Study On Wool Dyeing Process with Reactive Dyes

 

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Wool Dyeing Process

The somewhat late development of the fiber-reactive dyes was partly caused by a lack of appreciation of the considerable reactivity of fibers made of cellulose or proteins. Several dyes developed for wool with 2-sulfur ethyl sulphone or chloro acetylamino groups were not immediately recognized as reactive dyes. In fact, the development of major reactive dyes really started with the introduction of the production dyes for cotton by ICI in 1955.

Despite the many possible reactive groups in dyes capable of covalent bond formation with nucleophilic groups in wool, only a limited number of reactive dye types have been commercially successful. The dye chromophores are essentially those used for the cotton reactive group.

The most important reactive groups in wool are all nucleophilic and are found mainly in the side chain of amino acid residues. They are, in order of decreasing reactivity, thiol, amino, and hydroxyl groups. Difluoro-fluoropyrimidines undergo aromatic nucleophilic substitution of one or both fluorine atoms, the flooring between the two nitrogen atoms being the most reactive. Bromoacrylamido groups are stable in boiling water at Ph 7 and react by 

Chemical Structure of Reactive Group with Wool

                     

                            Dye-NHCOCH2Cl + Wool- SH   =       Wool-S-CH2CONH-Dye  + HCL

both nucleophilic addition to the double bond and nucleophilic substitution of the bromine atom. They can form a three-membered aziridine ring that can react further with the protein resulting in a new crosslink. The actual dyes are probably dibromo propion amides, which eliminate HBr on dissolving in hot water. Methyl-taurine ethyl sulphones and 2-sulfate ethyl sulphones form the vinyl sulphone reactive group relatively slowly at pH 5-6. This allows some leveling during dyeing before the vinyl sulphone group reacts with the wool and becomes immobilized. This is useful in hank and winch dyeing where the goods interchange is less favorable. In fact, all commercial reactive dyes for wool have absorption rates that are greater than the rate of reaction with the fiber to allow the migration. Chloro acetylamino groups react by an Sn2 mechanism.

Wool reactive dyes are applied like acid dyes in a weakly acidic solution. The degree of exhaustion and fixation are usually very high and clearing of unfixed dye from the group may only be needed for good shades. Reactive dyes for wool tend to be unlevel dyeing and are prone to give skitter dyeings. They are used mostly on loose fibers and slubbing on piece goods, where they accentuate fiber non-uniformly, poor and uneven fabric preparation. Several amphoteric or weakly cationic auxiliary products are available to assist level dyeing. Despite their good light fastness and very good wash fastness, they are not still widely used, partly because of their high cost. Red to maroon shades are very popular but there are no black reactive dyes available that can match the chrome blacks on wool.

Batch Dyeing of Wool with Reactive Dyes

Reactive dyes are usually applied to wool at 5-6 using ammonium salts, and acetic acid as required. At higher pH values, exhaustion is too low, and at lower values rapid dye uptake gives unlevel dyeings. Slightly higher pH values are used for dyeing pale shades pH (5.5-6.0)  and lower values pH (5-5.5) for deep shades. Reactive dyes often give quite good exhaustion at temperatures below the boil but the dyeing temperature will eventually be raised to 100C to ensure that the reaction with the wool is as complete as possible. Some procedures recommend a holding stage at an intermediate temperature (65-70) for (15-20) minutes to allow the dye to migrate before it reacts with the wool.  

Because they tend to give unlevel, skittery dyeings, reactive dyes are usually applied to wool in the presence of proprietary leveling agents. These are often amphoteric, having both cationic and anionic groups in the molecule. In contrast to most leveling agents, which decrease the dyeing rate, the auxiliary products for dyeing wool with reactive dyes accelerate dyeing. The anionic dye complexes with the cationic site in the auxiliary product but the remaining anionic site provides substantivity to the wool surface. The bulky dye auxiliary complex exhausts well onto the fiber surface at relatively low temperatures, better than the dye alone, but cannot penetrate into the fibers. the complex breaks down as the dyeing temperature increases so that the smaller liberated dye molecule can then absorb into the wool. The use of such products avoids unlevel, skittery dyeings and provides better compatibility with dye mixtures

Deeply dyed goods must be treated to remove unfixed dye to give good wet fastness. This is particularly important to ensure that there is no staining of adjacent undyed material during washing. After dyeing, the material can be washed at 80 C for about 15 min using a dilute ammonia solution at pH 8-8.5., and then rinsed in water with little acetic acid. To avoid any alkali damage to the wool, washing can be done with examine (hexa methylene tetra amine from formaldehyde and ammonia) at pH 6.5, or with sodium bicarbonate. Certain proprietary chemicals can be added to the dyedbath on completion of the dyeing and their hydrolysis increased the bath pH to around 7. For example, hydrolysis of sodium tri chloro acetate gives chloroform, and carbon dioxide, both of which are volatile and sodium hydroxide. The actual color removed may consist of unreacted dye, hydrolyzed dye, and products of the reaction of the dye with soluble wool hydrolysis products such as ammonia and hydrogen sulfite or amino acid

                         CCl3 - CO2Na +  H2O  =  HCCl3  + NaOH

Shrink-proof wool, which has been treated with resins in the thermoset process, remains cationic on the surface and gives rapid uptake of reactive dyes. The uneven auxiliary leveling agents may be less effective in this case. The deposited resin protects the wool from damage and the best fastness results for deep shades are obtained by dyeing at 110 C for 30 min.

Wool dyed in deep shades with reactive dyes is better protected from damage during dyeing. Several explanations for this have been proposed. These involve protein chain crosslinking, reaction with thiol groups that interfere with the reformation of disulfide links, and reaction with non-keratinous proteins in the cell membrane complex and endocuticle 

Semi-continuous Method for Reactive Dyes  on Wool

Reactive dyes can be applied to both wool clothes and tops by padding followed by batch for 24-74 hours. A thickener is added to the pad liquor to give a high solution pick-up. High concentrations of urea are used and the solution also contains sodium meta bi-sulfite. Urea breaks up dye aggregates of sodium meta bisulfite and reduces cystine crosslinks in the wool, generating more reactive thiol groups. This promotes more higher and uniform fixation. The combination of urea and meta bisulfite in the padding solution results in greater fiber swelling and better dye penetration. Dyes with activated double bonds react with bisulfite by nucleophilic addition and are converted into a non-reactive form. So, the pad solutions are not particularly soluble. The pad batch method can be used with reactive dyes normally applied to cotton, such as procion MX, and Drimarene K dyes. The dyed goods are treated with dilute ammonia, rinsed, and neutralized with acetic acid to remove the unfixed dye. This type of process is very economical and reduces fiber damage.