Sorting out the mess

An ecofriendly polymer solution can replace polluting salts used for dyeing

Published: Sunday 31 March 2002

it encompasses all vivid, vibrant and dazzling colours of life, but paradoxically, it is a messy affair. Yes, this is an apt description of the dyeing industry that uses excessive amounts of chemical salts. These salts, when carelessly dumped, not only contaminate the soil but also kill fish. But now, scientists from the University of Leeds, Yorkshire, the uk , have found a way out of this mess -- an ecofriendly polymer solution that can replace the salts (see: New Scientist , vol 172, no 2319).

The salts are used for bolstering dyes to stick to the cloth. Dyes and natural fibres both become negatively charged in water. They, therefore, repel each other, making it difficult for strong colours to bind to the cloth quickly. Adding salts such as sodium sulphate helps overcome this impediment. The salts separate into positive and negative ions when they dissolve. When positive ions come close to the negatively charged dye they reduce repulsion, allowing the dye molecules to get close to the fibres. Once in contact with the cloth, the dye binds strongly to it.

What leads to environmental degradation is the careless disposal of wastewater without removing the salts from it. At times the amount of salt used by dyers is equivalent in weight to the fabric itself. The us textile industry uses about 95,000 tonnes of sodium sulphate per year. Industry representatives assert that removing all salt residues is not economically viable. Therefore, all over the country, sodium-rich wastewater contaminates fresh water and makes soil too alkaline to support crops. "Salt is the most problematic chemical that is released when dyeing," says Richard Blackburn from the university.

According to him, such environmental degradation can be avoided by using a polymer solution instead of the salts. The process is simple: cotton has to be soaked in a bath of cellulose-like polymers before dyeing it. Since cellulose is the major constituent of plant carbohydrates, the polymer and the fibres have a similar physical structure. This and the polymer's positive charge supplement its binding with the fibres. This gives the cloth a positive charge, which then attracts the negatively charged dye and forms an even stronger chemical bond due to the action of the opposite charges. This stronger bonding means that the dyeing and rinsing process uses up to 80 per cent less water -- and wastes less dye.

However, there are some scientists who doubt the method's viability. Peter Hauser from North Carolina State University, usa , says that the polymer molecules are too big to penetrate the fibres, so the dye stays on the surface rather than soaking in and is, therefore, more likely to fade over time.

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