Direct Dye

Direct dyes are one of the most versatile classes of dyestuff applicable to cellulose, wool, nylon fibres. Although direct dyes have deficiency in wet colorfastness but it exhibits a wide range of fastness properties as shown by their use in apparel, upholstery, lining and automotive fabrics. Direct cotton dyes have inherent substantivity for cotton, and for other cellulosic fibres. Their aqueous solutions dye cotton usually in the presence of an electrolyte such as NaCl or Na2SO4. Direct dyes do not require the use of a mordant and, as their name implies, the dyeing procedure is quite simple. The goods go into the bath followed by the dissolved dyes. The bath is then gradually heated, usually to the boil, and additions of salt promote dyeing.

Dyeing mechanism of direct dyes:

The dyeing mechanism for the application of direct dyes to cellulose fibre involves the adsorption, diffusion and migration over fibre. Several factors persuade the dyeing mechanism; however, most important is the cellulose fibre structure, morphology and the use of electrolytes. When the cellulosic fibre is immersed into water the amorphous regions of the fibre swell to produce small pores in the order of 20-100A units- the smaller size dye molecules diffuse into the fibre structure through these pores. The addition of electrolytes (e.g. Sodium chloride, sodium sulfate) assists the diffusion and exhaustion of direct dye anionic by neutralizing the negative surface charge of cellulosic fibres. Then the dye anions become affixed to the cellulosic fibre through hydrogen bonding and van der waals forces.

Typical recipe of Reactive dyeing:

Wetting agent 1.0-2.0g/LSequestering agent 1.0-2.0g/LLeveling agent 0.5-1.0g/LLubricant 1.0-3.0g/LDirect dyes X%Soda ash 1.0-5.0g/LGlauber salt 5.0-20.0g/LTemperature 95±5°CTime 30-50minutespH Neutral to alkalineM:L 1:5-1:10

Process curve(Dyeing of cotton fabric with direct dyes):

Figure: dyeing cuve of direct dye

Working Procedure of dyeing with reactive dyes:

Set the dyebath with substrate at rom temperature. Add dye solution with auxiliaries and raise the temperature to 90°C. Run the bath for 15-20minutes and add salt step by step according to dye bath concentration, higher the depth of shade need more salt concentration. This is better to add this salt after reaching the temperature to boiling point since at this period the maximum penetration is achieved all over the substrate. Run the dyebath for 30-50minutes at 90-95°C for complete the dyeing cycle. Cool down the dyebath temperature to 60-70°C. Drop the bath and rinse. Carry on the aftertreatment process to improve wet fastness.

Aftertreatment process:

In after treatment process a suitable fixing agent is used for improving color fastness(wash, water, perspiration) properties; usually this process is done at 30-40°C, sometimes at 60°C for 15-20min or according to vender recommendation. After that a cationic softener is added to the last rinsing bath for improving handle properties of fabrics; this process is done at 40-50°C for 15-20min. Then wash with hot and cold water and dry to use.

Reactive Dye

Nowadays reactive dyes are very popular for textile coloration because of its some specific properties like colorfastness, wide range of shade, brilliance of shade, good reproducibility and simple application procedure. It reacts with fibre in presence of alkali and adheres as a part of fibre. Three principles steps are happened during reactive dyeing – exhaustion, fixation or reaction and aftertreatment or washing.

Mechanism of Reactive dyeing:

A typical exhaust dyeing process for cellulosic materials using reactive dyes hasthree distinct phases:

(1)the initial exhaustion phase. Dyeing is started in neutral solution so that there is little likelihood of the dye reacting with the cellulose. During this stage of dyeing, some reactive dye will be absorbed by the fibres, the amount depending upon its substantivity. This dye is capable of migration to promote level dyeing. Sodium chloride or sulphate will often be present initially or be added gradually to the dyebath during this phase to promote exhaustion. Thetemperature of the dyebath may be gradually increased to aid penetration of dye into the fibres and to assist migration.

