Effect of cold air plasma on the morphology and thermal stability of bleached hemp fibers

Abstract

The use of cold plasma surface modification techniques has lately gained increasing interest as a complementary option (nonpollutant and free of hazardous chemicals) to increase the thermal stability of bio-based fibers, and thus make them more suitable for polymer reinforcing applications. A considerable amount of studies has been performed to improve the thermal stability of “raw” lignocellulosic fibers by using cold plasma. Only few studies were done using “bleached” ones. Bleaching is mainly applied to increase the reinforcing capacity of fibers. Improving the thermal stability of bleached fibers is needed to reduce or avoid their degradation during the processing of bio-based composites. In this study, commercial bleached hemp (CBH) fibers were modified using low-pressure rotatory air plasma (LPRP) and atmospheric-pressure air plasma jet (APPJ) devices to be further characterized by Fourier transform infrared spectroscopy-attenuated total reflectance mode (FTIR-ATR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). FTIR analysis evidenced the oxidation and crosslinking of cellulose chains. XRD analysis revealed a slight increase in interplanar distances of cellulose chains, that was attributed to the interchain insertion of functional groups. SEM images displayed much rougher surfaces for the treated CBH fibers than for the untreated one. TGA showed that LPRP (30 min treatment) and APPJ increased the thermal resistance of CBH fibers, which exhibited an initial degradation temperature 15 and 30 °C higher than that of the untreated fiber, respectively. For LPRP, a longer exposition time (180 min) provoked significant eroding without improving the thermal resistance. Finally, the cold plasma surface modification of bleached hemp fibers may well assist with the mechanical interlocking and thermal resistance (during processing) when applied in polymer reinforcing.