The Action Mechanism of Acidic Dye Fixing AgentAcidic dye fixing agents are generally aromatic sulfonic acid condensates with high molecular weight, which can improve the water fastness, seawater fastness, washing fastness and perspiration fastness after fixing. High molecular weight aromatic sulfonic acid condensates are coated on the surface of the fibers. The negative charge of sulfonic groups on the surface of the fibers repels the acid dyes in the fibers, which prevents the dyes from coming out. The coating effect also reduces the swelling of the fibers, the chance of dyes binding with water molecules, and thus it improves the fastness.
Several problems should be paid attention in fixing acid dyes.
First, the fixing temperature is generally between 70 and 80 degrees. High molecular weight aromatic sulfonic acid condensates enter the fibers and lose their coating effect. As a result, the negative charge of sulfonic group decreases the repulsion of acid dyes in the fibers, resulting in a decrease in fastness.
Second, fixing time should not be too long. If the time is too long, high molecular weight aromatic sulfonic acid condensates will diffuse into the fiber, reducing the coating effect, resulting in a decline in fastness. Acidic dye fixing agents are generally high molecular weight aromatic sulfonic acid or carboxylic acid condensates, which are easy to precipitate under strong acid conditions, that is, precipitation occurs, it is easy to produce fixed spots when fixing.
Third, acetic acid should not be mixed with fixing agent to avoid precipitation.
Fourth, when fixing, it is better to use soft water to avoid precipitation of calcium and magnesium ions and dye fixing agents.
Fifth, before fixing, the cloth surface should be washed to avoid fixing spots.