Thermomechanical and Combustion Behavior of Post Cured Allyl and Furan Functionalized Bio‐Based Benzoxazine Composites

ABSTRACT Bio‐based benzoxazines represent a promising class of high‐performance thermosets for sustainable composite applications. Nonetheless, their thermomechanical characteristics are frequently not as robust as those found in traditional fossil‐derived materials. This study systematically investigates the influence of allyl and furan functional groups, on the crosslinking behavior and thermomechanical performance of bio‐based polybenzoxazine composites, with particular emphasis on the role of post curing. Allyl and furan groups were selected for their bio‐based origins and distinct crosslinking mechanisms: allyl groups undergo thermally induced radical crosslinking to enhance network stiffness, while furan moieties provide high aromatic content and electrophilic reactivity to increase crosslink density and thermal stability. Two benzoxazine monomers based on 4,4′‐diaminodicyclohexylmethane Ph‐dcda (phenol‐based), and AllylPh‐dcda (allylphenol‐based) were synthesized and compared with the commercially available furan functionalized FB602. Fiber‐reinforced polymers were manufactured via compression moulding using recycled carbon fiber nonwovens as reinforcement. The effects of post curing at 220°C on network formation, thermomechanical properties, and combustion behavior were evaluated revealing that post curing significantly enhanced the crosslink density ( V e ) for all systems. The structural effects, however, varied substantially depending on the functional groups present. For poly(AllylPh‐dcda), post curing nearly doubled the storage modulus and increased V e by a factor of 1.8, attributed to thermally activated crosslinking of allyl bonds as compared to non‐post‐cured samples. Poly(FB602) exhibited the highest V e and a substantial increase in glass transition temperature ( T g ) from 169°C to 203°C upon post curing, owing to the high aromatic content and furan mediated crosslinking. In contrast, poly(Ph‐dcda), lacking additional reactive functionalities, underwent network degradation during post curing, resulting in a decreased gel content. Combustion analysis demonstrated that functionalized benzoxazines exhibited reduced peak heat release rates and shorter burning times, compared to non‐functionalized systems, although all composites failed the UL‐94 vertical burning test, highlighting the limitations in flame retardancy. Future studies should focus on incorporating flame retardant additives to improve fire safety compliance. This work elucidates the structure property relationships governing crosslinking efficiency and thermomechanical performance in functionalized biobased polybenzoxazines, providing guidance for the rational design of sustainable high‐performance composites.