7 Disperse and Sulphur dyes

R. Sukanya Devi

Introduction

Disperse dyes are anionic in nature and insoluble in water.These dyes are mechanically ground to a very small particle size of approximately0.5 – 1 m. Disperse dyes are applied in the form of fine aqueous dispersion.The fibres most readily coloured by disperse dyes are cellulose acetate, polyester, acrylic and nylon. Cellulose acetate is dyed in the presence of a dispersing agent like soap. Dyeing of polyester, polyamide fibres are carried out at elevated temperature(130⁰C to 200⁰C).Chemically, the disperse dyes belong to various classes such as azo, anthraquinone, methylene, and diphenylamine. The dyes have -OH, -NO,, -CN, halogen and amine groups, but never have any polar groups.

Properties of Disperse Dyes

Though disperse dyes are insoluble in water, their solubility increases to some extent by increasing the temperature.

Since they do not have any polar groups they are insoluble in water Textile materials coloured with disperse dyes have fair to good light fastness. The average light fastness rating is 4 – 5.

The wash fastness properties of the disperse dyes are dependent on the solubility ofthe dye and the nature of fibre to which it is applied. The wash fastness of the disperse dyed textile materials posses moderate to good wash fastness, being 3 – 4 on the average.

Some of the disperse dyes are sensitive to nitrogenous fumes present in the atmosphere, especially that polluted with exhaust fumes from automobiles.

Disperse dyes have the ability to undergo sublimation, that is, they can be vaporized without significant change in their colour. Their ability to sublime is the result of a stable electron arrangement. This property is used to advantage in transfer printing.

Excessive hot pressing or ironing of disperse dyed or printed textiles may result in colour loss.
The dyes have low molecular weight in the range of 400 – 600.
They are available in the form of powder, liquid and micro-dispersed granules.

Classification of disperse dyes

• Based on dyeing properties and sublimation Class A to Class D

Class A – These dyes have low relative molecular mass and poor sublimation fastness but excellent dyeing properties

Class D- These dyes have high relative molecular mass and excellent sublimation fastness but poor dyeing properties

Class B and Class C – Properties in between two extremes

• Based on the chromogen Azo

Anthraquinone Methene Nitrodiphenylamine Xanthene and others

Mechanism of Dyeing

The dyeing mechanism involves three steps
Dispersion and Solubility
Adsorption
Fixation

Theory of Dyeing

The first step:

Disperse dyes are generally as insoluble, their dissolution in the dye bath depends on their dispersability and solubility. Dispersability is aided by the presence of dispersing agents and the solubility by the rise in temperature.

The second step:

This relates to the adsorption of the dye molecules on to the fibre surface. Dyestuff adsorption by the fibre surface is influenced by two characteristics, the solubility of the dye in the dye bath and that in the fibre.

The third step:

After adsorption, the next step in a dyeing process is penetration of the dyestuff molecules into the fibre structure. The overall rate of dyeing is governed by such penetration or diffusion of the dye from the fibre surface into the fibre. Under the normal dyeing conditions of time and temperature the rate of adsorption is always higher than the rate of diffusion and it is therefore the third step that governs the overall rate of dyeing.If the amount of dye present in the dye bath is not sufficient to saturate the fibre in the dyeing process, only the first two equilibria are established. If the fibre is saturated, the third equilibrium is attained and the dyeing is completed. Thus the aqueous solubility of the dye and the diffusion of the dye in the fibre are the two factors governing the dyeing process that are more important.

Auxiliaries Used in Polyester Dyeing

i) Dispersing agent

ii) Leveling agent

iii) Antifoaming agent

iv) Wetting agent

v) pH and redox buffers

Methods of Dyeing Polyester with Disperse Dyes

Polyester is difficult to dye because of a high degree of orientation and high cohesive forces and compact structure.

The dyeing methods are divided into exhaust methods and continuous methods.

i) Exhaust method

This method involves dye transfer from aqueous baths into the fibre. Exhaust dyeing is carried out at boil using carriers and high temperature conditions. Woven, knitted and texturised polyester fabrics are dyed by these method.

ii) Continuous method

The dye is mechanically fixed on to the fibre and then made to diffuse into the fibre by thermal treatment called “thermosoiling”, where the dye is applied to the fibre mechanically and then thermofixed at 200 – 220°C, when the fibre become thermoplastic and the dye diffuses into the fibre.

In thermofixation dyeing, the water in the pad liquor is completely removed by drying before the dye is fixed in the fibre.

Polyester fibre may be dyed by the following methods.

Carrier Method

Principle of dyeing

Certain chemicals called ‘carriers’ are added to the dye bath to act as swelling agents for polyester fibres which swells the fibre at boil temperature and enables the molecules to move easily.

This increases the inter-molecular distances so that there is more space to accommodate the dyestuff molecules.

The swollen fibres permit large dye molecules to diffuse more rapidly into the fibre. Some chemicals that have been used as carriers are:

o-phenylphenol; p-phenylphenol; dichlorobenzene, trichlorobenzene; DMT, etc.
Carriers enable dyeing to be carried out at boiling temperature under normal atmospheric pressure.
With regard to polyester, the dyeing temperature must be at about 130°C where its Tgis around90°C.
But the Carriers are swelling agents and openup the fibre structure, and thus lowers the Tg such that dyeing can be doneatlOO°C.
The carrier is usually added first to the dye bath or it is added slowly during the dyeing process.

