Sunday, September 22, 2013

Classification of vat colorants and principles of vat dye applications

Vat dye is an ancient class of natural coloring agents which is used to dye cotton, wool, and synthetics as well as other fabrics. Vat dyes have a Quinanone ring chemical structure in every single molecule.
Properties of theses coloring agents are:
  • Insoluble in water
  • Due to their insolubility feature, they can be used directly on color textile materials
  • The process of coloring is difficult
  • Various color shades can be induced
  • Vat dyes are quick to wash
Principles of vat colorants application:
  • Vatting is a process in which an insoluble commercial coloring agent is reduced. Chemicals like Sodium hydrosulphite and sodium hydroxide are used for vatting.
  • Dyeing is a process in which the desired colors are induced on fabrics of different types. During coloring, the soluble sodium salt of the vat coloring agents are absorbed by the fabric.
  • Oxidation is a process in which the soluble salt of the colorant is absorbed by the textile, and recovered into insoluble form by airing.
  • Soaping is a process in which the colored fabric is boiled in soap and detergent solution, to induce the proper shade and to get color fastness.
Grouping of vat coloring substances:
There is no single classification for quinone vat colorants. According to their properties and considerations, such as Vatting Temperature, Redox potential, Leuco solubility, Time etc., these colorants can be categorized into four groups which are; IN, IN special, IW and IK.

Lets’ have a brief understanding about these four groups:

IN, IN Special Group:

The IN and IN special group of Vat colorants, have high exhaustion features. Due to this property, these dyes do not require the addition of salts. They induce attractive and bright shades with color fastness characteristics. Due to its high exhaustion properties vat dyes show high strikes. This group of colorants requires leveling agents, and protective colloids for producing even colors on the substrate.
The shades like green and violet belong to the IN group, and shades of black belong to IN Special category. This group requires high alkalinity and high temperatures during dyeing.

IW group:

The browns and olives come under the IW category of vat dye groups. It is a property of the IW group coloring substances, which do not fully exhaust. It is suggested to add salts especially if the M to L ratio in the colorants is high. The coloring behavior of the colors that comes under IW category is controllable which can help induce even shades. The colors that fall under this group have slower oxidation features, than the colorants of IN group.
The alkalinity of IW colors is brought down when oxidized. In dark colors shades like olives and browns, high alkalinity results with oxidation patches on the fabric. Generally, the fabric goes under bicarbonate treatment after coloring and before oxidation, it results in an effective oxidation bath.

IK Group:

This group of colors has lower exhaustion characteristics, and needs less temperature during the coloring process.