Air permeability

1. Air Permeability

2. Contents
 What is Permeability
 Types of Permeability
 Air permeability
 Air resistance
 Air porosity
 Standard test method used for measuring air permeability
 Influence of different factor on air permeability
 Shirley air permeability tester
 Standard method description on basis of Shirley air permeability tester

3. Permeability
 Permeability may be defined as the rate at which gas or
liquid passes through a porous medium. Textile fabrics are
permeable substances.
 Fabric permeability is a property of fabric which is used to
assess the ability of fabric to allow a penetrant, such as a
gas, liquid or solid material, to pass through such a barrier
and then desorb into a specified medium

4. Types of fabric permeability
 The fabric needs air, water and vapour permeability so
that a person feels comfortable wearing it.
Types of permeability
 Air permeability
 Moisture permeability
 Liquid form
 Vapor form

5. Air Permeability
 It is a measure of how well a fabric allows the passage of air
through it that means the air will pass through the pores available
between the yarn or between the fibre.
 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.
 Apart from apparel comfort, it is also important for a number of
fabric end-uses, for example, industrial filters, tents, sail-cloths,
Fabric testing parachutes, air bags, vacuum cleaner bag and so on

6.  In another way
 Air permeability –The volume of air(in cc)which pass in one second
through 1 cm² of fabric under a pressure head of 1 cm of water.
 So effectively air permeability is that the rate of air, the volume of passing
through the fabric.
 Units of Measure available in mm/s, ft³/ft²/min, cm³/cm²/s, l/m²/s,
l/dm²/min, m³/m²/min, m3/m2/h.

7. Air resistance and Air porosity
 Air resistance:
The air resistance of a fabric is the time in seconds for 1 cm3 of air to pass through 1cm2 of the
fabric under a pressure head of 1 cm of water.
 Air resistance is the reciprocal of air permeability.
 Air porosity: described same as air permeability in some papers. But
 Skinkle’ gave a definition in ‘Textile Testing’:
“The porosity of a fabric is the ratio of air space to the total volume of the fabric expressed as a
percentage.”
 Let,
S = total volume of sample in cubic centimetre
F = Total volume of fibre in cubic centimetre
Air porosity, P = (S-F) / S * 100

8. Measurement of Air permeability
 Principle
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.
Relative testing methods and Standards
ASTM D737-96 Standard Test Method for Air Permeability of Textile Fabrics
BS EN ISO 9237 Textiles – Determination of the Permeability of Fabrics to Air
BS 3424-16:1995 Testing Coated Fabrics – Part 16: Method 18. Determination of Air
Permeability

9. Methods used for determining the air
permeability
 Methods used for determining the air permeability: Three principles can be
used to measure the ability of the fabric to allow the air passage through
it [5].
 Measurement of time required to pass the given volume of air through a given
area of fabric.
 Measurement of pressure differential required for air to pass at a given
constant volume rate through a given fabric area.
 Measurement of volume rate of flow of air through a given fabric area under a
given pressure differential. Most of the instruments are working on this
principle. (ASTM D737-96).

11. Shirley air permeability apparatus.
1. R1: 0.05-0.5
2. R2: 0.5-3.5
3. R3: 3-35
4. R4: 30 – 350
5.07 cm2

13. ASTM 737–96 and British standard
 The ASTM 737–96 procedure determines the volume rate of air fl ow 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 specifi ed 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.

14. Comparative analysis of different test method
Standard Method Test specimen
size
Pressure difference Air permeability unit
ASTM 737–96 38.3 cm2 125pa(12.7 mm water gauge)
From 100pa to 2500pa
ft³/ft²/min or
cm³/cm²/s
BS ISO 9237 508 mm2 (25.4 mm
diameter)
pressure head of 10 mm of
water
millilitres per 100
mm2 per second
BS 3424–16:1995 5 cm2,20 cm2, 50
cm2 or 100 cm2
50 Pa, 100 Pa, 200 Pa, 500 Pa
or 1 kPa
L/min
DIN 53387 20 cm2 200/160/100pa l/m²/s, l/dm²/min
Shirley test method 5.07 cm2 pressure head of 1 cm of
water
cm³/cm²/s
Germany DIN 53387 specifies that clothing fabric is defined as 100Pa (10mm water column),
parachute fabric as 160Pa (16mm water column), filter fabric and industrial fabric as 200Pa
(20mm water column),

