Effect of PH on cotton dyeing with reactive dyes

শনিবার, ১ জানুয়ারী, ২০১১

Reactive dye:
Reactive dye are so called because their molecules react chemically with the fiber polymers of some fiber to from a covalent bond between the dye molecules and fiber polymer. Reactive dye is a class of highly colored organic substances, primarily utilized for tinting Textiles that attach themselves to their substrates by a chemical reaction that forms a covalent bond between the molecule of dye and that of the fiber. The dyestuff thus becomes a part of the fiber and is much less likely to be removed by washing them are dyestuffs that adhere by adsorption. The very first fiber-reactive dyes were designed for cellulose fibers, and are still used mostly in this way. There are also commercially available fiber-reactive dyes for protein and polyamide fibers. In theory, fiber-reactive dyes have been developed for other fibers, but these are not yet practical commercially. The dyes contain a reactive group that, when applied to a fiber in a weakly alkaline dye bath, form a chemical bond with the fiber. Reactive dyes can also be used to dye wool and nylon, in the latter case they are applied under weakly acidic conditions.
A fiber-reactive dye will form a covalent bond with the appropriate textile functionality is of  great interest, since, once attached, they are very difficult to remove.
The first fiber-reactive dyes were designed for cellulose fibers, and they are still used mostly in this way. There are also commercially available fiber-reactive dyes for protein and  polyamide fibers. In theory, fiber-reactive dyes have been developed for other fibers, but these are not yet practical commercially.
Although fiber-reactive dyes have been a goal for quite some time, the breakthrough came 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.
The first fiber-reactive dyes contained the 1, 3-5-triazinyl groups, and were shown by Rattee and Stephen to react with cellulose in mild alkali solution. No significant fiber degradation occurred. ICI launched a range of dyes based on this chemistry, called the Portion dyes. This new range was superior in every way to vat and direct dyes, having excellent wash fastness and a wide range of brilliant colors. Portion dyes could also be applied in batches, or continuously. The general structure of a fiber-reactive dye is shown below:

Figure : General structure of a fiber-reactive dye.

The four different components of the dye:
1        The chromogen is azo, carbonyl or phthalocyanine class.
2        The water solubilising 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. This means that reactive dyes are not unlike acid dyes in nature.
3        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.
4        The fiber-reactive group is the only part of the molecule able to react with the fiber. The different types of fiber-reactive group will be discussed below.
A cellulose polymer has hydroxy functional groups, and it is these that the reactive dyes utilize as nucleophiles. Under alkali conditions, the cellulose-OH groups are encouraged to deprotonate to give cellulose-O" groups. 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.
Reactive dyeing:
Reactive dyeing directly links the colorant to the fiber by formation of a covalent bond. For years, the idea of achieving high wet fastness for dyed cotton by this method was recognized, but early attempts employed conditions so drastic that partial degradation of the fibers occurred. Studies at a Swiss dyeing company called Ciba in the 1920s gave promising results with wool using colorants having monochlorotriazine groups. (Triazines are heterocyclic rings containing three carbons and three nitrogens within the ring.) However, there was little incentive for further development because the available dyes were satisfactory. These new dyes, however, were sold as direct dyes for many years without recognition of their potential utility as dyes for cotton. In 1953 British chemists Ian Rattee and William Stephen at ICI in London found that

dyes with dichlorotriazinyl groups dyed cotton under mild alkaline conditions with no fiber degradation. Thus, a major breakthrough for the dye industry was made in 1956 when ICI introduced their Procion MX dyes—reactive dyes anchored to the fiber by covalent bonds—100 years after the discovery of the first commercial synthetic dye by Perkins. The generation and subsequent bonding of these three new dyes (a yellow, a red, and a blue) with fibers has a common basis, namely, the reactivity of chlorine on a triazine ring. It is readily displaced by the oxygen and nitrogen of -OH and -NH2 groups. Reaction of a dye bearing an amino group with cyanuryl chloride links the two through nitrogen to form the reactive dye. A second chlorine is displaced (in the dyeing step) by reaction with a hydroxyl group of cotton or an amino group in wool. A key feature of cyanuryl chloride is the relative reactivity of the chlorines: only one chlorine reacts at 0-5 =C (32-41 °F), the second reacts at 35-50 °C (95-122 °F), and the third reacts at 80-85 :C (176-185 °F). These differences were exploited in the development of series of related reactive dyes.
At the German company Hoechst Aktiengesellschaft, a different approach had been under study, and in 1958 they introduced their Remazol dyes. These dyes are the sulfate esters of hydroxyethylsulfonyl dyes, which, on treatment with mild base, generate the vinylsulfone group. This group, in turn, reacts with cellulose to form a unique dye-fiber bond.
In the Procion T series, marketed by ICI in 1979, particularly for dyeing cotton in polyester and cotton blends by the Thermosol process the reactive dye is bonded through a phosphonate ester. The introduction of reactive dyeing not only provided a technique to overcome inadequacies of the traditional methods for dyeing cotton but also vastly
icreased the array of colors and dye types that could be used for cotton, since almost any chromogen can be converted.

