The textile industry is one of the major contributors to water pollution in India. The various processes that contribute to water pollution are sizing, desizing, scouring, bleaching, mercerization, dyeing, printing and finishing. Each process uses various chemicals that are let out into effluents. Thus, by dumping chemical effluents the eco-balance of nature has been gradually disturbed. In recent years there has been a growing need for controlling pollution through industrial effluents and full fledged efforts are being made by governments the world over to draw up or to tighten the legislation pertaining to the controls on the types and extent of pollutants that could be passed onto nature.
The consumption of cotton is almost half of the world's total fibre consumption and it is expected to maintain this position during the current century. Surveys conducted on the consumption of dyes the world over, indicate that in the next five years, the sulphur, direct, vat and indigo dyes will remain constant, azoics will continue to decline, and reactive dyes will show increase. The lack of growth of dye classes other than reactives may be attributed to factors such as high price and limited supply, environmental problems and technological obsolescence.
Reactive dyes -promise of the future
It has been estimated that the use of reactive dyes would raise up to 50% by 2004. Though the reactive dyes can minimize the effluent problems, yet two problems need to be tackled viz.
- Colour in the effluent
- Minimization of chemical usage
Removal of colour in the effluent
The removal of colour from the effluent is rather expensive with the conventional technique. Unlike other classes of dyes, in the case of reactive dyes, sometimes as high as 30% dye remains unbound during primary treatment and needs to be treated either on site as a secondary process after biological oxidation or at a municipal sewage works.
Unrelenting govt. pressure for regulating industrial effluent discharge has led to the development of new reactive dyes, machinery and processes to minimize waste and colour in the effluent. The development of bi functional reactive dyes is important from this point of view.
Bi functional reactive dyes consist of two reactive groups capable of forming covalent bonds with the fibre. Dyes with two separate reactive groups can, in true sense, be called bi functional. These bi functional dyes can be of two types.
- Those consisting of two similar reactive groups (homo bi functional reactive dyes) and
- Those with two different reactive groups (hetero bi functional reactive dyes)
The Table 1 shows various homo bi functional and hetero bi functional reactive dyes, which are presently being marketed. The homo bi functional dyes were developed for high temperature ranges in exhaust dyeing with substantially increased exhaustion and fixation. This led to better dye utilization and resulted in less hydrolyzed dye and less effluent.
The hetero bi functional dyes possess various advantages such as excellent stability during storage, good fastness to light, perspiration & chlorine.