Solvent Effects in Polyurethane Cure: A Model Study

The effect of change of solvent type on the rate of cure of a polyol with an isocyanate was measured using a range of different techniques. The initial stages of the cure process were followed using viscosity and Fourier transform infrared spectroscopy [FTIR] measurements. The gelation point was observed using viscosity measurements and depends on the solvent used. FTIR measurements confirm that both acceleration and inhibition of the polyurethane formation occurs with change of solvent. Comparative studies carried using mixtures of ethyl acetate/toluene and n-butyl acetate/xylene are reported. Intrinsic viscosity measurements revealed that the size of the polyol changes with temperature in a different manner depending on the solvent used. In part, the size of the polyol influences the ability for reaction to occur. Measurements of the permittivity, refractive index and solution viscosity indicates that these solvent mixtures deviate from ideality. A model to describe the observed solvent effects is proposed which includes the influence of polarity on the transition state and viscosity on the diffusion of the reactants. Using the measured viscosity and permittivity data for the mixtures, it was possible to obtain a good fit of the experimental data. This study illustrates how the polyurethane reaction is sensitive to the type of solvent used and indicates how the reactivity may be influenced by change in solvent.