B a s e d o

Dyeing with Direct Dyes They directly dye the cellulosic materials without need of a mordant They have affinity for cellulosic materials andpossess property of dyeing the cellulosicmaterials with some resistance to removal bya subsequent washing Classification of DCD Based on Chemistry Azoic dyes Derived from amine diphenyleamine Derived from bezidine Thiozol dyes Based on the quantity of sodium carbonateneeded Dyes which dye in a neutral bath Dyes which dye in a slightly alkaline bath Dyes which dye in a strongly alkaline bath Based on leveling properties (SDC) Class A : self-leveling dyes Class B: dyes with average leveling salt controllable dyes Class C: dyes with poor leveling temperature controllable dyes Based on capacity to undergo furthertreatment T reatment with copper or chrome salts T reatment with formaldehyde Diazotization Finishing with resins, etc Based on capacity to undergo continuousdyeing Suitable for continuous dyeing or dyeing at lowliquor ratio Depends on limit of dye solubility, speed of fixing Not suitable for continuous dyes Based on suitability for HT dyeing Suitable for HT dyeing, above 100C Not suitable for HT dyeing Based on stability with respect to impurities oralkaline scouring Suitable for dyeing without scouring Suitable for dyeing after scouring Suitable for simultaneous scouring and dyeing Based on coverage of immature cotton Dyes with good coverage properties Dyes with poor coverage properties Based on commercial aspects Common direct dyes Light fast direct dyes Cationic fixable direct dyes Dyes treatable with metallic salts Dyes which can be diazotized and coupled General Properties

Solubility Direct dyes are generally water soluble Some require presence of sodium carbonate forsolubility Solubility increases with: Increase in number of sulphonic groups Increase in temperature Decrease in molecular size Diluted solutions are molecular Can form colloidal solutions when cold Effect of neutral electrolytes Salts like sodium chloride and sodium sulphate reduce solubility The higher the valency of the cation, the higher the effect Effect of alkaline electrolyte Products like sodium carbonate assist in solubility Effect of solvents Direct dyes dissolve in pyridine Effect of acids Mostly resist the action of acids Some acids may make the shade redder Some may result in dye extraction from the dyedfiber, e.g. oleic acid Effect of reducing agents Can cause break-up of dye molecule because of cleavage of the azo group Class A dyes properties Good leveling properties Can be used in exhaustion dyeing without anyspecial precautions, within the normal limits Class B dyes properties Medium leveling properties Salt controllable Dyeing is begun without salt, and as thetemperature rises adsorption is regulated bythe addition of sodium chloride or sodiumsulphate Class C dyes properties Low leveling properties T emperature controllable Dyeing is started at cold, and the exhaustion iscontrolled by gradual increase in temperatureas well as with controlled addition of the salt Effect of temperature on dyeing of direct dyes In the kinetic phase An increase in temperature produces an increasein exhaustion/absorption At the state of equilibrium An increase in temperature produces a decreasein absorption Effect of neutral electrolyte Sodium chloride or sodium sulphate Increases the rate and extent of exhaustion By neutralizing the negative charge of the fiber Effect is less pronounced in case of highaffinity dyes Increase dye aggregation

The higher the valence of cation of the salt, the higher is its absorption Therefore, the dyebath should preferably be free from high valence heavy metal, Mg, or Ca salts. Salt sensitivity The concentration of salt necessary to obtain 50% of exhaustion The less is this concentration, the more is the salt sensitivityof a dye Salt controllability The increase in exhaustion produced by an increase in salt concentration Effct of alkaline electrolyte Sodium carbonate or sodium hydroxide Improve dye disaggregation Reduce rate of dyeing Above 100C, azoic dyes may be hydrolyzed due to alkaline hydrolysis Effect of pH Neutral pH is normally used for most dyes Alkaline pH may be used for dyes whichexhaust too rapidly Acidic pH may be used for material whichcontain oxycellulose The acid more effectively diminishes the negative charge of the oxycellulose, increasing the affinity of the dye Effect of liquor ratio May range from 3:1 to 20:1 Influence of the dyeing kinetics is usually smallfor normal dyes However, for dyes which have poor solubility lower liquor ratio may result in dyebaggregation, slowing down rate of dyeing absorption If the liquor ratio is too high, the equilibriumexhaustion may be lower Effect of agitation Uniformity of dye concentration Uniformity of dyebath temperature Improvement in rate of dyeing Effect of fiber types Dyeability depends of degree of crystallinity Degree of absorption of viscose rayon> cottonmercerized without tension > cottonmercerized under tension> unmercerizedcotton> linen> hydrocellulose>oxycellulose Level of cotton fiber maturity also affects dyeability Increase in affinity can be obtained by mercerization or through cationization Formation of oxycellulose reduces affinity due to increased number of carboxylic groups; these increase negative charge of the cellulose The effect can be reduced by dyeing in slightly acidic pH, which neutralize the negative charge of the oxycellulose Y arns made from finer fibers appear to be lighter and vice versa Dyeing procedures Preparation of dye solution Dyeing After-treatment of the dyed material Preparation of dye solution

