VAT DYESBY:

IMRAN ANWAR06-NTU-112

Imrananwar_112ntu@yahoo.comEng.imrananwar@gmail.com

NATIONAL TEXTILE UNIVERSITY

FAISALABAD

PAKISTAN

IntroductionThe name vat was derived from the larger

wooden vessel from which vat dyes were first applied .

Vat dyes provide textile materials with the best colour fastness of all the dyes in common use.

Vat dyes: Vat dyes are mostly used for the natural and

man made cellulosic fibres.

Dyeing with vat dyes:The application of vat dyes to cellulosic

material occurs in five stage.Aqueous dispersion VattingAbsorption of dye molecule by fibreRe-oxidation of dye molecules within fibreSoaping-off vat dye.

Aqueous dispersion:The insoluble vat dye is dispersed in water.

Vatting:This step involves the chemical reduction of

the vat dye to produce the soluble, reduced or leuco form of the dye. This achieved by the sodium hydrosulphite, sodium hydroxide and wster. The sodium hydrosulphite chemically reduces the vat dye in the alkaline conditions created by the presences of sodium hydroxide .

Note:The vatting stage also temporarily alters the

original of the dye.

Absorption of dye molecules by the fibre:The vatted dye molecules are substantive to the

cellulosic material when this is introduced into the dye liquor. To achieve adequate exhaustion an electrolyte is added to the dye liquor and temperature may increased depending on the specific vat dye. The application of vat molecules to the fibre occur at temperature specific to a particular vat dye and occur in the rage from 20-60°c.the addition of the electrolyte molecule for the fibre.

During this stage of dye application the textile material must be kept immersed in the dye liquor to prevent premature oxidation of the lecuo compound.

Re-oxidation of dye mole:Once within the polymer system of the lecuo

form of the vat dye has to be oxidised and converted to its original colour and insoluble form of vat dye. Oxidation of the lecuo compound can be achieved by atmospheric oxygen although this is somewhat slow. In practice a mild oxidizing reagent such as sodium perborate is used to convert the soluble lecuo compound into the original insoluble vat dye.

Soaping-off vat dye:During the previous stage some insoluble vat

dye may be deposited on the surface of the textile material. This has to be removed to prevent poor rub-fastness as well as a possible change of shade due to the subsequent removal of this surface deposit. Soaping off which is the boiling of the dyed material in a liquor containing some suitable detergent, remove this surface dye. The term soaping-off was derived from the fact that before the development of detergents, soap was used to remove the surface dye.

Molecular configuration and characteristics:Vat dyes are bases on indigo and

anthraquinon. The excellent fastness properties of the textile materials coloured with vat dyes is attributed in part to the very large sixe of vat dye molecule and in part to its aqueous insolubility. In general, vat dyes based on anthraquinon have better fastness properties than vat dyes derived from indigo.

Solublized forms of vat dyes have been developed (it is the leuco ester part of the dye molecule that is responsible for the aqueous solubility of the vat dye).

Molecular configuration and characteristics:

Vat Dyes:Water insoluble and fast dyes applied along with

strong reducing agents (sodium hydro sulfite) and alkali to make the dye soluble.

The cloth is then exposed to air for oxidation.The excess alkali remaining on the cloth is neutralized

by scouring.Vat dyes have mainly anthraquinone (82%) or

indigoid/thioindigoid (9%) structures, with the former having better fastness properties

Vat dyes are easier to reoxidize than sulfur dyes and the oxygen in air is often the agent used. As would be anticipated, most vat dyes display high wash fastness

Example

Representative Anthraquinone vat dye structures (a) Vat Red 13, (b)

Vat Black 27, (c) Vat Orange 2, (d) Vat

Blue 4, and (e) Vat Green 1.

Dyeing Process:Dissolve dye into water (insoluble dye) Vatting process by reducing the insoluble dye

in alkali condition. (soluble dye) Absorb into fibers (soluble dye) After dyeing, oxidize the dye with oxidizing

agent (insoluble dye) Wash and soap the goods

Recipe of vat dye for 100% cottonDye =........%o.w.f (for exhuste method)Dye =.......g/l(for contineous method)Sodium hydrosulphite(reducing agent)Caustic sodaDispersing agentPeroxide(oxidizing agent)Acetic acid

Dyeing route We adop different route such as;

01

02

03

pad dry steam

pad dry pad steam

themofixpad dry

Dyeing Conditions:Continuous ProcessPad-dry-Pad (Chemical)The solution of the dye is prepared without

adding reducing agent to ensure levelling dyeing. After that, the goods are passed to chemical bath to reduce the dye into soluble dye and fix within the goods. Oxidizing agent is added to the goods and converted to insoluble dyes.

Dyeing Conditions:Pad-oxidize-pad-oxidize (many times)This will put the goods into the dye solution

and squeeze with 2 rubber rollers to get rid of excess water, oxidize with the air and then immerse into the dye solution again and again to allow the dye penetrate into the goods.

Properties of vat dyes:Light Fastness:Light fastness rating of vat dyes about 7. the

excellent light fastness of textiles coloured with vat dyes is attributed to the stable electron arrangement in the chromospheres of the vat dye molecule. The presence of the numerous benzene rings contributes to the stability of the vat dye molecule.

Properties of vat dyes:Wash Fastness:The wash fastness rating of vat dyes is about 4-

5. the excellent wash fastness of textile materials with vat dyes is attributed to the large vat dye molecules as well as its aqueous insolubility. The large vat dye molecule is trapped within the polymer system of the fiber because of its size and aqueous insolubility and is absorbed within the fiber polymer system by van der wall forces.

Textile Auxiliaries:Salt - reduce the negative charges on the fibersSodium hydroxide - adjust pH to the dyeing

bath and make the suitable conditions for reducing agent.

Sodium hydrosulfite - reducing agent for the vat dyes.

Water - dyeing mediaOxidizing agents - (Hydrogen peroxide or Acetic

acid or Air) oxidize the soluble to insoluble dye