Bio-based benzoxazine containing phthalonitrile: Nonsolvent synthesis, curing behavior and pyrolysis mechanism

Traditional benzoxazines are largely constrained by the dependence on non-renewable resources, solvent-based synthesis, and insufficient thermal resistance. This study presents a novel high-performance bio-based benzoxazine containing phthalonitrile (BZPN), synthesized via melt condensation using eugenol, 3-aminophenoxyphthalonitrile, and paraformaldehyde. The chemical formula of BZPN was validated by FTIR, ¹H and ¹³C NMR analyses. The curing of BZPN involves ring-opening polymerization of oxazine group and ring-forming polymerization from phthalonitrile unit, yielding polybenzoxazines and heteroaromatics respectively. Combined TG-DTG, TG-FTIR with TG-MS technique, the three-stage thermal pyrolysis mechanism of BZPN was proposed. The initial decomposition ( T dm1 =395 °C) occurs through the cleavage of C N, C O, and C C bonds in the polybenzoxazine, releasing NH₃, H₂O, CH₄, and CO₂. The second stage ( T dm2 =485 °C) involves the cleavage of C O and C C bonds in the heteroaromatic networks by releasing NH₃, H₂O, CH₄, and CO₂. The final carbonization stage ( T dm3 =615 °C) includes the transformation of highly aromatic compounds into graphite. With the facile solvent-less synthesis method and superior comprehensive properties (compressive strength: >220 MPa, T g : >375 °C, T d5 : >390 °C, and C y : 72 % at 800 °C), bio-based BZPN is a promising high-performance thermosetting resin for advanced industrial and engineering applications.