport wouid continue until the void became filled. At

this poht, osmotic pressures wouid increase causing adhesive crackhg and blister growth.

Ba adhesive @pm)

20040 adhesive (ppm)

-


Table 4-5 Concentration of ionic species in water containing Cybond 1126 and Cybond

4523GB

The morphology of the adherend surface at the center of blisters was examined using

an optical surface profilorneter (WYKO) with a resolution on the order of the wavelength of

white light. The samples used in profiiometry were as follows: B65 1 OONCT, a residue-fke and

not-purposely-contaminated Cybond 1 126 specimen aged at 65OC with 100% RH for

approximately 3 weeks; A65 1 OONCT, a residue-fiee and not-purposely-contaminated Cybond

4523GB specimen aged at 6S°C with 100 RH for approximately 3 weeks; 865 10016 1, a Cybond

1 126 specimen prepared for DP verification and aged at 6S°C with 100% RH for about 4 months;

A85 100 1 12, a Cybond 45323GB specimen prepared for DP verification and aged at 8S°C with

100% RH for about I month. Figures 4-15 and 4-16 show that on the specimens with only a very

remote possibiiity of etching solution residue and other kinds of contamination (A65 100NCT and

B65 100NCT) blisters developed centering on small pi&. But on the DP verifïcation specimens,

which had a greater possibility of carrying some etching solution residue, pits were not observed

undemeath the blisters (Figs. 4-1 7 and 4-1 8). It is interesthg to see that not only srnail pits but

also lumps were at the center of blisters (Fig. 4-19). An EDX analysis revealed that energy

spectra of O, Al, Mn, Cr, and Fe of the circular lump and of an area away fiom the lump were

essentially the same, showing that the lump was an alwninwn flaw on the adherend surface, not

the product of corrosion.

concentration of species generated by 1 g Cybond 4523GB in 20 ml water (ppm)

0.03

species

AI

concentration of species generated by 1 g Cybond 1126 in 20 ml water (ppm)

0.10


102 x me: OIM199 W: VS 3 0 PI& Ti; 15332

R.sri.lOlliu: T 5 Ta - Nor

Tl: Note: an adhamd side. sampk 065 100NCT

Fig. 4-15 Morphobgy of adhennd undemerth a blbter on a Cybond 1126 specimen

(6651 OONCT)

M y 53X

Mode: 99 3 0 Plot

I I Tie: The blirttr c e Note: on tbc adkw~d si&. sampic A65100NCT

Fig. 416 Morphobgy of adharend undamerth a bibi on r Cybond 452308 specimen

(A651 OONCT)


Mag 102X Mode: PSf 3 0 Plot

Tie: Rie blista centa Note: on adhcrmd &. d e B65100161

Fig. 44 7 Morphology of adherend underneath a blister on a Cybond 11 26 specimen

(B65100161)

O p r Tetas Rammd Tilt Fa*

Tic: The bhSea ctntcr Note. on the a&rend. suqAt A85100 112

Fig. 418 Morphokgy of adhennd underneath a blbter on a Cybond 452308 specimen


Fig. 4-19 Morphoiogy of adhemnd undemeath a blister on a Cybond a2306 specinrn

(A651 OONCT)

4.4 Mechanisms of Büstering in Adhesive Joints

4.4.1 BIhfering due tu Osmosis

The analysis presented in Section 4.3 suggests that the blisterhg observed on open-

fked specimens and in closed joints was due to osmosis. It has also been s h o w that the species

inducing osmotic pressure were impurities fiom the etching solution or ions leached fiom the

adhesive.

The foiiowing mode1 of the blisterkg process seems to fit o u . present observations:

1 . Voids-Voids are a prerequisite for water clustering. Pits on adhereods or air

bubbles in the adhesive will provide space to aiiow the formation of micro-droplets of water.

2. Water c l u s t e ~ B o u n d water in the adhesive joint aggregates to form

microscopie drops. This may require a high relative humidity in the environmental chamber

(higher than 85% RH observed in the present experiments). Hydrophitic sites in the voids may

serve to nucleate water clustering-

3. Leaching-In the case of Cybond 452368 and Cybond 1 126, Mg, Cl, and other

ions c m be leached h m the adhesive layer during diaision and dissolved in the water clusters.

4. Osmosis-The dnving force for water to enter and fiil the voids is osmosis caused

by ionic species fiom the leaching of the adhesive or fiom pretreatment residues on the

adherends-

5 . Intemal pressurization-Mer the void is fillecl, osmotic pressure will cause the

adhesive layer to crack either cohesively or interfacialiy.

6. Void growth-The growth of the blister provides additionai space for water,

causing a drop in the internai pressure, which in turn aliows osmosis to bring more water hto the

blister (void). At this point, the process goes back to step 4, and continues until the blister breaks

open.

It should be noted that ionic adherend contaminants (e-g., etching solution residues)

enhance osmosis and may ais0 promote water clustering, but mold release agent does neither. As

well, the ionic solution in the blisters may degrade the adhesive and the interface, decreasing Gc

and facilitating blister growth.

The blistering process may occur in open-faced specimens and in closed joints. Blister

growth in the latter will, of course, be slower because of the srnaller rate of water uptake and the

constraint imposed by the adherends. Nevertheless, blister growth rnay stiil occur due to the very

high pressure that results fiom osmosis. For example, the blisters s h o w in Figs- 4-6 to 4-10

developed in two specimem aged at 8S°C, 100% RH for 1 10 days and 78 days, respectively.

Osmotic pressure 17 (kPa) in a solution is found to obey a law similar in form to the

ideal gas law as shown in Eqn. (4-3)40:

B c R T (4-3)

where c is the concentration of the soluîion in moi& R is the gas constant (8.3 14 J/(mol- K)), and

T is temperature in K. This equation shows that ndepends mainly on the amount of solute

dissolved in a given qua~tity of solvent, irrespective the nature of the solute, and f7resulting

fiom different solutes in the same solvent is simply the addition of the contriiuîions of each

solute.

