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Textile coloration, dyeing and printing of textile materials

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gezuketema2009

The document deliberates on the dyeing of textile materials.

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1 Wolkite University College of Engineering and Technology Textile Engineering Department Textile coloration By: Gezu K. 2012
INTRODUCTION TO TEXTILE COLORATION
3  Textile coloration refers to the process of imparting color to textile materials. COLORATION DEFINED DYEING PRINTING Only certain area of the substrate gets colored based on design requirement . The whole substrate gets the color by immersing in to a solution of the color. The textile material may be in one of several forms such as fiber, yarn, fabric, garment, etc.
DYES/PIGMENTS Coloring matters are required to provide the desired color to textile substrates These substances are known as dyestuffs and pigments In general dyes/pigments are organic molecules A dye is soluble in the application media and is substantive to the textile substrate. A pigment is insoluble & is not substantive to the textile substrate binding [by adhesive agent]. No interaction with fiber
5 Historical Development of Dyes Dye development stages Driving forces Natural dyes Poor substantivity & poor fastness Use of mordant with natural dyes Long and difficult process Indigo [Water insoluble pigment] Good fastness but sill long process First synthetic dye [Mauveine] Affinity for few fibers only & poor light fastness Acid type azo dyes Acid dyeing of wool and silk Direct substantive dyes Poor wet/wash fastness Synthetic indigo vat dye Process difficulty Fiber reactive dyes Limited substantivity [Hydrophobic fibers] Disperse dyes For hydrophobic fibers [method of application] Dyes
6 SUBSTANTIVITY AFFINITY  Colored compounds which are absorbed by the fiber from a solution or suspension where they are subsequently fixed .
Colour and Constitution of dyes 7  chromogen:  chromogen:  a chemical compound that is either coloured or can be made coloured by the attachment of suitable substituents – the chromophore and the auxochrome(s) are part of the chromogen;  Chromophore (chromophoric group):  Chromophore (chromophoric group):  An organic compound appears color due to the presence of unsaturated groups in it, such groups are chromophores  responsible for the appearance of color  all dyes contained aryl rings bearing unsaturated groups which termed ‘chromophores
8 Some typical chromophores are,  The colour intensity increases with the number of chromophores or degree of conjugated.
 Auxochrome:  a substituent group in a chromogen that influences its colour. such as –OH or –NH– groups,  The presence of auxochroms in the chromogen molecule is essential to make it a dye auxochromes are acidic such as carboxylic and sulphonic groups and in basic auxochromes includes amino and hydroxyl groups.
10 Diazo group Diazo group Amino group chromogen chromophore auxochroms azobenzene yellow 4,4′-diaminoazobenzene
11 HISTORICAL DEVELOPMENT OF DYES Dye development stages Driving forces Natural dyes Poor substantivity & poor fastness Use of mordant with natural dyes Long and difficult process Indigo [Water insoluble pigment] Good fastness but sill long process First synthetic dye [Mauveine] Affinity for few fibers only & poor light fastness Acid type azo dyes Acid dyeing of wool and silk Direct substantive dyes Poor wet/wash fastness Synthetic indigo vat dye Process difficulty Fiber reactive dyes Limited substantivity [Hydrophobic fibers] Disperse dyes For hydrophobic fibers [method of application]
12  Scientific classification based on chemical structure Example: Anthraquinone dyes, Azo dyes etc.  Technical classification based on dyeing properties Example: Direct dyes, Acid dyes, Disperse dyes etc.  Commercial classification based on manufacturers’ aspects [Brand names based on fastness, method of dyeing and so on] Example: Indanthrene, Remazol, Procion, etc CLASSICATION OF DYES
13 BASIC DYES DISPERSE DYES MORDANT DYES METAL COMPLEX DYES OTHER DYES * [Sulphur, chrome etc] CLASSIFICATION BASED ON DYEING PROPERTIES DIRECT DYES REACTIVE DYES VAT DYES AZOIC DYES ACID DYES
DYE SELECTION Type of fiber present Form of textile material & degree of levelness Fastness properties required Dyeing method used Availability of machinery Customer color requirement 14 Cost & Environment
Interaction of matter with light  Light is a form of energy propagated at high speed in the form of electromagnetic waves.  Limited range of the electromagnetic wave detected by human eye  Response of light: matter interaction ABSORBANCE REFLECTION VISIBLE RANGE [400 – 700nm] COLOR & DYES
 Color is produced due to selective absorption of the visible light. The reflected portion of the visible light corresponds to the color of the object. ALL REFLECTED ALLABSORBED WHITE BLACK
17 Wavelength (nm) Absorbed Light Reflected Light 400 - 440 Violet Greenish-yellow 440 - 480 Blue Yellow 480 - 510 Blue-green Orange 510 - 540 Green Red 540 - 570 Yellowish-green Magenta 570 - 580 Yellow Blue 580 - 610 Orange Greenish-blue 610 - 700 Red Blue-green Colors of typical spectral bands and perceived colors after absorption by a material viewed in white light
 Preparation of dye solution [Dye and auxiliary chemicals]  Application of the dye  Fixation of the dye  Aftertreatment Batch Continuous DYEING METHODS SALIENT FEATURES OF DYEING PROCESS The stages/steps in dyeing process are: EXHAUSTION IMPREGNATION Semi continuous
 Padding and squeezing  Fixation [Batching , Steam or hot air ]  Rinsing  Drying CONTINOUS DYEING PADDER
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21 Semi Continuous The fabric is first passed though with the dye-liquor, that is called a padding machine or padding mangle. Then it is subjected to batch wise treatment in a jigger stored with a slow rotation for many hours dyeing consists of Pad-batch, at room temp. Pad-roll at increased temp. by employing a heating chamber This helps in fixation of the dyes on to the fiber The dye is applied continuously by a padding. The fixation and washing remains discontinuous
22 BATCH DYEING  Discontinuous system of dyeing  The dyebath is allowed to exhaust by providing the necessary condition  Dye fixation takes place in the dyebath CIRCULATING LIQUOR IN A STATIONARY MATERIAL MATERIAL MOVEMENT IN A STATIONARY LIQUOR CIRCULATION OF BOTH LIQUOR AND MATERIAL Th ree general types of BD machine
AFTERTREATMENT Washing in detergent at or near the boil [Soaping] Treatment with chemicals to improve fastness Application of simple finishing chemicals 23
24 DYEABILITY FACTORS LIQUOR RATIO DYE CHARCTERISTICS DYEBATH ADDITIONS DYEING CONDIIONS FIBER CHRACTERISTICS CRYSTALLINITY & HYDROPHILICITY STRUCTURE & DIFFUSEABILITY TIME & TEMPERATURE SALTS & OTHER AUXILARIES TYPE OF MACHINE
25  Exhaustion refers to the amount of dye transferred from dye-bath to the substrate in the dyeing process.  Co: initial concentration of dye in dye bath Cs= concentration during the process  Dye fixation means the reaction between the dye and fiber molecules. EXHAUSION AND FIXATION E % =(Co – Cs)/Co * 100 PROCESS TERMINOLOGIES
COLOUR YIELD DEPTH OF SHADE Paleness or dullness of a given shade Shade depth per a given amount of dye
27 DYE MIGRATION LEVELLING Tendency of dye transfer from heavily dyed region Uniformity of shade throughout the substrate
28 FASTNESS LIQUOR RATIO Resistance to color change or color removal Ratio of weight of material to volume of liquor COMPATIBILITY Dyes having same or similar rates of dyeing
THEORY OF DYEING
Fundamental steps in dyeing From molecular point of view dyeing process involves • Transport of dye from dye bath to fiber surface • Adsorption on the surface of fiber • Diffusion in the fiber interior • Fixation of dye 30
STUDIES ON THEORY OF DYEING 31 THERMODYNAMIC APPROACH KINETIC APPROACVH Dyeing equilibrium Rate of Dyeing
32  Exhaustion of the dye in solution takes place in dyeing process  After the initial increase in the degree of exhaustion, it eventually becomes constant.  No net transfer of dye from solution to the fiber. At this point dyeing equilibrium is established. DYEING EQUILIBRIUM Subsidiary equilibria is established between dye dissolved in the bath & dye adsorbed on fiber surface; between dyed adsorbed & dye diffused in the fiber. Nernst Isotherm Langmuir Isotherm Freundlich Isotherm In the case of dyeing equilibria, the most commonly encountered adsorption isotherms are the partition isotherm :
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34 Nernst Isotherm Dye is equally divided between solution and fiber which is typically displayed for the adsorption of disperse dyes on hydrophobic fibers such as PES.
35 Rapid sorption limited by accessibility of fiber surface sites Freundlich Isotherm the adsorption of direct dyes on cotton and other cellulosic fibers.
36 Sorption limited by fixed number of adsorption sites Langmuir Isotherm the adsorption of anionic dyes (e.g. non- metallized acid dyes) on wool, silk and PA fibers, as well as basic dyes on PAN fibers.
AFFINITY 37  The attraction of the dyes to fibers is generally expressed as AFFINITY.  During dyeing mass changes occur that change thermodynamic functions The chemical potential is defined as the change in free energy of a system that occurs when the composition changes by a unit molar amount of substance, all other variables such as the temperature, pressure and the amounts of other components remaining constant. CHEMICAL POTENTIAL µ = µ0 + RT ln (a)
38 In the case of dyeing, if the chemical potential of the dye in solution is higher than in the fiber, the dye will transfer to the fiber. The chemical potential in solution falls; that in the fiber increases. At equilibrium, the chemical potential of the dye in the fiber is equal to the chemical potential of the dye in the solution.  The standard affinity of a dye for a fiber is defined as the difference of the standard chemical potentials of the dye in the two phases.
39  The standard affinity is the change in chemical potential of dye when one mole is transferred from standard state in solution to standard state in the fiber.  The standard molar free energy change for dyeing Enthalpy of dyeing [-ΔHo] Entropy of dyeing [ΔSo] Immobilization of dye in the fiber MEASURE OF STRENGTH OF DYE-FIBER BONDS
40 EXOTHERMIC NATURE OF DYEING Establishment of equilibrium involves decrease in the total free energy  Hence heat is given out during dyeing  Exothermic reactions are favored at low temperatures
 Fick's Law of diffusion: The rate of transport of dye (dQ/dt) across a given area is proportional to concentration gradient RATE OF DYEING
• The rate of dyeing increases with increasing temperature. • The equilibrium uptake of dye decreases owing to the exothermic nature of dye adsorption. Effect of Temperature on Dye Diffusion
MODELS OF DYE DIFFUSION 43 FREE VOLUME
Cotton, flax and CLY Hydrophobic substrates such as PES
45 Rapid Dyeings @ shorter half dyeing time HALF DYEING TIME  The time required for the exhaustion to reach half [50%] of is equilibrium value.
FACTORS ON RATE OF DYEING  Type /construction of material  Dyebath temperature & pH  Type of dye-bath additions  Liquor ratio  Degree of agitation
DYE-FIBER INTERACTIONS 47  Ionic links  Hydrogen bonds  Covalent bonds  Physical forces
Cellulosic dyeing 48 Direct dye Reactive dye Vat/indigo dye Sulphur dye
Dyeing of Cellulosic Textiles Dyes for cellulosic fibers: • Cotton and other cellulosic fibers can be dyed with Reactive, Direct, Vat, Sulphur, Azoic, Phtalogen, Pigment and Mordant dyes. • Application methods, dyeing characteristics, cost, fastness properties and color range are differ for each dye. • They have their own particular advantages and disadvantages. 49
DIRECT DYEING
Dyeing processes which involve use of direct dyes Direct dyes are so called direct because they are applied to cellulosic fiber textiles directly with out the need for a mordant. Typical characteristics of Direct Dyes • Inherent substantivity to cotton and other cellulosics • Do not require mordant, dyeing procedure is quite simple. • Direct dyes are anionic in nature • Their aqueous solutions dye cotton usually in the presence of an electrolyte such as NaCl or Na2SO4. • Generally these dyes are water soluble • Relatively inexpensive, available in a full range of hues but not noted (known) for their color brilliance. Introduction
• Direct dyes usually have linear long coplanar molecular structure • Sulphonated azo dyes constitute the predominant group of direct dyes • Their major drawback is their poor to moderate fastness to washing. • Various treatments to improve fastness • Dye Solubility depends on Number of solublizing groups, Molecular weight and temperature of dyeing After treatment Solubilizing group
53 By bringing dye bath to boil gradually and holding at boil whilst the dye diffuses into the fiber. In absence of electrolyte some direct dyes will not dye cellulose at all. Usually applied with the addition of electrolyte at or near the boil. Dyeing with direct dyes is carried out in neutral solution Simple dyeing procedure Requirement of soft water for some direct dyes during application Application of direct dyes to cellulose
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CLASSIFICATION OF DIRECT DYES 56 CHEMICAL STRUCTURE OF CHROMOPHORE DIRECT AZOIC DYES DIRECT THIAZOLIC DYES Classification based on chemical structure is not significant for the dyer because dyes with similar constitution can have quite different application and fastness properties.
57 DYEING PROPERTIES [EQUALIZING] Class A: Self leveling Class B: Average leveling Class C: Poor leveling The most common classification of direct dyes is that of the Society of Dyers and Colorists (SDC), based on their levelling ability and their response to increase in the dyeing temperature and to added salt during exhaust dyeing.
58 Class A: Self leveling Low molecular weight => mono- and bis-azo dyes with several anionic sulphonate groups per molecule. Good migration, even in the presence of salt. Several sulphonated groups per molecule Good solubility and no aggregation Low substanivity Large amount of salt for exhaustion
59 Dyeing is started at 50 °C in the presence of added salt, the bath heated to the boil over 30–40 min, and dyeing continued at the boil for up to an hour. Several further salt additions, of increasing size, are required to promote exhaustion, the total amount of salt (5–20% owf NaCl) depending upon the depth of shade and the liquor ratio.
60 Class B: Salt Controllable Molecular weight higher than Class A Lower sulphonated groups than Class A dyes Medium solubility and degree of aggregation Low to moderate substanivity [Absence of salt] Salt sensitive & exhaust well with salt addition
61 Class C: Temperature controllable High molecular weight Few sulphonated groups per molecule Less solubility and high degree of aggregation High substantivity Very sensitive to salt addition Leveling by control of temperature & leveling agents
TYPE MIGRATION LEVELING FASTNESS Class A Good Self leveling Low Class B Medium Medium leveling Moderate Class C Poor Poor leveling High Additional Classification Parameters: Further treatment, continuous dyeing, high temperature dyeing COMPARISON OF DIRECT DYES
63 DIRECT DYE SUBSTANTIVITY FOR CELLULOSE Long, coplanar dye molecules can sit on top of a cellulose polymer chain with the aromatic rings parallel to the glucose rings.  Possibility of hydrogen bond formation  Vander walls and other secondary forces of interaction
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65 Their long flat (coplanar) structure enables them to lie along a cellulose chain in register with hydroxyl groups and results in effective vander waals forces.
66 Dyeing methods The selection of specific direct dyes for dyeing cellulosic fibres depends on Their dyeing properties, The particular fastness requirements, Any after treatments used to improve the washing fastness, and The particular finishing processes involved.
67 A typical dyeing process for a Class B direct dye on cotton EXHAUST DYEING
68 Immerse the wetted fabric NaCl NaCl 40oC 100oC 35 min 25min 35min
Salient Features of Batch Dyeing 69  Solution preparation starts with small dye paste to which is added sufficient hot water for dissolution Wetting agent assist penetration and level dyeing Depth enhancement by controlling salt addition & temperature  Examples  Hank and package dyeing for cotton yarns  Jigger for woven fabrics  Winch for knitted fabrics
PAD DYEING 70 Major pad dyeing operations:  Pad dyeing methods are only used on fabrics Pad with dye solution, pad with salt bath and roll Pad with dye solution, dry, pad with salt solution and steam Pad with salt, pad wet-on-wet with dye solution and steam Dyes with less strike rates are preferable.  Migration of Class A during steaming/drying & very rapid strike by Class C dyes  Class B dyes are better choice for continuous dyeing Less Suitable for continuous dyeing
71 FACTORS AFFECTING DIRECT DYEING EFFECT OF SALT  Cellulose immersed in water develops a negative surface potential  Repulsion between negative charge of cellulose surface & anionic dye molecule The added salt provides sodium ions to counteract the negative surface potential of the wet cotton Hence increase in exhaustion of direct dyes Commercial direct dyes already contain much electrolyte.
The nature of the anion of the added electrolyte has little influence on the amount of adsorbed dye. Therefore, NaCl and Na2SO4, at the same total sodium ion concentration, have about the same influence. A higher positive charge on the cation promotes increased adsorption because metal ions counteract the negative surface potential more effectively, thus decreasing the repulsion of approaching dye anions. 72
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75 B is more salt sensitive than A
EFFECT OF TEMPERAURE 76 • Temperature increases dye migration and rate of dyeing. • Equilibrium exhaustion decrease as dyeing temperature increases DYE DEAGGREGATION RATE OF DYEING EXHUASTION
These diverse variations of direct dye exhaustion depend on two opposing influences of the increasing dyeing temperature. 1. These are the usual effect of increasing temperature decreasing the dyebath exhaustion because dyeing is exothermic, and its enhancement of the dyeing rate particularly at lower temperatures. 77
2. Increasing temperature also promotes dye de-aggregation in the dyeing solution liberating more individual dye molecules to enter the fibre. The dyes CI Direct Yellow 12, Direct Red 81 and Direct Yellow 28 have maximum exhaustion at 30 °C (a), 60 °C (b) and 100 °C (c), Respectively. For example, open jig dyeing machines cannot achieve dyeing temperatures much above 85 °C. 78
 Greater negative potential under alkaline pH and retardation  Increasing dissociation of a number of cellulose hydroxyl groups  Oxycellulose (over bleaching) presence reduction in exhaustion and depth Higher proportion of carboxyl groups  Alkaline reduction of azoic dyes reducing color yield  Acid baths specially strong acid not suitable [Cellulose Damage]  Dyeing of cotton with direct dyes at neural pH 79 EFFECT OF DYEING PH Reduce exhaustion Carboxylate ions repel the dye anions of like charge.
EFFECT OF LIQUOR RATIO Lower liquor ratio enhances exhaustion for substantive dyes Dyeing at low liquor ratio  Decreases the amount of waste dye in the effluent.  Consumes less water and steam, and  Allows a given salt concentration with less added salt.
Inhibit the formation of larger aggregates and increase the proportion of dye in the monomolecular form. Hence solubilisation is increased. EFFECT OF SURFACTANT TIME OF DYEING The production of level and well-penetrated dyeings is usually favored by an increased time of dyeing, although prolonged dyeing at boil sometimes results in the decomposition of direct dyes.
82 DIRECT DYEING OF DIFFERENT CELLULOSIC FIBERS COTTON < MERCERIZED COTTON < LINEN < VISCOSE RAYON Dyeing of blends of these cellulose fibres with direct dyes, they do not absorb dyes at the same rate or to the same extent because of the differences in their morphology. With dyes of low substantivity, neps usually absorb less dye and appear as paler spots on the fabric. Since immature fibres give much greater rates of dye desorption, the paler dyed neps may not appear until after washing. Improved by mercerization before dyeing swells the immature fibres The rate of dyeing increases as the diameter of a fibre decreases
83 Direct dyes have higher substantivity for regular viscose than for cotton because viscose is less crystalline and oriented so it has a much greater internal surface. It is more accessible than regular cotton Mercerized cotton will absorb more dye and be darker in color than normal cotton dyed in the same bath.
