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Water durability of adhesive bonds between glass and polysulfide sealants
Water durability of adhesive bonds between glass and polysulfide sealants G.B. Lowe, T.C.P. Lee, J. Comyn* and K. Huddersman* (Morton International Ltd/*De Montfort University, UK)
Joints consisting of glass bonded with polysulfide sealants have been aged at 95% relative humidity and 60°C for up to 14 months. All were weakened and some with higher levels of plasticizer fell apart during exposure. The amount of interfacial failure increased with exposure, and was related to a discolouration which develops in the sealant and can be seen through the glass. The rate of joint weakening is greater than can be accounted for by the rate at which water diffuses into the sealant. Thermodynamic predictions based on measurements of contact angles show that the bonds are probably unstable in the presence of water.
Key words: adhesive joints; environmental testing; durability; polysulfide sealant; glass
Double glazing is a large market for flat glass and the amount used [ for this purpose in the U K in 1988 was 1.7 x l 0 7 m 2. The edges of such units are sealed with a polymer-based sealing material, the purpose of which is to hold the unit together and to provide a barrier against the permeation of water into the unit. It must also be resistant to glazing compounds and paints used in installation. Polysulfides are the dominant polymers in this application accounting for about 80% of the market; polyurethanes, pol~cisobutenes and polysiloxanes are also use&. Insulated glass units have anticipated service lives well above 10 years, but premature failure can occur. The manifestation of failure is misting of the internal surfaces of the unit. The major cause of failure is adhesion loss between sealant and glass. • A typical insulated glass unit consists of two panes of 4 mm thick glass separated by a hollow spacer-tube. The tube is generally of aluminium and filled with a desiccant, either silica gel or zeolite molecular seive. Holes on the inner face of the tube allow water vapour in the unit access to the desiccant. In the present work, the effect of water on polysulfide sealants, such as may be used in insulated
glass units, was studied. This information was used to assess their durability.
Materials The sealants consisted of two components, the base and the curative.
Base compound A number of bases were formulated using the following components: polysulfide, precipitated and ground calcium carbonate, bentonite, titanium dioxide, plasticizer and a silane coupling agent. The polysulfide was LP2C made by Morton International. It is made by reacting bis(2-chloroethyl formal) with sodium polysulfide and has the structure shown below; the average value of x is 2.1 and that of n is 22.
H S-(CH2-CH2-O-CH2-O-CH2-CH2-S_0n-H In addition, a small amount of trichloropropane (2 mol%) is added to give branch points, which will lead to crosslinks when the sealant is cured.
0143-7496/94/02/0085-08 © 1994 Butterworth-Heinemann Ltd INT.J.ADHESlON AND ADHESIVES VOL. 14 NO. 2 APRIL 1994
85
Fillers are added to the polysulfide to reduce cost and modify rheological properties. Precipitated calcium carbonates have much smaller particles than their ground counterparts. About 90% of the particles are smaller than 1 pm, in comparison to the ground which has a distribution in the range 2 10pm. A coat of stearic acid on the calcium carbonate particles increases the viscosity of the base compound and introduces thixotropy. The precipitated material Winnofil SP made by ICI and the ground material Polcarb S made by ECC were used; both were coated with stearic acid. Titanium dioxide is added as a whitening pigment and to assist in visualizing the complete mixing of the base and curative. The rutile grade used was supplied by BTP Ltd. Plasticizers are used to lower the viscosity of the base compound. Here, Santiciser 278 from Monsanto was used: it is Texanol benzyl phthalate. Texanol is 2,2,4trimethyl-pentan- 1,3-diol monoisobutyrate. Bentonite in the form of Bentone SD-2 (Bakers Castor Oil Co) is added as a thixotropic agent. 3Glycidoxypropyltrimethoxysilane is added as an adhesion promoter. All the bases contained 25 parts by weight of each type of calcium carbonate. The amount of titanium dioxide was 25 parts, and 3 parts of both bentonite and silane coupling agent were used. The amounts of polysulfide and plasticizer were varied and are shown in Table 1, Each sealant is referred to by a code which gives the proportion of polysulfide and plasticizer; e.g., the sealant with 70 parts of polysulfide and 20 parts of plasticizer is designated as S(3.5). The components were mixed by first adding the calcium carbonates and titanium dioxide to the liquid polysulfide in a high shear mixer over a period of 15 to 20 rain to form a viscous paste. The thixotrope and adhesion promoter were then added and mixing continued for 15 rain. Finally the plasticizer was added.
Glass-to-sealant adhesive joints were prepared in the following manner. Squares measuring 50 x 50ram were cut from 4 mm float glass and cleaned twice with a proprietary glass cleaner which was a mixture of water, 2-propanol and ammonia. No account was taken of the fact that the two faces of float glass are different, one lace being richer in tin than the other ~. The sealant [brmed a square prism 12 × 12mm between the two sheets of glass. These are known as 'H-bonds' and their preparation is fully described in a British Standard 7. Following assembly, the joints were allowed to cure at 23 C and 50% relative humidity (RH) for 7 days before the average strength was measured for four joints. The remaining joints were aged for up to 14 months in the environmental chamber set at 60'C and 95% RH. After ageing in the chamber for 12 months, some joints were dried at 23'C and 50% RH in an airconditioned laboratory for up to I year. Within 1 h of removal from the various storage conditions, the assemblies were tested in an lnstron 1026 tensometer using clamps designed for this purpose. The glass squares were separated at a speed of 6 mm rain ~ to failure. The load and extension at break, the load at 5% extension and the locus of failure were noted. Four .joints were used for each experimental point.
Curative
Preparation of cured sheets
The curative was a paste consisting of manganese(IV) dioxide, plasticizer and tetramethyl thiuram disulfide in the ratio of 20:20:1 parts by weight. Manganese dioxides used for this purpose are not pure compounds, but are activated by the addition of alkali metal salts or hydroxides. The product used here was manganese dioxide FA grade supplied by Riedel de Haen, which
Sheets of the sealants were made by first mixing the components such that the ratio of polysulfide to manganese dioxide was 10:1 by weight. After mixing for 10 rain the mix was placed on siliconized release paper, a metal former 2 m m thick was placed around the sample and a second sheet of siliconized paper placed on top. The sandwich was then placed between two sheets of 10ram thick glass and the whole sample pressed flat. The assembly was left to cure at room temperature overnight. After removing the siliconized paper, the sheets were left to cure for a further 6 days at 23:'C and 50% RH. The samples were trimmed to give specimens measuring 200 x 20 x 2 ram. Sheet thicknesses were measured with a micrometer.
Table 1. Amounts of polysulfide and plasticizer in base compounds Reference code 5 4.5 4 3.5 3 2.5 2 1.4 1.0 0.7
86
A m o u n t (parts by weight) Polysulfide
Plasticizer
100 90 80 70 60 50 80 70 60 50
20 20 20 20 20 20 40 50 60 70
Polysulfide Plasticizer 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.4 1.0 0.71
INT.J.ADHESION AND ADHESIVES APRIL 1994
was activated with sodium hydroxide ~. The role ,:,f tetramethyl thiuram disult]dc is to accelerate the cure reaction. The process involves oxidative coupling of two S H end groups on polysulfide molecules to form an S S , resulting in chain extension. The reaction is complex but has been shown to proceed by a freeradical mechanism 4' 5.
