Page 20 - New Cloth Market December 2022 Digital Edition

Multifunctional Auxiliary in Reactive Dyeing & Process Thereof

Dr. Naresh M. Saraf & Dr. Priti B. Tayade
AGES
Sarex Chemicals
A division of Saraf Chemicals Ltd., Andheri (W), India
Website: www.sarex.com, E-mail: [email protected]
Introduction
Water is the basic necessity for treating textile materials in wet processing industry. The textile industry is
one of the major consumers of water, consuming a huge amount of water in various processing steps,
such as pretreatment, dyeing, printing, and finishing. Rinsing and washing operations alone consume
enormous amounts of water. Almost all dyes and chemicals are applied to the textiles in water baths.
Water consumption is far greater than the amounts of fibres processed. Water from almost all sources
contains various types of contaminations like, calcium and magnesium ions (hardness), alkalinity, heavy
metal ions (iron, copper, manganese etc.), chlorine, various anions (sulphide, fluoride etc.), suspended
matter, dissolved solids etc. Hardness is generally referred to the presence of calcium and magnesium
ions in water. In the textile wet processing, water quality plays a significant role in determining the final
shade of the fabric, consistency and production efficiency. One of the essential factors in influencing the
EXECUTIVE P
dye house water quality is its hardness which is expressed in CaCO equivalent. The unit of measurement
3
is parts per million (ppm). If the amount of hardness in water is less than 60 ppm, it is regarded as soft
water. If the amount of hardness is more than 180 ppm, it is termed as hard water. Hard water is generally
undesirable in the dyeing process since it may cause dye precipitation, promoting dye aggregation, which
results in certain dyeing defects such as uneven dyeing, color specks, and loss of depth. In addition, hard
water may interfere with solubility of the dye, alter the color of the dye, and adversely affect color appear-
ance of textile products.
Amongst many classes of textile dyestuffs, the reactive dyes contribute about 50% of the total market
share due to their wide-ranging shade gamut, flexibility in application, and the outstanding fastness prop-
erties. Reactive dyeing essentially has two stages. In the first stage, the dye is adsorbed onto the
cellulosic substrate through hydrogen bonding and Vander Waals interactions. Dyes are applied from
aqueous solutions with electrolyte which is needed to overcome the longrange repulsion forces operating
between anionic dyes and the negative zeta-potential acquired by cotton surface in aqueous media.
Without electrolyte addition, adsorption of dye on the fibre will not occur, and with dyes such as reactive
dyes, which are very soluble in water, a high amount of electrolyte is required in the dyeing process.
Alkalinity is also one of the influential factors for conventional reactive dyeing of cotton fabrics. They rely
on alkaline condition to have a reaction with the hydroxyl group of cellulose, mostly by nucleophilic
substitution or addition, to form covalent bonds. However, the competing reaction of the dye with hydrox-
ide ions in the dye bath is prominent, which may produce a hydrolyzed, nonreactive form of the dye,
leading to serious environmental problems due to the production of colored effluent discharge after soap-
ing and washing-off processes. The wash-off and subsequent effluent treatments, to remove the resultant
color pollution, can account for up to 50% of the total cost of reactive dyeing. The amount of water used
in the wash-off process; a process involving over seven separate rinsing stages is not uncommon. High
volumes of water and numerous repeated individual wash-off stages are often required to dilute the
electrolyte and alkali concentration in the wash-off bath. A wash-off process might include a coldwater
wash, hot water wash, soaping followed by subsequent hot and coldwater rinses.
Right-first-time dyeing concept in textile dyeing sector was first introduced in the year 1970. This term is
used to define the dyeing efficiency and how one can dye a fabric. If the dyeing process is completed
properly without any fault and there is no need to put the dyed fabric into the bath to get the proper shade
then this dyeing is called RFT dyeing. The benefits of achieving the right-first-time are very significant
including, for instance, reduction in water and energy consumption, reduction in effluent generation, re-
duce time consumption, reduction in cost, increase productivity and increase profit. Right-first-time dye-
ing is a difficult and daunting task for a dyer and requires great control over dyeing parameters such as
quality of dyestuff and auxiliaries, concentration of salt and alkali, time and temperature, and water
quality. The hardness of water is particularly a critical factor in shade matching and it can prevent dye-
houses from achieving a high level of right-first-time production. The dissolved calcium, magnesium and
iron salts in hard water may react with dyes, forming undesirable precipitates, which yields impure shades
and give spots on the fabrics being dyed. Hard water, when used for washing purposes, does not lather
freely with soap. On the other hand, it produces sticky precipitates of calcium and magnesium soaps. The
formation of such insoluble, sticky precipitated continues, till all calcium and magnesium salts present in

NCM-DECEBER 2022
20