Applicability of the WLF equation to polyurethane polyols and film properties of their resins

Abstract

The need to reduce solvent content of coatings in compliance to air pollution and other Statutory Regulations has led to major efforts to develop high solids coatings. The reduction in molecular weight helps to achieve application viscosity by an increase in free volume. Various polyurethane polyols have been synthesised by reacting the diols with polyisocyanates. The molecular weight and polydispersity indexes of these polyols have been determined by gel permeation chromatography (GPC). The viscosity of 50% polyol solutions in various solvents has been determined at different temperatures. The temperature dependence on viscosity of solution of these polyols has been determined by the Williams, Landel and Ferry (WLF) equation. The non-linear regression analysis has been solved by using Levenberg Marquardt's algorithm. It is found that non-linear regression analysis gives high correlation coefficients and very low overall residual errors. The variation in the values of constants A and B for each WLF equation in the polyol solution indicates that these constants are system dependent. The values of constants A and B obtained for polyurethane polyols in a particular solvent have been used to calculate the values of glass transition temperature of the oligomer solution ( T gs) at different concentrations. The values of T gs are found to vary with concentration. As the concentration of polymer increases there is an increase in the T gs value. The tensile properties and resistance to scratch of resins prepared using these polyols and butylated melamine formaldehyde are better than those of conventional resins. The VOC content of these coating formulations is lower than the value prescribed by the Environmental Protection Agency (EPA).