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Durability in high humidity of glass-to-lead alloy joints bonded with an epoxide adhesive
Durability in high humidity of glass-to-lead alloy joints bonded with an ep0xide adhesive J. Comyn, C.L. Groves and R.W. Saville* (De Montfort University/*M.G. Bennett & Associates Ltd, UK)
Received 30 July 1993; revised 3 January 1994
Adhesive joints consisting of glass bonded to lead alloy with an epoxide adhesive have been aged at 50°C at relative humidities of 50 and 100% for up to 3 months. In some joints the glass had been pretreated with a silane coupling agent. Joints aged at 50% relative humidity show no weakening, and at 100% joints with silane pretreatment are not weakened as much as those without it; many of the latter samples fell apart during ageing. The diffusion coefficient of water in the adhesive has been obtained from an absorption experiment. The rate at which joints are weakened is higher than can be accounted for by diffusion of water in the adhesive. Water is supplied to intact regions of the adhesive bond by way of zones that have been disbonded.
Key words: adhesive-bonded joints; environmental testing; durability; pretreatment; failure mode; glass-to-lead joints; epoxy adhesive
Joints of glass bonded to lead alloy with an epoxide adhesive, similar to those which might be used in double glazing units, have been exposed to conditions of accelerated ageing. It is important that adhesive joints of this type do not fail in service, as this leads to the ingress of moisture and the formation of condensation on the inner glass surfaces. Such units have to be replaced. The conditions were warm and wet (50°C and 100% relative humidity (RH)) and warm and dry (50°C and 50% RH). Joints were made with both clear and bronzed glass and in some cases the glass was treated with 3-aminopropyltriethoxysilane coupling agent. Silane coupling agents are effective in improving the water durability of adhesive bonds to glass and metals 1. An investigation by Dukes and Greenwood 2 which has received much attention involved the Churchill Memorial Screen at Dudley in Worcestershire, UK. It was constructed of pieces of glass bonded with an epoxy adhesive, which unfortunately began to disintegrate on exposure to the weather; failure was at the interface between the adhesive and glass. Tests showed that the use of a
glycidoxypropylsilane greatly increased bond durability, and the screen has been reconstructed using this compound, giving it an anticipated life measured in decades rather than in months.
Experimental Materials The epoxide adhesive used was AT1 supplied by CibaGeigy Ltd; it is a one-part high-temperature-curing adhesive which is supplied as a powder. 3-Aminopropyltriethoxysilane coupling agent was obtained as A1100 from Union Carbide Ltd. Pilkington clear and Antisun bronze coloured float glass were 6 mm thick. A 6 mm sheet of lead alloy containing 5% tin was obtained from Alliance Lead Ltd, Manchester, UK.
Preparation of joints Strips of lead alloy measuring 12 x 80 mm were cut with a bandsaw, deburred with a file, and the area to be bonded brushed with a brass wire brush. Small pieces of glass measuring approximately 15 x 18 mm
0143-7496/94/01/0015-06 © 1994 Butterworth-Heinemann Ltd INT.J.ADHESlON AND ADHESIVES VOL. 14 NO. 1 JANUARY 1994
15
were cleaned by wiping the faces in a mixture of 2500 cm 3 distilled water, 125 cm -~ 880 ammonium hydroxide solution and 30 g ceri rouge. In addition, some samples were treated with 3-aminopropyltriethoxysilane by immersion for 1 rain at room temperature in a freshly prepared 2% by volume solution of the silane in distilled water. After draining they were placed in an oven set at 6 T C for 15 min. The two faces of a sheet of float glass are different in that one contains more tin than the other. It has been shown ~ that there are differences in the weathering properties of the two faces, and from this we would anticipate possible differences in the wet durability of the tin-rich and tin-lean faces of bonded glass. Hirthammer and Scheffler 4 have indicated that it is possible to differentiate between the two faces with an ultraviolet lamp. As we tried this and were unsuccessful, our selection of glass surfaces was random. This may be a factor which contributes to the scatter of experimental results. The thickness of the adhesive layer was controlled by wrapping two loops of copper wire, 0.125 m m in diameter, around the long axes of the lead alloy adherends. The lead alloy adherends were preheated in an oven set at 100°C and the powdered adhesive was sprinkled over the area to be bonded, whereupon it melted and spread to form a continuous liquid film. The glass adherends were placed squarely on the area to be bonded, and held in place with a clip. The joints were cured in an oven at 180°C for 75 min. A sketch of the joint assembly appears in Fig. 1. The bonded areas measured about 12 × 15 mm.
Joio,
I
1
\ \ \\
Fig. 2
/ z ~¸
Tensometerjews
Method of testing joints
the joints and apply a compressive force to the glass during testing. They were tested to destruction in a Monsanto W-type tensometer using a crosshead speed of 6 m m rain ]. Each joint was examined visually to ascertain the locus of failure.
Water uptake by the adhesive Detached films of the adhesive were prepared by spreading the powder on a sheet of glass which had been treated with a silicone release agent and heated to 100°C. A spreader was used to apply a layer of adhesive about 0.5 m m thick. The adhesive was then cured for 1 h at 185°C; it was easily removed from the glass after cooling. Thicknesses of two samples with dimensions of about 20 x 50 m m were measured at five points with a micrometer; the samples were then immersed in distilled water at 50°C. Periodically the samples were dried with paper tissues, weighed and re-immersed.
