Disperse dye is originally developed for the dyeing of cellulose acetate. They are substantially water insoluble. The dyes are finely ground in the presence of a dispersing agent then sold as a paste or spray dried and sold as a powder. They can also be used to dye nylon, triacetate, polyester and acrylic fibres. In some cases a dyeing temperature of 130 deg C is required and a pressurised dyebath is used. The very fine particle size gives a large surface area that aids dissolution to allow uptake by the fibre. The dyeing rate can be significantly influenced by the choice of dispersing agent used during the grinding.
Disperse dyes have low solubility in water, but they can interact with the polyester chains by forming dispersed particles. Their main use is the dyeing of polyesters, and they find minor use dyeing cellulose acetates and polyamides. The general structure of disperse dyes is small, planar and non-ionic, with attached polar functional groups like -NO2 and -CN. The shape makes it easier for the dye to slide between the tightly-packed polymer chains, and the polar groups improve the water solubility, improve the dipolar bonding between dye and polymer and affect the colour of the dye. However, their small size means that disperse dyes are quite volatile, and tend to sublime out of the polymer at sufficiently high temperatures.
The dye is generally applied under pressure, at temperatures of about 130oC. At this temperature, thermal agitation causes the polymer's structure to become looser and less crystalline, opening gaps for the dye molecules to enter. The interactions between dye and polymer are thought to be Van-der-Waals and dipole forces.
The volatility of the dye can cause loss of colour density, and staining of other materials at high temperatures. This can be counteracted by using larger molecules, or making the dye more polar (or both). This has a drawback, however, in that this new larger, more polar molecule will need more extreme forcing conditions to dye the polymer2.
The most important class is the azo class. This class of azo disperse dyes may be further sub-divided into four groups, the most numerous of which is the aminoazobenzene class. This class of dye can be altered as mentioned before, to produce bathochromic shifts. A range of heterocyclic aminoazobenzene dyes are also available. These give bright dyes, and are bathochromically shifted to give blues. The third class of disperse dye is based on heterocyclic coupling components, which produce bright yellow dyes. The fourth class are disazo dyes. These tend to be quite simple in structure. Other than these, there are disperse dyes of the carbonyl class, and a few from the nitro and polymethine classes. Below is an example of a disperse dye. It is the same as the chime molecule at the top of the page.