As EVs (Electric Vehicles) push toward longer range, faster charging, and higher energy density, thermal management has become one of the most significant engineering challenges in the industry. Battery packs generate heat during their charge and discharge cycles. Keeping battery cell temperatures within a narrow operating band is critical not just to performance but also to longevity and safety. At the center of the cooling system are the seals that keep coolant where it belongs.
Choosing the right seal material is not a minor detail, as it directly impacts warranty exposure, system reliability, and platform longevity. And PTFE (Polytetrafluoroethylene) has emerged as one of the most effective seal materials for EV battery cooling applications, and for good reason.
The Challenges of the EV Battery Cooling Environment
The current trend in coolant for EV battery cooling systems is immersion cooling with synthetic dielectric fluids for high-performance and fast-charging applications (800V architectures), since they allow much faster heat removal. However, for most mass-market EVs, glycol/water mixture (WEG) in cold-plate systems remains the most commonly used due to cost and simplicity.
The chemistry of glycol/water coolant is extremely aggressive toward a number of materials. Common issues that can develop include:
- Galvanic corrosion can occur where dissimilar metals meet
- Coolant pH drops and becomes acidic as the inhibitor package depletes
- Coolant degradation generates glycolic and oxalic acids
- Degraded coolant loses thermal efficiency
- Seal compression set
- Vibration-induced fatigue at fittings and crimped connections
- Thermal cycling causes repeated expansion/contraction
- Even a small internal leak can be catastrophic since coolant contacting cells can cause short circuits or thermal runaway
- Off-gassing from degraded coolant or seals
Seals intended for battery cooling systems are exposed to a wide operating temperature range of -40°C to 150°C. They are also exposed to pressure cycling and vibration in battery thermal loops. Finally, they have long service life expectations (150k–200k miles).
Why PTFE Performs Well for EV Battery Cooling Systems
PTFE is the most chemically inert polymer on the market, and that means it is resistant to the chemical effects of glycol-based coolants, especially as they degrade. It also has extremely low friction, which has a positive impact on pump efficiency and reduces seal wear (extending the life of the battery system). While PTFE is subject to creep, that can be addressed through the judicious use of common fillers (e.g., carbon fiber, graphite, glass).
PTFE liners and gaskets can electrically isolate dissimilar metals, interrupting the galvanic cell and preventing galvanic corrosion. As just alluded to, it is chemically inert across virtually the entire pH spectrum (pH 0–14). While acidic coolants attack rubber seals or metal surfaces, they have essentially no effect on PTFE. This inertness makes it a stable sealing material even in neglected systems. In addition, PTFE does not react with glycolic or oxalic acid. This means that seal integrity is maintained even as the coolant degrades.
While PTFE does not directly restore thermal efficiency, it does not shed any type of degradation byproducts into the coolant. This is an excellent feature, as it avoids contributing particulate or chemical contamination that reduces efficiency or fouls the cold plate channels. In addition, PTFE is thermally stable up to about 260°C and does not off-gas meaningful volatile compounds under normal or even moderately elevated operating conditions.
PTFE does have a very low creep resistance, which is actually a weakness when it comes to EV battery cooling systems. It is prone to cold flow under sustained load and does not recover well. However, this is typically managed by pairing PTFE with a spring energizer or backing elastomer to maintain a consistent sealing force over time.
Its low friction coefficient means it does not transmit vibrational micro-movement into fretting or abrasive wear at interfaces the way harder materials might. PTFE-lined fittings also tend to dampen rather than amplify vibration stress at the sealing face.
PTFE has a relatively high coefficient of thermal expansion, which is a known limitation. However, its expansion is highly consistent and predictable. When properly designed as a joint with appropriate preload, it accommodates cycling without cracking or permanent deformation.
Its low permeability and chemical stability mean it is less likely to develop micro-leaks or weep paths, and reducing seal-originated leak risk is highly significant in a battery pack where any coolant ingress near the battery cells is a serious safety event.
PTFE Seal Types Used in EV Thermal Management
The three most common types of PTFE seals are spring-energized seals for rotary and reciprocating pumps, lip seals for coolant circulation systems, and gaskets and diaphragms for valve and manifold assemblies.
In a spring-energized seal, a metal spring compensates for PTFE’s cold-flow tendency by maintaining a consistent sealing pressure. This pressure is maintained even when subject to temperature cycling and acidic coolant conditions. PTFE lip seals, in addition to PTFE’s other features, provide extremely low friction and are self-lubricating.
Finally, expanded PTFE gaskets will conform under bolt load across dissimilar metal flanges, all while resisting chemical attack. In fact, PTFE diaphragms provide an inert barrier that protects valve internals from coolant and prevents degradation products from entering the fluid.
Unfilled vs Filled PTFE
Unfilled PTFE (also referred to as virgin PTFE) has some serious limitations, like creep and wear. Because of this, unfilled PTFE is rarely used for seals unless it is for a static application. However, the use of additive fillers can address those issues and further enhance the performance of PTFE. Fillers can improve wear life, reduce creep, and increase stiffness, as well as introduce electrical conductivity. The drawback of fillers is that their use can compromise the natural chemical resistance of PTFE.
Glass, for example, reduces creep, increases compressive strength, and reduces how much PTFE deforms under compressive loads. It is, however, abrasive and is a poor choice for rotary applications or alkaline environments. To achieve the lowest friction of any PTFE compound, PI (Polyimide) is used. It also enhances wear and abrasion resistance and works extremely well in dry-running applications.
Carbon additives also increase compressive strength, but also enhance thermal conductivity, wear resistance, and hardness. Graphite, which is a crystal-modified high-purity carbon, decreases friction and increases the load-carrying capabilities of PTFE.
Carbon-filled and carbon/graphite blends are the most common choices for EV cooling seal specifications because they offer the best overall balance of wear life, chemical compatibility with glycol/water, and creep resistance. Glass-filled grades are the typical fallback where higher stiffness is needed.
Conclusion
Battery cooling systems are a critical aspect of EV functionality. And for those systems to do their job, they need seals that are reliable and engineered for harsh environments. Filled PTFE has risen to the top as one of the materials of choice for spring-energized seals, lip seals, gaskets, and diaphragms. To learn more about the options available for coolant systems, contact the seal experts at Advanced EMC today.