Extent of side reactions and gelation of polyether polyurethanes

Abstract

Gelation and gel build-up due to a side reaction in poly-(oxypropylene) diol — 4,4′-diisocyanatodiphenylmethane systems, catalyzed by dibutyltin dilaurate and containing excess NCO groups was studied at 60 to 140°C. Formation of allophanate groups from urethane and urea and biuret from traces of water were the main side reactions. The extent of these reactions was monitored by the determination of isocyanate reacted in excess to that consumed for the formation of urethane and urea groups. Two gel point were observed: one corresponding to the sol→gel transition at a low excess of NCO groups and the other one corresponding to the gel→sol transition at a high excess of NCO groups. The width of the gel and the maximum amount of gel decrease with increasing temperature.