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Molecules 2020, 25, 5572 15 of 26 3.4. Heat Capacity and Thermal Conductivity Although most data found in the literature concern the melting points of triglycerides, recently additional studies have been conducted on other thermal properties such as the heat capacity and thermal conductivity. Nevertheless, the data often prove to be unsatisfactory, in some cases with no mention of the purity of the sample investigated or of the uncertainty in the data reported. Data exist on the heat capacities of SST derived from fatty acids with even carbon numbers from 6 to 18, while it is non-existent for the remaining saturated triglycerides (odd and even) and for the unsaturated derivatives. Therefore, it is clear that in order to promote the introduction of triglycerides in the thermal energy storage field as novel PCMs, further studies need to be conducted on their thermal properties besides melting points and enthalpies of fusion. Table 4 summarizes the literature data collected for even numbered SST with carbon numbers between 6 and 18 concerning heat capacities and thermal conductivities. No thermal conductivity values could be found for tricaproin. The heat capacities retrieved are comparable to those of other organic materials typically used as PCM such as paraffins, and there is good agreement between the sources with the exception of tricaprin. On the other hand, discrepancies are observed between the thermal conductivity of trimyristin reported by Bogatishcheva [46] and Sari [40]. In general, the heat capacities increase linearly with increasing carbon numbers, while no clear trend is detected between the thermal conductivities and chemical structures. Morad et al. [47] analyzed the thermal properties of both simple and mixed triglycerides, and report that no pattern between specific heat capacity and carbon number is observed for mixed triglycerides. In addition, the specific heat capacities of the mixed triglycerides are lower than those of the simple triglycerides. Overall, the thermal conductivities of the triglycerides show rather low values around 0.2 W/(m· K) in agreement with those observed for other organic compounds. Table 4. Data collected on the heat capacities [J/(g·K)] and thermal conductivities [W/(m·K)] of some saturated symmetrical triglycerides. The abbreviation indicates the chain length of the precursor fatty acid; for example, tricaproin is derived from glycerol and three caproic acid molecules, whose carbon number is 6. Therefore, tricaproin is indicated as 6.6.6. Next to the abbreviations, the common given names of each triglycerides are indicated. The superscript numbers on each measurement value indicate the reference from which the value has been extracted. Missing values have been indicated with the “-“ sign. Abbreviation 6.6.6 8.8.8 10.10.10 12.12.12 14.14.14 16.16.16 18.18.18 Given Name tricaproin tricaprylin tricaprin trilaurin trimyristin tripalmitin tristearin Structure Heat Capacity Cp [J/(g·K)] (at Room Temperature) 1.88 [47] 1.94 [44,48], 1.96 [49] 1.97 [44,48], 2.08 [49] 2.06 [44,48], 2.05 [47] 2.14 [46,48], 2.10 [47] 2.20 [47] 2.23 [47] Thermal Conductivity λ [W/(m·K)] (at room Temperature) - 0.15 [46] 0.15 [46] 0.16 [46] 0.17 [46], 0.23 [40] 0.19 [40] 0.17 [40] 3.5. Thermal Degradation Limited data are available on the thermal stability of triglycerides in terms of thermal degradation. In general, due to the similarities in the chemical structure, triglycerides behave comparably to other organic molecules such as esters [44,50]. For example, as reported by Morad et al. [47] trilaurin shows an onset degradation at 150 ◦C, where its specific heat capacity drops suddenly in non-inert atmosphere. Additionally, the authors measured the onset degradation temperatures of trimyristin, tripalmitin and tristearin at 181, 184 and 187 ◦C respectively, which indicates that the thermal stability of triglycerides increases with increasing carbon number and chain length. This is in accordance with results reported by Ravotti et al. for fatty esters and diesters [44,50,51].PDF Image | Triglycerides as Novel Phase-Change Materials
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