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Dye
A dye can generally be described as a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and may require a mordant to improve the fastness of the dye on the fiber. Both dyes and pigments appear to be colored because they absorb some wavelengths of light preferentially. In contrast with a dye, a pigment generally is insoluble, and has no affinity for the substrate. Some dyes can be precipitated with an inert salt to produce a lake pigment, and based on the salt used they could be aluminum lake, calcium lake or barium lake pigments.
Dyed flax fibers have been found in the Republic of Georgia dated back in a prehistoric cave to 36,000 BP. Archaeological evidence shows that, particularly in India and Phoenicia, dyeing has been extensively carried out for over 5000 years. The dyes were obtained from animal, vegetable or mineral origin, with no or very little processing. By far the greatest source of dyes has been from the plant kingdom, notably roots, berries, bark, leaves and wood, but only a few have ever been used on a commercial scale.
The dyeing or wet processing flow chart is given below:
Grey Fabric Inspection
↓
Sewing or Stitching
↓
Singeing
↓
Desizing
↓
Scouring
↓
Bleaching
↓
Mercerizing
↓
Dyeing
↓
Printing
↓
Finishing
↓
Final Inspection
↓
Delivery

Characteristics of Textile Dyes:
Dye Class Description Method Fibers Typically Applied to Typical Fixation (%) Typical Pollutants Associated with Various Dyes
Acid water-soluble anionic compounds Exhaust/ Beck/ Continuous (carpet) wool, nylon 80-93 color; organic acids; unfixed dyes
Basic water-soluble, applied in weakly acidic dyebaths; very bright dyes Exhaust/ Beck acrylic, some polyesters 97-98 N/A
Direct water-soluble, anionic compounds; can be applied directly to cellulosic’s without mordants (or metals like chromium and copper) Exhaust/ Beck/Continuous cotton, rayon, other cellulosic’s 70-95 color; salt; unfixed dye; cationic fixing agents; surfactant; defoamer; leveling and retarding agents; finish; diluents
Disperse not water-soluble High temperature exhaust Continuous polyester, acetate, other synthetics 80-92 color; organic acids; carriers; leveling agents; phosphates; defoamers; lubricants; dispersants; delustrants; diluents
Reactive water-soluble, anionic compounds; largest dye class Exhaust/ Beck Cold pad batch/ Continuous cotton, other cellulosic’s, wool 60-90 color; salt; alkali; unfixed dye; surfactants; defoamer; diluents; finish
Sulfur organic compounds containing sulfur or sodium sulfide Continuous cotton, other cellulosic’s 60-70 color; alkali; oxidizing agent; reducing agent; unfixed dye
Vat oldest dyes; more chemically complex; water-insoluble Exhaust/Package/ Continous cotton, other cellulosic’s 80-95 color; alkali; oxidizing agents; reducing agents

SYNTHETIC DYES:
The discovery by Sir William Perkin, an English chemist in 1936 that a mauve coloring matter capable of dyeing silk could be prepared by the oxidation of aniline started a vast chain of events. It resulted in production of approximately 8,000 distinctly different dyestuffs being manufactured all over the world and sold under 40,000 trade names.
Synthetic dyes used in the textile industry are broadly split into 11 groups:
1. Basic dyes 7. Mordant dyes
2. Direct dyes 8. Acid dyes
3. Vat dyes 9. Disperse dye
4. Reactive dyes 10. Oxidation dyes
5. Azoic dyes 11. Mineral and pigment dyes
6. Sulphur dyes

Basic dyes (These dyes are also known as cationic dyes):
This a class of synthetic dyes, that act as bases and when made soluble in water, they form a colored cationic salt, which can react with the anionic sites on the surface of the substrate. The basic dyes produce bright shades with high tinctorial values, on textile materials.
Properties of basic dyes Basic Dyes are cationic soluble salts of colored bases. Basic dyes are applied to substrate with anionic character where electrostatic attractions are formed. Basic dyes are not used on cotton as the structures are neither planar nor large enough for sufficient substantively or affinity. Basic dyes are called cationic dyes because the chromophore in basic dye molecules contains a positive charge. The basic dyes react on the basic side of the isoelectric points. Basic dyes are salts, usually chlorides, in which the dyestuff is the basic or positive radical. Basic dyes are powerful coloring agents. It’s applied to wool, silk, cotton and modified acrylic fibres. Usually acetic acid is added to the dyebath to help the take up of the dye onto the fibre. Basic dyes are also used in the coloration of paper.
Ionic nature:-The ionic nature of these dyes is cationic.
Shade range:-These dyes exhibit an unlimited shade range with high tinctorial strength, brightness and many colors are having fluorescent properties.
Solubility:-The solubility of these dyes is very good in water, in the presence of glacial acetic acid.
Leveling properties: - These dyes have a very high strike rate, therefore leveling is poor.
Exhaustion: - cationic dyes exhaust at a variable rates, K values are used to define the exhaustion characteristics of the cationic dyes. K=1 means the fastest exhaustion, while K=5 means the slowest exhaustion. So while making the combination shades the dyes of similar K values must be used.
Affinity: - These dyes show a very affinity towards wool, silk and cationic dye able acrylic, but have no affinity towards cellulosic. To dye cellulosic with basic dyes the material must be treated with suitable mordanting agents.
Fastness properties: - The light fastness is poor to moderate, but wet fastness is good.
Dyeing mechanism with basic dyes?
Dyeing of acrylic with basic dyes
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-polymerization, or as the residues of anionic polymerization 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).

