The effect of humidity on the durability of aluminium-epoxide joints

The effect of humidity on the durability of aluminium-epoxide joints D.M. Brewis*, J. Comyn*, A.K. Raval* and A.J. Kinloch t (*Leicester Polytechnic, UK/*lmperial College, London, UK)

Single lap joints of sandblasted aluminium alloy have been prepared with an epoxide adhesive based on DGEBAand 1, 3-diaminobenzene. They have been exposed to air at 500C and to a range of humidities for up to 10080h, and then tested. After 10080h ageing the results give some evidence for a critical relative humidity at 65%, such that joints weaken to a greater extent i f this value is exceeded. Measurement of mass uptake of films of the adhesive suspended in air at various humidities has provided the water absorption isotherm and diffusion coefficients. The isotherm is gently curved and of BET type IV; there are no sharp changes that might account for a critical relative humidity. The presence of salt hydrates at the interface is a feasible explanation for this.

Key words: lap joints; epoxide adhesive; humidity; durability; diffusion; absorption isotherm; critical relative humidity

When adhesive joints with metallic adherends are exposed to air of high humidity (eg 80-100% relative humidity (Rn)), they weaken with time. This fact is widely known and has been reported in the literature and covered in a review I. In contrast, it has been frequently observed that joints can withstand exposure at lower humidities (eg 50% RH or less) for long periods without weakening. For example DeLollis 2 has referred to some epoxidealuminium joints which showed no loss of strength after exposure to laboratory humidity for up to 11 years. Gledhill, Kinloch and Shaw 3 exposed butt joints with an epoxide adhesive at 55% ~H and 20°C for 2500 h and found no weakening. In experiments on Ciba-Geigy Redux 312 and Cyanamid FM1000 adhesives both with and without carders, and on an unmodified epoxide adhesive, Comyn and Brewis and their coworkers*'7 found no significant weakening of joints after exposure for 10 000 h at about 45% RH and 20°C. Such information led to the proposal from Kinloch et al. 3 that there must be a critical concentration of water in the adhesive which demarcates conditions under which weakening will occur from those under which it will not. In a joint which is absorbing water there may be an outer zone where the critical water concentration is exceeded, and this zone can be

regarded as a crack in the bondline which can be dealt with by fracture mechanics. The hypothesis was tested with some butt joints bonded with an epoxide based on the diglycidylether of bisphenol-A (DGEBA) and the 2-ethylhexanoate of 2, 4, 6-tris(dimethylamino)-phenol, immersed in water at 20, 40, 60 and 90°C and also in air at 20°C and 55% RH. All the water-immersed joints became weaker, and it could be shown using the fracture mechanics approach that the strengths of the joints employed in this study could be correlated if the critical concentration of water in the adhesive was 1.35%. If an adhesive joint is stored in air at constant temperature and RH, then eventually the adhesive layer will reach a state of equilibrium with the wet air, such that there will be a uniform concentration of water in the adhesive. If there is a critical water concentration there will also be a critical RH, and the purpose of this investigation was to find the critical RH for some aluminium joints bonded with an epoxide adhesive. The adhesive chosen was the diglycidylether of bisphenol-A cured with 1, 3-diaminobenzene. Surface preparation of the aluminium alloy adherends was by sandblasting: this was chosen because of the poor durability of sandblasted joints, a factor which we thought would give rapid results which would be particularly sensitive to changes in relative humidity.

0143-7496/90/040247-07 © 1990 Butterworth-Heinemann Ltd INT.J.ADHESION AND ADHESIVES VOL. 10 NO. 4 OCTOBER 1990

247

Adhesive joints were stored in humid air at 50°C, a temperature which has been widely used in accelerated ageing experiments.