HIGH TEMPERATURE DYEING 84 Conventional direct dyeing process @ boil High temperature dyeing up to 130 C Reductive cleavage at higher temperature Decomposition of direct dyes REDUCTION OF DYES USED IN STRIPPING & DISCHARGE PRINTING POOR WET/WASH FASTNESS
AFTERTREATMENT OF DYEINGS WITH DIRECT DYES 85  Aftertreatment is carried out to improve washing fastness Basic principle involved is increasing the dye’s molecular weight  Adsorbed dye becomes insolublized & has low diffusion coefficient  Impact on hue and light fastness  Aftertreatments are difficult and costly to carry out  Replacements of direct dyes today reduce use of aftertreatment Reactive dye
AFTERTREATMENTS IN DIRECT DYEING • Metal complex formation • Aftertreatment with formaldehyde • Use of cationic fixatives • Based on resin crosslinkks 86
METAL COMPLEX FORMATION 87 Treatment with acidic copper sluphate solution for the formation of a complex between copper and adsorbed dye
88 FORMALDEHYDE TREATMENT Based on formation of methylene bridge between adsorbed direct dye molecules
89 TREATMENT WITH CATIONIC FIXATIVES Cationic agents of relatively high molecular weight form water- insoluble salt like molecule with anionic direct dye. Precipitation of anionic dyes in the cotton with a cationic surfactant or polymer in warm water Based on addition of crosslinking agents used for easy care finishing Direct dyed cellulose is crosslinked using crosslinking agents Surface dyes have less tendency of movement Wash fastness is improved due to strong anchoring effect RESIN TREATMENT Reduce the fastness light The change in hue is only slight Decrease the light fastness and give a hue change. Amino resins
90 Ionic bonds involve the attraction between oppositely charged chemical groups on the dye and fibre. Example
REACTIVE DYEING
INTRODUCTION • Poor washing fastness because only weak polar and dispersion forces bind the dye molecules to the cellulose polymer chains. • Weaker forces of interaction (Hydrogen and physical bond) [Dye – fiber] Immobilizing dye molecule by covalent bond formation with reactive groups in fiber Easily diffuse out of the cotton during washing. A dye which is capable of reacting chemically with a substrate to form a covalent dye substrate linkage Direct dye Reactive dye
Cont. … A L K A L I N E Mild The role of the alkali is to cause acidic dissociation of some of the hydroxyl groups in the cellulose, and it is the cellulosate ion (Cell–O–) that reacts with the dye. Reactive chlorine atom on the triazine ring Anionic and water soluble like direct dye
Simple molecular structure & low substantivity High degree of wash fastness Bright shade Complete color gamut /wide color range Relatively simple dyeing procedure [no redox system] Versatile in application [both bath & continuous] Characteristics of Reactive Dyes
95 STRUCTURE OF REACTIVE DYES Solubilizing group (S)  Chromophore (C)  Bridge group (B)  Reactive group (RG) with a leaving group (X) attached to it ≥1 sulphonic acid substituents Reacts with Cell-OH Color + Substantivity Links R with C
The reactive group must exhibit adequate reactivity towards cotton, but be of lower reactivity towards water that can deactivate it by hydrolysis. The dye–fibre bond, once formed, should have adequate stability to withstand repeated washing.
THE REACTIVE SYSTEM TRIAZINYL RING VINYL SULPHONE Reactive dyes look like direct, but with added reactive group Anionic and water soluble like direct dyes NH
98 MECHANISM OF DYEING WITH REACTIVE DYES Reactive groups are of two main types 1. Those reacting with cellulose by nucleophilic substitution of labile leaving groups such as chlorine, fluorine, methyl sulphone or nicotinyl leaving group activated by an adjacent nitrogen atom in a heterocyclic ring
99 In nucleophilic substitution, a mobile halogen atom of the dye is substituted by the ionized nucleophilic oxygen group of the cellulose (Cell-O).
100 2. Reaction with cellulose by nucleophilic addition to a carbon– carbon double bond, activated by an adjacent electron-attracting sulphone group. The vinyl sulphone group is usually generated by elimination of sulphate ion from sulphatoethylsulphone with alkali. CELLULOSE ETHER FORMATION
101 Covalent bond formation In nucleophilic addition, a proton and the ionized oxygen group of the cellulose are added to the active group of the dye. The best-known dyes of this group – the vinyl sulphones – carry a protective group, which is eliminated even at a fairly low pH and sets the reactive group free during dyeing.
102 Dyed sample Hydrolysis
103 Basis of Classification Based on chemical structure Based on dyeing property Commercial – Manufacturer aspect of brand name CLASSIFICATION OF REACTIVE DYES
104 [AZO & ANTHRAQUINONE] MONOFUNCTIONAL BIFUNCTIONAL HOMOBIFUNCTIONAL HETROBIFUNCTIONAL REACTIVE GROUP CHROMOPHORE GROUP Based on chemical structure POLY-FUNCTIONAL
Monofunctional Reactive Dyes Presence of reactive group (one or two) at a single location on dye molecule Reactive dyes developed at early stages were mono-functional typical examples are  Mono-chloro triazine  Di-chloro triazine (two reactive groups located on the same triazine ring)  Vinyl sulphone
DICHLOROTRIAZINYL [DCT] Monofunctional Reactive Dyes MONOCHLOROTRIAZINYL [MCT] VINYLSULPHONE [VS] No separation of reactive groups from each other Reactive group attached to a single Chromophore
107 DCT MCT VS Low reactivity Medium reactivity High reactivity
108 Other Monofunctional Reactive Dyes
BIFUNCTIONAL REACTIVE DYES • Presence of two reactive groups of same type (mono or dichloro triazine) or different types (monochloro triazine and vinyl sulphone). • At two different locations in the dye molecule. • Show high exhaustion, high fixation • Better color yield • Reduced pollution: less dye in effluent • Very popular for exhaust dyeing. • High exhaustion is due to high molecular weight similar to direct dyes, high affinity to cotton • React with cellulose with cross link formation.
BIFUNCTIONAL REACTIVE DYES BIS(AMINOCHLOROTRIAZINYL) AMINOCHLOROTRIAZINE-SULPHATOETHYLSULPHONE
111 BIS(SULPHATOETHYLSULPHONE)  Homobifunctional Reactive Dyes Consist of two similar reactive groups
112 Reactive Dye Reactivity Different reactive groups of reactive dyes show a wide range of reactivity The more reactive the dyes the less weaker the alkali required and The lower the reactivity of reactive group of the dye towards the alkaline cellulose, the higher the final dyeing temperature required.
113 DYE REACTIVITY DYEING TEMP. pH DCT HIGH 20 - 40 Weak alkali (NaHCO3 or Na2CO3) MCT LOW 80 - 85 Strong alkali (Na2CO3 or NaOH) MFT MODERATE 40 - 60 Moderately alkaline VS MODERATE 40 - 60 Moderately alkaline NT* MODERATE TO HIGH 100 - 130 Neutral ALKALIS USED: Sodium bicarbonate, sodium carbonate & sodium hydroxide/Sodium silicate
CLASSIFICATION BASED ON DYEING PROPERTY 114  Alkali controllable reactive dyes Salt controllable reactive dyes  Temperature controllable reactive dyes On the basis of dyeing temperature COLD BRAND, WARM BRAND & HOT BRAND
ALKALI-CONTROLLABLE REACTIVE DYES  Have high reactivity and only moderate substantivity  Applied at relatively low temperatures  Level dyeing requires careful control of the addition of alkali Examples: DCT, DFCP and VS
 These are dyes of relatively low reactivity  They have appreciable substantivity  Dyeing temperature as high as 85 °C  Requires careful addition of salt Examples: MCT and MFT SALT-CONTROLLABLE REACTIVE DYES
117 • At temperatures above the boil in the absence of alkali • No auxiliary product addition in dye bath • Control over rate of temperature rise Examples: NT reactive dyes TEMPERAURE-CONTROLLABLE REACTIVE DYES
 Cold BRAND dyes - fixation temperature of 30-40°C  Warm BRAND dyes - fixation temperature of 50-80°C  Hot BRAND dyes - fixation temperature of > 80°C • Lower substantivity dyes diffuse easily into fibers and are easy to wash out unfixed dyes but less exhaustion. • Higher substantive dyes give higher bath exhaustion, better reaction with fiber, but greater difficulty of removing unfixed dye
120 Reactive dyes in a mixture should all exhaust and react with the fiber at about the same rate.  Dyes with different reactive groups and different substantivity are available  Mix dyes with same type of reactive group having the same substantivity. Compatible dyeing behaviour is a function of all the process variables and requires careful control of the dyeing parameters. Dyeing compatibility of reactive dyes Ideally Different dyeing characteristics and reactivities. Requires careful control of the dyeing temperature, salt and alkali concentrations, the dyeing time and the liquor ratio.
PROBLEMS IN DYEING WIH REACTIVE DYES  Hydrolysis [particularly for high reactivity series, e.g. DCT]  Low level of fixation when dyeing using a high liquor ratio  Appreciable dye concentrations in the dyehouse effluent  High salt concentrations are also present Less than 70% of the original dye reacts with the fibre. For exhaustion
122 Precipitation or insolublization by metal ions in water supply Free chlorine hydrolyze and dye-fiber bond breakage Unreacted chlorine atoms in the dye’s reactive group may hydrolyze under warm humid storage conditions, liberating HCl. Catalyzes the hydrolysis of the dye–fibre bond. Calcium, magnesium or heavy metal ions
INITIAL EXHAUSTION [NEUTRAL PHASE] 123  Dyeing is started in neutral aqueous- little likelihood of dye-fiber reaction-capable of migration to promote level dyeing  Salt/ neutral electrolyte is added gradually to promote exhaustion (higher conc. Compared to direct dyeing).  Temperature is gradually increased to aid penetration of dye assist migration;  Dye absorption depending upon substantivity STAGES IN REACTIVE DYEING PROCESS
124 FIXATION (REACTION) [ALKALINE PHASE] Fixation at alkaline pH Alkali facilitates dissociation of cellulose hydroxyl groups; Nucleophilic cellulosate ions begin to react with the dye. Fixation of dyes results in additional dye absorption, re-establishes dyeing equilibrium. Dye absorption from solution and reaction with the fiber then progress until no further dye is taken up.
AFTERTREATMENT [POST – DYEING WASHING] 125  Unfixed and hydrolyzed dye on cotton  Residual alkali and salt As much as possible unfixed dye must be washed out of the dyeing If not, desorption of this dye during washing by the consumer can cause staining of other materials in wash. I. Successive rinsing in warm water II. Thorough washing using a boiling detergent solution (soaping) III. Final rinsing in warm water then cold wash DRYING
REACTIVE DYEING METHODS 126 Both exhaustion and padding methods can be used JIGGER WINCH PAD-BATCH PAD-DRY PAD-STEAM SOFT WATER
 Level well-penetrated dyeing's require careful preparation of the material. Material available for dyeing in neutral, uniform & readily absorbent  Residual size must always be removed from woven goods because of the risk of dye wastage by reaction with hydroxyl groups in size PREPARATION
128 ALKALI/SALT/SEQUESTERANT/WETTING AGENTS  All traces of residual chlorine or peroxy compounds must be removed prior to dyeing, otherwise loss of reactivity and destruction of dye  Variation in covering ability of neps of immature cotton need for effective mercerization or causticization
129 BATCH DYEING Alkaline
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131 a = dye in dye bath b = hydrolysed dye on the fiber c = fixed dye d = dye to be washed out
Problems Encountered in continuous dyeing (1) Dyes of low substantivity are desirable to avoid preferential dye absorption during padding and the initial color tailing that it causes. (2) High dye concentration for deep shades that may exceed limiting solubility of dye [use of urea for disaggregation] CONTINUOUS DYEING Very rapid fixation (< 60 s) at high temperatures that is possible in fully continuous dyeing gives high productivity for long runs of a given color. Tailing is shade variation along the length of the fabric Urea helps to break up dye aggregates
3] when using pre-prepared alkaline solutions of reactive dyes Stability of dye/alkaline mixture [hydrolysis dye’s reactive group in pad bath will result in a loss of fixed color.] [Pad – dry – Pad] 4) Migration control and heating uniformity [Anti-migrant] (5) Final rinsing and soaping of goods dyed @ high speed with less time of contact salt as an anti-migrant and also a thickening agent such as sodium alginate Counter current washing
134 The lower limit of liquor ratio in batchwise dyeing is about 5:1. Padding methods extend this further to the range 1:1 to 0.5:1. Enhanced exhaustion and fixation ADVANTAGE DYE SELECTION Excellent solubility moderate substantivity, and versatile reactivity
135 Single-pad sequence: dye and alkali are applied together Double-pad sequence: dye and alkali are padded separately, with or without an intermediate drying step. Padding PAD – DRY – WASH PAD – BATCH - WASH PAD – DRY – BAKE -WASH PAD – DRY – STEAM -WASH PAD – DRY – PAD - BATCH –WASH PAD – DRY – PAD - STEAM -WASH PAD – DRY – PAD - WASH SINGLE PAD DOUBLE PAD
1. Impregnation of the well-prepared dry fabric in a solution of dye and alkali at ambient temperature  A 4:1 dye-to-alkali solution volume ratio is very common.  The dye and alkali solutions are usually mixed just before padding using metering pumps to maintain the correct ratio. 2. Uniform squeezing of surplus liquor from the fabric as it passes through the mangle nip. PAD – BATCH DYEING 1000 – 10000m CPB[Cold Pad Batch]
137 3. Wrapping of the batched roll of wet fabric in polythene film 4. Washing-off 5. Drying SLOW ROTATION Cost Reduction Scheme To avoid drainage of the internal liquid within the batch. Storage at ambient temperature for a specified dwell time (2– 24 h), depending on dye reactivity and pH) The more reactive dyes give effective fixation within 2–6 h.
138 PAD–HEAT/DRY DYEING Suitable for reactive dyes with fairly high reactivity. The fabric is first padded with a dye solution containing sodium bicarbonate. During drying, bicarbonate is converted into carbonate, which gives a higher pH and more effective fixation The dye solution also contains a high concentration of urea (100 g/l). Retain water during drying Increases dye solubility Pollution problem Nutrients for algal growth
139 PAD – STEAM DYEING In this process, goods are padded with a solution containing reactive dyes, salt and appropriate alkali. Hot humid conditions during steaming tend to cause excessive hydrolysis of reactive group and lower color yield.  Two-stage wet-on-wet padding is used to avoid an intermediate drying step.  Sufficient pick up and minimum color bleeding must be ensured During dyeing of terry towel and other pile fabrics Selected dyes are suitable and the manufacturer’s recommendations should be followed.
140  The dye solution is quite stable, because there is no alkali Anti-migrants help to minimize migration during initial drying Intermediate drying ensures uniform & high pick-up of chemical solution & minimizes bleeding of dye  Sodium hydroxide is the preferred alkali Pad–dry–pad–steam The fabric is padded with a neutral solution of the reactive dyes, dried and then padded with the alkali solution containing salt before steaming
141 TEMPERATURE/TIME FACTORS AFFECTING REACTIVE DYEING PH [ALKALI] SALT & OTHER AUXILARY MLR
142 Dyeing temperature is chosen based on reactivity Time depending on exhaustion and leveling Selection of alkali based on reactivity & ease of hydrolysis Effect of salt is similar to direct dyeing process  Larger amount of salt due to low molecular weight and large number of sulphonic groups per reactive dye molecule Selection of MLR considering tendency of hydrolysis and solubility
143 It is not usual to exceed a dye-bath pH of 11: Hydrolysis. For polysulphonated dyes, one effect of dyeing at pH above 11 is the decrease in substantivity of the dye for the increasingly anionic dissociated cellulose. Some dyes actually desorb from the fiber into the dye-bath when the alkali is added at the start of the fixation stage giving a sudden decrease in the degree of exhaustion. LARGE AMOUNT OF SALT Longer LR m/c --- dyes of higher substantivity are preferred
144 Viscose fibers give higher fixation and exhaustion of reactive dyes than cotton. For identical conditions, exhaustion and fixation increase in the order: COTTON, MERCERISED COTTON, VISCOSE. Because of the ease of swelling of viscose, the dyeing pH and temperature for a given dye is different than for cotton
145  Dyeings are resistant to color stripping with hot aqueous pyridine, a solvent that removes direct dyes from cotton.  Dyeings of cotton obtained with bifunctional dyes exhibit reduced swelling and decreased solubility in cuprammonium EVIDENCE FOR COVALENT BOND FORMATION The good fastness to washing of dyeing's with reactive dyes on cellulosic fibres is a consequence of the stable covalent bond formed between the dye’s reactive group and the cellulose polymer.
146 VAT DYE Gezu K. 2019
Introduction  Vat dyes are one of the oldest types and water insoluble dyes.  Vat dyes ‐ a series of dyes of different chemical constitution … contain two or more keto (C=O) groups  They are called dyes because: i. Chemical reduction into a water-soluble leuco form ii. Substantivity in leuco form for cotton
 Insoluble in water  Have best overall fastness properties. .. often used for fabrics that will be subjected to severe washing and bleaching conditions (toweling, industrial and military uniforms)  Not bright colors  No substantivity for cellulose  Have a limitation with regards to use of strongly alkaline solutions  Mostly complex dye structures with no ionic groups CHARACTERISTICS OF VAT DYES
Cont. …  Vat dyes are used predominantly for dyeing cellulosic fibers.  Although leuco dyes have substantivity for wool and nylon, technical reasons (damage in wool and poor fastness on nylon) vat dye application is restricted to cellulose only.  The vat pigment and the leuco compound often have quite different colors  The primitive color is developed by subsequent oxidation
 Characterized by containing at two or more keto group [C=O]  Mostly complex dye structures with no ionic groups Chemical Characteristics
MECHANISM OF DYEING WITH VAT DYES Converting a water-insoluble keto-substituted colorant by reduction to a water-soluble leuco compound. This penetrates into the fiber, where it is reoxidised back to the original insoluble form. Fixation based on secondary forces and insolublization
The process of reduction of vat dye in aqueous alkaline solution is known as VATTING. Sodium dithionite [sodium hydrosulphite or hydros] is used as reducing agent and caustic soda as alkali . Hydros is a very strong reducing agent, more effective at high pH and at higher concentration. Control over concentration & temperature for optimum reduction Rate of reduction depends on pigment dispersion DISPERSION STABILITY FOR PROPER REDUCTION The process of reduction of vat dye in aqueous alkaline solution is known as VATTING. Sodium dithionite [sodium hydrosulphite or hydros] is used as reducing agent and caustic soda as alkali . Hydros is a very strong reducing agent, more effective at high pH and at higher concentration. Control over concentration & temperature for optimum reduction Rate of reduction depends on pigment dispersion DISPERSION STABILITY FOR PROPER REDUCTION Na2S2O4 & NaOH
CLASSIFICATION OF VAT DYES Based on chemical structure Method of application
Indigo Anthraquinonoid vat dyes: Based on anthraquinone which give leuco compounds of relatively high affinity Based on chemical structure a) Dyes derived from indigo – both natural and synthetic Anthraquinon e Indigoid vat dyes: Derivatives of indigo which give leuco compounds of relatively low affinity b) Dyes derived from anthraquinone – most of vat dyes Indigo dyes give more brilliant colors than anthraquinone But their light and wash fastness are not as good
Classification Based on Application/dyeing properties  IN [INDANTHRENE NORMAL]  IW [INDANTHRENE WARM]  IK [INDANTHRENE COLD {KALT}]
 Use of concentrated NaOH Have high molecular weight  High vatting temperatures (60 °C)  High dyeing temperatures (60 °C)  High substantivity  No salt addition to the dyebath THE IN (INDANTHRENE NORMAL)
IW [INDANTHRENE WARM] DYES  Moderate concentration of NaOH  Lower vatting temperature than IN dyes (50 °C)  Lower dyeing temperatures than IN dyes (50 °C)  Moderate substantivity  Some amount of salt addition to aid exhaustion
IK [INDANTHRENE COLD {KALT}] DYES  Low concentration of NaOH  Lower vatting temperature (40 °C)  Dyeing at room temperature (20°C).  Poor substantivity  Considerable amount of salt addition for good exhaustion
 The vatting temperature is often higher than the subsequent dyeing temperature. To avoid decomposition of hydros and decreases the risk of over-reduction
1) Reduction of the pigment to the soluble leuco compound 2) Absorption of the leuco compound by the cotton during dyeing 3) Oxidation of the absorbed leuco compound in the cotton, reforming the insoluble pigment inside the fibers. 4) Soaping to remove pigment loosely adhering to the fiber surfaces and to develop the true shade and fastness properties Key Steps in Vat Dyeing Of Cotton Air Oxidation Or Chemical Oxidants Recrystallization/Aggregation Preparation of the vat containing the leuco forms of the dyes Water-soluble leuco compound
Application of Vat Dyes on Cellulose i. Reduction of pigment to soluble leuco compound, a process called vatting; Converting a water-insoluble keto-substituted colorant by reduction to a water- soluble enolate leuco compound. ii. Absorption of leuco compound by cotton; water-soluble enolate leuco penetrates into the fiber, where it is reoxidised back to the original insoluble form. iii. Fixation based on secondary forces and insolublization. Oxidation of absorbed leuco compound in cotton, reforming insoluble pigment inside the fibers.