Results and discussion Ageing of joints
Joint strengths
Some joints were aged in air at 100% RH and 50°C. This was done by placing them in desiccators with a layer of water in the bottom, which were in turn placed in an air-circulating oven at 50°C. Other joints were aged in air at 50% RH and 50°C. Here the desiccators contained saturated solutions of sodium bromide 5 instead of water. After periods of about 1, 2 or 3 months, 10 joints were removed from the desiccators for testing.
The mean strengths with standard deviations are collected in Tables 1 and 2. Some joints fell apart during exposure at 100% RH and the numbers of these are indicated in Table 1. The following codes are used to indicate the modes of failure:
Testing of joints
•
•
This type of joint was selected because some early experiments revealed the difficulties inherent in testing lap joints made of a brittle and a ductile material. A special jig, illustrated in Fig. 2, was constructed to hold
•
• •
Tape1"ohold wires
~GIQss
~
"x, \\ /
") >//
\ t
Wires to control gluelinethickness Fig. 1 Assembly of adhesive joints
16
INT.J.ADHESlONAND ADHESIVESJANUARY 1994
M(Pb) material failure in the lead. This was seen as a thin layer of lead adhered to the epoxide adhesive on one side of the failed joint, and a bright lead surface on the other; I(Pb) interfacial failure between the lead and the epoxide adhesive; M(G) ...... material failure in the glass, manifested by pieces of glass remaining adhered to the epoxide adhesive; I(G) .... interracial failure between the glass and the epoxide adhesive; and Pb ductile here the lead adherend began to stretch and the bonded areas did not actually fail.
In no instance was cohesive failure of the epoxide adhesive observed. There was quite a large amount of experimental scatter in joint strengths. This probably arises mainly from irregularities in the shape of the glass adherends, causing differences in stress distribution in the joints. These irregularities arose from the manner of cutting the glass, which was with a diamond-tip glass-cutter. Strengths for joints with tinted glass are compared graphically in Fig. 3, but the data for clear glass (which are not shown graphically) are very similar.
Table 1. Strengths of joints aged at 100% RH and 50°C Type of glass
Ageing time (days)
Strength (N)
0 35 65 96 0 32 65 96
1440 ± 1340 ± 900 ± 960 ± 1460 ± 720 ± 680 ± 690 ±
760 400 530 470 610 320 350 330
0 30 57 97 0 30 57 97
1090 ± 420 ± 58 ± 0 1770 ± 460 ± 196 ± 56 ±
740 480 107
Number which failed during ageing
Silane-treated glass Bronze
Clear
0 1 0 0 0 0
Untreated glass Bronze
Clear
Table 2.
530 390 192 98
3 7 10 1 3 7
Strengths of joints aged at 50% RH and 50°C
Type of glass
Ageing time (days)
Strength (N)
0 33 68 100
1440 1690 1960 1800
2000
Silane-treated glass Bronze
Clear
± ± ± ±
0
1460 ±
32 65 100
1900 ± 2100 ± 1690 ±
760 670 430 660 610 350 170 520
0 30 57 98 0 30 57 97
1090 ± 1390 ± 760 ± 1160 ± 1770 ± 2190 ± 1640 i 1460 ±
740 650 390 630 530 290 370 500
Z J=
E
I ooo
Untreated glass Bronze
Clear
Joints were not weakened by exposure to air at 50% RH and 50°C for 3 months (Table 2). It has frequently been observed that joints can withstand exposure at lower humidities (e.g., 50% RH or less) for long periods without weakening. DeLollis 6, for example, has referred to some epoxide-aluminium joints which showed no loss in strength after exposure to laboratory humidity for up to 11 years. Such information led to the proposal from Gledhill e t al. 7 that there must be a critical concentration of water (and a corresponding critical relative humidity) below which the interface is not weakened. Experiments carried out in these laboratories s on ageing of epoxidealuminium joints at 50°C gave evidence that the critical relative humidity in this case was 65%.
80
ClIo
Time(days) Fig. 3 Failure loads of joints with bronzed glass on exposure to air at 100% RH and 50°C: O, glass treated with silane; e, untreated glass. For clarity, only one error bar is shown for the unaged joints
Joints are weakened by exposure to air at 100% RH and 50°C, and joints without the silane coupling agent are weakened most. Indeed, after 3 months most of the joints without silane had fallen apart in the desiccators. When silane is present strengths of exposed joints tend to level out at about 50% of their initial strength. There are no significant differences in strength for joints with clear and bronzed glass. Silane primer has no effect on the strengths of unaged joints. Failure modes for joints aged at 50% RH are mainly M(Pb) but there is a significant amount of Pb ductile, even after 3 months' exposure. When exposed at 100% RH, however, the failure patterns change. The dominant mode is summarized in Table 3 for each type of joint at each exposure period; where mixed failure modes have been observed, the dominant mode is given first. There are two points of interest arising from this.