Properties  of reactive Dyes: 


In  generally, textile material colored with reactive dye have a very good light-fastness, the   light fastness rating being about 6. These dyes have a very stable electron arrangement and provide very good resistance to the degarding effect of the ultraviolet component of the sunlight. There are some reactive dyes with only fair light fastness.

Wash fastness

Textile materials colored with reactive dyes have very good wash fastness, yhe washfastness rating is about 4-5. This is attribute to the very stable covalent bond that exists between the dye molecule and the fiber polymer.under the usual laundering and dry-cleanung conditions one finds in the home, there are few chemicals that have an effect on the covalent bond.


Textile materials which are colored with reactive dyes have to be throughly rinsed and scoured. Reactive dyes can react with the hydroxyl groups of the water molecule to produce dye molecules with poor sustantivity for the fiber. In fact it is thes molecules which have to remove by a washing-off process, involving scouring and rinsing. If thes molecules of dye are not removed, poor rub-fastness may result.

Effect of acids

The formation of the covalent bond between dye and fiber occurs under alkaline conditions. The presence of acids may reverse this process. Perspiration and atmospheric pollution which are both slightly acid may effect textile materials colored with reactive dyes and result in some fading.

Effect of chlorine

When reactive dyes were first introduced it was found that some of these were adversely affected by bleaches which contained chlorine.

Preconditioning Method of Reactive Dyeing:

Top of Form Bottom of Form
This is a very simple and beneficial way of dyeing medium and dark shades of Vinyl sulphone dyes. Although this process yields better results in the final production of shades by way of uniform and reproducible batches, this dyeing method has still not been followed popularly. For coarse variety of yarn, normal woven or knitted goods ( not suitable for very close and heavy fabrics like canvas or high GSM knits), this process is most economical and very useful.
The following few facts may be reason for avoid using this good method of dyeing.
·        Since it is applied in cold condition, everybody has a fear that some part of the fabric or yarn may not be get dyed properly.
·        The dyestuffs would not have reacted properly with the fiber due to cold conditions.
·        The application of salt, soda and even a part of caustic alkali added even before adding the dyestuff, may have created an illusion that this type of dyeing may result in streaky (strikes) dyeing due the presence of salt and alkali in the bath.
·        The doubt on the fastness properties of cold dyed material.
This process does not work well if the preparation of the material is not good. (But it is ::ue for all type of bad preparations). In reality, preconditioning is nothing but - .:t ionization of cotton and make it in to a more receptive substrate to yield deeper _nd uniform shades.

Meps in the Precondition method of Dyeing:
1        Set the dye bath with the basic auxiliary chemicals that we may have been using it, like a sequestering agent, leveling agent, lubricant, and de-foamer.
2          Load the RFD fabric in the machine.
3        Run blank in the auxiliary bath for 20 minutes.
4        A - Add 1/2 the amount of Salt, 1/2 the amount of Soda and 1/4 amount caustic soda - Run 10 minutes.
5        B - Add 1/2 the amount of Salt, 1/2 the amount of Soda and 1/4 amount caustic soda - Run 10 minutes.
6        C - Add 1/2 the amount of pre-dissolved and filtered dye solution - Run for 10 minutes
7        D - Add 1/2 the amount of pre-dissolved and filtered dye solution - Run for 10 minutes
8        Raise the temperature to 40 to 45°C on the fabric.
9        Run for 30 minutes - check the shade and make additions if necessary.
10. E - Add 1/4 amount of balance caustic soda - Run 10 minutes
11. F - Add 1/4 amount of the final amount of caustic soda - Run 90 minutes at 45°C
12. Drain — Cold Running Wash - Neutralize - Hot wash - Hot Wash -Cold Wash -Soap at Boil - Fix and do softening as required. (All hot washes should be done at min 60°C with good quality water).

Influence of pH in reactive dyeing at every stage of dyeing:

1        In the beginning of dyeing, the water bath should be carefully adjusted to a neutral to slightly acidic pH, as otherwise premature hydrolysis of dyestuff will take place and cause (a) uneven dyeing and (b) lighter depths than the previous batches or in other words batch to batch variation will occur.
2        If the fabric or yarn has not been neutralized properly the core alkali presence will adversely affect the dyeing, forming patchy uneven dyeing. The places were alkali residue was high have the tendency to make deeper dyeing.
3        Lower alkali dosages and hence lower pH leads to partial reaction of reactive dyes; most of the dye may remain in water; the dyestuff that has got absorbed in to the fiber would also have less tendency to get fixed on to it, leading to poor washing and rubbing fastness.
4        Higher dosage of alkali may cause hydrolysis of dyestuff in the water itself. Thus lower depth of shade and poor washing and rubbing fastness.
5        After dyeing is over, when the alkali still fully remain on the fiber, if we do not neutralize the alkali properly with adequate quantity of acid, that also leads to higher amount of dyestuff bleeding during subsequent soaping and hot wash operations.
6        Finally after completing the dyeing, before unloading, if we do not keep the pH neutral - alkaline pH will slowly hydrolyze the dyestuff in the fiber and acid pH will tender the cotton fiber itself.
7        Every dyestuff appears in different tone under different pH conditions. Bright Lemon yellow, if allowed dry under alkaline pH, it will turn to a dull redder yellow and similarly Turquoise blues and royal blues will appear yellowier and duller in alkaline pH and brighter and redder in acidic pH. So make sure that the pH is exactly neutral or slightly acidic during final drying process.
8        Final cationic fixation and cationic softening treatment if not done in acidic pH, that will leave higher tonal changes and improper dye fixation and improper softening effect.
Dyes with similar Exhaustion and Fixation values:

The Reactive dyeing takes place in three steps.

1.      Exhaustion ( primary and secondary)
2.          Fixation
3.        Wash off

Normally two types of exhaustion take place while dyeing. These are primary and secondary exhaustion.
Primary  exhaustion is the total amount of dye migrated on the substrate in the presence
f salt and alkali. While selecting a combination, one has to ensure that the Percentage Exhaustion (PE) and The Percentage Fixation (PF) of dyes should be similar.

Reactive ME Dyes                            PE is 60 to
Reactive HE dyes                              70% PE is 70 to
Reactive VS dyes                               80% PE is 40 to 50%

If is  always preferable to use dyes with PE about 60 to 70%, i.e., ME dyes. Patchy dyeing may occur if proper care is not taken while using dyes with higher PE, or lower PE.
The Chemistry of Dyeing: Reactive Dyes:

A molecule is much too tiny to see, but we can use models to show what the dye molecule is shaped like. Each of these balls represents a different sort of atom. These Cs are carbon atoms, like we see in charcoal. The Os are oxygen, like in the air we breathe, and these Cls are chlorine, like in bleach. This model shows we what the blue molecules in this bottle are shaped like. Different dye colors are made of different dye molecules. Here is a model of another dye molecule. The water containing that kind of dye molecule is red, as we can see. We can see that the models are shaped a little differently. Each different shape of dye molecule absorbs light differently. That's what makes the different colors! The fabric our clothing is made out of is also made of molecules. Cotton, which grows on a cotton plant, is made of long strands of cellulose molecules, all twisted together. If we put two molecules, the dye and the cotton, together, nothing will happen, unless we can get some of the atoms on the surfaces to come unstuck. If the H comes off of the cellulose, and the Cl comes off of one end of the dye molecule, the molecules will be able to react with each other and stick together. How do we get the H and the Cl to get off of the cellulose and the dye? We just add another chemical, called sodium carbonate: [model of Na2CC>3] what this does is increase the pH. That's how we say that it makes it less acid. An acid has a low pH. The opposite of acid is called a base. When we put baking soda in water, we get a high pH. A high pH is all that is needed to get the dye and the cellulose ready to react. Sodium carbonate is stronger than baking soda, so it works better for dyeing. All we have to do to make a permanent bond between the dye and the cotton is to put the dye on the cotton and add washing soda. We can put the sodium carbonate on the fabric before or after we put on the dye. After we put the dye and the sodium carbonate on the fabric, we just have to wait a while. While we wait, the reaction is happening - chlorines are coming off the dye molecules and hydrogen is coming off of the cellulose molecules. If they do this right next to each other, the dye then attaches to the cellulose, and a permanent bond is formed. If we leave it in a warm room for a few hours, we can then wash the excess dye out. We have to rinse it in cold water and wash it with detergent in hot water to get all the extra dye off. After all the excess dye is out, the dye left on the fabric is permanent.
The effect of different electrolytes on the sorption of the hydrolyzed form of four different reactive dyes has been investigated. The electrolytes studied were sodium, ammonium, Lithium, and magnesium chlorides, and ammonium sulfate, which differ widely in their ability to increase the sorption of hydrolyzed reactive dyes by cellulose. Their relative efficiencies were in the order: ammonium chloride  ammonium sulfate  sodium chloride lithium chloride ~ magnesium chloride.
The effect of the electrolytes has been discussed in terms of partial screening of the surface charge on cellulose by the crowding of the cat ions at the cellulose-water interface, pH of the bath, and the ability to modify the structure of water. The ability of the electrolytes to modify the pH of the solution plays a dominant role in sorption increase at lower concentrations of electrolytes, whereas at higher concentrations the ability of the electrolytes to modify the cellulose-water interface plays a decisive role.

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