Developing intrinsic halogen-free and phosphorus-free flame retardant biobased benzoxazine resins with superior thermal stability and high strength

Endowing intrinsic halogen-free and phosphorus-free flame retardant biobased benzoxazine resins (biobased content > 50 %) with ultrahigh thermal stability and high strength is still an important and challenging issue. Herein, starting from citraconic anhydride and para-aminophenol or ortho-aminophenol, two isomeric phenolic compounds containing citraconimide groups, pCI and oCI, were designed and synthesized, each of which then reacted with furfurylamine and paraformaldehyde to synthesize novel para-functionalized benzoxazine monomer (pCI???fa) or ortho-functionalized benzoxazine monomer (oCI???fa). Results show that pCI???fa and oCI???fa have excellent processability, represented by low melting point (pCI???fa: 150.0 ??C, oCI???fa: 127.7 ??C) and low curing temperature (pCI???fa: 217.6 ??C, oCI???fa: 205.5 ??C); moreover, interestingly, there is no Diels-Alder reaction be-tween citraconimide and furan groups. The cured resin from pCI???fa or oCI???fa, coded as poly(pCI???fa) or poly (oCI???fa), has outstanding thermal resistance, tensile properties and flame retardancy. Especially, poly(pCI-fa) has better integrated performance than poly(oCI???fa), and the initial thermal decomposition temperature (Tdi) of poly(pCI-fa) is as high as 391 ??C, obviously higher than the those of all halogen-free and phosphorus-free flame retardant biobased benzoxazine resins (the biobased content > 50 %) reported in SCI database by 3 August 2022. The mechanism behind the integrated performances was discussed.

Automated summary

In this study, novel phenolic compounds containing citraconimide groups were designed and synthesized, and para-functionalized and ortho-functionalized benzoxazine monomers were synthesized through reactions with furfurylamine and paraformaldehyde. The cured resins showed excellent processability and flame retardancy, with poly(pCI-fa) exhibiting better performance than poly(oCI-fa).