At the t h e when the blister water was collected, osmotic pressures built up by the

listed species in the blisters of Cytmnd 1 126 and Cybcmd 4523GB investigated in Table 4-3 were

estirnated at 36 kPa and 280 kPa using Eqn. (4-3), respectively.

Generally, as the blisters grow, the new adhesive constantly contributes to leaching,

adding more ions to the solution, but growth is pmiicted to slow since the exposed adhesive

surface area becomes retatively smail as biister volume increases. However, a membrane analysis

of blister gr~wth4' gives the foliowing equation relating the criticai pressure, Pm, for expansion to

the blister radius, r, and the adhesive critical energy release rate, Gc:

where v, E, and if are, respectively, Poisson's ratio, Young's modulus, and the thickness of the

adhesive layer. It is seen that for a constant Gc, as r increases P, decreases; therefore the osmotic

pressure required for continued blister p w t h will decrease.

It should be noted that Eqn. (4-4) is based on a mode1 of the thin membrane; therefore

it will only approximate the growth behavior of blisters developed on the open-faced specimens

Cbapter 4 Blisterkg in Adhesîve Joints

with a 0.4 mm thick adhesive layer. Neverthelers, it is of interest to calculate the P, for blisters

observed in the present study. It has been observed that r of a typical blister on Cybond 4523GB

open-faced specimens was 3 mm (Table 4-1). The adhesive was leathery in the 8S°C, 100% RH

chamber, suggesting the E of mbber (7 may k a g d approximation of the Young's

modulus. It was found in Chapter 5 that Gc (phase angle y604 of Cybond 4523GB joints aged

at 8S°C, 85% RH for one year dropped by 20%. Assumuig that crack propagation during blister

growth was prne mode 1 (@'), and the strength l o s observed in Chapter 5 applied to the

specimens aged at 85OC, 100% RH for 80 days ( t h e of s w e y reported in Table 4-1). with a Crc

of 2 16 ~/m' (Table 2-2), and a Poisson's ratio 0.5, accordhg to Eqn. (a), P, wouid be 400 kPa.

It is interesting to observe that this value is fairly close to the estimated osmotic pressure (280

kPa, using Eqn. (4-3)) based on the ion concentrations of blister liquid collected fiom a Cybond

4523GB plate specimen.

Figure 4-20 illustrates the blistering process in both open-faced specimens and closed

joints. It should be noted that the origin of blistering is not necessarily at the interface. For

exarnple, it seems that the blister shown in Figs. 4-7 and 4-8 originated in the adhesive layer and

then the crack path deflected to the interface.

The osmotic biistering mechanism has implications for both the understanding of

adhesive joint degradation and for the manufacture of joints in industry. First, blistering is

certainly a form of degradation for joints at high temperature and high nlative humidity. A

fûrther study of its role in the degradation of closed joints under other conditions of aging would

enrich our knowledge of degradation processes, and may yield a quantitative description of the

durability of adhesive joints by correlating the blister growth rate and the fkquency of blistering

in a joint to the speed of degradation. It was noticed that there may have existeci a critical dative

Cbapter 4 Blisterkg in Adhesive JO*

humidity (higher than 85% at both 6S°C and 8S°C for Cybond 1126 and Cybond 4523GB

specimens) for joints to blister in the present research. If blistering as a form of degradation is

applicable to other temperatures and adhesives, the critical water content observed by other

15.16 researchers would be strongly supported by the present work.

adhesive layer

crack tip, Gc

aüherend crack tip, Gc

adherend adesive adherend

a) open-faced specimen b) closed joint

Fig. 4-20 Blistering process in open-faced specimen and tlosed joint

Since the o@n of blistering is related to adherend surface pretreatment, flaws on the

adherends, and air bubbles in the joint, a s m d blistering test may be designed to check the

appropnateness of the process of joint fabrication. Using open-faced specimens, flaws and

defects on the adherend or in the suface pretreatment wil1 manifest on the specimens in the form

of blisterhg in a relatively short tirne. Further analysis of the blister liquid and adherend

morphology undemeath the blisters would provide information based on which the flaws or

defects of the fabrication codd be identified.

Chapta 4 BListering in Adhesive Joints

4.4.2 Blisterhg due to Sweiiing of Adhesive Suwounding Air Bubbfes

Blisters on the surface of aged open-faced adhesive specimens were observed with

different diameters and heights. Some blisters were relatively small end codd be sanded off

using ordinary sandpaper (220 grit). These blisters were not likely to have originated fiom

delamination because, afler sanding, the samples could still be closed to form normal joints

which carried reasonable loads; they were actuaily relatively large air bubbles entrapped inside

the bondine that were not yet filled with water.

To explain the mechanism by which these entrapped air bubbles blister, it is helpfùi to

consider a portion of the adhesive layer with an air bubble as show in Fig. 4-21. After the

adhesive reaches its equilibrium water content, it will have swelled by a hctionai amount #?(B

equals volume increase divided by the original volume). At the same tirne, the air in the bubble

will expand d e r the temperature rises fiom rwm temperature to that of the enviromnent

chamber. When the sample cools to room temperature, the condition when the blisters were

sanded off, the air pressurizing effect will disappear. Because of adhesion at the interface, the

expansion of the adhesive above the void (Fig. 4-2 1) is constrained to be in the vertical direction

only, prirnarily occurring as a type of buckling deformation.

The expansion of the adhesive can be estimated using Cs and its specific gravity p.

