## 3 Mechanism and Factors Affecting PET Degradation  
According to IUPAC, degradation of polymer materials is an alteration in the chemical structure of these materials that results in unwanted changes of their properties [75]. These changes may include a change in average molecular weight [25], color and transparency (plastics become yellow and opaque), as well as mechanical properties (polymers become brittle) [28]. Degradation, as shown in Figure 3, can be classified based on the factors that cause it [24]:  
* Thermal degradation (factor: temperature),
* Photo-degradation (UV radiation),
* Hydrolytic degradation (water),
* Chemical degradation (chemical substances),
* Mechanical degradation (mechanical factors and stress),
* Biochemical degradation (enzymes or microorganisms),
* Combination of factors above. Thermal degradation induced by high temperature in the absence of oxygen can be observed only in a controlled environment. The process primarily begins at the methylene group in the \(\beta\)-position relative to the carbonyl group, which serves as the primary site for \(\beta\)-scission. This reaction mainly leads to random scission of ester linkages, resulting in the formation of carbonyl-terminated units and carboxylate units terminated with a vinyl group. Intramolecular hydrogen shift leads to the formation of vinyl alcohol, which is unstable and rapidly converts into acetaldehyde through tautomerization. Meanwhile, intermolecular reaction results in the formation of ethylene. Additionally, various other degradation products have been identified, including CO, CO\({}_{2}\), aliphatic aldehydes, tetrahydrofuran, and unsaturated and aromatic hydrocarbons such as butadiene, benzene, and biphenyl [76; 77]. It is worth mentioning that [PERSON] [78] states that the thermal degradation rate is insensitive to moisture content, whereas the hydrolysis rate depends on it. Figure 2: Contamination pathways (degree I to IV) by which harmful substances (e.g., microplastics) can enter the human body. In the presence of oxygen and at elevated temperature, thermo-oxidative degradation occurs, known as Bolland's cycle. It begins with the formation of side groups in the form of hydroperoxides on the diester linkage of the polyester chain, which is initiated by the extraction of hydrogen by oxygen. Next, the hydroperoxides break down, leading to the formation of macroradicals [24; 79]. Photodegradation of PET plastics is initiated by ultraviolet (UV) light. Many studies have proven that photodegradation of PET occurs through several major mechanisms: Norrish type I, Norrish type II, and Photo-Fries reactions [24; 25; 26; 27; 28; 80; 81; 82; 83]. A Norrish type I reaction is a reaction involving radicals in a degradation process. The radicals are created around the ester bond of the PET polymer chain. These radicals are capable of recombination, hydrogen abstraction, or reaction with oxygen [27]. During a Norrish type II reaction, a cyclic intermediate product is created via intramolecular rearrangement of \(\gamma\)-hydrogen. This unstable product breaks down, which leads to the creation of polymer chains ended with carboxyl groups and alkenes. The photodegradation process can also cause cross-linking, which affects the material's structure. The creation of vinyl ester groups can further promote cross-linking, leading to the formation of polyenes and other colored compounds, causing PET to turn yellow [24]. PET is able to undergo hydrolytic degradation in the presence of water at high temperature and pressure. Hydrolysis leads to the formation of terephthalic acid (TPA) and ethylene glycol (EG). Further transformations may result in the formation of benzoic acid from TPA and acetaldehyde from EG [84]. There are many products of PET chain degradation, including carboxylic acids, aldehydes, formates, aliphatic alcohol ending groups, quinones, dimers, anhydrides, mono- and dihydroxyl PET derivatives, CO and CO\({}_{2}\), terephthalic acid, and ethylene glycol [26; 27; 80; 84; 85; 86].