Focus on automotive: Low-thickness thermoplastic composite passes strict thermal runaway tests for EV battery housings

26 September 2023

Envalior is offering a new composite under the Tepex brand that, even with very low test specimen thicknesses, passes the standard thermal runaway tests for electric vehicle battery housings. The high resistance of this composite to the extreme conditions of a battery cell fire can be attributed to the non- flammable long and continuous fibres that reinforce the material in a multi-layer structure. “It’s thanks above all to the fibres that our structural material is capable of withstanding the extreme pressures, temperatures well in excess of 1000°C, and bombardment by abrasive hot particles that occur during the thermal runaway of battery cells. This is why it’s also ideal for components inside the battery, such as the cell housing, holder and partitions,” says Dr. Dirk Bonefeld, Head of Tepex® Product Management.

"There were 26M electric cars on our roads worldwide in 2022, an increase of more than 9M over the year before. According to a forecast from the International Energy Agency (IEA), this figure will rise to more than 200M by 2030."

Effective containment of battery cell fires

Vehicle battery cells are at risk of exothermic chemical reactions leading to fires as a result of factors such as electrical malfunctions, overheating and mechanical damage. This process is known as “thermal runaway.” If the fire spreads from one cell to the next, this is known as “thermal propagation.” Every effort must be made to ensure that the fire does not spread to the entire vehicle and endanger the occupants. This is why the battery housing plays a key role in containing fires. Thermal runaway tests simulate the extreme stresses to which the battery housing is exposed when a fire breaks out. “Our new composite can pass the standard tests covering this eventuality – such as the BETR test to UL 2596 – with test specimen thicknesses of just two millimetres or even less,” says Bonefeld. BETR stands for “battery enclosure thermal runaway.”


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“Even upon exposure to particle bombardment, the Tepex® test specimen did not undergo burnthrough, neither when the temperature at the end of the test was as high as 1400°C nor for another 20 seconds after – and that’s without any additional protective measures in the material and without supporting metal sheets. So even with low wall thicknesses and, in turn, low weight, a high degree of safety is ensured,” says Bonefeld. The composite also acts as an effective barrier against external fire sources. In the fire pan test, which is based on UN regulation 180, 6.2.4, and simulates highly realistic battery fire scenarios in accident situations, burning fuel did not create holes in the material, and the fibres did not ignite.

Lightweight design suitable for mass production

Another benefit of the material is that it is much lighter than steel or aluminium. “The density of an exclusively glass-fibre-reinforced material variant is around 70% less than that of steel. And if the core layer of the composite is reinforced with carbon fibres, this difference in density is even greater. So compared with aluminium, our composite can be significantly more than one third lighter,” says Bonefeld. Envalior has completed development of this lightweight material and now offers it in quantities suitable for mass production.

The composite comprises several layers of long and/or continuous fibres. Depending on the requirements, each layer can be reinforced with special fibre textiles. The total fibre content in the composite is more than 50% by weight. Polyamides or other engineering plastics can be used as matrix materials. “In particular, the huge freedom of choice when it comes to fibres, fibre arrangements, and matrix materials is a major strength of our material. This means that it can be specially adapted to individual requirements,” says Bonefeld.

The composite is also available in a variant containing recycled carbon fibres, putting the proportion of recycled material in the composite as a whole at around 36% by weight. “This composite is especially good for housings that are subject to very high mechanical loads. And the carbon fibres make it the material of choice when the housing needs to be electromagnetically shielding,” says Bonefeld. This shielding ensures that any equipment in or near the battery is not exposed to electrical or electromagnetic interference.

The fibre of the outer layers are completely impregnated with matrix plastic, which creates a closed plastic surface. “This ensures that our material exhibits outstanding electrical properties such as high dielectric strength and surface resistance,” says Bonefeld. It also offers excellent tracking resistance (CTI A > 400 V, Comparative Tracking Index).

The composite is also resistant to immersion cooling fluids. Entire battery housings are often submerged in these electrically non-conductive and highly flame-retardant fluids as a means of direct cooling (immersion cooling), for example in order to dissipate the significant amount of heat that is generated when batteries undergo quick charging.

Long-term storage in standard dielectric immersion cooling fluids showed that even after 1500 hours, the composite does not undergo any changes to its mechanical properties or start to swell, nor does it release any substances into the cooling fluid. This means that it can be used in immersion cooling fluids without any problems.

Creating sustainable material cycles

As a thermoplastic material, the new composite – like all other products in the Tepex® organic sheet range – can be easily recycled, meaning that production waste such as offcuts can be easily shredded and then reused as quality-assured recycled compounds for injection moulding. Components can also be recycled in this way. Composites and components can also be recycled by means of solvolysis and depolymerisation. “Our new material is, therefore, perfect for creating sustainable material cycles,” says Bonefeld.

There were 26M electric cars on our roads worldwide in 2022, an increase of more than 9M over the year before. According to a forecast from the International Energy Agency (IEA), this figure will rise to more than 200M by 2030. This means that electric vehicles are playing an increasingly important role in private motorised transportation. The biggest market for electric vehicles is currently China, but numbers are growing  in countries such as Germany too, where at the start of 2023, more than 1M all-electric vehicles were registered. “With our materials, we want to do our bit to enhance safety and sustainability in the world of electric transportation too so that this eco-friendly form of transportation can finally achieve its breakthrough,” says Henrik Plaggenborg, head of Global Sales & Business Development Tepex at Envalior.

For more information on the products and services provided by Envalior, visit

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