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, p CI and o CI , were designed and synthesized, each of which then reacted with furfurylamine and paraformaldehyde to synthesize novel para -functionalized benzoxazine monomer ( p CI–fa ) or ortho -functionalized benzoxazine monomer ( o CI–fa ). Results show that p CI–fa and o CI–fa have excellent processability, represented by low melting point ( p CI–fa : 150.0 °C, o CI–fa : 127.7 °C) and low curing temperature ( p CI–fa : 217.6 °C, o CI–fa : 205.5 °C); moreover, interestingly, there is no Diels-Alder reaction between citraconimide and furan groups. The cured resin from p CI–fa or o CI–fa , coded as poly( p CI–fa) or poly( o CI–fa) , has outstanding thermal resistance, tensile properties and flame retardancy. Especially, poly( p CI-fa) has better integrated performance than poly( o CI–fa) , and the initial thermal decomposition temperature ( T di ) of poly( p CI-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.