Basics of dyeing principles

1. What is dyeing?

Dyeing is the process of dyeing fiber materials with color. It makes the entire textile a colored object by means of a physical, chemical or chemical combination of dyes and fibers, or by chemically generating dyes on the fibers. Dyeing products not only require uniform color, but also must have good dye fastness.

Depending on the objects of dyeing processing, dyeing methods can be divided into three types: fabric dyeing, yarn dyeing and loose fiber dyeing. Among them, fabric dyeing is widely used. Yarn dyeing is mostly used for colored fabrics and knitted fabrics, while loose fiber dyeing is mainly used for the production of blended or thick fabrics, both of which are mainly woolen fabrics.

The purpose of studying dyeing is to rationally select and use dyes, correctly formulate and carry out dyeing processing, and obtain high-quality dyed finished products.

2. Dyeing Theory

When the fiber is immersed in a dye aqueous solution at a certain temperature, the dye will move from the water phase to the fiber. At this time, the amount of dye in the water gradually decreases. After a period of time, , an equilibrium state is reached.

The dye reduced in the water is the dye that moves to the fiber. Take out the fiber at any time, and even if it is twisted, the dye will still remain in the fiber, and the dye cannot simply be completely separated from the fiber. This phenomenon of dye being combined in the fiber is called dyeing.

If the sponge is immersed in the dye solution, the dye solution can also enter the inside of the sponge. However, the concentration of the dye solution does not change even if it takes a long time. When the sponge is taken out and twisted, the dye and water will escape from the sponge at the same time. It is squeezed out, so the sponge is not stained.

3. The basic process of dyeing

According to the modern dyeing theory, the reason why dyes can dye fibers and have a certain fastness on fiber fabrics is because the dye molecules and There are various gravitational forces between fiber molecules. The dyeing principles and dyeing processes of various dyes are very different due to the respective characteristics of the dyes and fibers and cannot be generalized.

As far as the dyeing process is concerned, it can be roughly divided into three basic stages.

　1. Adsorption

When the fiber is put into the dye bath, the dye first diffuses to the fiber surface, and then gradually transfers from the solution to the fiber surface. This process is called adsorption. As time goes by, the dye concentration on the fiber gradually increases, while the dye concentration in the solution gradually decreases, and after a period of time, an equilibrium state is reached. The reverse process of adsorption is desorption, and adsorption and desorption exist simultaneously during the dyeing process.

　2. Diffusion

　The dye adsorbed on the surface of the fiber diffuses into the interior of the fiber until the dye concentration in each part of the fiber becomes consistent. Since the dye concentration adsorbed on the fiber surface is greater than the dye concentration inside the fiber, the dye is promoted to expand from the fiber surface to the inside of the fiber.scattered. At this time, the diffusion of the dye destroys the initially established adsorption equilibrium, and the dye in the solution will continue to be adsorbed to the fiber surface, and the adsorption and desorption will reach equilibrium again.

　3. Fixation

　It is the process of combining dyes and fibers. The combination methods vary with dyes and fibers.

The above three stages often exist at the same time in the dyeing process and cannot be completely separated. It’s just that a certain process is dominant during a certain period of time during dyeing.

4. How the dye is fixed in the fiber

The fixation of the dye in the fiber can be considered as the process of keeping the dye on the fiber. Different dyes and different fibers have different fixation principles. Generally speaking, there are two types of dyes fixed on fibers.

　1. Pure chemical fixation

It refers to the chemical reaction between the dye and the fiber, which fixes the dye on the fiber.

For example: reactive dyes dye cellulose fibers and form ether bonds with each other.

The general formula is as follows:

DRX+Cell-OH → DR-0-Cell+HX

DRX: reactive dye molecule

X: active group

Cell-OH: represents cellulose

　2. Physical and chemical fixation

Due to the mutual attraction between the dye and the fiber and the formation of hydrogen bonds, the dye is fixed on the fiber. Many dyes used to dye cotton, such as direct dyes, sulfur dyes, vat dyes, etc., rely on this attraction to be fixed on the fiber.