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Ind. Eng. Chem. Prod. Res. Dev. 1904, 2 3 , 288-297
Role of Processing on the Durability of Epoxy Composites in Humid Environments A. Aplcella” and L. Nlcolals Polymer Engineering Laborato% Engineering School, University of Naples, Piazzale Tecchio, 80 725 Naples, Italy
This work relates moisture sorption behavior and plasticization of two epoxy systems to hygrothermaland processing variables, a low cross-linked epoxy diglycidyl ether of bisphenol A (DGEBA) cured with linear amines (ethylenediamine, EDA, diethylenetriamine, DETA, and triethylenetetramlne, TETA) and a highly cross-linked system, tetraglycidyldlaminodiphen~~ne O M ) cured with diaminodiphenyl sulfone (DDS). Increases in the equilibrium moisture content were observed as a result of thermal cycling in liquid environments. The hygrothermal interactions produced changes both in the epoxy network structures and in the observed moisture sorption behavior. The intrinsic moisture sensitivity arises from the chemical characteristics(hydrophilic groups) as well as from microscopic and macroscopic defects (heterogeneous network structures and fillers). The modes of absorption of the water molecules have been found to atter differently the glass transtion temperatures of these two classes of thermosets: free volume changes are principally responsible for the plasticization of the low cross-linked DGEBA resin, while water-polymer bond formation and polymer-polymer intramolecular hydrogen bond rupture are responsible for the T, depression in the highly cross-linked TGDDM-DDS systems.
Introduction The chemical characterization and reproducibility of thermosetting epoxies, which are the dominant forms of polymeric matrices for carbon fiber composites, are an important area of concern. The degradation of the matrix associated with both moisture-induced plasticization and/or micromechanical damage (Browning, 1978; Loos and Springer, 1979) strongly dominates the composite response properties under temperature, humidity, and stress fatigue tests (McKague et al., 1975; Morgan and O’Neal, 1966; Apicella et al., 1979). Sorbed moisture, acting as a plasticizer and crazing agent for the epoxy, differently deteriorates the mechanical and chemical integrity of the composite under certain temperaturemoisture combinations. The damaging process is governed by the synergistic action of sorbed moisture and temperature leading to additional weight gains in samples exposed to cycling conditions of temperature (Apicella et al., 1981, 1982,1983). These additional weight gains were attributed to the entrapment of moisture during the microcracking of the resin, since they do not induce changes of the glass transition temperature (Browning, 1978; Apicella et al., 1979). Completed studies of graphite epoxy composites for aerospace applications have revealed two dominant classes of hygrothermal degradation mechanisms: microscopic defects in the molecular network structure of the matrix phase and macroscopic defects such as voids, bubbles, and debonded and broken fibers produced during the processing and life of the composite. The chemical structure, functionality, and composition of the matrix chemical constituents, as well as the processing conditions, influence the resulting networks and hence the properties of the cross-linked polymer. In fact, although the concept of homogeneous infinite network has been long erroneously applied to describe the morphology of all thermosetting polymers, the hypothesis of highly cross-linked nodules immersed in an internodular matrix of lower cross-linking density seems more reasonable (Morgan et al., 1982; Mijovic and Tsay, 1981; Dusek et al., 1978). The influence of the matrix nodular structure on durability in agressive environments, mode of failure, and mechanical properties of TGDDM-DDS-based composites has been reported in the literature by Morgan et al. (1979) 0196-4321/84/1223-0288$01.50/0
and Schneider et al. (1979). Complete cure of these high-performance matrices, however, can be only driven to completion at high temperatures, where chemical degradation or the formation of a heterogeneous network are favored both by diffusion constraints, due to the vitrification of the system (Schneider et al., 1979) and the presence of differently activated cross-linking mechanisms. Mechanical properties of partially cured systems have been, in fact, reported by Gillham (1979) and Lewis et al. (1979) to often represent the optimum. Incomplete cure, on the other hand, has been recognized to reduce the durability of these materials in long-term environment aging (Apicella et al., 1983). It then becomes difficult to establish close relationships between the chemical composition, processing variables, and the optimum characteristics in real life without accounting for the influence of the time-temperature path on the molecular structure during the early stages of cure, when gelation occurs, and in the final postcuring of these materials. The exposure of heterogeneous materials to humid environments induces different morphological changes of the polymeric structure, depending on their affinity to the water molecules and their mode of sorption (Weitsman, 1976; Apicella et al., 1983). The adsorbed species are present in the glassy polymer in two different forms; one is associated with an equilibrium content dissolved in the compact resin, while the second is stored in holes characteristic of the nonequilibrium glassy structure (Vieth et al., 1976; Barrer et al., 1958). A t low activities, sorption of gasses and vapors is usually described by a superposition of Henry’s law and a Langmuir isotherm (Michaels, 1963). The Henry’s law term is generally attributed to molecular solution of the penetrants in the glassy matrix, while the Langmuir sorption mode occurs as a result of the insertion of the sorbed molecules into a finite number of preexisting gaps in the polymeric matrix (Barrer et al., 1958). A t high activities, however, vapor sorption often involves strong positive deviations from Henry’s law which cannot be simply explained in terms of the Flory-Huggins solution theory, but conveniently interpreted by considering the tendency of the molecules to cluster (Zimm and Lunderberg, 1956; Apicella and Carfagna, 1983). The water molecules combine the tendency to cluster, craze, and plasticize the epoxy matrix with the charac0 1984
American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 2, 1984
