Direct dyes are defined as anionic dyes that have an affinity for cellulosic fibers and are typically used from water-based dye baths containing sodium chloride (NaCl) or sodium sulfate (Na2SO4) electrolytes. The dyeing process of direct dyes is very simple, direct dyeing is usually carried out at or near the boiling point in a neutral or weakly alkaline dye bath, but most direct dyeing requires a separate post-treatment, such as cationic dye fixing, to improve wet fastness.

Chemically speaking, they are complex sulfonates, and more than 75% of direct dyes are nonmetallic azo structures, the vast majority of which are diazo or polyazo types. Their ionic properties are anions and are soluble in water. They have an affinity for a variety of fibers, such as cotton, viscose fiber, silk jute, flax and so on. They do not form any permanent chemical bonds with cellulose fibers, but are connected to them by extremely weak hydrogen bonds and van der Waals forces. Their flat shape and length allow them to be placed side by side with cellulose fibers and maximize van der Waals, dipole, and hydrogen bonds.

Direct dyes are widely used in cotton, paper, leather, wool, silk and nylon. They can also be used as pH indicators and biological colorants.

Direct dyes are easy to apply and provide a vibrant coloring effect. For these reasons, direct dyes are some of the most commonly used dyes used to color paper. Direct dyes are also used in many soaps, detergents and cleaning products. Some direct dyes have been used as indicators and for special stains in the laboratory.

Most direct dyes are azo, phthalocyanine, stilbene or thiazole colorants. Direct dyes adhere directly to the fibers through van der Waals gravity and hydrogen bonding. This association with fibers is called directness. Direct dyes are often used with dye fixatives to enhance the adhesion of the dye to the fibers and to inhibit dye fading due to repeated washing.

What are the direct dyes:

There are two main types of direct dyes

Anionic direct dye

Anionic direct dyes are used to color paper, control tone and chroma, and correct the color of double-sided paper. The main structural characteristics of anionic direct dyes are: the planes change with each other, the structural single and double bonds are very extended, and contain one or more sulfonate groups. These sulfonate groups make them soluble in water.

Cationic direct dye

The main structural characteristics of cationic direct dyes are:

● Extended structural single and double bonds.

● The molecular structure is flat

● Positive charge is greater than negative charge

Disadvantages of direct dyes:

● Many direct dyes are dull in color.

● Direct dyes provide darker colors than fibrous reactive dyes.

● Washing fastness quality is also low.

● A few direct dyes have low light fastness.

What is the dyeing process of direct dyes

Adding salt to the dyeing solution can improve the dyeing rate and reduce the charge effect in the process of fiber dyeing, which is one of the important reasons for salt to promote direct dyeing. Cellulose fibers have a surface potential in neutral or weakly alkaline dyeing tanks.

After adding the electrolyte, a large number of positively charged sodium ions are generated in the dye solution, so that the dye and anion maintain charge neutral, eliminate the obstacles in the dyeing process, so that the dye molecules and the fiber surface close contact, reduce the concentration difference between the dye solution and the fiber interface, so as to improve the dyeing rate of the dye.

The dyeing process of direct dyes can be divided into adsorption, diffusion and fixing three stages, but in the adsorption stage can not be too fast coloring, if the electrolyte is added in the adsorption stage, it is possible to make the dye aggregate and cause uneven, adding electrolyte in the middle of the process, can avoid dyeing too fast, improve the dyeing rate.

Human skin is a protein component, the basic unit is amino acids, that is, carboxylic acids containing amino groups.

The direct dye itself can stain silk and wool, and its principle is to use the sulfonic acid group on the dye, and the electrostatic attraction of the positive, negative amino, positive and negative electric combination on the protein fiber to produce a binding force.

In addition to positive-negative binding, direct dyes have van der Waals gravity and hydrogen bonding forces generated by linear macromolecules, all of which can bind to protein or cellulose macromolecules.

Therefore, the skin encounters direct dyes, not only produce electrostatic attraction, but also produce van der Waals force and hydrogen bond force, color fastness is very good, of course, not easy to wash off.

Future development trend

1. The process of clean production and recycling in the industry has been accelerated

Clean production, recycling and continuous production, automation technology, generally by the production enterprises attach great importance. Dye enterprises continue to improve production equipment and process technology around energy saving and emission reduction, and continue to absorb high-tech achievements in combination with their own reality, and vigorously promote dye clean production and preparation technology, reducing the generation of waste water, COD and other pollutants. Enterprise operators gradually establish a full range of ecological, environmental and health concepts, increase investment in comprehensive environmental protection management, consciously and actively recycle and use waste, so as to achieve clean production and recycling.

2. Increasing market concentration

With the tightening of national environmental protection policies, the production and environmental protection costs of dye manufacturing enterprises have risen sharply, and some small and medium-sized dye manufacturing enterprises have experienced increased operating pressure and have been forced to withdraw from market competition. Strong dye manufacturing enterprises rely on the advantages of scale, technology, capital and other aspects to obtain the market share left by the exit enterprise, which further promotes the concentration of domestic dye manufacturing industry, which is more conducive to the healthy and orderly development of the industry.

3. Acceleration of product upgrading

From the long-term development trend of domestic printing and dyeing enterprises in the downstream of the dye industry, it will face higher quality requirements of customers, faster delivery time, small batch and multi-variety printing and dyeing processing requirements, and the pressure of energy saving and environmental protection is also increasing. Therefore, printing and dyeing enterprises will have an increasing demand for more cost-effective differentiation, medium and high-end dyes and supporting application processes. These downstream promotion factors have forced dye companies to accelerate the pace of product innovation.

Article source: Minggao dye, Jingyan Chemical, Zhongshang Industrial Research Institute