PRINTING AUXILIARIES

ð  Printing is localized dyeing. Printing is the most important of all the processes used at present to decorate textile materials.

ð  The main idea of printing is the deliberate and controlled application of dye to exactly defined locations on the fabric leaving the rest of the fabric essentially unaffected.

ð  The following auxiliaries are used in printing:-

a)      Wetting agent

b)      Dispersing agents or solvents

c)      Antifoaming agent

d)     Hygroscopic agent

e)      Oxidizing and reducing agent

f)       Carriers

g)      Binders

h)      After washing agents

i)        Miscellaneous auxiliaries

j)        Thickeners

Wetting Agent

ð  The use of wetting agent is generally beneficial in dissolving dyestuff eg. Terkey red oil (T.R.O.), Lissopol C, Igepoh T etc.

ð  During the penetration of printing paste the dyestuff which is usually in the powder form is dissolve in a small amount of water.

ð  In case of direct, acid, basic & reactive dyes, which are water soluble, lump formation can take place if water is poured over the dye powder or if the dye powder is added to water.

ð  The correct procedure is to add a little amount of water to the dye powder & to make a paste by proper stirring.

ð  When all the dye is pasted, further quantity of water is added & heated to the required temperature and stirred till complete solution is obtain.

ð  If the pasting is not proper prior to addition of water, small lumps of the dye powder remain undissolved & during printing, these small lumps get deposited on the fabric & produce dark spots.

ð  The high surface tension of water prevents wetting of the dye powder.

ð  Therefore it is necessarily used a suitable wetting agent while dissolving the dye.

ð  The wetting agent reduces the surface tension of water & facilitates the wetting of dye particles & ultimately results in solubilising of the dye by chemical reaction.

Solvents/Solubilising Agent

ð  They are used to prevent aggregation of dyestuff molecules in highly concentrated paste of the dye.

ð  Commonly used solvents include Acetin, Diethylene glycol, thiodiethylene glycol, glycin – A etc.

ð  Acetin is a mixture of mono, di & triacetates of glycerenes prepared by reacting acetic acid or acetic anhydride with glycerol.

ð  Monoacetate is completely miscible with water, diacetate is less soluble & triacetate only moderately soluble in water.

ð  All of them are freely miscible with alcohol & ether & other organic solvents, but not with aliphatic hydrocarbon.

ð  These are useful a solvents a dissolving for various class of dyes including basic, rapidogens, etc.

ð  These are used as additives to printing paste to produce better prints & high colour value.

ð  Soledon Developer GE (ICI) is a neutral colouless mobile liquid which is completely mobile with water & consists of glycol monoethylether.

ð  It is useful in printing vat & azoic for increasing solubility of dyes, so that deeper & better penetrated shade can be obtain.

ð  Glycine – A (BASF), Glycine – B (ICI) (thiodiethylene glycol) are excellent solvents for basic, direct, acid & disperse dyes.

Antifoaming Agent

ð  In the case of roller printing, wetting agents are used in the printing paste & simultaneously continuous agitation of the paste is also taking place in the colour box in which printing paste is kept.

ð  Thus a considerable amount of foam is produce in the colour box.

ð  Due to this overflowing of the colour takes place, which then falls on the other box.

ð  In roller printing; one colour may be blue & other may be red, the foam of one colour may fall on the other, sometimes mixing of colours may occur.

ð  The foam formed in the colour box produce faulty prints & to avoid this fault defoamer should be incorporated in the printing paste.

ð  Silicone defoamer readily emulsifiable hydrocarbon, sulphated oils, etc. may be used for this purpose.

ð  Perminal KB (ICI) is neutral yellowish brown liquid containing an aqueous emulsion of sulphated sperm oil & pine oil.

ð  It is anionic in nature, miscible with water, gives milky dispersion which is atable to alkalis.

ð  A mixture of ortho, para & meta -  methyl cyclohexanol has a power of reducing the foam & widely used in textile printing.

ð  Emulsified pine oil is readily miscible with water & can be used as defoamer.

ð  Triactyl phosphate is also a very good defoaming agent.

Hygroscopic Agent

ð  The function of hygroscopic agents used in the 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 fibre.

ð  Glycerine, diethylene glycol and urea are generally used as hygroscopic agents in printing.

Oxidizing and Reducing Agents

ð  In order to develop the final colour in steaming or in subsequent after treatment as in printing of solublised vat dye, aniline black, some oxidizing agents are added to the printing paste.

ð  They are also used in certain styles of printing such as colour discharge/oxidation discharge.

ð  Most commonly used oxidizing agents are:-

a)      Chlorates of Na, K, NH_3, or Al

b)      Dichromates of K & Na

c)      Chromates of K & Na

d)     Potassium ferrycynide (K₃FeCN₆)

e)      Nitrites, Nitrates

ð  The proportion of oxidizing agent should neither be less nor more. It should be such that it only discharges the colour.

