Introduction of Dyeing & Dyeing Process | Types of Dyes | Reactive dyes Disperse dyes
Dyeing is the process of adding color to
textile products like fibers, yarns, and fabrics. Dyeing is normally
done in a special solution containing dyes and particular chemical
material. After dyeing, dye molecules have uncut Chemical bond with
fiber molecules. The temperature and time controlling are two key
factors in dyeing. There are mainly two classes of dye, natural and
man-made.
For most of the thousands of
years in which dyeing has been used by humans to decorate clothing, or
fabrics for other uses, the primary source of dye has been nature, with
the dyes being extracted from animals or plants. In the last 150 years,
humans have produced artificial dyes to achieve a broader range of
colors, and to render the dyes more stable to resist washing and general
use. Different classes of dyes are used for different types of fiber
and at different stages of the textile production process, from loose
fibers through yarn and cloth to completed garments.
Acrylic
fibers are dyed with basic dyes, Nylon and protein fibers such as wool
and silk are dyed with acid dyes, polyester yarn is dyed with disperse
dyes. Cotton is dyed with a range of dye types, including vat dyes, and
modern synthetic reactive and direct dyes.
Yarn dyeing
There
are many forms of yarn dyeing. Common forms are the at package form and
the at hanks form. Cotton yarns are mostly dyed at package form, and
acrylic or wool yarn are dyed at hank form. In the continuous filament
industry, polyester or polyamide yarns are always dyed at package form,
while viscose rayon yarns are partly dyed at hank form because of
technology.
The common dyeing process of cotton yarn with reactive dyes at package form is as follows:
- The raw yarn is wound on a spring tube to achieve a package suitable for dye penetration.
- These softened packages are loaded on a dyeing carrier’s spindle one on another.
- The packages are pressed up to a desired height to achieve suitable density of packing.
- The carrier is loaded on the dyeing machine and the yarn is dyed.
- After dyeing, the packages are unloaded from the carrier into a trolly.
- Now the trolly is taken to hydro extractor where water is removed.
- The packages are hydro extracted to remove the maximum amount of water leaving the desired color into raw yarn.
- The packages are then dried to achieve the final dyed package.
After this process, the dyed yarn packages are packed and delivered.
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A dye is 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 requires 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 more than others. 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 widely 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.
Synthetic dye
The
first human-made (synthetic) organic dye, mauveine, was discovered
serendipitously by William Henry Perkin in 1856. Many thousands of
synthetic dyes have since been prepared.
Synthetic
dyes quickly replaced the traditional natural dyes. They cost less,
they offered a vast range of new colors, and they imparted better
properties to the dyed materials.[5] Dyes are now classified according to how they are used in the dyeing process.
Different types of Dyes
Acid dyes
Acid dyes are
water-soluble anionic dyes that are applied to fibers such as silk,
wool, nylon and modified acrylic fibers using neutral to acid dye baths.
Attachment to the fiber is attributed, at least partly, to salt
formation between anionic groups in the dyes and cationic groups in the
fiber. Acid dyes are not substantive to cellulosic fibers. Most
synthetic food colors fall in this category.
Basic dyes
Direct or substantive dyeing
It is
normally carried out in a neutral or slightly alkaline dyebath, at or
near boiling point, with the addition of either sodium chloride (NaCl)
or sodium sulfate (Na2SO4). Direct dyes are used
on cotton, paper, leather, wool, silk and nylon. They are also used as
pH indicators and as biological stains.
Mordant dyes
Mordant dyes require
a mordant, which improves the fastness of the dye against water, light
and perspiration. The choice of mordant is very important as different
mordants can change the final color significantly. Most natural dyes are
mordant dyes and there is therefore a large literature base describing
dyeing techniques. The most important mordant dyes are the synthetic
mordant dyes, or chrome dyes, used for wool; these comprise some 30% of
dyes used for wool, and are especially useful for black and navy shades.
The mordant, potassium dichromate, is applied as an after-treatment. It
is important to note that many mordants, particularly those in the
heavy metal category, can be hazardous to health and extreme care must
be taken in using them.
Vat dyes
Vat dyes are
essentially insoluble in water and incapable of dyeing fibres directly.
However, reduction in alkaline liquor produces the water soluble alkali
metal salt of the dye, which, in this leuco form, has an affinity for
the textile fibre. Subsequent oxidation reforms the original insoluble
dye. The color of denim is due to indigo, the original vat dye.
Reactive dyes
Reactive dyes utilize
a chromophore attached to a substituent that is capable of directly
reacting with the fibre substrate. 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.
Disperse dyes
Disperse dyes were
originally developed for the dyeing of cellulose acetate, and are water
insoluble. The dyes are finely ground in the presence of a dispersing
agent and sold as a paste, or spray-dried and sold as a powder. Their
main use is to dye polyester but they can also be used to dye nylon,
cellulose triacetate, and acrylic fibres. In some cases, a dyeing
temperature of 130 °C is required, and a pressurised dyebath is used.
The very fine particle size gives a large surface area that aids
dissolution to allow uptake by the fibre. The dyeing rate can be
significantly influenced by the choice of dispersing agent used during
the grinding.
Basic Knowledge of Wet Processing Pdf Ebook Bangla
Azoic dyeing
Azoic dyeing is a
technique in which an insoluble azo dye is produced directly onto or
within the fibre. This is achieved by treating a fibre with both diazoic
and coupling components. With suitable adjustment of dyebath conditions
the two components react to produce the required insoluble azo dye.
This technique of dyeing is unique, in that the final color is
controlled by the choice of the diazoic and coupling components. This
method of dyeing cotton is declining in importance due to the toxic
nature of the chemicals used.
Sulfur dyes
Sulfur dyes are
two part “developed” dyes used to dye cotton with dark colors. The
initial bath imparts a yellow or pale chartreuse color, This is
aftertreated with a sulfur compound in place to produce the dark black
we are familiar with in socks for instance. Sulfur Black 1 is the
largest selling dye by volume.
Food dyes
Food dyes One
other class that describes the role of dyes, rather than their mode of
use, is the food dye. Because food dyes are classed as food additives,
they are manufactured to a higher standard than some industrial dyes.
Food dyes can be direct, mordant and vat dyes, and their use is strictly
controlled by legislation. Many are azo dyes, although anthraquinone
and triphenylmethane compounds are used for colors such as green and
blue. Some naturally-occurring dyes are also used.
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