## 3 Results and Discussions
### Dynamical Mechanical Analysis (DMA)
Instead, the addition of SB in the ELO-based matrix decreases the value of \(E^{\prime}\) in this region, with the SB particles giving a plasticizing effect to the composite material. The damping factor (tan \(\delta\)) so-called also \(\alpha\) relaxation (cooperative motions of main chain) of the systems with HC and SB were investigated and are represented in Figure 6. The maximum values of the tan \(\delta\) peaks, which are associated with the macroscopic glass transition (\(T_{g}\)), were recorded from DMA analysis, and the obtained data are collected in Table 2. The tan \(\delta\) of the EDI resin is ~42 \({}^{\circ}\)C, while the addition of 1 wt.% HC decreased the tan \(\delta\) to ~24 \({}^{\circ}\)C. By increasing the HC%, the tan \(\delta\) values increase, and this became superior to that of neat resin at \(>\)15 wt.% HC (tan \(\delta\) = 43 \({}^{\circ}\)C), presenting the maximum value for the system with 20 wt.% HC (tan \(\delta\) = 45 \({}^{\circ}\)C). In contrast, the addition of SB has an opposite effect on the tan \(\delta\) values: the maximum value of the damping factor is reached for the composite with 1 wt.% SB; thereafter, tan \(\delta\) values gradually decrease with SB% increasing, with the lowest value being ~37 \({}^{\circ}\)C for the formulation with 20 wt.% SB. The hardness of the samples was studied by using the Shore hardness tests with a Shore D device. The hardness value of the EDI resin is ~63 SD, a value indicating a medium hard material. The addition of HC and SB produces an increase in the composites' hardness, with the composites showing values ranging between 65 and 85 SD for HC-based materials and between 64 and 84 SD for SB-based materials. Figure 5: Storage modulus (\(E^{\prime}\)) vs. temperature curves of the (**a**) HC-based samples and (**b**) SB-based samples. The mechanical properties of the composites' materials are influenced by various factors, such as the manufacturing and processing parameters, the filler nature and content, the polymeric matrix, the compatibility between the matrix and filler, etc. [49]. From Figure 7, we can observe that the EDI resin has good mechanical properties, showing a tensile stress value of about 10 MPa and a very high elongation at break of about 62%. With the addition of the filler, an increase in the tensile stress values can be observed only up to a certain load, such as for materials with 5 and 10 wt.% HC (\(\sim\)12-14 MPa) and for the ones filled with 5 wt.% SB (\(\sim\)11 MPa). For the systems filled with 15-20 wt.% HC and 10-20 wt.% SB, the tensile stress properties slightly decreased, varying between 8.6 and 9.2 MPa for HC-based samples and between 8.9 and 9.7 MPa for SB-based samples. The elongation at break of the materials also decreased considerably with the increase of the filler amounts (10-16% for HC systems; 20-23% for SB systems), thus giving rigidity to the samples. The stiffness of the developed materials, also called Young's modulus, is defined by the relationship between stress and deformation in the regime of linear elasticity of a uniaxial deformation, and the obtained values are tabulated in Table 3. Based on the obtained data, it can be seen that the natural filler addition improved the Young's modulus values. For example, the stiffness value for the EDI resin is about 180 MPa, while the addition of 15 wt.% HC led to an increase to the Young's modulus value to about 300 MPa. The addition of spruce bark powder also led to the improvement of the mechanical properties, with 20 wt.% SB improving the stiffness of the final materials to about 270 MPa. Based on the data obtained from tensile test an DMA analyses, the brittleness (\(B\)) of the designed materials was also calculated using the equation:  
\[B=1/\varepsilon_{b}E^{\prime} \tag{5}\]  
where \(\varepsilon_{b}\) is the elongation at break obtained by tensile testing and \(E^{\prime}\) represents the storage modulus value at 25 \({}^{\circ}\)C obtained by DMA.