List of Printing Paste Ingredients and Their Functions

Printing Paste Ingredients / Additives

Printing paste is principle element in textile printing, consisting of several key ingredients that work together to create vibrant designs on fabric. Printing paste is prepared by using printing ingredients. Each ingredient plays a specific role in ensuring that the printing process is effective and produces high-quality results. The composition of printing paste varies depending on the printing technique (e.g., screen printing, digital printing, block printing) and fabric type. The performance of printing depends on a well-prepared printing paste.

List of Printing Paste Ingredients and Their Functions with Examples

Printing paste additives are described below with functions and examples.

1. Thickeners:

Thickeners used in textile printing are high molecular weight compounds giving viscous pastes in water. A thickener is a thick viscous mass which imparts stickiness and plasticity to the printing colour, so that it may be applied. To the fibrous surface without bleeding and be capable of maintaining the design outlines, even at the application of a high pressure. The thickeners are prepared from gums which upon dissolving or swelling in water give thick, adhesive and viscous masses. The thickener should give a stable paste viscosity, which would allow an even quantity of the paste during the run so that no uneven printing happens.

The printing thickeners used depend on the printing technique, the fabric and dyestuff used. Typical thickening agents are starch derivatives, fl our, gum arabic, guar gum derivatives, tamarind, sodium alginate, sodium polyacrylate, gum senegal and gum tragacanth, British gum or dextrine and albumen.

The requirements of a thickener may vary depending upon the dye class being printed and the substrate. However the thickener should have the following properties:

• Compatibility with all the other components of the print paste
• Easy to dissolve in water
• Stable during the printing and fixation stages
• Capable of being removed in the final print washing stage

Thickeners may be divided chemically into three main types depending upon their chemical constitution: natural products, modified natural products and synthetic products.

2. Resist salt:

Resist Salt is a chemical compound used in textile printing and dyeing to inhibit or block dye penetration in specific areas, creating resist effects. One of the very commonly used mild oxidising agent is sodium metanitrobenzene which is commonly called as Resist salt (also called Ludigol).

Resist salt darkens on exposure to light and the print containing resist salt should be given adequate washing to ensure its complete removal from the fabric. However, the normal reactive print wash is sufficient for the removal of Resist salt. Resist salt is used in alkaline printing pastes. But in the case of acidic paste normally sodium or potassium chlorate is used in its place.

3. Urea/Hydrotropic agents:

Urea and hydrotropic agents are essential additives in textile printing pastes, primarily used to enhance dye solubility and penetration.

Printing is normally done with highly concentrated pastes that tendto lose moisture under adverse conditions which can affect the colour yield. Using an auxiliary that tends to increase the aqueous solubility of the dye, one can achieve better colour yield. Hydrotropes, come into play an important part in printing, which act as an amphiphilic bridge between the dye solubilisate and the aqueous medium. Even though some surfactants can function as a hydroptrope in dyeing dark shades where the dye quantity is much high, this may not be much useful in print paste due to its powerful wetting properties which can promote bleeding and haloing of the prints. Hence for printing hydrotropes with much less strong surface-active properties are more suitable for use in printing.

Urea, which is the most frequently used hydrotropic auxiliary, performs the following functions:

• Increases dye solubility in the discharge print paste or pad liquor
• Accelerates dye fixation during steaming
• Improves dye colour yield
• Prevents the fibre drying out during steaming

4. Hygroscoping agents:

The function of hygroscopic agents used in printing paste is to take up sufficient amounts of water during steaming to give mobility to dye molecules, to enable them to transfer to the fiber. Urea, glycerin, diethylene glycol and urea are generally used as hygroscopic agents in printing. For example, in printing with acid and metal-complex dyes, the print paste contains hygroscopic agents. Sodium chlorate can be used as a strong oxidising agent at the same time hygroscopic agent. Hygroscopic agents also help in absorbing the moisture from surroundings, etc., from steam in a steamer. If the hygroscopic agent is not deliquescent it can be used in print paste to avid spreading of the print when higher amount of moisture is absorbed thereby maintaining the clarity of the print. Generally, a hygroscopic agent functions as humidity controller and fibre swelling. Zinc chloride is used as a hygroscopic agent in textile printing pastes for white resists on bromine indigo, etc. In wool printing with acid dyes glycerol is used as a hygroscopic agent (20–30 g/kg).

