The past and present of direct dyes

Date:Apr 06, 2021

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. Cotton fibers must be treated with a mordant before they can be dyed, and the dye fastness is very low.

Congo red is soluble in water and can be directly dyed to cotton fibers without first treating with a mordant. The process is simple and meets the needs of the textile industry at that time. Similar dyes have developed rapidly. 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.

Since Boutig obtained the first direct dye-Congo red by chemical synthesis in 1884, the direct dye has gone through a long historical process for more than one hundred years. During this period, this kind of dyes are constantly developing and changing, and their dyeing theory is also constantly deepening and perfecting.

The early direct dyes were mostly benzidine azo dyes in terms of chemical structure, especially the structure of bisazo type. For example, Congo red is a symmetric benzidine bisazo dye. The discovery of Congo Red made the preparation of direct dyes through diazotization and coupling reactions with aromatic diamines as the only way to obtain direct dyes by chemical synthesis at that time.

During this period, it was mainly through the manufacture and use of different types of coupling components (various amino naphthol sulfonic acids) to obtain different color varieties of direct dyes, which has a large market share. With the development of dye synthesis technology, azo direct dyes of new diazo components such as anilide, stilbene, diarylurea, and melamine, as well as non-azo dyes of dioxazine and phthalocyanine series, have appeared. Heterocyclic direct dyes of similar structure.

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.

Their use makes the direct dyes have a certain improvement in the fastness of water immersion, washing, perspiration and other dyeing fastness, but the color fastness to light and rubbing are reduced to varying degrees. With the emergence of washing machines and their widespread entry into households, people have put forward higher requirements for the dye fastness of textiles. with

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; yes The color fastness of direct dyes is significantly 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.

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.