Continuous dyeing of cotton

Continuous dyeing of cotton/polyester and 100% polyester fabrics was performed using mixtures of reactive and disperse dyes, or disperse dyes alone, respectively, and achieving dye fixation by heating, using an electric infrared oven situated in front of an electric hot air unit. Generally, the colors of the thermally produced dyeings were reasonably similar to those of the respective exhaust dyeings obtained using the same recipes. As expected, the thermally produced dyeings usually

contained more unfixed dyes than the exhaust dyeings, largely a consequence of the quicker and less-efficient post-dyeing washing process. The results for pilot-scale dyeings are also compared with those obtained on an industrial scale in a finishing mill. Dyeing using infrared heating and hot air had no influence on the light stability of the colors or on the fabric handle. Most significantly, the negligible variation of color along the fabric length during continuous thermal dyeing illustrated that the process was well-controlled in all cases and could be valuable for the dyeing of small lots of fabric.

The working of a continuous dyeing process is described here. The textile substrates are feeded continuously into a dye range. The speeds can vary between 50 to 250 meters per minute. According to Industry estimates Continuous dyeing is a popular dyeing method and accounts for around 60% of total yardage of the products that are dyed.

A Continuous dyeing process typically consists the following. Dye application, dye fixation with heat or chemicals and finally washing. Continuous dyeing has been found to be most suitable for woven fabrics. Mostly continuous dye ranges are designed for dyeing blends of polyester and cotton. The step of padding plays a key role in the operation of continuous dyeing. Sometimes Nylon carpets are also dyed in continuous processes, but the design ranges for them is unlike that for flat fabrics. Warps are also dyed in continuous process. Very good examples of such warp dyeing are long chain warp dyeing and slasher dyeing using indigo.

A continuous dye range has been found useful and economically sustainable for dyeing long runs of a given shade. One important factor that separates continuous dyeing from batch dyeing is the tolerance factor for color variation. That is more for continuous dyeing as compared to batch dyeing. This is so because of two reasons a) the speed of the process. b) presence of a large number of process variables which affects dye application. The process that is illustrated below is designed for dyeing of blended fabric of polyester and cotton.

Some of the popular methods in continuous dyeing process are Pad-steam, Wet-steam, thermosol dyeing, TAK dyeing, space dyeing, and pad-steam dyeing long chain warp dyeing etc.

Continuous and to some extent semi-continuous dyeing processes both are less prone to water consumption than batch dyeing, but results in high concentration of residues. If some strict control measures are taken up it is possible to reduce this losses of concentrated liquor. The following steps may prove useful.

·        Applying low add-on liquor application systems along with minimising of volume capacity of the dip through when pad dyeing techniques are in operation.

·        Adoption of latest dispensing systems, where the chemicals get dispensed on-line as separate streams. They gets mixed only at the moment just before the delivery to the applicator.

·        Using any of the following systems for dosing of the padding liquor. Important to know that it should be strictly according to the measurement of the pick up:

o        A proper measurement of the dyeing liquor quantity consumption in comparison to the processed fabric. The resulting values thus obtained are processed automatically and applied in preparing the next comparable batch.

o        Application of the technique of rapid batch dyeing. Here the dyestuff solution is prepared just in time, with steps that are based on on-line measurement of the pick up. This proves better than those dyestuff that is kept prepared already for the whole batch before the commencement of the dyeing batch.

·        To increase washing efficiency based on the proven principles like reduction of carry-over and counter-current washing.