Volatile matter determination for power battery thermal conductive pads is a critical quality control test that quantifies the low-molecular-weight substances released by silicone and non-silicone thermal interface materials under elevated temperatures. Excessive volatilization in battery packs can cause contact contamination, insulation degradation, condensation, and loss of thermal contact pressure. This article explains the test principles, applicable standards, sample types, analytical methods, and industry applications of volatile content testing for EV battery thermal pads.
Technical Overview
Thermal conductive pads are elastomeric thermal interface materials (TIMs) formed by dispersing high-loading ceramic fillers—typically alumina, aluminum nitride, or boron nitride—into a silicone or non-silicone polymer matrix. In power battery systems, these pads are installed between prismatic or pouch cells, between cell stacks and liquid cooling plates, and between modules and structural enclosures to reduce thermal contact resistance and equalize temperature distribution across the pack.
During long-term service at typical operating temperatures of 40 °C to 90 °C, with transient excursions above 100 °C, low-molecular-weight species embedded in the pad matrix gradually migrate to the surface and volatilize. These volatiles mainly include cyclic siloxane oligomers (D3 to D10), residual crosslinking agents, platinum catalyst residues, plasticizers, process solvents, and absorbed moisture. The consequences of uncontrolled outgassing are significant:
- Condensation of silicone volatiles on cooler surfaces and enclosure walls, producing film contamination that is difficult to remove
- Deposition on relay contacts, PCB connectors, and current sensor terminals, increasing contact resistance and causing intermittent or permanent circuit failures
- Shrinkage and mass loss of the pad, reducing compression deflection and contact pressure, which increases thermal interface resistance over time
- Potential degradation of insulation resistance and dielectric performance of adjacent components
- Fogging and contamination of optical sensors and venting membranes in sealed pack environments
For this reason, battery manufacturers and material suppliers specify strict volatile content limits in pad datasheets—commonly less than or equal to 0.5% by mass for low-volatility grades, and less than or equal to 1.0% for general-purpose grades. Determination of volatile matter is therefore a mandatory incoming inspection and qualification item. The test framework draws on standards such as ISO 248 (volatile matter in rubber), ASTM E595 (total mass loss and collected volatile condensable materials), ISO 11358 (thermogravimetry of polymers), and ASTM E1131 (compositional analysis by thermogravimetry), supplemented by headspace and thermal desorption GC-MS procedures for volatile speciation.
Testing Samples
Silicone Thermal Conductive Pads and Gap Fillers
These are the most widely tested products, covering filled silicone pads with typical thicknesses of 0.5 mm to 10 mm and thermal conductivity ratings of 1 to 8 W/m·K. Both addition-cure (platinum-catalyzed) and peroxide-cure systems are tested, since cure chemistry strongly influences residual volatile levels. Specimens are normally die-cut to 50 mm × 50 mm sheets or prepared to a mass of 1 g to 5 g per replicate.
Silicone Foam and Blended Thermal Pads
Foamed thermal pads used for cell-to-cell buffering and expansion compensation contain blowing agent residues and cell gas that contribute to volatile loss. Testing focuses on mass loss after thermal conditioning as well as post-aging dimensional stability.
Non-Silicone Thermal Pads and Bonding TIMs
Acrylic, polyolefin, and polyurethane-based pads, including thermally conductive adhesive films and double-sided bonding pads, are tested for residual monomers, solvents, and plasticizer emissions, particularly where silicone contamination is prohibited by design.
Sample Preparation Requirements
- Condition specimens at 23 °C ± 2 °C and 50% ± 5% relative humidity for at least 24 hours prior to testing, in accordance with ISO 291
- Prepare a minimum of three replicates per sample lot
- Handle specimens with powder-free gloves and clean tweezers to avoid surface contamination
- Measure initial mass with an analytical balance of 0.1 mg readability
Testing Items
Typical volatile-related test items for power battery thermal conductive pads include:
- Volatile matter content (mass percentage) at specified temperature and dwell time, for example 150 °C for 2 hours
- Mass loss on heating at 105 °C, 125 °C, or 150 °C under atmospheric or vacuum conditions
- Thermogravimetric (TGA) mass-loss profile and volatile onset temperature
- Total mass loss (TML) and collected volatile condensable materials (CVCM) per ASTM E595
- VOC speciation and emission quantification by headspace or thermal desorption GC-MS, reported in µg/g
- Low-molecular-weight cyclic siloxane content (D3 to D10) for silicone-based pads
- Retention of thermal conductivity, hardness, and compression deflection after volatile-loss conditioning
Testing Methods
Gravimetric Oven Mass-Loss Method
The reference method for routine quality control follows the principle of ISO 248. Pre-weighed specimens are placed in a forced-ventilation oven set to the specified temperature—commonly 105 °C, 150 °C, or 200 °C—for a defined dwell time such as 1, 2, or 4 hours. After heating, specimens are cooled to ambient temperature in a desiccator and reweighed. The volatile matter content is calculated as (m0 − m1) / m0 × 100%, where m0 is the initial conditioned mass and m1 is the post-heating mass. Where specified by the customer, specimens are reconditioned at standard atmosphere for 24 hours before the final weighing to account for moisture readsorption.