(2)the fixation phase. After the initial exhaustion phase, the pH of the dyebathis increased by complete or gradual addition of the appropriate type and amount of alkali. This causes dissociation of some of the hydroxyl groups in the cellulose and the nucleophilic cellulosate ions begin to react with the dye.The fixation process then results in additional dye absorption, to re-establish the dyeing equilibrium. Dye absorption from solution and reaction with thefibre then progress until no further dye is taken up. Figure 16.4 illustrates the variations of the dyebath exhaustion and the extent of fixation during a typical exhaust dyeing.

(3)the post-dyeing washing. The rinsed dyeing contains dye bonded to the cellulose, absorbed but unreacted dye, as well as hydrolysed dye. There will also be residual alkali and salt. The latter are relatively easy to remove by successive rinsing in cold and then warm water. As much unfixed dye as possible must be washed out of the dyeing. If this is not done, desorption of this dye during washing by the consumer can cause staining of other materials in the wash. Some unfixed dye is eliminated during the initial rinsing that removes salt and alkali. Thorough washing of the dyeing using a boiling detergent solution (soaping) eliminates the remainder. The dyeing is then finally rinsed in warm water. Soaping must often be repeated for deep dyeings or the residual unfixed dye must be complexed with a cationic agent.

Typical recipe of Reactive dyeing:

Wetting agent 0.2-0.5 g/LSequestering agent 1.0-2.0 g/LAntifoaming agent 0.5-1.0 g/LDyes X%Glauber salt Yg/L (based on depth of shade, deeper shade higher amount of salt)Soda ash Zg/L (based on depth of shade, deeper shade higher amount of alkali)Lubricating agent 1.0-2.0 g/LTemperature 50±5°CTime 30-60 min pH 10.5±0.5M:L 1:5 – 1:10

Process curve for Reactive dyeing:

Working procedure:

Set the bath with substrate at room temperature and add wetting agent, sequestering agent, lubricant and antifoaming agent. Add dyes and half amount of salt. Raise the temperature to 50°C and add rest amount of salt and soda ash. Run bath for 30-60minutes at 50°C. Drop the bath and carry on aftertreatment process.

After-treatment:

For better colorfastness aftertreatment is very important. First need to complete removal of salts and alkali by rinsing. Usually several rinsing process is done to got better result. Then the substrate is neutralized by acid wash. After that it needs soap wash to confirm that the goods are totally free from unfixed dyes.

Soap wash Recipe:

Detergent 1.0-2.0 g/LSoda ash 0.5-1.0 g/LTemperature 95-100°CTime 15minutespH 8-9M:L with enough water

Vat Dye

Vat dye is the most popular dyes for achieving high colorfastness on cellulosic fibres. But it’s an expensive class of dyes compared to reactive or other dyes used for cotton dyeing. It is insoluble in water and has no or little affinity for cotton fibre. Before dyeing the dyes must be converted to solubilized form by being reacted with caustic and hydroze-this state is called leuco form of vat dyes. The leuco vat dyes have affinity for the fibre. After exhaustion of these leuco dyes into the fibre, it is oxidized to form insoluble colored pigment again inside the fibre, thus, the dye molecules locks the fibre.

Mechanism of Vat dye:

Vat dyeing process is carried out in two subsequent step- vatting and dyeing. But the whole process of cotton dyeing with vat dyes embraces several steps such as vatting, dyeing, oxidizing and aftertreatment.

Vatting is a process developed for reducing the vat dye into soluble form. It is done by treating the insoluble vat dyes with strong reducing agent in presence of alkali. Thus produce leuco form of vat dyes and the process is known leuco vat dyeing.

The predetermined amount of cotton goods is immersed into dye bath. In that time need to add additional caustic and hydroze mixture with required amount of hot water into dye bath to maintain liquor ratio. During dyeing process air contact may oxidize the dye bath, which can hinder the successful dyeing. Usually the dyeing temperature is raised to about 80°C or even high if the dyes are not sensitive to over-reduction and more stable to hydroz. The temperature of 100°C and little higher may also be possible.

