Multi-Component Coatings Enabling Low-Toxicity and Self-Extinguishing Polyurethane Foams with Potential for Railway Fire Safety.

This work presents the design of a novel multi-component surface-coating system incorporating expandable graphite (EG), ammonium polyphosphate (APP), aluminium tri-hydroxide (ATH), alginate, and D-glucosamine hydrochloride (DGH). Importantly, the coated polyurethane (PU) foams demonstrated performance within the corresponding Hazard Level 3 limits for the measured parameters, including Maximum Average Rate of Heat Emission (MARHE), smoke density, and toxicity, providing preliminary indications of their potential for railway fire safety applications. Systematic variations in EG loading revealed substantial improvements in flammability metrics, with the EG-rich formulation achieving a limiting oxygen index (LOI) of 73%, MARHE of 18 kW m-2, and significantly reduced smoke production. EG transformed the fire behaviour even at high external heat fluxes from flaming to self-extinguishing and only smouldering, promoting rapid formation of a dense, thermally insulating char. Combined interactions between EG, inorganic, and biobased additives reinforced char integrity, suppressed degradation rates, and enhanced condensed-phase protection. Thermogravimetric analysis confirmed increased residue yields (up to 52 weight percentage at 600 °C). Overall, this multi-functional coating offers a cost-effective, low-toxicity strategy for producing flame-resistant PU foams for demanding transportation and construction applications.