Dyes and color index( CI) numbers

The color index is produced jointly by SDC (Society of Dyers and Colourists) in the UK, and the AATCC (American Association of Textile Chemists and Colorists) in the USA. The index is split into two parts: one part gives the commercial names for the individual dyes; the other part of the index gives the color index number, and lists the commercial names for the dyes using that number. 

Color index number

Every dye is given a color index number based on its chemical type. The same number is given to the dyes with the same chemical structure. For example, Resolin Red FB and Dispersol Red B2B, both have the color index number “CI Disperse Red 60." The index number is divided into four sections: 

1) CI stand for color index and is displayed in every color index number 2) The next section is the dye type, e.g., Disperse, Acid, etc. 

3) The third section is the color, according to a defined list of color names, e.g., Red Yellow, Orange 

4) The last number is increased every time a new dye is added to the index; in the example above, “CI Disperse Red 60" is the 60th red disperse dye to be added to the index. 

As an extensive international compilation of the structure and properties of dyes and pigments, as well as a system of numbering these materials, the Colour Index is a potentially valuable tool for all users of colorants.

CAS numbers for chemicals and chemical compounds

CAS Registry Numbers are an important tool for substance searching. These are unique identifiers assigned by the Chemical Abstracts Service (CAS) to specific substances identified by them in the chemical literature.

Registry Numbers save you from having to think of all the different synonyms, trade names, or possible systematic names when you are doing a compound search by linking them all into a single search term.

For example, there are over 20 synonyms for formaldehyde, e.g., methanal, formalin, methyl aldehyde. The CAS Registry Number for it, 50-00-0, is specific to all those names for the same compound.
"The foam retracts above the legible symphony."

Indigo Dyeing

Indigo is the primary color of blue jeans , it is an important dyestuff with unique shade of blue color.The dye gives a brilliant and eye catching blue color to the fabric. Indigo has a low affinity  for cotton ,therefore deep blue dyeings are only possible when and oxidation is done several times.The color partially penetrates the yarn , and imparts the surface blue color.The dye then fades gradually fades from the surface of the fabric naturally.

For Manufacturing process of Indigo click here

Natural Indigo

Indigo dye is an important dyestuff with a distinctive blue color. The natural dye comes from several species of plants.A variety of plants have provided indigo throughout history, but most natural indigo is obtained from plants in the genus Indigofera, which are native to the tropics. In temperate climates indigo can be obtained from woad (Isatis tinctoria) and dyer's knotweed (Polygonum tinctorum), although the Indigofera species yield more dye. The primary commercial indigo species in Asia was true indigo (Indigofera tinctoria, also known as Indigofera sumatrana). In Central and South America the two species Indigofera suffructicosa and Indigofera arrecta (Natal indigo) were the most important.

Properties of  Indigo

Indigo is an insoluble dye ,a dark blue crystalline powder, which has to be reduced with suitable reducing agents to make it soluble in water,Indigo works by a chemical reaction called oxidation reduction.Indigo does not dissolve in water. It must be reduced — i.e. the oxygen must be removed— in the presence of alkali by a reducing agent such as thiourea dioxide (thiox), sodium hydrosulfite, Zinc, or bacteria. Upon reduction, indigo becomes colorless and water soluble. In this state, indigo has a high affinity for cellulosic fibers and enters the open spaces of the fiber. The dyed fibers are then exposed to air, which oxidizes the dye molecule back to its insoluble form. The insoluble dye particles are trapped inside the fiber, coloring them permanently blue. Unlike most dyes, indigo forms a mechanical, not
chemical, bond.

A.Indigo is a dark blue crystalline powder that melts at 390°–392°C.
B.It is insoluble in water, alcohol, or ether but soluble in chloroform, nitrobenzene, or concentrated sulfuric acid.
C.The chemical structure of indigo corresponds to the formula C16H10N2O2.

 

Click for Complete Chemistry of Indigo

D.The naturally occurring substance is indican, which is colorless and soluble in water. Indican can easily be hydrolyzed to glucose and indoxyl.
E.Mild oxidation, such as exposure to air, converts indoxyl to indigo. 

F.Indigo is a frequency range of visible light, from 440 to 420 nanometers in wavelength, placing it between blue and violet.

Indigo dye

Indigo has despite many other blue dyestuffs kept his popularity. This by no doubt is achieved by the fact that Indigo has a number of properties which have in this combination not yet been achieved by other single dyestuffs.

