# A Review of Degradation and Life Prediction of Polyethylene
## 1 Introduction  
Polyethylene is one of the most significant and useful polymers that has been extensively studied for use as a plastic material [1; 2; 3; 4; 5; 6; 7; 8]. The benefits of using polyethylene as a commercial plastic material include its excellent mechanical properties, good flexibility, good chemical resistance, lightweight properties, good thermal stability, and high-cost performance [9; 10]. The piping sector has been impacted by the trend of replacing steel with plastic during the past few decades, resulting in the steady replacement of metal-based pipes with plastic pipes. Polyethylene pipes are the most commonly utilized among them [11; 12]. Consider the case of high-density polyethylene pipes. Its market worth was USD 15.975 billion in 2018, and 9.283 million tons were consumed each year. The service life of polyethylene pipes will not be less than 50 years, and it will continue to grow at a rate of at least 5% annually in the upcoming years [13; 14].  
Polyethylene has numerous applications in a wide range of industries, including agriculture [15], manufacturing [16; 17], medicine [18; 19; 20], construction [21; 22], packaging [23], energy [24], outdoor items [25], and others, due to its technical benefits and low cost when compared to other materials. In China alone, the use of agricultural films reached 2.6 million tons in 2016, including 1.47 million tons of polyethylene films, covering 18.4 million hectares of land [26].  
Although polyethylene materials offer great resistance to microbial, peroxide, and degrading damage, they do have a tendency to alter their initial performance characteristicswith time [27; 28; 29; 30]. In general, three frequent and significant occurrences are bound to happen when polyethylene is present in a gaseous or liquid environment for an extended period of time. The first phenomenon demonstrates that the mobility of polyethylene molecules is altered by the diffusion of gas or liquid molecules, just as it is by changes in temperature, pressure, or time, which also have an impact on the material's mechanical properties. The second phenomenon relates to the fact that, due to the extremely adhesive nature of the amorphous phase attached to the crystalline skeleton, the physical aging of polyethylene materials is destined to occur over extended periods in glassy amorphous polymers or semi-crystalline polymers [31; 32]. The third phenomenon demonstrates that polyethylene is subject to aging and degradation processes when exposed to chemically active gases or liquids for extended periods. As a result, brittleness [33], fracture [34], bending [35], and other phenomena may occur, which may shorten the service life of polyethylene products.  
In general, it's critical to comprehend the mechanisms of degradation in addition to the elements that influence how polyethylene materials deteriorate over time. Researchers must investigate polyethylene's deterioration behavior and offer theoretical backing to enhance the material's performance and service life. However, there has not been a study conducted yet that completely explains the degradation mechanism of polyethylene.  
The aging degradation process of polymeric materials is described in this work as being influenced by several common external environmental conditions. First, it enumerates the common ways that polyethylene degrades in the environment and describes the effects that these different elements have on the process. Second, illustrations of the fundamental hypotheses and technical approaches employed in the experimental research carried out thus far are provided. Finally, some thoughts and views are shared along with predictions about polyethylene's future trends.