What is the mechanism of action of fixing agents?

Film-forming and sealing agents primarily function through three mechanisms, with the specific mechanism depending on the type of fixing agent and the type of dye:

Film Formation and Sealing

Description: This is the most straightforward mechanism. The fixing agent forms a colorless, dense film on the fiber surface. This film acts like a "protective suit," encapsulating the dye within the fiber and physically preventing dye molecules from contacting water and dissolving.

Application: This method is effective for a wide variety of dyes, especially direct and reactive dyes.

Adsorption and Cross-linking

Description: Fixing agent molecules typically carry a positive charge (cationic), while most dye molecules (especially anionic dyes such as direct, reactive, and acid dyes) and the fiber surface carry a negative charge in water. The positive and negative charges attract each other, causing the fixing agent to be firmly adsorbed onto the fiber and dye.

Furthermore: Many fixing agent molecules are "reactive." They not only adsorb but also chemically react with certain groups on dye and fiber molecules, forming strong "cross-linked bonds" (like bridges) that tightly network the dye, fiber, and fixing agent together.

Application: This is one of the most important and effective mechanisms for fixing reactive dyes.

Reducing Water Solubility

Description: Some dyes are inherently water-soluble (such as direct dyes). Fixing agents can react with dye molecules, increasing their molecular size or altering their properties, thereby reducing the dye's solubility in water. Difficulter-to-dissolve dyes are naturally more difficult to remove from fibers.

Application: Primarily used for direct dyes.

For Reactive Dyes:

Challenge: Reactive dyes bind to fibers through covalent bonds, but some dyes hydrolyze during dyeing. These hydrolyzed dyes are not firmly bound, which is the main cause of fading.

Fixing Function: Fixing agents adsorb and "capture" these hydrolyzed dyes through cationic charges and fix them through cross-linking, preventing them from staining the fabric or other parts.

For direct dyes:

Challenge: Direct dyes are highly water-soluble and bind to fibers primarily through van der Waals forces and hydrogen bonds, resulting in poor colorfastness.

Fixing: Colorfastness is improved mainly by forming a thin film and reacting with the dye to reduce its solubility.

Application in immersion vs. padding:

Immersion: Conducted in a dye bath, the fixing agent is uniformly adsorbed onto the fibers. Suitable for yarns, knitted fabrics, etc.

Padding: The fabric passes through a solution containing the fixing agent, is then pressed by rollers, and finally dried. Suitable for high-volume continuous processing of woven fabrics, offering high efficiency.