TEXTILE FINISHING PROCESSES

Textile finishing processes Basic methods and processes The term finishing includes all the mechanical and chemical processes employed commercially to improve the acceptability of the product, except those procedures directly concerned with colouring. The objective of the various finishing processes is to make fabric from the loom or knitting frame more acceptable to the consumer. Finishing processes include preparatory treatments used before additional treatment, such as bleaching prior to dyeing; treatments, such as glazing, to enhance appearance; sizing, affecting touch; and treatments adding properties to enhance performance, such as preshrinking. Newly formed cloth is generally dirty, harsh, and unattractive, requiring considerable skill for conversion into a desirable product. Before treatment, the unfinished fabrics are referred to as gray goods, or sometimes, in the case of silks, as greige goods. Finishing formerly involved a limited number of comparatively simple operations evolved over the years from hand methods. The skill of English and Scottish finishers was widely recognized, and much British cloth owed its high reputation to the expertise of the finisher. More sophisticated modern finishing methods have been achieved through intense and imaginative research. Preparatory treatments It is frequently necessary to carry out some preparatory treatment before the application of other finishing processes to the newly constructed fabric. Any remaining impurities must be removed, and additives used to facilitate the manufacturing process must also be removed. Bleaching may be required to increase whiteness or to prepare for colour application. Some of the most frequently used preparatory processes are discussed below. Production of fabric Knitted fabrics Knitted fabrics are constructed by the interlocking of a series of loops made from one or more yarns, with each row of loops caught into the preceding row. Loops running lengthwise are called wales, those running crosswise courses. Hand knitting probably originated among the nomads of the Arabian Desert about 1000 BC and spread from Egypt to Spain, France, and Italy. Knitting guilds were established in Paris and Florence by the later Middle Ages. Austria and Germany produced heavily cabled and knotted fabrics, embroidered with brightly coloured patterns. In The Netherlands, naturalistic patterns were worked on fabric in reverse stocking stitch, and several Dutch knitters went to Denmark to teach Danish women the Dutch skills. The craft of hand knitting became less important with the invention of a frame knitting machine in 1589, although the production of yarns for hand knitting has remained an important branch of the textile industry to the present day. The frame knitting machine allowed production of a complete row of loops at one time. The modern knitting industry, with its highly sophisticated machinery, has grown from this simple device. Knitted fabrics were formerly described in terms of the number of courses and wales per unit length and the weight of the fabric per unit area. This system is limited, however, and there is a shift to use of the dimensions and configuration of the single loop, the repeating unit determining such fabric characteristics as area, knitting quality, and weight. The length of yarn knitted into a loop or stitch is termed the stitch length, and in a plain knitted structure this is related to the courses per inch, wales per inch, and stitch density. The two basic equilibrium states for knitted fabrics are the dry-relaxed state, attained by allowing the fabric to relax freely in the air, and the wet-relaxed state, reached after static relaxation of the fabric in water followed by drying. Knitting machines The needle is the basic element of all knitting machines. The two main needle types are the bearded spring needle, invented about 1589, and the more common latch needle, invented in 1847. The bearded needle, made from thin wire, has one end bent, forming an operating handle; the other end is drawn out and bent over, forming a long flexible tipped hook resembling a beard. A smooth groove, or eye, is cut in the stem or shank of the needle just behind the tip. In use this needle requires two other units, a sinker to form a loop and a presser to close the needle beard, allowing the loop to pass over the beard when a new stitch is formed. Bearded needles can be made from very fine wire and are used to produce fine fabrics. The latch needle is composed of a curved hook, a latch, or tumbler, that swings on a rivet just below the hook, and the stem, or butt. It is sometimes called the self-acting needle because no presser is needed; the hook is closed by the pressure of a completed loop on the latch as it rises on the shaft. Needles differ greatly in thickness, in gauge, and in length, and appropriate types must be selected for specific purposes. A 4-gauge needle, for example, is used for heavy sweaters, but an 80-gauge needle is required for fine hosiery. Production of fabric Other interlaced fabrics Net and lace making The popularity of handmade laces led to the invention of lace-making machines. The early models required intricate engineering mechanisms, and the development of the modern lace industry originated when a machine was designed to produce laces identical with Brussels lace. In the Heathcot, or bobbinet, machine, warp threads were arranged so that the threads moved downward as the beams unwound. Other threads were wound on thin, flat spools or bobbins held in narrow carriages that could move in a groove or comb in two rows. The carriages carrying the bobbins were placed on one side of the vertical warp threads and given a pendulum-like motion, causing them to pass between the warp threads. The warp threads were then moved sideways, so that on the return swing each bobbin thread passed around one of them. Then the warp threads moved sideways in the opposite direction, thus completing a wrapping movement. In addition, each row of bobbins was moved by a rack-and-pinion gearing, one row to the left and one to the right. As these movements continued, the threads were laid diagonally across the fabric as the warp was delivered. Improvements on the Heathcot machine followed through the 19th century: Nottingham-lace machines, used primarily for coarse-lace production, employ larger bobbins, and the pattern threads are wound independently on section spools; in another type, the Barmens machine, threads on king bobbins on carriers are plaited together, sometimes with warp threads. Schiffli lace, a type of embroidery, is made by modern machines, evolved from a hand version, using needles with points at each end. Several hundred needles are placed horizontally, often in two rows, one above the other. The fabric to be embroidered is held vertically in a frame extending the full width of the machine, and the needles, supplied with yarn from individual spools, move backward and forward through the fabric. At each penetration a shuttle moves upward and interlaces yarn with the needle loop. Movement of both fabric and needles is controlled by Jacquard systems. Many types of machine-made laces are made, frequently with geometrically shaped nets forming their backgrounds. Formerly