The main properties are:


• Pleasant colour shade.


• Possibility to achieve by simple repeated dipping, deep marine blue shade.


• Possibility to dye cotton in cold dye bath.


• Competitive in price.

• Possibility to achieve an acceptable colour fastness and the exceptional advantage by repeated washing of fading colour, to keep the colour shade that always a clear, pleasant blue shade result.

Indigo Application
Three basic steps for indigo application are .

1.Dissolving the dye by reduction involving vatting and loosening up.

2.Dyeing from the vat.

3.Oxidising in the air.

Since indigo has a low affinity for cotton, deep blue dyeings are only obtained when dyeing and oxidising are repeated several times.

Dyeing Methods

Five major Indigo dyeing methods for the basic denim are

1- Indigo rope dyeing process


2- Indigo one sheet dye slashing

3- Indigo double sheet dyeing

4- Loop dye 1 for 6 (continuous dye slashing)

5- Loop dye 1for 6 with dyemer (continuous mercerization dyeing and sizing).

Indigo Dyeing on a 20 rope range 
Indigo Dyeing different systems
Indigo rope dyeing some important technical considerations
Dyeing with Indigo - Natural Fermentation Vat

The Blue Denim

Manufacturing Process Of Denim Enter Here
Manufacturing denims Click here
Denim Washing Process :-A step by step guide

New Developments In Indigo Dyeing

Improved system for denim dyeing Click Here
Indigogenius Denim Dyeing System click here


Read Articles on Indigo Denims
Indigo Dye facts Click here
All about developments of Indigo Click here
Garment Washing Techniques for cotton apparels Click here
A-comparative-study-of-different-stone-and-bio-stone-washing-of-denim Click here

Denim Finishes Click here
Bi-stretch denim manufacturing Click Here
Indigo Dyeing on a 20 rope range Click here
Modification on plain loom for Denim Click here
Notes on Yarn for Rope Dyeing in Denim Click Here
Denim Manufacturing Process- the presentation Click here

Proposed Inspection Process for Indigo Dyed Denim Click Here

Manufacturing Process of Denim Click here

Natural Dyes

For A complete guide to natural dyes and dyeing method,dyeing recipe and dyeing additives Click here

Complete guide to natural dyes click here

Natural dyes are a class of colorants extracted from vegetative matter and animal residues.Dyestuffs and dyeing are as old as textiles themselves. Nature provides a wealth of plants which will yield their colour for the purpose of dyeing, many having been used since antiquity.
Almost any organic material will produce a colour when boiled in a dye-bath, but only certain plants will yield a colour that will act as a dye. The plants names given below are a selection of plants that have stood the test of time, and are used widely and traditionally by natural dyers. Natural dyes fall into the following categories:

•Leaves and stems
                                  •Twigs and prunings
•Flower heads
•Barks
•Roots
•Outer skins, hulls and husks
•Heartwoods and wood shavings
•Berries and seeds
•Lichens
•Insect dyes

The choice of mordant for a particular plant is dependant upon the material with which it will be used. It is necessary to check a recipe before using a plant, or one can experiment to see what effect a mordant has for a particular application. It is recommended that plants be grown specifically for the purpose of dyeing. Harvesting plants from the wild on a non-sustainable basis can endanger the survival of the plant. Many lichens are registered as protected organisms and it is illegal to gather them from the wild.

Why natural dyes Here

Natural dye applications click here
Natural Dyes for Textiles and Clothes Click hereBook on Natural dyeing of textiles Click Here
How to use natural dyes click here
Natural dyeing of Textiles click here
Natural dye international click here
Natural dyes and mordants A students project
Natural dyes and thier application classes Click here
Natural dye process Click here
Natural plant dyes click here

