Air permeability

15 Air permeability J.Y. HU, YI LI AND K.W. YEUNG Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hong Kong

15.1

Introduction

The concept of ‘air permeability’ is widely used in the textile industry to interpret the intrinsic characteristics of fabric. Outdoor garment manufacturers in particular frequently employ this technical information to describe functional performance of products. Several existing standards can be used for air permeability evaluation with different testing conditions. Air permeability is significantly influenced by a fabric’s material and structural properties, such as shape and value of the pores of the fabric and yarn, which in turn are dependent on the structural parameters of the fabric, such as fabric weave, the raw material of the yarns, the set of yarns and others. In addition, as the results of McCullough’s research show, fabrics with hydrophilic components can change their air permeability properties under different humidity conditions.13 Construction factors and finishing techniques can also have an effect upon air permeability by causing a change in the length of air flow paths through a fabric. Fabrics with different surface textures on either side can have a different air permeability depending upon the direction of air flow. For woven fabric, yarn twist is also important. As twist increases, the yarn diameter and the cover factor are decreased, and this increases air permeability. Increasing yarn twist may also allow the more circular, high-density yarns to be packed closely together in a tightly woven structure with reduced air permeability. ASTM defined the term air permeability as the rate of air flow passing perpendicularly through a known area under a prescribed air pressure differential between the two surfaces of a material. It is generally expressed in SI units as cm3/s/cm2 or in inch-pound units as ft3/min/ft2.2 The term air permeability is often used in evaluating and comparing the ‘breathability’ of various fabrics for such end-uses as raincoats, tents and uniforms. It is also closely related to the terms fabric water vapor permeability and wind resistance performance which evaluate the performance of parachutes, sail cloth, sportswear and industrial filter fabrics.5,11,16,17 252

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Based on earlier research results, the clothing system needed to protect an individual in a cold environment would depend on the following main factors: (1) metabolic heat, (2) environment factors, like temperature, humidity and wind speed, and (3) fabric/garment properties, like thermal insulation, air permeability and moisture vapor transmission behaviours. The survival of a dressed human depends on the balance of heat losses due to (2) and (3) and heat generation due to metabolic heat (1). Prior research also points out that vapor transfer through clothing systems may occur due to diffusion (driven by vapor concentration gradients) and convection (driven by air pressure differences). Convective heat and mass transfer in textiles is often more important than transport due to diffusion, especially if such materials are used in conditions where a large pressure gradient is present.10 Therefore, the air penetration and vapor diffusion behaviour significantly influences the thermal comfort sensations and can even increase an individual’s ability to survive in critical cases. As an example, many types of functional fabrics like modern waterproof fabrics are designed for use in garments that provide protection from the weather, that is from wind, rain and loss of body heat. Therefore, in the modern textile and clothing industry, the fabric treatments such as finishes, coatings and film membranes are added to shell fabrics to reduce or prevent water and wind penetration into clothing layers. However, some of these treatments inhibit the evaporation of sweat from the body surface and its transportation through the fabric layers to the environment. If water vapor cannot escape, it may condense in the cooler outer layers of the clothing system or on the inner surface of the shell fabric. The accumulation of water vapor inside the clothing microclimate may cause discomfort to the wearer.13 Milenkovi et al.14 point out that the air permeability of a fabric can influence its comfort behaviour in several ways. 1. In the first case, a material that is permeable to air is also, in general, likely to be permeable to water, in either the vapor or the liquid phase. Thus, the moisture-vapor permeability and the liquid-moisture transmission are normally closely related to air permeability. 2. In the second case, the thermal resistance of a fabric is strongly dependent on the enclosed still air, and this factor is in turn influenced by the fabric structure, as also is the air permeability. A very open cloth can inflict serious wind chill problems on the wearer in cold climates with a breeze blowing and may thus affect survival chances in extreme cases. 3. Finally, a highly air-permeable fabric may be sheer or have a very open structure, so that aesthetic factors such as modesty, dimensional stability, drape, handle, etc. may result in discomfort of a psychological or physical nature in the wearer. Although air permeability in itself is

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Clothing biosensory engineering merely another effect, rather than a cause, associated with such manifestations of discomfort, it can nevertheless provide a convenient and readily measured way of quantifying the likely behaviour of a fabric in these other areas.

