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Dyeing:

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.

The primary source of dye, historically, has generally been nature, with the dyes being

extracted from animals or plants. Since the mid-18th century, however, 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.

Vat Dye

Vat dyes are a class of dyes that are classified as such because of the method by which they

are applied. Vat dyeing is a process that refers to dyeing that takes place in a bucket or vat.

Almost any dye, including fiber-reactive dyes, direct dyes, and acid dyes, can be used in a vat

dye. Cotton, wool, and other fibers can be all dyed with vat dyes. The original vat dye

is indigo, once obtained from plants but now produced synthetically

Why So Called Vat Dye?

The word vat means, ‘Vessel’. The dyes take their name from vatting. The vat dyes are naturally coloring matter and kept in wooden vat and make solubilise vat dyes by the process of formantation – so it is called vat dyes. They are applied in a special kind of a dyebath in which the dye is reduced to a soluble form by means of a strong reducing agent, such as hydrosulphite. The vat dyes are insoluble and cannot be used directly and requires vatting. Among all the dyes, it has the best fastness properties.History

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The vat dye is found amongst the oldest natural coloring matters used for textiles. Indigo

has been known in India since the earliest periods of which historical records exist.

According to the writings of Julius Caesar the ancient Britons used Wood to stain their

Bodies and faces and Tyrian purple was exported from Tyre to the Mediterranean countries

nearly 4000 years ago. Tyrian purple is extracted from a shell fish and is therefore of animal

origin but woad and indigo exist in plants, combined with glucose in the form of glucosides

the vat dyes are all insoluble in water and cannot be used for dyeing without modification

when treated with reducing agents they are converted into leuco compounds all of which

are soluble in water in the presence of alkalis. These leuco compounds are substantive

towards cellulose and reoxidize to the insoluble coloured pigment within the fibre when

exposed to air. The leuco compounds are often colourless or of quite a different colour from

the product of oxidation.

Vat dyes so named: the word vat means vessel. The dye takes their generic name from

vatting , the vat dyes are naturally obtained colouring matter from the ancient time and kept

into wooden vat and make solubilise in vat by the process of fermentation – so it is called

vat dye.

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Materials suited for vat dying

Although almost all dyeing can be done in a vat, the term vat dye is used to describe a

chemical class of dyes that are applied to cellulosic fibre (i.e. cotton) using a redox reaction

as described below. Because of the use of caustic soda, and the very high pH of the dye bath

in the dyeing process, wool cannot be dyed using vat dyestuffs. Wool is soluble in caustic

soda solutions. It is possible to dye wool at room temperatures with indigo (vat blue 1) and

other low substantive vat dyes using soda ash as the alkali source with very little strength

loss. Vat red 10, vat violet 13 and vat orange 1 can be applied in this manner as well.

Light-oxidized vat dyes

Inkodye is a type of vat dye that uses light rather than oxygen to "fix" the dye, with a wide

variety of possible effects. These dyes, which are chemically similar to vat dyes, are

developed by light instead of being applied in an oxygen-free bath and being developed in

the fabric by exposure to oxygen. Inkodyes are true dyes, not fabric paints. A dye itself

attaches to the fabric; fabric paint includes a glue-like binder, which imparts a stiffer feeling

to the fabric. The process is more difficult than tie-dyeing.

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Chemical structures

Many vat dyes are derivatives of indigo dye. For example, vat blue 2 and 3 are halogenated

or methylated derivatives and so are several violets. Many other vat dyes are derivatives

of anthroquinones.

Blue vat dyes

Vat Blue 1, commonly known asindigo dye

Vat Blue 4

Vat Blue 20, aka Violanthrone

Vat Blue 21

Vat Blue 25

Vat Blue 26

Vat Blue 30

Vat Blue 36

Vat Blue 64

Green vat dyes

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Vat Green 1

Vat Green 3

Vat Green 8

Vat Green 9

Vat Green 11

Vat Green 12

Orange vat dyes

Vat Orange 1

Vat Orange 2

Vat Orange 3

Vat Orange 9

Vat Orange 15

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Vat Orange 17

Violet vat dyes

Vat violet 15

Vat Violet 18

Red vat dyes

Vat Red 10

Vat Red 13

Vat Red 18

Vat Red 28

Vat Red 28

Vat Red 29

Brown vat dyes

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Vat Brown 1

Vat Brown 3

Vat Brown 45

Black vat dyes

Vat Black 25

Vat Black 27

Vat Black 29

Yellow vat dyes

Vat Yellow 1

Vat Yellow 4

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Vat Yellow 10

Vat Yellow 12

Vat Yellow 20

Vat Yellow 28

Classification of Vat Dyes

The vat dyes are to be divided into several groups, depending upon their chemical nature

and origin, as follows:

(a) Indigo, including both natural and Synthetic

(b) Thio-indigo dyes, containing sulphur.

(c) Indigo derivatives, such as the brom-indigos; usually not derived directly from indigo

itself, but built up synthetically.

(d) Anthraquinone derivatives, including the various Indanthrene, Marione, Algol dyes, some

Helindone, and others.