For Cybond 4523GB, at 100% RH, Cs is 3.69% at 65OC and 4.88% at W°C, and p is

approximately 1.3 g/cm3. Considering a cube of the adhesive of 100 g; then its volume will be 73

cm'. lf the void volume inside the adhesive and themial expansion rate of the adhesive are

negiigible, after the cube getting satunited at SOC, 100% RH, its volume will be increased by

4.88 cm3 (the density of water is 1 @cm3). So, the expansion rate of the adhesive 0, by volume,

Chapter 4 Blisrering in Adhesive Joints

is approximately 6.3% at 85OC, 100% RH. Likewise, the flvalue of the adhesive at 6S°C, 1 0 %

RH is approximately 4.8%-

<- adhesive layer i

C

//////////// /// adherend

Fig. 4-21 A blister due to swelling of adhesive surrounding an entrapped air bubble

The height e of a blister is determined by the volume increase of the adhesive disc

above the bubbte. Because of the lateral constraiat surrounding it, the volume increase of the disc

must be accommodatecl by vertical buckling deformation. The relationship between linear

expansion rate a and volume expansion rate can be obtained h m the a mit cube:

( i + al3= 1 +3 a+3 d+d=l +fl (4-9

Provided that magnitudes of both P and a are far less than unity; then the 1st two

ternis are negligible, i.e. ma. For Cybond 4523GI3, based on the estimated volume expansion

rate, a is calculated to be 1.6% at 6S°C and 2.1% at 85°C. The diameter of the disc before and

after deformation wiii form a chord (AB) and an arc (ACB) of a spherical cap, respectively (see

Fig. 4-21). The radius of the spherical cap R can be solved fkom foilowing equations:

R sin(@ / 2) = r (4-6)

R8 = 2(1+ a)r (4-7)

Chapter 4 BWerhg in Adhesive Joints

FinaUy, the height of a blister e can be calculated as

e = R[I - COS(@ / 211 (4-8)

For Cybond 4523GB aged at W°C, 100% RH, assuming that an air pocket of ZH.5

mm diameter; then, B is determined to be 0.7 1 rad (approximately 4 19, and e would be

0.045m.m. The heights of blisters of different diameters are listed in Table 4-6. It shows that the

blisters induced by swelling of adhesive above entrapped air bubbles of diameter less than 4 mm

have very s m d heights (Iess than 0.4 mm, the thicbess of the adhesive layer) and could be

sanded off ushg ordinary sandpaper without damaging the adhesive layer severely.

Table 4-6 Heights of blisters induced by swelling of adhesive above air bubbles of

different siws on Cybond 462308 specimens aged at 8S°C. 100% RH, assuming

that the adhesive Iayer has reached its equilibrium water content (4.88%)

The heights of air bubble induced blisters on samples aged at 6S°C are slightly less

than those of specimens aged at 85OC, given that other conditions are the same.

It is believed that blisterhg due to swelling is a relatively short-term phenomenon that

would occur well before the air bubble becomes filled with water as a resdt of osmosis.

Char,tcr 4 Biisterbz in Adhesive Joints

4.5 Conclusions

Based on the experimental obse~ations and analyses presented in this chapter, the

following conclusions regarding the blistering on Cybond 1 126 and Cybond 4523GB specimens

at both 65OC and 85°C were drawn:

1. Biisters develop both in open-faced specimens and closed joints.

2. Osmosis is the mechanism of blistering that causes cohesive or interfacial cracking

due to the pressure of the Liquid inside the blister, and blistering can occur at any location where

water clusters might initiate.

3. Water clusters, as a prerequisite of blistering, can f o m and grow at voids. Cluster

initiation, as weli as growth, may be assisted by the presence of ionic species on an adherend.

4. In osmotic blistering, the osmotic pressure is generated by ionic species leached out

fiom the adhesive or present as pretreatment residues.

5. Entrapped air bubbles of relatively large size in the bondline can induce blisters of

small height on open-faced specimens due to adhesive swelling. In general, this kind of blister

does not significantly affect the performance of joints made fiom open-faced specimens.

Chapter 5 Experimental Investigation of the Degradation

Parameter

5.1 Introduction

The mechanisms of degradation in adhesive joints due to water are not weil-

understood, but it is believed that the amount of water and its time in the adhesive are positively

correlated to the strength loss of the j ~ h $ ~ ~ * " * ' ~ . Wyide and spelt' proposed a concept to

quantitatively characterize the combined effect of water and its aging t h e in an adhesive joint

using a "degradation parameter" (DP).

Accordhg to the DP hypothesis, at a given temperature, if the spatial water

distribution over time at a point C(x, y, z, t) in an adhesive layer is known, the effects of water

content and time can be combined in a single measure of the degree of environmental exposure:

It was hypothesized that the amount of strengh degradation correlates with DP at a

given temperature. For an open-faced specimen aged at a specific temperature with equilibrium

water content CS and diffusion coefficient D, assurning that water diffusion occurs only in the z-

direction (Le., there is no diffusion at the interface, and diffusion fkom the edges is negligible), as

shown in Fig. 5-1, a simple expression of DP for the specimen can be obtained by integrating

Eqn. (4-2) holding z constant:

Chapter 5 Experhenfal Verincation of Degradation Parameter

adhesive lay& with Cs and D adherend

Fig. 5-1 One-dimensional diffusion model in an open-faced specimen

L€ the Cs and D of the adhesive layer are the same as the diffiision properties

measured fiom cast wafea as shown in Chapter 3, then the degree of degradation at any point z

in the adhesive layer can be characterized using a DP value calculated nom Eqn. (5-2) using the

diffusion properties of the bdk adhesive. This Chapter examines the DP concept using Cybond

4523GB and Cybond 1 126.


S. 2. I Experimental Procedures Adopted from Re$ [3]

Many of the experimental procedures were taken directly fiom ref. [3], and were

reproduced here for convenience. The following is a List of common procedures and material

between ref [3] and the present work.