Experimental Water uptake by films of the adhesive The adhesive used was the diglycidylether of bisphenol-A (Epikote 828) cured with 13.0% by weight of 1, 3-diaminobenzene (DAB). The molten DAB (melting point 60-65°C) was mixed into the resin and the mixture was warmed for a few minutes to allow any bubbles to disperse. Films of the adhesive were prepared by spreading the warm adhesive using a thin layer chromatography spreader, on to tin foil. The tin foil, 0.13 m m thick, had been rolled onto a sheet of glass which had been sprinkled with water, which leads to easier removal of the cured films. It was heated to 80°C before spreading the adhesive. Whilst supported by tin foil and glass, the adhesive was cured in a laborator~ oven for 2 h at 80°C followed by 2 h at 150°C °. After cooling, the adhesive films were peeled from the tin; the presence of rolling lubricants on the tin probably eases peeling. Samples measuring about 10 × 20 m m were cut, and their thicknesses measured at six places with a screw micrometer graduated to + 0.01 mm. A C.I. Electronics vacuum microbalance connected to a chart recorder was used for uptake experiments from water vapour 9. Film thicknesses were in the range 0.11-0.17 mm. The vapour pressure of water was lowered by adding glycerol, and was measured with a mercury manometer. The whole assembly was contained in an air thermostat operating at 50 + 0.2°C.

sand at a pressure of 760 kPa, wiping again with trichloroethene and drying at room temperature for 15 rain. The adhesive was prepared as described above, except that 1% of ballotini glass spheres (107-125 pm diameter) were added to control bondline thickness. Two alurninium panels were bonded together along their long edges with 12.5 m m overlap, with the use of specially constructed jigs 4 in which they were assembled and cured. Curing was at 80°C for 2 h followed by a further 2 h at 150°C in a Daniels press using a pressure of 0.05 MPa. The panels were left to cool overnight in the press and were then cut into individual single lap joints with 12.5 × 12.5 m m overlap using a bandsaw with compressed air as a coolant. The blade had 550 teeth m - ~, and a low band speed (2.5 m s -1) and slow cutting speed were used to avoid introducing stresses to the joints. The first three and last three joints from each panel were discarded, six were picked at random from the remainder and were tested as controls, and three were used for the measurement of bondline thickness. The rest were used in the humid ageing experiments. A Vickers projection microscope was used to measure bondline thickness and this was found to be in the range 124 + 3 pm. Joint testing was conducted using a Monsanto tensometer, using a cross-head speed of 0.08 m m s-I. Joint failure was apparently interfacial in all cases. Joints were aged in desiccators containing glyceroldistilled water mixtures, which gave relative humidities in the range 23-95%, or containing distilled water. Each joint was suspended vertically in a desiccator, and the latter were kept in an air-circulating oven held at 50 + I°C. At selected times six joints were removed from each desiccator for testing.

Mechanical properties of wet and dry adhesives A compression test was used to compare the mechanical properties of dry and wet adhesives. Cylindrical samples 8 m m in diameter were cast in a brass mould which had been coated with Ciba-Geigy Releasil 7 and Araldite Q213 mould release agents. The adhesive was mixed and cured as already described, and the cured cylinders were pressed out of the mould and turned flat at the ends so that the final lengths were about 12 mm. Samples were compressed using a Mayes M P U 500 mechanical testing instrument using a cross-head speed of 2 m m m i n - I . Six samples were tested dry but another six were exposed to water at 100°C in a Soxhlet apparatus for 500 h. Previous work l° has shown that D for water in this adhesive at 100°C is 4.9 X 10 -12 m 2 s - l and M E is 1.9%. Use of Equation (4) (see later) shows that the fractional uptake for axial diffusion into the cylinders is 0.803, and the Hill equation II shows that the fractional uptake for radial diffusion is 0.97. Treating these values by the same reasoning that leads to Equation (6) means that the overall fractional uptake is 1 - (1 - 0.803) (1 - 0.97) = 0.994, so that the cylinders are saturated with water for all practical purposes.

Adhesive joints Aluminium alloy BS3L152, 1.8 m m thick, was cut into panels 460 × 75 mm, and surface treated in the following manner: wiping with a tissue soaked in trichloroethene, sandblasted for 2--4 s with 60-80 mesh

248

INT.J.ADHESION AND ADHESIVES OCTOBER 1990

M a t h e m a t i c s of diffusion Fick's first law is the fundamental law of diffusion. It states (Equation (1)) that the flux in the x-direction (Fx) is proportional to the concentration gradient (de~ dx). D is the diffusion coefficient.

