Synthesis and studies on forward and reverse reactions of phenol-blocked polyisocyanates: an insight into blocked isocyanates

Blocked isocyanates are an important class of raw materials used in the polyurethane industry. In this study, the synthesis and kinetics of blocking and deblocking reactions of a series of phenol-blocked polyisocyanates have been studied in detail using a hot-stage FT-IR spectrophotometer adapting to neat conditions. The results were compared with an aim to resolve complex questions on the relationship between the forward and reverse reaction parameters. As a result, double Arrhenius plots for thermally reversible reactions were proposed for the first time. Using these plots, the most probable equilibrium temperatures for the forward and reverse reactions and the equilibrium rate constants of these reactions were assessed. It was found that the trend present in the rate constants of the forward reaction, reverse reactions and equilibrium were uniform i.e., the rate of these reactions decreased or increased with respect to the acidity of the blocking agent. A phenol with more acidity and less nucleophilicity, e.g., 2-chlorophenol, was found to be a better blocking agent; it blocks the isocyanate quickly, and at the same time, it cleaved off easily. The most probable temperatures assessed using the double Arrhenius plots were found in accordance with deblocking temperatures. The data, such as time required for conversion into product, the equilibrium temperature range for forward and reverse reactions and most probable equilibrium temperature in combination with deblocking temperature, reported in this work are very attractive from the manufacturing and application points of view.