ð  Oxidizing agents are added during the preparation of printing paste but it should be effective on colour only during steaming.

ð  Reducing agents are used mainly for printing of vat colours, sulphur colours, etc. & for production of reducing discharge effect on the goods dye with direct, vat, indigosol etc. & also for producing resist style in the application of insoluble azoic colours.

ð  Commonly used reducing agents are:-

a)      Na₂S₂O₄

b)      Sodium sulphoxylate formaldehyde (NaHSO₂.2H₂O.HCHO)

This is commonly available as Rongalite C, Formosol, Hydrosulphite NF etc.

c)      Sodium bisulphate (NaHSO₃)

d)     Glucose

e)      Tin compounds (SnCl₂, SnCOOCH₃)

f)       Ferrous sulphate

g)      Thiourea dioxide [NH₂C(SO₂)NH₂]

Carriers & Swelling Agents

ð  Dyeing & steaming processes are carried out at high temperature.

ð  The compact structure of synthetic fibres apecially polyester making it difficult for the dye molecule to penetrate in the fibre structure under condition of dyeing at boil & steaming at 100 – 102°C in ager.

ð  It is known that certain hydrocarbons, substituted hydrocarbons, phenol etc. accelerates the rate of dyeing of polyester with disperse dyes from an aqueous medium at about 100°C.

ð  These substances used are called carriers & one of their function is to swell the fibres so that the dye molecules can diffuse easily inside the fibre.

ð  Therefore they are also known as swelling agents.

ð  Generally urea is used as swelling agent for printing various dyes on different fibres.

ð  Some important carriers used for polyester are:-

a)      Ethyl alcohol

b)      Diethyl alcohol (DEG)

c)      Diethylene glycol diacetate (DEGDA)

d)     Polyethylene glycol

e)      Ammonium sulphocynide

f)       Thiodiethyl glycol

g)      Ethyl lactate

h)      Diethyl tartarate

i)        Phenols etc,

ð  These carriers should be completely removed from the fabric after dyeing or printing is over.

ð  Otherwise they may decrease the light fastness of dyed & printed goods.

ð  These carriers may also be toxic in nature & therefore they have to be used very carefully.

Binders

ð  Binders are used in pigment printing. Binder is a substance which can form a film at the printed portion.

ð  As the fastness properties of pigment prints are determined by the fastness of the binder film, the quality of a print depends on the quality of the binder.

ð  The binder film formed on the fibre must be colourless and clear.

ð  It must be of even thickness, smooth and neither too hard nor too soft. It should be elastic and should have good adhesion to the substrate without being tacky.

ð  Further, it should have good resistance to mechanical and chemical stresses, but should be readily removable from the printing rollers, screens, backgreys and blankets during operation or shortly afterwards.

ð  Albumin, casein, glue etc. were used earlier as binders cum thickeners. These were later replaced by cellulose esters such as cellulose acetate and cellulose nitrate.

ð  Recently, large numbers of synthetic binders have been developed.

ð  The important amongst them are vinyl resins, acrylic resins, melamine formaldehyde and urea formaldehyde precondensates, chlorinated rubber, etc.

ð  Helizarin binder UD (BASF) is formed from acrylic esters with styrene and gives prints having excellent fastness properties.

ð  The latest development is the combination of binders, white spirit emulsifier, leveling agent and catalyst.

ð  Special binders are also developed for printing polyester/cotton blended fabrics.

ð  These are based on styrene-butadiene and acrylonitrile-butadiene, which have good adhesion power for circular cross-section polyester.

After Washing Agents

ð  After washing or soaping of printed goods is an important operation.

ð  It serves two purposes:-

a)      It removes the thickener from the printed goods and thereby improves the handle of the cloth.

b)      It helps in developing true shades and also improves the fastness properties of certain dyestuffs.

ð  The auxiliaries used in after washing are described as in Dyeing Auxilaries.

Miscellaneous Auxiliaries

ð  Mordants:-

·         These are used in the printing of basic dyes on cotton.

·         A mordant is a substance having affinity for both the fibre and the dyestuff.

·         Tannic acid along with tartar-ematic and “Katenol O” which is thiophenol are generally employed as mordants.

ð  Leucotrope W:-

·         It is dimethyl phenyl benzyl ammonium chloride.

·         It is extensively employed in the discharge printing of vat dyes.

·         This substance forms an additive complex with leuco vat dye.

·         The complex so formed is not oxidized to parent vat dye by oxidation and being soluble in alkali, it is easily removed from the fibre during washing.

·         Thus, when vat dyed goods are printed with alkali, sulphoxylate formaldehyde and “Leucotrope W”, steamed and washed, one can easily remove dyestuff from the printed portion to get discharge prints.

