Air Permeability of Fabric:
Permeability may be defined as the rate at which gas or liquid passes through a porous medium. Textile fabrics are permeable substances. The fabric needs air, water and vapour permeability so that a person feels comfortable wearing it.
The concept of ‘air permeability’ is widely used in the textile industry to interpret the intrinsic characteristics of fabric. It is an important property of fabric which is frequently asked by retail buyers. The air permeability of a fabric is a measure of how well it allows the passage of air through it. This technical information to describe functional performance of garments. 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. Apart from clothing comfort, it is also important for a number of fabric end-uses, for example, industrial filters, tents, sail-cloths, parachutes, raincoat materials, shirtings, down proof fabrics, airbag and so on.
Air permeability is defined as the volume of air in millilitres which is passed in one second through 10Os mm2 of the fabric at a pressure difference of 10mm head of water. Air permeability of any material can be defined as the air passing at right angle through a known area of the specimen in a unit time under a prescribed air pressure differential between the two surfaces of a specimen.
In order to evaluate the air permeability of the specimen, air passing perpendicularly through the specific area of the fabric in a unit time is adjusted in order to obtain the predetermined air pressure difference between the two surfaces of the samples. This rate of air flow helps determine the air permeability of the fabric sample. This parameter directly influences the comfort parameter of the fabric.
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. 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.
The comfort property of the fabric is also influenced by air permeability. Any material which is permeable to air is also likely to be permeable to moisture vapor. Hence, the air permeability is in general closely related to moisture–vapor permeability and the liquid moisture transmission. Fabrics made from cotton or wool undergoes swelling upon absorption of water vapor from the atmosphere. This would close off the pores in the fabric and thereby increase the resistance to convective flow through the material. Changes in fabric air permeability as a function of relative humidity are important for chemical protective clothing point of view.
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.
The level of air permeability varies depending on the following:
Important fabric properties for maintaining thermal comfort include the following:
Air permeability
Water or moisture (vapor) permeability/transportation
Heat transmission
Measurement of Air Permeability: Air permeability affects the comfort aspect of a garment in terms of air passage through the fabric. High air permeability per unit area of a fabric gives lower protection against winds, especially for outerwear garments, whereas low air permeability causes heavy body perspiration.
Figure 1: Air permeability test for fabric
In Shirley air permeability tester the sample is clamped between two rubber gaskets, and a guard ring surrounding the test specimen ensures that all the measured airflow passes through the specimen with no leakage. The test area is a circle of 5.07 cm2. Airflow is measured when a pressure differential of 20 mm H20 (0-2 kPa) is applied. Ten measurements of airflow are made on each sample.
Figure 2: Relationship between air permeability and cloth cover
Procedure of air permeability test for fabric:
1. Handle the test specimens carefully to avoid altering the natural state of the material.
2. Place each specimen onto the test head of the test instrument and perform the test as specified in the manufacturer’s operating instructions.
3. Place coated test specimens with the coated side down (towards the low-pressure side) to minimize edge leakage.
4. Use a water pressure differential of 125 Pa (12.7 mm or 0.5 in. of water).
5. Read and record the individual test results in SI units as cm3/s/cm2 and in inch-pound units as ft3/min/ft2 rounded to three significant digits.
6. For special applications, the total edge leakage underneath and through the test specimen may be measured in a separate test, with the test specimen covered by an airtight cover, and subtracted from the original test result to obtain the effective air permeability.
7. Remove the tested specimen and continue testing until all the specimens have been tested for each laboratory sampling unit.
8. The number of tests may go up to 10 but the minimum required number of tests is 4.
References:
A Practical Guide to Textile Testing By K. Amutha
Physical Testing of Textiles by B. P. Saville
Textile Engineering – An Introduction Edited by Yasir Nawab
Emerging Research Trends in Medical Textiles by N. Gokarneshan, D. Anitha Rachel, V. Rajendran, B. Lavanya, Arundhathi Ghoshal
Clothing Biosensory Engineering Edited by Y. Li and A.S.W. Wong
Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.
