4 Acid and basic dyes

R. Sukanya Devi

Introduction

Most of the dyes used in the textile industry are organic synthetic colourants and very less amount of colourants obtained from the natural extraction, derived from vegetable and animal sources. Dyes are classified based on their ionic character, chemical constitution or application method. Specific dyes are used for specific fabrics based on their adsorption and interaction between the dye and the fibres.

Acid dyes

Acid dyes are a class of dyes used to dye wool, silk and nylon fibres. Acid dyes consist of the sodium salts of sulphonic acids. The acid dyes can be applied easily on the textile material without the application of mordant that are used as fixing agents. The acid dyes are rarely used for the dyeing of the cellulosic fibres, when usedfor this purpose, a basic mordant such as alum is required. Nitrophenols also belong to aciddyes.

Acid dyes are water soluble, and have better fastness to light than basic dyes.The acid dyes are widely used for natural protein fibers such as silk, wool and synthetic polyamides and modified acid dyeable acrylics. They contain sulphonic acid groups, which are usually present as sodium salts of sulphonate acids. These acid groups increases solubility in water, and gives the dye molecules a negative charge. In an acidic solution, the -NH2 functionalities in the fibres are protonated to give a positive charge -NH3+. Thesecharges interact with the negative dye charge of the dyes, allowing the formation of ionic interactions. Van-der-Waals bonds, dipolar bonds and hydrogen bonds are also formed between dye and fibre apart from ionic bonding.

Classification of acid dyes

Acid dyes are classified based on the following

1. Chemical structure
2. Levelling properties
3. Dyeing characteristics

Acid dyes are normally have large aromatic molecules and have very complex structure, having a sulphonyl or amino group which makes them soluble in water. Most of the acid dyes belongs to following three main structural molecules,

1.       Anthraquinon type

2.       Azo dye type

3.       Triphenylmethane type.

Classification based on the levelling properties

The acid dyes are classified into several groups , based on the leveling properties, economy of the dyeing and fastness properties, however generally these are classified into these three classes,

1. Neutral acid dyes:-

Neutral acid dyes are supra milling or fast acid dyes, many of these dyes are used for self shades only. These dyes are applied primarily with a weak acid or at neutral pH on the fabrics. These dyes possess medium to good wet fastness properties however , some dyes exhibit poor light fastness properties in pale shades.

2. Weak acid dyes

Weak acid dyes belongs to the gategory of milling class of dyes. These dyes possess good wet fastness properties but moderate to poorlight fastness

3. Strong acid dyes

As the name implies these dyes are applied in a strong acidic medium and also called as leveling dyes, however has a limited wet fastness property.These dyes are very good to produce the combination shades.

Classification according to dyeing characteristics

Acid dyes are commonly classified according to their dyeing behaviour, especially in relation to the dyeing pH, their migration ability during dyeing and their washing fastness. The molecular weight and the degree of sulphonation of the dye molecule determine these dyeing characteristics. The original classification of this type, based on their behaviour in wool dyeing, is as follows:

1.       Level dyeing or equalizing acid dyes;

2.       Fast acid dyes;

3.       Milling acid dyes;

4.       Super-milling acid dyes.

To promote felting in a woolen material, it is treated with weak alkaline solution with considerable mechanical action. This process is called milling. Dyes of good fastness to milling are essential to avoid colour bleeding during the process.

Properties of acid dyes

The solubilizing group present in acid dyes are sodium salt of sulphonic acid and these form large anion in the aqueous solution.

The main properties of acid dyes are ,

• Acid dyes are water soluble
• Acid dyes are anionic in nature
• Application of acid dyes are from strong acidic condition to neutral pH
• Acid dyes have excellent affinity towards protein and polyamide fibres like wool, silk and nylon, modified acrylics.
• They are not suitable for dyeing cellulosic fibres because the dye lack affinity towards cellulosic fibres
• The dye fibre bonding that normally occurs is by hydrongen bond, vanderWaal’s forces and mainly through ionic bonding.

Mechanism of dyeing with acid dyes

Dissolution of dyes in aqueous solvent, produces a colored anion,

The protein and polyamide fibers produce cationic sites in water under acidic conditions, as the acidity of the solution is increased more cationic sites are produced under these strongly acidic conditions. These cationic sites are thus available for the acid dye anions to combine with through hydrogen bonding, vander waals forces or ionic bonding. These linkages are strong enough to break , and thus dyeing produced are fast .

Reaction between an acid dye and wool can be represented by following equation

A-SO3- + H3N+- W-COOH  A-SO3- H3N+-W-COOH

Electrolyte in the acid dye bath acts as a retarding agent because of chlorides ions attracted by the positive sites at the fiber and in the competition between. Addition of acid acts as a n exhausting agent , because strongly acidic conditions makes more cationic sites available and thus available dye anions got combined with these.

