Characteristics, Types and Application of Acid Dyes

Last Updated on 10/09/2021

Acid Dyes:
The term “acid dye” derives from the dyeing process, which is carried out in an acidic aqueous solution (pH 2.6). Acid dyestuff is so called mainly due to two reasons. In the first place these classes of dyestuff were applied in a bath containing mineral or organic acids like sulphuric, acetic or formic acid and secondly most of them are sodium salts of organics acids. Acid dye was made by Nicholson in 1862 by treating an insoluble dye. They are commonly used for dyeing protein fibers (e. g. wool and silk) and nylon fibers. Acid dyes are anionic in nature, and their negatively charged anions are attracted by positively charged amino groups in wool under acidic conditions.

acid dyes

A common property of these dyes is their mechanism of interaction with the fiber polymers, which leads to strong binding of the dye to the material. The binding of the dye at least partly can be understood to follow an ionexchange mechanism. Ionic bonds are formed between charged groups present in the fiber polymer and the charged dyestuff molecule. The dyes are important for coloration of protein fibers (wool, silk) and polyamides, which contain ammonium groups at low pH. The amount of ammonium groups in wool can be assumed with 850 mmol/kg, in silk with 250 mmol/kg and in polyamide with 30– 50 mmol/kg.

These cationic groups serve as binding anchor to build an ionic bonding with negatively charged groups, for example, sulphonate groups in the acid dye and metal complex dye.

The anionic groups in such dyes thus have two functions:

  1. To achieve water solubility of the dye
  2. To build an ionic bond linkage to the fiber

Besides the ionic bond, also secondary attractive forces (Van der Waals forces) contribute to strengthening of the dyestuff–fiber interaction (Figure 1).

Binding of an acid dye
Figure 1: Binding of an acid dye

The chemistry of acid dye is complex and diverse. According to their structure, acid dye belong to the following chemical groups: azo; anthraquinone; triphenylmethane; pyrazolone; azine; nitro; and quinoline. Azo dyes represent the largest and most important group and are followed by antraquinone and traylmethane dyes. Of the other dye groups, very few products are of any commercial value.

chemical structure of acid dyes
Figure 2: Chemical structure of acid dye

Properties of Acid Dyes:
Since these are sold as a sodium salt, therefore these form a large anion in the aqueous medium.

The main properties of acid dyes are,

  1. Acid dyestuff is soluble in water.
  2. These dyes are anionic in nature.
  3. They are usually combined with basic dyes.
  4. These dyes are suitable for wool, silk, polyamide and modified acrylics.
  5. Acid dye have molecular weights in the range 300–1000 g mol-1.
  6. These dyes are generally applied in the presence of acids like sulphuric, acetic or formic acid to neutral pH bath
  7. These dyes have no affinity for cotton cellulose’s, hence not suitable for cellulosics.
  8. They have substantivity towards protein and polyamide fibers.
  9. These dyes combine with the fiber by hydrogen bonds, vander waals forces or through ionic linkages.
  10. They are in much case soluble in alcohol.
  11. The dye anion is the active colored component in these dyes.
  12. When acid dyestuffs are treated with a reducing agent they are generally decolorized.
  13. The dyed acid colors have good light fastness and moderate washing fastness.

Characteristics of the Various Types of Acid Dyes:

Characteristics of acid dyes
Table: Characteristics of acid dyes

Types and Classification of Acid Dyes:
In use, acid dyes are classified by their dyeing behavior and wet fastness properties, rather than chemical composition. Dyeing behavior is, however, determined to a great extent by relative molecular mass (r.m.m.) and degree of sulphonation. Acid dyes are generally divided into three classes which depend on fastness requirements, level dyeing properties and economy. The classes overlap and generally depend on type of fiber to be colored as well as the process used.

Classification of Acid dyes according to leveling characteristics,

1. Acid dyes with Good leveling characteristics: This type of acid dye is responsible for their good leveling characteristics. As the dye molecules have less attraction for the fiber they will migrate only slowly into the polymer systems of wool or nylon fibers. However to obtain sufficient substantive and to ensure, adequate exhausting agent (sulphuric acid) is added to the dye liquor, their lack of substantive is evidence by their poor wash fastness. However the light fastness is very good to excellent.

2. Acid dyes with average leveling characteristics: The moderate substantive of this type of acid dye is responsible for average leveling characteristics. To obtain sufficient substantive and to and ensure it, adequate exhausting agent, a week acid (Acetic acid or Formic acid) is added to the dye both. The washing fastness of these dyes is fair, whilst their light – fastness is good to very good.

3. Acid dyes with poor leveling characteristics: These dyes are also known as fast acid dye, and milling dyes or Natural dyeing acid dye. They have the best substantive of all the acid dyes, but have relatively poor leveling characteristics. Unless care is taken during, their relatively good substantive for the fiber may result in too rapid uptake and consequently unleveled dyeing.

The excellent substantive of these dyes require neutral bath in order to obtain slower exhaustion and more level dyeing. The wash fastnesses of these dyes are good to very good, while their light fastness is fair to good. The better wash-fastness, compared with the other two type of acid dye, is due to the greater number of sodium sulphate groups.

