Importance of Air Permeability/Fabric porous structure in production of technical textile fabrics
Fabric air permeability is a measure to what extent it gives air passing through the 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 level of air permeability differs depending on the following:
I. Fabric Structure: Woven fabric specifications
- Fabric construction: –warp count x weft count/ends per inch x picks per inch
- Fabric area density/GSM
- Cover factor—changing the area density and/or the cover factor may affect strength, stiffness, stability, porosity, filtering quality and abrasion resistance of fabric. Application of jammed fabrics or closely woven fabrics finds use in waterproof, windproof, bulletproof requirements.
- Type of weave
- Fabric width
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.
II. 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.
III. Yarn Structure
The tighter the structure of the yarn, the smaller the penetration within the yarn but the greater the penetration between the yarns. The material, twist and smoothness of the yarn contributes to permeability. The material type and amount of yarn twist, count and yarn structure manufactured by Ring spinning, Open end, Air textured, condenser spinning methods does impact on fabric air permeability. Some important parameters related are pore in the fabric were taken in to account like the cross-section of the pore, depth of pore or thickness of fabric and number of pores per unit area.
IV. Environmental Conditions
Under the constant temperature, the air permeability of the fabric decreases with the increase of relative humidity, due to the hygroscopic expansion of the fibres which reduces the internal voids of the fabric and some moisture can block the passage. Under the constant relative humidity, the air permeability of the fabric increases as the ambient temperature increases. Because when the ambient temperature rises, on the one hand, the thermal motion of the gas molecules is intensified, leading to the diffusion of molecules, which enhances the permeability. On the other hand, the thermal expansion of the fabric as a whole improves the permeability of the fabric.
V. Other Aspect
Besides the above, air permeability of material also hangs on parameters like the material cover and material permeability. Sum cover of material is known as the ratio area concealed by the covering and the stuffing fibres to the area concealed by the material. The kind of knit decides the way in which the fibres are twisted in the material. The air permeability of the materials can be altered by changing the way of knitting. When the size of the fibre changes, the same happen in the fibre of the material hence the permeability of the material changes.
|Fabric||Fabric Code||Types of Weave||Weight per Square meter.g||Cloth thickness, mm||Warp Crimp C,%||Weft Crimp C, %||Air permeability, Ft3/cm2|
|I||Standard||Plain 1/1||Plain 1/1||276.6||0.8280||23.4||13.6||5.2||4.8||38.0|
|II||Standard||Twill 3/3||Plain 1/1||252.4||0.9220||10.4||11.8||5.2||6.2||35.0|
|III||Standard||Satin 6 Weft||Plain 1/1||289.0||0.9800||11.6||12.8||3.0||3.2||35.0|
Factors influencing porosity in multi-layer/spacer woven fabrics
- Type of material
- The linear density of yarns “warp-weft”
- Warp and weft density per cm
- Twist factors
- Type of spinning
- The difference of denting system
- Type of stitches
- Form and relative porosity
- Type of woven construction
- Thickness & weight