# A Review of Degradation and Life Prediction of Polyethylene
## 2 Aging Degradation of Polyethylene
### General Mechanism of Degradation  
Various types of polymers have different degradation mechanisms. The deterioration of polymeric materials may involve multiple degradation pathways at once [67]. Bond fractures in the polymer's backbone are the main method by which they degrade, and these breaks can occur anywhere in the chain or at the ends of the chain due to random generation. A frequently used mechanism in the breakdown of polymers is the chain-break decomposition mechanism. A multi-step free radical chain reaction with the general properties of such reaction mechanisms as initiation, proliferation, branching, and termination is involved in the chain-breaking breakdown process [68].  
Free radicals are produced in both induced reactions--when a chain break happens at a random location in the main chain--and the terminal chain breaks reactions, where such a tiny unit or group is broken at the end of the main chain [59]. The following is the reaction sequence:  
\[\mathrm{R}-\mathrm{H}\overset{heat,light}{\rightarrow}R\cdot+H. \tag{1}\]  
The proliferative process begins with a free radical reaction with oxygen molecules, then produces a peroxide radical, a hydroperoxide group with hydrogen atoms, and finally, a peroxide radical with oxygen molecules [69]. The resulting groups are extremely unstable and readily decompose into renewable free radicals. The following is the reaction sequence:  
\[\mathrm{R}\cdot+\mathrm{O}_{2} \rightarrow R\mathrm{O}O\cdot \tag{2}\] \[\mathrm{R}\mathrm{O}O\cdot+RH \rightarrow R\cdot+R\mathrm{O}OH\] (3) \[\mathrm{R}\mathrm{O}OH \rightarrow \mathrm{R}\mathrm{O}\cdot+\mathrm{O}H\] (4) \[\mathrm{R}\mathrm{O}\cdot+RH \rightarrow R\cdot+ROH \tag{5}\]\[\text{OH}+\text{RH}\rightarrow\text{R}\cdot+H_{2}O \tag{6}\]  
Taking over a hydrogen atom or another atom on a carbon atom next to a radical from another chain is known as a \"termination reaction\". The following is the reaction sequence:  
\[\text{R}\cdot+R\cdot\rightarrow R-R \tag{7}\]  
\[2\text{ROO}\cdot\rightarrow ROOR+O_{2} \tag{8}\]  
\[\text{R}\cdot+ROO\cdot\rightarrow ROOR \tag{9}\]  
\[\text{R}\cdot+RO\cdot\rightarrow ROR \tag{10}\]  
\[\text{HO}\cdot+ROO\cdot\rightarrow ROH+O_{2} \tag{11}\]  
Both biotic and abiotic circumstances, such as photooxygenation, can cause the aforementioned degradation pathways to occur in polymeric materials. From a macro perspective, the biodegradation process can be broken down into three stages. In the first stage, a particular enzyme secreted by microorganisms can lead to the depolymerization of polyethylene molecular chains. In the second stage, the microorganisms absorb the products of the first stage and transform them into the energy they need. In the third stage, the microorganisms use these products to finish their own cellular metabolism and convert them into other compounds [47].  
In conclusion, the degradation of polymers under actual conditions is frequently a combination of various degradation mechanisms because the mechanism of degradation of polymers is quite complex and no one mechanism can fully describe the situation. For our investigation into the service life of polyethylene materials, it is crucial to comprehend the biotic and abiotic causes of degradation.