Dyeing Mechanism

Dyeing Mechanism
Affinity
It is the difference between the chemical potential of dye in its standard state in the fiber & the corresponding chemical potential in the dye bath i.e. tendency of a dye to move from dye bath into a substance. It is expressed in Joule or cal (per mole) and quantitative expression of substantivity.
Substantivity
The attraction between a substrate and a dye or other substance under the precise condition of test whereby the test is selectively extracted from the application medium of substrate. It is the qualitative expression of affinity. Substantivity depends on temperature, type of fiber, electrolyte concentration. Substantive dyes have affinity and are soluble.
Reproducibility of Shades
The shade of the dyes should be reproducible when required. Certain dyes have ability to overcome the factors like liquor ratio, pH, temperature etc. which affect the reproducibility.
Characteristics of highly reproducible dyes are:
Highly soluble
Medium substantivity
Medium reactivity
Good wash off properties
Highly diffusible
Optimization of Dye
The principle is to carry out dyeing in a manner in which the dyestuffs absorbed by substrate almost uniformly with less dye wastage.
Substrate
Affinity
Circulation speed
Action of chemicals before
Dyestuff
Depth of shade
Optimum quantity/yield
Diffusion ability and regularity
Color fastness
Combination & mixability
Chromphore percentage
Auxiliary Products
Optimum quantity
Compatibility with dyestuff and with each other
Levelness
Control of PH in final exhaustion
Reproducibility
No adverse effect
Temperature and time
Low initial temperature to avoid rapid absorption of dye
Control of critical temperature zone for maximum exhaustion
Sufficient time for penetration and fixing
Machine
Control of batch
Volume of flow
Temperature regulation
The actual dyeing theory can be obtained mathematically from kinetics of dyeing or dyeing equilibria. The dyeing phenomena found in principle of dyeing curve. The factors for uniform color & optimization of dye all are related to kinetic phenomena. Therefore kinetic dyeing is important in the dyeing process.
Functional Groups of fiber
Cotton: OH-, at higher pH it is ionizable
Wool: -COOH, -NH, -CONH2. At pH 3-4 ionized positively so acid dye is used to dyeing
Acrylic: -COOH, -SO3H, -O SO3H
Silk: -NH2, -CONH
Viscose: -OH, -COOH
Polyester: -OH, -COOH. No ionization effect, high temperature used for dyeing with dispersing.
Diacetate: -OH, -COOCH3
Triacetate: -COOCH3
Dyeing Medium
Aqueous medium
Water
Solvent
Foam
Vapor phase: cationic, anionic, nonionic
Dyeing Mechanism
The sequence of dyeing falls into four stages
Transfer of dye onto fiber surface
Adsorption
Diffusion into the fiber
Interaction
Transfer of dye onto fiber surface
The transfer of dye onto the fiber surface depends on:
Environment of the dyebath: environment of the bath refers to
Solvent and its type, nature, quantity: solvent may be water and or any other solvents which may be soft/hard, acidic, alkaline, ionic, nonionic etc.
pH
Dyeing assistants like electrolytes, leveling agents, carrier, dispersing agents etc.
temperature of the dyebath which depends on material type (cotton or polyester), type of dye (hot brand or cold brand), method of dyeing (padding or exhaust) Suitable environment ensures easy transference of dye on fiber surface.
Substantivity
Mechanical and physical force
Adsorption
The distribution process is called adsorption, if the substance which is to be distributed is retained by a surface. The assembly of dye molecules at the fiber surface is governed by:
Electropotential forces: All fiber when immersed into water or aqueous solution acquires an electric potential known as ‘zeta potential.’ Cellulosic fiber bears a negative charge while protein fibers at higher pH than its isoelectric point bears are negatively charged and at lower pH than isoelectric point is positively charged.
Temperature: most dyes in solution are either in molecular and partially ionized state or exist in the form of ionic micelles; increase in temperature tends to breakdown micelles into less aggregated units. Increase of temperature promotes vibrational activity accelerates the migration of the surface of the fiber.
