Importance of Air Permeability/Fabric porous structure in production of technical textile fabrics

The ease or otherwise of the passage of air is of importance for a number of fabrics end uses such as industrial filters, tents, sailcloth’s, parachutes, bulletproof, windproof, raincoat materials, shirting’s, down proof fabrics and airbags.

Fabric air permeability is a measure to what extent it gives air passing through the fabric. Air permeability, a given area in the vertical direction of the air flow rate, a given time period, as measured by the fabric test area inside the pressure difference of the fabric. Basically, it depends on weight, thickness and porosity of fabric. The porosity of fabric is the demonstration of the air gap as a percentage within the fabric. It has been important for especially the tent fabric and parachute.

The reciprocal of air permeability, air resistance, can be defined as the time in seconds for ImI of air to pass through 100s mm2 of fabric under a pressure head of 10mm of water. The advantage of using air resistance instead of air permeability to characterize a fabric is that in an assembly of a number of fabrics, the total air resistance is then the sum of the individual air resistance.

Fabrics, are porous materials which allow the transmission of energy and substances and are therefore interesting materials for different applications. In general, they are used for clothing, interior and wide range of technical applications

Garments must be characterized by good air circulation between the skin surface and the environment, good ventilation at skin level and the possibility of eliminating the excess humidity generated through perspiration.

Air permeability Definition

The air permeability of woven fabrics is important because it influences the comfort properties of the final product – garments. The air permeability can be controlled during the design stage through: the characteristics of the raw material (the type of fibres and blend ratio), the geometric characteristics of the yarns used, the structural parameters of the woven fabrics, the technology used to produce the fabrics and the finishing process.

Air permeability, simply a physical ability of a fabric to let certain air flow through under differential pressure between either surface, refers to the speed at which water vapour molecules transmit into the top layer. Fabrics with different surface textures on either side can have a different air permeability depending upon the direction of air flow. Air permeability and fabric porous structure are correlated and indicate the breathability which makes great differences in the performance of materials. That is to say, air permeability and porous fabric structure affect how breathable a garment is; besides, air permeability can be measured, whereas breathability is more subjective.

In the British Standard test, the airflow through a given area of fabric is measured at a constant pressure drop across the fabric of 10mm head of water. The specimen is clamped over the air inlet of the apparatus with the use of rubber gaskets and the air is sucked through it by means of a pump as shown in Fig.A. The air valve is adjusted to give a pressure drop across the fabric of 10mm head of water and the air flow is then measured using a flow meter.

Five specimens are used each with a test area of 508mm2 (25.4mm diameter) and the mean air flow in ml per second is calculated from the five results. From this, the air permeability can be calculated in ml per 100mm2 per second.

To obtain accurate results in the test, edge leakage around the specimen has to be prevented by using a guard ring or similar device (for example, efficient clamping). The pressure drop across the guard ring is measured by a separate pressure gauge. Air that is drawn through the guard ring does not pass through the flowmeter. The pressure drops across the guard ring and test area are equalized in order that no air can pass either way through the edge of the specimen. A guard ring of three times the size of the test area is considered sufficient.

Classification of fabrics: based on fabric type there are four types of fabrics

1. Woven fabric: this has been defined as the interlacing/ interlacement of warp and weft yarns where minimum two sets of yarns are needed and warp yarn stay in vertical and parallel to the selvedges.

2. Knitted fabrics: This has been defined as the interloping/interlocking/ intermeshing of warp yarn where minimum one set of yarn is needed.

3. Non-Woven fabrics: this has been defined as the mechanical/chemical/thermal bonding to make non-woven fabrics.

4. Braided fabrics: this is defined as the intertwining/diagonal/interlacement to make braided fabric where minimum three sets of yarns are needed

Factors that Affect Air Permeability/Fabric porous structure of textile fabrics

Correlation between porosity and air permeability of fabric is very complicated because changes of the textile structure (by the influence of the venting system), can be possibly classified as a horizontal increase in porosity.  A correlation relationship has been elaborated between the percentage of open porosity for double layer fabrics and air permeability, considering the use of the different system of reed denting.

Fabric porosity is an important parameter in the assessment of clothing comfort and physical properties of technical textiles and the porosity are defined by the ratio of free space to fibre in a given volume of fabric. The porous are by voids between weft and warp yarns in the fabrics. The air passes through the pores from the surface of the fabric. Tightness factor can be used for fabric air permeability forecasting. The high correlation between the permeability to air and the tightness factor confirms that. Porosity is affected by yarn number or yarn count number. … Increasing loop length, looser the structure and so the values of air permeability increases.