Exhaust Dyeing With Reactive Dyes
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Transcript of Exhaust Dyeing With Reactive Dyes
4/20/2012
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Exhaust Dyeing with Reactive Dyes
Dr. Tanveer Hussain
Dean Faculty of Engineering & Technology
National Textile University Faisalabad.
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Variables in Reactive Dyeing
• Dye variables
• System/process variables
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Dye variables in Reactive Dyeing
• Dye chemistry
• Substantivity
• Reactivity
• Diffusion coefficient
• Solubility
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Reactive Dye Chemistry
• Chromophore – Affects colour gamut, light fastness, chlorine/ bleach
fastness, solubility, affinity, and diffusion – Azo dyes are dischargeable. Disazo dyes have the
disadvantage of being much more sensitive to reduction and many of them are difficult to wash-off.
– Anthraquinone dyes exhibit relatively low substantivity and are easier to wash-off. Most of them possess excellent fastness to light and to crease-resistant finishes, but they are not dischargeable.
– Phthalocyanine dyes diffuse slowly and are difficult to wash-off
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Reactive Dye Chemistry
• Reactive Group
– Affects
• affinity
• efficiency of reaction with the fibre,
• dye–fibre bond stability,
– Determines
• Sat requirement
• Alkali requirement
• Temperature requirement
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Reactive Dye Chemistry
• S-triazine dyes
– Do not have good wet fastness properties in acidic media
– Due to their high substantivity, have poor wash-off properties.
– Monochlorotriazines have good fastness to light, perspiration and chlorine.
– The fluorotriazine groups form linkages with cellulose that are stable to alkaline media
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Reactive Dye Chemistry
• Reactive dyes of dichloroquinoxaline, monochlorotriazine and monofluorotriazine types show a tendency for lower resistance to peroxide washing and dye–fibre bond stability
• A lower sensitivity to changes in dyeing conditions (particularly temperature) is the most important characteristic feature of the monochlorotriazine-vinyl sulphone heterobifunctional dyes
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Reactive Dye Chemistry
• Vinyl sulphone reactive system – have poor alkaline fastness.
– chemical bond between the vinyl sulphone and the cellulosic fibre is very stable to acid hydrolysis.
– The substantivity of hydrolysed byproducts of vinyl sulphone is low, so washing off is easy
• The turquoise reactive dye shows an optimum dyeing temperature that is generally about 20 °C higher than that of other dyes with the same reactive group
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Dye Substantivity
• Substantivity is more dependent on the chromophore as compared to the reactive system.
• A higher dye substantivity may result in: – a lower dye solubility
– a higher primary exhaustion
– a higher reaction rate for a given reactivity
– less diffusion, migration and levelness
– a higher risk of unlevel dyeing,
– more difficult removal of unfixed dye
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Dye Substantivity
• An increase in the dye substantivity may be affected by:
– higher concentration of electrolyte,
– lower temperature,
– higher pH (up to 11)
– lower liquor to goods ratio
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Half dyeing time
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Dye Reactivity
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Dye Reactivity
• A high dye reactivity entails a lower dyeing time and a lower efficiency of fixation.
• Reactivity of a dye can be modified by altering the pH or temperature, or both.
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Diffusion Coefficient
• Dyes with higher diffusion-coefficients usually result in better levelling and more rapid dyeing.
• Diffusion is hindered by the dye that has reacted with the fibre and the absorption of active dye is restrained by the presence of hydrolysed dye.
• Different types of dyes have different diffusion characteristics. • For example, the order of decreasing diffusion is: unmetallised
dyes, 1:1 metal-complex dyes, 1:2 metal complex dyes; phthalocyanine dyes.
• An increase in the diffusion is affected by: – increasing temperature, – decreasing electrolyte concentration, – adding urea in the bath – using dyes of low substantivity.
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Dye Solubility
• Dyes of better solubility can diffuse easily and rapidly into the fibres, resulting in better migration and levelling.
• An increase in dye solubility may be affected by:
– increasing the temperature,
– adding urea
– decreasing the use of electrolytes.
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System Variables in Reactive Dyeing
• Temperature
• pH
• Electrolyte
• Liquor ratio
• Surfactants & other auxiliaries
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Effect of Temperature
• A higher temperature in dyeing with reactive dyes results in: – a higher rate of dyeing
– lower colour yield
– better dye penetration
– rapid diffusion
– better leveling
– a higher risk of dye hydrolysis
– lower substantivity
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Effect of pH…
• pH influences primarily the concentration of the cellusate sites on the fibre.
• Raising the pH value by 1 unit corresponds to a temperature rise of 20 °C.
• The dyeing rate is best improved by raising the dyeing temperature once a pH of 11–12 is reached.
• Further increase in pH will reduce the reaction rate as well as the efficiency of fixation
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Type of Alkali
• Different types of alkalis, such as caustic soda, soda ash, sodium silicate or a combination of these alkalis, are used in order to attain the required dyeing pH.
• The choice of alkali usually depends upon the dye used, the dyeing method as well as other economic and technical factors
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Effect of Electrolyte
• The addition of electrolyte results in: – increase in the rate and extent of exhaustion,
– increase in dye aggregation
– decrease in diffusion.
