Thermal stability of renewable diesters as phase change materials

Lipid derived diesters display excellent thermodynamic characteristics suitable for applications in phase change materials (PCMs), making them potential renewable alternatives suitable for replacing traditional petrochemical derived PCMs such as paraffins. In order to evaluate their abilities to be used as PCMs, however, it is important to understand how their structures influence the high temperature behaviour under both thermal and thermo-oxidative conditions. This study analyzed the thermal stability of three template diester systems under both inert and oxidative environments using thermogravimetric analysis (TGA) under nitrogen and air, respectively. The lipid derived diesters, synthesized from dialcohols and either oleic (unsaturated) or stearic acid (saturated), and from oleyl alcohol and various diacids, all possessed the form 18-n-18, where n represented the length of the dialcohol or diacid moiety (n=2, 4, 6, 8, 9 or 10 carbon atoms) and 18 gave the length of the fatty acid moiety on the diol derived diesters, and the fatty alcohol moiety on the dibasic diesters. Under inert atmospheres, none of the diesters break down and are stable until they evaporate. The introduction of unsaturation raises the evaporation temperature of the reference diesters, especially at large diol chain lengths, while the reversal of the ester group lowers the evaporation temperatures with respect to the diol derived diesters. These results reveal that saturated and unsaturated diol derived diesters and saturated dibasic diesters are able to function in a wide range of applications as high as their evaporation point depending on the atmosphere present, and above∼240°C in all instances. Specifically, the thermal and oxidative stabilities of these diesters indicate their suitability for use in thermal energy storage as phase change materials.