INT.J.ADHESlON AND ADHESIVES JANUARY 1994
17
Table 3.
Failure modes of joints exposed at 100% RH and 50 C
Exposure time (months)
Bronze glass Bronze glass + silane Clear glass Clear glass e silane
0
1
2
3
M(Pb) M(Pb) M(Pb) M(Pb)
M(Pb) 4-I(G) M(Pb) M(Pb) + I(G) M(Pb)
I(G) M(Pb) + I(G) I(G) + M(Pb) M(Pb)
I(G) I(G) ~ M(Pb) I(G) M(Pb)
Firstly there is a tendency for the litilure mode, which begins as M(Pb), to become I(G). This is exemplified by the clear glass case, where the failure is initially M(Pb), is a mixture of M(Pb) and I(G) after 1 month with M(Pb) being dominant, is the same mixture but with I(G) being dominant after 2 months, but there is only I(G) after 3 months. This is entirely consistent with water attacking the glass-epoxide adhesive bond. The phenomenon of water attacking the joints fi'om the edges is illustrated by a number of samples all aged at 100% RH (see Fig. 4), where the lililure pattern was a central disc of M(Pb), surrounded by an outer- zone of I(G). The second point is that the onset of I(G) [iii]ure is slower when silane is present, and in [itct when clear glass is used with silanc, I(G) does not appear at all. It thus appears that the use of silane increases the durability of the interface.
[
i~ 4 ~ ©
,')~
1
I
~:)
1
IO
1
2(;= i T~rne)l/2 { h)i/2
Fig. 5
Uptake of water by t w o films of cured AT1 adhesive at 50 C
where M.i and M E a r e the masses absorbed at time l and at equilibrium, respectively, D is the diffusion coefficient and L is the thickness of the film. At short times Equation (1) simplifies to:
MT/Mt = 4(Dl/rr) I 2//l.
Water uptake by the adhesive
Fig. 5 shows water uptake of two films of ATI adhesive, of thickness 0.45 ± 0.05 mm and 0.48 ± 0.02 mm, on immersion in water at 50 C. The behaviour is reasonably close to Fickian absorption ~ in that the plot of mass uptake against the square root of time is initially linear, and leads to equilibrium. If a thin film of a permeable material is immersed in a liquid or a vapour at constant pressure, then the mass absorbed at time t is given I° by:
(2)
This means that if mass uptake is plotted against the square root of time, then the initial part of the plot should be a straight line passing through the origin with slope 4ME(D/Tr)i;-/L. From this. the diffusion coefficient of water in the adhesive is 1.22 x 10 ~ m 2 s at 50~C and the equilibrium uptake is 5.15% by weight.
Comparison of joint strengths with water content
M,/M~:= 1 - ~ pl
8exp[
D ( 2 n + l):Tr:t/LZ!/
II
(2n+l)Z~:
(1)
[f a slab of adhesive is exposed to air at constant relative humidity, the concentration of water at points across the film (C) will be given l° by: C/C, = 1 - (4/7r) ~ sl
[-D(2n +
[(
I)"/(2n + 1)-~!exp
0
l):~2z/4/'~]cos[(2n F
1)mr/2~
(3)
where Ci is the concentration at equilibrium, t is time. The slab is located in Cartesian coordinates such that the origin is at its centre, and the faces are located at .r = ± l . The layer of adhesive in a rectangular lap joint can be considered to be the intersection of two slabs at right angles, one of which is along the x-axis and the other along the ),-axis. Equation (3) can then be abbreviated for the x-axis slab as:
(C,./C~) = f ( x )
(4)
and for the y-axis as: Fig. 4 Failure surfaces of joint with untreated glass after exposure to o RH for 2 m o n t h s 100%
18
INT.J.ADHESION AND ADHESIVES JANUARY 1994
(C,/G)
=
f(r)
(5)
Concentrations at points (x, y) in the lap joint are then given by: 5.0-
(1
-
Cx, y )
~-
(1
--
Cx/C1)(1
(6)
- Cy/Cl)
a= t~
Concentrations along the two axes calculated in this way are shown in Figs 6 and 7 for 30, 60 and 90 days' exposure. It can be seen that, even after 90 days' exposure, water has not entered the central zone by diffusion through/the adhesive. In fact, the plots show that there will be/a central zone measuring about 7 × 10 mm whict~ is free of water, even though after this time some o l the joints have fallen apart under their own weight. This shows that water displaces adhesive from the glass, and that the means by which water enters the joints is not via diffusion through the adhesive, but by 'wicking' along debonded zones. Comparison with other types of adhesive joint The diffusion coefficient of water in AT1 adhesive at 50°C is in the range that has been measured for some other epoxide adhesives, but amongst the lower values. Likewise, the equilibrium water uptake is amongst the higher values ]l . Previous work at these laboratories studied the durability of joints with aluminium alloy adherends bonded with epoxide and other structural adhesives. In most cases the lowering of joint strength was controlled by the rate of water diffusion into the adhesive, and plots of joint strength against the amount of water absorbed were linear. This behaviour was observed for aluminium alloy adherends etched in chromic acid and bonded with epoxide-polyamide TM13, modified epoxidel4, 15 and some modified phenolic adhesives 15, and for joints with an aliphatic-amine-cured epoxide used to bond aluminium alloy pretreated by a number of methods 16. Radioactive water 17 was used to show that the transport of water into adhesive joints is no faster than can be accounted for by diffusive transport into the adhesive layer, for some joints with aluminium and modified epoxide adhesives. Deviations did occur from the general pattern in a number of cases. Joints with a polyamide-epoxide
g
22.5
0 4
I 5 Distance from centre of joint (mm)
6
Fig. 7 Concentration of water along the minor centre-line of the adhesive layer after exposure at 100% RH and 50°C for 30, 60 and 90 days
adhesive showed a rapid fall in strength when they were almost saturated with water TM and joints with a nitrile-phenolic or vinyl-phenolic adhesive 15 did not give linear plots of strength against water content, and there was a large and fairly rapid drop in strength during exposure. The major difference with the present investigation is that the interface which is sensitive to water has glass as the adherend. It seems that water can displace the adhesive from untreated glass and this is the mechanism of failure. Water is made available by transport through debonded zones rather than by diffusion through the adhesive. The progressive development of failure at the epoxide-glass interface as these joints are aged at 100% RH supports this view strongly. Silane coupling agents prevent this displacement from taking place, presumably by the formation of interfacial covalent bonds I which are more durable than the van der Waals' attractions which they replace. A consequence is that even after 3 months exposure failure in the lead alloy is a major phenomenon.
