Morphology effect on the hydrothermal ageing of a thermoplastic modified epoxy thermoset.
Epoxy thermosetting polymers are widely used as structural adhesives and matrices for composite materials. Epoxy/thermoplastic binary blends have been extensively studied in the past decade because they offer the possibility of generating materials with improved properties [1, 2]. Most of the cases studied deal with an initially homogeneous mixture consisting of a thermoplastic that dissolves in the epoxy resin precursor (epoxy monomer + curing agent). On curing the initially miscible blends phase separate, and different morphologies are generated as a function of curing conditions and thermoplastic content [1, 2].
The behavior of epoxy resins based on diglycidyl ether of bisphenol A (DGEBA) modified with polyvinyl acetate (PVAc) has been previously reported [3-6]. Specifically we have studied the phase separation upon curing, cure kinetics, morphology, and dynamic-mechanical properties for epoxy/PVAc blends cured with 4,4'diaminodiphenyl-sulphone (DDS) [4, 5]. The thermoplastic initially soluble in the epoxy resin, becomes insoluble upon curing and phase separates forming a biphasic material. Thus cured epoxy/PVAc samples are constituted by an epoxy rich phase, and a second phase whose composition is mainly PVAc. The morphology of these samples is a function of the PVAc content. Samples having 5 wt% of PVAc show nodular morphology, in which the PVAc phase is dispersed as small spheres in an epoxy matrix. In samples having 15 wt% of PVAc the morphology is inverted, that is, the epoxy phase forms spheres that are surrounded by a thin continuous phase of PVAc. Samples with intermediate composition (10 wt% of PVAc) show a combined morphology, coexisting nodular and inverted regions.
It is well known that epoxy thermosets absorb small amounts of water [7-21], as a consequence, their thermomechanical properties worsen with hydrothermal ageing. Water acts as a plasticizer that lowers the glass transition temperature ([T.sub.g]). Because of the technical interest, there is a relative large amount of literature on water absorption in epoxy thermosets. Most of the studies are focused on neat epoxy networks. Works studying the water absorption in thermoplastic modified epoxies are scarcely reported, besides the effect of the morphology on water absorption has only been considered in few cases [20, 21].
In this work, we study the water uptake process in cured epoxy/PVAc blends containing 5, 10, and 15 wt% of PVAc and in neat epoxy. Scanning electron microscopy (SEM) and dynamic mechanical thermal analysis (DMTA) are used to study the effect of the hydrothermal ageing on the samples. The main objective is to correlate the behavior with the samples morphology.
Materials and Sample Preparation
The epoxy resin studied was based on the diglycidyl ether of bisphenol A (DGEBA) supplied by Uneco S.A. (Barcelona, Spain) under the commercial name of Araldit F. The number average relative molecular mass, [bar.M.sub.n] = 380 g/mol, was obtained by chemical titration of the end groups. The curing agent was 4,4'-diaminodiphenylsulfone (DDS) (98 wt%) supplied by Sigma-Aldrich Chemical (Madrid, Spain). Poly(vinyl acetate) (PVAc), with [bar.M.sub.n] = 9 x [10.sup.4] g/mol and polydispersity index 2, was purchased from Polysciences (Eppelheim, Germany). Acetone Panreac Quimica S.A. (Barcelona, Spain) QP (0.3 wt% water content) was used as solvent. Frekote from Loctite (Madrid, Spain) was employed as mould release product.
DGEBA + DDS and (DGEBA + DDS)/PVAc blends were obtained by mixing appropriate amounts of their respective solutions in acetone. The DGEBA/DDS ratio was stoichiometric in all the cases. Three blend compositions having different amounts of PVAc (5, 10, and 15 wt%) have been prepared together with the unmodified epoxy (0 wt% PVAc). These solutions were cast onto aluminum moulds of dimensions: 250 mm x 80 mm x 2 mm that were previously treated with mould release product. The acetone was carefully eliminated by degassing under vacuum at 80[degrees]C, then temperature was raised to the cure temperature ([T.sub.c] = 180[degrees]C). The cure conditions were established in previous papers [4, 5]: 3 h at [T.sub.c] = 180[degrees]C under atmospheric pressure. From the cured panels of thickness 1.5-2 mm, specimens for hydrothermal ageing (60 x 60 [mm.sup.2]) and for DMTA measurements (30 x 10 [mm.sup.2]) were cut.
