Thermal degradation, flame retardance and mechanical properties of polyurethane composite elastomers based on expandable graphite/aluminum hypophosphite/melamine cyanurate

This study develops a synergistic flame-retardant system for polyurethane elastomer (PUE) by combining expandable graphite (EG) with AM (a mixture of aluminum hypophosphite and melamine cyanurate in a 2:1 mass ratio). Thermogravimetric analysis (TGA) and thermogravimetric-infrared spectroscopy (TG-FTIR) demonstrates that the EG/AM system significantly enhances PUE's thermal stability through modified decomposition pathways. At the optimal EG/AM ratio of 3:1, the composite achieves exceptional flame retardancy, including a limiting oxygen index (LOI) of 31.8 %, an Underwriters Laboratories Standard 94 (UL-94) V-0 rating, and significant reductions of 73 % in peak heat release rate (PHRR), 26 % in total heat release (THR), and 49 % in total smoke release (TSR) during cone calorimetry tests (CCT). This formulation maintains a tensile strength reduction of <20 %, demonstrating a balance between flame safety and structural integrity. The synergistic mechanism involves two key pathways: (i) non-combustible gases from AM thermal decomposition (e.g., NH₃) dilute oxygen and quench free radicals; (ii) phosphorus compounds from AM catalyze char formation and hybridize with EG's expanded char layer, creating a dense barrier that effectively interrupts combustion reactions and thermal mass transfer. This physical-chemical dual protection strategy provides a practical solution for developing high-performance flame-retardant PUE composites, suitable for demanding applications.