After dyeing the reduced dye molecules are penetrated and distributed into the fibre. The exhausted dyes are then oxidized to convert it to insoluble form into the fibre. Before oxidation rinsing is very important for medium to deep shades to wash out the alkalies, reducing agent and loosely attached dye with heavy shades it might be necessary to include 1-2 g/L of both caustic and hydrosulfite and a little bit dispersing agent, for about 5 minutes at ambient temperature.

Then rinse the goods with cold-hot-cold water successively. If necessary neutralization rinsing is done with acetic acid. Finally soften the goods as requirements.

Recipe of Vat dyeing:

Wetting agent 0.5-1.0 g/LSequestering agent 1.0-2.0 g/LLevelling agent 1.0-2.0g/LDyes X%Caustic soda Y g/LHydroze Zg/LTemperature 70-80°CTime 30-50minM:L 1:10

Dyeing curve for Vat dye:

Working Procedure:

First set the dye bath with substrate a 40°C and add wetting agent, sequestering agent, leveling agent and then run for 5-10minutes. Add dyes according to substrate weight and run the bath for 5minutes. Add 2/3 of caustic soda requirements and run for 5minutes. Raise the temperature to 70-80°C at 2-3 °C/minute and run for 5minutes. Add sodium hydrosulfite to the bath with rest 1/3 amount of caustic requirements. Run the bath for 30minutes and maintain reduction by checking with vat reduction paper. Overflow rinse until clear with cold water.

After dyeing cycle penetrated and distributed dye molecules are oxidized to convert insoluble form into the fibre. The oxidization process can be done in contact of air or other oxidizing agent. For example oxidization process can be carried out treating the dyed goods with 0.5-1.0g/L hydrogen peroxide at 30-50°C for 10minuets.

Aftertreatment:

Aftertreatment is done by soaping.

Recipe:

Detergent 1.0-2.og/LSoda ash 1-2g/LGlucose 3-6g/LTemperature 95-100°CTime 15-30minM:L 1:10-1:20

Then rinse the goods with cold-hot-cold water successively.

Azoic Dye

Azoic colors are not ready-made dyes like reactive dyes. The insoluble azoic colors are synthesized inside the fibre by coupling of two components neither of which is a dye itself. These two components are aromatic diazonium salt and an aromatic hydroxy compound. First the cotton goods are impregnated with napthol solution called naptholation and these goods are trated with diazo compound base component called diazotization. After diazotization the actual color is developed in the substrate.

Mechanism of Vat dyeing:

The whole dyeing process is divided into three stages-

1. Naptholaton

2. Diazotization

3. Aftertreatment

Naptholation:

First make napthol paste with wetting agent. After that add necessary amount of caustic soda solution and boil it to prepare a clear solution. According to napthol type and substantive nature, common salt may be used to accelerate the process. The substrate is then immersed into napthol solution and ensures that the goods properly impregnated with the solution. This process is done at normal temperature for 30sec to 1minute.

Diazotization:

Diazotization is carried out by pasting the base with a little amount of water and required amount of concentrated hydrochloric acid and water are added. The mixture is stirred till a clear solution of salt of the base is obtained. Diazotization reaction takes place between the hydrochloride of the base and the nitrous acid and it takes 20-30minutes for the reaction to go to completation.

Development or coupling:

The reaction between the napthol when present of the textile material and the diazonium chloride is called development or coupling by which the azoic pigment is formed. Thus a bright red pigment is formed when a cotton fabric impregnated with Napthol AS is coupled with diazotized Fast Red TR bases.

Aftertreatment:

Immediately after diazotization the substrate is squeeze and wash with cold water and soak in 2-3ml/L HCl and then wash with the following solution:

Soap 1.0-2.0 g/LSoda ash 1.0-2.0 g/LTemperature 65-95°CTime 15-30minutesM:L 1:10

Finally apply successive rinsing with hot and cold water and if necessary treat with acetic acid to neutralize the residual alkalies.

