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What are the effects of pH on the dyeing and finishing process?

Date:Jan 22, 2021

Online measurement and control of pH during the dyeing process is particularly important for improving the quality of textiles.

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1. The pH value of denim "ring dyeing" must be controlled


Indigo dye is a vat dye, which needs to be reduced to a soluble leuco body by alkaline reduction before dyeing the fiber. Since the affinity of the dye and fiber is only 1/60~1/10 of that of general dyes, if the redox potential value (-760mV) is not reached, the affinity will be worse and the coloring rate will be lower.


Therefore, the control of oxidation-reduction potential or pH value in the dyeing process is the key to whether the denim can be dyed well. Monophenol sodium ion type indigo leuco body and bisphenol sodium ion type indigo leuco are the main forms absorbed by cotton fiber, but from the aspects of color yield, color fastness, and luster, monophenol sodium ion type indigo leuco Body is more important.


In order to produce more monophenol sodium ion-type indigo leuco body during the reduction process of the dye solution, the pH value of the dye bath is controlled at 10.8~11.2, so that the ionization of dye and fiber is minimized, and there is almost no ion repulsion. Under the circumstances, the relative affinity of the dye to the cotton fiber is increased, resulting in a higher instant dyeing rate of the dye to the cotton fiber, and the ring dyeing of the denim yarn required by the process is obtained.


As the pH value of the dye liquor drops, the rubbing fastness generally shows a downward trend. When the pH value is 7.5, the dye solution is basically a reduced non-ionic leuco body, which is slightly soluble in water and cannot be dyed;


With the increase of pH value, the monophenol sodium ion type indigo leuco body increases continuously. When the pH value reaches 11.0, the bisphenol sodium ion type indigo leuco body begins to appear. When the pH value rises to 13.5, it is basically Sodium bisphenol ionic indigo leuco body, at this time, the dye uptake rate will decrease, the color fastness will decrease, and the color will be gray.


If fast washing and fading of denim is required, the pH value of the dye bath should be controlled at 11.0~11.5; if fast washing and fading is not required, the pH value can be increased to 12.5~13.5, reducing dye affinity and improving permeability.


When the pH value is between 11.0 and 11.4, the absorbance reflectance is the highest, which is inconsistent with the pH value when the dye's maximum absorption is between 9.5 and 10.5. This shows that when the pH value is greater than 10.5, at least part of the indigo dye will precipitate, but it has no effect on increasing the color depth.


During the indigo dyeing process, the pH value changes at any time. In order to ensure the stability and reproducibility of the dyeing quality, it is urgent to carry out online automatic measurement and control of the pH value.



2. The pH value affects the fiber reaction rate


When promoting the application of the two-phase printing process, people often ask why the fast steaming takes only ten seconds?


The reaction rate between the dye and the fiber is the product of KF [DF] [Cello -]. When the concentration of the dye on the fiber [DF] is fixed, the reaction rate between the fiber and the dye is mainly related to the cellulose ion concentration [Cello -].


Increasing a unit of pH value can increase the reaction rate by approximately 10 times. The conventional alkali agent is baking soda, which is converted into soda ash during steaming, and its pH value is 11;


The two-phase method uses caustic soda as the alkali agent, and the pH value is above 13. According to Table 1, the reaction rate value is 40 times that of soda ash. Therefore, the normal method of steaming is 8 min, and the two-phase method only needs to be steamed for 12 s. This is the main basis for the rapid evaporation of reactive dyes after two-phase printing. It can be seen that the pH value changes slightly, and the reaction rate changes greatly, which directly affects the effect of dyeing and fixing.


To implement steam saving, urea, or urea-free steaming process after printing, it is essential to implement online control of pH.



3. The influence of pH value of dyeing bath on medium temperature reactive dyes


The uptake of reactive dyes in dip dyeing is different from that of direct dyes. The dyeing process of direct dyes is mainly the adsorption of dyes by fibers, which is a physical change; while the dyeing process of reactive dyes has chemical changes, that is, the bonding reaction between the dye and the cellulose occurs, and the dye-water is Hydrolysis reaction will occur. Therefore, when reactive dyes are dyed, the pH value of the dye bath has a greater impact on the dyeing results than direct dyes.


1. The dye solution is weakly acidic when it absorbs color


At this time, the absorption depth of most dyes is significantly reduced, but it has little effect on the final fixation depth. However, it must be noted that due to the decrease in the amount of primary color absorption, the dye concentration in the residual liquid is higher, which will undoubtedly lead to faster dye adsorption and dyeing at the initial stage of alkali fixation (initial secondary color absorption). This will have a negative impact on uniform color absorption and uniform color fixation.


Therefore, in actual production, the semi-finished fabric must be pickled (or neutralized) before dyeing, and the remaining pickling should be done to avoid affecting the leveling effect.


2. The dye solution is alkaline when it absorbs color


At this time, the color absorption of the dye varies with the alkalinity of the dye solution. When the dye liquor is relatively weak in alkalinity, the color absorption rate and amount of the dye will not change significantly compared with the neutral bath. When the pH value of the dye solution is greater than 9, the absorption rate and amount of the dye will be greatly increased.


