DYEING AUXILIARIES

ð  Dyeing is the process of adding colour to textile products like fibres, yarns, and fabrics.

ð  Dyeing is normally done in a special solution containing dyes and particular chemical material.

ð  The temperature and time controlling are two key factors in dyeing.

ð  In order to get satisfactory results in dyeing, textile auxiliaries are invariably employed.

ð  The use of particular auxiliary in dyeing will depend upon the type of dyestuff and also on the type of fibre.

ð  The important auxiliaries used in dyeing can be broadly grouped into the following classes:-

(a)    Wetting and penetrating agents.

(b)   Dispersing agents.

(c)    Levelling agents.

(d)   Sequestering agents.

(e)    Antifoaming agents.

(f)    Accelerators.

(g)   Migration Inhibitors.

(h)   Dye fixing agents.

(i)     After-washing agents.

(j)     Stripping agents.

Wetting and Penetrating Agents

ð  Wetting agents are added to the dye bath to ensure that the entering goods are thoroughly and uniformly wetted with the dye solution.

ð  In this capacity they are often referred to as Penetrating agents, since they cause the dye solution to penetrate into the interior of the yarn.

ð  It should be noted that they have little effect on the rate at which dye molecules migrate into the fibre substance.

ð  Wetting agents reduce the surface tension of water by adsorbing at the liquid-gas interface.

ð  They also reduce the interfacial tension between oil and water by adsorbing at the liquid-liquid interface. Many surfactants can also assemble in the bulk solution into aggregates.

ð  Examples of such aggregates are vesicles and micelles. The concentration at which wetting agent begin to form micelle is known as the critical micelle concentration (CMC).

ð  When micelles form in water, their tails form a core that can encapsulate an oil droplet, and their (ionic/polar) heads form an outer shell that maintains favourable contact with water.

ð  When wetting agents assemble in oil, the aggregate is referred to as a reverse micelle.

ð  In a reverse micelle, the heads are in the core and the tails maintain favourable contact with oil.

ð  In the preparation of solutions of naphthols and vat dyes wetting agents are used for pasting the dyestuff.

ð  Sulphated oils have proved to be satisfactory wetting agents in the application of vat and azoic dyes by padding techniques.

ð  The wetting agents help to prevent dusting off of the materials. The wetting power further increased by adding electrolytes.

ð  The dyeing of polyester and polyester/cellulosic blends with disperse dyes, the use of wetting agents is desirable, since polyester fibres being hydrophobic are difficult to wet.

ð  Anionic as well as certain non-ionic wetting agents of polyethylene glycol type are effective in the dyeing methods wherein exhaustion takes place.

ð  In the continuous thermofixation dyeing of polyester and P/C blends, the selection of a proper wetting agent is of the utmost importance.

ð  Anionic highly sulphonated oils such as ‘Calsolene oil HSI” have been found to be efficient wetting agents, while non-ionic wetting agents are less satisfactory.

ð  Non-ionic gives less colour yield because of the formation of complexes with the dye, which have a lower rate of vapourisation as compared to the dye alone.

ð  Sulphated Vegetables Oils:- Sulphated castor oil (Turkey Red Oil) is a good wetting agent. Its main constituent is sulphated rincinoleate.

 Chemical Formula of Turkey Red Oil

ð  Sulphated Fatty Acid Esters:- Sulphated methyl oleate (or butyl oleate) is also a good anionic wetting agent.

ð  The most powerful wetting agent is Dioctyl sulphosuccinate (Octyl being obtained by 2 – ethyl hexanol).

Me = Methyl (CH₃)

Chemical Formula of Dioctyl Sulphosuccinate

ð  Fatty acid amides and their derivatives, Alkyl phenol condensates are also good wetting agents.

Dispersing Agents

ð  The function of a dispersing agent is to prevent agglomeration of individual dye particles during dyeing.

ð  Dispersing agents enhance the dispersion and ensure a fine particle size. It enhances the processes, by effectively dispersing finely divided solids and liquids in aqueous environment.

ð  The nature of dispersing agent depends upon the properties desired and class of the dyes. It can be of both high and low molecular weights.