Advantages of Basic Dyes
•High Tinctorial strength
•Moderate substantively
•Relatively economical
•Wide shade range
•Includes some of the most brilliant synthetic dyes
•Shows good brightness
Limitations of Basic dyes Poor shade stability
•High acid content
•Colored backwaters
•Very poor light fastness
•Preferential dyeing
Modified Basic dyes
These dyes, generally based on the chemistry of basic dyes, have longer molecular structures than traditional basic dyes, and thus have significantly improved properties.
Though still cationic in nature, modified basic dyes exhibit improved fibre coverage and substantively on many furnishes, making them ideal for dyeing applications. Light fastness is also improved considerably over traditional basic dye.
Direct Dyes
Direct Dyes are molecules that adhere to the fabric molecules without help from other chemicals. Direct dyes are defined as anionic dyes with substantively for cellulosic fibres, normally applied from an aqueous dyebath containing an electrolyte, either sodium chloride (NaCl) or sodium sulfate (Na2SO4).. The dyeing process with direct dyes is very simple, direct dyeing is normally carried out in a neutral or slight alkaline dyebath, at or near boiling point, but a separate after treatment such as cationic dye fixing; to enhance wet fastness has been necessary for most direct dyeing.
Direct dyes are used on cotton, paper, leather, wool, silk and nylon. They are also used as pH indicators and as biological stains.
Chemicals nature of direct dyes
Chemically they are salts of complex sulfonic acids.
Structure:-More than 75% of all direct dyes are unmetalized azo structures, great majority of them are disazo or polyazo types.
Ionic Nature:-Their ionic nature is anionic.
Solubility:-They are soluble in water.
Affinity:-They have an affinity for a wide variety of fibers such as cotton, viscose, silk jute, linen etc. They do not make any permanent chemical bond with the cellulosic fibers but are attached to it via very week hydrogen bonding as well as Vander Waals forces. Their flat shape and their length enable them to lay along-side cellulose fibers and maximize the Vander-Waals, dipole and hydrogen bonds.
Types of direct dyes:
The SDC classification of direct dyes is follows
(1) Class A – dyes that are self-leveling, i.e. dyes of good migration or leveling properties.
(2) Class B – dyes that are not self-leveling, but which can be controlled by addition of salt to give level results; they are described as salt-controllable.
(3) Class C – dyes that are not self-leveling and which are highly sensitive to salt, the exhaustion of these dyes cannot adequately be controlled by addition of salt alone and they require additional control by temperature; they are described as temperature-controllable.
Application of Direct Dyes:
Direct dyes are usually applied with the addition of electrolyte at or near the boil in the machines capable of running at atmospheric pressure .But in HTHP dyeing machines it is carried out at temperatures above the boil in case of pure as well as blended yarns.
An addition of alkali, usually sodium carbonate, may be made with acid-sensitive direct dyes and with hard water as well as to enhance the dye solubilisation. When cellulose is immersed in a solution of a direct dye it absorbs dye from the solution until equilibrium is attained, and at this stage most of the dye is taken up by the fibre. The rate of absorption and equilibrium exhaustion varies from dye to dye. The substantively of the dye for cellulose is the proportion of the dye absorbed by the fibre compared with that remaining in the dyebath.
Dyeing Method
The color is pasted well and dissolved in boiling water to get a lump free solution .An addition of 0.5–2 g l–1 sodium carbonate may be advantageous when applying dyes of only moderate solubility in full depths.
The dyebath is set at 40°C,
Rise to the boil at 2 degC min–1.
Hold at the boil for 30–45 min,
During hold add 10–15 g l–1 of sodium chloride or calcined Glauber’s salt. Light shades are dyed without or lesser addition of salt.
Improved yields can be achieved when applying full depths by cooling to 80°C at the end of the period at the boil, adding a further 5 g l–1 salt and rising to the boil again.

Dye bath variables which must be considered for level dyeing,
1. Temperature of Dyeing and rate of heating
2. Electrolyte concentration and addition
3. Time
4. Dye solubility
5. Use of leveling agent
After treatment of Direct dyed material
The wet fastness properties (particularly washing, water and perspiration) of virtually all dyeing of direct dyes are inadequate for many end uses but notable improvements can be brought about by after treatments.
1. Diazotization and development
2. Metal salt treatments
3. Cationic fixing agents
4. Formaldehyde treatment
5. Cross linking agents and resin treatments
Stripping: Most direct dyes can be stripped of the use of stripping salts (Sodium Hydrosulphite) and/or by using a chlorine bleaching agent such as sodium hypochlorite, without harmful effects on the fibers.
Color fastness properties of Direct Dyed material: -
Generally these dyes are used where high wash fastness is not required.
Wash Fastness:- poor unless treated with suitable dye fixing agent and/or fastness improving finishing agent.
Light Fastness:-Good
Rubbing Fastness:- Moderate to Good
Chemical Wash Fastness:- Poor