ð  Mild Oxidants:-

·         m – Nitrobenzene sulphonic acid sold under the trade names, “Ludigol” or “Resist Salt” is used as a mild oxidant in printing vat discharge on direct colour ground.

·         It prevents the adverse effect of the reducing agent on unprinted ground during steaming.

·         The method of application involves incorporation of “Ludigol” in the padding liquor used for dyeing direct colour.  

Thickeners

ð  The main function of thickeners in printing are :-

(a)    To act as a vehicle for carrying the dye onto the cloth

(b)   To prevent the spreading of the colour on the cloth by capillary action beyond the limit of the defined portion in the motif.

ð  A large number of thickeners are available for printing.

ð  The choice of thickener will depend upon the class of dye to be printed and style of printing.

ð  The compatibility of thickener with other ingredients in the printing paste should also be considered during its selection.

ð  Carbohydrates form the most important and largely used varieties of thickening agents.

ð  Wheat starch and maize starch have been used as thickeners in printing for a very long time.

ð  Starch is not suitable as a thickener in the preparation of paste in which acids are to be added.

(A) Gums

ð  Gum Tragacanth or Gum Dragon:-

·         It is obtained from the ligneous plant.

·         The advantage of this gum is that it leaves the cloth quite soft.

·         Generally 4 – 5 % gum paste is used in printing.

·         It is stable under mildly alkaline conditions but not under strongly alkaline conditions.

ð  Gum Senegal or Gum Arabic:-

·         It is obtained from the exudation of Acasia plant.

·         30 – 50 % of gum paste is required for printing.

·         It is stable under both strongly alkaline and strongly acidic conditions.

ð  Locust Bean Gum:-

·         It is obtained from the hard seeds of the locust bean of carob tree.

·         Only 2 – 3 % of this gum gives print paste of required viscosity.

ð  Tragasol Gum:-

·         It is a special product of locust bean.

·         It has better stability to mild alkalis and acids than locust bean gum.

ð  Gum Karaya:-

·         This gum is obtained from the Karaya tree.

·         It is not much used at present.

(B)  Modified Starches as Thickeners

ð  Dextrin or British Gum:-

·         Dextrin or dark British gum is a degradation product of starch obtained by heating it with mineral acid or roasting at 160°C till it becomes completely soluble in water.

·         Generally 20 – 50 parts of dextrin per 100 parts of printing paste is used.

(C)  Cellulose Derivatives as Thickeners

ð  Methyl Cellulose:-

·         Methyl ether of cellulose is prepared from wood pulp or cotton linters.

·         The ether is insoluble in boiling water and also in presence of alkali but soluble in cold water.

·         It retains its consistency satisfactorily during storage.

ð  Sodium Carboxy Methyl Cellulose (CMC):-

·         It is prepared by reacting alkali cellulose with monochloro acetic acid.

·         This thickener is stable to alkali and hence can be used for printing vat colours.

·         The removal of CMC from the cloth after printing is very easy.

(D) Sodium Alginate

ð  It is a sodium salt of alginic acid which is extracted from seaweed.

ð  Sodium alginate has the following advantages:-

·         It is soluble in cold, warm or hot water

·         Only small amounts of sodium alginate are required in the preparation of printing pastes

·         Due to its wetting power the penetration of the dye in the fabric is superior

·         It can be used with most dyes and is particularly suitable in the printing of reactive dyes

·         Removal of thickener is easy and thus the cloth remains soft after printing.

(E)  Emulsion Thickeners

ð  Emulsion thickeners came into existence with the rapid development of pigment colours.

ð  However, today emulsion thickeners are also employed for printing other classes of dyestuff.

ð  In each system of emulsion there is a dispersed phase and a dispersing medium.

ð  When very stable emulsions are required, emulsification is brought about in the presence of a third component called emulsifier.

ð  For textile printing, oil – in – water type of emulsion is popularly used.

ð  Kerosene – water emulsions have been widely accepted.

ð  Different types of emulsifiers are available such as anionic, cationic and non – ionic.

ð  At present the non – ionic type of emulsifier is preferred and alkyl – phenol – ethylene – oxide condensates have proved most useful.

(F)   Thickeners for Printing Reactive Dyes

ð  In the printing of reactive dyes the choice of a thickener is very important.

ð  Thickeners based on starches cannot be used as reactive dyes react with these thickeners.

ð  Sodium alginate has proved to be the most suitable.

ð  Sometimes emulsion thickeners are also used either alone or in combination with sodium alginate.

ð  Guar gum is another thickener which is sometimes used in printing reactive dyes.

ð  It is generally employed along with either sodium alginate or an emulsion thickener.

ð  Recently, various modified starches and gums have been developed for printing reactive dyes.