## 3 Results and Discussions  
### Recycled PET Composites' Development and Characterization  
With a significant number of scientific, technical, and common industrial applications, polymers are part of our everyday routines [22]. According to [PERSON], et al. [23], PET, PP, and HDPE are 3 of the 15 most common polymers found in the environment. Their main applications are bottles and gear clothing for PET, rope and bottle caps for PP, and plastic bags, bottles, clothing, and cages for HDPE. In this study, six different materials--based on the above-mentioned polymers, along with Al nanopowder as a reinforcing material--were prepared using an injection-molding machine, their codification and composition being listed in Table 2, together with two basic properties: hydrostatic density (HD) and swelling degree (Q). The polymers had different density values: 0.96-1.45 g/cm\({}^{3}\) for PET, 0.02-0.83 g/cm\({}^{3}\) for PP, and 0.94-0.98 g/cm\({}^{3}\) for HDPE; these values are comparable with those obtained for recycled PET composites, as shown in Table 2. The highest HD of the polymer mixtures (M1_4, M6_4, M11_4) without Al nanopowder was obtained for PET alone [23]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Codification**} & **Recycled** & **PP** & **HDPE(\%)** & **Al Nanopowder** & **HD** & **Q 168 h (\%)** & **Q 1200 h (\%)** \\ \cline{2-7}  & **PET (\%)** & **(\%)** & **HDPE(\%)** & **(\%)** & **(g/cm\({}^{3}\))** & **Q 168 h (\%)** & **Q 1200 h (\%)** \\ \hline M1\_4 & 100 & 0 & 0 & 0 & 1.318 \(\pm\) 0.0004 & 0.22 & 0.65 \\ \hline M2\_4 & 95 & 0 & 0 & 5 & 1.347 \(\pm\) 0.0009 & 0.32 & 0.68 \\ \hline M6\_4 & 70 & 30 & 0 & 0 & 1.186 \(\pm\) 0.0016 & 0.24 & 1.57 \\ \hline M7\_4 & 66.5 & 28.5 & 0 & 5 & 1.395 \(\pm\) 0.2833 & 0.39 & 2.07 \\ \hline M11\_4 & 70 & 0 & 30 & 0 & 1.180 \(\pm\) 0.0004 & 0.25 & 1.01 \\ \hline M12\_4 & 66.6 & 0 & 28.5 & 5 & 1.210 \(\pm\) 0.0000 & 0.41 & 3.80 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Composition, hydrostatic density, and swelling degree of recycled PET composites. Figure 1: DNA identification on recycled PET composites—workflow. The structure of semicrystalline polymers has two phases: thin lamellar crystal layers, and disordered amorphous layers. The amorphous phase is composed of entanglements retained during the crystallization process [24]. Crystallinity is a key property of synthetic polymers, as it is strongly related to their mechanical properties, degradation, and behavior in liquid chemicals. PET, PP, and HDPE are all semicrystalline polymers, with PP and HDPE having 50% crystallinity, while the crystallinity of PET is between 0 and 50%, e.g., for PET beverage bottles it is higher than 30% [25]. The small values obtained for all of the materials, even at 50 days of complete immersion, can be explained by the hydrophobic character of the three polymers, attributed to their semicrystallinity, as also observed in other studies [26; 27]. The values increased over time, as a consequence of the changes in the surface wettability of the polymeric materials immersed in an aqueous medium for an extended period [28]. As can be seen in Table 2, the highest values were obtained for the materials containing Al nanopowder, which created voids in the manufacturing process, facilitating the penetration of water molecules into the materials' structure [28]. Q values can also be correlated with the morphology of the materials, as shown in Figure 2, where we can observe a compact structure without cracks, and Al nanoparticles distributed in the synthetic polymer matrix. In Table 3 are the values obtained from the transformation processes (glass transition, crystallization, and melting) that resulted from the analysis of the DSC variation curves (Figure 3), as a function of temperature. Following the thermal analysis, it was found that all of the studied composite materials can be used at up to 300 \({}^{\circ}\)C without mass loss.