In view of the differences in dyeing characteristics and types of different reactive dyes and the composition, properties and performance of alkali substitutes, the chemical properties and characteristics of traditional soda ash and trisodium phosphate are compared. Various analyses during color fixation, detailing the effects and application advantages of various alkaline agents on reactive dyes.
Common alkaline agents for printing and dyeing
In the dyeing of reactive dyes, the traditional alkali agents used in the fixation reaction are relatively mild soda ash, trisodium phosphate, and some alkali substitutes.
As a special alkaline agent for color fixing of reactive dyes, it is used to replace traditional alkaline agents such as soda ash. However, in the specific application of the alkali substitute, because its chemical composition, properties and performance are different from those of the traditional alkali agent, it has formed its own characteristics, and then has different applications.
Chemical properties and characteristics of traditional alkaline agents
The fixation reaction of reactive dye dyeing needs to be carried out under certain alkaline conditions. The selection of alkali agents has traditionally been dominated by soda ash and trisodium phosphate, and strong alkalis such as caustic soda are rarely used alone.
1. The alkalinity of soda ash is relatively mild, and its chemical properties are suitable for dyeing with reactive dyes. Its aqueous solution has little change in pH value within a relatively wide unit concentration range; trisodium phosphate also has this feature, but its alkaline stronger than soda ash.
2. The changing characteristics of the pH value of soda ash in aqueous solution are related to its chemical properties. After soda ash is dissolved in water, it reacts reversibly with water to generate sodium hydroxide and sodium bicarbonate, in which sodium bicarbonate will gradually release carbon dioxide under humid and hot conditions to generate soda ash and water, thus making the pH value of the solution It maintains a fairly constant state; when it encounters caustic soda, it generates soda ash and water, so it has an anti-alkali effect in some cases, so sodium bicarbonate is also called acid sodium carbonate. Therefore, within a certain unit concentration range, the pH value of the solution does not change much, which is very suitable for the dyeing of reactive dyes, and the fixation reaction process of dyes and fibers can be carried out smoothly and continuously.
3. The chemical characteristic of trisodium phosphate is the change of its pH value in aqueous solutions of different unit concentrations. Although the influence and mechanism of action are not the same as those of soda ash, they are similar to those of soda ash. Although its alkalinity is stronger than that of soda ash, it is weaker than that of caustic soda, and it is within the range suitable for reactive dyeing, so it is often used in dye fixation reactions, especially for reactive dyes with relatively high pH requirements. Sometimes it is also mixed with soda ash according to the specific variety.
4. Strong alkali such as caustic soda, due to its strong alkalinity, a small change in unit concentration will also cause a significant change in pH value, and due to the lack of buffering properties such as soda ash and trisodium phosphate, although the dosage reaches the specified pH value Rarely, except for a very small number of dye varieties, it is difficult to achieve good coordination and control of the dyeing reaction with most reactive dye varieties. Therefore, in actual large-scale production, it is mainly mixed with trisodium phosphate in an appropriate proportion. Some varieties used for thermosetting dyes are basically not used alone. If the pH value of the dye bath is too high (most varieties cannot be higher than 12), the proportion of dyes hydrolyzed will increase linearly.
Dyeing Characteristics and Type Differences of Reactive Dyes
Usually, soda ash is used for dichlorotriazine reactive dyes with strong reactivity, and stronger alkaline agents such as trisodium phosphate, trisodium phosphate and The mixed alkali agent of caustic soda, of course, it is not impossible to use soda ash, but the amount needs to be increased. At this time, attention should be paid to the problem of alkali flower when dyeing dark colors. If caustic soda is involved, the temperature can be appropriately lowered.
When the pH value of the dye bath of dichlorotriazine reactive dyes is less than 9.0, the fixation rate is very low, and the fixation rate starts to rise sharply when the pH value exceeds 9.0.
The monochlorotriazine reactive dyes did not start to increase significantly until the pH value reached 10.0, and the maximum value appeared at the pH value of 11.0-11.5.
The degree of activity of vinyl sulfone reactive dyes is between dichlorotriazine and monochlorotriazine. The alkali concentration of the fixing bath is generally similar to that of dichlorotriazine, but the temperature should be higher than that of monochlorotriazine. Triazine low, in between. There are also individual varieties that should increase the fixing reaction temperature to the conditions of a chlorotriazine type dye, so as to increase the color quantity.
B-type, M-type and other isomeric double reactive group reactive dyes, different brands, different origins and different varieties, their
The isomeric ratios may not be the same, but it is necessary to consider and take into account the characteristics of both monochloro-s-triazine and vinyl sulfone, and it should not be too biased towards one side. The fixing bath temperature can also be slightly higher than the vinyl sulfone type.
Due to the reaction process of reactive dyes fixing to cellulose fibers, it must first be carried out under certain alkaline conditions, but it will gradually release acid during the reaction process. Under certain conditions, it is easy to react with the hydroxyl group (primary alcohol group) in the molecular structure of cotton fiber to form a cellulose ether and release hydrochloric acid at the same time.
