Generally, the air permeability of a fabric can influence its comfort behaviours in several ways. In the first case, a material that is permeable to air is, in general, likely to be permeable to water in either the vapour or the liquid phase. Thus, the moisture-vapour permeability and the liquid-moisture transmission are normally related to air permeability. 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.
Air permeability is defined as the volume of air in millilitres which is passed in one second through 100mm2 of the fabric at a pressure difference of 10mm head of water. The air permeability of a fabric is a measure of how well it allows the passage of air through it .The ease or otherwise of passage of air is of importance for a number of fabric end uses such as industrial filters, tents, sailcloths, parachutes, raincoats, shirtings, downproof fabrics and airbags. In outdoor clothing it is important to have air permeability as low as possible for achieving better wind protection .
It was noted that the stitch length, porosity and air permeability increase and the thermal retaining property decreases for dry relaxed cotton 1 x 1 rib knitted fabrics. Thermal properties and thermal behavior of cellulosic textile fabrics air permeability, porosity were investigated previously and found that air permeability and heat transfer through fabrics is closely related to both the capillary structure and surface characteristics of yarns .
The effect of fibre composition and yarn type on the wickability, air permeability and thermal insulation of knitted fabrics was studied previously and found that the air permeability of fabrics decreases with the increase in cotton content. The air permeability of a fabric is affected by the fabrics material such as fibre fineness, structural properties such as shape and value of pores of the fabric and the yarn and fabric thickness.
The range of values obtained is significant, ranging from 287 to 690 cm3/cm2/s for viscose single jersey fabric and from 331 to 725 cm3/cm2/s for excel. The porosity determines the variation of air permeability. Fabrics having low porosity values shows the lowest value for air permeability. Increasing fabric tightness by machine setting decreased the air permeability in both fabrics. Coarser yarn produce fabrics with more intra-yarn air spaces but with fewer inter yarn air spaces resulting in lower air permeability in the single jersey knitted fabrics. Air permeability increases for the fabrics made from finer yarns as expected. The lower thickness and mass per square meter also facilitate the passage of air through the fabric. The lower hairiness of the finer yarn may be another contributing factor towards the better air permeability. As the loop length increases, the air permeability value also increases.
