Page 96 - Shawl Weaving Artisan - TT

2 Selection of brand: Brand selection is important. It Acid dyes are water soluble, and have better fastness
may be to light than basic dyes. The acid dyes are widely
used for natural protein fibers such as silk, wool and
a Hot brand – Less reactive dye (temp 72-93)
synthetic polyamides and modified acid dyeable acrylics.
b Medium brand – Medium reactive dye. They contain sulphonic acid groups, which are usually
c Cold brand – Most reactive dye. present as sodium salts of sulphonate acids. These acid
groups increases solubility in water, and gives the dye
3 Economy of production molecules a negative charge. In an acidic solution, the
4 Availability of dyes -NH2 functionalities in the fibres are protonated to give a
positive charge -NH3+. These charges interact with the
5 Storage of dyes. negative dye charge of the dyes, allowing the formation
6 Bond stability i.e. kind of bonding of ionic interactions. Van-der-Waals bonds, dipolar bonds
and hydrogen bonds are also formed between dye and
7 Fastness of dye i.e. washes, light, rubbing fastness fibre apart from ionic bonding.
8 Re – producibility Classification of acid dyes

Dyeing mechanism of reactive dye Acid dyes are classified based on the following
The dyeing mechanism of material with reactive dye 1 Chemical structure
takes place in 3 stages 2 Levelling properties

1 Exhaustion of dye in presence of electrolyte or dye 3 Dyeing characteristics
absorption. Acid dyes are normally have large aromatic molecules
2 Fixation under the influence of alkali. and have very complex structure, having a sulphonyl or
amino group which makes them soluble in water. Most of
3 Wash-off the unfixed dye from material surface.
the acid dyes belongs to following three main structural
Now they are mentioned below: molecules.
1 Dye absorption: When fibre is immersed in dye 1 Anthraquinon type
liquor, an electrolyte is added to assist the exhaustion 2 Azo dye type
of dye. Here NaCl is used as the electrolyte. This
electrolyte neutralize the negative charge formed in 3 Triphenylmethane type.
the fibre surface and puts extra energy to increase dye Classification based on the levelling properties: The
absorption. So when the textile material is introduces acid dyes are classified into several groups, based on the
to dye liquor the dye is exhausted on to the fibre. levelling properties, economy of the dyeing and fastness
properties, however generally these are classified into
2 Fixation: Fixation of dye means the reaction of these three classes.
reactive group of dye with terminal –OH or-NH2 group
of fibre and thus forming strong covalent bond with the 1 Neutral acid dyes: Neutral acid dyes are supra milling
fibre and thus forming strong covalent bond with the or fast acid dyes, many of these dyes are used for
fibre. This is an important phase, which is controlled self shades only. These dyes are applied primarily
by maintaining proper pH by adding alkali. The alkali with a weak acid or at neutral pH on the fabrics.
used for this purpose depends on brand of dye and These dyes possess medium to good wet fastness
dyeing temperature. Here generally caustic soda, properties however , some dyes exhibit poor light
soda ash or NaHCO3 is used as alkali depending fastness properties in pale shades.
upon reactivity of dye. They create proper pH in dye 2 Weak acid dyes: A weak acid dye belongs to the
bath and do as the dye-fixing agent. category of milling class of dyes. These dyes possess
3 Wash-off: As the dyeing is completed, a good wash good wet fastness properties but moderate to poor
must be applied to the material to remove extra and light fastness
unfixed dyes from material surface. This is necessary 3 Strong acid dyes: As the name implies these dyes
for level dyeing and good wash-fastness. It is done are applied in a strong acidic medium and also called
by a series of hot wash, cold wash and soap solution as levelling dyes, however has a limited wet fastness
wash. property. These dyes are very good to produce the
Acid dyes combination shades.
Classification according to dyeing characteristics:
Acid dyes are a class of dyes used to dye wool, silk Acid dyes are commonly classified according to their
and nylon fibres. Acid dyes consist of the sodium salts dyeing behaviour, especially in relation to the dyeing pH,
of sulphonic acids. The acid dyes can be applied easily their migration ability during dyeing and their washing
on the textile material without the application of mordant fastness. The molecular weight and the degree of
that are used as fixing agents. The acid dyes are rarely sulphonation of the dye molecule determine these dyeing
used for the dyeing of the cellulosic fibres, when used for characteristics. The original classification of this type,
this purpose, a basic mordant such as alum is required. based on their behaviour in wool dyeing, is as follows:
Nitrophenols also belong to acid dyes.
86 Textile & Handloom: Shawl Weaving Artisan : (NSQF - 2024) R.T. Ex.No 1.6.55-65