Isothermal Water Absorption Measurements
Hydrothermal ageing was carried out by immersion in distilled water at three temperatures 20, 40, and 60[degrees]C [+ or -] 0.3[degrees]C. Three specimens were tested for each blend composition (0, 5, 10, and 15 wt% PVAc) and ageing temperature. The specimens were periodically removed from water and carefully wiped to remove excess of water. The change in weight of the specimens was monitored as a function of the immersion time. Weight was determined with accuracy [+ or -]0.5 mg. The total period for which the sample was out of water was ~1 min. The water uptake content was calculated as the amount of absorbed water per unit weight of dry specimen: water uptake (wt%) = ([DELTA]M/[M.sub.0]) x 100, being [M.sub.0] the initial weight of the specimen and [DELTA]M = M - [M.sub.0], where M is the weight of the hydrothermally aged specimen.
Dynamic Mechanical Thermal Analysis Measurements
Dynamic mechanical thermal analysis (DMTA) was performed in dual cantilever bending mode, using a DMTA V Rheometric Scientific instrument. All the measurements were done at 1 Hz frequency, with temperature increasing from -40 to 240[degrees]C at a heating rate of 2[degrees]C/min. The maxima of tan [delta]-temperature plots were determined to identify the [alpha]-relaxation associated to the glass transition.
A Mettler Toledo mod.821e differential scanning calorimeter was used to measure the glass transition temperatures, [T.sub.g], of the samples. All measurements were done at a heating rate of 20[degrees]C/min, from -40 to 300[degrees]C, under nitrogen atmosphere. The instrument was calibrated with indium and zinc. Samples of 8-10 mg were used. The glass transition temperature was taken at the midpoint of the heat capacity change.
Environmental Scanning Electron Microscopy Measurements
Environmental scanning electron microscopy (ESEM) was used to study the morphology of the samples. Because of the non conducting nature of the samples, the environmental SEM mode was selected. A Phillips XL30 instrument was used with beam energy of 20 kV, verifying that this did not produce severe damage on the sample. The water vapor pressure was 0.6-0.7 Torr that corresponds to a relative humidity of ~5%.
RESULTS AND DISCUSSION
The water sorption of cured epoxy samples having different PVAc content (0, 5, 10, and 15 wt%) at 20, 40, and 60[degrees]C was gravimetrically monitored. To eliminate the specimen thickness effect, the water uptake weight percentage, [DELTA]M/[M.sub.0] (%), has been plotted versus [t.sup.[1/2]]/h, where t is the absorption time and h is the specimen thickness. Three specimens were tested for each composition and temperature, and they followed the same pattern. Figure 1A shows the water absorption curves at 20[degrees]C for the three specimens of different thickness corresponding to epoxy/5 wt% PVAc. As it can be seen, the three curves are essentially coincident and linear in the early stage of absorption, this behavior is typical of systems in which the absorption process is predominantly diffusion controlled and can be described by Fick's diffusion law [22, 23]. However, for epoxy/15 wt% PVAc samples after long immersion times (>100 h), the data of the three specimens were not coincident, as it can be seen in Fig. 1B, where the absorption curves at 40[degrees]C are plotted.
Figure 2 shows the absorption curves for epoxy/PVAc samples containing 5, 10, and 15 wt% PVAc and for the neat epoxy (0 wt% PVAc) at 20, 40, and 60[degrees]C. The curves correspond to one of the tested specimens. Neat epoxy and epoxy/5 wt% PVAc samples present similar behavior at the three temperatures. However, when the percentage of PVAc overrides 5 wt% PVAc, the absorption curves are clearly dependent on the composition, that is, the higher PVAc content, the more water uptake is. The absorption curves are linear from the beginning to a few per cent weight gain (2-3 wt%), after the first linear region, the diffusion rate decreases, becoming the behavior linear again. The slope of this second linear region for neat epoxy and epoxy/5 wt% PVAc is close to zero, indicating that the water equilibrium uptake is almost reached. Epoxy/10 wt% PVAc samples behave somewhat similar to the epoxy/5 wt% PVAc samples although they do not reach full saturation during the studied period of time.
