Introduction to Textile Dyeing

Introduction to Textile Dyeing By Dr. Abu Yousuf Department of Chemical Engineering & Polymer Science Shahjalal University of Science & Technology Sylhet-3114, Bangladesh

Basic Textile Terms and Definitions Dyestuff is organic or inorganic substances which can absorb light and reflect some lights to show color. Actually, the dyestuff is water soluble substances. Pigmentis a substance that can absorb light and reflect some lights to show color but it is water insoluble substances. Normally it is used for printing (with the presence of binder) or mass-coloration of the synthetic fibers.

Choice of dye You can not assume that to dye any piece of fabric to a given colour, all you need to do is use a dye of that particular colour.No dye will dye all textile fabrics satisfactorily.This means, simply, that you must choose a dye that will suit the material (or a material that will suit the dye). • Cost consideration • Shade and fastness requirements • Kind of material to be dye

Classification of Dyestuffs Dyestuffs can be classified by many methods as follows: 1. by their ionic (e.g. cationic, anionic and non-ionic) 2. by their fiber dyeabilities (e.g. on cellulosic, protein, polyester, polyamide etc.) 3. by their names (e.g. acid, direct, disperse, reactive, basic etc.) 4. by their chemical structures (e.g. azo, anthraquinone, stilbene etc.) 5. by their origins (e.g. natural, synthetic) 6. by their colors (e.g. red, green, black etc.)

Dyeing Processescan be applied in many stages such as • 1.Mass-colorationof the molten fibers • This method is for dyeing the molten fibers or plastic chips or textile polymers with pigment dyes. After that, the molten or polymers will extrude from a spinneret to form fibers. • Normally, the synthetic fibers are added with white pigment in order to give a hiding power (non-see through fabrics). • Advantage: give excellent fastness • Disadvantage: very difficult to clean

2.Fiber Dyeingis the method of dyeing fibers before blending with other colors to give fancy yarns or fabrics. • Note: This is used for special purposes only. • 3. Yarn Dyeing is the method of dyeing yarns in forms of hanks or packages dyeing. This will give Scottish’s style fabrics, carpet with many colors and styles. • Note: This is used in hand loom weaving in the Northern and North-eastern parts of Thailand.

4. Fabric Dyeing is the method after weaving, knitting, or non-woven to make fabrics. This is very popular method of dyeing as the dyed fabrics will be processed further to garment industries very easily. • Dyeing forms of the fabric dyeing can be used in 2 ways: • a. Open width form using the fabrics to spread without any creases and dye them. • b. Rope form using the fabrics with the form like a rope (many creases and look like “a rope”)

Some people classify into: • 1. Exhaustion Process • This method is using lot of water as shown in “Liquor Ratio (ratio between water and goods)” This should immerge the goods into dye solution for a long time in order to let the dye penetrate into the goods. This will lead to produce more waste water than the continuous process. • Advantage: inexpensive, no need to train the worker to look after and run them properly. • Disadvantage: lots of water needed, very slow process (60-120 min/batch.)

2. Continuous Process This method is designed by putting different machinery into a sequence so that it can produce the dyed fabric in one pass. Advantage: very fast process (10-100 m/min), small amount of water in the process. Disadvantage: very expensive, need to train the worker to look after and run them properly.

6.Garment Dyeing • This method is the last process of the dyeing of goods. However, the penetration of the dye solution may not be completely passed to the fibers such as between the seams, buttons, zippers etc. Normally, it is used for lingerie, socks, sweater dyeing etc.

Name of DyestuffsFibers that can be dyed Acid Silk, wool, polyamide, leather Basic Acrylic Direct Cellulosic, viscose Disperse Acetate, triacetate, polyamide, polyester, acrylic Reactive Cellulosic, viscose, protein Vat Cellulosic Sulphur Cellulosic

Factors that give some choices of the dyestuffs • 1. Cheap • 2. Non-toxic • 3. Compatible to other dyes and chemicals • 4. High color strength • 5. Better brightness • 6. Better fastness • 7. Good levelness on the materials

Dyes for Cellulosic Fibers Direct Dyes (Anionic) They can be dyed directly on cellulosic fibers. Without the presence of salt, when the fibers are immerged in water, it will show anionic charge which repels the dyes. Adding salt into the dye bath, it will reduce anionic on the fibers so the dyes can get closer and adsorb into the fibers. Advantage: cheap, easily dyed on fibers Disadvantage: poor wet fastness and some dyes have poor light fastness.

Textile Auxiliaries 1. Salt - reduce negative charges on the fibers 2. Water - dyeing media 3. Fixing Agent - enhance wet fastness but usually reduce light fastness Dyeing Conditions (Exhaustion) dyeing 100oC x 30-90 min fixing 60oC x 20 min (Continuous) pad --> dry --> steam --> wash --> soap --> dry

Reactive Dyes (Anionic) They can be dyed on cellulosic fibers. The mechanism is nearly the same as described in Direct dyes. Advantage: high wet fastness due to covalent bonding (Chemical Bonding) between fibers and dyes, easily dyed on fibers Disadvantage: Expensive

Dyeing Conditions 1. Continuous Process 1.1 Pad-dry-bake This is very easy method for cotton or cotton blends fibers. 1.2 Pad-dry-Pad (Chemical)-Steam This will give brighter and more intense color than method 1.1. 1.3 Pad-batch This will put the goods into the dye solution and squeeze with 2 rubber rollers to get rid of excess water and then batch at room temperature for 1-2 days.

