Intumescent fire protective coating: Toward a better understanding of their mechanism of action

The aim of this work is to better understand the role and the mechanism of action of boric acid and of coated ammonium polyphosphate (pure ammonium polyphosphate coated with THEIC) used as flame retardants in a commercial intumescent epoxy-based formulation using analytical techniques including thermogravimetric analyses (TGA) and solid-state NMR. In a previous paper, we detected that some reactions took place during the intumescence phenomenon between boric acid and ammonium polyphosphate upon heating. The paper focuses on the analysis of the degradation of those sole components and on the study of their interaction. It is first shown that the THEIC increases the thermal degradation rate of ammonium polyphosphate. This enables the degradation products of boric acid and coated ammonium polyphosphate to react together, resulting in the formation of borophosphate. It is suggested that the formation of this product provides the superior mechanical resistance of the char and promotes the adhesion of char on the steel plate.