Conclusions 5.0-
.c
2.5-
0
5
6 Distance from centre of joint (mm)
I
7
Fig. 6 Concentration of water along the major centre-line of the adhesive layer after exposure at 100% RH and 50°C for 30, 60 and 90 days
1) Silane coupling agent has no effect on the strengths of unaged joints or those aged at 50% RH, which do not show interfacial failure. 2) The adhesive joints are not weakened on ageing in air at 50% RH and 50°C, for 3 months. 3) Joints are weakened on ageing in air at 100% RH and 50°C, and the weakening is less when the glass has been treated with silane coupling agent. Without the use of silane, most joints fall apart after 3 months in these conditions. 4) As water enters the adhesive joints it changes the dominant mode of failure from material failure in the lead alloy to interfacial failure at the glass. 5) The rate of weakening cannot be accounted for by the rate at which water enters the adhesive by diffusion. In the absence of silane coupling agent the mechanism of failure is by water physically displacing the adhesive from glass, and water being supplied through debonded regions.
INT.J.ADHESlON AND ADHESIVES JANUARY 1994
19
References 1
2 3 4 5 6 7
8 9 10
11 12
20
Comyn, J. in 'Structural Adhesives: Developments in Resins and Primers" edited by A.J. Kinloch (Elsevier Applied Science Publishers, London, 1986) Chapter 8 Dukes, W.A. and Greenwood, L. in 'Aspects ofAdhesion E edited by K.W. Allen (Transcripta Books, UK, 1975) p 92 Shelby, J.E., Vitko Jr, J. and Pantano, C. G. Solar Energy Mater 3 (1980) p 97 Hirthammer, M. and Scheffler, I. Plastics and Rubber Int 12 No 6 (1987) p 22 Wink, W.A. and Sears, G.R. TAPPI33 (1950) p 96A De/ollis, N.J. Nat SAMPE Symp Exhib 22 (1977) p 673 Gledhill, R.A., Kinloch, A.J. and Shaw, S.J. J Adhesion 11 (1980) p3 Brewis, D.M., Comyn, J., Raval, A.K. and Kinloch, A,J. Int J Adhesion and Adhesives 10 (1990) p 247 Fujita, H. Adv Polym Sci 3 (1961) p 1 Crank, J. "Mathematics of Diffusion" 2nd edition (Oxford University Press, Oxford, UK, 1975) Brewis, D.M., Comyn, J., Shalash, R.J.A. and Tegg, J.L Polymer 21 (1980) p 357 Brewis, D.M., Comyn, J. and Shalash, R.J.A. Int J Adhesion and Adhesives 2 (1982) p 215
INT.J.ADHESlON AND ADHESIVES JANUARY 1994
13 14 15 16 17 18
Brewis, D.M., Comyn, J., Cope, B.C. and Moloney, A.C. Polymer 21 (1980) p 345 Brewis, D.M., Comyn, J., Cope, B.C. and Moloney, A.C. Po/ym Engng Sci21 (1981) p 797 Comyn, J., Brewis, D.M. and Tredwell, S.T. J Adhesion 21 (1987) p 59 Brewis, D.M., Comyn, J. and Tegg, J.L. Int J Adhesion and Adhesives 1 (1980) p 35 Brewis, D.M., Comyn, J., Moloney, A.C. and Tegg, J.L. Europ Po/ym J 17 (1981) p 127 Brewis, D.M., Comyn, J., Cope, B.C. and Moloney, A.C. Polymer 21 (1980) p 1477
Authors
J. Comyn, to whom correspondence should be addressed, and C.L. Groves are with the Department of Chemistry at De Montfort University, Leicester LEI 9BH, UK. R.W. Saville is with M.G. Bennett & Associates Ltd, Bennett House, Pleasley Road, Whiston, Rotherham $60 4HQ, UK.
