## 4 Conclusions  
In this study, XLEP, which is used as the main insulator for underground transmission cables, was artificially exposed to heat and its characteristic changes were analyzed. XLEP changed from colorless to red and subsequently dark brown as deterioration progressed. This phenomenon was due to the red shift of the UV-vis spectra from the ultraviolet region to a longer wavelength. According to FTIR confirmation, the degradation of the C-C or C-H single-bond XLEP structure also resulted in the generation and increase in C=O and C=C bonds. The conjunction also occurred due to the resulting double bond. Thus, the bandgap, which is the interval between HOMO and LUMO, was reduced and could be quantified using Tauc's equation.  
The crystallinity of XLEP was obtained through XRD and DSC analyses, which confirmed that both results decreased similarly as the deterioration progressed. It was also possible to obtain a change in the thickness of the lamellar structure that also decreased with the change in crystallinities.  
Figure 13: Correlation between tensile strength and various factors: (**a**,**b**) degree of crystallinity by XRD and DSC; (**c**,**e**) carbonyl index-1 and -2; (**d**) _Tn; (**f**) _Tc; (**g**) lamellar thickness; (**h**) yellowness index ASTM DI925; (**i**) bandgap; and (**j**) degree of swelling.  
The tensile strength of XLPE also decreased with exposure to heat. Tensile strength was correlated with various property changing factors, such as crystallinity, carbonyl index, yellownness index, \(Tm\), \(Tc\), lamellar thickness, and bandgap. All showed a high linear and nonlinear correlation, and the coefficient of determination of the lamellar thickness, \(R^{2}\), was 0.9250, showing the highest linear correlation. Negative slope tensile strength factors included the carbonyl and yellownness indexes, while those with positive slopes were crystallinity, \(Tm\), \(Tc\), lamellar thickness, and bandgap.  
XLPE has excellent properties as an insulation material, so it is currently used in high voltage power cables and demand is increasing. Additionally, as usage increases, the occurrence of breakdowns is also increasing. The results of this study are expected to be helpful in evaluating the condition of XLPE in power cables dismantled due to failure and to be instrumental in establishing insulation material management standards for determining the stable operation condition of XLPE cables. Additionally, the results of this study are expected to be used as basic data in the development of a diagnostic device for the condition of XLPE insulation in the future.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app131810516/s1](https://www.mdpi.com/article/10.3390/app131810516/s1)]([https://www.mdpi.com/article/10.3390/app131810516/s1](https://www.mdpi.com/article/10.3390/app131810516/s1)), Figure S1: DSC heating thermograms of XLPE samples depending on aging time and temperature: (a) 100 \({}^{\circ}\)C, (b) 120 \({}^{\circ}\)C, (c) 140 \({}^{\circ}\)C, and (d) 160 \({}^{\circ}\)C; Figure S2: DSC cooling thermograms of XLPE samples based on aging time and temperature: (a) 100 \({}^{\circ}\)C, (b) 120 \({}^{\circ}\)C, (c) 140 \({}^{\circ}\)C, and (d) 160 \({}^{\circ}\)C. Table S1: The correlation parameters between tensile strength and factors.  
The authors gratefully acknowledge contributions as follows: B.S.K., T.J., A.-R.K., [PERSON], [PERSON]-K.K., Y.M. and H.P. conceived and designed the experiments; B.S.K. performed the experiments, analyzed the data, and wrote the paper. All authors have read and agreed to the published version of the manuscript.  
This work was funded by the Research Fund for the Korea Electric Power Research Institute.  
Not applicable.  
The data will be shared upon reasonable request.  
The authors declare no conflict of interest.