sulphur dyes

Prepared by: Karamat Ali Saif

Sulphur dyes

Sulphur dyes are so called because sulphur dyes contain disulphide ( S-S) linkage in their chemical structure.

Sulphur black is the most important class of sulphur dyes

Commercially successful black sulphur was first prepared in 1893.

Applicable to cellulosic materials, not to wool and protein fibers due to strong alkaline condition.

Chemical structure

• The exact chemical structure of sulphur dye is not known

characteristics of sulphur dyes

These are water insoluble dyes and have no affinity for the cellulosic fibers

Have to be converted into water soluble substantive form (leuco form) before application to the textile materials.

This conversion is carried out by a treatment with a reducing agent like dilute aqueous Na2S.

Dye.S-S.dye + 2H→ Dye.SH+SH.Dye This leuco form is substantive to cellulosic

materials.

characteristics of sulphur dyes

They are absorbed on the fiber surface Then they are reconverted original water

insoluble form of dye by oxidation Oxidation is carried out by “airing” or by using an

oxidizing agent like sodium dichromate ( Na2Cr2O7).

Dye.SH+dye.SH → dye.S-S.dye + H2O gives best result when they are used to

produced black, black and brown shades but red shades cant be obtained by sulphur dyes.

General properties

Contain sulphur linkages within their molecules. Highly colored water insoluble dyes Some dyes are partially soluble in water. Good light fastness Excellent wash fastness This good wash fastness is due to larger

molecular size and insolubility in water.

Classification of sulphur dyes

• May be classified in may ways .• But according to their solubality there are

essentially three classes.1. Conventional or water insoluble dyes :

which have no substantivity to cellulosics 2. leuco sulphur dyes : which are water soluble

and non substantive to cellulosics

3. solubilised sulphur dyes : which are substantive to cellulosics

Steps in dyeing

Dissolving the Dyestuff reduction: conversion of insoluble form to its

soluble form

dyeing: dyeing of cotton with its solubalised leuco form

oxidation: reconversion of soluble sulphur dye inside the fiber to its original insoluble form.

soaping

Common Problems and Corrective Actions

1. Poor Washing and Rubbing Fastness

poor washing and rubbing fastness is caused by improper color dissolution, color precipitation, poor solubality of dyes, insufficient washing after dyeing etc.

Corrective actions

• The dye dissolution must be complete • It should be filtered befored adding to the dye

bath.• Becouse insoluble dye particles if present will

cause unlevel dyeing and poor wash and rub fastness.

• The color should be dissolved in sufficient quantity of water, by keeping in mind the maximum solubality of dye.

Corrective actions

• The water and salt should be free from calcium and magnesium, which if present make insoluble inert salt.

• The washing after dyeing and soaping must be efficient to clear all the unused chemicals and insoluble dye particles

2. Bronziness

Sulphur black has tendency to show bronze appearance ( dull shades)

There are several reasons of bronzinessInsufficient quantity of sodium sulphide or reducing

agent resulting into quick oxidation of surface dyeing.High concentration of common salt for exhaustionDelay between dropping of bath and washing.

Corrective actions

Proper dissolution of dye stuff Using sufficient and calculated quantity of

reducing agent Using appropriate quantity of common salt After treatment in bath containing Na2S solution

(1-2 g/l) at room temperature After treatment in a liquor containing soap and

olive oil ( 1-2 g/l ) at temperature 60 c for 30 minutes.

Drying without rinsing.

3. Tendering

Tendering means the loss of strength or degradation of cellulosic materials upon storage.

Tendering is caused by the acid formation from the free sulphur present in the dyed materials by the action of moisture and air.

The acid produced reacts with cellulose and degrade it, resulting in loss of strength

Corrective actions

The tendering can be minimized by giving after treatment with acid neutralizing agent

Treatment of dyed fabric with sodium acetate Drying without rinsing Presence of sodium acetate on dyed fabric

converts harmful sulphuric acid to harmless acetic acid

THE END

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