• The electrolyte efficiency increases in the order: KCl < Na2SO4 < NaCl
• There may be impurities present in the salt to be used, such as calcium sulphate, magnesium sulphate, iron, copper and alkalinity, that can be a source of many dyeing problems
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Effect of Liquor Ratio
• At lower liquor ratios, there is:
– Higher exhaustion, and
– Higher colour strength
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Effect of surfactants & auxiliaries
• Some anionics may enhance colour yield
• Some non-ionics may decrease exhaustion and colour yield
• Some non-ionics may slow down dye hydrolysis
• Triethanolamine (TEA) is known to enhance colour strength by enhancing the swellability and accessibility of the cellulose structure
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Reactive Dye Exhaustion
• Primary exhaustion – Occurs before addition of alkali
• Secondary exhaustion – Occurs after addition of alkali
• Rate of exhaustion can be increased by selecting dyes of high substantivity, increasing the temperature and increasing the electrolyte concentration.
• Degree of exhaustion can be increased by selecting dyes of high substantivity, lowering a bit the equilibrium temperature and increasing the electrolyte concentration and dyeing time.
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Reactive Dye Migration
• The intrinsic properties of a reactive dye that affect migration are: – substantivity, – molecular structure, – physical chemistry and stereochemistry.
• The higher the dye substantivity, the lower is the migration. • The external factors that affect migration are:
– concentration of the dye, – temperature, time, – liquor ratio, – liquor circulation – the form of the textile material
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Reactive Dye Levelness
• Levelness of dyeing may be inhibited by: – high substantivity,
– lower dye migration
– too much salt in the dyebath
– too high rate of exhaustion
– too high concentration of alkali
– a rapid shift of dyebath pH,
– too high rate of fixation
– too high rate of rise of temperature
– poor liquor agitation.
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Approaches to Obtain Level Dyeing
Controlled absorption can be obtained by salt dosing, alkali dosing, and/or controlling the rate of heating.
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Obtaining Level Dyeing in Light Shades
• During the primary exhaustion, the dye is free to migrate. • During the secondary exhaustion stage, dye migration is
poor. • For light dyeing shades (less than 1 % o.w.f.) the degree of
primary exhaustion is over 80% and the degree of secondary exhaustion is very small.
• Therefore control of the primary exhaustion stage is very important if level dyeing is to be obtained.
• The rate of primary exhaustion is dependent on the amount of electrolyte used.
• Dosing or split addition of salt is recommended to obtain level dyeing.
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Obtaining Level Dyeing in Medium & Dark Shades
• For medium shades, both primary and secondary exhaustion steps are important for obtaining level dyeing.
• Both controlled salt and alkali addition are important in this case.
• In the case of deep shades, the all-in salt addition may be possible, but during the secondary exhaustion, alkali dosing is important
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Obtaining level dyeing in general
• Dyes with high substantivity, low secondary exhaustion, and low MI (Migration Index) values require controlled addition of electrolyte after the addition of the dye.
• In contrast, dyes with low substantivity, high secondary exhaustion, and medium to high migration index values require precise control of liquor ratio, concentration of electrolyte, and addition profile of the fixation alkali
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Ways to Enhance Dye Fixation & Colour Yield
• Use of fixation accelerators • Use of shorter liquor ratio • Dyeing at low temperature (with decreasing
temperature the substantivity for fibre increases, causing increased exhaustion)
• Modification of chromophore and reactive group • Use of dyes with high substantivity and high reactivity • Treating cellulosic fibres with swelling agents • Modification in appearance techniques • Changing the morphology of fibre by chemical
modification.
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Approaches for Uniform Rise in Rate of Fixation
• Controlling the temperature of the dyeing process suitably (possible for hot dyeing dyes only);
• Adding alkali in stages (it is virtually impossible, however, to prevent a sharp rise in fixation rate whenever alkali is added);
• Starting with a weaker alkali such as soda ash, and following this with a stronger alkali, but only after a higher degree of fixation has been achieved;
• Progressive metering of alkali (such as the Remazol automet process);
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Exhaust dyeing method – example 1
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Exhaust dyeing method – example 2
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Washing-off of Hydrolyzed Dye
• Phases
– dilution of dye and chemicals (salt, alkali) in solution and on the surface of the cellulose;
– diffusion of the deeply-penetrated, unfixed, hydrolysed dye to the fibre surface; and
– dilution and removal of the diffused-out dye
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Washing-off of Hydrolyzed Dye
• Goods are rinsed cold twice to remove electrolyte & alkali
• then rinsed hot to desorb some hydrolysed dye from the fibre, prior to a
• ‘soaping process’ at or near the boil.
• A subsequent cold rinse completes the task of removing un-reacted and hydrolysed dye
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Washing-off Factors
• Dye substantivity • Diffusion behaviour • Liquor ratio • Washing temperature • Electrolyte concentration • pH • Presence of calcium and magnesium ions in the ‘boiling
soap’/hardness of water • Amount of unfixed dye • Washing time • Number of washing cycles/washing baths/Filling and draining • Washing auxiliary employed • Mechanical action
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Dye-fiber Bond Stability
• Dyes that react by a nuceophilic displacement mechanism show good stability to alkali and, to different degrees, less stability to acid.
• Dyes that react by nucleophilic addition give dye–fibre bonds with good stability to acid, but are less stable to alkali.
• The triazine–cellulose bond is generally resistant to oxidative breakdown in the presence of perborate, whereas this is a serious defect of some of the pyrimidine based systems
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Fastness of Reactive Dyes
• Factors
– The chromophoric group,
– the stability of the dye–fibre bond
– the completeness of the removal of the unfixed dye.
• To maximise wet fastness, particularly in deep shades, it is advisable to apply cationic after-treatments.
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Sustainable Reactive Dyeing
• Shorter/more robust dyeing procedures
• Reduced water consumption
• Reduced energy consumption
• Reduced effluent discharge
• Improved ecological image
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BASF has developed a new polymer which combines with a reactive dye hydrolysate to eliminate its substantivity for the substrate in the presence of salt.
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