Received 30 July 1993; revised 3 January 1994
Adhesive joints consisting of glass bonded to lead alloy with an epoxide adhesive have been aged at 50°C at relative humidities of 50 and 100% for up to 3 months. In some joints the glass had been pretreated with a silane coupling agent. Joints aged at 50% relative humidity show no weakening, and at 100% joints with silane pretreatment are not weakened as much as those without it; many of the latter samples fell apart during ageing. The diffusion coefficient of water in the adhesive has been obtained from an absorption experiment. The rate at which joints are weakened is higher than can be accounted for by diffusion of water in the adhesive. Water is supplied to intact regions of the adhesive bond by way of zones that have been disbonded.
Key words: adhesive-bonded joints; environmental testing; durability; pretreatment; failure mode; glass-to-lead joints; epoxy adhesive
Joints of glass bonded to lead alloy with an epoxide adhesive, similar to those which might be used in double glazing units, have been exposed to conditions of accelerated ageing. It is important that adhesive joints of this type do not fail in service, as this leads to the ingress of moisture and the formation of condensation on the inner glass surfaces. Such units have to be replaced. The conditions were warm and wet (50°C and 100% relative humidity (RH)) and warm and dry (50°C and 50% RH). Joints were made with both clear and bronzed glass and in some cases the glass was treated with 3-aminopropyltriethoxysilane coupling agent. Silane coupling agents are effective in improving the water durability of adhesive bonds to glass and metals 1. An investigation by Dukes and Greenwood 2 which has received much attention involved the Churchill Memorial Screen at Dudley in Worcestershire, UK. It was constructed of pieces of glass bonded with an epoxy adhesive, which unfortunately began to disintegrate on exposure to the weather; failure was at the interface between the adhesive and glass. Tests showed that the use of a
glycidoxypropylsilane greatly increased bond durability, and the screen has been reconstructed using this compound, giving it an anticipated life measured in decades rather than in months.
Experimental Materials The epoxide adhesive used was AT1 supplied by CibaGeigy Ltd; it is a one-part high-temperature-curing adhesive which is supplied as a powder. 3-Aminopropyltriethoxysilane coupling agent was obtained as A1100 from Union Carbide Ltd. Pilkington clear and Antisun bronze coloured float glass were 6 mm thick. A 6 mm sheet of lead alloy containing 5% tin was obtained from Alliance Lead Ltd, Manchester, UK.
Preparation of joints Strips of lead alloy measuring 12 x 80 mm were cut with a bandsaw, deburred with a file, and the area to be bonded brushed with a brass wire brush. Small pieces of glass measuring approximately 15 x 18 mm
0143-7496/94/01/0015-06 © 1994 Butterworth-Heinemann Ltd INT.J.ADHESlON AND ADHESIVES VOL. 14 NO. 1 JANUARY 1994
15
were cleaned by wiping the faces in a mixture of 2500 cm 3 distilled water, 125 cm -~ 880 ammonium hydroxide solution and 30 g ceri rouge. In addition, some samples were treated with 3-aminopropyltriethoxysilane by immersion for 1 rain at room temperature in a freshly prepared 2% by volume solution of the silane in distilled water. After draining they were placed in an oven set at 6 T C for 15 min. The two faces of a sheet of float glass are different in that one contains more tin than the other. It has been shown ~ that there are differences in the weathering properties of the two faces, and from this we would anticipate possible differences in the wet durability of the tin-rich and tin-lean faces of bonded glass. Hirthammer and Scheffler 4 have indicated that it is possible to differentiate between the two faces with an ultraviolet lamp. As we tried this and were unsuccessful, our selection of glass surfaces was random. This may be a factor which contributes to the scatter of experimental results. The thickness of the adhesive layer was controlled by wrapping two loops of copper wire, 0.125 m m in diameter, around the long axes of the lead alloy adherends. The lead alloy adherends were preheated in an oven set at 100°C and the powdered adhesive was sprinkled over the area to be bonded, whereupon it melted and spread to form a continuous liquid film. The glass adherends were placed squarely on the area to be bonded, and held in place with a clip. The joints were cured in an oven at 180°C for 75 min. A sketch of the joint assembly appears in Fig. 1. The bonded areas measured about 12 × 15 mm.
Joio,
I
1
\ \ \\
Fig. 2
/ z ~¸
Tensometerjews
Method of testing joints
the joints and apply a compressive force to the glass during testing. They were tested to destruction in a Monsanto W-type tensometer using a crosshead speed of 6 m m rain ]. Each joint was examined visually to ascertain the locus of failure.
Water uptake by the adhesive Detached films of the adhesive were prepared by spreading the powder on a sheet of glass which had been treated with a silicone release agent and heated to 100°C. A spreader was used to apply a layer of adhesive about 0.5 m m thick. The adhesive was then cured for 1 h at 185°C; it was easily removed from the glass after cooling. Thicknesses of two samples with dimensions of about 20 x 50 m m were measured at five points with a micrometer; the samples were then immersed in distilled water at 50°C. Periodically the samples were dried with paper tissues, weighed and re-immersed.