teristic of easily diffusing in the polymer through the composite fibers. The morphology, surface, and type of fiber surface treatments are adversely influenced by the presence of the sorbed moisture. The polymer, in fact, does not present an effective barrier to the diffusion of the water through the fibers, and leads to the degradation of the matrix, fibers, and interfaces. In order to evaluate the effect of hygrothermal fatigue on the physical and mechanical properties of composites in actual service, it is crucial to resolve the basic phenomena driving the complex water sorption behavior and degradation mechanisms in various combinations of moist environment and temperature. This publication presents the results of an extensive study on the influence of processing parameters, prepolymer composition, and type of epoxy resin on the environmental aging of widely utilized matrices for composites. In particular, the water sorption behavior and properties of the DGEBA epoxy, cured with linear amines, and of the TGDDM epoxy, cured with an aromatic amine (DDS), characterized respectively by low and high cross-linking densities, are critically related to the morphological and the chemical modifications induced by different processing and aging conditions. Experimental Section Commercial grade tetraglycidyldiaminodiphenylmethane, TGDDM, (MY-720,CIBA)and diglycidyl ether of bisphenol A, DGEBA (epikote 828, Shell It.) epoxy resins were used. Mixtures of TGDDM with 20 to 50 PHR of diaminodiphenyl sulfone (DDS) and of DGEBA with 5,14, and 25 PHR of triethylenetetramine (TETA) (PHR = parts per hundred of resin) and stoichiometric amounts of ethylenediamine(EDA) and diethylenetriamine (DETA) have been cast in films of 0.3-0.5 mm thickness and cured by using the following schedules: (a) Heating at 2.5 "C/ min from room temperature up to 177 "C, holding 90 min at 177 "C and cooling at 10 "C/min for the TGDDM-DDS systems; (b) cast and gel at room temperature for 24 h; postcure for 24 h at 80 "C and for 5 days at 100 "C for the DGEBA linear amine systems. A differential scanning calorimeter, DSC-2 Perkin-Elmer, was used to determine the glass transition temperatures of the water-conditioned and desiccated samples. Sealed steel pans were used to scan 5.0 to 10 mg of resin at 10 "C/min. Liquid and vapor water sorption tests were carried out at 60 and 20 "C with distilled water. Gravimetric liquid sorption measurements were performed by repeatedly weighing 3.0 X 3.0 X 0.03 cm3 samples on an analytical balance following immersion in water maintained at constant temperature (fO.l "C). Vapor sorption experiments were performed with a temperaturehumidity controlled quartz spring balance. Tensile tests were carried out with an Instron machine equipped with a thermostatic chamber and strain gage extensimeters. The samples were temperature equilibrated and tested in the thermostatic chamber saturated with water vapors. Dynamic mechanical properties were determined by using a Rheovibron DDVII as previously described by Mikols (1982). All experiments were performed at a frequence of 11 Hz over the temperature range beginning near -150 "C to just above the specific sample TB'A heating rate of 1 "C/min was employed for all experiments. Results and Discussion It has been described in previous publications by Apicella and Nicolais (1981) that higher levels of saturation in epoxies exposed to humid environments were determined by the highest temperature experienced by the
289
teta
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6
7
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Figure 1. Epoxide-aromatic IR adsorbance ratios as a function of the cure time for DGEBA cured with TETA, DETA, and EDA.
sample in the presence of sorbed water. The differences in the water uptakes for the same external conditions of temperature and humidity, observed for samples with different hygrothermal histories, were explained in terms of formation of microcavities that have been induced by solvent crazing during the sorption process at high temperatures. The dependence of microvoiding on temperature was experimentally observed by Apicella et al. (1979) to be progressively more relevant at relatively high temperatures where lower energies for craze formation are required (Andrews et al., 1975). Influence of Network Homogeneity on Hygrothermal Stability. The influence of the network homogeneity on hygrothermal stability has been tested on the DGEBA epoxy resin cured with polyfunctional amines of different molecular weight and length having the same distance between the active sites in order to produce equivalent cross-linked structures in reactions of different rates. Reaction rates for the three systems were extimated from the decrease of the ratio between the intensities of the epoxide and aromatic IR adsorbance at 915 cm-l and 805 cm-l reported in Figure 1. The rates of reaction of the amines in the system cured with EDA, DETA, and TETA were progressively lower, reflecting the lower mobilities of the molecules of higher molecular weight. The greatest reaction rate and diffusivities of the EDA molecules suggest that the network grew with a large number of little nodules immersed in an internodular matrix of almost the same characteristics, whereas the network of the system cured with the TETA probably grew with highly crosslinked nodules immersed in an internodular matrix of much lower cross-linkingdensity. The nodular morphology of thermosets has been discussed for a long time and finally recognized to be induced by diffusion constraints of the species involved in the cure reactions (Morgan et al.,1982; Mijovic and Koutaky, 1979; Dusek et al., 1978). The lower mobility of the highly hindered TETA molecules limits, in fact, their diffusion toward regions of lower hardener concentration, increasing the probability that the reaction principally interests limited areas of higher cross-linking density. The system cured with the DETA will present intermediated characteristics of network homogeneity. Figure 2 compares the liquid water sorption behavior in thin samples of the three systems first equilibrated at
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 2, 1984
200
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60 "C and then brought back to 20 "C to those of samples equilibrated exclusively at 20 "C. Water sorption is initially controlled by ordinary Fickian diffusion, while it becomes anomalous approaching equilibrium. The samples previously saturated at 60 "C sorb additional water at 20 "C even if the saturation level achieved in a test carried out exclusively at 20 "C is apparently lower. The difference between the two asymptotic equilibrium values, which has been considered the excess water trapped in the hygrothermally induced voids, is progressivly reduced for the systems cured respectively with EDA, DETA, and TETA. Previous investigations by Apicella et al. (1979) and Apicella and Nicolais (1981) have also shown that this effect is less evident at low temperatures and that water desorption reversibly takes place when the sample is brought back to the higher temperatures of the thermal cycle, leading to the same initial equilibrium value reached in the first sorption at that temperature. The anomalous increase of the water uptake observed in Figure 2a when approaching equilibrium at 60 "C has been associated with the damaging process. The abrupt upturn of the sorption curve may be explained in terms of crazing of the low
