18 Auxiliaries for Printing

S. Karpagam Chinnammal

1. Introduction

Most of the operations in textile processing such as scouring, bleaching, dyeing, printing and finishing are carried out by the use of basic chemicals like acid, alkalis, oxidizing and reducing agents, dyestuff etc. In addition to this other chemicals are also used in relatively small quantities to increase the efficiency of the above said processes. These substances are called textile auxiliaries. Auxiliaries help by speeding up the processes or carry out the processes in a more efficient manner. They are necessary to obtain the desired effect.

2. Learning Objectives

To know about the different printing auxiliaries
To learn the function of printing auxiliaries

3. Printing auxiliaries

Textile printing is the process of applying colour to fabric in definite pattern or design. Printing involves deposition of viscous paste containing the dyestuff, a thickening agent, a small amount of water and other printing assistants. It is immediately dried to prevent the spreading of the colour beyond the boundaries of the design. The actual transfer of the dyestuff from the thickener film into the fiber of the fabric is carried out by steaming, curing or sometimes chemical fixation.

The chemicals added to printing paste that facilitate production of effective and quality printing are called printing auxiliaries. There are different printing auxiliaries. They are classified according to their functions and various uses. The auxiliaries are selected based on the method of printing, dye class , fabric to be printed and compatibility with other ingredients in the paste. Some typical printing auxiliaries are briefly discussed below

Dyestuffs and pigments
Binders
Thickening agents
Wetting agents
Acids and alkalies
Hygroscopic agents
Oxidising and reducing agents
Defoamers/anti-foaming agents
Carriers and swelling agents
Solvents and dispersing agents/solution aids and humectants
After washing agents
Miscellaneous auxiliaries

3.1.Dyestuffs and pigments

According to the fabric content ie natural or synthetic, fastness property (light, washing, perspiration, rubbing etc) required for the printed fabric dyes are selected from different classes of dye ie acid, basic, cationic, direct, disperse, reactive, sulphur , vat etc. For example reactive for cotton, disperse for polyester and acid for wool and silk. Then there are pigments which can be applied to any fiber. Dyes are used in printing for two purposes i) to colour the printed area ie direct style and ii) to colour the ground fabric and then to print with discharging agents ie discharging style. And the dye used for dyeing the ground material should be easily dischargeable.

3.2.Binders /fixtures

In pigment printing insoluble pigments which have no affinity for fibres have to be fixed on to the fibers with binders. Binders are polymeric film, which holds the pigment on one hand and the textile substrate on the other. In other words the pigment is embedded in the polymeric film of the binder, which has adhesion to the fiber surface. Pigments have no groups which can react with the fiber. Fastness property of the pigment print therefore depends on the forces of adhesion between the fiber surface and the binder. Binders form a thin film on the printed portion and they must be colourless and clear. The sharpness and quality of the print depends on the binders quality. Good binders should provide quality such as even thickness, smooth, elasticity, adhesion to substrate, resistance to chemical and physical stress and lastly easily removable after steaming Fixtures are substances like binders which help in achieving optimum fastness properties.

Prints of good fastness property on cellulosic fibres can be achieved using binders/fixers such as melamine formaldehyde, vinyl resins, urea formaldehyde precondensates, chlorinated rubber and acrylic resins. The latest chemicals used are Acramin and Lutexal HD. Acrylic binders can affect crocking fastness, colour yield, handle and print appearance.

3.3.Thickening agents

Thickeners as its name implies are mainly employed in giving consistency to colouring matter. Thickeners are high molecular weight compounds giving viscous paste in water. They impart stickiness and plasticity. Viscosity of printing paste is very important as it influences the clarity and appearance of the printed pattern. The viscosity should be stable not only during printing on machine but also while storing for weeks.

The main function of thickener is to

a. act as a vehicle for carrying the dye on to the cloth

b. To prevent the spreading of the colour on the cloth by capillary action beyond the limits of the defined portion in the pattern

The adhesive nature of the thickener holds the dye particles during steaming. Fabric becomes saturated with steam and the chemical reactions takes place. The soluble dye in the thickener gets absorbed by the fabric giving the dye good fastness property.

The thickener film containing dyes and chemicals should not be dried with too much of heat which will lead to colour mark off on the fabric. Thickeners should not turn into brittle flakes when cured by steaming.

