Nanobubble Processing Method for Improved Surface Properties of Recycled Carbon Fibre

Recycled carbon fibres frequently exhibit degraded surface functionality owing to prior matrix removal processes, limiting their compatibility with contemporary epoxy resin systems. This study proposes a nanobubble-based surface treatment route designed to restore and enhance the surface characteristics of recycled carbon fibres without aggressive chemical oxidation. The study generated ozone and carbon dioxide nanobubbles in aqueous media and experimentally investigated the effects of nanobubble treatment on the surface properties and adhesive behaviour of recycled carbon fibres. Surface chemical changes were examined using X-ray photoelectron spectroscopy, which revealed an increase in oxygen-containing functional groups due to the nanobubble treatment, indicating improved surface polarity and potential for chemical interaction with epoxy networks. The practical effectiveness of the treatment was assessed via a pinhole pull-out test that served as an indirect measure of interfacial adhesion with epoxy resin, especially the combination of ozone nanobubbles and recycled carbon fibres. Notably, the nanobubble-treated recycled carbon fibres exhibited an increase in the adhesion compared with untreated recycled carbon fibres, rising from 84.5 ± 11.5 MPa to 138.5 ± 14.8 MPa, reflecting enhanced wetting behaviour and stronger fibre–matrix interfacial bonding. Overall, the proposed nanobubble processing route offers a mild, scalable, and environmentally favourable method for restoring surface reactivity in recycled carbon fibres, supporting their reintegration into high-performance composite applications.