Evaluating cure of a pMDI-wood bondline using spectroscopic, calorimetric and mechanical methods

The cure of polymeric diphenylmethane diisocyanate (pMDI)/wood bondline in a controlled saturated steam environment was monitored using micro-dielectric analysis (μDEA). Saturated steam environments were produced between 110° and 140°C. The degree of cure calculated from μDEA was a basis for further spectroscopic, calorimetric, and mechanical evaluation. Interpretation of calorimetric and spectroscopic analysis revealed large consumption of isocyanate early in cure. However, mechanical strength, as revealed by lap-shear tests, did not develop until late in cure. Low lap-shear strengths and a plateau in conversion rates were detected for samples pressed at 110° and 120°C. Several components of the analysis suggest that low temperature cure may result in crystal formation, leading to diffusion controlled cure.