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The effect of thermal spiking on moisture absorption, mechanical and viscoelastic properties of carbon fibre reinforced epoxy laminates
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 65 (2005) 1299–1305 www.elsevier.com/locate/compscitech
The effect of thermal spiking on moisture absorption, mechanical and viscoelastic properties of carbon fibre reinforced epoxy laminates James A. Hough a, Sunil K. Karad b, Frank R. Jones a
a,*
The ceramics and composites laboratory, Department of Engineering Materials, University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, UK b Maharashtra Academy of Engineering, Pune, Alandi, India Received 2 September 2003; received in revised form 3 January 2005; accepted 4 January 2005 Available online 10 March 2005
Abstract Moisture absorption by carbon fibre reinforced epoxy laminates may result in a reduction in mechanical properties and a lower maximum service temperature. Thermal cycling of a laminate coupled with a humid environment, often encountered by military jet aircraft components, results in a higher concentration of absorbed moisture. The effect of moisture absorption as a result of thermal spiking on the mechanical properties of the three composite systems Narmco Rigidite 5245C, Fibredux 927 and Fibredux 924 has been studied. Maximum moisture enhancement occurred at a spike temperature of 140 and 160 °C. For the 5245C laminates, thermal spiking and resultant increased moisture absorption resulted in a reduction in transverse flexural strength relative to un-spiked control specimens. Transverse flexural stiffness was unaffected by thermal spiking in all three systems. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: A. PMCs; B. Hygrothermal effect; B. Mechanical properties; B. Thermomechanical properties; Thermal spiking
1. Introduction Epoxy resins are the most common matrices for high performance carbon-fibre composites because of the ease with which the curing process can be controlled. However, they tend to absorb moisture readily, resulting in a lower service temperature. Under fixed ambient conditions, absorbed moisture contents in epoxy laminates are found to depend on the relative humidity of the environment only. Certain aircraft components can be exposed to temperatures in excess of 100 °C for only a few minutes [1]. Under such conditions of rapid heating followed by rapid cooling, studies have shown that the moisture absorption of epoxy/carbon laminates can increase significantly [2–4] relative to the control *
Corresponding author. Tel.: +44 114 222 5477; fax: +44 114 222 5943. E-mail address: f.r.jones@sheffield.ac.uk (F.R. Jones). 0266-3538/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2005.01.009
samples under constant hygrothermal conditions. The mechanism for this enhancement when the laminates are thermally spiked is not clear. Resin manufacturers have attempted to improve the hygrothermal properties of epoxy resins by blending them with other thermosetting and thermoplastic components of higher glass-transition temperatures and lower water absorption. Recent matrix formulations have combined tetra-functional epoxy resins with polyimide and cyanate ester resins. Epoxy resins are inherently brittle because of their high degree of crosslinking. More established composite systems have employed thermoplastics, such as polyethersulphone, to improve the fracture toughness of epoxy systems. However, the resultant phase morphologies of these blends influence the moisture absorption under thermal spiking. Moisture absorption by carbon fibres is negligible and the mechanical properties of fibres are unaffected. Thus water diffusion into a laminate is a matrix-
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dominated phenomena. As a result, the matrix glasstransition temperature [5] may be reduced, possibly accompanied by a reduction in stiffness and strength. These properties are carried over into the laminate. Thermal spiking and the resultant enhanced moisture absorption of epoxy/carbon composites have been studied extensively, but usually using a specific system for each analysis. However, the effect of different epoxy blends, curing agents, fibre sizes, has not and as such may change the effect, thermal spiking has on the proper-ties of different systems. This study attempts to identify some common characteristics of the phenomena of enhanced moisture absorption by comparing three different epoxy/carbon composite systems.
Table 1 The thermal spiking times employed Thermal spike temperature (°C)
Spike time for O16 5245C laminates (min)
Spike time for O8 924 and 927 laminates (min)
100 120 140 160 180 200
5 6 7 8 – –
3.5 4 4.5 5 5.5 6
turned to the humidity chamber (control samples) or subjected to a thermal spike (spiked specimens). The moisture absorbed was determined using an electro-balance accurate to 105 g.
2. Experimental
2.3. Thermal spike program
2.1. Materials and sample preparation
Coupons were placed in metal racks, to ensure both major surfaces were heated evenly, and placed in an air-circulating oven pre-heated to the spike temperature. Time spent at the spike temperature was calculated so that the specimen was maintained at this temperature for 1 min. Table 1 lists the spiking times for 16-ply 5245C specimens determined by Xiang et al. [4], and for 924 and 927 8-ply laminates. Subsequently, they were removed from the oven and allowed to cool in air before replacement in the conditioning chambers.
Three carbon fibre reinforced epoxy resin systems have been examined; 1. Narmco Rigidite 5245C (BASF) – a bismaleimide modified epoxy resin. 2. Fibredux 927 (Ciba Geigy) – a high temperature, modified epoxy blend. 3. Fibredux 924 (Ciba Geigy) – a thermoplastic/epoxy blend.
2.4. Glass-transition temperature All systems were received from the manufacturers as rolls of prepreg tape. The tape was cut to the dimensions of the laminate required. Sixteen plies were stacked to form 16-ply laminates of 5245C, while eight plies were stacked to form 8-ply laminates of 924 and 927. The laminates were then cured in a press-clave for 2 h, either at 177 °C for 5245C, or 180 °C for 924 and 927. Vacuum bagging procedures and cure cycles recommended by the manufacturers were followed. For conditioning 5245C coupons were cut to dimensions of 60 mm by 50 mm using a water cooled diamond saw. The 924 and 927 coupons had dimensions of 20 mm by 140 mm. The edges of all samples were polished to a, 1200 grit finish. 5245 and 924/927 coupons were postcured for 4 h at 210 °C and 180 °C for 2 h, respectively, as recommended by the manufacturers. 2.2. Conditioning The coupons were dried to constant weight in a vacuum oven at 50 °C, prior to conditioning. They were then placed on racks above a saturated salt solution of potassium sulphate (K2SO4) in distilled water, which has a relative humidity of 96% in a sealed humidity chamber in an air-circulating oven at 50 °C. Laminate samples were removed intermittently, weighed and re-
Dynamic mechanical thermal analysis (DMTA) was performed in dual cantilever bending mode using a Polymer Laboratories Mk II analyzer. Samples of dimensions 10 mm by 40 mm were cut from the conditioned specimens, with the fibres at 90° to the DMTA clamp faces. A frequency of 1 Hz was used over a temperature range of 50–300 °C. 2.5. Mechanical testing The transverse flexural strength and modulus of the laminates were measured in three-point bend, on a Mayes Universal testing machine. The samples were cut from the fully saturated laminates, with the fibres parallel to the axis of the pivots. This was done to minimize the contribution of the fibres to the coupon strength. The dimensions and span between rollers were chosen in accordance with B.S. 2782: part 3: method 335A: 1978. For the 016 laminates from 5245C prepreg, the specimen dimensions were 10 mm by 50 mm. A span of 40 mm was used between the bottom rollers. For the O8 laminates from 924 and 927 prepreg, the specimens of dimension 20 mm by 30 mm, with a span of 20 mm were used.
