MATERIALS IN PAINTING PASTE
A set of materials are combined together to make the printing paste. The combination of materials used to make the printing paste is dependent upon the type of fiber in the fabric to be printed and type of dyestuff or pigment being used.
The material used for colouring of fabric or the material that actually releases colour to fabric. In printing colouring matter used is of mainly 2 categories Dyes and Pigments. The choice of particular dyestuff or pigment is dependent on the particular colour(red/blue/cherry red/peacock blue) required and the type of fiber (cotton/silk/polyester).
- Dyes: Are soluble in water making. In a form of aqueous solution, dye molecules enter each fiber of the fabric and colour the fiber completely. Different types(classes) of Dyes are used for different types of fabric. Similarly different types of dyes are used to achieve different colours of fabric. A solvent is used with dyes.
- Pigments: Are microscopic colour particles, not soluble in water. These are made to adhere (stick) to fiber surface with the help of Binders. Different types of Pigments are used for different types of fabric.
Dyes and Pigments colour Cotton & Linen fibres – Direct, Reactive, Basic with mordant, Vat, Vat soluble, Azoic(Naphthols), Mineral (gives Khaki colour) and Sulphur (gives Black colour).
Dyes and Pigments that colour Wool & Silk fibres – Direct, Reactive, Basic, Acid, Acid mordant and Metal complex.
Dyes and Pigments that colour Acetate, Rayon & Polyester fibres – Disperse dyes.
In preparing print paste, the dye stuff has tobe dissolved in a small amount of water. Therefore, solvents are used to prevent precipitation of the dyestuff in the paste. (increase solubility of the dye stuff). Commonly used solvents are Tetralene, Decalene and Ethylene glycol.
Assists in fixing of dye to fabric and facilitate subsequent washing of the thickener. Hygroscopic agents absorb sufficient amount of water during steaming and enable the dye molecules to diffuse into the fiber; they have tobe used in correct quantities, if too much of the substance is used, it leads to spreading of the dye beyond the boundaries of the design, if too little of it is used, it results in lighter prints. Some common hygroscopic agents are Urea and Glycerin.
To obtain a smooth paste of the dyestuff without formation of any lumps, which if allowed to remain, get deposited on the cloth during printing producing dark spots. In the case of soluble dye stuffs, wetting agents are used to facilitate wetting of dye stuff and subsequent dissolution. In the case of soluble dyes wetting agent is not normally required. Turkey red oil and Monopol Brilliant are 2 common wetting agents used.
- Soluble dyes – Direct, Indirect, Acid, Basic, Reactive
- Insoluble dyes – Vat, Sulphur, Disperse
These are used in printing in the form of a paste or emulsion in order to prevent spreading of the dye beyond the boundaries of the design and hold the dye particle in the printed portion until they are fixed into the fabric. The thickener has tobe compatible with other ingredients of the paste and should have no affinity for the dye. It should be capable of being removed easily during washing, the paste should be firm and stable. Some commonly used thickeners are; Starches, Gums, Sodium Alginate, Carboxy Methyl Cellulose, Emulsion thickeners, Synthetic thickeners.
These are used to develop final colour during steaming or to develop baths in case of Vat and Aniline dyes. Commonly used Oxidising agents are Sodium di chromate, chlorates, nitrates, sodium bromate, potassium ferrocyanide.
These are used mainly in Discharge printing, in printing with Vat dyes by re-reduction method. These are used as discharging agent for Azoic ground, Acid and Reactive dyes. Common reducing agents are Sodium sulphite, Formaldehyde (sodium sulphoxylate), Sodium bisulphite and Tin salts.
These are used to speed up the development of colour by oxidising agents to reduce the risk of their attacking the fibre. Commonly catalysts are Potassium ferrocyanide and Copper sulphide.
DEFOAMING AGENTS OR DEFOAMERS
Many colours froth during printing, producing lighter shades due to a sort of churning action. In roller printing, the wetting agent in the paste, with continuous agitation produces considerable foam in the colour box. The foam produces faulty prints. To avoid formation of foam, defoamers are used in the print paste. When added to printing paste, it has an antifoaming effect. Used with Direct, Acid, Vat or Disperse dyes. Common defoamers are KB of ICI, Methyl cyclohexanol – ortho/meta/para, Emulsified pine oil, silicones.
ACIDS AND ALKALI
The purpose of using acids or alkali in Printing paste is:
- Weak acids like citric acid, tartaric acid are used to make the paste acidic if required or as required.
- Alkali (acid liberating agents) like ammonium chloride, aluminium sulphate, ammonium sulphocyanide may be used if required or as required in the print paste.
Mainly used for fixing disperse dyes on polyester or polyester blends at temperatures below 105ᵒC.
TYPES OF THICKENERS
- Gum Tragacanth (Gum Dragon)
- Starch-Tragacanth paste
- Gum Arabica (Gum Senegal)
- Locust Bean Gum
- Guar Gum
- Indalca Gum
- Nafka Crystal Gum
- British Gum or Dextrin
- Sodium Alginate
How to Make STARCH?
Wheat or maize(corn) starch can be used. Most commonly used is wheat starch.
10% Paste = 100gms starch + 900ml cold water + 4ml castor oil
Mix to smooth slurry consistency. Boil with continuous stirring till thick paste, stir continuously till cold.
How to Make Gum Tragacanth (Gum Dragon)?
This paste is not affected by alkali.
6% Paste = 60gms of gum + 940ml cold water. Pour water over gum and stand for 2-3days. Stir occasionally. If paste is not obtained, then boil with continuous stirring and cool with continuous stirring. Strain in fine sieve.
How to Make Starch-Tragachanth Paste?
100gms wheat paste + 300ml water + 600gms of 6% gum tragacanth paste. Stir lightly, boil for 30mins with continuous stirring. Stir till cold. Strain in fine sieve.
How to Make Gum Arabica (Gum Senegal)?
Gum paste is slightly acidic, neutralise before use. Most common thickener used in Block and Screen printing.
How to Make Locust Bean Gum?
Coagulates by alkali. Used in FLASH AGE process in Vat Dyes Printing.
3% gum = sprinkle gum over cold water, when it swells, boil till paste is formed and cool.
How to Make Guar Gum?
Obtained from seeds of the Gaur plant, is cheaper than Gum Tragacanth, is non-ionic in nature. Gives constant viscosity. Derivative is used in printing of synthetic fibres. Modified gum tragacanth is resistant to acids and alkali.
How to make Indalca Gums?
Are modified gaur gum thickeners. Good for screen printing, manual and machine. A series of Indalca gums are available. Indalca gums are mostly alkaline, they are made acidic by adding acetic/citric/formic/tartaric acid.
5% - 9% paste = add powder to cold water at highspeed stirring for 30mins. Stand overnight.
How to Make Nafka Crystal Gum?
Similar to gum Arabica, but has 3-4times more thickening power that natural gums. The paste is very stable. Used in printing of polyester, nylon and acetate rayon in Roller and Screen printing.
20% paste = Dissolve in cold water and stand overnight.
How to Make British Gum or Dextrin?
Obtained from starch, by converting through hydrolysis to soluble dextrin.
How to Make Sodium Alginate?
Obtained from sea weed, is extensively used as it does not react with reactive dyes. Gives a viscous solution in water, has low adhesive power. Acid makes the paste thin hence is used when neutral or alkaline thickening is required.
6% paste = little powder is sprinkled on cold water or warm water under high stirring.
Available under commercial names such as Lanitex, Protokyp, Manutex, Algogel.