The dyeing of supermicrofibre nylon with acid and vat dyes

Abstract

5% omf dyeings of three 1:2 pre-metallised acid dyes displayed poor fastness to repeated washing at 60 °C on 0.05 dtex ‘supermicrofibre’ artificial nylon suede. Although fastness was improved by an aftertreatment with the full backtan, considerable dye loss still occurred during repeated washing, which was attributed to the large surface area of the microfibre substrate. In contrast, the wash fastness of 4% omf dyeings of three vat dyes, applied as both the alkali leuco and acid leuco variants, was substantially better than that of the acid dyes, despite the considerably higher colour strength of the vat dyeings. Although the rub fastness of the vat dyeings was higher than that of the acid dyeings, the level of rub fastness, especially wet, was only moderate. The colour and colour strength of the acid leuco dyeings were generally different to those of the alkali leuco dyeings.

Introduction

Polyamide fibres are dyed predominantly using acid dyes, even though their wash fastness properties on such fibres leaves much to be desired [1]. Although 1:2 pre-metallised acid dyes generally display higher levels of wash fastness on nylon than their non-metallised counterparts, an aftertreatment of the dyed fibre with either a synthetic or a natural tanning is commonly used in order to achieve highest levels of wash fastness, especially in moderate to deep depths of shade [1]. The two-stage treatment of dyed nylon with tannic acid and potassium antimony tartrate constitutes the classic full backtan aftertreatment which is very effective in improving the wash fastness of acid dyes on nylon [1]. However, owing to many reasons, the use of the toxic antimony salt has been superseded in recent times by that of protease enzymes [2], [3], [4] and metal salts [5], [6].

Polyamide microfibres are typically <1 dtex and enjoy manifold apparel applications owing to their soft handle, high lustre and excellent drapeability [1]. Although the polymer used in nylon microfibre is often the same as that employed in conventional dtex fibres and, therefore, microfibre can be dyed in a similar manner to its conventional counterpart, microfibre has a larger surface area than conventional decitex fibre per unit mass of substrate, resulting in greater reflection of light from the microfibre surface which, in turn, results in higher amounts of dye being required to achieve the same visual depth of shade as that on conventional decitex fibres. For example, to produce the same depth of shade on 0.5 dtex microfibre as that obtained using 1% omf dye on 3.5 dtex conventional fibre, requires 2.65% omf dye [1]. This requirement to use higher amounts of dye on polyamide microfibre coupled with the inherently low/moderate wash fastness properties of acid dyes on nylon results in comparable depth dyeings on microfibre polyamide displaying lower wash fastness than on conventional decitex nylon fibres [1], [7], [8]. In addition, the larger surface area of microfibre results in a greater rate and extent of dye desorption during washing and, therefore, reduced levels of wash fastness [8]. This particular situation is exacerbated in the case of polyamide artificial suede [9], which was introduced some 40 or so years ago and which comprises ‘supermicrofibre’ of ∼1.1 × 10⁻⁴ to 0.33 dtex and which is considered to be the origin of textile microfibres [1]. Using the above example of a 1% omf depth of shade on 3.5 dtex fibre, in order to achieve the same depth of shade on 0.05 supermicrofibre suede would require the application of 8.7% omf dye.

Although vat dyes are widely used on cellulosic fibres on which they exhibit characteristically excellent light and wet fastness properties, this dye class is not routinely used on polyamide fibres (with the exception of their minor usage on nylon/cotton blends) owing to their low substantivity and the generally pale shades that accrue from their limited extent of diffusion within the substrate [1]. The dyes contain at least two conjugated carbonyl groups which, during their conventional application to cellulosic fibres, are converted by reduction under alkaline conditions to the corresponding, water soluble, ‘alkali leuco’ form which is applied to the substrate. At the end of dyeing, the alkali leuco form is oxidised, so as to regenerate the insoluble, parent vat dye in situ within the fibre (Fig. 1). However, the alkali leuco form is readily converted to the sparingly water-soluble ‘acid leuco’ variant (Fig. 1), which has been used to dye nylon fibres [10], [11], [12]. In effect, the acid leuco variant resembles a disperse dye in terms of its adsorption characteristics and, when oxidised at the end of dyeing, is converted into the insoluble parent vat dye in situ within the nylon substrate.

This paper concerns the dyeing of artificial nylon suede with both the alkali and acid leuco forms of vat dyes and compares the resulting dyeings with those obtained using 1:2 pre-metallised acid dyes which had been aftertreated with a modified full backtan, in terms of depth of shade and fastness to repeated washing.