Reactive Dyes

In the simplest terms, all reactive dyes are made up of three basic units, a chormophore , a bridge and a reactive group/ groups (either a haloheteocycle or an activated double bond). These dyes are used for dyeing of cellulosic fibers and when these are applied to a cellulosic fiber in an alkaline dye bath, they form a covalent bond with hydroxyl group of the fiber by chemically reacting with fiber. The covalent bond formed between the dye molecule and fiber make dye molecule a part of the fiber molecule.
The covalent bonds that attach reactive dye to natural fibers make them among the most permanent of dyes. "Cold" reactive dyes, such as Procion MX, Cibacron F, and Drimarene K, are very easy to use because the dye can be applied at room temperature. Reactive dyes are by far the best choice for dyeing cotton and other cellulose fibers at home or in the art studio.
Reactive dyes are categorized by functional group:
Functional group Fixation Temperature Included in Brands
Monochlorotriazine
Haloheterocycle 80˚ Basilen E & P
Cibacron E
Procion H,HE
Monofluorochlorotriazine Haloheterocycle 40˚ Cibacron F & C
Dichlorotriazine Haloheterocycle 30˚ Basilen M
Procion MX
Difluorochloropyrimidine Haloheterocycle 40˚ Levafix EA
DrimareneK & R
Dichloroquinoxaline Haloheterocycle 40˚ Levafix E
Trichloropyrimidine Haloheterocycle 80-98˚ Drimarene X & Z
Cibacron T
Vinyl sulfone activated double bond 40˚ Remazol
Vinyl amide activated double bond 40˚ Remazol

Types of the reactive dyes

Basically there are three types of the reactive dyes, which are classified as per the functional group present,

1.mono functional reactive dyes
2.bi functional reactive dyes(homo and hetero bifunctional)
3. Poly functional or multifunctional reactive dyes

These dyes are also classified based on their exhaustion properties as well as on their application temperature such as cold brand and hot brand reactive dyes.
These are water soluble dyes and hence applied from aqueous bath under neutral to weakly acidic conditions, electrolyte is added to exhaust the dyes on the fiber from the dye bath and then exhausted dye is fixed on the fiber by adjusting the pH suitable for making a covalent bond between the dye molecule and fiber , with the help of alkali. The unexhausted dye which remains in the liquor is hydrolysed ( hydrolysed dye is formed by the reaction of dye molecule with water under alkaline conditions) and loses its reactivity with cellulosic fiber , that too is absorbed by the fiber . Therefore washing and soaping treatment is done to remove the hydrolysed and excess dye to improve the color fastness of the dyed substrate. The overall colorfastness achieved is good when the material is dyed and washed properly.
Dyeing cycle and Important factors/phases in reactive dyeing
1. pretreatment of the substrate
2. pH of the substrate prior to dyeing
3. pH of the dyebath
4. solubility of the dyestuff
5. dyeing temperature
6. electrolyte concentration
7. dyeing time
8. M:L
9. Type of alkali
10. Washing off sequence
11. Quality of water and salt

The detail of impact of above parameters,

1. Pretreatment of the substrate
The process of removing natural impurities like oils, fats, waxes, pectin, protein, amino acids and hydrophobic impurities from a yarn or fabric is called “Scouring”. Scouring is done to improve absorbency of the textile material by removing the oils and fat in the yarn or fabric by boiling.
The pretreatment includes the scouring and bleaching of the substrate prior to dyeing. The main objective of the scouring treatment is removing the major impurities from the cotton and improving the absorbency. The material shall be free from any contaminants and natural coloring matter in the scouring and bleaching treatment. The extent of the pretreatment such as ground whiteness depends upon the target shade, as brighter and whiter ground whiteness is required for light and bright shades and dark and dull shades can be dyed on scoured ground without any difficulty.

2. pH of the substrate
pH of the substrate prior to dyeing must be controlled and it should be either neutral or slightly acidic because if the pH is alkaline at the beginning , the dye molecule may form a permanent bond or premature fixation leading to unlevel dyeing.

3. pH of the dye bath
It should be weakly acidic to neutral in the dyeing bath in migration and exhaustion phase, before addition of alkali. It must be checked and regulated because presence of bicarbonates in the water may increase the pH with increase in temperature, which results into partial fixation of the dye molecules resulting in unlevel dyeing. The pH in the fixation stage must be kept at 10.8-11.0, which shall be achieved by either soda ash alone or a mixture of soda ash and caustic soda.
4. Solubility of the dyestuffs
It is better to consider the dye solubility chart of individual colors provided by the manufacturer, the dyes with higher solubility are more suitable for better shade and color fastness control.