Naphthols and Bases

Azoic Dyes (Naphthol Dyes)
Naphthol dyes are insoluble azo dyestuffs that are produced on the fiber by applying a Naphthol to the fiber and then combining it with a diazotized base or salt at a low temperature to produce an insoluble dye molecule within the fiber. Naphthol dyes are classified as fast dyes, usually slightly cheaper than Vat dyeings; the methods of application are complex and the range of colors limited.
Azoic combinations are still the only class of dye that can produce very deep orange, red, scarlet and Bordeaux shades of excellent light and washing fastness.The pigments produced have bright colors, and include navies and blacks, but there are no greens or bright blues. Crocking fastness varies with shades but washing fastness is equal to Vat dyeings, generally with less light fastness than the Vats.
Naphthols :-The Naphthols are phenols, soluble in alkaline solution and substantive to cotton, particularly in the presence of salt. The anilides of BON acid(beta-oxynaphthoic acid or BON acid) are soluble in dilute NaOH solution and form the corresponding naphtholate ion. These relatively small molecules are of only low to moderate substantivity for cotton, but they diffuse rapidly into the fibres. In general, the higher the substantivity the better the rubbing fastness as less azo pigment forms on the fibre surfaces. The naphtholate ions are always coplanar and preferably have elongated molecular structures. They behave essentially as colorless, low molecular weight direct dyes. The substantivity increases with increase in the molecular size of the naphtholate ion, but the diffusion rate in the fibres and solubility in dilute aqueous alkali decrease. Addition of salt promotes better exhaustion of the bath, more being needed for Naphtols of lower substantivity.
Bases:-These are available as the free amine base or as amine salts such as the hydrochloride.Many of the amines used are simple substituted aniline derivatives with no ionic substituents. The so-called Fast Colour Bases require diazotisation. This usually involves reaction of the primary aromatic amine in acidic solution or dispersion with sodium nitrite, at or below room temperature. Successful diazotisation requires careful weighing of all the chemicals and regard for the supplier’s recommendations. Diazotisation of a primary aromatic amine is often difficult and solutions of diazonium ions are inherently unstable. They undergo decomposition even at low temperature and particularly on exposure to light. Storing prepared diazonium ion solutions is not usually possible.
General Dyeing Procedure

The application of the naphthols is consists of following steps,
1.Dissolution of the naphthol component.
2.Exhaustion of the naphthol dolution onto the substrate or absorption of the naphtholate ion by the cotton;
3.Removal of excess naphthol from the material by squeezing, partial hydroextraction or brine washing.
4.Diazotization of the base component.
5.Development or treatment with the diazonium ion solution to bring about coupling.
6. Neutralisation ,Soaping at the boil to remove superficial pigment, followed by rinsing and drying.
The process can be carried out in almost any type of dyeing machine determined by the form of the goods.
Dyeing Methods

Precautions in Naphthol Dyeing

1.The alkalinity of the naphthol bath shall not drop below the prescribed limit , otherwise the naphthol may presipitate.

2.Formaldehyde shall not be used when working at more than 50 Deg C or when the material is to be dried after naphthol application.

3.Material shall be protected from water spotting,steam,acid and chlorine fumes , and exposure to sunlight after naphthol application.

4.Use of excess salt in naphthol bath may result into precipitation of the bath.

5.The temperature is very important in base preparation stem , otherwise diazotization may not take place.

6.Sodium acetate must be added to the developing bath just before the use , otherwise base will become unstable due to fall in concentration of HCl.

7.Hydroextraction time must not be too long , which may result into light spots after development.

8.Material shall be rinsed without delay after developing , otherwise the mechanically held excess developing liquor will undergo some decomposition and cause deposition of dark colored spots , which will be difficult to remove.

9.It is important to use sufficient amount of alkali binding agents , otherwise it will result into presipitation of developing bath.

Stripping of Faulty Dyeing

-Treat the dyed material with Non ionic detergent and 3-5 gpl caustic soda at boil for 15 min. cool to 85 degC

-Add 3-5% sod. hydrosulphite for 30-45 min at 85 deg.

-Rinse hot and cold

-Bleach with 1-2 Gpl Available chlorine for 20 min.

-Antichlore and neutralise.

-Soap at Boil for 15-20 min.

-Cold rinse.
Role of Different chemicals

T.R. oil :-Wetting agents for naphthol pasting and dissolution and penetrating agent in fiber in naphthol application.

Caustic Soda:-For solubilising of naphthols and keeping proper alkalinity of naphthol bath.

Formaldehyde:-Protective agent of naphthol impreganated material from effect of air.

Salt:-Electrolyte for exhaustion of naphthol during naphtholation and to prevent the desorption of naphthol in the bath during brine rinsing and development phase.

HCl acid:- Dissolution of base and to produce nitrous acid in diazotization phase.

Sodium Nitrite:- Producing nitrous acid in diazotization process.