(e) Carbazol derivatives, of which Hydron Blue is the chief representative

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Properties:

Here are some highlights of the common properties of Vat dye:

Superior color fastness properties, which are exceptional in other dye classes

Needs vatting for application

Water insoluble dyes

A number of colors can be produced

Rubbing fastness is not good, but is can be easily lessened with special treatments to

the fabric

Especially used for cellulose fiber with PH (measure of hydrogen ion concentration)

control.

Classification of Vat Dyes depend on properties:

Depending upon the properties one popular classification of vat dyeing substances has been

developed which is given below:

The 1N (indanthrene normal)-group of dyeing temperatures of 600C. In such dyeing

substances no salt is added to the dye bath because they have high portion of leuco

dyes for cotton.

The 1W (indanthrene Warm) - group of dyeing temperatures of 500C. In such dyeing

substances some addition of salt is required so to aid exhaustion.

The IK- group of dyeing substances requires low concentration of NaOH with dyeing

temperature of 200C.

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Equipment and Chemicals:

Bleached 100% cotton ...................(8" x 24")

C. I. Vat Green 1........................... (20 g/L) Indanthrene Green FFG

Dispersing agent ............................(10 g/L)

Alginate NVS ................................(sodium alginate, 1 g/L)

Soap solution .................................(50 g/L)

NaOH solution ..............................(100 g/L)

Sodium hydrosulfite (hydros).......... (100 g/L)

Sodium chloride solution .................(100 g/L)

Application of Vat Dye

Stages in Dying :

(i)Vatting

(ii) Dying

(iii) Oxidation

Theory of Vat Dyeing:

The simplest arrangement for dyeing fabric is to pull the textile material through

the dyebath so that the dye can exhaust on to the fabric surface. Low liquor ratios and

the addition of common salt or Glauber's salt both promote such exhaustion. In some

cases, the addition of acid also promotes exhaustion. If the dye is only partially soluble in

water and likely to be exhausted unevenly, the addition of soap or sodium carbonate may

promote leveling. A dyeing is considered to be level if all parts of all fibers have

been penetrated evenly and completely. Machines for this type of dyeing are called

batch machines.

Package and beam machines for yarns.

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Jigs for open width fabrics

Winches for woven and knitted fabrics in rope form

Jet dyeing machines for knitted fabrics in rope form.

Paddle machines for sewn products like bedspreads

Smith drums for nylon hosiery or special machines for nylon hosiery

If a dye is not soluble in water, as is the case with vats, it may be applied to the fabric as

a dispersion by a padder. Once the insoluble vat dye has been uniformly applied to

the fabric surface, usually with the aid of special dispersing agents (detergents), it can

be solubilized by reaction with a reducing agent, e.g., sodium hydrosulfite

("hydros", Na2S2O4) in dilute NaOH. Once it has been converted to its soluble (LEUCO)

form, the vat dye can penetrate into the cotton fibers. After adequate time for penetration

to occur, the fabric is withdrawn from the bath and oxidized by air or an oxidizing agent

such as sodium perborate or hydrogen peroxide. This process is schematically represented

below.

pad hydros

Vat dye (insol) → on fabric surface → LEUCO form (soluble)

NaOH

Before chemical reducing agents were readily available, vat dyes were converted to

their soluble leuco form by fermentation of organic matter in wood tubs called vats.

This method of reduction and application is the source of the name for this class of

dyes. Once the vat dyes have been regenerated inside the fiber, they are very insoluble.

This accounts for their excellent wash fastness. Because they can be applied as a dispersion

by padding, solubilized by reduction, and finally reoxidized when inside the fibers, vats are

well- suited to continuous dyeing operations. Such treatments exhibit a number of

advantages:

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a) Very efficient use of the dye. All that is made up can be applied.

b) The insoluble vat is very evenly distributed over the fabric surface, leading to level

dyeings

c) Continuous processes are normally more economical processes than batch processes

Problems with anthraquinone dyes

A number if chemical problems aruse with some quinone vat dyes . these include:

1. Multiple reduction steps for poluquinones such as iudanthrone;

2. Isomerusm of leuco compounds to oxanthrones;

3. Hydrolysis of amide groups;

4. Over-oxidation after dyeing;

5. Dehalogenation of some dyes,

To minimize these types of problems , the supplier’s recommendations for vatting and

dyeing must be followed.

Indanthrone (Cl var blue 4) and some of its derivatives show a number of these problems

indanthrone has two anthraquinoe residues in its molecule. The normal blue leuco

compound used in dyeing is that corresponding to the reduction of one of the

anthraquinone groups (5, in figure 17.5). If both anthraquinone groups are reduced. The

final product (6) gives a brownish yellow solution. Has poor substantivity for cotton and is

more difficult to oxidize. Such over-reduction produces duller blue dyeing of lower colour

yield.

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Referneces:

http://royceintl.jimdo.com/2013/02/05/vat-dyes-properties-classification-much-more/

http://textilelearner.blogspot.com/2012/03/why-so-called-vat-dye-classification-

of.html#ixzz37ArEfGAm

http://textilelibrary.blogspot.com/2009/02/vat-dye.html

http://en.wikipedia.org/wiki/Vat_dye

http://en.wikipedia.org/wiki/Dyeing