Important factors for dyeing cellulose fibre
(with cold brand reactive dye in batching
process):
1) pH of the dye bath:
The optimum pH for fixing cold brand
reactive dyes on cotton and viscose rayon depends on individual dyes, the
temperature and time of dyeing. pH decreases with increasing temperature and
time of dyeing. For most of the dyes the optimum pH is 10.8 to 11 at 20o
to 25oC. Soda ash has been the best alkali for dyeing at 30oC
for cotton, mercerized cotton and linen. Increased fixation (due to higher
temperature) and increased dye bath stability and better reproducibility are
the advantages of soda ash as the fixing agent.
For viscose rayon the optimum pH is
10.3 at 20o to 25oC.
2) Amount of alkali:
The amount of alkali used for fixing
depends on the depth of shade dyed and the liquor ratio employed. Some
quantities of alkali required for fixing the reactive dyes are given table 28.
3) Dyeing temperature:
Since increase in temperature affects
the rate of physical and chemical processes involved in dyeing, it is important
in dyeing reactive dyes also. The affinity of the dye for the fibre decreases
with increases in temperature and at the same time the rate of hydrolysis of
the dye increases and adversely affects the fixation of color yield. However
the rate of diffusion of the dye in the fibre increases with increased
temperature. At temperatures lower than 20oc, the rate of fixation
is very low. Hence for most of the dyes a temperature of 20o to 25oC
is the recommended temperature while for some other dyeing at 50o to
60oC with sodium bicarbonate as the alkali gives maximum color
value.
4) Electrolyte concentration:
Since reactive dyes have low affinity
for cellulose exhausting the dye bath by adding common salt or Glauber’s salt
prior to fixation can increase the fixation. The amount of salt required
producing adequate exhaustion decreases with decreasing liquor ratio. Thus for
pale shade on cotton and viscose rayon 15 and 10 g/l of common salt used. The
quantities may be increased to 30 and 20 to 30 g/l for medium and deep shades
on these fibres.
5) Time of dyeing:
Generally
the dye may be added in two portions. The salt may also be added in two lots.
The exhaustion takes place in 20 to 30 min. There is generally no advantage in
extending the period beyond 30 min. The alkali is then added and the dyeing
continued for 30 to 90 min. The depth of shade and reactivity of the dye decide
the time of dyeing. For deeper shades larger times are required.
6) Liquor ratio:
With
decreased liquor ratio, both exhaustion and fixation take place to increased exert.
However the rate of fixation of most of the dyes is not significantly affected.
As the liquor ratio is decreased, the effectiveness of increasing salt addition
also decreases. Hence lower amount of salt are sufficient to get optimum
exhaustion.
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