فارسیFixing Agents: Definition and Mechanism of Action
Reactive dyes are among the most commonly used dyes in the textile industry for dyeing cotton fabrics, due to their advantages and wide range of applications. Reactive dyes are the only type of dyes that form covalent bonds with fibers, giving them high stability. They are also widely used because they offer a broad range of colors.
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After the dyeing process, the fabric surface often covered with unabsorbed or hydrolyzed dyes that have not formed any bonds with the fiber, these dyes must be removed during washing; otherwise, they can cause significant problems in subsequent processes, such as reduced wash fastness or uniformity and color shifts.
After washing with hot water and a soaping agent the fastness process on the fabric begins. Fixing agents are commonly applied in these types of finishing processes.
Fixing agents belong to a group of substances that enhance dye fastness on fabric surfaces and, during the washing process, reduce the number of rinsing cycles required after dyeing. They improve the final quality by forming a thin protective layer on the fabric.
In simple terms, the reactive dyes that do not form covalent bonds or react with the fiber remain loosely attached to the fabric surface. By applying a fixing agent, these unfixed dyes are stabilized, which significantly improves both dry and wet rubbing fastness for most reactive dyes.
Fixing Agent: ionic structure and Mechanism on the fabric
Some of fixing agents, such as aliphatic polyamines, create ionic interactions between the fabric surface and unfixed reactive dyes due to their ionic structure. These agents do not react with cationic dyes; they only form a weak surface interaction without creating a strong bond.
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Fixing molecules combine with unfixed dye molecules on the fabric surface and forms insoluble colored precipitates; this occurs due to the presence of cationic groups in the molecular chain of the fixing agents. When these cationic groups interact with dye molecules, electrostatic bonds will form to improve the dye fastness on the fibers; This happens due to the reduction of dye solubility in the aqueous environment. The fixing component in this mechanism is known as the Active Cationic Surface. Researches has shown that the higher the number of cationic groups in the fixing agent, the greater the improvement in the dye rubbing fastness.
Moreover, the stability of dyes in terms of rubbing, friction, and overall fastness can be improved by the properties of fixing agents and their ability to reduce the fabric’s friction factor. The fixing agent with this mechanism is usually called a resin fixing agent. The more effectively it forms film, the better the dye fastness will be.
For instance, most polymerized compounds can exhibit a special dye-fixing effect with small Elphin molecules; even though the molecular chain lacks reactive groups. This occurs because the polymer has film-forming properties and creates a layer over the dyed fabric, thereby reducing the dye’s solubility in water. Meanwhile, the molecular weight of the fixing agent can slightly influence its effect on dye fastness.
For instance, dye-fixing substances with polyamine bases improve dye fastness by increasing molecular weight. Some researchers indicate that adding active groups during polymer synthesis not only promotes film formation but also enables dye complexation, making the dye insoluble in water, which in turn enhances dye fastness.