Biomass-Derived Ester-Rich Insulating Fluids from Soybean and Canola Oils: Route-Specific Synthesis and Preliminary Performance Screening

The valorization of vegetable-oil biomass into bio-based functional fluids offers a sustainable route for replacing petroleum-derived insulating liquids in power equipment. In this study, soybean and canola oils were used as renewable lipid feedstocks and converted into biomass-derived ester fluids through acid-catalyzed transesterification with methanol, ethanol, 1-propanol, and 1-butanol. The obtained ester-rich products were subjected to a combined physicochemical, dielectric, and thermal screening workflow, including kinematic viscosity at 40 °C (ν40), acid value, breakdown voltage (BDV), differential scanning calorimetry (DSC; 2–8 °C min−1 under N2), and oxygen bomb calorimetry. Transesterification effectively upgraded the vegetable oils into low-viscosity ester-rich product fluids for most alcohol routes, with soybean methyl ester (SME) reaching 4.41 ± 0.02 mm2 s−1 and selected canola-derived esters showing viscosities of 5.81–6.81 mm2 s−1. However, the functional performance of the biomass-derived fluids was strongly governed by the alcohol route. SME exhibited the most favorable balance between dielectric and physicochemical properties, delivering the highest BDV of 64.90 ± 9.74 kV, exceeding the IEC 60156 threshold of 30 kV, while maintaining a low acid value of 0.0103 ± 0.0006 mg KOH g−1. In contrast, propyl- and butyl-derived esters showed substantially lower BDV values of ≤14.98 kV, whereas ethanol-derived products retained near-neat-oil viscosities and were unsuitable for BDV testing under the applied conditions. Although propyl- and butyl-derived ester-rich products reduced kinematic viscosity, their markedly lower BDV values were likely associated with route-dependent product heterogeneity, lower alcohol–oil miscibility, possible residual polar impurities, and moisture sensitivity; therefore, they were regarded as non-optimized screening outcomes rather than IEC-compliant transformer-fluid candidates. DSC analysis provided comparative thermal-response descriptors under nitrogen, with methylation producing more coherent endothermic features. The combustion heats of the ester-rich products were concentrated at approximately 39–41 MJ kg−1, lower than that of the mineral-oil reference in this dataset, suggesting combustion heat was used only as a preliminary energy-density descriptor and was not interpreted as direct evidence of improved fire safety. From an engineering-safety perspective, the lower combustion heat of the bio-esters may reduce the potential fire-load contribution during fault-related fire scenarios, although full fire-safety qualification requires additional flash-point, fire-point, and aging evaluations. Overall, this work demonstrates that alcohol route selection is a critical factor in converting vegetable oil biomass into high-value bio-based insulating fluids. Among the tested formulations, soybean methyl ester is the most promising baseline candidate for further development as a biodegradable, sustainable transformer fluid.