Soft water or preferably de-ionized water Solubility may be affected by the presence of salts or polyvalent ions Limits of dye solubility needs to be considered inmaking dye solution Dye is mixed with a small quantity of ambient waterto make a homogenous dough Then hot water (80C) is added along with stirring tomake-up the solution In few cases boiling may be necessary Dyeing processes Exhaust or batch dyeing methods Pad dyeing methods Semi-continuous Pad-jig Pad-batch (cold) Pad-batch (hot) Continuous Pad-steam Pad-dry Pad-salt Exhaust dyeing General Material is dyed in batches of lots Lot-to-lot variation may occur Suitable for dyeing loose stock, sliver, yarn and fabrics Exhaust dyeing in neutral bath Used for dyes with better leveling propertieswhich do not need retarder alkali Sodium chloride or sodium sulphate is used The role of salt is to promote/controlexhaustion Amount of salt may vary from 1-25% o.w.f.depending on dye affinity and dye amount Higher affinity dyes need less salt Higher amount of dyes need more salt Dyeing started at 40-50C with of electrolyte;the rest is added after reaching 100C, whichis maintained for further exhaustion, diffusionand reaching equilibrium T otal dyeing time 1-4 hours Depending on shade depth and material GSM For shade correction, temp. is lowered for dyeaddition Lastly the material is washed with water Exhaust dyeing in alkaline bath Usually used for dyes with very high dyeing rate Alkali acts as a retarding agent Usually sodium carbonate is used from 0.5-2%o.w .f according to the depth ; neutralelectrolyte is used from 1-25% o.w.f. Dyeing is started at ambient conditions Dye and sodium carbonate are usually added at the beginning After 30 min. temperature is raised to 60C Then neutral electrolyte (salt) is added in 2-3portions Then temperature is further raised slowly to the optimum dyeing temperature Finally, the material is washed with water

Pad dyeing Mainly used for fabrics The fabric to be dyed must have good absorbency and uniform moisture content Fabric pick-up is usually kept 60-80% For deep shaded, addition of urea (100-200 g/l) helps in improving dye solubility Other auxiliaries may include: Wetting agent Anti-foaming agent Anti-migrant Pad jig The fabric is padded in bath containing: Dye Wetting agent (if fabric has poor absorbency) Anti-foamer (if there is any chance of foaming) Urea (to improve solubility for deep shades) The fabric is squeezed for 60-80% pick-up Intermediate drying (optional) Development in jigger containing 5-15 g/l salt for L:R 3:1-5:1 10-40 ml/l of pad dyebath (for deep shaded to avoid excessivedye bleeding in the bath) T emperature of jig is raised to boil and maintained for 20-30 min. or for 4-8 passages Pad batch The fabric is padded in the dyebath Then squeezed for pick-up less than 100% Then the fabric batch is wrapped in polythene sheet For cold batching, the fabric batch is left rolling at ambient temperature for 8-24 hours depending on the shade depth For hot batching, the fabric is first heated up to 80-90C with IR, then the fabric batch is left rolling in a closed chamber for 1-8 hours Humidity is to be kept constant to avoid drying of upper fabric layers Pad steam After padding, the fabric is subjected to saturatedsteam at 100-105C Addition of urea (as disaggregating agent) in equalamount of dye, in padding helps in achieving dyesolubility as well as eases dye diffusion during steaming If steaming temperature is below 100C, quantity of urea should be increased and anti-migrant (e.g. sodiumalginate) should be specially included to avoidmigration In case of deep shades, after steaming, the fabric ispassed through 1-2 boiling water baths containing 10-20 g/l salt, then washed. Pad dry After padding the fabric in the dyebath, it is dried

Drying may be in two steps First IR drying (with 5-10% residual moisture) Then hot-flue drying/fixation (100-150C for 1/2-2min. depending on shade depth and fabric GSM) Pad saline bath For dyes which quickly fix in hot saline bath The fabric is padded and then passed through hot water/washing baths containing 10-20 g/lsalt Washing off Usually the dyed fabric is rinsed in plain water after dyeing If dyeing was done in alkaline bath, then the washing bath may contain small quantity of acid for fabric neutralization Detergents are rarely used in washing-off of direct dyes except for deep shades e.g. of marine/navy blue and black Aftertreatment T reatment with cationic agents T reatment with copper sulphate T reatment with chrome compounds T reatment with potassium dichromate and copper sulphate T reatment with formaldehyde treatment with cationic agents Basis Cationic agents form bond with anionic dye in the fabric Dye molecular weight increases and solubility decreases Col SO3- + Aux-NR3 Col SO3-NR3-Aux Exhaust treatment Give the dyed fabric a short mild rinse T reat the fabric in new bath containing 0.5-4%o.w.f. cationic agent at neutral or slightly acidic pH for 20-30 min. at 20-70C temp. depending on the type of cationicagent used Followed by rinsing and drying Continuous treatment 5-25 g/l cationic agent 40-60C Contact time 20-40 sec. Followed by rinsing and drying Treatment with copper sulphate Copper sulphate 1-4% o.w .f . 0.5-1.5% acetic acid (80%) or 0.5-1% formicacid (85%) 50-70C 30 min. Followed by washing or soaping at 40-50Cfor 10 min.

R-N=N-R-OH + CuSO4 R-N=N-R Cu O Treatment with chrome compound Sodium/potassium dichromate or chrome fluoride or alum chrome 3-4% o.w.f. 100C 30 min. 2 HO-R-N=N-R-OH + CR+6 R - N= N R T reatment with potassium dichromate and copper sulphate Also called chromatation Potassium dichromate 1-2% o.w .f . Copper sulphate 1-2% o.w .f . Acetic acid (6Be) 2-4% o.w.f. 60-70C 30-60 min. T reatment with formaldehyde Mostly suited for dyes containing diamine groups 1-2% o.w.f formaldehyde (30%) Slightly acidic pH 60C 30 min. 2Col-NH2+ CH2O Col-NH-CH2-NH-Col