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

摘要

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 90deg C. Reactivity ratios of vinyl-ester and styrene vinyl groups were evaluated suign the copolymer composition equation. The results indicate that the ratio of vinyl-ester to styrene vinyl groups wer evaluated using the copolymer composition equation. The results indicate that the ratio of vinyl-ester to styrene double bonds incorproated into the network is greater for 30 than for 90deg C cure. Mechanical properties were obtained for systems subjected to isothermal cures at 30 and 90deg C and postcured above ultimate T_g. The results show that the inital 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 30deg C are significantly higher than those obtianed for samples cured isothermally at 90deg C. Examination of fracture, surfaces using atomic ofrce microscopy revealed the existence fo 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 interction 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 entropic factors, the changes in enthalpic ocntribution 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.