Sodium Acetate:-For neutralization of excess HCl in developing bath.

Acetic Acid:-As an alkali binding agent in developing bath.

Non Ionic Dispersing agent:-To keep the azoic pigments in fine dispersion phase , which are formed by the coupling of free naphthol in developing bath. Also helps in better color fastness during soaping operation.

Fastness properties on cotton

Correctly prepared dyeings with azoic combinations on cotton have fastness properties often comparable, or only slightly inferior, to those produced using quinone vat dyes. They complement the vat dyes because of the wide range of orange, red and Bordeaux shades that they provide.
The fastness to washing of azoic combination dyeings on cotton is usually very good to excellent but only after careful elimination of particles of azo pigment loosely adhering to exposed fiber surfaces. Intermediate drying or rinsing of fabric containing the Naphtol, and the soaping of the final dyeing, are key processes ensuring optimum fastness. The same argument applies to rubbing fastness. Deep dyeing that have not been well soaped easily transfer color onto adjacent white fabric, even under conditions of gentle rubbing.
There are two other problems associated with the fastness properties of azoic combinations on cotton. In pale shades, the dyeings often have much reduced light fastness, particularly under humid conditions. Some sensitive azoic combinations also give dyeings of only fair resistance to chlorine and peroxide bleaching.

Vat vs Azoics

A.Azoics are economical than Vat dyes.

B.Bright reds , oranges and deep maroon shades are possible in Azoics.

C.Limited Shade range in Azoics.

D.Bright pinks , bright Blue . olive green , green and blacks are not possible in Naphthols.

E.No fear of photodegradation in Naphthols.

F.Rubbing na dlight fastness is inferior in Naphthol dyeing.

G.More Suitable for Tie-dye and space dyeing.

Reactive Vs Azoics

A.Limited shade range in Azoics.

B.Dyeing method is very complex with too many variables in naphtholation , diazotisation and developing baths.

C.Color fastness to Chlorine is better in naphthols.

D.rubbing and Light fastness are inferior in azoics.

E.Dark and bright reds , maroons and ornages can be produced in azoics which are difficult in reactives.

F.Azoics are suitable for Tie-dye and space dyeing.
Space dyeing and Tie dye with naphthol dyes

Metal Complex Dyes

Metal complex dyes
Metal complex dyes are premetalised dyes , in which one or two dye molecules are co-ordinated with a metal ion. The dye molecule is typically a monoazo structure containing additional groups such as hydroxyl, carboxyl or amino, which are capable of forming a strong co-ordination complexes with transition metal ions such as chromium, cobalt , nickel and copper.
Types of metal complex dyesThe classification of metal complex dyes is dyes is based on the basis of number of dye molecules which are complexed with one metal ion , these are
1. 1:1 metal complex dyes :-in 1:1 metal complex dyes one metal ion is complexed with one dye molecule.
2. 1:2 metal complex dyes :-in 1:2 metal complex dyes there are two dye molecules which are complexed with one metal atom.
Properties of the metal complex dyes
Fastness:-The metal complex dyes exhibit good fastness towards exposure to light , however the wet fastness properties are moderate , particularly in darker shades. However the fastness is also depends upon the fiber type and dye type.
Solubility:-These are water soluble dyes.
Application:-These dyes are applied from neutral , weakly acidic to a very strongly bath depending upon the dye sub class.
Applicability:-These dyes are suitable for dyeing of wool, silk, polyamides and leather.
Application of the metal complex dyes
a.1:1 metal complex dyes
These dyes have good leveling and penetration properties ,and are particularly suitable for application on carbonized wool. These dyes are applied under a strongly acidic bath at a pH of 1.8 –2.5 with sulfuric acid or at a pH of 3-4 with formic acid, there fore these are not suitable for the blends having cotton component. Glauber salt is used as exhausting agent and organic leveling agents are used to assist the level dyeing.
A typical dyeing cycle for wool dyeing is as given below,

