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Effect of hydrostatic pressure on the water absorption of glass fibre-reinforced epoxy resin
Effect of hydrostatic pressure on the water absorption of glass fibrereinforced epoxy resin A. A VENA and A. R. BUNSELL (Ecole Nationale Sup~rieure des Mines de Paris, France) The use of glass fibre-reinforced epoxy resin immersed at great depths in water has been simulated by immersion tests under high hydrostatic pressures. The effect of these high pressures on water uptake has been studied and any changes in properties due to this environment revealed. Two types of composites differing only in the presence or absence of a size (coupling agent) on the glass fibres have been examined. The absence of a size increased water uptake significantly at all pressures considered. The coefficient of diffusivity of the unsized fibre composite decreased with increasing pressure whereas no change was seen when a size was present. A slight decrease in water uptake was observed with increasing hydrostatic pressure indicating a closing of spaces in which water can be lodged under low pressures. Changes in properties were associated with water absorption and the high hydrostatic pressure was seen to play only a secondary role. Key words: composite materials; glass fibres; epoxy resins; water absorption; immersion tests; high hydrostatic pressure
Considerable attention has been given to the effect of water penetration on the behaviour of fibre-reinforced composite materials. Although most organic matrix composites absorb only one or two percent by weight of water its presence can provoke the failure of secondary bonds ensuring the fibre-matrix interface on which many of the composite properties depend. Interest has been concentrated on the effects of exposure to humid conditions and it has been shown that, particularly at raised temperatures, degradation of the composite can occur.
1-3
Little attention seems to have been given to the possibility of degradation of composite structures used at great depths in the ocean however the use of composites for submersibles, and submarine pipelines is increasing. The results of hydrothermal ageing obtained under one atmosphere are not necessarily applicable to a composite structure destined to be immersed at great depths for prolonged periods of time. It should be noted that at a depth of 6000 m the hydrostatic pressure is 60 MPa. In addition it has been noted that high pressures can affect the rate of material ageing. 4 The combined effects of high hydrostatic pressure and ageing in water have been studied in order to evaluate possible problems for the use of composites at great depths.
EXPERIMENTAL DETAILS Two types of composite laminate have been studied both supplied by HEXCEL-GENIN. The reinforcements in both cases were E-type glass fibres in the form of a balanced woven cloth (200 g m -2) embedded in a diglyaidyl-ether-biphenol - A (DGEBA)-epoxy resin with a dicyandiamide hardener. The fibre volume fraction was 60%. The laminates only differed in the fibre surface treatment; the composite FV1 consisted of fibres from which the size had been removed by heating and the composite FV2 consisted of fibres which had been treated with an organosilane size. The glass transition temperatures for the two composites were respectively 113°C and 115°C. 5 The specimens tested were in the form of rectangular plates 150 x 25 mm having a thickness of 0.73 mm cut parallel to one of the fibre directions. All specimens were dried for 14 days at 100°C. The effects of hydrostatic pressure on water uptake were examined by immersing the specimens in distilled water for 180 days at 23°C inside pressure chambers capable of pressures up to 30 MPa. Water uptake was monitored by gravimetric measurements. Changes in mechanical properties were monitored by three point bending tests.
0010-4361/88/090355-03 $3.00~1988 Butterworth & Co (Publishers) Ltd COMPOSITES. VOLUME 19. NUMBER 5. SEPTEMBER 1988
355
EXPERIMENTAL RESULTS Water uptake at time t shown as a percentage increase in weight M (%) defined as a percentage of the dry weight Mo so that M(%) -- 100 ((Mr - Mo)/Mo), is shown for both materials in Fig. 1. It can be seen that water uptake showed classicial Fickian behaviour irrespective of the hydrostatic pressure and could be
The flexure tests showed no change in forcedisplacement curves nor in rigidity as a function of hydrostatic pressure. However, failure stress ORVwas affected. Fig. 3 shows changes in ORe as a function of water absorption after 180 days immersion at different hydrostatic pressures. The decrease in strength was seen to depend on the amount of water absorbed and to be independent of the applied pressure. As the effect of the size in the FV2 composites was to reduce the amount of water absorbed irrespective of the pressure, these composites also showed less change in strength.
