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
§ According to chemical groups
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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
§ 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 binder
Trade 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