Saminathan Ratnapandian

Indigo dyeing and printing

1. Introduction

Indigo, the blue dye known since ancient times, is a widely known natural vat dye. Textile colouration using indigo involves a reversible reduction-oxidation process, mentioned in Section 1.2.1. A combination of sodium hydroxide (caustic soda) as alkali and sodium dithionite (sodium hydrosulfite or hydros) as reducing agent is widely employed in commercial production processes for indigo dyeing. Section 2.2.2.2 outlines the disadvantages of the above industrial process. In spite of the various alternatives developed for the reduction of indigo, scope for a simpler method exists.
This chapter details the investigations into the use of chemicals having lesser impact on the environment in the indigo reduction process. The chemicals investigated are thiourea dioxide as the reducing agent and sodium carbonate as the alkali.

1. Thiourea dioxide (TUD)

TUD or formamidine sulfinic acid is an established reducing agent for vat and sulfur dyes. The use of TUD has been proposed for exhaust dyeing and printing with vat dyes especially for silk and cellulose acetate [192, 193]. At an alkaline pH of 11, a 1% solution of TUD has a reduction potential of (-965) mV as compared to (-800) mV for a similar solution of sodium dithionite [50, 194]. TUD is stable both as a solid and in aqueous solution up to 40^○C and reaches full reduction potential at 100^○C. The degradation products are less harmful as compared to those of sodium dithionite [113, 129, 195, 196]. The above features of TUD make it an attractive alternative to replace sodium dithionite.

2. Sodium carbonate

Sodium carbonate (washing soda or soda ash) is the sodium salt of carbonic acid. The textile processing industry uses it as a buffer to maintain alkaline conditions. Soda ash with its lower degree of alkalinity has a much reduced environmental impact as compared to sodium hydroxide [126, 197].

The following sections describe the methods adopted in colouring cotton fabrics using indigo that was reduced utilising the abovementioned chemicals. The effectiveness of the methods was evaluated further by comparing the results obtained from indigo of both natural and synthetic origins.

2.