The molecule attracts to the positive part of fabric

The chemistry of dyeing is highly
complex, and intricate. There are many steps of production, and each is crucial
for the final colour to emerge with and retain full intensity.

A dye is an organic compound, which can
be a natural or synthetic substance,  that is used to add or change the colour of
something. In the past, mostly all dyes have come from natural sources such as
plants and animals. More recently, chemists have begun to replicate these
colours found in nature, to synthetic dyes. These synthetic dyes are generally more
intensely coloured, and have better colour fastness.

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Molecular Structure of a Dye:

A dye has 2 main parts. The first one is
a chromophore. Each dye has one chromophore. It’s a group of atoms that control
the colour of the dye and this is their main function. They hold on tightly to
their electrons.

Most dyes also contain 2 auxochromes.
These are used to intensify the colour and to form chemical bonds with the
fibres, so that the dye particles can attach to the fibre. They give away their
electrons. Chromophores and auxochromes are connected by a conjugated system.

Types of Dyes:

There
are four main types of dyes. Direct dyes form chemical bonds with fabric. You
immerse the fabric in a solution and following there is an “uptake” where the
dye molecules attach to the fabric molecules. Due to “polar” structure of dye
molecule and fabric molecule, sections of the molecules can have a slightly positive
or negative charge. The negative part of dye molecule attracts to the positive
part of fabric molecule. There is no sharing of electrons, and there is a
weaker bond called “Van Der Waals”, and the color can wash away easily. An
example of a direct dye, would be tie dye. The second type of dye is a
dispersed dye. They are unique in the sense that they are the only insoluble
dyes, meaning they are not soluable in water. Disperse dyes, are most effective
at dying polyester. The molecules that make up disperse dyes are the smallest
of all the dye molecules. These dyes are mostly used with dye bath solutions,
and to ensure that the process goes smoothly, dispersing products are used to
intensify the water soluability of the dye. The third type of dye is a fibre
reactive dye. These dyes can react chemically to create covalent bonds. These
are the strongest dyes. The actual chemical reaction can strip the electrons or
protons from fabrics. This process opens up sites for new bonds, between the
dye molecule and the fabric. This reaction produces very strong, covalent
bonds.

The original fibre-reactive dyes
were created for cellulose fibres, and until today they are mostly used for
that purpose. Fibre-reactive dyes have been anticipated for a long while, and weren’t
really created until 1954. Before that, they tried many times to react the dye
and fibre, but it only resulted in the material being ruined. The fourth type of dye is the vat dyes. In vat dyes,
the colour is formed within the fibre. The colour does not appear unless it’s
given the proper treatments. An example of a vat dye is indigo. Another example
would be tyrian purple. This colour doesn’t form until the dye is exposed to
air and sunlight.

First Synthetic Dye:

The first synthetic dye was created by,
William Henry Perkins. It was actually discovered by mistake. Perkins was
trying to synthesize quinine, to attempt to find a cure for Malaria, he was
using coal tars and when he heated it, it became a black gunk and it didn’t
absorb water. He dissolved that in alcohol and discovered the first synthetic
dye – purple. This colour is called “mauveine”, or Perkins violet.  He went into mass production funded by his
family. He built a factory to synthesize Perkin’s violet and other dyes on a
large scale.

Types
of Fabrics used in Dyeing:

            Silk/wool = proteins, amino acids.
Ionic groups, bonding works super well

Cotton
– cellulose, hydroxyl group, polar

Polyethelyne
(CH2) not receptive to dyes,

 Ways to Dye Material:

1) Aqueous
or conventional dying – most important and most widely used method. Process is
use of dyestuffs. Treatment of textile material in aqueous water solutions

2)            
Solution dyeing
– this is part of manufactured fibre production. It involves adding micro sized
colored pigments to man made fibre during manufacturing.

A Useful Dye Should Include the
Following:

The dye should have an intense colour (how the dye imparts colour to
textile materials). Its solubility in water is crucial. This is talking about during
the process of dyeing, so that water can carry dye particles towards inner molecular
structure of fibre. Another important element is the substantivity to fiber –
how the dye molecule will penetrate the inner molecular structure. The dyes durability
to wet treatments is another important part. Once the dye is penetrated is
inside fibre molecular structure, the kind of chemical bond takes place in
between fibre and dye molecules is important. Meaning, the colourfastness and property
of dye, STRONG = record durability to further treatment. Like washing. Safe,
easy to handle, and reasonably priced.

Why Dyeing With and Without Mordants Produces
a Different Colour:

            A
mordant is basically glue for the dye. It’s a chemical that attaches itself to
the molecular bond, between the dye and the fibre. Using a mordant is crucial
in achieving the desired colour result when dyeing. If a fabric is dyed without
a mordant, the colour will eventually fade with each wash. Soaking the fibre in
water with a mordant, will ensure that the colour stays intense even with
washing. Without a mordant, the colour will be very dull, and wash out easily.
With a mordant, the colour will be intense, and will stay attached to fibre
even after washing.

Why Varying Acidity of Dyebaths Affects
Final Colour:

            Whereas
most dyes dye better in alkaline baths, acid dyes bond better in acidic
dyebaths. Acidic dyes are highly soluable in water, and they donate their H+
(hydrogen). The solution is acidic so there are lots of positive charges. Acid
dyes are used to dye certain protein fibres. This includes, feathers, silk and
wool. The reason they are called “acid” dyes, is because a very light acid such
as lemon juice or vinegar is added to decrease the pH of the dyebath, making it
a little acidic, thereby causing the dye to make bonds with the protein fibres.
The varying acidity levels, may affect the final colour. In part 1 of the
assignment, the pieces of fabric with lemon juice added came out much lighter
than those without an acid used.

            In
conclusion, dyeing is a very useful process, and has made many advances in
recent years. The basic process of dyeing is central and it is heavily relied
in our everyday lives, without even realizing it.