Sulfur Dye

Sulfur dyes are a type of vat dye used for cellulosic fibres. The insoluble pigment is converted into the substantive leuco compound by reduction with sodium sulphide and the leuco form is subsequently oxidized inside the fibre. The chemistry of sulfur dyes is very complex and little known of the molecular structure of the dyestuff produced. A variety of structural units have been produced for various sulfur dyes. It is generally accepted that these dyes are polymeric having sulfur containing aromatic heterocyclic units such as thiazoles linked by di or polysulphide bonds.

Mechanism of Sulfur Dyeing:

Initially the goods are wet out in the bath. Since the dyeing liquor contains appreciable amounts of sulphide, copper fittings must be avoided. If wetting or penetrating agents are used these should be of the anionic type since non-ionic surfactants form stable, non-substantive complexes with the leuco thiols. An anionic product such as phosphated 2-ethylhexanol is suitable. The bath may then be set at 40 °C with some sodium polysulphide and some EDTA. Polysulphides in the leuco dyebath prevent premature oxidation of the dye and reduce the tendency to bronziness of deep dyeings of blues, navies and blacks. An addition of a sequestrant such as EDTA avoids precipitation of the leuco thiolate by calcium and magnesium ions. The leuco dye is then added slowly and, since the leuco dyes only have low to moderate substantivity for cellulose, some salt may be added initially, or in portions during dyeing, to promote exhaustion. After dyeing the goods are rinsed, the leuco dye oxidised and the dyeing is soaped as for a conventional vat dye. Sulphur dyes usually have acceptable substantivity, particularly in the presenceof salts, so that stripping in a fresh reducing bath is not easy. Dyeing is often conducted at the boil but this decreases the degree of exhaustion. Sulphur dyes require less salt than reactive dyes and usually have reasonable exhaustion. Low sulphide leuco dyes require more salt and no polysulphide. They do not give good exhaustion in heavy shades and the use of a low liquor ratio is recommended. Popular shades such as black, it has long been common practice to use a standing bath. This is a dye bath that is re-used for subsequent dyeings after addition of more reduced dye. Any free sulphur that tends to accumulate is dissolved by addition of sodium sulphite to give thiosulphate. This prevents it sticking to the goods. The actual dyeing temperature can vary. At higher temperatures around the boil, the bath exhaustion is less but penetration of the leuco dye into the fibres is better than at lower temperatures.

Recipe of Sulfur dyeing:

Wetting agent 0.5-1.0 g/LSequestering agent 1.0-2.0 g/LSulfur dyes X%Sodium sulphide 3-6 g/LGlauber salt 10-20 g/LSoda ash 1-3 g/LTime 1hourM:L 1:10-1:20

Dyeing curve of Sulfur dyeing:

Working procedure:

Prepare the dye bath with substrate at room temperature. Add necessary amount of soda ash with water and raise the temperature to 60°C and then add the prepared dye solution into the dyebath. Add required amount of salt into the dyebath. Close the dyebath and raise the temperature to 90-95°C and run for 1hour. Cool downj the bath and drop. Carry out the aftertreatment process to improve the colorfastness. To improve the colorfastness to washing the dyed fabric is then squeeze and treat with 2-4 g/L soda ash at 70-80°C for 10-15minutes. But to improve both light and wash fastness, the dyed fabric is then squeeze and treat with the following solution at 70-75°C for 10-15minutes.

Potassium dichromate 0.5-1.0 g/LCopper sulfate 1.0-2.0 g/LAcetic acid 1.0-3.0 g/L

`

Oxidation is done with 1-2 g/L hydrogen peroxide at 60°C for 15-20minutes. Rinse and wash to improve colorfastness.