5. Dyeing temperature

The temperature and rate of rise of the dye bath affects the affinity of the dye molecules towards fiber, rate of hydrolysis, migration and covalent bond formation. Therefore the dyeing temperature selected must be as per the dye sub class. Effect of temperature on the build up and fixation of individual dyes must be studied to form dyes groups having similar characteristics and then these groups must be selected for making combination shades.

6. Electrolyte concentration
The dyes must be exhausted by addition of salt or glauber salt before starting the fixation of color. For electrolyte concentration in the dye bath to be used please refer to tables provided by the dye manufacturers. The electrolyte used must be free from unwanted impurities such as metal salts (iron, copper etc) calcium content, insoluble material, and hardness creating salts.
7. Dyeing time
The dyeing time must be selected based upon the depth of the shade. The timings must be optimized to get maximum exhaustion as well as maximum fixation of the color in the dyebath. Based on the exhaustion and fixation curves of the individual dyestuffs, an optimum time can be selected. There is no advantage of increasing the fixation time than desired because that will not help either in exhaustion or fixation. Generally darker shades need more time in fixation phase than the lighter shades.
8. material to liquor ratio
The M: L of dyebath affects the dye or shade performance to a large extent, as for as possible robust dye combinations must be used, which are unaffected by the change in liquor ratio. The chemical concentrations must be changed with the change in liquor ratios. If a dye house is having different capacity machines with different M: L ratio then it shall be taken into consideration from the lab recipe stage.
9. Type of alkali for fixation
There are different methods applied to achieve the right pH for the fixation of reactive dyes. Normally soda ash alone is used for the fixation purpose; however a mixed alkali system of soda ash and caustic soda is also used, particularly in the case of dark shades. A gradual pH change is preferred over the shock change for better dyeing results, therefore a dosing system is strongly recommended in the reactive dyeing to achieve consistent shade reproducibility.

10. Washing off sequence for reactive dyes
The hydrolyesd and unfixed dyes which are present in the dye bath as well as on the fiber after the completion of dyeing cycle, these hydrolyesd dyes have no affinity for the fiber but still they act as direct dyes and in the presence of electrolyte penetrates inside the fiber , with the rise in temperature of washing and soaping. If these are not removed before soaping and washing these can severely affect the color fastness properties. Such trouble can be reduced or eliminated by following an optimized washing off sequence after dyeing.
The dyed goods must be free from any inorganic salt before going for a soaping treatment. Generally a non ionic soaping agent is used for soaping purpose to get good washing fastness. An organic acid such as acetic acid is must be used to neutralize the dyebath. The soaping treatment can be done up to a boiling temperature to remove the unfixed and hydrolysed dye effectively. A higher soaping temperature can be selected because unfixed dye has practically no affinity for the fiber and the loosely held dye rapidly diffuse out. The soaping treatment is recommended in a neutral bath because under alkaline soaping conditions at higher temperature the dye fiber bond may break and result into loss of color value due to rupture of dye fiber bond. If the soaping treatment is carried out efficiently and carefully then there is no need of cationic dye fixing treatment at the end of dyeing cycle. A typical soaping treatment recommended for Procion XL and XL+ dyes is as shown below
Mordant Dyes
mordants are substances, which are used to fix a dye to the fibres. They also improve the take up quality of the fabric and help to improve color and light fastness. The term is derived from the Latin word ‘mordere’ which means to bite mrdants are prepared solution, often with the addition of an ‘assistant’ which improves the fixing of the mordant to the yarn or the fibre. These are special acid dyes in which certain metal atom can be introduced during dyeing. These are water soluble dyes and affinity for silk, wool and polyamides. Mordant dyes require a mordant in their application and these dyes upon cobination with the mordant deposit on the fiber in the form of insoluble color. Most commonly dyes have hydroxyl or carboxyl groups and are negatively charged (anionic) in nature.
The most commonly used mordants are ‘alum, which usually used with cream of tartar as assistant. Other mordants are:
• Iron( ferrous sulphate)
• Chrome( stannous chloride )
• Copper sulphate
• Oxalic acid etc.

Types of mordant dyes

Properties of mordant dyes:
These dyes are economical dyes and are generally used to produce dark shades such as dark greens, dark blues and blacks.
These dyes have good leveling and color fastness properties.
The interaction between fiber and dye is established through very strong ionic bonds, which are formed between the anionic groups of the colorant and ammonium cations on the fiber. Chromium or the metal ion acts as bridge between the dye and fiber, which gives rise to a very strong linkage, resulting into excellent fastness properties.
However there are disadvantages of the chrome dyes also such as longer dyeing cycles, difficulties in shading, and risk of chemical damage to the fiber and the potential release of chromium in the waste water.
Mechanism of dyeing:
Since these are a special class of acid dyes, which are soluble in water and applied to the fiber from an acidic bath. When a solution of an acid mordant dye is mixed with a solution of potassium dichromate in the presence of sulfuric acid, chromium ion from dichromate forms a complex with the dyes, this complex is insoluble in water, and hence precipitates on the fiber.

