What`s it all about?
classification of dyesM
Dyes can be classified according to their chemical structure or according to their use. However, classifications vary from agency to agency though there are some fundamental categories that are common to all.
According to the CPCB, there are approximately a million known dyes and dye intermediates out of which about 5,000 are produced commercially. Based on their use-based application, the dyes are divided into 15 groups:
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pigments
“Pigments,” defines the Coloured Pigment Manufacturers Association of the US, “are coloured, black, white or fluorescent particulate organic or inorganic solids which usually are insoluble in, and essentially physically and chemically unaffected by, the vehicle or substrate in which they are incorporated. They alter appearance by selective absorption and/ or by scattering light. Pigments are usually dispersed in vehicles or substrates for application, for instance in inks, paints, plastics or other polymeric materials. Pigments retain a crystal or particulate structure throughout the colouration process.”
Pigments differ from dyes in that they:
• Remain insoluble during application
• Have no affinity for the fibres
• Require binders
• Do not react with the fibres
• Pigments are usually mixed with a vehicle that hardens upon drying, forming an opaque coating.
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unhealthy dyes
Research has revealed the toxic effects of dyes and pigments on people and the environment. Regulators have therefore banned or restricted the use of colourants classified as highly toxic. The eu banned more than 20 different kinds of azo dyes in the mid 1990s. In December 2003, the us Environmental Protection Agency (usepa) listed process-related solid waste, including sludge from producing certain dyes, plus pigments, food, drug, and cosmetic colourants as hazardous waste, for they “may pose unacceptable risks to human health and the environment when improperly disposed in quantities above specific levels”.
In December 2004, the eu banned sale and use of textile dye “Navy blue 018112”. Tests showed it highly toxic for fish: the “level of concentration that can be reached in rivers from the sewage of the dyeing processes is so high as to suspect damage to aquatic organisms”. Dark coloured dyes block sunlight from entering the water, thus inhibiting photosynthesis and killing aquatic flora and fauna. Impact of dyes on human health is also worrying and has led to regulatory action in several countries.
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