Rheokinetic investigations on the thermal cure of phenol-formaldehyde novolac resins

The curing behavior of commercial phenol-formaldehyde novolac resins with hexamethylenetetramine (HMTA) under non-isothermal and isothermal conditions was followed by dynamic rheological measurements. Changes in parameters of the viscoelastic material such as storage modulus (G′) and complex viscosity (η*) during the cure process were evaluated accurately. The isothermal kinetics reaction was described using a phenomenological equation based on the Malkin and Kulichikhin model. The systematic increase in (E_a) values showed that the increase in bulk viscosity during the reaction alters the resin curing process by reducing diffusion of molecules and hindering network growth. Higher HMTA contents lead to faster growing rates in the crosslinking network, hence there are more crosslinked bonds after a shorter period of reaction. This fact implies that active species need more energy to form new crosslinking bonds, changing the curing reaction mechanism.