 The most important reducing agent in vat dyeing is sodium dithionite (Na2S2O4), referred to as hydrosulphite or hydros. Reduction of Vat dyes  Vat dyes are available as fine powders or grains, and as liquid dispersions or pastes.  The insoluble pigment is extensively milled with dispersants such as sodium ligno-sulphonates to produce very small particles In the presence of NaOH- alkali
Control over concentrations & temperature for optimum reduction Rate of reduction depends on pigment dispersion Dispersion stability for proper reduction
 Some Quinone vat dyes can be over-reduced  reduction of more than one pair of keto groups (C=O)  Over-reduced vat dyes give have poor substantivity for cotton; more difficult to oxidize; produces duller shades; gives lower color yield.
Others ... sodium sulphoxylate formaldehyde and sodium sulphoxylate acetaldehyde, particularly in printing and in continuous dyeing processes. Cont’d Others auxiliaries .... neutral salts (to increase substantivity), wetting, sequestering and dispersing agents, leveling agent. Help to overcome dyeing problems caused by inadequate pretreatment.
ALKALI PREVENT VAT ACID FORMATION Caustic soda ‐ is used for: 1. The formation of the leuco: pH = 12‐13 needed. 2. The transformation of cellulose into alkali‐cellulose , 3. Neutralization of acid products of decomposition of hydros Cont’d
Absorption and levelling  Alkaline leuco vat dye exhausts very fast onto cellulose until equilibrium is attained.  The higher the substantivity the higher is the exhaustion  High concentration of Na+ from sodium hydrosulfite and sodium hydroxide facilitate exhaustion
Because of this, many dyes have a rapid strike; dye bath may be 80–90% exhausted within 10 minutes. The more rapidly dye exhausts, the greater is the risk of obtaining an unleveled dyeing. High electrolyte content Use of retardants
Adsorption Isotherm For Vat Dyes On Cotton
Substantivity of Leuco Vat Dyes for Cotton  H-bonds b/n hydroxyl groups and phenolate ion groups, or amino or amide substituents are important  Molecules of leuco vat dyes are large and also coplanarity of structure is essential.
Oxidation and subsequent process/soaping  Once the dye has been absorbed by the fiber in the form of a leuco derivative, its oxidation takes place. The leuco derivative must be oxidized Any excess hydrosulphite must be eliminated The excess of alkalinity in the fiber must be neutralized  Leuco dye is then reconverted into its original form by oxidation.  Carried out with oxidizing agents like hydrogen peroxide, sodium perborate, Sodium dichromate or percarbonate.  The higher the temperature and the lower the pH, the higher the rate of oxidation reactions
Dyes that are easy to reduce are more difficult to oxidize, and vice versa. Chemical oxidants preferred because they give more rapid and uniform oxidation throughout the material.
Soaping  Removes pigment deposited on fiber surface that would wash or rub off in use.  Involves thoroughly washing goods in a detergent solution at or near boiling point.  It improves the light, washing and rubbing fastness of the dyeing.  Soaping probably involves re-crystallization of oxidized and dispersed vat dye particles  Crystallization results in slight change in hue and improvements in fastness properties. Neutralizing with acetic acid solution and final rinsing.
Two methods: 1. Dyeing with reduced vat ‐ the dye in the form of soluble leuco is put into contact with the cellulosic fiber, is absorbed by it and then oxidized in the fiber itself. 2. Dyeing by pigmentation ‐ the insoluble dye in the form of a fine dispersion is deposited on the surface of the fiber; reduced in an alkaline solution which causes rapid absorption, finally, the process is continued as in the reduced vat system. Dyeing Processes
DYEING METHODS Both batch and continuous methods are used Batch – (hot method, warm method and cold method) ‐ leveling problem due to high dyeing rate Continuous method – mostly used method (most common is pad‐steam process)
Exhaust Dyeing: Typical Vat Dyeing Cycle Leuco Process In the leuco process the material to be dyed is entered into a prepared dye liquor that contains the fully vatted dye, alkali (caustic soda) and reducing agent (hydrosulphite), together with various amounts of  Salt,  Dispersing agent,  Sequestering agent and  Levelling agent, as required.
High Degree Of Fastness Pre-oxidation Ancillary Chemicals [Chelating Agents, Wetting Agents, Sequesterants , Dispersants] Over-reduction Over-oxidation Effective Pretreatment General features Of Vat Dye Washing fastness Light fastness
VAT DYEING FAULTS  Unlevellness  [inadequate oxidation, salt, temperature]  Poor penetration  (rapid heating result surface deposition)  Dull shade  due to over-reduction  Staining  [inadequate vatting, rinsing & circulation]  Poor rub fastness  [inadequate vatting, poor soaping]  Incorrect shade  [Soaping too short or low energy]  Fiber damage
DENIM AND INDIGO VAT  DENIM is a warp faced twill fabric made from cotton yarns  The warp predominates on the surface  Dyed yarns in the warp and undyed yarns in the weft  Small white flecks distributed in a darker basic color  JEANS made from blue denim is most popular  Inexpensive, durable, versatile
 Indigo is applied in a series of ‘dips’, with intermediate squeezing and atmospheric oxidation.  Reduction of indigo to soluble leuco-indigo  Multiple dip-squeeze-oxidize of the yarn  Rinsing, soaping & drying  Blue-dyed warps wash down to an attractive blue  No staining on the white yarns  STONE WASHING FOR FADED LOOK
The goods are threaded through each box and skyed The first box is used to wet out the material. In subsequent boxes, the goods are immersed in the leuco Indigo solution for 10–30 s, squeezed and skyed for 2 min to oxidise the leuco dye to Indigo. This process of several dips and oxidations is then repeated in a second series of boxes, and so on. Deep shades built up by repeated dipping in the dye bath after each oxidation. Several rinsing and washing boxes complete the process.
SULPHUR DYES SULPHUR dyes constitute the largest class of dyes in terms of quantity Like vat dyes applied to cellulosics by redox mechanism The insoluble pigment is converted into the substantive leuco compound by reduction and the leuco form is subsequently oxidized inside the fiber. Chromophore based on sulphur containing compounds The dyes contain THIAZINES AND THIOZOLES Limited color gamut [Known for black] SULPHUR BLACK
THIAZOLES THIAZINES Sulphur containing aromatic heterocyclic linked by sulphide bonds
 On treatment of an aqueous dispersion of the insoluble pigment with sodium sulphide, the sulphide links are reduced forming individual heteroaromatic units with thiol groups. DYEING PROCESS WITH SULPHUR DYES These are soluble in the alkaline solution in the form of thiolate ions that have substantivity for cellulose. After dyeing, the thiolate ions in the fibers can be re-oxidized back to the polymeric pigment. Low to moderate substantivity for cellulose Salt addition to promote exhaustion RINSED, OXIDIZED SOAPED
Reduce d Sulphur dyes are important for black, navy, brown, olive and green colors in medium to heavy depths, being relatively inexpensive.
 SODIUM SULPHIDE [Na2S] IS USED AS THE REDUCING AGENT
Sulphur dyes have the dullest range of colors of all dye classes but are relatively inexpensive. When black color and dull shades are needed, with good fastness at reasonable cost, sulphur dyes are irreplaceable. Cotton dyed with some sulphur blacks becomes tendered on storing under warm humid conditions. Formation of sulphuric acid due oxidation of dye. Jig and pad steam for woven fabrics Large amounts of sodium sulphide used in the application of sulphur dyes, pose a significant environmental problem. OVER-REDUCTION
CATIONIC [BASIC] DYEING  Dyes containing basic functional groups that are protonated  Organic cations  Dye fibers with anionic sites by a process of ion exchange  Mostly applied for acrylic, wool and silk  Low substantivity for cotton
 Dye adsorption by acrylic fibers involves interaction between anionic sulphonate/sulphate polymer end groups and cationic dye molecules TRIPHENYL METHANE ANTHRAQUINONE
ACID DYEING
 Acid dyes are applied in acidic media • Acid dyes are usually sodium salts of sulphonic acids or less frequently of carboxylic acids • Anionic in aqueous solution • Affinity for fibers with cationic sites • Usually substituted ammonium ion groups in fibers • Cationic character by acid absorption • Affinity for wool, silk and nylon INTRODUCTION
MECHANISM OF DYEING WITH ACID DYES Ionic interaction between dye anions and amino groups of fibers Van der waals forces exerted between hydrophobic dye anion & hydrophobic parts of fiber adjacent to amino groups
In wool, the numbers of amino and carboxylic acid groups are almost equal (820 and 770 mmol kg–1, respectively) ISOELECTRIC POINT OF WOOL: Equal numbers of cationic and anionic groups [pH ~5]
Based on chemical structure of chromophore CLASSIFICATION OF ACID DYES AZO DYES ANTHRAQUINONE
Based on dyeing characteristics GROUP I: LEVELLING ACID DYES GROUP II: MILLING ACID DYES GROUP III: SUPERMILLING ACID DYES Milling is the process by which wool is treated, in weakly alkaline solution, with considerable mechanical action to promote felting.
201 MOLECULAR WEIGHT OF DYE SOLUBILITY OR DEGREE OF SULPHONATION  DYEING PH  DEGREE OF AGGREGATION  DEGREE OF MIGRATION/LEVELLNESS  DEGREE OF WET FASTNESS COMPARISON FEATURES
203 LEVELLING ACID DYES 1. Add Glauber’s salt plus sulphuric acid 2. Add dye 3. Rinse
204  Strong acid are needed to achieve good exhaustion Glauber’s salt acts as a retarding/leveling agent  Level dyeing controlled by using less sulphuric acid initially Decreasing exhaustion on increasing dyebath pH above 4, and with increasing temperature  Small molecular size for good penetration into pores
205 MILLING ACID DYES 1. Ammonium sulphate/acetate + Acetic acid 2. Add dye 3. Rinse
206  Higher molecular weights and greater substantivity for wool than levelling acid dyes Weak acids or their salts are required (sodium acetate or ammonium sulphate) Have one sulphonate group per dye molecule and lower water solubility  Dyeing starts in the presence of ammonium sulphate  Addition of acetic acid to promote exhaustion  Both ion exchange and hydrophobic interactions Glauber’s salt is never added High concentration of sodium ions suppresses fiber negative charge and results high exhaustion with minimum leveling
207 SUPERMILLING ACID DYES 1. Ammonium sulphate/acetate 2. Add dye 3. Rinse
208  Ammonium acetate or sulphate is used at nearly neutral pH  More hydrophobic than milling acid dyes  Characterized by presence of long alkyl chains  Lower pH results rapid exhaustion and unlevellness  Rate control by temperature increase  High degree of aggregation even at boil
209 Auxiliaries are used in wool dyeing to promote leveling. Two main types of levelling agents: ANIONIC AND CATIONIC/NON-IONIC. Wool adsorbs anionic leveling agents and they retard dye absorption by initially blocking the cationic ammonium ion sites.  More substantive dye anions eventually displace anionic product. Anionic surfactants with long alkyl chains have higher fibre substantivity. LEVELLING AGENTS Cationic polyethoxylated amine
210 Cationic agents form a complex with the anionic dye in the dye-bath and prevent its uptake by the wool. The non-ionic portion of this type of product keeps the auxiliary–dye complex dispersed in solution. Avoid its precipitation in the bath or on the material surface. Free dye molecules are liberated as the dyeing temperature increases since the complex is less stable at higher temperatures
211 PROBLEMS IN ACID DYEING OF WOOL  Temperature and pH control  Skittery dyeing or tippy dyeiing  Ring dyeing  Shade variation
DISPERSE DYEING
DISCOVERY • Cellulose triacetate fibers and poor dyeability • Hydrophobic character; could not be penetrated by water, & not be dyed by water soluble dyes. • There was little point in treating fibers with a mixture of pure dye and water because particles would not be distributed uniformly. • Incorporating surface-active agent with dye ensures uniform distribution of dye in the dye-bath, allowing uniform dyeings DISPERSE DYES
INTRODUCTION  Disperse dyes are non-ionic and slightly soluble in water  Applied in the form of fine aqueous dispersions  Dye particles are able to penetrate fiber during dyeing in a molecularly dispersed state and are held in fiber  Disperse dyes have affinity for synthetic fibers  Polyester, nylon, cellulose acetate and acrylic fibers COMPACTNESS [CRYSTALLINITY/ORIENTATION] HYDROPHOBICITY
Dyeing of polyester with disperse dye Polyester is hydrophobic and characterized by compact physical structure PET is non ionic and thermoplastic fibers Because of non ionic it needs non ionic dye Dyeable only with disperse dye
Chemical constitution  Simple low molecular non ionic mono azo and anthraquinone with polar groups for slight solubility required in dyeing. DISPERSION STABILITY DYE SOLUBILITY PARTICLE SIZE Hydrophobic dye ‘dissolving’ in hydrophobic fiber
 Stable suspension of the dyes is achieved by using dispersing agent in the preparation of the dyes.  The solubility (slight) depends on the presence of polar groups such as OH, NO2, -CH2–CH2–OH and the particle size.  In general the particle size of dye must be very small. This is influenced by the degree of grinding and affects disaggregation of dye hence degree of absorption.
REASONABLE RATE OF DYEING HIGH TEMPERATURE IN REFERENCE TO Tg DYEING ACCELERATORS [CARRIERS] FREE VOLUME THEORY
POLYESTER DYEING METHODS  DYEING WITH CARRIER AT BOIL Benzoic and phenolic compounds unpleasant odours and are toxic  HIGH TEMPERATURE PRESSURE DYEING  DYEING BY THERMOSOL PROCESS Exhaust and continuous
Carrier dyeing method A carrier is an organic compound dissolved in the dye bath and which increase the rate of dyeing Mechanism :  The polyester fibers absorb the carrier and swell.  Swelling can impede liquor flow in packages causing unlevelness.  The overall effect seems to be lowering of the polymer (Tg), thus promoting polymer chain movements and creating free volume.  This speeds up the diffusion of the dye into the fibers.
221 Dyeing Temperature 100 oC Alternatively, the carrier may form a liquid film around the surface of the fiber in which the dye is very soluble, thus increasing the rate of transfer into the fiber
HTHP dyeing method  The dyeing of polyester with disperse dyes at the boil is slow because of the low rate of diffusion of the dyes into the fiber. The activation energy for diffusion is quite high and raising the dyeing temperature from 100 to 130 °C considerably increases the rate of dye diffusion.
223 HTHP DYEING Dyeing Temperature ~ 130 oC
224  Padding fabric with a dispersion of disperse dyes (use of a migration inhibitor in the pad bath)  Drying using hot flue air dryer or by infrared radiation  Heating in air, or by contact with hot metal surface, to a temperature in the range of 190–220 °C for 1–2 min.  The fabric approaches the maximum temperature, the disperse dyes begin to sublime and the polyester fibers absorb the vapours. THERMOSOL DYEING
225 Dyeing Temperature ~ 220 C Drying is used to minimize the migration during heating During heating dyes will be converted into gaseous state and PET fibers has very high affinity to thee gaseous disperse dyes
 In case of dyeing with disperse dye, temperature plays an important role.  For the swelling of fiber, temperature above 130°C is required if high temperature dyeing method is applied. Again in case of carrier dyeing method, this swelling occurs at 90-100°C.  If it is kept for more time, then dye sublimation and loss of fabric strength may occur. Effect of Temperature
227 For disperse dyeing the dye bath should be acidic and pH should be in between 4.5-5.5. For maintaining this pH, generally acetic acid is used at this pH dye exhaustion is satisfactory. During color development, correct pH should be maintained otherwise fastness will be inferior and color will be unstable. Effect of pH
Textile Auxiliaries 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
229 MODIFICATION OF POLYESTER IMPROVED DYEABILITY Cationic Dyeable Polyester [CDP] Copolymerization for lower Tg
230 DYEING BLEND FIBERS (UNION DYEING)
Why blending ???  Achieving economic advantages via blending of expensive fibers with cheaper ones  Enhancing performance and quality properties via combination of desirable properties of both fiber components  Prolonging the durability of textile product via incorporation of a more durable fiber component, etc
Fiber classes according to affinity to dyes  Group A: Acid and pre-metallized acid dyes for dyeing wool, silk, nylon and polyurethane (electrostatic bond)  Group B: Basic dyes for acrylics and modacrylics, cationic dyeable polyesters (electrostatic bond)  Group C: Cellulosic dyes, e.g. reactive, direct, vat, sulphur, etc. via formation of physical and/or covalent bonds  Group D: Disperse dyes for polyester, cellulose acetate, nylon and polyurethanes via hydrophobic bonding
Color effects obtained by dyeing blends  Solid effect: All fiber components are identically colored  Reserve effect: One type of fiber is kept white  Shadow effect:(tone in tone ) dyeing All components colored to the same hue but of varying depth of shades.  Contrast effect: fiber components are dyed in contrast hues. , orange-black blue-yellow
Methods of dyeing blends  1-bath 1-step method: reduces cost of dyeing drastically due to nearly 50% reduction in use of water, heat, chemicals, time and manpower  1-bath 2-step method: material handling is remarkably less, as the blend once loaded remain there till completion of dyeing for both components; the same bath is used for dyeing both these fibers saving water and energy  2-bath 2-step method: to dye blends made fibers of different nature-more handling of material, huge water consumption, doubled loading/unloading time, more manpower
IMPORTANT FACTORS IN DYEING BLENDS  DYEBATH CONDITIONS  dye auxiliaries  dyebath temperature  stress on fabric  CROSS-STAINING OF DIFFERENT FIBERS  The colour effects and fastness properties required;  The process costs.
