## 3 Results and Discussion
### Fibre/Matrix Interfacial Assessment by Single Yarn Fragmentation Test  
The interfacial adhesion analysis focused on two specific treatment conditions. In fact, according to the wetting test results and tensile strength values, single yarn fragmentation tests were performed on flax yarns treated using 5 wt % and 10 wt % Peclyve enzymatic preparation for 4.5 h and 6 h. The 5 wt %, 4.5 h condition was selected because it allowed changes in the wetting properties of the flax yarns without significantly affecting their mechanical performance. Indeed, this enzymatic treatment condition produced a decrease in the tensile strength of the yarns of approximately 27%. On the other hand, the condition of 10 wt % for 6 h was selected in order to assess the effects of the maximum concentration of Peclyve solution and of the maximum treatment duration on the adhesion quality between the flax yarns and thermoset matrices. At least 10 fragmentation tests were carried out for each family of single yarn composites. The experimental procedure has been thoroughly described in previous publications [8; 23; 31]. A comparison between the critical filament lengths measured for the untreated and the enzymatically treated single yarn composites is shown in Table 6.  
The results highlight that following the enzymatic treatments, a modest reduction in the critical yarn length in both epoxy and vinylester matrices occurred. In particular, the best adhesion quality was obtained after the treatment performed using the maximum concentration value of 10 wt % and the maximum treatment duration of 6 h. In contrast, the flax/epoxy and the flax/vinylester systems where the flax yarn was treated using 5 wt % Peclyve solution for 4.5 h showed a critical yarn length almost equal to that obtained for the untreated single yarn composites. Unfortunately, the yarn critical length cannot be used as the exclusive parameter for evaluating the interfacial adhesion, as it strictly depends on the yarn strength that has been affected by the biochemical treatment [50]. Nevertheless, a comparison between the two matrix systems is possible, and the results suggest that flax/vinylester samples are characterized by larger values of critical lengths than the flax/epoxy ones. As already described for alternative flax yarn treatments [8; 23; 31], this behaviour is related to the significant \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Matrix** & **Flex Yarn** & **I\({}_{\text{c}}\) [\(\mu\)m]** & **I\({}_{\text{debonding}}\) [\(\mu\)m]** \\ \hline \multirow{3}{*}{Epoxy} & Neat & 2687 \(\pm\) 631 & 444 \(\pm\) 49 \\  & Peclyve 5 wt \%-4.5 h & 2628 \(\pm\) 531 & 353 \(\pm\) 89 \\  & Peclyve 10 wt \%-6 h & 2511 \(\pm\) 203 & 365 \(\pm\) 46 \\  & Neat & 3942 \(\pm\) 807 & 830 \(\pm\) 343 \\ \multirow{3}{*}{Vinylester} & Peclyve 5 wt \%-4.5 h & 3962 \(\pm\) 243 & 628 \(\pm\) 128 \\  & Peclyve 10 wt \%-6 h & 3554 \(\pm\) 776 & 529 \(\pm\) 65 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Critical fragment length and debonding length values for the different single yarn composites. Figure 8: Weibull distribution plots of strength measured for neat flax yarns and enzymatically modified flax yarns for a duration of 4.5 h and 6 h.  
incompatibly between the highly hydrophilic flax yarns and the fully dispersive vinylester resin used in the present study. Another parameter usually reported in interfacial studies is the interfacial shear strength (IFSS), which was calculated in accordance with the Kelly and Tyson model [33]. The strength value at the gauge length was obtained through the extrapolation of the Weibull cumulative distribution function [31], and the corresponding values are summarized in Table 7. By comparing the results found after the enzymatic treatment with those obtained for the as-received flax yarns, it is possible to highlight that a reduction in the IFSS values was produced after both surface modification treatments.