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Spinning & Weaving
  Fabric structures for protective textiles

Protective textiles are a part of technical textiles that are defined as comprising all those textile-based products that are used principally for their performance or functional characteristics rather than their aesthetic or decorative characteristics. Textile materials in the form of woven, knitted, nonwoven and braided structures or a combination of these structures are being used for protective textiles (Figure 1). Most fabrics are two-dimensional but an increasing number of three-dimensional protective textile structures are being developed and produced. The properties of fabrics depend on the characteristics of the constituent yarns or fibres and on the geometry of the formed structure.

Woven fabrics

Woven fabrics are the most common type of textile structures used for protective applications. Depending on the degree of interlacement, basic weave structures may be classified as plain, matt, twill and satin/sateen with thousands of derived structures. Woven fabrics consist of two sets of yarns mutually interlaced into a textile fabric structure. The threads that run along the length of the fabric are called warp or ends, while the threads that run along the width of the fabric from selvedge to selvedge are referred as weft or picks. Warp and weft yarns are mutually positioned under the angle of 90°.

The number of warp and weft yarns per unit length is called warp and weft density. The warp and weft yarns in a woven fabric could be interlaced in various ways that is called a weave structure. A crimp is the ratio of the yarn actual length to the length of the fabric it traverses. The crimp influences the fibre volume fraction; fabric thickness and fabric mechanical properties. A cover factor is the fraction of the total fabric area that is covered by the component yarn. Fabric area density and cover factor influence strength, thickness, stiffness, stability, porosity, filtering quality and abrasion resistance of fabrics.

Plain weave

It is the simplest weave; it has the minimum repeat size of 2. The type of fibre in the yarn used for making fabric will influence the fabric’s characteristics. The yarn used for might be flat, monofilament, twisted or textured multifilament or it has been spun from natural or synthetic staple fibres. The stiffness of the raw material used for producing the yarn and the twist factor of the yarn affect the weavability and stiffness of the fabric. Very highly twisted yarns are used in cases that require certain special characteristics in plain weave yarns. Fabrics produced from such yarns will have a good amount of extensibility or will be semi opaque.

Plain fabrics can be woven as warp or weft faced fabrics. The warp faced plain fabrics have much higher warp cover factor than weft cover factor. Also the warp thread density is twice the weft thread density for same linear densities of warp and weft yarns. There will be a great difference in the crimp values of warp and weft (warp has much higher crimp). Thus by choosing suitable cover factors and yarns, most of the abrasion on the fabric will be taken by the warp yarn, thereby protecting the weft yarns.

The weft faced plain fabrics have much higher weft cover factor than the warp cover factor. The weft crimp far exceeds the warp crimp. Weft faced plain fabrics have the disadvantage that they are more difficult and expensive to weave and hence are restricted.



Satin weave

Triaxial woven structure

Rib knit structure

Triaxial braid structure

Figure 1. Examples of woven (top left), knitted (top right), 3D Woven (bottom left) and braided (bottom left) structures used for protective textiles.

Rib weaves

These are derivatives of plain weaves. They may warp, weft or matt ribs according to the dominance of the float of threads in warp or weft or both. These weaves have lesser number of interlacements. Therefore, more number of threads can be inserted in a given space to obtain a higher cover without jamming the threads.

In warp rib fabrics the warp ends/cm exceed those of the weft yarns. And thus have a high warp crimp and low weft crimp. The situation reverses in weft ribs. Rib weaves with long floats face the problem of overlapping adjoining yarns. Weft ribs are uneconomical to weave because of higher weft thread density per unit length. The possibilities are better with two picks inserted at the same time.

Simple matt weaves tent to imitate plain weaves due to equal dominance of warp and weft floats.

The warp and weft threads in matt weaves can be floated unevenly to obtain special technical effects. Large repeats of matt weaves are avoided, as the structure tends to become unstable. If they are to be used, some kind of fancy weave or binding is required. Matt weaves can have higher cover factors due to lesser binding points. When constructed closely they can have better abrasion and filtration properties and greater resistance to water penetration. When constructed more openly matt weaves have a greater tear resistance and bursting strength. Weaving costs can also be reduced if two or more picks can be inserted simultaneously.

Twill weaves

These weaves give a diagonal line effect in the fabric, which runs from one selvedge end to the other. The minimum repeats size of the weave is 3. The direction of the twill line may be ‘Z’ or ‘S’. The twill angle can be easily varied. Twills have longer floats, fewer intersections and more open construction than plain weave fabrics with identical construction details. Weft faced twills cause less strain in weaving as compared with warp faced twills, since lesser number of warp ends has to be lifted so as to allow weft picks to pass below them. If the twills are woven upside down, it becomes difficult to inspect the warp yarns during weaves. More combination of twills can be had with increase in size of repeat.



Satin sateen weaves

These weaves are produced either warp or weft faced. Unlike simple twills they do not show continuous diagonal lines in fabric and tend to give smooth surface due to more floats. The warp thread density/cm is greater than weft in case of satin. In case of sateen weaves it is vice versa. The weaves are constructed on a minimum repeat size of 4. Though satins / sateen’s can be constructed with many repeat sizes, 5 end satins/sateen’s are most common, as they give moderate cover factors and firm fabrics.

