The physical aging of an epoxy resin based on diglycidyl ether of bisphenol‐A cured by a hardener derived from phthalic anhydride has been studied by differential scanning calorimetry. The isothermal curing of the epoxy resin was carried out in one step at 130°C for 8 h, obtaining a fully cured resin whose glass transition was at 98.9°C. Samples were aged at temperatures between 50 and 100°C for periods of time from 15 min to a maximum of 1680 h. The extent of physical aging has been measured by the area of the endothermic peak which appears below and within the glass transition region. The enthalpy relaxation was found to increase gradually with aging time to a limiting value where structural equilibrium is reached. However, this structural equilibrium was reached experimentally only at an aging temperature of Tg‐10°C. The kinetics of enthalpy relaxation was analysed in terms of the effective relaxation time τeff. The rate of relaxation of the system given by 1/τeff decreases as the system approaches equilibrium, as the enthalpy relaxation tends to its limiting value. Single phenomenological approaches were applied to enthalpy relaxation data. Assuming a separate dependence of temperature and structure on τ, three characteristic parameters of the enthalpic relaxation process were obtained (In A = −333, EH = 1020 kJ/mol, C = 2.1 g/J). Comparisons with experimental data show some discrepancies at aging temperatures of 50 and 60°C, where sub‐Tg peaks appears. These discrepancies probably arise from the fact that the model assumes a single relaxation time. A better fit to aging data was obtained when a Williams‐Watts function was applied. The values of the nonexponential parameter β were slightly dependent on temperature, and the characteristic time was found to decrease with temperature. © 1994 John Wiley & Sons, Inc.

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ABSTRACT:The cure reaction kinetics of epoxy resin, with organically modified montmorillonite loadings of up to 20 wt % and with stoichiometric conditions, has been studied by differential scanning calorimetry with a view to understanding further the fabrication of epoxy-based polymer layered silicate nanocomposites. The kinetic analysis of isothermal and nonisothermal cure shows that the autocatalytic model is the more appropriate to describe the kinetics of these reactions, and it is observed that a dominant effect of the montmorillonite is to catalyze the curing reaction. However, it was not possible to model the reactions over the whole range of degrees of conversion, in particular for nonisothermal cure. This attributed to the complexity of the reactions, and especially to the occurrence of etherification by cationic homopolymerization catalyzed by the onium ion of the organically modified montmorillonite. The homopolymerization reaction results in an excess of diamine in the system, and hence in practice the reaction is off stoichiometric, which leads to a reduction in both the heat of cure and the glass transition temperature as the montmorillonite content increases. Small angle X-ray scattering of the cured nanocomposites shows that an exfoliated nanostructure is obtained in nonisothermal cure at slow heating rates, whereas for nonisothermal cure at faster heating rates, as well as for isothermal cure at 708C and 1008C, a certain amount of exfoliation is accompanied by the growth of dspacings of 1.4 nm and 1.8 nm for dynamic and isothermal cure, respectively, smaller than the d-spacings of the modified clay before intercalation of the resin. A similar nanostructure, consisting of extensive exfoliation accompanied by a strong scattering at distances less than the d-spacing of the modified clay, is also found for resin/clay mixtures, before the addition of any crosslinking agent, which have been preconditioned by storage for long times at room temperature. The development of these nanostructures is attributed to the presence of clay agglomerations in the original resin/clay mixtures and highlights the importance of the quality of the dispersion of the clay in the resin in respect of achieving a homogeneous exfoliated nanostructure in the cured nanocomposite.

