Thermal material selection inside a battery pack is a different problem from insulation elsewhere in the vehicle. The foam has to hold a cell in place for the pack's life, survive the temperature it actually sees, avoid absorbing anything that could reach the cell, and stay non-conductive. This guide walks through how to choose closed-cell cross-linked foam for battery pack thermal insulation and cell spacing, using measured material data rather than marketing claims.
What the Foam Has to Do
Inside a pack, one foam layer usually serves several functions at once:
- Cell spacing and location — holding cells at a defined gap so the module stays dimensionally correct.
- Thermal insulation — slowing heat transfer between cells and to the enclosure, which shapes how a thermal event propagates.
- Vibration and impact damping — protecting welds and busbars from road-induced fatigue.
- Dielectric separation — keeping a non-conductive barrier between live parts and the case.
Closed-cell cross-linked polyethylene foam (XPE and IXPE) covers most of this. IXPP, a cross-linked polypropylene foam, is chosen where the operating temperature is higher.
Material Choice: XPE, IXPE or IXPP
- XPE — chemical cross-linking, single layer 3–15 mm, heat-bonded 15–200 mm, ratio 3–40x. Denser, stiffer, the workhorse for module spacers and structural pads.
- IXPE — radiation cross-linking, single layer 0.5–14 mm, laminated 15–200 mm, ratio 3–40x. Thinner and finer-celled, so it conforms to curved cell cans and prismatic cell faces.
- IXPP — cross-linked polypropylene, single layer 1–6 mm, laminated 6–100 mm, ratio 5–25x. Selected for better temperature resistance than PE foam; confirm the service temperature with your supplier for the specific grade.
All three are closed-cell. That is not a cosmetic detail: an open-cell or felt material in a pack can retain moisture and electrolyte residue, and it sheds fibre into an environment you cannot service.
Thermal Conductivity and Thickness
Thermal conductivity in the XPE and IXPE ranges improves as the foam gets lighter (higher expansion ratio), because more of the volume is gas rather than polymer. Measured to GB/T 10297:
| Expansion ratio | Density (kg/m³) | Thermal conductivity (W/m·K) | Compression set, 23°C/22h |
|---|---|---|---|
| 3x | 300±40 | 0.095 | ≤10% (GB/T 6669) |
| 5x | 200±30 | ≤0.095 | |
| 8x | 125±15 | ≤0.084 | |
| 10x | 100±10 | ≤0.073 | |
| 15x | 66.7±8 | ≤0.065 | ≤10% |
| 20x | 50±6 | 0.055 | |
| 25x | 40±4 | ≤0.049 | |
| 30x | 33.3±3 | 0.045 | ≤10% |
| 35x | 28.6±2 | ≤0.040 | |
| 40x | 26.8±3 | ≤0.040 |
The practical tension: lighter foam insulates better but is softer and compresses more, so it holds cells less firmly. Lighter foam is also more affected by compression set. For a cell spacer, start around 33–67 kg/m³ (15x–30x) and tune against a measured compression-force requirement rather than picking the lowest conductivity.
Moisture, Dimensional Stability and Ageing
- Water absorption — ≤0.2% (3x–15x), ≤0.3% (20x–25x), ≤0.4% (30x–40x), to GB/T 1034 (method 4). Low absorption keeps the pad from swelling or holding condensate.
- Dimensional stability — at 70±2 °C for 22 hours, XPE dimensional change is typically around 2% or less for most grades (GB/T 8811), with 15x at 2–6%.
- Compression set — ≤10% at 23±2 °C for 22 hours. A spacer that takes a set stops locating the cell.
None of these values should be read as a maximum service temperature. Continuous temperature capability depends on grade and on how long the exposure lasts; state your worst-case condition to the supplier and get the material confirmed for it rather than extrapolating from a 22-hour test.
Electrical Behaviour
Polyethylene foam is a non-polar dielectric and is used as a spacer and separation layer inside packs. Two points to be clear about:
- Foam is not solid polymer. Cell structure and thickness drive breakdown behaviour, so validate the finished part at your system voltage and creepage rather than relying on a generic figure.
- If you need a defined resistivity, specify it. Anti-static and conductive foam grades exist for controlled dissipation; standard XPE/IXPE is not a substitute for them.
How to Specify Battery Pack Foam
- Function — spacer, insulator, damper, or a combination.
- Material — XPE, IXPE or IXPP, with the cross-linking process stated.
- Density and tolerance — e.g. 40 ±4 kg/m³.
- Thickness — single layer or heat-bonded, with tolerance.
- Compression force — the requirement that actually locates the cell.
- Electrical requirement — insulating, or a specified resistivity grade.
- Temperature and ageing — worst-case condition, plus flammability and restricted-substance requirements your programme demands, declared by the supplier.
Common Questions
Which foam goes between prismatic cells? A thin IXPE or a mid-density XPE sheet, chosen for compression force and thickness tolerance; the goal is a defined, stable gap rather than maximum insulation.
Is a lighter foam always better for insulation? Its conductivity is lower, but it compresses further and takes a set sooner. If the layer also locates the cell, density is usually set by the mechanical requirement.
Can the same foam be used for the pack lid? Often yes, but the lid sees different heat and load, so treat it as a separate specification rather than reusing the cell-spacer grade.
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Related Reading
See also: XPE vs IXPE vs IXPP · Automotive foam solutions · All foam materials