Importance of technical textiles is great and increasing. Experts estimate that annual rising ratio of this application of textile materials is 3.8 % in average and consumption in each filed of this group of applications is anticipated as growing. Roughly one third of the quantity of the world’s fibre consumption is used in production of technical textiles.
Range of technical textiles is very wide.There are various definitions of this term from which we usually use the list disseminated by the well known international fair of these products, Techtext i l , which contains 11 groups. Fig. 1 presents them with their weight concerning the anticipated ratios of products used in these applications.
Thus, we can say that the term “technical textiles” can be defined as do not belong to them the usual underwear and outerwear products as well as products called ordinary home textiles and household textiles (table linen, bed linen, dishcloth, curtains, etc.) except the ones used in furniture and upholstery. Most of technical textiles are made of non-conventional materials, they are usually man-made fibres, in many cases special types developed for specific applications. Metallic yarns (thin wires) alone or parallel to other yarns are also used in some technical knitted fabrics. However, traditional materials, including those of natural origin, play important role in some fields.
Though each production technologies are involved in these fields, from spinning to the various kinds of fabric manufacture, including manufacture of ropes and twines, we concentrate in this paper to the role of knitting technologies.
Use of Knitting technologies in manufacture of technical textiles
Studying the objects exhibited on fairs and the articles published in technical papers and on the Internet we have the experience that various kinds of warp knitted fabrics play the most important role among knitted technical textiles. These fabrics are made on tricot, raschel, crochet and knit braiding machines. Products of these machines can be used in themselves, like nets or bandages, but also as reinforcement materials in composites or backing materials for laminated or coated
fabrics.
However, besides warp knitting technology important products are made also on weft knitting, mainly on circular knitting machines but V-bed flat knitting must not be neglected either.are also available. Raschel machines with two needle bars are able to produce wide tubular net fabrics. To manufacture tubular nets of smaller diameters (from 1 or 2 centimetres to about 20 cm) knit breading machines can be used very effectively but their final diameter can be extended in the practice if they contain elastane yarns.
Knitted fabrics with orientated behaviours
Knitted fabrics with orientated behaviours are made usual ly wi th lots of yarns laid lengthwise, crosswise and/or diagonally into the fabric. Their keeping together is performed by warp knitted loops. Aim of these structures is mainly to reduce the stretch and/or to increase the forth of the fabric in one or more directions. If this effect is realized only in one direction (lengthwise or crosswise) the fabric is called “unidirectional” or “monoaxial”. If this behaviour asserts itself in both directions the fabric is called “biaxial”. “Multiaxial” or “multidirectional” fabrics have almost the same behaviour in every direction (Fig. 3.).
To manufacture such fabrics special tricotand raschel machines have been developed completed by equipment to prepare and lead the lots of reinforcing yarns into the place of loop formation. In these fabric constructions the laid-in yarns play the main role, the loops only link them together.
There are a great number of end uses for these fabrics. Biaxial fabrics with PVC coating, for instance, are used for manufacture of stressed roof constructions by hich very wide spaces can be covered. Fabrics with directed behaviours are available also as reinforcement materials of composites or for geogrids.
Spacer fabrics
Among knitted fabrics perhaps the most spectacular development can be registered in case of spacer fabrics. This is already the product of the 21st century, their development began only several years ago but it has made great progress since then. Though the principle of the fabrication is not new, it goes back to the manufacture of plush fabrics on raschel machine, the adaptation of this technology to make a completely new type of fabric is very ingenious. At the beginning they were developed really on two needle bar raschel machines but now there are also circular knitting machines on the market for manufacturing of such products and, of course, V-bed flat knitting machines are also able to make spacer fabrics of some kinds (Fig. 4).
The two surface layers of spacer fabrics are usually linked by relatively thick monofilaments which makes the fabric elastic when pressed in thickness direction. This is the most important reason why spacer fabrics have found many fields of application. They can substitute foam in seats or beds, in orthopaedic support devices, in bras and shoes. It can serve in smart clothes as heat insulation or for forming of ventilation passages. As a type of geotextiles spacer fabrics can be used to lead off water from the soil. In manufacturing of composites used in the motor industry or ship building they can work as reinforcement inlay. Using proper yarns or with application of special treatment they can be electrically conductive, flame retardant, antibacterial, etc.
As we see at this moment, most spacer fabrics used are made on raschel machines or, in recent times, also on crochet machines. Needle bed distance on these machines can be varied within wide range and fabric thickness can reach even 60 mm. On circular knitting machines needle bed distance is much more limited, only thinner (thickness of some millimetres) spacer fabrics can be made on them. However, these variants are also very important and can be found in many products.
