Acid dyeing of protein fibers, in addition to the composition, chemical properties and physical state of the fibers and dyes themselves have a direct impact on the dyeing, external conditions such as pH value of the dye solution, temperature and the addition of electrolytes also directly affect the dyeing process.
1. The pH value of the dye bath
When the pH value of the solution is low, the wool is positively charged, and there are many opportunities for the wool to combine with inorganic anions (chloride ions or sulfate ions) and dye anions. At this time, wool is easy to dye.
Therefore, controlling the pH value of the dye bath also controls the amount and speed of dye adsorption on wool. Adding acid during the dyeing process increases the concentration of hydrogen ions in the dyeing bath, making the wool easier to dye. For the purpose of leveling, it can be added in stages.
Choose different acids according to the type of dye, such as sulfuric acid for strong acid bath dyeing, acetic acid for weak acid bath dyeing, and ammonium acetate or ammonium sulfate for neutral bath dyeing.
2. The addition of electrolytes
The effect of adding electrolytes in wool dyeing is related to the pH value of the dyeing bath. That is, when the pH value of the dye bath is below the isoelectric point of wool, the wool and the dye are mostly bonded by salt, and the electrolyte is added to retard the dyeing effect; when the pH value of the dye bath is higher than the isoelectric point of the wool, the wool and the dye are mostly hydrogenated. The bond and van der Waals force are combined, and the addition of electrolyte plays a role in promoting dyeing.
The relative molecular mass of acid dyes is about 300-800. At typical staining concentrations and room temperature, the dyes rarely aggregate. However, the acid dyes dyed in weak acid bath and neutral bath will aggregate at room temperature. Only higher temperature, or even boiling dyeing, can be used to reduce the aggregation tendency of dyes.
The greater the tendency of dyes to aggregate, the more significant the effect of temperature. Increasing the temperature can also promote the bulking of wool, which is conducive to the diffusion of dyes into the fiber. When the temperature is above 50 ℃, increasing the temperature has a significant effect on the bulking of the fiber.
The scale layer of wool greatly hinders the diffusion of dyes, so dyeing requires boiling and prolonged time. In order to avoid uneven dyeing, at the beginning of dyeing, the dyeing temperature should be low, and then gradually heat up to boiling. Controlling the temperature rise is very important for level dyeing.