There are three different methods of application of chrome dyes on the fiber,
Chrome – Mordant method
After chrome method
Meta chrome method

Application of mordant dyes
Chrome-mordant process

In chrome mordant process, the fiber is first treated with potassium dichromate in the neutral bath or in the presence of either sulfuric or formic or oxalic acid. When sufficient amount of chromium is taken up by fiber, it is taken out, squeezed and entered in the dye bath containing acid mordant dye. The dye forms an insoluble complex with chrome present on the fiber.

After chrome method

In this method the substrate is first treated with the dye, the dye is exhausted by the addition of an acid, and after complete exhaustion the material is taken out squeezed and then run in a solution containing potassium dichromate and an acid. Metal dye complex is formed on the fiber, which is insoluble.

Meta Chrome process:

This is a single bath process, in which the material is treated with in a bath containing acid mordant dye, potassium dichromate and ammonium sulphate.

The dye along with potassium dichromate and ammonium sulphate got absorbed by the fiber and evenly distributed but no complex is formed because the pH is not suitable for the chemical reaction to take place.

In the second step of the Meta chrome process, when the dye bath is heated, ammonium sulphate is converts into ammonia and sulphuric acid, which makes the bath strongly acidic and potassium dichromate in the presence of strong acid now react with the dye molecule forming an insoluble complex on the fiber.

A dyeing cycle for dyeing of wool with chrome dyes is shown below

HERE A=GALAUBER SALT+ACETIC ACID+LEVELING AGENT

B= DYE

C=POTASSIUM DICHROMATE

D=ACID

Sulfur Dyes
Sulfur dyes are synthetic organic substantive dyes for cellulosics.
Chemical structure: - The exact chemical structure of the dyes is not known, but these dyes contain sulfur as an integral of the chromophore as well as in the polysulphide side chains. These are produced by thionisation or sulphurisation of organic intermediates containing nitro and amino groups.

Properties of sulfur dyes
these are water insoluble dyes and have no affinity for the cellulosics as such, but solubilised when treated with a weak alkaline solution of sodium sulphide or any other reducing agent to form a leuco compound. These leuco compounds are water soluble and have affinity for the cellulosic materials such as cotton, viscose, jute and flex etc. These dyes are absorbed by the cellulosic material in the leuco form from aqueous solution and when oxidized by suitable oxidizing agents, got converted into insoluble parent dye, which is fast to normal color fastness parameters.

Main properties of the sulfur dyes are as follows:

1. Economical dyeing with excellent tinctorial value and good build up properties.
2. Good overall colorfastness properties such as wash fastness, light fastness, perspiration fastness etc. Moderate fastness to crocking and poor fastness to chlorines bleaching agents such as bleaching powder and sodium hypochlorite.
3. Limited shade range to produce only dull shades and there is no true red dye in the range.
4. These dyes can be applied by exhaust, semi continuous or continuous dyeing methods on garment, yarn, knits, fabric as well as loose stock etc.
5. It s Available in powder, granules and liquid forms.
6. Sulphur black 1 is the major black dye used world vide for dyeing of cellulosics.
7. The conventional dyeing process is not environment friendly due to pollution problems of sodium sulphide as well as sod/pot. Dichromates.
8. When dyed by using none polluting reducing and oxidizing agents the process is environment friendly.
Types of sulfur dyes
There are three classes of sulfur dyes, which are available commercially,
1.Conventional water insoluble dyes which have no substantivity to cellulosics.
2. Solubilised sulfur dyes, which are water soluble and non substantive to cellulosics.
3. Pre-reduced sulfur dyes, in the stabilized leuco compound form, which are substantive to cellulosics.

APPLICATION
Mechanism of the sulfur dyeing
the application of the sulfur dyes involves several steps, which are described as given below:
1. Dissolving the dyestuff.
The dye is taken in an SS vessel (size of the vessel should be selected as per the quantity and solubility of the dyes) and pasted well with a good alkali stable wetting agent and small quantity of soft water. A required quantity of soda ash may be added to neutralize any acid formed in the dyestuff during storage.( if the acid is not neutralized , it will react with the sodium sulphide , resulting into formation of H2S gas, which will result into incomplete and poor reduction of the dyes). It is very important that the dye dissolution must be complete otherwise particles of the undissolved dyes may deposit on the surface of the substrate resulting into patchy dyeing and poor rubbing / washing fastness.

2. Reducing the dyes to form a leuco compound.
Chiefly sodium sulphide is used as a reducing agent for the sulfur dyeing. The quantity of the reducing agent depends upon the shade depth and M: L of the bath. For complete reduction the required quantity of the sodium sulphide is dissolved in a separate container and solution is allowed to settle for 10-15 min. before decanting the clear solution into the dye dissolving vessel. Further boiling water is to be added to make up the required volume, and then heated to boil for 10-15 minutes either by live steam or indirect heating, for complete reduction of the dyestuff.

