Influence of thermoplastic fibre-epoxy adhesion on the interlaminar fracture toughness of interleaved polymer composites

We present an experimental study using surface modification of polyetherimide (PEI) and polyphenylene sulfide (PPS) nonwoven fibrous veils to probe their performance as interleaves to improve the interlaminar fracture toughness (IFT) of carbon fibre-epoxy composites. Veil fibre surfaces were modified with ozone and a posttreatment with ultraviolet (UV) light (ozone + UV). From surface characterisation, mechanical testing of composites and fractography we show that for the PEI veil, these surface modifications resulted in a decreased mode I IFT attributable to decreased fibre-epoxy adhesion and hence, fibre/matrix debonding. In contrast, an increase in sulfinyl functional groups on the surface of PPS fibres after ozonation was observed alongside an increase in PPS veil-epoxy adhesion. The strong bond between fibre-matrix resisted crack propagation across veils, compelling the crack to divert through weaker carbon fibre-epoxy interfaces in adjacent layers. The mode I fracture toughness during crack propagation GIprop decreased, confirming the level of veil-epoxy adhesion to be a significant contributor to the IFT that can be associated with specific functional groups on fibre surfaces.