The active group of dichlorotriazine reactive dyes - two active chlorine atoms in chlorotriazine, only one active chlorine atom is easy to bond with cellulose under weaker alkaline agent and lower temperature; At pH value and higher temperature, two active chlorine atoms will bond with cotton fibers at the same time to form cross-links. Of course, only one reactive chlorine atom can bond to cotton fibers with a chlorotriazine-type reactive dye.
The active chlorine atom in chlorotriazine, in the presence of alkaline agent and under certain conditions, is also prone to covalent bonding reaction (hydrolysis) with water molecules to generate hydrolyzed dyes, and also release hydrochloric acid. β-Vinylsulfone sulfate as the active group. Its chemical structure is as follows:—SO2—CH2—CH2—OSO3H
In the presence of alkali agent and certain conditions, β-vinyl sulfone sulfate first generates vinyl sulfone, and then undergoes an addition reaction with the hydroxyl group in the cellulose molecular structure to generate cellulose ether and release sulfuric acid at the same time.
β-Vinyl sulfone sulfate is also prone to addition reaction (hydrolysis) with water molecules in the presence of alkaline agent and under certain conditions to generate hydrolyzed dyes and release sulfuric acid at the same time.
The acid release phenomenon will gradually consume the alkali in the dye bath by means of neutralization reaction. When the unit concentration of the alkali agent stored in the dye bath is consumed, once the pH value is lower than the minimum required for the fixation reaction, the fixation reaction will be changed. Slowly weakened, and finally the reaction was terminated.
Because the pH value of different unit concentrations of soda ash can be almost stabilized at the same value, such as 5g/L, 15g/L, 25g/L such a large difference, the pH value is kept between 11.0 and 11.2, so The fixation reaction remains normal even with consumption of up to 20 g/L of soda ash. A corresponding increase in the electrolyte in the dyebath is bound to follow the neutralization reaction. From another point of view, soda ash is also a basic salt, so the secondary dyeing phenomenon of reactive dyes in the fixation reaction stage has a corresponding relationship with this.
Alkali replacement performance
Although there are many varieties of alkali substitutes on the market, each with its own name, from the perspective of its physical and chemical parameters, their chemical compositions are not the same, but it is certain that they are all non-single chemical substances monomers. , which are basically mixtures of strong bases and buffered alkaline substances. Strictly speaking, it should be called the mixed alkali reagent specially used for the fixation reaction of reactive dyes.
The design and chemical composition of alkali substitutes are basically based on the following framework:
1. Select a suitable strong alkali as the alkali agent for pH adjustment, so that the dosage per unit concentration can be reduced to a small amount, but it must have good coordination with reactive dyes.
2. Select a variety of buffer-type alkali agents, alkaline salts, etc. with high solubility, good aqueous solution stability and miscibility to form the components of the buffer system together. Although the total unit concentration is small, it can stabilize the dye bath at within the required pH range. In particular, with the increase of temperature, the extension of time, and the gradual release of acid in the fixation reaction, the buffering effect and efficacy can still be maintained.
The difference of the effect of caustic soda and soda ash on the secondary dyeing of dyestuffs
In practical applications, the alkali-replacing special-purpose alkali-fixing agents are used for light colors with less use of dyes and dye-promoting electrolytes. It is found that the secondary dyeing of dyes is lower than that of soda ash. Caused by the structural composition of the alkaline agent.
The strong alkali component of the alkali substitute makes the total mass of alkali substances in the dye bath greatly reduced compared with that of soda ash when the dye bath reaches the same pH value required for the fixation reaction. This should be said to be an advantage that everyone is willing to accept in terms of operation, storage and transportation costs, and washing after dyeing. However, on the other hand, the source of substances that can be used to generate electrolytes is also greatly reduced. When the amount of substances that can generate electrolytes is significantly lower than the parameter value of the original soda ash, the secondary dyeing of the dye will be more than that of soda ash. decrease accordingly. Especially for light colors, the final total concentration of electrolytes in the dyeing bath is low, so depending on the situation, an appropriate amount of electrolytes can be added. In addition, due to the small absolute value of the amount of light-colored dyes, the method of adding an appropriate amount of dyes can also be changed from a different angle.
When dyeing intermediate colors, with the addition of dye-promoting electrolytes in the dyeing bath and the corresponding increase in the amount of dyes, the difference between the secondary dyeing of alkali substitutes and soda ash gradually decreases.
In the dark color, after the electrolyte dosage reaches 30 g/L, there is basically no difference (in actual dyeing, it is generally higher than 30 g/L excessive use).
The amount of dye used (which also determines the source and amount of acid), together with alkali substances, constitutes the source of new electrolytes, and the dye-promoting electrolyte added in the original dyeing bath together affects the final dyeing of dyeing. total.