cross-linked internodular matrix induced by the differential swelling stresses that can arise, at high water contents, between areas of different cross-linking density. Therefore, the more the system is heterogeneous the more considerable the damaging process should be. The hypothesis that the systems cured with amines of lower molecular weight are the most homogeneous seems to be confirmed by the sorption tests. The water uptakes for the three systems in the test carried out exclousively at 20 "C was almost the same, i.e., 3.0%, as expected at low temperature. The rate of formation of the microvoids, probably generated in the regions of lower cross-linking density where crazing and relaxation are possible (Andrews, 1975) is sufficiently fast at high temperatures so that while the sorption proceeds to the final equilibrium, the damage also reachs equilibrium. However, due to the different activation played by the temperature on the two modes of degradation (by diffusion and crazing), tests carried out at low temperatures should be only influenced by diffusion if short times are required to reach equilibrium. The use of thin films, in fact, has been suggested by Berens (1975) and Hopfenberg et al. (1977) to resolve the influence of the diffusion and relaxation (crazing) in complex mechanisms of sorptions such as those of the n-alkanes in polystyrene or vinyl chloride monomers in PVC. The samples used in our experiment were sufficiently thin to separate the two effects. Finally, the time needed to craze the material is strongly influenced by the test temperature due to the relaxation nature of the hypothesized damage. Activation energies for relaxation are, in fact, significantly higher than those for diffusion, leading to a different influence of the two components of the sorption process as the temperature is varied. A sorption test, carried out at 20 "C for more than 2 years, is compared with the sorption behavior at 60 "C in Figure 3, where a logarithmic scale has been used for the times: while for the test performed at 60 "C the formation of the damage is evident from the inflection of the sorption curve (the possible Fickian curve is reported for comparison in Figure 2a, the water uptake for the test at 20 "C slowly moves to the same final value determined at 20 "C for the sample previously conditioned at 60 "C. Fatigue test programs can utilize the above characterization to predict the effect of the hygrothermal aging in accelerated tests once the kinetics of the two sorption processes are determined. The water actually dissolved in the bulk material should be lower than the overall uptake determined in the sorp-
Ind. Eng. Chem. prod. Res. Dev.. VoI. 23. No. 2. 1984 291
Table 1. Influence of the Water Conditioning at Different Temperatures on the Mechanical Properties of DGEBA-TETA System at 25 "C water conditioning
temp. "C 2 20 50 70 dry ref
Sann.% 2.58 2.96 3.22 4.00 -rr
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E, kelcm' 139 149 156 179 216
oh. - kg/cm' .
3.40 3.58 3.80 4.05 4.40
tion test since it also contains the contribution of the water trapped in the microcavities. The increase of the apparent solubility, observed when a sample conditioned at a high temperature is equilibrated at a lower temperature, reflects the slight exothermic process of the water sorption reported in a previous work by Apicella and Nicolais (1981) although the equilibrium water uptakes of samples conditioned at increasing temperatures seem to indicate the opposite behavior; see Table I. This intringuing aspect of the sorption behavior is a consequence of the corresponding increased tendency of the material to readily craze a t high temperatures. Mechanical properties and glass transition temperatures, however, should be only influenced by the plasticization due to the water actually dissolved. The results of tensile tests performed at 20 and 50 "C on wet DGEBA-TETA samples of different hygrothermal histories are compared in Table I with the apparent water uptakes determined from water sorption tests. The values of the elastic moduli and stresses a t break for dry and wet samples indicated that the higher degrees of plasticization were reached by the systems conditioned a t the lower temperatures irrespective of the corresponding higher values of the water uptakes. The same conclusions have been previously reported by Apicella et al. (1979) from the comparison of the glass transition temperatures of differently conditioned samples of the DGEBA system. Influence of Macroscopic Heterogeneity on Hygrothermal Stability. Recognition that crazing, which is responsible for the micromechanical instability of the epoxy matrices, requires void formation leads to the suggestion that the phenomenon should be favored in the presence of a dilatational component of the stress. Sorptions on tensile loaded and unloaded samples immersed in liquid water confirmed this hypothesis (Apicella et al., 1981). As a consequence, intemal tensile stresses, such as those arising in a composite from the shrinkage of the glassy matric around the fillers, should further increase locally the tendency of the matrix to craze. A DGEBA-TETA based composite containing 7.5% by volume of 3 W u M glsas beads has been used to evaluate the influence of the filler on the mechanical performance during moist thermal ageing. Tensile tests were performed a t 20 "C on net resin and composite samples previously equilibrated in liquid water a t 70 "C. The liquid water sorptions for the net resin and the composite are compared in Figure 4 for samples first equilibrated a t 70 "C (left side) and then brought back to 20 "C (right side) and for a sample of net resin equilibrated exclusively a t 20 O C . The water uptakes for the composite are referred to the dry weight of polymer only. The two curves at 70 "C show a difference of about 1% ( b > d ) in the equilibrium water uptakes (4.0% for the resin and 5.0% for the composite) which is maintained when the same samples are subsequently conditioned at 20 "C (a > e, 4.6% for the resin and 5.6% for the composite). The water uptake for the net resin conditioned directly a t 20
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Figure 5. SEM of the fracture surfaces far low temperature (a, 4 "C)and high temperature (h. 70 'C) DGEBA-TETA water-aged
bead composites.
