## 3 Results and Discussion
### Scanning Electron Microscopy (SEM) Analysis of the Recovered Fibers  
Scanning electron microscopy (SEM) was used to examine the appearance/state of the fibers and their possible damage and also to determine their diameters. Thus, the diameters of the rCFs measured using SEM allowed us to determine whether the fibers \begin{table}
\begin{tabular}{c c c c} \hline \hline
**t (min)** & **T**(\({}^{\circ}\)**C**) & **SF** & **DR (\%)** \\ \hline
35 & 300 & 7.43 & 51 \\
80 & 300 & 7.79 & 95 \\
120 & 300 & 7.97 & 98 \\
35 & 400 & 10.38 & 98 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Severity factor (SF). Figure 7: Comparison between the DRs achieved when operating at supercritical (400 \({}^{\circ}\)C; 243 bar) or subcritical (300 \({}^{\circ}\)C; 100–130 bar) conditions using an approximately 300 \(\mathrm{ml_{solv}}/\mathrm{g_{comp}}\) ratio and IPA 5 wt.% (except for P11, where pure water was used). retained their sizing layer or whether, on the contrary, their sizing had been degraded by the hydrothermal process. Sizing is a thin, homogeneous coating that is applied to the surface of fibers during the manufacturing process to protect them during their handling and processing. In addition, sizing improves the chemical, mechanical, and thermal properties of the fibers and also improves the bond between the fibers and the resin matrix [37]. Figure 8a shows a raw composite sample before being subjected to a hydrothermal treatment. It can be seen in the SEM image of this part in Figure 8d that the fibers of the raw composite piece are embedded in the epoxy resin. Figure 8b shows the fibers recovered through the P3 test (300 \({}^{\circ}\)C; 100 bar; 120 min; IPA 5 wt.%; KOH 0.1 M) and its SEM image; Figure 8e shows that theses fibers have a fairly clean surface with hardly any traces of epoxy resin, which is in agreement with the 98% DR registered for this test. On the other hand, Figure 8c,f show the fibers recovered through the P12 test (400 \({}^{\circ}\)C; 240 bar; 120 min; IPA 5 wt.%; KOH 0.1 M), which was run under supercritical conditions. Their corresponding SEM image, Figure 8f, shows a rougher and more irregular surface that also presents adhered residues of a larger size. This may be related to the fact that P12 also uses a catalyst under supercritical conditions. The diameter of the carbon fibers in the original composite sample (Figure 8d) as well as the diameter of the recovered carbon fibers were measured by SEM. The difference between the measured diameter of the CFs in the original composite material and the diameter specified by the manufacturer (5 \(\upmu\)m) is considered to represent the average thickness of the sizing that covers the CFs. Table 3 includes the measured average diameters and the estimated average sizing thickness for each of the tests. Based on the results presented in Table 3, it can be concluded that the hydrothermal treatment corresponding to the P3 test (300 \({}^{\circ}\)C; 100 bar; 120 min; IPA 5 wt.%; KOH 0.1 M) and P7 tests (300 \({}^{\circ}\)C; 130 bar; 80 min; IPA 5 wt.%; NaOH 0.1 M) did not damage either the fibers or their sizing, as they exhibited thicker sizing than the original fibers. The rest of the tests resulted in fibers with a marginally lower thickness than that corresponding to the fibers in the raw sample. This might suggest that their sizing had not been damaged at all or that only minimal sizing damage had occurred. Figure 8: Appearance of a piece of composite material before and after the hydrothermal treatment. Photography of the raw sample (**a**) and of the fibers recovered through the P3 (**b**) and P12 (**c**) tests. SEM images of the raw sample (**d**) and of the fibers recovered through the P3 (**e**) and P12 (**f**) tests.