Method of dyeing polyester using carriers

Bath Preparation : The bath is set with an anionic wetting agent, a dispersing agent, a carrier and acetic acid to adjust the pH at 5.5 – 6.5.

Dyeing

The fabric is run in the blank bath with carrier for 30 minutes at 60°C and then previously dispersed dye is added to the bath after filtration.
The dispersion is added in two lots at 60°C.
The temperature is then slowly raised to the boil and dyeing continued for 90 minutes.

After the dyeing: The bath is run off and the fabric is treated with boiling water or scoured with 1 g/1 non-ionic detergent to remove loosely-held carrier.

Reduction Clearing: The dyed material is then given a reduction-clearing treatment to remove the unfixed colour from the fabric. This is with

Caustic soda –	2 gpl
Sodium hydrosulphite –	2gpl
Temperature –	60°C
Time-	30 minutes

Washing: The fabric is now washed thoroughly with hot and cold water.

Removal of Carrier: A heat treatment is given at about 150 – 180°C for 30 – 60 seconds, depending upon the type of carrier used.

Recipe
Disperse dye	-X%
Anionic wetting agent	-0.1 to 0.5%
Dispersing agent	-0.1 to 0.5%
Carrier	-Y%
Acetic acid	– 0.1 to 0.5% (pH = 5.5 – 6.5)

High Temperature High Pressure (HTHP) Dyeing Method Principle of HTHP dyeing

In this method the polyester fibre is dyed at high temperature (above 100°C) and high pressure in an aqueous bath.

The HTHP conditions help to bring the fibre to a temperature well above the Tg of the fibre, so excellent diffusion can take place and good dyeing results are obtained.

Types of HTHP dyeing machines

Pressure jigger
Pressure winch
Package dyeing machine
Beam dyeing machine
Jet / Soft flow dyeing machine.

Dyeing of polyester fabric using high temperature dyeing method Batching High pressure high temperature jet dyeing machine

The jet dyeing machine is an extension of the HPHT winch dyeing machine. It is looked upon as a welcome improvement over the winch machine, which has many disadvantages.

Four forms of energy are involved in jet dyeing machines. These are,

1. Chemical

2. Thermal

3. Mechanical (fabric transport)

4. Fluid (movement of the dye liquor).

Types of jet dyeing machine

Jet dyeing machines with different shapes and sizes of dyeing vessels are available. The machines are either vertical, horizontal or inclined.
They are also classified as partially flooded, fully flooded and soft overflow type machines.
The jet dyeing machines can usually be operated up to 140°C under high pressure and have capacities capable of dyeing 100 to 150 kg of fabric at a time.

Soft flow machines

In order to overcome the problem of a conventional jet dyeing machine, new horizontal jet dyeing machines based on soft flow systems have been designed to minimise undue tension on the fabric and in which the fabric follows the path similar to that in a conventional winch.

Various developments are taking place in jet dyeing machines to meet the requirement of the dyer for defective-free dyeing.

Air flow jet dyeing machine

This is based on the aerodynamic system where the fabric is propelled by means of an air system, to which finely disperse dyes and auxilliary products are fed in the jet systems. Higher fabric speed can be used in gas flow system with shorter dyeing time and low liquor ratios.

Thermosol Dyeing Technique Principle

The Du Pont thermosol process is a continuous process in which dry heat is utilized effectively in the dyeing of synthetic textiles.

It involves padding a fabric through a dispersion of the dye drying and heating for about one minute at 200°C.

At this temperature many disperse dyes are molten whilst the fibre is plastic with its chain molecules vibrating vigorously.

Thus, the dyeing consists essentially of a liquid dye dissolving in a fibre, which itself is like a ‘viscous liquid’; hence the name “thermosol process”.

Basically, the mechanism of thermosol fixation of the dye is the same as for exhaust dyeing.

The dye is dissolved directly in the fibre when exposed to temperatures of the order of 205°C. Complete penetration is obtained in 60 seconds.

Sulphur Dyes

Introduction

Sulphur dyes being relatively inexpensive are widely used for dyeing of cellulosic fibres and their blends especially with polyester/cotton yarn is dyed in- a package or hank dyeing machine. Sulphur dyes are widely used for black, blue, maroon, olive and green colours in medium to heavy depths.

Fastness properties of sulphur dyes vary markedly throughout the range, e.g. light fastness of yellow dyes is almost 3 whereas black is about 7″. Wash fastness is generally good. Fastness to bleaching is poor except in the case of a few dyes. These dyes are available in powder, pre-reduced powder, grains, paste and liquid form.

Properties of Sulphur Dyes

Sulphur dyes are insoluble in water.
They are soluble in a solution of sodium sulphide to which sodium carbonate may or may not be added.
The sodium sulphide acts as a reducing agent, severing the sulphur -linkage and breaking down the molecules into simpler components, which are soluble in water.
The wash fastness is good (3 – 5) and the light-fastness is satisfactory (3 – 7).These dyes have poor fastness to chlorine.