ð  Today, not only do these dispersing agents have a strong dispersing effect, but also exhibit very high temperature stability throughout the dyeing cycle.

ð  Dispersing agents are of particular interest in the application of vat dyes by vat acid and pigment padding methods.

ð  This product is variously described as sodium salt of a naphthalene sulphonic acid condensation product (Setamol WS of BASF India Ltd.), aromatic sulphonic acid condensation product (Uniperse P), disodium salt of methylene dinaphthalene disulphonic acid (Dadamol V).

Chemical Formula of Dadamol V

ð  These are available in the form of yellowish brown powder, light beige coloured powder or dark brown mobile liquid.

ð  The powder brand is a non-hygroscopic powder that is readily soluble in water with any degree of hardness.

ð  It is resistant to alkalis, acids and salts and has dispersing and protective colloid properties. It is not a surface active agent and hence has no wetting, foaming and detergent properties.

ð  Being anionic in nature, it is compatible with anionic and non-ionic products. When mixed with cationic products, it may form precipitates.

ð  A 10 % solution of the powder has a pH of 6.5 to 7.5 and has good storage stability.

ð  It finds use in the dyeing of vat, disperse, solubilised vat and azoic colours.

ð  Disperse dyes are characterised by low solubility in water and in order to ensure application of these dyes from aqueous liquors dispersing agents are added to the dyes as they are marketed.

ð  In addition to this, it is the usual practice to add some dispersing agent to the dye bath.

ð  Because of its non-foaming nature, it is recommended for high temperature dyeing.

ð  Anionic agents such as sodium dinaphthylmethane sulphonates and lingo sulphonates are generally used as dispersing agent in polyester dyeing. 

ð  When added to the developing baths in azoic dyeing, the dispersing agent increase the clarity of these baths and yields dyeing of improved rubbing fastness.

Levelling Agents

ð  The levelness of a dyeing is generally governed by two properties of the dye.

(a)    The exhaustion behaviour

(b)   The levelling out capacity (migration power)

ð  One of the important objectives in dyeing is to secure level or uniform dyeing.

ð  Many dyestuffs have a high initial rate of dyeing and show a tendency to rush on to the fibre, causing uneven dyeing.

ð  Levelling agents are, therefore, added to the dye bath to regulate the process of dyeing and to get uniform results.

ð  A levelling agent therefore act on the basis of the following two methods:-

(a)    Allow the dye to go into the fibre, regardless of the initial unlevelness. During the course of the further dyeing process, the unlevelness is eliminated by migration.

(b)   Control the rate of strike of the dye from the beginning so that it does not go on to the fibre too rapidly.

ð  However, during actual dyeing a levelling agent generally acts in both ways. These agents are generally used in dyeing polyester with disperse dye and cellulosics with vat dyes.

ð  In the dyeing of wool with neutral dyeing acid dyes, it is usually difficult to obtain even dyeing.

ð  For a long time anionic surfactant such as fatty alcohols sulphates, fatty amide sulponates and alkyl aryl sulphonates were used as levelling agents.

ð  These agents compete with the anionic dyestuffs for the available sites in the fibre and thus procedure uniform dyeing.

ð  Fatty alcohol – ethylene oxide condensates like cetyl alcohol – ethylene oxide, available in the liquid form are useful as levelling agents in vat dyeing where the dyeing rate is very large, as in the case of IN class of vat dyes.

ð  Anionic products like dodecylbenzene sulphonates (Na), sulphated fatty acid esters of lower alcohols, Turkey Red Oil, fatty alcohol sulphates, etc. may be used as levelling agents in dyeing acid dyes on nylon.

ð  They being anionic in nature compete with the dye anions for the cationic sites in the fibre and thereby reduce the effective concentration of the dye anions in the dye bath.

ð  However, at the later stages of dyeing, especially at higher temperatures, the levelling  

      agents anions are replaced by the dye anions and good colour value is obtained.

ð  Cationic retarders, like Quaternary ammonium compounds generally described as higher alkyl trimethyl ammonium chloride are useful as levelling agents in dyeing cationic dyes acrylic fibres.

ð  In the dyeing of cotton by direct dye levelling agents are added which promote levelling by breaking the dye aggregates which migrate slowly.

