The mordant dyeing of wool using tannic acid and FeSO₄, Part 1: Initial findings

Abstract

C.I. Mordant Black 8 produced reddish-brown dyeings on wool of good light fastness, moderate rub fastness but poor fastness to washing at 40 °C. Subsequent afterchroming with Na₂Cr₂O₇ provided deep, black dyeings of much improved fastness. The substitution of Na₂Cr₂O₇ with FeSO₄ also improved the fastness of the unmordanted dyeings but the ensuing brown shades displayed lower, all-round fastness to their afterchromed counterparts. When the dye was applied in combination with tannic acid, subsequent mordanting with ferrous sulfate yielded brown/greyish-brown dyeings of high colour strength and high all-round fastness. Of two methods that were used to produce such dye/tannic acid/FeSO₄ ‘combination’ dyeings, namely, (dye + tannic acid)/FeSO₄ and dye/(tannic acid + FeSO₄), the latter yielded dyeings of superior fastness.

Introduction

The pretreatment of textile fibres with a mordant (salts of various metals or tannins) in order to achieve dyeings of improved fastness, depth of shade or different colours, has been practised since time immemorial. Nowadays, the use of mordant dyes is mostly confined to wool, on which the dyes, in conjunction with a metallic mordant, provide deep shades of characteristically excellent wet and light fastness. Although, essentially, mordant dyes resemble non-metallised acid dyes, they contain ligands (OH, NH₂, COOH) that enable them to form a stable, coordination complex with a metal ion in situ within the wool fibre, this being accompanied by a dramatic improvement in both the fastness of the dyeing to light and wet agencies, as well as a marked change in the colour of the dyeing. Whilst, in theory, various transition metals can function as mordants and chromium predominates because of the superior fastness of dyeings obtained using salts of this metal (commonly Na₂Cr₂O₇ or K₂Cr₂O₇); consequently, the term ‘chrome dye’ is often incorrectly used. In the case of wool, for which mordant dyes were first devised, three methods of application can be used namely, pre-chrome, metachrome and afterchrome, depending on whether the mordant is applied before, together with or after the dye; the afterchrome method predominates currently. However, the use of mordant dyes has decreased in recent years, owing to environmental concerns regarding the use of heavy metals, including chromium. In this context, the replacement of chromium with less environmentally challenging Fe(II) and Fe(III) salts wa found to improve the light fastness [1], [2] and the wash fastness [2] of various mordant dyes on wool, although the colour of the dyeings obtained using iron salts differed to that achieved using dichromate. It was shown that the use of iron salts resulted in the formation of 1:2 dye:metal complexes within the dyed substrate [1], [2].

Vegetable tannin extracts are large M_r, water-soluble phenolic compounds that have been used on textiles for several hundreds of years, as exemplified by the dyeing of cotton and silk with natural dyes obtained from ‘dyewoods’, in which the tannin ‘fixed’ the metal salt (e.g. CuSO₄) that was employed as a mordant for the dye. Natural tannins continue to enjoy textile usage, as represented by the application of tannic acid as an aftertreatment to improve the wet fastness of dyeings on nylon fibres and as a stain resist treatment for nylon carpet [3], [4]. The combination of tannins with iron salts has enjoyed varied applications over many centuries, such as the use of tannic acid and iron sulfate in the preparation of iron gall inks and, in the context of the work described herein, the use of copperas (FeSO₄·7H₂O) in conjunction with tannins to produce greyish-brown shades on cotton, silk and wool, which has been practised since medieval times.

This paper concerns the feasibility of obtaining deep, black shades on wool using tannic acid and iron_(II) sulfate. This part of the paper concerns initial findings made using C.I. Mordant Black 8 whilst the second part of the paper discusses the nature of the interactions between various mordant dyes, tannic acid and FeSO₄.