Agitation: when a fiber is immersed in the dye a large no of molecules tend to enter the fabric at once, thus creating a layer called ‘Barrier.’ If the dye molecules are to reach the fiber surface then the barrier should be broken which is done by agitation.
Dye adsorption has affect on fastness properties.
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Labels: Theory of Dyeing

Pigment Printing
In pigment printing, insoluble pigments, which have no affinity for the fiber, are fixed on to the textile with binding agents in the pattern required. This description is perhaps oversimplified, but it does obviously set pigments apart from dyes that are absorbed into the fiber and fixed there as a result of reactions specific to the dye.
Historical Development of Pigment Printing
§ Until 1937 natural polymers as binders and thickeners (starch, glue)
§ Around 1937 emulsion thickening
§ Around 1960 use of aqueous self-crosslinking dispersions as binders
§ Around 1970 development of synthetic thickening agents based on acrylic acid
§ After 1980 ecological improvements (e.g., emission)
Why Pigment Printing is Important
§ The pigment can be applied to all fibers potentially and it is the only coloration to glass fiber, fabric and polyester
§ No wet treatment is required, so drying and curing is applicable to all fiber.
§ Extensive color range of highly light fast colors
§ Possible to produce good combination shades on blended fiber in one padding operation
§ Application procedure is simple
§ No change of hue of colorant throughout processing
§ Less expensive
A good quality pigment print is characterized by
§ Brilliance and high color value relative to the pigment concentration in material
§ Minimum stiffening in the handle of the textile
§ Generally acceptable fastness properties.
Components of a pigment printing system
A pigment printing system consists of three essential components:
§ Pigment dispersion: Specific pigments are treated in a grinding mill in the presence of suitable non-ionic surfactants. A particle size of 0.1-3 μm is typical. Generally, the pigment pastes are aqueous based and contain the dispersing agent, humectants (to prevent evaporation and drying out).
§ Binders and cross-linking agents (polymers): The binders used in pigment printing systems are film-forming substances made up of long-chain macro molecules which, when heated with a suitable acid-donating catalyst, form a three-dimensional structure in the pigment.
§ Thickeners and auxiliary agents: These give the required print thickening power (rheology).
Binder
The binder is a film forming substance made up of long‑chain macromolecules which, when applied to textile together with the pigment, produce a three dimensionally linked network.
Binder- CH2-OR + HO-Textile Binder –O- textile + HOR
Where R is H or CH3.
The links are formed during some suitable 'fixing' process, which usually consist of dry heat and a change in pH value, bringing about either self-crosslinking or reaction with suitable crosslinking agents.
The degree of cross linking should be limited, to prevent the macromolecules becoming too rigidly bonded, thus preserving some extensibility. The important criteria, which ensure that the pigment within the crosslinked binder film is fast to wear and cleaning, are elasticity, cohesion and adhesion to the substrate, resistance to hydrolysis, as little thermoplasticity as possible and absence of swelling in the presence of dry cleaning solvents.
Required properties for Binders
§ Should be film forming
§ Should be water swell
§ Should not be too thermoplastic
§ Should have atmospheric stability
§ Should be colorless and clear
§ Should be of even thickness and smooth; neither too hard nor too stiff.
§ Should have good adhesion to substrate without being tacky.
§ Should possess good resistance to chemical and mechanical stress
§ Should be readily removable from equipments
§ Should provide good color yield
§ Should be non toxic
Types of Binders
§ According to the origin
Natural: glue, gelatine etc
Synthetic: acramin binders
§ According to chemical groups
Acrylic binders: These are normally an aqueous dispersed co-polymer of butyl acrylate and styrene, having N-methylol acrylamide groups for cross-linking purposes.