[FIGURE 1 OMITTED]
For epoxy/15 wt% PVAc, the slope of the second linear region is clearly positive pointing out that the water absorption continues. This kind of absorption behavior, in which the weight continuously increases, has been sometimes attributed to the existence of irreversible processes, such as chemical hydrolysis or physical damage [2, 23]. The main chemical modification that could take place in these samples would be the hydrolysis of PVAc. To clarify this point, PVAc was hydrothermally aged under the same ageing conditions and subsequently dried. The Raman and RMN spectra as well as DSC [T.sub.g] were coincident with those corresponding to unaged samples. Therefore no PVAc hydrolysis had taken place. Accordingly, [T.sub.g] of the PVAc phase takes similar values in epoxy/PVAc unaged and in dried after ageing samples.
The water absorption behavior for epoxy/15 wt% PVAc can be explained taking into account that in this blend the morphology is inverted, that is, the epoxy phase forms spheres that are surrounded by a thin continuous phase of PVAc. The hydrothermal ageing causes debonding between the two phases in the specimen (see morphology section) being more susceptible to water sorption. The different water absorption profiles for the other blend compositions are also related to their morphology. Thus, in epoxy/5 wt% PVAc samples that have nodular morphology (small PVAc spheres dispersed in an epoxy matrix), the water absorption is governed by the rigid crosslinked epoxy phase, and the absorption curves of neat epoxy and epoxy/5 wt% PVAc samples are alike. Samples containing 10 wt% PVAc have a combined morphology (nodular and inverted regions coexist), accordingly their behavior is intermediate between those observed in the other two morphologies.
[FIGURE 2 OMITTED]
Although the sorption processes of liquids and vapors in glassy polymers follow complex diffusion mechanisms, water sorption in epoxy thermosets has been frequently described by Fick's diffusion model [2, 8-10, 22, 23]. In the blends here studied two features are concordant with Fickian diffusion: the absorption curves are linear in the initial stages and specimens of different thickness generate super-imposable absorption curves. However, the fact that specimens containing 10 and 15 wt% of PVAc does not reach the equilibrium uptake is not in accordance with the Fickian diffusion model.
Following Fick's second law, the total water uptake for one-dimensional diffusion, in a sheet of thickness h exposed on the two sides to the same environment during a time t, is given by:
[DELTA]M = (1 -[8/[[pi].sup.2]][[infinity].summation over (j=0)][[exp[-(2j + 1)[.sup.2][[pi].sup.2](Dt/[h.sup.2])]]/(2j + 1)[.sup.2]])[DELTA][M.sub.[infinity]] (1)
where [DELTA]M = M - [M.sub.0], and [DELTA][M.sub.[infinity]] = [M.sub.[infinity]] - [M.sub.0], being M the weight of the sample at a time t, [M.sub.0], and [M.sub.[infinity]] the initial weight and the final equilibrium weight, and D is the diffusion coefficient. The infinite series of Eq. 1 is often simplified by the relationship :
[DELTA]M = (1 - exp[-7.3(Dt/[h.sup.2])[.sup.0.75]])[DELTA][M.sub.[infinity]]. (2)
Besides, at short times, where [DELTA]M/[DELTA][M.sub.[infinity]] is less than 0.6. Eq. 1 can be approximated by:
[DELTA]M = [4/h] (Dt/[pi])[.sup.1/2] [DELTA][M.sub.[infinity]]. (3)
Equation 3 is easily rearranged to reveal that the initial slope in a plot of [DELTA]M/[M.sub.0] versus [t.sup.[1/2]]/h is related to the diffusion coefficient, D, through:
D = [pi](1/[4[DELTA][M.sub.[infinity]]/[M.sub.0]])[.sup.2][[[DELTA]M/[M.sub.0]]/[(t)[.sup.1/2]/h]][.sup.2]. (4)
For real specimens the "edge effect" that takes into account the water entering through the edges should be considered , in such a case Eq. 4 becomes:
D = [pi](1/[4[DELTA][M.sub.[infinity]]/[M.sub.0]])[.sup.2][[[DELTA]M/[M.sub.0]]/[(t)[.sup.1/2]/h]][.sup.2] [[beta].sup.-1] (5)
[beta] = (1 + [h/a] + [h/b])[.sup.2] (6)
where a and b are the other two specimen dimensions. For the specimens here studied the correction factor is [beta] ~ 1.06.
