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.
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