ð  Cationic surfactants and non-ionic ethylene oxide condensates levelling agents are used for vat dye.

ð  The non-ionic levelling agents are more widely used today. These products from colloidal aggregates with part of the leuco vat dye present in the dye liquor and thus slow down the dyeing process.

ð  Glue as well as lignin sulphonate products prepared from cellulose sulphite waste liquors, a by product in the paper – making industry, are also sometimes used.

ð  The levelling agent has to be chosen with reference to the dye used.

Sequestering Agents

ð  The presence of metals as salts of iron, copper, zinc, manganese, tin, aluminium etc., in the dye bath is highly undesirable as it adversely affects the tone and brightness of the colour.

ð  A very small amount of these metals can cause appreciable effect on dyeing. For example, little as one part of iron or aluminium in 10 million parts of water cause a detectable shade change in certain metallised dyes.

ð  Sequestering agents are useful in any textile operation in which metallic impurities generally present in hard water interfere in the processing.

ð  The functions of sequestering agents are

(a)    Combine with metallic impurities like calcium, magnesium and other heavy metal ions in hard water.

(b)   They form molecules in which the ions are held so securely (sequestered) that they can no longer react.

(c)    The sequestering agents prevent salts from recontaminating parts.

(d)   The sequestering agents may also tie up the active chemicals in a detergent that may decrease the cleaning efficiency and life of a wash bath.

ð  Common sequestering agents are orthophosphate, orthosilicate and phosphates. These are available in powder form.

ð  Ideal sequestering agent is H.E.D.P. (Hydroxyehtylene phosphonic acid), as the chelation value does not drop at scouring temperature, while the worst performer is citric acid.

Chemical Formula of EDTA

ð  The most effective sequestering agent in dyeing is ethylene diamine tetraacetic acid (EDTA).

ð  Calgon which is sodium hexa – metaphosphate is often used as a sequestering agent.

Antifoaming Agents

The use of wetting agents in textile processing liquors, coupled with the rapid movement either fabric or liquor causing extensive agitation results in the formation of foam or froth.

Chemical formula of Trimethyl Cyclohexanol

ð  This creates problems in the application of dyes by dyeing, specifically; high speed padding mangles are prone to foaming.

ð  In order to overcome this problem, antifoaming agents are added to these liquors.

ð  In early days, benzene, pyridine, and turpentine were used as antifoaming agents. However, their efficiency was found to be inadequate.

ð  Most of antifoaming agents which are more efficient are based on silicones.

ð  Dimethyl polysiloxane, trimethyl polysiloxane are used as antifoaming agents.

ð  Yet another new type of antifoaming agents is based on polyaminoalkyl substituted organo polysiloxanes in which there are 5-10 amino substituted hydrocarbon radicals distributed over each 100 silicon atoms in the chain molecules.

ð  Fatty alcohol and lower alcohols, cyclohexanols etc., are used as antifoaming agents.

ð  n – Octanol, 2 – ethyl – hexanol, n - Octyl alcohol, n – Decanol, lauryl alcohol, oleyl alcohol, trimethyl cyclohexanol are most efficient antifoaming agents.

Accelerators or Carriers

ð  Carriers are substances which accelerate the rate of dyeing of disperse dyes on polyester fibre materials when dyed at boil at atmospheric pressure.

ð  They alter the dispersing properties of the dyes and the physical characteristics of the fibre so that more of the dyestuff can be transferred from the dye bath to the fibre than in the absence of the carriers.

ð  An ideal carrier should be :-

(a)    Availability at low cost.

(b)   Sufficiently effective in increasing rate of dyeing.

(c)    Absence of unpleasant odour.

(d)   Non – toxic.

(e)    Easily removable after dyeing.

(f)    Compatible with dyestuffs.

(g)   Low volatility

(h)   Unlikely to cause any undue shrinkage of the material.

(i)     Little or no effect on the fastness of the final dyeing.

(j)     Preferably biodegradable.

ð  Carriers are mainly based on phenyl phenol (ortho and para) and di – or trichlorobenzenes.