The diffusion coefficients were obtained from the slopes of the first linear region of the absorption curves, S = ([DELTA]M/[M.sub.0])/([t.sup.[1/2]]/h), using Eq. 5. Table 1 collects the values of S, [DELTA][M.sub.[infinity]]/[M.sub.0], and D for neat epoxy and epoxy/5 wt% PVAc blends, for each temperature. As expected S, [DELTA][M.sub.[infinity]]/[M.sub.0], and D take similar values in neat epoxy and epoxy/5 wt% PVAc blend. For epoxy(DGEBA + DDS) thermosets cured under schedules that lead to cured samples similar to the ones studied here ([T.sub.g] ~ 180-200[degrees]C), the values of D reported for hydrothermal ageing at 20[degrees]C are D = 1.1 x [10.sup.-7] - 1.5 x [10.sup.-7] [mm.sup.2]/s and [DELTA][M.sub.[infinity]]/[M.sub.0] = 2.8-3.6 wt% , which are in agreement with the results given in Table 1. Moreover, these values are in the range of the reported for the typical epoxy-amine cured systems [9, 10].
No equilibrium uptake plateau was reached in epoxy/10 wt% PVAc and more clearly for epoxy/15 wt% PVAc samples (see Fig. 2). The continued increase in weight after a first diffusion controlled process has been often reported for glassy polymers [7, 8, 22, 23]. This effect may be originated from changes in the polymer structure upon absorption (widening of the relaxation time distribution) . This is consistent with a two-stage diffusion model: after a first Fickian diffusion process, a slower second absorption stage appears, which is associated to the relaxation processes of the glassy polymer, that is, as the polymer chains rearrange in the presence of the absorbed molecules, an additional absorption takes place. In the system here studied, even considering the plasticization effect of the water absorbed, that lowers the [T.sub.g] of the epoxy (see DMTA results), the new [T.sub.g] values are still higher than the ageing temperatures, so the epoxy relaxations should occur very slowly. Therefore, as in other epoxy systems  [DELTA][M.sub.[infinity]]/[M.sub.0] reaches a constant value for neat epoxy and epoxy/5 wt% PVAc samples. The continuous water uptake for 10 and 15 wt% PVAc modified epoxy is too high to be caused by relaxations of the epoxy network. As it has been discussed above, this two-stage behavior can be related to the morphology of the specimens. Following similar schemes to those developed previously to explain moisture absorption in bismaleimides , we have fit the absorption curves to a two-stage absorption model with two separated stages. The first one, Fickian in nature, would be described by Eqs. 1 and 2 where D represents apparent mean diffusion coefficient of the heterogeneous system. Since the weight gain during the second stage increases linearly with [t.sup.1/2] (see Fig. 2) it would be described by (1 + kx), being x = [t.sup.1/2]/h. The water uptake in the complete time scale is given by :
[FIGURE 3 OMITTED]
[DELTA]M = [DELTA][M.sub.[infinity]][1 - exp (-7.3(D[x.sup.2])[.sup.0.75])](1 + kx) (7)
where [DELTA][M.sub.[infinity]] is the equilibrium water uptake of the Fickian stage and k represents the rate of absorption of the second stage.