I am convinced that spacer fabrics will have high importance in the future among technical end uses, including healthcare.
Stitch bonding
Stitch bonding machines combine knitting and sewing. They have grown from warp knitting technology and their products occupy an intermediate position between knitted and nonwoven fabrics. This fabric contains a carded web which is reinforced by yarns (Fig. 5.) or loops formed from fibres pulled out from the web itself. Needles of the warp knitting machine pierce holes through the web and work like a sewing needle when forming a seam of chain stitching (called pillar stitch in warp knitting technology) or made of polypropylene foil tapes.
Knitted fabrics can be used also to reinforce wall coverings, both outside and inside. Some types of geotextiles and geogrids are also knitted structures, as entioned above.(Fig. 7)
Many buildings, and not only provisional ones, have roofs made of textile fabrics (sports stadiums, air terminals, halls for various functions, etc.). If this roof is made from knitted fabric bi- or multiaxial knitted structures are used with waterproof and weatherproof coating. Huge areas, many hundreds of square metres can be covered by such fabrics.
Another possible application of knitted fabrics in construction is textile reinforced concrete. There are warp knitted structures developed especially for this purpose like the one shown in Fig. 8. Textile reinforced concrete has the advantage that it is much lighter than the one reinforced by steel bars.
Medical treatment
Many kinds of textiles are used in medical treatment. It is not surprising that a great part of clothing worn by doctors and nurses in hospitals and clinics is product of the knitting industry (e.g. undershirts, socks). But sometimes they are not conventional ones, they are made from yarns or with finishing that make them antibacterial against infections or against of rising of unpleasant sweaty smell. Various types of bandages (both rigid and elastic), surgical stockings, certain parts of orthopaedic
equipment (ortheses) (like knee-, wrist- and elbow-braces, calf and lumbar supports, etc.) are also made by knitting technology. An important application field for pacer fabrics is manufacturing of mattresses for beds, operating tables and wheel-chairs. Knitted products find application field also among implants: artificial blood vessels (they can be circular knitted or warp knitted, the latter is made on double needle bar raschel machine and can be made also in Y form), surgical meshes (made on tricotmachine), coverings of artificial heart valves, etc. (Fig. 9).
Thus, development and application of textiles open interesting possibilities for medical sciences and vice versa: manufacture of textiles for medical treatments offer important possibilities for the textile industry. Cooperation of doctors and technical experts of the textile industry can lead to development of new surgical technologies. Structure of the textiles used as implants is determined by its material composition, fibres’ behaviour and features of degradation. Materials of sutures and implants having biologically good properties, designable absorption and degradability and that endure the sterilization process are continuously subjects of research. At the same time, continuous development of textile technologies and machines enables to develop newer and newer methods in surgery and medical treatment. For this mutual development textile technologists and doctors must closely cooperate, while all the administrative procedures concerning manufacturing and trading of such products must be strictly respected.
Functional clothes
Knitted fabrics may be important components of functional clothes, too. For example, spacer fabrics can be used here as lining that, due to its hollow structure, enables ventilation inside the garment or, due to its elastic behaviour in thickness direction, protects against pressure or hit. This is why this fabric is a penchant for lining of motorcyclists’ protective garments. The speciality of a spacer fabric that there is a distance between its two surfaces but they can be springily pressed together enables to use them as electric switch if electrically conductive yarns are used in the two isolated surfaces. When they connect to each other under pressure an electric signal can be created. Also pressure sensor can be built in between the surfaces. Cables can be led in the inside hollow.
Undershirts, trousers, socks made from elastic knitted fabrics fit close to the body and if sensors are fastened on them they can transport signals of the movements and the state of the body (perspiring, pulse, breath frequency, etc.).
Remarkable developments are going on – also in Hungary – with knitted fabrics containing etallic fibre components for manufacture of protective underwear against electromagnetic radiation as well as with other types of underwear that contain modacrylic or carbon fibres to make the knitted garments flame retardant.
Socks made from heat resistant aramide fibres do good service on hot workplaces. On the recent Techtextil in 2009 a knitting factorypresented a complete set of knitted underwear (long-sleeve shirt, trousers, socks , hood) made of a special blending of modacrylic, aramide and cotton fibres to be worn in hot work environment. An other set of knitted underwear was made of antistatic polyester completed by antibacterial treatment.
No comments:
Post a Comment