Selection Of Dyeing Process For Dyeing Of Textile Material

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.

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

Textile Dyeing Process / Fabric Dyeing Process / Textile Fabric Dyeing

Dyeing Process:

Dyeing is the process of imparting colors to a textile material through a dye (colour). Dyes are obtained from flowers, nuts, berries and other forms of vegetables and plants as well as from animal and mineral sources. These are known as natural dyes.

The other class of dyes is known as synthetic dyes. These are based on a particular type of chemical composition. Some of these dyes are- Acid ( Anionic) dyes, Basic ( Cationic) dyes, Neutral- Premetalized dyes, sulfur dyes, vat dyes, reactive dyes, pigment dyes etc.

Dyeing Methods:

Colour is applied to fabric by different methods of dyeing for different types of fiber and at different stages of the textile production process. These methods include Direct dyeing; Stock dyeing; Top dyeing; Yarn dyeing; Piece dyeing; Solution pigmenting or dope dyeing; Garment dyeing etc. Of these Direct dyeing and Yarn Dyeing methods are the most popular ones.

Direct Dyeing:

When a dye is applied directly to the fabric without the aid of an affixing agent, it is called direct dyeing. In this method the dyestuff is either fermented (for natural dye) or chemically reduced ( for synthetic vat and sulfur dyes) before being applied. The direct dyes, which are largely used for dyeing cotton, are water soluble and can be applied directly to the fiber from an aqueous solution. Most other classes of synthetic dye, other than vat and sulfur dyes, are also applied in this way.

Yarn Dyeing:

When dyeing is done after the fiber has been spun into yarn, it is called Yarn dyeing. There are many forms of yarn dyeing- Skein (Hank) Dyeing, Package Dyeing, Warp-beam Dyeing,and Space Dyeing.

Skein (Hank) Dyeing:

The yarns are loosely arranged in skeins or hanks. These are then hung over a rung and immersed in a dyebath in a large container. In this method, the colour penetration is the best and the yarns retain a softer, loftier feel.

Package Dyeing:

The yarns are wound on spools, cones or similar units and these packages of yarn are stacked on perforated rods in a rack and then immersed in a tank. In the tank, the dye is forced outward from the rods under pressure through the spools and then back to the packages towards the center to penetrate the entire yarn as thoroughly as possible.

Warp-beam Dyeing:

It is similar to package dyeing but more economical. Here, yarn is wound on to a perforated warp beam, immersed in a tank and dyed under pressure.

Space Dyeing:

In this method, the yarn is dyed at intervals along its length. For these two procedures- knit- deknit method and OPI Space-Dye Applicator- are adopted. In the first method, the yarn is knitted on either a circular or flat-bed knitting machine and the knitted cloth is then dyed and subsequently it is deknitted. Since the dye does not readily penetrate the areas of the yarn where it crosses itself, alternated dyed and undyed spaces appear. The OPI Space-Dye Applicator technique produces multi coloured space- dyed yarns. The yarns are dyed intermittently as they run at high speeds of upto 1000 yards (900 m) per minute through spaced dyebaths with continuous subjection to shock waves produced by compressed air assuming supersonic velocities.

Colorfastness Of Cotton Textiles

INTRODUCTION:

Today’s consumer is more sophisticated than ever. They are conscious not only of style and comfort, but also of care and durability. They demand a quality product. Market studies show that consumers make many purchase choices based on color. Therefore, a fabric’s ability to retain its original color is one of the most important properties of a textile product.

The colorfastness or color retention of cotton textiles is influenced by a number of variables that occur both pre-consumer and post-consumer. This report summarizes how variations in raw materials, chemicals, manufacturing processes and consumer practices all have an effect on the performance characteristics of a fabric. Manufacturers must understand how the many variables affect colorfastness to achieve the ultimate goal of consumer satisfaction.

COLORFASTNESS AND TEST METHODS:

Colorfastness is defined by the American Association of Textile Chemists and Colorists as “the resistance of a material to change in any of its color characteristics, to transfer its colorant(s) to adjacent materials, or both, as a result of the exposure of the material to any environment that might be encountered during the processing, testing, storage, or use of the material.” In other words, it is a fabric’s ability to retain its color throughout its intended life cycle. There are many types of colorfastness properties that must be considered to provide the consumer with an acceptable product. The American Association of Textile Chemists and Colorists has over thirty test methods that evaluate different colorfastness properties. These include, but are not limited to wash, light, crock, dry cleaning, perspiration, abrasion and heat. The type of product being manufactured determines which types of colorfastness are important and therefore which test methods are relevant. For example, upholstery fabrics must have excellent lightfastness and crockfastness properties, whereas washfastness is important for clothing fabrics. Manufacturers must know a fabric’s intended end use in order to make processing decisions that will produce a product of acceptable performance.

