Vanillin-derived phosphorus-containing compounds and ammonium polyphosphate as green fire-resistant systems for epoxy resins with balanced properties

Flame retardants from vanillin when utilized together with ammonium polyphosphate (APP) yield excellent synergistic flame retardancy toward epoxy resins. Bisphenol A epoxy resins have been widely used due to their excellent mechanical properties, chemical resistance, electrical properties, adhesion, etc., while they are flammable. Environment-friendly and bio-based flame retardants have captured increasing attention due to their ecological necessity. In this paper, 3 bio-based flame retardants were synthesized from abundant and more importantly renewable vanillin, and their chemical structures were determined by H-1 NMR and C-13 NMR. They were used together with APP (an environment-friendly commercial flame retardant) to improve the fire resistance of bisphenol A epoxy resin. With the addition APP content of 15 phr, the modified bisphenol A epoxy resin could reach UL-94V0 rating during vertical burning test and limit oxygen index values of above 35%, but reducing APP content to 10 phr, the flame retardancy became very poor. With the total addition content of 10 phr, the epoxy resins modified by 7 to 9 phr APP and 1 to 3 phr bio-based flame retardants with epoxy groups or more benzene rings showed excellent flame retardancy with UL-94V0 rating and limit oxygen index values of around 29%. The T(g)s of the epoxy resins could be remained or even increased after introducing bio-based flame retardants, as the control; those of APP alone-modified epoxy resins compromised a lot. The green synergistic flame-retardant systems have a great potential to be used in high-performance materials.