## 3 Results and Discussion
### Characterization of Flax Yarns After Enzymatic Treatment
As shown in Table 5, the tensile strength and failure strain decreased after enzymatic treatments, in line with other studies on hemp fibres [26; 29]. The reduction in mechanical performance can be linked with the enzymatic effect of yarn fibrillation observed by the FE-SEM analysis. A marked loss of tensile strength of about 58.5% was obtained after a treatment with the 10 wt % Pelyve solution for 6 h. This tensile strength drop can be connected to the progressive breakage of the fibres present at the outer surface of the yarn produced at a high solution concentration (Figure 4). Another phenomenon able to explain the reduction in the mechanical performance of the yarns is likely to be the enzymatic removal of non-cellulosic constituents of flax, such as pectin, hemicellulose and lignin. Although they are not structural constituents of the fibre like cellulose, the non-cellulosic components have to keep the cellulose microfibrils linked together. A similar result was found by [PERSON] et al. [26]. A reduction in the tensile properties of hemp fibres was reported after an enzymatic treatment with polygalacturonase. The authors proposed a \"composite model\" to explain the mechanical behaviour of plant fibres and the effect of the enzymatic preparation on their properties. The hemp fibre can be assimilated to a composite material, in which the cellulose microfibrils are the reinforcing fibres and the non-cellulosic components are the matrix. According to this model, the mechanical behaviour of the composite is controlled not only by the cellulose microfibrils but also by their adhesion with the non-cellulosic matrix. The enzymatic degradation of the non-cellulosic matrix produced during the treatment resulted in a reduction in the fibre/matrix interface strength and in an overall decrease in the tensile properties of the fibres. Similar conclusions have been found for pectinase-treated hemp fibres by [PERSON] et al. [46]. In addition, lignin is a structural component and its degradation can result in a decrease in fibre tensile strength. The removal of amorphous non-cellulosic components from flax fibres has been confirmed by both FT-IR and thermogravimetric analysis. Several studies in literature investigated the dependence of tensile properties on the cellulose crystallinity index for plant fibres [47; 48]. In general, an increase in tensile strength and Young's modulus with the increased crystallinity index was reported. As described previously, in this study an increase in the cellulose crystallinity index was observed after the different enzymatic treatments. The reduction in the tensile strength of enzymatically treated flax yarns can be related to an excessive degradation of the structural components of fibre, such as lignin. A similar behaviour was found by [PERSON] et al. [49] for sisal fibres subjected to a mercization treatment. The tensile strength and Young's modulus of sisal fibres were found to decrease with an increasing crystallinity index, a behaviour ascribed to the alkali treatment severity that resulted in a severe degradation of the fibres. The results of the single yarn tensile tests were analysed using the Weibull distribution (Table 5) and the corresponding curves are reported in Figure 8. Both the untreated and enzymatically treated flax yarns displayed a quasi-linear trend, pointing out that the final failure was controlled by a unique population of defects and that the treatment had an influence on the concentration and severity of the flaws but not on their nature.