As it is illustrated in Fig. 3, for 10 and 15 wt% PVAc modified epoxy hydrothermally aged at 40[degrees]C a good description of the overall process is achieved. Similar fittings were produced for all the samples, Table 2 collects the fit parameters: k, [DELTA][M.sub.[infinity]]/[M.sub.0], and D together with the residual variance ([chi square]) obtained, all the fits have correlations [R.sup.2] > 99%. For 10 and 15 wt% PVAc modified epoxy k > 1, in these cases the model describes a process in which after a certain period of time the epoxy phase would reach the final equilibrium water uptake, but water would continue being absorbed in the thermoplastic-epoxy interfaces. However, when the PVAc is dispersed into the epoxy matrix, as it occurs in epoxy/5 wt% PVAc samples, the water absorption would be limited by the rigid epoxy matrix, and the equilibrium water uptake is almost reached as it occurs in unmodified epoxy, therefore the values of k for these compositions are close to 0. In these cases the small positive values of k can be related to the slow relaxation of the epoxy network which is in the glassy state. It is worthy to note that PVAc is in the [T.sub.g] zone at the ageing temperatures, therefore the relaxations of the thermoplastic occur quickly and they must not affect the sorption process.
[FIGURE 4 OMITTED]
For all the compositions studied the diffusion coefficients follow an exponential dependence with temperature. Although only three temperatures have been studied, the activation energy for diffusion, Q, has been obtained by plotting lnD versus the reciprocal temperature 1/T, according to the Arrhenius relationship:
D = [D.sub.0] exp(-Q/RT) (8)
where [D.sub.0] is the diffusion constant, R the universal gas constant, and T the absolute temperature. The Q values obtained are 43 [+ or -] 4, 42 [+ or -] 4, 40 [+ or -] 7, and 38 [+ or -] 4 kJ/mol for 0, 5, 10, and 15 wt% of PVAc, respectively. The variation of the Q values with the PVAc content is within the estimated error; nevertheless, there is a slight trend to decrease Q on raising the PVAc percentage. These results refer only to the first stage of diffusion, and it could be assumed that temperature changes have major influence on the diffusion process when PVAc content is increased. Similar values of the activation energy for diffusion have been reported in the literature for various epoxy-amine systems [10-12, 15].
[FIGURE 5 OMITTED]
To study the morphological changes induced by hydrothermal ageing, the ESEM micrographs of unaged samples were compared to the micrographs of samples that had been hydrothermally aged more than 2000 h and subsequently dried. Figure 4 shows the micrographs of fractured surfaces of epoxy samples containing 5 wt% of PVAc (epoxy matrix with PVAc particles dispersed). No changes are observed in morphology after hydrothermal ageing, the mean diameter of the PVAc particles is 1.5 [+ or -] 0.4 [micro]m, for both unaged and aged samples. The hydrothermal ageing seems not to affect the adhesion between phases. Moreover, the appearance of the fractured surfaces is similar.
[FIGURE 6 OMITTED]
As it has been discussed above, samples containing 10 wt% of PVAc show combined morphology, they are formed by distinct regions: epoxy domains with spherical PVAc particles dispersed and regions with inverted morphology (epoxy particles surrounded by a thin PVAc matrix). Figure 5 shows the micrographs of these regions for unaged and aged samples. The PVAc particles dispersed in the epoxy domains have mean diameters of 1.6 [+ or -] 0.4 [micro]m both for unaged and aged samples. (Fig. 5 A, C, and E). The morphology of these domains is similar to the morphology of epoxy samples containing 5 wt% PVAc and is not affected by hydrothermal ageing. The domains with inverted morphology show epoxy spheres with broad size distribution (diameters between 2-50 [micro]m). After hydrothermal ageing slight changes are observed, that is, only the epoxy particles seem to be more exposed.
However, important changes were detected in 15 wt% PVAc modified epoxy, after hydrothermal ageing, as illustrated in Fig. 6. Aged samples show a certain loose of the spheres array and the PVAc phase that initially adheres and covers the epoxy spheres (mean diameter = 140 [+ or -] 50 [micro]m) becomes unstuck and slackened. Therefore, it seems that the interface PVAc/epoxy is the weaker region of the specimen. Debonding between the two phases can explain the water absorption profile of these samples. The interaction between cured epoxy and PVAc is mainly van der Waals, so poor bonding will exist between the phases, water tends to interact through hydrogen bonding replacing the thermoplastic-epoxy interface and water transport may be enhanced by pathways open along the interface. The absorption through the interface would also take place for PVAc dispersed morphology but this would have a minor effect over the whole absorption process, because the epoxy network is a rigid phase that envelopes the thermoplastic and does not favor the creation of extra pathways in the interface.