9 Adhesives (Cybond 4523GB and Cybond 1126);

Chapter 5 Ercperimental Verification of Degradation Parameter

Surface pretreatment (wiping with kimwipe tissue soaked with acetone and

degreasing with P3 Almeco 18 cleaner for 30 s, then etching with sodium dichromate/sulfiuic

acid solution (ASTM D265 1-79) at 70°C for 25 min);

Bondline thickness (0.4 mm primary, 0.1-0.125 mm secondary) and its controt

(0.375 mm thick Teflon spacers for primary; giass beads buiit-in the Cybond 4523GB for

secondary and clamp pressure when Cybond 1 126 was used as secondary bond);

Open-faced specimen fabrication and procedure (using miiied and mold release

agent baked plates);

C u ~ g (one and half hours at 1 50°C for Cybond 452368 and room temperature for

Cybond 1 126);

Aging temperatures (65OC and 85OC for Cybond 4523GB specimens, 65OC for

Cybond 1 126 specimens);

Specimen and its dimensions (saw-cut double cantilever bearn @CB) specimens of

20 mmx27Omm);

Fracture testing (ATM Mode1 TCS-100 software contrdIed load h e with phase

angle (y ) achieved ushg a load jig). Of the 48' and 60' setthgs, only 60' testing was conducted

because it has a higher potential to drive the crack to the targeted bond (the primary bond in the

present research);

DP calculation. Using Eqn. (5-2), z was chosen at 0.4 mm for Cybond 4523GB

specimens because they were tending to crack at the interface between the primary bond and the

adherend; and z was chosen at 0.2 mm for Cybond 1 126 specimens because they were tending to

crack at the middle plane of the adhesive layer.

Certain modifications were made to these experimental techniques as described below

in Sections 5.2.2 and 5.2.3.

5.2.2 Specimen Fabrication

Compared with the experimental procedures of r d 131, in the present work, 1.5 inch

thick AA6O6 1-T65 i plates were used as adherends instead of AA7075-T6 plates to reduce

material costs. Based on the conclusion drawn in Chapter 2, Cybund 4523GB and Cybond 1 126,

the primary bond adhesives, were also used as secondary bond adhesives in al1 specimens except

for A65030090, A650301 80, A85030090, A850301 80, A85(60+80)80, and A85(80+60)80

which were closed with a Hysol9309EA secondary bond. Sandpaper of 200 grit uistead of 300

grit was used to clean the aged prinaary bond surface before secondary bonding.

In surface pretreatment, acetone wiping was tirne-consuming; it took about 20-30 min

to clean a plate. During secondaty bonding, the bonding surface was sanded using a sander with

1A-X Pl 20 sandpaper to reduce the acetone wiping time (about 3-5 min to clean a sanded

surface). It was estimated that the sanding removed an duminum layer of approximateiy 30 p

thick, thus it did not change the thickness of the adherend signincantly. The surface sanding

before pretreatment was inspired by the fact that flaws on the adherend surface assist the

initiation of blistering (Section 4.4). A sanded or milled surface may, therefore, enhance the

resistance to blistering, although this was not proven.

When making Cybond 4523GB bonds, the adhesive was applied at the center of the

plates in the form of a ridge, and it was squeezed out by the backing plates which were pretreated

by mold release agent on the contacting surface. The joint was then clamped with 10-12 ordinary

paper clamps (2" size) and aiiowed to corne to an equilibrium thickness as the adhesive flowed

out the edges of the joint Due to the high viscosity of Cybond 1 126, the backing plate was

clamped after the adhesive had been spread out using a spanila. These same procedures were

adopted during secondary bonding with these adhesives.

The width of the open-faced plates was 90 mm, and one plate was designed to be cut

into three DCB specimens using a table saw after the plate was formed into a closed joint. The

extra 30 mm was the aliowance for saw blade width (4x3 mm) and spacer zone trim-off (2x8

mm) -

5.2.3 Fracture Tests

To avoid adhesive plasticîzation effects, al1 the sampks were tested in dry state. The

aged specimens were dned at 60°C for approximately two days to half a month before they were

sanded using 220 grit sandpaper and rinsed using acetone. The cleaned open-faced plates were

then dried again at 50-6S°C for a few hours. Then, the plates were closed with secondary bond,

and cut into specimens with a table saw.

An aluminurn foii insert served as a pre-crack for each open-faced specimen to ensure

the crack would propagate at or near the interface between the primary adhesive layer and the

adherend. Because the aluniinum foil insert does not form an ideal sharp crack tip, DCB samples

were then M e r pre-cracked using a chisel, and the length of the pre-crack was controiled by a

clamp applied to the DCB (Fig. 5-2). Because a crack tends to propagate near the more highly

strained adherend, during chisel pre-crackiug, the direction of the chisel was tilted toward the

prirnary bond adherend as shown in Fig. 5-2.

secondary bond

intended initial crack Length

==- loading holes

- primary bond P

(The other end of the specimen was clamped in a vice)

Fig. 5-2 Pmracking a DCB specimen

The mode ratio (v) of the hcture tests was chosen to be 60' such that the upper

adherend in Fig. 5-3 carried a greater load, driving the crack to propagate in the primary bond. In

the present work, DCB specimens were always clamped in such a way that the primary bond was

on top of secondary bond and FpF2. The specimens were tested at 60' mode ratio so that the

results could be compared with those of ref. [3].

Chapter 5 Experimental Verification of Degradation Parameter

Fig. 5-3 DCB sample clamped on load jig showing crack path associated with

The load jig conditions for stable crack propagation were derived analyticaily in ref.

[22] . For a DCB specimen clamped on the load jig shown in Fig. 5-3, stable crack propagation

requires

a2(P-13)+2a(3+P)+(p-l3) s= 8(l+ a') + P(l+ a)' <O,

where

Forces F I , Fz, dimensions A, b, and initial crack length a are defmed in Fig. 5-3.

In the present work, A and b (Fig. 5-3) were set at 200 mm and 40 mm, respectively.

