direct dyes
TRANSCRIPT
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Dyestuff Chemistry
Direct Dyes
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Introduction
Direct dyes are mainly applied on Cellulosic fibers
Few examples are cotton, viscose rayon Easily applied on cellulosic fibers
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They can be directly died from simple solutions in water
That’s why these dyes are called direct dyes They have an affinity for cellulose so can
also be known as substantive dyes
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Congo red was the first direct dye which was discovered in 1894
Introduction of reactive dyes was welcomed because its was not expensive to implement.
Direct dyes are marketed under different brand names by different dyestuff manufactures.
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Properties of a solution
direct dyes are water soluble They ionize in water That give dye anions (negative ions) and
sodium cations (positive ions) Dyes are manufactured as sulphonic acids
and are converted into their sodium salts
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Sulphonic acids are less soluble in water and have less affinity for cellulose than their sodium salts.
Small amount of soda ash is added to the solution to convert sulphonic acid to convert into sodium salt.
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Reason to convert sulphonic acid into salt is that then some of the dye will be wasted during their application to the textiles
The dye will not be fixed properly on the fiber
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Chemical Structure
DYE SO3H
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Direct dyes attaches themselves to the fiber through the formation of large number of weak attractions
These consist of hydrogen bonds and van der waals forces
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Cellulose is not ionic in nature so ionic bonds that exist between acid dyes and wool are not possible between direct dyes and cellulose.
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Classification of direct dyes
Depending on the effect of salt and temperature on the dyeing , direct dyes are classified into three groups
Group A Group B Group C
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Group A Migrate well Have high leveling power When dyeing these dyes, dyeing can be
uneven first but continued dyeing levels the shade
These do not need salt for exhaustion
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Group B Poor leveling properties ( not self leveling
dyes) Need controlled addition of salt for the
exhaustion Also known as salt controllable dyes
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If these are not taken up uniformly by the fiber in initial stage of dyeing then its very difficult to even out the shade
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Group C These are temperature controllable dyes Have poor leveling power ( not self- leveling
dyes) These are highly sensitive to salt
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Exhaustion cannot be controlled by the addition of salt alone
Need temperature control
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Application of Direct dyes
They can applied by two methods Exhaust method Continuous method
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Dyeing Method for Cotton and other Cellulosics
Exhaust Application Application of Azonine and Durantine Dyes to Cellulosic
fiber Azonine dyes are an economical range of direct dyes with
good color values for users where specific fastness properties are not the prime requirement.
Durantine dyes may be used to dye most cellulosic fibres and its blend by exhaust, continuous and printing techniques.
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At A add Azonine or Durantine Dye (predissolved)0.1 to 0.5g/l Neutrasol CR (for shades difficult to level)
At B add y g/l Glaubers salt (anhydrous)(1/5th of total amount required).
At C add z g/l Glaubers salt (anhydrous)(4/5th of total amount required).
At D rinse twice cold. Thoroughly rinse after dyeing to remove loose colour
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All direct dyes perform rather poorly with respect to wash fastness. Without an appropriate after-treatment, direct dyes bleed a little with every washing, losing their brightness and endangering other clothes washed in the same load. However, there are special after-treatments which may be used to solve this problem
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Dyeability of cellulosic fibers
The addition of electrolyte to a solution of direct dye tends to lowers the electrostatic repulsion between the negatively charged dye anions and promote aggregation or exhaustion.
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The effect of the presence of an inorganic salt when dyeing cellulosic is to overcome the long-range forces of repulsion between the dye anions and the negatively charged fiber surface
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The closer approach then allows the hydrogen bonding and other short-range attractive forces to operate between the dye molecules and the glucoside units of the fibrous polymer
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Hydrogen bonding that forms between the hydroxyl groups of cellulose and centers of electronegativity ( nitrogen, oxygen and sulphur atoms) in the dye molecule.
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The most hydrogen bonds that a dye can form with the glucosidic polymer, the more readily it can compete with and rupture the fiber-fiber hydrogen bonds in order to penetrate more deeply into the amorphous structure of the polymer
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Especially those substituted with hydrogen atoms such as (=N-NH-, NH2, -OH,)
This is widely acknowledged as it contributes to adsorption and the retention of dye molecule.