Classification of Sulphur Dyes

The sulphur dyes can be classified into the following groups

1. Sulphur dyes

2. Leuco sulphur dyes

3. Solubilised sulphur dyes

4. Condensed sulphur dyes

Sulphur dyes

These are water insoluble dyes, normally applied in alkaline-reduced (leuco) form. Sodium sulphide is normally used as reducing agent, and after dyeing, the dye is oxidised back into its original insoluble form in the fibre.

Leuco sulphur dyes

These are powders or liquid-based dyes containing the soluble leuco form of the parent dye. A reducing agent, usually sodium sulphide or hydro sulphite, is used insufficient quantity to make the dye suitable for application either directly or with the addition of only small amount of extra reducing agent.

Solubilised sulphur dyes

These are thiosulphuric acid derivatives of the parent dyes, and are non-substantive to cellulose but are converted to substantive alkali soluble thiol form during dyeing.

Condensed sulphur dyes

These are sodium-S-alkyl or S-aryl thiosulphates. They do require sodium sulphide for dyeing, but conventional sulphur dyeing methods are unsuitable for them.

Application of Sulphur Dyes to Cotton

The dyestuff (usually 5 to 25% owf) is pasted with soda ash and sodium sulphide. The quantity of soda ash is equal to the amount of dyestuff taken and the sodium sulphide is between half and twice the weight of the dye. The following table shows the quantities of sodium sulphide required for some sulphur dyestuffs.

Dye
Sulphur Yellow
Sulphur Browns
Sulphur Red
Sulphur Blues
Weight of sodium sulphide required
1 –l1/2 times the weight of the dye
1/2the weight of the dye
3/4the weight of the dye
1/2–3/4 the weight of the dye

Add sufficient quantity of hot water to the paste and if necessary, boil it; then filter the dye solution to remove insoluble impurities.

Dyeing

Maintain the temperature of the bath at the boil. Enter the goods in the dye liquor and add common salt or Glauber’s salt (from 10 to 50% owf). The amount will vary according to the depth of shade. The salt should be added in portions while keeping the bath temperature at the boil.

Dissolving sulphur dyes

The dyestuff is pasted with soda ash and sodium sulphide. 2 g/l of soda ash willnormally be adequate. The quantity of sodium sulphide is usually twice the weight ofthe dye. Boiling water is then added to the paste until the dye dissolves. Strain the dissolved dye before it is added to the dye bath because the sulphur dyes are liable to contain more insoluble impurities than other classes of dyes.

Dyeing

The dye bath is made up with dyestuff and 5 to 25 percent of the weight of the goods of common salt or 10 to 50 percent of crystalline Glauber’s salt, the actual amount varying according to depth of shade.

The salt may be added at the commencement, but if there is any risk of uneven dyeing it is preferable to add it after the temperature has reached 100°C, and that too in several portions.

The addition of a surface-active penetrating agent is recommended.

With most sulphur dyes the liquor is brought to the boil and dyeing continued in a cooling liquor for a further half an hour. There are some sulphur dyes, however, which exhaust best at 70 to 75°C.The sulphur dyes do not give very good exhaustion, especially in heavy shades.

A machine with a low liquor ratio is therefore preferable. It is common practice to use a standing bath for the black shade.Since the reduced dye readily oxidises in the presence of air it is desirable that during application the goods are totally immersed during dyeing.

It is also important that the excess liquor should be squeezed out immediately and rinsed after dyeing, to prevent deposition of the insoluble product of oxidation on the surface of the fibres.

Oxidation step

Since some sulphur dyes oxidise slowly, it is better to wait for the true shade to develop. The procedure is to run them, after rinsing, in a solution sodium perborateof 0.5 to 1 g/1 concentration, at 40 to 50°C for 20 minutes.

Soaping

The process is important for removing superficial dyestuff and traces of free sulphur from the fabric.
The washing and rubbing fastness properties of the dyed goods are also improved.
Soaping is carried out in a bath containing 2 – 4% soap solution at the boil for 20 – 30 minutes. The goods are washed thoroughly with hot and cold water respectively.
The washing is carried out in running water so as to ensure maximum removal of the undesirable superficial products.

After-treatment

The washing and light fastness, and the brilliance of shade for some of the sulphurdyes can be improved by using one of the following after-treatments.

i) Treatment with metallic salts e.g. dichromates and copper sulphate either alone or in combination.
ii) Topping with basic dyes.

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REFERENCES and URLs

Clark M, Handbook of textile and industrial dyeing: principles, processes and types of dyes, wood head Publishing, 2011
Richard Aspland J, Textile Dyeing and Coloration, AATCC, 1997, ISBN 0-9613350-1-7
Arthur D Broadbent, Basic Principles of Textile Coloration, Society of Dyers and Colourists, 2001
Wilfred Ingamells, Colour for textiles A user’s handbook, Society of Dyers and Colourists,1993
Shenai V A, Technology of Textile Processing : Technology of Dyeing, Sevak Publications, 1985