Knitted fabrics

Knitted fabrics are textile structures assembled from basic construction units called loops. There exist two basic technologies for manufacturing knitted structures: weft and warp knitted technology.

Weft knitted fabrics

The repeating unit of the knitted fabric is called loop. The feature of the weft knitted fabric is that the loops of one row of fabric are formed from the same yarn. A horizontal row of loops in a knitted fabric is called a course and vertical row of loops is called a wale. The stitch density is the number of stitches per unit area in the knitted fabric. The stitch length is the length of a yarn in a knitted loop and is an important factor that determines the properties of the weft knitted fabric.

The cover factor is a number that indicates the extent to which the area of a knitted fabric is covered by the yarn. The higher cover factor indicates a more tight structure and vice-versa. The fabric area density is a measure of the mass per unit area of the fabric. In weft knitted fabrics the loops are formed successfully along the fabric width. The yarn is introduced more or less under the right angle regarding the direction of the fabric formation.

The feature of the weft knitted fabric is that the neighboring loops of one course are created of the same yarn. The simplest weft knit structure produced by the needles of one needle bed machine is called plain knit or jersey knit. The plain knit has different appearance of both sides of the fabric. A structure produced by the needles of both needle beds is called rib structure or double jersey having the same appearance on both sides of the fabric.

Weft knitted fabric can be produced on a number of different types of knitting machines. Circular or flat bar machines using a latch needle can produce both fabrics and knitting garments. Straight bar or circular machines using a bearded needle can produce shaped knitwear. Many machines can produce a double fabric structure with differing knitted structure on each fabric face. Spacer yarns can be inserted between the front and back fabric, thus creating a complex three layer structure. The properties of each layer are determined by the fibre, yarn properties and structure of that layer. These structures can be tailored for specific applications and are useful in the protective textile field.

Warp knitted fabrics

In warp knitted technology every loop in the fabric structure is formed from a separate yarn called warp mainly introduced in the longitudinal fabric direction. The most characteristic feature of the warp knitted fabric is that neighboring loops of one course are not created from the same yarn. To accomplish the warp knitted structure every needle along the width of the fabric must receive yarn from the individual guide. The function of the guide is to lead and wrap the warp yarn around the knitting needle during the knitting process. The loop structure in the warp knitted and weft knitted structure is similar in appearance. The warp knitted structure is very flexible and regarding construction it can be elastic or inelastic. The mechanical properties are in many cases similar to those of woven structures. The best description of warp knitted fabrics is that they combine the technological, production and commercial advantages of woven and weft knitted fabrics.



Warp knitted fabrics can be produced on a number of different types of knitting machine. Raschel machines using a latch or compound needle produce high pile upholstery fabrics, industrial furnishing fabrics and bags for vegetables. Tricot machines using bearded or compound needle produce lace, nets, and outerwear fabrics. Weft insertion with, for example, elastic yarns or fleeces can produce directionally orientated fabrics. Warp knitted fabrics are commonly used in linings for protective clothing and laminated with polyurethane foams to provide a strong flexible base for the foam.

Nonwoven fabrics

Nonwovens are a class of fabric that are produced directly from fibres, and in some cases directly from polymers, thereby obviating a number of intermediate processes such as spinning, winding, warping, weaving/knitting. Hence nonwovens can be produced inexpensively for both single use and durable applications. Nonwovens are produced in two distinct steps:

1. Web formation: arrangement of fibres into a 2D sheets, and

2. Consolidation: bonding the fibres together to create a nonwoven fabric.

Web formation methods

Web formation may be classified into dry laid, spun laid and wet laid processes.

Dry laid process

Dry textile fibres are carded, using a carding machine similar to the once used in the spinning industry, to arrange the fibres in a 2D sheet with fibre orientations predominantly in the machine direction. The web is subsequently folded using a cross – lapping machine to increase the web thickness and to achieve transverses fibre orientation. In some cases, conventional carded and cross-lapped webs are combined to produce a web with bi-directional fibre orientation. Alternatively, an aerodynamic system is used for creating a web random fibre orientation.

Spun laid process

This is a method of producing fabrics directly from polymer chips, hence eliminating the entire textile supply chain. Fibres are extruded from a spinneret similar to conventional melt spinning process. These fibres are attenuated (stretched) using high-velocity air streams before depositing on a conveyer in a random manner. The spun laid process is the most commonly used method for producing both disposable and durable nonwovens for protective application.

There are other related systems such as flash spinning, melt blowing and electro-spinning. Flash spinning involves extrusion of a polymer film dissolved in a solvent; subsequent evaporation of the solvent and mechanical stretching of the film results in a network of very fine fibres. These fibres are subsequently bonded to create a smooth, microporous textile structure used for protective applications. The melt-blowing process produces microfibres by attenuating the polymer jet, coming out of the spinneret, using high-velocity air jet. Since the polymer is stretched in the molten state, extremely fine fibres can be produced. Because of the lack of molecular orientation, melt-blown fibres are weak and hence are generally used in conjunction with other type of non-weaves. For example, a composite nonwoven consisting of melt-blown layer and a spun-bond layer is becoming popular for medical protective applications

Wet laid process

Developed from the traditional paper making process, relatively short textile and wood fibres are dispersed in large quantities of water before depositing on as inclined wire mesh. These materials find application in hospital drapes and filters.