ð  Orthophenyl phenol may be used in the concentration range of 3 to 6 gpl. The free phenol is insoluble in water and difficult to disperse in water.

ð  To overcome this defect, its sodium salt is used, but has no carrier action. The free phenol should be liberated in the dye bath by the addition of acid – liberating agent.

ð  This carrier adversely affects the light fastness of the final dyeing if not completely removed from the dyeing.

ð  Di – and trichlorobenzenes are good carriers which promote dye absorption by the fibre and the residual carriers do not affect the fastness of the final dyeing.

ð  These water insoluble carriers have to emulsify in the dye bath using an emulsifying agent such as anionic detergent (alkyl benzene sulphonate).

ð  These carriers are toxic in nature and their use involves use of enclosed dyeing machine.

ð  Monomethyl naphthalene has less powerful carrier action and is suitable only for dyeing rapidly diffusing dyes. It is readily biodegradable.

ð  Dimethyl ester or Terephthalic acid possesses many desirable properties of an ideal carrier but it has only moderate odour and it is costly.

ð  Methyl cresotinate is an efficient carrier and has no adverse effect on the light fastness of the dyes. It is however expensive and volatile.

ð  Butyl benzoate is only moderate efficient in increasing the uptake of disperse dyes. It is employed for promoting levelling in the high temperature dyeing of polyester. It has a powerful odour.

ð  Methyldichlorophenoxy acetate is a very efficient carrier with little odour, can be easily removed from the dyed material. It is however very expensive.

Me = Methyl

Chemical Formula of Methyldichlorophenoxy acetate

ð  N – Alkylphthalimide derivative has good efficiency, little odour and low toxicity. Due to its ease of removal, it is especially useful in garment dyeing.

Migration Inhibitor

ð  Today, continuous dyeing of textile fabrics are becoming more popular compare to batch methods due to higher rate of production.

ð  Continuous dyeing methods require fabric to be dried after padding and before developing. This step is known as intermediate drying.

ð  Intermediate drying is carried out on hot air machine, or drying cans. These machines for drying operate by evaporating water from the fabric surface.

ð  During the passage of the fabric through the machine, the surface water is removed as vapour and more water is drawn from the interior to the surface to restore equilibrium between liquid and vapour.

ð  Migration of the dye particles to the surface may occur as a result of this movement of water and lead to lack of penetration.

ð  Also differences in the rate of drying of the two faces of a fabric would give a two – sided appearance because of unequal amount of dye, on the two surfaces.

ð  This difficulty of uneven dyeing due to migration can be avoided by the use of migration inhibitors and also by careful drying.

ð  In the dyeing of vat dyes by the continuous pigment pad method migration inhibitors such as common salt, glauber’s salt, sodium bisulphite, sodium acetate and various thickeners such as gum tragacanth, sodium alginate are added to the padding liquor.

ð  Sodium alginate, when neutral and very carefully prepared in concentration up to 1 g/l is probably the most satisfactory. Sometimes carboxy methyl cellulose is employed.

ð  ICI has recommended migration inhibitor V, which is said to be an organic anionic polyelectrolyte. It produces level results and better colour yield.

ð  Reactive dyes are widely applied on cellulosic fabrics by continuous methods. In this case migration inhibitors recommended are based on non – ionic polyelectrolytes.

ð  In the thermosol process of applying disperse dyes to polyester/cellulosic fabrics, it is absolutely necessary to have even distribution of the dyestuff on the fabric prior to thermofixation, so migration inhibitors are added to the padding liquor to prevent migration of dyestuffs during drying.

ð  An inhibitor to be suitable for thermosol process, should have the following properties:-

(a)    The solid content of the additive should be as low as possible.

(b)   It should not affect the solubility or dispersion of the dyestuff.

(c)    It should prevent migration but should not hinder the diffusion of dye into fibre.

(d)   It should be resistant to high temperature.

(e)    It should be easily removable after dyeing.

Dye Fixing Agents

ð  The main drawback of direct cotton dyes is that their fastness to washing and soaping is poor.

ð  Attempts have been made to develop some after-treatments which can improve the fastness properties of these dyes on cellulosic fibres.

