Carbon fibres have been extensively used for improving mechanical properties of polymer composite materials due to their high strength, high toughness and light weight. An epoxy resin is widely used as a host matrix since it shows excellent mechanical properties, bondability to a variety of materials, and resistance to moisture. Good adhesion between the fibre surfaces and a polymer matrix is a key issue for improving the longitudinal tensile strength of carbon fibre reinforced polymer (CFRP) composites. However, due to the non-polar nature of carbon fibres they are difficult to wet and chemically bond to epoxy. Adhesion can be improved by surface treatment of carbon fibres, mainly by oxidation of the surfaces, introducing chemically reactive functional groups onto the fibre surfaces. Plasma surface modification is attractive for this application due to its environmental compatibility, and high treatment effects without affecting the textural characteristics of the bulk material. This kind of plasma is generally obtained at low pressure. These plasma treatments at low pressures, however, suffer from the drawbacks that they require expensive vacuum systems, and methods are only well-developed for batch or semi-batch treatments. To overcome these drawbacks, an atmospheric pressure plasma treatment system can be used, which not only avoids the need for vacuum equipment but also permits continuous treatment in a production line.
In the present work, electrochemically-treated carbon fibres (ECFs) are continuously treated with helium dielectric barrier discharge plasma at atmospheric pressure for adhesion improvement with an epoxy resin matrix. The plasma treated ECFs showed better wetting to epoxy than the original ECFs. An X-ray photoelectron spectroscopic analysis indicated that oxygen is effectively introduced onto the ECF surfaces by the plasma treatment. CFRP composite plates were prepared using both sized and plasma-treated ECFs and subsequently fractured. The plasma treated specimens showed higher fracture energies, indicating that adhesion between the fibres and the epoxy was improved by the plasma treatment.
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