Generally, azo dyes have low light fastness.
This is because the electron cloud density of the nitrogen atom on the azo group (-N= N-) in the dye is high, the light stability is poor, and it is relatively easy to break due to the photooxidation reaction. However, in the molecular structure of azo dyes, electron-withdrawing groups (-Cl, -CN, -Br, etc.) are introduced in the ortho position of the azo group, and their electron-withdrawing effect is used to make the electron cloud density of the azo nitrogen atom Decreased, light stability is improved, so as to obtain good light fastness.
For example, Disperse Orange S-4RL (C.I. Disperse Orange 30)
Due to the introduction of an electron-withdrawing group (-Cl) in the ortho position of the azo group, when dyeing polyester in an amount of 1% (o.w.f), the light fastness can reach 6 to 7 grades (ISO 105-B02 blue label).
The aminoanthraquinone-based disperse dyes also have advantages and disadvantages in light fastness.
The influencing factors are mainly based on the density of α-amino electron cloud. The lower the electron cloud density of α-amino group, the less prone to photooxidation reaction, the better the light stability and the higher the light fastness.
For example, Disperse Red E-3B (C.I. Disperse Red 60)
Because it is an electron-withdrawing group, the density of -NH2 electron cloud can be greatly reduced, so when disperse red E-3B 1% (o.w.f) dyes polyester, it has a fastness to sunlight of grade 7.