Influence of ammonium polyphosphate flame retardant on the flammability and mechanical behavior of glass fiber reinforced Elium® composites

Composite materials offer a significant advantage for electric vehicle (EV) battery enclosures due to their high strength-to-weight ratio. A critical factor influencing their performance, both mechanically and in terms of fire safety, is the integration of effective flame retardants. In this study, an emerging class of recyclable, acrylic-based thermoplastic resin systems (Elium®), for which fire response behavior remains largely unexplored in the literature, was reinforced with woven glass fibers and modified by incorporating varying concentrations (5–20 wt%) of the non-halogenated flame retardant ammonium polyphosphate (APP). The resulting composites were manufactured using vacuum-assisted resin transfer molding. Mechanical, thermal, rheological, and flammability properties were systematically evaluated. The addition of APP significantly improved thermal degradation resistance and reduced the total heat of combustion. In UL 94 testing, the classification improved from not rated (NR) for the neat composite to a V-0 rating for the 20 wt% APP formulation. Furthermore, torch-based fire tests showed that the 20 wt% APP loading reduced the mass loss ratio from 29.1% to 13.1%. Rheological analysis showed that all formulations exhibited shear-thinning behavior, which is advantageous for resin infusion during composite fabrication. It was observed that 5 and 10 wt% APP loadings led to modest enhancements in tensile and flexural strengths, ranging from 2.8% to 11.6%. Conversely, higher loadings (15–20 wt%) adversely impacted mechanical performance; specifically, flexural and interlaminar shear strengths decreased by 8.5–24.1% and 21.0–30.9%, respectively. These findings demonstrate the potential of APP-modified Elium composites for EV battery applications, with a need for careful optimization of APP loading to balance fire resistance and mechanical integrity.