Essential characteristics required of thickeners are easy after wash. The unexhausted dyes and other by products present on the fabric should be removed from the fabric without staining the ground and affecting the other colours., it must retain the sharp outline of print ie avoid spreading of the colour beyond the outline of the design while curing or steaming . Also thickener should not react or get precipitated by the action of chemicals, dyes and pigments which are added to printing paste.

3.3.1. Classsification of thickeners

Thickeners are classified as

3.3.1.1.Natural thickener

These are of natural origin widely distributed throughout the plant kingdom, they are easily available and present in abundance since these are renewable, they can be sustainable raw materials for the textile industry. As the ingredients of natural thickeners are purely natural, they are non allergic and non toxic to our body and cause no health hazard.

3.3.1.1.1. Starch– are the most serviceable of thickeners. It is a vegetable substance manufactured from cereals and tubers such as rice, wheat, maize, potato etc. It is insoluble in cold water . Starch has the property of swelling when heated with water to give a viscous paste. This glutinous mass readily absorbs and retains colours and it is therefore the very best thickener.

3.3.1.1.2. Plant exudates (Gums)

Several gums are used in printing as a thickening agent or binding agent by which colour connects with fiber, the chief of which are gum Arabic and gum tragacanth.

3.3.1.1.2.1. Gum tragacanth: Gum tragacanth is the most powerful of thickening agents It is extracted from leguminous plant like Astragalus gummifer as a dried up exudate and is a natural gum. It is difficult to dissolve in cold water, but readily melts in boiling water and gives a thick smooth paste.4-5 % gives a thick paste useful for printing. The gum when removed leaves the cloth soft .The gum is stable under mild alkaline but unstable in strong alkaline condition

3.3.1.1.2.2.Gum Senegal or gum Arabic: Gum Arabic is the more useful, possessing the advantage over most other thickening agents of being soluble in cold water, but the cost of the gum restricts its use .It is mainly derived from trees of the acacia genus . For printing paste gum 30-50 % is required. The gum is stable in strongly alkaline and strongly acidic condition. It is commonly used in block and screen printing

3.3.1.1.3. Roots and seeds

3.3.1.1.3.1. Guar gum- This gum is cheaper than gum tragacanth and is obtained from the seed of the guar plant. It forms viscous colloidal solution when hydrated in cold water. It gives maximum viscosity at low temperatures at 25-40°C.It is compatible with dyes and chemicals. Its derivatives are used in printing synthetic fabric.

3.3.1.1.3.2. Locust bean gum– gum gatto-it is derived from the seeds of locust bean of the carab tree

3.3.1.1.4. Sea weed

3.3.1.1.4.1.Sodium alginate–It is sodium salt of alginic acid which is found in sea weeds. Small amount of sodium alginate is required in printing paste. It solublises in cold as wells as in hot water, has good wettability, so absorption of dye by fiber is excellent. It is used with most of the dyes and gives best results with reactive dyes. It can be washed easily from the fabric and leaves the fabric soft

3.3.1.2.Modified natural thickener

3.3.1.2.1.Starch derivatives

3.3.1.2.1.1. Dextrin or british Gum: it is degradation product of starch made by heating it with mineral acid (nitric acid) or roasting at 160°C till it becomes soluble in water completely. For 100 parts of printing paste 20-50 parts dextrin is used.

3.3.1.2.2. Cellulose derivatives

Cellulose, which is normally very insoluble in water by chemical modification produce valuable thickeners

3.3.1.2.2.1.Methyl cellulose : methyl cellulose ether is obtained from wood pulp or cotton linters. Ether is insoluble in the presence of alkali and boiling water but soluble in cold water. Its consistency is retained during storage

3.3.1.2.2.2.Sodium carboxy methyl cellulose (CMC) is obtained by reacting monochloro acetic acid and cellulose. The thickener is stable to alkali and so can be used for printing with vat dyes, it can be easily washed from fabric.

3.3.1.2.3. Gum derivatives-Indalca Meyprogam is a modified guar gum which is resistant to acids and alkalies

3.3.1.3. Synthetic thickener

Synthetic thickening agents are molecular substances generally co polymers of unsaturated organic acids such as acrylic and maleic anhydride. They have the following properties- high degree of purity, rapid preparation of stock thickening, simple printing recipe, good running properties, optimum depth of shade and brilliance of the prints, excellent stability.

Synthetic thickeners have the ability to produce prints with better smoothness , levelness and sharpness as compared to natural and modified thickeners and are suitable for photographic and multicolour prints. They are used as a substitute for emulsion thickening in pigment printing.

