Hydrolytically Capped Schiff Base‐Functionalized Ammonium Polyphosphate: A Synergistic P, N, Si Flame Retardant Containing a Three‐Dimensional Crosslinked Network to Achieve Epoxy Resin Flame Retardancy and High Charring Properties

ABSTRACT Compared with traditional flame retardants, highly carbonaceous flame retardants show comprehensive advantages in terms of efficient flame retardancy and multi‐scenario applicability. In order to address the compatibility and carbon formation effect of ammonium polyphosphate (APP) in epoxy resins (EPs), an efficient P‐N‐Si synergistic flame retardant SCAP was synthesized in this study by a hydrolysis‐covering process based on the presence of Si‐bearing Schiff bases in flame retardants. Added to EP, SCAP/EP can achieve excellent fire safety and shows good self‐extinguishing off‐fire capability and inhibition of heat and smoke release during testing. The material's PHRR decreased from 1089 kW/m 2 of pure EP to 796 kW/m 2 with the addition of 7 wt% of SCAP, according to the data, and it was rated V‐0 in the UL‐94 rating. In addition, EP's mechanical, tensile, and flexural strengths were increased by adding SCAP. Mechanistic analysis revealed that when exposed to heat, the Schiff base promoted a high degree of aromatization of the carbon skeleton, producing a high‐quality carbon layer that had a good flame‐retarding action. Furthermore, APP and other gas‐phase flame‐retardant components were crucial to gas‐phase flame retardancy.