Experimental Adhesive joints
Water uptake Four sheets of each sealant were placed in a humidity cabinet set at 60~'C and 95% RH, on shelves made of stainless steel wires so that both sides were exposed. The samples were periodically removed for weighing. Thermal analysis Samples measuring 10 x 5 x 2 m m were placed in a Polymer Laboratories D M T A Mark 2 dynamic mechanical thermal analyscr. Frequencies of both I
Table 2.
Failure loads of joints on ageing at 60°C and 95% RH
Ageing period (months) 0 1 2 3 4 6 8 10 12 14
Failure load (N) S(5)
S(4.5)
S(4)
S(3.5)
S(3)
S(2.5)
S(2)
474 ± 48 359i40 3704-22 312 ± 20 260 ± 63 210±75 154± 25 1 2 1 ± 19 106± 10
441 + 36 342±48 235i6 198 ± 4 205±34 185±23 134±4 121± 17 130±30
368 ± 8 320±51 235±51 222 ± 41 199±40 157±14 131±5 122±31 119±19
421 4- 10 314+73 208+115 178 ± 20 173 ± 32 129±24 151±39 118±20 69±25
367 ± 22 318±52 216±48 213 ± 55 173±75 134±21 2 0 2 ± 17 96±16 83±4
370 ± 28 201±98 111±15 148 ± 48 125±71 80±72 . . .
139 ± 28 197±18 1 5 8 i 18 150 ± 15 132±25 103±8 83±6 68±10 68±3
.
.
147 ± 15
.
126± 51
.
55i8
-'X- and S(1.4) and S(O.7), all joints fell apart during exposure
and 10 Hz were used and temperature was scanned from - 1 0 0 to +100°C at a heating rate of I°C min -1. The samples examined included unaged ones, others aged at 60°C and 95% RH for 3 or 12 months, and samples aged in these wet conditions and then dried at 45°C for 28 days. Sealants were examined by differential scanning calorimetry (DSC) using either a Mettler DSC or a Stanton Redcroft DSC 700 at a heating rate of IO°C min -I. X-ray photoelectron spectroscopy (XPS)
XPS spectra were obtained using a VG ESCA Lab using AI K~ X-rays; atomic analyses were obtained from peak areas. The electron take-off angle was 90 ° . Measurement of contact angles
The liquids used for the contact-angle measurements were triply distilled water and ethanediol (ED), dimethylsulfoxide (DMSO) and dimethylformamide (DMF) obtained from Romil Chemicals Ltd, Shepshed, UK. 'Gold quality' n-hexadecane (n-HEX) was obtained from the Aldrich Chemical Company. The syringes which were used to place small drops of the liquids on the film surfaces were each dedicated to handle only one of the liquids. Each syringe was rinsed five times with liquid before finally being filled. The drops were placed on areas of the sealants which were flat and free from defects. The volumes of the drops were about 3 #1. Contact angles were measured using a Krfiss G40 Contact Angle Measuring System. Generally three drops of liquid were used, but more drops were used if the results were variable. Measurement of surface interfacial tensions
The surface tension of plasticizer and the interfacial tension between the plasticizer and a silicone oil was measured by the ring-pull method using a torsion balance and ring made by the White Electrical Instrument Co Ltd. The corrections described by Harkins and Jordan 8 were applied. Results and discussion Joint strengths
The failure strengths of all joints which survived to be
tested are shown in Table 2; all joints of the S(1.4) and S(0.7) series failed very quickly during exposure. It can be seen that all joints are weakened by exposure to air at 60°C and 95% RH, and that joints bonded with sealants S(5), S(4.5), S(3.5) and S(2.5) had zero strength after 14 months' exposure. With sealant S(2.5) zero strength was reached after only 8 months. The strengths of joints fall more rapidly during the first few months but most tended to level out after 10 months. The data for joints with S(3.5) are typical of this and are plotted in Fig. 1. Loads at 5% extension showed a small but steady fall over the period of exposure; the data for S(3.5) appear in Fig. 1. Extension at break fell rapidly over the first few months but then levelled out; the data for S(4.5) are shown in Fig, 2. The error bars shown in Figs 1 and 2 are standard deviations, where none are shown error was small. Extensions at break for the unaged joints are given in Table 3. The amount of apparent interfacial failure was assessed visually and it spread from the periphery of the bonded region with exposure time. The amount of interfacial failure is compared with joint strength for one sealant in Fig. 3, showing that weakening of the interface is the primary cause of weakening. The natural colour of the adhesive was grey. However, a brown colour developed at the sides of the joints and spread inwards during exposure. This could 500
250 -
0
I.
I
I
1
0
z,
g
12
TIN~
/ HONTHS
Fig. 1 Force at break (O) and at 5% extension ( 0 ) for joints made with sealant S(3.5) on exposure at 60°C and 95% RH
INT.J.ADHESlON AND ADHESIVES APRIL 1994
87
600 120
1
lOO L,00
f
•
80 50
2O0
L.0
0
I
0
Z,
~ 12
8
0
TIME / MONTHS
0
I
I
f
I
0
a,
B
12
T.IME / MONTHS
Fig. 2 Extension at break for joints made with sealant S(4.5) on exposure at 60°C and 95% RH
be seen through the glass before the joints were tested. The distance which the discoloured area had spread into the side of the joints was measured with a ruler. Fig. 4 shows the growth of the discolouration for joints made with sealant S(5); it is compared with the amount of apparent interfacial failure, indicating that the two parameters are closely related. In fact the discoloured area became the region of interfacial failure when the joint was tested. Joints which were wet-aged and then allowed to dry showed little change in strength over the first 92 days of drying, but distinct increases were seen after drying for 213 and 365 days. The data for joints with sealant S(3.5) (which are typical) are shown in Fig. 5. The mode of failure did not change as the joints dried. In most cases there was a central area of the bond which had not discoloured and this failed cohesively on testing. There was no recovery of the discoloured area. This indicates that the discoloured area is permanently damaged by water and does not recover when the water is removed. The ageing of the joints shows that water plasticizes the sealant, causing the load at 5% extension to fall. Furthermore, water attacks the bonded region quite quickly as shown by the rapid fall in extension at break, the increase in interfacial failure and the growth of the discoloured region. Joints with higher levels of
Fig. 3 Force at break (O) and amount of interracial failure (0) for joints made with sealant S(5) on exposure at 60'C and 95% RH
plasticizer are initially weaker and have higher extensions at break. Joints with very high levels of plasticizer fell apart before they could be tested. Water uptake by the sealants
Measured weight increases for sheets of sealants exposed to air at 60°C and 95% RH were plotted in the form of mass uptake against the square root of time. The diffusion coefficient, D, was obtained from the time for mass uptake to reach half the equilibrium value, t(112), using Equation (1), where L is the thickness of the sampleg: D = O.049L2/t(l.2)
The data for sealant S(2.5), which are typical, are shown in Fig. 6. This plot is typical of Fickian diffusion I° in that the plot is initially linear and leads to water uptake equilibrium. Table 4 lists the diffusion coefficients and levels of water uptake at equilibrium. The behaviour of the joints on ageing cannot be accounted for by water diffusing through the sealant to the interface and weakening it, because the rate of ilO0
°
o/ /-
Table 3. Extension at break and load at 5% extension for unaged joints Sealant
Extension at break (%)
Load at 5% extension (N)
S(5) S(4.5) S(4) S(3.5) S(3) S(2.5) S(2)
57 ± 1 90±5 t08 ± 9 118 ± 7 131 ± 10 147 ± 5 148 ± 8
137 ± 7 116±5 120 ± 8 102 ± 5 82 ± 3 84 ± 2 45 ± 0
88
INT.J.ADHESION AND ADHESIVES APRIL 1994
( 1)
0
~,
0
50
I
I
I
L,
8
12
0
Fig. 4 Width of discoloured region at the edge of the bond (O) compared with the amount of interfacial failure (0), for joints with sealant S(5)
Table 4. Water uptake by the sealants
200
100
0
0
I
I
I
t,
8
12
DRYING
Fig. 5
TIME /
MONTHS
Recovery of joint strength on drying for joints with sealant S(3.5)
water diffusion is too low. This point is illustrated by the information in Table 5, which presents the calculated water concentration at the inner edge of the zone of interfacial failure for joints with sealant S(5). The table includes the value of Dt/l 2 for the joint, the value of C/C] at the edge of the zone and the actual water concentration at this point. If water diffusion were an important factor in debonding, we would expect that the concentration at the edge of the zone would be constant and that the actual concentration of water would be much higher. In fact it would be expected to be a significant proportion of the 45% water absorbed by this sealant at equilibrium. Values of C/CI were calculated from Dt/l 2 using Equation (2) 9 which gives concentrations C within a lamina of thickness 2l immersed in vapour or liquid so that the surface concentration of diffusant C] remains constant, t is time. The origin of coordinates is at the centre of the lamina.