Classification according to dyeing characteristics,
Acid dyes are commonly classified according to their dyeing behavior, 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 behavior in wool dyeing, is as follows:

  1. Level dyeing or equalizing acid dye;
  2. Fast acid dye;
  3. Milling acid dye;
  4. Super-milling acid dye.

1. Equalizing/leveling acid dyes: Highest level dyeing properties. Quite combinable in trichromatic shades. Relatively small molecule therefore high migration before fixation. Low wet fastness therefore normally not suited for apparel fabric.

2. Fast acid dyes: This group, known also as half-milling or perspiration-fast dyes, are generally monosulphonated and of r.m.m. about 500–600, and are exemplified by the Sandolan P (S) and Supramin GW (BAY) dyes. As their name implies, they exhibit superior wet fastness properties to level-dyeing acid dyes but retain some of the migration properties.

3. Milling acid dyes: Medium to high wet fastness. Some milling dyes have poor light fastness in pale shades. Generally, not combinable. Used as selfshades only.

4. Metal complex acid dyes: More recent chemistry combined transition metals with dye precursors to produce metal complex acid dyes with the highest light fastness and wet fastness. These dyes are also very economical. They produce, however, duller shades.

Acid dyes are classified according to acidic power:

1. Neutral acid dyes: These are supra milling or fast acid dyes, having medium to good wet fastness properties, some of the dyes have poor light fastness in pale shades. Many of the dyes are used as self-shades only. These are applied to the fiber in a weakly acid or neutral pH.

2. Weak acid dyes: These dyes belongs to the milling class of dyes. These dyes have good fastness properties but light fastness is moderate to poor.

3. Strong acid dyes: These dyes are applied in a strongly acidic medium and also called leveling dyes; however, there wet fastness properties are a limitation. These dyes are very good to produce the combination shades.

Stripping Process of Acid Dyes:
Stripping becomes necessary when uneven dyeing occurs. It is a critical procedure and it is required to match the shade of the dyed fabric with the buyer’s requirement. Stripping process of acid dyes are given below:

  1. With the molecularly – dispersed acid dyes, on account of their good migrating properties, continued boiling in the same liquor is often effective. It must, how ever be borne in mind that wool felts if it so boiled form too long.
  2. Boiling in fresh liquor with 20% of Glauber‟s salt will strip some of the color, and this can be exhausted again by cautious addition of acid or, preferably, ammonium salt.
  3. If treatment with Glauber’s salt is not successful, more of the color can be stripped by boiling with 0.5% ammonia, or in liquor containing 2% pyridine.
  4. Another method is to boil the unevenly-dyed material with 3 to 5% of a cationic, non-ionic mixture such as Tine gal W or Lyogen SMK. When sufficient color has been removed by any of these compounds the wool is rinsed and redeye.

Application of Acid Dyes:
Because of their excellent level-dyeing properties, these dyes are most suitable for dyeing fabric and yarn in hank form, particularly carpet yarns. Acid dyes can be used for dyeing natural fibers such as wool and silk. They are also used for dyeing polyamide. Acid dyes are anionic, and in solution they will ionise into sodium ions plus large negatively charged colored ions. For this reason, they will dye any fibers which have positively charged sites, In comparison to wool, polyamide has fewer sites available for dyeing, and thus the shades produced on polyamide are generally weaker than on wool but of higher fastness.

Typical applications for acid dyes:

Acid Dye Class Potential Applications
Levelling or equalising
  • Carpet yarn, particularly high twist types for which penetration is an issue.
  • Woollen and worsted fabrics, particularly for bright shades on carbonised goods and for tightly constructed fabrics, such as gabardines.
  • Fashion and interior textile products for which washing and perspiration fastness are not required.
Fast acid, half-milling or optimised half-milling Piece goods, carpet yarn and knitting yarn that will not carry a washable care claim (pale-medium shades may meet hand-washable standards).
Milling Medium-heavy bright shades on piece goods, weaving, carpet and knitting yarns
Super milling and optimised milling Bright shades on loose stock, sliver, yarns that require high wet fastness to milling and similar processes. May even be used in pale-medium shades for machine-washable performance

References:

  1. Chemical Principles of Synthetic fiber Dyeing by S. M. Burkinshaw
  2. Textile Dyes by N. N. Mahapatra
  3. Textile Dyes By Mansoor Iqbal
  4. Physico-chemical Aspects of Textile Coloration by Stephen M. Burkinshaw
  5. Textile Chemistry by Thomas Bechtold, Tung Pham
  6. Handbook of Textile and Industrial Dyeing, Volume 1: Principles, Processes and Types of Dyes Edited by M. Clark
  7. The Coloration of Wool and other Keratin fibers Edited by David M. Lewis and John A. Rippon
  8. Industrial Dyes: Chemistry, Properties, Applications Edited by Klaus Hunger
  9. A Novel Green Treatment for Textiles: Plasma Treatment as a Sustainable Technology By Chi-wai Kan

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