Wednesday, November 2, 2011

SELECTION OF DYEING PROCESS FOR DYEING OF TEXTILE MATERIAL

SELECTION OF DYEING PROCESS:
Even dyes that belong to the same class can have differing degrees of colorfastness to the same condition, so that the consumer has no real guarantee of color permanence unless a label specifies that a particular fabric is colorfast. Dye performance labeling is not required by any form of legislation or regulation. Some manufacturers do, however, include colorfastness information on labels. Such labels will generally describe the conditions under which the fabric is colorfast, such as “colorfast to laundering, but not to chlorine bleaching” or “colorfast to sunlight.” A few terms may be found on labels that carry an assurance of colorfastness, such as trademarks that have been applied to solution-dyed synthetic fibers. The colorfastness of one class of dyes, the vat dyes, is so consistently good for laundering that the term “vat dyed” on labels has come to be accepted as an assurance of good colorfastness.

Textile may be dyed at any stage of their development from fiber into fabric or certain garments by the following methods:

• Stock dyeing, in the fiber stage
• Top dyeing, in the combed wool sliver stage
• Yarn dyeing, after the fiber has been spun into yarn
• Fabric/ Piece dyeing, after the yarn has been constructed into fabric
• Solution pigmenting or dope dyeing before a manmade fiber is extruded through the spinneret
• Garment dyeing after certain kinds of apparel are knitted /Woven


Stock Dyeing:

Mass Coloration
Mass coloration is the addition of color to manufactured fibers before they are extruded. These fibers have been variously known as spun-dyed, solution-dyed or doped.:; ed. iib.,is extruded, it carries the coloring material as an integral part of the

fiber.This “locked-in” color is extremely fast to laundering (that is, it will not diminish); however, such colors can be sensitive to light and bleaching or may fade. The range of colors in which solution dyeing is done is rather limited for economic reasons.

The fiber manufacturer must produce substantial quantities of fiber to justify the expense of adding an extra step during the manufacturing process. Furthermore, fiber production takes place well in advance of the time when fabrics reach the market.

Fashion color trends may change fairly rapidly, so that, by the time a mass colored fabric reaches the market, the color may be out of fashion and not salable. For this reason, spun-dyed fabrics are generally produced in basic colors. Mass coloration is used on acetate to prevent gas fading. Gas fumes in the air may turn some blue or green dyes used for acetate to pink or brown.

Dyeing Fibers
When color is added at the fiber stage, this process is known as fiber dyeing or stock dyeing. It is a batch process in which loose (usually staple) fibers are immersed in a dyebath. dyeing takes place, and the fibers are dried. Exhaustion is quicker in fiber dyeing because the dye liquor has better access to fiber surfaces.

Levelness may be a problem but its effect can be minimized by blending fibers later during yarn processing. Stock-dyed fibers are most often used in tweed or heather effect materials in which delicate shadings of color are produced by combining fibers of varying colors. The yarns in Harris Tweed fabrics are a distinctive example of fiber dyeing. Fiber-dyed fabrics can be identified by untwisting the yarns to see whether the yarn is made up of a variety of different colored fibers. In solid-colored yarns untwisted stock-dyed fibers will be uniform in color, with no darker or lighter areas. Stock dyeing refers to dyeing a staple fiber before it is spun.

There are two methods.

The first method, bale dyeing, applied mostly on wool and all types of manmade fibers, is that of splitting the bale covering on all six sides, placing the entire bale in a specially designed machine (the covering and straps need not be removed), and then forcing the dye liquor through the bale of fiber. In stock dyeing, which is the most effective and expensive method of dyeing, the color is well penetrated into the fibers and does not crock readily.

**yarn & fabric dyeing **

Yarn Dyeing
When dyeing is done after the fiber has been spun into yarn, it is described as yarn dyeing. Cloth made of dyed yarns is called yarn-dyed. Yarn-dyed fabrics are usually deeper and richer in color. Yarn-dyed fabrics intended for laundering must be quite colorfast, or bleeding could occur. The primary reason for dyeing in the yarn form is to create interesting checks, stripes, and plaids with different-colored yarns in the weaving process.

If color has not been added either to the polymer or the fiber, it can be applied to the yarns before they are made into fabrics. Yarns may be dyed in skeins, in packages, or on beams. Special dyeing equipment is required for each of these batch processes. In skein dyeing, large skeins of yarn are loosely wound on sticks and placed in a vat for dyeing. In package dyeing, the yarn is wound onto a number of perforated tubes or springs. The dye is circulated through the tubes to ensure that the yarns have maximum contact with the dye. Beam dyeing is a variation of package dyeing, which uses a larger cylinder onto which a set of warp yarns is wound.