C/C, = - ( 4 / ~ ) ~
[(-1)"/(2n + 1)]
n--0
exp[-D(2n + 1)27z2t/412]cos[(2n + 1)nx/21]
(2)
Formulation
Equilibrium mass uptake (%)
D
S(5) S(4.5) S(4) S(3.5) S(3) S(2.5) S(2) S(1.4) S(0.7)
45 63 70 52 58 50 65 92 63
5.9 6.1 6.8 5.7 4.4 4.4 4.5 4.2 11.0
x 10 -14
(m 2 s -1)
It might have been (but is not the case) that in the polysulfide-glass joints there is a critical water concentration above which the interface is significantly weakened. This has been demonstrated in some other systems. When adhesive joints with metallic adherends are exposed to air of high humidity (e.g., 80-100%), 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 (e.g., 50% RH or less) for long periods without weakening. For example, DeLollislZhas referred to some epoxidealuminium joints which showed no loss of strength after exposure to laboratory humidity for up to 11 years. Giedhill et al. ]3 exposed butt joints with an epoxide adhesive at 55% RH and 20°C for 2500h and found no weakening. In experiments on Ciba-Geigy BSL 312 and Cyanamid FM1000 adhesives both with and without carriers, and on an unmodified epoxide adhesive, Comyn, Brewis and co-workers ]4-17 found no significant weakening of joints after exposure for 10000h at about 45% RH and 20°C. Such information led to the proposal from Gledhill et al. ]3 that there must be a critical concentration of water in the adhesive. 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
60 O
O-"
40
20
I
0 0
30
60
90
(TI~E~/HOURS) T/2
Fig. 6 Water uptake by sealant S(2.5) in air at 95% RH at 60°C
INT.J.ADHESION AND ADHESIVES APRIL 1994
89
Table 5.
Calculations on the amount of water at the edge of the zone of interfacial failure for joints with sealant S(5)
Ageing time (months)
Dt/I 2
Distance of zone f r o m edge (mm)
C/C~
Concentration (%)
2 3
8.6 × 10 3 1.3 × 10 2
0.5 2.0 3.5 5.0
2.3 3.5 4.6 6.9
0.12 0.16 0.21 0.31
4
1.7 × 10 2
6
2.6 × 10 2
with some butt joints bonded with an epoxide adhesive, immersed in water at 20, 40, 60 and 90~C and also in air at 2 0 C and 55% RH. All the water-immersed joints became weaker, and it could be shown using fracture mechanics 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. Brewis et al. ~s found a critical RH of 65% for some aluminium joints bonded with an epoxide adhesive. This means that the critical concentration of water was 1.45%, a value very similar to that of 1.35% obtained by Gledhill et al. l-~. Because polysulfides absorb much more water than epoxides 11 about 10 times as much, it might be expected that critical water concentrations for the sealants would be about 1(~15%, much more than the very low values calculated in Table 5. Examination of failure surfaces
The glass surface from the ruptured bond with sealant S(1.4), which contained a high proportion of plasticizer, was rinsed with diethyl ether; the material removed was examined by infra-red spectrophotometry. The spectrum obtained was identical with that of the plasticizer. Weakening of the interface could be due to plasticizer diffusing to the interface. Indeed, joints with sealants S(1.4) and S(0.7), which contained the greatest proportion of plasticizer, all failed catastrophically on humidity ageing. X-ray photoelectron spectra were obtained on glass surfaces taken from joints with sealant S(3) after various periods of ageing and are shown in Table 6. Ignoring hydrogen which is not detected by xps, the atomic composition of the polysulfide is 53.7% C,
:,, 10 3 :< 10 3 × 10 3 x 10 3
32.8% S and 13.4% O, and that of the plasticizer is 81.8% C and 18.2% O. The composition of the matrix of sealant S(3) would be 60.8% C, 24.6% S and 14.6% O. The analysis of surfaces for aged samples show an increase in oxygen and a reduction in sulfur, which is consistent with plasticizer displacing polymer at the interface. As the amounts of the elements Mg, Ca, Si and Na (which are present in glass) generally increase on ageing, the layer of organic substances left on the glass becomes thinner. Interfacial sulfur decreases but oxygen increases on ageing, which also indicates that the amount of plasticizer at the failure surface increases. It is possible, however, that the migration of plasticizer to the interface occurs after it has failed. Thermal analysis
Dynamic mechanical thermal analysis (DMTA) Fig. 7 shows the DMTA d a t a f o r sealant S(4.5) after wet
ageing. The unaged sealants all showed a single peak in the region - 2 4 to - 3 0 : C which is the glass transition temperature of the unplasticized sealant. This shows that although termed a plasticizer, the material used does not plasticize in the usual manner, that is by lowering the glass transition temperature. On wet ageing the position of this peak did not change but a second peak appeared in the region 3 5 C . The latter peak is due to the melting of water and was absent in samples that had been wet-aged and then dried. The data show that water does not plasticize the sealant, and that most is isolated as droplets.
I
',,-F25
.5
L
1
A-.'
[
i
/,/!~2<'A, ! ' \ x ,'
ii } ' , \
/
Table 6. Elemental analysis (atomic %) by XPS of glass surface from joints with sealant S{3) A g e i n g t i m e (months) Element
Peak
1
3
10
C O S Mg Ca
ls ls 2p A 2p
72 20 5.3 2.1 0.9 0.0 0.0
67 20 3.0 4.6 1.3 3.0 1.1
62 22 0.0 3.3 1.6 7.7 3.0
~L k
i' /
Xx \
2p
Na
A
90
INT.J.ADHESION A N D ADHESIVES APRIL 1994
\
!
.~_
-- ~
/]
,'.5~
[ F
'I
i
Si
-~
i
F
. . . . . . . . . .