Results and discussion Ageing of joints
Joint strengths
Some joints were aged in air at 100% RH and 50°C. This was done by placing them in desiccators with a layer of water in the bottom, which were in turn placed in an air-circulating oven at 50°C. Other joints were aged in air at 50% RH and 50°C. Here the desiccators contained saturated solutions of sodium bromide 5 instead of water. After periods of about 1, 2 or 3 months, 10 joints were removed from the desiccators for testing.
The mean strengths with standard deviations are collected in Tables 1 and 2. Some joints fell apart during exposure at 100% RH and the numbers of these are indicated in Table 1. The following codes are used to indicate the modes of failure:
Testing of joints
•
•
This type of joint was selected because some early experiments revealed the difficulties inherent in testing lap joints made of a brittle and a ductile material. A special jig, illustrated in Fig. 2, was constructed to hold
•
• •
Tape1"ohold wires
~GIQss
~
"x, \\ /
") >//
\ t
Wires to control gluelinethickness Fig. 1 Assembly of adhesive joints
16
INT.J.ADHESlONAND ADHESIVESJANUARY 1994
M(Pb) material failure in the lead. This was seen as a thin layer of lead adhered to the epoxide adhesive on one side of the failed joint, and a bright lead surface on the other; I(Pb) interfacial failure between the lead and the epoxide adhesive; M(G) ...... material failure in the glass, manifested by pieces of glass remaining adhered to the epoxide adhesive; I(G) .... interracial failure between the glass and the epoxide adhesive; and Pb ductile here the lead adherend began to stretch and the bonded areas did not actually fail.
In no instance was cohesive failure of the epoxide adhesive observed. There was quite a large amount of experimental scatter in joint strengths. This probably arises mainly from irregularities in the shape of the glass adherends, causing differences in stress distribution in the joints. These irregularities arose from the manner of cutting the glass, which was with a diamond-tip glass-cutter. Strengths for joints with tinted glass are compared graphically in Fig. 3, but the data for clear glass (which are not shown graphically) are very similar.
Table 1. Strengths of joints aged at 100% RH and 50°C Type of glass
Ageing time (days)
Strength (N)
0 35 65 96 0 32 65 96
1440 ± 1340 ± 900 ± 960 ± 1460 ± 720 ± 680 ± 690 ±
760 400 530 470 610 320 350 330
0 30 57 97 0 30 57 97
1090 ± 420 ± 58 ± 0 1770 ± 460 ± 196 ± 56 ±
740 480 107
Number which failed during ageing
Silane-treated glass Bronze
Clear
0 1 0 0 0 0
Untreated glass Bronze
Clear
Table 2.
530 390 192 98
3 7 10 1 3 7
Strengths of joints aged at 50% RH and 50°C
Type of glass
Ageing time (days)
Strength (N)
0 33 68 100
1440 1690 1960 1800
2000
Silane-treated glass Bronze
Clear
± ± ± ±
0
1460 ±
32 65 100
1900 ± 2100 ± 1690 ±
760 670 430 660 610 350 170 520
0 30 57 98 0 30 57 97
1090 ± 1390 ± 760 ± 1160 ± 1770 ± 2190 ± 1640 i 1460 ±
740 650 390 630 530 290 370 500
Z J=
E
I ooo
Untreated glass Bronze
Clear
Joints were not weakened by exposure to air at 50% RH and 50°C for 3 months (Table 2). It has frequently been observed that joints can withstand exposure at lower humidities (e.g., 50% RH or less) for long periods without weakening. DeLollis 6, for example, has referred to some epoxide-aluminium joints which showed no loss in strength after exposure to laboratory humidity for up to 11 years. Such information led to the proposal from Gledhill e t al. 7 that there must be a critical concentration of water (and a corresponding critical relative humidity) below which the interface is not weakened. Experiments carried out in these laboratories s on ageing of epoxidealuminium joints at 50°C gave evidence that the critical relative humidity in this case was 65%.
80
ClIo
Time(days) Fig. 3 Failure loads of joints with bronzed glass on exposure to air at 100% RH and 50°C: O, glass treated with silane; e, untreated glass. For clarity, only one error bar is shown for the unaged joints
Joints are weakened by exposure to air at 100% RH and 50°C, and joints without the silane coupling agent are weakened most. Indeed, after 3 months most of the joints without silane had fallen apart in the desiccators. When silane is present strengths of exposed joints tend to level out at about 50% of their initial strength. There are no significant differences in strength for joints with clear and bronzed glass. Silane primer has no effect on the strengths of unaged joints. Failure modes for joints aged at 50% RH are mainly M(Pb) but there is a significant amount of Pb ductile, even after 3 months' exposure. When exposed at 100% RH, however, the failure patterns change. The dominant mode is summarized in Table 3 for each type of joint at each exposure period; where mixed failure modes have been observed, the dominant mode is given first. There are two points of interest arising from this.
INT.J.ADHESlON AND ADHESIVES JANUARY 1994
17
Table 3.