Many types of fabrics utilize yarn of differing colors to achieve a particular design. Stripes in which contrasting sections of color alternate in the length or crosswise direction, chambrays in which one color is used in one direction and another color is used in the other direction, complex dobby or jacquard weaves, and plaids may all require yarns to which color has already been added.

Yarn-dyed fabrics may be identified by unraveling several warp and several filling yarns from the pattern area to see whether they differ in color. Not only will each yarn be a different color, but the yarns will have no darker or lighter areas where they have crossed other yarns.

Usually yarns are dyed to one solid color, but in a variant of the technique called space dyeing, yarns may be dyed in such a way that color-and-white or multicolored effects are formed along the length of the yarn.

Skein (Hank) Dyeing:
Yarn may be prepared in skein, or hank, form and then dyed. The loose arrangement of the yarn allows for excellent dye penetration. The skeins are hung over a rung and immersed in a dye bath in a large container.

Piece Dyeing
Fabrics that are to be a solid color are usually piece dyed. In piece’ dyeing, the finished fabric is passed through a dye bath where the fibers in the fabric absorb the dye. A number of different methods are used for piece dyeing, each of which differs slightly in the way in which the fabric is handled. Fabrics may be dyed in either continuous or batch processes. In continuous dyeing, the cloth continually passes through the dyebath. This is the cheaper process and, where possible, is used for dyeing large yardages. Batch dyeing is used for shorter fabric lengths.

Some fabrics are dyed in open, Rat widths. Knitted fabrics and those woven materials that are not subject to creasing are handled in “rope” form, that is, bunched together and handled as a narrower strand. They are usually attached at the ends to form a continuous loop. Some dyeing methods are especially suitable for certain types of fabrics and unsuitable for others. Many different kinds of machines can be used for piece dyeing. The great bulk of dyed fabric on the market is dyed in the piece.

Small lots of fabrics of all fibers are dyed in batches. Piece dyeing is thoroughly satisfactory as regards levelness, penetration, and overall fastness, assuming that the proper dyes have been used. Fabric may be piece-dyed whether it is composed of only one kind of fiber or yarn or of blends of different fibers or combinations of different yarns. When the fabric is made of one kind of fiber or yarn, then dyeing is relatively uncomplicated because the one appropriate dye is used. However, when the fabric contains a blend of fibers or combination of different yarns, then special procedures are required which employ different dyes that are each specific for the particular fibers used. These procedures are called union dyeing and cross dyeing.

Union Dyeing:
This process of dyeing piece goods made of different fibers or yarns in one color may be readily accomplished. Although different fibers may require different dyes to obtain the same color, this may be done by putting the appropriate color dye that is specific to each type of fiber into one dye bath.

Cross Dyeing:
One method is a combination of stock dyeing or of yarn dyeing with subsequent piece dyeing. Cross dyeing produces varied effects. For instance, either the warp or the filling yarns may be stock-dyed or yarn-dyed, one set of yarns being left undyed. The fabric is piece-dyed after weaving; thus, color is given to the undyed yarn in a second dyebath, and the yarns that were originally stock-dyed or yarn-dyed acquire some additional coloring, which blends with the piece-dyed portion of the fabric. If yarns of vegetable fibers have been combined with yarns of animal fibers in a fabric that is to be piece-dyed, two separate dye baths must be used. The fabric is dipped into both solutions, each of which affects the fiber for which it has an affinity. This provides colorful effects. Still another method of cross-dyeing is to immerse a fabric composed of two different types of fibers into one dye bath containing two different dyes, one specific for each of the fibers. One of methods of piece dyeing is described below.

Beck Dyeing(Beam dyeing)
Long lengths of cloth that are to be dyed on a continuous process are very often beck-dyed, or box-dyed, by passing the fabric in tension-free rope form through the dyebath. The rope of cloth moves over a rail onto a reel, which immerses it into the dye and then draws the fabric up and forward to the front of the machine. The process is repeated as long as necessary to dye the material uniformly to the desired intensity of color.

Beam dyeing, which is used for lightweight, fairly open-weave fabrics, utilizes the same principle as beam dyeing of yarns. The fabric is wrapped around a perforated beam and immersed in the dyebath. Tightly woven fabrics would not allow sufficient dye penetration; hence, this method must be applied to loosely woven cloth. It has the added advantage of not putting tension or pressure on the goods as they are processed.