3. Dyeing with the reduced dyes.
It is advantageous that the goods are scoured well before dyeing, to have a satisfactory absorbency for better pe*******on. The dye bath is kept ready with small quantity of the alkali stable and aompatible wetting agent, a dye bath stabilizer, sodium sulphide and caustic soda or soda ash to maintain the alkalinity of the dye bath. The dye solution is then added through a filter cloth slowly over 15-25 minutes and then run for another 15 minutes at 40-50 oC, then temperature is raised to 60 oC and electrolyte is added in at least 3 portions. The quantity of salt added is depends upon the type of shade , depth and dyestuffs , however a maximum quantity does not exceed more than 15 gpl .The temperature is then raised to above 80 oC or even boil depending upon the dyes and kept for sufficient time to get the desired shade.
A typical dyeing cycle is as shown below


Here A = Wetting agent ,Dye bath stabiliser ,Sodium sulfide ,Soda ash
B= Reduced dyes slow addition
C= common salt or Galuber salt
D= Oxidizing agent
E= Neutral Soap+ soda ash
F= Dye fixer/ softener /final chemical rinse
After getting the correct shade the bath is either dropped by draining the contents or by collecting it in the storage tanks for reuse after replenishing with fresh dyestuffs.

4. Washing off the un-exhausted dyestuff.
With an objective of achieving the highest possible color fastness results such as washing, rubbing, light and perspiration, the material is washed and rinsed several time with fresh water to remove maximum possible loose residual dye as well as sodium sulphide from the material. At the end of the washing process the water should be clear, with no further leaching out color. After washing the material is given a hot wash at 70C.

5. Oxidation back to the parent dye.
The oxidation is done to reconvert the leuco compound back to insoluble parent dye. There are number of methods available for oxidizing the leuco compound which are used either independent or in combination, such as

a. Oxidation by exposing the dyed material to atmospheric oxygen.
b. Oxidation by the dissolved oxygen in the fresh water.
c. Chemical oxidation, by employing different oxidizing chemicals, such as

6. After treatment
After oxidation and hot wash, the material is neutralized with soda ash to adjust the pH and then soaping treatment is done with a neutral soap and soda ash at boil. Followed by a hot wash at 85 0C

7. Dye fixing treatment optifix F (clariant) is a cationic dyefixing agent, which is applied in alkaline conditions (at a pH of 10-11) , and is a suitable dyefixer for sulphur dyed material to improve the color fastness.

8. Softening:-
A suitable (compatible) softener can be applied to the dyed material as per the intended end use and dyestuff applied.

9. Final treatment:-
To avoid the tendering of the dyed material final wash is given to maintain a slight alkaline pH by a weak base or acid neutralizing agent at the end without further washing. Following treatments are recommended,
a. Soda ash wash 2-3 g/l
b. Sodium Acetate 2-3 g/l
c. Tetra sodium pyrophosphate 5.0 g/l
d. Lime and tannic acid treatment

10. Use of standing bath.
Since a large quantity of the dye always present in the unexhausted form in the spent liquor , this remaining dye can be reused , after replenishing with fresh dye. This system is particularly suitable when producing repeated lots of the same shade with a single dye , such as black.
The dye liquor at the end of dyeing cycle is collected in the tanks, to replenish the bath a separately made dye solution is added and calculated quantities of sodium sulphide , soda ash as well as salt are added. The final volume is made up to the required level and reused. Usually a 50 –70% dye is replenished in case of blacks.
The spent bath use is not recommended in case of mixture shades, due to difference in the exhaustion and fixation of individual dyes.

Common problems and corrective action

1. Poor wash and rubbing fastness
Poor washing and rubbing fastness is generally caused by improper color dissolution, color precipitation, poor solubility of the dyes, poor and insufficient washing after dyeing of unexhausted dyes and poor or insufficient soaping treatment.
To get overall good fastness properties:
a. The dye dissolution must be complete and it should be filtered before adding to the dye bath, because insoluble dye particles ,if present , will stick at the outer surface of the substrate causing unleveled dyeing and poor wash and rub fastness.
b. The color should be dissolved in sufficient quantity of water, by keeping in mind the maximum solubility of the dye.
c. The water and the salt should be free from calcium and magnesium, which, if present will make insoluble inert salts, which precipitates especially in the closed dyeing machines, in the form of sludge.
d. The washing after dyeing and soaping treatment must be efficient to clear all the unused dye as well as chemicals, before going to the next operation such as oxidation and neutralization respectively.

2. Bronziness
There are various reasons for bronziness in the sulphur dyed material such as, in sufficient quantity of sodium sulphide or reducing agent , resulting into quick oxidation of surface dyeing. The presence of excess dyestuff on the material caused by high concentration of dye or electrolyte, delay between dropping of bath and washing, oxidation step. Following are the corrective actions for correcting and avoiding the bronziness problem,
a. proper dissolution of the dyestuff.
b. Thorough washing and treatment with reducing agent before oxidation.
c. Use of surfactants, sequestering agents, dispersing agents , dye bath stabilizers, and anti oxidants in reducing bath.
d. Using sufficient and calculated quantity of reducing agents.
e. Using appropriate quantity of electrolyte e.g. less than 15 g/l.
f. After treatment witj 2-3 g/l TR oil+ 1-2 cc/ltr of ammonia in luke warm bath, to overcome the problem.
g. Treatment with soap solution at boiling temperature.
h. Using a blank bath of sodium sulphide .