In most cases, method of blend dyeing is similar to that of dyeing individual component fibers with particular types of dyes selected  This is so because most binary blends are mixtures of a synthetic and a natural fiber. In such cases, there is minimal cross-staining.
Dyeing cotton/polyester blends  Dyeing properties of polyester and cotton are quite different- mostly the two fibers dyed separately  Polyester component is invariably dyed first with of disperse dyes  For cotton, there is a choice of dyes, actual selection depending on desired color, type of finishing required, demanded fastness properties, costs etc  Cotton is usually dyed with reactive, direct, sulphur, vat, or azoic dyes  Reactive/disperse dye combination is the most popular
 If cotton is dyed first dyeing of polyester at 120–130 °C can change shade of dyed cotton- cotton dyes are less stable at high temperatures.  Dyeing polyester before cotton allows reduction clearing of any disperse dye on the polyester surface or staining cotton-cross staining of cotton by disperse dyes can be a problem.  Once polyester is dyed any detrimental effects of cotton dyeing assistants such as alkali and salt on disperse dyes are avoided.
major areas of concern in dyeing of cotton/polyester  Degree of cross-staining  Disperse dyes stain cotton but anionic cotton dyes usually completely reserve polyester.  Disperse dyes selected should be those that give minimal cotton staining  Interactions between dyes and auxiliaries  When present in same bath many disperse dyes are not stable under alkaline reducing conditions used in leuco vat dyeing.  Salt and alkali for dyeing cotton with reactive dyes often have a harmful effect on dispersing agent for disperse dyes and cause particle aggregation;
Conditions for fixation or aftertreatment Cotton dyes must be stable to high temperatures in Thermosol process- if to be applied simultaneously or before disperse dyes. Not possible to clear disperse dyes staining cotton with an alkaline hydros solution if cotton is already dyed. All types of dyes used to color cotton will be reduced under these conditions and color destroyed.
 Disperse and reactive dyes are added, temperature raised to 130°C at acidic conditions to fix disperse dye.  Dyebath is then cooled to 80°C and sodium carbonate is added to fix the reactive dye, before washing.  Reactive dye fixation and clearing of surface disperse dyes is combined here, making PC single bath dyeing possible Developments in PC Blend Dyeings
TEXTILE PRINTING Gezu k Wolkite University 2012
Textile printing is related to dyeing but, whereas in dyeing process whole fabric is uniformly covered with one color, in printing one or more colors are applied to it in certain parts only, and in sharply defined patterns. A TYPE OF COLOURATION FOR IMPARTING COLOR IN SPECIFIC AREA ON A SUBSTRATE BASED ON PATTERN OR DESIGN REQUIREMENT. Textile Printing Defined
DIFFERENCES BETWEEN DYEING AND PRINTING
PRINTING Printing involves localized coloration This is achieved by applying thickened pastes containing dyes or pigments onto fabric surface according to a given color design The paste must color all the visible fibers on the printed surface So it must penetrate some what into the yarn structure If the paste is too thin it will spread, giving poor print definition and penetrate too far into the yarns decreasing the color yield
PRINTING PROCESS PREPARATION OF PRINT PASTE APPLICATION OR PRINTING DRYING FIXATION
DYE OR PIGMENT THICKENER BINDER AUXILIARIES PRINT PASTE INGREDIENTS
 Impart the required color of design as per requirement  Dyes are chosen with respect to their affinity Example: Reactive dyes for cotton, acid dyes for wool, disperse dyes for polyester and so on  Pigments are applied for all kinds of fiber textiles DYE OR PIGMENT
 To localize the printing paste on the desired area of the fabric. Enables the print paste to stay in place once it is deposited onto the fabric  Clear cut design and proper outline definition of print  Natural and synthetic thickeners are available  Based mainly on polysaccharides and polyacrylic acids THICKENER Optimum Viscosity
Desirable properties of thickeners used in pigment printing: low solid content clear and colorless don’t decrease the fastness and hand of textile prints good leveling power without spreading sideways.
 Pigments are fixed by binders hence binders provide durability of the pigment on the fabric.  A pigment binder is the resin that forms a three-dimensional film on the surface of the fiber.  This film contains the dispersion of textile pigment and will act to adhere the pigment to the surface of the substrate.  The permanence of this film is dependent on the polymer type and the application conditions. BINDER
 Crosslinking agents to improve fastness  Softeners to improve handle of pigment printed fabric  Defoamers to prevent foam generation  Humectants to increase water content of the paste  catalyst and so on AUXILIARIES
Different techniques can be used to transfer the paste TO textile substrate. The transfer can be by means of blocks, screens, rollers: reproduction of image. There is possibility of digitizing design and transfer image without reproduction. APPLICATION OF PRINT PASTE
DRYING AND CURING (STEAMING)  The main purpose of drying is to control migration  Curing/steaming for fixation of pigment/dye  Cylinder dryers and stenters can be used  Curing temperatures depend on the fiber 180 oC for pigment, 205 oC for dispersed dye
 In the case of dye prints, the printed fabric is thoroughly washed then dried after fixation.  Necessary to remove thickener, alkali, other ingredients of print paste left on fabric after fixation. Could interfere with subsequent finishing processes. Pigments are often printed on finished AFTERWASHING
 Reactive dyes for cotton fibers has given the possibility of using only one type of dye and simple application conditions, in place of the complex permutations necessary  Reactive dyes produce brilliant shades of very good fastness and leveling properties on cellulosic fibers.  Next to pigment printing the most important printing method is printing with reactive dyes. REACTIVE DYE PRINTING
Selection of thickeners is very important for reactive dye printings; and the selection of proper reactive dyes also plays a major role. Normally low reactive dyes are preferred for printing on cotton. Because the stability of the low reactive dyes is high in printing paste. Furthermore for obtaining good effect proper pretreatment of fabric before printing is very important.
Requirements of an ideal thickener for reactive printing • Good compatibility and no affinity or reactivity with dye • Good flow property • Good swelling and moisture absorption capacity during steaming for dye fixation • Quick drying to prevent bleeding & good water solubility for easy removal
Conventional reactive printing thickener The usual polysaccharide thickeners such as starch and gums used in printing are not suitable for printing with reactive dyes As all these contain primary hydroxyl groups in their molecular structure and react with reactive dyes. Sodium alginate is widely used for reactive dye printing. It is an anionic polymer, has only two free OH groups.
Polysaccharide illustration The reaction between alginate and dye is limited by anion repulsion of alginate's carboxyl groups and dye's sulfonic acid groups.
Moderate to low reactivity dyes such as MCT and VS Reactive dyes in printing Dyes having less substantivity towards cellulose are preferred for printing. Because of greater ease of “washing off” and avoidance of staining of adjacent white areas. Printing pastes made from less reactive dyes have better storage stability and this is of considerable importance. The reactive print paste normally contains reactive dye, sodium bicarbonate/carbonate, urea, sodium alginate and water. DCT is not recommended
PIGMENT PRINTING DIFFERENCE BETWEEN DYE AND PIGMENT PIGMENT  NO AFFINITY TO FIBRE  INSOLUBLE IN WATER  NEED BINDER FOR FIXATION ONTO FIBRE DYE  AFFININTY TO FIBRE  WATER SOLUBLE OR CAN BE MADE WATER SOLUBLE  HELD ON FIBRE DYE-FIBRE INTERACTIVE FORCES MOST WIDELY USED
MECHANISM OF BINDING SEQUENCE: PRINT > DRY > CURE (DRY HEAT)  DURING CURING BINDER POLYMERIZES AND FORMS A STRONG FILM.  BINDER FILM EMBEDES PIGMENT COLOUR AND ALSO STRONGLY ADHERES TO THE FIBRE. BINDER IS A PRE-POLYMER AVAILABLE IN THE FORM OF AQUESOUS EMULSION CROSSLINKING
CHOICE OF THICKENER DYE PRINTING THICKENERS ARE NOT SUITABLE. INTERACT WITH BINDER, REDUCE INTERATION WITH FIBRE. AFFECTING THE FASTNESS PROPERTIES SUITABLE THICKENERS  EMULSION THICKENER  SYNTHETIC THICKENER DEVELOPMENT OF VISCOSITY HMW COPOLYMERS OF ACRYLIC ACID OIL + WATER EMULSIONS
ADVANTAGES OF PIGMENT PRINTING  PRACTICALLY ALL TYPES OF FIBRES AND BLENDS CAN BE PRINTED  PIGMENT COLOURS CAN BE READILY MIXED WITH EACH OTHER TO GET QUICK SHADE MATCHING  NO AFTERTREATMENT SUCH AS WASHING & SOAPING REQUIRED DISADVANTAGES  HARSH FABRIC FEEL DUE TO BINDER FILM  LOW RUBBING FASTNESS
FIBER CHARACTERISTICS YARN TYPES FABRIC CONSTRUCTION CONSIDERATIONS ON PRINTING PROCESS
STYLES OF PRINTING There are two basic styles of printing Direct printing and Indirect printing DIRECT PRINTING  The dye or the pigment is printed on the fabric in the paste form and any desired pattern may be produced  Designs printed directly using pigments or dyes on white or colored background  Is the most important printing style and the most popular as compared to indirect style
INDIRECT PRINTING IP is classified into two categories Discharge printing the dye. In discharge style, the fabric is dyed to the required ground color. Next, the fabric is printed with a chemical that selectively destroys the dye. This allows printing of white designs on a ground color of any depth with a pattern definition that is much higher than would be possible by direct color printing. If the paste contains dyes resistant to the discharging agent, these dyes, called illuminating colors, will color the printed areas.
Resist is the term that describes prevention of dyeing process by application of a physical or chemical substance to fabric to prevent a dye’s access to fabric. Resist agents can be waxes, thickeners, surfactants, organic acids, oxidizing agents, or reducing agents. In discharge style, the fabric is dyed to the required ground color. Next, the fabric is printed with a chemical that selectively destroys the dye. Resist printing
BATIK TIE-DIE
In batik effect the fabric is coated with wax and then dyed with suitable dyes depending on the fiber. In tie-die effect first the fabric is tied in certain portions to lower the penetration of dye into the fiber and then dyeing is carried out conventionally.
Methods of Printing Non contact printing Contact printing
STENCIL PRINTING BLOCK OR SURFACE PRINTING ENGRAVED ROLLER PRINTING SCREEN PRINTING TRANSFER AND DIGITAL PRINTING TECHNIQUES OR METHODOF PRINTING Refers to the technical means by which pastes are put onto design areas
Stencil printing  Stencil printing is the process of cutout the required designs on the plastic papers  The surface of the papers should be smooth  Colorants are smaller molecular weight (no need of washing)  Pastes are forced out through cutting out shapes from papers onto fabric underneath  The idea of screen printing is originated from stencil printing
BLOCK PRINTING  It is the oldest and simplest way of printing  In this method a wooden block with a raised pattern on the surface was dipped into the printing colorant and then pressed face down on to fabric.  The desired pattern was obtained by repeating the process using different colors.  Generally the wooden block is carved out of hand  Printing is done manually
Screen is made of woven fabrics firmly attached to a suitable frame  Mostly of nylon/silk/polyester  The screen is the image carrier made from a porous mesh stretched tightly over a metal or wooden frame.  Reproduce patterns as per design requirement. SCREEN PRINTING SCREEN, INK, SQUEEGEE
281 (Major step in screen printing) (1) the selection of the fabric and the printed design (2) the choice of the size and arrangement of design’s repeat rectangle (3) the necessary color separation of the design Color separation involves reproduction of the pattern for each color on separate clear films with opaque tracer. Screen production
Single color Multi color
285  The screen is coated with a photo-sensitive emulsion. A typical polymer is polyvinyl alcohol, its crosslinking sensitized by ammonium dichromate. The coated screen is dried  Exposed beneath diapositive for the given color All operations take place in a dark-room!! Screen Preparation Steps Screen Preparation Steps Lacquer
 The rest hardens (become insoluble)  The inked zones, corresponding to particular color pattern, do not transmit ultraviolet light used for exposure.  The layer of polymer on the screen beneath the pattern is thus not exposed and does not crosslink.  The rest hardens (become insoluble)  The non-exposed polymer remains soluble and can be washed out, leaving the screen open in those areas.  Any small holes in hardened areas on screen are painted over.
 MIXING PHOTO-EMULSION  COATING  DRYING  PREPARING POSITIVE  EXPOSING  WASHING AND DRYING
288 288 Flat screen printing can be carried out by hand and automatically by using different machines. Rotary screen printing is carried out by fully continuous machines. TYPES OF SCREEN PRINTING FLAT SCREEN PRINTING In flat screen printing, a screen on which print paste has been applied is lowered onto a section of fabric. A squeegee then moves across the screen, forcing the print paste through the screen and into the fabric. ROTARY SCREEN PRINTING The tubular screens rotate at the same velocity as the fabric, the print paste is distributed inside a tubular screen, which is forced into the fabric as it is pressed between the screen and a printing blanket (a continuous rubber belt).
• Printing is carried out on a flat, solid table covered with a layer of resilient felt and a washable blanket. • Heat for drying the printed fabric may be provided either under the blanket or by hot air fans above the table. • Fabric movement or shrinkage must be avoided in order to maintain registration of the pattern. HAND SCREEN PRINTING
The process consists of forcing print paste through the open areas of the screen with a synthetic rubber squeegee (rubber blade contained in wooden/metal support)  The rubber blade is drawn steadily across the screen at a constant angle speed and pressure.  The screen is washed immediately after use. If this is not done, paste dries on screen and clogs up design.
S C R E E N P R I N T I N G MANUAL
SEMI-AUTOMATIC FLAT SCREEN PRINTING 296 The manual process has been semi-automated by mounting the screen in a carriage. In semi-automated screen printing, a mechanically driven squeegee transfers the color. Long tables, typically 20–60m long, are used, and some provision is usually made for drying the printed fabric. Very popular where the scale of production is not large, or where capital investment is limited.
FULLY AUTOMATIC FLAT-SCREEN PRINTING • To increase the speed of flat-screen printing, it was necessary to devise a method of printing all the colors simultaneously. • All the screens for the design (one screen for each color) are positioned accurately along top of a long endless belt (blanket) on top of which is the fabric to be printed. • The fabric is gummed to the blanket at the entry end and moves along with the blanket in an intermittent fashion, one repeat distance at a time.
• All colors in design are printed simultaneously while the fabric is stationary; then the screens are lifted and the fabric and blanket move on. • When the fabric approaches the turning point of the blanket, it is pulled off and passes into a dryer. • The soiled blanket is washed and dried during its return passage on the underside.
PRODUCTIVITY
 Correct fabric placement is vital for accurate registration of the different colored patterns.  A slight pattern overlap prevents a white gap between two printed colors.
301 The main fault in screen printing is poor pattern registration  Inaccurate screen placement  Inaccurate fabric movement  Fabric slippage on the blanket (poor adhesion)  Distortion of screen mesh by drag of squeegee PROBLEMS IN FLAT SCREEN PRINTING SLOW PRINTING PROCESS [5 -10m/min]
302  Number of squeegee passes LOW PRODUCTION RATE More than one pass is used to achieve uniformity and adequate penetration, especially in blotch areas, for thick fabrics or irregular surface Squeegee takes longer to move along the screen where the repeat distances are large  Efficiency of the dryer If the dryer is short, or if temperature in dryer is too low, printing speed will have to be reduced in order to ensure the printed fabric is adequately dried.
ROTARY SCREEN PRINTING The process involves initially feeding fabric onto the rubber blanket. As the fabric travels under the rotary screens, the screens turn with the fabric. Print paste is continuously fed to the interior of the screen through a color pipe.
• As the screen rotates, squeegee device pushes print paste through the design areas of the screen onto the fabric. • As in flat-bed screen printing, only one color can be printed by each screen. After paste application, the process is the same as flat screen printing.
305 By converting the screen-printing process from semi- continuous to continuous, higher production speeds are obtained. Typical speeds are from 45-100 mpm for rotary screen printing depending upon design complexity and fabric construction. Continuous patterns such as linear stripes are possible. Rotary screen machines are more compact than flat screen machines for the same number of colors in the pattern (less floor space).
306  Glue streaks – from the rubber blanket Color out – from a lack of print paste  Creased fabric  Pinholes in any screen Damage to the screen leading to misprints Lint on the fabric causes pick-off With print designs, color application must be correct the first time, because printing defects cannot be repaired. General screen printing defects
307 Long process set up time for color and pattern change Screen production is slow and expensive Screens require considerable storage space Limitations Specific to Rotary Screen Printing
ROLLER PRINTING  Engraved roller printing is a modern continuous printing.  The design is engraved on the surface of a metal roller, to which dye is applied, and the excess is scraped off the roller's surface, leaving dye in the engraved sections.  When it rolls across the fabric, the dye on the roller transfers to the fabric.
TRANSFER PRINTING  Heat transfer printing is a technique where paper is printed, followed by the transfer of the design from the paper onto the textile fabric.  Commercial process involves printing release paper with pigments. The design on paper is placed onto the fabric, heated so that the pigment binder softens, releases from the paper, and adheres to the fabric.  Heat transfer printing is clean and environment friendly  The only by-product is the paper carrier. Transferring an image to fabric from a paper carrier. When heat and pressure are applied to this paper the inks are transferred.
DIGITAL PRINTING  It is the more advanced type of printing.  This includes :-  Jet spray printing  Electrostatic printing  Photo printing  Differential printing
 Rotary Screen Printing 60%  Automatic Flat Bed 18%  Other methods 22% WORLD PRODUCTION SHARE

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Textile coloration, dyeing and printing of textile materials

  • 1. 1 Wolkite University College of Engineering and Technology Textile Engineering Department Textile coloration By: Gezu K. 2012
  • 3. 3  Textile coloration refers to the process of imparting color to textile materials. COLORATION DEFINED DYEING PRINTING Only certain area of the substrate gets colored based on design requirement . The whole substrate gets the color by immersing in to a solution of the color. The textile material may be in one of several forms such as fiber, yarn, fabric, garment, etc.
  • 4. DYES/PIGMENTS Coloring matters are required to provide the desired color to textile substrates These substances are known as dyestuffs and pigments In general dyes/pigments are organic molecules A dye is soluble in the application media and is substantive to the textile substrate. A pigment is insoluble & is not substantive to the textile substrate binding [by adhesive agent]. No interaction with fiber
  • 5. 5 Historical Development of Dyes Dye development stages Driving forces Natural dyes Poor substantivity & poor fastness Use of mordant with natural dyes Long and difficult process Indigo [Water insoluble pigment] Good fastness but sill long process First synthetic dye [Mauveine] Affinity for few fibers only & poor light fastness Acid type azo dyes Acid dyeing of wool and silk Direct substantive dyes Poor wet/wash fastness Synthetic indigo vat dye Process difficulty Fiber reactive dyes Limited substantivity [Hydrophobic fibers] Disperse dyes For hydrophobic fibers [method of application] Dyes
  • 6. 6 SUBSTANTIVITY AFFINITY  Colored compounds which are absorbed by the fiber from a solution or suspension where they are subsequently fixed .
