Successful synthesis of blocked polyisocyanates, using easily cleavable phenols as blocking agents, and their deblocking and cure studies

TLDR: In this article, a series of blocked polyisocyanates were prepared using phenol, 2,4-dichlorophenol and 2-chloro-4-nitrophenol; their blocking and deblocking kinetics, deblocking temperatures, equilibrium temperatures, and cure-times were studied using a hot-stage FT-IR spectrophotometer.

Abstract: Phenols with electron withdrawing substituents at the 2,4-positions are important for use as blocking agents for isocyanates. Blocked polyisocyanates derived using such blocking agents are attractive for producing heat-cured polyurethane products at relatively low temperatures, i.e., below 160 °C. In this study, a series of blocked polyisocyanates were prepared using phenol, 2,4-dichlorophenol, 2-chloro-4-esterphenol and 2-chloro-4-nitrophenol; their blocking and deblocking kinetics, deblocking temperatures, equilibrium temperatures, equilibrium rate constants and cure-times were studied using a hot-stage FT-IR spectrophotometer, adapting neat conditions. Double Arrhenius plots for these thermally reversible systems were reported with an aim to understanding the relationship between forward and reverse reactions. It was found that the rates of forward and reverse reactions and equilibrium rates increased with increasing the acidity of phenol, except in the case of 2-chloro-4-nitrophenol; correspondingly, the deblocking temperature and cure-time of blocked polyisocyanates decreased. Blocked polyisocyanates obtained using unsubstituted phenol showed equilibrium temperature as a range in the double Arrhenius plot, whereas, in the case of 2,4-dichlorophenol and 2-chloro-4-nitrophenol, the Arrhenius plots showed distinct equilibrium temperatures. The equilibrium temperature range or equilibrium temperature of 2-chloro-4-esterphenol-blocked polyisocyanate was not determined, as extrapolation of its plot was found to extend out of the temperature range studied. Importantly, as expected with strong background, all three di-substituted phenols were found to deblock remarkably below 55–70 °C, compared to unsubstituted phenol, which deblocks at 135 °C. More importantly, 2-chloro-4-nitrophenol deblocks at 65 °C and its blocked polyisocyanate cures with polyol within 25 minutes at 110 °C.