Cure and properties of unfoamed polyurethanes based on uretonimine modified methylene-diphenyl diisocyanate

The curing behaviour of a series of polyurethanes based on modified methyle ne-diphenyl diisocyanate (MDI) and poly(propylene oxide) polyols was studie d using isothermal Fourier-transform infrared spectroscopy (FTIR), temperat ure-ramped differential scanning calorimetry (DSC) and adiabatic exotherm e xperiments. The effects of catalyst type and content, and of polyol molecul ar weight and functionality on the curing behaviour of the material were in vestigated. Increasing catalyst concentration or decreasing the polyol mole cular weight raised the rate of reaction and shifted the DSC peak exotherm temperature to lower temperatures, but: the heat of reaction was effectivel y constant. A marked increase in reaction rate was observed when a 1 degree s-alcohol-based polyol (from ethylene oxide end-capping) was used in place of the standard poly(propylene oxide) end-capped 2 degrees-polyols. FTIR is ocyanate conversion during polyurethane formation for a range of dibutyltin dilaurate (DBTDL) concentrations was satisfactorily fitted to second-order kinetics. An approximately linear relationship between DBTDL catalyst conc entration and reaction rate constant was found, but increasing the concentr ation of DBTDL was found to have no significant effect on the magnitude of the activation energy. The activation energy for polymerization was found t o be independent of the molecular weight of the diol or triol systems. Dyna mic mechanical thermal analysis revealed a linear increase of the glass tra nsition temperature with decreasing triol weight fraction, and was in good agreement with a theoretical model based on copolymer and crosslinking effe cts. (C) 2000 Society of Chemical Industry.