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 between 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.