Theory/Mechanism/Chemistry of Wool Dyeing

Wool fiber is made from the hair of sheep. Other animal hair fibers, such as mohair, angora, cashmere, and camel's hair, are in most respects dyed the same as wool. As wool readily accepts dye colors, dyeing can occur at almost any stage of the wool processing.

You can also like: Wool Fiber: Production, Classification, Properties and Application

Wool dyeing
Fig: Wool dyeing
Mechanism of wool dyeing:
Acid dye is most appropriate for wool fiber, that’s why it is called wool dye. To understand the dyeing mechanism, it’s first important to understand the wool molecule structure. The macro molecules of wool are linked by “peptide linkage” to each other. Again the polymers are attached by different bonds. A wool keratin can be represented as follows:
H2N – W – COOH

Under certain conditions, the hydrogen ion attached to the carboxylic ion is transferred to the amino group at the other end of the macromolecule so that the two ends of the keratin molecular chain acquire opposite electric charges.
H2N – W – COOH → H3N(+) – W – COO(-)

When a protein containing such molecules with electrically charges ends is entered into a solution of an acid (e.g. dilute HCl), some of the negatively charged carboxylic group (-COO-) take up (H+) released from HCl into the solution and become neutral (COOH).

HCl → H(+) + Cl(-)

H3N(+) – W – COO(-) + H(+) + Cl(-) → ClH3N – W – COOH

And the Cl- chloride ions, released from HCl are absorbed by the positively charged amino ends of the keratin macro molecule.

Therefore, when wool is entered into an acidic solution, the concentration of acid is decreased by the absorption of some of acid. It continues until the equilibrium is achieved.

The dye-fibre reaction can be represented as follow in the dye bath.

D – SO3(-) Na(+)→ D – SO3(-) + Na(+)

H3N(+) – W – COOH + D – SO3(-) + Na(+)Cl(-) → D – SO3(-) H3N(+) – W – COOH + NaCl

The dye take up by the fiber depends upon the power and concentration of acid used in the dye bath.

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