# Stability Study of Erythrhitol as Phase Change Material for Medium Temperature Thermal Applications
## 3 Results and Discussion
### Chemical Degradation
A slight yellowing of the polyvinyl chloride (PVC) was observed due to the formation of conjugated double bonds, which when increasing turned the PVC dark brown to finally black [53]. Color formation was also related to the increase of conjugated double bonds during thermal degradation of poly(l-lactic acid) (PLLA), as its UV-Vis spectra became stronger in intensity while the processing temperature increased [54]. [PERSON] and [PERSON] [37] also observed an increase of absorption in the UV-Vis spectra of D-mannitol when kept for longer times at 180 \({}^{\circ}\)C under air. In addition, they confirmed the occurrence of \(\alpha\)-\(\beta\) unsaturated carbonyl groups, such as 4-methylpent-3-en2-one, which display absorption bands between 240 nm and 320 nm, as a likely indicator of D-mannitol degradation. 4-methylpent-3-en-2-one is a well-known conjugated ketone generated in the aldol condensation of acetone on dehydration [55]. As mentioned before, erythroid became steadily brown with increasing temperature and time of thermal treatment, which is in good agreement with the time/temperature dependency of the formation of \(\alpha\), \(\beta\)-unsaturated carbonyl groups. In addition, [PERSON] [56] claimed that color build up in furfural is due to an increasing number of conjugated double bonds triggered by oxygen. Oxygen triggers discoloration by producing radicals, which attack the double bond of a furfural molecule leading to the formation of an uninterrupted sequence of conjugated double bonds. As already mentioned, pure  
Figure 9: Proposed mechanism of oxidation of erythroid under air. Pathway A: abstraction of hydrogen atom to form alkyl radicals, which in the presence of oxygen are turned into peroxy radicals. Peroxy radicals can undergo the unimolecular HO\({}_{2}\) elimination process yielding carbonyl compounds. Pathway B: abstraction of hydrogen atoms to form alkoxy radicals, which undergo decomposition via C-C bond scission. erythritol under air had the biggest color change after heating treatment, which suggests the formation of conjugated carbonyl systems from the oxidation products. Therefore, it is suggested that erythroid browning is rather related to the formation of conjugated carbonyl compounds when it oxidizes/dehydrates than to the caramelization process. On the other hand, dehydration of erythritol at high temperatures has been reported [15; 52; 57]. The experimental method that [PERSON] et al. [15] described is closer to the one used for this study, as erythroid under air was also heated up to 140 \({}^{\circ}\)C and kept there for a period of time. Even though the experimental conditions reported by the above-mentioned authors differ from each other, all of them confirmed 1,4-anhydroerythritol as the first reaction product of the dehydration of erythritol. Thus, not only oxygen concentration, but also temperature and heating time affect the dehydration process of erythritol. [PERSON] and [PERSON] [37] mentioned the formation, not only of cyclic ethers, but also of other products through different reaction pathways of thermal dehydration of polyols, giving place to the \(\alpha\), \(\beta\)-unsaturated carbonyl (or enones) groups. Intramolecular dehydration of polyols leads to cyclic ether production via an SN\({}^{2}\) substitution process [58]. Intermolecular dehydration can lead to, among others, carbonyl compounds via a carbocation intermediate (SN\({}^{1}\) mechanism) [59; 60]. [PERSON] [61] found the occurrence of carbonyl compounds via enolic intermediate by dehydration at the C-2 position of erythritol. Due to the occurrence of carbonyl compounds, many further possible subsequent reactions can follow, such as aldol additions and condensations, which in turn can give place to the \(\alpha\), \(\beta\)-unsaturated carbonyl groups mentioned also by [PERSON] and [PERSON] [37]. Figure 10a shows the FT-IR spectra of samples of pure erythroid under air before and after being heated. As temperature was raised, intermolecular hydrogen bonds weakened, and hence, the hydroxyl bonds strengthened [32].