Thermally conductive silicone cloth is a critical thermal interface material in electric vehicle battery packs, providing both heat dissipation and electrical insulation between cells, modules, and cooling plates. Comprehensive performance testing verifies thermal conductivity, dielectric strength, mechanical durability, and long-term reliability under demanding automotive conditions. This article outlines the key testing samples, test items, methods, standards, and application scenarios for thermal silicone cloth used in automotive battery systems.
Technical Overview
Thermally conductive silicone cloth (also referred to as silicone-coated fiberglass fabric or thermal silicone pad fabric) is a composite material consisting of a fiberglass substrate coated on one or both sides with silicone rubber filled with thermally conductive particles such as aluminum oxide, boron nitride, or aluminum nitride. In automotive battery applications, this material serves a dual function: it conducts heat away from lithium-ion cells toward cooling plates or heat spreaders, while simultaneously maintaining electrical insulation to prevent short circuits between adjacent cells and grounded metal structures.
Because battery packs operate across wide temperature ranges (-40 °C to +150 °C in some designs), experience vibration and mechanical compression, and must meet strict safety requirements, manufacturers and vehicle OEMs require verified material performance data. Performance testing of thermal silicone cloth therefore covers thermal properties, electrical insulation characteristics, mechanical strength, flame retardancy, environmental durability, and aging behavior. Testing is typically performed by accredited third-party laboratories or in-house quality departments in accordance with international and national standards published by ISO, IEC, ASTM, UL, EN, GB, and JIS organizations.
Testing Samples
Standard Sheets and Rolls
Unprocessed silicone cloth in sheet or roll form is tested to evaluate the intrinsic material properties. Typical sample dimensions for thermal testing are 50 mm × 50 mm discs or squares, while tensile testing uses dumbbell-shaped specimens die-cut along both the machine direction (MD) and transverse direction (TD) to capture anisotropic behavior introduced by the fiberglass reinforcement.
Die-Cut Parts and Custom Components
Finished components, such as cell-to-cell pads, gap fillers, insulating barriers, and laser-welded or adhesive-backed configurations, are tested to verify that converting processes (cutting, lamination, bonding) have not degraded performance. Critical dimensions, edge quality, and adhesive peel strength are examined in addition to bulk material properties.
Aged and Environmental-Conditioned Specimens
To assess long-term reliability, samples are conditioned before testing through heat aging, humidity exposure, thermal cycling, compression under load, and immersion in battery coolants or electrolytes. Post-conditioning measurements reveal changes in thermal conductivity, dielectric strength, hardness, and mechanical integrity.
Testing Items
Performance evaluation of automotive battery thermal silicone cloth typically includes the following categories:
- Thermal properties: thermal conductivity (W/m·K), thermal impedance (K·cm²/W or °C·in²/W), specific heat capacity, and thermal diffusivity.
- Electrical properties: dielectric breakdown voltage (kV or kV/mm), dielectric strength, volume resistivity (Ω·cm), surface resistivity, and insulation resistance.
- Mechanical properties: tensile strength, elongation at break, tear resistance, puncture resistance, hardness (Shore A or Shore C for fabric-coated materials), flexibility and mandrel bend performance, and compression deflection characteristics.
- Physical properties: thickness and thickness uniformity, areal density, basis weight, density, and dimensional stability.
- Flame retardancy: burning behavior ratings such as UL 94 V-0/V-1, horizontal and vertical burning tests, and limiting oxygen index (LOI).
- Environmental durability: heat aging resistance, low-temperature flexibility, damp-heat aging, thermal shock and thermal cycling resistance, coolant and oil resistance, and hydrolysis resistance.
- Reliability indicators: compression set, stress relaxation, outgassing and volatile content (important for optical sensors and electronics), and long-term thermal endurance (relative temperature index, RTI).
Testing Methods
Testing is conducted in accordance with recognized international and national standards. The most commonly applied methods are summarized below:
Thermal Conductivity Testing
- ASTM D5470 – steady-state heat flow method measuring thermal impedance and effective thermal conductivity of thermally conductive electrical insulation materials under controlled compression pressure; the primary industry method for thermal interface materials.
- ASTM E1461 / ISO 22007-4 – laser flash method and transient hot-wire (or hot-disk) techniques for rapid determination of thermal diffusivity and conductivity.
- ISO 22007-2 – transient plane heat source method suitable for flexible sheet materials.
Electrical Insulation Testing
- ASTM D149 / IEC 60243-1 / GB/T 1408.1 – dielectric breakdown voltage and dielectric strength at power frequency using short-time, step-by-step, or slow rate-of-rise methods.