Fx = - D ( d c / d x )

(1)

Flux is the amount of substance diffusing across a unit area in unit time. The first law can only be directly applied to diffusion in the steady state, that is where concentration is not varying with time. Fick's second law of diffusion describes the nonsteady state and it has several forms. The form of relevance here is

dc/dt = D(d2c/dx 2 + 02clOy 2 + d2c/dz 2)

(2)

This describes the build up or decay of diffusant with time (dc/dt) at a point in Cartesian space. Under circumstances where diffusion is limited to the x-direction it simplifies to Equation (3):

dc/dt = Dd2c/dx 2

(3)

The problem now is to seek solutions to Fick's second equation, and these can be found in Crank's book 12 on the mathematics of diffusion, or in that by Carslaw and Jaeger 13 on the analogous situation of heat flow in solids. One solution (Equation (4)) is for a thin film of a permeable material immersed in a liquid or a vapour

15

at constant pressure. M T is the mass absorbed at time t and ME is the mass absorbed at equilibrium; the ratio MT/ME is known as the fractional uptake. L is the film thickness.

MT/ME = 1 -- Z

8 e x p ( - D ( 2 n + l)2zr2t/4L2)/

n=0

(2n + 1)2zr2

(4)

For short times Equation (4) simplifies to Equation (5)

MT/M E = 4(Dt/n) l/2/L

(5)

Equation (4) describes unidirectional diffusion normal to the surfaces of a film or slab, but it can be used for two-dimensional diffusion into a square adhesive layer in a joint by using Equation (6). [1 - (MT/ME)2D ] = [1 -- (MT/ME)ID] 2

% 10 T O

o----"

(6)

Here (MT/ME)ID is the fractional uptake for a slab of thickness equal to the width or length of the adhesive layer, which can be calculated from Equation (5), and (MT/ME)2D is the fractional mass uptake for the adhesive layer.

l

5 0

50 Relative humidity (%)

100

Fig. 1 Mean diffusion coefficient of water into films of epoxide adheisve, as a function of relative humidity at 50°C

Results and discussion Water uptake by adhesive films

All uptake plots from water vapour in adhesive films were Fickian 14 with a single uptake stage. The plots of MT/ME against the square root of time were linear over the range 0
Relative humidity (%)

0 ( 1 0 -13 m 2 s -1 )

ME(% )

23 42 66 72 83 86 95 1 O0

7.2 7.4 8.1 8.5 9.7 11.0 12.0 14.0

0.54 0.84 1.5 1.6 2.0 2.0 2.1 2.1

2.0

1.0

/

/

/

/

/

/

/

I

50 Relative humidity (%) Fig. 2

100

Absorption isotherm of water by the epoxide adhesiveat 500C

for by Schroeder's paradox 15. The latter is observed when equilibrium mass uptake by samples of the same polymer is greater from the liquid than from the saturated vapour at the same temperature. The source of the paradox is the difficulty in obtaining truly saturated vapours. D increases slightly with water concentration (Fig. 1) and the sorption isotherm (Fig. 2) is a BET type IV r6. A number of water absorption isotherms have been published for epoxide adhesives. For example, that of DGEBA cured with di(1-aminopropyl-3-ethoxy)ether at 48°C is gently curved 9 with the slope increasing with m~. The isotherm at 50°C for the epoxide based on

INT.J.ADHESION AND ADHESIVES OCTOBER 1990

249

DGEBA cured with triethylene tetramine has a similar shape up to 80% RH, but then the slope decreases slightly so that the whole curve is sigmoid 17. The water absorption at 25°C of the epoxide from DGEBA and diaminodiphenylsulphone is linear 18. The point to be taken from this information is that water absorption isotherms of epoxide adhesives do not show any sharp changes that might be the cause of the critical gn. Even if the isotherms did show any discontinuities, they would not account for a critical RH. This is because the joints all failed interfacially, so the main role of the adhesive in durability is in transmitting water to the adhesive interface. Water in the surroundings would eventually reach equilibrium with water in the adhesive, and also with water at the interface. Hence the equilibrium distribution of water between the surroundings and the interface will be independent of the adhesive. This is a consequence of the zeroth law of thermodynamics. Mechanical properties of wet and dry adhesives

