# Toughened Vinyl Ester Resin Reinforced with Natural Flax Fabrics
## 3 Results and Discussion
### Resin Characterization  
3.1.1. Mechanical Results  
In Table 3 is reported the mechanical proprieties of Dion 9400. The samples exhibited brittle behavior with low elongation at break and smooth fracture patterns. The modulus increased with temperature, peaking at 3.02 GPa for D9400-25-90-1, indicating enhanced crosslinking. D9400-25-90-1 also showed the highest ultimate stress, confirming the need for high post-cure temperatures for optimal mechanical properties. Conversely, D9400-25-70-3 showed a slight decrease in ultimate stress, suggesting embrittlement with prolonged exposure. The samples showed low Charpy impact strength values, indicating the necessity to improve their toughness. Temperature positively affected crosslink density, but extended post-curing reduced impact strength. All samples of Dion 9500 showed improved mechanical properties (as it possible to see in Table 4) compared to D9400; indeed, a remarkable stress at break was registered, assuming that a reinforcement mechanism occurred. The D9500-2-25-70-1 sample exhibited the highest average ultimate stress value of 45.76 MPa, indicating that post-curing can enhance material strength. In contrast, D9500-2-25-90-1 and D9500-2-25-70-3 showed lower ultimate stress compared to samples without post-curing, suggesting that high temperatures or prolonged post-curing at intermediate temperatures can lead to excessive crosslinking and material embrittlement. Comparatively, D9500-1-25 demonstrated a significantly lower Young's modulus than D9500-2-25, implying that a higher catalyst concentration (1%) requires elevated temperatures to achieve sufficient crosslink density. Although D9500-1-25-70-3 had the highest modulus among the 1% catalyst samples, it remained lower than D9500-2-25-90-1, with average values of 2.66 GPa and 2.82 GPa, respectively. Table 4 illustrates that D9500-1-25-90-1 \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Resin Identification**} & \multicolumn{3}{c}{**Tensile Tests**} & \multirow{2}{*}{
\begin{tabular}{c} **Chary Impact** \\ **Strength (kJ/m2)** \\ \end{tabular} } \\ \cline{2-3}  & **Stress at Break (MPa)** & & **Young’s Modulus** \\ \hline D9400-25 & \(18.07\pm 1.98\) & \(1.30\pm 0.61\) & \(1.24\pm 0.24\) \\ \hline D9400-25-70-1 & \(23.13\pm 1.23\) & \(2.35\pm 0.14\) & \(2.56\pm 1.33\) \\ \hline D9400-25-70-3 & \(22.95\pm 6.06\) & \(2.71\pm 0.74\) & \(0.98\pm 0.25\) \\ \hline D9400-25-90-1 & \(30.99\pm 4.40\) & \(3.02\pm 0.92\) & \(2.33\pm 0.57\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Dion 9400 mechanical characterization. achieved the highest stress at break, strengthening the argument for the necessity of high temperatures for producing high-quality material. However, excessive post-curing time appears detrimental to material strength, as evidenced by the performance of D9500-1-25-90-1. Regarding impact strength, the highest was observed in D9500-2-25-90-1, with a value of 2.16 kJ/m\({}^{2}\), representing a 55% improvement over D9500-2-25. This enhancement is likely due to increased network density at higher temperatures and potential rubber activation. Moreover, Table 4 illustrates the ability of the material to absorb and dissipate energy under impact. D9500-1-25-90-1 exhibited the best compromise between impact properties and tensile ones. Although D9500-1-25-70-1 had the lowest value, it still outperformed the D9500-2 and D9400 series. This formulation, with a limited catalyst amount, enhances impact strength without requiring high post-curing temperatures, thereby reducing energy consumption during production. The fracture toughness (KIC) of the molded resin samples was also evaluated to measure the material's resistance to brittle fracture in the presence of a crack. The results are summarized in Figure 3. Among their respective groups, D9500-2-25-90-1 and D9500-1-25-90-1 exhibited the highest KIC values, surpassing those reported in the recent literature [35].