Vat Dyes:
Vat dyes have been used for many decades to color cotton and other cellulose fibers. Despite their high cost and their muted colors, these dyes are extremely important for certain textiles because of their superior fastness. Vat dyes are insoluble and cannot be used directly on textile materials and requires vatting. Vat dye mainly used for cellulose fiber dyeing but in protein fiber dyeing pH should be controlled. Dyeing process is difficult from other dye. Among all the dyes, vat dye has the best fastness properties though rubbing fastness is not good. In this article I will discuss mechanism of dyeing with vat dyes and I also focus different application methods of vat dyes and uses.
Stages or Chemistry or Mechanism of Dyeing with Vat Dyes:
Generally, the application of vat dyes to textile materials involves five distinct steps.
a) Aqueous dispersion: The insoluble vat dyes, in this stage dispersed in water.
b) Vatting: Vatting in which the insoluble commercial dye is reduced and solidities (vatted) by using Sodium hydrosulphite (hydrose) and Sodium hydroxide (NaOH). Sodium hydrosulphite work as a reducing agent. Again salt formation by neutralising their hydroxide to give a water soluble product. Sodium hydroxide is used as a solubilising agent.
c) Dye absorption: Dyeing, in which the soluble sodium salt of the leuco vat dye is absorbed by the textile material from an alkaline reducing medium in the presence of either a retarding agent or an exhausting agent depending on the rate of dyeing. In this stage, the textile material must be immersed in dye liquor to prevent oxidation of leuco compound.
d) Reoxidation of vat dye: Oxidation, in which the soluble form of the dye absorbed by the fiber, is reconverted into the original insoluble dye by atmospheric oxygen (Airing) or by the use of “chemical oxidation.” (i.e. involving the use of a chemical like sodium per borate or potassium dichromate or Hydrogen per oxide).
e) After treatment: During the previous stage some insoluble vat dye may be disposited on the surface of the textile material. Soaping – off, in which the dyed material is subjected to a treatment either boiling soap or other detergent solution in order to get the proper tone by way of aggregation of smaller dye particles into bigger ones and also to get the optimum fastness, especially rubbing fastness by removing the surface deposited dye particles.
Vat Dye Application Methods: Vat dyes can be applied to cotton in almost any stage of manufacture, such as:
(a) Batch-wise exhaust dyeing process: Yarn in hank or package form, jigger dyeing, jet dyeing, soft flow dyeing machine are recommended for fabric dyeing.
(b) Semi-continuous dyeing process: It is well known that the usual method of vat dyeing in jigger by exhaust process is not satisfactory for thick fabric and also due to the consumption of higher amount of chemicals. It is therefore necessary to modify the method of application as below:
Pad-jig process (Pigmentation)
Vat acid process
Cold pad-batch process
(c) Continuous dyeing process: Various other suggestions for the continuous dyeing of cellulosic fabrics with vat dyes have been used. Continuous dyeing is economical particularly where long yardages have to be dyed in the same color. Following are the various applications:
Pad-steam process
Wet-steam process
Pad-dry-chemical pad process
Uses of Vat Dyes: Vat dyes are ideal for colored woven goods such as handkerchiefs, shirtings and toweling. They are used for all outlets where the highest level of fastness is required such as:
Work wear and uniforms for armed forces, police, fire, nursing services, etc., which are subjected to severe laundry washing and occasional bleaching with hypochlorite and long exposure to sunlight.
Clothing and household articles such as shirtings, sportswear, toweling, bed sheets, tablecloths.
Furnishing requiring high fastness to light.
Outdoor fabrics requiring high weather fastness such as parasols, tenting, tarpaulins.
Yarns like sewing threads and for colored threads for weaving which are subsequently piece – bleached with hypochlorite.
Yarns used for border of dhoti, towel, etc.
Shirting fabrics with white stripe go to mercerizing in fabric stage after weaving with dyed and grey yarn.
References:
Textile Dyes by N. N. Mahapatra
Industrial Dyes: Chemistry, Properties, Applications Edited by Klaus Hunger
Textile Chemistry by Thomas Bechtold, Tung Pham
Cellulosics Dyeing Edited by John Shore
Textile Engineering-An Introduction Edited by Yasir Nawab
Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.