1.0 FV1 0.8
~0.6 t-
"~ 0.4
-
0.2
o Atmospheric pressure • 20 MPa I
I
I
I
DISCUSSION
I
Water penetration in composite materials depends on the combined effect of three processes: diffusion in the resin, capillary action along the fibre-matrix interface and entrapment of water molecules in microdefects consisting of porosities, microcracks and interfacial debonds. Thermoactivated diffusion is determined by factors such as the mobility of the diffusing molecules, the mobility of the macromolecular structure and the free volume. It has been shown that molecular migration is prevented in unreinforced resins if the free volume Vf is less than a critical value Vf* and that the diffusivity D is related to Vf by the expression: 6
1.0 FV2 0.8
o
-'~-~-
~" 0 . 4 b "
described by the two parameters D, the coefficient of diffusion and Mm, the increase in weight at saturation. Both these parameters can be seen to have been little affected by the hydrostatic pressure Pv see Fig. 2. The presence of the silane size of the glass fibres in the FV2 composites can be seen to have led to a reduction in saturation level from 0.71 to 0.52% at one atmosphere and this difference was maintained at all pressures.
0.2 0
I
0
1000
2000
3000
I
I
4000
5000
D = A e x p ( - BVT~
\
6000
vf/
where A and B are constants. Fig. 1 Water uptake as a percentage increase in weight, M(%) for composites FVl and FV2, the former consisting of unsized glass fibres and the latter of glass fibres treated with an organosilane size, both types in an A (DGEBA) epoxy resin
3L I
'
The compressibility of polymers and resin matrix composites at pressures less than 104 atmospheres is due to the closing of the free volume and
I 0.7
"E
E O v
C3
2.5
._>
~" 0
0.6
"O
O3
.u O
2-
c)
2Jl
1
I
I
100
I
200
o.5 _ 1 _ ~ ± 1
L
100
l___
200
Pv (bar) Pv (bar) The coefficient of diffusivity D and the saturation level M m as a function of water pressure for composites FV1 (A) and FV2 (&) Fig. 2
356
COMPOSITES.
SEPTEMBER
1988
0.55
-4t....._
defects and made them less susceptible to being sites for water to be lodged.
FV2
The reduction in strength observed with composites after water absorption has been noted by other authors. 2 It is often largely reversible, as in this case. a.
:5
CONCL USlON
0.45 FV1 Oo 0.35
I
0
0.2
I
I
i
I
I
0.4 0.6 Increase in weight, M (%)
FV2
0.55 ~
0.50
G 0.45 FV1
$
0.40 0.1
I 5
I 10
t 20
Hydrostatic pressure does not seem to have been a significant factor in producing degradation in the two glass fibre-reinforced epoxy resins studied. It does not play a direct role in determining their behaviour nor does it produce mechanical degradation of the resin or the reinforcement. The effect of the pressure on water uptake is secondary as its tends to compress the matrix and close microvoids or defects which are present in the composite such as microporosities and interfacial defects. As a consequence, an increase in hydrostatic pressure during immersion, as would occur during a descent in deep water, would, if anything, have a beneficial effect on degradation processes. Degradation in this type of composite is solely due to water uptake even at great depths and this can lead to a decrease in ultimate mechanical properties. It has been seen that water absorption can be greatly reduced by the use of the size on the glass fibres and that its effect is beneficial irrespective of the hydrostatic pressure. These results should remove doubts about the use of glass fibre-reinforced epoxy resins for under water applications.
Pv (MPa) Fig. 3 Influence of water uptake and pressure on failure stress, measured by a three-point bending test of the two composite systems FV1 and FV2. The group of four symbols in the upper curve represent results obtained under hydrostatic pressure of 5, 10, 20 and 29MPa
microdefects.5, 8 The free volume is therefore a decreasing function of the hydrostatic pressure. As a direct result, the diffusivity also decreases with increasing pressure. Studies on unreinforced epoxy resin have shown that the diffusivity decreases by 30% when the applied pressure increases from one atmosphere to 29 MPa.5 The present study on fibre-reinforced epoxy resin has shown that the rate of water uptake was most influenced by the quality of the interface. The absence of a size can be presumed to result in not only a poor bond between fibre and matrix but also other microdefects at the interface. The effect of the hydrostatic pressure can be seen to have closed these
C O M P O S I T E S . SEPTEMBER 1988
REFERENCES 1 Springer, G. S. Advanced Engineering and Science (13th Soc Eng Sci Vol 1) (Springfield, 1976) pp 147-156 2 Dewimille, B. and Bunsell, A. R. Composites 14, No 1 (January 1983) pp 35-40 3 Anstice, P. D. and Beaumont, P. W. R. J Mater Sci 18 (1983) pp 3404-3408 4 Filyanov, E. M. Tarakanov, O. G. and Hamov, I. V. S. Mekhanika Polimeror No 1 (January 1974) pp 163-165 5 Avena, A. Doctoral Thesis (Ecole des Mines de Paris, October 1987) p 344 6 Turnbnll D. and Cohen, M. H. J Chem Phys 34 No 1 (January 1961) pp 120-125 7 Crank, J. and Park, G. S. 'Diffusion in Polymers' (Academic Press, London, 1968) p 108 8 Warfield R. W. Polym Eng Sci (April 1966) pp 176-180
A U THORS The authors are with the Ecole Nationale Sup6rieure des Mines de Paris, Centre des Mat6riaux P M Fourt, B P 87, 91003 Evry C6dex, France. Enquiries should be addressed to Dr Bunsell.
