The receding ground water levels, the high water pollution due to effluents from textile processing units and the great awareness of the people and the law enforcing authorities, all put together are the driving force for finding out a real, cost effective solution to eradicate the use of huge quantities of common salt, soda ash and other basic chemicals such as neutralizing acids.
There are lot stories of accomplishments of this magic task. And there are many patents pending registration in this regard.
The physico-chemical necessity of the use of salt and soda in reactive dyeing is well known to all technicians. The salt is an exhausting agent to push the dyestuff towards cellulose
molecules and the alkali (soda ash) is hydrolyzing/fixing agent for the reactive
dyestuff. In current practice, cellulosic fibers are predominantly dyed with reactive dyes in the presence of a considerable amount of salt and fixed under alkaline conditions. However, dye fixation efficiency on cellulosic
fibers is generally low (varying from 50 – 90%). This, results in a highly colored
dye effluent, which is unfavorable on environmental grounds. Furthermore, the high concentrations (40 – 100g/I) of electrolyte and alkali (5 – 20 g/I) required in cellulose fiber dyeing may pose additional effluent problems.
There are trials being conducted throughout the world and I came across one
paper presented by M/s R. Nithyanandan and M.
Subramanian Senthil Kannan, which is really very interesting. You can see
their original
article here.
There is no need for salt and soda.
The RFD fabric was padded with a chemical called 'polyacrylamide', followed
by drying and curing. The resultant cationized fabric was dyed using regular
reactive dyestuffs at their stipulated dyeing temperatures, but with salt and
soda. The dyed fabric was subjected regular after treatments like hot wash,
soaping, hot wash and cold washes.
The light and wash fastness properties, they claim that they were almost equivalent
to the regular processed fabric.
Practical feasibility and economy:
After reading their interesting article, I searched for the availability of
polyacryamide and I got the following message.
Polyacrylamide (PAM) is a synthetic, high molecular
weight organic polymer. Depending on the difference in molecular
structure, PAM can be classified as linear or cross-linked polymers.
Linear PAM dissolves in water; cross-linked PAM is a granular crystal that
absorbs hundreds of times its weight in water.
PAM can be manufactured as a neutral, cationic, anionic, or amphoteric
polymer of varying chemical and physical properties, molecular weights and
lengths. There are actually hundreds of specific PAM formulations.
Machinery requirement:
Fabric may be padded using any conventional padding mangle and a
stenter drying. But in the case of hank and cheese yarn dyeing,
suitable machinery fabrications has to be done. Pretreatment
of cotton with polyacrylamide enhances the possibility of dyeing cotton at
neutral pH with various commercial reactive dyes. Such pretreatment, as
applied through pad – dry – cure process, brings about some chemical
changes in the treated fabric.
Economy:
Since salt, soda ash and neutralizing acid have been completely eradicated
from this process, huge cost on chemical saving would be there. But the
pretreatment and pre-drying costs along with labor and cost of
polyacrylamide have to be found out and a reasonable process route has to
be fixed to solve the long standing problems of reactive dyeing.
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