Thermogravimetric Analysis (TGA)
TGA per ISO 11358 and ASTM E1131 provides a continuous mass-loss record. A specimen of 5 mg to 10 mg is heated under high-purity nitrogen from ambient temperature to 600 °C at 10 °C/min. The mass loss occurring below approximately 200 °C is attributed to volatiles, allowing clear discrimination between volatile loss and thermal decomposition of the polymer matrix. The method requires only milligram-scale samples and returns the volatile onset temperature, which is valuable for comparing post-cure completeness between production lots.
Outgassing Test (TML and CVCM)
Following ASTM E595, pre- and post-conditioned specimens are heated at 125 °C for 24 hours in a vacuum of 6.7 Pa or lower while a chromium-plated collector plate maintained at 25 °C faces the specimen. Total mass loss is calculated from specimen weighing, and CVCM is determined from the mass gain of the collector. Automotive and electronics specifications typically require TML below 1.0% and CVCM below 0.1%, confirming that emitted species do not recondense on nearby surfaces.
VOC Identification and Quantification by GC-MS
Where the source and chemistry of volatiles must be understood, headspace GC-MS or thermal desorption GC-MS is applied. Specimens are held in sealed vials at elevated temperature, and the evolved gases are analyzed to identify and quantify cyclic siloxanes, residual solvents, and decomposition markers against calibration standards. Results are reported as individual species concentrations in µg/g and as total VOC emission, supporting both compliance verification against OEM limits and root-cause investigation of formulation issues.
Method Selection Criteria
- Use the gravimetric oven method for routine incoming inspection and lot release against datasheet limits
- Use TGA when volatile content must be separated from thermal degradation or when sample quantity is limited
- Use ASTM E595 outgassing when condensation risk on electrical or optical components is the design concern
- Use GC-MS when volatile identification, supplier comparison, or failure analysis is required
Applications
Volatile matter determination supports the entire lifecycle of thermal interface materials in power battery programs:
- Incoming material quality control at battery pack plants, verifying that each pad lot meets the specified volatile limit before assembly
- Supplier qualification and benchmarking of competing pad materials, including comparison between addition-cure and peroxide-cure silicone systems and between standard and low-volatility grades
- Formulation and process optimization by material manufacturers, evaluating extended post-cure schedules, vacuum degassing, and raw-material purity improvements
- Thermal aging and reliability studies per IEC 60216-1 and ISO 188, correlating volatile loss with long-term changes in thermal conductivity, hardness, and compression set
- Design validation for cell-to-pack (CTP) and cell-to-body architectures, where pads are installed adjacent to sensitive electronics with limited ventilation
- Failure analysis of field returns, distinguishing material outgassing from other contamination sources by GC-MS fingerprinting
- Independent verification and certification support through accredited third-party laboratories for OEM homologation dossiers
FAQ
What is the typical volatile content limit for power battery thermal pads?
Most automotive specifications require volatile matter below 0.5% by mass for low-volatility silicone pads used near electronics, with 1.0% as the common upper limit for general-purpose grades. Specific limits depend on pad location, pack ventilation design, and OEM requirements.
Why are silicone volatiles a particular concern in battery packs?
Cyclic siloxane oligomers can migrate and deposit on relay contacts, connector terminals, and current sensors, forming an insulating film that increases contact resistance and can cause circuit faults. Because the deposits are silicone-based, they resist cleaning and may propagate to components far from the pad itself.
Which test temperature should be selected for the oven method?
The temperature is defined by the product specification and intended service condition. Common conditions are 105 °C for moisture-dominated loss, 150 °C for 2 hours as the industry default for pad qualification, and 200 °C for short-term screening of high-temperature grades. TGA can be run first to identify a temperature window below the decomposition onset.
How does TGA differ from the oven gravimetric method?
The oven method measures the integrated mass loss of a full-size specimen under realistic conditioning and is directly comparable to datasheet limits. TGA uses milligram samples under controlled heating and provides the full mass-loss curve, distinguishing volatiles from decomposition and revealing the volatile onset temperature, but it is less representative of bulk material averaging.
How can pad manufacturers reduce volatile content?
Effective measures include selecting low-volatile-matter silicone base polymers, extending post-cure at elevated temperature, applying vacuum degassing during mixing, using high-purity fillers with controlled surface treatment, and switching from peroxide-cure to platinum-catalyzed addition-cure systems that leave no peroxide by-products.
What sample quantity and turnaround are typical?
Standard gravimetric and TGA tests require only a few grams of material and can usually be completed within three to five working days, including conditioning. GC-MS speciation and ASTM E595 outgassing programs may require an additional week for calibration and data review, depending on the scope defined by the client.
