The effects of cure temperature and time on the isocyanate-wood adhesive bondline by N-15 CP/MAS NMR

A 99% N-15-labelled polymeric methylenebis(phenylisocyanate) (pMDI) resin was used to make a series of wood-N-15-pMDI composites cured as a function of temperature and time. Bondline chemistry was monitored using N-15 CP/MAS NMR. Urea linkages were identified as the major chemical moieties at low cure temperatures, while biurets were predominant at higher temperatures. Bondline chemistry did not change with time when curing at 120 degrees C. However, cure chemistry at 185 degrees C did change with time, from mostly biuret to urea. This transition involved the thermal decomposition of biurets, producing urea and isocyanate. It is believed that urethane formation has been detected under the conditions that cleave biurets. This suggests that urethanes result from the thermal liberation of isocyanate via biuret cleavage. The N-15 technique has proven valuable for the elucidation of this complex cure chemistry.