SYNOPSISIsothermal curing of an epoxy resin based on diglycidyl ether of bisphenol A, using a hardener derived from phthalic anhydride, has been performed a t curing temperatures between 30 and 130°C. Samples were cured isothermally at various intervals of time and analyzed by differential scanning calorimetry (DSC ) , the glass transition temperature Tg, and the conversion degree being determined by the residual enthalpy technique. The vitrification phenomenon and a further structural relaxation process, occurring at curing temperatures ( T,) lower than the maximum Tg ( 109"C), at which Tg equalizes T,, have been studied at curing temperatures between 30 and 100°C. The structural relaxation process is analyzed by the endothermic peak that appears superposed on Tg in dynamic DSC scans.The area of this peak ( Q ) is a measure of the recovery enthalpy, and thus of the extent of the relaxation process. This process begins at higher curing times ( t , ) when T, decreases because the vitrification of the system starts later. Both the enthalpy recovery ( Q ) and the temperature of the endothermic peak ( T,) increase with the annealing time ( t o ) , calculated as the difference between t, and the time in which vitrification occurs, and tend to have a limiting value due to the fact that the system loses mobility when the free volume decreases during its asymptotic approach toward the metastable equilibrium state. Furthermore, the dependence of Q and T,,, on t. at different T, shows that the relaxation process in partially cured resins depends on the conversion degree of the system and consequently on the crosslinking density of the network.

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SYNOPSISThe cure kinetics of a diglycidyl ether of bisphenol A (DGEBA)-based epoxy resin with methyltetrahydrophthalic anhydride and an accelerator was studied by nonisothermal DSC data. The systems were uncured resin and partially cured with the following extents of cure measured by the residual heat method (aDsc): 0.37,0.63,0.81, and 0.90. The activation energy calculated by the Kissinger method increases from 63 kJ/mol for the uncured epoxy to 77 kJ/mol for the partially cured with aDsC = 0.90. Additionally, the activation energy calculated by the isoconversional method shows a dependence on the conversion degree a. The activation energy tends to decreases initially with the conversion degree, possibly due to the autocatalytic effect; then, it passes through a minimum about a = 0.4 and, finally, increases slightly due to the increase of crosslinks which reduce the mobility of the unreacted groups. A simple, consistent method of kinetic analysis was applied. This method enables one to select the most convenient model and the calculation of kinetic parameters. A twoparameter (m, n) autocatalytic model (Sesthk-Berggren equation) was found to be the most convenient model to study the curing of epoxy systems. The results show a dependence of the kinetic parameters on the initial degree of crosslinking of the partially cured epoxy. The exponent m tends to decrease with the extent of cure, while the exponent n remains practically invariable. These results show a change of the kinetic when the initial extent of cure of the epoxy system increases. The In A data, A being the preexponential factor in the Arrhenius dependence of the temperature on the rate of conversion, increase with the extent of cure, showing a correlation with the calculated activation energy values. The nonisothermal DSC curves theoretically calculated show a very good agreement with the experimental data. The two-parameter (m, n) autocatalytic model gives a good description of the curing kinetics of epoxy resins with different extents of cure. 0 1995 John Wiley & Sons, Inc.

Various methods of preparation of epoxy resin/clay mixtures, before the addition of the crosslinking agent and curing to form epoxy-based polymer layered silicate (PLS) nanocomposites, have been investigated to determine their effect on the nanostructure. Organically modified montmorillonite clay was used, and the mixtures were prepared by both simple mixing and solvent-based methods. X-ray diffraction shows that intercalation of the resin into the clay galleries occurs for all clay loadings up to 25 wt % and for both preparation methods, but the dispersion of the clay in the resin, observed by optical microscopy, is significantly better for the solvent preparation method. Differential scanning calorimetry (DSC) shows that the intercalated resin has the same molecular mobility as the extra-gallery resin, but suggests that the intercalated resin does not penetrate completely into the galleries. Prolonged storage of the resin/clay mixtures at room temperature leads to changes in the DSC response, as well as in the response to thermogravimetry, which are interpreted as resulting from homopolymerization of the epoxy resin, catalyzed by the onium ion in the modified clay. This confirms and explains the earlier observation of Benson Tolle and Anderson (J Appl Polym Sci 2004, 91, 89) that ''conditioning'' of the resin/clay mixtures at ambient temperature has a significant effect when the crosslinking agent is subsequently added, and indicates that the preparation method has important consequences for the nanostructure development in the PLS nanocomposites.