Some of the more important properties of this type of binder are:
§ Good resistance to ageing by light
§ Good heat stability
§ Generally a harsh handle
§ Good solvent resistance
Butadiene co-polymer binders : They are made by emulsion co-polymerisation with acrylonitrille and N-methylolmethacrylamide. Some of the more important properties of this type of binder are:
§ Poor resistance to ageing by light
§ Susceptible to yellowing on heat treatment
§ Generally a soft handle, particularly on synthetic fibers
§ Generally the highest binding action on synthetic fibers
§ good solvent resistance
Trade names of binderTrade name Manufacturer Origin
Acramin Bayer Germany
Tinolite, Microfix, orema Ciba Switzerland
Helizarine BASF Germany
Imperon Hoechst Germany
Thickening Systems
There is a wide range of thickener materials available including alginates, natural vegetable gums, synthetic polymers, or even foams. These materials show sensitivity to factors such as temperature, pH, and salt content.
§ Ionic thickener (alginates): Better color yield
§ Nonionic thickener (cellulose ether): stable to pH variation and electrolyte content.
§ Natural and semi synthetic hydrophilic thickeners: should not used in pigment printing because:
- When entrapped in binder film, are either soluble in water or swell in presence of water even after fixation.
- They contain large no of polar groups like hydroxyl group and produce a hard film and stiff handle.
- Aftertreatment to remove them is not effective since they are enclosed in the binder film.
Emulsion Thickener
Two mutually immiscible liquids (oil and water) are stirred to produce an emulsion with the presence of emulsifier. The nature of the emulsifier and the ratio of the two immiscible liquids determine which liquid will be dispersed (the disperse phase) in the other (the outer, continuous phase)
The emulsifier forms a film between the two liquids, reducing interfacial tension. The emulsion stability depends on
- The degree of dispersion
- Type and quality of emulsifying used
- The substance dissolved or dispersed in the dispersed or dispersion medium
Two types of emulsion thickener
§ Oil in water (o/w): kerosene/white spirit in water
§ Water in oil (w/o): water in kerosene or white spirit
Synthetic thickeners
§ A thickener that is made artificially. Synthetic thickeners are typically designed to offer high viscosity at low concentrations, high yield value, shear thinning, stability, integrity over a wide temperature range, and ease of use.
§ Synthetic thickeners are efficient at only 1-3 % concentration level while approximately 10% of a natural thickener is needed to give the required viscosity in the print paste.
§ Other advantages of synthetic thickeners include rapid make-up since they require no waiting for hydration to occur, sharp print boundaries, and controlled penetration which usually provides greater color value and levelness.
Other Auxiliaries
§ Catalysts
§ Diammonium phosphate: - most widely used acid catalyst
§ used in conc. of 0.5% and 0.8% in screen and roller printing respectively
§ when used in correct proportion produces a pH of 3 in fabric and brings a cross linking reaction
§ Ammonium salts: sulphocyanide, sulfate and chloride are suitable. Ammonium nitrate: not recommended and it turns polyamide fiber yellow
§ Urea
These are agents that are added to improve “runnability” on printing machines. Owing to their low volatility these auxiliaries are used sparingly, maximum amounts of 20 parts/1000 being common; otherwise the fastness properties may be adversely affected.
§ Softening agents
After curing fixation the resultant “handle” of the printed fabric depends on a number of factors:
- monomer composition of the binder
- presence of water-soluble protective colloids (e.g. alginates, etc.)
- extent and type of cross-linking.
By the addition of certain compounds (usually termed “plasticisers”) improves the handle of printed goods.
§ Cross-linking agents
These agents are universally based on either urea-formaldehyde types (e.g. dimethylolurea) or melamine-formaldehyde types. They are incorporated into printing compositions in an attempt to increase various aspects of fastness, particularly rub and scrub fastness with synthetic fibers. A maximum addition of 10-20 pts/1000 is normally encountered: larger amounts can have a quite marked effect on the “handle” of the fabric
Pigment Printing Recipe and Procedure
Typical Recipe:
Pigment: 10-20gm
Binder: 40-50 gm
Thickener: 35-50 gm
Catalyst: 5 gm
Dispersing agent 2 gm
Water x ml
Procedure:
§ Preparation of printing paste using dispersing agents and thickener and catalyst.
