NETWORK FORMATION OF POLYURETHANES DUE TO SIDE REACTIONS

A theoretical approach based on the theory of branching processes has been developed to describe gelation and network formation in polyurethane systems as a result of side reactions: formation of allophanate, urea, biuret, and isocyanurate groups. The branched and cross-linked structures are assembled of structural fragments containing these and unreacted hydroxyl and isocyanate groups and urethane groups. Relations for calculating the gel point, sol fraction, and concentration of elastically active network chains are derived. For a diol-diisocyanate system, two gel points are predicted: a sol-gel transition at a low excess of isocyanate and a gel-sol transition at a high excess. Critical excess of isocyanate groups and development of the gel were studied experimentally in systems composed of α, ω-dihydroxypoly(oxypropylene) and bis(4-isocyanatophenyl)methane at 60-140 °C. The reaction was catalyzed with dibutyltin dilaurate. Experimentally, the fractions of urea groups and of the sum of allophanate and biuret groups were determined by titration. The agreement with theory was good. At higher temperatures, an unknown side reaction not accounted for in the theory made the experimental gel fractions somewhat lower. © 1990, American Chemical Society. All rights reserved.