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.
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
- Fibre Shape
Factor (S):
- Ratio of
the bimolecular rate constant (RF)
- In Control of Dyers (Fully/Partially)
- 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 |
- 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.
- 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.
- 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.
- 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.
- 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