A typical load-displacement curve for the compression test is shown in Fig. 3, and Table 2 gives the various parameters with standard deviations for the wet and dry samples, and, as would be expected, it can be seen that water has a plasticizing action on the adhesive. The 1.9% of water which is absorbed increases yield strain, but decreases yield stress, modulus and rupture stress. The rupture strain is unaltered. The fact that the properties of an adhesive change due to plasticization arising from uptake of water does mean that joints which have been exposed to moisture may well exhibit different failure strengths due to this effect, apart from the aspect of environmental attack and weakening of the interface. J o i n t strengths

The strengths of joints as a function of RH, with error bars giving standard deviations, are shown in Figs 4-9, for exposure times of 192 to 10080 h. From both a visual assessment and scanning electron microscopy studies, the position of failure was interfacial between the adhesive and aluminium oxide. Joints stored for

Table 2. Effect of water on the mechanical properties of the adhesive Property

Dry sample

Wet samples

Modulus (GPa) Yield stress (MPa) Yield strain Rupture stress (MPa) Rupture strain

1.40 124 0.090 169 0.42

0.80 104 0.130 163 0.41

+ 0.06 +_ 1.2 + 0.004 + 8 + 0.02

+ + _ _ +

0.05 1.2 0.010 12 0.02

192, 504 and 1008 h do not weaken with increasing RH. A slight weakening is evident with the 2016 h joints, and this becomes greater at 5040 and 10080 h. At 2016 and 5040 h it is possible to place a straight line through the data, but at 10080 h the line has a kink at 65% RH. The values of (MT/ME)2Dfor the adhesive joints at different times and relative humidities are shown in Table 3. These have been calculated from the values of D in Table l using Equations (6) and (4). The values in Table 3 are a measure of the mean level of joint saturation, such that a joint is saturated with water at (MT/ME)2D 1. The value of (MT/ME)2Dnever exceeds 0.65 in the first 1008 h of exposure; this means that the average level of water never quite reaches that attained by a joint which has equilibrated with water at 55% RH, although the mean level will exceed the value of 0.65 in the outer regions of the joints. With joints exposed for 2016-10080 h the value of 0.65 is exceeded and weakening now takes place. The plots in Figs 4-9 are collected together and compared in Fig. 10. This shows that the data at 192, 504 and 1008 h have the same strengths. However, joints become progressively weaker after 2016 and 5040 h, and after 10080 h there is a clear discontinuity at 65% Rn. This means (see Fig. 2) that the critical concentration of water in the DGEBA-DAB adhesive is =

Rupture J< "10 ¢3 o

== o

o

Y o

LL

O O Displacement Fig. 3 Typical load-displacement plot for compression testing of dry and wet adhesives

250

INT.J.ADHESION AND ADHESIVES OCTOBER 1990

I 50 Relative humidity (%)

I 10(

Fig. 4 Dependence of joint strength upon relative humidity, after 192 h exposure

adhesive may be changed by the formation of cracks or crazes, by hydrolysis or more generally by plasticization. Water may also be the cause of swelling stresses. Plasticization will occur to some extent at all relative humidities (the evidence for this is Fig. 2). However, as demonstrated by the absence of any kink in Fig. 2, a critical relative humidity does not exist in the absorption properties of the bulk adhesive. Attack at the interface will involve the displacement or weakening of van der Waal's interactions, hydrogen

J¢ "o o

o

_= .m I.l_

0

t 50

1 100

"O O

_o

Relative humidity (%) Fig. 5 Dependence of joint strength upon relative humidity, after 5 0 4 h exposure

1B LL

0

O

I 50 Relative humidity (%)

I 100

Fig. 7 Dependence of joint strength upon relative humidity, after 2 0 1 6 h exposure

o

LL

0

I 50 Relative humidity (*/.)

1 100

Fig. 6 Dependence of joint strength upon relative humidity, after 1 0 0 8 h exposure

..to "10

_90 *,= w

,,o

1.45%, a value which is very similar to that of 1.35% obtained by Kinloch and his coworkers 3. In Table 3 a line has been drawn to separate those joints in which the mean water concentration exceeds 1.45% from those where this limit is not exceeded. The limit is not exceeded during ageing for 192, 504 and 1008 h, or at 23 and 42% ~H and it can be seen from Figs 4-9 that it is these joints which do not weaken. Water may weaken joints by either modifying the properties of the adhesive or the interphase; the subject has been reviewed by one of us 1. The properties of the

0 0 Relative

I 50 humidity

I 100 {%)

Fig. 8 Dependence of joint strength upon relative humidity, after 5040 h exposure

I N T . J . A D H E S I O N A N D A D H E S I V E S OCTOBER 1 9 9 0

251

192 and 504 h

(3 _9 O

==

1008 h

•. - - ~ . . - . : ~ . . . .

