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240 ACID, PRE-METALLISED AND MORDANT DYES

CHAPTER 13

Acid, pre-metallised and mordant dyes

13.1 GENERAL DESCRIPTION OF ACID DYES

This chapter deals with acid, mordant and pre-metallised dyes. Despite their
different names, these three types of dye have many features in common. They dye
both protein and polyamide fibres using similar dyeing methods. In the Colour
Index, they are classified as ‘acid dyes’ or ‘mordant dyes’. The name ‘acid dye’
derives from the use of an acidic dyebath. Most pre-metallised and mordant dyes
are acid dyes. In the case of mordant dyes, the dyeings are aftertreated with a
suitable metal ion mordant, usually chromium. In fact, mordant dyes are often
referred to as chrome dyes. The metal in pre-metallised dyes is incorporated into
the dye molecule during the manufacturing process.

The dyeing properties of acid dyes vary widely (Section 13.2.2). The acids used
in the dyebath range from sulphuric acid (dyebath pH < 2.0) to ammonium
acetate (dyebath pH > 6.5). Acid dyes are usually sodium salts of sulphonic acids,
or less frequently of carboxylic acids, and are therefore anionic in aqueous
solution. They will dye fibres with cationic sites. These are usually substituted
ammonium ion groups in fibres such as wool, silk and nylon. These fibres absorb
acids. The acid protonates the fibre’s amino groups, so they become cationic.
Dyeing involves exchange of the anion associated with an ammonium ion in the
fibre with a dye anion in the bath (Scheme 13.1).

Acid dyes have molecular weights in the range 300–1000 g mol–1. The dyes
with larger molecules have higher substantivity for wool or nylon. Such dyes have
slower diffusion in the fibre and therefore less ability to migrate and dye level. The
more hydrophobic, high molecular weight dyes therefore have better fastness to
wet processes. Their absorption by wool and nylon also involves the interaction of

Fibre NH2(s) + H (aq) + HSO4 (aq) Fibre NH3 HSO4 (s)

Fibre NH3 HSO4 (s) + Dye SO3 (aq) Fibre NH3 Dye SO3 (s) + HSO4 (aq)

Scheme 13.1

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