made only of cotton, they are now frequently made from man-made fibre yarns. Bobbinet lace, essentially a hexagonal net, is used as a base for appliqu work for durable non-run net hosiery, and, when heavily sized, for such materials as millinery and veilings. Barmens lace has a fairly heavy texture and an angular pattern; flowing lines, heavy outline cords, and fine net backgrounds are not usually made on Barmens machines. The introduction of light-resistant polyester yarns led to a revival of Nottingham machine-made curtains. Leavers lace is available in an infinite variety of patterns, since the manufacturing technique allows use of almost any type of yarn. The high strength and comparatively low cost of man-made fibre yarns has made sheer laces widely available. Net, an open fabric having geometrically shaped, open meshes, is produced with meshes ranging from fine to large. Formerly made by hand, the various types are now made on knitting machines. Popular types include bobbinet, made with hexagonal-shaped mesh and used for formal gowns, veils, and curtains, and tulle, a closely constructed, fine net having similar uses. Fishnet, a coarse type with knots in four corners forming the mesh formerly made by fishermen, is now a popular machine-made curtain fabric. Braiding or plaiting Braid is made by interlacing three or more yarns or fabric strips forming a flat or tubular narrow fabric. It is used as trimming and for belts and is also sewn together to make hats and braided rugs. Plaiting, usually used synonymously with braiding, may be used in a more limited sense, applying only to a braid made from such materials as rope and straw. 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 interlacing, such as weaving or knitting. Weaving, currently the major method of fabric production, includes the basic weaves, plain or tabby, twill, and satin, and the fancy weaves, including pile, Jacquard, dobby, and gauze. Knitted fabrics are rapidly increasing in importance and include weft types and the warp types, raschel and tricot. Other interlaced fabrics include net, lace, and braid. Nonwoven fabrics are gaining importance and include materials produced by felting and bonding. Laminating processes are also increasing in importance, and fairly recent developments include needle weaving and the sewing-knitting process. Woven fabrics Woven fabrics are made of yarns interlaced in a regular order called a binding system, or weave. Weaving is the process of combining warp and weft components to make a woven structure. The components need neither be parallel to each other nor cross each other at right angles, but most woven structures are composed of two sets of components, both flexible and crossing at right angles. Weaving is differentiated from warp and weft knitting, braiding, and net making in that these latter processes make use of only one set of elements. In addition, there are geometrical differences, one of the most significant being the small angles through which the components of a woven structure are, in general, bent, in contrast with the components of other structures. Weaving is a widely used constructional method because it is cheap, basically simple, and adaptable. Woven fabrics have valuable characteristics resulting partly from the geometrical conformation of their components and partly from the fact that the components are held in position not by rigid bonding but by friction set up at the areas where they make contact. Woven fabrics are used in household, apparel, and industrial textiles. Textile designers can produce a very large variety of cloths by their selection of yarns, finishing processes, and binding systems. Yarns vary in thickness, smoothness, fibre content, twist, and colour, all of which have a profound influence on the finished cloth. Finishing processes range from such simple treatment as brushing up the nap on a woven fabric to such a complicated chemical process as that employed to change opaque cotton fabric to transparent, permanently stiffened organdy. The binding system, or weave, however, is the basic factor in determining the character of a woven fabric. The three basic systems are plain or tabby, twill, and satin. In complex binding systems, the basic weaves are combined or enriched by hand manipulation or mechanical loom attachments; these include multiple-plane, pile, inlaid, and gauze weaves. Regardless of the binding system, other devicesmanipulation of warp spacing, beating in, or tensioncan be used to alter the appearance of any weave, to make it looser or more compact, to make it more or less regular. As musical notation conveys a composer's ideas, so weave drafts or point paper plans communicate a textile designer's directions for constructing woven fabrics. The draft is a plan on graph paper showing at least one repeat or weave unit of the fabric to be woven. This information enables the weaver or mill specialist to plot the drawing in of the warp, tie up of harnesses to shedding mechanism, and shedding order. Production of yarn Yarn is a strand composed of fibres, filaments (individual fibres of extreme length), or other materials, either natural or man-made, suitable for use in the construction of interlaced fabrics, such as woven or knitted types. The strand may consist of a number of fibres twisted together; a number of filaments grouped together but not twisted; a number of filaments twisted together; a single filament, called a monofilament, either with or without twist; or one or more strips made by dividing a sheet of material, such as paper or metal foil, and either twisted or untwisted. The properties of the yarn employed greatly influence the appearance, texture, and performance of the completed fabric. Textile fibres Raw materials Fibres are units of matter having length at least 100 times their diameter or width. Fibres suitable for textile use possess adequate length, fineness, strength, and flexibility for yarn formation and fabric construction, and for withstanding the intended use of the completed fabric. Other properties affecting textile fibre performance include elasticity, crimp (waviness), moisture absorption, reaction to heat and sunlight, reaction to the various chemicals applied during processing and in the dry cleaning or laundering of the completed fabric, and resistance to insects and microorganisms. The wide variation of such properties among textile fibres determines their suitability for various uses. The first fibres available for textile use were obtained from plant and animal sources. Over a long period of experimentation with the many natural fibres available, cotton, wool, jute, flax, and silk have become recognized as the most satisfactory. The commercial development of man-made fibres began late in the 19th century, experienced much growth during the 1940s, expanded rapidly after World War II, and in the 1970s was still the subject of extensive research and development. This group includes regenerated fibres, such as rayon, made from fibre-forming materials already existing in nature and manipulated into fibrous form, and synthetic fibres, with the fibre-forming substance produced from chemicals derived from such sources as coal and oil, and then made into such fibres as nylon and polyester.