TEXTILE MANUFACTURING PROCESSES AFFECTING COLORFASTNESS:

1. Preparation:

Many aspects in the textile manufacturing process of taking a loom state fabric to a finished product have an effect on the colorfastness properties. Preparation is the first stage of textile wet processing. Cotton fibers are approximately 95% cellulose. The non-cellulosic portion consists of natural products such as waxes, sugars, metals, and man-made products such as processing aids, grease, plastic, and rubber. To achieve optimum dyeing and finishing conditions, it is important that these impurities are thoroughly removed with minimal damage to the cotton fiber.

2.Dye Selection:

Dyeing is the crucial step in determining the colorfastness performance of a fabric. The American Association of Textile Chemists and Colorists define a dye as “a colorant applied to or formed in a substrate, via the molecularly dispersed state, which exhibits some degree of permanence.” Dyeing is accomplished by immersing the textile in a dye bath, applying heat and chemicals to drive the dye onto the textile, and then rinsing the substrate to remove the surface dye. These principles are illustrated below.

Different dye classes are used for each fiber type. The table below shows which dyes can be used for which fibers.

Dye Classes Available for Different Fibers

Fiber	Dyestuffs
Cotton & manmade cellulosics	Direct, Vat, Sulfur, Naphthol, Reactive, Pigment
Polyester	Disperse, Basic
Nylon	Disperse, Acid, Premetallized
Acetate	Disperse
Wool & Silk	Acid, Premetallized
Acrylic	Dispersed, Basic

Dye selection must be based on desired performance criteria, manufacturing restrictions and the costs a market can bear for each end product. Every dye has unique colorfastness properties. Some dyes are known for their excellent washfastness characteristics and others are known for their lightfastness properties. The structure of the dye, the amount of dye, its method of bonding to the fabric and dyeing procedures all contribute to a dye’s performance characteristics. Dye combinations in a specific formulation must also be evaluated for their effect on colorfastness. Heavy shades often have reduced fastness properties. When high concentrations of dye are required, proper rinsing and washing off procedures are essential. However, due to entrapped dye particles within the cellulose structure, some unbound dye molecules can still remain and contribute to color loss and dye transfer

DYES FOR COTTON:

Dyes can be categorized based on the mechanism by which they become fixed to a fiber. Dyes used for cotton fibers can be categorized into the surface bonding, adhesion, or covalent bonding mechanisms.

Pigments are sometimes used to color cotton fabrics, however they are not considered dyes. They are completely insoluble in water and have no affinity for cotton fibers. Some type of resin, adhesive, or bonding agent must be used to fix them to the cotton fiber. Typically, they exhibit good colorfastness to light and poor colorfastness to washing.

Direct dyes are water soluble and categorized into the surface bonding type dye because they are absorbed by the cellulose. There is no chemical reaction, but rather a chemical attraction. The affinity is a result of hydrogen bonding of the dye molecule to the hydroxyl groups in the cellulose. After the dyestuff is dissolved in the water, a salt is added to control the absorption rate of the dye into the fiber. Direct dyes are fairly inexpensive and available in a wide range of shades. Typically, they exhibit good lightfastness and poor washfastness. However, by applying a fixing agent after dyeing the washfastness can be improved dramatically.

Vat, sulfur, and naphthol dyes are fine suspensions of water insoluble pigments, which adhere to the cotton fiber by undergoing an intermediate chemical state in which they become water-soluble and have an affinity for the fiber. Typically, vat dyes exhibit very good colorfastness properties. Sulfur dyes are used to achieve a low cost deep black. They exhibit fair colorfastness properties, although the lighter shades tend to have poor lightfastness. Naphthol dyes are available in brilliant colors at low cost, but application requirements limit their use. They exhibit good lightfastness and washfastness, but poor crockfastness.

Reactive dyes attach to the cellulose fiber by forming a strong covalent (molecular) chemical bond. These dyes were developed in the 1950’s as an economical process for achieving acceptable colorfastness in cellulosic fibers. Bright shades and excellent washfastness properties are the trademark of reactive dyes. One concern regarding reactive dyes is their susceptibility to damage from chlorine. Another is that lighter shades tend to have reduced lightfastness properties.

