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Characteristics:
These dyes derived their name from their insoluble aqueous properties and the need to apply them from an aqueous dispersion.
While they are not soluble in water, they are supplied in a finely ground form that will disperse in water.
Disperse dyes have the smallest molecules of all the common dye classes. A feature of disperse dye molecule is their lack of polar groups, evidenced by the insolubility of disperse dyes.
Suitability to textiles:
- These dyes were developed originally for dyeing acetate fibres and have since found wide use in dyeing triacetate, manmade ester-cellulose and synthetic fibres like, polyester, modacrylic and olefin as well as cellulose fibres.
- Acrylics and nylon are dyed with disperse dyes less oftenly.
- These dyes have wide use in apparel, decorative fabrics used for dyeing and printing.
Method of application:
To form an aqueous dispersion disperse dyes are added to water with a surface active agent. The insolubility of disperse dyes enable them to leave the dye liquor as they are more substantive to the organic fibre than to the relatively inorganic aqueous dye liquor. The application of heat increases the energy of the dye molecule which accelerates the dyeing of textile fibre. Heating the dye liquor swells the fibre to an extent and assists the dye penetration in to fibre polymer system which results the dye to be located in the amorphous region of the fibre. Once the dye enters the fibre polymer system the dye molecules are held by hydrogen bonds and Vander walls forces.
It is difficult to obtain medium or dark shades on polyester fibres even by dyeing at the boil because polyester fibres are extremely crystalline and hydrophobic. For this polyester fibres are dyed using carriers or by using high temperature dyeing techniques. By using carriers, they swell the polyester fibre polymer and in so doing allow the disperse dye molecule to enter the polyester fibre more readily.
By using high temperature dyeing technique, this is carried out at temperatures above the boil (in the range 100-1300c) and under pressure ranging from 0 to 170 kpa. This method of dyeing is also called pressure dyeing: it is generally used for the highly crystalline synthetic fibres or for fibre blends containing their fibres. This method eliminates the need for carriers which add extra cost.
Fastness properties:
- Light fastness of textile materials which have been coloured with disperse dyes have a fair to good light fastness rating about 4-5. Any significant loss of colour occurs in a disperse coloured textile material is only with prolonged exposure to the U.V component of the sun light.
- Wash fastness of textile materials dyed with disperse dyes have moderate to good wash fastness rating from 3-4.
- Gas fading can be observed in the textile materials dyed with certain blue and violet disperse dyes in the presence of nitrous oxide.
- Disperse dyes have the ability to undergo sublimation; that is, they can be vaporized without a significant change in the colour, light and wash fastness. This property is used as an advantage in the transfer printing.
- The ability of the disperse dyes to undergo sublimation may also be a disadvantage. Excessive hot pressing or ironing of disperses dyed or printed textiles may result in colour loss because of sublimation. Fading is apparent after the application of the heat.
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