b. 1:2 metal complex dyes
These dyes are subdivided into two subgroups based on the solubilising groups present in the dye molecule, these dyes shows moderate migration properties on nylon but very good overall fastness properties , because metal complex dyes not only attatch to nylon with ionic linkages , but also with co-ordinate bonds.
1.weakly polar dyes :-These dyes exhibit excellent light and wet fastness properties with excellent penetrating power. These dyes are solubilised by the inherent anionicity of the complex or containing non ionic, hydrophilic substitutes such as methylsulphone. Weakly polar dyes are applied in the range of pH 4-7 adjusted with acetic acid in the presence of ammonium acetate.
2.Strongly polar dyes:-These dyes have lower to moderate leveling properties , but have excellent fastness properties. These are solubilised by one or more sulphonic acid or carboxylic acid residues. These dyes are mainly used for the dyeing of polyamide , wool and their blends with cotton. These dyes are applied to the substrate in the range of pH 5-6 adjusted with the help of acetic acid in the presence of ammonium sulphate.
Polyamides(nylon)
On polyamides dyes containing sulphonic groups are preferred because they are more water soluble and produce better wet fastness .As the degree of sulfonation increases , lower pH are required for complete bath exhaustion and at the same time critical temperature is higher and at the same time with lower degree of sulfonation the exhaustion is better. To improve the absorption of low affinity dyes dyeing is carried out in weakly acidic bath, high affinity dyes are applied in neutral to slight alkaline bath in the presence of non ionic or amphoteric leveling agents.
Here A= Leveling agent + pH adjustment
B= pre dissolved dye

Mordant Dyes

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 requires 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.

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 , 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 form 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

Basic 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 coloured 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 substantivity 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 colouring 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 shows a very affinity towards wool , silk and cationic dye able acrylic, but have no affinity towards cellulosics. To dye cellulosics 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-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).
Dyeing of wool

Advantages of Basic Dyes
•High Tinctorial strength
•Moderate substantivity
•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
•Coloured backwaters
•Very poor lightfastness
•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 substantivity on many furnishes,making them ideal for dyeing applications. Lightfastness is also improved considerably over traditional basic dye.
Key advantages over conventional basic dyes:-
•Excellent substantivity
•Better Lightfastness
•Covers all fibres
•Clear backwaters

Read More abou Basic dyes Click Here

Soluble Vat Dyes

Solublized Vat Dyes (Indigosol Dyes)

Vat dyes are are insoluble in water and leuco vat is not a stable product , however leuco esters of vat dyes are stable and readily soluble in water, leuco ester of vat dyes are available in powder form known as Indigosol Dyes or solubilized vat dyes.Being water soluble these dyes are readily applied to cellulosic fibers and are subsequently converted to parent vat dye by oxidation treatment.

Properties of Soluble Vat colors

1.Stability

The stability of leuco esters is good and varies with different types . The decomposition may occur by action of light , moisture and acid fumes, Which results into auto decomposition. The auto decomposition may be prevented by adding alkaline salts during dye standardization , which acts as buffer during storage.

2.Solubility

The leuco ester powders are readily soluble in water , but boiling must be avoided , which may result into premature oxidation of the dye.

3.Principles of application

The application process of soluble vat colors consists of two steps

a.The exhaustion of color from alkaline , neutral or acidic conditions.

b.The oxidation or development of exhausted leuco ester in the presence of an acid and oxidizing agent , changing the color into original dyestuff.

4.Affinity

The affinity for cellulosic materials is low , but the dye exhaustion is assisted by addition of electrolytes , however the affinity for animal fibers such as wool is good and can be increased by temperature.

5.Sensitivity to Light

Most of the soluble vats are sensitive to light before development and exposure to light can affect the leucoester in powder or paste , dye stuff solution , on dyeing machine before development, during dyeing and after dyeing and drying. In order to avoid all difficulties which may occur through the action of light , it is advisable to reduce the exposure to minimum.

6.Development or Oxidation

The ease of regeneration of leucoesters to original vat dye is most important .The potential of oxidizing medium depends upon ,

1. Choice of Oxidizing agent.


2. Temperature


3. pH of the developing bath.

However over oxidation must be avoided , because excess oxidizing agent may result into over oxidation and may attack the developed dye.

Dyeing method

Due to the higher cost and low substantivity towards cellulosics , these dyes are used only for light and pale shades or on the substrates such as mercerized cotton where penetration of normal vat dye is a problem.

