Why does the color change after finishing the dyed fabric? How to deal with it?

1. Causes

After dyeing, the color of the fabric will change to a certain extent after finishing. This is a common phenomenon in dyeing. After analysis, the following factors are closely related to color fluctuations.

① After dyeing, many dyes are not in a stable state on the fiber, but in a metastable state (reduced dyes are the most prominent). After wet and heat treatment, the following changes may occur:

a. The dye molecules in the fiber that are in a highly dispersed state will further aggregate, which will cause a certain change in the crystal state of the dye.

b. The dye molecules in the fiber will change from the parallel state of the fiber molecular chain to the vertical state, changing the existence orientation of the dye.

c. Dyes with a tendency to molecular isomerization (cis-trans) will undergo configuration changes.

Once the dyes in the fiber undergo changes in crystallization, orientation, configuration, etc., the light absorption properties of the dyes will inevitably change, thereby causing color fluctuations.

② Commonly used finishing agents (softeners, waterproofing agents, antibacterial agents, anti-ultraviolet agents, etc.) will produce the following problems during wet and heat treatment:

a. There will be some complex chemical reactions between cationic additives and cationic dyes.

b. The additive film on the fiber will have a certain degree of "yellowing".

c: The pH value (mainly acidity) of the finishing liquid will cause the color of the dye to change.

d. The finishing agent will cause the disperse dye to produce different degrees of "thermal migration" and "thermal sublimation" under high temperature conditions. This complex relationship between the finishing agent and the dye will inevitably affect the dyeing color.

③ The color of the dyed cloth after finishing may change during the placement process. The main influencing factors are the following three points:

a. Commonly used dyes generally have different "three-sensitivity" (humidity-sensitive, heat-sensitive, and light-sensitive) phenomena. That is, the dyeing color will vary with the amount of fiber moisture content, the temperature, and the intensity of light. Therefore, once the moisture content, temperature, and light of the fabric change, its color will be different.

b. Minerals and heavy metal compounds (mainly from water and finishing agents) remaining on the fabric will slowly react with the dyes in a complex chemical reaction during the placement of the colored cloth, thus causing changes in color.

c. During the placement of the colored cloth, the acidic, alkaline gases or oxidizing, reducing gases in the surrounding environment will also cause the stability of the dye to decrease, and then the color will change.

2. The countermeasures are as follows

① After dyeing, it is necessary to wash (soap) thoroughly. There are two reasons:

First, sufficient washing (soaping) can promote the dye in the fiber to achieve a stable state in terms of crystallization, orientation, configuration, etc.;

Second, floating color (including staining) dyes are more sensitive to moisture, heat, light, acid, alkali, and chemicals, and are more likely to cause changes in color.

Thoroughly washing (soaping) the floating color and removing various dyeing chemicals as much as possible will undoubtedly improve the stability of the color.

② The dye should be selected. The "three-sensitivity" phenomenon is not obvious, and the color is less affected by the finishing agent and acid and alkali. Disperse dyes should be selected with good "thermal migration" and "thermal sublimation" fastness.

③ Use the best finishing agent. Choose the one with less yellowing and slight effect on color. For this purpose, a pioneer test must be done in advance.

④ The production workshop should strengthen ventilation and improve the cleanliness of the air. In addition, the cloth box should be covered and the finished product should be timely.

Implementing the above comprehensive measures can significantly improve the color stability of dyed cloth.

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