Dynamic Mechanical Thermal Analysis
To study the influence of the hydrothermal ageing on the mechanical properties, DMTA measurements were performed for samples that had been hydrothermally aged more than 2000 h at 20, 40, and 60[degrees]C.
Figure 7 shows the E' versus temperature plots for unaged and aged samples at 20[degrees]C. The similar behavior of neat epoxy and 5 wt% modified epoxy is apparent, that is, unaged samples maintain the modulus value up to ~170[degrees]C and aged samples up to ~145[degrees]C, beyond these temperatures the storage modulus drops two decades as result of entering in the glass transition zone of the epoxy network. For unaged 10 and 15 wt% PVAc modified epoxy the modulus drops at temperatures close to the PVAc [T.sub.g] (~45[degrees]C), this drop is much more important in the 15 wt% modified epoxy as a consequence of the inverted morphology. The hydrothermal ageing worsens even more the mechanical behavior of these samples that begin to soften from ~17[degrees]C.
In Figure 8, the tan [delta] versus temperature plots for neat and 5 wt% modified epoxy are shown. A tan [delta] peak appears at high temperature (around 195-210[degrees]C) that is assigned to [alpha]-relaxation of the neat cured epoxy network. The hydrothermally aged specimens show a split in the epoxy [alpha]-relaxation peak, this behavior has been previously detected for other epoxy systems [14-19]. The new peak in the aged samples is clearly defined around 160[degrees]C, and could be assigned to the [alpha]-relaxation of the epoxy network that has been plasticized by water. Thus, water is an effective epoxy plasticizer that lowers the [T.sub.g] of the epoxy network [DELTA][T.sub.g] ~ 40[degrees]C). The Fox relationship  can be used to describe the [T.sub.g]-composition dependence of ideal random blends:
[FIGURE 7 OMITTED]
1/[T.sub.g] = [[w.sub.1]/[T.sub.g1]] + [[w.sub.2]/[T.sub.g2]] (9)
On applying this rule to the epoxy-water mixtures, [T.sub.g1], [T.sub.g2], and [T.sub.g] refer to the [T.sub.g] values of water, dry, and wet epoxy respectively, and [w.sub.1] = 1 - [w.sub.2] represents the water weight fraction in the mixture. Using the experimental DMTA value of [T.sub.g] (160[degrees]C for the water plasticized epoxy and taking [T.sub.g1] = 100 - 150 K [27-29] and [T.sub.g2] = 473 K, a value of [w.sub.1] ~ 0.02-0.04 is predicted. This is in accordance with the experimental equilibrium water uptake (see Table 1).
[FIGURE 8 OMITTED]
A small tan [delta] peak for 5 wt% PVAc modified epoxy is located at ~40[degrees]C which corresponds to [alpha]-relaxation of the PVAc phase (4, 5). After hydrothermal ageing this peak moves to lower temperature ~20[degrees]C, indicating that the PVAc dispersed phase into the epoxy matrix has been also water plasticized. Applying Fox rule to water-PVAc mixture, it results that PVAc phase in these samples has absorbed 3-6 wt% water.
The water absorption of cured epoxy(DGEBA+DDS)/PVAc blends having 0, 5, 10, and 15 wt% PVAc, at three temperatures (20, 40, and 60[degrees]C) is reported. Differences in the water uptake process were observed as a function of the PVAc content. These differences were related to the different morphology of the samples that is a function of PVAc content. The absorption curves for neat epoxy and samples having PVAc dispersed phase in the epoxy matrix (5 wt% PVAc modified epoxy) are similar. However, samples containing 15 wt% PVAc are more susceptible to water absorption, displaying a continuous water uptake as a consequence of the inverted morphology. The diffusion coefficients were estimated according to Fick's law for neat and 5 wt% modified epoxy, and the activation energy for diffusion for each blend composition was calculated. The continued increase in water uptake for blends with PVAc content [greater than or equal to]10 wt% is consistent with a two-stage diffusion model.
No changes in morphology were detected in neat and 5 wt% modified epoxy samples after hydrothermal ageing, minor changes were detected for 10 wt% PVAc modified epoxy samples. However, debonding between thermoplastic and thermoset phases was observed in 15 wt% PVAc modified epoxy samples after hydrothermal ageing.
