Thermal insulation performance and char formation and degradation mechanisms of boron-containing hydrocarbon intumescent coatings

Boron compounds are effective ingredients, widely used in passive fire protection. In this work, epoxy-based hydrocarbon intumescent coatings, containing one of three boron compounds, zinc borate (ZB), ammonium pentaborate (APB), and boric acid (BA), were compared in thermal insulation performance at UL1709 heating conditions, char morphology, viscoelastic behavior, thermal degradation, and composition of the char surface layer. Results show that the char structure provides the most reliable indication of the thermal insulation performance. For APB- and BA-containing coatings, results were similar, while the ZB-containing ones showed a distinctive performance, especially the ones with the high borate concentration. The latter was attributed to the role of ZB in the intumescence and oxidation reactions, where the zinc, in the presence of boron, led to a compact char with less oxidation and high residual mass. In the char surface layer of one sample with a high concentration of ZB, a crystalline phase of boron trioxide, anticipated to be one important origin of the compact char, was detected. The present work contributes with an understanding of the essential roles of boron compounds, as well as the binder and pigment contents, and provides guidelines for seeking more sustainable and environmentally friendly alternatives.