J.A. Hough et al. / Composites Science and Technology 65 (2005) 1299–1305
Thermal spike temperature (°C)
5245C (O16 laminates)
927 (O8 laminates)
924 (O8 laminates)
Control (unspiked) 100 120 140 160 180 200
0.82 0.87 1.26 1.98 1.50 – –
0.94 1.03 1.19 1.34 1.44 1.38 1.32
1.72 2.37 2.84 2.98 2.88 2.43 2.08
Moisture content (wt.%)
Table 2 Moisture content (wt%) in the 5245C, 927 and 924 laminates, after thermal spiking and conditioning at 96% R.H. for 10,000 h (5245C and 927) and 5100 h (924)
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3
2
1
0 0
50
100
150
200 °
Thermal spike temperature ( C) 5245C
The force and deflection at failure were recorded from six samples subjected to each spike-temperature. The mean transverse flexural failure strengths and moduli were calculated.
3. Results In Table 2, the final moisture contents of 5245C, 927 and 924 laminates after spiking and conditioning for 10,000 h (5245C and 927) and 5100 h (924) at 96% R.H. are given. Fig. 1 shows the moisture content plotted as a function of spike temperature for the three systems. It is seen that a maximum moisture enhancement exists for the 5245C and 924 systems at a thermal spike temperature of 140 °C, and for the 927 system at 160 °C. Under static conditions, the 5245C laminates absorbed 0.82 wt% moisture, the 927 laminates absorbed 0.94 wt% moisture and the 924 laminates absorbed 1.72 wt%, which was the highest. The 5245C laminates spiked at 120 °C absorbed 1.26 wt% moisture, a significantly higher concentration than in the control laminates. For the 927 and 924 laminates, a significant increase in absorbed moisture was observed for the samples spiked at 100 °C . More moisture is absorbed by the 5245C laminates than the 927 laminates, when spiked at or above 120 °C. At the maximum enhancement spike temperatures, the 927 lami-
927
924
Fig. 1. Moisture content in the 5245C, 927, and 924 laminates, after thermal spiking and conditioning at 96% R.H. for 10,000 h (5245C/ 927) and 5100 h (924).
nates absorbed 1.44 wt% moisture, whereas the 5245C laminates absorbed 1.98 wt% moisture and 924 laminates absorbed 2.98 wt%. It was observed that the temperature at which modulus begins to decrease rapidly, the softening point, is reduced on absorbing moisture under static conditions. This is further reduced when the laminates are thermally spiked. Table 3 lists the changes in tan d peak temperatures with conditioning and thermal spiking. The primary tan d peak (Tg1) was not reduced by spiking, however a secondary peak emerged (Tg2), which, although evident in the wet control samples, became better defined with increasing thermal spike temperature and the content of absorbed moisture. The changes in Tg1 and Tg2 with spike temperature are plotted in Figs. 2 and 3, respectively. The sensitivity of Tg2 to the thermal spike temperature and moisture content is shown to be higher for the 927 and 5245C materials. For the 5245C laminates, Tg2 decreased from a temperature of 222 °C (Tg1) by 98 to 124 °C after thermal spiking at 160 °C . For the 5245C and 927 laminates, Tg2 decreases almost linearly
Table 3 Changes in tan d peak temperatures for 5245C, 927 and 924 laminates as a result of thermal spiking and conditioning Spike temperature
Dry Control 100 120 140 160 180 200
5245C O16 laminates
927 O8 laminates
924 O8 laminates
H2O content (wt%)
Tg1 (°C)
Tg2 (°C)
H2O content (wt%)
Tg1 (°C)
Tg2 (°C)
H2O content (wt%)
Tg1 (°C)
Tg2 (°C)
0 0.82 0.87 1.26 1.98 1.50 – –
222 200 205 198 200 200 – –
– 174 172 157 146 124 – –
0 0.98 1.03 1.19 1.34 1.44 1.38 1.32
223 198 197 196 197 190 195 196
– 183 172 162 152 140 126 125
0 1.72 2.37 2.84 2.98 2.88 2.43 2.08
234 222 216 216 216 215 216 216
– 179 155 151 152 150 152 153
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250
200
Tg1 ( °C)
180 160
200
140 120 100
150 0
50
100
150
200
250
80 0
°
Spike temperature ( C) 5245C
927
50
100
150
200
250
Spike temperature (˚C)
924
5245C
Fig. 2. Effect of spiking on Tg1 of 5245C, 927 and 924 laminates conditioned at 96% R.H./50 °C h (5245C/927) and 5100 h (924).
with increase in spike temperature, but not with moisture content. The effect of moisture on Tg2 for 924 exhibited a different trend as shown in Table 3. Tg2 decreased from a temperature of 234 °C by 55 to 179 °C after isothermal conditioning and by a further 24 °C to 155 °C on thermal spiking at 100 °C. Spiking to higher temperatures does not result in a further drop in Tg2. Fig. 4 is a plot of the glass-transition-onset temperature ðTg0E Þ, determined as the temperature at which the slope of the storage modulus (log E 0 ) curve decreased rapidly, against thermal spike temperature. It can be seen that thermal spiking and moisture absorption reduces the Tg0E for all of the laminates. The 924 laminates exhibit a reduction in Tg0E from 214 °C when dry to 132 °C, when spiked at 100 °C. The 924 laminates showed no further reduction in the Tg0E after thermal spiking to higher temperatures. On absorbing moisture under static conditions, 5245C laminates exhibited a reduction in Tg0E from a dry value of 184 °C to 150 °C. Thermal spiking reduced this further, to a minimum Tg0E of 103 °C in laminates spiked to 160 °C . Sim-
927
924
Fig. 4. Effect of spiking on glass-transition-onset temperature, Tg0E of 5245C, 927 and 924 laminates conditioned at 96% R.H./50 °C h (5245C/927) and 5100 h (924).
ilarly for 927 laminates, Tg0E was reduced from a dry value of 197 to 172 °C on absorbing moisture under static conditions. Tg0E was reduced further with increased spike temperature to a minimum of 93 °C for laminates spiked to 200 °C. From thermomechanical data obtained, it can be inferred that whereas 924 appears to be subjected to a physical plasticisation, the other systems appear to be subjected to non-reversible effects on spiking above 100 °C. In previous study by Karad et al. [6] a similar trend was attributed to a small degree of hydrolysis. Figs. 5 and 6 show the effect of thermal spike temperature on the transverse flexural strength and modulus of wet 5245C, 927 and 924 laminates. Transverse flexural strength of each laminate system appears to be affected differently. The 5245C laminates showed a small decrease in flexural strength with increased spike temperature up to 140 °C. At 160 °C, flexural strength was reduced from a control value of 103 to 73 MPa. This 190 Flexural Strength (MPa)
250
Tg2 ( °C)
200
150
170 150 130 110 90 70 50 50
100 0
50
100
150
200
250
Spike temperature ( °C) 5245C
927
100
120
140
160
Thermal spike temperature 5245C
927
180
200
( °C) 924
924
Fig. 3. Effect of spiking on Tg2 of 5245C, 927 and 924 laminates conditioned at 96% R.H./50 °C h (5245C/927) and 5100 h (924).
