Synergistic enhancement of flame retardancy and interfacial adhesion in glass fiber-reinforced polyamide 6 composites via DOPO-functionalized maleic anhydride copolymer

Glass fiber-reinforced polyamide 6 (PA6/GF) composites are widely used due to their excellent mechanical and thermal properties, but the glass fibers exacerbate flammability via the “candlewick effect”. Conventional flame retardants require high loadings (>15 wt%), which degrade mechanical performance and processing, limiting practical use. Herein, we developed a dual-functional interfacial flame-retardant strategy through incorporating a DOPO-functionalized compatibilizer, poly(DOPO-hydroxyethyl acrylate- co -maleic anhydride) [P(DH-co-MA)]. P(DH-co-MA) covalently bonds to the glass fiber surface via maleic anhydride groups, significantly improving fiber-matrix interfacial adhesion and composite integrity. With a minimal addition of 0.6 wt% P(DH-co-MA) and 10 wt% base flame-retardant system, the modified composites achieve a limiting oxygen index (LOI) of 33.8 %, UL-94  V -0 rating, and a 30.2 % reduction in peak heat release rate (pk-HRR). Moreover, tensile strength, flexural modulus, and impact strength increase by 4.2 %, 5.5 %, and 17.4 %, respectively. Thermogravimetric analysis and char residue characterization indicate that P(DH-co-MA) promotes interfacial char formation and gas-phase radical quenching, exhibiting a dominant gas-phase flame-retardant mechanism. This study offers a viable and efficient route for developing high-performance, interface-engineered flame-retardant polymer composites, achieving simultaneous improvements in mechanical robustness and fire safety at low additive levels. Notably, the DOPO-functionalized compatibilizer can be readily applied for surface-tailored sizing of glass fibers, enabling in situ modification without altering existing processing protocols.