As the batteries for electric vehicles increase in power and energy density, their materials need to become even more fire-resistant. Find out the key methods for testing and measuring fire resistance, and the information developers can gain from them.
In the mobility sector, the transition from internal combustion engine to electric powertrain not only presents a huge opportunity for OEMs, but also a series of challenges. One of those challenges is maintaining the integrity and safety of the battery – something that is vital for the increased ranges consumers are demanding, as well as the industry’s confidence in the safety of a vehicle’s driver and passengers.
With the majority of electric vehicles using Li-ion batteries, the main issue for manufacturers is the risk of thermal runaway. Li-ion batteries may offer higher power and energy density, but they also generate a lot of heat, which if not controlled, can dramatically increase and eventually cause thermal runaway, resulting in the battery catching fire or even exploding. Not only will this cause extensive damage to the car, but potentially put the vehicle’s occupants in danger.
Therefore, one of the key requirements for sealing materials in the batteries of electric vehicles to contribute to overall thermal management is fire resistance. As the batteries have evolved, the legislation around fire safety has been continuously changing, and now all modern battery seals are required to have a V0 rating, which means the materials are self-extinguishing and can survive exposure to fire for certain time.
However, by their very nature, organic elastomers that make up standard sealing compounds are highly flammable, thus fire resistance must be achieved by other means, such as using fire-retardant additives or fire-retardant coatings.
At Datwyler, material developers combining their expertise in elastomer compounding, surface treatment and testing developed elastomer compounds meeting the harshest fire resistance requirements.
In this article we focus on introduction of various methods evaluating fire resistance of materials.
The four major fire resistance tests
While the physical properties of rubber compounds such as tensile strength, elongation, and compression set are relatively simple to assess and compare, testing their fire resistance is more difficult to quantify. In addition, the tests have certain practical difficulties and potential health and safety hazards.
That said, several key tests have emerged, each one offering different piece of information about the compound that add up to a larger picture of fire resistance.
#1 Limiting Oxygen Index
Defined as the minimum concentration of oxygen in a mixture of oxygen and nitrogen that is needed to support the flaming combustion of a material, the Limiting Oxygen Index (LOI) is expressed in volume percent of oxygen. Materials with LOI values less than 21% are classified as combustible, since their combustion can be sustained at ambient temperature without any external energy contribution. Those with an LOI greater than 21% are classified as self-extinguishing. The higher the percentage, the greater the fire resistance.
The test involves supporting a sample of the material vertically in a transparent chimney while a mixture of oxygen and nitrogen flows upwards. The upper end of the sample is ignited, and the subsequent burning of the specimen is observed to compare how long the burning continues or the length of the sample burnt, both within specified limits. By testing a series of samples in different oxygen concentrations, the minimum oxygen concentration is determined.
#2 Maximum Average Rate of Heat Emission
One of the more sophisticated tests of fire resistance, the Maximum Average Rate of Heat Emission (MARHE) provides key information about how a material behaves when it burns. During the test, a sample of material is heated and starts to emit composition gases. These gases are ignited by an electric spark, with the emitted gases collected and transported away through a ventilation system. By measuring the type, flow rate and other parameters of gas, the heat release over time can be calculated.
The advantage of the MARHE test is that it offers the ability to measure the propensity for fire development under real conditions. Along with the Average Rate of Heat Emission (ARHE) test, MARHE is used to accurately predict how the material will behave in the final application, giving researchers the data required for correlation or mathematical models.
#3 UL 94
One of the most well-known and requested tests for fire resistance is UL 94, which Datwyler can conduct in-house. It has two major components: horizontal burn (HB) and vertical burn (VB). Horizontal burn provides information about the speed of the flame spread, and vertical burn is all about self-extinguishing ability of a material. While the HB is less challenging and even standard elastomer formulations can pass, passing VB is more challenging and normally requires special development.
During the VB test, a sample of material is held in a steel fireproof hood and a flame is applied for ten seconds. If the sample catches fire, the length of time it continues to burn is measured. This is ‘Afterflame 1’. The flame is then applied again and the duration of time the sample burns for is measured (‘Afterflame 2’).
For the highest classification – V0 – the accumulative afterflame time must be less than 10 seconds for ten samples, which means the samples must either not catch fire or simply flicker and extinguish themselves immediately.
#4 Specialized tests
Next to LOI, MARHE, and UL 94 tests, there are a few more specialized fire resistance tests. Since smoke is a particularly dangerous consequence of fire, the measurement of smoke density is a key measurement in determining the safety of a material. In such a test, a sample is burned in a large chamber while a beam of light is transmitted through. Sensors will then measure the absorbance of light before the toxic fumes are analyzed.
Other specialized tests include the burning of cable bundles to measure the effects of fire on cable wiring in a building, and the isolation integrity test, which involves burning a cable over a gas burner while sprinkling it with water to simulate a building’s sprinkler system.
As electrification of the vehicle sector becomes more widespread and the demand for larger and more powerful batteries increases, the need for effective and reliable tests for fire resistance will become more important.
These tests not only provide manufacturers and OEMs with the assurance of fire resistance and safety, but enable them to develop new battery technologies that will drive the sector forward.