'C was 3.0%. The large excess of water uptake found for the thermally aged composite, 90% more than the net resin conditioned directly a t 20 "C, could be only partially attributed to the damage relative to the polymer network, while it should be inferred that additional damage was c a d by the presence of the filler. The electron scanning micrographs of the fracture surfaces of composites conditioned in water at 4 and 70 "C are reported in Figure 5a and b, respectively. The low temperature conditioned sample shows "clean" bead surfaces while the thermally aged composite has fragments of the bulk resin adherent to the fillers. For the sample conditioned a t low temperature the adhesion between the matrix and the filler is weaker than the bulk strength while it becomes "relatively" stronger for the thermally aged samples since the matrix is locally deteriorated by crazing. Light transmission microscope observation showed, in fact, extensive whitening of the aged samples due to the debonding of the glass beads. The residual stresses induced around the inclusions by the shrinkage of the resin during the cure favor a localized crazing when the composite was exposed to more severe conditions of temperature. The plasticization due to the sorbed water and the matrix-filler interface failure are both responsible for the mechanical strength reduction observed for composites exposed to severe conditions of temperature and humidity. The role of the water plasticization and matrix-filler interface failure on the reduction of the mechanical properties of the net resin and the glass-bead composite aged at 70 O C are summarized in Table 11. Whereas the presence of the inclusion increases the elastic modulus a t 20 ' C of the dry sample, first column in Table 11, it
292
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 2, 1984
Table 11. Influence of t h e Filler Volume Fraction, 9,o n the Mechanical Properties of Samples Equilibrated at 70 "C. Test Temperature, 25 "C
E , kg/cmZ a b , kg/cm2 Cb,
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Table 111. Weight Losses for TGDDM-DDS Samples Equilibrated at 20 and 70 "C
7.5 175 3.2 3.0
7.5 127 2.7 2.4
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strongly decreases and embrittles the composite conditioned in liquid water at 70 "C. In fact, while only the plasticization due to the sorbed water lowers the elastic modulus of the net resin (reduction of 17%), the wet composite is subjected to both the influence of plasticization and filler debonding due to the localized crazing (reduction of 34%). The ultimate mechanical properties show the same behavior. The strength losses observed for composites exposed to severe environmental conditions of temperature and humidity should be in part attributed to the lower number of fillers acting as effective reinforcement and in part to the plasticization induced by the sorbed water, depending on the particular history experienced by the sample. Influence of Prepolymer Composition. The molecular characteristics of the polymer networks have been recognized in the previous discussion to strongly influence the aging resistence of a thermoset. In this section the influence of the prepolymer composition, cure schedule, and type of epoxy prepolymer will be discussed by comparing the liquid and vapor water sorptions of a highly cross-linked epoxy matrix, TGDDM-DDS, and of a low cross-linking epoxy matrix, DGEBA-TETA. The liquid water sorption behaviors of the TGDDMDDS and DGEBA-TETA epoxy polymers cross-linked with different concentrations of hardener are reported in Figure 6. The information reported there constitutes first cycle water sorptions for dry "as cast" films exposed to a liquid environment at 20 "C. The noted differences in the sorption behaviors are to be expected in view of the network differences associated with each system. Results for the TGDDM-DDS samples shown in this figure suggest anomalous diffusion and increasing resistance to moisture sorption for samples cured with higher PHR of curing agent. Similar behavior is inferred when sorption in the less cross-linked DGEBA-TETA resin is compared with any of the TGDDM-DDS sample sorptions. Kinetics of the DGEBA-TETA sorption curves follow Fickian behavior, while the TGDDM-DDS system seems strongly anomalous. Extensive literature exsists to support this observation for numerous highly cross-linked epoxy systems (Illinger and Schneider, 1980; Vinson, 1977). This
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Figure 7. DSC thermograms of "as cast" and thermally aged in liquid water TGDDM-DDS systems (desiccated).
behavior becomes more pronounced as the DDS concentration increases from 19 to 31 PHR. While systems having the greater PHR contents form higher cross-linked networks, the mobility of the chain segments in the curing resin is impaired as the gel point is approached. This causes an increased fraction of the molecular constituent species to become trapped in the unreacted state. I t is possible that these trapped species occupy free volume in the polymer network. Consequently, moisture sorption could be impaired. Swelling of the network at 20 "Cshould results in an increased mobility for such trapped species that, without elevating the temperature to permit additional curing, may be free to diffuse out of the polymer. The weight losses for first sorption-desorption at 20 and 70 "C of the TGDDM-DDS systems are given in Table 111. These data agree with the proposed behavior. Subsequent resorption and desorption do not cause any further permanent weight loss for all the samples examined. The secondary but nevertheless important role played by the hygrothermal aging on the activation of the unreacted species has been investigated by means of differential scanning calorimetry. The residual reactivity of the "as cast" cured resins is evident from the thermograms reported in Figure 7 (full lines) where liquid water aged and unaged samples of different DDS content are compared in the desiccated state. The exothermic peak occurring at temperatures above the cure temperature (marked in the figure) indicates the incomplete cure of the material. Moreover, this exotherm was strongly reduced for the system containing the lower DDS content and completely disappeared for the system of higher DDS content upon water aging, leading at the same time to an appreciable increase of the glass transition temperature of the former. The influence of the degree of cure on the moisture sensitivity for a system of the same composition has been tested in liquid water sorptions at 20 and 60 "C for mixtures containing 20 and 50 PHR of hardener (DDS). A set of TGDDM-DDS samples was cured according to the schedule reported in the Experimental Section, while a second set of samples were directly extracted from the oven
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 2, 1984 293
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and quenched after the isothermal cure at 177 "C. The liquid water sorption kinetics are compared in Figure 8 for the specimens containing 20 and 50 PHR of DDS. The influence of the cooling procedures becomes more important at higher test temperature and for samples of lower hardener concentration. The additional cure achieved in the final step of the process by the sample of lower DDS content and hence lower glass transition temperature significantly lowers the final equilibrium water uptakes, especially in more severe conditions of temperature. The additional water uptakes are probably confined in a crazed material of lower cross-linking density. Significant weight losses were also observed upon drying of the water conditioned samples of lower degree of cure. The removal of the unattached or hydrolyzed low molecular weight species from the matrix should reduce the observed concentration dependence of the transport process. Subsequent sorption-desorption data shown in Figure 9a verified this. These data are normalized to the final sample dry weight. Therefore, the initial weight losses in Table I11 are incorporated into this figure. Sorption data taken after the initial moisture cycle exhibit a theoretical Fickian type behavior over the entire range of thicknesses and times investigated. In addition to the noted transient transport effect of the various epoxy networks, temperature a d s a3 a confounding variable for the TGDDM-DDS systems. Figure 9b indicates how this water-conditioned epoxy system respond to changes in temperature. As previously discussed for the DGEBA resins, samples first equilibrated in liquid environment at 70 "C and then allowed to reequilibrate at 20 "C respond to the temperature reduction by absorbing
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Figure 9. Liquid water sorptions of TGDDM-DDS systems: (a) Moisture sorption-desorption-resorption at 20 OC;(b) hygrothermal hystory effect of samples equilibrated in water at 20 and 70 OC. The 70 "C samples were subsequently reequilibrated at 20 O C .