Another related term is ‘breathability’, which indicates that a fabric is actively ventilated. For the functional breathable fabric, water vapor produced by the body is required to easily diffuse through it into the surrounding environment and to prevent the penetration of liquid water from outside into the garment in order to maintain a dry feeling during wear. These are the essential requirements for maintaining comfort sensations during wear. Because perspiration is one of the main mechanisms for cooling the body, if the water vapor is unable to escape into the surrounding atmosphere easily, it will increase the relative humidity of the microclimate inside the clothing. Such increases cause damp and clammy perceptions and also a corresponding increase in thermal conductivity of the insulating air, which can affect coolness perceptions. In extreme cases hypothermia can result if the body loses heat more rapidly than it is able to produce, for example when physical activity has stopped, causing a decrease in core temperature. Unlike modern waterproof breathable fabric, prior research indicated that conventional woven fabrics display a proportional relationship between vapor permeability and air permeability.12 Wind resistance is usually assessed by measuring air permeability. This is the rate of air flow per unit area of fabric at a standard pressure difference across the faces of the fabric and can significantly influence the wearing comfort behavior. If a fabric is permeable to air, it also means that the water vapor or liquid moisture can transfer from the fabric’s inner surface to its outer surface and evaporate into the environment. Therefore, water vapor or liquid moisture transmission are closely related to the material air permeability and thermal comfort sensations during wear. On the other hand, the fabric thermal resistance is strongly dependent on the enclosed still air; this is also influenced by fabric structure and fabric permeability. As an example, it is generally more stressful for a worker operating in a warm or hot environment than in a neutral environment.15

15.2

Measurement of air permeability

Air permeability is normally measured on apparatus designed to force air through the test specimen, usually classified into two types. In one system, the pressure difference between the opposite faces of a test specimen is fixed and measurement is made of the resulting air flow through the material. In the other type, the rate of movement of air through the fabric is

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Table 15.1 Relative testing methods and standards Developer

Document number

Title

ASTM

D 737-96

ASTM

F 2298-03

BS EN ISO

9237

ASTM

D 6476

BS

3424-16:1995

Standard Test Method for Air Permeability of Textile Fabrics Standard Test Methods for Water Vapor Diffusion Resistance and Air Flow Resistance of Clothing Materials Using the Dynamic Moisture Permeation Cell Textiles – Determination of the Permeability of Fabrics to Air (Supersedes BS 5636-90) (L) Standard Test Method for Determining Dynamic Air Permeability of Inflatable Restraint Fabrics Testing Coated Fabrics – Part 16: Method 18. Determination of Air Permeability

adjusted to a fixed value and the pressure difference that must be developed across the fabric in order to maintain this air flow is then measured.14 In the textile industry, the principle of the test to determine fabric air permeability is that air is drawn through a specified area of fabric. The rate of air flow is adjusted until a specified pressure difference between the two fabric surfaces (face and back) is achieved. The air flow is measured and the air permeability is calculated. Several relevant published standards are summarized in Table 15.1. The ASTM 737–96 procedure determines the volume rate of air flow per unit area of fabric in cubic centimetres per square centimetre per second. The British, European and International standard procedure determines the velocity of air of a standard area, pressure drop and time, in millimetres per second. The standard pressure specified in the ASTM standard procedure is 125 Pa (12.7 mm water gauge) whereas that specified in the British Standard procedure is 100 Pa for apparel fabrics and 200 Pa for industrial fabrics. Results obtained using the two procedures are, therefore, not comparable. In general, the testing environment must be set up in a standard conditioned laboratory, so that the air being drawn through the specimen is at standard conditions, i.e. 20 ± 2 °C and RH 65 ± 2% following ASTM D1776. A steady-state air permeability test apparatus consists of: • a clamping device for securing the test specimen in a flat, tensionless state;

