## 3 Results
### SLS 3D Printing of TPU-Based Materials Containing Recycled TR  
Going further in the feasibility of the printing process and the valorization of the recycled TR powder, we explored the interest of TR incorporation in the TPU polymer matrix for its soft and elastomeric characteristics. In this series of specimens, the TR content was also fixed at 10, 20, and 30 wt% with respect to the total polymer content, and the specimens were coded as TPU/TRP10, TPU/TR20, and TPU/TR30. In contrast to the PA12 composites, the thermal stability of TPU/TR-based samples was not impacted by the addition of the elastomeric filler (Figure 7). In fact, TR powder presented higher thermal stability than the pristine TPU (Figure 7). In the case of here studied TPU, a final residue of about 5% was collected and corresponded to black fillers used by Sinterit(r) in order to enhance laser absorption during the printing process. Once more, the residue at the end of the TGA analysis allowed us to evaluate the amount of the incorporated rubber fraction within the produced materials, revealing the successful incorporation of the rubber particles by SLS (Table 3). The TR content was found to be 13 \(\pm\) 0.7 wt%, 20 \(\pm\) 1.2 wt%, and 33.5 \(\pm\) 2 wt% for the TPU/TRP10, TPU/TR20, and TPU/TR30 samples, respectively. The presence of the TR powder in the TPU induced a serious drop in the composite mechanical properties, as presented in Figure 8. In detail, the Young's modulus of the reference TPU sample was 63 \(\pm\) 2 MPa, accompanied with an elongation at a break of 230%, while the incorporation of 30 wt% TR dropped the mechanical properties progressively to 6.3 MPa with an elongation at break of only 55%. In the same as  
Figure 7: TGA thermograms of TPU, TR, and TPU/TR20 printed samples. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multirow{2}{*}{**Td (\({}^{\circ}\)C)**} & \multicolumn{2}{c}{**Td after 5 wt\% Weight**} & \multirow{2}{*}{**Residue at 800 \({}^{\circ}\)C (\%)**} & \multicolumn{2}{c}{**TR Amount in the**} \\  & & **Loss (\({}^{\circ}\)C)** & & **Printed Samples (\%)** \\ \hline TR & 386 and 426 & 308 & 41.2 \(\pm\) 1.3 & n.a. \\ TPU powder & 378 and 408 & 320 & 5.5 \(\pm\) 0.16 & n.a. \\ TPU printed & 363 and 404 & 313 & 5.1 \(\pm\) 0.13 & n.a. \\ TPU/TR10 & 385 & 314 & 10.3 \(\pm\) 0.4 & 13 \(\pm\) 0.7 \\ TPU/TR20 & 376 & 313 & 15.2 \(\pm\) 0.5 & 20 \(\pm\) 1.2 \\ TPU/TR30 & 373 & 312 & 18.5 \(\pm\) 0.9 & 33.5 \(\pm\) 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: TGA analysis of TR, TPU/TR-based samples, and TR content in the printed materials. decreased more than 50%, where the TR content was greater than 10 wt%, as presented in Table 4.  
To understand the reason behind the significant drop in the TPU mechanical properties, we started to evaluate the morphological characteristics of the printed materials. As presented in Figure 9, the composite samples presented suitable dispersion of the rubber particles and a significant degree of interconnected pores. It was found that the printed TPU samples have a degree of porosity of 11%, which increased to 15.4% and even up to 40.2% for TR content of 10 and 20 wt%, respectively (Table 5). Since the volume and the shape of the printed and tested specimens is the same for all compositions, the increase in the