## 3 Results and Discussion
### Thermal Properties  
The thermal stability at high temperatures of FF, BioEP resin, and different BioEP/FF composites was obtained by thermogravimetric analysis (TGA). The temperature effect on the mass of each sample is shown in Table 2, while Figure 4 shows the corresponding TGA and DTG curves for FF, BioEP resin and BioEP/FF with varying CFF and FFF content from 10 to 40 wt%. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Code** & \(\mathbf{T_{0}}^{1}\)**(\({}^{\circ}\)**C**) & \(\mathbf{T_{\text{max}}}\)(\({}^{\circ}\)**C**) & **wt\% Residual Mass** \\ \hline BioEP & 306.0 \(\pm\) 2.1 & 327.5 \(\pm\) 1.7 & 8.5 \(\pm\) 0.3 \\ BioEP\_10 CF & 292.7 \(\pm\) 2.8 & 322.2 \(\pm\) 2.5 & 12.9 \(\pm\) 0.4 \\ BioEP\_20 CFF & 273.7 \(\pm\) 1.9 & 322.2 \(\pm\) 1.9 & 13.5 \(\pm\) 0.5 \\ BioEP\_30 CFF & 268.0 \(\pm\) 3.1 & 322.0 \(\pm\) 2.5 & 15.0 \(\pm\) 0.6 \\ BioEP\_40 CFF & 249.2 \(\pm\) 2.0 & 323.2 \(\pm\) 1.4 & 18.7 \(\pm\) 0.6 \\ BioEP\_10 FFF & 279.2 \(\pm\) 1.0 & 322.2 \(\pm\) 1.8 & 10.1 \(\pm\) 0.5 \\ BioEP\_20 FFF & 270.1 \(\pm\) 2.4 & 323.0 \(\pm\) 3.2 & 12.7 \(\pm\) 0.4 \\ BioEP\_30 FFF & 264.0 \(\pm\) 1.6 & 321.1 \(\pm\) 1.8 & 15.7 \(\pm\) 0.3 \\ BioEP\_40 FFF & 244.2 \(\pm\) 2.2 & 319.0 \(\pm\) 2.1 & 20.3 \(\pm\) 0.4 \\ \hline \hline \end{tabular} \({}^{1}\)T\({}_{0}\) calculated at 5% mass loss.  
\end{table} 
Table 2: Thermal parameters of BioEP and BioEP/FF composites reinforced with different contents of coarse (CFF) and fine (FFF) flaxseed flour, obtained by thermogravimetry (TGA). As shown in Figure 4a, the FF showed four typical degradation stages of lignocellulosic materials. In the first stage, produced at 50-150 \({}^{\circ}\)C, the moisture contained in the material evaporated, which was reflected by a mass loss of around 2.3 wt% [64]. In the second stage thermal depolymerization of hemicelluloses took place in the temperature range from 150 to 375 \({}^{\circ}\)C with a weight loss of about 41.4% [65]. In the third stage, which was located at 375-450 \({}^{\circ}\)C, cellulose degradation occurred, this stage being observed by a weight loss of 18.9% [61]. Finally, the lignin degradation was detected, which begins at around 250 \({}^{\circ}\)C, but due to its complex structure, it degraded more slowly, producing a progressive weight loss up to 500 \({}^{\circ}\)C. Its decomposition being overlapped with that of other compounds [14]. As shown in the TGA graph, the residual mass of FF was high, which can be due to its high mineral content [66]. On the other hand, as can be seen in Table 2, the thermal degradation of the BioEP resin occurred in a single step, starting its degradation (\(T_{0}\)) around 306 \({}^{\circ}\)C and with a maximum degradation temperature (\(T_{\max}\)) of 327.5 \({}^{\circ}\)C. Regarding BioEP/FF composites degradation, it can be seen that the FF filler addition to the matrix results in a slight decrease of the composites' thermal stability, which is reflected by a \(T_{0}\) decrease. This is due to the low thermal stability of the lignocellulosic reinforcement, whose degradation onset temperature begins around 234 \({}^{\circ}\)C, which affects the overall thermal stability of composites negatively. Besides, the increased filler content in composites results in a reduced weight fraction of the BioEP resin, causing a more significant decrease in \(T_{0}\) as the filler content increases [20]. Comparing the two types of composites obtained according to the filler size, it can be seen that the thermal stability was slightly lower, i.e., lower \(T_{0}\), for composites with different FFF contents. This lower thermal stability was more evident for composites with 10 wt% FFF, where a decrease in \(T_{0}\) of 27 \({}^{\circ}\)C compared to the \(T_{0}\) of the BioEP resin can be observed, while this decrease is only 13 \({}^{\circ}\)C for the same composite containing 10 wt% EFF. By observing the DTG curves (Figure 4b) it can be seen that the thermal degradation of BioEP/FF composites takes place in two stages.