Using Hydrophobic Membranes
to Protect Gas Sensors

Microporous membranes can seal out liquids and heavy particulates while allowing unrestricted flow of air and gases. Combine these hydrophobic media with gas sensors, and you have a device that functions in hostile environments.

Mark Stuart, Pall Corp.

Photo 1. The Hydrolon nylon 6,6 membranes from Pall Corporation allow the flow of air and gases while resisting the passage of liquids, making the membranes an effective means of protecting sensors exposed to moisture and heavy particulates.

ydrophobic membranes are a class of microporous materials that allow the flow of air and gases while resisting the passage of liquids. This feature makes the membranes an effective means of protecting sensors exposed to moisture, cleaning-in-place operations, or heavy particulates (see Photo 1). An understanding of the properties of these membranes will help you expand the utility of sensors into areas where environmental considerations previously limited their use.

Typically, membranes are thin, porous films made of either a single polymer or a combination of polymers. The polymers provide a 3D structure containing pores, or passages, through the membrane. Companies and laboratories manufacture these membranes using various techniques, all of which produce membranes with consistent characteristics. The degree to which a membrane can protect sensors from environmental hazards is determined by such characteristics as thickness, airflow rate, and water intrusion pressure.

Membrane Thickness
Thickness is usually measured with micrometers and expressed as mils or microns. The proper thickness for a given application is the optimum between two often conflicting needs. Thinner membranes offer better flow rates, but this is at the expense of sealing and handling convenience.

To maximize airflow and minimize handling and sealing difficulties, the manufacturer combines membranes with support materials (which are typically made from nonwoven media, such as polypropylene or polyester). These materials allow the membranes to be sealed by a variety of methods.

Airflow Rate
The airflow rate indicates the ease with which air flows through the membrane. You measure this by calculating the time it takes a volume of air to pass through a defined area of membrane at a defined differential air pressure, usually at room temperature and atmospheric pressure downstream. To accurately compare two membranes, you must consider the same factors for both.

Differential pressure (P_d), or P, is defined as the pressure difference between a pressure gauge upstream (P_i) and a pressure gauge downstream (P_o) of the membrane:

High differential pressure indicates the membrane is creating resistance to airflow. This can be caused by small pores, few pores, particulates clogging the membrane, or fluid trapped on or in the membrane.

It can be seen that at atmospheric air pressure downstream P_d = P_i.

P is frequently used when describing real-world applications of airflow rates and differential pressure. For example, P is used when a volume of air must pass through a membrane at a given rate at the lowest cost or pressure.

Water Intrusion Pressure
Water intrusion pressure, also called water breakthrough, is a measure of the force required to push water through a membrane. This characteristic is expressed in units of pressure (e.g., psi, mbar, or bar), which are easily converted from one unit to another. Water intrusion is particularly relevant when the sensor is exposed to water under pressure caused by depth of water or high-pressure water streams. A membrane's water intrusion characteristic is a function of various membrane properties. The two principal contributing factors are pore size and the nature of the hydrophobic force.

Photo 2. The PTFE membrane is a low-surface-energy membrane with good water intrusion characteristics and high airflow. The membrane provides support polymers to maximize sealing options.

Membranes with small pores resist fluid flow because the fluid flow path is constricted. When a hydrophobic force is added, resistance to high-surface-tension fluids is increased. Therefore, a small-pore membrane tends to have a higher water intrusion pressure than a large-pore membrane and higher resistance to airflow (see Photo 2). A balance between the airflow rate and water intrusion pressure should be considered.

As mentioned above, the nature of the hydrophobic force itself is a contributing factor in determining a membrane's water intrusion characteristics. There are two types of hydrophobic forces that should be considered. The first is based on polar/nonpolar chemical properties. Nonpolar membranes tend to resist passage of polar compounds. An example of this type of hydrophobic interaction is best demonstrated by mixing a polar liquid with a nonpolar oil (e.g., vinegar and oil).

Polyvinylidene fluoride (PVDF) membranes are based on this type of hydrophobic force and are useful in molecular biology studies for binding hydrophobic portions of nucleic acids and proteins. However, PVDFs allow passage of nonpolar liquids and wet out (i.e., liquids fill the membrane pores, creating a liquid path through the membrane) with polar/nonpolar mixtures, allowing passage of any liquid once they are wet out. In addition, these membranes can bind nonpolar compounds, potentially clogging the membrane.

The second hydrophobic force is based on surface-tension characteristics. Only liquids with surface energy lower than that of the membrane can wet out the membrane. As demonstrated by the principle of water intrusion, any fluid can be forced through a membrane—regardless of its surface energy—if sufficient pressure is applied to the fluid. In fact, the lower the surface energy of a fluid, the lower the pressure needed to force the liquid through the membrane.

Membranes can be manufactured using polymers with low surface energy, or they can be post-treated with low-surface-energy chemistry after manufacturing. Examples are polytetrafluoroethylene (PTFE), a low-surface-energy polymer, or Repel treatment, which places fluorocarbons on exposed membrane surfaces, imparting low-surface-energy characteristics to virtually any membrane polymer.

Selection of a Membrane
Choosing the proper membrane for a sensor application involves performance and engineering considerations. As mentioned earlier, airflow rate and water intrusion pressure typically display an inverse relationship. However, the relationship of airflow rate to water intrusion pressure varies with the type of membrane.

Table 1 gives examples of membranes with various pore sizes, airflow rates, and water intrusion pressures. As you can see, PTFE offers the best combination of airflow rate, water intrusion pressure, and pore size characteristics.

Pore size indicates the size particle that will be retained by the membrane. This is usually expressed in microns for microporous membranes and is measured indirectly. The most common method is to measure the air pressure required to force a known fluid from the pores of a membrane wetted in that fluid. Smaller pores require higher pressure than larger pores to force fluid out of the pores and allow air passage. The relationship between the pressure required to force the fluid out of the pores and pore size is constant for a given fluid and membrane type.

Pore size deserves consideration as an independent variable for sensor applications in which a particle of a known size is present and must be excluded from the sensor. The relationship between pore size, particle size, and retention is based on a liquid system.

In an air stream, other more complicated mechanisms are involved in membrane particle retention. The mechanisms allow a membrane with larger pores to capture particles smaller than expected from the relationship of the liquid pore size rating to the particle size.

Incorporating Membranes into Sensors
The final consideration is sealing the membrane to the sensor. All the membranes listed in the table can be sealed by a variety of methods. Heat sealing, sonic welding, and use of adhesives are some of the more common methods. The various membrane polymers and support materials that are available increase the sealing options. These options include media compatible with various temperatures, energies, times, adhesive types, and housing materials.

The flexibility of incorporating membranes into sensors and the characteristics described in this article make membranes a viable consideration when designing sensors. The combination of air flow, fluid resistance, and ease of handling makes membranes ideal for use with gas sensors. Hydrophobic membranes allow sensors to be used in areas exposed to moisture, cleaning fluids, and particulates where they otherwise would be restricted from use.

Mark Stuart is Technical Marketing Manager, Pall Corp., 25 Harbor Park Dr., Port Washington, NY 11050; 516-484-3600, fax 516-484-3651.

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