# Stability Study of Erythrhitol as Phase Change Material for Medium Temperature Thermal Applications
## 3 Results and Discussion
### Chemical Degradation
This caused the -OH absorption band of unheated sugar alcohol (\(\sim\)3225 cm\({}^{-1}\)) to move to higher wave numbers (up to 3240 cm\({}^{-1}\)) at higher temperatures. The new peak that appeared at \(\sim\)1716 cm\({}^{-1}\) in the thermally treated samples indicates the likely presence of carbonyl groups. In addition, a C=O stretching of a conjugated double bond of a ketone appeared at around 1640 cm\({}^{-1}\) only for the heated samples [62]. This confirms the formation of conjugated carbonyl groups after heating treatment under air. As proposed, erythritol undergoes oxidation/dehydration reactions when heated under air. Both reactions give rise to carbonyl compounds and conjugated double bonds with similar absorptions bands. Hence, it is hard to confirm which one of the processes was the one that generated the products shown in the FTIR spectra. Irganox 1010\({}^{\text{\text{\textregistered}}}\) contains ester linkages [63], which are represented by the absorption peak at \(\sim\)1730 cm\({}^{-1}\) of the unheated samples in Figure 10. Heated samples with antioxidant under air (Figure 10c) show weak peak intensities at \(\sim\)1730 cm\({}^{-1}\). This is most likely due to the consumption of antioxidant during thermal treatment, since there was high oxygen availability. In contrast, Figure 10d shows stronger peak intensities at \(\sim\)1730 cm\({}^{-1}\) for all heated samples. This could be related to a higher amount of antioxidant remaining in samples, as these were under argon atmosphere, and therefore, there was less oxygen to scavenge. Unlike samples of pure erythritol under air, the FT-IR spectrum recorded for samples under argon atmosphere (Figure 10b) did not show significant changes either in the shape or in the wave numbers of absorption bands of carbonyl functional groups. Lastly, Figure 11a depicts that after around 935 h of thermal treatment, in the long-term tests, two peaks appeared at \(\sim\)1643 cm\({}^{-1}\) and \(\sim\)1725 cm\({}^{-1}\) in samples of erythroid with antioxidant under air. As previously mentioned, absorption in this region corresponds to carbonyl groups. The C=O stretching of the conjugated ester is no longer visible (\(\sim\)1730 cm\({}^{-1}\)), since the absorption bands corresponding to the oxidation/caramelization products overlap. However, it is likely that the antioxidant has been depleted during the long-term test, since as shown in Figure 10c, heated samples with antioxidant did not show absorption in the carbonyl groups region, while Figure 11a shows similar absorption bands as Figure 10a, which indicates an oxidation/dehydration process. Heated samples with antioxidant under air show only weaker peaks in the Irganov 1010\({}^{\otimes}\) absorption region after 100 h of thermal treatment compared to their corresponding unheated sample (Figure 10c). In addition, as mentioned in Section 3.2, during the long-term test and only after 260 h, the degradation rate started to increase, which could be another hint about the antioxidant depletion. On the contrary, Figure 11b shows no chemical changes in pure erythroid under argon atmosphere after the long-term thermal treatment. Figure 10: FT-IR spectra of the short-term-tests for (**a**) pure erythroid under air; (**b**) pure erythroid under argon atmosphere; (**c**) erythroid with antioxidant under air and (**d**) erythroid with antioxidant under argon atmosphere.