A Bird’s Eye View of Dyeing

           The colouration of cotton is in regular practice since the ancient era. This colouration is termed “Dyeing” which is the decorating of textile materials by applying basic principles of chemistry. So, it can also be defined as “Applied Chemistry”. Cotton must be well pretreated for achieving perfect dyeing. Well pretreated means, well desized, scour and bleach. Also, whiteness is majorly concentrated for dyeing operations. One would select full white fabric for dyeing pale shades. Dark shades application do not require full bleach fabric, so the supervisor can proceed for half bleach only.

            The textile materials are coloured with dye or pigments with desired colour fastness is termed as “Dyeing”. The said operation was performed with natural dyes prior, but nowadays natural dyes have been replaced by synthetic dyes. The first synthetic dye was discovered by Perkin in 1856. Travelling from ancient to today’s world, cotton has locked its popularity. Therefore, the dyeing of cotton is also at the same level as industrial practices. Cotton dyeing can be performed in various forms. 

Various terms related to dyeing operation are commonly used, therefore, what they mean is important to know.

Self-Shade

            When cotton, in any form, is dyed with a single dye to produce full shade, is known as “Self-Shade”.

Compound Shade

             When cotton, in any form, is dyed with a mixture of two or more dyes to produce full shade, is known as “Compound Shade”.

Solid Shade

            When the warp and weft, both yarns are of the same textile material, then the obtained shade due to dyeing is known as “Solid Shade”

Per cent Shade

            The amount of dye present on the textile material after dyeing is termed as “Percent Shade”. Thus, if 1 g of dye is utilized for 100 g of textile material, then it is said to be 1% shade. This does not suggest that the material has 1 g of dye on it for its 100 g of weight.

Per cent Exhaustion

             At the end of dyeing, the amount of the dye taken up by the textile material from the dye liquor is calculated as “Percent Exhaustion”. Thus, if a dyer wants to produce 1% shade on 200 g fabric, he required 2 g of dye which is the concentration at starting of dyeing. At the end of dyeing, if liquor contains 0.2 g of dye in it. That means 1.8 g dye has been taken up by textile, so the exhausted dye is 90%. As the exhaustion is higher, dye waste is lower.

Material to Liquor (M:L) Ratio

            The requirement of the total quantity of water depends on the weight of the textile material. On the weight of the material, the dyer takes the water. This total quantity of water on the weight of the material is known as “Material to Liquor Ratio”. If dyer is writing 1:30 M:L ratio, means 1 g of textile material requires 30 ml of water to dye it or 1 kg of the same requires 30 kg of water. A low ratio must be preferred as it reduces water consumption.

Standing Bath

              During dyeing, when the exhaustion is very low, a major amount of dye is not used and so it is wasted. Dyer utilizes the same bath again for another lot with the same shade requirement. Such stored dye baths are termed “Standing Bath”. Particularly, this type of bath is used in sulphur, indigo, and an azoic class of dyes.

Cross Dyeing

            When manufactured textile material contains different kinds of fibres, like cotton and polyester blend, then the dyeing is performed with a single bath with the mixture of dyes or with two baths by dyeing each component in a separate bath. This dyeing is termed “Cross Dyeing”.

Reserve Dyeing

            Dyeing of a blend like cotton and polyester is done by reactive dye will result in the dyeing of cotton only, and polyester will be undyed. This happens due to the affinity of reactive dye towards cotton only, not to the polyester. Thus polyester is reserved for dyeing. This operation is known as “Reserve Dyeing”.

Topping

            The dyed fabric is over-dyed with another dye of a different class or same class to obtain a deeper shade or brighter shade for generating a multicolour effect. This over-dyeing process is termed “Topping”. It helps in producing deeper shade by reducing the cost.

Tailing Effect

            The shade of dye during dyeing is weakened on some specific machines. So the shade becomes lighter than the dyed area of the material. So the dyeing results in dark and light shades. This effect is “Tailing Effect”

Stripping

            After dyeing, if the material is uneven dyed or shade is darker than required is achieved, dyestuff present on the material has to be removed completely. Removal of dyestuff is termed as “Stripping”.

