17 Auxiliaries for dyeing

S. Karpagam Chinnammal

epgp books

 

 

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. Auxillaries 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 dyeing auxiliaries
  • To learn the function of  dyeing auxiliaries

   3. Basic functions of Auxillaries

  • To develope or improve the substrate for coloration To transform the absorption properties of dyes
  • To stabilize the application medium To protect or transform the substrate To improve the colourfastness of dyeing

4. Dyeing Auxiliaries

 

Dyeing is the process of imparting colour onto a substrate. This imparting of colour is done by use of wet processing techniques. The chemicals added to dyebath that facilitate production of effective and quality dyeing ie required shade and evenness are called dyeing auxillaries. Dyeing auxiliaries provide high dyeing quality, process safety and increased reproducibility on all kinds of substrates and machines, in a variety of processes and with all classes of dyestuffs. There are different dyeing auxillaries. They are classified according to their functions and various uses. Some typical dyeing auxillaries are briefly discussed below

   4.1.Water Softening/Sequestering Agents/Chelating agents

 

These compounds are used basically where hard water is used in dyeing process. The presence of metal salts such as iron, zinc, copper, manganese, tin, aluminium etc will affect the tone and brightness of colour. A very small amount present in the dyebath makes significant effect on dyeing. They react with metal salts in such a waythattheybecomepartofacomplexion.CommonlyusedsofteningagentsareCalgon(sodiumhexametaphosphate)andethylene-diamine-tetraoften abbreviated as EDTA.The formation of complexes is referred to as ‘chelation’ and is manifested by what are called chelating compounds.The sequestering agents are helpful in reducing the effect of metal salts.

Calgon reacts with calcium or magnesium in the following way:

 

Na2[Na2(PO3)6] + Ca++ Na2[Ca(PO3)6] + 2Na++

 

4.2.Exhausting Agents

 

Exhausting agents are important from the point of view of economical dyeing because they facilitate the movement of the dye molecules to the fibre and thereby “exhaust” the dye bath. Electrolytes like Common salt or Glauber’s salt (sodium sulphate) is predominantly used as an exhausting agent in the dyeing of cellulose fibres with direct, azoic, vat, sulphur and reactive dyes.

 

In all these cases, the dyes act as anionic dyes (splitting up into dye anions and sodium or other metal cations in water). When a cellulose fibre is immersed in water it acquires a negative electrical charge called zeta potential. As a result, the negatively charged fibre surface repels the negatively charged anions present in the water .

 

Addition of sodium chloride or sodium sulphate to the dyebath causes release of sodium ions which reduce or neutarlise the negative charges of the fiber surface, as a result reducing the repulsion of the dye anion at the fiber surface. Hence, the approach of the dye anions towards the fibre surface is facilitated. In other words, the fibre takes up more dye in the presence of salt than in its absence. However, the extent of increase in exhaustion brought about by salts varies from dye to dye. Also, with other dye-fibre combinations, other chemicals are used as exhausting agents.

 

The quantities of electrolyte present may vary upto 100g/l depending on depth of shade required, dye structure and recipe.

 

4.3. Dyeing Carriers

 

 The crystalline nature of polyester fibres makes it difficult to obtain dark shades even at high temperature. To overcome this problem carriers or accelerators are used to increase the rates of dyeing of polyester with disperse dyes. It is also possible to dye at lower temperature ie 100 °C and obtain deep shades in the presence of carriers.

 

Carriers are organic compounds- hydrocarbons, phenols, amino acids, amides, alcohols, esters, ketones, nitriles etc. These dyeing assistants when added to the dyebath alter the dispersing properties of the dyes and the physical characteristics of the fibre so that more dye can be transferred from the dye bath to the fibre.

 

There are a number of carriers commonly used for polyester fibres. Some of the good dye carriers are o-phenyl phenol and it’s monochloro derivative, p-phenyl phenol, diphenyl, monomethyl naphthalene, trichlorobenzene, dimethyl terephthalate and methyl salicylate. Other dye carriers in common usage are o-dichlorobenzene, diphenyl ether, n-butyl phthalimide, chloromethoxy ethanol, methyl cresotinate, alkyl and aryl benzoates and tetralin.