The following table summarizes the fastness properties of the dye categories or classes available for dyeing cotton fabrics. Keep in mind that these are generalizations. Every dye is unique and some dyes within a particular class may behave differently.

Finishing:

Finishing is the final stage of textile wet processing. Different types of finishes can be utilized depending on the desired performance characteristics of the end product. Resin and enzyme treatments are common finishing techniques that can influence the colorfastness of textile fabrics. Crosslinking resins are used to improve the durable press or wrinkle resistance of a fabric. Generally, resin treated fabrics demonstrate improved color retention to laundering. However, this increase in color retention comes at the expense of reduced physical properties of the fabric. Silicone softeners incorporated into the resin finish bath may further improve color retention for some fabrics. Softeners and resins play a key role in reducing surface abrasion and therefore improved overall wash performance. Cellulase enzymes are used to remove surface fibers that can create a fuzzy appearance on the surface of a fabric. Generally, enzyme treated fabrics show improved ability to maintain their original color and appearance after multiple home launderings. The degree of improvement from any of these finishing techniques is highly dependent on the individual dyes used in a particular formulation to achieve a given shade

CONSUMER PRACTICES:

Manufacturers can follow every recommendation and precaution to produce a fabric with optimum performance characteristics. However, colorfastness properties are also influenced by consumer practices. These include laundry detergent selection and wash procedures. Therefore, when evaluating colorfastness properties of a product it is important to use the appropriate test method that accurately reflects the consumer laundry practices. Due to higher energy costs consumers are laundering clothes at lower temperatures. For this reason detergent with “color safe” or activated peroxy bleaching agents, which improve cleaning efficacy at lower wash temperatures, are one of the fastest growing segments of the home laundry market. Some fabrics may fade a little when home laundered with standard detergent, but fabrics laundered with detergents containing activated bleach can show significant losses in color strength as determined by the sensitivity of the dye to those detergents. Another type of detergent available to consumers is those containing enzymes, which remove surface cellulosic fibers from the fabric. Many times the loss or apparent loss of color can be attributed to surface changes in the fabric caused by abrasion during laundering. Detergents containing enzymes generally reduce the color change associated with home laundering by decreasing the fuzziness of a fabric’s surface. Wash procedures also influence a fabric’s ability to retain its color. Consumer practices such as washing clothes inverted, reducing the wash load size, adding softener to the final rinse and reducing the tumble dry time minimize color loss.

CONCLUSIONS:

The colorfastness of cotton textiles can be a complicated subject. Fiber quality, yarn formation, fabric construction, textile wet processes and consumer practices can all have an influence on the performance characteristics of a fabric. Of these variables, the choices made during textile wet processing have the most significant effect on the colorfastness properties. Dye selection is of the utmost importance. Consumer practices such as detergent selection and laundering techniques also play a major role in the color retention of a fabric. Customer satisfaction should improve as manufacturers gain experience and knowledge in understanding and controlling the many aspects that influence colorfastness.

Classification of Dyes

There are several ways for classification of dyes. It should be noted that each class of dye has a very unique chemistry, structure and particular way of bonding. While some dyes can react chemically with the substrates forming strong bonds in the process, others can be held by physical forces. Some of the prominent ways of classification is given hereunder.

• Classification based on the source of materials
• Chemical classification of the Dyes- Based on the nature of their respective chromophores.
• Dyes according to the nuclear structure
• Industrial Classification of the Dyes

Classification based on the source of materials:

A very common classification of the dyestuff is based on the source from which it is made. Accordingly the classification could be:
• Natural Dyes
• Synthetic Dyes

1.Natural dyes:

Colouring materials have been used for many thousands of years by man. Leather, cloth, food, pottery and housing have all been modified in this way. The two old ways were to cover with a pigment (painting), or to colour the whole mass (dyeing). Pigments for painting were usually made from ground up coloured rocks and minerals, and the dyes were obtained from animals and plants. Today, many of the traditional dye sources are rarely, if ever, used (onion skins, for instance). However, some of our most common dyes are still derived from natural sources. These are termed natural dyes. The Colour Index uses this as a classification and naming system. Each dye is named according to the pattern:

–Natural + base colour + number

These dyes are thereby specifically identified as dyes of the stated colour, and which may still be derived from animals or plants. Note that this is a classification based on the dye’s source and colour. It contains no chemical information; neither does it imply that dyes with similar names but unique numbers are in any way related. It gives no information about the mechanism by which staining occurs.
Natural dyes are often negatively charged. Positively charged natural dyes do exist, but are not common. In other words, the coloured part of the molecule is usually the anion. Although the molecular charge is often shown on a specific atom in structural formulae, it is the whole molecule that is charged. Many, but by no means all, natural dyes require the use of a mordant.
The use of dyes is very ancient. Kermes (natural red 3) is identified in the bible book of Exodus, where references are made to scarlet coloured linen. Similar dyes are carmine (natural red 4) and lac (natural red 25). These three dyes are close chemical relatives, obtained from insects of the genus Coccus. All require a mordant. The most commonly used natural dye is undoubtedly hematein (natural black 1), obtained from the heartwood of a tree. This dye also requires a mordant.

Saffron (natural yellow 6), is obtained from the stigmata of Crocus sativus, and is used without a mordant, staining as an acid dye. Although its use is very ancient, it is more common now as a colouring and spice for food than for dyeing, due to its expense.

Synthetic Dyes
Dyes derived from organic or inorganic compound are known as synthetic dyes. Examples of this class of dyes are Direct, Acid, Basic, Reactive, Mordant, Metal complex, Vat, Sulphure , Disperse dye etc. However using general dye chemistry as the basis for classification, textile dyestuffs are grouped into 14 categories or classes:

Group:	Application:
Direct	Cotton, Cellulosic and Blends
Vat dyes	Cotton, Cellulosic and Blends
Sulphur	Cotton, Cellulosic fibers
Organic pigments	Cotton, Cellulosic, Blended Fabrics, paper
Reactive	cellulosic fibers and fabric
Dispersed dyes	Synthetic fibers
Acid Dyes	Wool, Silk, Synthetic fibers, leather
Azoic	Printing inks and pigments
Basic	silk, wool,cotton
Oxidation dyes	Hair
Developed Dyes	Cellulosic fibers and Fabric
Mordant dyes	Cellulosic fibers and Fabric, Silk, Wool
Optical/Fluorescent Brighteners	synthetic fibers, leather, cotton, sports goods
Solvent dyes	Wood Staining, solvent inks, waxes, colouring oils

Chemical classification of the Dyes:

According to a system of chemical classification, dyes can be divided according to the nature of their Chromophore:

Chromophoric Group	Textiles, leather
Acridine dyes, derivatives of acridine >C=N-and >C=C	Texties
Anthraquinone dyes, derivatives of anthraquinone >C=O and >C=C
Arylmethane dyes; Diarylmethane dyes, based on diphenyl methane, Triarylmethane dyes, based on triphenyl methane Azo dyes, based on a -N=N- azo structure Cyanine dyes, derivatives of phthalocyanine Diazonium dyes, based on diazonium salts Nitro dyes, based on the –NO2 nitro functional group
Nitroso dyes, are based on a –N=O nitroso functional
Phthalocyanine dyes, derivatives of phthalocyanine >C=N	Paper
Quinone-imine dyes, derivatives of quinine	Wool and paper
Azin dyes; -Eurhodin dyes, -Safranin dyes, derivatives of safranin -C-N=C- -C-N-C	Leather and textile
Xanthene dyes, derived from xanthene -O-C6H4-0	Cotton, Silk and Wool
Indophenol dyes, derivatives of indophenol >C=N-and >C=O	Colour photography
Indophenol dyes, derivatives of indophenol >C=N-and >C=O	Colour photography
Oxazin dyes, derivatives of oxazin -C-N=C =C-O-C=	Calico printing
Oxazone dyes, derivatives of oxazone
Thiazin dyes, derivatives of thiazin
Thiazole dyes, derivatives of thiazole >C=N- and -S-0=	Intermediate
Fluorene dyes, derivatives of fluorine	Intermediate
Rhodamine dyes, derivatives of rhodamine Pyronin dyes

Dyes according to the nuclear structure:

Though not very popular but dyes can be categorized into types by using this method of classification:
• Cationic Dyes
• Anionic Dyes

Industrial Classification of the Dyes:

As globally majority of the dyestuff is primarily consumed by the textile industry. So, at this level a classification can be done according to their performances in the dyeing processes. Worldwide around 60% of the dyestuffs are based on azo dyes that gets consumed by in the textile finishing process. Major classes of dyes in textile finishing are given here. Major Dye classes and the substrates:
• Protein Textile Dyes
• Cellulose Textile Dyes
• Synthetic Textile Dyes

Cellulose Textile Dyes:

• Direct dyes:

The name ‘direct dye’ alludes to the fact that these dyes do not require any form of ‘fixing’. They are almost always azo dyes, with some similarities to acid dyes. They also have sulphonate functionality, but in this case, it is only to improve solubility, as the negative charges on dye and fibre will repel each other. Their flat shape and their length enable them to lie along-side cellulose fibres and maximise the Van-der-Waals, dipole and hydrogen bonds. Below is a diagram of a typical direct dye. Note that the sulphonate groups are spread evenly along the molecule on the opposite side to the hydrogen bonding -OH groups, to minimise any repulsive effects.

