Differential scanning calorimetry of the effects of temperature and humidity on phenol-formaldehyde resin cure

Abstract

Phenol-formaldehyde (PF) resin is a widely used adhesive in the manufacture of wood composites. However, curing behaviour of the resin under various environmental conditions is not well known. A differential scanning calorimeter was employed to characterize the degree of resin cure in this study. Resin-impregnated glass cloth samples with varied moisture contents (0, 31, 71%) were exposed to an environment of controlled temperature (105, 115, 125, 140°C) and relative humidity (41, 75, 90%) for a series of time periods (0–20 min). The samples were next conditioned to different levels of relative humidity (0, 64, 89%) before being sealed in d.s.c. capsules. The degree of cure of these partially cured samples was then determined by d.s.c. by measuring the residual heat of cure. The results show that the degree of resin cure increased with increasing precure temperature and time. The rate of cure increased for dry samples as precure humidity increased. However, the rate of cure decreased with an increase in the initial moisture content of samples when precured at 90% relative humidity. The effect of moisture put into a sample immediately before analysis by d.s.c. on the subsequent calculation of resin precure became less significant with increasing precure temperature and degree of resin cure.