But it has little effect on the final dyeing result (the fixation rate has a tendency to increase). Under this condition, the color absorption of the dye becomes stronger because at this time, part of the dye has a bonding reaction with the primary hydroxyl group (—CH2OH) on the C6 position of the glucose residue on the cellulose macromolecular chain. (The primary hydroxyl group at C6 position is easier to ionize than the secondary hydroxyl group at C2 and C3 position, the chemical activity is stronger, and it has a certain reaction ability under weak alkaline conditions).


Obviously, when reactive dyes are impregnated, first absorb the color in an appropriate alkaline dyeing solution (so-called pre-alkali dyeing) to increase the amount of color absorption. This will effectively reduce the secondary color absorption rate of the dye in the initial stage of alkali fixing. , Improve the leveling effect.


Especially for some dyes whose primary color absorption is low but the "sudden coloring phenomenon" of secondary color absorption is prominent, such as reactive brilliant blue KN-R, reactive brilliant blue A(B)-RV, reactive black KN-B, reactive Blue BRF and Remazol Blue RGB etc. When these dyes are used to dye light-colored fabrics or fabrics with poor levelling properties, pre-alkaline dyeing is used, and the levelling effect is the most significant.


3. Fixing


Since the reaction between the medium temperature reactive dye and the cellulose fiber is an acid release reaction, it can proceed smoothly to the maximum extent only under alkaline conditions. The β-hydroxyethyl sulfone sulfate active group contained in the dye will only undergo an elimination reaction under alkaline conditions and become a vinyl sulfone group, resulting in strong reactivity.


Therefore, the fixation of reactive dyes must be carried out under alkaline conditions. It is worth noting that in the alkaline bath, the bonding reaction between dye and cellulose and the hydrolysis reaction between dye and water proceed simultaneously.


Under certain dyeing conditions, the rate of these two reactions mainly depends on the pH value of the dye solution.


The pH value of the dyeing liquor is low, and the hydrolysis reaction rate between the dye and water is low, but the bonding reaction ability between the dye and cellulose is also low, neither the high dyeing rate nor the deep dyeing ability. The pH value of the dyeing solution is high, the bonding ability between the dye and the cellulose is strong, and the fixing rate is fast; but the hydrolysis reaction rate between the dye and the water will be faster, so instead, the color depth will be reduced due to the large amount of hydrolysis of the dye. It can be seen that in order to obtain a faster fixation rate and a higher fixation depth, it is necessary to accurately grasp the optimal pH balance point of fixation (that is, the optimal fixation pH value) to achieve the highest final fixation rate purpose.


4. The best fixing pH value of different middle temperature reactive dyes is not the same.


Some dyes require lower fixing pH value, such as Reactive Red M-3BE; some dyes require higher fixing pH value, such as Reactive Turquoise Blue B-BGFN. Obviously, this is because the structure of the dye is different, and its reactivity is different.


5. The optimal pH value of commonly used medium temperature reactive dyes is not a point, but within a narrow pH range.


For example, the optimal pH value of Reactive Red M-3BE is 10.60~10.71, Reactive Blue M-2GE is 10.71~11.08, and Reactive Turquoise Blue B-BGFN is 11.08~12.05 (this refers to 60℃ medium temperature dyeing, not 80℃ high temperature dyeing).


That is to say, a certain dye has the highest fixation rate and the lowest hydrolysis rate within its optimal fixation pH range. If the fixing pH value is larger or smaller than this range, the fixing rate (color depth) will decrease significantly.


6. Except for a few dyes, the best fixation pH value for most moderate temperature reactive dyes is between 10.5 and 11.0.


However, this does not mean that the fixation pH of different dyes is within this range, and the highest fixation rate can be obtained.

This is because the optimal fixation pH range of each reactive dye is relatively narrow. Once the pH exceeds its optimal range (even if the deviation is small), the fixation rate will decrease.


7. Commonly used reactive turquoise blue, such as reactive turquoise blue A(B)-BGFN, the optimal pH value is far higher than most other dyes, 11-12. This is because reactive turquoise blue belongs to copper phthalocyanine dye, with large relative molecular mass and lack of linear structure, not only poor diffusion ability, but also weak reactivity.


8. Commonly used active bright yellows, such as active bright yellow A(B)-6GLN, although the optimal fixing pH value is within the conventional range (pH=10.71~11.08), there are abnormal performances, that is, once the fixing pH value exceeds In the best range, the color depth will drop sharply.


This indicates that Reactive Bright Yellow A(B)-6GLN is very sensitive to fixing pH and must be controlled precisely.


As explained above, the optimal fixing pH value of medium temperature reactive dyes is different due to different dyes, and must be accurately controlled, which cannot be done with test paper measurement.


Comparing different alkali agents with different concentrations using test paper and pH meter, the pH value error is as high as 2.0-2.5. According to Table 1, the difference in the reaction rate is very high. Therefore, human experience is not necessarily reliable, and it is not advisable to control the process.