Main hygroscopic agents used in print pastes are urea and glycerine. Hygroscopic agents are required to absorb the condensed steam (water) during the steaming process otherwise it may spread the colour and affect the clarity of the print. It is essential to use the optimum quantities of the hygroscopic agent. When lower quantities of hygroscopic agents are used all the dye present in the thickener may not get enough water to dissolve ending up in poor colour yield. The larger amount of hygroscopic agent than optimum amount, due to more water supplies to thickener film may cause spreading of the print as mentioned earlier.

5. Anti-foamers and defoamers:

Wetting agents are sometime added to the print paste for various reasons like penetration of the print, etc. or other surface active agents or dissolution agents, causes foam generation due to the stirring, agitation, squeegee action, etc. and this can cause faulty prints. To avoid this, defoaming agents should be incorporated in the print paste. Usually silicone defoamers, readily emulsifiable hydrocarbons, sulphated oils etc., may be used for this purpose. Another common defoamer used in printing is Pine Oil which is readily miscible with water to give a milky dispersion.

The main requirement for an effective defoamer is that the agent should be insoluble in the foaming system in order that, at low concentrations, it can be retained at the phase interface. The entry coefficient must be positive, and the agent should form quasi-gaseous boundary layers.

6. Alkalis:

Various alkalis are used in printing taking into consideration the pH requirements of the dye fibre reaction, the ability of the fibre involved to withstand the alkalinity, etc. The main strong alkalis used are sodium hydroxide, potassium hydroxide. As milder alkalis sodium carbonate, potassium carbonate, sodium bicarbonate, sodium silicate, disodium hydrogen phosphate, trisodium phosphate, sodium acetate, triethanolamine, ammonium hydroxide, etc. are used. Milder alkalis are used in the case of fibres like wool, silk, etc., where strong alkali cannot be used.

When a paste has to be acidic while printing and needs alkalinity while fixation Sodium acetate can be suitably used. Sodium acetate being a salt of a weak acid and strong alkali, after printing with an excess of acetic acid remains as acidic preventing the dissociation into acetic acid and sodium hydroxide, while steaming at higher temperature it dissociates and acetic acid being volatile escapes and the medium becomes alkaline.

7. Oxidising agents:

Oxidising agents are required to develop final colour in steaming or after treatments in case of certain dyes like solubilised vat dyes, aniline black, etc. In most cases it should not be active in the print paste as it may cause premature development of the dye preventing the subsequent fixation of the dyes. Potassium chlorate, sodium chlorate are common oxidising agents used in printing. It is not active in neutral and alkaline medium and hence can be used a volatile base like ammonia.

Sodium nitrite or sodium chromate is also used as mild oxidising agent.

In some cases where the dye used is susceptible for reduction, a mild oxidising agent like resist salt. But it is active in alkaline medium in a reducing atmosphere only. If a reducing action has to be prevented in an acid medium sodium chlorate may be used.

8. Reducing and discharging agents:

Reducing agents or discharging agents are used in various occasions in printing pastes. These are used to prevent precipitation of dye particles as the concentration of the dyestuff in the printing paste is high. A major requirement is in discharge printing where the already dyed ground has to be destroyed and made white and leaves it as white or another colour can be fixed at the discharged area. For example, on a ground dyed with a dischargeable azo dye has to be discharged by printing, a reducing agent is incorporated into the print paste and printed so that the white ground of the fabric is developed on steaming.

The main discharging (reductive) agents used in printing are:

• Sodium formaldehyde sulphoxylate
• Zinc formaldehyde sulphoxylate
• Calcium formaldehyde sulphoxylate
• Thiourea dioxide
• Tin (II) chloride

Selection of a reducing agent for a printing primarily depends on the fibre to be printed and of course, based on the dyes involved. Water soluble sulphoxylates can give haloing problems on the synthetic fibres, due to the movement of solution along the yarns. This problem can be overcome by using the insoluble formaldehyde sulphoxylates or thiourea dioxide.

9. Solvents, Dissolution aids, Dispersing agents:

In some print pastes solvents are used for the easier diffusion of dyes into fibres. Examples are diethylene glycol diethyl ether (diethyl “Carbitol”) and diethylene glycol dibutyl ether (dibutyl “Carbitol”) used for cationic dyes, also pasting auxiliaries for vat dyes, print pastes which helps in the dissolution of leuco esters dyes. Glycerol is used in printing pastes as a hygroscopic agent and solvent. In polyamide printing with acid dyes urea, inter alia, is used as a dye solvent. Benzyl alcohol and ethanol also are used a solvent in print pastes.