additional moisture. The apparent anomalous behavior has been previously described in terms of internal network structure changes caused by the synergistic effect of moisture and temperature. The subsequent thermal cycling, however, was indicated as completely reversible for the DGEBA resins while it appears to further change the internal network structure of the highly cross-linked TGDDM-DDS resins; see Figure 10. In this test the temperature was cycled between 20 "C (full circles) and 70 "C (open circles). In going from 20 to 70 "C, the equilibrium solubility content of moisture in the epoxy is expected to decrease, forcing the water which is dissolved in the polymer to enter the free volume associated with the network structure. The result is a rapid release of moisture from the network if enough free volume is available as in the case of the low cross-linked DGEBA based thermosets. However, this is not the case of the highly cross-linked TGDDM-DDS resins which do not have enough free volume at 70 "C to incorporate all the moisture absorbed in the previous step. If the kinetics for moisture release from the dissolved state are faster than the diffusion process for release from the network, then internal stressing of the system may result in creation of additional free volume, microvoids, or other network changes. The progressive increases of the water uptakes observed under cycling conditions of temperature in Figure 10 may be related to low diffusivity of the water molecules in the more stiff and dense TGDDM-DDS resins as compared to the DGEBA-TETA system, where as reversible behavior was, in fact, observed. Results of dynamic mechanical experiments performed on these epoxy samples exposed to moist environments are presented in Figure 11. These tan 6 vs. temperature curves exhibit the primary and secondary transitions observed in many polymer systems. The primary CY transition is readly associated with large-scale molecular movement in the polymer network at the glass transition. The secondary low-temperature p transition in glassy polymers is often a result of segmental chain mobility triggered by
284
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23,No. 2, 1984
3.51
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Figure 10. Influence of the thermal cycling between 20 and 70 " C for TGDDM-DDS systems previously equilibrated at 70 O C . 10O
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TEMPERATURE, ('(2) TEMPERATURE,("c) Figure 11. Dynamic mechanical tan (6) for (a) DGEBA-TETA and (b) TGDDM-DDS (25 PHR), samples exposed to moist environments.
the system's thermal characteristics. This low-temperature peak is quite broad, indicating that a wide spectrum of motion types and/or activation energies are contributing to the transition. While the a transition requires large-scale movements, the secondary transition is often a combination of molecular rotation of some main chain side groups, motion of some segment of the main chain side group, or motion of a small molecule dissolved in the polymer. Thus, changes in polymer network structure are manifest through transitions exhibited in dynamic mechanical spectra. Figure 11 compares the dynamic mechanical behavior of soaked and desiccated samples of the two epoxy systems. The slight increase in magnitude of the low-temperature transition peak can be rationalized due to the plasticization of the epoxy network. However, the significant differences which arise above 30 "C are not entirely attributable to plasticization. The shift is in fact a tertiary w dynamical mechanical transition indicative of structural or molecular rearrangements within the network as a result of moisture sorption (Mikols et al., 1982). It is interesting to note that previous sorption works by Apicella et al., 1979) with
DGEBA-TETA indicates that a moisture altered sample raised beyond its TBapparently recovers the previous dry state characteristics. The TGDDM-DDS shows almost the same behavior of the DGEBA-TETA system with a tertiary w transition occurring near 50 "C which also demonstrates strong moisture dependence (Mikols et al., 1982). The previous sorption analysis of these systems has indicated that some network modifications, which influence the apparent water solubility, occur in ordinary test times only a temperatures higher than 20-40 "C (Apicella et al., 1979; Apicella and Nicolais, 1981; Mikols et al., 1982). In particular, the moisture-induced structural changes causing differences in the sorption behavior for these amine-linked epoxy systems may be correlated with the observed dynamic mechanical tertiary w transition. The network structure molecular change responsible for the excess moisture accumulation is also linked to the molecular conformation responsible for the dynamic mechanical transition. The Nature of the Epoxy Water Interactions. The fact that property differences between soaked and desic-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 2, 1984
295
Table IV. Water Uptakes, Glass Transition Temperatures for Dry Tgd, and Wet Resins, Tgw, Densities and Expansion Coefficients at the Glass Transition for TGDDM-DDS Samples of Different Composition. In the Last Two Columns the Experimental and Theoretically Calculated Glass Transition Temperature Depressions (Eq 1)Are Compared 20 30 50
5.0 4.5 5.0
150 175 200
110 152 140
cated samples exist reinforce the concept that a physical modification has been introduced into the network by the moisture-temperature aging. While the exact nature of this change has not yet completely identified, several possibilities exist. The previous discussion formulates an explanation related to microvoiding and crazing in the network structure. Another possibility may postulate a dual state of the water in the epoxy which would retain water even at elevated temperatures due to some bond formation. Although preliminary NMR evidence reported by Moy and Karazs (1980) supports both these explanations, the nature of the association process between the water and the polymer is not yet clearly understood. The intrinsic moisture sensitivity of the epoxy resins is traceable directly to the molecular structure. The presence of polar and hydrogen bonding groups, such as hydroxyls, amines, sulfones and tertiary nitrogen, provides the chemical basis for moisture sensitivity, while the available free volume and nodular network structure represent the physical aspects. The evidence given by the broad line NMR analysis indicates that the plasticization effect of the water on a cross-linked epoxy may also be related to the strong interactions between the dissolved molecules