The development of expanded PTFE (ePTFE) membranes unleashed a material that was unmatched in terms of properties. The breadth of applications that have been – and continued to be – found to benefit from ePTFE membranes is significant. With membranes, ePTFE’s exceptional properties such as chemical resistance, thermal stability, and high tensile strength were combined with micro-porosity to find applications in a wide range of fields, including filtration, medical devices, textiles, automotives, chemical plants, and many more.
The overarching tagline for ePTFE membranes is this: they repel dust and water but allow vapours and moisture to pass. This seemingly simple property can be further fine tuned to block not only water, but oils and mineral spirits as well, turning them from plain hydrophobic membranes to oleophobic membranes.
However, to understand how the membrane functions, and indeed to test the efficacy of the membrane, two fundamental parameters need to be considered.
The two important parameters that characterize the performance of ePTFE membranes are air permeability and water entry pressure. In this article, we will delve into the significance of air permeability testing and water entry pressure testing in expanded PTFE membranes.
Understanding Expanded PTFE Membranes
Expanded PTFE (ePTFE) is a remarkable material made by rapidly stretching extruded PTFE under high temperatures. The stretching process – which may be either uni-axial or bi-axial – imparts a micro-porosity to the material. These pores are much smaller than a molecule of water, but far larger than a molecule of steam. As a result, the membranes can act as a sort of valve, allowing only vapours through, but no liquids or dust. This feature – often called breathability – is somewhat unique to ePTFE and when combined with the ability to withstand high temperatures and corrosive chemicals, makes the material very useful.
However, to truly develop a tailored solution, one must be able to tweak the parameters to suit the end-application. While air and moisture can pass through the material, an application will have specific requirements as to the volume of air or moisture that should pass at a given pressure. Similarly, while water will be repelled by the membrane, certain applications would also have requirements pertaining to the pressure at which water might pass through the membrane.
Air Permeability Testing
Air permeability, also known as air porosity, is a crucial property of ePTFE membranes that influences their performance in applications where gas flow is involved. It refers to the ease with which air can pass through the membrane's pores. Air permeability testing is essential to assess the suitability of a membrane for specific applications, particularly in industries such as outdoor clothing, medical devices, and industrial filtration systems.
The air permeability test involves measuring the volume of air that passes through a unit area of the membrane under a defined pressure difference. The unit of measurement is usually cubic feet per minute per square foot (CFM/ft²) or litres per minute per square meter (LPM/m²). The test apparatus typically consists of a controlled air source, a sample holder, and a differential pressure gauge. The sample is securely clamped between two chambers, and air is allowed to flow through one side of the membrane while the pressure drop across it is measured. The standard ASTM D737 is used to test the material for air permeability.
The air permeability value provides valuable information about the membrane's breathability, which is crucial in applications like outdoor clothing and medical textiles. Membranes with higher air permeability values allow for better moisture vapor transmission while maintaining water resistance.
Water Entry Pressure Testing
Water entry pressure, also known as bubble point pressure or hydro testing, is another essential parameter in assessing the performance of ePTFE membranes, particularly in applications involving liquid filtration and moisture management. It determines the pressure at which liquid begins to pass through the pores of the membrane. Water entry pressure testing helps determine the membrane's resistance to liquid penetration and is crucial in applications like medical filtration devices, fuel filtration, and waterproof textiles.
To conduct a water entry pressure test, the ePTFE membrane is subjected to increasing pressure on one side while the other side is exposed to a wetting liquid, often water. As the pressure increases, the point at which liquid starts to penetrate through the membrane's pores is the water entry pressure. This pressure is usually measured in psi or bar. The standard ASTM D751 is used to test the material for water entry pressure.
A higher water entry pressure indicates that the membrane can withstand greater hydrostatic pressure before allowing liquid to pass through, making it suitable for applications that require liquid resistance. Conversely, membranes with lower water entry pressures are ideal for applications that require controlled liquid transport.
Importance of Testing
Air permeability and water entry pressure testing are vital for ensuring that ePTFE membranes meet the specific requirements of their intended applications. Different industries have different needs, and these tests help manufacturers and engineers select the most appropriate membrane for their products.
For example, membranes used in automotive vents are required to have a very high-water entry pressure, as these vents protect the circuitry. Even under very harsh weather – including events where a vehicle might find itself partially submerged – the membrane cannot succumb to the pressure. Similarly, in case water does find a way into the circuitry, the heat within the system should be sufficient to turn the water into vapour so it can escape through the vent.
In medical applications, such as wound dressings or drug delivery devices, a balance between air permeability and water entry pressure is crucial to ensure optimal patient comfort and controlled liquid absorption. In industrial filtration, membranes with high air permeability and suitable water entry pressure values are chosen to maximize filtration efficiency and minimize pressure drop.
Expanded PTFE membranes have revolutionized various industries due to their exceptional properties. Understanding their air permeability and water entry pressure characteristics is crucial for their successful deployment. The ability to manufacture, test, and fine tune membranes is something that Poly Fluoro has been developing over the past few years. With in-house testing facilities for air permeability and water entry pressure, our engineering can pinpoint the exact process needed to ensure the end-properties are exactly what are needed by the end-user.