Various acid dyes

Various acid dyes

In the wet dyeing and finishing process, it is necessary to control the concentration of acid, alkali and salt in the working fluid to ensure the normal operation of the process and the quality of the product. The use of acids, bases and salts in the dyeing and finishing process is selected mainly based on their physicochemical properties and cost. The following discusses the important acids in the dyeing and finishing process.

The actual purpose of using acid is to generate a higher hydrogen ion concentration than pure water, and the hydrogen ion concentration of pure water is 1×10-7mol/L.

Strong acids are fully ionized in water, such as hydrochloric acid, sulfuric acid; weak acids are partially ionized in water, such as acetic acid. Acid is used for pH adjustment and pH control and for neutralization after various alkali pretreatment, dyeing and finishing, and acid can also be used for impurity removal. In many cases, a buffer system with acid as the single component to control pH is appropriate.

Hydrochloric acid is an aqueous solution of hydrogen chloride, which is a strong acid and completely ionized in water. Hydrochloric acid is mainly used to neutralize bases, for example: in the production of color-fixing agents, hydrochloric acid is used to neutralize amino groups to form cationic groups. During color fixing, the cationic groups are electrostatically combined with anionic dyes to form water-insoluble salts deposited on the fibers, which will greatly improve the color fastness of the fabric.

Sulfuric acid is a strong acid and completely ionizes in water. Mainly used for pickling, such as the carbonization of wool, carbonization is based on the different resistance of wool fibers and cellulose substances to strong inorganic acids. The plant impurities in the raw wool, such as grass seeds, grass stalks, grass leaves and hemp shavings, are often entangled with the wool.

The existence of these impurities not only brings many difficulties to carding and spinning, but also reduces the quality of wool yarn, affects the appearance of the fabric, and easily causes dyeing defects, especially when dyeing dark colors, which must be removed by carbonization. The basic substance of vegetable impurities in raw wool is cellulose, and the fiber rope will be hydrolyzed when treated with sulfuric acid at a certain temperature to form brittle oligomeric cellulose, which is then crushed to remove dust and wool.

Citric acid is a medium-strong acid, which is partially ionized in water. It is mainly used for anti-shrinkage and anti-wrinkle finishing. Under the action of external force, the cellulose molecular segments are displaced, and hydrogen bonds and van der Waals forces are formed in new positions, and no longer return. to the original position, resulting in wrinkles.

In the anti-shrinkage and anti-wrinkle finishing of pure cotton fabrics, citric acid is dehydrated at high temperature to form acid anhydride, and the acid anhydride forms an ester bond with the hydroxyl group of cellulose to fix the cellulose molecular segment and prevent the displacement of the molecular segment. But at the same time, carboxylic acid will be produced, which will cause the hydrolysis of cellulose molecules, and the strength of pure cotton fabric will decrease. Therefore, some alkalis, such as urea and sodium acetate, should be added to the finishing solution.

Formic acid, also known as formic acid, is a colorless and transparent liquid with a density of 1.22g/cm3, a boiling point of 100.6°C, a melting point of 8.2°C and a flash point of 68.89°C. Soluble in water, ether, ethanol, glycerin, etc. Formic acid is a medium-strong acid, which is partially ionized in water and is mainly used for antisepsis

Acetic acid is a sick acid that is partially ionized in water and is used to adjust the pH of the dye solution. For example: adjust the pH value when dyeing protein fibers with weak acid dyes; adjust pH value when dyeing ester fibers with disperse dyes; adjust pH value when dyeing acrylic fibers with cationic dyes.

It is used to adjust the pH value of the oxidizing solution when vat dyes or sulfur dyes are used to dye cellulose fibers for oxidation. Weak acid dyes have a complex molecular structure and are negatively charged after ionization in water, and have large hydrogen bonds and van der Waals forces with fibers.

The isoelectric point of wool is 4.2-4.8, and the isoelectric point of silk is 3.5-5.2, which is neutral to negatively charged under weakly acidic conditions. Weak acid dyes mainly dye wool or silk by hydrogen bonds and van der Waals forces, and the acidity is enhanced. When the pH value is lower than the isoelectric point, the wool or silk will be partially positively charged, and the dyeing rate will increase. That is to say, the initial dyeing rate is high, and it is easy to dye flowers. When the pH value is high, the wool or silk will have a large amount of negative charge, and the electrostatic repulsion between it and the weak acid dye will increase, making it difficult to dye. Therefore, the pH value of the dye bath is adjusted with acetic acid to make it slightly higher than the isoelectric point, which meets the requirements of uniform dyeing.

When dyeing polyester fibers with disperse dyes, adjust the pH value with acetic acid; when using hydrosulfite for reduction cleaning, adjust the pH value with acetic acid.

The acid group contained in the acrylic fiber is ionized in water, which makes the surface of the fiber negatively charged, and the cationic dye dissolves in the water to form a positively charged pigment ion. Due to electrostatic attraction, the dye is adsorbed on the surface of the fiber. When the temperature of the dye bath rises to the glass transition temperature of acrylic fiber, the dye diffuses from the surface of the fiber to the inside, and is fixed with the acid group on the fiber by ionic bond. Due to the high rate, it is easy to dye flowers. Acetic acid binds quickly to the acid group on acrylic fiber, but the cationic dye binds firmly to the acid group. Therefore, adding acetic acid can play a role in retarding dyeing.

When vat dyes dye cellulose fiber piles, first, the vat dyes generate leucobody under the action of sodium hydroxide and sodium hydrosulfite, and the leucobody dyes the fiber, and then oxidizes the color in hydrogen peroxide solution. At this time, the oxidizing solution needs to be used Acetic acid adjusts pH.

When sulfur dyes dye cellulose fibers, first sulfur dyes generate leucobody under the action of sodium polysulfide, leucobody dyes the fiber, and then oxidizes in sodium dichromate solution to develop color. At this time, the oxidizing solution needs to adjust the pH with acetic acid Due to the harm of sodium dichromate to the human body, it has been limited, and now hydrogen peroxide is gradually replaced by sodium dichromate.