Enhanced flexural properties despite volatile-induced porosity in biobased benzoxazine and recycled carbon fibre composites

Biobased benzoxazines offer sustainable alternatives to conventional thermosets but release significant volatiles during curing, causing porosity in out-of-autoclave processing. This study investigates recycled carbon fibre (rCF) nonwoven fabrics made from offcut (oCF) and pyrolysed carbon fibre (pCF) feedstocks in compression-moulded composites using FB602, the first commercially available biobased benzoxazine. It establishes a compression moulding route and presents a parametric study on initial compaction pressure (5 to 30 bar) establishes processing-porosity-property relationships for this novel material combination. The two rCF types exhibit fundamentally different porosity responses. The oCF composites show monotonically increasing porosity from 3.3% at 5 bar to 25.5% at 30 bar with interconnected pore networks (negative Euler numbers, high percolation). In contrast, pCF composites display non-monotonic behaviour, ranging from 0.1% at 5 bar to a maximum of 19.1% at 10 bar, with isolated pores throughout (positive Euler numbers, low percolation). These differences are attributed to fibre-dependent packing densities affecting resin infiltration and volatile release. Contrary to conventional expectations, both systems achieve their highest flexural modulus of 14.9 ± 1.6 GPa (oCF) and 16.9 ± 0.6 GPa (pCF) at 30 bar despite elevated porosity. Notably, pCF composites exhibit superior stiffness despite the reduced intrinsic fibre properties associated with pyrolysis, suggesting enhanced fibre-matrix interaction. Lightweight performance analysis favours oCF for strength-limited and pCF for stiffness-limited designs. The enhanced mechanical performance at high pressure suggests improved fibre-matrix interaction due to the presence of entrapped benzoxazine volatiles.