INTRODUCTION:
Dyes are coloured substances which
can adhere to the surface of
materials and are used to give
colour to paper, foodstuffs, and
various textiles such as cotton, wool,
synthetic fibres, silk etc. For
example, alizarin, indigo, Congo red,
etc. Chemically, a dye contains:
Some group (such as azo,
indigoid, triphenylmethyl,
anthraquinone, etc.) which is
responsible for the colour of the
dye.
Some groups (such as –NH2, -
SO3H, -COOH, etc) which makes
the dye stick to the fabric by
formation of some salt.
Dyeing is the process of adding
colour to textile products like fibres,
yarn and fabrics. The temperature
and time controlling are two key
factors in dyeing.
The primary source of dye,
historically has been nature, with the
dyes being extracted from plants
and animals. Since the 18th century,
humans produced artificial dyes to
achieve a broader range of colours
and to render the dyes more stable
to resist washing and general use.
The dyed fabrics appear to be
coloured because a particular dye
absorbs radiations of some specific
wavelengths from the visible region
of electromagnetic radiations which
fall on the surface. The remaining
radiations (complementary colours)
of light are reflected. The colour
which we observe is due to the
reflected light. For example, if a dye
absorbs the light in the wavelength
region corresponding to red, then it
would appear green, which is the
complementary colour of red.
Similarly, if a dye absorbs blue
colour, it would appear orange.
Methods to apply dye:
Dyes are applied to textile goods by
dyeing from dye solutions and by
printing from dye pastes.
Methods include:
1. Direct application
2. Yarn dyeing
Characteristics of a dye:
1. It must have a suitable colour.
2. It must be capable of being fixed
to the material.
3. When fixed it must be fast to
detergents, soaps, water, dry-
cleaning solvents, light and dilute
acids.
Types of dye:
The dyes are classified by dye
manufacturers for marketing into the
following types:
1. Acid dyes:
These are azo dyes and are
characterized by the presence of
acidic groups. The presence of
soluble and serves as the reactive
points for fixing the dye to the fibre.
They are chiefly used for dyeing
wool, silk and nylon. For example,
Orange I and Orange II.
2. Basic dyes:
These dyes contain NH2 or NR2. In
acidic solutions, these form water
soluble cations and use the anionic
sites on the fabric to get used for
dyeing wool, silk and nylon. For
example, aniline yellow, butter
yellow.
3. Direct dyes:
These are also azo dyes and are
used to dye fabrics directly by
placing in aqueous solution of the
dye. These dyes attach to the fabrics
by means of hydrogen bonding.
4. Disperse dyes:
These dyes are applied in the form
of dispersion of minute particles of
the dye in a soap solution in the
presence of phenol or benzoic acid.
These dyes are used to dye rayons,
Dacron, nylon, polyesters etc. For
example, celliton fast pink B and
celliton fast blue B.
5. Fibre reactive dyes:
These dyes are linked to the fibre by
–OH or –NH2 group present on the
fibre. These dyes induce fast colour
on fabrics which is retained for a
longer time. These dyes are used for
dyeing cotton, wool and silk.
6. Insoluble dyes:
These dyes are directly synthesized
on the fibre. The fabric to be
coloured is soaked in an alkaline
solution of phenol and then treated
with a solution of diazotized amine
to produce azo dye. The colour
induced by such dyes is not so fast.
These dyes are used for dyeing of
cotton, silk, polyester nylon, etc. For
example, nitroaniline red.
7. Vat dyes:
These dyes are water-insoluble and
before dyeing these are reduced to
colourless compounds in wooden
vats by alkaline reducing agents.
The fibre is then soaked in the
solution of the dye. Fibre is then
exposed to air or an oxidizing agent.
By doing so the colourless
compound gets reoxidized to
coloured dye on the fabric. For
example, indigo.
8. Mordant dyes:
These dyes are applied after
treating the fabric with precipitates
of certain substances (mordant
material) which then combines with
the dye to form a coloured complex
called lake. Some of the mordants
are salts of aluminium, iron and
tannic acids. Depending on the
mordant used, the same mordant
dye can give different colours and
shades. For example, alizarin gives
red colour with aluminium and black
violet with iron mordant. Mordant
dyes are used for dyeing of wool, silk
and cotton.
OBJECTIVE:
To dye wool and cotton with
malachite green.
REQUIREMENTS:
500 ml beakers, tripod stand, wire
gauze, glass rod, spatula, wool cloth
and cotton cloth.
Chemicals required:
Sodium carbonate, tannic acid,
tartaremetic acid, and malachite
green dye.
PROCEDURE:
1. Preparation of sodium carbonate
solution:
Take about 0.5 g of solid sodium
carbonate and dissolve it in 250 ml
of water.
2. Preparation of tartaremetic
solution:
Take about 0.2 g of tartaremetic and
dissolve it in 100 ml of water by
stirring with the help of glass rod.
3. Preparation of tannic acid
solution:
Take 100 ml of water in a beaker
and add about 1.0 g of tannic acid to
it. Heat the solution. On heating a
clear solution of tannic acid is
obtained.
4. Preparation of dye solution:
Take about 0.1 g of malachite green
dye and add to it 4oo ml of water.
On warming aclear solution of the
dye results.
5. Dyeing of wool:
Take about 200 ml of dye solution
and dip it in the Woolen cloth to be
dyed. Boil the solution for about 2
minutes. After that remove the cloth
and wash it with hot water 3-4 times,
squeeze and keep it for drying.
6. Dyeing of cotton:
Cotton does not absorb malachite
green readily, therefore it requires
the use of a mordant. For dyeing a
cotton cloth dip it in sodium
carbonate solution for about 10
minutes and then rinse with water.
Then put the cloth in hot tannic acid
solution for about 5 minutes. Now
take out the cloth from tannic acid
solution and keep it in tartaremetic
solution for about 5 minutes.
Remove the cloth and squeeze it
with spatula to remove most of the
solution. Now place the cloth in
boiling solution of the dye for about
2 minutes. Remove and wash the
dyed cloth thoroughly with water,
squeeze and keep it for drying.
7. Dyeing of cotton directly:
Take another piece of cotton cloth
and pit it directly into boiling
solution of the dye. Keep it dipped
for about 2 minutes. Remove the
cloth, wash with water, squeeze and
keep it for drying. Compare the
colour of this cloth with that dyed by
using mordant.
OBSERVATIONS:
1. The colour of wool cloth dyed
directly by dipping in hot solution of
malachite green dye is fast.
2. The colour of cotton dyed cloth
directly (without using mordant) by
dipping in hot solution of malachite
green is not so fast to washing and
is of low intensity.
3. The colour of cotton cloth dyed
indirectly by using mordant and then
by dipping in hot solution of
malachite green is fast to washing
and is of high intensity.
BIBLIOGRAPHY:
1. Comprehensive practical
chemistry (class 12)
2. www.wikipedia.org/wiki/Dyeing