The main problem with direct dyes is their lack of fastness during washing. However, they are cheap, so are popular for items which are less likely to require fastness during washing. Wash fastness may be improved, though, by the application of direct and developed dyes, which contain -NH2 functional groups as well as sulphonate groups. In this process, the dyed fabric is treated with sodium nitrite, which causes the dye to be converted to a diazo salt. It is then treated with a coupling compound such as 2-napthol. The resultant larger azo molecule now has more affinity for the fibres, and is less soluble.

• Vat dyes:

Vat dyes are a good example of the cross-over between dyes and pigments. Large, planar and often containing multi-ring systems, vat dyes come exclusively from the carbonyl class of dyes (for example, indigo). The ring systems of the vat dyes help to strengthen the Van-der-Waals forces between dye and fibre.

Vat dyes are insoluble in water, but may become solublised by alkali reduction, for example sodium dithionite (a reducing agent) in the presence of sodium hydroxide. For this reason, they tend not to contain many other functional groups which may be vulnerable to oxidation or reduction. The leuco form produced by alkali reduction is absorbed by the cellulose and, once there, can be oxidised back to its insoluble form. Oxidation is usually performed using hydrogen peroxide, but occasionally with atmospheric oxygen under the correct conditions. Treating the dyed textile with a soap completes the process, since the soap molecules encourage the dye molecules to clump together and become crystalline.

The other types of dyes, for example the azo class, undergo a non-reversible change on reduction.

• Basic dyes:

Basic dyes possess cationic functional groups such as -NR3+ or =NR2+. The name ‘basic dye’ refers to when these dyes were still used to dye wool in an alkaline bath. Protein in basic conditions develops a negative charge as the -COOH groups are deprotonated to give -COO-Basic dyes perform poorly on natural fibres, but work very well on acrylics. A general structure of an acrylic type polymer is shown below. It is simplified, and doesn’t show any anionic groups which are often present.

The most common anionic group attached to acrylic polymers is the sulphonate group, -SO3-, closely followed by the carboxylate group, -CO2-. These are either introduced as a result of co-polymerisation, or as the residues of anionic polymerisation inhibitors. It is this anionic property which makes acrylics suitable for dyeing with cationic dyes, since there will be a strong ionic interaction between dye and polymer (in effect, the opposite of the acid dye-protein fibre interaction). An example of a basic dye is shown below:

• Fibre-Reactive Dyes:

A fibre-reactive dye will form a covalent bond with the appropriate textile functionality. This is of great interest, since, once attached, they are very difficult to remove.
Early fibre-reactive dyes; The first fibre-reactive dyes were designed for cellulose fibres, and they are still used mostly in this way. There are also commercially available fibre-reactive dyes for protein and polyamide fibres. In theory, fibre-reactive dyes have been developed for other fibres, but these are not yet practical commercially. Although fibre-reactive dyes have been a goal for quite some time, the breakthrough came fairly late, in 1954. Prior to then, attempts to react the dye and fibres involved harsh conditions that often resulted in degradation of the textile.

The first fibre-reactive dyes contained the 1,3-5-triazinyl group, and were shown by Rattee and Stephen to react with cellulose in mild alkali solution. No significant fibre degradation occurred. ICI launched a range of dyes based on this chemistry, called the Procion dyes. This new range was superior in every way to vat and direct dyes, having excellent wash fastness and a wide range of brilliant colours. Procion dyes could also be applied in batches, or continuously.
The general structure of a fibre-reactive dye is shown below:

A cellulose polymer has hydroxy functional groups, and it is these that the reactive dyes utilise as nucleophiles. Under alkali conditions, the cellulose-OH groups are encouraged to deprotonate to give cellulose-O- groups. These can then attack electron-poor regions of the fibre-reactive group, and perform either aromatic nucleophilic substitution to aromatics or nucleophilic addition to alkenes.

