The problem of dyeing polyester with disperse dyes

Polyester fibres are essentially undyeable below 70–80 °C, leaving only a 20– 30 °C range for increasing the dyeing rate before reaching the boiling temperature. At any temperature, the rate of dyeing of polyester with a given disperse dye is very much lower than for cellulose acetate or nylon fibres. The rate of diffusion of disperse dyes into the polyester below 100 °C is so low that dyeing at the boil does not give reasonable exhaustion. The rate of dyeing is higher for dyes of small molecular size that have higher diffusion coefficients. Dyeing is faster when using fibre swelling agents called carriers to improve the fibre accessibility, or when dyeing at higher temperatures above 100 °C to increase the dye diffusion rate.

Fibres of the most common polyester, polyethylene terephthalate (PET or PES), are quite crystalline and very hydrophobic. Hot water does not swell them and large dye molecules do not easily penetrate into the fibre interior. Polyesters have no ionic groups and are dyed almost exclusively with disperse dyes. The better diffusion at the boil of low molecular weight dyes results in moderate migration during dyeing but then the washing fastness is only fair. Many of the more recent disperse dyes are specifically for dyeing polyester. These are of higher molecular weight to provide adequate fastness to sublimation during heat treatments. Some of these produce a reasonable depth of shade by dyeing at the boil. Most, however, require higher dyeing temperatures or carriers for satisfactory results. Dyeings of polyester with disperse dyes have good light fastness. This does not always correlate with the light fastness on other fibres such as cellulose diacetate. The disperse dyes provide a full range of colours with adequate to good build-up on PET fibres. Uneven filament texturising or heat setting can lead to barré but higher dyeing temperatures, or addition of some carrier, will promote migration to minimise this. Again, a full black requires aftertreatment of the dyeing by diazotisation of an amino disperse dye and coupling with a suitable component, often BON acid. Concurrent dyeing with a mixture of the amino disperse dye and dispersed BON acid, followed by treatment with sodium nitrite and hydrochloric acid, is a common procedure. Some blacks are mixtures of dull yellow, red and blue dyes.