3. Tendering
Tendering means the loss of strength or degradation of cellulosic materials upon storage. The tendering is caused by the acid formation from the free sulphur present in the dyed material by the action of moisture and air. The acid produced reacts with cellulose and degrade it , resulting in loss of strength. The tendering can be minimized by giving after treatments with acid neutralizing agents or by weak alkaline washing at the end of dyeing process.

4. Poor color value
Poor color value is caused by insufficient amount of reducing agent , presence of calcium salts in water and salt, over reduction of dyestuff , over oxidation etc.

5. Correction of faulty dyeing.If the dyeing results are unlevel , then these can be corrected by
a. Leveling the dyed material by running in a blank bath containing excess sodium sulphide, dispersing ,sequestering agent, wetting agent at a temperature of 80-90 degress, this treatment will partially strip the color , which can be adjusted in a fresh bath. Or alternatively the partial stripping can be done by using caustic soda 5 gpl and hydros 5 gpl at a higher temperature than the dyeing temperature.
b. For poorly leveled material, the material is treated with sodium or calcium hypochlorite, in which it is treated with 2-3 gpl available chlorine at room temperature, followed by thorough wash and neutralization and antichlore treatment.
Water quality for sulphur dyeing
the use of soft water with less than 50 ppm hardness is preferred which should be free from calcium salts, but in case only hard water is available, a sequestering agent based on sodium hexametaphosphate or EDTA should be used. These chemicals avoid the formation of insoluble metal-dye complexes which cause poor rubbing fastness and uneven dyeing.

Other recommended chemicals in dyeing

1. Wetting agents:-
Normally 1-2 g/l wetting agent is used for good pe*******on; in the dyeing bath. Wetting agents used must be compatible with the dyestuff, particularly in combination shades. The wetting agents must be low foaming and alkali stable at high temperature. Unsuitable wetting agents adversely affect the dye bath, inhibiting the dye uptake or precipitating the leuco compound of the dye.
Normally 1-2 gpl of wetting agents are used in the dyeing bath for good pe*******on.

2. Dispersing or dye bath conditioner: - These are used to impart the leveling effect as well as to keep the dye in dispersed form, to avoid the dye aggregation and precipitaion.
Generally naphthalenesulphonic acid –formaldehyde condensate, ligninsulphonates and sulphonated oils are used in sulphur dyeing.

Dyeing with pre-reduced liquid sulfur dyes:
The pre-reduced sulphur dyes are available as stabilized liquids, which have substantivity for the cellulosic, materials.
Main properties of pre –reduced dyes:
a. No dissolution required, therefore cleaner environment and working conditions.
b. No use of sodium sulphide, therefore lesser smell.
c. Low salt additions are required.
d. Lesser pollution loads.
e. Easier washing off of the reducing agents, therefore easy oxidation.
f. Less staining and contaminations of the dyeing machines.
g. Good storage stability and water solubility.
These dyes are recommended for exhaust dyeing of cotton in loose fiber, yarn, fabric and continuous dyeing, such as rope dyeing.
All about Diresul RDT Liquid sulfur dyes
Dyeing cycle of black dyeing in package form
Environmental problems and sulfur dyeing
The use of non polluting chemicals and by Reusing the spent liquor dye bath , the dyeing becomes less polluting and environmental friendly.There are two major pollutants generated in classical sulfur dyeing procedure ,
a. sodium sulfide in the reducing step
b. Potassium/ sodium dichromate in the oxidation step.
Both these chemicals are potentially hazardous for the environment , but can be replaced by environment friendly , less polluting chemicals such as,For reducing baths,1. sodium sulfhydrate and alkali( soda or caustic)
2. sodium hydrosulphite and caustic soda.
3. sodium hydrosulphite in glucose/caustic.
4. glucose and caustic soda.
5. alkaline sodiumformaldehyde sulphoxylate.
For oxidation baths
1.Hydrogen peroxide and liquid ammonia.
2.sodium perborate .
3.sodium bromate and acetic acid.
4.alkaline solution of sodium chlorite at pH 10.
5.Air oxidation , wherever possible.
New green chemical techniques in textile coloration
Use of spent dye bath in dyeing
A major consumption of the sulfur dyes is in the dyeing of black shades and a large amount of dye is used to produce a good black. Due high concentration of dye in the dyeing bath , al the dye is not transferred to the substrate and a large amount of dye is always remains unexhausted at the end of dyeing. Which if drained creates problem at water treatment plants and increase the cost of treatment. The unexhausted dye in these cases can be reused , after replenishing with fresh dye, when repeated lots of a particular shades has to be produced (say black).
The dye liquor at the end of dyeing cycle is collected in the tanks made for this purpose, the volume is made up for the lost liquor in dyeing ,the dye which is to be replenished is separately and added to it. Similarly the quantities of the electrolytes , and reducing agents are calculated and replenished. This bath then can be used as a fresh dye liquor.
The spent dye bath reuse is recommended for the self shades and blacks only , and not in combination shades because , where a mixture of dyes is used the exhaustion properties of the dyes is different and it is not possible to replenish the bath , for producing the exact shade.
Acid Dyes
What are acid dyes?
Acid dye class is a water soluble class of dyes with anionic properties. The textile acid dyes are effective for protein fibers such as silk, wool, nylon and modified acrylics. Acid dyes fix to the fibers by hydrogen bonding, vander waals forces and ionic linkages.

