What types of disperse dyes are there?

Disperse dyes can be classified in two ways: by application performance or by chemical structure.

I. Classification by application:

The application performance classification method divides disperse dyes into three main categories based on dyeing performance: E-type, S-type, and SE-type.

1. E-type disperse dyes:

E-type disperse dyes are also known as low-temperature disperse dyes.

They are characterized by a small molecular weight, good leveling, migration, and covering properties, low sublimation fastness, and high low-temperature fixation, which decreases with increasing baking temperature. These dyes are suitable for high-temperature, high-pressure, and carrier-based dyeing, and are also used in transfer printing.

2. S-type disperse dyes:

S-type disperse dyes are also known as high-temperature disperse dyes.

They are characterized by a large molecular weight, poor leveling, migration, and covering properties, high sublimation fastness, and low low-temperature fixation, which increases with increasing baking temperature, reaching their highest fixation at 220°C. These dyes are suitable for hot-melt dyeing. 

3. SE-Type Disperse Dyes:

SE-type disperse dyes, also known as medium-temperature disperse dyes, are a type of disperse dye between E-type and S-type.

They are characterized by a moderate molecular weight, moderate leveling, migration, and covering properties, moderate sublimation fastness, and a flat fixation curve with increasing baking temperature, without variations in fixation due to baking temperature fluctuations. These dyes are suitable for all three types of disperse dye dyeing processes and represent the most diverse of the three types.

II. Classification by Chemical Structure:

Based on their chemical structure, disperse dyes can be divided into monoazo, anthraquinone, heterocyclic, and methine types.

1. Monoazo Disperse Dyes: 

Monoazo disperse dyes cover the entire spectrum of yellow, orange, red, violet, blue, green, brown, and black. They consist of a p-nitroaniline derivative as the diazo component and N-alkyl and N,N-dialkylanilines as coupling components. Their simple chemical structure meets the requirement for a low molecular weight. Their simple synthesis process and high conversion rate contribute to their low price. The chemical structure of monoazo disperse dyes is based on a diazo component as an electron-withdrawing group and a coupling component as an electron-donating group. This lowers the excited-state energy level of the dye, resulting in a darker color effect. According to molecular orbital theory, the lone pair of electrons on the nitrogen atom of the amino derivative in the coupling component can form a non-bonding orbital. This electron excitation will excite the higher-energy non-bonding orbital to the excited state, reducing the energy level difference. Therefore, the electron-donating substituents on the amino group and N,N-substituted amino groups have a strong darker and more intense color effect. Because the primary nitroamine in the diazo component is a strong electron-withdrawing substituent, the overall polarity of the dye molecule is enhanced, the excited-state energy level is lower, and the darker and more intense color effect is more pronounced.

2. Anthraquinone-type disperse dyes:

Anthraquinone-type disperse dyes have vibrant colors, primarily in dark colors such as red, purple, blue, and turquoise. Anthraquinone-type disperse dyes have a simple chemical structure similar to monoanthraquinone, and their color-producing ability is most pronounced in anthraquinone-type disperse dyes. If two electron-donating groups, such as -NH2 and -OH, are introduced at the a-position, a darkening effect is more pronounced, especially if both substituents are on the same benzene ring. If electron-donating groups are introduced at all four a-positions, the darkening effect is even more pronounced.

3. Compared with aromatic amine-based azo disperse dyes, heterocyclic disperse dyes offer brighter colors, higher fastness, and better build-up.

Heterocyclic disperse dyes are further divided into heterocyclic azo disperse dyes (with heterocyclic rings as diazo components or coupling components) and heterocyclic condensation disperse dyes (with heterocyclic rings linked together by other conjugated systems rather than azo groups). Although heterocyclic disperse dyes have been recognized for nearly three decades and described in numerous patents, and many have been commercially produced, only a few molecular structures of these dyes have been publicly disclosed in the Colour Index to date. 

4. Methanogen-Type Disperse Dyes:

Methanogen I structured disperse dyes offer exceptionally vibrant yellows with excellent lightfastness. They can be used to dye not only polyester, but also acetate and nylon, with excellent leveling and lift. These dyes are primarily composed of one or more methine (methine) groups forming a conjugated system, with an electron-withdrawing group and an electron-donating group attached to either end. They are primarily used as optical sensitizers and cationic dyes. Disperse dyes have a particularly simple molecular structure and are limited in variety, with only about a dozen known structures.