§ Application of pigment paste and binding resin together
§ Drying at 140 – 150°C
§ Curing to fix the resin pigment
Affect of curing on PET
Temperature Time Strength loss
205°C 1 min 0%
220°C 1 min 0%
235°C 1 min 2%
245°C 1 min 5%
260°C 1 min 13%
Problems of Pigment Printing
§ Adverse effect due to binder as it changes texture of fabrics.
§ The quality of printing or dyeing depends on the characteristics of binder used to affix the pigment even more than the properties of pigment.
§ Some solvents used in emulsion like kerosene, white spirit cause problem like flammability.
§ The chemical and physical influences on the binder and print paste can interfere during production and processing resulting in sticking especially in roller printing.
§ The gumming up of equipments, odor, air and water pollution
§ Difficulty in obtaining the necessary wet treatment fastness and abrasion resistance with certain products, may not be obtained pigment printing or dyeing.
Pigment Dyeing on Fabric
Typical Recipe
Pigment: 10-20gm/L
Binder: 40-50 gm/L
Thickener: 35-50 gm/L
Catalyst: 5 gm/L
Thickener: 2 gm/L
Dispersing agent: 2 gm/L
Procedure:
§ Binder is weighted and diluted with cold water
§ Pigment and thickener is added with cold water
§ Catalyst solution is added
§ Dispersing agent is added
§ The dyeing liquor is well filtered and stirred; material is padded
§ The material is dried at 70 -100°C in hot flue steam but no use of cylinder dryer.
§ Curing is done at 150°C, 2-3 min
Precautions:
§ No alkalinity: The fiber to be dyed should not be alkaline
§ No OBA: OBA may produce faulty shade
§ No formation of skein: Binder should not be allowed to form skein which ultimately give specky shade
Typical procedure for Garment dyeing
§ First bleach the material then treat with a synthetic mordant cationising agent at pH 7
§ Rinse at 60°C at a rate of 2°C/ min for 20 min
§ Cold rinse
§ Apply pigment at 7O°C (pH 5) for 20 min
§ Add salt, acid and raise temperature when necessary
§ Now use binder 4% for 10 min at 70°C
§ Cold rinse with 1 gm/L soap wash for 10 min at 65°C
§ Cold rinse and dried
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Labels: Printing

Pigment
Pigments implied general insolubility and complete insolubility in water.
Difference between dye and pigment
The difference between dye and pigment is not a clear one. Most organic pigments are closely related to dyes with respect to their chemical structure and there are dyes which become pigments after application. Vat dye is a dye when used in dyeing but a pigment when used in printing.
Dye
Pigment
Solubility in water
All dye must be soluble during process
Almost insoluble
Affinity
Possess a specific affinity towards fiber
Have no affinity but used as coating
Chemical nature
Organic and few are metallic
Most are metallic or organometallic.
Application
Through water medium
Through adhesive or binder
Uses of Pigment
Pigments are used for coloration of a very broad and diverse number of materials
Surface coating for interior, exterior, automotive and other application
Paints based on olegoresinous liquid and water emulsion
Printing ink for papers (lithographic, rotogravure and flexographic systems (and for other materials such as metal plates, foils, artists and writing material)
Coloration of plastics and rubber
Textile printing
Coloration of manmade fibers by mass pigmentation before fiber formation (dope dyeing) etc.
Required Properties of pigments
They should have covering power which is influenced by particle size
Should be inert, stable and have long life
Should have capability of mixing
Good wet fastness, light fastness and abrasion resistance
Good resistance to acid, base, perspiration, chlorine, peroxide and gas fading
Good solvent resistance (insoluble in water, CCl4, Cl2C=CHCl)
Suitable brilliance, hardness and stability
Suitable characteristics for good dispersion including particle size and distribution, electrical charge (most are negatively charged particle), specific gravity, purity and crystalline structure, conditions of precipitation of the pigments
Should be applicable to all fibers.
