Fluoropolymer PTFE Vent Membranes
Whether they contain chemical fluids, marine navigation lights, or electronic equipment, enclosures and containers are vulnerable to forces unseen by the naked eye. Internal pressure buildup due to changes in altitude or temperature can stress container seals, causing leakage or even complete enclosure failure. A container’s contents can also outgas or scavenge oxygen, furthering the pressure buildup.
A vent provides an effective solution to the pressure problem, allowing containers and enclosures to breathe or egress vapors while preventing ingress of unwanted contamination or liquids. Vents are critical features and may go unnoticed by the untrained eye. But most vents need a membrane so that harmful contaminants and liquids remain outside the device while gas flow continues.
Consider some common cleaning products, many of which contain chlorine or peroxide. These liquids must be packaged well enough to protect them from fluids, other chemicals, and dirt particles while in storage, but they can easily outgas and cause an internal pressure buildup. The packaging, therefore, requires a membrane that effectively repels fluids but allows air from within the container to exit through the membrane.
The Importance of Hydrophobicity for Venting Applications
The first step in determining an appropriate membrane for a venting application is to consider the vent’s environment and the contaminants potentially affecting the membrane. For applications that require gas transfer and may involve exposure to water—including marine/automotive electronics and lighting, gas sensors, and liquid product packaging—a hydrophobic membrane is an ideal choice.
Hydrophobicity refers to the physical property of a material that causes it to naturally “repel” water. It is a geometric phenomenon, relying on surface tension and contact angle.
Surface tension is an internal force caused by the imbalance between molecular forces when two different materials make contact. Water has a relatively high surface tension when compared to other non-polar liquids, as shown in the table below. Thermodynamics demand that systems migrate to the lowest energy state, the higher surface energy or
Surface Tension of Liquids
| Liquid | Surface Tension @ 22° C (dynes / cm) | ||||
|---|---|---|---|---|---|
| Mercury | 484 | ||||
| Water | 73 | ||||
| Oil | 30 | ||||
| Unleaded gasoline | 19 | ||||
Surface tension is an internal force caused by the imbalance between molecular forces when two different materials make contact. Water has a relatively high surface tension when compared to other non-polar liquids, as shown in the table below. Thermodynamics demand that systems migrate to the lowest energy state, the higher surface energy or tension surface is minimized.
Surface Tension of Hydrophobic Materials
| Material | Surface Tension (dynes / cm) | ||||
|---|---|---|---|---|---|
| Polytetrafluorethylene (PTFE) | 18 | ||||
| Polypropylene | 29 | ||||
| Polyethylene | 31 | ||||
| Polyvinyl chloride (PVC) | 39 | ||||
| Cellulose | 44 | ||||
The surface tension of the solid contacted by the liquid determines whether the liquid will wet, or spread across the surface, or bead into discrete droplets on the surface. A material exhibiting the first characteristic is referred to as hydrophilic, while one exhibiting the second is hydrophobic. Hydrophobic materials are classified as those having surface tensions below 70 dynes/cm, while hydrophilic ones have surface tensions above that point. The table below lists some common hydrophobic materials, with highly hydrophobic ones appearing at the top.
Contact Angle
The contact angle (indicated as θ in the diagram above) between a liquid and solid is also a good indicator of hydrophobicity. If a droplet forms a sphere that barely touches the surface, the angle between the droplet’s edge and the surface is greater than 90 degrees and the surface is hydrophobic. If the droplet spreads and wets a large area, the contact angle will be less than 90 degrees and the material is considered hydrophilic.
Pore Size for Venting Membranes
Pore size determines which contaminants a membrane rejects, which is also an important consideration when selecting a venting membrane. Filter membranes are classified by pore size, as shown in the table below.
Filtration Classification Guide
| Filter Classification | Pore Size Range (µm) | Rejected Particles | ||
|---|---|---|---|---|
| Microfiltration (MF) | 0.1 - 5 | Bacteria, blood cells, fine dusts | ||
| Ultrafiltration (UF) | 0.01 - 0.1 | Viruses, silica, proteins, smog | ||
| Nanofiltration (NF) | 0.001 - 0.01 | Non-aqueous, salts, sugars, synthetic dyes | ||
| Reverse Osmosis (RO) | 0.0001 - 0.001 | Most molecules, except water |
Each membrane classification is ideal for certain applications. For example, microfiltration is often ideal for venting applications because the large pore sizes ensure that the filter operates effectively under lower pressures found in containers and enclosures. At the other end of the spectrum, the tiny pores in RO filters make it ideal for water filtration but require significant osmotic pressure to force filtration.
One of several methods for determining the largest pores in a hydrophobic material is the bubble point test. This involves placing a membrane in a housing and covering it with an alcohol. Gas pressure is applied to the underside of the membrane, gradually increasing until rising bubbles appear in the liquid. A membrane’s bubble point is specified by the pressure (in psi) at which the bubbles appear. Standards such as ASTM F316 provide best practices for bubble point testing.
ZITEX G Meets the Venting Challenge
Saint-Gobain’s ZITEX G Microporous PTFE is ideal for venting applications and others, including electrochemical gas sensors, gaskets, and chemical filtering in harsh environments. Based on a porous form of PTFE, it takes advantage of that fluoropolymer’s outstanding non-wetting attributes due to its low surface tension.
ZITEX G has a number of other excellent qualities for use as a venting membrane:
- ZITEX G is inherently hydrophobic, making it ideal for protecting packaged liquids from contamination while allowing outgassing. It can be affixed to plastic closures and substrates for use as a container cap liner.
- The ZITEX G line comes in a variety of pore sizes and thicknesses within the microfiltration range. Products range from G-104, with ~5-6 μm pore size and 0.004” thickness, to G-115, which has ~1-2 μm pores and 0.015” thickness.
- ZITEX G showcases the excellent thermal stability and chemical inertness of PTFE. As such, it’s resistant to virtually all acids, bases, and solvents and can be used over a wide temperature range from -450° F to 500° F (-268° C to 260° C).
With over 350 years of experience in high-performance materials, Saint-Gobain provides tailored expertise and unmatched industry-leading capabilities. From initial consultation and existing product support to future product opportunities, selecting a Saint-Gobain product is always a solid choice.
This article was produced by IEEE GlobalSpec. Visit the Saint-Gobain Specialty Films' Engineering360 page on IEEE Global Spec.