Ammonium Pentaborate: an Intumescent Flame Retardant for Thermoplastic Polyurethanes

Ammonium pentaborate (APB) is shown to be an effective intumescent, char- forming additive for thermoplastic polyurethanes (TPU). As little as 5 to 10 parts of APB, added to a flame retarded TPU, provides a 7 to 10-fold improve ment in burn-through resistance. The APB/TPU char is characterized by its glassy, multicellular structure which provides virtually instantaneous thermal protection to heat-sensitive substrates and is able to resist thermo-oxidative degradation. In contrast to its behavior in polyurethanes, APB is found to be somewhat less effective as an intumescent additive for several non-urethane polymers such as natural rubber, polyester resin, polyamide and polyvinyl chloride. A mechanism is proposed to account for the unique nature of the APB/TPU system. It is suggested that, via a series of chemical reactions, APB redirects the thermal decomposition of the polyurethane thereby producing less volatiles and increased char.