Nucleophilic substitution; Aromatic rings are electronically very stable, and will attempt to retain this. This means that instead of the nucleophilic addition that occurs with alkenes, they undergo nucleophilic substitution, and keep the favourable -electron system. However, nucleophilic subsitutions are not very common on aromatics, given their already high electron density. To encourage nucloephilic substitution, groups can be added to the aromatic ring which will decrease the electron density at a position and facilitate attack. For example:

But this requires harsh conditions. To improve the rate under mild conditions, powerful electron-withdrawing groups such as -NO2 may be added.

However, this will only work if there is a good leaving group, such as -Cl or -N2. The major fibre-reactive group which reacts this way contains six-membered, heterocyclic, aromatic rings, with halogen substituents. For example, the Procion dye:

Where X = Cl, NHR, OR. Nucleophilic substitution is facilitated by the electron withdrawing properties of the aromatic nitrogens, and the chlorine, and the anionic intermediate is resonance stabilised as well. This resonance means that the negative charge is delocalised onto the electronegative nitrogens:

One problem is that instead of reacting with the -OH grous on the cellulose, the fibre-reactive group may react with the HO- ions in the alkali solution and become hydrolysed. The two reactions compete, and this unfavourable because the hydrolysed dye cannot react further. This must be washed out of the fabric before use, to prevent any leakage of dye, and not only increases the cost of the textile, but adds to possible environmental damage from contaminated water.

Nucleophilic addition;Alkenes are quite reactive due to the electron-rich -bond. They normally undergo electrophilic addition reactions. Again, nucleophilic additions are less favoured generally, because of the repulsion between the Nu- and the electron-rich bond. However, they will occur if there are sufficient electron withdrawing groups are attached to the alkene, much as before, with aromatic substitution. In this case, the process is known as Michael addition or Conjugate addition.

For this reaction type, the most important dye class is the Remazol reactive dye. This dye type reacts in the presence of a base such as HO-. The mechanism for the reaction of one of these dyes is shown below:

As before, the intermediate is resonance stabilised, but this has not been shown.

Protein Textile Dyes:

Acid dyes:

Acidic dyes are highly water soluble, and have better light fastness than basic dyes. They contain sulphonic acid groups, which are usually present as sodium sulphonate salts. These increase solubility in water, and give the dye molecules a negative charge. In an acidic solution, the -NH2 functionalities of the fibres are protonated to give a positive charge: -NH3+. This charge interacts with the negative dye charge, allowing the formation of ionic interactions. As well as this, Van-der-Waals bonds, dipolar bonds and hydrogen bonds are formed between dye and fibre. As a group, acid dyes can be divided into two sub-groups: acid-leveling or acid-milling.

Acid-leveling dyes: 

These planar dyes tend to be small or medium sized, and show moderate inter-molecular attractions for wool fibres. This means that the dye molecules can move fairly easily through the fibres and achieve an even colour. This is somewhat similar to the process that occurs during chromatography- the molecules with the strongest affinity for the substrate move the least distance from the point of origin whereas molecules with less affinity move much further. However, the low affinity means that these dyes are not always very resistant to washing.

Acid-milling dyes: 

Acid-milling dyes are larger than acid-leveling dyes, and show a much stronger affinity for wool fibres. Because of this, the resultant colour may be less even (see explanation above), but they are much more resistant to washing. As well as intermolecular interactions, intramolecular interactions play an important part in the properties of the dye. Compare the two molecules shown below. They are isomers, but the one on the right (with hydrogen bonding) shows a much greater resistance to washing in alkali, and much increased light fastness.

Acid dye colours: 

Usually, yellow, orange and red acid dyes are azo compounds, with blues and greens often come from the carbonyl class, particularly anthraquinones (see the example below).An example of an acid dye is Alizarine Pure Blue B. It is a sulphonated aminoanthraquinone

• Mordant dyes:

Mordant is a Latin word meaning ‘to bite’. Mordants act as ‘fixing agents’ to improve the colour fastness of some acid dyes, which have the ability to form complexes with metal ions. Mordants are usually metal salts; alum was commonly used for ancient dyes, but there is a large range of other metallic salt mordants available. Each one gives a different colour with any particular dye, by forming an insoluble complex with the dye molecules. Chromium salts such as sodium or potassium dichromate are commonly used now for synthetic mordant dyes. The diagrams below show C.I. Mordant Black 1 with and without a chromium (III) ion. Chromium (III) forms 6-coordinate complexes, so two Mordant Black molecules would attach to one ion. Only one is shown below for clarity.

