Tailoring the Fire-Retardant Efficacy of Intumescent Coatings via Optimal Selection of Epoxy Binder Molecular Weight

Epoxy resins are widely used as binders in intumescent fire-retardant coatings; however, the effect of their molecular weight on the fire-retardant performance of coatings with different intumescent mechanisms remains insufficiently understood. In this study, five commercial bisphenol A-based epoxy resins with number-average molecular weights (Mn) ranging from 375 to 1520 g/mol were characterized by HPLC, NMR, DSC, and TGA, and subsequently formulated into intumescent coatings of two types: classical ammonium polyphosphate/pentaerythritol/melamine (APP/PER/MEL) systems and expandable graphite (EG)-based systems. The fire-retardant efficacy and expansion factor were evaluated using a standard fire test and thermal shock exposure. In the APP/PER/MEL coatings, the optimal fire-retardant efficacy of 55 min was achieved at an intermediate Mn (645 g/mol) and an optimal pore structure (mean equivalent pore diameter deq = 41 μm), whereas both low- and high-molecular-weight resins led to distinct structural defects—macrocavities and through-thickness cracks, respectively, as revealed by SEM. In the EG-based coatings, a monotonic decrease in both the expansion factor (from ~2000% to ~1000%) and fire-retardant efficacy (from 65 to 52 min) was observed with increasing Mn, with the bisphenol A-based epoxy resin having a molecular weight of 375 g/mol providing the best performance. The observed differences were attributed to the effect of melt viscosity on char expansion and structural integrity. These findings demonstrate that the molecular-weight range of the epoxy binder can be selected according to the expansion capacity of the intumescent system, providing a practical guideline for the formulation of high-performance epoxy-based fire-protective coatings.