Crosslinking by polycarbodiimides

Polycarbodiimides are effective crosslinkers for carboxyl functional polymers. The reaction between carbodiimides and carboxyl groups can follow two main courses, one leading to N-acyl urea groups, and one leading to anhydride and urea groups. In this paper, we show that both routes are relevant for crosslinking of urethane resins with polycarbodiimide crosslinkers. It was found that if the concentration of carboxyl groups is high, anhydride and urea formation is the dominant route, but if the concentration of carboxyl groups is low N-acyl urea formation is dominant.