Mechanical behavior of chemically-treated hemp fibers reinforced composites subjected to moisture absorption

Natural Fibers Reinforced Composites (NFRC) are finding much interest as substitutes for glass- or carbon-reinforced composites thanks to their lightness, easy handling, processing and recyclability. However, their polarity makes them incompatible with hydrophobic thermoplastic matrices, leading to extended moisture adsorption which causes the debonding between fibers and matrix, affecting, thus, the mechanical properties of NFRCs. In the present work, NFRCs were manufactured using hemp fibers previously chemically treated with NaOH alkali solutions or (3-Glycidyloxypropyl) trimethoxysilane (GPTMS) solutions of various concentrations. To assess the effectiveness of the used chemical treatments in hindering the moisture adsorption and the entailed mechanical failure of the NFRCs, untreated and treated hemp fibers based composites were subjected to moisture adsorption test and then to tensile testing as a function of the chemical treatment, temperature and concentration of reagents. The results show that the treatments with 5 wt% of both NaOH and GPTMS are the most effective, reducing composites' water uptake from 7.74% to 6.46% and 5.58% respectively at room temperature, and from 9.67% to 8.19% and 8.13% respectively at 50 °C. Moreover, the comparison between the mechanical testing results carried out before and after the moisture adsorption test, shows that the water uptake induces mainly a stiffness decrease (about 50% when alkali treatments were used and about 60% using silane treatment), while not significantly affect the loading capability of the composites regardless of chemical treatment. However, the specimen obtained using 5 wt% GPTMS is more effective in the prevent the failure of the composite induced by water uptake.