Another purpose of dispersing agents and solvents are preventing aggregation of dyestuffs in the printing pastes. It is more important where dye concentration is higher.

10. Whitening agents:

In discharge print pastes, often auxiliaries are added which enhances the discharged grounds. Examples are optical whitening agents, white pigments like zinc oxide, titanium dioxide. These auxiliaries should be resistant to the discharging agents in the print paste.

12. Wetting agents:

The high surface tension of water prevents wetting of the dye powder. Therefore, it is necessary to use a suitable wetting agent while dissolving the dye to obtain a smooth paste of the dyestuff without formation of any lumps. Wetting agents are used, especially in discharge prints, to reduce the surface tension between water and air and enable the aqueous medium to penetrate into the air-filled capillary spaces in the fibre easily.

Wetting agents have two purposes in the print paste – to wet out the fibre and helping in dissolving the dyes. Water is the main solvent used in the paste preparation. Because of the surface tension of water (73 dynes/cm) it has a resistance to wet out the hydrophobic surface of the fibre or dyes. There are substances which is soluble in water and reduces the surface tension of water and thus helping in the wetting out the surfaces which are non-wettable (e.g., synthetic fibres, insoluble dyes like vat dyes) or difficulty wettable (e.g., natural fibres, some soluble dyes) – such chemicals are called wetting agents. Wetting agents comes in the group of surface active chemicals.

Common wetting agents are sodium salts of sulphated vegetable and animal oils and other completely synthetic wetting agents. Sodium salt of sulphated castor oil (Turkey Red Oil) is one of the most common wetting agents used in dissolving insoluble dyes.

13. Acids and acid donors:

Acids are used for maintaining the pH of the pastes and for providing acidic atmosphere in the fixation period. In the former case, normally non-volatile acids are used so that pH changes should not happen during drying or steaming due to the volatility of acids like acetic acid. For example, since several disperse dyes are sensitive to alkalis, the pH of the print pastes must be adjusted on the weakly acidic side by the addition of a non-volatile acid such as tartaric, citric, lactic or glycolic acid. Acid medium is required in the fixation of several dyes like solubilised vat dyes. In pigment printing Binding agents and crosslinking agents require an acid pH value for their fixation.

Additional acid donors such as diammonium phosphate are included in the printing paste to guarantee optimum fixation even if the conditions are unfavourable. In certain cases, acid pH is required to activate oxidising agents like sodium chlorate which is included in the printing paste.

14. Fixation accelerants:

These are used to improve dyestuff fixation in printing as well as to shorten fixation time by swelling compounds. They are also effective in preventing fixation uneven­ness that may be caused by fluctuation of conditions for dyestuff fixation such as time, temp, humidity and so on.

Examples of fixation accelerators are:

• P-phenyl phenol is applied for polyester.
• Thiourea is applied for polyamide.
• Resorcinol is applied for polyamide, cotton and acrylic fibers.

In case of fixation of disperse dyes on polyester by the thermosol or HT-steam process, it is often advantageous to add fixation accelerator in the print pastes for the achievement of good colour yields. In the case of half-emulsion thickeners, no addition of fixation accelerator is necessary. But when printing triacetate with disperse dyes fixation accelerator is necessary even if half emulsion is used as thickener.

15. Carriers and swelling agents:

Since the synthetic fibres, especially polyester, which is difficult for the fibre to penetrate, carriers are used as swelling agents in dyeing to help dyeing of synthetic fibres, at boiling temperature. Under the same principle, carriers are used in printing also for fixing dyes by pressure steaming at 120-1300C. They basically swell the fibre so that the pores in the fibre are enlarged so that the dye molecules can enter the fibre. Therefore, they are also known as swelling agents. Generally, urea is used as swelling agent for printing various dyes on different fibers. The suitable swelling agents for this purpose are diphenyl (polyester) and phenol, resorcinol, benzoic acid, thiourea, diethylene glycol acetate (DEGDA), Polyethylene glycol [HO(CH2CH2O) nH], ammonium sulphocyanide (NH4CNS), ethyl lactate [CH3CH(OH) COOC2H5] (cellulose triacetate). Phenol, Resorcinol acts as swelling agents for polyamide fibres and at the same time dye solvents for acid and metal complex dyes.

