# Recycled PP for 3D Printing: Material and Processing Optimization through Design of Experiment
## 3 Results  
### Preliminary Characterization of PPt/r-PP Blends: Optimization of the Material Formulation  
The main objective of this work is the evaluation of the suitability of PP derived from mechanical recycling processes for a FFF processing. To this aim, we first tried producing a filament that was fully based on r-PP. Unfortunately, due to the high heterogeneity of the starting material, this attempt resulted in an unsuccessful processing. Therefore, we decided to blend r-PP with different amounts of a first use PP filled with 20 wt.% of talc, that previous studies [16; 26] demonstrated have the proper rheological and thermal properties for an FFF process. Then, the first step concerned the formulation of PPt/r-PP blends, characterized by different weight ratios of the two components, aiming at optimizing the formulation of the material for the subsequent FFF processing. Unfortunately, formulations containing more than 50 wt.% of r-PP resulted in unsuccessful processing since, due to the high heterogeneity of the recycled material, extrudates with irregular morphology were obtained. For this reason, it was decided to keep the amount of r-PP in the blends lower than 50 wt.%. The composition of the formulated blends is reported in Table 3; after the melt compounding step, the rheological and thermal properties of the as-obtained materials were evaluated. Figure 2 reports the complex viscosity curves as a function of frequency for all the formulated blends. In addition, the rheological curves of the two components (i.e., PPt and r-PP) and of unfilled PP are also depicted. Looking at the behavior of PPt and r-PP, that were evaluated either at 230 (Figure 2a) or 260 (Figure 2b) \({}^{\circ}\)C, it is clearly observable that both polymers show a marked non-Newtonian behavior, involving a continuous decrease of the complex viscosity as a function of frequency for the whole investigated frequency range. More specifically, both materials show an apparent yield stress behavior at low frequencies, and a well-developed shear thinning in the high frequency region. Both these features can be attributed to the presence of talc particles in PPt and of some inorganic fillers in r-PP (as demonstrated by TGA analyses reported in Figure S2) which are able to interfere with the relaxation dynamics of PP macromolecules, hence causing the disappearance of the typical Newtonian behavior of PP. The rheological behavior of the PPt/r-PP blends is very similar to those of the two base polymers, and also in this case, significant non-Newtonian features (i.e., yield stress and shear thinning) can be recognized. In particular, looking at the curves that were recorded at 230 \({}^{\circ}\)C and depicted in Figure 2a, it is possible to observe that the blend containing 10 wt.% of r-PP presents a similar behavior to PPt. Otherwise, the blends containing 20 or 30 wt.% of r-PP show complex viscosity trends intermediate between those of the neat components. Finally, the behavior of the systems 40r-PP and 50r-PP is quite close to that of PPt. On the other hand, Figure 2b shows that all the blends exhibit a rheological behavior more similar to that of r-PP. Furthermore, it can be noticed that the non-Newtonian behavior of all the investigated materials is more pronounced at 260 \({}^{\circ}\)C, \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **r-PP Amount** & \(\sigma_{y}\)@230 \({}^{\circ}\)C** & \(\sigma_{y}\)@230 \({}^{\circ}\)C** & \multirow{2}{*}{**Total \(\Delta\)Hm[J/g]**} \\  & **[wt.\%]** & **[Pa]** & **[Pa-s]** & \\ \hline r-PP & 100 & 344 & 608 & 65.5 \\
50r-PP & 50 & 326 & 487 & 67.0 \\
40r-PP & 40 & 368 & 537 & 68.7 \\
30r-PP & 30 & 293 & 520 & 65.1 \\
20r-PP & 20 & 269 & 446 & 66.5 \\
10r-PP & 10 & 182 & 267 & 65.9 \\ PPt & 0 & 135 & 104 & 79.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Composition of PPt/r-PP blends, yield stress values, and total melting enthalpy of the studied materials. Data for base PPt and r-PP are also reported.