Cure characterization of 4-4' diphenylmethane diisocyanate in a saturated steam environment

The cure of polymeric 4-4' diphenylmethane diisocyanate (pMDI) in wood composite manufacturing has been the subject of much research. The exact contribution of polyurethane, polyurea, and polyurete formation with pMDI/wood bonding is still debated. This study forgoes the mechanism controversy and studies the cure a whole. Dielectric analysis (DEA) was utilized to monitor the cure of pMDI in controlled heat, steam, and pressure representing those conditions encountered during wood composite manufacturing. A small steam generating press was mounted to a universal testing machine producing saturated steam environments between 110° and 140°C. The degree of conversion calculated from DEA was the basis for further spectroscopic, calorimetric, and lap-shear analysis. DSC and FTIR techniques revealed a large consumption of isocyanate early in cure. However, mechanical strength, as revealed by lap-shear analysis, developed late in cure. Low lap shear strengths and a plateau in conversion rates were detected for 110° and 120°C and may indicate a diffusion-controlled reaction and crystallization effect. A phenomenological approach to kinetics was utilized to model the reaction. Isothermal DEA and dynamic DSC data were fit to a first order autocatalyzed model. ASTM E698 was modified to fit an autocatalyzed model. The modifications were compared to a method of performing a single dynamic temperature ramp to obtain kinetic parameters. Higher activation energies were obtained for DEA than DSC methods. In addition, ASTM methods for DSC produced higher activation energies than single dynamic ramps and compared favorably with data obtained from the previous DSC study. The observed differences in activation energy relate to differing mechanisms in chemical and physical cure.