Effects of oxygen and tetravinylsilane plasma treatments on mechanical and interfacial properties of flax yarns in thermoset matrix composites

This work is focused on the assessment of the effect of oxygen and polymer plasma tetravinylsilane (pp-TVS) treatments on the adhesion of flax yarns with epoxy and vinylester thermoset matrices. These low temperature plasma processes have been selected as more environmentally friendly alternatives to traditional chemical treatments. Tensile tests performed on single flax yarns revealed a reduction in their mechanical properties after plasma treatments. In particular, a tensile strength reduction of 36.4% was detected after the oxygen plasma treatment using 100 W of plasma power. The morphological analysis highlighted that this result is mainly ascribed to the ablation action produced by oxygen plasma process. In the case of pp-TVS, both morphological and Fourier transform infrared spectroscopy analysis confirmed the presence of a homogeneous tetravinylsilane film on the surface of the yarns. The interfacial adhesion of untreated, oxygen plasma treated, and plasma-polymer coated flax yarns has been determined by single fibre fragmentation test. The plasma polymer deposition can produce a significant improvement of the adhesion property of flax yarns with both epoxy and vinylester matrices. An increase of the interfacial shear strength values of 114% and 71% was found after the TVS film deposition in epoxy and vinylester composites, respectively. These results were confirmed by high-resolution micro-CT, photoelasticity analysis and FE-SEM observations.