Effects of coating ingredients on the thermal properties and morphological structures of hydrocarbon intumescent coating chars

Mapping the performance of intumescent coatings as a function of concentration of ingredients is of great significance for formulation optimization. In this work, the effects of ammonium polyphosphate (APP), melamine (MEL), TiO₂, CaCO₃, and vitreous silicate fiber, were investigated on the fire-resistance performance of selected zinc borate (ZB)-containing and ZB-free hydrocarbon intumescent coatings with exposure to the hydrocarbon fire testing curve UL 1709. Owing to the promoted interactions of APP with epoxy binder and zinc borate, increasing levels of APP in the ZB-containing coatings enhanced the critical times of steel substrate and the formation of macroporous and compact char layers. In contrast, when increasing the content of MEL, the performance of the ZB-containing coatings generally declined, due mainly to the restricted char formation resulting from the decreased binder content. Among the investigated formulations, the coating with 25 wt.% APP or 5 wt.% MEL obtained the best performance. The oxides TiO₂ and CaCO_3, and the fiber barely had effects on the performance of the ZB-containing intumescent coatings, but the critical times and char appearance were strongly affected for the ZB-free coatings. Increasing the content of any of the three inorganics led to reduced expansion of the ZB-free coatings, but improved the chars with more mechanically stable structures. The longest critical times (e.g. to 550 oC) were found with the ZB-free formulations containing 1.5 wt.% TiO₂ (87.7 min), 2.5 wt.% CaCO3 (86.3 min), and 5 wt% vitreous silicate fiber (85.1 min).