Natural vs Synthetic Esters for Immersion Cooling
Both ester types are used as dielectric coolants. Their chemistry leads to different strengths – and to different requirements for the tank.
Updated 9 November 2026 · Published by IsoTec
Short answer
Synthetic esters are more resistant to oxidation, have a lower pour point and suit tanks with air contact. Natural esters are vegetable-oil based and have a very high fire point, but need covered or inerted tanks, antioxidants and close monitoring. Both tolerate moisture far better than hydrocarbons.
Chemistry
Natural esters are triglycerides – glycerol esterified with fatty acids from vegetable oils such as rapeseed, soybean or sunflower. Part of these fatty acids is unsaturated, which makes the molecule more reactive with oxygen.
Synthetic esters for dielectric use are mostly polyol esters, made by reacting polyols such as pentaerythritol with selected saturated fatty acids. The saturated structure gives high oxidation and thermal stability. The fatty acids can be of vegetable origin.
Comparison
| Property | Natural ester | Synthetic ester |
|---|---|---|
| Base | Refined vegetable oil (triglyceride) | Polyol ester |
| Oxidation stability | Lower – limit air contact | Very high |
| Suitable tank | Sealed, tightly covered or inert-gas blanketed | Covered open bath or sealed |
| Viscosity | Higher | Lower to moderate, depending on grade |
| Pour point | Higher – check for cold start | Low |
| Fire point | Very high | High |
| Moisture tolerance | High | High |
| Hydrolysis | Possible with water and heat | Possible with water and heat |
Typical tendencies; individual products differ.
Why air contact matters in data center tanks
Many single-phase tanks are open baths with lids. Each time a server is lifted out, air reaches the fluid surface. Oxygen, heat and metal surfaces such as copper accelerate oxidation. For natural esters, this can lead to rising acidity and viscosity and, in the long term, to deposits. Synthetic esters are much less affected. The OCP Immersion Requirements recommend a lid on every open bath to limit the air-to-liquid interface.
When natural esters make sense
- Closed chassis-level (precision) immersion with little air exchange
- Tanks with nitrogen blanketing or tight lids
- Projects where a vegetable-oil base is a requirement
- Applications with a strong focus on a very high fire point
In all cases, a suitable antioxidant package and a monitoring plan for acidity and viscosity are part of the solution.
When synthetic esters make sense
- Open-bath tanks that are opened regularly for service
- Higher operating temperatures, for example to enable heat reuse
- Locations with cold start conditions
- Long service life with limited maintenance
IsoTec ester fluids
IsoTec works with both ester types: IsoTec SE 300 is a synthetic ester and IsoTec NE 350 a natural ester for electrical equipment. For immersion cooling projects we select and test the ester fluid for your tank design and components. See the data center immersion cooling guide.
Frequently asked questions
Are natural esters suitable for data center immersion cooling?
Yes, if air contact is limited – for example in sealed chassis or tanks with tight lids or nitrogen blanketing – and the fluid contains suitable antioxidants and is monitored.
Why do natural esters oxidise faster?
Part of their fatty acids is unsaturated. The double bonds react with oxygen, especially at higher temperatures and in contact with copper.
Are synthetic esters made from vegetable raw materials?
They can be. Many polyol esters use fatty acids of vegetable origin, combined with a synthetic polyol.
Natural or synthetic – which ester fits your tank?
We test both ester types with your components and operating conditions. IsoTec products are supplied to customers outside Germany.