Effects of Temperature on Cure Kinetics and Mechanical Properties of Vinyl-Ester Resins

The relationships among cure temperature, chemical kinetics, microstructure, and mechanical performance have been investigated for vinyl-ester resins. Fourier transform infrared spectroscopy was used to follow the reactions of vinyl-ester and styrene during isothermal curing of Dow Derakane 411-C-50 at 30 and 90°C. Reactivity ratios of vinyl-ester and styrene vinyl groups were evaluated using the copolymer composition equation. The results indicate that the ratio of vinyl-ester to styrene double bonds incorporated into the network is greater for 30 than for 90°C cure. Mechanical properties were obtained for systems subjected to isothermal cures at 30 and 90°C and postcured above ultimate T_g. The results show that the initial cure temperature significantly affects the mechanical behavior of vinyl-ester resin systems. In particular, values of strength and fracture toughness for postcured samples initially cured isothermally at 30°C are significantly higher than those obtained for samples cured isothermally at 90°C. Examination of fracture surfaces using atomic force microscopy revealed the existence of a nodular microstructure possessing characteristic nodule dimensions that are affected by the temperature of cure. Such features suggest the existence of phase separation during cure. A binary interaction model in conjunction with chemical kinetic data and estimated solubility parameters was used to evaluate enthalpic interactions between the growing polymer network and monomers of the vinyl-ester system. The results indicate that the interaction energy becomes increasingly endothermic as cure progresses and that this energy is affected by the temperature of cure through differences in copolymerization behavior. Hence, in addition to entropie factors, the changes in enthalpic contribution to the Gibbs free energy suggest that the probability of phase separation increases with extent of cure and that its onset is potentially affected by cure temperature.