Surface modification of extended chain polyethylene fibres to improve adhesion to epoxy and unsaturated polyester resins

Extended chain polyethylene fibres have been treated in ammonia and oxygen lo‐pressure gas discharges (plasmas) in order to enhance adhesion to epoxy and unsaturated polyester resins, respectively, and thus significantly improve fibre/resin interfacial properties in fibre‐reinforced polymer composites. Ammonia plasma treatment results in the incorporation of amine functional groups onto the fibre suface. The treated fibre surface has been analysed using XPS and spectrophotometric techniques. Extended chain polyethylene/epoxy composites made from ammonia, plasma‐treated fibres show a marked increase in interlaminar shear strength over composites made from untreated, corona‐treated or oxygen plasma‐treated fibres. The increase in fibre/resin adhesion after ammonia plasma treatment is confirmed by SEM observations of fracture surfaces, which show clean interfacial fracture surfaces in composites made from treated fibres. Fibres modified by oxygen plasma treatment contain a significant concentration of carbon‐oxygen functionalities, which contribute to the polarity of the surface and hence increase wet‐out by unsaturated polyester resins. The concentration and nature of carbon‐oxygen species on the fibre surface have been determined by XPS. Pull‐out tests on multifilament yarns embedded in a polyester resin confirm the high fibre/matrix adhesion achieved with the oxygen plasma‐treated fibres compared to corona‐treated or untreated fibres. Tensile properties of the fibres are reduced significantly after prolonged treatment in an oxygen plasma, while in an ammonia plasma the fibre strength is unaffected.