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 100W 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 (FT-IR) 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 (SFFT). 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 (IFSS) 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.
In an attempt to improve mechanical properties of basalt fibre/epoxy composites, the present work provides a comparison between the effects of a commercial coupling agent, a thermal de-sizing treatment and a plasma polymerization process on the fibre/matrix interfacial strength. The different basalt fibres were characterized in terms of surface morphology, by FE-SEM observations, and chemical composition, performing FT-IR analysis. The interfacial adhesion has been investigated by single fibre fragmentation test (SFFT) on single fibre composite samples. The plasma polymerization process was able to produce a homogeneous tetravinylsilane (pp-TVS) coating on the surface of basalt fibres, which resulted in a significant increase in the adhesion between basalt fibre and epoxy resin. The surface roughness of the untreated and treated basalt fibres has been measured by Atomic Force Microscopy (AFM) and a relationship between the surface roughness and the adhesion quality between the basalt fibres and the epoxy matrix was found. High-resolution microtomography (µ-CT) has been used to support the analysis of the damage modes during fragmentation tests.
High-performance fibre-reinforced polymer composites are important construction materials based not only on the specific properties of the reinforcing fibres and the flexible polymer matrix but also on the compatible properties of the composite interphase. First, oxygen-free (a-CSi:H) and oxygen-binding (a-CSiO:H) plasma nanocoatings of different mechanical and tribological properties were deposited on planar silicon dioxide substrates that closely mimic E-glass. The nanoscratch test was used to characterize the nanocoating adhesion expressed in terms of critical normal load and work of adhesion. Next, the same nanocoatings were deposited on E-glass fibres, which were used as reinforcements in the polyester composite to affect its interphase properties. The shear properties of the polymer composite were characterized by macro- and micromechanical tests, namely a short beam shear test to determine the short-beam strength and a single fibre push-out test to determine the interfacial shear strength. The results of the polymer composites showed a strong correlation between the short-beam strength and the interfacial shear strength, proving that both tests are sensitive to changes in fibre-matrix adhesion due to different surface modifications of glass fibres (GF). Finally, a strong correlation between the shear properties of the GF/polyester composite and the adhesion of the plasma nanocoating expressed through the work of adhesion was demonstrated. Thus, increasing the work of adhesion of plasma nanocoatings from 0.8 to 1.5 mJ·m−2 increased the short-beam strength from 23.1 to 45.2 MPa. The results confirmed that the work of adhesion is a more suitable parameter in characterising the level of nanocoating adhesion in comparison with the critical normal load.
All reinforcements for polymer-matrix composites must be coated with a suitable material in the form of a thin film to improve compatibility and interfacial adhesion between the reinforcement and the polymer matrix. In this study, plasma nanotechnology was used to synthetize such functional nanocoatings using pure tetravinylsilane (TVS) and its mixtures with oxygen gas (O2) as precursors. The plasma-coated glass fibers (GFs) were unidirectionally embedded in a polyester resin to produce short composite beams that were analyzed by a short-beam-shear test to determine the shear strength characterizing the functionality of the nanocoatings in a GF/polyester composite. The developed plasma nanocoatings allowed controlling the shear strength between 26.2–44.1 MPa depending on deposition conditions, i.e., the radiofrequency (RF) power and the oxygen fraction in the TVS/O2 mixture. This range of shear strength appears to be sufficiently broad to be used in the design of composites.
Unsized single-end rovings are oxygen plasma pretreated and organosilicon plasma coated using plasma nanotechnology to optimize the interphase in glass-fiber-reinforced polyester composites and to determine the achievable range of their shear strength for potential applications. This surface modification of the fibers allows us to vary the shear strength of the composite in the range of 23.1 to 45.2 MPa at reduced financial costs of the process, while the commercial sizing corresponds to 39.2 MPa. The shear strength variability is controlled by the adhesion of the interlayer (plasma nanocoating) due to the variable density of chemical bonds at the interlayer/glass interface. The optimized technological conditions can be used for continuous surface modification of rovings in commercial online fiber-processing systems.
Basalt fibres are becoming a promising alternative to synthetic fibres as a green reinforcement phase in polymeric matrix composites, showing excellent mechanical, chemical and thermal properties. In this work we synthetized tetravinylsilane (TVS) or a mixture formed by tetravinylsilane and different percentages of oxygen on the surface of unsized basalt fibres through the Plasma-Enhanced Chemical Vapor Deposition (PECVD) technique for improving the fibre/matrix adhesion. Single fibre tensile test proved the effectiveness of the process, without any degradation of the mechanical properties of modified basalt fibres. Finally, through pull out tests, the interfacial properties of basalt fibres were studied, measuring increases up to 80% of the IFSS for modified fibres compared to neat fibres. This result is the consequence of a greater chemical compatibility between the fibres and the matrix, thanks to the presence of a higher number of Si-O-C groups, and of a mechanical interlocking effect promoted by the increased surface roughness of the plasma-modified fibres.
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