## 3 Results and Discussion
### Analysis of PET Degradation  
#### 3.2.1 Fourier Transform Infrared Spectroscopy Results  
In this analysis, PET samples from the olive oils subjected to different temperature treatments and storage times were used. The interpretation of the spectra results was carried out using the OPUS 8.7.31 software, and the absorption peaks of the characteristic FT-IR spectra of PET were described according to the various authors listed in Table 7. This identification and description were common for all the olive oils analyzed in Experiments A and B. In addition to the individual identification of the peaks described above, a set of absorption bands was observed that as a consequence of the thermal treatments applied to the samples, caused changes in the PET structure (Table 8). The PET samples were subjected to thermal treatments at 40 \({}^{\circ}\)C and 60 \({}^{\circ}\)C. During the thermal induction period, material degradation may have occurred, as one of the main causes is the presence of heat [61]. Generally, the synthesis of PET takes place at high temperatures, under vacuum conditions, and in the presence of effective catalytic conditions. As the temperature of PET increases, it degrades more easily [62]. Initially, the degradation process due to thermal action occurs in the cleavage of the ester bond chain, forming terminal carboxyl groups and a vinyl ester. The transesterification of the latter results in vinyl alcohol, which quickly converts into acetaldehyde. This process causes the polyester chain to regenerate, maintaining an average polymerization. As a result of this reaction, the terminal hydroxyl groups are replaced by terminal carboxyl \begin{table}
\begin{tabular}{c c c} \hline \hline
**Wavenumber (cm\({}^{-1}\))** & **Functional Group Description** & **Reference** \\ \hline
1714 & Stretching of the C=O bond of the carbonyl group & [46; 47; 48; 49] \\
1338 & Stretching of the C–O bond and bonds of the ethylene glycol compound & [37; 50] \\
1242 & Associated with terephthalate (OOCC\({}_{6}\)H\({}_{4}\)–COO) & [37] \\
1096 & Symmetric vibration of the ester bond and methylene group (CH\({}_{2}\)) & [51; 52] \\
1017 & Stretching of the C–H bond and aromatic rings & [53; 54; 55] \\
873 & Associated with aromatic rings & [52; 53; 54; 55; 56; 57; 58] \\
724 & C–H bond rocking movements & [58] \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Characteristic PET wavenumbers found in the literature. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Spectral Band (cm\({}^{-1}\))** & **Functional Group Description** & **Reference** \\ \hline
3200–2850 & Symmetric and asymmetric stretching of the methylene group (CH\({}_{2}\)) & [58] \\
1850–1600 & Stretching of the C=O bond and appearance of the carbonyl group & [59] \\
1450–1350 & Presence of alkenes & [59] \\
1300–1200 & Stretching of the C(=O)–O bond and the presence of terephthalate & [50] \\
1095–1040 & Symmetric and asymmetric stretching of the C–O–C bond and the presence of & [52; 60] \\
900–800 & Stretching of the C–H bond and the presence of aromatic rings & [53; 55] \\
780–650 & Out-of-plane bending and the presence of aromatic rings & [58] \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Spectral bands of PET identified as susceptible to changes after the applied treatments. groups, producing the same amount of acetaldehyde. Additionally, due to the generation of macroradical areas by hydrogen atom abstraction, thermally and photochemically unstable peroxide and hydroperoxide radicals may appear, leading to further degradation [63]. The absorption bands and the most notable peaks in the spectral range of 400-4000 cm\({}^{-1}\) described in Tables 7 and 8 are consistent across all PET containers analyzed for O04 and O1, confirming that the containers are made of this material. The structure is represented only once for the olive oils in Figure 2, which corresponds to the superposition of spectra resulting from the treatments applied to O04 in Experiment A.