Air permeability

Measuring air permeability can help in product development for such items as automotive filters and performance clothing.

Air permeability is an important factor in the performance of many products, including filters, clothing, tentage, sails, parachutes, building structures and acoustical suppressers such as car head liners (the lining to the interior of the car roof). The required performance level will vary from very high flow rates to materials expected to block flow completely. In extreme cases, such as gaskets and air bags, the material may need to be resistant to the passage of air even at the molecular level.

Air permeability is a measure of how easily air can pass through a material. The main influences on this property are the density of the material and its structure. Many fabrics are coated in order to modify their permeability to both air and water vapour, while maintaining comfort and insulation properties.

There are a number of applications where it is desirable to have a construction that allows air to pass through. Filters for combustion engines and air conditioning units must form a barrier against very small particles (in some instances even against bacteria and pollen), yet have high enough air permeability to maximise the efficiency of the mechanism they are protecting. The process of drawing air into an engine, for example, requires energy – this reduces the power available and increases fuel costs. Air permeability measurements can be used to examine the decreasing efficiency of filters over time in order to predict a filter's working life. Correct filter design can increase both engine life and performance.

However, some products may have a requirement for a low or zero air permeability. Outdoor performance clothing should have a very low permeability in order to prevent the effects of wind chill. Fleece-type materials can exploit this to their advantage, being able to more effectively entrap air where there is low permeability. Air is an excellent insulator – when wearing clothing, the air is trapped within the weave or knit of the fabric. This trapped air will be warmed by, for example, heat from the wearer’s body. If cold air can blow through the garment, this will displace the warm air trapped between the layers, making the wearer feel cold. Fleece materials may also be designed to absorb moisture. If the warm air is removed, so is the moisture, further cooling the wearer.

Air permeability should not be confused with moisture vapour transmission. Moisture vapour transmission is where moisture (water) vapour passes through from one side of a material to the other, and can occur in materials that have no apparent air permeability. A number of membranes, for instance, claim to have no air permeability and yet still transmit moisture vapour.

Testing for air permeability

Air permeability is determined by measuring the rate of airflow through a known area of material. This is achieved by applying a known pressure differential across a set area of material. Both these factors can be selected for the particular type of material being tested. Many models of test machine are designed to reduce the pressure on one side of the fabric with the other open to the laboratory atmosphere.

The mounting of the specimen is extremely important. Thinner fabrics are usually clamped over an orifice. However, this clamping method is unsuitable for very thick, compressible materials such as filters, as it may change their properties. In these cases, there are special mounting techniques available. It is important to ensure that all of the flow actually being measured is due to air passing through the specimen, since due to the relatively small size of the test piece, air can pass through the cut edge – commonly referred to as ‘edge leakage’.

This can be countered by introducing a so-called ‘guard ring’ around the test piece. The guard ring sits on top of a second, outer chamber (see figures 1 and 2), and the pressure drop in this outer chamber is equalised with that in the test chamber, thus reducing the edge leakage to zero. A second and simpler method is to run the test with an impermeable membrane above the test specimen in order to measure the edge leakage. This value is then deducted from the result obtained without the impermeable membrane present.

Air permeability results are usually expressed in terms of air velocity (mm/sec) – for example, BS 3424 part 16:1995 – but can be direct instrument readings of volume of air permeated per second for the given pressure difference and area of the test piece. In all cases, it is important to quote the pressure differential used and the area of the test piece for the results obtained.

---