Fig. 5. Transverse flexural strengths of wet 5245C laminates after spiking and conditioning at 96% R.H./50 °C h (5245C/927) and 5100 h (924).
Transverse Flexural Modulus (GPa)
J.A. Hough et al. / Composites Science and Technology 65 (2005) 1299–1305 12 11 10 9 8 7 50
100
120
140
160
Spike temperature 5245C
927
180
200
( ° C) 924
Fig. 6. Transverse flexural modulus of wet 5245C laminates after spiking and conditioning at 96% R.H./50 °C h (5245C/927) and 5100 h (924).
is consistent with the trend in Tg0E , which seemed to suggest that small degree of hydrolysis occurs at the spike temperature. The 927 laminates appeared to show an increase in transverse flexural strength when spiked to temperatures between 140 and 180 °C. However, this increase was within experimental error. Severe blistering had occurred during the thermal spiking of samples at 200 °C and may explain the decrease in flexural strength for these samples. Thus for the 927 samples the flexural strength showed no significant change with spike temperature. The 924 laminates showed a reduction in flexural strength to a minimum of 120 MPa from a control value of 165 MPa, when spiked to 100 °C. Thermal spiking at temperatures higher than 100 °C did not reduce the flexural strength further. The 5245C laminates showed no significant change from the control transverse flexural modulus of 9.58 GPa on being thermal spiked at all temperatures. The 927 laminates on spiking between 100 and 140 °C showed no significant change in modulus compared to a control value of 8.33 GPa. The 927 samples appear to show an increase in modulus on spiking at 160 and 200 °C. However, the experimental error suggests that in reality the stiffness of these samples is also unaffected. Thus the transverse flexural stiffness of the 5245C and the 927 laminates is not changed by thermal spiking. For the 924 laminates, it is seen that spiking at 100 °C, reduces the flexural modulus from 11.2 to 8.34 GPa. Spiking to temperatures higher than 100 °C does not reduce the transverse flexural modulus further.
4. Discussion A comparison of the levels of moisture absorbed by the three systems revealed that the incorporation of bismaleimide and other thermosetting modifiers into
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epoxy resin to form the matrix system, does reduce the maximum absorbed moisture by these laminates. Under static conditions, the 5245C laminates absorbed the least moisture, whilst the 924 laminates absorbed the most. However, thermal spiking of laminates resulted in increased moisture contents for all three systems. The 927 laminate system absorbed the least water as a result of spiking, relative to the 924 and 5245C systems. There was a maximum moisture enhancement spike temperature for each system. Although the moisture contents are lower for the 5245C and 927 systems than for the 924 system, thermal spiking at the maximum moisture enhancement temperature resulted in an increase in absorbed moisture relative to the control specimens of 141% for 5245C laminates and 53% for 927 laminates. Microscopic examination of the spiked samples revealed no significant damage such as microcracks or delaminations. This suggests that the additional absorbed moisture is not resident in microcracks and voids which are created during spiking, as suggested by some authors [7,8]. The 927 laminates spiked at 200 °C did show severe blistering, however the moisture concentration was less than in samples spiked at lower temperatures. Considering the reduction in moisture concentration with the addition of thermosetting resins, such as a bismaleimide to the epoxy, the mechanisms responsible for the moisture enhancement during thermal cycling are likely to be associated with the epoxy component. Analysis of the DMTA tan d traces revealed that a primary peak existed for the dry laminates (Tg1) at 220 °C for the 927 and 5245C and 230 °C for the 924 resins. On absorbing moisture under static conditions, Tgl was reduced by approximately 20 °C in all cases. The Kelly and Beuche [9] free volume theories argue that the Tg of a polymer system decreases by an amount proportional to the amount of diluent present, as the free volumes of the two components are additive. This cannot be the only mechanism for the reduction of Tg in the three systems studied because the laminates spiked at the maximum moisture enhancement temperature between 140 and 160 °C absorbed the most moisture yet did not show a similar reduction in Tg as the laminates spiked at higher temperatures which contained less moisture. A secondary tan d peak (Tg2) emerged at lower temperatures after the absorption of moisture. This peak appeared in the spectrum at a lower temperature when the coupons were thermally spiked, such that it appeared 50 °C lower than in the spectra for the un-spiked coupons. Fig. 4 infers that the decrease in Tg2 is dependent upon the temperature at which 5245C and 927 laminates were spiked. In the 924 system, Tg2 was not lowered by thermal spiking at temperatures above 100 °C. The polymer diluent free volume theory often fails with epoxy systems as interactions between the dissolved molecules and segments of the polymer network are ignored.