Synthetic acrylic based polymers are grouped into two classes : alkali swellable or soluble emulsions , hydrophobically modified alkali swellable emulsions.

3.3.1.4. Emulsion thickener

Emulsion thickening derives its viscosity from a dispersion of tiny droplets of one liquid in another with a dispersing agent. The two liquids must not mix or be soluble. With development of pigments colours emulsion came into existence. And emulsions are also employed for dyes. Emulsions are available in disperse phase and dispersing medium. For printing textile material, oil in water type of emulsion is popularly used. Widely accepted is kerosene based emulsion. Alkyl-phenol-ethylene-oxide condensates are also used.

Choice of a thickener will depend upon the class of dye to be printed and style of printing for ex British gum and starch tragacanth thickener for direct dye, British gum for vat ,Sodium alginate for disperse and synthetic for reactive

3.4. Wetting agents

Wetting agent is used 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. It is used to dissolve the dye powder. It is not required in case of water soluble dyes. In case of insoluble dyes like napthol and vat wetting agents are used to facilitate wetting of the dye and its subsequent dissolution. Turkey red oil, monopol soap, casoline oil are few examples of wetting agents. Turkey red oil is the commonly used agent.

3.5. Acids

The acids are used when acidic condition is required during steaming or curing to fix the colour on the fabric. The acids are not required while printing but required during the steaming so such acids which liberate during steaming should be used. The acids are required for the development of some class of dye and also to activate oxidisng agent during steaming or curing

For this purpose two types of acid liberating agents are used. Firstly organic and inorganic salts of ammonium salts such as oxalate, chloride, ammonium acetate, sulphate, nitrate, phosphate etc Ammonium chloride and diammonium hydrogen phosphate are examples. Secondly ester of organic acids such as diethyl tartarate which on heating splits up into tartaric acid and ethyl alcohol. Tartaric acid will produce required amount of acidity.

To prepare printing paste with reactive dye free acids can be added such as formic, acetic, glycolic, tartaric, lactic and citric acids

3.6. Alkalies

Alkalies like sodium carbonate , sodium bi carbonate and sodium silicate are necessary for fixing reactive colours in printing. While preparing printing paste if strong alkalies are required then sodium hydroxide and potassium hydroxide are preferred. For mild alkalies sodium silicate, sodium and potassium carbonate, trisodium phosphate. Triethanolamine, disodium phosphate. Trisodium phosphate, triethanolamine, ammonium hydroxide etc can be used

Sodium acetate is the salt of acetic acid (weak acid) and sodium hydroxide (strong base). When sodium acetate is dissolved in water it forms little amount of sodium hydroxide. If excess amount of acetic acid is added neutralizing of alkali takes place and the solution becomes acidic. When the solution is boiled or the printed fabric is steamed the acetic acid being volatile evaporates and wet thickener will acquire alkaline property.

3.7. Hygroscopic agents

Hygroscopic agents are added in printing paste to enhance the transfer of the dye or pigment from the thickener film to fabric and facilitate subsequent washing off of the thickener. Hydroscopic agents are especially used for certain class of dye like vat dyes. These are used to absorb sufficient amount of water during steaming and enable the dye molecules to diffuse into the fibre thereby resulting in good colour yield.

Commonly used hygroscopic agents are glycerin, urea, sodium lactate , diethylene glycol a 40/60 mixture of urea and formamide,

3.8. Oxidising agent

To develop final colour during steaming or in developing dye baths in case of solublised vat dyes and aniline black, oxidising agents are required to be added. Some of the oxidizing agents are chlorates, nitrates, nitrites and potassium ferrocyanide

The oxidizing agent should be inactive at room temperature (25-30° C). Oxidising agent like sodium chlorate, potassium chlorate etc do not act in neutral or alkaline solution so they are added with volatile base ammonia. The paste also contains acid liberating agents such as ammonium chloride, ammonium phosphate etc. While steaming at 100 °C ammonia volatilizes and ammonium salt decomposes into acid and ammonia. Ammonia escapes leaving behind Acid achieves chlorate at high temperature and create oxidizing condition and colour gets developed

3.9. Reducing agent

In discharge printing reducing agents are used as discharging agents to chemically destruct the dye and create a white pattern on a coloured background. The liberated hydrogen obtained by the decomposition of discharging agent during steaming process causes the decomposition of dischargeable dye into colourless water soluble products which are removed in subsequent washing off process after printing producing white effect on dyed fabric.

