## 3 Results
### Mechanical Properties of the Biochar-Based Composites  
Addition of nanostructured carbon to an epoxy matrix involves some issues related to the dispersion [34]. As an example, good dispersion of graphene was possible only after derivatization as graphene oxide or reducing its concentration [35; 36]. In this research, composites were prepared dispersing biochars into epoxy monomer using ultrasonication up to 2 wt.%. According to [PERSON] et al. [10], 2 wt.% resulted the best percentage to exploit the beneficial properties of biochar fillers. Dog-bone samples were produced according to the methods described above using a filler concentration of 2 wt.% and analyzed through stress test producing the curves reported in Figure 6 while the main mechanical properties are summarized in Figure 7.  
Neat resin has a Young's modulus (YM) of \(1286\pm 185\) MPa, an ultimate tensile strength (UTS) of \(17.0\pm 1.4\) MPa, a maximum elongation of \(4.08\pm 0.08\)%, and a toughness of \(0.565\pm 0.029\) MJ/m\({}^{3}\). Composites showed two different behaviors leading either to the simultaneous increment of UTS and YM, or to the enhancement of maximum elongation with lesser impact on YM and UTS. OSR and WSP induced a drastic increase of YM (up to \(2731\pm 113\) MPa and \(3074\pm 137\) MPa, respectively) and UTS up to around 33 MPa. Compared with these last two samples, MSP and SWP showed lower YM (\(1669\pm 89\) MPa and \(1456\pm 79\) MPa, respectively) and UTS did not significantly differ from neat resin. Differently, MSP and SWP induced a maximum elongation increment of up to 1.8% compared with the neat resin, while OSR and WSP decreased it to \(2.46\pm 0.08\)% and \(1.90\pm 0.44\)%, respectively. Compared to the properties of OSR or WSP composites, RH composite showed lower improvement of YM of up to  
Figure 5: Biochar particles size distribution of (**a**) RH, (**b**) MSP, (**c**) SWP, (**d**) OSR, and (**e**) WSP after grinding. \(2166\pm 129\) MPa with a comparable maximum elongation of \(2.70\pm 0.14\%\). According with data reported in Table 2, it is possible to regroup the composites in brittle (OSR, WSP, and RH) characterized by both high YM and UTS and ductile (MSP and SWP) with improved elongation. These different behaviors could be ascribed to different interactions between fillers and epoxy matrix. According to FT-IR spectra (Figure 3), all biochars showed similar residual groups, but looking at FES-SEM images (Figure 2), we found clear differences among the various fillers. Furthermore, the particle size distributions were quite close to each other, with exception of RH, to induce such different behaviors. Based on such evidences, we hypothesized the following explanation for the impact of the fillers on the mechanical properties of the composites. Considering SWP and MSP, neat or slightly channeled surface particles (Figure 1a,b,g,h) could move together with the resin under longitudinal stress. This could induce plane rearrangement and a quasi-laminar flow with an increase in ductility. On the other hand, OSR and WSP showed the massive presence of pores that can act as anchoring sites for the polymer matrix creating high tensioned structures. As a consequence, these materials showed lower yield points and plastic deformation range prior to the breakage than neat resin. Composites were analyzed through FE-SEM after fracture as shown in Figure 8. MSP containing composites underwent a delamination process after the break. In this case, resin was detached from the channel structures (Figure 8a,b in white circles). OSR based composites showed a different behavior. As shown in Figure 8c,d, the resin entirely filled the OSR porous structure showing a remarkably continuity on the interphases. In this case, cracks propagated themselves all around the particles while using MSP they moved preferentially along the channeled structures. Additional considerations were required in the case of RH based composites that showed both plane surfaces and spikes and pores.