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
It is defined as “the volume of air in cubic centimetres (cm3) which is passed through in one second through 100cm2 of the fabric at a pressure difference of 10cm head of water” — Wikipedia.
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
- 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.
- Knitted fabrics: This has been defined as the interloping/interlocking/ intermeshing of warp yarn where minimum one set of yarn is needed.
- Non-Woven fabrics: this has been defined as the mechanical/chemical/thermal bonding to make non-woven fabrics.
- 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.
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
Technical Fabric architectural design, production considerations and applications.
Advanced fabric production project demands developing strategies with regard to new fabric constructions and it should have the desired end-usage properties as per there applications. For specified Fabric, we need to have complete knowledge and understanding of porous barrier between the human body and environment. This should support heat and water vapour exchange between the body and environment in order to keep the body temperature within the homeostasis range.
Besides thermo-physiological protection, fabrics also play an important role by heat protection due to the flames or convection heat, contact heat, radiant heat as well as due to the sparks and drops of molten metal, hot gases and vapours.
Moreover, Fabrics protect users against micro-organisms, pesticides, chemicals, hazardous particles and radiations (radioactive particles, micro-meteorites, X-rays, microwaves, UV radiation, etc.). They act very important role also by environmental protection as filters for air and water filtrations, sound absorption and isolation materials against noise pollution, adsorption materials for hazardous gas pollution, etc.
By all mentioned applications dedicated to absorption, desorption, filtration, drainage, vapours transmission, etc., the essential constructional parameter that influences fabric efficiency to protect human or environment is porosity. The fabric in a dry state is a two-phase media physical property of the material and describes the fraction of void space in the material.
The porosity (or void volume fraction) is expressed as a coefficient ranging between 0 and 1 or a percentage ranging between 0% and 100% (by multiplying the coefficient by 100). Mathematically, the porosity is defined as the ratio of the total void space volume to the total (or bulk) body volume which consists of the fibrous material – solid component and void spaces containing air – gas (void) component.
The porosity of material has different porous structures as the consequence of different manufacturing techniques needed to interlace the fundamental structural elements, e.g. fibres, yarns or layers, into a fibrous assembly. Fabric porosity strongly determines important physical, mechanical, chemical, and thermal properties of the fabrics such as mechanical strength, thermal resistance, permeability (resistance to wind, breathability), absorption and adsorption properties (wicking, wetting), translucence, soiling propensity, UV light penetration, sound absorption ability, etc.
Knowledge about the fabric’s porous structure is, therefore, an important step when characterizing fabrics, in order to predict their behaviour under different end-usage conditions regarding a product. Hence, if porosity is estimated or predicted then when developing a new product, the desired porosity parameters can be set in advance on the basis of selecting those fabric constructional factors that have an effect on porosity and, in this way sample production trials could be reduced.
Types of pores according to the air/fluid accessibility
- cylindrical pores
- slit-shape pores
- cone-shape pores
- ink bottle pores
Four groups of pore descriptors, e.g. size, shape, orientation, and placement, are defined as important parameters. Pores can be mathematically assessed on the basis of a known model of pores geometry and constructional parameters of the material with the following parameters: the number of pores, pore size, pore volume, pore surface area, pore length, etc.
On the basis of an ideal geometrical model of a porous structure, the pore size distribution which is also an important parameter of the material porous structure cannot be assessed while the pores in the geometrical model are usually assumed to be the same sizes. Such a situation rarely occurs in real fabrics. The further considerations of ideal geometrical models of material porous structures and porosity parameters will be focused on different types of fabrics.
Fabrics are flat textile materials which are produced by different manufacturing techniques using different fibrous forms of input material (or structural element), and consequently having different porous structures.
When a woven fabric is treated as a three-dimensional formation, different types of pores are detected
- Inter-pores, e.g. the pores which are situated between warp and weft yarns (micropores, inter-yarn pores) and pores which are situated between fibres in the yarns (mesopores, interfiber/intra-yarn pores)
- Intra-pores, e.g. the pores which are situated in the fibres (micropores, intra-fibre pores). The structure and dimensions of the inter- or intra-yarn pores are strongly affected by the yarn structure and the density of yarns in the woven structure.As fibrous materials, woven fabrics have, with regard to knitted fabrics or nonwovens, the most exactly determined an ideal geometrical model of a macro-porous structure in the form of a tube-like system, where each macropore has a cylindrical shape with a permanent cross-section over all its length.Because the warp density is usually greater than the weft density, the elliptical shape of the pore cross-section is used to represent the situation in Figure 6. Macropores are opened to the external surface and have the same cross-section area. They are separated by warp or weft yarns and are uniformly distributed over the woven fabric area.
Production of fabric
Fabric construction involves the conversion of yarns, and sometimes fibres, into a fabric having characteristics determined by the materials and methods employed. Most fabrics are presently produced by some method of interlacings, such as weaving or knitting.
The future of textiles is very unpredictable due to day by day innovations in the various fields of textiles such as composite textiles and one such example is Three-Dimensional Fabrics such as Multi-Layer, Orthogonal, Domed, Nodal, Spacer Fabrics, etc. These 3d fabrics have versatile physical & structural attributes and various application scopes in speciality industrial fabrics, medical technology, aerospace applications and so on.
3D fabrics have wide methods in terms of manufacturing processes like Stitching Operation, Multilayer Principle, Orthogonal Principle, Angle Interlock Principle and Dual Direction Shedding Method.
The fabrics used in composites manufacture are referred to as Preforms and are specially engineered as a single-fabric system to impart reliability and performance. 3D Preforms appear to be better than the most conformable 2D fabrics.
The flexural, tensile and compressive stiffness and strength are better in laminates made from 3D preforms than those made from comparable 2D woven or knitted fabrics mainly due to the absence of in-plane crimp of yarns in materials.
Acknowledgement: Technical and technological Facts in this write up has been selected from various sources