ACID DYE

Definition

Dye which is mostly sulphuric or carboxylic acid salt and essentially applied from acidic or neutral dye bath known as Acid dye. The dye anion is the active colored component in these dyes, they are synthesized as sodium salts as free dye acids are more difficult to isolate.

Properties

·         These dyes are anionic in nature.

·         These dyes are soluble in water.

·         These dyes are suitable for wool, silk, polyamide and modified acrylics.

·         These are applied from a strongly acidic to neutral pH bath.

·         Generally, the molecular weights of acid dye ranges from 200-900.

·         Generally, dyes contain 1 to 4 sulphonic groups present in the structures.

·         These dyes have no affinity for cotton cellulose’s, hence not suitable for cellulosics.

·         These dyes combine with the fiber by hydrogen bonds, vander waals forces or through ionic linkages

Classification:

Structure

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

1.       Anthraquinon type

2.       Azo dye type

3.       Triphenylmethane type

Dyeing characteristics

	Types of Dye
Property	Self – Levelling	Milling	Super-milling
Color brightness	Good	Lower than leveling	Fair (deep shade)
Levelling	Very good	Poor to fair	Very poor to fair
Affinity	Less	High	Very high
Migration	Excellent	Poor	Very poor
Wet fastness	Low	Good	Very good
Light fastness	Very good	Good	Good
Dyebath additives (pH adjusting agent)	Sulphuric or formic acid	Acetic acid	Ammonium acetate
Dyeing pH	2 – 4	4 – 6	6 – 7
Molecular weight (g/mole)	Low (200-400)	High (500-900)	High (500-900)
Molecular Size	Very small	Relatively bigger	Biggest
Water solubility (g/l)	High (40-80)	Moderate (13-30)	Low (3-20)
Solution behaviour	Ionizes	Aggregates	Aggregates
Protein affinity	Low	High	Very high
Cellulose staining	None	Stains	Stains

Mechanism

(1)   Dye: On the addition of dye in aqueous solution, it produced colored anion as follows

(2)   Fiber: When protein and polyamide fibers immersed in water, H-atom attached to the carboxylic group at one end of fiber and transferred to –NH₂ group at the other end of the macromolecule so that the two ends of fiber chain acquire opposite electrical charges, called zwitter ions:

 

(3)   Dye and Fiber: These cationic sites are thus available for the acid dye anions to combine with through hydrogen bonding, vander waals forces or ionic bonding in acidic condition. These linkages are strong enough to break, and thus dyeing produced is fast.  

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

 

Problems with acid dye

           

·    Unequal access of the fibers to the dye solution, resulting from densely packed fibers or yarns and from poor agitation of the dyebath.

·       Variation of the temperature throughout the dyebath and the goods.

·       Uneven pH in the bath and the material.

References:

1.      Chemical Technology in the Coloration of Textiles, S.R.Karmakar(2007), 92-96

2.      Fundamental & Practices in Coloration of Textiles, J.N.Chakraborty(2010), 166-169

3.      Textile Preparation & Dyeing, AKR Choudhury(2011), 470-475

4.      Handbook of Textile & Industrial Dyeing, M.Clark(2011), 164-167