Samples were tested at three phase angles ((v): 0' and 48' to test for the effect of the secondary

bond (Chapter 2); 60' to test the DP verification specimens. The conditions for stable

propagation for the three phase angles are:

v O O , a=-1, a can be arbitraqq

@go, a=8, a296 mm;

v6O0, a=3, a2106 mm.

5.2.4 Tesiing Sckedules

Tables 5- 1 and 5-2 present the testing schedules of Cybond 4523GB and Cybond

I 126 specknens, respectively. 100% RH aging was eliminated because blisters had been

observed on those speciwns at both 6S°C and 85OC, and 60% RH aging of Cybond 1126

specimens was abandoned for as discussed in Appendix A. The 85% RH cases were added in

lieu of the 100% RH tests. 30% RH environment chambers were kept, but the specirnens aged in

these chambers refiected virtually only the effect of temperature because zero water absorption

was detected in the adhesives (for Cybond 1 126, see Appendix A).

The testing schedule shown in Table 5-1 was designed to address the following

aspects of DP vaiidation. First, different combinations of relative humidity and aging time yield a

series of different DP values, at which the hcture strength loss of the joints was measured.

Second, a certain DP value can be realized through different combinations of relative humidity

and aging time (e-g., specimens A65060260 and A65085093; specimens A85060235 and

A86085093). If the same DP value through different time and RH paths corresponds to the sarne

strength loss, this would be a f i d e r proof that DP uniquely correlates with the loss of hcture

strength. Third, certain specimens were aged through difEerent sequences of relative humidity;

e-g., a sample aged at 60% relative humidity for a period of time and then aged at 85% relative

humidity for another period (specimens A85(60+85)80 and A85(85+60)80). If this combined DP

corresponded to the same effect on the strength Ioss as a test at a single relative burnidity, and if

Chanter 5 Exmrhental Verifkation of Denradation Parameter

the order of aging stages does not affect the degree of degradation to joints, the DP concept

would be fiuther shown to be path independent.

Table 5-1 Testing schedule for Cybond 4523GB specimens, DP was calculated based on

the diffusion properties measured from fresh. a s t wafers (at 66%. D 4 . 9 ~ 1 ~ ~ '

m%, Ca=2.20)C with 86% RH and -.78% with 60% RH; at 8S0C, ~ 2 . 3 ~ 1 0 " '

m21sl Cs=2.90?+6 with 85% RH and Cs=1.15% with 60% RH)

Although it was expected that temperature has its own effect on the degradation

process of adhesive joints, this may be incorporated in its effect on the d i h i o n coefficient and

equilibrium water concentration. To check this possibility, the DP values were arranged to be

comparable between 65OC aged specimens and 85OC aged specimens in Table 5-1.

It has been noted that, due to the thin layer of adhesive on the open-faced specimens,

Cs is reached at a relatively short t h e ; thus, the complicated summation terni in the DP

expression, Eqn. (5-2), which is the characterization of the water and its time effect before

6S°C 85OC

DP (dayx%)

O

O

200

201

269

267

473

547

613

specimen

A65030090

A65030180

A65060260

A65085093

A65060346

A65085123

A65085216

A65085250

A65085280

specimen

A85030090

A850301 80

A85(60+85)80

A85(85+60)80

A85060235

A85085093

A85060294

A85060330

A85085 129

RH (%) 30

30

60

85

60

85

85

85

85

RH (%) 30

30

6W85

85+60

60

85

60

60

85

agingtime (&YS)

90

180

40+40

40+40

aging time (&YS)

90

180

260

93

346

123

216

250

280

DP (dayx%)

- -

199

199

235 1 269

93

294

267

337

330 378

129 1 377

Chapter 5 ExperÎmentai Vai6ication of Degradation Parameter

saturation, was negligible. In other words, DP=CSt is a good approximation for long-term and

short-diffusion-path aging. For instance, with an adhesive layer thickness of 0.4 mm, for Cybond

4523 GB specimens aged at 6S°C and 8S°C, the difference between DP caicdated using Eqn. (5-

2) and the value simply calculated fiom Cst will be less than 10% after approxirnately 8 days of

asinse

table 5-2 Testing scheduk for Cybond 1126 specimens ageâ rt 6S°C, DP was calculateâ

based on the diffusion properties measured from cast wafers ( ~ 1 0 . 2 ~ 1 0 - ' * m2/s,

Cs=3.62@h with 8Sah RH)

1 specimen 1 R H (%) 1 time (day) 1 DP (dayx%) (

Cybond 1 126 specimens were aged only at 6S°C (Table 5-2) because 85OC was well

beyond its glas transition temperature (T''a I0C for Cybond 1 126). Because diffuçion properties

of the adhesive at 60% RH, 6S°C were not available (see Appendix A), the same DP value fiom

different combinations of RH and aging time could not be attained.

Chapter 5 Expcrimental Verification of Degradation Parameter

5.3 Results and Discussion

5.3. I ControC Values of Gc for Joints of Cybond 4523GB and Cybond Il26

As mentioned in Chapter 2, fksh Cyband 4523GB joints were tested at phase angles

of 0" and 48'. The results of Gc at these two mode ratios were 2 16 ~ l r n ~ ( 5 9 ~ 1 4 . 2 J/m2, N=7),

and 347 .J/m2 (S.D.=lS.2 ~/rn?, N=6), respectively (Table 2-2). It is reported36 that Gc at 0°, 4g0,

and 60' were approximately 213 3/m2, 333 J/m2, and 405 Nm', respectively, and in ref. [3], Gc

values of undegraded specimens were found to be 343 ~/m' and 369 Urn2 at 48O and 60°,

respectively. This relatively good agreement shows that Gc values of undegraded C ybond

4523GB specimens are a robust measurement. One fiesh specimen (A5) of the same adhesive

batch as the secondary bond specimeas was tested at 60' and its undepded hcture strength Gc

was fomd to be 394 ~ / m ~ (S.D.=30.6 Um2, N=11) (Fig. 5-4). It should be noted that the adhesive

batch used in the fiesh joint testing in Chapter 2 was different fiom the batch used for DP

verification. But as shown above in the cornparison of the results of the present work and r&.