.a¢

5040 h~ " ~ ..~"" " ~ . . .

1.5

,oo

o LL

o0 t.

1.0

1-

_=

[

0

50 Relative humidity (%)

o LI.

Comparison of plots in Figs 4 - 9

Fig. 10

0

I

I 50

0

0.8 m m Hg as CuSO4.H20 loses water, and finally fails to zero when all the water is lost. These changes are illustrated in Fi~. 11. Calcium chloride behaves in a similar manner 2° with the following equilibria and vapour pressures at 25°C.

100

CaC12.6HzO = CaCI2.4H20 + 2 H 2 0 CaCI2.4H20 = CaCI2.2H20 + 2 H 2 0 CaC12.2H20 = CaCI 2 + 2 H 2 0

Relotive humidity l%) Fig. 9

Dependence of joint strength upon relative humidity, after

10080 h exposure

bonds or ion-pairs. Alternatively the oxide layer itself might be subject to hydration and loss of mechanical integrity. If any of these mechanisms occur only when the relative humidity exceeds 65%, then we have an explanation for the kinks in Fig. 9. The critical relative humidity in turn could be explained by a discontinuous p h e n o m e n o n at the interface. The hydration of salts is such a phenomenon, and this is now described by reference to the example of copper sulphate 19. At 25°C the vapour pressure of water in equilibrium with CuSO4.5H20 is 7.8 m m Hg; this corresponds to RH = 32.9% at this temperature, where the saturated vapour pressure of water is 23.73 m m Hg. As CuSO4.5H20 dehydrates to CuSO4.3H20 the vapour pressure remains constant, but it then falls sharply to 5.6 m m Hg (art = 23.6%). It stays constant at this value as CuSO4.3H20 dehydrates to the monohydrate, but then falls to 0.8 m m Hg (RH = 3.4%). It remains at Table joints

3.

Values

of

100

(MT/ME)2Df o r

the

5.1 m m Hg 3.4 m m Hg 2.0 m m Hg

Glasstone m has emphasized that 'the existence of a definite vapour pressure of a salt-hydrate system requires the presence of two solid phases in addition to the vapour, for only then is the system univariant'. He adds 'the vapour pressure for the system is then definite and independent of the amounts of the two hydrates'. Salts, or more specifically cations which are much more susceptible to hydration, might arise at the surface of aluminium-epoxide joints in a number of ways. These are as sodium chloride byproduct from the synthesis of the diglycidylether of bisphenol-A, aluminium ions or ions from the water used to rinse the adherends. The m a n n e r in which a critical relative humidity would occur are as follows. If joints are in air at a low relative humidity/vapour pressure, which is lower than one of the definite 10 - 40

adhesive -r

E

23 42 66 72 83 86 95 100

E ==

Exposure time (h)

RH (%)

192

504

1008

2016

5040

10080

0.24 0.24 0.25 0.26 0.27 0.29 0.30 0,32

0.37 0.37 0.39 0.40 0.42 0.44 0.46 0.49

0.50 0.50 0.52 0.53 0.56 0.59 0.61 0.65

0.65 0.87 0.98 0.66 0,88 0.98 0.69 0.90 ~ 0.98 0.70 [ 0.91 0.99 0.73 j 0.93 0.99 0.76 0.95 1.00 0.78 0.96 1.00 0.82 0.97 1.00

E

5-

- 20

o lZ:

g

C 5

I

, t

I

I

4

3

2

1

Moles To the left of the dotted line mean water concentration in joints does not exceed the critical level of 1.45%

252

INT.J.ADHESION AND ADHESIVES OCTOBER 1990

==

0 0

H z O to 1 mole C u S 0 4

Vapour pressur~9of water in equilibrium with copper sulphate hydrates, after Glasstone Fig. 1 1

vapour pressures the salt hydrate, then as the relative humidity of the air rises to above the next definite vapour pressure, water will be absorbed by the salt hydrate and the latter will be converted to the next hydrate. This process will be repeated as each level is filled, but once the highest hydrate has been formed, water molecules can no longer be absorbed and they will be free to attack and weaken the interface.