Chemical structure of acid dyes
These dyes are normally very complex in structure but have large aromatic molecules, having a sulphonyl or amino group which makes them soluble in water. Most of the acid dyes belongs to following three main structural molecules,
1.Anthraquinon type
2.Azo dye type
3.Triphenylmethane type.

Different types of acid dyes
The basic dyes are classified into several groups , based on the leveling properties, economy of the dyeing and fastness properties, however generally these are classified into these three classes,
1.Neutral acid dyes :-
These are supra milling or fast acid dyes, having medium to good wet fastness properties , some of the dyes have poor light fastness in pale shades . many of the dyes are used as self shades only. These are applied to the fiber in a weakly acid or neutral pH.
2.Weak acid dyes
These dyes belongs to the milling class of dyes. These dyes have good fastness properties but light fastness is moderate to poor.
3.Strong acid dyes
These dyes are applied in a strongly acidic medium and also called leveling dyes, however there wet fastness properties is a limitation. These dyes are very good to produce the combination shades.

Classification according to dyeing characteristics
Acid dyes are commonly classified according to their dyeing behaviour, especially in relation to the dyeing pH, their migration ability during dyeing and their washing fastness. The molecular weight and the degree of sulphonation of the dye molecule determine these dyeing characteristics. The original classification of this type, based on their behaviour in wool dyeing, is as follows:
(1) Level dyeing or equalising acid dyes;
(2) Fast acid dyes;
(3) Milling acid dyes;
(4) Super-milling acid dyes.
Milling is the process in which a woollen material is treated, in weakly alkaline solution, with considerable mechanical action to promote felting. Dyes of good fastness to milling are essential to avoid colour bleeding during the process.

Properties of acid dyes
The main properties of acid dyes are ,
Since these are sold as a sodium salt, there fore these form a large anion in the aqueous medium. 1.These dyes are anionic in nature.
2.These dyes are suitable for wool, silk, polyamide and modified acrylics.
3.These are applied from a strongly acidic to neutral pH bath.
4.These dyes have no affinity for cotton cellulose’s , hence not suitable for cellulosics.
5.These dyes combine with the fiber by hydrogen bonds , vander waals forces or through ionic linkages.

Mechanism of dyeing with acid dyes
Dissolution of dyes in aqueous solvent, produces a colored anion,

The protein and polyamide fibers produce cationic sites in water under acidic conditions, as the acidity of the solution is increased more cationic sites are produced under these strongly acidic conditions. These cationic sites are thus available for the acid dye anions to combine with through hydrogen bonding, vander waals forces or ionic bonding. These linkages are strong enough to break , and thus dyeing produced are fast .
Reaction between an acid dye and wool can be represented by following equation
ON WOOL
Electrolyte in the acid dye bath acts as a retarding agent because of chlorides ions attracted by the positive sites at the fiber and in the competition between. Addition of acid acts as a n exhausting agent , because strongly acidic conditions makes more cationic sites available and thus available dye anions got combined with these.

Dyeing temperature
the dyeing is generally carried out at boiling temperature for 30- 60 minutes depending upon the depth of the shade and dyestuffs used.

Dyeing leveling agents
In the case dyeing with acid dyes, mainly cationic agents such as ethoxylated fatty amines are used as leveling agents.

Heating rates
Heating rate is generally kept 1-30C/Min

Washing off process
A typical dyeing cycle of nylon filament dyeing with acid dyes is shown in the above chart,
Wool dyeing method with acid dyes

Method 1

At A set bath at 50° with:
4% Sulphuric Acid (96%)
5% Glaubers Salt anhydrous,
pH 2.5 to 3.5

At B add required amount to dyestuff.
Method 2

At A set bath at 50° with:
2% Formic Acid (85%)
5% Glaubers Salt anhydrous,
pH 3.5 to 4.5

At B add required amount of dye.

At C add 2% Sulphuric Acid (96%) or 2% Formic Acid (85%).

Thoroughly rinse after dyeing to remove loose colour.

A dyeing cycle for nylon filament dyeing


Fastness properties of acid dyes?
The wet and light fastness properties of the acid dyes varies from poor to excellent , depending upon the molecular structure of the dyes.

The fastness properties as per the category are as follows
Neutral acid dyes:-since these dyes have very good leveling and migration properties, and have a low affinity for the fiber, therefore the wet fastness properties of this class are generally poor.
Weak acid dyes or half milling dyes :- These dyes have a medium to good affinity for the fiber and are generally applied in a weakly acidic bath, shows medium to good wet fastness properties. Strong acid dyes or super milling dyes :- These dyes have poor exhaustion properties, therefore applied under very strong acidic condition , exhibit good fastness properties.