## 4 Discussion
\begin{table}
\begin{tabular}{c c c} \hline
**Tested Oil** & \
\begin{tabular}{c} **Fresh Oil Onset** \\ **Temperature (\({}^{\circ}\)C)** \\ \end{tabular} &
\begin{tabular}{c} **Aged Oil Onset** \\ **Temperature (\({}^{\circ}\)C)** \\ \end{tabular} \\ \hline Therminol 66 [29] & 215 & 223 \\ Therminol SP [30] & 244 & 177 \\ Bluesil & 376 & 185 \\ \hline \end{tabular}
\end{table} 
Table 4: Onset temperatures of fresh and aged oils obtained from TGA. \begin{table}
\begin{tabular}{c c c} \hline
**Thermally Stressed Oils** & **Fresh Oil Onset Fitting Equations** & **Aged Oil Onset Fitting Equations** \\ \hline Therminol 66 [29] & Cp [K]/Kg K] = 1.55 + 3.34 \(\times\) 10\({}^{-3}\) T [\({}^{\circ}\)C] & Cp [K]/Kg K] = 1.54 + 3.21 \(\times\) 10\({}^{-3}\) T [\({}^{\circ}\)C] \\ Therminol SP [30] & Cp [K]/Kg K] = 1.91 + 3.04 \(\times\) 10\({}^{-3}\) T [\({}^{\circ}\)C] & Cp [K]/Kg K] = 1.96 + 3.47 \(\times\) 10\({}^{-3}\) T [\({}^{\circ}\)C] \\ Bluesil & Cp [K]/Kg K] = 1.53 + 1.81 \(\times\) 10\({}^{-3}\) T [\({}^{\circ}\)C] & Cp [K]/Kg K] = 1.62 + 1.36 \(\times\) 10\({}^{-3}\) T [\({}^{\circ}\)C] \\ \hline \end{tabular}
\end{table} 
Table 5: Fitting equations based on calorimetric measurements on fresh and aged Therminol 66 and Therminol SP. Figure 7: Comparison between fresh (black) and aged (red) Bluesil specific heat. No significant changes were detected within the experimental error. strong increase was observed for Therminol 66, while Therminol SP underwent a viscosity decrease. Both the obtained results agree with the TGA degradation onset temperatures; viscosity increasing together with the volatility drop indicate that Therminol 66 thermal aging occur mainly through polymerization processes, while thermal cracking prevails for Therminol SP. Bluesil viscosity is much higher with respect to the other oils both in fresh and aged samples; the above citated molecular mechanisms explain the TGA and rheometric results. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**T** & **Fresh\_Bluesil** & **Aged\_Bluesil** & **Diff** \\ \hline \({}^{\circ}\)C & Pa*s & Pa*s & \% \\
40 & \(6.85\times 10^{-2}\) & \(8.56\times 10^{-2}\) & 20.0 \\
60 & \(3.81\times 10^{-2}\) & \(4.33\times 10^{-2}\) & 11.9 \\
80 & \(2.43\times 10^{-2}\) & \(2.54\times 10^{-2}\) & 4.30 \\
100 & \(1.77\times 10^{-2}\) & \(1.59\times 10^{-2}\) & 11.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Rheometric comparison between fresh and aged Therminol 66 with percentage differences. Figure 8: Comparison between the viscosities of fresh and aged Bluesil; the literature data are also reported. A detectable viscosity increase in aged Bluesil can be observed, and differences between fresh and aged samples are particularly marked at lower temperatures (red circle). Figure 9: Comparison of viscosity measurements performed on fresh (**left**) and aged (**right**) oil samples. Error bars, according to experimental error, are also reported. Regarding the IR analysis of fresh and aged Bluesil, the infrared spectra are reported together in Figure 10, showing a small increase in the Si-O-Si peak intensity, which can be realistically associated with the formation of cross-linked bonds during the degradation bond, as discussed above; this result is congruent with the increase in dynamic viscosity for thermally stressed samples. Additionally, the overall chemical structure can be considered to be only slightly affected due to the minor extent of chemical degradation.