## 4 Properties of Treated Liquids from Waste Cooking Oil as an Alternative Insulating Liquid
### Influence of WCO Treatment on Thermal Properties
#### 4.1.1 Cooling Efficiency  
As presented in Table 3, waste cooking oil exhibits a relatively higher viscosity compared to commercially available natural ester liquids such as Midel 1204 and Midel 1215. Although its viscosity remains within the acceptable range for transformer insulating liquids, the elevated value limits its potential and reduces its effectiveness as a high-performance insulation fluid. The higher viscosity in waste cooking oil is primarily attributed to the presence of long-chain triglycerides, polymerized compounds formed during repeated heating, and residual impurities such as food remnants, free fatty acids, and oxidation by-products [137]. These components increase the internal friction within the fluid, thereby \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Properties**} & **Homogeneous Base** & **Heterogeneous Base** & **Enzyme Catalysis (e.g., NaOH, KOH)** & **Supercritical Method (e.g., MgO, SrO, TiO2)** & **Supercritical Methanol** \\ \hline \multirow{3}{*}{Industrial Viability} & Widely used, but purification is a challenge & Promising, but the temperature requirements are high & Limited by cost, still under development & Technically efficient but expensive \\ \cline{3-5}  & \multirow{3}{*}{Limited} & Metal-doped & Immobilized enzymes, membrane reactor, ultrasound/microwave-assisted & Use of ultrasound, reactive distillation, and membrane reactors \\ \cline{3-5}  & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: _Cont._ \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Possible Variable**} & \multicolumn{1}{c}{**Sub-Categories/Conditions**} & \multicolumn{1}{c}{**Remarks/Effects**} & **Reference** \\ \hline Temperature & Ambient (20 to 25 \({}^{\circ}\)C) & & \\ Moderate (60-100 \({}^{\circ}\)C) & Temperature range affects reaction rate & [130; 131; 132] \\  & High (>100 but \(\sim\)200) & & \\ Pressure & Ambient (101.3 kPa) & & \\ High (10 MPa) & Higher pressure may improve reaction yield & [20] \\  & Supercritical (>100 MPa) & & \\ Mixing rate & Low, Medium, High, Static, & \\  & Ultrasonic & & \\ Reaction time & Total, Continuous, and Batch & Influences the extent of conversion & [133; 134] \\  & Alcohol-to-Glycerol, Alcohol-to-Oil, & & \\  & Alcohol-to-Reagent & & \\ Catalyst type & Acid, Alkali, and Enzyme & Different catalysts influence conversion & [136] \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Possible process parameters for optimization of FAME yield. reducing its flowability. In transformer applications, high viscosity negatively impacts the cooling performance by limiting the oil's ability to circulate and transfer heat efficiently [11]. Consequently, high-viscosity oil may compromise transformer thermal management and increase the risk of overheating due to high pumping power [138]. Therefore, viscosity reduction becomes essential. Transesterification is a general method for reducing the viscosity of waste cooking oil as it breaks down triglycerides into smaller methyl esters, significantly reducing the fluid's viscosity and enhancing its cooling efficiency. Table 8 presents findings from the literature on the viscosity of transesterified waste cooking oils for transformer application. It was observed that transesterification effectively reduces the viscosity of waste cooking oil, bringing the modified oils within the range of cooling liquids specified by ASTM D445 at 40 \({}^{\circ}\)C [83,139]. This reduction in viscosity can be attributed to the removal of glycerol, which forms the backbone of the triglyceride structure. By eliminating glycerol, the molecular structure becomes less complex, reducing intermolecular friction and thereby enhancing fluidity. The transesterified oils exhibit viscosities comparable to or lower than those of mineral insulating liquids and traditional natural esters, suggesting enhanced cooling performance in transformer applications.