Mordants do not have to be metal salts. Organic molecules such as tannic acid and tartaric acid can be used as well.

2.Synthetic Textile Dyes:

• Disperse dyes:

Disperse dyes have low solubility in water, but they can interact with the polyester chains by forming dispersed particles. Their main use is the dyeing of polyesters, and they find minor use dyeing cellulose acetates and polyamides. The general structure of disperse dyes is small, planar and non-ionic, with attached polar functional groups like -NO2 and -CN. The shape makes it easier for the dye to slide between the tightly-packed polymer chains, and the polar groups improve the water solubility, improve the dipolar bonding between dye and polymer and affect the colour of the dye. However, their small size means that disperse dyes are quite volatile, and tend to sublime out of the polymer at sufficiently high temperatures.

The dye is generally applied under pressure, at temperatures of about 130°C. At this temperature, thermal agitation causes the polymer’s structure to become looser and less crystalline, opening gaps for the dye molecules to enter. The interactions between dye and polymer are thought to be Van-der-Waals and dipole forces.

The volatility of the dye can cause loss of colour density, and staining of other materials at high temperatures. This can be counteracted by using larger molecules or making the dye more polar (or both). This has a drawback, however, in that this new larger, more polar molecule will need more extreme forcing conditions to dye the polymer.

Classes of disperse dye: The most important class is the azo class. This class of azo disperse dyes may be further sub-divided into four groups, the most numerous of which is the aminoazobenzene class. This class of dye can be altered as mentioned before, to produce bathochromic shifts. A range of heterocyclic aminoazobenzene dyes are also available. These give bright dyes, and are bathochromically shifted to give blues. The third class of disperse dye is based on heterocyclic coupling components, which produce bright yellow dyes. The fourth class are disazo dyes. These tend to be quite simple in structure. Other than these, there are disperse dyes of the carbonyl
class, and a few from the nitro and polymethine classes.

• Solvent dyes:

Dyes are generally defined along the lines of being coloured, aromatic compounds that can ionise. One class of dyes is an exception to this. These colours are applied by dissolving in the target, which is invariably a lipid or non-polar solvent.

The Colour Index uses this as a classification and naming system. Each dye is named according to the pattern: – solvent + base colour + number

These dyes are thereby specifically identified as dyes of the stated colour, and whose primary mechanism of staining is by dissolving in the target. Note that this is a functional and colour classification. It contains no chemical information; neither does it imply that dyes with similar names but unique numbers are in any way related. It should also be noted that the classification refers to the primary mechanism of staining. Other mechanisms may also be possible, but are rare.

As a general principle, solvent dyes do not ionise. Many are azo dyes which have undergone some molecular rearrangement and lost the ability to ionise. In the process they gained the ability to dissolve in non-polar materials such as triglycerides. They are commonly used to stain such materials in sections. They are frequently called lysochrome dyes. The prefix lyso means dissolve, and chrome means colour. Sudan III (solvent red 23), sudan IV (solvent red 24) and oil red O (solvent red 27) are commonly used for demonstrating fat in sections. Sudan black B (solvent black 3) is also very effective, but can also stain ionically under some circumstances.

Other important dyes:

A number of other classes have also been established, based among others on application that includes the following:

• Leather Dyes – Used for leather.
• Oxidation Dyes – Used mainly for hair.
• Optical Brighteners – Used primarily for textile fibres and paper.
• Solvent Dyes – For application in wood staining and production of coloured lacquers, solvent inks, waxes and colouring oils etc.
• Fluorescent Dyes – A very innovative dye. Used for application in sports good etc.
• Fuel Dyes – As the name suggests it is used in fuels.
• Smoke Dyes – Used in military activities.
• Sublimation Dyes – For application in textile printing.
• Inkjet Dyes – Writing industry including the inkjet printers.
• Leuco Dyes – Has a wide variety of applications including electronic industries and papers.

Dye Identification

I was looking for simple methods to identify class of dyes on fabric. Broadly I work with cotton, linen, silk and union fabrics of silk and viscose. The dyes that I am looking for to identify are direct, reactive, acid ( on silk), sulphur, vat, aniline black and napthols.

Here are some methods to test these dyes. I got the idea from this document. However I have not tested them so far.

1. Direct Dyes:

                   This test of direct dye holds valid for both cotton and silks.

A 100-300 mg portion of the dyed sample is placed in a 35 ml test tube, 5-10 ml of water and 0.5 to 1 ml of conc. ammonia are added and the mixture is boiled in order to bleed off a sufficient amount of dye for redyeing a piece of white cotton cloth.