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• a device to prevent air leaking from the edges of the test area, usually called a guard ring; • a pressure gauge or manometer to measure the pressure drop from one side of the specimen to the other; • an air pump to draw a steady flow of air through the clamped specimen; • a means of adjusting the rate of airflow to achieve and hold the specified pressure drop from one side of the specimen to the other; • a flow-meter to measure the actual rate of air flow through the specimen. In British Standard BS 9237, ‘Determination of Permeability of Fabrics to Air’, a standard testing method is given to test the airflow through a given area of fabric at a constant pressure drop across the fabric of 10 mm head of water. During the testing the specimen is clamped over the air inlet of the apparatus using gaskets and air is sucked through it by means of a pump as shown in Fig. 15.1.3 The air valve is adjusted to give a pressure drop across the fabric of 10 mm head of water and the air flow is then measured using a flow-meter. Several flow-meters with different ranges are usually incorporated to enable the instrument to deal with a wide range of fabrics. At least five specimens are used, each with a test area of 508 mm2 (25.4 mm diameter) and the mean air flow in millilitres per second is calculated from the five results. From this the air permeability can be calculated in millilitres per 100 mm2 per second. Then fabric air resistance can be defined as the time in seconds for 1 ml of air to pass through 100 mm2 of fabric per second under a given pressure head of 10 mm of water. A widely employed testing standard is ASTM D737–96, ‘Standard Test Method for Air Permeability of Textile Fabrics’, which gives a test method for measuring the air permeability of textile fabrics. This test method applies to most fabrics including woven fabrics, non-woven fabrics, air bag fabrics, blankets, napped fabrics, knitted fabrics, layered fabrics and pile fabrics. The fabrics may be untreated, heavily sized, coated, resin-treated or otherwise treated. BS 3424–16:1995, ‘Testing Coated Fabrics – part 16: Method 18. Determination of Air Permeability’, provides another method to determine fabric air permeability. The principle of this method is that the rate of air flow through a known area of coated fabric is adjusted so that there is a known pressure drop across the fabric. According to this standard the main components of testing apparatus include a rigidly mounted circular specimen holder, having an orifice 5 cm2, 20 cm2, 50 cm2 or 100 cm2 (i.e. 25.23 ± 0.05 mm, 50.05 ± 0.05 mm, 79.79 ± 0.05 mm or 112.84 ± 0.05 mm diameter). A suction pump draws a steady flow of air through the test specimen with

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15.1 Typical apparatus for air permeability testing.

adjustable suction rate so that the pressure differential across the test specimen can be maintained at a constant 50 Pa, 100 Pa, 200 Pa, 500 Pa or 1 kPa. During the measurement, the test specimen is mounted in the circular specimen holder with the coated surface to the low-pressure side if single-faced with sufficient tension to eliminate wrinkles. The suction pump is started and the rate of flow adjusted until the required pressure differential is obtained. The air flow rate in litres per minute is then recorded. The pressure differential should be maintained for a further one minute and the air flow rate in litres per minute measured again. Finally, the fabric air permeability R can be calculated according to equation (15.1): R=

r 167 A

[15.1]

where r is the arithmetic mean of the air flow rate in L/min and A is the area of orifice of the test assembly in cm2.

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15.3

Humidity-dependent air permeability

Another important property of a fabric is the way in which it allows water vapor to pass through it. This property is known as ‘permeability of a fabric to moisture vapor’ and is closely related to fabric air permeability behavior. Moisture-vapor permeability in fabrics is achieved or lost at either the manufacturing or the finishing stage of the production process. Although heat transmission may be critical to survival in cold weather, it is incontestable that moisture-vapor transmission is crucial to comfort in both cold and hot weather. Free movement of water to the fabric surface is essential if perspiration discomfort, causing fabric wetness with resulting freezing in winter or clamminess in summer, is to be prevented. Porous hygroscopic materials, like pure cotton woven fabrics, often exhibit humidity-dependent air permeability due to the swelling of the solid matrix as it takes up water vapor from the environment. These effects are most evident in materials such as tightly woven fabrics, low-porosity hygroscopic membranes and non-woven fiber mats. Humidity-dependent air permeability is usually evident from the volumetric flow rate versus pressure drop plot. The plot will no longer have a line of constant slope, but will show some curvature according to the relative humidity of the gas flowing through the sample. It would be very appealing to have a test method available that can determine both diffusion and convection properties, and is able to directly compare the results obtained between materials as different as air-impermeable membrane laminates, very air-permeable knitted fabrics, woven fabrics, and complicated non-woven and polymeric foam structures.7,9 ASTM F2298-03, ‘Standard Test Methods for Water Vapor Diffusion Resistance and Air Flow Resistance of Clothing Materials Using the Dynamic Moisture Permeation Cell’, proposed a device for measuring water vapor transport and air permeability of porous materials like textiles.1 The reasons for carrying out this testing are due to some functional fabrics like Gore-Tex, Sympatex, etc., having much better water vapor transport properties when they are in a humid environment than when they are in a dry environment.6 All the testing is carried out on an apparatus called a dynamic moisture permeation cell (DMPC).7 Nitrogen streams consisting of a mixture of dry nitrogen and watersaturated nitrogen are passed over the top and bottom surfaces of the sample. The relative humidity of these streams is varied by controlling the proportions of the saturated and the dry components. By knowing the temperature and water vapor concentration of the entering nitrogen flows, and by measuring the temperature, water vapor concentration and flow rates of the nitrogen leaving the cell, one may measure the fluxes of gas and

