Learning Point │ Application of Buffer Solution in Dyeing and Dyeing [Effect of PH Value on Dyeing]

In fabric dyeing, the pH of the dye bath fluctuates due to the following factors:

(1) The pre-treatment of the fabric is not thoroughly cleaned, so that a small amount of alkaline or acidic material is introduced into the dye bath.
(2) When dyeing, non-neutral additives (such as auxiliaries) are added to affect the pH of the bath.
(3) The pH of some dyes is too high or too low. If added to the bath, the pH of the dye bath will change. For example, the pH value of the disperse dye Fulong Yellow SE26GFL is 8.60; the pH value of Fulong Dahong S23GFL is 10.60.
(4) When steam is supplied to the boiler, negative pressure may sometimes occur. The alkaline or acidic solution in other containers is poured into the steam pipe. When heating the dye bath, alkaline or acidic substances are brought into the dyeing bath.
Most dyes are sensitive to the pH value of the dyebath. The pH control of the dye bath is slightly inappropriate, and color, light, color, and color stains will appear. In order to make the pH of the dyeing solution have good stability, the corresponding buffer solution must be properly prepared in the dyeing bath.

2. 1. 1 disperse dyeing

Take high-temperature and high-pressure overflow dyeing machine to disperse dye-dyed polyester fabric as an example. Since many disperse dyes are sensitive to the pH value of the dyeing bath, staining at different pH values ​​results in a large difference in color, only staining in a weakly acidic (pH = 5 ± 0.5) dye bath, resulting in a color shade The most pure and pure. In order to maintain a good stability of the dye bath at the time of dyeing, a sodium acetate 2 acetate buffer solution can be prepared.

The actual amount of sodium acetate used to control pH:
(1) The ratio of acetic acid to sodium acetate sets the initial concentration of HA c to Ca mo l/L, the initial concentration of N aA c to Cbmo l/L, and the partial concentration of HA c to x mo l/L.

Then there is:
HA c -----H+ + A c-
N aA c= N a+ + A c-
[HA c ]= Ca - x ≈ Ca
[A c- ]= Cb + x ≈ Cb
Ionization constant K a = [H+ ]·[A c- ]/[HA c ] ≈x Cb/Ca
Since pH is controlled at about 5, pH = 4.80±0.20 is suitable.
pH= - lg [H+ ]= 4.80
[H+ ]= 1. 59×10-5 mol/L
[H+ ]= x = K a ·Ca/Cb= 1. 8×10 - 5 mo l/L ·(Ca/C)b= 1. 59×10 - 5 mo l/L
Cb = 1. 13 Ca
That is acetic acid: sodium acetate = 1:1. 13
Due to the high temperature and high pressure conditions, the ionization equilibrium constant K a of the weak acid is still very small, and the pH of the acetic acid 2 sodium acetate buffer system has little effect, and can still be calculated as acetic acid K a = 1.80×10 −5 .

(2) The buffer concentration of acetic acid and sodium acetate is directly proportional to the buffer volume. According to the actual production requirements for the buffer volume, the actual concentration of sodium acetate 2 is calculated.
In the dyeing process, the possibility of acidic substances entering the dye bath is small, and alkali resistance is generally the main reason.
The amount of alkali introduced into the bath of the high-temperature and high-pressure overflow by various factors is 150 g, and 150 g is the weight of all alkaline substances hydrolyzed into NaOH.
If the overflow dyer bath is 3000 L, the alkali concentration in the dye bath is
150/3000÷40= 0. 00125 mo l/L.
Since the pH requirement is 4.80±0. 2< 5, and Cb = 1.13 Ca , according to the ionization equilibrium constant:
[H+ ]= K a ·Ca/Cb
1 × 10 - 5 = 1. 8 × 10 - 5 × (Ca - 0. 00125/1. 13Ca + 0. 00125)
Ca = 0. 00522 mo l/L
Cb = 1. 13× Ca = 0. 0059 mo l/L
That is, the concentration of acetic acid was 0.005222 mol/L, the concentration of 100% acetic acid was 0.3132 g/L, the concentration of sodium acetate was 0. 0059 mol/L, and the concentration of sodium acetate was 0. 484 g/L.

In the high-temperature and high-pressure overflow dyeing, if 3000L of the dye bath, brought into the alkali concentration of 0. 00125mo l? L (assuming caustic soda concentration), it can be 100% acetic acid 0. 3132g / L and 100 % Sodium Acetate 0. 484 g?L Buffering solution is added to the dyebath to effectively control the pH of the dye bath within the required range.

2. 1. 2 Dyeing of wool with weak acid dyes and reactive dyes

When dyeing wool fibers with weak acid dyes or high-temperature reactive dyes L anasol, the pH of the entire dye bath must be strictly controlled under weakly acidic conditions (the pH of the bath is determined by the depth of dyeing and the auxiliary used for dyeing). Due to the acid-promoting dyeing effect, if the acidity is strong, dyes in the dye bath are easily adsorbed on the surface of the wool fiber, and even the superequivalent dyeing on the surface of the fiber causes the dye molecules to aggregate to stain the flower. Generally, the stronger the acidity of the dyeing medium, the faster the dye uptake, and the poorer the leveling (especially the temperature above 80 °C is more prominent). When using NH3·H2O2(NH4)2SO4 or HA c2N aA c as dyeing agents to control the pH value of the dyeing bath, it can be kept within the required range, so that the wool fiber can obtain higher Exhaustion rate and levelness (especially light color).

2.1.3 Reactive dye cold pad batch dyeing

The reactive dye cold pad batch dyeing adopts buffered mixed alkali as the fixing alkali, which has great benefit for improving the stability of the dye liquor and eliminating the wind print.
Dichloro-s-triazine dyes are more active. Cold pad batch dyeing uses Na2CO3-2N aHCO3 as the fixing alkali. Dyeing and uneven dyeing of the dyed fabric surface is better than using Na2CO3 alkalinity alone. Improvements have been made, especially for dyes in excess of 30 g/L and for individual poorly dyed and hard-to-dye varieties, the leveling of Na 2 CO 3 -2N aHCO 3 (ratio 2:1) provides good levelness. Improve dye fixation rate.
In addition, the sulfur black in the sulphur dyes is susceptible to oxidation during the dyeing process, resulting in spotting. The addition of Na2CO32N aHCO3 (2.2% to 4.2%) to control the pH of the dyeing bath can reduce the oxidation rate and have a good effect on alleviating the stain.