Direct dye

Direct dyes are dyes that can be ionized into colored anions in water and can be directly dyed on cellulose fibers or protein fibers without the aid of mordants. This dye was developed after the discovery of Congo Red in 1884. Congo red is a combination of hydrazine and 1-naphthylamine-4-sulfonic acid. By the end of the 19th century, various aminonaphthol sulfonic acids began to be industrially produced, and the dye varieties were greatly enriched, and green and black direct dyes appeared one after another. Direct dyes have a higher affinity for cellulose fibers and can be dyed in neutral or weakly alkaline media. When dyeing protein fibers, it is generally carried out in a neutral or weakly acidic medium. Most of the direct dyes are sodium sulfonate salts of aromatic compounds, which are easily soluble in water. Their solubility in water increases with increasing temperature and dissociates into anions in water. The direct dye has a large molecular weight and poor permeability, and generally does not easily penetrate into the interior of the leather, and is easily colored on the surface of the leather. A small amount of ammonia water is often added before the dyeing to promote penetration, so that it binds slowly to the leather. Direct dyes are very sensitive to acids, which can be precipitated by the addition of acid. After direct dye dyeing, if it is treated with a fixing agent, an organic acid or the like, the fastness can be improved. The direct dye production method is simple, the chromatogram is complete, the price is low, and the application is convenient. However, such dyes have poor light fastness and soap fastness. Direct dyes have previously been widely used for the dyeing of cellulosic fibers, protein fibers, synthetic fibers and their blended fabrics. The main intermediate used in many varieties of direct dyes is benzidine and its derivatives. It has been proposed in the early 1970s to be carcinogenic and has ceased production and use. To date, major companies producing direct dyes in the world have stopped the production and application of dyes. In the application, direct dyes are divided into four categories: general direct dyes, direct fast dyes, direct copper salt dyes and direct diazo dyes. According to the chemical structure types, there are azo type, stilbene type, phthalocyanine and dioxazine type, among which azo type is the most important. Generally, the direct dye structure types are mainly azo type (including monoazo and polyazo), and many varieties use benzidine and its derivatives as diazo components. For example, the first direct dye Congo red (CI Direct Red 28) synthesized by N. Bottiger in 1884, due to poor light fastness, has not been used for fiber dyeing, can be used for coloring biological materials and paper, and is also used as an acid-base indicator. Agent. Direct lightfast dyes are direct dyes with a light fastness of 4 or higher. The molecular structure is more complicated than the general direct dye. For example, direct fast blue (CI Direct Blue 106). The direct copper salt dye contains a group capable of forming a complex with a copper salt such as a hydroxyl group, a carboxyl group or an alkoxy group. They are adjacent to each other in the azo group. After the dye is treated with copper salt, a copper salt complex of the dye is formed, which can improve the light fastness. For example, direct copper salt blue 2R (CI Direct Blue 151). The direct diazo dye molecule contains an amino group. After the dye is dyed on the fiber, the amino group can be diazotized and coupled with the coupling component on the fiber to deepen the dye color depth and improve the dye fastness. Fastness. Because of the large variation in the color of such dyes, it is difficult to control, and it is basically not used now. For example, direct diazo black BH (CI Direct Blue 2).