Immersion Cooling Fluid Compatibility: EPDM, NBR, Silicone, Plastics and TIMs

A dielectric immersion cooling fluid can meet electrical and thermal specifications and still damage a server.

The problem begins when the fluid enters a polymer or pulls part of the polymer formulation into the fluid. A seal may swell and lose compression. A cable jacket may lose plasticizer and become hard. A thermal grease may separate or wash away from the interface. The affected material changes, but the extracted compounds can also change the color, acidity and dielectric performance of the cooling fluid.

This is why immersion compatibility cannot be assigned to a fluid or a material alone. It belongs to a specific fluid–material combination at a specific temperature and exposure time.

Compatibility Is More Than “The Material Did Not Dissolve”

Most immersion-related polymer failures follow one of two paths.

In the first, the material absorbs the cooling fluid. Its mass and volume increase, and its hardness may fall. An O-ring that still looks intact can become too soft or too large for its groove. A hose can deform around a fitting. The Open Compute Project material compatibility guide identifies excessive EPDM hose swelling as a possible cause of leakage.

In the second path, the cooling fluid extracts mobile compounds from the material. Plasticizers, process oils, uncured oligomers or part of a TIM binder move into the fluid. The original component may shrink, harden, crack or lose cohesion. Meanwhile, the fluid can become hazy, darker or less electrically stable.

Temperature accelerates both processes. A material that changes little during a short room-temperature soak may behave very differently after months beside a high-power CPU or GPU.

The word “compatible” therefore needs a measurable meaning. It should cover dimensional change, mass change, hardness, mechanical function and appearance of the material. It should also cover changes in the fluid, including breakdown voltage, dielectric dissipation factor, acid value and contamination.

EPDM, NBR and Silicone Do Not Behave as One Group

EPDM, NBR and silicone are all elastomers, but their interaction with immersion fluids can be very different.

For a synthetic hydrocarbon fluid, the OCP guide lists nitrile rubber containing more than 35% nitrile as acceptable in its tested system. The same chart classifies EPDM and silicone rubber as unacceptable because of weight or volume increase and, for EPDM, dissolution. In its PAO chart, NBR remains acceptable while EPDM and silicone move into the case-by-case category.

The pattern has a chemical basis. EPDM has good resistance to water, steam and many polar fluids, but non-polar hydrocarbon oils can enter its polymer network and cause heavy swelling. NBR contains polar acrylonitrile groups that improve its resistance to many oils. A higher acrylonitrile content usually improves oil resistance, which is why a label such as “NBR” is incomplete without the actual compound or grade.

Silicone rubber is also not automatically safe because it is heat resistant. Its open siloxane structure can absorb oils and lose dimensional stability. The result depends on the crosslink density, fillers, post-curing and other parts of the compound. A silicone hose, silicone O-ring and silicone thermal pad may therefore behave differently in the same fluid.

The practical starting point is:

Plastics and TIMs Can Fail Without an Obvious Leak

Plastic parts create a different problem because the resin name describes only part of the material. A finished cable jacket, connector, fan part or clip may also contain plasticizers, flame retardants, pigments, stabilizers and reinforcing fibers.

Plasticized PVC is a clear example. The immersion fluid can extract its plasticizer. The cable jacket then loses mass and gradually becomes harder or more brittle, while the released plasticizer remains in the cooling loop. Polyethylene and polypropylene may show the opposite response: they can absorb hydrocarbon fluid, gain mass and change dimensions.

Related Plasticizers：

Plasticizer extraction can change the weight, hardness and flexibility of PVC components exposed to immersion cooling fluids. Foconsci supplies plasticizers for PVC and other polymer formulations, including : DOP CAS 117-81-7; DOTP CAS 6422-86-2; DOA Plasticizer.

Even plastics that perform well as unstressed coupons may fail as real parts. Molded-in stress, snap-fit load, thin sections and elevated temperature can turn moderate absorption into warping or environmental stress cracking. Compatibility data for an unfilled resin should not be applied directly to a glass-filled connector or a flame-retardant housing.

TIMs create an even less visible risk. Thermal grease, putty, phase-change material and polymer pads contain an organic phase that holds thermally conductive fillers in place. If the immersion fluid dissolves or dilutes that phase, the TIM can bleed, swell, separate or move away from the contact area. The processor may then run hotter even though the heat sink still appears firmly mounted.

The released binder and low-molecular-weight material do not disappear. They enter the tank, where they may discolor the fluid, load the filter or deposit elsewhere. The OCP design guidelines for immersion-cooled IT equipment therefore warn that thermal compounds and pastes may dissolve, reduce heat transfer and pollute the dielectric fluid.

A silicone thermal pad is not automatically compatible simply because it is solid. Its silicone polymer, mobile silicone oil and surface treatment must still survive the chosen fluid. A metal TIM such as indium foil avoids dissolution of an organic binder, but it introduces a different set of requirements: sufficient mounting pressure, suitable surface flatness, controlled bond-line thickness and correct metal selection.

The real TIM question is not “Does the sample still exist after soaking?” It is “Does the assembled interface retain its thermal resistance after aging in the fluid?”

Test the Exact Fluid, Compound and Finished Part

A generic compatibility chart is useful for removing clearly unsuitable candidates. It cannot qualify the final system. Fluid composition varies within the same chemical family, and commercial polymers with the same name use different additives and cure systems.

The OCP guideline proposes an accelerated fluid-aging test at 80°C for 336 hours, followed by measurements of material dimensions, mass, color and Shore hardness. It also calls for checking the fluid’s color, breakdown voltage, dielectric dissipation factor and acid value. Its suggested classification treats changes below 10% as acceptable, 10–20% as case-by-case and above 20% as unacceptable. These limits provide a common screening framework; the actual component may require tighter limits.

A useful qualification sequence is:

1. Screen material coupons. Compare mass, volume, hardness and appearance before and after exposure.
2. Test finished components. Age the actual O-ring, hose, connector, cable, plastic clip or TIM rather than relying only on a generic resin sample.
3. Check function under load. Measure sealing force, retention strength, flexibility, electrical performance or TIM thermal resistance as appropriate.
4. Test the fluid as well. Look for extracted material, sediment, haze, acidity change and loss of dielectric strength.
5. Repeat with aged fluid and mixed materials. A tank contains many polymers at once, and contamination released by one material can affect another component.

For two-phase fluorinated systems, ordinary static soaking may miss the stronger extraction created by repeated evaporation and condensation. OCP recommends adding a Soxhlet-type extraction test for this service.

Material selection should follow the same direction as the test: start with the actual cooling fluid, identify every wetted material, and qualify the combination under realistic temperature and time. EPDM, NBR, silicone, plastics and TIMs do not carry universal “compatible” or “incompatible” labels. Their behavior becomes meaningful only after the fluid, formulation and component function are defined together.

Looking for Materials for Immersion Cooling Applications?

Foconsci supplies silicone oils, plasticizers, polymer additives and other chemical materials for industrial formulations.

If you are evaluating materials for an immersion cooling system, tell us the cooling fluid type, material or formulation, operating temperature, and the problem you are trying to solve. Our team can help you identify suitable raw materials for further compatibility testing.

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