1. COST REDUCTION IN DYEING OF COTTON WITH
REACTIVE DYES
R.B.Chavan
Department of Textile Technology
Indian Institute of Technology
Hauz-Khas, New Delhi 110016
E-mail: rbchavan@hotmail.com
Introduction
Among the various fibres and dye classes, cotton and reactive dye system is the most
popular. Reactive dye molecules are characterised by a chromophore, to impart colour,
attached to a reactive group capable of reacting with cellulose, and thereby binding the
chromophore to the cellulose covalently.
The total world-wide production of cotton is estimated at 21 x 106 te p.a. (metric tonnes
per annum). The corresponding amount of reactive dye used for cotton coloration is
estimated to be 120,000 te p.a. Of this about two thirds is dyed by exhaust technology.
Salient statistics, which follow from these figures, include:
 4 x 106 te p.a. of cotton is exhaust dyed with reactive dyes;
 4 x 108 te p.a. of fresh water is used in the overall process, and all of this is ultimately
discarded in a contaminated state;
 2.8 x 106 te p.a. of salt is used in the process; and all of this is ultimately discarded in
the aqueous effluent;
 8 x 104 te p.a. of reactive dye is applied, with an average fixation yield of 70%, thus:
 2.4 x 104 te p.a. of dye is discarded in the aqueous effluent.
The reactive dye business, due to its maturity, is very competitive and price sensitive. As
older dye technologies have moved out of patent, these have been taken up by
manufacturers in the developing world, driving down manpower costs and total

2. production costs. Therefore, a large proportion of cotton processing has also moved to the
cheaper and less environmentally conscious economies of the developing world.
In the present paper a modified process for rinsing after dyeing has been suggested for
cost reduction and environment benefits.
Reactive dyeing process
The process for reactive dyeing of cotton can be divided into three steps: the pre-
treatment, the dyeing and the rinsing after dyeing. Traditionally, the consumption of
energy, chemicals and water in rinsing is crucial; approximately half of the total energy
consumption and of the total water consumption are attributed to the rinsing process
after dyeing. Therefore there is much scope for cost reduction in rinsing operation.
Water consumption in reactive dyeing.
During the pre-treatment, the cotton fabric is scoured and bleached and washed. After
some rinses, the dyestuff is poured into the dye bath and a diffusion of the dyestuff
molecules between the cellulose fibres takes place. After some time, salt is added to
obtain adsorption of the dyestuff to the cellulose fibre. After this, adjusting temperature
(50-80°C) and pH (10,5-11,5) completes the reaction between the dyestuff and the
cellulose. Some of the dyestuff will be hydrolysed during this dyeing process, and the
adsorbed hydrolysate must be removed in the succeeding rinsing after dyeing.
The rinsing traditionally consists of several baths, as in table 1.
Table 1 Typical water consumption in reactive dyeing (100 kg
batch)
Step Process Water
Litres
1 Dyeing 700
2 Overflow rinse 7300
3 Warm rinse 700

3. 4 Neutralisation 700
5 Overflow rinse 7300
6 Hot soaping 700
7 Warm rinse 700
8 Overflow rinse 4300
9 Hot soaping 700
10 Warm rinse 700
11 Overflow rinse 4300
12 Neutralisation & softening 700
Total water 20800
The large water consumption in the rinsing after dyeing is primarily caused by the large
number of baths but also by the common use of overflow rinses. Before the temperature
is raised in the rinse, the dyestuff producers recommend neutralisation to pH around 8,
when dyestuffs with vinyl sulphone reactive groups are used. This neutralisation has,
however, in some dye-houses, become usual practice for all sorts of reactive dyestuffs.
After neutralisation, the rinsing consists of a number of soaping sequences: hot soaping,
warm rinse and overflow rinse. In the hot soaping steps 6 and 9 in table1 soaping
additives are used, in the form of surface active agents (detergents), complexing agents
and dispersing agents. The reasons for the use of these auxiliary agents are protection
against hardness in the water and/or the cotton, and keep the unfixed dye in dispersed
form.
The process is completed with neutralisation to pH around 7 and treatment with
softening agents, necessary for the subsequent sewing process.

