Acid dye

Acid Dyes

Acid dyes are water-soluble dyes. They are generally used for dyeing protein fibers such as:

) Animal hair fibers like wool , alpaca and mohair )silk and )synthetic fibers such as nylon.

Acid Dyes produce the most vibrant colors possible on protein fibers and nylon.

They are represented as RSO3Na and the dissociation of the dyes occurs only in the presence of some activating agents such H2SO4 and HCl etc.

STRUCTURE The chemistry of acid dyes is quite complex. Dyes are normally

very large aromatic molecules consisting of many linked rings. Acid dyes usually have a sulphonyl or amino group on the molecule making them soluble in water. Water is the medium in which dyeing takes place. Most acid dyes are related in basic structure to the following:

Anthraquinone

First way of classification

Characteristics Molecularly split acid dyes

Aggregated acid dyes

Dissociation Complete Incomplete

Leveling properties

Good Poor

Affinity Less High

Washing Fastness Poor Good

Presence of acids H2SO4, HCl CH3COOH

Second way of classification

Characteristics Levelling Dyes Milling Dyes Supermilling Dyes

Additive used H2SO4 CH3COOH CH3COOH

pH 2-4 4-6 6-7

Leveling properties

Good Medium Poor

Fastness Poor Good Very Good

Solubility Good Poor Poor

Form in water Moleculer Colloidal Colloidal

Affinity Less High High

Mechanism of Acid Dyeing

• The wool gets protonated in the presence of the acid activator.

• HOOC-W-NH2 HOOC-W-NH3+Cl-

• Then this protonated wool reacts with acid dye in which NaCl is formed.

• This is a reversible process and if more of NaCl is formed, the dyeing process is retarded.

• A little of NaCl is thus added in the starting which retards the rate of reaction of dyeing and hence prevents uneven dyeing.

Dyeing method for acid levelling dyes on wool

Method

At A set bath at 50° with:2% Formic Acid (85%)5% Glaubers Salt anhydrous,pH 3.5 to 4.5

At B add required amount of dye.

At C add 2% Sulphuric Acid (96%) or 2% Formic Acid (85%).

Thoroughly rinse after dyeing to remove loose colour.

Dyeing Method for Cotton and other Cellulosics

At A add Azonine or Durantine Dye (predissolved)0.1 to 0.5g/l Neutrasol CR (for shades difficult to level)

At B add y g/l Glaubers salt (anhydrous)(1/5th of total amount required).

At C add z g/l Glaubers salt (anhydrous)(4/5th of total amount required).

At D rinse twice cold. Thoroughly rinse after

dyeing to remove loose colour.

Advantages of acid dyes• Found in bright colours. • Widely used on silk.• Used for protein fibres, nylon, spandex, special

type acid-dyeable acrylic.

Disadvantages of acid dyes• Poor washing fastness.

HEALTH & SAFETY Any dyes including acid dyes have the ability to induce

senstisation in humans due to their complex molecular structure and the way in which they are metabolised (usually in the liver )in the body.

the dye CI Acid red 128 is banned in Europe as it was found to metabolise in the body back to ortho-toluidine, one of its chemical intermediates.

Many intermediates used in dye manufacture such as o-toluidine, benzidine etc. were found to be carcinogenic.

All the major chemical companies have now ceased to market these dyes.