Dyeing temperature

Dyeing temperature is normally boiling temperature and the time required for dyeing varies from 30 to 60 minutes which is dependant on the depth of shade and dyestuffs used. The heating rate is generally kept as 1-300C/Min

Dyeing leveling agents

Levelling of dyes is very important factor and mainly cationic agents like ethoylated fatty amines are added to the dye bath as levelling agents.

Wool dyeing method with acid dyes

There are two methods of dyeing the fabrics with acid dyes. The difference is the use of different acids at different concentrations.

Method 1

A bath is set at 500C at point A with 4% Sulphuric Acid (96%) and5% Glaubers Salt anhydrous, maintain pH 2.5 to 3.5. At B add required amount to dyestuff.

Method 2

At A set bath at 50° with 2% Formic Acid (85%) and 5% Glaubers Salt anhydrous,

pH 3.5 to 4.5. At B add required amount of dye.

Once the bath reaches 75 to 90 min i.e., at C add 2% Sulphuric Acid (96%) or 2% Formic Acid (85%).

The fabric is rinsed thoroughly after dyeing to remove any loose dye.

Dyeing cycle for nylon filament with acid dyes

Fastness properties of acid dyes

The wet and light fastness properties of the acid dyes varies from poor to excellent , depending upon the molecular structure of the dyes.

The fastness properties as per the category are as follows

Neutral acid dyes:-since these dyes have very good leveling and migration properties ,and have a low affinity for the fiber, therefore the wet fastness properties of this class are generallypoor.

Weak acid dyes or half milling dyes :-

These dyes have a medium to good affinity for the fiber and are generally applied in a weakly acidic bath, shows medium to good wet fastness properties. Strong acid dyes or super milling dyes :- These dyes have poor exhaustion properties, therefore applied under very strong acidic condition , exhibit good fastness properties.

  Basic dyes

Basic dyes or cationic dyes as their name implies they are salts of organic bases and in solution form the dye molecules split up into positively charged coloured cations and negatively charged colourless anions. The chemical structures of two basic dyes are shown in Figure.1.

Figure 1 Chemical structure of basic dyes

The basic dyes are generally insoluble in water and they are soluble when they are converted into their hydrochlorides, sulphates or double salts. Wool, silk, acrylic and modacrylic fibres are normally dyed with basic dyes.

Properties of Basic Dyes

1.  Basic dyes are well known for their brilliance and intensity of the colours. They also possess high tinctorial value (high colour value).

2.  Basic dyes exhibit poor fastness to light and poor to moderate washing fastness.

4. Basic dyes can be converted to an insoluble compound when they are combined with tannic acid, in the absence of any mineral acid.

5.Hence this property of converting soluble to insoluble form of dyes by addition of tannic acid during after treatment can be adopted in protein fibres to enhance the wet fastness properties. This process is called back tanning. This consists of after treating the dyed material with tannic acid in order to form an insoluble complex and thereby to reduce the tendency of the dye to migrate.

6. Basic dyes are used when there is a requirement of bright shade which are not available in acid dyes. Wool fibres are dyed to obtain brighter shades with basic dyes. Silk is normally dyed with basic dyes.

7.  Basic dyes do not have any affinity for cellulosic fibres. Mordanting is a pre-process used to apply basic dyes on cellulosic fibres. Generally mordanting is done with tannic acid. This cumbersome dyeing is done to obtain brighter shades in cotton or other cellulosic fibres.

8. Acrylic fibres dyed with basic dyes exhibit excellent all round fastness on these fibres.

Classification of Basic Dyes

Basic dyes can be classified based on the chemical structure as azo, methane, diphenylmethane, triphenylmethane, acridine, xanthane, azine, induline, oxazine, thiazole, thiazine, etc.

Dye fibre interaction between protein fibre and basic dyes

In the case of protein fibres (wool, silk:, etc.), the carboxyl groups present at the ends as well as in the interior of the macro molecules play an important role in fixing the basic dyes. When these carboxyl groups are’ionised, negative sites are created in the fibre substance. It is at these sites that the cations of basic dyes are held by electrostatic attraction as shown in Figure 2

It can be seen from the figure that hydrochloric acid is formed as a by-product of the reaction between the dye and the fibre. From this it follows that if hydrochloric (or any other similar) acid is added to the dye bath, the take-up of dye by the fibre can be retarded. In most cases, the tendency of basic dyes is to rush on to the fibre, and the use of a retarding agent would be a step that contributes to level dyeing. Further, when an acid such as hydrochloric acid is present in the dye bath, the ionisation of the carboxyl groups of the protein fibre is slowed down i.e. the number of anionic or negative sites in the fibre substance decreases. As a result, the dye uptake or the exhaustion of the dye bath too decreases.

