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Thursday, March 19, 2009

Dyeing and printing

Textile finishing processes » Dyeing and printing »Dyeing

Most forms of textile materials can be dyed at almost any stage. Quality woollen goods are frequently dyed in the form of loose fibre, but top dyeing or cheese dyeing is favoured in treating worsteds. Manufacturers prefer piece dyeing, which allows stocking of white goods, reducing the risk of being overstocked with cloth dyed in colours that have not been ordered.

The dye used depends on the type of material and the specific requirements to be met. For some purposes, high lightfastness is essential; but for others it may be inconsequential. Factors considered in dye selection include fastness to light, reaction to washing and rubbing (crocking), and the cost of the dyeing process. Effective preparation of the material for dyeing is essential.

Textile finishing processes » Dyeing and printing »Dyeing » Types of dyes

Textile dyes include acid dyes, used mainly for dyeing wool, silk, and nylon; and direct or substantive dyes, which have a strong affinity for cellulose fibres (see Table). Mordant dyes require the addition of chemical substances, such as salts, to give them an affinity for the material being dyed. They are applied to cellulosic fibres, wool, or silk after such materials have been treated with metal salts. Sulfur dyes, used to dye cellulose, are inexpensive, but produce colours lacking brilliance. Azoic dyes are insoluble pigments formed within the fibre by padding, first with a soluble coupling compound and then with a diazotized base. Vat dyes, insoluble in water, are converted into soluble colourless compounds by means of alkaline sodium hydrosulfite. Cellulose absorbs these colourless compounds, which are subsequently oxidized to an insoluble pigment. Such dyes are colourfast. Disperse dyes are suspensions of finely divided insoluble, organic pigments used to dye such hydrophobic fibres as polyesters, nylon, and cellulose acetates.

Fibres and Dyes

fibres                                  application classes    Natural fibres         Animal    Wool                                  acid, basic, mordant, reactive,                                            (solubilized vat)    Wool blends (wool-cotton,             acid, direct, mordant, reactive      wool-viscose, etc.)    Silk                                  acid, basic, direct, mordant, (reactive),                                            (solubilized vat)  Vegetable    Cotton                                azoic, basic, direct, mordant, oxidation,                                            reactive, sulfur, vat    Bast (linen, flax, hemp,              acid, direct, (disperse), reactive, vat,      jute, ramie)                          solubilized vat    Modified cellulose fibres           Viscose                               direct, mordant, pigment, reactive, sulfur,                                            vat, solubilized vat    Secondary acetate                     disperse    Triacetate                            disperse    Synthetic fibres           Polyamide (nylon, Perlon, Rilsan)     acid, disperse, mordant, pigment, reactive    Polyester (Dacron, Terylene)          disperse, pigment    Polyacrylonitrile (Acrilan,           basic, disperse, pigment      Courtelle, Orlon)    Polyvinyl chloride (Envilon,          basic, disperse      Thermovyl)    Polyolefines (Meraklon, Prolene)      disperse    Elastomers (Glospan, Lycra)           acid, disperse, reactive, (wool), vat  

Reactive dyes combine directly with the fibre, resulting in excellent colourfastness. The first ranges of reactive dyes for cellulose fibres were introduced in the mid-1950s. A wide variety is now available.

Textile finishing processes » Dyeing and printing »Dyeing » Application process

The dyeing of a textile fibre is carried out in a solution, generally aqueous, known as the dye liquor or dyebath. For true dyeing (as opposed to mere staining) to have taken place, the coloration must be relatively permanent; that is, not readily removed by rinsing in water or by normal washing procedures. Moreover, the dyeing must not fade rapidly on exposure to light. The process of attachment of the dye molecule to the fibre is one of absorption; that is, the dye molecules concentrate on the fibre surface.

There are four kinds of forces by which dye molecules are bound to fibre: (1) ionic forces, (2) hydrogen bonding, (3) van der Waals’ forces, and (4) covalent chemical linkages. In the dyeing of wool, which is a complex protein containing about 20 different α-amino acids, the sulfuric acidadded to the dyebath forms ionic linkages with the amino groups of the protein. In the process of dyeing, the sulfate anion (negative ion) is replaced by a dye anion. In the dyeing of wool, silk, and synthetic fibres, hydrogen bonds are probably set up between the azo, amino, alkylamino, and other groups, and the amido -CO-NH-, groups. Van der Waals’ forces (the attractive forces between the atoms or molecules of all substances) are thought to act in the dyeing of cotton between the molecular units of the fibre and the linear, extended molecules of direct dyes. Covalent chemical links are brought about in the dyebath by chemical reaction between a fibre-reactive dye molecule, one containing a chemically reactive centre, and a hydroxy group of a cotton fibre, in the presence of alkali.

