Insulation Tubing

ePTFE membranes in battery vents: Quality assurance tests

Particularly to ensure safe, long-term use of lithium-ion batteries in motorized vehicles, functional venting systems are essential to compensate for temperature and pressure fluctuations. Battery housings sealed to protect against external influences are highly susceptible to pressure or temperature changes due to the lack of replaceability options. If these are not compensated for, damage to the batteries may result. Adequate ventilation solutions can solve this problem by allowing gases to escape, as well as preventing the development of severe pressure and temperature gradients. Especially in passive ventilation systems, GREMCO ePTFE membranes have been used very successfully. Made of expanded polytetrafluoroethylene, these prevent the ingress of external particles, while tiny openings allow gases to escape, thus ensuring the functionality of the ventilation in the long term. In order to ensure the necessary performance indicators as part of quality assurance, various test procedures are performed. These are:

• The Water Entry Pressure Test
• The Airflow Test

Water Entry Pressure (WEP) Test

The Water Entry Pressure Test is a measurement to determine the water entry pressure. Water entry pressure, in turn, represents the minimum pressure required to force water through the largest of the tiny openings in the ePTFE membrane. ePTFE membranes are generally hydrophobic and resistant to water penetration. However, under elevated pressure conditions, it is possible to force water through them. Hence, the water inlet pressure is an indicator of the membrane’s suitability to act as a hydrophobic filter in an industrial context. This suitability depends on the pore size, the surface free energy as well as the thickness of the membrane, the sizes of the openings in the membrane surface, and the surface tension of the liquid.

In general, it can be stated: The smaller the nominal pore size of the membrane, the more pressure needs to be applied to allow water to enter and therefore the higher the water entry pressure. The Water Entry Pressure Test is performed on numerous batches of ePTFE membranes. In addition to the quality assurance described above, this further serves as an assurance of an adequate bond between the membrane and the substrate material. The principle of the WEP testing is illustrated in the figure below.

The Airflow Test for ePTFE membranes

The Airflow test measures the airflow passing through the membrane over a predetermined period at given pressure gradients. The air escaping through the openings in the ePTFE membrane is therefore determined under standardized test conditions. In addition to the pressure difference, the resulting value depends, among other things, on the size of the openings in the membrane, the air properties as well as the pore size, the surface free energy, and the thickness of the membrane. A value of airflow within the desired limits resulting from the Airflow Test guarantees complete functionality of the membrane for deaeration and thus its protective effect to avoid serious pressure and temperature gradients. The principle of the Airflow Test can be seen in the figure below.