Isoconversional kinetic analysis of novolac-type lignophenolic resins cure

Several phenomenological models (including simple models by Ozawa and Kissinger and the Kissinger–Akahira–Sunose isoconversional method) have been used to compare the cure kinetics of two lignin-based novolac-type phenolic resins with those from a commercial novolac system. When 45 wt% phenol is substituted by a sulfonated kraft lignin, an important reduction in the activation energy is obtained. This behaviour has been attributed to the incorporation of an extra amount of hydroxymethyl groups in the formulation, as they are present in important quantities in the original lignin structure. KAS isoconversional model shows that the rise in viscosity derived from lignin introduction leads to a moderate change in the limiting stage from a kinetic to a diffusion regime, while condensation reactions, which are favoured by the abundance of lignin hydroxymethyl groups, acquire high relevance in LPF-45 system cure. Finally, competition with other mechanisms initiated at high temperature is reported at high conversion grades for all cases.