  • 7. Colour and Constitution of dyes 7  chromogen:  chromogen:  a chemical compound that is either coloured or can be made coloured by the attachment of suitable substituents – the chromophore and the auxochrome(s) are part of the chromogen;  Chromophore (chromophoric group):  Chromophore (chromophoric group):  An organic compound appears color due to the presence of unsaturated groups in it, such groups are chromophores  responsible for the appearance of color  all dyes contained aryl rings bearing unsaturated groups which termed ‘chromophores
  • 8. 8 Some typical chromophores are,  The colour intensity increases with the number of chromophores or degree of conjugated.
  • 9.  Auxochrome:  a substituent group in a chromogen that influences its colour. such as –OH or –NH– groups,  The presence of auxochroms in the chromogen molecule is essential to make it a dye auxochromes are acidic such as carboxylic and sulphonic groups and in basic auxochromes includes amino and hydroxyl groups.
  • 10. 10 Diazo group Diazo group Amino group chromogen chromophore auxochroms azobenzene yellow 4,4′-diaminoazobenzene
  • 11. 11 HISTORICAL DEVELOPMENT OF DYES Dye development stages Driving forces Natural dyes Poor substantivity & poor fastness Use of mordant with natural dyes Long and difficult process Indigo [Water insoluble pigment] Good fastness but sill long process First synthetic dye [Mauveine] Affinity for few fibers only & poor light fastness Acid type azo dyes Acid dyeing of wool and silk Direct substantive dyes Poor wet/wash fastness Synthetic indigo vat dye Process difficulty Fiber reactive dyes Limited substantivity [Hydrophobic fibers] Disperse dyes For hydrophobic fibers [method of application]
  • 12. 12  Scientific classification based on chemical structure Example: Anthraquinone dyes, Azo dyes etc.  Technical classification based on dyeing properties Example: Direct dyes, Acid dyes, Disperse dyes etc.  Commercial classification based on manufacturers’ aspects [Brand names based on fastness, method of dyeing and so on] Example: Indanthrene, Remazol, Procion, etc CLASSICATION OF DYES
  • 13. 13 BASIC DYES DISPERSE DYES MORDANT DYES METAL COMPLEX DYES OTHER DYES * [Sulphur, chrome etc] CLASSIFICATION BASED ON DYEING PROPERTIES DIRECT DYES REACTIVE DYES VAT DYES AZOIC DYES ACID DYES
  • 14. DYE SELECTION Type of fiber present Form of textile material & degree of levelness Fastness properties required Dyeing method used Availability of machinery Customer color requirement 14 Cost & Environment
  • 15. Interaction of matter with light  Light is a form of energy propagated at high speed in the form of electromagnetic waves.  Limited range of the electromagnetic wave detected by human eye  Response of light: matter interaction ABSORBANCE REFLECTION VISIBLE RANGE [400 – 700nm] COLOR & DYES
  • 16.  Color is produced due to selective absorption of the visible light. The reflected portion of the visible light corresponds to the color of the object. ALL REFLECTED ALLABSORBED WHITE BLACK
  • 17. 17 Wavelength (nm) Absorbed Light Reflected Light 400 - 440 Violet Greenish-yellow 440 - 480 Blue Yellow 480 - 510 Blue-green Orange 510 - 540 Green Red 540 - 570 Yellowish-green Magenta 570 - 580 Yellow Blue 580 - 610 Orange Greenish-blue 610 - 700 Red Blue-green Colors of typical spectral bands and perceived colors after absorption by a material viewed in white light
  • 18.  Preparation of dye solution [Dye and auxiliary chemicals]  Application of the dye  Fixation of the dye  Aftertreatment Batch Continuous DYEING METHODS SALIENT FEATURES OF DYEING PROCESS The stages/steps in dyeing process are: EXHAUSTION IMPREGNATION Semi continuous
  • 19.  Padding and squeezing  Fixation [Batching , Steam or hot air ]  Rinsing  Drying CONTINOUS DYEING PADDER
  • 20. 20
  • 21. 21 Semi Continuous The fabric is first passed though with the dye-liquor, that is called a padding machine or padding mangle. Then it is subjected to batch wise treatment in a jigger stored with a slow rotation for many hours dyeing consists of Pad-batch, at room temp. Pad-roll at increased temp. by employing a heating chamber This helps in fixation of the dyes on to the fiber The dye is applied continuously by a padding. The fixation and washing remains discontinuous
  • 22. 22 BATCH DYEING  Discontinuous system of dyeing  The dyebath is allowed to exhaust by providing the necessary condition  Dye fixation takes place in the dyebath CIRCULATING LIQUOR IN A STATIONARY MATERIAL MATERIAL MOVEMENT IN A STATIONARY LIQUOR CIRCULATION OF BOTH LIQUOR AND MATERIAL Th ree general types of BD machine
  • 23. AFTERTREATMENT Washing in detergent at or near the boil [Soaping] Treatment with chemicals to improve fastness Application of simple finishing chemicals 23
  • 24. 24 DYEABILITY FACTORS LIQUOR RATIO DYE CHARCTERISTICS DYEBATH ADDITIONS DYEING CONDIIONS FIBER CHRACTERISTICS CRYSTALLINITY & HYDROPHILICITY STRUCTURE & DIFFUSEABILITY TIME & TEMPERATURE SALTS & OTHER AUXILARIES TYPE OF MACHINE
  • 25. 25  Exhaustion refers to the amount of dye transferred from dye-bath to the substrate in the dyeing process.  Co: initial concentration of dye in dye bath Cs= concentration during the process  Dye fixation means the reaction between the dye and fiber molecules. EXHAUSION AND FIXATION E % =(Co – Cs)/Co * 100 PROCESS TERMINOLOGIES
  • 26. COLOUR YIELD DEPTH OF SHADE Paleness or dullness of a given shade Shade depth per a given amount of dye
  • 27. 27 DYE MIGRATION LEVELLING Tendency of dye transfer from heavily dyed region Uniformity of shade throughout the substrate
  • 28. 28 FASTNESS LIQUOR RATIO Resistance to color change or color removal Ratio of weight of material to volume of liquor COMPATIBILITY Dyes having same or similar rates of dyeing
  • 30. Fundamental steps in dyeing From molecular point of view dyeing process involves • Transport of dye from dye bath to fiber surface • Adsorption on the surface of fiber • Diffusion in the fiber interior • Fixation of dye 30
  • 31. STUDIES ON THEORY OF DYEING 31 THERMODYNAMIC APPROACH KINETIC APPROACVH Dyeing equilibrium Rate of Dyeing
  • 32. 32  Exhaustion of the dye in solution takes place in dyeing process  After the initial increase in the degree of exhaustion, it eventually becomes constant.  No net transfer of dye from solution to the fiber. At this point dyeing equilibrium is established. DYEING EQUILIBRIUM Subsidiary equilibria is established between dye dissolved in the bath & dye adsorbed on fiber surface; between dyed adsorbed & dye diffused in the fiber. Nernst Isotherm Langmuir Isotherm Freundlich Isotherm In the case of dyeing equilibria, the most commonly encountered adsorption isotherms are the partition isotherm :
  • 33. 33
  • 34. 34 Nernst Isotherm Dye is equally divided between solution and fiber which is typically displayed for the adsorption of disperse dyes on hydrophobic fibers such as PES.
  • 35. 35 Rapid sorption limited by accessibility of fiber surface sites Freundlich Isotherm the adsorption of direct dyes on cotton and other cellulosic fibers.
  • 36. 36 Sorption limited by fixed number of adsorption sites Langmuir Isotherm the adsorption of anionic dyes (e.g. non- metallized acid dyes) on wool, silk and PA fibers, as well as basic dyes on PAN fibers.
  • 37. AFFINITY 37  The attraction of the dyes to fibers is generally expressed as AFFINITY.  During dyeing mass changes occur that change thermodynamic functions The chemical potential is defined as the change in free energy of a system that occurs when the composition changes by a unit molar amount of substance, all other variables such as the temperature, pressure and the amounts of other components remaining constant. CHEMICAL POTENTIAL µ = µ0 + RT ln (a)
  • 38. 38 In the case of dyeing, if the chemical potential of the dye in solution is higher than in the fiber, the dye will transfer to the fiber. The chemical potential in solution falls; that in the fiber increases. At equilibrium, the chemical potential of the dye in the fiber is equal to the chemical potential of the dye in the solution.  The standard affinity of a dye for a fiber is defined as the difference of the standard chemical potentials of the dye in the two phases.
  • 39. 39  The standard affinity is the change in chemical potential of dye when one mole is transferred from standard state in solution to standard state in the fiber.  The standard molar free energy change for dyeing Enthalpy of dyeing [-ΔHo] Entropy of dyeing [ΔSo] Immobilization of dye in the fiber MEASURE OF STRENGTH OF DYE-FIBER BONDS
  • 40. 40 EXOTHERMIC NATURE OF DYEING Establishment of equilibrium involves decrease in the total free energy  Hence heat is given out during dyeing  Exothermic reactions are favored at low temperatures
  • 41.  Fick's Law of diffusion: The rate of transport of dye (dQ/dt) across a given area is proportional to concentration gradient RATE OF DYEING
  • 42. • The rate of dyeing increases with increasing temperature. • The equilibrium uptake of dye decreases owing to the exothermic nature of dye adsorption. Effect of Temperature on Dye Diffusion
  • 43. MODELS OF DYE DIFFUSION 43 FREE VOLUME
  • 44. Cotton, flax and CLY Hydrophobic substrates such as PES
  • 45. 45 Rapid Dyeings @ shorter half dyeing time HALF DYEING TIME  The time required for the exhaustion to reach half [50%] of is equilibrium value.
  • 46. FACTORS ON RATE OF DYEING  Type /construction of material  Dyebath temperature & pH  Type of dye-bath additions  Liquor ratio  Degree of agitation
  • 47. DYE-FIBER INTERACTIONS 47  Ionic links  Hydrogen bonds  Covalent bonds  Physical forces
  • 48. Cellulosic dyeing 48 Direct dye Reactive dye Vat/indigo dye Sulphur dye
  • 49. Dyeing of Cellulosic Textiles Dyes for cellulosic fibers: • Cotton and other cellulosic fibers can be dyed with Reactive, Direct, Vat, Sulphur, Azoic, Phtalogen, Pigment and Mordant dyes. • Application methods, dyeing characteristics, cost, fastness properties and color range are differ for each dye. • They have their own particular advantages and disadvantages. 49
  • 51. Dyeing processes which involve use of direct dyes Direct dyes are so called direct because they are applied to cellulosic fiber textiles directly with out the need for a mordant. Typical characteristics of Direct Dyes • Inherent substantivity to cotton and other cellulosics • Do not require mordant, dyeing procedure is quite simple. • Direct dyes are anionic in nature • Their aqueous solutions dye cotton usually in the presence of an electrolyte such as NaCl or Na2SO4. • Generally these dyes are water soluble • Relatively inexpensive, available in a full range of hues but not noted (known) for their color brilliance. Introduction
  • 52. • Direct dyes usually have linear long coplanar molecular structure • Sulphonated azo dyes constitute the predominant group of direct dyes • Their major drawback is their poor to moderate fastness to washing. • Various treatments to improve fastness • Dye Solubility depends on Number of solublizing groups, Molecular weight and temperature of dyeing After treatment Solubilizing group
  • 53. 53 By bringing dye bath to boil gradually and holding at boil whilst the dye diffuses into the fiber. In absence of electrolyte some direct dyes will not dye cellulose at all. Usually applied with the addition of electrolyte at or near the boil. Dyeing with direct dyes is carried out in neutral solution Simple dyeing procedure Requirement of soft water for some direct dyes during application Application of direct dyes to cellulose
  • 54. 54
  • 55. 55
  • 56. CLASSIFICATION OF DIRECT DYES 56 CHEMICAL STRUCTURE OF CHROMOPHORE DIRECT AZOIC DYES DIRECT THIAZOLIC DYES Classification based on chemical structure is not significant for the dyer because dyes with similar constitution can have quite different application and fastness properties.
  • 57. 57 DYEING PROPERTIES [EQUALIZING] Class A: Self leveling Class B: Average leveling Class C: Poor leveling The most common classification of direct dyes is that of the Society of Dyers and Colorists (SDC), based on their levelling ability and their response to increase in the dyeing temperature and to added salt during exhaust dyeing.
  • 58. 58 Class A: Self leveling Low molecular weight => mono- and bis-azo dyes with several anionic sulphonate groups per molecule. Good migration, even in the presence of salt. Several sulphonated groups per molecule Good solubility and no aggregation Low substanivity Large amount of salt for exhaustion
  • 59. 59 Dyeing is started at 50 °C in the presence of added salt, the bath heated to the boil over 30–40 min, and dyeing continued at the boil for up to an hour. Several further salt additions, of increasing size, are required to promote exhaustion, the total amount of salt (5–20% owf NaCl) depending upon the depth of shade and the liquor ratio.
  • 60. 60 Class B: Salt Controllable Molecular weight higher than Class A Lower sulphonated groups than Class A dyes Medium solubility and degree of aggregation Low to moderate substanivity [Absence of salt] Salt sensitive & exhaust well with salt addition
  • 61. 61 Class C: Temperature controllable High molecular weight Few sulphonated groups per molecule Less solubility and high degree of aggregation High substantivity Very sensitive to salt addition Leveling by control of temperature & leveling agents
  • 62. TYPE MIGRATION LEVELING FASTNESS Class A Good Self leveling Low Class B Medium Medium leveling Moderate Class C Poor Poor leveling High Additional Classification Parameters: Further treatment, continuous dyeing, high temperature dyeing COMPARISON OF DIRECT DYES
  • 63. 63 DIRECT DYE SUBSTANTIVITY FOR CELLULOSE Long, coplanar dye molecules can sit on top of a cellulose polymer chain with the aromatic rings parallel to the glucose rings.  Possibility of hydrogen bond formation  Vander walls and other secondary forces of interaction
  • 64. 64
  • 65. 65 Their long flat (coplanar) structure enables them to lie along a cellulose chain in register with hydroxyl groups and results in effective vander waals forces.
  • 66. 66 Dyeing methods The selection of specific direct dyes for dyeing cellulosic fibres depends on Their dyeing properties, The particular fastness requirements, Any after treatments used to improve the washing fastness, and The particular finishing processes involved.
  • 67. 67 A typical dyeing process for a Class B direct dye on cotton EXHAUST DYEING
  • 68. 68 Immerse the wetted fabric NaCl NaCl 40oC 100oC 35 min 25min 35min
  • 69. Salient Features of Batch Dyeing 69  Solution preparation starts with small dye paste to which is added sufficient hot water for dissolution Wetting agent assist penetration and level dyeing Depth enhancement by controlling salt addition & temperature  Examples  Hank and package dyeing for cotton yarns  Jigger for woven fabrics  Winch for knitted fabrics
  • 70. PAD DYEING 70 Major pad dyeing operations:  Pad dyeing methods are only used on fabrics Pad with dye solution, pad with salt bath and roll Pad with dye solution, dry, pad with salt solution and steam Pad with salt, pad wet-on-wet with dye solution and steam Dyes with less strike rates are preferable.  Migration of Class A during steaming/drying & very rapid strike by Class C dyes  Class B dyes are better choice for continuous dyeing Less Suitable for continuous dyeing
  • 71. 71 FACTORS AFFECTING DIRECT DYEING EFFECT OF SALT  Cellulose immersed in water develops a negative surface potential  Repulsion between negative charge of cellulose surface & anionic dye molecule The added salt provides sodium ions to counteract the negative surface potential of the wet cotton Hence increase in exhaustion of direct dyes Commercial direct dyes already contain much electrolyte.
  • 72. The nature of the anion of the added electrolyte has little influence on the amount of adsorbed dye. Therefore, NaCl and Na2SO4, at the same total sodium ion concentration, have about the same influence. A higher positive charge on the cation promotes increased adsorption because metal ions counteract the negative surface potential more effectively, thus decreasing the repulsion of approaching dye anions. 72
  • 73. 73
  • 74. 74
  • 75. 75 B is more salt sensitive than A
  • 76. EFFECT OF TEMPERAURE 76 • Temperature increases dye migration and rate of dyeing. • Equilibrium exhaustion decrease as dyeing temperature increases DYE DEAGGREGATION RATE OF DYEING EXHUASTION
  • 77. These diverse variations of direct dye exhaustion depend on two opposing influences of the increasing dyeing temperature. 1. These are the usual effect of increasing temperature decreasing the dyebath exhaustion because dyeing is exothermic, and its enhancement of the dyeing rate particularly at lower temperatures. 77
  • 78. 2. Increasing temperature also promotes dye de-aggregation in the dyeing solution liberating more individual dye molecules to enter the fibre. The dyes CI Direct Yellow 12, Direct Red 81 and Direct Yellow 28 have maximum exhaustion at 30 °C (a), 60 °C (b) and 100 °C (c), Respectively. For example, open jig dyeing machines cannot achieve dyeing temperatures much above 85 °C. 78
  • 79.  Greater negative potential under alkaline pH and retardation  Increasing dissociation of a number of cellulose hydroxyl groups  Oxycellulose (over bleaching) presence reduction in exhaustion and depth Higher proportion of carboxyl groups  Alkaline reduction of azoic dyes reducing color yield  Acid baths specially strong acid not suitable [Cellulose Damage]  Dyeing of cotton with direct dyes at neural pH 79 EFFECT OF DYEING PH Reduce exhaustion Carboxylate ions repel the dye anions of like charge.
  • 80. EFFECT OF LIQUOR RATIO Lower liquor ratio enhances exhaustion for substantive dyes Dyeing at low liquor ratio  Decreases the amount of waste dye in the effluent.  Consumes less water and steam, and  Allows a given salt concentration with less added salt.
  • 81. Inhibit the formation of larger aggregates and increase the proportion of dye in the monomolecular form. Hence solubilisation is increased. EFFECT OF SURFACTANT TIME OF DYEING The production of level and well-penetrated dyeings is usually favored by an increased time of dyeing, although prolonged dyeing at boil sometimes results in the decomposition of direct dyes.
  • 82. 82 DIRECT DYEING OF DIFFERENT CELLULOSIC FIBERS COTTON < MERCERIZED COTTON < LINEN < VISCOSE RAYON Dyeing of blends of these cellulose fibres with direct dyes, they do not absorb dyes at the same rate or to the same extent because of the differences in their morphology. With dyes of low substantivity, neps usually absorb less dye and appear as paler spots on the fabric. Since immature fibres give much greater rates of dye desorption, the paler dyed neps may not appear until after washing. Improved by mercerization before dyeing swells the immature fibres The rate of dyeing increases as the diameter of a fibre decreases
  • 83. 83 Direct dyes have higher substantivity for regular viscose than for cotton because viscose is less crystalline and oriented so it has a much greater internal surface. It is more accessible than regular cotton Mercerized cotton will absorb more dye and be darker in color than normal cotton dyed in the same bath.
