exhaust dyeing with reactive dyes

4/20/2012

1

Exhaust Dyeing with Reactive Dyes

Dr. Tanveer Hussain

Dean Faculty of Engineering & Technology

National Textile University Faisalabad.

4/20/2012 Dr. Tanveer Hussain 1

Variables in Reactive Dyeing

• Dye variables

• System/process variables


4/20/2012

2

Dye variables in Reactive Dyeing

• Dye chemistry

• Substantivity

• Reactivity

• Diffusion coefficient

• Solubility


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


4/20/2012

3


• Reactive Group

– Affects

• affinity

• efficiency of reaction with the fibre,

• dye–fibre bond stability,

– Determines

• Sat requirement

• Alkali requirement

• Temperature requirement



• 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


4/20/2012

4


• 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



• 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


4/20/2012

5

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



4/20/2012

6

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


Half dyeing time


4/20/2012

7

Dye Reactivity


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.


4/20/2012

8

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.


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.


4/20/2012

9

System Variables in Reactive Dyeing

• Temperature

• pH

• Electrolyte

• Liquor ratio

• Surfactants & other auxiliaries


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


4/20/2012

10

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


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


4/20/2012

11

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


Effect of Liquor Ratio

• At lower liquor ratios, there is:

– Higher exhaustion, and

– Higher colour strength


4/20/2012

12

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


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.


4/20/2012

13

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


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.


4/20/2012

14

Approaches to Obtain Level Dyeing

Controlled absorption can be obtained by salt dosing, alkali dosing, and/or controlling the rate of heating.


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.


4/20/2012

15

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


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


4/20/2012

16

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.


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);


4/20/2012

17

Exhaust dyeing method – example 1


Exhaust dyeing method – example 2


4/20/2012

18

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


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


4/20/2012

19

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


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


4/20/2012

20

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.


Sustainable Reactive Dyeing

• Shorter/more robust dyeing procedures

• Reduced water consumption

• Reduced energy consumption

• Reduced effluent discharge

• Improved ecological image


4/20/2012

21



4/20/2012

22



4/20/2012

23



4/20/2012

24



4/20/2012

25



4/20/2012

26

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.


exhaust dyeing with reactive dyes

Documents