[36, 31, the batch-to-batch variance was not significant. Thetefore, 394 ~ / m ~ , the Gc value at 60'

measured fiom the adhesive batch used for secondary bond testing, was chosen to be the initial

value of the batch used for the DP verification.

Three single-bonded fresh specimem of Cybond 1 126 yielded the following Gc result

at a phase angle of 60': 2230 J/m2 (S.D.=S 1 ~ / r n ~ , N=3). It should be noted that the R-cwes of

Cybond 1 126 specimens did not show a plateau as seen in the undegraded Cybond 452368

cases. All three specïmens demonsû-ated an ascending trend of Gc with crack growth as shown in

Fig. 5-5. Gc of each specimen was taken as the average of al1 the data points measured after 12-

15 mm of crack propagation (&er precrack), therefore the S.D. of Gc within each specimen was

much greater (ranging nom 78-1 77 J/m2) than that among the three specimens.

Chapter 5 Experimeatal Verification of Degradation Parameter

e -- I crack tip ARmage zone development

- -2. - - - -2- !

Fig. 5 4 R-turve of an undegraded Cybond 4523GB DCB specimen (#AS, -0')

Fig. 5-5

a (ml

R-curve of an undegraded Cybond 1126 DCB specimen (-4, ~ 6 0 ' )

Chapter 5 Experimental Verifkation of Degradation Parameter

In cornparison, ref. [3] gave 595 ~ / r n ~ as undegraded Gc at +Hl0 (using Hysol

9309EA secondary-bonded specimeas). The data for @O0 are not available h m this reference.

Both undegraded and degraded results at yFO" from ref [3] are still cited in Fig. 5- 10 to compare

the trend of degradation at different phase angles.

5.3.2 Results of Degruded Cybond 4523GB and Cybond I I 2 6 Specimens

Compared with undegraded specimens, the crack tip development length

correspondhg to the rising part of the R-curve (the crack growth before reaching the critical

resistance, Gc, at the plateau) was generally slightly longer for aged Cybond 4523GB joints. A

typical hcture mistance c w e of a degraded Cybond 452368 specimen is shown in Fig. 5-6.

As seen in Fig. 5-4, the crack tip damage zone in an undegraded Cybond 4523GB joint

developed over a crack length of about 5-7 mm, while in a degraded joint, the crack length of

crack tip damage zone development was approximately 10 mm.

Some Cybond 4523GB specirnens, aged both at 65°C and 85°C with both low and

high RH (30% and 85%), did not show a clear plateau (Fig. 5-7). The Gc values of these

specirnens not having a Gc plateau on the R-curve was the average of the data points after

approximatelyl O mm crack growth after the initial chisel precrack.

Chapter 5 I3pimental Vdlcation of Degradation Paramaer

crack tip damage zone 1

Fig. 5-6 R-cuwe of a degnded Cybond 452308 DCB specimen (#A85085280-3, tp60°)

\

crack propagation

0 \ crack tip damage zone deveiopment .

Fig. 5-7 Rcuwe of a degraded Cybond 4S2MB DCB specimen (#A60085216-2, @oO)

Like mdegraded Cybond 1 126 speciwns, the degraded ones also demoastrated an R-

curve without a well-defined plateau. It was also observed with the degraded specimens that the

slope of the R-cwe did not always decrease over the measurement length. A typical R-cuve of

a degraded Cybond 1126 specimen is shown in Fig. 5-8. The Gc values for these specimens were

taken as the average of data points d e r the crack had propagated 12-1 5 mm fiom the initial

chisel precrack-

Fig. 5-8 Rcurve of r degraded Cybond 1126 DCB spacimen (#B65085250-2, p60°)

It therefore seems that aging changed the fracture resistance patterns of both Cybond

4523GB and Cybond 1 126 specimens, although the mechanisms of these changes are Imknown.

Tables 5-3 and 5-4 present the average Gc comsponding to various DP for Cybond

4523GB and Cybond 1126 specimens, respectively. The Gc sbown in the tables are the averages

of the average Gc values for the Uidicated number of DCB specimens cut fiom the same plate.

Chapter 5 Experimental Verscation of Degradation Parameter

Table 5-3 DP and average Oc of Cybond 4523GB specimens degraded at 6S°C and 8S°C

and tested at d y sta1e and -0' (Oc of undegraded Cybond 4523308 spcimens

was 394 ~ l r n * )

. - -

Figures 5-9 and 5- 10 show the relationships between normalized Gc (G~/G=', where

G ~ O is the undegraded value of Gc at p60° ; in the present work, 394 ~ / r n ~ for Cybond 4523GB

and 2230 ~ l r n ~ for Cybond 1 126) and DP of Cybond 4523GB specimens and Cybond 1 126

specimens, respectively. Data at a 60' phase angle are not avaiiable for Cybond 1 126 fiom ref.

[3]; instead, results for a O0 phase angle were presented for cornparison ( G ~ O at @O0 was 595

~/rn') in Fig. 5-10.

