Isothermal transitions of a thermosetting system

Abstract

A study of the curing reactions of a cycloaliphatic epoxy resin/anhydride system by torsional braid analysis showed the existence of two critical isothermal temperatures. These are T_g∞ (the maximum glass transition temperature of the thermoset system) and T_gg (the glass transition temperature of the material at its gel point). Two rheologically active kinetic transitions occur during isothermal cure which correspond to gelation and vitrification. Three types of isothermal behavior occur: if T_cure > T_g∞, only gelation is observed; if T_g∞ > T_cure > T_gg, both gelation and vitrification are observed; if T_cure < T_gg, only vitrification is observed. T_gg corresponds to the isothermal cure temperature at which gelation and vitrification occur simultaneously. Methods for determining the time to gel and the time to vitrify, and also T_g∞ and T_gg, have been developed. The time to gel obeyed the Arrhenius relationship, whereas the time to vitrify passed through a minimum. Application of these results to thermosetting systems in general is discussed in terms of the influence of molecular structure on the values of T_g∞ and T_gg.