Interaction of Polyisocyanate Adhesive with Wood

Some physico-chemical aspects of the polyisocyanate-wood reaction were studied to provide insight into the. structural integrity of the adhesive system. Differential scanning calorimetry showed the reaction between polyisocyanate and wood to differ from that between phenol-formaldehyde (PF)and wood. The polyisocyanate-wood reaction is altered by the presence of water. Not only the carbohydrate, but also the lignin component of wood is highly reactive with polyisocyanate. Infrared and thermal softening data indicate the curing reaction is both time- and temperature-dependent and involves a two-stage reaction. The initial reaction takes place at 25? to 50 C, with a further cross-linking reaction occurring near 200?C, which is higher than the corresponding temperature required for PF cure. Although this initial polyisocyanate cure apparently provides good bond properties, the high temperature requirements for full cross-linking may create problems under normal particleboard and waferboard pressing conditions. Specifically, if cross-linking is insufficient, creep may occur under long term loading. Thus creep-data compilation should be an important part of future polyisocyanate-wood bond evaluation.

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