In any dyeing process, whatever the chemical class of dye being used, heat must be supplied to the dyebath; energy is used in transferring dye molecules from the solution to the fibre as well as in swelling the fibre to render it more receptive. The technical term for the transfer process is exhaustion. Evenness of dyeing, known as levelness, is an important quality in the dyeing of all forms of natural and synthetic fibres; it may be attained by control of dyeing conditions, that is, by agitation to ensure proper contact between dye liquor and substance being dyed, and by use of restraining agents to control rate of dyeing, or strike.

Serious consideration has recently been given to methods of dyeing in which water as the medium is replaced by solvents such as thechlorinated hydrocarbons used in dry cleaning. There are a number of technical advantages in solvent dyeing, apart from the elimination of effluent (pollution) problems associated with conventional methods of dyeing and finishing. Advantages include more rapid wetting of textiles, less swelling, increased speed of dyeing per given amount of material, and savings in energy, because less heat is required to heat or evaporate perchloroethylene, for example, than is needed for water.

For each application the dyer selects the combination of dyes best suited to the particular fibre or blend he plans to dye and best able to withstand the conditions the textile will encounter in further processing and in use in the finished article. In general, the higher the standard of fastness, the more expensive the dye, and the final choice may be a compromise between the desired fastness standards and the cost of the dyes. Fastness tests and standards have been the subject of work by the American Association of Textile Chemists and Colorists (AATCC), Europäisch-Continentale Echtheitsconvention (ECE), and the Society of Dyers and Colourists (SDC), Bradford, West Yorkshire. Efforts have been made to set up a unified system by the International Organization for Standardization (ISO). Lightfastness is assessed on a scale of 8; 1 represents the poorest fastness and 8 the best. Fastness to other agents, among them water, bleach, acid, alkali, detergent solution, and perspiration, is measured on a scale of 5.

Dyes are generally used in combination to achieve a desired hue or fashion shade. If the substance to be dyed consists of only one type of fibre, such as wool, the dye mixture will be made up solely of wool dyes. But if the fabric contains more than one kind of fibre and they differ in dyeing properties, then mixtures of different application classes of dyes are used.

Textile finishing processes » Dyeing and printing »Dyeing » Forms in which textiles are dyed

Loose stock consists of randomly distributed wool or cotton fibres; tow is the corresponding term for synthetic fibres. Sliver is a more orderly arrangement of fibres in a loosely connected, continuous form suitable for spinning. It is wound into either hanks or tops, loose balls about one foot in diameter. After spinning, the yarn is either made up into hanks or into packages weighing about two pounds each, by winding the yarn round perforated metal tubes. The packages are curiously named, some according to their shapes; for example, cones, cheeses, cakes, beams, and rockets. Piece goods, woven cloth or textiles knitted in rope form, and garments, a term that includes stockings, tights, hose, and half hose, are also dyed as such.

Textile finishing processes » Dyeing and printing »Dyeing » Machinery and equipment

Modern dyeing machines are made from stainless steels. Steels containing up to 4 percent molybdenum are favoured to withstand the acid conditions that are common. A dyeing machine consists essentially of a vessel to contain the dye liquor, provided with equipment for heating, cooling, and circulating the liquor into and around the goods to be dyed or moving the goods through the dye liquor. The kind of machine employed depends on the nature of the goods to be dyed. Labour and energy costs are high in relation to total dyeing costs; the dyer’s aim is to shorten dyeing times to save steam and electrical power and to avoid spoilage of goods.

A widely used machine is the conical-pan loose-stock machine; fibres are held in an inner truncated-conical vessel while the hot dye liquor is mechanically pumped through. The fibre mass tends to become compressed in the upper narrow half of the cone, assisting efficient circulation. Levelling problems are less important because uniformity may be achieved by blending the dyed fibres prior to spinning.