I£
I I I
i o
~
1oo
Fig. 7 D y n a m i c m e c h a n i c a l t h e r m a l a n a l y s i s of s e a l a n t S(4.5) after a g e i n g at 60~C and 9 5 % RH, at 1 Hz (solid line) and 10Hz (broken line)
DSC
Samples which had been wet-aged showed a prominent peak slightly above 0°C, which is due to the melting of absorbed water. Thermodynamic work of adhesion
Contact angles were measured for three drops on the cured sealant surfaces and were reproducible to within 2°; values appear in Table 7. Values of the dispersive and polar components of surface free energies (7D and 7sP) of the sealants, polysulfide and glass have been calculated from these and are shown in Table 8. Thermodynamic work of adhesion of sealants to glass have also been calculated in dry conditions (WA) and in the presence of water (WA, w) and are given in Table 9. Details of the methods of calculation and of estimating errors appear in the literature 19. Contact angles of the four liquids on the sealant surfaces are very similar, and the values of the polar and dispersive components of surface free energy for them are the same within experimental error. This obviously leads to the situation that values of thermodynamic work of adhesion in dry and wet conditions do not vary for the sealants. The latter values show that all sealant-glass bonds are stable in dry conditions (i.e., WA is positive). Because of the experimental errors there is doubt about whether they are unstable in the presence of water, even though the mean values of WA, w are actually negative. Displacement by plasticizer
The surface tension of a liquid is the sum of dispersive and polar components (Equation (3)) and the interfacial tension (h 2) between two liquids (1 and 2) is given by Equation i4)2°: ?L = 7D L + 7cP
(3)
?12 = 71 q- 72 -- 2(71D72D)1/2 -- 2(71PYP)1/2
(4)
Interfacial tension measured between silicone fluid and plasticizer was 4.0 mN m ~. The surface tension of silicone was 18.5 mN m J and that of the plasticizer was 37.0 mN m -~. By making the assumption that the polar component of surface tension for the silicone is zero, Equation (4) gives a value of the dispersive Table 7. Contact angles in degrees of liquids on cured sealant and glass surfaces Liquid Sealant
DMF
DMSO
ED
Water
S(5) S(4.5) S(4) S(3.5) S(3) S(2.5) S(2) S(1.4) S(1) Cured PS only Glass
0 0 0 0 0 0 0 0 0 0 10
28 17 18.5 15 20 18 17 12 12 29 17.5
52 64 59 57.5 56 61 44 60 53 43 28
71 70 59 75 70.5 78 62.5 72 65 75 17.5
Table 8. Dispersion and polar components of surface free energies of sealants and glass Sealant
7~ (mJ m 2)
7e (mJ m -2)
S(5) S(4.5) S(4) S(3.5) S(3) S(2.5) S(2) S(1.4) S(1) Cured PS only Glass
26 ± 3 2 5 ± 10 1 9 ± 10 29 ± 7 26 ± 7 31 ± 8 23 ± 4 27 ± 9 24±7 30 + 0.3 10 ± 5
11 ± 5 11 ± 8 21 ± 11 7.4 ± 4.0 11 ± 5 5.6 ± 3.7 17 ± 4 9.4 ± 5.7 15±6 8.1 ± 0.2 60 ± 13
Table 9. Thermodynamic work of adhesion of sealant to glass interfaces in dry conditions and in the presence of water Sealant
WA (mJ m -2)
WA,W (mJ m -2)
S(5) S(4.5) S(4) S(3.5) S(3) S(2.5) S(2) S(1.4) S(1) Cured PS only
83 ± 82 ± 98 ± 76 ± 82 ± 80 ± 95 ± 80 ± 90± 78 ±
- 6 ± 23 - 5 ± 32 - 2 ± 33 - 7 ± 24 - 6 ± 25 - 8 ± 25 - 4 ± 21 - 6 ± 28 4±26 7 ± 17
15 21 22 15 16 16 13 18 17 10
component for the plasticizer of 35.8 mN m ~; hence the polar component is 1.2 mN m '. The work of adhesion between polymer and glass in the presence of plasticizer, calculated from these values, is 26 :t: 14 mN m -1, showing that plasticizer cannot displace the polymer from glass. Conclusions
1) Sealant-to-glass joints are weakened on exposure to air at 95% RH and 60°C, and some with higher levels of plasticizer fall apart during exposure. 2) The amount of visually assessed interfacial failure for tested joints increases with exposure, and is closely related to a discolouration which develops in the sealant and can be seen through the glass. This area is permanently damaged and does not recover on drying. 3) The sealants absorb large amounts of water (4592%) but most of this is located in droplets. Water attacks the interface. 4) Values of diffusion coefficients of water in the various sealants are very similar. The rate of joint weakening is greater than can be accounted for by the rate at which water diffuses into the sealant. 5) Thermodynamic work of adhesion of sealant-toglass bonds is positive in dry conditions (about 80 -4- 20 mJ m -t) but probably slightly negative in the presence of water (about - 5 ± 25 mJ m-Z),
INT.J.ADHESION AND ADHESIVES APRIL 1994
91
indicating that water may displace the sealant from glass. Plasticizer cannot displace polysulfide from glass as the work of adhesion in its presence is positive (26 ± 14mJ m :). References 1 2
t2
Garrido, M.J. G/ass Age (September 1988) p 90 "European Insu/ated G/azing Market" (Morton International Ltd, Coventry, UK, 1988) US Patent 4 104 189 (1 August 1978) Coates, R.J. DPhi/Thesis (University of York, UK, 1993) Capozzi, G. and Modena, G. in "The Chemistry of the Thio/Group; Part 2 edited by S. Patai (John Wiley & Sons, Chichester, 1974) chapter 7 Shelby, J.E., Vitko Jr, J. and Paritano, C.G. So~at Energy Mater 3 (1980) p 97 'Specification for polysulphide based construction sealants' BS 4254 (British Standards Institution, London) Harkina, W.D. and Jordan, H.F. J Amer Chem Soc 52 (1930) p 1751 Crank, J. "Mathematics of Diffusion; 2nd edition (Oxford University Press, 1975) Fujita, H. Adv Po/ym Sci 3 (1961) p 1 Comyn, J. in "Durability of Structura/Adhesives" edited by A.J. Kinloch (Applied Science Publishers, London, 1983) chapter 3 DeLollis, N.J. Nat SAMPE Symph Exhib 22 (1977) p 673
92
INT.J.ADHESION AND ADHESIVES APRIL 1994
3 4 5
6 7 8 9 10 11
13 14 15 16 17 18 19 20
Gledhill, 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) p 344 Brewis, D.M., Comyn, J., Cope, B.C. and Moloney, A.C. Polymer 21 (1980) p 1477 Brewis, D.M., Comyn, J., Cope, B.C. and Moloney, A.C. Polym Engng Sci21 (1981) p 797 Brewia, D.M., Comyn, J. and Tegg, J.L. Int J Adhesion and Adhesives 1 (1980) p 35 Brewia, D.M., Comyn, J., Raval, A.K. and Kinloch, A.J. Int J Adhesion and Adhesives 10 (1990) p 247 Blackley, D.C., Comyn, J. and Harding, L.M. Int J Adhesion and Adhesives 13 (1993) p 163 Fowkes, F.M. Ind Eng Chem 56 No 12 (1964) p 40
Authors
G.B. Lowe and T.C.P. Lee are with Morton International Ltd, University of Warwick Science Park, Sir William Lyons Road, Coventry CV4 7EZ, UK. J. Comyn, to whom correspondence should be addressed, and K. Huddersman are with the Department of Chemistry at De Montfort University, Leicester LE1 9BH, UK.
