Why is the blackness of polyester fabric dyed black not ideal?

When pure polyester fabrics are dyed with disperse dyes, especially filament fabrics, they always look bright and white after dyeing. Increasing the dye concentration will not help, and the color fastness will be greatly reduced.

There are two reasons for the low blackness:

1. The cross-section of polyester fiber is smooth and circular, and the reflection of light is more specular, that is, the white light is more concentrated, so it does not look black.

2. The refractive index constant of polyester fiber is the largest among all fibers. For example, the refractive index of wool is 1.553, silk is 1.598, viscose fiber is 1.577, nylon is 1.580, and polyester is 1.725. An object with a large refractive index has a large reflectivity. The reflected light is reflected in white light, so it looks white.

The high reflectivity means that the amount of light absorbed is reduced, so the color depth is not large.