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The systems investigated are blends of epoxy resins with either thermoplastic or other thermosetting resins. Therefore, it seems likely that the Tg2 peak results from the plasticisation of either one of the components or of the products of the curing reaction and/or the reaction of the epoxy with the thermosetting modifiers. Alternatively, partial hydrolysis of the network may be occurring with the formation of a moisture sensitive phase, as indicated by the increased separation of Tg2 from Tg1 with spiking temperature. For plasticisation to occur, the moisture disrupts the intermolecular forces between the molecules or segments of the network chains [10], increasing the free volume of the polymeric resin. However for the 924 system, which is essentially an epoxy-based system, the moisture concentration is increased by thermal spiking above 100 °C, but Tg1 and Tg2 are not reduced further. In contrast, for 5245C and 927 laminates spiked at 160 °C and above, less moisture is absorbed, yet Tg2 is reduced further. This suggests that thermal spiking results in polymer network/moisture interactions differing from those discussed above. Thus effects other than plasticisation take place. Some studies suggest that network chains can be hydrolytically broken in the presence of moisture [11]. It is possible that hydrolytic degradation of one of the components of the network can occur during thermal spiking of 5245C and 927 laminates, increasing their susceptibility to plasticisation. This would result in the reduction in Tg2. The 924 laminate matrix comprises an epoxy and a thermoplastic component, and as such only plasticisation is expected (assuming epoxy crosslinks are hydrolytically stable). Thus in this case thermal spiking at high temperatures has not led to chain scission of the network, and a further reduction in Tg2 is not observed. The DMTA data presented supports this argument. Free volume theories have been used to explain an increase in unoccupied volume during thermal spiking, which can accommodate the additional absorbed moisture [3,12]. These theories rarely take into account the behavior of the polymer network near to the glass-transition region, where the free volume is generally accepted to increase. Xiang et al. [4] proposed that thermal spikes close to the wet Tg of the matrix-induced partial relaxation of the molecular network. Plasticisation and hydrolysis of network chains during thermal spiking in the presence of moisture leads to a reduction in glass-transition temperature of the matrix. If the temperature at which a wet laminate is spiked approaches that of the matrix glass-transition temperature, an increase in free volume is expected. Simultaneously, moisture present will diffuse into this additional free volume. On rapid cooling, the moisture can be trapped or frozen into the extra free volume. The original, pre-spike free volume contains less moisture than it did prior to the spike, so that more moisture
diffuses into this on further conditioning. Thus with each thermal spike, additional free volume is created in which the moisture can be accommodated. Thermal spiking of the laminates at temperatures above the glass-transition point would result in molecular relaxations taking place to such a degree, that diffusion of moisture through the network would be hindered and thus slowed down. The 5245C and 924 laminates spiked at 160 °C and 927 laminates spiked at 180 °C are above the wet Tg of the matrix, and absorption curves show slower moisture uptake for these samples compared to samples spiked at lower temperatures. The onset of the glass transition ðTg0E Þ, determined from the temperature at which the slope in the storage modulus log E 0 curves decreased, follows the same trend as Tg2. Thus the Tg0E temperature appears to be directly related to Tg2. This is of particular interest since resin manufacturers have blended thermosetting resins such as bismaleimide resins with epoxy, to produce matrices with improved temperature and moisture resistance. Although the resultant 5245C and 927 laminates absorb lower levels of moisture than the 924 system, a greater reduction in Tg0E with increasing spike temperature is observed. The 924 laminates showed a reduction in Tg0E when spiked at 100 °C, but at higher spike temperatures no further change occurred. For 5245C and 927, Tg0E continued to decrease, as the spike temperature was increased, to values well below the manufacturers stated service temperature. It would appear therefore that the introduction of thermosetting components to the epoxy matrix formulations reduces the softening point of their laminates, after conditioning and thermal spiking. This suggests degradation mechanisms as well as plasticisation by moisture are taking place. The degree of degradation is dependent on the thermal spike temperature, and not on moisture concentration, although previous studies have shown that moisture must be present. Thus hydrolysis of the polymer network and possibly the crosslinks formed from the reaction of the bismaleimide or other thermosetting resin with epoxy resin during cure, may be responsible for the reduction in Tg0E at the thermal spike temperature. The transverse flexural strength of wet 5245C laminates, shown in Fig. 5, were reduced by thermal spiking and enhanced moisture absorption. Specimens spiked at 140 °C, showed the highest moisture concentration but not the lowest flexural strength, suggesting that it is not merely the concentration of moisture which reduces strength. Matrix degradation through hydrolysis, evident from DMTA analysis, may play a more influential role. Inspection of the fractured surfaces revealed a transition from failure at the interface between fibres and matrix to matrix yielding with spike temperature. This suggests, that the degree of hydrolysis of the polymer network with spike temperature increases, resulting in a lower yield strength and a lower transverse flexural
J.A. Hough et al. / Composites Science and Technology 65 (2005) 1299–1305
strength of the laminates which is a matrix-dominated property. A similar result might be expected with the wet 927 laminates, but the flexural strength appeared to increase with spike temperature. Epoxy resins are inherently brittle due to their high degree of crosslinking. Other thermosetting resins with lower moisture absorption and lower degree of crosslinking are blended with epoxy resin to improve toughness. If thermal spiking and moisture absorption causes partial network chain scission within the phases present in the matrix, then, with a reduction in Tg0E , a slight improvement in the toughness of the resin could occur. Fig. 6 shows that laminate modulus is unaffected by thermal spiking and enhanced moisture absorption. The transverse flexural modulus is largely a function of the matrix modulus in the glassy state, which is a complex function of overall resin density and is neither directly related to the crosslink density [6,13] nor to the moisture content. It is therefore relatively insensitive to the effects reported here.
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the matrix an appropriate biphasal morphology to the 927 and 5245C systems, respectively. Thermally accelerated hydrolysis of the network chains is believed to be responsible for the reduction of the temperature at which this phase relaxed. No microcracks or voids were observed in the wet laminates that had been spiked, therefore the additional absorbed moisture must be present in the polymer network. Transverse flexural modulus was not significantly affected by thermal spiking or enhanced moisture absorption. Structural changes in the chemical properties of the thermosetting components in the 927 and 5245C matrices appear to be responsible for reductions in flexural strength of the laminates after thermal spiking.
Acknowledgements We thank DERA, BAc systems Ltd and EPCRC for funding.
5. Conclusions Thermal spiking of 5245C, 927 and 924 laminates conditioned in humid environments increased the concentrations of absorbed moisture. A spike temperature occurred for maximum moisture enhancement at 140 °C for the 5245C and 924 laminates, and at 160 °C for the 927 laminates. The addition of thermosetting components such as bismaleimide to the epoxy matrix reduced the total moisture absorbed relative to the epoxy/thermoplastic blend (924). However, DMTA analysis revealed that the temperature at the onset of the glass transition was reduced more rapidly with increasing spike temperature for the 5245C and 927 laminates than for 924 laminates. Analysis of DMTA Tan d curves revealed the splitting of the main relaxation peak. For the latter, Gumen et al. [14] have recently attempted to model this phenomenon and attributed the lower temperature peaks to regions of the network, which have a higher affinity to plasticisation by water molecules. For the former, the thermosetting resin components added to the epoxy blend are intended to give
References [1] Whiteside JB, Delasi RJ, Schulte RL. Long-term behavior of composites. ASTM STP 813. Philadelphia, USA: American Society for Testing Materials; 1983. p. 192–205. [2] Clark G, Saunders DS, van Blaricum TJ, Richmond M. Compos Sci Technol 1990;39:355. [3] Adamson MJ. Long-term behavior of composites. ASTM STP 813. Philadelphia, USA: American Society for Testing Materials; 1983. p. 192–205. [4] Xiang ZD, Jones FR. Compos Sci Technol 1997;57:451. [5] Ankara A, Weisgerber D, Vilsmeier J. Mater Sci Technol 1986;2:1081. [6] Karad S, Jones FR, Attwood D. Polymer 2002;43(19):5209. [7] Wolf EG. SAMPE J 1993;29:11. [8] Pritchard G, Speake SD. Composites 1987;18(3):227. [9] Beuche F, Kelley FN. J Polym Sci 1960;45:267. [10] Lee MC, Peppas NA. J Appl Polym Sci 1993;47:1349. [11] Kosfeld R, Brostow W, Hess M, Marzi T, Schroeder U. Makromol Chem Macromol Symp 1993;76:269. [12] El-SaÕad L, Darby MI, Yates B. J Mater Sci 1990;25(5):3577. [13] Aronhime MT, Peng X, Gillham JK, Small RD. J Appl Polym Sci 1986;32(2):3589. [14] Gumen VR, Jones FR, Attwood D. Polymer 2001;42(13):5717.