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water vapor transported through the test sample. With no pressure difference across the sample, transport of water vapor proceeds by pure diffusion, driven by vapor concentration differences. If a pressure difference across the sample is present, transport of vapor and gas includes convective transport, where the gas flow through the sample carries water vapor with it, which may add to or subtract from the diffusive flux, depending on the direction of the convective gas flow.7 Results may be shown in terms of water vapor flux (g/m2/day) or resistance to the diffusion of water vapor (units of s/m). The resistance units make comparing results obtained at different environmental conditions much easier. The lower the diffusion resistance, the more water vapor gets through the material.8,9,11 With the advantage of DMPC, this apparatus also can be used to determine the following material properties, except for the steady-state airpermeation properties:1,4 1. 2. 3. 4. 5.

concentration-dependent permeability; temperature-dependent permeability; combined convection/diffusion; humidity-dependent air permeability; transient sorption/desorption.

15.4

Acknowledgements

The authors would like to thank the Hong Kong Polytechnic University for funding this research through projects G-V987, A188 and ITF project ITS-023-03.

15.5

References

1. ASTM, ASTM F2298-03 Standard Test Methods for Water Vapor Diffusion Resistance and Air Flow Resistance of Clothing Materials Using the Dynamic Moisture Permeation Cell. 2003: West Conshohocken, PA, ASTM. p. 10. 2. ASTM, ASTM D6476 Standard Test Method for Determining Dynamic Air Permeability of Inflatable Restraint Fabrics. 2002: West Conshohocken, PA, ASTM. 3. BSI, Textiles – Determination of the Permeability of Fabrics to Air. BS EN ISO 9237:1995. 1995: London, British Standards Institution. 4. U.S. Army Soldier Systems Center, http://www.natick.army.mil/soldier/media/ fact/ss&t/DMPC.htm, 2004. 5. Epps, H.H., Prediction of Single-layer Fabric Air Permeability by Statistical Modeling. Journal of Testing and Evaluation, 1996. 24(1): p. 26–31. 6. Gibson, P., http://www.verber.com/mark/outdoors/gear/breathability.pdf, 2004. 7. Gibson, P., D. Rivin, and C. Kendrick, Convection/Diffusion Test Method for Porous Materials Using the Dynamic Moisture Permeation Cell, in Final rept.

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Jun–Nov 97. 1998: MA, Army Natick Research and Development Center, p. 60. Gibson, P., D. Rivin, C. Kendrick, and H. Schreuder-Gibson, Humiditydependent Air Permeability of Textile Materials. Textile Research Journal, 1999. 69(5): p. 311–317. Gibson, P.W., Factors Influencing Steady-State Heat and Water-Vapor Transfer Measurements for Clothing Materials. Textile Research Journal, 1993. 63(12): p. 749–764. Gibson, P.W. and M. Charmchi, Coupled Heat and Mass Transfer through Hygroscopic Porous Materials – Application to Clothing Layers. Sen-i Gakkaishi, 1997. 53(5): p. 183–194. Gibson, P.W., A.E. Elsaiid, C.E. Kendrick, D. Rivin, and M. Charmchi, A Test Method to Determine the Relative Humidity Dependence of the Air Permeability of Woven Textile Fabrics. Journal of Testing and Evaluation, 1997. 25(4): p. 416–423. Holmes, D.A., Waterproof Breathable Fabrics, in Handbook of Technical Textiles, S.C. Anand and A.R. Horrocks, Eds, 2000: Cambridge, UK, Woodhead Publishing Limited p. 283–314. McCullough, E.A., M. Kwon, and H. Shim, A Comparison of Standard Methods for Measuring Water Vapour Permeability of Fabrics. Measurement and Textile Technology, 2003. 14: p. 1402–1408. Milenkovi, L., P. Kuudri. Sokolovi, and T. Nikoli, Comfort Properties of Defense Protective Clothings. Working and Living Environmental Protection, 1999. 1(4): p. 101–106. Olsuskiene, A. and R. Milasius, Dependence of Air Permeability on Various Integrated Fabric Firmness Factors. Materials Science, 2003. 9(4): p. 401–404. Oxtoby, E., Air-permeability Measurement of Open Fabrics by Using Superimposed Fabric Layers. Journal of the Textile Institute, 1970. 61(3): p. 153–156. Szosland, J., A. Babska, and E. Gasiorowska, Air-penetrability of Woven Multi-layer Composite Textiles. Fibres & Textiles in Eastern Europe, 1999. 7(1): p. 34–37.