## 4 Properties of Treated Liquids from Waste Cooking Oil as an Alternative Insulating Liquid
### Influence of WCO Treatment on Thermal Properties
#### 4.1.2 Fire Safety  
The flash point of insulating liquids is a critical safety parameter, particularly for power equipment operating near residential areas and essential facilities. Transformer explosions can lead to catastrophic consequences, including loss of life and extensive property damage. As shown in Figure 9, a tragic transformer explosion emphasizes the importance of using insulating liquids with high thermal and fire safety margins [143]. A higher flash point reduces the risk of fire and explosion, enhancing the overall safety and reliability of transformers [144]. This not only protects surrounding environments but also improves the economic viability of utilities by reducing maintenance costs, minimizing fire-related risks, and extending equipment lifespan. Additionally, insulating liquids with higher flash points contribute to better thermal stability, further ensuring the safe and efficient operation of transformers under varying load conditions. Natural ester insulating liquids are classified as Class K insulating materials due to their high fire points exceeding 300 \({}^{\circ}\)C, providing a significant safety advantage over mineral insulating liquids [145]. The flash point temperatures of treated waste cooking oil fatty acid methyl esters are presented \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**References**} & **Viscosity of** & **Viscosity After** & \multirow{2}{*}{**Catalyst**} & **Enhancement (\%)** \\  & **Waste WCO** & **Modification** & & **CJ0 (\%)** \\  & **(cSt) (40 \({}^{\circ}\)C)** & & & \\ \hline
[82] & 70 & 27 & NaOH & 61.43 \\
[76] & 42.8 & 6 & CaO & 85.98 \\
[46] & 40.84 & 14.19 & KOH & 65.25 \\
[140] & 41.85 & 15.23 & CaO & 63.61 \\
[100] & 42.41 & 4.52 & Coal ash & 89.34 \\
[141] & 24.3 & 14.76 & - & 39.26 \\
[142] & 32.85 & 4.2 & KOH & 87.21 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Viscosity of treated and transesterified waste cooking oils. in Table 9. A reduction in flash points is generally observed after transesterification, primarily due to the removal of glycerol, the central backbone of triglycerides, which increases the volatility of the resulting esters compared to the original waste cooking oil [101]. For instance, the flash point dropped from 290 \({}^{\circ}\)C to 220 \({}^{\circ}\)C with NaOH as the catalyst [82], representing a 24.14% decrease. A more pronounced reduction was observed in Ref. [76], where the flash point fell from 230 \({}^{\circ}\)C to 140 \({}^{\circ}\)C using CaO, equivalent to a 39.13% decrease. The highest reduction occurred in Ref. [142], with a 59.42% decrease from 308.03 \({}^{\circ}\)C to 125 \({}^{\circ}\)C when KOH was used. Conversely, an anomalous increase in flash point, from 129.6 \({}^{\circ}\)C to 235.7 \({}^{\circ}\)C, was reported in Ref. [141], suggesting the influence of other process variables such as alcohol recovery, feedstock purity, and transesterification efficiency. Among the various factors contributing to flash point variation, the chemical nature of the base oil and the type of catalyst used are important. However, the most critical determinant is the presence of residual short-chain alcohols (e.g., methanol or ethanol) in the final product [146]. Incomplete purification following the transesterification reaction can leave behind traces of these alcohols, which are highly volatile and can significantly depress the flash point of the resulting esterified insulating fluid [147]. This makes the liquid more prone to vaporization and ignition, thus compromising fire safety. Therefore, implementing rigorous post-reaction purification, such as vacuum distillation, washing, or adsorption, is crucial to ensure that bio-based insulating liquids derived from waste oils meet the safety standards required for use in high-voltage equipment.