# Experimental Tests on Solvent Solutions for the Recycling of Coated Polypropylene in Food Packaging
## 3 Results and Discussion
### ATR-IR Spectrophotometer  
In order to gain a higher precision level on the alteration of the thin surface layer, especially for the lacquered sample, another analysis was carried out on the specimens using the ATR spectrophotometer for the measurements of the absorbance of the coated surface, for which the thickness is roughly 1 \(\upmu\)m.  
Figure 4: Transmissivity and reflectance spectra in the solar and the long-wave infrared of the AC specimen. Figure 5: Transmissivity and reflectance spectra in the solar and the long-wave infrared of the AC_SMET specimen. Depending on the composition of the specimens' surface layers, absorbance graphs with typical peaks were chosen. The polypropylene films are characterized by several absorbance peaks (Figure 6) attributable to vibrational modes of different functional groups [27]. Table 2 shows the wave numbers and vibrational modes of chemical bonds corresponding to the absorbance peaks of polypropylene. Thanks to this characterization of the polypropylene composition, it is possible to compare the resulting curves to assess the results of the effectiveness of the solvents used for the surface treatments. \begin{table}
\begin{tabular}{c c} \hline
**Wavenumber (cm\({}^{-1}\))** & **Bond Vibrational Mode** \\ \hline
839 & CH(CH\({}_{2}\)) Rocking \\ \hline
972 & CH(CH\({}_{3}\)) Rocking \\ \hline
997 & CH(CH\({}_{3}\)) Rocking \\ \hline
1169 & C-C Stretching \\ \hline
1375 & CH\({}_{3}\) bending \\ \hline
1452 & CH\({}_{2}\) bending \\ \hline
2839 & CH\({}_{2}\) Symmetrical stretching \\ \hline
2870 & CH\({}_{2}\), CH Symmetrical stretching \\ \hline
2918 & CH\({}_{3}\) Asymmetrical stretching \\ \hline
2951 & CH\({}_{3}\) Asymmetrical stretching \\ \hline \end{tabular}
\end{table} 
Table 2: Identification of functional groups’ vibrational movements corresponding to the absorbance peaks in the BF specimen. Figure 6: ATR absorbance spectrum of the BF specimen. Labels in the figure indicate the wavenumber of polypropylene characteristic peaks. Therefore, the approach used refers to a comparison of the spectrophotometric analysis of metallized films' specimens before the addition of the non-polymeric layer (BF), after the addition (AC and ACRC), and after a selective dissolution treatment (AC_SH2O, AC_SKOH, AC_SMET, ACRC_SH2O, ACRC_SKOH, and ACRC_SMET). Deionized water was used as the control treatment. The spectrum obtained from the ATR analysis of the film specimens treated with KOH's 1M solution (AC_SKOH) shows that the surface layer has the same chemical characteristics as the polypropylene base film (BF). In fact, similar absorbance values were found in correspondence to the characteristic peaks of polypropylene previously observed (Table 3, Figures 6-8). Likewise, similar absorbance values were found for the untreated metallized film specimen (AC) and the film specimen treated by selective dissolution in deionized water (AC_H2O) (Table 3, Figures 7 and 8). Water treatment was used as the control, and the results obtained suggest that the action of mechanical stirring alone is not sufficient to alter the aluminum layer. The film specimens treated with \(\geq\)99.9% methanol (AC_SMET) have a spectrum profile similar to that of the metallized film but with different absorbance values (Table 3, Figures 7 and 8). Indeed, as can be observed from Figure 9B, the surface layer remains covered with aluminum which, according to the visual analysis, is only slightly abraded. From the visual observation of the treated films and the analysis of the spectra obtained through ATR-FTIR, KOH's solution seems to be the one that brings the metallized film back to a more similar state to that of the base film. Applying the Pearson correlation index [28] to the measured absorbance values for the various spectra, the coefficients shown in Table 4 were obtained.