Fabric architecture outweighs local fibre–matrix adhesion in recycled carbon fibre nonwoven benzoxazine composites

Recycled carbon fibre (rCF) nonwovens provide a promising route towards more sustainable composite reinforcements, yet their interphase behaviour in high-performance thermoset matrices remains insufficiently understood. This study systematically compares pyrolysed (pCF) and offcut (oCF) carbon fibre nonwovens in a bisphenol-F benzoxazine matrix using a multi-hierarchical framework spanning from single-fibre characteristics to macroscopic composite performance. Single-fibre analysis showed that pyrolysis causes surface roughening, increases moisture uptake (0.30 vs. 0.03 wt%), and reduces tensile strength by 21 % relative to oCF (3.27 ± 0.95 GPa vs. 4.14 ± 1.13 GPa). Despite these reductions in intrinsic fibre properties, pCF exhibited superior local fibre–matrix adhesion evident with a 35 % higher apparent interfacial shear strength (92.8 vs. 68.9 MPa), and a correspondingly lower critical fibre length (0.10 vs. 0.15 mm). Dynamic mechanical analysis showed comparable storage modulus for pCF and oCF composites (8.06 ± 0.73 GPa vs. 7.81 ± 0.92 GPa), despite the distinct local adhesion behaviour. At the composite scale, however, oCF-reinforced laminates achieved significantly higher flexural strength, despite comparable fibre volume fractions (~23 %) and porosity levels (~5.5 %). A rule-of-mixtures-based efficiency analysis incorporating orientation- and length-dependent factors revealed that macrostructural parameters, particularly fibre length distribution, nonwoven architecture, and fibre–fibre cohesion, govern performance more strongly than local interphase advantages. These findings identify bisphenol-F benzoxazine as a suitable matrix for rCF composites from different feedstocks, with pCF showing especially strong interfacial compatibility. Crucially, they demonstrate that optimising the performance of rCF nonwoven composites requires prioritising reinforcement architecture over microscale adhesion engineering.