Determination of isocyanate group content in polyurethane prepolymer: optimization of solvent systems and amine reagents

This study develops a novel composite solvent system to replace highly toxic traditional toluene for the accurate determination of isocyanate (–NCO) content in polyurethane prepolymers, where acetone serves as the prepolymer dissolution solvent and isopropanol acts as the solvent for organic amines. By systematically comparing the reaction behaviors of various organic amines in acetone and three graded alcohol solvents (primary ethanol, secondary isopropanol, and tertiary tert-butanol), combined with Fourier transform infrared spectroscopy analysis, this work elaborates the isocyanate reaction mechanisms in ketone-amine and alcohol-amine systems. In the ketone-amine system, acetone condenses with primary amines to form Schiff bases and inhibit their –NCO reactions, while secondary amines (di-n-butylamine) show inertness toward acetone, guaranteeing measurement reliability. In the alcohol-amine system, –NCO groups undergo two competing reactions with organic amines (primary reaction) and alcohols (secondary reaction), with the interference of alcohol-induced side reactions following the order: ethanol > isopropanol > tert-butanol. Under the reaction conditions of 25 °C and 3 min, the detection accuracy of the acetone-isopropanol-di-n-butylamine method is comparable to that of ASTM D5155-19. Infrared spectroscopy further verifies the chemical stability of the alcohol-amine system. This green, eco-friendly solvent system eliminates highly toxic toluene and additional cosolvents, providing an economical and precise alternative for routine –NCO content determination of polyurethane prepolymers.