## 3 Results and Discussions  
Figure 2 illustrates the color change of an XLPE sample when heated. Without exposure to heat, XLPE is typically colorless or white. However, it was confirmed that XLPE undergoes a color change when exposed to heat. The sample exposed to 100 \({}^{\circ}\)C acquired a bright yellow color, while that exposed to 120 \({}^{\circ}\)C showed a darker yellow color. When exposed to 140 \({}^{\circ}\)C for an extended period of time, the XLPE changed color from orange and red to brown. The sample exposed to 160 \({}^{\circ}\)C turned briefly red before gradually changing to a dark brown color. The differences in the morphologies of unaged and deteriorated XLPE were investigated using SEM, as shown in Figure 3. In Figure 3a, the unaged XLPE was confirmed to have a smooth surface. However, when exposed to heat, cracks were observed to form on the surface. In addition, it was confirmed that the XLPE surface changed to fine particles when aged at high temperatures for a long period of time, and micro  
Figure 2: Discoloration of XLPE samples depending on aging time and temperature. The compositions of XLPE were analyzed using SEM EDX, and the results are shown in Table 1. The majority of XLPE is composed of carbon. However, it has been found that the oxygen content in XLPE increases as it ages because of oxygen and heat. And in XLPE after 60 days of degradation at 160 \({}^{\circ}\)C, the atomic percentage was confirmed to increase to about 11.3%. Figure 3: SEM images of XLPE samples depending on aging time and temperature: (**a**) unaged XLPE, (**b**) 100 \({}^{\circ}\)C, (**c**) 120 \({}^{\circ}\)C, (**d**) 140 \({}^{\circ}\)C, and (**e**) 160 \({}^{\circ}\)C.  
Figure 4 shows the tensile strength measurement results of the deteriorated XLPE. The tensile strength of the unheated XLPE sample was 22.36 MPa, which decreased gradually with exposure to heat, accelerating as the temperature increased. After 60 days of exposure, the tensile strengths at 100, 120, 140, and 160 \({}^{\circ}\)C were 19.08, 17.62, 7.00, and 4.24 MPa, respectively. This experiment confirmed that the XLPE insulating material deteriorated when exposed to heat, resulting in a decrease in mechanical strength. Figure 5 depicts the XRD patterns of the new and deteriorated XLPE. Since XLPE is a crystalline polymer, its crystal peak can be confirmed through XRD. The XLPE peaks were observed at 20 = 23.13\({}^{\circ}\) and 24.45\({}^{\circ}\) and assigned to the (110) and (200) planes, respectively [22]. As deterioration progressed, the intensity of this crystal peak decreased. To quantify this pattern, the degree of crystallinity (%) was calculated via XRD using the following equation [23]:  
\[x_{XRD}(\%)=\frac{area\ 2+area\ 3}{area\ 1+area\ 2+area\ 3} \tag{3}\]  
where \(x_{XRD}\) is the degree of crystallinity (%); \(area\ 1\), \(area\ 2\), and \(area\ 3\) are the peak areas of the amorphous portion, (110) plane, and (200) plane, respectively. New XLPE was confirmed to have a degree of crystallinity (%) of approximately 37.14%. In addition, samples exposed to heat showed an overall decrease in the degree of crystallinity. The degree of crystallinity decreased and then slightly increased at both 100 and 120 \({}^{\circ}\)C (relatively low temperatures). This phenomenon was due to the additional crystallization of uncrystallized XLPE molecules by heat exposure. However, XLPE exposed to temperatures of 140 and \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Atomic Percent** & **Unaged** & **100 \({}^{\circ}\)C,** & **120 \({}^{\circ}\)C,** & **140 \({}^{\circ}\)C,** & **160 \({}^{\circ}\)C,** \\
**(\%)** & **Unaged** & **60 Days** & **60 Days** & **60 Days** & **60 Days** \\ \hline C & 99.6 & 97.1 & 97.4 & 95.5 & 84.7 \\ N & 0.0 & 1.7 & 1.4 & 1.0 & 4.0 \\ O & 0.3 & 1.2 & 1.2 & 3.5 & 11.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Atomic ratio via SEM EDX analysis. Figure 4: Tensile strength of XLPE samples depending on aging time and temperature.