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.
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