The reducing agent such as Rangolite C (sodium sulphoxylate formaldehyde ), thiourea dioxide are helpful as reducing agents for discharge style printing. Formaldehyde sulphoxylate (NaHSo2.Ch2O.2H2o) is one of the most powerful discharging agent, however it is toxic and produces formaldehyde. Sodium borohydride has also been tried as an alternative. Zinc sulphoxylate formaldehyde is slightly soluble in water. This is used to discharge printing of polyester and is mainly suited for disperse dye printing paste. For this purpose stannus chloride, stannous acetate is also used.

For vat printing sodium hydrosulphite is used.

3.10. Defoamers /Anti –foaming Agents

In case of roller printing the use of a wetting agent in the printing paste, coupled with continuous agitation of the paste in the colour box during printing produces foam which leads to defects in printing like lighter shade. So to avoid faulty printing defoaming agents are used. Defoamers such as silicone defoamers , octyl alcohol, turpentine, emulsifiable hydrocarbons etc can be used. Silicolapse and perminal KBI are commercial product

3.11. Carriers

The carriers are organic compounds used for dyeing polyester because they increase the rate of dyeing. By incorporating carriers deep shades can be achieved on polyester at 100° C; without carriers the deep shade cannot be obtained. Carriers cause swelling of the fiber and allow big molecules to diffuse faster into the fibre.

Some of the carriers used are Methyl Salicylate, o-Phenylphenol, monochloro-o-Phenylphenol, Diphenyl, p-Pheylphenol and Butyl Benzoate

3.12. Solvents and dispersing agents

After an extent water soluble compounds are not soluble in water. At a particular concentration compounds are soluble and further addition of compound does not dissolve in water, it settles down in bottom. This is known as saturated solution compound in water. If temperatures is raised the water dissolves some more compounds and solution is saturated at low temperatures. At each temperature there is saturation or solubility compound above which solution cannot dissolve. To increase the solubility of dye certain solvents and solution are added.

In printing paste only a limited amount of water has to be used, in which dyes reach saturation. In such cases certain solvents are used to increase the solubility of the dyes and prevent agglomeration of the dyestuff in the highly concetrated paste .Acetin, diethylene glycol, thiodiethylene glycol, etc are commonly used solvents.

3.13. After washing agents

The printed fabric after steaming has to be washed with soap solution. Washing with the soap solution makes the fabric soft ie soap will remove the thickener. It helps to achieve true colour for some dyes

3.14. Miscellaneous Auxiliaries

i) Mordants are used on cotton to print with basic dyes, because basic dyes and cotton do not have affinity for each other but mordants has affinity for both fiber and dye, Commonly used mordants is tannic acid as cotton readily absorbs this acid and colurs also are attracted to it

ii) Leucotrope W (dimethyl phenyl benzyl ammonium chloride) is used for discharge printing of vat dye. It forms a complex with leco vat dye. It does not get oxidised when the oxidation of the vat dye takes place, so it can be easily washed and discharge can be obtained

iii) Mild oxidants Ludigol or resist salt (m-Nitrobenzene sulphonic acid) are used as mild oxidant on direct dye in ground fabric printing with vat discharge. This substance saves the unprinted ground fabric from damage while steaming. Usually the ludigol is used in the direct dye liquor

iv) Sequestering agents: are intended for blocking the activities of metallic ions present in the printing paste. Its recommended amount is 0.5 % OWP Eg sodium hexametaphosphate

v) Preservatives: When thickening paste is preserved for a long time, it may be spoiled by action of bacteria and its viscosity may get lower. Preservatives like Salicilic acid, phenol, cresol sodium benzoate etc can be used to prevent this

Summary

Textile printing is the most versatile and common method used for decorating of any textile fabric. The technology of printing along with dyeing has been the main stay of textiles. In this module we got an insight into the printing auxiliaries and their role in printing. Textile auxiliaries play a vital role in the final outcome of a good quality printed product.

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REFERENCE

Hemalatha jain-Techniques of dyeing and printing, An books Pvt Ltd, New delhi. 2010, p.179-186
C.N.Sivaramakrishnan, Reducing agents in textile printing, Colourage, may 2016 p 58
C.N.Sivaramakrishnan, Binders in textile printing, Colourage, December 2015 p 48