Acid Dye

Nylon molecules have amino groups at the end of the polyamide chains. Nylon,like wool, can therefore be dyed with all types of acid dyes by an ion exchange process. Nylon filaments are sensitive to mechanical, thermal and chemical stresses during filament production as well as during their assembly into fabric and its preparation for finishing. These stresses modify both the chemical and physical structure of the nylon filaments and result in changes in dyeing rate and dye uptake. If the modification of the nylon is not uniform along the length of the filament, the final colour will be uneven. Disperse dyes, which dye most synthetic fibres, migrate readily when dyeing nylon and cover such chemical and physical irregularities in the filaments. Unfortunately, the washing fastness of disperse dyes on nylon is only poor to moderate, particularly in heavy shades. They are used mainly for ladies’ hosiery and lingerie. The washing fastness of non- metallised acid dyes on nylon is moderate to good and the light fastness is usually good. Milling or metal-complex dyes give dyeings of superior washing fastness on nylon. Their unlevel dyeing behaviour, however, results in poor coverage of filament irregularities. The 2:1 pre-metallised dyes, in particular, give dyeings of high light fastness, as required for example for nylon automotive fabrics. Besides hosiery and apparel, carpets are a major consumer of nylon, particularly in the USA, and are usually dyed with acid dyes.

Typical Recipe of Acid dye:

Wetting agent 0.5-1.0 g/LAntifoaming agent 0.1-2.0 g/LLevelling agent 1.0-2.0 g/LAnticreasing agent 1.0-2.0 g/LAmmonium sulfate 2.0-4.0 g/LAcetic acid Y g/LAcid dyes X% Temperature 100°CTime 30-50 minuteM:L 1:8 to 1:10

Working procedure:

Set the dyebath with substrate at 40°C and add wetting agent, leveling agent and buffer. Run the bath for 10minutes at same temperature. Add dyes and run for 10minutes again. Raise the temperature to 95-100°C over 30-45minutes. Run the bath at 95-100°C for 30-50minutes. Cool down the bath to 60°C at 2-3°C/minute. Drop the bath and carry on aftertreatment process.

Rinse twice with hot and cold water successively. For improvement of polyamide dyeing with acid dyes can improve by treating with a suitable fixing agent at 60-70°C for 15-20minute at 4-5pH level. Rinse again to clean unexpected chemicals any more.

Disperse Dye

The dyeing of nylon with disperse dyes is therefore limited mainly to pale shades for lingerie fabrics and sheer hose that do not require repeated or severe washing. Disperse dyes on nylon are also more sensitive to fading by ozone and nitrogen dioxide.They are, however, economical and easy to apply. Most nylon filaments are oriented by drawing but both undrawn and partially oriented yarns can be dyed with disperse dyes. With increasing draw ratio, the increased polymer chain orientation decreases the rate of dyeing (decreased fibre accessibility) but not the extent of dye absorption at equilibrium (unchanged fibre availability). It is only at the very high draw ratios typical of strong industrial yarns that the equilibrium dye absorption decreases. Uniform drawing of filaments is essential. Accessibility differences in dyeing can be minimised provided that the selected dyes and conditions are conducive to levelling. Disperse dyes on nylon are much better in this respect than acid dyes.

Mechanism of disperse dyes to nylon:

The disperse dye is pasted in warm water and the dispersion slowly diluted. Hot water and concentrated dispersant favour the formation of large dye particles. The concentrated dispersion is then strained into the dyebath that usually also contains additional dispersing agent. The bath is gradually heated and dyeing continued at the boil. The disperse dyes used for nylon are usually level dyeing. The exhaustion rates of individual disperse dyes on nylon are not overly high. They do vary from dye to dye so that selection of compatible dyes is necessary. Although some dyes have good migration and build up well, deep shades are rarely dyed with disperse dyes because of their inferior washing fastness. Many of the simple disperse dyes developed for dyeing acetate at 85 °C are not particularly fast to heat and can sublime from the

