Cure kinetics of PF/PVAc hybrid adhesive for manufacturing profiled wood-strand composites

In thermoforming of profiled wood-strand composites, an adhesive system is needed to provide a weak initial bond to maintain mat integrity and architecture during the forming process and eventually a durable bond when the final cross-sectional shape is achieved. A hybrid adhesive composed of phenol formaldehyde (PF) and poly( vinyl acetate) (PVAc) is proposed in this study. The cure kinetics of this hybrid adhesive and bond development in a multi-step hot-pressing is discussed. Cure kinetics studied by differential scanning calorimetry indicated that adding PVAc slowed down the curing reaction of PF resin; however, the full cure of PF was not inhibited. The nth-order Borchardt Daniels (nth-BD) model provided good prediction for the curing of adhesives with a PF/PVAc ratio lower than 1:1. To simulate roll forming of wood-strand mats, a hot-pressing schedule at low temperature combined with multi-stage closing and opening was developed. The nth-BD model was able to predict the actual bond development for composites made with neat PF resin. The results indicated that cure kinetics of a PF/PVAc hybrid adhesive would not significantly differ from neat PF resin for blend ratios of 1:1 or lower, thus potentially providing a resin system for roll forming or matched-die forming of wood-strand composites.