Flame retardant effect and mechanism of benzoxazine as synergist in intumescent flame‐retardant polyoxymethylene

Benzoxazine monomers of traditional bisphenol‐A benzoxazine (BA‐a) and bisphenol‐A benzoxazine containing trialkoxysilane (BA‐a‐Si) were synthesized and incorporated into polyoxymethylene/ammonium polyphosphate/melamine (POM/APP/ME) to improve the fire retardancy of POM. The flame retardation, thermal, and mechanical properties of POM composites were evaluated by cone calorimeter, limiting oxygen index (LOI), and the Underwriters Laboratories‐94 (UL‐94) vertical burning tests, as well as thermogravimetric analysis and mechanical tests. Addition of BA‐a‐Si (2 wt%) into POM could simultaneously improve its flame retardancy (UL‐94 vertical burning rating to V‐0 rating and LOI value reached 52.1%) and notched impact strength (increased by 33.3% and 100.0% compared with that of POM/APP/ME and POM/APP/ME/BA‐a composites). Notably, compared with POM/APP/ME composite, the smoke productions of both the composites with benzoxazines were significantly suppressed. In contrast, the peak heat release rate and total heat release of composite with 2 wt% BA‐a‐Si were 33.3% and 32.9% lower than those of composite with 2 wt% BA‐a, which were closely related to the formation of high quality and quantity of charred residue. In summary, benzoxazine, especially siloxane‐containing benzoxazine, plays an effective role in flame retarding POM acting as charring agent and synergistic flame retardant.