## 3 Results and Discussion
### Analysis of FTIR Spectroscopy  
The characterization of virgin materials by Fourier-transform infrared spectroscopy (FTIR) establishes a reference baseline for interpreting chemical changes due to thermal aging and electrical stress. The primary materials investigated, insulating oils (natural and mineral) and cellulose paper, exhibit distinct spectral signatures that reflect their molecular composition. Natural ester oil, primarily composed of fatty acid esters, is characterized by prominent absorption bands corresponding to ester functional groups (C=O, C-O-C) and aliphatic chains (Table 2). In contrast, mineral oil, derived from petroleum refining, consists mainly of paraffinic, naphthalene, and aromatic hydrocarbons and lacks ester-related features  
Figure 5: Dielectric spectra of the conductivity for fresh and aged paper samples impregnated with different oils, after breakdown. Figure 6: Dielectric spectra of the loss tangent Tan (Delta) for fresh and aged paper samples impregnated with different oils, after breakdown. (Table 3). Cellulosic paper shows characteristic bands associated with O-H and C-H bonds, as well as the carbohydrate backbone of cellulose (Table 4). These reference spectra serve as a basis for identifying degradation phenomena, such as hydrolysis, oxidation, impregnation, or molecular structure loss, in aged or electrically stressed materials. Figures 7-9 present the ATR-FTIR spectra of the fresh and aged samples before and after dielectric breakdown. The examination of the evolution of FTIR spectra of the oils after thermal aging reveals chemical transformations that are specific to each insulating fluid:  
For NE 1204 and NE 1215, a progressive attenuation of the ester carbonyl band (C=O \(\sim\)1740 cm\({}^{-1}\)) occurs alongside the emergence or enhancement of a carboxylic acid band (\(\sim\)1710 cm\({}^{-1}\)), particularly from stage S2 onward. These changes reflect partial hydrolysis of triglycerides [33]. The quantitative analysis revealed that the relative intensity of the 1710 cm\({}^{-1}\) band increases more significantly in NE 1204, reaching a ratio of 0.82 at S3, compared to 0.60 for NE 1215. This finding suggests a more pronounced acid formation in NE 1204. For mineral oil, characteristic oxidation bands in the 1650-1750 cm\({}^{-1}\) range appear as early as S1, with increasing intensity through to S3. It was observed that a peak at 1705 cm\({}^{-1}\), associated with ketones and weak acids, becomes predominant, with a ratio of 1.21 at S3, indicating advanced and early oxidation processes that are not solely moisture-induced. \begin{table}
\begin{tabular}{c c c} \hline
**Wavenumber (cm\({}^{-1}\))** & **Functional Group** & **Vibration Type** \\ \hline \(\sim\)3330 & O–H (cellulose) & Broad stretching (H-bonded hydroxyl) \\ \(\sim\)2900 & Aliphatic C–H & Stretching \\ \(\sim\)1640 & Adsorbed H\({}_{2}\)O & Stretching of bound water \\ \(\sim\)1430 & CH\({}_{2}\) (amorphous cellulose) & Bending \\ \(\sim\)1375 & C–H/CH\({}_{3}\) (carbohydrate backbone) & Bending (symmetric) \\ \(\sim\)1160–1030 & C–O/C–O–C (carbohydrate structure) & Stretching (cellulose polysaccharide network) \\ \hline \end{tabular}
\end{table} 
Table 4: FTIR bands characteristic of cellulose paper [31; 32]. \begin{table}
\begin{tabular}{c c c} \hline
**Wavenumber (cm\({}^{-1}\))** & **Functional Group** & **Vibration Type** \\ \hline \(\sim\)1740 & C=O (ester) & Carbonyl stretching \\ \(\sim\)2920 & Aliphatic CH\({}_{2}\) & Asymmetric stretching \\ \(\sim\)2850 & Aliphatic CH\({}_{2}\) & Symmetric stretching \\ \(\sim\)1465 & CH\({}_{2}\)/CH\({}_{3}\) & Bending (scissoring) \\ \(\sim\)1230–1170 & C–O–C (ester linkage) & Stretching \\ \(\sim\)3400 & O–H (alcohols, acids, water) & Broad stretching band (hydrogen bonding) \\ \hline \end{tabular}
\end{table} 
Table 2: Characteristic FTIR absorption bands of virgin natural ester oil [27; 28; 29].