Failure modes of joints exposed at 100% RH and 50 C
Exposure time (months)
Bronze glass Bronze glass + silane Clear glass Clear glass e silane
0
1
2
3
M(Pb) M(Pb) M(Pb) M(Pb)
M(Pb) 4-I(G) M(Pb) M(Pb) + I(G) M(Pb)
I(G) M(Pb) + I(G) I(G) + M(Pb) M(Pb)
I(G) I(G) ~ M(Pb) I(G) M(Pb)
Firstly there is a tendency for the litilure mode, which begins as M(Pb), to become I(G). This is exemplified by the clear glass case, where the failure is initially M(Pb), is a mixture of M(Pb) and I(G) after 1 month with M(Pb) being dominant, is the same mixture but with I(G) being dominant after 2 months, but there is only I(G) after 3 months. This is entirely consistent with water attacking the glass-epoxide adhesive bond. The phenomenon of water attacking the joints fi'om the edges is illustrated by a number of samples all aged at 100% RH (see Fig. 4), where the lililure pattern was a central disc of M(Pb), surrounded by an outer- zone of I(G). The second point is that the onset of I(G) [iii]ure is slower when silane is present, and in [itct when clear glass is used with silanc, I(G) does not appear at all. It thus appears that the use of silane increases the durability of the interface.
[
i~ 4 ~ ©
,')~
1
I
~:)
1
IO
1
2(;= i T~rne)l/2 { h)i/2
Fig. 5
Uptake of water by t w o films of cured AT1 adhesive at 50 C
where M.i and M E a r e the masses absorbed at time l and at equilibrium, respectively, D is the diffusion coefficient and L is the thickness of the film. At short times Equation (1) simplifies to:
MT/Mt = 4(Dl/rr) I 2//l.
Water uptake by the adhesive
Fig. 5 shows water uptake of two films of ATI adhesive, of thickness 0.45 ± 0.05 mm and 0.48 ± 0.02 mm, on immersion in water at 50 C. The behaviour is reasonably close to Fickian absorption ~ in that the plot of mass uptake against the square root of time is initially linear, and leads to equilibrium. If a thin film of a permeable material is immersed in a liquid or a vapour at constant pressure, then the mass absorbed at time t is given I° by:
(2)
This means that if mass uptake is plotted against the square root of time, then the initial part of the plot should be a straight line passing through the origin with slope 4ME(D/Tr)i;-/L. From this. the diffusion coefficient of water in the adhesive is 1.22 x 10 ~ m 2 s at 50~C and the equilibrium uptake is 5.15% by weight.
Comparison of joint strengths with water content
M,/M~:= 1 - ~ pl
8exp[
D ( 2 n + l):Tr:t/LZ!/
II
(2n+l)Z~:
(1)
[f a slab of adhesive is exposed to air at constant relative humidity, the concentration of water at points across the film (C) will be given l° by: C/C, = 1 - (4/7r) ~ sl
[-D(2n +
[(
I)"/(2n + 1)-~!exp
0
l):~2z/4/'~]cos[(2n F
1)mr/2~
(3)
where Ci is the concentration at equilibrium, t is time. The slab is located in Cartesian coordinates such that the origin is at its centre, and the faces are located at .r = ± l . The layer of adhesive in a rectangular lap joint can be considered to be the intersection of two slabs at right angles, one of which is along the x-axis and the other along the ),-axis. Equation (3) can then be abbreviated for the x-axis slab as:
(C,./C~) = f ( x )
(4)
and for the y-axis as: Fig. 4 Failure surfaces of joint with untreated glass after exposure to o RH for 2 m o n t h s 100%
18
INT.J.ADHESION AND ADHESIVES JANUARY 1994
(C,/G)
=
f(r)
(5)
Concentrations at points (x, y) in the lap joint are then given by: 5.0-
(1
-
Cx, y )
~-
(1
--
Cx/C1)(1
(6)
- Cy/Cl)
a= t~
Concentrations along the two axes calculated in this way are shown in Figs 6 and 7 for 30, 60 and 90 days' exposure. It can be seen that, even after 90 days' exposure, water has not entered the central zone by diffusion through/the adhesive. In fact, the plots show that there will be/a central zone measuring about 7 × 10 mm whict~ is free of water, even though after this time some o l the joints have fallen apart under their own weight. This shows that water displaces adhesive from the glass, and that the means by which water enters the joints is not via diffusion through the adhesive, but by 'wicking' along debonded zones. Comparison with other types of adhesive joint The diffusion coefficient of water in AT1 adhesive at 50°C is in the range that has been measured for some other epoxide adhesives, but amongst the lower values. Likewise, the equilibrium water uptake is amongst the higher values ]l . Previous work at these laboratories studied the durability of joints with aluminium alloy adherends bonded with epoxide and other structural adhesives. In most cases the lowering of joint strength was controlled by the rate of water diffusion into the adhesive, and plots of joint strength against the amount of water absorbed were linear. This behaviour was observed for aluminium alloy adherends etched in chromic acid and bonded with epoxide-polyamide TM13, modified epoxidel4, 15 and some modified phenolic adhesives 15, and for joints with an aliphatic-amine-cured epoxide used to bond aluminium alloy pretreated by a number of methods 16. Radioactive water 17 was used to show that the transport of water into adhesive joints is no faster than can be accounted for by diffusive transport into the adhesive layer, for some joints with aluminium and modified epoxide adhesives. Deviations did occur from the general pattern in a number of cases. Joints with a polyamide-epoxide
g
22.5
0 4
I 5 Distance from centre of joint (mm)
6
Fig. 7 Concentration of water along the minor centre-line of the adhesive layer after exposure at 100% RH and 50°C for 30, 60 and 90 days
adhesive showed a rapid fall in strength when they were almost saturated with water TM and joints with a nitrile-phenolic or vinyl-phenolic adhesive 15 did not give linear plots of strength against water content, and there was a large and fairly rapid drop in strength during exposure. The major difference with the present investigation is that the interface which is sensitive to water has glass as the adherend. It seems that water can displace the adhesive from untreated glass and this is the mechanism of failure. Water is made available by transport through debonded zones rather than by diffusion through the adhesive. The progressive development of failure at the epoxide-glass interface as these joints are aged at 100% RH supports this view strongly. Silane coupling agents prevent this displacement from taking place, presumably by the formation of interfacial covalent bonds I which are more durable than the van der Waals' attractions which they replace. A consequence is that even after 3 months exposure failure in the lead alloy is a major phenomenon.
