Crosslinking and Network Formation in Polyurethane Systems with Excess Isocyanate

The article reviews authors’ recent work on the intensity of side reactions and network formation in polyurethane systems when isocyanate groups are in excess. Urethane and allophanate groups are formed, and when traces of water are present as are urea and biuret groups. The rate of formation and stability of biuret groups are higher than those of allophanate groups. The formation of trifunctional allophanate, biuret and isocyanurate groups may lead to gelation and network formation in originally linear polyurethane systems, which was shown to be the case in poly(oxypropylene) diol - MDI systems. With increasing the excess of isocyanate, two gel points were observed one occurring at a low excess and the other at a high excess of isocyanate. Application of the branching theory can predict the conditions of gelation and can characterise quantitatively the amount of soluble material. The theory can predict well the number of elastically active network chains and other structural features of the gel.