Table 5-3 and Fig. 5-9 show that the aged Cybond 4523GB specimens prduced

relatively large scatter. with larger variation at 85OC than at 6S°C. Large variability could be seen

even in a single plate (#A85060294, DP-337 %-days). This is due to the nature of bcture and

aging tests and was expected. Cs also contributes to the scatter since it is sensitive to the relative

humidity, and fluctuations of RH in the environment chambers were up to 5%. in Chapter 3, it

plate specimen

A65030090

Gc (l/m2) 452

DP (&yx%) -

plate specimen

A85030090

A65030180

A65060260

S.D. (l/m2)

5.7

S.D. (J/mZ)

4

DP (day~o/)

-

No. ofDCB specimens

2

No. of DCB specimens

2

- 16

8.5

21

64

9.7

Gc (J/m2)

393

- 200

76

27

2.8

44

16

479

334

3

2

2

3

3

318

317

398

364

354

288

A65085093

A65060346

A65085123

A65085216

A65085250

A65085280

1

3

2

3

2

3

201

269

267

473

547

613

- 199

199

269

267

337

A85030 180

A85(60+85)80

A85(85+60)80

A85060235

A85085093

A85060294 112

16

18

411

495

265

237

312

279

378

377

A85060330

A85085 129

3

3

2

40

23

309

338

3

3

Chapter 5 Jkperhental Verification of Degradation Parameter

was observed that the ratios of Cs for Cybond 4523GB at 100% RH, 85% RH, and 60% RH were

approximately 4.5:2.7:1 at both 6S°C and 8S°C. Moreover, Cs itselfmay have increased with

t h e and forniaton of closed joints, M e r complicating the calculation of DP; ic., DP in these

tables and figures was calculated using the nominal d u e of Cs fiom fksh cast wafers (Tables 3-

8 and 3-1 O), not the actuai joint values.

Table 5 4 DP and average Ge of Cybond I l 2 6 specimens degraded at 6S°C and tested at

dry state and p60° (The Gc value of undegraded Cybond 1426 specimens was

*AU of the three specimens cracked at the secondary bond

Another general trend seen in Fig. 5-9 is that Gc of the aged specirnens decreased

with DP (Gc dropped to approximately 75% of G=O at 6S°C for the highest DM 13 %&YS, and

to 80% of at 85OC for the bighest DP377 %-&YS. Path independence was observed at 6S°C,

DP=200 %-&YS (specimens A65060260 and A65085093) and 8S°C , DP=378 %-days

(specimens A85060330 and A85085 129). Path independence was, however, not seen at 85'C,

DP499 %-days (specimens A85(60+85)80 and A85(85+60)80), and 85'C, D e 2 6 9 %-days

(specimens A85060235 and A85085093). A t-test assuming that the variance of each group is

No. of DCB specimens

1

DP (Aayx%)

- Gc (~lm') 1345

S.D. (~lrn~)

-

Chapter 5 Experimental Vcrificaîion of Degradation Parameter

unknown and unequal confümed that the means of Gc of specimens A85(60+85)80 and

A85(85+60)80 are different at 95% of confidence level, and those of specimens A85060235 and

A85085093 are different at 90% of confidence level.

Fig. 5-9 The nlationship between G ~ G : and DP of Cybond 452308 specimens degradeci

at 6S°C and 8S°C, and tested at d y state at yp60°. Each date point corresponds to

the resuît of a DCB specimen (The of the present work was 394 ~lrn', and the GC*

of ref. [3] was 405 ~lm'. bpresent woik, 6S°C; x-present work, 85OC; A-mf. [3],

6S°C)

Cbapter 5 Expaimentai Verindon of Degradation Parameta

It can be seen from the resuits of Cybond 1126 specimens (Fig. 5-1 0) that the

correlation between DP and the loss of fracture strength is clear and, except for DP=529 %-&YS

(specirnen B65085 153, Table 5-4) foilowed the trend of ref. [3]. As expected, the data for (v=OO

fiom ref. [3] Lie below those for yr=60°.

It should be noted that aU the specimens tested for DP verification were non-blistered,

Le., aged at 85% RH or 65% RH.

Fig. 5-10 The relationship between Gc and DP of Cybond 1126 specimens degraded at

6S°C and tested at dry state at @O0 and p O O . Each data point hom the present

work corresponds to a DCB specimen (x-present work, -O0, 0='=2230 ~lrn'; A-

ref. [3], p=OO, 0c0=595 ~ l m ' )

Chapter 5 Experhental Verification of Degradation Paramder

Specimens aged at 30% RH were effectively subject to only a temperature effect

because Cs was close to zero at this RH. Of the Cybond 45323GB specimens aged at 30% RH,

A65030090 and A65030180 (both aged at 65OC) reached a higher Gc (452 ~ / r n ~ and 479 J/m2)

than the control value (394 ~/m'), while the 85OC specimens yielded an unchanged Gc; 393 Urn2

and 41 1 ~/rn~com~ared with ~='=394 J/m-

in addition, the specimens which had the same DP but were aged at different

temperatures (data of DP=200,201,199 %-&YS, and -267,269 %&YS at both temperatures

Table 5-3 and Fig. 5-9) did not seem to show any Gc pattern whiçh could attribute to

temperature.

It shouid be noted that the secondary bond specimens tested in Chapter 2 were al1

6esh ones, and so it is only assumed that the conclusions drawn in Chapter 2 are valid for the

testing of the aged specimens in the dry state.

5.4 Conclusions

The present experimental study has led to the following conclusions conceming the

validity of DP concept and the use of open-faced specimens:

1. Gc decreases with DP but more data, especiaiiy, fiom other adhesives for longer

times, are required to fûrther establish the correlation.

2. 100% RH at 65OC and 8S°C leads to blistering and open-faced approach cannot be

used to measure Gc degraded by bound water.

3. Large scatter complicates the correlation between DP md Gc.

4. Path independence may exist but more data are needed to confirm this.

Chapter 6 Conciusions and Future Work


The present work has dealt with several aspects of the assessment of adhesive joint

durability of aqueous environments using accelerated aging approaches. The accelerated aging

techniques adopted in the present work included elevated temperature, raised relative humidity,

and shortened water ciBûsion path (direct exposure of adhesive layer using open-faced

specimens). To validate the open-faced specimen technique, secondary bond effecl on the

measurement of Gc were investigated. The concept of the degradation parameter was studied in

greater detail to reveal the correlation between hcture strength lose of adhesive joints and the

combined effect of water and its aging tirne. More impottantly, the patterns of water diffusion in

b o t . buk adhesives and closed joints under short and long-tenn aging were examined to gain

knowledge about water transport and distribution in adhesive joints. Blisterhg was

systematically studied as a form of degradation in adhesive joints.

