Classification of disperse dye for polyester

The SDC classification of disperse dyes is based on migration ability during exhaust dyeing, colour build-up, sensitivity to changes in temperature and the rate of dyeing.

This type of dye is often classified on the basis of dyeing rate and sublimation fastness, particularly for polyester dyeing. These two properties are a function of molecular weight and the number of polar groups in the dye molecule. Table shows the most common classification. It applies to the dyeing of acetate, of polyester with or without carrier, and of polyester/cotton, but is somewhat arbitrary.

Classification	Molecular  weight	Polarity	Dyeing Rate	Sublimation
Low energy	Low	Low	High	Low
Medium energy	Moderate	Moderate	Moderate	Moderate
High energy	High	High	Low	High

Disperse dyes can be further classified as-

1. No-suffix dye: Not suitable for polyester but for acetate & nylon.
2. Group A dye: poor sublimation fastness suitable for acetate & nylon.
3. Group B dye: Moderate sublimation fastness.
4. Group C dye: Good sublimation fastness; suitable for all method of polyester.
5. Group D dye: Maximum sublimation fastness; exclusive use for PET(Polyethylene terephthalate) dyeing.

Most dyeing and fastness properties change gradually with increase in molecular size. Small dye molecules with low polarity are leveling, rapid dyeing dyes with poor heat resistance. These are called low energy disperse dyes.

More polar, higher molecular weight dyes have low dyeing rates, poor migration during dyeing but good heat and sublimation fastness. These constitute the high energy disperse dyes. The development of disperse dyes of improved sublimation fastness required dye molecules with relatively polar and hydrophilic substituents to reduce their vapour pressure at high temperatures. This promotes somewhat higher solubility in water but the increase in molecular size reduces the dyeing rate at a given temperature. The high energy disperse dyes are those requiring a higher Thermosol temperature. The light fastness does not depend on the molecular size of disperse dye.

Dyes in a mixture are usually selected from the same energy class. Build-up of the colour on shade requires that the dyes all have about the same dyeing rate. Testing of dye recipes is essential because many disperse dyes, even dyes of the same dyeing group, are incompatible in mixtures. This is true even though they may have the same dyeing rates and build-up properties when tested separately. The dye manufacturers provide considerable information assisting the dyer to select appropriate dyes for a given application.