## 3 Results
### SLS 3D Printing of PA12-Based Materials Containing Recycled TR
Therefore, particle loading higher than 50% is often related to unsatisfying physicochemical characteristics. The general tendency is that the tensile and the impact strength performances are deteriorated with the addition of the rubber particles and even more with the increase in average particle size. This is often accompanied by poor interfacial adhesion between the polymer matrix and the rubber domains, leading to both high interface tension and failure cracks. This was attested in several thermoplastic matrices containing recycled TR powders. For example, a clear decrease in the tensile modulus and elongation at break up to 93% in HDPE blends [25]. The same material behavior was found while using recycled HDPE containing 50 wt% TR [26], in PP containing 30 wt% TR [20], and in EVA containing 20 wt% TR [22]. The stress-strain curves of the developed PA12(/TR) samples are presented in Figure 5A. It was found that Young's modulus of the PA12 specimens was 1270 \(\pm\) 71 MPa with 7% elongation at break (Table 2). The incorporation of the TR powder, up to 20 wt%, did not lead to a significant deterioration of the composite mechanical properties. In fact, both Young's modulus and elongation at break remained close in values to those of the pristine PA12 in the case of TR loading of 10 and 20 wt%. The Young's modulus values were: 1280 \(\pm\) 12 MPa and 1210 \(\pm\) 20 MPa for PA12/TR10, PA12/TR20, respectively. Lower values were obtained in the case of the PA12/TR30-1110 \(\pm\) 100 MPa. A slight decrease in the strain at break values was recorded at 5.8 \(\pm\) 0.3%, 5.0 \(\pm\) 0.2%, and 5.4 \(\pm\) 0.2% for PA12/TR10, PA12/TR20, and for PA12/TR30 as compared to unfilled PA12. The presence of tire phase also induced some decrease in the stress at yield from 37 MPa for pristine PA12 down to 30 MPa, 27.5 MPa, and 23.5 MPa for 10, 20, and 30 wt% of loaded TR, respectively. This effect was explained by the softness of the tire powder, which reduces the stiffness of the printed composites. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Young’s Modulus** & \multirow{2}{*}{**Stress at Yield (MPa)**} & **Elongation at Break** & **Impact Resistance** \\  & **(MPa)** & & **(\%)** & **(kJ/m\({}^{2}\))** \\ \hline PA12 & 1270 \(\pm\) 71 & 36.7 \(\pm\) 0.3 & 7.0 \(\pm\) 0.3 & 2.3 \(\pm\) 0.2 \\ PA12/TR10 & 1280 \(\pm\) 12 & 30.0 \(\pm\) 1.0 & 5.8 \(\pm\) 0.3 & 2.9 \(\pm\) 0.2 \\ PA12/TR20 & 1210 \(\pm\) 20 & 27.5 \(\pm\) 1.5 & 5.0 \(\pm\) 0.2 & 2.7 \(\pm\) 0.2 \\ PA12/TR30 & 1110 \(\pm\) 100 & 23.5 \(\pm\) 2.3 & 5.4 \(\pm\) 0.2 & 3.0 \(\pm\) 0.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Young’s modulus, elongation at break and impact resistance values of PA12(/TR) printed materials. Figure 5: Stress-strain curves of PA12 and PA12/TR samples (**A**) and digital images before and after tensile and impact strength tests (**B**). The positive effect of the soft rubber particles is also verified on the composite impact properties. In fact, the incorporation of TR in PA12 matrix did not induce any fall of the composite impact resistance that is in the opposite slightly increased: from 2.3 \(\pm\) 0.2 kJ/m\({}^{2}\) for the PA12 sample up to 3.0 \(\pm\) 0.5 kJ/m\({}^{2}\) when 30 wt% of TR is incorporated (Table 2). Digital images of the printed samples before and after the tensile and impact strength tests are presented in Figure 5B. This result is in suitable accordance with the literature that support similar effects. For example, recycled TR was incorporated in HDPE, where a clear increase in the impact strength was obtained at TR content up to 50 wt% depending on the type of the used vulcanization agent (sulfur or peroxide), or to the addition of a compatibilizer [27]. However, in this system, the tensile strength was deteriorated (ca. 70 wt%) with the increase in the rubber content. The same tendency was reported after adding recycled and grounded TR to PP, where the higher impact strength was found for the sample after the dynamic sulfur vulcanization of the rubber [23].