Kinetic analysis of phenol-formaldehyde bonded wood joints with dynamical mechanical analysis

Cure development of phenol-formaldehyde (PF) resins has been extensively modeled based on chemical advancement. However, it is in situ mechanical development of wood-adhesive systems that is most relevant with process optimization such as hot-pressing of wood-based panels. The objective of this research is to examine the feasibility of applying common model-fitting kinetic analyses to describe cure development based on storage modulus development recorded with dynamic mechanical analysis (DMA). Dynamic three-point bending tests were conducted on a sandwich specimens composed of two wood adherends bonded with an adhesive layer. Two commercial PF resins of different molecular weights distributions (labeled as PF-high and PF-low, respectively) were used as adhesives. In addition, PF-high bonded wood joints were also wrapped by aluminum foil to investigate the effect of moisture loss. The specimen curing process was monitored using various isothermal and linear heating regimes. The results showed that the PF-low joints cured more slowly than the PF-high joints. The foil-wrapped PF-high joints displayed slower curing process than the unwrapped joints and rendered two peaks in the tan δ curves. These peaks were attributed to gelation and vitrification, respectively. Overall, model-fitting kinetic analyses were effective to describe the mechanical development of wood-adhesive systems.