The detailed conclusions of the individual topics were reported în theu respective

chapters. The following conclusions and recommendations are made to sum up the present work

fkom the broader perspective of the assessment of adhesive joint durabiliîy using accelerated

aging approaches.

Chauter 6 Conclusions and Fidure Work

6.1 Conclusions

1. As an accelerated aging technique, open-faced specimens can be w d to test the

bcture strength of the primary bond closed using a secondary bond. SpecSdly , the extra

curing cycle, at a temperature lower than that of the primary bond, and the increased bondline

thickness does not significantly affect the h c t w e behavior of the primary bond.

2. The formation of closed joints changes the pattern of water diaision in the

adhesive. However, water transport and distri'bution in adhesive joints can be, within reasonable

accuracy, predicted using Fick's law and diffusion properties measured fiom the closed joints.

Generaily, closed joint has a higher equilibriurn water concentration and a iarger water diffusion

coefficient.

3. Blistering is a form of degradation in which delamination occurs in both open-

faced specimens and closed joints exposed to high relative humidities (greater than

approximately 85% in the present work). The cause of blistering is osmotic pressure due to

impurities or ionic species leached fkom the adhesive iayer. Voids on the adhrends or in the

adhesive, such as pits or air bubbles, serve as sites for water clustering and subsequent osmotic

blister growth.

4. The degradation parameter concept proposed in ref. [3] requires M e r evaluation

with other adhesives over longer exposure periods.


6.2 Future Work

1. The effect of long-term aging on the diaision properties of adhesives should k

examined M e r .

2. Water transport and distribution in closed joints should be M e r investigated as

a function of themial or swelhg stress in the adhesives, and a f'unction of aging tirne.

3. The possible existence of interfacial diaision should be investigated using non-

destructive techniques such as neutron radiography.

4. The investigation of blistering should be extended to the nequency of blistering

and the rate of blister growth in open-faced specimens and closed joints.

5. The degradation parameter should be explored M e r over longer tirnes with

other adhesives.

References

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Adhesion, 1980, Vol. 1 1, pp3-15.

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Appendix A: Water Diffusion into Cybond 1126 at 65OC

under 30% REC and 60% RH

The Cybond 1 126 was also tested at 65OC with 30% and 60% RH. Figures A-1 and A-2

are the plots of water uptake in percentage of dry adhesive mass (MJMo) versus rwt time at these

two conditions.

mot time (s'O)

Fig. A-1 Water uptake of a Cybond 1126 wafer at 65%. 30% RH

It is understandable that the adhesive absorbs virtudy no water at 30% RH; Fig. A-1

shows that the mass gain was fluctuating within the range of *O.4%. What seemed inexplicable was

the decreasing mass of the adhesive at 60% RH. This raiseci the question as to whether the adhesive

had a zero initial water content d e r 24 hour dryhg at 30-50°C.

To eliminate the effect of potential initial water content on the absorption behavior of

Cybond 1 126, five 0.4 mm thick w a k samples were tested at bth 30% and 60% RH after 24 hour

drying at a higher tempera- (65OC). The resuits (Table A-1) suggest that the new process

produced a drier state of the adhesive, so that the wafer absorbed a significant amount of water at

30% and 60% RH. The samples tested at the same relative humidity were exposed in the same

environment chamber; the reason why sample 5 had a higher water content than sampies 3 and 4 is

unknown.

O 300 600 900 1200 1500 1800

mot time (sIR)

Fig. A-2 Water uptake of a Cybond 11 26 wafer at 6S°C, 60% RH

The dinusion propeity testiag samples were cirieci at 30-50°C (see Sdon3-3); therefore

the initiai water content of those sarnple couid be different due to the changing temperature- If it

happened that the initial water content of the sample shown in Fig. A- 1 was lower than that of the

samples shown in Fig. A-2, this may explain why the 30% RH sample did not lose mass, while the

60% RH sarnple did.

The results shown in Table 3-1 1 might have been aEected by the iower temperature

drying, but as ali the degradation speciimens were dried befDre aging m the same way, the diffusion

properties obtained h m these tests should be valid for the calculation of degradation parameter.

Table A-1 Low relative humidity (30% and 60°h) water uptake of Cybond 1126 wafers

exposed a 6S°C after 24 hour 65'C drying

treatment and t h e

mass (g)

24 hours of curing at room temperame 24 hour cfrying at 65°C (starting of water absorption test) 33.5 hours of water

60?! sample 3 sample

30% RH

%of maSS

absorbing 1 1 1

sample 1

mas (g)

428500

420542

4.22527 1 1

72.5 hours of water absorbing 98.5 hou- of water

sampte 2

mass (g) %of aiass

absorbing 123.5 hours of water

%of maSS

-

O

4-21 863

4 3 1426

absorbing 1 5 1 hours of water

4.21 272

435900

428943

0.3 1

0.21

4-21 000

244 fiours of water absorbing 3 15.5 hours of water absorbing

0.47 14.3071 1

0.17

-

-

-

-

-

O

4.30302

4.30003

O. 1 1

0.41

4.29842

2.6518

2.60129

0.32

0.25

4.293 15 I

-

O

2.6356

0.21

2.62935

2.64588

-

-

1-32

2.62982

2.62885

0.09

1 .O8 2-87548

1.71 2.87614

- - - - -

-

1.10

1 .O6

2630% 1.14

2.62971 1.09

Appendùr A

Conceming the testing of Cybond 4523GB, because this is a 150°C cured adhesive and

its a i o n coefficient is aboui 10 time as that of the Cybond 1 126, the zero-water-content

assumption should be a very good approximation of its real initiai state.

WATER ABSORPTION AND DEGRADATION IN ADHESIVE …

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