## 4 Discussion  
According to the literature data, polymeric siloxane compounds under thermal stress react through two main competitive degradation routes: a molecular mechanism which implies Si-O bond scission takes place with the formation of cyclic oligomers, and the rearrangement of the silicone-oxygen bonds leads to volatile byproducts. Also, a radical mechanism, which prevails at high temperatures, occurs by a homolytic Si-CH\({}_{3}\) bond scission and leads to methane (Figure 5) and hydrogen release, with the formation of cross-linked materials by oxidation with peroxyl compounds increasing fluid viscosity [36; 37; 38]. Therefore, methane and hydrogen production can be considered as processes occurring in series. It is important to note that no solid byproducts were detectable in the aged oil, indicating the absence of fouling. Thermogravimetric curves acquired on fresh and aged Bluesil samples are shown in Figure 6; initial decomposition temperatures are very high for fresh Bluesil, with values of 375.5 \({}^{\circ}\)C under nitrogen and 374 \({}^{\circ}\)C under air. A very interesting result was obtained for the aged Bluesil under nitrogen, where the onset temperature strongly decreased to 185 \({}^{\circ}\)C, probably due to siloxane chain fragmentation processes. In previous work [29; 30], the same analysis was performed for Therminol 66 and Therminol SP. The onset temperatures are summarized in Table 4. Although fresh Therminol 66 seems to be less stable than fresh Therminol SP, the volatility of the latter undergoes to a strong decrease with the thermal aging processes, and this explains the pressure values reached during the heating process. Fresh Bluesil, as expected, given the higher dissociation energy of Si-O bond compared to C-C bond [39] (\(\Delta\)Hf\({}_{\text{Si-O}}\) = 798 kJ/mol, \(\Delta\)Hf\({}_{\text{C-C}}\) = 607 kJ/mol), has a much higher stability with respect to aromatic oils [21]; however, a strong volatility increase was observed in the aged sample. Finally, thermal stability was slightly changed in the thermally stressed Therminol 66. Figure 5: Reaction mechanism for methane production. Figure 6: On the left: the TGA of fresh Bluesil both under 20 Nml of nitrogen (continuous curve) and air (dashed curve), onset temperatures are highlighted (red circles); the oxygen presence effects on the Bluesil volatility are limited. On the right: Fresh (continuous curve) and aged (dashed curve) Bluesil thermogravimetric curves under nitrogen, onset temperatures are highlighted (red circles); the volatility decreases significantly in the aged sample. Specific heat measurements, along with a confrontation regarding the literature data, are reported in Figure 7, with a resulting relative error of below 5%. It can be concluded that, considering the experimental error, specific heat does not change significantly during the aging process. Calorimetric measurements for fresh and aged Therminol 66 and Therminol SP [29; 30] are summarized in Table 5. In addition to their specific heat values, their dependance in regard to temperature does not significantly change in the temperature range, with Bluesil presenting the smaller specific heat values and the lower dependence on temperature. Table 6 and Figure 8 show the viscosity values for the fresh and thermally stressed Bluesil samples. Dynamic viscosity increases for the thermal stressed sample; although the effect is limited, it is still detectable considering 10% of the measured experimental error, especially at lower temperatures. The reason for this was explained above, and it is due to the formation of cross-linked high-viscous polymers [36]. Fresh Therminol 66 [29] and Therminol SP [30] show similar viscosity values (Figure 9) in the tested temperature range but an opposite trend with respect to thermal aging; a