## 3 Results and Discussions  
### Reactivity Studies by DSC Analyses  
DSC analyses were performed on fresh mixtures to investigate the crosslinking reaction and the influence of the fillers' nature or ratio on reactions' enthalpy and temperature interval. In Figure 1 is represented the evolution of heat flow as a function of temperature for the neat ELO-based thermosetting resin and for its formulations with SB and HC fillers. The obtained DSC data are tabulated in Table 1.  
The epoxy-anhydride crosslinking process is displayed as a broad exothermic peak, with its first peak at around 165 \({}^{\circ}\)C. These thermograms reveal the presence of complex epoxy-anhydride reactions initiated by a tertiary amine [35,36]. The copolymerization reaction of ELO-DDSA starts at around 106 \({}^{\circ}\)C, reaching the maximum reactivity at 194 \({}^{\circ}\)C and finishing at around 280 \({}^{\circ}\)C. Based on the DSC thermograms from Figure 1a,b, it can be seen that the addition of both SB and HC fillers greatly influences the shape of curing reaction curves. The reaction enthalpy of the systems was determined by integrating the area under the exothermic peak and normalizing it to the mass of resin. According to the literature studies [37,38], the \(T_{\text{peak}}\) temperature obtained by DSC analyses is considered a criterion for evaluating the reactivity of the systems; the lower the value of \(T_{\text{peak}}\), the higher the reactivity. Figure 2 displays the evolution of the reaction enthalpy (\(\Delta H\)) and maximum temperature of the reaction (\(T_{\text{peak}}\)) of the systems as a function of the filler's nature and ratio. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{1}{c}{\(T_{\text{peak}}\) (\({}^{\circ}\)C)} & \multirow{2}{*}{**\(\Delta H\) (\(\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{ \mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{    }}}}}}}}}}}}}  
From Figure 1 and Table 1, we can observe firstly that the formulations with the two fillers, SB and HC, react following the EDI system behavior, i.e., with a complex multipeak evolution of the heat flow. In contrast, we can observe that the fillers' presence modifies the reactions' temperature intervals, enthalpy and the amplitude of the first peak at around 160 \({}^{\circ}\)C. With the increase of the amount of HC and SB, the curing reactions start at lower temperatures. For example, if the crosslinking reaction of EDI formulation begins at about 106 \({}^{\circ}\)C, the addition of 30 wt.% natural filler decreases the reaction onset to 52 \({}^{\circ}\)C for the EDI70_HC30 system and 50 \({}^{\circ}\)C for EDI70_SB30. The temperature at which the systems reach a quasi-linear response and the ends of their reactions is not influenced by the HC addition, but with the increase of the spruce bark amount, the \(T_{\text{end}}\) temperature is significantly reduced to 218 \({}^{\circ}\)C for the formulation with 30 wt.% SB. As can be seen in Figure 2, the maximum temperature of the reaction for the systems with low amounts of HC (1, 5 and 10 wt.%) is shifted to higher temperatures (205-210 \({}^{\circ}\)C), but with the increase of HC quantity at 15, 20 and 30 wt.%, a constant decreasing of \(T_{\text{peak}}\) and increasing of reactivity can be observed. For the formulations with spruce bark powder, the maximum temperature of the reaction increased only for the mixture with 1 wt.% filler (\(T_{\text{peak}}\) = 206 \({}^{\circ}\)C), while for the other systems (from 5 to 30 wt.%), \(T_{\text{peak}}\) was shifted to lower temperatures, reaching up to 148 \({}^{\circ}\)C for the formulation with 30 wt.% SB. Based on the DSC data listed in Table 1, the reaction enthalpy of ELO-based resin is about 92 J\(\cdot\)g\({}^{-1}\), while the addition of only 1 wt.% HC doubled the reactivity of the system, reaching values of about 608 J\(\cdot\)g\({}^{-1}\) for the formulation with 30% by weight HC.