Parameters of Dyeing

            Cotton dyeing is complicated chemistry compare to synthetic dyeing. As the natural fibre, cotton has many variables affecting the dyeing procedure. Generally, the dyer tries to exhaust maximum dye onto the material. Such exhaustion is affected by various parameters. Such parameters are:

• Not in Control of Dyers

1. Fibre Shape Factor (S):
2. Ratio of the bimolecular rate constant (R_F)

• In Control of Dyers (Fully/Partially)

1. Dye Dissolution: The dye must be pasted with cold water and then pouring warm water with high-speed stirring. This solution must be prepared before utilizing it in dyeing. The pH of the water should be neutral, and the water must be soft. The recommended level for the water used in dyeing.

Total Hardness	50 – 55 ppm
pH	7.0 + 0.5
Copper	0.05 mg/l
Iron	0.05 mg/l
Chloride ions	300 mg/l

 

2. Exhaustion of Dye: Exhaustion (substantivity) of the dyes affected by dye structure,  concentration of the dye, temperature of the dye bath, and pH. If the dye structure is planner then the exhaustion will be high compare to the complex structure. As the exhaustion is high, migration will decrease, and uneven dyeing increases. So the substantivity between dye and fibres must be proper.
3. Temperature: Temperature affects the dyeing process based on the class of dyes utilized for the dyeing. It depends essentially on the heat of dyeing of various dyes. An increase in the temperature will result in a low substantivity ratio and reactivity will increase which reduces the efficiency. 
4. pH: As dyer change the pH, the substantivity of individual dye is affected. pH profile will decide the fixation which required sufficient time required for reacting the dye with fibre.  
5. Liquor Ratio: M:L ratio is largely under the control of dyer. As the amount of liquor reduces, depth of shade, concentration of dye must increases. So the ratio of substantivity decreases.
6. Electrolyte concentration: The rate of reaction and efficiency increases with the increase in electrolyte concentration. It also affects the reactivity of dye in little amount with precipitation or aggregation. So there must be some limit on the electrolyte concentration.

Classification of Dyes

ð According to Chemical Constitution: The colour index listed several chemical classes. A dye may have a single or multiple functional or chemical groups in its structure. They are unlimited and regularly developed by dye manufacturers. Some reported classes are:

Nitro	Diphenylmethane	Thiazole	Aminoquinone
Nitroso	Triphenylmethane	Indamine	Hydroxyketone
Monoazo	Anthraquinone	Indophenol	Indigoid
Disazo	Xanthene	Azine	Phthalocyanine
Triazo	Acridine	Oxazin	Chorotiazinyl
Polyazo	Quinolone	Sulphur	Vinyl sulphone
Stilbene	Methine	Lactone

 

ð According to Methods of Application: The dyes are classified into two broad categories, viz. readymade and ingrain dyes. Readymade dyes are further classified into water-soluble and insoluble categories. Ingrain dyes are developed on the surface or insitu by coupling intermediate compounds which are not true dyes. Classification is as follows:

 

Mechanism of Dyeing

            The dyeing mechanism deals with various theories, such as kinetic theory, molecular theory, and thermodynamic theory. These theories are utilized for explaining the physic-chemical principle involved in dyeing. So, the mechanism of dyeing is divided into four fundamentals phenomena which are:

1)      Exhaustion: Migration of dye particles from dye solution to the fibre surface.

2)      Adsorption: Dye particles are locking their position on the surface of the fibre.

3)      Absorption: Dye particles travelled/diffused to the inner structure of the fibre from its surface.

4)      Fixation: Dye attached with the fibre and permanently locked its position in the fibre.

 

By keeping all the above parameters in mind, dyes are selected for the dyeing based on:

1)      Compatibility

2)      Consistency

3)      Reproducibility

4)      Right first time