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Many direct dyes with high affinity for cellulose are disazo or triazo azo structures
The molecules of almost all direct dyes possess flexible chains of aryl nuclei linked by azo or other groups
Such structures can readily adopt a non-linear spatial conformation
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Ability to cover neps in cotton fabric
Pale flecks in dyed cotton fabrics can be caused by immature or dead cotton
This causes poor dyeability Poor dye penetration may leave undyed
areas if the neps of loosely attached immature fiber changes position after dyeing
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Direct dyes containing more than one amino or amide group in their structure were found most likely to achieve relatively good coverage of neps.
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Dye enters cellulose as single molecules and then aggregates inside the cellulose
A decisive factor in determining the rate of dyeing of many direct dyes from aqueous salt solutions is their tendency to aggregate
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Application of direct dyes
Direct dyes can be applied to cellulosic fibers by the following methods
Batchwise Semi and fully continuous methods
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Batchwise method
Direct dyes are water soluble dyes A solution can be made by adding them into
cold water and then put hot water while stirring
A wetting agent should be added to the dye bath to assist penetration and level dyeing
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They are normally applied at the boil Dye-bath is set at 40C and the temperature
is raised 2C per min and maintained at the boil for 35-45 minutes
During which salt is added according to the recipe
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Pastel shades are preferably dyed without the addition of salt
Many direct dyes suitable for application by combined scouring and dyeing of either woven fabrics on jigs or knitted fabrics on jets or winches
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In this process usual practice is to employ soda ash and a nonionic detergent.
Combined peroxide bleaching and dyeing with selected direct dyes is another long established process
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It offers savings of a process time and energy but more care is necessary to ensure good results
Sodium carbonate is preferred over caustic soda as there is a risk of oxidative degradation of direct dyes at higher PH.
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Semi and Fully Continuous method
They are less suitable for continuous application than for batchwise dyeing
Wet fastness of direct dyes may be lowered when they are applied by continuous processes
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This can be overcome by adding electrolyte or by increasing the impregnation temperature
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The pad-roll process is probably the most suitable semi-continuous method for dyeing cellulosic fibers with direct dyes
The temperature range should be 80-100C
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It is very important to control the fabric moisture content and pad-liquor temperature, dye selection for optimum compatibility and the use of appropriate auxiliaries
Its more versatile in production than the steaming methods
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After treatment processes
The wet fastness properties of all direct dyes are inadequate for many end uses but it can be improved by different types of after treatments.
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Diazotization and development
Many long established direct dyes containing primary amino groups could be diazotized and coupled onto the fiber with a variety of developers, including napthols, diamines and phenols to give larger molecules with improved wet fastness properties.
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Metal salt treatments
Treatment with acidified copper salt solution (0.25-2% copper sulphate and 1% acetic acid for 20-30 minutes at 60C) results in a marked improvement in the light fastness of certain direct dyes.
Washing and alkaline treatments removes the copper and light fastness becomes normal
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Cationic fixing agents
These compounds interact with the sulphonate groups present in direct dyes, conferring increased wet fastness in all tests at temperatures below 60C
They will also precipitate direct dyes from solution and therefore the dyed material must be cleared of loosely held dye before treatment.
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Color changes may occur and in some cases light fastness may be reduced
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Formaldehyde treatment
Treatment of certain direct dyes mainly blacks, with 2-3% formaldehyde (30%) 1% acetic acid (30%) for 30 minutes at 70-80C improves the wet fastness to both water and washing.
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A drop of light fastness may occur
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Cross linking agents and resin treatments
Improvements in wet fastness properties can be ensured by treatment with cellulose reactants or amide-formaldehyde resins
Subsequent removal of resin by acid hydrolysis (formic acid at 90C or HCL at 60C) leaves the unfixed direct dye on the fiber with its originally low level of wet fastness.
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Treatment with cross linking agents in resin finishing improves the wet fastness but color and light fastness may be affected
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Stripping of direct dyes
It is very easy to strip the direct dyes if these have not been after treated.
The colors can be destroyed by boiling with sodium dithionite Na2S2O4.H2O
Or by bleaching with hypochlorite solution containing 1-2gm/l of available chlorine
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If a cationic agent has been used as an after treatment, it is removed by boiling with 1-2% formic acid before destroying color with either reduction or oxidation
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For materials after treated with metal salts, the metal is first removed by boiling with a metal sequestering agent like sodium salt of ethylene diamine tetra acetic acid (EDTA) in a concentration of 3g/l and then the dye is decomposed by the usual treatment with hydros or chlorine