The effect of thermal spiking on moisture absorption, mechanical and viscoelastic properties of carbon fibre reinforced epoxy laminates James A. Hough a, Sunil K. Karad b, Frank R. Jones a
a,*
The ceramics and composites laboratory, Department of Engineering Materials, University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, UK b Maharashtra Academy of Engineering, Pune, Alandi, India Received 2 September 2003; received in revised form 3 January 2005; accepted 4 January 2005 Available online 10 March 2005
Abstract Moisture absorption by carbon fibre reinforced epoxy laminates may result in a reduction in mechanical properties and a lower maximum service temperature. Thermal cycling of a laminate coupled with a humid environment, often encountered by military jet aircraft components, results in a higher concentration of absorbed moisture. The effect of moisture absorption as a result of thermal spiking on the mechanical properties of the three composite systems Narmco Rigidite 5245C, Fibredux 927 and Fibredux 924 has been studied. Maximum moisture enhancement occurred at a spike temperature of 140 and 160 °C. For the 5245C laminates, thermal spiking and resultant increased moisture absorption resulted in a reduction in transverse flexural strength relative to un-spiked control specimens. Transverse flexural stiffness was unaffected by thermal spiking in all three systems. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: A. PMCs; B. Hygrothermal effect; B. Mechanical properties; B. Thermomechanical properties; Thermal spiking
1. Introduction Epoxy resins are the most common matrices for high performance carbon-fibre composites because of the ease with which the curing process can be controlled. However, they tend to absorb moisture readily, resulting in a lower service temperature. Under fixed ambient conditions, absorbed moisture contents in epoxy laminates are found to depend on the relative humidity of the environment only. Certain aircraft components can be exposed to temperatures in excess of 100 °C for only a few minutes [1]. Under such conditions of rapid heating followed by rapid cooling, studies have shown that the moisture absorption of epoxy/carbon laminates can increase significantly [2–4] relative to the control *
Corresponding author. Tel.: +44 114 222 5477; fax: +44 114 222 5943. E-mail address: f.r.jones@sheffield.ac.uk (F.R. Jones). 0266-3538/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2005.01.009
samples under constant hygrothermal conditions. The mechanism for this enhancement when the laminates are thermally spiked is not clear. Resin manufacturers have attempted to improve the hygrothermal properties of epoxy resins by blending them with other thermosetting and thermoplastic components of higher glass-transition temperatures and lower water absorption. Recent matrix formulations have combined tetra-functional epoxy resins with polyimide and cyanate ester resins. Epoxy resins are inherently brittle because of their high degree of crosslinking. More established composite systems have employed thermoplastics, such as polyethersulphone, to improve the fracture toughness of epoxy systems. However, the resultant phase morphologies of these blends influence the moisture absorption under thermal spiking. Moisture absorption by carbon fibres is negligible and the mechanical properties of fibres are unaffected. Thus water diffusion into a laminate is a matrix-
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dominated phenomena. As a result, the matrix glasstransition temperature [5] may be reduced, possibly accompanied by a reduction in stiffness and strength. These properties are carried over into the laminate. Thermal spiking and the resultant enhanced moisture absorption of epoxy/carbon composites have been studied extensively, but usually using a specific system for each analysis. However, the effect of different epoxy blends, curing agents, fibre sizes, has not and as such may change the effect, thermal spiking has on the proper-ties of different systems. This study attempts to identify some common characteristics of the phenomena of enhanced moisture absorption by comparing three different epoxy/carbon composite systems.
Table 1 The thermal spiking times employed Thermal spike temperature (°C)
Spike time for O16 5245C laminates (min)
Spike time for O8 924 and 927 laminates (min)
100 120 140 160 180 200
5 6 7 8 – –
3.5 4 4.5 5 5.5 6
turned to the humidity chamber (control samples) or subjected to a thermal spike (spiked specimens). The moisture absorbed was determined using an electro-balance accurate to 105 g.
2. Experimental
2.3. Thermal spike program
2.1. Materials and sample preparation
Coupons were placed in metal racks, to ensure both major surfaces were heated evenly, and placed in an air-circulating oven pre-heated to the spike temperature. Time spent at the spike temperature was calculated so that the specimen was maintained at this temperature for 1 min. Table 1 lists the spiking times for 16-ply 5245C specimens determined by Xiang et al. [4], and for 924 and 927 8-ply laminates. Subsequently, they were removed from the oven and allowed to cool in air before replacement in the conditioning chambers.
Three carbon fibre reinforced epoxy resin systems have been examined; 1. Narmco Rigidite 5245C (BASF) – a bismaleimide modified epoxy resin. 2. Fibredux 927 (Ciba Geigy) – a high temperature, modified epoxy blend. 3. Fibredux 924 (Ciba Geigy) – a thermoplastic/epoxy blend.
2.4. Glass-transition temperature All systems were received from the manufacturers as rolls of prepreg tape. The tape was cut to the dimensions of the laminate required. Sixteen plies were stacked to form 16-ply laminates of 5245C, while eight plies were stacked to form 8-ply laminates of 924 and 927. The laminates were then cured in a press-clave for 2 h, either at 177 °C for 5245C, or 180 °C for 924 and 927. Vacuum bagging procedures and cure cycles recommended by the manufacturers were followed. For conditioning 5245C coupons were cut to dimensions of 60 mm by 50 mm using a water cooled diamond saw. The 924 and 927 coupons had dimensions of 20 mm by 140 mm. The edges of all samples were polished to a, 1200 grit finish. 5245 and 924/927 coupons were postcured for 4 h at 210 °C and 180 °C for 2 h, respectively, as recommended by the manufacturers. 2.2. Conditioning The coupons were dried to constant weight in a vacuum oven at 50 °C, prior to conditioning. They were then placed on racks above a saturated salt solution of potassium sulphate (K2SO4) in distilled water, which has a relative humidity of 96% in a sealed humidity chamber in an air-circulating oven at 50 °C. Laminate samples were removed intermittently, weighed and re-
Dynamic mechanical thermal analysis (DMTA) was performed in dual cantilever bending mode using a Polymer Laboratories Mk II analyzer. Samples of dimensions 10 mm by 40 mm were cut from the conditioned specimens, with the fibres at 90° to the DMTA clamp faces. A frequency of 1 Hz was used over a temperature range of 50–300 °C. 2.5. Mechanical testing The transverse flexural strength and modulus of the laminates were measured in three-point bend, on a Mayes Universal testing machine. The samples were cut from the fully saturated laminates, with the fibres parallel to the axis of the pivots. This was done to minimize the contribution of the fibres to the coupon strength. The dimensions and span between rollers were chosen in accordance with B.S. 2782: part 3: method 335A: 1978. For the 016 laminates from 5245C prepreg, the specimen dimensions were 10 mm by 50 mm. A span of 40 mm was used between the bottom rollers. For the O8 laminates from 924 and 927 prepreg, the specimens of dimension 20 mm by 30 mm, with a span of 20 mm were used.
