Cure-induced Normal Force Development in Unfilled and Filled Epoxy Resins

Abstract

The present study is devoted to assessing the development of normal force (Fn) in a bisphenol A-type epoxy resin (EP) with (about 50wt%) and without filler when cured by anhydride under isothermal conditions, however, at various temperatures (T=75 up to 150°C). The evoking Fn was measured in a plate—plate rheometer capable of following the contraction of the cross-linking EP. Supplementary information on the conversion, glass transition temperature, and gel time was received from differential scanning calorimetry (DSC) and oscillatory rheological measurements. Up to gelation no Fn was detected. Between gelation and vitrification the normal force rate (dFn/dt) proved to be constant at a given temperature and followed an exponential function as a function of temperature. F n plotted as a function of conversion confirmed that its onset is linked with the gelation. The buildup of Fn after gelation remained independent of the curing temperature in a given conversion range. At higher conversion, however, the Fn values measured in low temperature isothermal tests became higher than at high temperature tests due to the earlier onset of vitrification in the former case. The results were basically similar for both the neat and filled EPs except that the gelation and vitrification transitions were less sharp for the filled compared to the unfilled EP version.