Direct dye was developed after Bottiger discovered Congo Red in 1884. Before this, cotton fibers were dyed with indigo and other natural dyes, which was troublesome. At that time, synthetic dyes were just beginning to be developed, and cotton fibers could not be dyed directly. The cotton fibers must be treated with a mordant before they can be dyed, and the color fastness is very low. Because dyes can directly dye cotton fibers without a mordant, they are called direct dyes. The ability of dyes to directly dye fibers is called directness.
By the 1960s and 1970s, the medical profession discovered that benzidine had a serious carcinogenic effect on the human body, and countries have banned the production of benzidine one after another. At present, in the chemical structure of direct dyes, most of the early varieties developed with benzidine have been eliminated.
Another important factor that promotes the development of direct dyes is that people continue to put forward new requirements for color fastness. Around the 1930s, vat dyes and insoluble azo dyes appeared in cotton dyes. The reactive dyes for cotton that appeared in the 1950s have won the favor of consumers for their good dyeing fastness, which has formed a huge impact on direct dyes.
As we all know, the biggest disadvantage of direct dyes is poor color fastness. To solve this problem, people have made unremitting efforts for hundreds of years. In the early days, improvements were made in the post-dyeing treatment. The more mature and widely used methods are copper salt post-treatment and diazotization post-treatment, forming two types of direct dyes called direct copper salt dyes and direct diazo dyes.
Then came the azo direct dyes with copper complex structure and some non-azo direct dyes with heterocyclic structure. Their main feature is that the light fastness is above grade 4. For the convenience of application, people added the varieties of direct dyes that do not require post-treatment and have better light fastness, forming direct light fast dyes. From the 1930s to the 1970s, people's interest in the dyeing of cellulose fibers seems to be concentrated on reduced, insoluble azo and reactive dyes. During this period, there was no significant progress in the structure of direct dyes, mainly using the development of organic chemistry to develop cationic fixing agents. The more widely used varieties are fixing agent Y and fixing agent M.
At the same time, in order to adapt to the same bath dyeing of cotton-polyester blended products, in the 1970s and 1980s, dye researchers at home and abroad carried out the research and development of new direct dyes. These new dyes have characteristics that are different from the previous direct dyes: they are stable under high temperature conditions above 130°C, do not degrade, and can resist acid dyeing conditions; under high temperature acid conditions, they still have high directness and dye uptake; The color fastness of direct dyes is obviously higher than that of the previous direct dyes, especially the wet processing fastness.
In order to meet these requirements, two methods are used in the molecular design of dyes:
(1) Metal atoms are introduced into the dye molecule to form a chelating structure, which improves the bending resistance of the molecule, and contains a very active hydrogen atom nucleophilic group. At the same time, a special fixing agent is designed. After dyeing, it forms a multi-dimensional structure cross-linked state with the dye and fiber after the fixing treatment, and achieves a higher color fastness.
(2) In the dye molecule, introduce a melamine group with strong hydrogen bond forming ability. The dye molecule does not contain metal ions, but a mixture of cationic polyamine polymer and special metal salt is designed as a fixing agent to improve the color fastness.
The Swiss company Sandoz used the former method to research and produce a new set of direct dyes: IndosolSF dyes, the Chinese name is direct fastenin dyes. This set of dyes entered my country in the 1980s, which aroused interest in the application, research and production of dyes. After years of hard work, the domestic similar varieties, namely direct cross-linked dyes, have been developed and produced. This set of dyes also has the advantages of new direct dyes and is suitable for the same bath dyeing of polyester-cotton blended fabrics. Through production practice, people found that this set of dyes and disperse dyes often cause discoloration of disperse dyes when they are dyed in the same bath. After research, this is because the free copper in the dye causes the structure of some disperse dyes to be destroyed. In order to solve this problem, a new polymer metal complexing agent was developed to make up for the defects of this type of dye.
The Japanese chemical company adopted the second method and introduced a new type of direct dye Kayacelon C. According to this idea, the dye workers in our country have developed and produced a set of direct blend dyes called D-type after years of research. This type of dye also has the advantages of the aforementioned new direct dyes. Because the melanoyl group introduced into the dye molecule does not contain metal atoms, it will not affect the disperse dyes during dyeing, and is especially suitable for dyeing polyester-cotton blended fabrics. It is called blended dyes, which means that the dyes have good intermixing properties and can be dyed in the same bath with various disperse dyes, but this set of dyes contains disperse dyes.
The birth of a new type of direct dyes reinjects new vitality into direct dyes, provides a new means for the dyeing of cellulose fibers, and further expands the application range of direct dyes.