and segments or groups of the polymer, although the exact sorption sites to which the water may be bonded are still uncertain. The dependence of the water sorption as a function of the prepolymer composition of amino hardened epoxy resins will be discussed further on to resolve the sorption process into effect arising from morphological (microcavitation and solution) as well as chemical origins (hydrogen bonding). In the cure reactions of an epoxy system, primary and secondary amines give a cross-linked structure through the addition with the epoxide rings. The hydroxyls formed in the previous reactions are further able to add to the epoxides through homopolymerization. Both types of reactions increase the number of hydrophilic sites, namely hydroxyls and unreacted secondary amines, which are able to bond the sorbed water molecules. The saturation of the hydrogen bond sites has been found to occur in the low range of water activities (Apicella et al., 1983; Moy and Karasz, 1980). Water vapor sorptions in TGDDM-DDS systems of different composition are characterized by the isotherms reported in Figure 12. The epoxy cured with 50 PHR of DDS shows a tendency to sorb by hydrogen bonding higher than that of the system cured with 20 PHR of DDS. The sorption isotherm of the former, in fact, clearly indicates that the saturation of the hydrophilic sites (negative curvature) occurred at relatively higher values of the water uptakes. The mentioned increase in the tendency to sorb by hydrogen bond formation in DDS-rich systems may be principally related to the presence of unreacted secondary amines rather than to the presence of other potential bond sites. The overall concentration of the potential hydrogen bonding sites, namely sulfones, amines, and hydroxyls, is in fact almost unaffected by the prepolymer composition (Apicella et al., 1982). The formation of polymer-water intermolecular bonds and the rupture of polymer-polymer intramolecular hydrogen bonds, which stiffen the polymer network, strongly
1.262 1.213 1.284
0.63
-40 -23 -60
1.08 2.94
-50 -32 -21
e
Figure 12. Water vapor sorption isotherms at 60 OC for TGDDMDDS systems of different composition.
depresses the glass transition temperature. The glass transition temperatures of wet and desiccated samples, determined from the DSC thermograms, and the corresponding equilibrium water uptakes at 60 OC, are compared in Table IV for three compositions of TGDDM-DDS mixtures. The glass transition temperature depression observed for the system containing 50 PHR of DDS is 20 OC greater than that of the resin cured with 20 PHR of hardener even if the same amount of water was absorbed at equilibrium, i.e., 5.0%. It is interesting to note that the system of intermediate composition, 30 PHR, showed the lowest plasticization, although it sorbed almost the same amount of water, i.e., 4.5%. The anomalous effect may be considered from the influence that the molecular solution and hydrogen bond formation may have in systems of different chemical and physical characteristics. As indicated before, plasticization may depend on the dilution process as well as on the rupture of the polymer-polymer hydrogen bonds. However, while the former is governed by the diluent volume fraction, the second depends on the effective concentration of the hydrophilic sites. The occurrence of two mechanisms of plasticization that can be inversely influenced by the increase of the DDS concentration, namely decrease of the available free volume and increase of hydrophyllic sites, determined the lower depression of the glass transition observed for the system of intermediate composition. The glass transition temperature of a dilute system, according to the free volume changes, is determined by the diluent volume fraction v d , and changes of the thermal expansion coefficient, a, at Tgby Tg,wet = To,pA + (1) where A = vp(.p/(vpap
+ V,(Y,);
B =
Vdad/(Vpap
+ vdad)
The subscripts p and d refer to the polymer and the diluent, respectively. The glass transition temperature of the wet sample is then influenced by both diluent volume fraction and change of its volume expansion coefficient at Tr The higher the change of the expansion coefficient, the lower is the influence of the diluent volume fraction. The changes of the thermal expansion coefficients, densities, and glass transition temperatures of the three sys-
296
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 2, 1984
Table V . Actual Water Uptakes, Glass Transition Temperatures for Dry, T g d , and Wet, T g w ,Resins, Densities, and Expansion Coefficients at the Glass Transition for DGEBA-TETA Samples of Different Composition. In the Last Two Columns the Experimental and Theoretically Calculated (Eq 1 ) Glass Transition Temperature Depressions Are Compared
'c
TETA,PHR
SaCt, Y'
7
100
109
14
2.13 8.66
142
25
Tgd,
95
Tgw, "C IO5 109 ?, 9
tems investigated are also reported in Table IV. The more dense and stiffer resin cross-linked with 50 PHR of DDS should be, in principle, less sensitive to the water plasticization, if the single mechanism of molecular solution is effective. The changes in the glass transition experimentally observed are opposed to the free volume changes predictions; the values of the glass transition depression calculated from eq 1 and reported in the last column of Table IV are progressively impaired as the DDS concentration is increased. The observation of the unexpected trend confirms the presence of the secondary mechanism of plasticization, namely, the intramolecular hydrogen bond rupture. The liquid water sorption and calorimetric behavior have been also investigated for the DGEBA cured with different concentration of TETA. The densities, glass transition temperatures of the wet and dry samples, and the actual equilibrium water uptakes at 70 and 20 "C for as cast resins and the glass expansion coefficients at the glass transition are summarized in Table V. The resin cross-linked with an excess of hardener (25 PHR) sorbs considerably more than the other two systems. The increase of the hardener concentration increases both the equilibrium water uptakes and plasticization. The strong influence of the prepolymer composition on the water solubility and plasticization may also be related for this system to the increased number of unreacted amines which are able to bond the water molecules. In these low cross-linked networks, however, the influence of the second mechanism of plasticization is much less relevant. The glass transition temperature depressions, calculated by considering only molecular solution (eq l),favorably compares the experimental determinations.