16. Binders:

The binder is a film-forming substance. Binder component in print paste is used only for Pigment printing. Pigments are fixed on the fabric by means of binder. During fixation (curing) binder creates a film, which surrounds the colour pigment and is responsible for fixing to the substrate. Pigments are of the size 0.2-1 μm but a very good quality pigment size is of 0.2 – 0.5 μm. In a pigment, dyes from manufacturer may contain only 25–45% pigments remaining will be additives like dispersants, water, etc.

As explained the binder acts as an adhesive between the pigment particle and substrate. since the pigment does not have any affinity or reaction with the substrate, and can remain as far as the binder is present, there are many requirements for a binder. Also, it can greatly affect the handle of the substrate and fastness properties of the pigment on the substrate, the quality of dyeing greatly depends on the binder. Hence the binder should have following requirements for resulting in a good quality prints:

• It should have minimum or no effect on the fastness characteristics of the pigment.
• It should be resistant to acids, alkalis and solvents
• It should have swelling resistance
• It should be resistance to weather ageing and heat
• It should have minimum or no effect on the handle of the fabric.
• The binder should be easily removable from the screen and other parts of the machines like conveyor, rubber blankets.
• It should form film only at higher temperatures. Film formation at lower temperature gives problems like choking of the screen, etc.
• The film formed must be colourless, clear, of uniform thickness, neither too soft nor too hard, i.e. possessing elastic properties.
• It must coat and adhere to the pigment well and possess resistance to both mechanical and chemical loads.

The listed requirements of the binder make it clear that the development of the pigment printing or pigment dyeing process followed the development of suitable binders.

Binders are selected high molecular compounds, which are built up from single monomers by polymerisation, polycondensation or polyaddition. In textile printing, primarily weak dispersion binders are used, which are manufactured by emulsion polymerisation.

17. Mordants:

These are used in the printing of basic dyes on cotton or poor wash-fast acid dyes on wool and silk. A mordant is a substance having affinity for both the fiber and the dyestuff. Tannic acid along with tartar emetic was once very popular as mordant. “Katenol O”, which is thiophenol, is also employed as mordant.

18. Resist agents:

Resist agents are different from resist salts, which are used for resisting the fixation of a dye in a resist printing method. Discharge method of printing has a limitation of using only fully dischargeable dyes on which discharge is planned. In many cases fully white discharge in not possible where the dye is not fully dischargeable. Resist prints can produce effects of discharge prints especially when a dye is not dischargeable or partly dischargeable in a print.

In many dischargeable prints, when a dye has to be fixed in alkaline medium (e.g., Reactive dyes) and acid, acid donor or a dye (white resist) or pigment, which fixes in acid medium, along with the acid or acid donor can be used as a resist (coloured resist) or vice versa. Examples of acid resists are acetic acid, citric acid, lactic acid, tartaric acid, glycolic acid, etc., and acid donors are ammonium chloride, ammonium Sulphate, ammonium citrate, ammonium nitrate, ammonium sulphocyanide etc. These chemicals are used depending on the fixation conditions like steaming, curing, drying etc. where these agents should be able to perform in that condition. For example, a volatile acid like acetic acid cannot work in curing/baking conditions.

19. After-Washing Agents:

After-washing or soaping of printed goods is an important operation. It serves two purposes, namely:

1. It removes the thickener from the printed goods and thereby improves the handle of the cloth.
2. It helps in developing true shades and improves the fastness properties of certain dyestuffs.

Several after-washing chemicals are available which are probably a mixture of various chemicals like dye-fxing chemicals, anti-staining agents and cleansing agents.

Other printing paste components:

After treatment agents or fixing agents, levelling agents, complexing agents, etc. are also used in printing paste at different occasions. A special complexing agent is used in the discharging of dischargeable vat dyes called Leucotrope W.

Conclusions

Printing paste ingredients play a pivotal role in achieving vibrant, durable, and eco-friendly textile designs. The choice of ingredients depends on the type of printing method, fabric, and desired finish. Proper selection and balance of these ingredients ensure sharpness, colorfastness, and durability in printed textiles.

References

[1] Kolanjikombil, M. (2024a). Printing of Textile Substrates: Machineries and Methods. CRC Press.

[2] Choudhury, A. K. R. (2022). Principles of Textile Printing. CRC Press.

[3] Purushothama, B. (2018). Handbook of Value Addition Processes for Fabrics. Woodhead Publishing.

[4] Wardman, R. H. (2017). An introduction to textile coloration: Principles and Practice. John Wiley & Sons.

[5] Miles, L. W. C. (2003a). Textile Printing. Amer Assn of Textile.

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