Temperature effect and salt effect of direct dyeing

(1) Temperature effect

Temperature has different effects on the dyeing rate of direct dyes.

The structure is relatively simple, easy to dissolve, low affinity to fibers, and high diffusion rate.

The structure is relatively complex, difficult to dissolve, has a high affinity for fibers, and has a low diffusion rate.

The diffusion rate is a characteristic of the dye. The diffusion rate is high, the migration performance is good, the level dyeing property is good, and the washing fastness is poor. For example, the diffusion rate of the A dye, the B and C dyes is low. Once uneven, it is difficult Dyeing is corrected, the washing fastness is high, the high temperature increases the diffusion rate, but the equilibrium dyeing percentage is reduced. The dyes with high diffusion rate and low affinity should be dyed at a lower temperature to obtain a higher dyeing percentage. On the contrary, a higher dyeing percentage should be used. High temperature dyeing is beneficial to obtain higher dye uptake in a short time. Regenerated cellulose fibers such as viscose have different amorphous areas and different orientations, which will cause uneven dyeing, and increasing the temperature will promote dye migration. Reduce uneven dyeing.

(2) The role of salt

Dissociate into pigment anions in water and dye cellulose fibers.

Cellulose fibers are negatively charged in neutral or weakly alkaline dyeing baths. The effect of sodium sulfate or table salt on direct dyes is called dye promotion. Through the dye-promoting effect of salt, the dye uptake rate and the dye uptake percentage can be increased. Van der Waals force is inversely proportional to the 6th power of the intermolecular distance, and the effective distance is very small. Coulomb gravitation is inversely proportional to the second power of the distance. far. When the dye anion approaches the fiber interface, it is first affected by the fiber repulsion. Only those dye anions that have higher kinetic energy in an instant due to molecular collisions and can overcome the Coulomb repulsion can break through the barrier and enter a certain distance. At this time, Van der Waals force exceeds the Coulomb repulsion force, and adsorption occurs. Sodium ions of salt (sodium sulfate or table salt) are adsorbed on the surface of the fiber, which can reduce the charge repulsion, increase the dyeing rate and the dyeing percentage. The effect on the salt with more sulfonic acid groups Dyes have obvious dye-promoting effects.