## 1 Introduction
### Chemical Properties  
#### 2.2.1 Fourier Transform Infrared Spectroscopy Test  
Fourier transform infrared spectroscopy (FTIR) has been widely adopted in material analysis to obtain an infrared spectrum of absorption of samples. In this paper, the existence or absence of certain functional groups, as well as their abundance, was indicated by FTIR. Key absorption peaks corresponding to functional groups in silicone rubber material are listed in Table 4.  
Figure 3: Static contact angle of samples under different years: (**a**) A\({}_{1}\); (**b**) A\({}_{2}\); (**c**) A\({}_{3}\); (**d**) A\({}_{4}\); (**e**) A\({}_{5}\); (**f**) A\({}_{6}\). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Samples** & **Aging Time/Year** & **Standards of Division** & \(S_{em}\) \\ \hline A\({}_{1}\) & 1 & Some cracks and voids & 1 \\ A\({}_{2}\) & 3.5 & Some cracks and voids & 1 \\ A\({}_{3}\) & 6 & Some cracks and voids & 1 \\ A\({}_{4}\) & 8 & Penetrating cracks and voids & 2 \\ A\({}_{5}\) & 10 & Penetrating cracks and voids, some rough bumps & 3 \\ A\({}_{6}\) & 12 & Penetrating cracks and voids, some rough bumps & 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Aging level of composite insulators of Factory A. The results of wave numbers of the functional groups of the composite insulators sampled are plotted in Figure 4.  
Figure 4 shows the following:  
(1) The main functional groups of samples in FTIR are mainly consistent, and no new key functional groups appear in the surficial in aging process. (2) With the increase of aging time, the Si-O-Si absorption peak altitude at wave number 1100-1000 cm\({}^{-1}\) gradually decreases, which indicates that the content of the main Si-O-Si bond decreases. In the aging process, UV radiation could interrupt the main Si-O-Si bond in the surface material of silicone rubber, which would result in an organosilicon molecule in silicone rubber transferring to pollution layers, along with the decrease of the Si-O-Si absorption peak altitude. (3) The Si-(CH\({}_{3}\))\({}_{2}\) absorption peak altitude at wave number 840-790 cm\({}^{-1}\) decreases gradually with the increase of aging time, resulting in the silicone rubber tending to be polar and gradually hydrophobic in the aging process. (4) For aging composite insulators, the -OH absorption peak altitude at wavenumber 3700-2800 cm\({}^{-1}\) of surface materials rises gradually with the increase of aging time, and there are two main reactions occurring in -OH during this process. One is that water, the catalyst in the hydrolysis reaction, promotes mutual combination of diversified free radicals under high temperature conditions in the aging process, which results in the rise of the -OH absorption peak altitude. The other is that a large amount of free O-H groups in the surface material of silicone rubber volatilizes after contact with air, leading to a decrease of the -OH absorption peak altitude. In Figure 4, the -OH absorption peak altitude increases with the increase of aging time, showing that the first factor has greater influence than the second factor, leading to an increase of the content of the -OH bond. Previous research [23] indicates that the decrease of Si-O-Si and C-H absorption peak altitudes, which results in the materials being polar, has greater influence than the rise of -OH absorption peak \begin{table}
\begin{tabular}{c c} \hline
**Functional Groups** & **Wave number/cm\({}^{-1}\)** \\ \hline O-H & 3700–3200 \\ CH\({}_{3}\)(C-H) & 2960 \\ C-H & 1440–1410 \\ Si-CH\({}_{3}\)(C-H) & 1270–1255 \\ Si-O-Si(Si-O) & 1100–1000 \\ O-Si(CH\({}_{3}\))\({}_{2}\)-O(Si-O) & 840–790 \\ Si(CH\({}_{3}\))\({}_{3}\) & 800–700 \\ \hline \end{tabular}
\end{table} 
Table 4: Key absorption peaks of silicone rubber in Fourier transform infrared spectroscopy (FTIR) analysis. Figure 4: FTIR analysis results of samples under different years: (**a**) wave number between 500 and 2000 cm\({}^{-1}\); (**b**) wave number between 2000 and 4000 cm\({}^{-1}\). altitude, which adds some hydrophilic groups resulting from the hydrolysis reaction.