Effect of low‐power air plasma treatment on the mechanical properties of carbon fibres and the interfacial shear strength of carbon fibre—epoxy composites

A low‐power plasma treatment has been adapted to work on a pilot plant scale to continuously treat a tow of carbon fibres. This treatment successfully increases fibre–resin adhesion to levels comparable if not higher than those obtained by commercial electrochemical treatments. One advantage is that the plasma treatment does not appear to affect the tensile properties of the fibre, unlike the electrochemical oxidation. We have also shown that the chemical groups introduced during air plasma treatment play a significant role in fibre–resin adhesion whereas those introduced by electrochemical oxidation do not. A possible explanation for this is that plasma treatments introduce chemical groups onto the immediate surface of the fibres that have the potential to react with the resin, whereas electrochemical oxidation occurs predominantly at the edge sites and proceeds into the sublayers of the fibre. The vast majority of the functional groups are therefore inaccessible to the resin. This type of plasma treatment, however, is unlikely to remove all of the weakly bound layers as in the case of electrochemical treatment because the sputtering capability of the plasma is very low. The fact that these conclusions can be drawn from both the single‐fibre fragmentation test and the short‐beam shear test is encouraging and goes to prove that both these tests are suitable for looking at trends in fibre–resin adhesion.