## 3 Results and Discussion
### Analysis of PET Degradation
#### 3.2.1 Fourier Transform Infrared Spectroscopy Results
As observed by other authors, the degradation of PET occurs with the increase in temperature, resulting in a decrease in genuine functional groups [55] in favor of the appearance of new functional groups such as ketones, aldehydes, and alkenes, among others [58]. This degradation can be quantified through the carbonyl index, which is a widely used parameter to monitor degradation phenomena. In the case of PET, it allows for calculating the presence of carbonyl groups contained in the degradation products of acetaldehyde and carboxyl generated by this polymer. The carbonyl index is expressed using the following equation [64]:  
\[\text{Carbonylindex}=\frac{\text{Absorbance 1715 cm}^{-1}}{\text{Absorbance 974 cm}^{-1}}\]  
In this determination, the absorption band 1750-1700 cm\({}^{-1}\) is considered since, according to various authors, it is the region in which degradation products such as carbonyl are shown [46; 47; 48; 49]. The most widely used peak is 1715 cm\({}^{-1}\), while 974 cm\({}^{-1}\) is the peak that does not undergo alterations in the degradation processes [65]. Table 9 shows the obtained results for the carbonyl index for O04 and O1 after exposure according to Experiments A and B. In A for the PET O04 samples, there were no significant differences after applying the treatments, with the most extreme mean values found between the control sample and the most severe treatment (Table 9). An analysis of the results showed that there was no clear trend in the increase in the carbonyl index compared to the control sample; however, in the most severe treatment, this index increased by 5.87%. Slight changes were observed in  
Figure 2: Overlay of spectra for mild olive oil samples of Experiment A. Key wavenumbers included. the evolution of the carbonyl index after the application of the treatments. In this case, the influence of temperature on the increase in the carbonyl index occurred starting from T1. Here, the carbonyl index began to rise at the beginning of the less severe treatments, then stabilized and finally increased. For the PET O1 samples, there were significant differences after applying the treatments, with the most extreme mean values found between the control sample and the most severe treatment (T6; Table 9). The analysis of the results showed a slightly increasing trend in the carbonyl index compared to the control sample in all phases of the applied treatments, with the most severe treatment (T6) showing the highest percentage increase at 2.55%. An evolution in the increase in the carbonyl index was observed from the beginning to the end of the treatments. Likewise, the influence of temperature on the increase in the carbonyl index occurred starting from the first phase of the T1 treatment, in which the carbonyl index began to rise slowly. Generally, PET containers of both oils showed an increase in the carbonyl index compared to the control sample in some phases of the applied treatments. PET O04 also showed stages of stabilization or a slight decrease in the carbonyl index, coinciding with the T3 and T5 treatments; however, this \"valley\" was always above the result obtained for the control sample. In this regard, the observations described agree with those described by other authors, which would demonstrate that the degradation is in an initiation stage [55; 66; 67; 68]. Furthermore, it can be observed that PET O04 has the highest carbonyl index during all stages of the process compared to PET O1, including the control sample, except for the T4 treatment, in which a decrease occurs. This fact could indicate better quality in the material, as it is more resistant to the onset of degradation. In B for the PET O04 samples, there were no significant differences after applying the treatments, with the most extreme mean values between the control sample and the most severe treatment (T12; Table 9).