Conclusions 5.0-
.c
2.5-
0
5
6 Distance from centre of joint (mm)
I
7
Fig. 6 Concentration of water along the major centre-line of the adhesive layer after exposure at 100% RH and 50°C for 30, 60 and 90 days
1) Silane coupling agent has no effect on the strengths of unaged joints or those aged at 50% RH, which do not show interfacial failure. 2) The adhesive joints are not weakened on ageing in air at 50% RH and 50°C, for 3 months. 3) Joints are weakened on ageing in air at 100% RH and 50°C, and the weakening is less when the glass has been treated with silane coupling agent. Without the use of silane, most joints fall apart after 3 months in these conditions. 4) As water enters the adhesive joints it changes the dominant mode of failure from material failure in the lead alloy to interfacial failure at the glass. 5) The rate of weakening cannot be accounted for by the rate at which water enters the adhesive by diffusion. In the absence of silane coupling agent the mechanism of failure is by water physically displacing the adhesive from glass, and water being supplied through debonded regions.
INT.J.ADHESlON AND ADHESIVES JANUARY 1994
19
References 1
2 3 4 5 6 7
8 9 10
11 12
20
Comyn, J. in 'Structural Adhesives: Developments in Resins and Primers" edited by A.J. Kinloch (Elsevier Applied Science Publishers, London, 1986) Chapter 8 Dukes, W.A. and Greenwood, L. in 'Aspects ofAdhesion E edited by K.W. Allen (Transcripta Books, UK, 1975) p 92 Shelby, J.E., Vitko Jr, J. and Pantano, C. G. Solar Energy Mater 3 (1980) p 97 Hirthammer, M. and Scheffler, I. Plastics and Rubber Int 12 No 6 (1987) p 22 Wink, W.A. and Sears, G.R. TAPPI33 (1950) p 96A De/ollis, N.J. Nat SAMPE Symp Exhib 22 (1977) p 673 Gledhill, R.A., Kinloch, A.J. and Shaw, S.J. J Adhesion 11 (1980) p3 Brewis, D.M., Comyn, J., Raval, A.K. and Kinloch, A,J. Int J Adhesion and Adhesives 10 (1990) p 247 Fujita, H. Adv Polym Sci 3 (1961) p 1 Crank, J. "Mathematics of Diffusion" 2nd edition (Oxford University Press, Oxford, UK, 1975) Brewis, D.M., Comyn, J., Shalash, R.J.A. and Tegg, J.L Polymer 21 (1980) p 357 Brewis, D.M., Comyn, J. and Shalash, R.J.A. Int J Adhesion and Adhesives 2 (1982) p 215
INT.J.ADHESlON AND ADHESIVES JANUARY 1994
13 14 15 16 17 18
Brewis, D.M., Comyn, J., Cope, B.C. and Moloney, A.C. Polymer 21 (1980) p 345 Brewis, D.M., Comyn, J., Cope, B.C. and Moloney, A.C. Po/ym Engng Sci21 (1981) p 797 Comyn, J., Brewis, D.M. and Tredwell, S.T. J Adhesion 21 (1987) p 59 Brewis, D.M., Comyn, J. and Tegg, J.L. Int J Adhesion and Adhesives 1 (1980) p 35 Brewis, D.M., Comyn, J., Moloney, A.C. and Tegg, J.L. Europ Po/ym J 17 (1981) p 127 Brewis, D.M., Comyn, J., Cope, B.C. and Moloney, A.C. Polymer 21 (1980) p 1477
Authors
J. Comyn, to whom correspondence should be addressed, and C.L. Groves are with the Department of Chemistry at De Montfort University, Leicester LEI 9BH, UK. R.W. Saville is with M.G. Bennett & Associates Ltd, Bennett House, Pleasley Road, Whiston, Rotherham $60 4HQ, UK.