## 3 Results
### FTIR Spectroscopy  
Figure 9 shows the FTIR spectra of epoxy resin and epoxy resin/ZnO samples before and after 2880 h of AWE treatment. Table 1 summarizes the assignments for the main absorption bands of these spectra, according to the literature [27; 47; 48]. However, the Zn-O stretching vibration, at approximately 500 cm\({}^{-1}\), is not discrete in the composite samples. The transmittance drop after 750 cm\({}^{-1}\) is explained by KBr presence, which was used as a carrier material in the tablets for the FTIR measurements. Additional measurements of KBr carrier alone revealed the aforementioned tendency.  
Figure 7: (**a**,**b**) Percentages of (**a**) weight loss of reconditioned samples relative to previous interval, (**b**) sorbed moisture relative to the average sample group average weight.  
To study the photo-degradation phenomena and the effect of ZnO content, or the irradiated samples, the intensities of hydroxyl (I\({}_{\text{OH(3400)}}\)), etheric (epoxide) (I\({}_{\text{COC(1245)}}\), I\({}_{\text{COC(1035)}}\)) and the C-H (I\({}_{\text{CH(2920)}}\)) absorption bands were calculated, and the ratios IOH(3400)/ICH(2920), I\({}_{\text{COC(1245)}}\)/I\({}_{\text{CH(2920)}}\) and I\({}_{\text{COC(1035)}}\)/I\({}_{\text{CH(2920)}}\) were determined. The results of those calculations are presented in Figure 10a-c.  
\begin{table}
\begin{tabular}{l l} \hline
**Wave Number (cm\({}^{-1}\))** & **Functional Groups** \\ \hline
3415 & O–H stretching and symmetric stretching of primary amine \\
2960, 2920, 2872 & C–H stretching of CH\({}_{2}\) and CH\({}_{3}\) aromatic and aliphatic \\  & C=C stretching vibration of aromatic ring, N–H bending of \\  & primary amine \\
1512 & C–C stretching vibration of aromatic ring \\
1384 & C–N stretching vibration of imide \\
1245 & C–C–O–C stretching vibration of ether linkage \\
1180 & C–O stretching vibration of aromatic ring \\
1110 & Aromatic stretching, C–N \\
1038 & C–O–C stretching vibration of ether linkage \\
831 & H–C= out-of-plane bending \\ - & Zn–O stretching vibration \\ \hline \end{tabular}
\end{table}
Table 1: Band assignments of FTIR spectrum with common functional groups.  
Figure 8: (**a**,**b**). Transmittance of reconditioned samples (DS) at (**a**) INT0 and (**b**) INT2880.  
Figure 9: FTIR spectra of bare and ZnO composite samples.  
According to Figure 10a, the ratio \(\mathrm{I_{OH(3400)}/I_{CH(2920)}}\) of the non-irradiated C0 sample was 1.14. After its irradiation, this ratio took a lower value (i.e., 0.76), indicating that photo-degradation of the resin takes place, eliminating -OH groups. The C0.1 sample exhibited a higher ratio value (i.e., 1.22), indicating that photo-oxidation reactions occur, creating -OH groups on the resin. By increasing the ZnO content up to C2, this ratio increased almost linearly, indicating that ZnO was responsible for withholding OH moieties [49] during high moisture processing. As a result, ZnO prevented the resin from extensive photo-oxidation or photo-degradation reactions related to -OH groups.  
According to Figure 10b,c, the ratios \(\mathrm{I_{COC(1245)}/I_{CH(2920)}}\) and \(\mathrm{I_{COC(1035)}/I_{CH(2920)}}\) exhibited quite similar behaviour. The non-irradiated C0 samples exhibited higher ratios than the corresponding irradiated ones, which exhibit the lowest values. The latter showed that photo-degradation of the resins takes place, eliminating -C-O-C groups. By increasing the ZnO content (C0.1, C0.5 or C2), these ratios had lower values than the non-irradiated sample with 0 wt% ZnO; however, they were higher compared to C0 irradiated samples and the ratios increased with the increasing ZnO content. Thus, the incorporation of 2 wt% ZnO prevented the resin from extensive photo-degradation reactions related to -C-O-C groups.