Screening biodegradable alternatives to mineral oil coolants

Abstract

Mineral oils have been used extensively as cooling liquids, particularly when electrical insulation and high thermal stability are required. However, their use is increasingly scrutinized as they biodegrade slowly and accumulate in the environment. While alternatives including vegetable oils have been studied, viable options would need to provide comparable thermal and physical properties while being biodegradable, renewable, and cost-effective. In this study, candidate fluids, including mixtures of non-edible oils, essential oils, and synthetic esters were screened for thermophysical properties specifically density, specific heat capacity, viscosity, and thermal conductivity. To accommodate a wide range of fluids, fast multiplexed measurements were employed – an experimental campaign involving 2,500 tests performed at 50-fold the rate of conventional testing (that would require over 1,000 h). Density and viscosity were measured using a flow-through resonance quartz sensor, specific heat capacity was determined using a microfluidic calorimeter, and thermal conductivity was predicted using empirical models tailored to different molecular structures. With this approach, we identified biodegradable alternative mixtures – among a broad spectrum of vegetable oils, non-edible oils, essential oils, and natural and synthetic esters – with overall performance exceeding that of mineral oils. The best-performing formulations consisted of 65% jojoba or cottonseed oil (non-edible oils), 15% pine or tea tree oil (essential oils), and 20% synthetic ester (MIDEL 7131), achieving a balance of viscosity, oxidative stability, and thermal efficiency. These results confirm the feasibility of developing viable, biodegradable, renewable, and cost-effective alternatives that avoid the environmental costs of mineral oils.