## 3 Results
### SLS 3D Printing of TPU-Based Materials Containing Recycled TR
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Young’s Modulus** & **Stress at Yield (MPa)** & **Elongation at Break** & **Impact Resistance** \\  & **(MPa)** & **Stress at Yield (MPa)** & **(\%)** & **(kJ/m2)** \\ \hline \multicolumn{5}{c}{Classic printing conditions} \\ \hline TPU & \(63\pm 2\) & \(13\pm 0.9\) & \(228\pm 5\) & \(22.6\pm 2.4\) \\ TPU/TR10 & \(44\pm 1\) & \(8.1\pm 0.5\) & \(148\pm 6\) & \(14.8\pm 1.8\) \\ TPU/TR20 & \(14.5\pm 1\) & \(2.7\pm 0.2\) & \(76\pm 3\) & \(5.0\pm 0.5\) \\ TPU/TR30 & \(6.3\pm 1\) & \(1.2\pm 0.1\) & \(55\pm 3\) & \(2.8\pm 0.2\) \\ \hline \multicolumn{5}{c}{‘Rigid mode’ printing conditions} \\ \hline TPU rigid mode & \(62.5\pm 1\) & \(12.9\pm 2.2\) & \(235\pm 11\) & \(26.6\pm 3\) \\ TPU/TR20 rigid mode & \(31\pm 1\) & \(5.4\pm 0.4\) & \(100\pm 8\) & \(12\pm 1\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Tensile and impact strength tests results of TPU/TR printed materials. Figure 8: Stress-strain curves of TPU(/TR) samples produced by flex (**A**) and rigid (**B**) mode printing. Digital images after tensile and impact strength tests (**C**). material porosity imposed a decrease in the content of material per volume, thus leading to a decrease in the amount of material stressed during the mechanical test. The presence of such voids also plays the role of a structural defect that may readily induce mechanical failure. An improvement in the TPU/TR sample mechanical properties requires additional densification of the material, meaning a decrease in the degree of porosity. Therefore, we focused our attention on the system containing 20 wt% TR powder and prepared a series of samples (pristine TPU and TPU/TR20) using a new printing mode, described as 'rigid mode'. In fact, Sinteriri(r) printing software offers the possibility of increasing laser power while conserving the same printing layer thickness and conditions. As a result, the applied 'rigid mode' enabled us to reduce the porous volume for the TPU/TR20 from 40% to 29% (Table 5). This change improved the material mechanical properties where the Young's modulus values were increased from 14.5 MPa to 31 MPa, accompanied by a respective increase in the elongation at break (from 76% to 100%) and a significant enhancement of the material impact resistance from 5 kJ/m\({}^{2}\) to 12 kJ/m\({}^{2}\). It is important to mention that applying 'rigid mode' did not affect neither the porosity nor the mechanical properties of the pristine TPU. Porosity remained around 12%, while the impact strength increased slightly from ca. 23 to 26 kJ/m\({}^{2}\). In summary, for this section, it came out that SLS as an additive manufacturing approach can also be applied to produce elastomeric TPU-based materials with a high loading ratio of the recycled TR (up to 20 wt%) with a fine surface finish of the samples. In addition to the suitable distribution of the TR within the polymer matrix, a certain degree of porosity was however observed throughout the sample volume. Overall, the amount \begin{table}
\begin{tabular}{c c} \hline \hline
**Sample** & **Porosity (\%)** \\ \hline \hline \multicolumn{2}{c}{Classic printing conditions} \\ \hline TPU 1 & 11.0 \\ TPU/TR10 2 & 15.4 \\ TPU/TR20 3 & 40.2 \\ \hline \multicolumn{2}{c}{‘Rigid mode’ printing conditions} \\ \hline TPU 1 & 12.0 \\ TPU/TR10 2 & n.a. \\ TPU/TR20 3 & 29.1 \\ \hline \hline \end{tabular} The degree of porosity is referenced to: 1 injected TPU; 2 injected TPU containing 10 wt% TR powder; 3 injected TPU containing 20 wt% TR powder.  
\end{table} 
Table 5: TPU-based printed sample porosity. Figure 9: SEM micrographs of TPU/TR20 (TR particles enclosed in orange) (**A**) and TPU (**B**) printed sample. of the incorporated rubber particles and the thermal stability of the samples were not impacted after the printing procedure. The composite materials presented suitable impact strength properties that can be easily tuned with the applied printing conditions.