Joints consisting of glass bonded with polysulfide sealants have been aged at 95% relative humidity and 60°C for up to 14 months. All were weakened and some with higher levels of plasticizer fell apart during exposure. The amount of interfacial failure increased with exposure, and was related to a discolouration which develops in the sealant and can be seen through the glass. The rate of joint weakening is greater than can be accounted for by the rate at which water diffuses into the sealant. Thermodynamic predictions based on measurements of contact angles show that the bonds are probably unstable in the presence of water.
Key words: adhesive joints; environmental testing; durability; polysulfide sealant; glass
Double glazing is a large market for flat glass and the amount used [ for this purpose in the U K in 1988 was 1.7 x l 0 7 m 2. The edges of such units are sealed with a polymer-based sealing material, the purpose of which is to hold the unit together and to provide a barrier against the permeation of water into the unit. It must also be resistant to glazing compounds and paints used in installation. Polysulfides are the dominant polymers in this application accounting for about 80% of the market; polyurethanes, pol~cisobutenes and polysiloxanes are also use&. Insulated glass units have anticipated service lives well above 10 years, but premature failure can occur. The manifestation of failure is misting of the internal surfaces of the unit. The major cause of failure is adhesion loss between sealant and glass. • A typical insulated glass unit consists of two panes of 4 mm thick glass separated by a hollow spacer-tube. The tube is generally of aluminium and filled with a desiccant, either silica gel or zeolite molecular seive. Holes on the inner face of the tube allow water vapour in the unit access to the desiccant. In the present work, the effect of water on polysulfide sealants, such as may be used in insulated
glass units, was studied. This information was used to assess their durability.
Materials The sealants consisted of two components, the base and the curative.
Base compound A number of bases were formulated using the following components: polysulfide, precipitated and ground calcium carbonate, bentonite, titanium dioxide, plasticizer and a silane coupling agent. The polysulfide was LP2C made by Morton International. It is made by reacting bis(2-chloroethyl formal) with sodium polysulfide and has the structure shown below; the average value of x is 2.1 and that of n is 22.
H S-(CH2-CH2-O-CH2-O-CH2-CH2-S_0n-H In addition, a small amount of trichloropropane (2 mol%) is added to give branch points, which will lead to crosslinks when the sealant is cured.
0143-7496/94/02/0085-08 © 1994 Butterworth-Heinemann Ltd INT.J.ADHESlON AND ADHESIVES VOL. 14 NO. 2 APRIL 1994
85
Fillers are added to the polysulfide to reduce cost and modify rheological properties. Precipitated calcium carbonates have much smaller particles than their ground counterparts. About 90% of the particles are smaller than 1 pm, in comparison to the ground which has a distribution in the range 2 10pm. A coat of stearic acid on the calcium carbonate particles increases the viscosity of the base compound and introduces thixotropy. The precipitated material Winnofil SP made by ICI and the ground material Polcarb S made by ECC were used; both were coated with stearic acid. Titanium dioxide is added as a whitening pigment and to assist in visualizing the complete mixing of the base and curative. The rutile grade used was supplied by BTP Ltd. Plasticizers are used to lower the viscosity of the base compound. Here, Santiciser 278 from Monsanto was used: it is Texanol benzyl phthalate. Texanol is 2,2,4trimethyl-pentan- 1,3-diol monoisobutyrate. Bentonite in the form of Bentone SD-2 (Bakers Castor Oil Co) is added as a thixotropic agent. 3Glycidoxypropyltrimethoxysilane is added as an adhesion promoter. All the bases contained 25 parts by weight of each type of calcium carbonate. The amount of titanium dioxide was 25 parts, and 3 parts of both bentonite and silane coupling agent were used. The amounts of polysulfide and plasticizer were varied and are shown in Table 1, Each sealant is referred to by a code which gives the proportion of polysulfide and plasticizer; e.g., the sealant with 70 parts of polysulfide and 20 parts of plasticizer is designated as S(3.5). The components were mixed by first adding the calcium carbonates and titanium dioxide to the liquid polysulfide in a high shear mixer over a period of 15 to 20 rain to form a viscous paste. The thixotrope and adhesion promoter were then added and mixing continued for 15 rain. Finally the plasticizer was added.
Glass-to-sealant adhesive joints were prepared in the following manner. Squares measuring 50 x 50ram were cut from 4 mm float glass and cleaned twice with a proprietary glass cleaner which was a mixture of water, 2-propanol and ammonia. No account was taken of the fact that the two faces of float glass are different, one lace being richer in tin than the other ~. The sealant [brmed a square prism 12 × 12mm between the two sheets of glass. These are known as 'H-bonds' and their preparation is fully described in a British Standard 7. Following assembly, the joints were allowed to cure at 23 C and 50% relative humidity (RH) for 7 days before the average strength was measured for four joints. The remaining joints were aged for up to 14 months in the environmental chamber set at 60'C and 95% RH. After ageing in the chamber for 12 months, some joints were dried at 23'C and 50% RH in an airconditioned laboratory for up to I year. Within 1 h of removal from the various storage conditions, the assemblies were tested in an lnstron 1026 tensometer using clamps designed for this purpose. The glass squares were separated at a speed of 6 mm rain ~ to failure. The load and extension at break, the load at 5% extension and the locus of failure were noted. Four .joints were used for each experimental point.
Curative
Preparation of cured sheets
The curative was a paste consisting of manganese(IV) dioxide, plasticizer and tetramethyl thiuram disulfide in the ratio of 20:20:1 parts by weight. Manganese dioxides used for this purpose are not pure compounds, but are activated by the addition of alkali metal salts or hydroxides. The product used here was manganese dioxide FA grade supplied by Riedel de Haen, which
Sheets of the sealants were made by first mixing the components such that the ratio of polysulfide to manganese dioxide was 10:1 by weight. After mixing for 10 rain the mix was placed on siliconized release paper, a metal former 2 m m thick was placed around the sample and a second sheet of siliconized paper placed on top. The sandwich was then placed between two sheets of 10ram thick glass and the whole sample pressed flat. The assembly was left to cure at room temperature overnight. After removing the siliconized paper, the sheets were left to cure for a further 6 days at 23:'C and 50% RH. The samples were trimmed to give specimens measuring 200 x 20 x 2 ram. Sheet thicknesses were measured with a micrometer.
Table 1. Amounts of polysulfide and plasticizer in base compounds Reference code 5 4.5 4 3.5 3 2.5 2 1.4 1.0 0.7
86
A m o u n t (parts by weight) Polysulfide
Plasticizer
100 90 80 70 60 50 80 70 60 50
20 20 20 20 20 20 40 50 60 70
Polysulfide Plasticizer 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.4 1.0 0.71
INT.J.ADHESION AND ADHESIVES APRIL 1994
was activated with sodium hydroxide ~. The role ,:,f tetramethyl thiuram disult]dc is to accelerate the cure reaction. The process involves oxidative coupling of two S H end groups on polysulfide molecules to form an S S , resulting in chain extension. The reaction is complex but has been shown to proceed by a freeradical mechanism 4' 5.