357
Considerable attention has been given to the effect of water penetration on the behaviour of fibre-reinforced composite materials. Although most organic matrix composites absorb only one or two percent by weight of water its presence can provoke the failure of secondary bonds ensuring the fibre-matrix interface on which many of the composite properties depend. Interest has been concentrated on the effects of exposure to humid conditions and it has been shown that, particularly at raised temperatures, degradation of the composite can occur.
1-3
Little attention seems to have been given to the possibility of degradation of composite structures used at great depths in the ocean however the use of composites for submersibles, and submarine pipelines is increasing. The results of hydrothermal ageing obtained under one atmosphere are not necessarily applicable to a composite structure destined to be immersed at great depths for prolonged periods of time. It should be noted that at a depth of 6000 m the hydrostatic pressure is 60 MPa. In addition it has been noted that high pressures can affect the rate of material ageing. 4 The combined effects of high hydrostatic pressure and ageing in water have been studied in order to evaluate possible problems for the use of composites at great depths.
EXPERIMENTAL DETAILS Two types of composite laminate have been studied both supplied by HEXCEL-GENIN. The reinforcements in both cases were E-type glass fibres in the form of a balanced woven cloth (200 g m -2) embedded in a diglyaidyl-ether-biphenol - A (DGEBA)-epoxy resin with a dicyandiamide hardener. The fibre volume fraction was 60%. The laminates only differed in the fibre surface treatment; the composite FV1 consisted of fibres from which the size had been removed by heating and the composite FV2 consisted of fibres which had been treated with an organosilane size. The glass transition temperatures for the two composites were respectively 113°C and 115°C. 5 The specimens tested were in the form of rectangular plates 150 x 25 mm having a thickness of 0.73 mm cut parallel to one of the fibre directions. All specimens were dried for 14 days at 100°C. The effects of hydrostatic pressure on water uptake were examined by immersing the specimens in distilled water for 180 days at 23°C inside pressure chambers capable of pressures up to 30 MPa. Water uptake was monitored by gravimetric measurements. Changes in mechanical properties were monitored by three point bending tests.
0010-4361/88/090355-03 $3.00~1988 Butterworth & Co (Publishers) Ltd COMPOSITES. VOLUME 19. NUMBER 5. SEPTEMBER 1988
355
EXPERIMENTAL RESULTS Water uptake at time t shown as a percentage increase in weight M (%) defined as a percentage of the dry weight Mo so that M(%) -- 100 ((Mr - Mo)/Mo), is shown for both materials in Fig. 1. It can be seen that water uptake showed classicial Fickian behaviour irrespective of the hydrostatic pressure and could be
The flexure tests showed no change in forcedisplacement curves nor in rigidity as a function of hydrostatic pressure. However, failure stress ORVwas affected. Fig. 3 shows changes in ORe as a function of water absorption after 180 days immersion at different hydrostatic pressures. The decrease in strength was seen to depend on the amount of water absorbed and to be independent of the applied pressure. As the effect of the size in the FV2 composites was to reduce the amount of water absorbed irrespective of the pressure, these composites also showed less change in strength.
1.0 FV1 0.8
~0.6 t-
"~ 0.4
-
0.2
o Atmospheric pressure • 20 MPa I
I
I
I
DISCUSSION
I
Water penetration in composite materials depends on the combined effect of three processes: diffusion in the resin, capillary action along the fibre-matrix interface and entrapment of water molecules in microdefects consisting of porosities, microcracks and interfacial debonds. Thermoactivated diffusion is determined by factors such as the mobility of the diffusing molecules, the mobility of the macromolecular structure and the free volume. It has been shown that molecular migration is prevented in unreinforced resins if the free volume Vf is less than a critical value Vf* and that the diffusivity D is related to Vf by the expression: 6
1.0 FV2 0.8
o
-'~-~-
~" 0 . 4 b "
described by the two parameters D, the coefficient of diffusion and Mm, the increase in weight at saturation. Both these parameters can be seen to have been little affected by the hydrostatic pressure Pv see Fig. 2. The presence of the silane size of the glass fibres in the FV2 composites can be seen to have led to a reduction in saturation level from 0.71 to 0.52% at one atmosphere and this difference was maintained at all pressures.
