Application of disperse dyes to nylon

The disperse dye is pasted in warm water and the dispersion slowly diluted. Hot water and concentrated dispersant favour the formation of large dye particles. The concentrated dispersion is then strained into the dyebath that usually also contains additional dispersing agent. The bath is gradually heated and dyeing continued at the boil. The disperse dyes used for nylon are usually level dyeing.

The exhaustion rates of individual disperse dyes on nylon are not overly high. They do vary from dye to dye so that selection of compatible dyes is necessary. Although some dyes have good migration and build up well, deep shades are rarely dyed with disperse dyes because of their inferior washing fastness. Many of the simple disperse dyes developed for dyeing acetate at 85 °C are not particularly fast to heat and can sublime from the nylon during processes such as boarding. This is a form of heat setting used to stabilise the shape of ladies’ hosiery after dyeing. Dyes of higher fastness to sublimation are invariably of greater molecular size and therefore have lower rates of dyeing. The usual temperature for rapid dyeing disperse dyes on nylon is 85–100 °C. If slow dyeing heat fast dyes are used, dyeing under pressure at up to 120 °C may be useful. The disperse dyes used for dyeing nylon will also colour spandex (segmented polyurethane) filaments in stretch hose but the washing fastness is only fair. As for cellulose acetates, blacks are produced by diazotisation of a disperse dye containing a primary amino group and coupling of the generated diazonium ion with a suitable coupling component. Simple dyeing tests evaluate the migration, temperature range characteristics and dyeing rates of disperse dyes on nylon [3]. With rapid dyeing dyes, the dyeing rate increases with increasing temperature but the equilibrium exhaustion decreases. The more rapid dyeing dyes also migrate better and tend to be less temperature sensitive so that dyeings at different temperatures are close in shade. Nylon 6 is more amorphous and has a lower melting point than nylon 6.6. Disperse dyes dye nylon 6 using the same method as for nylon 6.6. Dyeing is usually faster than for nylon 6.6 under the same conditions and the dyes will usually show better migration. This usually means that the washing fastness is somewhat lower on nylon 6. One advantage of nylon 6 is that heat setting using hot air or steam is at lower temperatures than for nylon 6.6.