Experimental Adhesive joints
Water uptake Four sheets of each sealant were placed in a humidity cabinet set at 60~'C and 95% RH, on shelves made of stainless steel wires so that both sides were exposed. The samples were periodically removed for weighing. Thermal analysis Samples measuring 10 x 5 x 2 m m were placed in a Polymer Laboratories D M T A Mark 2 dynamic mechanical thermal analyscr. Frequencies of both I
Table 2.
Failure loads of joints on ageing at 60°C and 95% RH
Ageing period (months) 0 1 2 3 4 6 8 10 12 14
Failure load (N) S(5)
S(4.5)
S(4)
S(3.5)
S(3)
S(2.5)
S(2)
474 ± 48 359i40 3704-22 312 ± 20 260 ± 63 210±75 154± 25 1 2 1 ± 19 106± 10
441 + 36 342±48 235i6 198 ± 4 205±34 185±23 134±4 121± 17 130±30
368 ± 8 320±51 235±51 222 ± 41 199±40 157±14 131±5 122±31 119±19
421 4- 10 314+73 208+115 178 ± 20 173 ± 32 129±24 151±39 118±20 69±25
367 ± 22 318±52 216±48 213 ± 55 173±75 134±21 2 0 2 ± 17 96±16 83±4
370 ± 28 201±98 111±15 148 ± 48 125±71 80±72 . . .
139 ± 28 197±18 1 5 8 i 18 150 ± 15 132±25 103±8 83±6 68±10 68±3
.
.
147 ± 15
.
126± 51
.
55i8
-'X- and S(1.4) and S(O.7), all joints fell apart during exposure
and 10 Hz were used and temperature was scanned from - 1 0 0 to +100°C at a heating rate of I°C min -1. The samples examined included unaged ones, others aged at 60°C and 95% RH for 3 or 12 months, and samples aged in these wet conditions and then dried at 45°C for 28 days. Sealants were examined by differential scanning calorimetry (DSC) using either a Mettler DSC or a Stanton Redcroft DSC 700 at a heating rate of IO°C min -I. X-ray photoelectron spectroscopy (XPS)
XPS spectra were obtained using a VG ESCA Lab using AI K~ X-rays; atomic analyses were obtained from peak areas. The electron take-off angle was 90 ° . Measurement of contact angles
The liquids used for the contact-angle measurements were triply distilled water and ethanediol (ED), dimethylsulfoxide (DMSO) and dimethylformamide (DMF) obtained from Romil Chemicals Ltd, Shepshed, UK. 'Gold quality' n-hexadecane (n-HEX) was obtained from the Aldrich Chemical Company. The syringes which were used to place small drops of the liquids on the film surfaces were each dedicated to handle only one of the liquids. Each syringe was rinsed five times with liquid before finally being filled. The drops were placed on areas of the sealants which were flat and free from defects. The volumes of the drops were about 3 #1. Contact angles were measured using a Krfiss G40 Contact Angle Measuring System. Generally three drops of liquid were used, but more drops were used if the results were variable. Measurement of surface interfacial tensions
The surface tension of plasticizer and the interfacial tension between the plasticizer and a silicone oil was measured by the ring-pull method using a torsion balance and ring made by the White Electrical Instrument Co Ltd. The corrections described by Harkins and Jordan 8 were applied. Results and discussion Joint strengths
The failure strengths of all joints which survived to be
tested are shown in Table 2; all joints of the S(1.4) and S(0.7) series failed very quickly during exposure. It can be seen that all joints are weakened by exposure to air at 60°C and 95% RH, and that joints bonded with sealants S(5), S(4.5), S(3.5) and S(2.5) had zero strength after 14 months' exposure. With sealant S(2.5) zero strength was reached after only 8 months. The strengths of joints fall more rapidly during the first few months but most tended to level out after 10 months. The data for joints with S(3.5) are typical of this and are plotted in Fig. 1. Loads at 5% extension showed a small but steady fall over the period of exposure; the data for S(3.5) appear in Fig. 1. Extension at break fell rapidly over the first few months but then levelled out; the data for S(4.5) are shown in Fig, 2. The error bars shown in Figs 1 and 2 are standard deviations, where none are shown error was small. Extensions at break for the unaged joints are given in Table 3. The amount of apparent interfacial failure was assessed visually and it spread from the periphery of the bonded region with exposure time. The amount of interfacial failure is compared with joint strength for one sealant in Fig. 3, showing that weakening of the interface is the primary cause of weakening. The natural colour of the adhesive was grey. However, a brown colour developed at the sides of the joints and spread inwards during exposure. This could 500
250 -
0
I.
I
I
1
0
z,
g
12
TIN~
/ HONTHS
Fig. 1 Force at break (O) and at 5% extension ( 0 ) for joints made with sealant S(3.5) on exposure at 60°C and 95% RH
INT.J.ADHESlON AND ADHESIVES APRIL 1994
87
600 120
1
lOO L,00
f
•
80 50
2O0
L.0
0
I
0
Z,
~ 12
8
0
TIME / MONTHS
0
I
I
f
I
0
a,
B
12
T.IME / MONTHS
Fig. 2 Extension at break for joints made with sealant S(4.5) on exposure at 60°C and 95% RH
be seen through the glass before the joints were tested. The distance which the discoloured area had spread into the side of the joints was measured with a ruler. Fig. 4 shows the growth of the discolouration for joints made with sealant S(5); it is compared with the amount of apparent interfacial failure, indicating that the two parameters are closely related. In fact the discoloured area became the region of interfacial failure when the joint was tested. Joints which were wet-aged and then allowed to dry showed little change in strength over the first 92 days of drying, but distinct increases were seen after drying for 213 and 365 days. The data for joints with sealant S(3.5) (which are typical) are shown in Fig. 5. The mode of failure did not change as the joints dried. In most cases there was a central area of the bond which had not discoloured and this failed cohesively on testing. There was no recovery of the discoloured area. This indicates that the discoloured area is permanently damaged by water and does not recover when the water is removed. The ageing of the joints shows that water plasticizes the sealant, causing the load at 5% extension to fall. Furthermore, water attacks the bonded region quite quickly as shown by the rapid fall in extension at break, the increase in interfacial failure and the growth of the discoloured region. Joints with higher levels of
Fig. 3 Force at break (O) and amount of interracial failure (0) for joints made with sealant S(5) on exposure at 60'C and 95% RH
plasticizer are initially weaker and have higher extensions at break. Joints with very high levels of plasticizer fell apart before they could be tested. Water uptake by the sealants
Measured weight increases for sheets of sealants exposed to air at 60°C and 95% RH were plotted in the form of mass uptake against the square root of time. The diffusion coefficient, D, was obtained from the time for mass uptake to reach half the equilibrium value, t(112), using Equation (1), where L is the thickness of the sampleg: D = O.049L2/t(l.2)
The data for sealant S(2.5), which are typical, are shown in Fig. 6. This plot is typical of Fickian diffusion I° in that the plot is initially linear and leads to water uptake equilibrium. Table 4 lists the diffusion coefficients and levels of water uptake at equilibrium. The behaviour of the joints on ageing cannot be accounted for by water diffusing through the sealant to the interface and weakening it, because the rate of ilO0
°
o/ /-
Table 3. Extension at break and load at 5% extension for unaged joints Sealant
Extension at break (%)
Load at 5% extension (N)
S(5) S(4.5) S(4) S(3.5) S(3) S(2.5) S(2)
57 ± 1 90±5 t08 ± 9 118 ± 7 131 ± 10 147 ± 5 148 ± 8
137 ± 7 116±5 120 ± 8 102 ± 5 82 ± 3 84 ± 2 45 ± 0
88
INT.J.ADHESION AND ADHESIVES APRIL 1994
( 1)
0
~,
0
50
I
I
I
L,
8
12
0
Fig. 4 Width of discoloured region at the edge of the bond (O) compared with the amount of interfacial failure (0), for joints with sealant S(5)
Table 4. Water uptake by the sealants
200
100
0
0
I
I
I
t,
8
12
DRYING
Fig. 5
TIME /
MONTHS
Recovery of joint strength on drying for joints with sealant S(3.5)
water diffusion is too low. This point is illustrated by the information in Table 5, which presents the calculated water concentration at the inner edge of the zone of interfacial failure for joints with sealant S(5). The table includes the value of Dt/l 2 for the joint, the value of C/C] at the edge of the zone and the actual water concentration at this point. If water diffusion were an important factor in debonding, we would expect that the concentration at the edge of the zone would be constant and that the actual concentration of water would be much higher. In fact it would be expected to be a significant proportion of the 45% water absorbed by this sealant at equilibrium. Values of C/CI were calculated from Dt/l 2 using Equation (2) 9 which gives concentrations C within a lamina of thickness 2l immersed in vapour or liquid so that the surface concentration of diffusant C] remains constant, t is time. The origin of coordinates is at the centre of the lamina.