0.2 0
I
0
1000
2000
3000
I
I
4000
5000
D = A e x p ( - BVT~
\
6000
vf/
where A and B are constants. Fig. 1 Water uptake as a percentage increase in weight, M(%) for composites FVl and FV2, the former consisting of unsized glass fibres and the latter of glass fibres treated with an organosilane size, both types in an A (DGEBA) epoxy resin
3L I
'
The compressibility of polymers and resin matrix composites at pressures less than 104 atmospheres is due to the closing of the free volume and
I 0.7
"E
E O v
C3
2.5
._>
~" 0
0.6
"O
O3
.u O
2-
c)
2Jl
1
I
I
100
I
200
o.5 _ 1 _ ~ ± 1
L
100
l___
200
Pv (bar) Pv (bar) The coefficient of diffusivity D and the saturation level M m as a function of water pressure for composites FV1 (A) and FV2 (&) Fig. 2
356
COMPOSITES.
SEPTEMBER
1988
0.55
-4t....._
defects and made them less susceptible to being sites for water to be lodged.
FV2
The reduction in strength observed with composites after water absorption has been noted by other authors. 2 It is often largely reversible, as in this case. a.
:5
CONCL USlON
0.45 FV1 Oo 0.35
I
0
0.2
I
I
i
I
I
0.4 0.6 Increase in weight, M (%)
FV2
0.55 ~
0.50
G 0.45 FV1
$
0.40 0.1
I 5
I 10
t 20
Hydrostatic pressure does not seem to have been a significant factor in producing degradation in the two glass fibre-reinforced epoxy resins studied. It does not play a direct role in determining their behaviour nor does it produce mechanical degradation of the resin or the reinforcement. The effect of the pressure on water uptake is secondary as its tends to compress the matrix and close microvoids or defects which are present in the composite such as microporosities and interfacial defects. As a consequence, an increase in hydrostatic pressure during immersion, as would occur during a descent in deep water, would, if anything, have a beneficial effect on degradation processes. Degradation in this type of composite is solely due to water uptake even at great depths and this can lead to a decrease in ultimate mechanical properties. It has been seen that water absorption can be greatly reduced by the use of the size on the glass fibres and that its effect is beneficial irrespective of the hydrostatic pressure. These results should remove doubts about the use of glass fibre-reinforced epoxy resins for under water applications.
Pv (MPa) Fig. 3 Influence of water uptake and pressure on failure stress, measured by a three-point bending test of the two composite systems FV1 and FV2. The group of four symbols in the upper curve represent results obtained under hydrostatic pressure of 5, 10, 20 and 29MPa
microdefects.5, 8 The free volume is therefore a decreasing function of the hydrostatic pressure. As a direct result, the diffusivity also decreases with increasing pressure. Studies on unreinforced epoxy resin have shown that the diffusivity decreases by 30% when the applied pressure increases from one atmosphere to 29 MPa.5 The present study on fibre-reinforced epoxy resin has shown that the rate of water uptake was most influenced by the quality of the interface. The absence of a size can be presumed to result in not only a poor bond between fibre and matrix but also other microdefects at the interface. The effect of the hydrostatic pressure can be seen to have closed these
C O M P O S I T E S . SEPTEMBER 1988
REFERENCES 1 Springer, G. S. Advanced Engineering and Science (13th Soc Eng Sci Vol 1) (Springfield, 1976) pp 147-156 2 Dewimille, B. and Bunsell, A. R. Composites 14, No 1 (January 1983) pp 35-40 3 Anstice, P. D. and Beaumont, P. W. R. J Mater Sci 18 (1983) pp 3404-3408 4 Filyanov, E. M. Tarakanov, O. G. and Hamov, I. V. S. Mekhanika Polimeror No 1 (January 1974) pp 163-165 5 Avena, A. Doctoral Thesis (Ecole des Mines de Paris, October 1987) p 344 6 Turnbnll D. and Cohen, M. H. J Chem Phys 34 No 1 (January 1961) pp 120-125 7 Crank, J. and Park, G. S. 'Diffusion in Polymers' (Academic Press, London, 1968) p 108 8 Warfield R. W. Polym Eng Sci (April 1966) pp 176-180
A U THORS The authors are with the Ecole Nationale Sup6rieure des Mines de Paris, Centre des Mat6riaux P M Fourt, B P 87, 91003 Evry C6dex, France. Enquiries should be addressed to Dr Bunsell.
357