Conclusions This study reveals the need for separate investigative tools for quantitatively characterizing the influence of manufacturing defects and chemical characteristics on the hygrothermal fatigue response of epoxy based composites. The degradation effects of the matrix and interfaces in a moist environment strongly dominate the composite response properties under temperature, humidity, and stress fatigue tests. The intrinsic moisture sensitivity of epoxy matrices arises directly from the resin chemical structure, such as the presence of hydrophilic polar and hydrogen grouping, as well as from microscopic and macroscopic defects of the network structure, such as the heterogeneous cross-linking densities, bubbles, or debondened fillers. The experimental results presented relate calorimetric, mechanical, and dynamic mechanical properties to the hygrothermal history of two classes of widely utilized epoxy systems: DGEBA cross-linked with linear amines and the TGDDM cross-linked with an aromatic amine, DDS. The major factors influencing the matrix degradation in both systems are the network heterogeneity and the presence of hydrophilic and unreacted species. Liquid water sorptions under thermal cycling have been demonstrated to be particularly sensitive to the structural changes occurring during the hypothermal aging. The two classes of epoxy matrices, characterized by low and high cross-
p,
gicm'
1.174 1.219 1.107
01
x
lo3,"c-* A T,, 0.3 0.4 0.3
, "C
-9 -27 -43
A Tth, "C
-4 -33 -36
linking densities, have been hypothesized to craze on a microscopic level when exposed to relatively high temperatures (60-70 "C) in moist environments. This damage was probably confined in the internodular low cross-linked matrix generated by the reagent diffusion constraints during the cure, since they were less relevant when low molecular weight hardeners were utilized. Swelling stresses a t high water saturations between the regions of different cross-linking densities were probably the driving force for crazing. On a macroscopic level, the presence of fillers further favors this type of degradation process, especially at the interfaces where the stresses induced by the shrinkage of the matrix during the cure are concentrated. Dynamic mechanical spectra confirmed the diluent induced bulk property changes. The two amine-linked epoxies used in this study reveal difference in apparent moisture solubility that are readily traced by a tertiary w transition in the dynamic mechanical spectra. Changes of prepolymer chemistry and cure behavior, however, should be carefully considered since while modifying the chemorheology of the system, and hence the network morphology, they are also introducing modifications of the chemical structure. The second major source of moisture sensitivity has been, in fact, recognized to arise from the mode in which water is absorbed. Plasticization of the matrix, in particular, is extremely sensitive to the presence of hydrophilic sites. The cure reactions of the epoxy-amine systems held to networks containing a large number of hydroxyls, unreacted amines, and other polar groups. The mechanisms of plasticization are driven both by the free volume available to the penetrant and the rupture of the polymer-polymer intramolecular hydrogen bonds. The more stiff and dense TGDDM-DDS systems are much more sensitive than DGEBA-TETA to the changes in the prepolymer composition, especially at high DDS contents. The significant reductions of the glass transition temperature observed for these systems were not attribuable to free volume changes only. Polymer hydrogen bond saturation in vapor sorption tests has been, in fact, observed to occur at higher values of the vater uptakes in samples of higher DDS contents. Finally, incomplete cure, which is induced by diffusion constraints and vitrification of the highly cross-linked TGDDM-DDS systems, has been also described to alter the polymer morphology and moisture sensitivity during hygrothermal aging. Acknowledgment The authors express their appreciation to the Aeritalia Aircraft Company (Grant No. 2/P-B2R0350) and the CNR Proggetto Finalizzato Chimica Fine e Secondaria (Grant No. 82.00661) for the partial financial support. Registry No. Poly(TGDDM) (homopolymer), 31305-94-9; (bisphenol A)((chloromethyl)oxirane) (copolymer), 25068-38-6; water, 7732-18-5.
Literature Cited Andrews, E. H.; Levy, J. H.; Willis, J. J. Mater. Sci. 1975, 8 , 1000. Apicella, A.: Carfagna, C. J. Appl. folym. Sci. 1983. Aplcella, A,; Nicolals. L. Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 138.
Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 297-300 Apicella, A.; Nicolais, L.; Astarlta, G.; Drloil, E. Polymer 1979, 20, 9. Aplcella, A.; Nlcolais, L.; Carfagna, C.; de Notaristefani, C.; Voto, C, Proceedings of the 27th National SAMPE Meeting, San Diego, CA, 1982; "The Effect of the Prepolymer Composition on The Environment Aging of Epoxy Based Resins". Apicella, A.; Nlcolais, L.; Halpin, J. C., Proceedings of the 28th National S A M E Meeting, Anaheim, CA, 1983; "The Role of the Processing Chemorheology on the Environmental Ageing Behavlour of High Performance Epoxy Matrices". Aplcella, A.; Tessierl, R.; de Cataldls, C. J. Memb. Sci. 1983, in press. Barrer, R. M.; Barrle, J. A.; Slater, J. J . Polym. Sci. 1958, 2 7 , 177. Berens, A. R. Angew. Makromol. Chem. 1975, 47, 97. Browning, C. E. Polym. Eng. Sci. 1978, 18, 16. Chi-Hung; Springer, G. S. J . Compos. Mater. 1978, 10, 2. Dusek, K.; Plestie, J.; Lendnicky, F.; Lunak, J. Polymer 1978, 19, 393. Enscore, D. J.; Hopfenberg, H. E.; Stannett, V. T.; Berens, A. R. Po/ymer 1877, 18, 1005. Gillham, J. K. Polym. Eng. Sci. 1979, 19, 676. Illinger, J. R.; Schnider, N. S. Polym. Eng. Sci. 1980, 20, 310. Lewis, A. F.; Doyle, M. J.; Gillham, J. K. Polym. Eng. Scl. 1979, 19, 687. Loos, A. C.; Springer, G. S. J . Compos. Mater. 1979, 13, 17. Kwei. T. K. ; Zupko, H. M. J. Polym. Sci. 1989, A2(7), 876.