J.A. Hough et al. / Composites Science and Technology 65 (2005) 1299–1305
Thermal spike temperature (°C)
5245C (O16 laminates)
927 (O8 laminates)
924 (O8 laminates)
Control (unspiked) 100 120 140 160 180 200
0.82 0.87 1.26 1.98 1.50 – –
0.94 1.03 1.19 1.34 1.44 1.38 1.32
1.72 2.37 2.84 2.98 2.88 2.43 2.08
Moisture content (wt.%)
Table 2 Moisture content (wt%) in the 5245C, 927 and 924 laminates, after thermal spiking and conditioning at 96% R.H. for 10,000 h (5245C and 927) and 5100 h (924)
1301
3
2
1
0 0
50
100
150
200 °
Thermal spike temperature ( C) 5245C
The force and deflection at failure were recorded from six samples subjected to each spike-temperature. The mean transverse flexural failure strengths and moduli were calculated.
3. Results In Table 2, the final moisture contents of 5245C, 927 and 924 laminates after spiking and conditioning for 10,000 h (5245C and 927) and 5100 h (924) at 96% R.H. are given. Fig. 1 shows the moisture content plotted as a function of spike temperature for the three systems. It is seen that a maximum moisture enhancement exists for the 5245C and 924 systems at a thermal spike temperature of 140 °C, and for the 927 system at 160 °C. Under static conditions, the 5245C laminates absorbed 0.82 wt% moisture, the 927 laminates absorbed 0.94 wt% moisture and the 924 laminates absorbed 1.72 wt%, which was the highest. The 5245C laminates spiked at 120 °C absorbed 1.26 wt% moisture, a significantly higher concentration than in the control laminates. For the 927 and 924 laminates, a significant increase in absorbed moisture was observed for the samples spiked at 100 °C . More moisture is absorbed by the 5245C laminates than the 927 laminates, when spiked at or above 120 °C. At the maximum enhancement spike temperatures, the 927 lami-
927
924
Fig. 1. Moisture content in the 5245C, 927, and 924 laminates, after thermal spiking and conditioning at 96% R.H. for 10,000 h (5245C/ 927) and 5100 h (924).
nates absorbed 1.44 wt% moisture, whereas the 5245C laminates absorbed 1.98 wt% moisture and 924 laminates absorbed 2.98 wt%. It was observed that the temperature at which modulus begins to decrease rapidly, the softening point, is reduced on absorbing moisture under static conditions. This is further reduced when the laminates are thermally spiked. Table 3 lists the changes in tan d peak temperatures with conditioning and thermal spiking. The primary tan d peak (Tg1) was not reduced by spiking, however a secondary peak emerged (Tg2), which, although evident in the wet control samples, became better defined with increasing thermal spike temperature and the content of absorbed moisture. The changes in Tg1 and Tg2 with spike temperature are plotted in Figs. 2 and 3, respectively. The sensitivity of Tg2 to the thermal spike temperature and moisture content is shown to be higher for the 927 and 5245C materials. For the 5245C laminates, Tg2 decreased from a temperature of 222 °C (Tg1) by 98 to 124 °C after thermal spiking at 160 °C . For the 5245C and 927 laminates, Tg2 decreases almost linearly
Table 3 Changes in tan d peak temperatures for 5245C, 927 and 924 laminates as a result of thermal spiking and conditioning Spike temperature
Dry Control 100 120 140 160 180 200
5245C O16 laminates
927 O8 laminates
924 O8 laminates
H2O content (wt%)
Tg1 (°C)
Tg2 (°C)
H2O content (wt%)
Tg1 (°C)
Tg2 (°C)
H2O content (wt%)
Tg1 (°C)
Tg2 (°C)
0 0.82 0.87 1.26 1.98 1.50 – –
222 200 205 198 200 200 – –
– 174 172 157 146 124 – –
0 0.98 1.03 1.19 1.34 1.44 1.38 1.32
223 198 197 196 197 190 195 196
– 183 172 162 152 140 126 125
0 1.72 2.37 2.84 2.98 2.88 2.43 2.08
234 222 216 216 216 215 216 216
– 179 155 151 152 150 152 153
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J.A. Hough et al. / Composites Science and Technology 65 (2005) 1299–1305 220
250
200
Tg1 ( °C)
180 160
200
140 120 100
150 0
50
100
150
200
250
80 0
°
Spike temperature ( C) 5245C
927
50
100
150
200
250
Spike temperature (˚C)
924
5245C
Fig. 2. Effect of spiking on Tg1 of 5245C, 927 and 924 laminates conditioned at 96% R.H./50 °C h (5245C/927) and 5100 h (924).
with increase in spike temperature, but not with moisture content. The effect of moisture on Tg2 for 924 exhibited a different trend as shown in Table 3. Tg2 decreased from a temperature of 234 °C by 55 to 179 °C after isothermal conditioning and by a further 24 °C to 155 °C on thermal spiking at 100 °C. Spiking to higher temperatures does not result in a further drop in Tg2. Fig. 4 is a plot of the glass-transition-onset temperature ðTg0E Þ, determined as the temperature at which the slope of the storage modulus (log E 0 ) curve decreased rapidly, against thermal spike temperature. It can be seen that thermal spiking and moisture absorption reduces the Tg0E for all of the laminates. The 924 laminates exhibit a reduction in Tg0E from 214 °C when dry to 132 °C, when spiked at 100 °C. The 924 laminates showed no further reduction in the Tg0E after thermal spiking to higher temperatures. On absorbing moisture under static conditions, 5245C laminates exhibited a reduction in Tg0E from a dry value of 184 °C to 150 °C. Thermal spiking reduced this further, to a minimum Tg0E of 103 °C in laminates spiked to 160 °C . Sim-
927
924
Fig. 4. Effect of spiking on glass-transition-onset temperature, Tg0E of 5245C, 927 and 924 laminates conditioned at 96% R.H./50 °C h (5245C/927) and 5100 h (924).