Conclusions • The water absorption isotherm of the DGEBA-DAB adhesive at 50°C is a gently curved type IV BET. • Joints prepared using sandblasted aluminium alloy exposed at 50°C and relative humidities in the range 23-100% are not weakened after 192, 504 and I008 h, but some weakening sets in after this time at higher humidities. • At 10080 h there is a clear critical relative humidity of 65%, which corresponds to a critical water concentration in the adhesive of 1.45%. This critical level of water concentration is supported by the results from the 192, 504 and 1008 h ageing tests and at 23 and 42% relative humidity, since in these joints it has been calculated that the mean water concentration does not exceed 1.45%. • The mechanism of environmental attack which only operates in these joints above a critical water concentration of 1.45% is suggested to be associated with attack and weakening of the interphase regions of the joint. Salt hydration is a feasible explanation for this mechanism and for the existence of a critical relative humidity.

References 1

Comyn, J. Durability of Structural Adhesives (Ed. A.J. Kinloch) (Applied Science Publishers, 1983) chap 3

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

DeLollis, N.J. Nat SAMPE Symp Exhib 22 (1977) p 673 Gledhin, R.A., Kinloch, A.J. and Shaw, S.J. J Adhesion 11 (1980) p 3 Brewis, D.M., Comyn, J., Cope, B.C. and Moloney, A.C. Polymer 21 (1980) p344 Brewi$, D.M., Comyn, J., Cope, B.C. and Moloney, A.C. Polymer 21 (1980) p 1477 Brewis, D.M., Comyn, J., Cope, B.C. end Moloney, A.C, Polymer Eng Sci 21 (1981) p 797 Brewis, D.M., Comyn, J. end Tegg, J.L. Int J Adhesion Adhesives 1 (1980) p35 Dannenberg, H. and May, C.A. Treatise on Adhesion and Adhesives (Ed. R.L. Patrick) (Marcel Dekket Inc, 1972) Vol 2, chap 2 Brawis, D.M., Comyn, J. and Tegg, J.L. Polymer 21 (1980) p 134 Brawls, D.M., Comyn, J., Shalash, R.J.A. and Tegg, J.L. Polymer 21 (1980) p357 HilI, A.V. P r o c R o y S o c 1 0 4 B ( 1 9 2 8 ) p 3 9 Crank, J. Mathematics of Diffusion 2nd Ed (OUP, 1975) Carslew, H.S. and Jaeger, J.C. Conduction of Heat in Solids 2nd Ed (OUP, 1959) Fujita, H. Adv Polym Sci 3 ( 1961 ) p 1 Musty, J.W.G., Pattie, R.E. and Smith, P.J.A. J Appl Chem 16 (1966) p221 Brunauer, S., Emmett, P.H. and Teller, E. J Amer Chem Soc 60 (1938) p309 Brettle, J., Brewis, D.M., Cornyn, J., Cope, B.C., Goose¥, M.T. and Hurditch, R.D. Int J Adhesion Adhesives 3 (1983) p 189 Enns, J.B. andGillham, J.K. J A p p l P o l y m S c i 2 8 ( 1 9 8 3 ) p2831 Glasstone, S. Textbook of Physical Chemisty 2nd Ed (Macmillan Co, 1960) p 782 Rainy, H. Treatise on Inorganic Chemistry (Elsevier Publishing Co, 1956) Vol 1, p 74

Authors

D.M. Brewis, J. Comyn and A~K. Raval are in the Department of Chemistry, Leicester Polytechnic, Leicester, LE1 9BH, UK (D.M. Brewis is presently at the Institute of Surface Science and Technology, University of Technology, Loughborough, LEI 1 3TU, UK). AJ. Kinloch is in the Department of Mechanical Engineering, Imperial College, London, UK.

INT.J.ADHESION AND ADHESIVES OCTOBER 1990

253