4. Cost saving rinsing operation
New water saving, chemical free, high temperature and high speed rinsing steps are
shown in Table 2
Table 2 Modified rinsing process
Step Process water (Litre)
1 Dyeing 700
2 Hot rinse 700
3 Hot rinse 700
4 Hot rinse 700
5 Neutralisation & Softening 700
Total 3500
50 industrial scale trials with the new recipe documented that a chemical free, high
temperature rinse, using a reduced number of rinses, and thus saving water, chemicals
and process time can be implemented in the dye house with no adverse effect on product
quality. When implementing the water saving, chemical free, high temperature and high
speed rinse after reactive dyeing of cotton in batch, the following cost reduction and
cleaner production options should be considered:
A) Change from overflow rinsing to stepwise rinsing.
Rinsing by overflow, i.e. pouring clean cold water directly into the process water in the
machine while excess water is drained out of the machine, is used both for rinsing and for
cooling purposes. Overflow is quick but causes unnecessary water consumption.
Table 3 Stepwise rinsing as substitute for overflow rinse.
A Fill the machine according to liquor ratio
B 10 minutes rinsing

5. C Discharge rinsing water
D 5 minutes draining
B) Omit the use of detergents in the rinsing after reactive dyeing of cotton.
Surplus and un-fixed reactive dyestuffs are highly water-soluble, in spite of this ,
detergents are often used during rinsing after dyeing. In international literature, it has
been documented that detergents do not improve removal of hydrolysed reactive
dyestuffs from the fabric. More than 50 full-scale dyeings carried out at various
dye-houses without the use of detergents. All have successfully proven that
detergents can be omitted without negative impact on product quality.
C) Omit the use of complexing agents in the rinsing
If soft water with a quality of below 5h dH is used, complexing agents can be omitted
without any negative effects on dyed fabric. However, if hardness builders e.g. calcium
and magnesium are present in the dyeing processes and in the rinsing after dyeing, they
might have a negative effect on the dyeing result, e.g. change in shade or problems with
reproducibility. For that reason, soft water is recommended as standard procedure in the
dyeing processes. However, water softening in the dyeing machine by using complexing
agents, forming bonds with the hardness-builders, are both economically and
environmentally a bad solution. Water softening can profitably be done in a separate
plant by the ion-exchange technique or the membrane filtration technique.
D) Use only neutralisation after dyeing when using Vinyl sulphone(VS)
reactive dyestuffs.
Neutralisation in the first rinse after dyeing can be restricted to the vinyl sulphone (VS)
reactive groups. Some VS dyestuffs have poor alkaline washing fastness (low bond
stability) and thus sensitive to high pH and high temperature simultaneously.
Nevertheless, it is not uncommon that all recipes for reactive dyeing in a dye-house

6. include neutralisation in the first rinse after dyeing, whether VS reactive dyestuffs are
used or not.
The dyeing can be successfully carried out without the use of neutralisation in the first
rinse after dyeing. This in spite of the fact that more than half of the dyeings were carried
out with dyestuffs based on VS-groups. As it is not possible to put forward general
guidelines on when to neutralise dyestuffs based on VS-groups, it is recommended
always to neutralise these. There is no reason to neutralise in this step when all other
reactive dyestuffs are used, e.g. based on monochlorotrazine (MCT), dichlorotriazine
(DCT), trichloropyrimidine (TCP) or difluorochloropyrimidine (DFCP).
In general, it is recommended to select dyestuffs with a superior alkaline washing
fastness when selecting VS-dyestuffs.
E) Chemical-free high speed rinsing after reactive dyeing of cotton.
Tests have shown that rinsing is more effective and faster at elevated temperatures – e.g.
around 30% more unfixed hydrolysed reactive dyestuff is rinsed out after 10 minutes at
95°C than at 75°C.
Tests using hot 90-95°C rinsing after reactive dyeing of cotton have proved that the
technique has no negative effects on the dyeing results. Most often the fastness of the
goods were better after the hot rinsing than after the traditional rinsing with overflow,
detergents, complexing agents and neutralisation in the first rinse. Furthermore, when
using 90-95°C rinsing water, a few stepwise rinses (table 2) can reduce the rinsing time
by around 50% compared to a standard recipe (table 1).
Main achieved cost and environmental benefits
The benefits of the new rinsing operation are shown in Figure 1

7. Fig. 1 Benefits of step wise rinsing operation
1. Reduction in water consumption and wastewater generation.
2. Cost saving in chemical consumption and reduction in pollution load of waste
water
3. Time and energy saving.
2.1.4 Applicability
1. The new process can be implemented in all types of textile companies involved in
reactive dyeing of cotton.
2. The new process can only be implemented if the company do have availability of soft
groundwater or is operating with a soft-water system (which is normally the case).
3. It is recommended always to neutralise in the first rinse when dyestuffs based on
VS-groups are used. There is no reason to neutralise in this step when all other
reactive dyestuffs based on monochlorotrazine (MCT), dichlorotriazine (DCT),
trichloropyrimidine (TCP) or difluorochloropyrimidine (DFCP).

8. Economics
1. The economic feasibility is obvious. 50-70% reduction in the consumption of water
for rinsing. Total savings will depend on the number of reactive dyeings at the
company.
2. Omit the use of detergents, complexing agents and acetic acid. Savings will depend
on the number of reactive dyeings at the company.
Driving force for implementation
1. High costs for water and wastewater discharge and/or low availability for water of
appropriate quality.
2. High costs for chemicals and wastewater load.
3. A desire for reduced operation time per lot and increased capacity per machine.