Exhaust dyeing method for dyeing of hanks with soluble vat dyes

(cotton yarn in hank form)

Advantages of Soluble vat dyes
1.They offer extremely good coverage of yarn irregularities .
2.They are especially suitable for cotton and viscose rayon for pale shades.
3.Since no alkali is being used in the dyeing process these can be used for over dyeing of shirting materials dyed with vat or naphthol dyes, without affecting them. These dyes can be used for wool and silk dyeing also.
4.Soluble vat dyes can be easily used for mercerized sewing threads , viscose rayon and cotton embroidery threads for hank dyeing.
5.These can be applied knitted goods on winch machines and suitable for piece goods by padding and jigger dyeing.

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 dyesThese 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 dyesDissolution 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.

Sulfur Dyes

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.( read about sulphur dyes)

  

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. 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 non 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 , 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 penetration. 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 70 0C .
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
I. acetic acid
II. sodium perborate in cold at neutral pH.
III. Hydrogen peroxide and acetic acid.
IV. Potassium or sodium bicarbonates and acetic acid.
6.Aftertertreatment

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 gpl
b. Sodium Acetate 2-3 gpl
c. Tetrasodium pyrophosphate 5.0 gpl
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 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 gpl.
f. After treatment witj 2-3 gpl 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:-
Normaly 1-2 gpl wetting agent is used for good penetration, 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 penetration.
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.

Read Articles
Black is beautiful
Sulphur dyes
Cleaner production with sulphur dyes
Defects in sulphur dyeing
Sulphur dyeing of blends on different machines

Pigment Dyeing

Pigment dyeing is not really "dyeing" in it's truest form because the pigments stick on the fabric with the help of binders.. Pigments are insoluble in water. They exist in the form of finely ground molecules, milled for garment dyeing purposes into a paste. When anionic dispersing agents are added, a slightly negative charge is present, thus the foundation for pigment dyeing is born. When a positively charged cationic pre-treat is added to the fiber a magnetic bond is formed. The process is complete when a cationic binder is added to "lock" the pigment into place.In pigment dyeing no actual chemical reaction takes place between the dye and the fabric.

 
The pigment dyeing process is generally a three step process such as

1.Cationisation of the fabric or giving electric charge to the fabric by a  powerful cationic pretreatment.

Pigment dyeing is an electrical process whereby the goods to be dyed are given an electrical charge opposite that of the pigment.  When the pigment is added to the bath, the opposite electrical charges attract each other, much like the north and the south poles of two magnets.  Because of the electrical nature of the process.

A typical recipe for cationizing Step
• Cationic Imparting Agent
• Wetter
• Acetic Acid for pH control
The cationic agent is applied with a wetting agent and acetic acid onto the fabric in the batch or commercial dyeing machine. After a period of time, the fabric is rinsed.

2.Pigment exhaustion on the fabric

The pigments are first mixed with water and a dispersing agent that imparts an anionic charge to the pigments. This solution is added gradually to the dyeing machine. The temperature of the bath is slowly increased and the machine is held at the dyeing temperature for up to 20 minutes. The fabric is held at an elevated temperature to increase the adsorption of the pigments.

A typical recipe for this step

• Pigment colorants
• Dispersing agent

3.Binder application :-A binder used in the exhaust pigment dyeing procedure for fixing   pigment colors.Binders are commonly acrylic polymers with nonionic and cationic nature. It improves crock and wash fastness.After the pigment is exhausted , the binder is fed into the dyeing machine and exhausted onto the fabric over a period of time. Acetic acid is added to the bath, which facilitates binder polymerization, then the fabric is rinsed, the dyeing machine is drained, and the fabric is extracted.

Dyeing Process Flow Diagram

A Process flow diagram for pigment Garment dyeing process 

 
Advantages of pigment dyeing

1.Its applicable to a wide range of textiles and wide gamut of colors available. Some brilliant fluorescent pigments are available for special applications and fashion items. 

2.The process of pigment dyeing can also be relatively cheap.

3.The process chemicals are less toxic , environmentally friendly and least polluting.

4.The color fades with the repeated use , just like denims.

Limitations of Pigment Dyeing

1.Color fading could be a problem if not desired, and can only be kept intact with further chemical treatments.

2.The abrasion resistance and hand could be unsatisfactory depending upon the pigment, binder and softener used.

3.Dark and deep shades are better produced with other dyestuffs rather than pigments.

Further Readings

What is a pigment

Types of Pigments

Faq on pigments

Pigment Dyeing

Garment dyeing with pigments

Chemistry of Binders

Pigment printing

Chemistry of printing ink pigments

Dyes and pigments

Yarn dyeing with blue 3 GK
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