- ASTM D257 / IEC 62631-3 / GB/T 1410 – volume and surface resistivity measurement using guarded electrode configurations.
- IEC 60167 / GB/T 10064 – insulation resistance measurement of solid insulating materials.
Mechanical and Physical Testing
- ASTM D412 / ISO 37 / GB/T 528 – tensile strength and elongation at break of the silicone coating and composite fabric.
- ASTM D882 / GB/T 1040.3 – tensile properties of thin plastic sheeting for coated fabrics.
- ASTM D2240 / ISO 7619-1 / GB/T 531.1 – Shore hardness (durometer) measurement.
- ASTM D374 / GB/T 6672 – thickness measurement of solid electrical insulating materials.
- ASTM D5733 / ASTM D5735 – tear resistance of coated fabrics (trapezoid and trouser methods).
Flame Retardancy Testing
- UL 94 – vertical and horizontal burning tests for material flammability classification (V-0, V-1, V-2, HB).
- GB/T 2408 / IEC 60695-11-10 – burning characteristics of solid materials using horizontal and vertical specimens.
- ASTM D2863 / GB/T 2406.2 / ISO 4589-2 – limiting oxygen index determination.
Environmental and Aging Testing
- ASTM D573 / GB/T 3512 / ISO 188 – accelerated aging in air oven, with post-aging evaluation of tensile, hardness, and dielectric properties.
- IEC 60068-2-14 / GB/T 2423.22 – thermal shock and temperature cycling (e.g., -40 °C to +125 °C).
- IEC 60068-2-78 / GB/T 2423.3 – damp heat steady-state testing (e.g., 85 °C / 85% RH).
- UL 746B – long-term properties evaluation for polymeric materials, including relative temperature index.
- ASTM D471 / GB/T 1690 – resistance to liquids, including battery coolants, oils, and electrolyte simulant immersion.
Applications
Thermal silicone cloth and pads are used throughout electric and hybrid vehicle battery systems, as well as adjacent power electronics:
- Cell-to-cell thermal management: inserted between cylindrical, prismatic, or pouch cells to equalize temperature and conduct heat to cooling structures while preventing cell-to-cell electrical contact.
- Module-to-cold-plate interfaces: filling air gaps between battery modules and liquid cooling plates to reduce thermal contact resistance.
- Insulation barriers: providing high-voltage insulation between busbars, sensing harnesses, and the battery enclosure.
- Battery enclosures and covers: serving as thermally conductive, electrically insulating layers in pack lids and structural plates.
- Power electronics: thermal interface layers between IGBT modules, inverters, onboard chargers, and heat sinks, where dielectric strength and long-term thermal stability are essential.
- Energy storage systems (ESS): stationary battery cabinets for grid storage employing the same silicone cloth materials, tested to comparable performance criteria.
FAQ
Q: What thermal conductivity value is typical for automotive-grade thermal silicone cloth?
A: Commercial materials generally range from approximately 0.8 W/m·K to 4.0 W/m·K depending on filler loading. Because fiberglass reinforcement creates anisotropy, through-plane (perpendicular to the fabric plane) conductivity is the value most relevant for battery interface applications and is the value reported under ASTM D5470 testing.
Q: Why is dielectric breakdown testing important for this material?
A: In battery packs operating at 400 V or 800 V, the silicone cloth must simultaneously transfer heat and block electrical current. Dielectric strength testing verifies that the material withstands specified operating and surge voltages without breakdown, which is a core safety requirement in EV design validation.
Q: How is long-term reliability confirmed?
A: Reliability is demonstrated through accelerated aging programs combining heat aging, thermal cycling, damp-heat exposure, and compression under load, followed by re-measurement of thermal, electrical, and mechanical properties. Materials that retain a high percentage of their initial dielectric strength and tensile performance after aging are considered suitable for the vehicle service life.
Q: Should testing be performed in-house or by an external laboratory?
A: Incoming material inspection and routine quality control can be performed in-house using standardized instruments. For design validation, regulatory compliance, and customer acceptance, testing at accredited third-party laboratories with ISO/IEC 17025 accreditation provides independent, internationally recognized results.
Q: Does flame retardancy certification apply to battery thermal materials?
A: Yes. Most automotive specifications require a flame retardancy rating such as UL 94 V-0 or the equivalent GB/T 2408 classification, verified through vertical burning tests. This requirement limits fire propagation risk in the event of thermal runaway and is a mandatory item in most battery pack material specifications.