ilarly for 927 laminates, Tg0E was reduced from a dry value of 197 to 172 °C on absorbing moisture under static conditions. Tg0E was reduced further with increased spike temperature to a minimum of 93 °C for laminates spiked to 200 °C. From thermomechanical data obtained, it can be inferred that whereas 924 appears to be subjected to a physical plasticisation, the other systems appear to be subjected to non-reversible effects on spiking above 100 °C. In previous study by Karad et al. [6] a similar trend was attributed to a small degree of hydrolysis. Figs. 5 and 6 show the effect of thermal spike temperature on the transverse flexural strength and modulus of wet 5245C, 927 and 924 laminates. Transverse flexural strength of each laminate system appears to be affected differently. The 5245C laminates showed a small decrease in flexural strength with increased spike temperature up to 140 °C. At 160 °C, flexural strength was reduced from a control value of 103 to 73 MPa. This 190 Flexural Strength (MPa)
250
Tg2 ( °C)
200
150
170 150 130 110 90 70 50 50
100 0
50
100
150
200
250
Spike temperature ( °C) 5245C
927
100
120
140
160
Thermal spike temperature 5245C
927
180
200
( °C) 924
924
Fig. 3. Effect of spiking on Tg2 of 5245C, 927 and 924 laminates conditioned at 96% R.H./50 °C h (5245C/927) and 5100 h (924).
Fig. 5. Transverse flexural strengths of wet 5245C laminates after spiking and conditioning at 96% R.H./50 °C h (5245C/927) and 5100 h (924).
Transverse Flexural Modulus (GPa)
J.A. Hough et al. / Composites Science and Technology 65 (2005) 1299–1305 12 11 10 9 8 7 50
100
120
140
160
Spike temperature 5245C
927
180
200
( ° C) 924
Fig. 6. Transverse flexural modulus of wet 5245C laminates after spiking and conditioning at 96% R.H./50 °C h (5245C/927) and 5100 h (924).
is consistent with the trend in Tg0E , which seemed to suggest that small degree of hydrolysis occurs at the spike temperature. The 927 laminates appeared to show an increase in transverse flexural strength when spiked to temperatures between 140 and 180 °C. However, this increase was within experimental error. Severe blistering had occurred during the thermal spiking of samples at 200 °C and may explain the decrease in flexural strength for these samples. Thus for the 927 samples the flexural strength showed no significant change with spike temperature. The 924 laminates showed a reduction in flexural strength to a minimum of 120 MPa from a control value of 165 MPa, when spiked to 100 °C. Thermal spiking at temperatures higher than 100 °C did not reduce the flexural strength further. The 5245C laminates showed no significant change from the control transverse flexural modulus of 9.58 GPa on being thermal spiked at all temperatures. The 927 laminates on spiking between 100 and 140 °C showed no significant change in modulus compared to a control value of 8.33 GPa. The 927 samples appear to show an increase in modulus on spiking at 160 and 200 °C. However, the experimental error suggests that in reality the stiffness of these samples is also unaffected. Thus the transverse flexural stiffness of the 5245C and the 927 laminates is not changed by thermal spiking. For the 924 laminates, it is seen that spiking at 100 °C, reduces the flexural modulus from 11.2 to 8.34 GPa. Spiking to temperatures higher than 100 °C does not reduce the transverse flexural modulus further.
4. Discussion A comparison of the levels of moisture absorbed by the three systems revealed that the incorporation of bismaleimide and other thermosetting modifiers into
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epoxy resin to form the matrix system, does reduce the maximum absorbed moisture by these laminates. Under static conditions, the 5245C laminates absorbed the least moisture, whilst the 924 laminates absorbed the most. However, thermal spiking of laminates resulted in increased moisture contents for all three systems. The 927 laminate system absorbed the least water as a result of spiking, relative to the 924 and 5245C systems. There was a maximum moisture enhancement spike temperature for each system. Although the moisture contents are lower for the 5245C and 927 systems than for the 924 system, thermal spiking at the maximum moisture enhancement temperature resulted in an increase in absorbed moisture relative to the control specimens of 141% for 5245C laminates and 53% for 927 laminates. Microscopic examination of the spiked samples revealed no significant damage such as microcracks or delaminations. This suggests that the additional absorbed moisture is not resident in microcracks and voids which are created during spiking, as suggested by some authors [7,8]. The 927 laminates spiked at 200 °C did show severe blistering, however the moisture concentration was less than in samples spiked at lower temperatures. Considering the reduction in moisture concentration with the addition of thermosetting resins, such as a bismaleimide to the epoxy, the mechanisms responsible for the moisture enhancement during thermal cycling are likely to be associated with the epoxy component. Analysis of the DMTA tan d traces revealed that a primary peak existed for the dry laminates (Tg1) at 220 °C for the 927 and 5245C and 230 °C for the 924 resins. On absorbing moisture under static conditions, Tgl was reduced by approximately 20 °C in all cases. The Kelly and Beuche [9] free volume theories argue that the Tg of a polymer system decreases by an amount proportional to the amount of diluent present, as the free volumes of the two components are additive. This cannot be the only mechanism for the reduction of Tg in the three systems studied because the laminates spiked at the maximum moisture enhancement temperature between 140 and 160 °C absorbed the most moisture yet did not show a similar reduction in Tg as the laminates spiked at higher temperatures which contained less moisture. A secondary tan d peak (Tg2) emerged at lower temperatures after the absorption of moisture. This peak appeared in the spectrum at a lower temperature when the coupons were thermally spiked, such that it appeared 50 °C lower than in the spectra for the un-spiked coupons. Fig. 4 infers that the decrease in Tg2 is dependent upon the temperature at which 5245C and 927 laminates were spiked. In the 924 system, Tg2 was not lowered by thermal spiking at temperatures above 100 °C. The polymer diluent free volume theory often fails with epoxy systems as interactions between the dissolved molecules and segments of the polymer network are ignored.