nylon during processes such as boarding. This is a form of heat setting used to stabilise the shape of ladies’ hosiery after dyeing. Dyes of higher fastness to sublimation are invariably of greater molecular size and therefore have lower rates of dyeing. The usual temperature for rapid dyeing disperse dyes on nylon is 85–100 °C. If slow dyeing heat fast dyes are used, dyeing under pressure at up to 120 °C may be useful. The disperse dyes used for dyeing nylon will also colour spandex (segmented polyurethane) filaments in stretch hose but the washing fastness is only fair. As for cellulose acetates, blacks are produced by diazotisation of a disperse dye containing a primary amino group and coupling of the generated diazonium ion with a suitable coupling component. Nylon 6 is more amorphous and has a lower melting point than nylon 6.6. Disperse dyes dye nylon 6 using the same method as for nylon 6.6. Dyeing is usually faster than for nylon 6.6 under the same conditions and the dyes will usually show better migration. This usually means that the washing fastness is somewhat lower on nylon 6. One advantage of nylon 6 is that heat setting using hot air or steam is at lower temperatures than for nylon 6.6.

Recipe of disperse dyeing:

Dispersing agent 0.5-1.0 g/LSequestering agent 1.0-2.0 g/LLevelling agent 1.0-2.0 g/LDyes X%Acetic acid 0.5-2 g/LTemperature 80-100°CTime 30-60minuteM:L 1:10

Working procedure:

Set the dyebath with substrate at 50°C temperature and add dispersing agent, acetic acid and other auxiliaries then run the dyebath for 5-10minutes. Add dye solution and raise the temperature to 80-100°C at 1-3°C/min. the maximum dyeing temperature depends on product type and form. Some of higher energy polyester disperse dyes may be applied at temperatures of as much as 110°C on nylon 6 and 120°C on nylon 6:6. Run the bath for 30-60minutes at the same temperature. Lower down the bath temperature to 50-60°C at 1-2°C/min. Drop the dyebath and carry on the after treatment process.Rinse the fabric twice with hot and cold water. Treat the fabric with alkali solution at 70-80°C for 10-15minutes. Rinse twice with hot and cold water and then neutralize with acetic acid.

Printing of cotton fabric with reactive dye:

A stock thickening using alginate or a emulsion thickening with alginate is first prepared:

Stock thickening:

450 parts sodium alginate (6%)

320 parts water

200 parts urea

30 parts sodium bicarbonate

1000 parts

Emulsion thickening:

345 parts water5

25 parts emulsifier W

210 parts urea

200 pars kerosene

200 parts sodium alginate (6%)

20 parts sodium bicarbonate

1000 parts

The printing paste is then prepared as:

50 parts reactive dye

950 parts stock or emulsion thickener

1000 parts

After printing and drying the fabric is steamed for 5-10min in rapid ager or 15min on a star ager or steamed under high pressure at 130°C for 30 sec.

Printing of cotton fabric with sulfur dye:

Alkaline thickening is prepared as:

500 parts British gum

350 parts water

150 parts NaOH

1000 parts

Printing paste:

50 Parts dye

50 parts glycerin

900 parts alkaline thickening

1000 parts

After printing and drying the fabric is steamed at 103°C for 5 min in a rapid ager.

Printing of cotton fabric with azoic dye:

The fabric is first padded with napthol and then dried. Then fabric is printed with a paste containing:

60 parts Fast Red TR salt

320 parts water

600 parts starch tragacanth paste

20 parts acetic acid (40%)

1000 parts

After printing and drying the cloth is steamed for 5 minutes.

Printing of polyester fabric with disperse dye:

The fabric is printed with a paste containing:

100 parts disperse dye

175 parts water

700 parts locust bran gum (6%)

25 parts acetic acid

1000 parts

After printing fabric is dried carefully and the dye is fixed by one of the following three methods:

1. Thermo Fixation: the dye is fixed by subjecting the print to hot air at 210°C for 1 min.

2. Super heated steaming: live steam at atmospheric pressure is raised from 100°C to 180°C by bringing it in contact with radiators which are heated to 200°C by oil heating system or electrical heating. The time of fixing is 7 to 8 min.

3. High pressure steaming:

The steaming is carried out under pressure in a kier at 0.2kg/cm² pressure at temp 128°C.

Deep shades are reduction cleared at 50°C for 20 min in a bath containing2g/l caustic soda, 2g/l sodium hydrosulphite and 2g/l non-ionic wetting agent. Then rinse and dry.