Jig Dyeing:
This method utilizes the basic procedure of beck dyeing. However, in jig dyeing, the fabric is held on rollers at full width rather than in rope form as it is passed through the dye bath. The rope of cloth moves over a rail onto a reel, which immerses it into the dye and then draws the fabric up and forward to the front of the machine. The process is repeated as long as necessary to dye the material uniformly to the desired intensity of color. Batch processes that dye fabric in flat widths are jig and beam dyeing. Jig dyeing is a process that places greater tension on the fabric than the beck and jet machines. Fabrics are stretched across two rollers that are placed above a stationary dyebath. The fabric is passed through the dyebath and wound on one roller. The motion is then reversed until the desired exhaustion or depth of shade is achieved. The tension created by placing the fabric on the rollers means that this process must be reserved for fabrics with a fairly close weave that will not lose their shape under tension.

Jig dyeing
Jet dyeing: - Jet dyeing is a newer method that uses propulsion of the dye liquor through the fabric to improve dye penetration. Dyeing takes place in a closed system that carries a fast-moving stream of pressurized dye liquor. A fluid jet of dye penetrates and dyes the fabric. After it passes through this jet, the fabric is floated through an enclosed tube in which the fluid moves faster than the fabric. This prevents the fabric from touching the walls, keeping it constantly immersed in the dyebath. Turbulence is created by locating elbows in the tube. The turbulence aids in diffusing dyes and dyebath auxiliaries. Since no pressure is put on the fabric, even delicate fabrics can be dyed by this process. Jet dyeing has the advantage of being economical in operation and at the same time allowing a high degree of quality control

1. Fabric guide roll
2. Loading & unloading port
3. Header tank
4. U tube
5. Suction control
6. Suction control
7. Suction control
8. Delivery control
9. Main control
10. Filter
11. Heat exchanger
12. Service tank

Solution Pigmenting, or Dope Dyeing
During the production of manmade fibers, a great deal of time and money can be saved if the dye is added to the solution before it is extruded through the spinnerets into filaments. This method also gives a greater degree of colorfastness. A process called solution pigmenting, or dope dyeing, has been used for manmade fibers ranging from rayon through saran and glass fiber.

Garment Dyeing
Certain kinds of non-tailored apparel, such as hosiery, pantyhose, and sweaters can be dyed as completed garments because they are each made of a single component and will not be readily distorted. However, allowance must be made for anticipated shrinkage. A number of garments are loosely packed into a large nylon net bag. The bags are then put into a paddle dyer, which is a tub with a motor-driven paddle that agitates the dye bath. Except for dyeing socks and narrow fabrics, garment dyeing, is the process of dyeing completed garments, remained a rather unimportant novelty until the second half of the 1980s; Industry sources credit two factors with a sharp increase in the amount of garment-dyed apparel. First, fashion demanded small lots of garments from fabrics with stonewashed, ice-washed, tie-dyed, overdyed, and distressed effects. These effects were more readily achieved through garment dyeing than traditional dyeing methods. The second factor was the ability of manufacturers to achieve Quick Response or Just-In-Time production through garment dyeing.

The lead time required for delivery of orders in the traditional dyeing system is about eight weeks. For garment-dyed products lead time is about two weeks. Although the process of garment dyeing is more costly than traditional piece dyeing (estimated at $1 to $3 per item), savings are achieved in the long run because manufacturers and retailers need not maintain large inventories. If undyed merchandise is left from one season, it can be dyed for sale the following season. However, if it has already been dyed and a different color is wanted, it must be overdyed, given a second dyeing to a different color. Manufacturers can be more responsive to fashion trends by producing small dye lots.

Garment dyeing is primarily applied to cotton fabrics; however, high-pressure equipment can be used to process polyester and cotton blends. To achieve consistently good results with garment dyeing, manufacturers must exercise care in a number of areas.

1. Fabric. All fabric used in one garment must come from the same bolt of fabric. If, for example, one trouser leg of a pair of jeans is cut from one bolt of fabric, and the other from another bolt, each leg may dye to a different shade. The result would be jeans in which the legs do not match.

2. Shrinkage. Fabric must also be tested for shrinkage before cutting of garments, and garments must be cut large enough to allow for shrinkage so that sizes will be accurate.

3. Thread.
Thread must be chosen carefully and tested to be sure it will accept the dye in the same way as the fabric. One hundred percent cotton thread is preferred, but even with allcotton thread there may be problems. For example, mercerized thread will dye to a darker shade than unmercerized garment fabric. This will make the stitching stand out from the background fabric.

4. Labels, button, zippers.
All of these supplies must be compatible with the garment fabric in terms of reaction to the dye and shrinkage. The machines used for garment dyeing are called paddle machines. To avoid entanglement during dyeing, garments are generally placed inside bags. Paddles in the machine rotate, changing directions periodically, to make sure that all pieces being dyed are equally exposed to the dye liquor. Garments are generally washed before dyeing, to remove any finishing materials that would interfere with dyeing, and after dyeing to remove excess dye.

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