Direct DyeDirect dyes are one of the most versatile classes of dyestuff applicable to cellulose, wool, nylon fibres. Although direct dyes have deficiency in wet colorfastness but it exhibits a wide range of fastness properties as shown by their use in apparel, upholstery, lining and automotive fabrics. Direct cotton dyes have inherent substantivity for cotton, and for other cellulosic fibres. Their aqueous solutions dye cotton usually in the presence of an electrolyte such as NaCl or Na2SO4. Direct dyes do not require the use of a mordant and, as their name implies, the dyeing procedure is quite simple. The goods go into the bath followed by the dissolved dyes. The bath is then gradually heated, usually to the boil, and additions of salt promote dyeing. Dyeing mechanism of direct dyes: The dyeing mechanism for the application of direct dyes to cellulose fibre involves the adsorption, diffusion and migration over fibre. Several factors persuade the dyeing mechanism; however, most important is the cellulose fibre structure, morphology and the use of electrolytes. When the cellulosic fibre is immersed into water the amorphous regions of the fibre swell to produce small pores in the order of 20-100A units- the smaller size dye molecules diffuse into the fibre structure through these pores. The addition of electrolytes (e.g. Sodium chloride, sodium sulfate) assists the diffusion and exhaustion of direct dye anionic by neutralizing the negative surface charge of cellulosic fibres. Then the dye anions become affixed to the cellulosic fibre through hydrogen bonding and van der waals forces. Typical recipe of Reactive dyeing: Wetting agent Sequestering agent Leveling agent Lubricant Direct dyes Soda ash Glauber salt Temperature Time pH M:L 1.0-2.0g/L 1.0-2.0g/L 0.5-1.0g/L 1.0-3.0g/L X% 1.0-5.0g/L 5.0-20.0g/L 955C 30-50minutes Neutral to alkaline 1:5-1:10
Process curve(Dyeing of cotton fabric with direct dyes):
Figure: dyeing cuve of direct dye Working Procedure of dyeing with reactive dyes: Set the dyebath with substrate at rom temperature. Add dye solution with auxiliaries and raise the temperature to 90C. Run the bath for 1520minutes and add salt step by step according to dye bath concentration, higher the depth of shade need more salt concentration. This is better to add this salt after reaching the temperature to boiling point since at this period the maximum penetration is achieved all over the substrate. Run the dyebath for 30-50minutes at 90-95C for complete the dyeing cycle. Cool down the dyebath temperature to 60-70C. Drop the bath and rinse. Carry on the aftertreatment process to improve wet fastness. Aftertreatment process: In after treatment process a suitable fixing agent is used for improving color fastness(wash, water, perspiration) properties; usually this process is done at 30-40C, sometimes at 60C for 15-20min or according to vender recommendation. After that a cationic softener is added to the last rinsing bath for improving handle properties of fabrics; this process is done at 4050C for 15-20min. Then wash with hot and cold water and dry to use.
Reactive DyeNowadays reactive dyes are very popular for textile coloration because of its some specific properties like colorfastness, wide range of shade, brilliance of shade, good reproducibility and simple application procedure. It reacts with fibre in presence of alkali and adheres as a part of fibre. Three principles steps are happened during reactive dyeing exhaustion, fixation or reaction and aftertreatment or washing. Mechanism of Reactive dyeing: A typical exhaust dyeing process for cellulosic materials using reactive dyes hasthree distinct phases: (1)the initial exhaustion phase. Dyeing is started in neutral solution so that there is little likelihood of the dye reacting with the cellulose. During this stage of dyeing, some reactive dye will be absorbed by the fibres, the amount depending upon its substantivity. This dye is capable of migration to promote level dyeing. Sodium chloride or sulphate will often be present initially or be added gradually to the dyebath during this phase to promote exhaustion. Thetemperature of the dyebath may be gradually increased to aid penetration of dye into the fibres and to assist migration. (2)the fixation phase. After the initial exhaustion phase, the pH of the dyebathis increased by complete or gradual addition of the appropriate type and amount of alkali. This causes dissociation of some of the hydroxyl groups in the cellulose and the nucleophilic cellulosate ions begin to react with the dye.The fixation process then results in additional dye absorption, to reestablish the dyeing equilibrium. Dye absorption from solution and reaction with thefibre then progress until no further dye is taken up. Figure 16.4 illustrates the variations of the dyebath exhaustion and the extent of fixation during a typical exhaust dyeing. (3)the post-dyeing washing. The rinsed dyeing contains dye bonded to the cellulose, absorbed but unreacted dye, as well as hydrolysed dye. There will also be residual alkali and salt. The latter are relatively easy to remove by successive rinsing in cold and then warm water. As much unfixed dye as possible must be washed out of the dyeing. If this is not done, desorption of this dye during washing by the consumer can cause staining of other materials
in the wash. Some unfixed dye is eliminated during the initial rinsing that removes salt and alkali. Thorough washing of the dyeing using a boiling detergent solution (soaping) eliminates the remainder. The dyeing is then finally rinsed in warm water. Soaping must often be repeated for deep dyeings or the residual unfixed dye must be complexed with a cationic agent.