Figure 2. Dye fibre interaction between protein fibre and dye

Application of Basic Dyes to Silk

Dissolution of basic dyes

Basic dyes are insoluble in water. The procedure for dissolution of basic dyes is to paste the dye powder with equal amount of water and 40% acetic acid. Then the bath is added with boiling water to the paste with constant stirring. Silk is often dyed with basic dyes in ‘broken’ degumming liquor, the major constituents of which are alkali and soap, besides the ‘silk gum’. The dye bath is prepared as follows.

A dye bath containing one litre of boiled-off liquor from the degummingof silk is diluted to four litres with soft water. Then an amount of acetic acid that is required to just give a positive test with litmus paper is added to the bath. The acetic acid neutralises the alkali and also liberates fatty acids from the soap. The latter rises to the surface and must be skimmed off before the dyeing commences.

Dyeing in a Bath of Broken Degumming Liquor

The bath, prepared as above, is kept at 35°C and the silk material is immersed in it and wetted out. The material is then lifted out of the bath and the filtered dye solution is added and mixed well to give a homogeneously coloured solution. The silk is returned to the bath and worked in it for the next 20 minutes. The temperature is then raised slowly to 85°C and the material is worked in the dye bath at this temperature for half an hour.

Since silk has very good affinity for basic dyes, this method may result in uneven dyeing. To overcome this , a slower dyeing process can be made by dividing the dye solution into three equal portions. Similar to the above process silk is wetted out and first protion of the dye is added and stirred well. Silk is worked for 15 minutes in this dye bath and the second and third portions are added after a time interval of 20 minutes each. The dye bath temperature is next raised to 35°C, and the dyeing is continued for a further 20 minutes. The temperature is raised slowly to 80°C while working the material in the dye bath. After dyeing, the goods are rinsed, hydroextracted and dried. If a brighter fmish to the dyed material is desired. Finally , the dyed silk is immersed in a suspension of 2% olive oil soap or acetic acid on the weight of silk is given. Then without any further washing, the material is hydro-extracted and dried, to give ‘scrooped’silk.

Dyeing in a Bath of a Neutral Soap

10 – 15% of olive oil soap is added to the bath containing soft water. The temperature is raised to 60°C, silk is entered in the bath and worked in the liquor until it is thoroughly wetted out. It is then lifted out of the bath and a portion of the dye solution is added. The temperature is raised to 80°C over a period of one hour, during which time the remaining dye solution is added in several small portions, the material being lifted out of the dye bath before each addition.

Dyeing in a Liquor Acidified with Acetic Acid

This method may be used practically to all the basic dyes because, acetic acid acts as a retarding agent. The bath is set with 1 g/l of acetic acid. The goods are entered cold and the temperature is raised gradually to 85°C. The dyestuff solution is added in portions during the  dyeing operation. No after-treatment is necessary for improvement of brilliance by this method.

After-treatment

Basic dyes generally has moderate to poor wash fastness. The wash fastness of basic dyes can be improved by treating with tannic acid after dyeing, in order to convert the dyestuff into its comparatively insoluble tannic acid salt. The fastness can be further improved by the action of an antimony salt that forms an even more insoluble dye-tannic acid-antimony complex. The most convenient antimony compound used is ‘tartar emetic’, which is a popular name for potassium antimony tartrate. This treatment improves both light as well as wash fastness properties.

Dyeing Wool with Basic Dyes

A bath is made up containing 1 – 3% of 40% acetic acid and 10% sodium sulphate on the weight of the wool. The dye solution, after being strained through a filter, is added, and the goods, which have previously been scoured, are entered. The dye bath is slowly raised to 85°C within 30 – 45 minutes and the dyeing is continued for 30 minutes.

Conclusion

Acid and Basic dyes are best suitable for natural and synthetic polyamide fibres. Due to their affinity towards the polyamides, ease of application and satisfactory fastness properties , these dyes are widely used in the industry.

REFERENCES and URLs

1. Clark M, Handbook of textile and industrial dyeing: principles, processes and types of dyes, wood head Publishing, 2011
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3. Arthur D Broadbent, Basic Principles of Textile Coloration, Society of Dyers and Colourists, 2001
4. Wilfred Ingamells, Colour for textiles A user’s handbook, Society of Dyers and Colourists,1993
5. Shenai V A, Technology of Textile Processing : Technology of Dyeing, Sevak Publications, 1985