# A Review of Degradation and Life Prediction of Polyethylene
## 2 Aging Degradation of Polyethylene
### Types of Degradation
#### 2.2.2 Non-Biodegradable  
The abiotic degradation of polyethylene is influenced by environmental and molecular factors. The breakdown of polyethylene is promoted and accelerated by environmental elements such as sunlight's UV radiation, oxygen, heat, water, certain animals, and contaminants. The combined action of these factors may have a synergistic effect on the degradation rate of polyethylene [49]. Photoreactions and thermal oxidation reactions, which result in the creation of new products during chain breakage, hydrogen atom detachment, or cage effects, are the main environmental drivers of polymer degradation [50]. The interaction between oxygen and UV light causes polyethylene to begin to photodegrade. While photodegradation is the process by which molecules produce free radicals, photooxidation is the process by which polymers are destroyed by absorbing photons of visible, \begin{table}
\begin{tabular}{c c c c} \hline \hline _Fungtidae_ & _Fungal Species_ & _Bacteriaceae_ & _Bacterial Species_ \\ \hline \multirow{4}{*}{_Penicillum_} & _Simplicissimum_ & & _Amyloidiquefaciens_ \\  & _Frequentans_ & _Bacillus_ & _Brevies_ \\  & _Pinophilum_ & & _Cereus_ \\ \hline \multirow{2}{*}{_Aspergillus_} & _Versicolor_ & _Streptomyces_ & _Badius_ \\  & _Flawus_ & & _Viridosporus_ \\ \hline _Phaneroache_ & _Chrysosporium_ & _Rahtrella_ & _Aquatilis_ \\ \hline _Verticillium_ & _Leanii_ & _Rhodococcus_ & _Rhodochromus_ \\ \hline _Cladosporium_ & _Cladosporioides_ & _Brevibacillus_ & _Borstelensis_ \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Bacterial_ and _fungal strains_ linked to polyethylene biodegradation [48]. ultraviolet, or infrared light in the presence of oxygen [51]. Random chain breakage and photooxidation are the primary outcomes of photodegradation, and these processes in turn cause secondary crystallization and the creation of several degradation products, including carboxylic acids, ketones, and aldehydes, which are collectively known as carbonyl compounds [52; 53]. The Norrish reaction can result in the synthesis of vinyl groups (such as unsaturated bonds and conjugated systems), and it is crucial to realize that hydroperoxides are byproducts of the free radical formation process [54]. In addition to the breakdown of hydrogen peroxide, Norrish types I and II processes involving ketone groups can also start the photooxidation of polyethylene [55], as shown in Figures 1 and 2. Chain breakage and cross-linking are the primary and secondary outcomes of these three starting processes, respectively. The process by which heat or high temperatures are applied to a material, product, or component and where the outcome is a loss of physical, chemical, or electrical qualities is referred to as \"thermal degradation\" [58]. Free radical chains that are engaged in thermal and photodegradation have fundamentally identical processes. Typically, the degree to which the reaction with oxygen takes place has a significant impact on the mechanism and rate of degradation. The molecular amplification reactions are mostly chain-breaking reactions when oxygen is present [47]. Depending on the physical and chemical makeup of the polymer, for which many thermal degradation mechanisms exist, thermal degradation may cause molecular deterioration. The most frequent is the polymer's intermolecular links being broken or unchained, releasing oligomers and monomer units. Some polymer backbone and side chain reactions will also contribute to the polymer's final decomposition [59]. One of the crucial components of abiotic degradation's parameters is chemical degradation. The characteristics of polyethylene macromolecules may change as a result of reactions with atmospheric contaminants and some agrochemicals. Many materials must come into contact with air when used in daily life, making reactions with oxygen in the air simple. Free radicals are created when the covalent bonds in polyethylene react with the oxygen molecules in the air.