Different dynamic-mechanical behavior has been found for samples that have epoxy matrix and samples with inverted morphology, both before and after hydrothermal ageing. Hydrothermally aged samples show reduction on the glass transition temperature of both epoxy and thermoplastic phases and typical double peak shape for the epoxy network [alpha]-relaxation.
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C. Arribas, (1) L. Sepulveda, (1) C. Salom, (1) R.M. Masegosa, (2) M.G. Prolongo (1)
(1) Dpt. Materiales y Produccion Aeroespacial. E. T. S. I. Aeronauticos, Universidad Politecnica Madrid, Spain
(2) Dpt. Fisica y Quimica Aplicadas a la Tecnica Aeronautica. E. U. I. T. Aeronautica, Universidad Politecnica Madrid, Spain
Correspondence to: M.G. Prolongo: e-mail: email@example.com
Contract grant sponsor: MEC Spain; contract grant number: MAT 2003-1591; Contract grant sponsor: CAM; contract grant number: S-0505/MAT-0227-INTERFASES project.
TABLE 1. Slopes of the first linear region of the absorption curve, S, equilibrium weight, [DELTA][M.sub.[infinity]]/[M.sub.0], and diffusion coefficient, D, for neat epoxy and epoxy/5 wt% PVAc blend. Temperature S x [10.sup.5] Thermoset ([degrees]C) (mm [s.sup.-1/2]) Epoxy 20 2.8 (DGEBA + DDS) 40 5.2 60 6.9 Epoxy 20 2.9 (DGEBA + DDS) + 40 5.6 5 wt% PVAc 60 7.6 [DELTA][M.sub.[infinity]]/ D x [10.sup.7] Thermoset [M.sub.0] (wt%) ([mm.sup.2] [s.sup.-1]) Epoxy 3.2 1.3 (DGEBA + DDS) 3.1 4.7 2.8 10.6 Epoxy 3.2 1.4 (DGEBA + DDS) + 3.4 4.7 5 wt% PVAc 3.1 11.0 Mean values are given for the three specimens aged at each temperature. TABLE 2. Two-stage absorption model: k, [DELTA][M.sub.[infinity]]/ [M.sub.0]. D and [[gamma].sup.2] fit parameters. Temperature k x [10.sup.4] Thermoset ([degrees]C) (mm [s.sup.-1/2]) Epoxy(DGEBA+DDS) 20 0.04 40 0.01 60 0.01 Epoxy(DGEBA+DDS) + 20 0.04 5 wt% PVAc 40 0.07 60 0.03 Epoxy (DGEBA+DDS) + 20 1.1 10 wt% PVAc 40 2.5 60 2.9 Epoxy (DGEBA+DDS) + 20 5.5 15 wt% PVAc 40 11 60 16 [DELTA][M.sub.[infinity]]/ D x [10.sup.7] Thermoset [M.sub.0] (%) ([mm.sup.2] [s.sup.-1]) Epoxy(DGEBA+DDS) 3.3 1.8 3.2 4.4 2.9 10.0 Epoxy(DGEBA+DDS) + 3.3 1.6 5 wt% PVAc 3.5 4.9 3.1 19 Epoxy (DGEBA+DDS) + 3.4 3.3 10 wt% PVAc 3.3 16 3.2 28 Epoxy (DGEBA+DDS) + 3.2 5.7 15 wt% PVAc 3.2 29 3.2 49 Thermoset [chi square] Epoxy(DGEBA+DDS) 0.028 0.002 0.017 Epoxy(DGEBA+DDS) + 0.008 5 wt% PVAc 0.004 0.008 Epoxy (DGEBA+DDS) + 0.035 10 wt% PVAc 0.010 0.013 Epoxy (DGEBA+DDS) + 0.052 15 wt% PVAc 0.015 0.047
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|Author:||Arribas, C.; Sepulveda, L.; Salom, C.; Masegosa, R.M.; Prolongo, M.G.|
|Publication:||Polymer Engineering and Science|
|Date:||Jun 1, 2007|
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