# Stability Study of Erythrhitol as Phase Change Material for Medium Temperature Thermal Applications
## 3 Results and Discussion
### Chemical Degradation  
Samples of pure erythroid under air suffered the most evident physical changes after the thermal treatment. Pure erythroid experienced a significant consistency change, from a loose white powder to a brown sticky paste by the end of the tests. Several samples of pure erythroid under air did not directly solidify after being placed at room temperature. Figure 5: (a) Plot of degradation of latent heat \(-\)ln(\(1-\alpha\)) versus time t for pure erythiritol under air at three different temperatures. Data points were subjected to a least squares fitting. (b) Arrhenius plot for latent heat thermal degradation of pure erythroid under air. Data points were subjected to a least squares fitting. Figure 6: (a) Plot of degradation of latent heat \(-\)ln(\(1-\alpha\)) versus time t for erythroid with antioxidant under air at three different temperatures. Data points were subjected to a least squares fitting. (b) Arrhenius plot for latent heat thermal degradation of erythroid with antioxidant under air. Data points were subjected to a least squares fitting. Erythiritol under air became brown with increasing temperature and time under thermal treatment, as can be seen in Figure 7.  
No exothermic peak was observed in the cooling scan of the sample heated at 141 \({}^{\circ}\)C for 78 h, which also did not solidify at room temperature (Figure 8a). This indicates that erythroid became a glass and not a crystal during cooling. The occurrence of a glass transition can be confirmed in the heating scan of the same sample (Figure 8b), in which an exothermic peak can be seen around 6 \({}^{\circ}\)C. The latter agrees with the results obtained by other authors [18; 37; 38]. This exothermic peak corresponds to the cold crystallization of the amorphous phase created in the previous cooling (crystalline phase A). As erythroid was further heated up, a solid/solid transformation took place at around 74 \({}^{\circ}\)C, depicted by another small exothermic peak. During solid/solid transformation another crystalline phase similar to the single crystal of meso-erythtritol is formed (crystalline phase B) [38]. Afterward, the final crystalline structure melts at around 120 \({}^{\circ}\)C. A solid/solid transformation could take place at three different temperature ranges: about 65 \({}^{\circ}\)C, between 15 and 43 \({}^{\circ}\)C and about 75 \({}^{\circ}\)C. All heating curves of Figure 8b show exothermic peaks around the aforementioned temperature ranges. Each type of solid/solid transformation that takes place is related to the different paths of the erythroid's crystallization process. The solid/solid transformation has been reported as an entropically driven process, since heat is not involved [18; 37]. However, the exothermic peaks that appeared in the heating scans suggest that the solid/solid transformation that occurred involved heat. Similar results were obtained by [PERSON] et al. [38]. In addition, melting peaks shifted to lower temperatures while increasing the heating treatment time of the sample. Other sugar alcohols have showed similar melting temperature shifting as treatment time or temperature was increased [31; 37]. The presence of four hydroxyl groups in erythroid gives rise to a system of intermolecular and intramolecular hydrogen bonds. Molecular structure and properties, such as melting point, are related to hydrogen bonding [33]. The length and strength of hydrogen bonds are sensitive to temperature, as both vary with temperature quadratically [39]. Thus, the melting point could be reduced by weakening the hydrogen bonding when heating the material. Erythiritol with antioxidant under air suffered a slight browning compared with the samples without additive. Samples that were kept longer in the heating chamber suffered the most noticeable changes. On the other hand, pure erythroid under argon barely changed its color along the heating treatment, however, one sample at the highest temperature did not solidify immediately.