 

Depending on the color and shade, the dye carrier used is in a variable range of 2-10% of the textile weight. The use of Carrier at 100°C increases the amount of dye absorbed and decrease staining on other fibers. Carrier normally have unpleasant odour that require the dyebath to be enclosed and they may also be toxic

4.3.1.    Mechanism of Carrier Action

 

The carriers are found to assist the disperse dyes to enter the polyester polymer, enabling dark shades to be produced. The carriers swell the polyester fibres and ultimately cause relaxation. They open up the internal fibre structure ie it increases inter polymer space and let the dyemolecules to enter the polymer system easily and rapidly. Its action may be described as below:

  • It creates dye film on fibre surface.
  • Carrier takes dye inside the fibre from dye carrier association.
  • It increases the solubility of dye in the dye bath.
  • Carriers penetrate inside the fibre polymer chain and thereby reduce inter-chain attraction. Thus polymer chains become movable and so dye molecules may enter the polymer system of fibre.
  • It increases fibre swelling.
  • The absorbed carrier increases the rate of dye uptake by creating liquid co-fibre.
  • It increases the absorbency power of fibre.
  • It lubricates the thermally agitated fibre molecules.

  4.3.2. Advantages of Carrier Dyeing

  •  In the presence of carriers it is possible to obtain medium to dark shades on polyester fibres .
  • Materials can be dyed with simple equipments at atmospheric pressure and temperature below 100°C.
  • Staining of wool while dyeing polyester wool blends can be reduced in the presence of carriers
  • Rate of dyeing is accelerated by use of carriers
  • Improves fastness properties of fabric except light fastness.

4.3.3. Disadvantages of carrier dyeing

  • Carriers add to production cost of dyeing. Firstly, for dyeing it is used which is costly and secondly for its removal alkali is required.
  • Carriers are unhygienic and toxic. It creates skin diseases.
  • Carriers affect the light fastness property of dyed material. This effect may be reduced by treating the material with hot air for 30 min.
  • Some carriers are dye specific. They posses different efficiencies with different dyes; others have compatibility with certain dyes

  4.3.4. Factors Considered For Selecting a Carrier

  • Efficiency.
  • Cost
  • Effect on light fastness.
  • Odor.
  • Toxicity.
  • Discolouration of fibre.
  • Ease of removal after dyeing
  • Stability during dyeing
  • Compatibility with dye
  • Ease of dispersion
  • Even absorption by fibre

  4.4.Leveling Agents

 

While dyeing the desirable end result is to achieve even or uniform dyeing. Many dyes have a tendency to rush on the fibres causing uneven dyeing and shade variation. Levelling or retarding agents when added to dyebath ensure even dyeing by bringing about retardation in the movement of dye into the fibre .

 

The degree of retardation varies from dye to dye. The quantity of the leveling agent must not exceed 0.5 g/l of the dye liquor. Leveling agents are surface active agents and are chemically related to soaps, synthetic detergents and wetting agents.

 

 4.4.1. Types (or) classification of leveling agents:

    Leveling agents are classified into two main groups based on ionization and structure wise

 

4.4.1.1.According to the ionic nature

 

They may be

  • Anionic
  • Cationic
  • Non-ionic organic compounds

    4..4.1.1.1.Anionic agent:

 

The molecules of these compounds are negatively charged .Anions of the dye and anions of the leveling agents compete to get attached to the positive fiber site thus avoiding direct absorption of anionic dyes to the fiber. As the bath is heated the molecules develop sufficient energy to overcome the repulsive forces between the retarder and the dye thereby leading to uniform dyeing.

 

4.4.1.1.2.Cationic agents:

 

The molecules of these compounds are positively charged. The cationic agents when added to the dyebath forms complex with the anionic dye and hence reduces the dye uptake by the fibre. The complex acts as a reservoir of dyes. When heated the energy causes the complex to break down and gradually releases the dye and ensures a uniform absorption by the fiber.

 

Application of leveling agents based on ion type

Levelling agent based on ion type Fiber
Anionic silk, wool, nylon, Bast.
Cationic wool, silk, acrylic, nylon, cellulosic
Non-ionic synthetics & blends

 

4.4.1.2. According to the chemical structure

  • Fatty Acid (Ethylene Urea)
  • Fatty alcohol (Sulphates)
  • Sulphated (Fatty amine)
  • Alkyl aryl (Sulphonates)
  •  Fatty alcohol ethylene (oxide condensate)

4.4.2.    Mechanism of functioning of leveling agents:

 

The two fundamental mechanisms that contribute to level dyeing are

  • Control of exhaustion of dye so that it is taken up evenly
  • Migration of dye after initial unlevel absorption on the fiber.

  i)  The leveling agents possess affinity for the fiber but this affinity is less than that of the dye, but they compete with the dye molecules to enter the fiber. So, lesser amount of dye sites are available for the dye limiting the uptake of the dye by the fiber. As the dyeing proceeds and temperature increases, dye molecules replace the leveling agents at dye sites .So uniform dyeing results.