Physical/Chemical Properties of Pigments
Chemical Structure
Inorganic oxide, salts, organometallic toners, organic insoluble azo pigments, phthalocyanine metal complexes
Physical state
Very important, decreasing particle size increase color value but decreasing hiding power
Particle size
5-7 micron
Density
Sp gravity range from 1.17- 1.37 for most cases
Melting points
Usual range 110 -175°C
Boiling point
Decompose at 195- 345°C. phthalocyanine pigments sublimes at 500°C
Water solubility
Insoluble for all practical purposes.
Other solubility
Inorganic pigments are insoluble in most solvent
Spectra
Very strong and high, though not comparatively sharp peaks
Application Properties of Pigments
Fabric dyed
Any fiber can be dyed by selecting a suitable binder, quality greatly depends on binder used to affix the pigment
Fabrics printed
Any fiber by suitable binder even hard to print polyester blends and glass fibers
Disposable fabrics
Well suited for non woven fabrics
Dischargeability
Some pigments are suitable for discharge printing
Alkali fastness
Poor for organometallic azo toners, good for insoluble azo
Heat resistance
Extremely varied. Some are stable up to 200°C and some up to 300°C. optimum for inorganic pigments
Light fastness
Generally very good. Optimum for inorganic pigments
Wash fastness
Generally good to very good
Useful colors
Diarylide yellows and oranges, Hasna yellow, azoic reds, phthalocyanine blues and greens, carbon black, TiO2 white, violet and browns.
Processes used
Padding for dyeing
Aftertreatment
None required
Classification of pigments
According to origin
Natural/Mineral: Iron ores, clays, chalk etc
Synthetic/chemical: white lead, ZnO, TiO2 and large number of inorganic and organic color
According to Reactivity
Reactive pigment: some pigments on account of the chemical character react with oil, fatty acids and soaps. These are called reactive pigments e.g. ZnO, red lead
Inert pigment: TiO2
According to Chemical Nature
Organic pigment: appx 25% (by wt.) of the world production of organic colorant is accounted for organic pigments. They account for only 4% of total pigment production. Of the total organic pigments production yellow, red and blue tones accounted for 89%.
Most organic pigments exhibit a small solubility, typically in polar solvent. All the organic pigments are soluble in one or more of the four chemical: Chloroform (CHCl3), Methyl alcohol (CH3OH), Dimethyl formamide (DMF) and concentrated H2SO4. Organic pigment consists of:
Azo pigment:
Strong tinctorial strength
Good alkali resistance
Excellent brightness
Cover a wide range with regard to other application properties
Poor alkali resistance of certain organometallic pigments make them unsuitable for printing
Diarylide orange and yellows:
Extremely bright color
Inferior light fastness
Phthalocyanine
Blue, greens are dominant shade especially in plastic coloration
Offer low migration
Good temperature stability
Excellent light fastness
Good heat resistance
Excellent alkali resistance
Good solvent resistance
Used extremely in printing, pad dyeing and dope dyeing
Hasna yellow
Good light fastness
Have migration tendency
Inorganic pigment:
They account for 96% (by wt.) of total production. More than half of their production volume is accounted for a single production, TiO2, the most important white pigment
H2SO4 is a good solvent for many inorganic pigments
They are opaque
Less expensive
More weather resistant
More chemical resistant
Insoluble in most organic solvents
Highest degree of light fastness
Excellent heat resistance
They consist of
Salts: Sulfates, carbonates, silicates and chromates of many metal elements like, Ti, Zn, Ba, Pb, Sb, Zr, Ca, Al, Mg, Cd, Fe, Mo, Cr etc.
Oxides of Ti, Zn, Ba, Pb, Sb, Zr, Ca, Al, Mg, Cd, Fe, Mo, Cr etc.
Metal Complexes: Naturally occurring oxides and silicates
Difference between organic and inorganic pigment
Organic
Inorganic
Solubility
Soluble in organic solvent
Soluble in inorganic solvent
Tinctorial strength
Higher
Lower
Brightness
Higher
Lower
Purity
Higher
Lower
Transparency
Opaque
Transparent
Weather resistance
Less
More
Chemical resistance
Less
More
Fastness
Good
Excellent
Cost
Expensive
Cheap
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Labels: Printing
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