Tg as an index of conversion in PMDI-impregnated wood

It is well established that the glass transition temperature (Tg) is a sensitive measure of cure in neat thermosets. As cure advances, network mobility declines and the Tg rises in a systematic fashion. This study sought to determine if such a relationship exists for polymeric isocyanate adhesives (pMDI) cured in the presence of wood. Yellow-poplar (Liriodendron tulipifera) specimens were impregnated with neat pMDI and then isothermally cured for various periods in two different differential scanning calorimeters (DSCs). After this isothermal cure period, the Tg and residual heat of cure were determined. These thermal scans were performed using either constant (conventional) or modulated (MDSC) heating rates. For both methods, the degree of resin cure varied significantly under identical isothermal curing conditions; nevertheless a strong relationship was found between the degree of resin cure and the associated Tg. While the conventional DSC method yielded slightly improved sensitivity and reproducibility, results from both methods compared favorably.