 

ii)    They form large complexes with the dye molecules at low temperatures in the dye bath. The large-sized dye-agent complex finds it difficult to enter the fibre structure. When the temperature of the dye bath is raised, the complex breaks up, releasing the retarding agent. The dye is now taken up by the fibre.

 

Leveling agent (a) allows the dye to get in to the fiber regardless of initial Unevenness. During the course of further dyeing levelness is achieved through migration. For this selection of dyes for dyeing is of utmost importance or (b) it controls/retards the rate of dyeing throughout the dyeing cycle. Sometimes the leveling agent acts in both the ways.

4.4.3. Limitation of leveling agent:

 

Depth of the shade is reduced by 10 to 15%. Therefore, in case of Leveling agent add 10 to 15% more dye from the recipe.

 

4.5.Dispersing Agents

 

Disperse dyes are insoluble in water and form aqueous dispersion in water. At first these dye molecules are formed as large particles and they are made smaller particles by grinding. But as they are water insoluble, they will give uneven dyeing if they are directly used in dye bath. So to ensure uniform and trouble free dyeing the dye should be present in dye bath in an uniform and very fine form and should give an stable dispersion. This is the reason for which a special chemical is used in dye bath named dispersing agent. Dispersing agents are substances that promote the more or less uniform and stable suspension of relatively small particle in a given matrix. The function of dispersing agent is to prevent agglomeration of individual dye particle during dyeing. Dispersing agents are much preferred for Vat dyeing, Disperse dye and pigment padding. The anionic product ionizes water and form an electrical double layer. The mutual repulsion between these groups keep dye particle apart and prevent agglomeration. Dispersing agents are known to improve the migration and leveling of disperse dye on polyester during HT dyeing. Dispersing agents should be effective under dyeing conditions, stable to hard water, high temperature and other dyeing assistants. Soap powder, Turkey Red oil, alkali sulphates, alkyl aryl sulphonates,sulphated fatty esters etc.are some surface active agents which are recommended as dispersing agents in disperse dyeing.

Dispersing agents are described as sodium salts of any of the following.

 

i) a naphthalene sulphonic acid condensation product

ii) an aromatic sulphonic acid condensation product

iii)highly condensed NS acid

iv) disodium salt of methylene dinaphthalene disulphonic acid, etc.

 

These are available in the form of yellowish brown powder, light beige coloured powder, dark brown mobile liquid, and so on.

 

The powder brand is a non-hygroscopic powder that is,

 

i)  readily soluble in water with any degree of hardness (it is stable to hard water),

ii)   resistant to alkalis, acids and salts and has dispersing and protective colloid properties.

It finds use in the dyeing of vat dyes by the vat-acid process; also in dyeing with disperse dyes, solubilised vat dyes and azoic (naphthol) colours.

 

In the dyeing of polyester and polyamide with disperse dyes, the use of a dispersing agent leads to the production of a fine dispersion of the dye. Because of its non-foaming nature, it is recommended for the high temperature dyeing of piece goods or yarn under pressure

 

4.5.1.    Functions of Dispersing Agent

 

1.       It assists in the process of particle size reduction of dye.

2.       It enables the dye to be formed in powder form.

3.       It facilitates the recon version of the powder into a dispersion which is required for dyeing.

4.       It maintains the dispersion in a fine form in the dye bath through out the process.

5.       It increases the solubility of disperse dyes in water.

 

4.6.       After washing agents

.

After dyeing most dyes are given after treatment with surfactants known as soaping. Along with soaps there are a number on anionic , non ionic and neutral surfactants available for soaping or after treatment. Soaping causes aggregation of dye particles which helps in developing true colour of dye. In reactive dyeing it removes unfixed dye from material and in azoic dye rubbing fastness is improved because soaping removes dye particles not coupled from the fabric surface.

 

4.7 Anti foaming agents

 

Presence of surfactants in dyebath and continuous stirring lead to foaming. Formation of foam in the dyebath is undesirable and a problem to dyers as it leads to uneven or specky dyeing. Foam is suppressed by applying defoaming or deaerating agents.Anti foaming agents are silicon based and are active at low concentration. To suppress the foam for million parts of dye liquor 20-200 parts of agent is enough

 

4.8. Dye fixing agents

 

Cotton dyed with direct dyes have poor washing fastness and also colour bleeds while soaping. To overcome this draw back dye fixing agents are used. They develop a complex between the dye anion and cation agent which improves the washing fastness.

