Dyes

A dye is defined as the organic, inorganic or combination of both substance which are easily absorbed retained by fibre, impart color and withstand drying, washing and exposure to light over a long period of time. 

The first synthetic dye was discovered by Henry Perkin when he accidentally colored silk during the oxidation of Aniline.  

What do dyes do? #

• Impart color
• Soluble in water/organic solvent/ or both.
• Ability to get absorbed and retained on the fiber.
• Should be able to withstand color fastness while washing and drying

Pigments #

• Pigments are also organic molecules or metal organic complexes which impart color but they are different from dyes because they are not soluble in water/organic solvent/or both.
• they do not reach the bulk of the fiber upon their application.
• Pigments are used only to color the outer surface especially that of polymeric chains or fibres.

WITT’s Theory #

According to Witt’s theory of dyes all organic dyes have two important groups

1. Chromophore
   1. They are the functional groups which are responsible for the color of the dye
2. Auxophore
   1. They are functional groups which deepens the color of the dye

Mordents: Chemical comppounds which help the dye to adhere to the fabric by forming a bridge between the two. Most of the times they are heavy metal salts.

Classification of dyes #

Based on source #

1. Natural dyes: Dyes derived/extracted from natural sources such as plant, microbes, animals and minerals
2. Suffer from many disadvantages
   1. Poor to moderate light fastness
   2. Very less affinity for synthetic fibres
   3. They require heavy metal salts as mordants
   4. Natural dyes are extracted most of the time
   5. Overall extraction yield is very low considering the fact it requires huge amount of land and water
3. Advantage: Safe to use and environment friendly
4. Natural dyes can be monogenetic or polygenetic
   1. Monogenetic dyes: Dyes which produce only one color irrespective of the moderant used.
   2. Polygenetic dyes: Dyes whose color depends upon the moderant used.
   3. Can be further classified as:
      1. Adjective (additive) dyes: They do not bind to the fabric without the use of moderant. Hence the fibre must be pre treated with moderant. eg: Indigo, alkanate, Archil (orchid)
      2. Substansive dyes: They can be used without pre treatment. eg: Safflower, cochineal, black walnut
5. Synthetic dyes: Are the dyes which are synthesized from organic compounds
   1. Ex: Alizarin, Fast green, Eosin, Picric Acid

On the basis of chromophore #

1. Azodyes: where the chromophore is -N||N-. Ex: Methyl orange
2. Triphenyl methane. Ex: Malachite Green, Leucomalachite Green
3. Pthalein dyes are derivatives of Pthalic anhydrides.

Based on method of application #

10/2/2023 

1. Reactive dyes: they adhere very well on to the fabric. Very rarely mordents are used in this case. The dye solution prepared in water or any other organic solvent. Then the fabric is dipped into the solution, agitated well for a few hours and then taken out, washed with water and dried. Examples: Procion, Orange Mx2r. These dyes show very good light and washing fastness. 

2. Acid dyes: these dyes require pretreatment with slightly acidic solutions. Most of the nylon based fabrics and acrylics are dyed using acid dyes. They show good light fastness but their washing fastness is not so good and hence regular fabrics are not dyed using acid dyes. Ex: acid blue 78 

3. Premetallized dyes: these types of dyes are complex dyes which are used along with acidic solution and few molecules/atoms of metals(Cr) 

4. Direct dyes: they are applied to cellulose based fibres like jute, cotton. These dyes are directly used in a hot dye bath without any mordents. Eg: Congo red, Direct fast brown M, Brilliant Yellow. These dyes produce bright colors. However these dyes do not have any fastness towards light washing or drying.  

5. Azoic dyes: these kind of dyes result in formation of azo bonds (n||n) upon their application onto the fabric. These dyes might be initially colorless or very light colored but upon binding with the fabric, deep and bright colors. Eg: 4 chloro2nitroaniline, 3 chloro2methylaniline, 1amino2methylanthragnione. These dyes are used for natural fibres like cellulose and silk.  

6. Dispersed dyes: these kind of dyes have very low water solubility. They are non ionic in nature and they are dyed hot. Most of the acetate based fibres are dyed with dispersed dyes. Eg: disperse red, disperse yellow. These dyes show very good fastness and bright colors.  

7. Vat dyes: oldest type of dye used. They are water insoluble dyes. Their solution is prepared in water in presence of NaOH+ NaHSO3. The temperature of the solution is maintained at 50 deg C and the fabrics are dipped and agitated vigorously for 15-20 min. Cotton and most cellulose based fabrics are suitable for these dyes. They are available in different shades and show relatively good fastness. Eg: indigo, tyrian purple. 

8. Basic dyes: these kind if dyes are applied under basic conditions. Most of them are cationic dyes and their color can be improved by steaming. Eg: rhodamine, crystal violet. 

9. Sulphur dyes: they are known to produce blackish and dull shades. Most cellulose based fibres are treated with mordents and then dipped into a solution containing the sulphur dyes to produce different shades of black color. Advantage: they show good fastness and are very economical. Disadvantage: only black and dull shades. 

Mechanism of dying  #

The dying mechanism involves four steps 

1. Transfer of the dye to the fibre surface. 

2. Adsorption of the dye onto the fibre surface. 

3. Diffusion of the dye from the surface of the fibre to the bulk. 

4. Interaction of the dye molecules through chemical reaction or physical forces with the fibre. Types of interactions between dye and fibre: 

   1. Ionic 

   2. Covalent  

   3. Coordinate 

   4. Hydrogen bond 

   5. Dipole-dipole or ion-dipole 

   6. Wanderwaal  

Factors affecting dyeing mechanism #

1. Temperature of the dye bath
   1. Decides the solubility and even distribution of the dye throughout the solution.
   2. Also increases the ionic nature of the dye.
   3. Special cases:
      1. Cellulose based fibres: Temperature of the dye bath swells the fibre and helps in even distribution of the dye molecules throughout the fibre.
      2. Disperse dyes: Increase in temperature helps in breaking down the micellarr aggregates into smaller groups which can be easily adsorbed by the fibre
   4. Increased temp. also promotes the vibrational activity which increase the rate of migration of the dye onto the fibre.
2. Environment: Type of solvent, quantity of solvent
3. Presence of suitable pH, plectrolytes (moderants), dispersing agents
4. Substantivity of the dye: Dye0fibre interaction under a given set of conditions
5. Physical/chemical/mechanical forces applied during dyeing
6. Rate of adsorption
   1. The dye is transferred onto the fibre via adsorption
   2. Chemical adsorption: the forces of attraction are much stronger and form a permannetn bond between the dye and fibre.
   3. Physical adsorption is often reversible in nature.
   4. When a fibre is dipped into a aqueous solution the fibre acquires a electrostatic potential which helps in retaining the dye molecules on the surface of the fibre. Such kind of electrostatic potential is known as zeta potential. Eg:
      1. Cellulose based fibres gain -ve charge on their surface.
      2. Protein based fibres like silk may gain -ve or +ve charge based on the pH of the solution and isoelectric point of the protein.
         1. Isoelectric point: pH which contains the switter ionic structure and the overall charge is 0

Kinetics of Dye mechanism #

It is the study of the rate of dying, measurement of the rate and the mechanism of the dyeing process at different temperatures. The rate of the reaction will depend on:

• Concentration of dye
• Collision frequency