  • 84. HIGH TEMPERATURE DYEING 84 Conventional direct dyeing process @ boil High temperature dyeing up to 130 C Reductive cleavage at higher temperature Decomposition of direct dyes REDUCTION OF DYES USED IN STRIPPING & DISCHARGE PRINTING POOR WET/WASH FASTNESS
  • 85. AFTERTREATMENT OF DYEINGS WITH DIRECT DYES 85  Aftertreatment is carried out to improve washing fastness Basic principle involved is increasing the dye’s molecular weight  Adsorbed dye becomes insolublized & has low diffusion coefficient  Impact on hue and light fastness  Aftertreatments are difficult and costly to carry out  Replacements of direct dyes today reduce use of aftertreatment Reactive dye
  • 86. AFTERTREATMENTS IN DIRECT DYEING • Metal complex formation • Aftertreatment with formaldehyde • Use of cationic fixatives • Based on resin crosslinkks 86
  • 87. METAL COMPLEX FORMATION 87 Treatment with acidic copper sluphate solution for the formation of a complex between copper and adsorbed dye
  • 88. 88 FORMALDEHYDE TREATMENT Based on formation of methylene bridge between adsorbed direct dye molecules
  • 89. 89 TREATMENT WITH CATIONIC FIXATIVES Cationic agents of relatively high molecular weight form water- insoluble salt like molecule with anionic direct dye. Precipitation of anionic dyes in the cotton with a cationic surfactant or polymer in warm water Based on addition of crosslinking agents used for easy care finishing Direct dyed cellulose is crosslinked using crosslinking agents Surface dyes have less tendency of movement Wash fastness is improved due to strong anchoring effect RESIN TREATMENT Reduce the fastness light The change in hue is only slight Decrease the light fastness and give a hue change. Amino resins
  • 90. 90 Ionic bonds involve the attraction between oppositely charged chemical groups on the dye and fibre. Example
  • 92. INTRODUCTION • Poor washing fastness because only weak polar and dispersion forces bind the dye molecules to the cellulose polymer chains. • Weaker forces of interaction (Hydrogen and physical bond) [Dye – fiber] Immobilizing dye molecule by covalent bond formation with reactive groups in fiber Easily diffuse out of the cotton during washing. A dye which is capable of reacting chemically with a substrate to form a covalent dye substrate linkage Direct dye Reactive dye
  • 93. Cont. … A L K A L I N E Mild The role of the alkali is to cause acidic dissociation of some of the hydroxyl groups in the cellulose, and it is the cellulosate ion (Cell–O–) that reacts with the dye. Reactive chlorine atom on the triazine ring Anionic and water soluble like direct dye
  • 94. Simple molecular structure & low substantivity High degree of wash fastness Bright shade Complete color gamut /wide color range Relatively simple dyeing procedure [no redox system] Versatile in application [both bath & continuous] Characteristics of Reactive Dyes
  • 95. 95 STRUCTURE OF REACTIVE DYES Solubilizing group (S)  Chromophore (C)  Bridge group (B)  Reactive group (RG) with a leaving group (X) attached to it ≥1 sulphonic acid substituents Reacts with Cell-OH Color + Substantivity Links R with C
  • 96. The reactive group must exhibit adequate reactivity towards cotton, but be of lower reactivity towards water that can deactivate it by hydrolysis. The dye–fibre bond, once formed, should have adequate stability to withstand repeated washing.
  • 97. THE REACTIVE SYSTEM TRIAZINYL RING VINYL SULPHONE Reactive dyes look like direct, but with added reactive group Anionic and water soluble like direct dyes NH
  • 98. 98 MECHANISM OF DYEING WITH REACTIVE DYES Reactive groups are of two main types 1. Those reacting with cellulose by nucleophilic substitution of labile leaving groups such as chlorine, fluorine, methyl sulphone or nicotinyl leaving group activated by an adjacent nitrogen atom in a heterocyclic ring
  • 99. 99 In nucleophilic substitution, a mobile halogen atom of the dye is substituted by the ionized nucleophilic oxygen group of the cellulose (Cell-O).
  • 100. 100 2. Reaction with cellulose by nucleophilic addition to a carbon– carbon double bond, activated by an adjacent electron-attracting sulphone group. The vinyl sulphone group is usually generated by elimination of sulphate ion from sulphatoethylsulphone with alkali. CELLULOSE ETHER FORMATION
  • 101. 101 Covalent bond formation In nucleophilic addition, a proton and the ionized oxygen group of the cellulose are added to the active group of the dye. The best-known dyes of this group – the vinyl sulphones – carry a protective group, which is eliminated even at a fairly low pH and sets the reactive group free during dyeing.
  • 103. 103 Basis of Classification Based on chemical structure Based on dyeing property Commercial – Manufacturer aspect of brand name CLASSIFICATION OF REACTIVE DYES
  • 104. 104 [AZO & ANTHRAQUINONE] MONOFUNCTIONAL BIFUNCTIONAL HOMOBIFUNCTIONAL HETROBIFUNCTIONAL REACTIVE GROUP CHROMOPHORE GROUP Based on chemical structure POLY-FUNCTIONAL
  • 105. Monofunctional Reactive Dyes Presence of reactive group (one or two) at a single location on dye molecule Reactive dyes developed at early stages were mono-functional typical examples are  Mono-chloro triazine  Di-chloro triazine (two reactive groups located on the same triazine ring)  Vinyl sulphone
  • 106. DICHLOROTRIAZINYL [DCT] Monofunctional Reactive Dyes MONOCHLOROTRIAZINYL [MCT] VINYLSULPHONE [VS] No separation of reactive groups from each other Reactive group attached to a single Chromophore
  • 109. BIFUNCTIONAL REACTIVE DYES • Presence of two reactive groups of same type (mono or dichloro triazine) or different types (monochloro triazine and vinyl sulphone). • At two different locations in the dye molecule. • Show high exhaustion, high fixation • Better color yield • Reduced pollution: less dye in effluent • Very popular for exhaust dyeing. • High exhaustion is due to high molecular weight similar to direct dyes, high affinity to cotton • React with cellulose with cross link formation.
  • 111. 111 BIS(SULPHATOETHYLSULPHONE)  Homobifunctional Reactive Dyes Consist of two similar reactive groups
  • 112. 112 Reactive Dye Reactivity Different reactive groups of reactive dyes show a wide range of reactivity The more reactive the dyes the less weaker the alkali required and The lower the reactivity of reactive group of the dye towards the alkaline cellulose, the higher the final dyeing temperature required.
  • 113. 113 DYE REACTIVITY DYEING TEMP. pH DCT HIGH 20 - 40 Weak alkali (NaHCO3 or Na2CO3) MCT LOW 80 - 85 Strong alkali (Na2CO3 or NaOH) MFT MODERATE 40 - 60 Moderately alkaline VS MODERATE 40 - 60 Moderately alkaline NT* MODERATE TO HIGH 100 - 130 Neutral ALKALIS USED: Sodium bicarbonate, sodium carbonate & sodium hydroxide/Sodium silicate
  • 114. CLASSIFICATION BASED ON DYEING PROPERTY 114  Alkali controllable reactive dyes Salt controllable reactive dyes  Temperature controllable reactive dyes On the basis of dyeing temperature COLD BRAND, WARM BRAND & HOT BRAND
  • 115. ALKALI-CONTROLLABLE REACTIVE DYES  Have high reactivity and only moderate substantivity  Applied at relatively low temperatures  Level dyeing requires careful control of the addition of alkali Examples: DCT, DFCP and VS
  • 116.  These are dyes of relatively low reactivity  They have appreciable substantivity  Dyeing temperature as high as 85 °C  Requires careful addition of salt Examples: MCT and MFT SALT-CONTROLLABLE REACTIVE DYES
  • 117. 117 • At temperatures above the boil in the absence of alkali • No auxiliary product addition in dye bath • Control over rate of temperature rise Examples: NT reactive dyes TEMPERAURE-CONTROLLABLE REACTIVE DYES
  • 118.  Cold BRAND dyes - fixation temperature of 30-40°C  Warm BRAND dyes - fixation temperature of 50-80°C  Hot BRAND dyes - fixation temperature of > 80°C • Lower substantivity dyes diffuse easily into fibers and are easy to wash out unfixed dyes but less exhaustion. • Higher substantive dyes give higher bath exhaustion, better reaction with fiber, but greater difficulty of removing unfixed dye
  • 119.
  • 120. 120 Reactive dyes in a mixture should all exhaust and react with the fiber at about the same rate.  Dyes with different reactive groups and different substantivity are available  Mix dyes with same type of reactive group having the same substantivity. Compatible dyeing behaviour is a function of all the process variables and requires careful control of the dyeing parameters. Dyeing compatibility of reactive dyes Ideally Different dyeing characteristics and reactivities. Requires careful control of the dyeing temperature, salt and alkali concentrations, the dyeing time and the liquor ratio.
  • 121. PROBLEMS IN DYEING WIH REACTIVE DYES  Hydrolysis [particularly for high reactivity series, e.g. DCT]  Low level of fixation when dyeing using a high liquor ratio  Appreciable dye concentrations in the dyehouse effluent  High salt concentrations are also present Less than 70% of the original dye reacts with the fibre. For exhaustion
  • 122. 122 Precipitation or insolublization by metal ions in water supply Free chlorine hydrolyze and dye-fiber bond breakage Unreacted chlorine atoms in the dye’s reactive group may hydrolyze under warm humid storage conditions, liberating HCl. Catalyzes the hydrolysis of the dye–fibre bond. Calcium, magnesium or heavy metal ions
  • 123. INITIAL EXHAUSTION [NEUTRAL PHASE] 123  Dyeing is started in neutral aqueous- little likelihood of dye-fiber reaction-capable of migration to promote level dyeing  Salt/ neutral electrolyte is added gradually to promote exhaustion (higher conc. Compared to direct dyeing).  Temperature is gradually increased to aid penetration of dye assist migration;  Dye absorption depending upon substantivity STAGES IN REACTIVE DYEING PROCESS
  • 124. 124 FIXATION (REACTION) [ALKALINE PHASE] Fixation at alkaline pH Alkali facilitates dissociation of cellulose hydroxyl groups; Nucleophilic cellulosate ions begin to react with the dye. Fixation of dyes results in additional dye absorption, re-establishes dyeing equilibrium. Dye absorption from solution and reaction with the fiber then progress until no further dye is taken up.
  • 125. AFTERTREATMENT [POST – DYEING WASHING] 125  Unfixed and hydrolyzed dye on cotton  Residual alkali and salt As much as possible unfixed dye must be washed out of the dyeing If not, desorption of this dye during washing by the consumer can cause staining of other materials in wash. I. Successive rinsing in warm water II. Thorough washing using a boiling detergent solution (soaping) III. Final rinsing in warm water then cold wash DRYING
  • 126. REACTIVE DYEING METHODS 126 Both exhaustion and padding methods can be used JIGGER WINCH PAD-BATCH PAD-DRY PAD-STEAM SOFT WATER
  • 127.  Level well-penetrated dyeing's require careful preparation of the material. Material available for dyeing in neutral, uniform & readily absorbent  Residual size must always be removed from woven goods because of the risk of dye wastage by reaction with hydroxyl groups in size PREPARATION
  • 128. 128 ALKALI/SALT/SEQUESTERANT/WETTING AGENTS  All traces of residual chlorine or peroxy compounds must be removed prior to dyeing, otherwise loss of reactivity and destruction of dye  Variation in covering ability of neps of immature cotton need for effective mercerization or causticization
  • 130. 130
  • 131. 131 a = dye in dye bath b = hydrolysed dye on the fiber c = fixed dye d = dye to be washed out
  • 132. Problems Encountered in continuous dyeing (1) Dyes of low substantivity are desirable to avoid preferential dye absorption during padding and the initial color tailing that it causes. (2) High dye concentration for deep shades that may exceed limiting solubility of dye [use of urea for disaggregation] CONTINUOUS DYEING Very rapid fixation (< 60 s) at high temperatures that is possible in fully continuous dyeing gives high productivity for long runs of a given color. Tailing is shade variation along the length of the fabric Urea helps to break up dye aggregates
  • 133. 3] when using pre-prepared alkaline solutions of reactive dyes Stability of dye/alkaline mixture [hydrolysis dye’s reactive group in pad bath will result in a loss of fixed color.] [Pad – dry – Pad] 4) Migration control and heating uniformity [Anti-migrant] (5) Final rinsing and soaping of goods dyed @ high speed with less time of contact salt as an anti-migrant and also a thickening agent such as sodium alginate Counter current washing
  • 134. 134 The lower limit of liquor ratio in batchwise dyeing is about 5:1. Padding methods extend this further to the range 1:1 to 0.5:1. Enhanced exhaustion and fixation ADVANTAGE DYE SELECTION Excellent solubility moderate substantivity, and versatile reactivity
  • 135. 135 Single-pad sequence: dye and alkali are applied together Double-pad sequence: dye and alkali are padded separately, with or without an intermediate drying step. Padding PAD – DRY – WASH PAD – BATCH - WASH PAD – DRY – BAKE -WASH PAD – DRY – STEAM -WASH PAD – DRY – PAD - BATCH –WASH PAD – DRY – PAD - STEAM -WASH PAD – DRY – PAD - WASH SINGLE PAD DOUBLE PAD
  • 136. 1. Impregnation of the well-prepared dry fabric in a solution of dye and alkali at ambient temperature  A 4:1 dye-to-alkali solution volume ratio is very common.  The dye and alkali solutions are usually mixed just before padding using metering pumps to maintain the correct ratio. 2. Uniform squeezing of surplus liquor from the fabric as it passes through the mangle nip. PAD – BATCH DYEING 1000 – 10000m CPB[Cold Pad Batch]
  • 137. 137 3. Wrapping of the batched roll of wet fabric in polythene film 4. Washing-off 5. Drying SLOW ROTATION Cost Reduction Scheme To avoid drainage of the internal liquid within the batch. Storage at ambient temperature for a specified dwell time (2– 24 h), depending on dye reactivity and pH) The more reactive dyes give effective fixation within 2–6 h.
  • 138. 138 PAD–HEAT/DRY DYEING Suitable for reactive dyes with fairly high reactivity. The fabric is first padded with a dye solution containing sodium bicarbonate. During drying, bicarbonate is converted into carbonate, which gives a higher pH and more effective fixation The dye solution also contains a high concentration of urea (100 g/l). Retain water during drying Increases dye solubility Pollution problem Nutrients for algal growth
  • 139. 139 PAD – STEAM DYEING In this process, goods are padded with a solution containing reactive dyes, salt and appropriate alkali. Hot humid conditions during steaming tend to cause excessive hydrolysis of reactive group and lower color yield.  Two-stage wet-on-wet padding is used to avoid an intermediate drying step.  Sufficient pick up and minimum color bleeding must be ensured During dyeing of terry towel and other pile fabrics Selected dyes are suitable and the manufacturer’s recommendations should be followed.
  • 140. 140  The dye solution is quite stable, because there is no alkali Anti-migrants help to minimize migration during initial drying Intermediate drying ensures uniform & high pick-up of chemical solution & minimizes bleeding of dye  Sodium hydroxide is the preferred alkali Pad–dry–pad–steam The fabric is padded with a neutral solution of the reactive dyes, dried and then padded with the alkali solution containing salt before steaming
  • 141. 141 TEMPERATURE/TIME FACTORS AFFECTING REACTIVE DYEING PH [ALKALI] SALT & OTHER AUXILARY MLR
  • 142. 142 Dyeing temperature is chosen based on reactivity Time depending on exhaustion and leveling Selection of alkali based on reactivity & ease of hydrolysis Effect of salt is similar to direct dyeing process  Larger amount of salt due to low molecular weight and large number of sulphonic groups per reactive dye molecule Selection of MLR considering tendency of hydrolysis and solubility
  • 143. 143 It is not usual to exceed a dye-bath pH of 11: Hydrolysis. For polysulphonated dyes, one effect of dyeing at pH above 11 is the decrease in substantivity of the dye for the increasingly anionic dissociated cellulose. Some dyes actually desorb from the fiber into the dye-bath when the alkali is added at the start of the fixation stage giving a sudden decrease in the degree of exhaustion. LARGE AMOUNT OF SALT Longer LR m/c --- dyes of higher substantivity are preferred
  • 144. 144 Viscose fibers give higher fixation and exhaustion of reactive dyes than cotton. For identical conditions, exhaustion and fixation increase in the order: COTTON, MERCERISED COTTON, VISCOSE. Because of the ease of swelling of viscose, the dyeing pH and temperature for a given dye is different than for cotton
  • 145. 145  Dyeings are resistant to color stripping with hot aqueous pyridine, a solvent that removes direct dyes from cotton.  Dyeings of cotton obtained with bifunctional dyes exhibit reduced swelling and decreased solubility in cuprammonium EVIDENCE FOR COVALENT BOND FORMATION The good fastness to washing of dyeing's with reactive dyes on cellulosic fibres is a consequence of the stable covalent bond formed between the dye’s reactive group and the cellulose polymer.
  • 147. Introduction  Vat dyes are one of the oldest types and water insoluble dyes.  Vat dyes ‐ a series of dyes of different chemical constitution … contain two or more keto (C=O) groups  They are called dyes because: i. Chemical reduction into a water-soluble leuco form ii. Substantivity in leuco form for cotton
  • 148.  Insoluble in water  Have best overall fastness properties. .. often used for fabrics that will be subjected to severe washing and bleaching conditions (toweling, industrial and military uniforms)  Not bright colors  No substantivity for cellulose  Have a limitation with regards to use of strongly alkaline solutions  Mostly complex dye structures with no ionic groups CHARACTERISTICS OF VAT DYES
  • 149. Cont. …  Vat dyes are used predominantly for dyeing cellulosic fibers.  Although leuco dyes have substantivity for wool and nylon, technical reasons (damage in wool and poor fastness on nylon) vat dye application is restricted to cellulose only.  The vat pigment and the leuco compound often have quite different colors  The primitive color is developed by subsequent oxidation
  • 150.  Characterized by containing at two or more keto group [C=O]  Mostly complex dye structures with no ionic groups Chemical Characteristics
  • 151. MECHANISM OF DYEING WITH VAT DYES Converting a water-insoluble keto-substituted colorant by reduction to a water-soluble leuco compound. This penetrates into the fiber, where it is reoxidised back to the original insoluble form. Fixation based on secondary forces and insolublization
  • 152.