4.9. Migration inhibitors

 

In continuous dyeing methods fabrics are dried after padding and before developing. This type of intermediate drying is carried out in hot air machine and they operate by evaporating water. Initially surface water is removed as vapour and more water is drawn from interior of the fabric. Along with water dyes also migrate to surface and would lack penetration leading to uneven dyeing. To avoid uneven dyeing migration inhibitors can be added to dyebath and carefully dried. Migration inhibitors for vat dyeing are common salt, sodium bisulphate, glaubers salt, sodium acetate and for reactive dyeing migration inhibitors are based on non ionic polyelectrolyte and are superior to common salt or glaubers salt.

 

4.10. Wetting agents

 

A small quantity of wetting agent in the dye bath facilitates the dye liquor to be absorbed uniformly and quickly. The dyes like vat and azoic which are not soluble in water can be pasted with wetting agents such as sulphate oil or sulphated esters. Wetting agents are incorporated in the dyebath, so that the fabric are uniformly wetted with dye solution. They also penetrate to the interior of the yarn. Wetting agents such as anionic and non ionic agents of polyethylene glycol are used in polyester dyeing as the fibres are hydrophobic in nature. By using wetting agents, effective dyeing and excellent exhaustion can be achieved.

 

5.   Auxiliaries used for various dyes 5.6. Direct dyes

 

1.  Salt – reduce negative charges on the fibers

2.  Water – dyeing media

3.  Fixing Agent – enhance wet fastness but usually reduce light fastness

 

5.2. Reactive dyes

 

1.  Salt – reduce negative charges on the fibers

2.  Water – dyeing media.

3.  Soda Ash – excite the dye to link with the fibers with covalent bonding. Therefore, it enhances wet fastness

4.  Fixing Agent – enhances wet fastness for heavy shade but usually reduce light fastness.

 

5.3. Vat dyes

 

1.  Salt – reduce the negative charges on the fibers

2.    Sodium hydroxide – adjust pH to the dyeing bath and make the suitable conditions for reducing agent.

3.  Sodium hydrosulfite – reducing agent for the vat dyes.

4.  Water – dyeing media

5.   Oxidizing agents – (Hydrogen peroxide or Acetic acid or Air) oxidize the soluble to insoluble dye

 

5.4. Disperse dye

 

1.  Dispersing Agent – makes the dye solution stable and disperses in the dye bath.

2.  Acid – adjust pH to the suitable condition for the dye bath.

3.  Carrier – swell the fiber and dissolve the dye to make the dye getting into fiber.

4.  Leveling agent – make more leveling dyeing (some will have adverse effect on slower dyeing)

5.  Water – dyeing media

 

5.5 Acid and basic dye

  • Leveling agent – for levelness dyeing.
  • Retarding agent – for inhibiting the dye not to attach to fibers too fast.
  • Acid – adjust the dyeing condition and make the fibers to show positive charges.(Acid dye)/negative charges(Basic dye)
  • Water – dyeing media
  1. Summary

In this module we got an insight into the dyeing auxiliaries and their role in dyeing. Textile auxiliaries play a vital role in the final outcome of a good quality dyed product.

you can view video on Auxiliaries for dyeing

REFERENCE

 

  1. Hemalatha Jain,Technique of dyeing and printing , Ane Books Pvt.Ltd, Newdelhi 2010. P.17-22
  2. http://www.textiletoday.com.bd/leveling-agents-chemistry-and-performance/
  3. http://www.jofamericanscience.org/journals/am-sci/am0610/15_3028am0610_109_127.pdf
  4. http://textilelearner.blogspot.in/2012/01/carrier-dyeing-method-of-disperse-dye.html
  5. http://textilelearner.blogspot.in/2012/01/dyeing-carriers-some-commercial-name-of.html
  6. http://textileinformation.blogspot.in/2008/04/dye-carriers-are-accelerators-that.html
  7. http://appchemistry.blogspot.in/2012/10/polyester-dyeing-with-carrier.html
  8. http://textilelearner.blogspot.in/2012/01/dispersing-agent-functions-of.html
  9. http://nptel.ac.in/courses/116104046/24.pdf