Disperse dyes are classified in two ways: based on application performance or based on chemical structure.
1. Classification by Application:
Classification by application performance categorizes disperse dyes into three main types: Type E, Type S, and Type SE.
Type E Disperse Dyes
Type E, also known as low-temperature disperse dyes, are characterized by their small relative molecular mass, good uniformity in dyeing, transferability, and coverage; low sublimation fastness, high fixative rate at low temperature baking, which decreases as the baking temperature increases. These dyes are suitable for high-temperature high-pressure and carrier method dyeing, partially used in transfer printing.
Type S Disperse Dyes
Type S, also known as high-temperature disperse dyes, are characterized by their large relative molecular mass, poor uniformity in dyeing, transferability, and coverage; high sublimation fastness, low fixative rate at low temperature baking, which increases with the temperature, reaching the highest fixation rate at 220°C. These dyes are suitable for thermo-fusion dyeing.
Type SE Disperse Dyes
Type SE, also known as medium-temperature disperse dyes, fall between Type E and S. They are characterized by their moderate relative molecular mass, uniformity in dyeing, transferability, and coverage; medium sublimation fastness, with the fixation rate not significantly increasing with baking temperature — resulting in a flat fixation rate curve, unaffected by fluctuations in baking temperature. These dyes are suitable for all three disperse dyeing processes and have the widest variety among the three types.
This classification method was first introduced by Sandoz (now Clariant, currently Archroma). Foron-branded disperse dyes were named following this classification; British ICI (later Zeneca) company's Dispersol-branded disperse dyes were divided into five categories: A, B, C, D, and P, with Type A suitable for acetate and nylon fibers, Type P for printing, and types B, C, and D for polyester fibers (corresponding to Types E, SE, and S).
2. Classification by Chemical Structure:
Disperse dyes can be classified based on chemical structure into monoazo, anthraquinone, heterocyclic, and cationic types.
Monoazo Disperse Dyes
Monoazo disperse dyes cover the entire color spectrum including yellow, orange, red, purple, blue, green, brown, and black. They are comprised of para-nitroaniline derivatives as the diazo components and N-alkyl or N,N-dialkylaniline as the coupling components. Their chemical structures are simple, aligning with the requirement for small relative molecular mass. They have simple synthesis processes and high conversion rates, hence are economically priced. Monoazo disperse dyes' chemical structures have the diazo component serving as an electron acceptor and the coupling component as an electron donor, causing a deep color effect. According to molecular orbital theory, the lone pair electrons on the nitrogen atom in the amine derivatives of the coupling component can form non-bonding orbitals, with electron excitation moving from a higher-level non-bonding orbital to the excited state. Since the diazo component, para-nitroaniline, is a strong electron-withdrawing group, it endows the entire dye molecule with stronger polarity and a lower excited state energy level, resulting in a more pronounced deep and intense color effect.
Anthraquinone Disperse Dyes
Anthraquinone disperse dyes are vibrant, mainly in deep shades like red, purple, blue, and bright blue. Anthraquinone disperse dyes have a simple chemical structure consisting only of anthraquinone, with their coloring ability most pronounced at the anthraquinone moiety. If two electron-donating groups, such as -NH2 and -OH, are introduced at the a position, a significant deep color effect is observed, especially when both substituents are on the same benzene ring. If all four a positions are substituted with electron-donating groups, the deep color effect is even more pronounced.
Heterocyclic Disperse Dyes
Compared with aromatic amine azo disperse dyes, heterocyclic disperse dyes exhibit brighter colors, higher fastness, and better uplifting power. Heterocyclic disperse dyes are further divided into heterocyclic azo disperse dyes (with heterocycles as either diazo or coupling components) and heterocyclic condensation disperse dyes (which connect the heterocycles using other conjugated systems instead of azo groups). Though there has been a certain understanding of heterocyclic disperse dyes over the past thirty years, described in many patents, and many have been industrially produced, to this day, only a small number of heterocyclic disperse dyes have their molecular structures publicly disclosed in the "Dye Index".
Cationic Disperse Dyes
Cationic disperse dyes with methine structure feature vibrant yellow shades with excellent lightfastness. In addition to dyeing polyester fibers, they can also dye acetate and nylon, displaying excellent uniformity and uplifting power. The main structure of these dyes consists of one or more methine (methine) conjugated systems, connected at both ends by an electron-withdrawing group and an electron-donating group, mainly used for optical brighteners and cationic dyes. The molecular structures of cationic disperse dyes used as disperse dyes are particularly simple, with only a dozen or so varieties publicly disclosed.