C/C, = - ( 4 / ~ ) ~
[(-1)"/(2n + 1)]
n--0
exp[-D(2n + 1)27z2t/412]cos[(2n + 1)nx/21]
(2)
Formulation
Equilibrium mass uptake (%)
D
S(5) S(4.5) S(4) S(3.5) S(3) S(2.5) S(2) S(1.4) S(0.7)
45 63 70 52 58 50 65 92 63
5.9 6.1 6.8 5.7 4.4 4.4 4.5 4.2 11.0
x 10 -14
(m 2 s -1)
It might have been (but is not the case) that in the polysulfide-glass joints there is a critical water concentration above which the interface is significantly weakened. This has been demonstrated in some other systems. When adhesive joints with metallic adherends are exposed to air of high humidity (e.g., 80-100%), 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 (e.g., 50% RH or less) for long periods without weakening. For example, DeLollislZhas referred to some epoxidealuminium joints which showed no loss of strength after exposure to laboratory humidity for up to 11 years. Giedhill et al. ]3 exposed butt joints with an epoxide adhesive at 55% RH and 20°C for 2500h and found no weakening. In experiments on Ciba-Geigy BSL 312 and Cyanamid FM1000 adhesives both with and without carriers, and on an unmodified epoxide adhesive, Comyn, Brewis and co-workers ]4-17 found no significant weakening of joints after exposure for 10000h at about 45% RH and 20°C. Such information led to the proposal from Gledhill et al. ]3 that there must be a critical concentration of water in the adhesive. 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
60 O
O-"
40
20
I
0 0
30
60
90
(TI~E~/HOURS) T/2
Fig. 6 Water uptake by sealant S(2.5) in air at 95% RH at 60°C
INT.J.ADHESION AND ADHESIVES APRIL 1994
89
Table 5.
Calculations on the amount of water at the edge of the zone of interfacial failure for joints with sealant S(5)
Ageing time (months)
Dt/I 2
Distance of zone f r o m edge (mm)
C/C~
Concentration (%)
2 3
8.6 × 10 3 1.3 × 10 2
0.5 2.0 3.5 5.0
2.3 3.5 4.6 6.9
0.12 0.16 0.21 0.31
4
1.7 × 10 2
6
2.6 × 10 2
with some butt joints bonded with an epoxide adhesive, immersed in water at 20, 40, 60 and 90~C and also in air at 2 0 C and 55% RH. All the water-immersed joints became weaker, and it could be shown using fracture mechanics 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. Brewis et al. ~s found a critical RH of 65% for some aluminium joints bonded with an epoxide adhesive. This means that the critical concentration of water was 1.45%, a value very similar to that of 1.35% obtained by Gledhill et al. l-~. Because polysulfides absorb much more water than epoxides 11 about 10 times as much, it might be expected that critical water concentrations for the sealants would be about 1(~15%, much more than the very low values calculated in Table 5. Examination of failure surfaces
The glass surface from the ruptured bond with sealant S(1.4), which contained a high proportion of plasticizer, was rinsed with diethyl ether; the material removed was examined by infra-red spectrophotometry. The spectrum obtained was identical with that of the plasticizer. Weakening of the interface could be due to plasticizer diffusing to the interface. Indeed, joints with sealants S(1.4) and S(0.7), which contained the greatest proportion of plasticizer, all failed catastrophically on humidity ageing. X-ray photoelectron spectra were obtained on glass surfaces taken from joints with sealant S(3) after various periods of ageing and are shown in Table 6. Ignoring hydrogen which is not detected by xps, the atomic composition of the polysulfide is 53.7% C,
:,, 10 3 :< 10 3 × 10 3 x 10 3
32.8% S and 13.4% O, and that of the plasticizer is 81.8% C and 18.2% O. The composition of the matrix of sealant S(3) would be 60.8% C, 24.6% S and 14.6% O. The analysis of surfaces for aged samples show an increase in oxygen and a reduction in sulfur, which is consistent with plasticizer displacing polymer at the interface. As the amounts of the elements Mg, Ca, Si and Na (which are present in glass) generally increase on ageing, the layer of organic substances left on the glass becomes thinner. Interfacial sulfur decreases but oxygen increases on ageing, which also indicates that the amount of plasticizer at the failure surface increases. It is possible, however, that the migration of plasticizer to the interface occurs after it has failed. Thermal analysis
Dynamic mechanical thermal analysis (DMTA) Fig. 7 shows the DMTA d a t a f o r sealant S(4.5) after wet
ageing. The unaged sealants all showed a single peak in the region - 2 4 to - 3 0 : C which is the glass transition temperature of the unplasticized sealant. This shows that although termed a plasticizer, the material used does not plasticize in the usual manner, that is by lowering the glass transition temperature. On wet ageing the position of this peak did not change but a second peak appeared in the region 3 5 C . The latter peak is due to the melting of water and was absent in samples that had been wet-aged and then dried. The data show that water does not plasticize the sealant, and that most is isolated as droplets.
I
',,-F25
.5
L
1
A-.'
[
i
/,/!~2<'A, ! ' \ x ,'
ii } ' , \
/
Table 6. Elemental analysis (atomic %) by XPS of glass surface from joints with sealant S{3) A g e i n g t i m e (months) Element
Peak
1
3
10
C O S Mg Ca
ls ls 2p A 2p
72 20 5.3 2.1 0.9 0.0 0.0
67 20 3.0 4.6 1.3 3.0 1.1
62 22 0.0 3.3 1.6 7.7 3.0
~L k
i' /
Xx \
2p
Na
A
90
INT.J.ADHESION A N D ADHESIVES APRIL 1994
\
!
.~_
-- ~
/]
,'.5~
[ F
'I
i
Si
-~
i
F
. . . . . . . . . .