2. Exhaustion Process This method will be used in small factory. Dyeing conditions will be 40-80oC for 30-90 min depending to the types and structures of the dyes.

Textile Auxilaries 1. Salt - reduce negative charges on the fibers 2. Water - dyeing media 3. Soda Ash - excite the dye to link with the fibers with covalent bonding. Therefore, it enhances wet fastness 4. Fixing Agent - enhance wet fastness for heavy shade but usually reduce light fastness.

Vat Dye (Anionic when soluble) The dye is named from the container (Vat) that used for rotting the dye with alkali solution. This crucial process will reduce the dye from insoluble to soluble dye (suitable for exhaustion in the cellulosic fibers). Now the manufacturer can synthesize man-made vat dye.

Dyeing Process 1. Dissolve dye into water (insoluble dye) 2. Vatting process by reducing the insoluble dye in alkali condition. (soluble dye) 3. Absorb into fibers (soluble dye) 4. After dyeing, oxidize the dye with oxidizing agent (insoluble dye) 5. Wash and soap the goods

Dyeing Conditions 1. Continuous Process 1.1 Pad-dry-Pad (Chemical) The solution of the dye is prepared without adding reducing agent to ensure leveling dyeing. After that, the goods are passed to chemical bath to reduce the dye into soluble dye and fix within the goods. Oxidizing agent is added to the goods and converted to insoluble dyes.

1.2 Pad-oxidize-pad-oxidize (many times) This will put the goods into the dye solution and squeeze with 2 rubber rollers to get rid of excess water, oxidize with the air and then immerse into the dye solution again and again to allow the dye penetrate into the goods.

Textile Auxilaries 1. Salt - reduce the negative charges on the fibers 2. Sodium hydroxide - adjust pH to the dyeing bath and make the suitable conditions for reducing agent. 3. Sodium hydrosulfite - reducing agent for the vat dyes. 4. Water - dyeing media 5. Oxidizing agents - (Hydrogen peroxide or Acetic acid or Air) oxidize the soluble to insoluble dye

Sulfur Dye (Anionic when soluble) The dyeing process has the same process as described in the vat dye except that using Sodium sulfide instead of Sodium hydrosulfide.

Dyes for Synthetic Fiber Disperse Dye (polyester, nylon, and acetate) (non-ionic) The dye is named from less water soluble and normally appeared in dispersion in water. The dye shows no charge due to the groups presented in the dye molecules. Dyeing Conditions Insoluble dye in water Less water soluble Water insoluble in polyester fibers

1. Exhaustion Process 1.1 At boil with carrier This process is suitable for acetate fiber and pale shade polyester fiber dyeing. Carrier is an auxiliary for swollen fiber in order to allow more dyes absorbing in. Now the use of carrier is reduced because it may be a carcinogen. 1.2 At 130̊C without carrier With the high temperature, the dyes will be dissolved into smaller molecules and the void in fiber structure will be opened. This will give heavy shade.

2. Continuous Process Pad --> Pre-dry (100oC x 30 sec) --> Thermofixation (180oC x 30 sec) --> reduction clearing (RC) --> wash --> dry Printing with Disperse Dyes With the sublimation property, the dyes can be printed on paper, place the fabric with a printed paper and then using an iron with high temperature pressed on the paper. The dyes will transfer from paper to polyester fiber. This is called “Transfer Printing”.

Textile Auxilaries 1. Dispersing Agent - make the dye solution stable and disperse in the dye bath. 2. Acid - adjust pH to the suitable condition for the dye bath. 3. Carrier - swell the fiber and dissolve the dye to make the dye getting into fiber. 4. Leveling agent - make more leveling dyeing (some will have adverse effect on slower dyeing) 5. Water - dyeing media

Reduction Clearing (R.C.) After dyeing process, the dyed goods should be removed excess dyes on the fiber surface. R.C. will give higher washing and rubbing fastness of the dyed goods. Using alkali solution, sodium hydrosulfide and dispersing agent, the R.C. will be done at 60oC x 20 min. Note: Do not use more than the recommended temperature because sodium hydrosulfide can be decomposed.

Acid Dye (Anionic) The dye is called acid because it needs acidic dyeing condition. It can be dyed on protein fibers (silk, wool, other animal fibers) and on polyamide fibers. Acidic condition will give the fibers showing positive charge. As the negative charge will attach directly to positive charge, and penetrate into the fibers. Dyeing conditions 100oC x 30-60 min

Textile Auxiliaries 1. Leveling agent - for levelness dyeing 2. Retarding agent - for inhibiting the dye not attach to fibers too fast 3. Acid - adjust the dyeing condition and make the fibers to show positive charges 4. Water - dyeing media

Basic Dye (Cationic) The dye shows positive charge.Normally, the dye can be applied on wool, silk and polyacrylonitrile (acrylic). Dyeing Conditions 100oC x 30-60 min

Textile Auxiliaries 1. Leveling agent - for levelness dyeing 2. Retarding agent - for inhibiting the dye not attach to fibers too fast 3. Acid - adjust the dyeing condition and make the fibers to show negative charges 4. Water - dyeing media

Dyes Dyeing Condition (pH) Acid (Wool) 2-4, 4-6, 6-8 (depends on types of dyes) Acid (Nylon) 4.5-5.5, 6-7 (depends on types of dyes) Basic 3.5-4 Direct 7.0 Disperse (Polyester) 5-6 Disperse (Acetate) 6.5-7.0 Disperse (Triacetate) 4.5-6.5 Reactive 7.0 (Exhaustion) 11.0 (Fixing) -------------------------

Introduction to Textile Dyeing