Why So Called Vat Dye? | Classification of Vat Dyes | Application of Vat Dyes in Textile

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.

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 
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.
  • 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)

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:

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

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)

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Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company as a country agent. He is also a contributor of Wikipedia.

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