I£
I I I
i o
~
1oo
Fig. 7 D y n a m i c m e c h a n i c a l t h e r m a l a n a l y s i s of s e a l a n t S(4.5) after a g e i n g at 60~C and 9 5 % RH, at 1 Hz (solid line) and 10Hz (broken line)
DSC
Samples which had been wet-aged showed a prominent peak slightly above 0°C, which is due to the melting of absorbed water. Thermodynamic work of adhesion
Contact angles were measured for three drops on the cured sealant surfaces and were reproducible to within 2°; values appear in Table 7. Values of the dispersive and polar components of surface free energies (7D and 7sP) of the sealants, polysulfide and glass have been calculated from these and are shown in Table 8. Thermodynamic work of adhesion of sealants to glass have also been calculated in dry conditions (WA) and in the presence of water (WA, w) and are given in Table 9. Details of the methods of calculation and of estimating errors appear in the literature 19. Contact angles of the four liquids on the sealant surfaces are very similar, and the values of the polar and dispersive components of surface free energy for them are the same within experimental error. This obviously leads to the situation that values of thermodynamic work of adhesion in dry and wet conditions do not vary for the sealants. The latter values show that all sealant-glass bonds are stable in dry conditions (i.e., WA is positive). Because of the experimental errors there is doubt about whether they are unstable in the presence of water, even though the mean values of WA, w are actually negative. Displacement by plasticizer
The surface tension of a liquid is the sum of dispersive and polar components (Equation (3)) and the interfacial tension (h 2) between two liquids (1 and 2) is given by Equation i4)2°: ?L = 7D L + 7cP
(3)
?12 = 71 q- 72 -- 2(71D72D)1/2 -- 2(71PYP)1/2
(4)
Interfacial tension measured between silicone fluid and plasticizer was 4.0 mN m ~. The surface tension of silicone was 18.5 mN m J and that of the plasticizer was 37.0 mN m -~. By making the assumption that the polar component of surface tension for the silicone is zero, Equation (4) gives a value of the dispersive Table 7. Contact angles in degrees of liquids on cured sealant and glass surfaces Liquid Sealant
DMF
DMSO
ED
Water
S(5) S(4.5) S(4) S(3.5) S(3) S(2.5) S(2) S(1.4) S(1) Cured PS only Glass
0 0 0 0 0 0 0 0 0 0 10
28 17 18.5 15 20 18 17 12 12 29 17.5
52 64 59 57.5 56 61 44 60 53 43 28
71 70 59 75 70.5 78 62.5 72 65 75 17.5
Table 8. Dispersion and polar components of surface free energies of sealants and glass Sealant
7~ (mJ m 2)
7e (mJ m -2)
S(5) S(4.5) S(4) S(3.5) S(3) S(2.5) S(2) S(1.4) S(1) Cured PS only Glass
26 ± 3 2 5 ± 10 1 9 ± 10 29 ± 7 26 ± 7 31 ± 8 23 ± 4 27 ± 9 24±7 30 + 0.3 10 ± 5
11 ± 5 11 ± 8 21 ± 11 7.4 ± 4.0 11 ± 5 5.6 ± 3.7 17 ± 4 9.4 ± 5.7 15±6 8.1 ± 0.2 60 ± 13
Table 9. Thermodynamic work of adhesion of sealant to glass interfaces in dry conditions and in the presence of water Sealant
WA (mJ m -2)
WA,W (mJ m -2)
S(5) S(4.5) S(4) S(3.5) S(3) S(2.5) S(2) S(1.4) S(1) Cured PS only
83 ± 82 ± 98 ± 76 ± 82 ± 80 ± 95 ± 80 ± 90± 78 ±
- 6 ± 23 - 5 ± 32 - 2 ± 33 - 7 ± 24 - 6 ± 25 - 8 ± 25 - 4 ± 21 - 6 ± 28 4±26 7 ± 17
15 21 22 15 16 16 13 18 17 10
component for the plasticizer of 35.8 mN m ~; hence the polar component is 1.2 mN m '. The work of adhesion between polymer and glass in the presence of plasticizer, calculated from these values, is 26 :t: 14 mN m -1, showing that plasticizer cannot displace the polymer from glass. Conclusions
1) Sealant-to-glass joints are weakened on exposure to air at 95% RH and 60°C, and some with higher levels of plasticizer fall apart during exposure. 2) The amount of visually assessed interfacial failure for tested joints increases with exposure, and is closely related to a discolouration which develops in the sealant and can be seen through the glass. This area is permanently damaged and does not recover on drying. 3) The sealants absorb large amounts of water (4592%) but most of this is located in droplets. Water attacks the interface. 4) Values of diffusion coefficients of water in the various sealants are very similar. The rate of joint weakening is greater than can be accounted for by the rate at which water diffuses into the sealant. 5) Thermodynamic work of adhesion of sealant-toglass bonds is positive in dry conditions (about 80 -4- 20 mJ m -t) but probably slightly negative in the presence of water (about - 5 ± 25 mJ m-Z),
INT.J.ADHESION AND ADHESIVES APRIL 1994
91
indicating that water may displace the sealant from glass. Plasticizer cannot displace polysulfide from glass as the work of adhesion in its presence is positive (26 ± 14mJ m :). References 1 2
t2
Garrido, M.J. G/ass Age (September 1988) p 90 "European Insu/ated G/azing Market" (Morton International Ltd, Coventry, UK, 1988) US Patent 4 104 189 (1 August 1978) Coates, R.J. DPhi/Thesis (University of York, UK, 1993) Capozzi, G. and Modena, G. in "The Chemistry of the Thio/Group; Part 2 edited by S. Patai (John Wiley & Sons, Chichester, 1974) chapter 7 Shelby, J.E., Vitko Jr, J. and Paritano, C.G. So~at Energy Mater 3 (1980) p 97 'Specification for polysulphide based construction sealants' BS 4254 (British Standards Institution, London) Harkina, W.D. and Jordan, H.F. J Amer Chem Soc 52 (1930) p 1751 Crank, J. "Mathematics of Diffusion; 2nd edition (Oxford University Press, 1975) Fujita, H. Adv Po/ym Sci 3 (1961) p 1 Comyn, J. in "Durability of Structura/Adhesives" edited by A.J. Kinloch (Applied Science Publishers, London, 1983) chapter 3 DeLollis, N.J. Nat SAMPE Symph Exhib 22 (1977) p 673
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Gledhill, 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) p 344 Brewis, D.M., Comyn, J., Cope, B.C. and Moloney, A.C. Polymer 21 (1980) p 1477 Brewis, D.M., Comyn, J., Cope, B.C. and Moloney, A.C. Polym Engng Sci21 (1981) p 797 Brewia, D.M., Comyn, J. and Tegg, J.L. Int J Adhesion and Adhesives 1 (1980) p 35 Brewia, D.M., Comyn, J., Raval, A.K. and Kinloch, A.J. Int J Adhesion and Adhesives 10 (1990) p 247 Blackley, D.C., Comyn, J. and Harding, L.M. Int J Adhesion and Adhesives 13 (1993) p 163 Fowkes, F.M. Ind Eng Chem 56 No 12 (1964) p 40
Authors
G.B. Lowe and T.C.P. Lee are with Morton International Ltd, University of Warwick Science Park, Sir William Lyons Road, Coventry CV4 7EZ, UK. J. Comyn, to whom correspondence should be addressed, and K. Huddersman are with the Department of Chemistry at De Montfort University, Leicester LE1 9BH, UK.