  • 153. The process of reduction of vat dye in aqueous alkaline solution is known as VATTING. Sodium dithionite [sodium hydrosulphite or hydros] is used as reducing agent and caustic soda as alkali . Hydros is a very strong reducing agent, more effective at high pH and at higher concentration. Control over concentration & temperature for optimum reduction Rate of reduction depends on pigment dispersion DISPERSION STABILITY FOR PROPER REDUCTION The process of reduction of vat dye in aqueous alkaline solution is known as VATTING. Sodium dithionite [sodium hydrosulphite or hydros] is used as reducing agent and caustic soda as alkali . Hydros is a very strong reducing agent, more effective at high pH and at higher concentration. Control over concentration & temperature for optimum reduction Rate of reduction depends on pigment dispersion DISPERSION STABILITY FOR PROPER REDUCTION Na2S2O4 & NaOH
  • 154. CLASSIFICATION OF VAT DYES Based on chemical structure Method of application
  • 155. Indigo Anthraquinonoid vat dyes: Based on anthraquinone which give leuco compounds of relatively high affinity Based on chemical structure a) Dyes derived from indigo – both natural and synthetic Anthraquinon e Indigoid vat dyes: Derivatives of indigo which give leuco compounds of relatively low affinity b) Dyes derived from anthraquinone – most of vat dyes Indigo dyes give more brilliant colors than anthraquinone But their light and wash fastness are not as good
  • 156. Classification Based on Application/dyeing properties  IN [INDANTHRENE NORMAL]  IW [INDANTHRENE WARM]  IK [INDANTHRENE COLD {KALT}]
  • 157.  Use of concentrated NaOH Have high molecular weight  High vatting temperatures (60 °C)  High dyeing temperatures (60 °C)  High substantivity  No salt addition to the dyebath THE IN (INDANTHRENE NORMAL)
  • 158. IW [INDANTHRENE WARM] DYES  Moderate concentration of NaOH  Lower vatting temperature than IN dyes (50 °C)  Lower dyeing temperatures than IN dyes (50 °C)  Moderate substantivity  Some amount of salt addition to aid exhaustion
  • 159. IK [INDANTHRENE COLD {KALT}] DYES  Low concentration of NaOH  Lower vatting temperature (40 °C)  Dyeing at room temperature (20°C).  Poor substantivity  Considerable amount of salt addition for good exhaustion
  • 160.  The vatting temperature is often higher than the subsequent dyeing temperature. To avoid decomposition of hydros and decreases the risk of over-reduction
  • 161. 1) Reduction of the pigment to the soluble leuco compound 2) Absorption of the leuco compound by the cotton during dyeing 3) Oxidation of the absorbed leuco compound in the cotton, reforming the insoluble pigment inside the fibers. 4) Soaping to remove pigment loosely adhering to the fiber surfaces and to develop the true shade and fastness properties Key Steps in Vat Dyeing Of Cotton Air Oxidation Or Chemical Oxidants Recrystallization/Aggregation Preparation of the vat containing the leuco forms of the dyes Water-soluble leuco compound
  • 162. Application of Vat Dyes on Cellulose i. Reduction of pigment to soluble leuco compound, a process called vatting; Converting a water-insoluble keto-substituted colorant by reduction to a water- soluble enolate leuco compound. ii. Absorption of leuco compound by cotton; water-soluble enolate leuco penetrates into the fiber, where it is reoxidised back to the original insoluble form. iii. Fixation based on secondary forces and insolublization. Oxidation of absorbed leuco compound in cotton, reforming insoluble pigment inside the fibers.
  • 163.  The most important reducing agent in vat dyeing is sodium dithionite (Na2S2O4), referred to as hydrosulphite or hydros. Reduction of Vat dyes  Vat dyes are available as fine powders or grains, and as liquid dispersions or pastes.  The insoluble pigment is extensively milled with dispersants such as sodium ligno-sulphonates to produce very small particles In the presence of NaOH- alkali
  • 164. Control over concentrations & temperature for optimum reduction Rate of reduction depends on pigment dispersion Dispersion stability for proper reduction
  • 165.  Some Quinone vat dyes can be over-reduced  reduction of more than one pair of keto groups (C=O)  Over-reduced vat dyes give have poor substantivity for cotton; more difficult to oxidize; produces duller shades; gives lower color yield.
  • 166. Others ... sodium sulphoxylate formaldehyde and sodium sulphoxylate acetaldehyde, particularly in printing and in continuous dyeing processes. Cont’d Others auxiliaries .... neutral salts (to increase substantivity), wetting, sequestering and dispersing agents, leveling agent. Help to overcome dyeing problems caused by inadequate pretreatment.
  • 167. ALKALI PREVENT VAT ACID FORMATION Caustic soda ‐ is used for: 1. The formation of the leuco: pH = 12‐13 needed. 2. The transformation of cellulose into alkali‐cellulose , 3. Neutralization of acid products of decomposition of hydros Cont’d
  • 168. Absorption and levelling  Alkaline leuco vat dye exhausts very fast onto cellulose until equilibrium is attained.  The higher the substantivity the higher is the exhaustion  High concentration of Na+ from sodium hydrosulfite and sodium hydroxide facilitate exhaustion
  • 169. Because of this, many dyes have a rapid strike; dye bath may be 80–90% exhausted within 10 minutes. The more rapidly dye exhausts, the greater is the risk of obtaining an unleveled dyeing. High electrolyte content Use of retardants
  • 170. Adsorption Isotherm For Vat Dyes On Cotton
  • 171. Substantivity of Leuco Vat Dyes for Cotton  H-bonds b/n hydroxyl groups and phenolate ion groups, or amino or amide substituents are important  Molecules of leuco vat dyes are large and also coplanarity of structure is essential.
  • 172. Oxidation and subsequent process/soaping  Once the dye has been absorbed by the fiber in the form of a leuco derivative, its oxidation takes place. The leuco derivative must be oxidized Any excess hydrosulphite must be eliminated The excess of alkalinity in the fiber must be neutralized  Leuco dye is then reconverted into its original form by oxidation.  Carried out with oxidizing agents like hydrogen peroxide, sodium perborate, Sodium dichromate or percarbonate.  The higher the temperature and the lower the pH, the higher the rate of oxidation reactions
  • 173. Dyes that are easy to reduce are more difficult to oxidize, and vice versa. Chemical oxidants preferred because they give more rapid and uniform oxidation throughout the material.
  • 174. Soaping  Removes pigment deposited on fiber surface that would wash or rub off in use.  Involves thoroughly washing goods in a detergent solution at or near boiling point.  It improves the light, washing and rubbing fastness of the dyeing.  Soaping probably involves re-crystallization of oxidized and dispersed vat dye particles  Crystallization results in slight change in hue and improvements in fastness properties. Neutralizing with acetic acid solution and final rinsing.
  • 175. Two methods: 1. Dyeing with reduced vat ‐ the dye in the form of soluble leuco is put into contact with the cellulosic fiber, is absorbed by it and then oxidized in the fiber itself. 2. Dyeing by pigmentation ‐ the insoluble dye in the form of a fine dispersion is deposited on the surface of the fiber; reduced in an alkaline solution which causes rapid absorption, finally, the process is continued as in the reduced vat system. Dyeing Processes
  • 176. DYEING METHODS Both batch and continuous methods are used Batch – (hot method, warm method and cold method) ‐ leveling problem due to high dyeing rate Continuous method – mostly used method (most common is pad‐steam process)
  • 177. Exhaust Dyeing: Typical Vat Dyeing Cycle Leuco Process In the leuco process the material to be dyed is entered into a prepared dye liquor that contains the fully vatted dye, alkali (caustic soda) and reducing agent (hydrosulphite), together with various amounts of  Salt,  Dispersing agent,  Sequestering agent and  Levelling agent, as required.
  • 178. High Degree Of Fastness Pre-oxidation Ancillary Chemicals [Chelating Agents, Wetting Agents, Sequesterants , Dispersants] Over-reduction Over-oxidation Effective Pretreatment General features Of Vat Dye Washing fastness Light fastness
  • 179. VAT DYEING FAULTS  Unlevellness  [inadequate oxidation, salt, temperature]  Poor penetration  (rapid heating result surface deposition)  Dull shade  due to over-reduction  Staining  [inadequate vatting, rinsing & circulation]  Poor rub fastness  [inadequate vatting, poor soaping]  Incorrect shade  [Soaping too short or low energy]  Fiber damage
  • 180. DENIM AND INDIGO VAT  DENIM is a warp faced twill fabric made from cotton yarns  The warp predominates on the surface  Dyed yarns in the warp and undyed yarns in the weft  Small white flecks distributed in a darker basic color  JEANS made from blue denim is most popular  Inexpensive, durable, versatile
  • 181.
  • 182.  Indigo is applied in a series of ‘dips’, with intermediate squeezing and atmospheric oxidation.  Reduction of indigo to soluble leuco-indigo  Multiple dip-squeeze-oxidize of the yarn  Rinsing, soaping & drying  Blue-dyed warps wash down to an attractive blue  No staining on the white yarns  STONE WASHING FOR FADED LOOK
  • 183.
  • 184.
  • 185. The goods are threaded through each box and skyed The first box is used to wet out the material. In subsequent boxes, the goods are immersed in the leuco Indigo solution for 10–30 s, squeezed and skyed for 2 min to oxidise the leuco dye to Indigo. This process of several dips and oxidations is then repeated in a second series of boxes, and so on. Deep shades built up by repeated dipping in the dye bath after each oxidation. Several rinsing and washing boxes complete the process.
  • 186. SULPHUR DYES SULPHUR dyes constitute the largest class of dyes in terms of quantity Like vat dyes applied to cellulosics by redox mechanism The insoluble pigment is converted into the substantive leuco compound by reduction and the leuco form is subsequently oxidized inside the fiber. Chromophore based on sulphur containing compounds The dyes contain THIAZINES AND THIOZOLES Limited color gamut [Known for black] SULPHUR BLACK
  • 187. THIAZOLES THIAZINES Sulphur containing aromatic heterocyclic linked by sulphide bonds
  • 188.  On treatment of an aqueous dispersion of the insoluble pigment with sodium sulphide, the sulphide links are reduced forming individual heteroaromatic units with thiol groups. DYEING PROCESS WITH SULPHUR DYES These are soluble in the alkaline solution in the form of thiolate ions that have substantivity for cellulose. After dyeing, the thiolate ions in the fibers can be re-oxidized back to the polymeric pigment. Low to moderate substantivity for cellulose Salt addition to promote exhaustion RINSED, OXIDIZED SOAPED
  • 189. Reduce d Sulphur dyes are important for black, navy, brown, olive and green colors in medium to heavy depths, being relatively inexpensive.
  • 190.  SODIUM SULPHIDE [Na2S] IS USED AS THE REDUCING AGENT
  • 191. Sulphur dyes have the dullest range of colors of all dye classes but are relatively inexpensive. When black color and dull shades are needed, with good fastness at reasonable cost, sulphur dyes are irreplaceable. Cotton dyed with some sulphur blacks becomes tendered on storing under warm humid conditions. Formation of sulphuric acid due oxidation of dye. Jig and pad steam for woven fabrics Large amounts of sodium sulphide used in the application of sulphur dyes, pose a significant environmental problem. OVER-REDUCTION
  • 192. CATIONIC [BASIC] DYEING  Dyes containing basic functional groups that are protonated  Organic cations  Dye fibers with anionic sites by a process of ion exchange  Mostly applied for acrylic, wool and silk  Low substantivity for cotton
  • 193.  Dye adsorption by acrylic fibers involves interaction between anionic sulphonate/sulphate polymer end groups and cationic dye molecules TRIPHENYL METHANE ANTHRAQUINONE
  • 195.  Acid dyes are applied in acidic media • Acid dyes are usually sodium salts of sulphonic acids or less frequently of carboxylic acids • Anionic in aqueous solution • Affinity for fibers with cationic sites • Usually substituted ammonium ion groups in fibers • Cationic character by acid absorption • Affinity for wool, silk and nylon INTRODUCTION
  • 196. MECHANISM OF DYEING WITH ACID DYES Ionic interaction between dye anions and amino groups of fibers Van der waals forces exerted between hydrophobic dye anion & hydrophobic parts of fiber adjacent to amino groups
  • 197. In wool, the numbers of amino and carboxylic acid groups are almost equal (820 and 770 mmol kg–1, respectively) ISOELECTRIC POINT OF WOOL: Equal numbers of cationic and anionic groups [pH ~5]
  • 198. Based on chemical structure of chromophore CLASSIFICATION OF ACID DYES AZO DYES ANTHRAQUINONE
  • 199.
  • 200. Based on dyeing characteristics GROUP I: LEVELLING ACID DYES GROUP II: MILLING ACID DYES GROUP III: SUPERMILLING ACID DYES Milling is the process by which wool is treated, in weakly alkaline solution, with considerable mechanical action to promote felting.
  • 201. 201 MOLECULAR WEIGHT OF DYE SOLUBILITY OR DEGREE OF SULPHONATION  DYEING PH  DEGREE OF AGGREGATION  DEGREE OF MIGRATION/LEVELLNESS  DEGREE OF WET FASTNESS COMPARISON FEATURES
  • 202.
  • 203. 203 LEVELLING ACID DYES 1. Add Glauber’s salt plus sulphuric acid 2. Add dye 3. Rinse
  • 204. 204  Strong acid are needed to achieve good exhaustion Glauber’s salt acts as a retarding/leveling agent  Level dyeing controlled by using less sulphuric acid initially Decreasing exhaustion on increasing dyebath pH above 4, and with increasing temperature  Small molecular size for good penetration into pores
  • 205. 205 MILLING ACID DYES 1. Ammonium sulphate/acetate + Acetic acid 2. Add dye 3. Rinse
  • 206. 206  Higher molecular weights and greater substantivity for wool than levelling acid dyes Weak acids or their salts are required (sodium acetate or ammonium sulphate) Have one sulphonate group per dye molecule and lower water solubility  Dyeing starts in the presence of ammonium sulphate  Addition of acetic acid to promote exhaustion  Both ion exchange and hydrophobic interactions Glauber’s salt is never added High concentration of sodium ions suppresses fiber negative charge and results high exhaustion with minimum leveling
  • 207. 207 SUPERMILLING ACID DYES 1. Ammonium sulphate/acetate 2. Add dye 3. Rinse
  • 208. 208  Ammonium acetate or sulphate is used at nearly neutral pH  More hydrophobic than milling acid dyes  Characterized by presence of long alkyl chains  Lower pH results rapid exhaustion and unlevellness  Rate control by temperature increase  High degree of aggregation even at boil
  • 209. 209 Auxiliaries are used in wool dyeing to promote leveling. Two main types of levelling agents: ANIONIC AND CATIONIC/NON-IONIC. Wool adsorbs anionic leveling agents and they retard dye absorption by initially blocking the cationic ammonium ion sites.  More substantive dye anions eventually displace anionic product. Anionic surfactants with long alkyl chains have higher fibre substantivity. LEVELLING AGENTS Cationic polyethoxylated amine
  • 210. 210 Cationic agents form a complex with the anionic dye in the dye-bath and prevent its uptake by the wool. The non-ionic portion of this type of product keeps the auxiliary–dye complex dispersed in solution. Avoid its precipitation in the bath or on the material surface. Free dye molecules are liberated as the dyeing temperature increases since the complex is less stable at higher temperatures
  • 211. 211 PROBLEMS IN ACID DYEING OF WOOL  Temperature and pH control  Skittery dyeing or tippy dyeiing  Ring dyeing  Shade variation
  • 213. DISCOVERY • Cellulose triacetate fibers and poor dyeability • Hydrophobic character; could not be penetrated by water, & not be dyed by water soluble dyes. • There was little point in treating fibers with a mixture of pure dye and water because particles would not be distributed uniformly. • Incorporating surface-active agent with dye ensures uniform distribution of dye in the dye-bath, allowing uniform dyeings DISPERSE DYES
  • 214. INTRODUCTION  Disperse dyes are non-ionic and slightly soluble in water  Applied in the form of fine aqueous dispersions  Dye particles are able to penetrate fiber during dyeing in a molecularly dispersed state and are held in fiber  Disperse dyes have affinity for synthetic fibers  Polyester, nylon, cellulose acetate and acrylic fibers COMPACTNESS [CRYSTALLINITY/ORIENTATION] HYDROPHOBICITY
  • 215. Dyeing of polyester with disperse dye Polyester is hydrophobic and characterized by compact physical structure PET is non ionic and thermoplastic fibers Because of non ionic it needs non ionic dye Dyeable only with disperse dye
  • 216. Chemical constitution  Simple low molecular non ionic mono azo and anthraquinone with polar groups for slight solubility required in dyeing. DISPERSION STABILITY DYE SOLUBILITY PARTICLE SIZE Hydrophobic dye ‘dissolving’ in hydrophobic fiber
  • 217.  Stable suspension of the dyes is achieved by using dispersing agent in the preparation of the dyes.  The solubility (slight) depends on the presence of polar groups such as OH, NO2, -CH2–CH2–OH and the particle size.  In general the particle size of dye must be very small. This is influenced by the degree of grinding and affects disaggregation of dye hence degree of absorption.
  • 218. REASONABLE RATE OF DYEING HIGH TEMPERATURE IN REFERENCE TO Tg DYEING ACCELERATORS [CARRIERS] FREE VOLUME THEORY
  • 219. POLYESTER DYEING METHODS  DYEING WITH CARRIER AT BOIL Benzoic and phenolic compounds unpleasant odours and are toxic  HIGH TEMPERATURE PRESSURE DYEING  DYEING BY THERMOSOL PROCESS Exhaust and continuous
  • 220. Carrier dyeing method A carrier is an organic compound dissolved in the dye bath and which increase the rate of dyeing Mechanism :  The polyester fibers absorb the carrier and swell.  Swelling can impede liquor flow in packages causing unlevelness.  The overall effect seems to be lowering of the polymer (Tg), thus promoting polymer chain movements and creating free volume.  This speeds up the diffusion of the dye into the fibers.
  • 221. 221 Dyeing Temperature 100 oC Alternatively, the carrier may form a liquid film around the surface of the fiber in which the dye is very soluble, thus increasing the rate of transfer into the fiber
  • 222. HTHP dyeing method  The dyeing of polyester with disperse dyes at the boil is slow because of the low rate of diffusion of the dyes into the fiber. The activation energy for diffusion is quite high and raising the dyeing temperature from 100 to 130 °C considerably increases the rate of dye diffusion.
  • 224. 224  Padding fabric with a dispersion of disperse dyes (use of a migration inhibitor in the pad bath)  Drying using hot flue air dryer or by infrared radiation  Heating in air, or by contact with hot metal surface, to a temperature in the range of 190–220 °C for 1–2 min.  The fabric approaches the maximum temperature, the disperse dyes begin to sublime and the polyester fibers absorb the vapours. THERMOSOL DYEING
  • 225. 225 Dyeing Temperature ~ 220 C Drying is used to minimize the migration during heating During heating dyes will be converted into gaseous state and PET fibers has very high affinity to thee gaseous disperse dyes
  • 226.  In case of dyeing with disperse dye, temperature plays an important role.  For the swelling of fiber, temperature above 130°C is required if high temperature dyeing method is applied. Again in case of carrier dyeing method, this swelling occurs at 90-100°C.  If it is kept for more time, then dye sublimation and loss of fabric strength may occur. Effect of Temperature
  • 227. 227 For disperse dyeing the dye bath should be acidic and pH should be in between 4.5-5.5. For maintaining this pH, generally acetic acid is used at this pH dye exhaustion is satisfactory. During color development, correct pH should be maintained otherwise fastness will be inferior and color will be unstable. Effect of pH
  • 228. Textile Auxiliaries 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
  • 229. 229 MODIFICATION OF POLYESTER IMPROVED DYEABILITY Cationic Dyeable Polyester [CDP] Copolymerization for lower Tg
  • 230. 230 DYEING BLEND FIBERS (UNION DYEING)
  • 231. Why blending ???  Achieving economic advantages via blending of expensive fibers with cheaper ones  Enhancing performance and quality properties via combination of desirable properties of both fiber components  Prolonging the durability of textile product via incorporation of a more durable fiber component, etc
  • 232. Fiber classes according to affinity to dyes  Group A: Acid and pre-metallized acid dyes for dyeing wool, silk, nylon and polyurethane (electrostatic bond)  Group B: Basic dyes for acrylics and modacrylics, cationic dyeable polyesters (electrostatic bond)  Group C: Cellulosic dyes, e.g. reactive, direct, vat, sulphur, etc. via formation of physical and/or covalent bonds  Group D: Disperse dyes for polyester, cellulose acetate, nylon and polyurethanes via hydrophobic bonding
  • 233. Color effects obtained by dyeing blends  Solid effect: All fiber components are identically colored  Reserve effect: One type of fiber is kept white  Shadow effect:(tone in tone ) dyeing All components colored to the same hue but of varying depth of shades.  Contrast effect: fiber components are dyed in contrast hues. , orange-black blue-yellow
  • 234. Methods of dyeing blends  1-bath 1-step method: reduces cost of dyeing drastically due to nearly 50% reduction in use of water, heat, chemicals, time and manpower  1-bath 2-step method: material handling is remarkably less, as the blend once loaded remain there till completion of dyeing for both components; the same bath is used for dyeing both these fibers saving water and energy  2-bath 2-step method: to dye blends made fibers of different nature-more handling of material, huge water consumption, doubled loading/unloading time, more manpower
  • 235. IMPORTANT FACTORS IN DYEING BLENDS  DYEBATH CONDITIONS  dye auxiliaries  dyebath temperature  stress on fabric  CROSS-STAINING OF DIFFERENT FIBERS  The colour effects and fastness properties required;  The process costs.
