# Experimental Tests on Solvent Solutions for the Recycling of Coated Polypropylene in Food Packaging
## 3 Results and Discussion
### UV-VIS-NIR and LWIR Spectrophotometer  
Both specimen sets were analyzed using the UV-VIS-NIR and the LWIR spectrophotometers to measure transmittance and reflectance values in the wavelength ranges of the solar and LWIR (from 200 to 25,000 nm). Table 1 shows the values of transmittance and reflectance obtained for the basic film (BF), films with a non-polymer layer (AC and ACRC), and films treated with selective dissolution. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Specimens** & \multicolumn{4}{c}{**Radiometric Coefficients**} \\ \hline  & \multicolumn{2}{c}{**Wavelength Range**} & \multicolumn{2}{c}{**Transmissivity**} & **Reflectance** \\ \cline{2-5}  & & **nm** & **\%** & **\%** \\ \hline  & Solar & 200–2500 & 91.0 & 7.7 \\ \cline{2-5}  & VIS & 380–760 & 91.0 & 7.7 \\ \cline{2-5} BF & Solar IR & 760–2500 & 91.0 & 7.7 \\ \cline{2-5}  & UV & 280–380 & 89.7 & 8.7 \\ \cline{2-5}  & LWIR & 2500–25,000 & 85.5 & 7.9 \\ \hline  & Solar & 200–2500 & 0.3 & 91.0 \\ \cline{2-5}  & VIS & 380–760 & 0.3 & 88.3 \\ \cline{2-5} AC & Solar IR & 760–2500 & 0.3 & 93.0 \\ \cline{2-5}  & UV & 280–380 & 0.6 & 91.9 \\ \cline{2-5}  & LWIR & 2500–25,000 & 0.1 & 86.1 \\ \hline  & Solar & 200–2500 & 90.9 & 7.8 \\ \cline{2-5}  & VIS & 380–760 & 90.9 & 7.8 \\ \cline{2-5} AC\_SKOH & Solar IR & 760–2500 & 91.0 & 7.7 \\ \cline{2-5}  & UV & 280–380 & 89.8 & 8.9 \\ \cline{2-5}  & LWIR & 2500–25,000 & 85.4 & 7.8 \\ \hline  & Solar & 200–2500 & 0.5 & 94.0 \\ \cline{2-5}  & VIS & 380–760 & 0.4 & 92.6 \\ \cline{2-5} AC\_SMET & Solar IR & 760–2500 & 0.5 & 95.4 \\ \cline{2-5}  & UV & 280–380 & 0.7 & 97.0 \\ \cline{2-5}  & LWIR & 2500–25,000 & 0.2 & 85.6 \\ \hline \end{tabular}
\end{table} 
Table 1: Transmittance and reflectance values obtained by UV-VIS-NIR and LWIR spectrophotometers. Since the results with ATR showed no significant difference among triplicate repeats, UV-VIS-NIR and LWIR spectrophotometric tests were conducted on one specimen per solution for a total of ten specimens. The LWIR spectrophotometer analyses the radiometric characteristics of the entire thickness of the samples that varies between 18 and 30 \(\upmu\)m. The thickness of the non-polymer surface layer, measured by the company's Quality Control Center, is not superior to 10 nm. For this reason, the presence or lack of a lacquered surface layer does not affect the transmissivity and reflectance values in a manner detectable by the instrument. Instead, since aluminum has values of transmittance and reflectance opposite to the transparent film in pp, its presence/lack can be effectively detected with the LWIR spectrophotometer. It is possible to observe that the transmittance and reflectance values obtained for the basic film specimen (BF) and for the AC_SKOH differ by less than 0.5% in the wavelength range of 200 nm to 25,000 nm. In fact, looking at the graphs in Figures 2 and 3, it is clear that the spectra of the two specimens are comparable. For the specimen AC_SKOH, as for the basic film in polypropylene, there are transmittance values greater than 80% and reflectance values lower than 10%. In contrast, the untreated metallized specimen (AC) and the specimen treated with 99.9% methanol (AC_SMET) both have transmittance values less than 1% and reflectance values greater than 85% in the wavelength range of 200 nm to 25,000 nm (Table 1, Figures 4 and 5). This proves that the aluminum surface layer used to metallize the polypropylene film has been removed by the solvent KOH 1M and did not undergo significant alterations if treated with methanol.