When a sufficient amount of dye has bled from the sample, the sample is removed, a piece of white cotton cloth weighing 10-30 mg is placed in the test tube and 5-30 mg of common salt is added. After boiling gently for 40-80 seconds and cooling to room temperature, the cotton is removed, rinsed and examined.

Redyeing on cotton in an ammonical solution in the presence of salt to shade and strength comparable to the shade and strength of the original sample is a direct evidence of direct dye.

2. Acid Dyes:

                     For wool and Silk

A 100-300 mg portion of the dyed sample is placed in a 35 ml test tube, 5-10 ml of water and 0.5 to 1 ml of conc. ammonia are added and the mixture is boiled in order to bleed off a sufficient amount of dye for redyeing a piece of white cotton cloth.

The above part is same as acid dyes. However, if the sample in the direct dyes bled but left the test cotton white or only slightly stained, the colored extract is neutralized with 1 ml of 10% sulphuric acid solution and a few drops of acid are added in excess. A 20-30 mg of wool is added and the mixture is boiled for 1 to 2 min. The wool is rinsed and examined.

Redyeing of wool from an acid bath indicates the presence of acid dyes provided the presence of direct dyes is not shown.

3. Sulphur Dyes:

                   For Cotton

A 100-300 mg of dyed sample is placed in a 35 ml test tube and to it are added 2 to 3 ml of water, 1 to 2 ml of 10% sodium carbonate solution and 200-400 mg of sodium sulfide chips.

The mixture is raised to a boil and boiled for 1 to 2 minutes. The sample is removed and to the test tube are added 25-30 mg of white cotton and 10-20 mg of common salt. After boiling for 1 to 2 minutes, the cotton sample is removed and placed on filter paper and allowed to reoxidize.

Under these conditions, sulfur dyes redye cotton in a shade which differs from the original only in strength. A few easily reducable vat dyes will color the white cotton but in a shade markedly different from the original dyeing.

4. Vat Dyes:

    For Both Cotton and Protein ( For protein also follow the portion in Red font)

Vat dyes are to be tested after the sulphur dye test has come negative.

A 100-300 mg dyed sample is placed in a 35 ml test tube to which are added 2 to 3 ml of water and 0.5 to 1 ml of 10% caustic soda solution (The mixture is boiled till all the fiber is dissolved). After being brought to boil a 10-20 mg portion of sodium hydrosulphite  is added and boiling continued for another 0.5 to 1 minute. The sample is removed( Not Needed) and 25-50 mg of white cotton cloth and 10-20 mg of salt is added. Boiling is continued for 40-80 seconds, followed by cooling to room temperature. The cotton is removed and placed on filter paper to oxidize (then in a bath containing sodium nitrite and acetic acid).

Redyeing of cotton to a shade differing only in strength from the original dyeing indicates the presence of vat colors.

5. Napthol and Insoluble Azo Dyes:

The most characteristic property of this class is bleeding in pyridine. A 10-50 mg dyed sample is placed in a 10-15 ml test tube, 1-2 ml of pyridine added and the sample boiled. All napthol bleed to certain extent.

6. Reactive Dyes:

Boil in water with a temperature more than 60 deg in Sodium hydrosulphite and caustic soda as in vat dyes. First the color will come out and then that color will decolorize.

Industrial Garments Washing

Industrial Garments Washing is one of the major important parts for Textile sector. By industrial garments washing we can remove dust, dirt and infections material. For improving special look on garments as per fashion requirement.

There are many types of Industrial Wash. These are:

1.     Normal/Detergent Wash(Normal Detergent Wash)

2.     Bleach Wash

3.     Stone Wash

4.     Acid Wash

5.     Enzyme Wash

6.     Caustic Wash

7.     Super-White Wash

8.     Combined Wash.

There is some physical pretreatment related to industrial  garments washing. These are Hand Scraping, Sand Blasting, Whiskering, Tagging, Crinkle Effect, Grinding, and Destroying.

There is also some chemical pretreatment related to industrial garments washing. These are Potassium per Manganate spray, Color Spray. Color spray is also known as tinting.

For garment dyeing and washing plant some machines and equipments are necessary. These are:

1.     Dyeing and washing machine.

2.     Hydro extractor.

3.     Drying machine.

4.     Spray boote.

5.     Brushing mannequin.

6.     Sand blasting unit.

7.     Grinding machine.

8.     Hot pressing machine.

9.     Tag gun.

10.   Minor sewing facilities.