## 3 Mechanism and Factors Affecting PET Degradation
Figure 3 presents the types of degradation mechanisms along with some of their products. Figure 3: PET degradation mechanisms and some of their products. The degradation of PET in the natural environment is influenced by multiple factors, including UV radiation, temperature, oxygen, humidity, and mechanical damage. These factors do not act independently; instead, they accelerate one another, leading to the simultaneous occurrence of different degradation mechanisms. PET is known to be relatively resistant to hydrolysis and even more to thermo-oxidative and thermal degradation, which typically does not occur under natural conditions. However, exposure to certain environmental factors can significantly increase the degradation rate [79; 87]. The most important factor influencing PET degradation is UV radiation, as its presence significantly enhances the process. [PERSON] et al. [83] showed that PET that is exposed to UV light generates significantly more microplastic fibers (MPFs) than PET that is kept in the dark. PET strongly absorbs short wavelengths of UV light compared to longer wavelengths. According to [PERSON] et al. [26], this polymer shows high absorbance at \(\lambda\) = 302 nm. Radiation at this wavelength does not penetrate deeper into the plastic; degradation occurs primarily on its surface. In comparison, UV radiation at wavelength \(\lambda\) = 365 nm is less efficiently absorbed. This property makes long-wavelength light able to penetrate the plastic and cause degradation not only on the surface but also deeper within the material. The presence of water is another crucial factor that significantly influences the degradation of PET. A high water content accelerates the degradation process of the material [25; 84; 28]. In a low-humidity environment, PET that undergoes photodegradation does not show any visible signs of damage, such as yellowing or loss of transparency; there are only minimal changes in its chemical composition [28]. In wet conditions, PET degradation undergoes intense degradation. Moisture leads to the hydrolysis of PET, resulting in the creation of TPA and EG as main products [28; 84]. It was proven that PET is not resistant to alkaline solutions. The study of [PERSON] et al. [82] shows that PET particles would degrade when extracted using solutions of bases (potassium hydroxide and sodium hydroxide). The strongest degradation effect was observed for a concentrated NaOH solution. On the other hand, alkaline conditions do not have a great impact on the deeper parts of PET plastics, as degradation with NaOH solution is confined to the polymer surface [88]. The way plastics are exposed to water, such as constant submergence or wetting by sprinkling, influences the mechanisms and types of surface defects. Different surface changes are observed depending on the form of PET plastic and its production history [25]. As PET degrades over time, its surface contact angle decreases. This change is expected as the carboxyl groups, which are hydrophilic, are being created by exposure to UV radiation [26]. Interestingly, at a certain point during degradation, the contact angle increases instead of continuing to decrease. The roughness and heterogeneity of the degraded surface proved to be the cause of this change. The temperature at which degradation takes place influences the rate of the process. A study on degradation at different temperatures (30, 40, and 50 \({}^{\circ}\)C) conducted by [PERSON] et al. [28] has shown that degradation at 50 \({}^{\circ}\)C initiates photodegradation faster. The study has proven that the creation of carboxylic acid is much faster at 50 \({}^{\circ}\)C than at 40 \({}^{\circ}\)C and 30 \({}^{\circ}\)C. The work of [PERSON] et al. [84] has shown that higher temperatures promote PET hydrolysis to TPA and EG. In this case study, increased temperature to 350 \({}^{\circ}\)C also promotes decarboxylation of TPA, leading to the creation of benzoic acid and the degradation of EG to acetaldehyde.