  • 236. In most cases, method of blend dyeing is similar to that of dyeing individual component fibers with particular types of dyes selected  This is so because most binary blends are mixtures of a synthetic and a natural fiber. In such cases, there is minimal cross-staining.
  • 237. Dyeing cotton/polyester blends  Dyeing properties of polyester and cotton are quite different- mostly the two fibers dyed separately  Polyester component is invariably dyed first with of disperse dyes  For cotton, there is a choice of dyes, actual selection depending on desired color, type of finishing required, demanded fastness properties, costs etc  Cotton is usually dyed with reactive, direct, sulphur, vat, or azoic dyes  Reactive/disperse dye combination is the most popular
  • 238.  If cotton is dyed first dyeing of polyester at 120–130 °C can change shade of dyed cotton- cotton dyes are less stable at high temperatures.  Dyeing polyester before cotton allows reduction clearing of any disperse dye on the polyester surface or staining cotton-cross staining of cotton by disperse dyes can be a problem.  Once polyester is dyed any detrimental effects of cotton dyeing assistants such as alkali and salt on disperse dyes are avoided.
  • 239. major areas of concern in dyeing of cotton/polyester  Degree of cross-staining  Disperse dyes stain cotton but anionic cotton dyes usually completely reserve polyester.  Disperse dyes selected should be those that give minimal cotton staining  Interactions between dyes and auxiliaries  When present in same bath many disperse dyes are not stable under alkaline reducing conditions used in leuco vat dyeing.  Salt and alkali for dyeing cotton with reactive dyes often have a harmful effect on dispersing agent for disperse dyes and cause particle aggregation;
  • 240. Conditions for fixation or aftertreatment Cotton dyes must be stable to high temperatures in Thermosol process- if to be applied simultaneously or before disperse dyes. Not possible to clear disperse dyes staining cotton with an alkaline hydros solution if cotton is already dyed. All types of dyes used to color cotton will be reduced under these conditions and color destroyed.
  • 241.  Disperse and reactive dyes are added, temperature raised to 130°C at acidic conditions to fix disperse dye.  Dyebath is then cooled to 80°C and sodium carbonate is added to fix the reactive dye, before washing.  Reactive dye fixation and clearing of surface disperse dyes is combined here, making PC single bath dyeing possible Developments in PC Blend Dyeings
  • 243. Textile printing is related to dyeing but, whereas in dyeing process whole fabric is uniformly covered with one color, in printing one or more colors are applied to it in certain parts only, and in sharply defined patterns. A TYPE OF COLOURATION FOR IMPARTING COLOR IN SPECIFIC AREA ON A SUBSTRATE BASED ON PATTERN OR DESIGN REQUIREMENT. Textile Printing Defined
  • 245.
  • 246. PRINTING Printing involves localized coloration This is achieved by applying thickened pastes containing dyes or pigments onto fabric surface according to a given color design The paste must color all the visible fibers on the printed surface So it must penetrate some what into the yarn structure If the paste is too thin it will spread, giving poor print definition and penetrate too far into the yarns decreasing the color yield
  • 247. PRINTING PROCESS PREPARATION OF PRINT PASTE APPLICATION OR PRINTING DRYING FIXATION
  • 249.  Impart the required color of design as per requirement  Dyes are chosen with respect to their affinity Example: Reactive dyes for cotton, acid dyes for wool, disperse dyes for polyester and so on  Pigments are applied for all kinds of fiber textiles DYE OR PIGMENT
  • 250.  To localize the printing paste on the desired area of the fabric. Enables the print paste to stay in place once it is deposited onto the fabric  Clear cut design and proper outline definition of print  Natural and synthetic thickeners are available  Based mainly on polysaccharides and polyacrylic acids THICKENER Optimum Viscosity
  • 251. Desirable properties of thickeners used in pigment printing: low solid content clear and colorless don’t decrease the fastness and hand of textile prints good leveling power without spreading sideways.
  • 252.  Pigments are fixed by binders hence binders provide durability of the pigment on the fabric.  A pigment binder is the resin that forms a three-dimensional film on the surface of the fiber.  This film contains the dispersion of textile pigment and will act to adhere the pigment to the surface of the substrate.  The permanence of this film is dependent on the polymer type and the application conditions. BINDER
  • 253.  Crosslinking agents to improve fastness  Softeners to improve handle of pigment printed fabric  Defoamers to prevent foam generation  Humectants to increase water content of the paste  catalyst and so on AUXILIARIES
  • 254. Different techniques can be used to transfer the paste TO textile substrate. The transfer can be by means of blocks, screens, rollers: reproduction of image. There is possibility of digitizing design and transfer image without reproduction. APPLICATION OF PRINT PASTE
  • 255. DRYING AND CURING (STEAMING)  The main purpose of drying is to control migration  Curing/steaming for fixation of pigment/dye  Cylinder dryers and stenters can be used  Curing temperatures depend on the fiber 180 oC for pigment, 205 oC for dispersed dye
  • 256.  In the case of dye prints, the printed fabric is thoroughly washed then dried after fixation.  Necessary to remove thickener, alkali, other ingredients of print paste left on fabric after fixation. Could interfere with subsequent finishing processes. Pigments are often printed on finished AFTERWASHING
  • 257.  Reactive dyes for cotton fibers has given the possibility of using only one type of dye and simple application conditions, in place of the complex permutations necessary  Reactive dyes produce brilliant shades of very good fastness and leveling properties on cellulosic fibers.  Next to pigment printing the most important printing method is printing with reactive dyes. REACTIVE DYE PRINTING
  • 258. Selection of thickeners is very important for reactive dye printings; and the selection of proper reactive dyes also plays a major role. Normally low reactive dyes are preferred for printing on cotton. Because the stability of the low reactive dyes is high in printing paste. Furthermore for obtaining good effect proper pretreatment of fabric before printing is very important.
  • 259. Requirements of an ideal thickener for reactive printing • Good compatibility and no affinity or reactivity with dye • Good flow property • Good swelling and moisture absorption capacity during steaming for dye fixation • Quick drying to prevent bleeding & good water solubility for easy removal
  • 260. Conventional reactive printing thickener The usual polysaccharide thickeners such as starch and gums used in printing are not suitable for printing with reactive dyes As all these contain primary hydroxyl groups in their molecular structure and react with reactive dyes. Sodium alginate is widely used for reactive dye printing. It is an anionic polymer, has only two free OH groups.
  • 261. Polysaccharide illustration The reaction between alginate and dye is limited by anion repulsion of alginate's carboxyl groups and dye's sulfonic acid groups.
  • 262. Moderate to low reactivity dyes such as MCT and VS Reactive dyes in printing Dyes having less substantivity towards cellulose are preferred for printing. Because of greater ease of “washing off” and avoidance of staining of adjacent white areas. Printing pastes made from less reactive dyes have better storage stability and this is of considerable importance. The reactive print paste normally contains reactive dye, sodium bicarbonate/carbonate, urea, sodium alginate and water. DCT is not recommended
  • 263. PIGMENT PRINTING DIFFERENCE BETWEEN DYE AND PIGMENT PIGMENT  NO AFFINITY TO FIBRE  INSOLUBLE IN WATER  NEED BINDER FOR FIXATION ONTO FIBRE DYE  AFFININTY TO FIBRE  WATER SOLUBLE OR CAN BE MADE WATER SOLUBLE  HELD ON FIBRE DYE-FIBRE INTERACTIVE FORCES MOST WIDELY USED
  • 264. MECHANISM OF BINDING SEQUENCE: PRINT > DRY > CURE (DRY HEAT)  DURING CURING BINDER POLYMERIZES AND FORMS A STRONG FILM.  BINDER FILM EMBEDES PIGMENT COLOUR AND ALSO STRONGLY ADHERES TO THE FIBRE. BINDER IS A PRE-POLYMER AVAILABLE IN THE FORM OF AQUESOUS EMULSION CROSSLINKING
  • 265.
  • 266. CHOICE OF THICKENER DYE PRINTING THICKENERS ARE NOT SUITABLE. INTERACT WITH BINDER, REDUCE INTERATION WITH FIBRE. AFFECTING THE FASTNESS PROPERTIES SUITABLE THICKENERS  EMULSION THICKENER  SYNTHETIC THICKENER DEVELOPMENT OF VISCOSITY HMW COPOLYMERS OF ACRYLIC ACID OIL + WATER EMULSIONS
  • 267. ADVANTAGES OF PIGMENT PRINTING  PRACTICALLY ALL TYPES OF FIBRES AND BLENDS CAN BE PRINTED  PIGMENT COLOURS CAN BE READILY MIXED WITH EACH OTHER TO GET QUICK SHADE MATCHING  NO AFTERTREATMENT SUCH AS WASHING & SOAPING REQUIRED DISADVANTAGES  HARSH FABRIC FEEL DUE TO BINDER FILM  LOW RUBBING FASTNESS
  • 268. FIBER CHARACTERISTICS YARN TYPES FABRIC CONSTRUCTION CONSIDERATIONS ON PRINTING PROCESS
  • 269. STYLES OF PRINTING There are two basic styles of printing Direct printing and Indirect printing DIRECT PRINTING  The dye or the pigment is printed on the fabric in the paste form and any desired pattern may be produced  Designs printed directly using pigments or dyes on white or colored background  Is the most important printing style and the most popular as compared to indirect style
  • 270. INDIRECT PRINTING IP is classified into two categories Discharge printing the dye. In discharge style, the fabric is dyed to the required ground color. Next, the fabric is printed with a chemical that selectively destroys the dye. This allows printing of white designs on a ground color of any depth with a pattern definition that is much higher than would be possible by direct color printing. If the paste contains dyes resistant to the discharging agent, these dyes, called illuminating colors, will color the printed areas.
  • 271.
  • 272. Resist is the term that describes prevention of dyeing process by application of a physical or chemical substance to fabric to prevent a dye’s access to fabric. Resist agents can be waxes, thickeners, surfactants, organic acids, oxidizing agents, or reducing agents. In discharge style, the fabric is dyed to the required ground color. Next, the fabric is printed with a chemical that selectively destroys the dye. Resist printing
  • 274. In batik effect the fabric is coated with wax and then dyed with suitable dyes depending on the fiber. In tie-die effect first the fabric is tied in certain portions to lower the penetration of dye into the fiber and then dyeing is carried out conventionally.
  • 275. Methods of Printing Non contact printing Contact printing
  • 276. STENCIL PRINTING BLOCK OR SURFACE PRINTING ENGRAVED ROLLER PRINTING SCREEN PRINTING TRANSFER AND DIGITAL PRINTING TECHNIQUES OR METHODOF PRINTING Refers to the technical means by which pastes are put onto design areas
  • 277. Stencil printing  Stencil printing is the process of cutout the required designs on the plastic papers  The surface of the papers should be smooth  Colorants are smaller molecular weight (no need of washing)  Pastes are forced out through cutting out shapes from papers onto fabric underneath  The idea of screen printing is originated from stencil printing
  • 278. BLOCK PRINTING  It is the oldest and simplest way of printing  In this method a wooden block with a raised pattern on the surface was dipped into the printing colorant and then pressed face down on to fabric.  The desired pattern was obtained by repeating the process using different colors.  Generally the wooden block is carved out of hand  Printing is done manually
  • 279.
  • 280. Screen is made of woven fabrics firmly attached to a suitable frame  Mostly of nylon/silk/polyester  The screen is the image carrier made from a porous mesh stretched tightly over a metal or wooden frame.  Reproduce patterns as per design requirement. SCREEN PRINTING SCREEN, INK, SQUEEGEE
  • 281. 281 (Major step in screen printing) (1) the selection of the fabric and the printed design (2) the choice of the size and arrangement of design’s repeat rectangle (3) the necessary color separation of the design Color separation involves reproduction of the pattern for each color on separate clear films with opaque tracer. Screen production
  • 283.
  • 284.
  • 285. 285  The screen is coated with a photo-sensitive emulsion. A typical polymer is polyvinyl alcohol, its crosslinking sensitized by ammonium dichromate. The coated screen is dried  Exposed beneath diapositive for the given color All operations take place in a dark-room!! Screen Preparation Steps Screen Preparation Steps Lacquer
  • 286.  The rest hardens (become insoluble)  The inked zones, corresponding to particular color pattern, do not transmit ultraviolet light used for exposure.  The layer of polymer on the screen beneath the pattern is thus not exposed and does not crosslink.  The rest hardens (become insoluble)  The non-exposed polymer remains soluble and can be washed out, leaving the screen open in those areas.  Any small holes in hardened areas on screen are painted over.
  • 287.  MIXING PHOTO-EMULSION  COATING  DRYING  PREPARING POSITIVE  EXPOSING  WASHING AND DRYING
  • 288. 288 288 Flat screen printing can be carried out by hand and automatically by using different machines. Rotary screen printing is carried out by fully continuous machines. TYPES OF SCREEN PRINTING FLAT SCREEN PRINTING In flat screen printing, a screen on which print paste has been applied is lowered onto a section of fabric. A squeegee then moves across the screen, forcing the print paste through the screen and into the fabric. ROTARY SCREEN PRINTING The tubular screens rotate at the same velocity as the fabric, the print paste is distributed inside a tubular screen, which is forced into the fabric as it is pressed between the screen and a printing blanket (a continuous rubber belt).
  • 289.
  • 290. • Printing is carried out on a flat, solid table covered with a layer of resilient felt and a washable blanket. • Heat for drying the printed fabric may be provided either under the blanket or by hot air fans above the table. • Fabric movement or shrinkage must be avoided in order to maintain registration of the pattern. HAND SCREEN PRINTING
  • 291. The process consists of forcing print paste through the open areas of the screen with a synthetic rubber squeegee (rubber blade contained in wooden/metal support)  The rubber blade is drawn steadily across the screen at a constant angle speed and pressure.  The screen is washed immediately after use. If this is not done, paste dries on screen and clogs up design.
  • 293.
  • 294.
  • 295.
  • 296. SEMI-AUTOMATIC FLAT SCREEN PRINTING 296 The manual process has been semi-automated by mounting the screen in a carriage. In semi-automated screen printing, a mechanically driven squeegee transfers the color. Long tables, typically 20–60m long, are used, and some provision is usually made for drying the printed fabric. Very popular where the scale of production is not large, or where capital investment is limited.
  • 297. FULLY AUTOMATIC FLAT-SCREEN PRINTING • To increase the speed of flat-screen printing, it was necessary to devise a method of printing all the colors simultaneously. • All the screens for the design (one screen for each color) are positioned accurately along top of a long endless belt (blanket) on top of which is the fabric to be printed. • The fabric is gummed to the blanket at the entry end and moves along with the blanket in an intermittent fashion, one repeat distance at a time.
  • 298. • All colors in design are printed simultaneously while the fabric is stationary; then the screens are lifted and the fabric and blanket move on. • When the fabric approaches the turning point of the blanket, it is pulled off and passes into a dryer. • The soiled blanket is washed and dried during its return passage on the underside.
  • 300.  Correct fabric placement is vital for accurate registration of the different colored patterns.  A slight pattern overlap prevents a white gap between two printed colors.
  • 301. 301 The main fault in screen printing is poor pattern registration  Inaccurate screen placement  Inaccurate fabric movement  Fabric slippage on the blanket (poor adhesion)  Distortion of screen mesh by drag of squeegee PROBLEMS IN FLAT SCREEN PRINTING SLOW PRINTING PROCESS [5 -10m/min]
  • 302. 302  Number of squeegee passes LOW PRODUCTION RATE More than one pass is used to achieve uniformity and adequate penetration, especially in blotch areas, for thick fabrics or irregular surface Squeegee takes longer to move along the screen where the repeat distances are large  Efficiency of the dryer If the dryer is short, or if temperature in dryer is too low, printing speed will have to be reduced in order to ensure the printed fabric is adequately dried.
  • 303. ROTARY SCREEN PRINTING The process involves initially feeding fabric onto the rubber blanket. As the fabric travels under the rotary screens, the screens turn with the fabric. Print paste is continuously fed to the interior of the screen through a color pipe.
  • 304. • As the screen rotates, squeegee device pushes print paste through the design areas of the screen onto the fabric. • As in flat-bed screen printing, only one color can be printed by each screen. After paste application, the process is the same as flat screen printing.
  • 305. 305 By converting the screen-printing process from semi- continuous to continuous, higher production speeds are obtained. Typical speeds are from 45-100 mpm for rotary screen printing depending upon design complexity and fabric construction. Continuous patterns such as linear stripes are possible. Rotary screen machines are more compact than flat screen machines for the same number of colors in the pattern (less floor space).
  • 306. 306  Glue streaks – from the rubber blanket Color out – from a lack of print paste  Creased fabric  Pinholes in any screen Damage to the screen leading to misprints Lint on the fabric causes pick-off With print designs, color application must be correct the first time, because printing defects cannot be repaired. General screen printing defects
  • 307. 307 Long process set up time for color and pattern change Screen production is slow and expensive Screens require considerable storage space Limitations Specific to Rotary Screen Printing
  • 308. ROLLER PRINTING  Engraved roller printing is a modern continuous printing.  The design is engraved on the surface of a metal roller, to which dye is applied, and the excess is scraped off the roller's surface, leaving dye in the engraved sections.  When it rolls across the fabric, the dye on the roller transfers to the fabric.
  • 309. TRANSFER PRINTING  Heat transfer printing is a technique where paper is printed, followed by the transfer of the design from the paper onto the textile fabric.  Commercial process involves printing release paper with pigments. The design on paper is placed onto the fabric, heated so that the pigment binder softens, releases from the paper, and adheres to the fabric.  Heat transfer printing is clean and environment friendly  The only by-product is the paper carrier. Transferring an image to fabric from a paper carrier. When heat and pressure are applied to this paper the inks are transferred.
  • 310. DIGITAL PRINTING  It is the more advanced type of printing.  This includes :-  Jet spray printing  Electrostatic printing  Photo printing  Differential printing
  • 311.  Rotary Screen Printing 60%  Automatic Flat Bed 18%  Other methods 22% WORLD PRODUCTION SHARE