Application of dielectric analysis for monitoring the cure process of phenol formaldehyde adhesive

Dielectric analysis (DEA) was conducted for continuous in-process monitoring of the curing of phenol formaldehyde (PF) adhesive. The influence of wood moisture content (MC) on the dielectric signal obtained in the PF adhesive bond was examined. The cure kinetics of the PF adhesive was evaluated at five different press temperatures, ranging between 120 and 200 °C. The results showed that dielectric changes monitored by a LCR meter (impedance analyzer) and fringe-field sensor were related to the PF adhesives cure—which involves chemical reaction and water movement/diffusion from the adhesive bond line—but not related to the changes of the moisture in the wood. The rate of cure was strongly affected by the press temperature. The higher press temperatures induced a faster temperature rise in the PF adhesive bond, and thus accelerated the curing process. An nth-order kinetic equation was fit to the DEA degree of cure data to demonstrate the utility of DEA to characterize PF adhesive cure.