Consolidation process

Fibrous webs can be consolidated using a number of techniques depending on the area density and the desired properties. They can be classified into mechanical, chemical, thermal and stitch bonding processes.

Mechanical bonding

Needle punching and hydro-entanglement are two complementary mechanical processes. Relatively thick webs (150 to 1000 g/m2) are felted with the aid of oscillating barbed needles. The hydroentanglement process uses high velocity water jets to consolidate relatively thin webs (< 140 g/m2). The resulting spun laced fabrics are highly drapable and hence popular for medical protective clothing.

Chemical bonding

Fibres are bonded with a suitable adhesive and subsequently cured under heat. Saturation bonding is seldom used for protective applications, as this process results in a relatively stiff non-porous material. Spray and print bonding instead of saturation bonding improves the flexibility and permeability.

Thermal bonding.

Relatively thin webs are passed through a heated calender, resulting in partial melting and bonding of fibres. Thermal bonding is a high-speed process and hence commonly used in conjunction with spun laids.

Stitch bonding

Cross-laid webs are stitched together with a relatively large number of needles across the width. Alternatively, stitch bonding is also used to bond a series of non-interlaced thread systems.

3D woven fabrics

Protective textiles, especially produced for ballistic applications, consist of a number of fabric layers stitched or quilted together. An alternative and cost effective method would be to weave all the layers together. Relatively thick fabrics consisting of a number of wrap and weft layers can be produced on conventional and specialised 3D weaving machines. The warp and weft yarns are held together with interlacing z-yarns: orthogonal and angle-interlocked are the two prominent structures used. In theses structures, most of the yarns remain non-crimped and hence these structures have high in-plane modulus and high longitudinal wave velocity.

Potentially, 3D weaves have a number of advantages over broad cloth:

● Fabrics can be woven with much higher cover factors since there are only small percentages of interlacing yarns.

● Warp and weft yarns have very little or no crimp at all in 3D weaves. Hence, coarser yarns can be used as opposed to fine yarns being used in 2D fabrics, to minimize the effect of crimp.

● Labour cost can be reduced as a result of using coarser yarns and eliminating subsequent stitching processes.

Braided textile structures

Braided textile structures are manufactured with mutual interwining of yarns in a tubular form. There are three typical braid structures: diamond, regular and hercules. Diamond structure is obtained when the yarns cross alternatively over and under the yarns of opposite direction. The repeat notation is 1/1. Regarding this way of notification, the regular braid structure has notation 2/2 and hercules 3/3.

The braids are mostly produced in a regular structure. Generally braids are produced in a tubular form of biaxial yarns direction. By insertion of longitudinally oriented yarns (middle-end-fibre) into the structure the 3 axial braids is obtained. Moreover in the centre of the tubular braid, additional fibres called axial fibres can be inserted. When the number of braiding fibre bundles is the same, the tubular braid increases the fibre volume fraction more than the flat braid.

The main feature of the braid is the angle of interwining that can vary between 10–80° and depends on: the yarn fineness, the type of the structure (biaxial or triaxial), cover factor (tightness of the structure) and the volume ratio of the longitudinal yarns. Since the braids have tubular form, they are often replaced with the filament winding structures. In this respect it has been proven that the braids can be competitive regarding the price. The braid is a flexible product and can be adjusted to various shapes. With the special device called mandrel the braids can be shaped into various forms directly on the machine at the manufacturing stage.

Assembled fabrics

In some cases of protective textile application there is a need for connecting several layers of different textile structures. The reason could be increasing the thickness of composite perform before impregnation with resin. Several layers of the fabric are stacked upon each other till desired thickness is achieved, and then, the fabrics are assembled by sewing, stitching, needle punching or knitting. Depending on the thread size and stitching density, the latter can contribute to increasing through thickness strength, stiffness and delimitation resistance of the composite.

References

1. N Gokarneshan: Textile Asia, July 2007, 48-50.

2. R A Scott: Textiles for Protection, Woodhead Publishing, Cambridge, 2005.

3. G Demboski, and G B Gaceva: Bulletin of the Chemists and Technologists of Macedonia, 2005, 24, 77-86.

4. A R Horrocks, and S C Anand: Handbook of Technical Textiles, Woodhead Publishing, Cambridge, 2000.

5. A Ormerod, and W S Sondhelm: Weaving -- Technology and Operations, Textile Institute, Manchester, 1999.

6. D J Spencer, Knitting Technology, Pergamon Press, Oxford, UK, 1986.

Note: For detailed version of this article please refer the print version of The Indian Textile Journal May 2008 issue.

S Viju

Department of Textile Technology,

SSM College of Engineering,

Komarapalayam,Tamil Nadu 638 183.

Email: vijutext@yahoo.co.in.

published May , 2008
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