ð  Cationic surface active agents such as Fixanol C, Sandofix, Lyofix DF, Sapamines and Hicofix GL are commonly used for this purpose.

ð  This after-treatment improves the fastness to water, acids and alkalis, but the treated dyeing shows little or no improvement in solution of soaps or detergent.

ð  It is known that these products function by forming a complex between the dye anion and the cation of the agent.

ð  The complex being less soluble than the dye anion shows increased wet fastness.

ð  This complex, however, is broken down by anionic surface active agents leading to poor fastness to soaping.

ð  During the dyeing of cotton with reactive dyes, two reactions proceed simultaneously under alkaline conditions.

ð  One is the reaction of the dye with hydroxyl groups of cellulose, leading to the fixation of dyestuffs, and the other is the reaction with hydroxyl groups of water leading to the formation of the hydrolysed dye.

ð  This hydrolysed dye should be completely removed by soaping, as otherwise wash fastness properties will be poor.

ð  An after-treatment with cationic agents such as Fixanol PN, has been suggested to increase the fastness of the hydrolysed dye.

ð  In this case, the colour yield is also increased because the hydrolysed dye is attached to the fibre by the cationic agent.

ð  Recently, an after-treatment with a base containing at least one primary or secondary amino group e.g. diethanolamine has been also suggested for this purpose.

After Washing Agents

ð  On completion of dyeing, with most dyes it is usual to give an after-treatment with a surfactant which is termed soaping, since soap was used for this purpose.

ð  Today along with soap, a number of anionic and non – ionic surfactants are available for this purpose.

ð  In the dyeing of cotton with vat dyes, soaping causes the aggregation of dye particles, helps in developing true shades and removes surface dyestuff from the fibre.

ð  Soaping is very important in azoic dyeing as it helps to improve the rubbing fastness.

ð  In case of the reactive dyes, soaping is important in order to remove unfixed hydrolysed dye from the material and to obtain dyeing with satisfactory fastness properties.

Stripping Agents

ð  It is sometimes necessary to remove the dye from dyed material in order to correct the faulty dyeing. This process is called stripping.

ð  The primary requirements of a successful stripping methods are:-

(a)    The process must effectively remove the dyestuff

(b)   The fibre must remain substantially unimpaired so that it can be redyed to a saleable item

(c)    The cost must be low.

ð  Stripping of direct dye on cotton is simple and consists essentially of treatment of the material with either alkaline hydrosulphite or sodium chlorite.

ð  Stripping of vat dyes is difficult. An effective stripping agent for vat dyes must remove the leuco compound as it is formed and leach it out into the bath, thus preventing a re-adsorption on the fabric.

ð  Cationactive, long chain quaternary ammonium salts such as Lissolamine A and Lissolamine V are found to be effective in stripping of vat dyes.

ð  A new agent based on polyvinyl pyrrolidone sold under the trade name Albigen A is recommended for stripping vat, sulphur and direct dyes.

ð  Stripping of vat dyes is also achieved by using alkali and formamidine sulphinic acid, prepared by the action of hydrogen peroxide on thiourea at near boiling point.

ð  In the stripping of azoic dye, certain quaternary ammonium compounds such as cetyl trimethyl ammonium bromide in presence of alkaline reducing agent have been found very effective.

ð  Acid dyes can be stripped from wool by treatment with acidified sodium sulphoxylate formaldehyde, often in presence of a little formaldehyde.

ð  Disperse dyes from nylon are stripped by using either a mixture of non – ionic detergent and polyvinyl pyrrolidone or sulphoxylate formaldehyde activated by acetic or formic acid in presence of benzyl alcohol.

ð  The acid dye from nylon is stripped by using a mixture of trisodium phosphate and non – ionic detergent, often in presence of benzyl alcohol.

ð  Stripping of disperse dyes from polyester is extremely difficult. Treatment with carrier such as chlorinated benzene or salicylic acid is useful for partial stripping.

ð  For stripping deep shades on heat-set polyester, the following method is recommended:- treat with carrier, zinc sulphoxylate and acetic acid at boil, for 60 minutes, followed by a wash and another treatment with carrier sodium chlorite and oxalic acid at boil for 45 minutes.