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McKague, E. L.; Halkias, J. E.; Reynolds, J. D. J. Compos. Mater. 1975, 9 , 2. Michaels, A. S.; Vieth, W. R.; Barrie, J. A. J. Appl. h y s . 1983, 2 4 , 1. Mlkols, W. J.; Seferls, J. C.; Aplcella, A.; Nlcolais, L., Polym. Composites 1982, No. 3 , 118. Mijovlc, J.; Koutsky, J. A. Po/ymer 1979, 20, 1095. MIJovIc, J.; Tsay, L. Po/jmer 1981, 22, 902. Moy, P.; Karasr, F. E. Polym. Eng. Scl. 1980, 2 0 , 315. Morgan, R. J.; Mones, E. T.; Steele, W. J. Polymer 1982, 23, 295. Morgan, R. J.; O'Neal, J. J. Mater. Sci. 1977, 12, 1966. Morgan, R. J.; O'Neal, J.; Miller, D. E. J . Meter. Sci. 1979, 14, 109. Schneider, N. S.; Sprouse, J. F.; Hagnouer, G. L.; Gillham, J. H. Po/ym. Eng. Scl. 1979, 19, 304. Vleth, W. R.; Howell, J. M.; Hoselh, J. H. J . Membr. Sci. 1976, 1 , 177. Vinson, J. D., ASTM-858 1977, 43C. Zimm, E. H.; Lundemberg, J. J . Chem. Whas. 1958, 6 0 , 425. Weitsman, Y. J. Compos. Mater. 1978, 10, 193.
Received for review June 2, 1983 Revised manuscript received October 31, 1983 Accepted November 23, 1983
Substrate Effects on the Oxidative Cross-Linking of a Polybutadiene Coating Ray A. Dlckle,' Roscoe 0. Carter, 111, John S. Hammond,+John L. Parsons, and Joseph W. Holubka Research Staff, Ford Motor Company, Dearborn, Michigan 48 12 1
Fourier transform infrared spectroscopy has been employed to compare the oxidative cross-linking of a high vinyl content polybutadiene on various substrates. Spectra were obtained by transmission on salt plates and by grazing angle absorption-reflection from gold, chromium, and cold rolled steel substrates. I n the initial unoxldized state, essentially identical spectra were obtained independent of substrate. After Oxidative cross-linking (accomplished by baking the specimens in air at 180 O C for 30 min) substantial differences in the type and relative amounts of oxidized species were observed. Relative to the essentially bulk specimen observed in transmission, the thin film specimens cured on steel and chromium were more oxidized, while the specimen cured on gold was less oxidized.
Introduction The synthesis and properties of liquid polybutadiene resins and their use as coatings have been discussed (Auckett and Luxton, 1977). Typically these materials cross-link by oxidative processes upon heating in air to form tough, solvent-resistant films. Polybutadiene coatings have been extensively used in model studies of the corrosion of painted steel (e.g., Touhsaent and Leidheiser, 1972; Kendig and Leidheiser, 1976; Leidheiser and Kendig, 1976, 1978; Leidheiser and Wang, 1981; Castle and Watts, 1981; Dickie et al., 1981). XPS studies of the composition of the interfacial surfaces generated as a result of corrosion induced adhesion loss from steel substrates suggest that the coating near the substrate may be more oxidized than is the bulk of the coating film (Dickie et al., 1981). These results, and the results of a subsequent study of the interfacial chemistry of humidity-induced adhesion loss (Holubka et al., 1984) suggest that the nature of the substrate may influence the composition and properties of the organic coating in the interfacial region, and in particular that the normal oxide surface of the cold rolled steel may promote the oxidation of polybutadiene coatings in the interfacial region. To explore this possibility, the autoxidation of thin films (ca. 0.1 pm) of polybutadiene on polished steel has been investigated by infrared spectroscopy by use of a grazing angle absorption-reflection Physical Electronics, 6509 Flying Cloud Drive, Eden Prairie,
M N 55344. 0196-4321/84/1223-0297$01.50/0
method. These results have been compared with those of similar studies using gold and chromium substrates and of transmission infrared studies of thicker films (ca. 5 pm) on NaC1. Experimental Section Materials. The polybutadiene coatings used in this study were prepared from Lithene RH, a high vinyl content (49% 1,2-, 22% trans-1,4-, and 29% cis-1,4- addition) low molecular weight (M,= 2500) polybutadiene supplied by Lithium Corporation of America, Bessemer City, NC. For preparation of thin polymer films, a 1% solution of Lithene RH in toluene was used. Immersion of the substrate in this solution, followed by evaporation of solvent at room temperature and (for the cured films) baking in an air circulating oven at 180 "C for 30 min resulted in formation of cross-linked films ca. 0.1 pm thick. For conventional transmission spectra, films ca. 5 pm thick were prepared by drawing down and curing neat polymer on NaCl plates. Gold mirrors were prepared by vacuum deposition of gold on glass microscope slides (metal thickness, ca. 70 nm). Steel mirrors were prepared by metallographic polishing of SAE 1010 cold rolled steel paint test panels purchased from OMI-Parker; 0.3-pm alumina was used for the final polishing stage. The chromium substrates were electrodeposited chromium on an aluminum substrate. Infrared Spectroscopy. Spectra were obtained with a Nicolet 7001 interferometer equipped with a broad band mercury-cadmium telluride detector interfaced to a Digital 0 1984 American Chemical Society