15. Common Unit Conversion of Air Permeability
 1 cm³/cm²/s = 1.9685 ft3/ft2/min = 1.9685 ft/min
 1 cm3/cm2/s = 10 mm/s
 1 L/m2/s = 1 mm/s
 1 L/dm2/min = 1.667 mm/s
 1 m3/m2/min = 16.667 mm/s
 1 m3/m2/h = 1000.02 mm/s
 1 mm/s = 0.19685 cfm

16. 1.Flow rate between warp and weft yarn(Q1)
2.Flow rate through the interaction point of warp &weft
yarn(Q2)
3.Flow rate through the fibers of warp yarn(Q3)
4.Flow rate through the fibers of weft yarn(Q4)

17. Effects of Air permeability on Fabric
Properties
 Air Permeability and cloth cover
 Air Permeability and twist factor
 Air Permeability and thermal properties
 Environmental factor
 Fibre properties
 Thickness
 Comfort characteristics
 Yarn crimp and weave structure
 Fabric Finishing

18. Air Permeability and cloth cover
Air Permeability vs cloth cover

19. Air Permeability and twist factor

20. Weave Structure
 Under the same tightness of the fabric, the air permeability of the fabric is
inversely proportional to the yarn density; from the aspect of the texture of
the fabric, under the same arrangement density and tightness, the air
permeability is ranked as plain weave/twill/satin/porous structure; the fabric
with a larger volume fraction has a lower air permeability.

21. Fibre Properties
 Type of interlace, type of fibre (spun or strand), size of the fibre (Linear toughness), twist
factor in the fibre, strand toughness (ends and picks) and fold are other material parameters
that affect the air permeability of a material. Moreover, the moisture regains of the fibre has a
significant effect on the air permeability.
 When the wool fabric increases with the moisture regain, the air permeability drops
significantly due to the radial expansion of the fibre. The surface shape and cross-sectional
shape of the fibre will increase the resistance of the airflow due to the increase of the shape
barrier and the specific surface machine: The shorter the fibre, the greater the rigidity as well
as the probability of product hairiness, hence the poorer the air permeability.

22. Fabric Finishing
 the air permeability of a fabric can influence its comfort behaviour in several
way
Fabric Finishing: Construction factors and finishing techniques can have an
appreciable effect upon air permeability by causing a change in the length of
airflow 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.
Hot calendaring can be used to flatten fabric components, thus reducing air
permeability.

23. Yarn crimp and tension
 Yarn crimp
Yarn crimp and weave influence the shape and area of the interstices between
yarns and may permit yarns to extend easily. Such yarn extension would open up
the fabric,increase the free area, and increase the air permeability
Yarn tension
.Warp and weft filling tension also have the influence on the air permeability. More
the tension, higher will be the air permeability due to better penetration of air
through the larger openings produced by yarn. So it is important to maintain the
tension variation as low as possible.

24.  Laundering and Finishing
 Pressure head on testing instrument

25. Environmental factor
 When the temperature is constant, the air permeability of fabric decreases
with the increase of air relative humidity. After the fabric absorbs moisture,
the fiber expands and the pores contract, which reduces the fabric pores and
blocks, then leads to the decrease of the air permeability.
 When the relative humidity is constant, the air permeability of the fabric
increases with the increase of ambient temperature. Because the thermal
movement of gas molecules will increase the diffusion ability of molecules.
 When the temperature and relative humidity are constant, the change of air
pressure on both sides of the fabric will also affect the permeability of the
fabric, and it is nonlinear. The larger the air pressure difference is, the faster
the air flow rate through the pores of the fabric is, the greater the air
resistance is, which leads to the bending deformation of the fabric and thus
affects the air permeability.