Typical recipe of Reactive dyeing: Wetting agent 0.2-0.5 g/L Sequestering agent 1.0-2.0 g/L Antifoaming agent 0.5-1.0 g/L Dyes X% Glauber salt Yg/L (based on depth of shade, deeper shade higher amount of salt) Soda ash Zg/L (based on depth of shade, deeper shade higher amount of alkali) Lubricating agent 1.0-2.0 g/L Temperature 505C Time 30-60 min pH 10.50.5 M:L 1:5 1:10 Process curve for Reactive dyeing:
Working procedure: Set the bath with substrate at room temperature and add wetting agent, sequestering agent, lubricant and antifoaming agent. Add dyes and half amount of salt. Raise the temperature to 50C and add rest amount of salt and soda ash. Run bath for 30-60minutes at 50C. Drop the bath and carry on aftertreatment process. After-treatment: For better colorfastness aftertreatment is very important. First need to complete removal of salts and alkali by rinsing. Usually several rinsing process is done to got better result. Then the substrate is neutralized by acid wash. After that it needs soap wash to confirm that the goods are totally free from unfixed dyes.
Soap wash Recipe: Detergent 1.0-2.0 g/L Soda ash 0.5-1.0 g/L Temperature 95-100C Time 15minutes pH 8-9 M:L with enough water
Vat DyeVat dye is the most popular dyes for achieving high colorfastness on cellulosic fibres. But its an expensive class of dyes compared to reactive or other dyes used for cotton dyeing. It is insoluble in water and has no or little affinity for cotton fibre. Before dyeing the dyes must be converted to solubilized form by being reacted with caustic and hydroze-this state is called leuco form of vat dyes. The leuco vat dyes have affinity for the fibre. After exhaustion of these leuco dyes into the fibre, it is oxidized to form insoluble colored pigment again inside the fibre, thus, the dye molecules locks the fibre.
Mechanism of Vat dye: Vat dyeing process is carried out in two subsequent step- vatting and dyeing. But the whole process of cotton dyeing with vat dyes embraces several steps such as vatting, dyeing, oxidizing and aftertreatment. Vatting is a process developed for reducing the vat dye into soluble form. It is done by treating the insoluble vat dyes with strong reducing agent in presence of alkali. Thus produce leuco form of vat dyes and the process is known leuco vat dyeing. The predetermined amount of cotton goods is immersed into dye bath. In that time need to add additional caustic and hydroze mixture with required amount of hot water into dye bath to maintain liquor ratio. During
dyeing process air contact may oxidize the dye bath, which can hinder the successful dyeing. Usually the dyeing temperature is raised to about 80C or even high if the dyes are not sensitive to over-reduction and more stable to hydroz. The temperature of 100C and little higher may also be possible. After dyeing the reduced dye molecules are penetrated and distributed into the fibre. The exhausted dyes are then oxidized to convert it to insoluble form into the fibre. Before oxidation rinsing is very important for medium to deep shades to wash out the alkalies, reducing agent and loosely attached dye with heavy shades it might be necessary to include 1-2 g/L of both caustic and hydrosulfite and a little bit dispersing agent, for about 5 minutes at ambient temperature. Then rinse the goods with cold-hot-cold water successively. If necessary neutralization rinsing is done with acetic acid. Finally soften the goods as requirements.
Recipe of Vat dyeing: Wetting agent 0.5-1.0 g/L Sequestering agent 1.0-2.0 g/L Levelling agent 1.0-2.0g/L Dyes X% Caustic soda Y g/L Hydroze Zg/L Temperature 70-80C Time 30-50min M:L 1:10
Dyeing curve for Vat dye:
Working Procedure: First set the dye bath with substrate a 40C and add wetting agent, sequestering agent, leveling agent and then run for 5-10minutes. Add dyes according to substrate weight and run the bath for 5minutes. Add 2/3 of caustic soda requirements and run for 5minutes. Raise the temperature to 7080C at 2-3 C/minute and run for 5minutes. Add sodium hydrosulfite to the bath with rest 1/3 amount of caustic requirements. Run the bath for 30minutes and maintain reduct