Integrating ReaxFF-MD simulations into fire safety performance evaluation of polypropylene/ammonium polyphosphate composites

Polypropylene (PP) is widely used in consumer and engineering applications but is highly flammable and prone to melt-dripping. Here, we investigate how the polymerisation degree and water solubility of ammonium polyphosphate (APP) control the fire behaviour of PP/APP composites, combining UL-94, limiting oxygen index (LOI) and cone calorimetry with TG-FTIR, tensile testing and multi-scale morphology/chemistry characterisation (SEM/EDS, Raman and XPS) and ReaxFF reactive molecular dynamics (ReaxFF-MD) simulations. Two APP grades are examined: a highly polymerised, poorly soluble APP (WR-APP) and a low-polymerised, water-soluble APP (WP-APP). The fire tests show that WR-APP attains higher LOI values and UL-94V-0 ratings at lower loadings, forms denser P/N-rich char layers and reduces smoke production, whereas WP-APP leaves porous residues and requires higher loadings to reach comparable performance. SEM of unburned composites indicates APP-related accumulations and pull-out cavities, which become more evident at higher loadings and are consistent with the loss of ductility/strength in tensile tests. ReaxFF-MD simulations further show that WR-APP extends the induction period of PP pyrolysis, delays hydrocarbon release and promotes phosphate-mediated char-network formation, consistent with a delayed and localised decomposition process. TG-FTIR provides experimental support by tracking the evolution of key volatile species, strengthening the micro–macro linkage between simulated product trends and measured fire behaviour. Relating these atomistic results to the macroscopic fire behaviour underscores the roles of APP polymerisation degree and solubility in the condensed-phase flame-retardant action of PP and provides guidance for optimising halogen-free flame retardants for polyolefin systems.