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Vat Yellow-molecular structure.

The practical details of using vat dyes in both batch and continuous processes are covered in this article. Vat dyes generally offer superior fastness properties. Potential draw backs to selecting vat dyes include a tendency for photo tendering and a limited colour gamut. bases on the substantivities of leuco vat anions, the traditional classification system for vat dyes offers batch dyers a better means for selecting vat dyes to be used  successfully in combinations. Leuco vat dyeing, pre-pigmentation, semi pigmentation and batch after treatments are covered in this article. While continuous dyeing with vat offers economic advantages, several precautions are required to ensure a satisfactory result. The chemical structure and commercial forms of vat dyes are also covered.

Vat dye classification: The products in all dye application classes may be differentiated in several ways. e.g., by their chemical natures, by the fastness properties of the dying ( and consequently, their end uses), by their dyeing characteristics, or even by their colour. Because vat dyes are sold as pigments. It is important to be aware that the dissolution and other dyeing related properties can be affected by the particle size distribution of the products.

Vat dye synthesis: Vat dyes are difficult chemicals to make and their structures are complex. Some of these will be shown the sub-heading vat dyes, The different chemical structures affect the solubility of the sodium leuco vat, its stability towards over reduction and over oxidation, its substantivity, its rate of diffusion in to and out of the fiber ad the ultimate properties of the dyeing. But But these application properties cannot be readily deduced from the dye structures alone, classification by structure will not generally be of much help in the dye house, although in indanthrene are susceptible to over reduction and over oxidation.

Fastness properties: In exchange of their cost, vat dyes can deliver dyeing with highest levels of washing and wet fastness properties. Indigo, as you are aware, is a typical in this regard. For staining of adjacent cotton goods and shade changes on washing, vat dyeing are rarely rated less than 4-5 on a scale of 5.

Fastness to industrial laundering with hypochlorite and towards peroxide and hypochlorite bleaching are generally outstanding, as are the fastness properties towards sunlight, weathering and perspiration. Even the paler shades of vat dyes are generally have high light fastness ratings, in the order of 5-6 on a scale of 8 and 7-8 at heavy depths. It is the limited variability of these ratings, as well as their high values, which distinguishes vat dyeing from direct dyeing. However some combinations of yellow and blue or green vat dyes do show anomalous behaviors in which the blue or green components is lost more readily on exposure to light than expected. This is often referred to as catalytic fading, and is, unfortunately, not very predictable.

Another occasional problem is photo tendering, in which some vat dyes, such as Vat Orange 2, are partly reduced under the influence of light. In the dark, in the presence of atmospheric oxygen and the cellulose, they are regenerated; but oxidative tendering of the fiber by peroxides formed as by-products.

In spite of some drawbacks, which can be circumvented by proper dye selection, vat dyeing have several good properties which make them very difficult to overlook for dyeing some industrial yarns, terry cloth, industrial work clothing and shirting fabrics. They are the only class of dyes which does not suffer severe shade changes during the application of topical, cellulose reactive, flame retardant finishes.

Color Characteristics: All surface colors can be characterized in terms three parameters - lightness (or darkness), chroma and hue. Chroma is also called colorfulness, saturation or brightness ( or dullness). We will use the word brightness here, even though it is not the ideal synonym and not to be confused with lightness. Hue is the property of appearing red, yellow, green or blue or something in between.

The colour distribution of vat dyes available in U.S. is as follows: 25% blue, 18% brown, 14% black and olive, 11% orange, 10% red, 10% green, 6% violet and 6% yellow. There is good palette for blue, green and earth tones of very good fastness, with similar dye cost to normal fiber reactive dyes. The range of colors, which can be dyed with any one application class of dyes is known as colour gamut, which is a column of colour space within which these dyes are useful. The colour gamut of vat dyes is restricted by the absence of very bright colors, particularly in the red and turquoise hue areas, and only sulfur and azoic combinations have smaller colour gamut.
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