Reactive Dyes
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Transcript of Reactive Dyes
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Dyestuff Chemistry
Reactive Dyes
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Introduction to Reactive Dyes
Although fiber-reactive dyes have been a
goal for quite some time, the breakthroughcame fairly late, in 1954.
Prior to then, attempts to react the dye and
fibers involved harsh conditions that often
resulted in degradation of the textile.
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The best dyes, by far, to use for cotton and
other cellulose fibers are the fiber reactivedyes.
They are much brighter
longer-lasting, and easier-to-use
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Fiber reactive dye is the most permanent of
all dye types.Unlike other dyes, it actually forms a covalent
bond with the cellulose or protein molecule.
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Once the bond is formed, what you have is
one molecule, as the dye molecule hasbecome an actual part of the cellulose fiber
molecule
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The first fiber-reactive dyes were designed
for cellulose fibers, and they are still usedmostly in this way.
There are also commercially available fiber-
reactive dyes for protein and polyamide
fibers.
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In theory, fiber-reactive dyes have been
developed for other fibers, but these are notyet practical commercially.
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The first fiber-reactive dyes contained the
1,3-5-triazinyl group, and were shown byRattee and Stephen to react with cellulose in
mild alkali solution.
No significant fiber degradation occurred. ICI
launched a range of dyes based on thischemistry, called the Procion dyes.
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This new range was superior in every way to
vat and direct dyes, having excellent washfastness and a wide range of brilliant colors.
Procion dyes could also be applied in
batches, or continuously.
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Chemistry of Reactive dyes
Chromogen Fiber Reactivegroup
water solubilizing
group
Bridging group
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Reaction with Dye
This reaction takes place in presence of a base
Dye-Cl + H-O-cellulose base Dye-OH +salt
The bond formed is covalent which provides
excellent wash fastness and is stronger then the
hydrogen bonds of a direct dye on cellulose.
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Reaction with water
In reaction with water the reactive group ishydrolyzed and the dye looses its ability to
react with hydroxyl group of cotton or amino
groups of poly amide
Dye-Cl +H-OH base dye-OH + salt
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The chromogen is as mentioned before (Azo,
carbonyl or phthalocyanine class). The water solubilizing group (ionic groups,
often sulphonate salts), which has the
expected effect of improving the solubility,
since reactive dyes must be in solution for application to fibers.
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The bridging group links the chromogen and
the fiber-reactive group. Frequently the bridging group is an amino,
-NH-, group.
This is usually for convenience rather than
for any specific purpose.
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The fiber-reactive group is the only part of
the molecule able to react with the fiber.
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A cellulose polymer has hydroxyl functional
groups, and it is these that the reactive dyesutilize as nucleophiles.
Under alkali conditions, the cellulose-OH
groups are encouraged to deprotonate to
give cellulose-O- groups.
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These can then attack electron-poor regions
of the fiber-reactive group, and perform either aromatic nucleophilic substitution to
aromatics or nucleophilic addition to alkenes
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Reactive dyes can react both with water and
fiber but the main reaction is with fiber for 2reasons
Rate of chemical reaction with cellulose is
much faster than the reaction with water
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the probability of a diffused dye molecule
reacting with an OH group on cellulose ismuch larger than the cellulose group of water
In dyeing of cellulose with reactive most of
the dye reacts with fiber but some dye do
react with water.
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Unfixed dye composes of hydrolyzed dye
and unreacted dye The amount of unfixed dye can be 20-30% of
the total amount of dye used
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Properties of reactive dyes
Advantages
Available in a complete range of colors Very good wash fastness
Good to very good light fastness
High flexibility in the choice of method of application
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Easy to obtain level dyeing
Readily soluble in water
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Disadvantages
Cost of using reactive dyes is high Loss of substantial dye during application
Excessive time required for the dyeing process
Sensitive to oxidation
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Auxiliaries used in dyeing with
reactive
In the dyeing of cellulose with reactive dyes,
alkali is necessary it acts as a catalyst in the reaction between
the dyestuff and the fiber.
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The important point is not the type or amount
of alkali but rather the pH of the dye bath,which must be closely supervised.
Adding base increases the exhaustion of the
dye on the fiber
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It increases the rate of chemical reaction and
faster the rate of reaction, the faster the rateof exhaustion
Ionic nature of base further enhances the
exhaustion
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The most suitable pH for dyeing varies with
the temperature being approx. 11.5 for common warm dyeing
(dyeing at approx. 60 degrees )
10-11 for hot dyeing (80 )
12.5 for cold dyeing (40 degrees)
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Auxiliaries used in Reactive Dyeing
contd
Inorganic salts have two main functions in
exhaustion dyeing with reactive dyestuffs: Improving the affinity of the dyestuff
Acceleration of the dyestuff's association and
lowering of its solubility.
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Improving the affinity and thus the exhaustion
of dyestuff during primary exhaustion servesto raise the exhaustion rate of reactive
dyestuffs. Because reactive dyestuffs have a
lower affinity than direct dyestuffs, more
inorganic salt is required when using reactivedyestuffs in order to accelerate absorption
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The following graph displays the amount of
Glauber's salt used, the exhaustion rate and
the fixing rate for a conventional dye and a
high-fixation dye (Sumifix Supra E-XF).
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Because inorganic salts accelerate the
association of water-soluble dyestuffs and
lower the solubility of dyestuffs, an excess of
inorganic salt may in fact impede the
absorption of some dyestuffs, and so care is
necessary when one is used.
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The effectiveness of the inorganic salt is not
decided by the ratio of its quantity to the
quantity of fabric (o.w.f.) but rather by its
concentration in the dye bath (g/l)
so it is by lowering its concentration that the
effect of an inorganic salt can be reduced.
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Suitable fiber substrate
Cellulosic fibers
Protein fibers Polyamide fibers
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Methods of application
Exhaustion method
It consists of three stages Exhaust Dyeing
Fixation
After scouring
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Cold Pad-batch method
Semi- continuous method Fabric is padded with a solution of dyes and
the base and then wound onto a roller. which
are covered in plastic sheets
Left for 2hours to overnight depending on therate of fixation
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Continuous dyeing with reactive dyes
There are 2 methods currently in use byindustry
Two-Bath Method
the dyes and base are applied separately
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One-bath method
In this the dyes and the base are appliedtogether onto the fabric from the same
padding bath
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Chemical striping of reactive dyes
Once reactive dyes have been applied and
the unfixed, hydrolyzed dye has beenremoved.
there is no longer a possibility of
leveling or stripping by simple physicalmeans.
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If a reactive dyeing is unsatisfactory,
economics will probably dictate whatsteps must be taken to deal with the problem.
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For heavy shades, a reduction of about
20-40% in depth of shade can sometimesbe achieved by hydrolysis of the dye-fiber
bond with alkali in the presence of salt at
95C (200F)
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Depending on the individual dyes, complete
chemical stripping (color destruction)may be carried out in one of the
following four ways
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reduction only;
Reduction followed by oxidation;Oxidation only
oxidation followed by reduction.
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all traces of chemicals should be removed
prior to any re-dyeing attempts on a sampleof the stripped goods.
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General fastness properties
light fastness- good to very good on most
fibers
Washing- good, sensitive to chlorine bleach
Dry cleaning- good
Perspiration- good
Crocking- good