Environmentally responsible dye application in textile

Environmentally responsible dye application involves the principles of pollution prevention that were developed and promulgated in the early 1990s with the hierarchy of ‘reduce, reuse, recycle’

Successful dyeing depends on achieving the following as efficient as possible.

1. The correct shade
2. Level dyeing
3. Fastness

Issues of environmental responsibility are thus encountered in trying to achieve each of the basic requirements of shade, fastness, levelness and efficiency. Holding fewer dyes in stock may be more efficient, and selecting dyes that are considered more environmentally benign might be environmentally responsible, but reduced dye choice may make a level, fast or non-metameric match harder to achieve. The efficient use of water, dye, energy and chemicals is promoted by using low liquor ratios, but as these decrease the likelihood of unlevel dyeing increases. Chemical auxiliaries might reduce dyeing time, promote levelness or increase exhaustion, but ultimately represent a burden in the waste stream.

Reduced water and energy consumption can bring economic as well as environmental benefits, and can be achieved in many ways: more efficient machinery, heat recovery, elimination of redundant processes.

Many options exist for the replacement or reduced use of chemicals, particularly those that are environmentally questionable. More advanced methods for reducing the environmental burden of dyeing such as dye bath reuse, enzymatic preparation or dyeing from supercritical carbon dioxide, have been tested on a commercial scale. Many others have been the subjects of detailed research.

Machinery

Whether batch or continuous, machinery can become contaminated with color and at times, machines have to be taken off-line to be cleaned. The need for such cleaning can be minimized by sequencing the colors or patterns being produced from light to dark.

Utilities

water is by far the most common medium from which dyeing is carried out. Water has a high specific heat that makes heating and boiling it energy intensive, and the recovery of heat from waste streams a realistic and sensible thing to do.

The use of supercritical carbon dioxide as a medium for dyeing has been the subject of considerable research and semi commercial application.19 The technique has been suggested for virtually all dye–fiber combinations. While it is environmentally attractive, and eliminates drying, the method requires highly specialized equipment and dyeing auxiliaries and seems a long way from wide-scale implementation.

Reactive dyes

In recent years, the ‘default’ choice has come to be reactive dyes for their generally good fastness to wet treatments, and good range/brightness of shades.

Textile printing

The use of ink-jet printing is increasing. This technology does use process colors that mix on the substrate, so each pattern and color is generated from the same inks, and waste is minimal.

Waste minimization in textile dyeing

1. Maximizing the exhaustion from dye bath: In order to maximize the dye exhaustion, a knowledge of the relationship between the exhaustion, affinity and liquor ratio is important. These are the three main parameters, which control the dyeing process.

2. Maximizing fixation and wash off: For fixation of dyes, different mechanisms are used, which include chemical reaction of the dye with the fibre to form a covalent bond, insolublisation of the dye within the fibre, formation of an ionic bond, formation of solid solution, or the use of cationic dye fixing agents. These different mechanisms determine the colour retained by the fibre material and that released into the wash liquor on completion of dyeing. The better the fixation mechanism, the lower is the colour present in the wastewater.

3. Use of optimum amount of salt:

The general approaches to minimize the use of salt during dyeing are as given below:

· Use of the lowest possible liquor ratio in batch dyeing

· Optimization of salt use for each individual dyeing

· Use of continuous dyeing or pad-batch dyeing wherever possible

· Minimization of color changes and discards in continuous dyeing

· Reuse of dye baths

· Ensuring proper handling of dyes and fabrics

· Selection of dyes that exhaust with minimum salt

· Optimization of dyeing temperature individually for each recipe.

Source: R.M. Christie,2007, Environmental aspects of textile dyeing, Woodhead publishing, Cambridge, England