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.
