Vat dyes are still the first choice among all kinds of dyes due to their superior fastness. However, the dyeing of cotton yarn with vat dyes is affected by many factors, and it is not easy to increase the rate of level dyeing products, which brings a lot of troubles to the dyeing circles at home and abroad.

Measures to solve the level dyeing defects of cotton yarn with vat dyes

1. Pre-treatment

At present, the enzymatic hydrogen peroxide scouring and bleaching process is mostly used in foreign countries to use pectinase to add compatible non-ionic penetrants, catalysts, buffers and complexing agents at a certain pH value and temperature for pretreatment. The pectin on the primary cell wall of cotton fiber is degraded and removed. As the pectin acts as a bond between the cotton fiber and the cotton wax, the cotton wax can be easily removed in the subsequent saponification and emulsification after the pectin is removed. The pretreatment of cotton yarn directly affects the dyeing effect. You should always check whether there are spots, maculas, alkali spots, and calcium spots during scouring; whether there are floating spots when bleaching; whether there are defects such as mercerizing spots when mercerizing. It is not enough for the scouring effect to rely solely on the capillary effect to meet the standards. It is necessary to regularly check whether the residual amounts of waxy substances, nitrogen-containing substances and pectin substances meet the standards.

2. Dyeing water

If the dyeing water has a high hardness, the sodium salt of the vat dye leuco will form insoluble calcium and magnesium salts with the calcium and magnesium ions in the hard water, resulting in uneven dyeing.

The softening methods of dyeing water mainly include ion-exchange resin softening method, alkali softening method and complex softening method. If the factory does not set up an ion exchange resin softening workshop, the simple and easy sodium hexametaphosphate complex softening method can be used to soften the dyeing water.

Hard water becomes soft water after 20 min cold treatment.

The vat dyes that are particularly sensitive to calcium hardness are: vat blue RSN, GCDN, BC vat yellow G: vat orange RRT; vat dyes that are particularly sensitive to magnesium hardness are: vat orange 4R; thioindigo red B, 3B; thioisatin Lotus RRN; Reduced brilliant green lotus 3B, BBK, ZR, 4R: reduced blue BC; reduced brilliant green B, FFB, ZG, 4G: reduced gray M, RRH; reduced red brown GR.

3. Dyeing equipment and operation

Regardless of the cheese flow dyeing machine or the hank-cage liquid flow dyeing machine, the pump of the dyeing machine must be checked frequently and pay attention to whether the angle, speed, power, head, and flow of the pump meet the requirements.

The cross section of the triangular roller of the hank lifting dyeing machine should be 9 cm on each side, and the roller distance should be 20 cm. The speed is 30r/min for fast rotation, 15r/min for slow rotation, 10 cm for swing, 30 pendulum/min for fast swing, and 15 for slow swing Pendulum/min. Each packet of yarn (5kg) should occupy 6 to 7 feet on the triangular roller. Avoid the skein from being too thick or entangled on the triangular roller. The steering of the triangle roller should be freely controlled.

When dyeing on a cheese stream dyeing machine, first notice that the density of the cheese should be appropriate and even, and the reversal is the main alternative when dyeing. When dyeing on the hank hanging cage liquid flow dyeing machine, first pay attention to the cage operation. The skein should be straightened and loosened in advance, and the skein should be packed into the cage while flushing, and compacted with a yarn press. When the cover is installed and reversed, the skein in the cage will shrink, and the cage cover should be tightened again without leaving a gap to prevent short-circuit. Be sure to install it evenly. When dyeing, the positive and negative rotations are alternately mainly forward.

4. Compatibility of dyes

Many uneven dyeing defects are caused by improper dye selection during color matching. If the initial dyeing speed difference of the selected color matching dyes is too large, there are compatibility or compatibility problems in the dyeing method, which will inevitably lead to serious uneven dyeing defects.

5. Reduction potential of the dye itself

The negative value of the reduction potential of individual vat dyes is extremely high, which is not easy to reduce and is easy to prematurely oxidize, and precipitates precipitation. When dyeing on a lifting or hank dyeing machine, the yarn is exposed to the air, and it is very easy to produce "black skin" color dyeing defects. When dyeing, pay attention to avoid wind blowing. It is best to add a cover on the dyeing tank to avoid this disadvantage. For these dyes, special attention should be paid to the mixing and dry vat operation. It is best to grind the dye powder with a ball mill in advance. When the dye powder is added to the dye vat, stir it out through the screen.

The reduction potential of the vat dye leuco body is the potential when the leuco body begins to oxidize in the solution to precipitate a precipitate. The reduction potentials of several vat dyes are as follows: thioindigo lotus RH is -720 mV, thioindigo red R is -730 mV, thioindigo RF is -780 mV, reduced ash M is -760 mV, and vat blue RSN is -850 mV, the reduced brilliant green FFB is -865 mV, the reduced gray BG is -910mV, the reduced brown R is -920 mV, the reduced olive R is -927 mV, the reduced brown G, ZG, and 3G are made of brown R and olive R. The reduction potential is also very negative.

The dyeing of cotton yarn with vat dyes has relatively high requirements on the quality of cotton yarn pretreatment, dyeing water, dyeing equipment and operation requirements. In addition, the compatibility between the vat dyes and the oxidation-reduction potential of the vat dyes directly affect the dyeing effect.