## 3 Chemical Properties of Waste Cooking Oils and Purification Process  
In this section, the properties of waste cooking oil are examined, along with the purification methods required to meet the standard criteria for use as insulating liquids. The chemical composition of WCO is shown in Table 2[66], with oleic acid being the predominant component. Several studies have confirmed that the properties of used cooking oils differ significantly from those of unused oils, particularly in their physicochemical  
Figure 5: Correlation between reused cooking oil and cardiovascular disease incidence [68]. This Figure was reproduced with permission from Elsevier, Copyright 2025.  
characteristics. Table 3 compares the physicochemical properties of both used and unused cooking oils with those of standard natural ester insulating liquids. The results indicate that WCO possesses inferior values in key parameters such as moisture content, acidity, flash point, pour point, appearance, and breakdown voltage, rendering it unsuitable for transformer insulation in its raw form. Therefore, proper purification and treatment are essential to upgrade the quality of waste cooking oil before it can be considered for insulation applications in power transformers. It is to be mentioned that there are several modes of recycling waste cooking oils, which are BET (Biodiesel Enterprise Takeback) and TPT (Third-party Takeback), with the United States of America using the TPT and countries like Brazil using the BET [79]. The different approaches used by different countries in the collection of these materials have been extensively documented in Refs. [79; 80; 81]. During the collection of waste cooking oil, it is crucial to maintain the properties of the oil and avoid contamination, which could interfere with the properties of the oil and increase the cost of purification. To ensure the quality of WCO during transportation for insulation or other applications, proper storage and handling measures must be adopted. The use of sealed, leak-proof, and corrosion-resistant containers, such as food-grade drums or tanks, prevents contamination from external impurities, moisture, or microbial growth. Additionally, strict handling protocols, including proper labeling, temperature control, and hygiene standards, help maintain the oil's integrity, ensuring its suitability for reuse or further processing [82]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Fatty Acid Type** & **Fatty Acid Name** &
\begin{tabular}{c} **Percentage of Waste** \\ **Cooking Oil** \\ \end{tabular} \\ \hline Saturated Fatty Acids & Palmitic acid & 38.35 \\  & Stearic acid & 4.33 \\  & Myristic acid & 1.03 \\  & Lauric acid & 0.34 \\  & Heneicosaonic acid & 0.07 \\ Sub-Total & & 44.12 \\ Monounsaturated Fatty Acids & Oleic acid & 43.67 \\  & Cis-\(11\)-Eicosenoic acid & 0.16 \\ Sub-Total & & 43.83 \\ Polyunsaturated Fatty Acids & Linoleic acid & 11.39 \\  & \(\gamma\)-Linolenic acid & 0.37 \\  & Linolenic & 0.29 \\ Sub-Total & & 12.05 \\ Total & - & 100 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Fatty acids composition of waste cooking oil [43; 66]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Properties** & **Waste Cooking Oil** & **Unused Vegetable Oil** &
\begin{tabular}{c} **Natural Ester** \\ **Insulating Oils** \\ \end{tabular} & **Standard** \\ \hline Density (g/cm\({}^{3}\)) & 0.94 & 0.89 [66] & \(\leq\)0.92 & ASTM [84] \\ Viscosity (mm\({}^{2}\)/s) @ 40 \({}^{\circ}\)C & 44.1 & 39.99 [66] & \(\leq\)50 & IEEE [85] \\ Moisture (ppm) & 1130 & 105.70–125.8 [67] & \(\leq\)200 & ASTM [86] \\ Acid value (mgKOH/g) & 4.03 & 0.3 [66] & Max. 0.6 & IEC [87] \\ Flash point (\({}^{\circ}\)C) & 269 & \(>\)220 [43] & \(\geq\)275 & ASTM [86] \\ Pour point (\({}^{\circ}\)C) & 9 & \(-\)18 [82] & \(\leq\)\(-\)10 & IEC [87], ASTM [86] \\ Breakdown voltage (kV) & 27 & 40 [82] & \(\geq\)35 & ASTM [88] \\  & & Clear and free from & & \\ Physical color rating & L5.0 & suspended particles & Max. 1.0 & IEEE C57.147 [85] \\  & [43] & & & \\ \hline \hline \end{tabular}
\end{table}