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The systems investigated are blends of epoxy resins with either thermoplastic or other thermosetting resins. Therefore, it seems likely that the Tg2 peak results from the plasticisation of either one of the components or of the products of the curing reaction and/or the reaction of the epoxy with the thermosetting modifiers. Alternatively, partial hydrolysis of the network may be occurring with the formation of a moisture sensitive phase, as indicated by the increased separation of Tg2 from Tg1 with spiking temperature. For plasticisation to occur, the moisture disrupts the intermolecular forces between the molecules or segments of the network chains [10], increasing the free volume of the polymeric resin. However for the 924 system, which is essentially an epoxy-based system, the moisture concentration is increased by thermal spiking above 100 °C, but Tg1 and Tg2 are not reduced further. In contrast, for 5245C and 927 laminates spiked at 160 °C and above, less moisture is absorbed, yet Tg2 is reduced further. This suggests that thermal spiking results in polymer network/moisture interactions differing from those discussed above. Thus effects other than plasticisation take place. Some studies suggest that network chains can be hydrolytically broken in the presence of moisture [11]. It is possible that hydrolytic degradation of one of the components of the network can occur during thermal spiking of 5245C and 927 laminates, increasing their susceptibility to plasticisation. This would result in the reduction in Tg2. The 924 laminate matrix comprises an epoxy and a thermoplastic component, and as such only plasticisation is expected (assuming epoxy crosslinks are hydrolytically stable). Thus in this case thermal spiking at high temperatures has not led to chain scission of the network, and a further reduction in Tg2 is not observed. The DMTA data presented supports this argument. Free volume theories have been used to explain an increase in unoccupied volume during thermal spiking, which can accommodate the additional absorbed moisture [3,12]. These theories rarely take into account the behavior of the polymer network near to the glass-transition region, where the free volume is generally accepted to increase. Xiang et al. [4] proposed that thermal spikes close to the wet Tg of the matrix-induced partial relaxation of the molecular network. Plasticisation and hydrolysis of network chains during thermal spiking in the presence of moisture leads to a reduction in glass-transition temperature of the matrix. If the temperature at which a wet laminate is spiked approaches that of the matrix glass-transition temperature, an increase in free volume is expected. Simultaneously, moisture present will diffuse into this additional free volume. On rapid cooling, the moisture can be trapped or frozen into the extra free volume. The original, pre-spike free volume contains less moisture than it did prior to the spike, so that more moisture
diffuses into this on further conditioning. Thus with each thermal spike, additional free volume is created in which the moisture can be accommodated. Thermal spiking of the laminates at temperatures above the glass-transition point would result in molecular relaxations taking place to such a degree, that diffusion of moisture through the network would be hindered and thus slowed down. The 5245C and 924 laminates spiked at 160 °C and 927 laminates spiked at 180 °C are above the wet Tg of the matrix, and absorption curves show slower moisture uptake for these samples compared to samples spiked at lower temperatures. The onset of the glass transition ðTg0E Þ, determined from the temperature at which the slope in the storage modulus log E 0 curves decreased, follows the same trend as Tg2. Thus the Tg0E temperature appears to be directly related to Tg2. This is of particular interest since resin manufacturers have blended thermosetting resins such as bismaleimide resins with epoxy, to produce matrices with improved temperature and moisture resistance. Although the resultant 5245C and 927 laminates absorb lower levels of moisture than the 924 system, a greater reduction in Tg0E with increasing spike temperature is observed. The 924 laminates showed a reduction in Tg0E when spiked at 100 °C, but at higher spike temperatures no further change occurred. For 5245C and 927, Tg0E continued to decrease, as the spike temperature was increased, to values well below the manufacturers stated service temperature. It would appear therefore that the introduction of thermosetting components to the epoxy matrix formulations reduces the softening point of their laminates, after conditioning and thermal spiking. This suggests degradation mechanisms as well as plasticisation by moisture are taking place. The degree of degradation is dependent on the thermal spike temperature, and not on moisture concentration, although previous studies have shown that moisture must be present. Thus hydrolysis of the polymer network and possibly the crosslinks formed from the reaction of the bismaleimide or other thermosetting resin with epoxy resin during cure, may be responsible for the reduction in Tg0E at the thermal spike temperature. The transverse flexural strength of wet 5245C laminates, shown in Fig. 5, were reduced by thermal spiking and enhanced moisture absorption. Specimens spiked at 140 °C, showed the highest moisture concentration but not the lowest flexural strength, suggesting that it is not merely the concentration of moisture which reduces strength. Matrix degradation through hydrolysis, evident from DMTA analysis, may play a more influential role. Inspection of the fractured surfaces revealed a transition from failure at the interface between fibres and matrix to matrix yielding with spike temperature. This suggests, that the degree of hydrolysis of the polymer network with spike temperature increases, resulting in a lower yield strength and a lower transverse flexural
J.A. Hough et al. / Composites Science and Technology 65 (2005) 1299–1305
strength of the laminates which is a matrix-dominated property. A similar result might be expected with the wet 927 laminates, but the flexural strength appeared to increase with spike temperature. Epoxy resins are inherently brittle due to their high degree of crosslinking. Other thermosetting resins with lower moisture absorption and lower degree of crosslinking are blended with epoxy resin to improve toughness. If thermal spiking and moisture absorption causes partial network chain scission within the phases present in the matrix, then, with a reduction in Tg0E , a slight improvement in the toughness of the resin could occur. Fig. 6 shows that laminate modulus is unaffected by thermal spiking and enhanced moisture absorption. The transverse flexural modulus is largely a function of the matrix modulus in the glassy state, which is a complex function of overall resin density and is neither directly related to the crosslink density [6,13] nor to the moisture content. It is therefore relatively insensitive to the effects reported here.
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the matrix an appropriate biphasal morphology to the 927 and 5245C systems, respectively. Thermally accelerated hydrolysis of the network chains is believed to be responsible for the reduction of the temperature at which this phase relaxed. No microcracks or voids were observed in the wet laminates that had been spiked, therefore the additional absorbed moisture must be present in the polymer network. Transverse flexural modulus was not significantly affected by thermal spiking or enhanced moisture absorption. Structural changes in the chemical properties of the thermosetting components in the 927 and 5245C matrices appear to be responsible for reductions in flexural strength of the laminates after thermal spiking.
Acknowledgements We thank DERA, BAc systems Ltd and EPCRC for funding.
5. Conclusions Thermal spiking of 5245C, 927 and 924 laminates conditioned in humid environments increased the concentrations of absorbed moisture. A spike temperature occurred for maximum moisture enhancement at 140 °C for the 5245C and 924 laminates, and at 160 °C for the 927 laminates. The addition of thermosetting components such as bismaleimide to the epoxy matrix reduced the total moisture absorbed relative to the epoxy/thermoplastic blend (924). However, DMTA analysis revealed that the temperature at the onset of the glass transition was reduced more rapidly with increasing spike temperature for the 5245C and 927 laminates than for 924 laminates. Analysis of DMTA Tan d curves revealed the splitting of the main relaxation peak. For the latter, Gumen et al. [14] have recently attempted to model this phenomenon and attributed the lower temperature peaks to regions of the network, which have a higher affinity to plasticisation by water molecules. For the former, the thermosetting resin components added to the epoxy blend are intended to give
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