The Hussong machine is the traditional apparatus; it has a long, square-ended tank as dyebath into which a framework of poles carrying hanks can be lowered. The dye liquor is circulated by an impeller and moves through a perforated false bottom that also houses the open steam pipe for heating. In modern machines, circulation is improved especially at the point of contact between hank and pole. This leads to better levelling and elimination of irregularities caused by uneven cooling.

In package-dyeing machines dye liquor may be pumped in either of two directions: (1) through the perforated central spindle and outward through the package, or (2) by the reverse path into the outer layers of the package and out of the spindle. In either case levelness is important. In the case of soluble dyes the dye liquor must be free of suspended matter. In the case of disperse dyes, in which particles of dye are dispersed in, rather than dissolved in, the solution, no gross aggregates can be allowed; otherwise the packages would retain undesirable solids on the outer and inner surfaces. Some package-dyeing machines are capable of working under pressure at temperatures up to 130° C.

The winch is the oldest piece-dyeing machine and takes its name from the slatted roller that moves an endless rope of cloth or endless belt of cloth at full width through the dye liquor. Pressurized-winch machines have been developed in the U.S. In an entirely new concept, the Gaston County jet machine circulates fabric in rope form through a pipe by means of a high-pressure jet of dye liquor. The jet machine is increasingly important in high-temperature dyeing of synthetic fibres, especially polyester fabrics.

Another machine, the jig, has a V-shaped trough holding the dye liquor and guide rollers to carry the cloth at full width between two external, powered rollers; the cloth is wound onto each roller alternately, that is, the cloth is first moved forward, then backward through the dye liquor until dyeing is complete. Modern machines, automatically controlled and programmed, can be built to work under pressure.

Solutions or suspensions of colorants or their precursors may be padded onto piece goods by passing the cloth through a trough containing the liquor and then between rollers under pressure. Development and fixation processes such as steaming or dry-heat treatment can be carried out in other apparatus. The method is used in semicontinuous and continuous operations.

Textile finishing processes » Dyeing and printing »Printing

Printing is a process of decorating textile fabrics by application of pigments, dyes, or other related materials in the form of patterns. Although apparently developed from the hand painting of fabrics, such methods are also of great antiquity. There is evidence of printing being carried out in India during the 4th century bc, and a printing block dated at about ad 300 has been unearthed in the burial grounds of Akhmīn inUpper Egypt. Pre-Columbian printed textiles have been found in Peru and Mexico. Textile printing has become highly sophisticated and has involved the skills of many artists and designers.

The four main methods of textile printing are block, roller, screen, and heat transfer printing. In each of these methods, the application of the colour, usually as a thickened paste, is followed by fixation, usually by steaming or heating, and then removal of excess colour by washing. Printing styles are classified as direct, discharge, or resist. In direct printing, coloured pastes are printed directly on the cloth. For discharge printing, the cloth is first dyed with a background colour, which is destroyed by reagents, or reducing agents, carried in a print paste. This action may leave the discharged design white on a coloured background, although print pastes may also contain colouring matters not destroyed by the discharging agent, producing a coloured design. In the resist process, the cloth is first printed with a substance called a resist, protecting these printed areas from accepting colour. When the cloth is dyed or pigment padded only those parts not printed with the resist are dyed. A special application of this technique, imparting plissé effects, is the printing of the fabric with a resist, followed by treatment with caustic soda.

Textile finishing processes » Dyeing and printing »Printing » Block printing

Wooden blocks, carved with a design standing out in relief, are made from solid pieces of wood or by bonding closely grained woods with cheaper ones. When designs include large areas, these are recessed and the space filled with hard wool felt. Fine lines are usually built up with copper strips, and other effects are obtained with copper strips interleaved with felt. To facilitate registration of successive prints, or lays, each block has several pitch pins arranged to coincide with well-defined points in the pattern. Cloth is printed on a table covered with several thicknesses of fabric or blanket, the whole covered with a thick sheet of tightly stretched synthetic rubber. The cloth to be printed is spread on the rubber, either gummed in position or pinned to a backcloth attached to the table. Colour is applied evenly to the block, and the pattern is stamped on the fabric to be printed, using the handle of a small heavy hammer, or maul, to aid penetration of the paste. More colour is then applied to the block and the process is repeated using the pitch pin to obtain true registration. After the fabric has been entirely printed with one colour, other colours are applied in the same way until the design is complete. Although block printing is becoming too laborious and costly for commercial use, some of the most beautiful prints have been made in this way.


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