Direct Dyes

Direct dyestuff or substantive dyestuff (Ger. Direktfarbstoff, Fr. colorant direct; Eng. direct dyestuff; substantive dyestuff) is a dye class that is water-soluble and can dye collagen and cellulose fibers directly without the need for mordanting or any other process.

Properties of Direct Dyes:
Direct dyes are dyes that can be dyed directly without requiring any pretreatment in the dyeing of cellulosic fiber. They are also called substantive dyes. They are distinguished from basic and acid dyes by their high substantivities against cellulosic fiber. Congo red is the first direct dye. They are drawn directly from the liquor. With these dyes that can be leveled well, dyes that penetrate into the material are obtained. They are soluble in water. Due to ionization, these dyes are anionic dyes. The wet fastness of these dyes is not good. However, wet fastness can be improved with the help of suitable substances. Some direct dyes have excellent light fastness and these are copper complexes. However, these dyes have low boiling and chlorine fastness.

Advantages of direct dyes: They are cheap, water soluble, dyeing processes are very simple, do not require strong pH values ​​during dyeing and give good dyeing results in dark colors.

Disadvantages of direct dyes: Direct dyes have low wet fastness, their wet fastness is low and a significant portion of these dyes have carcinogenic effects.

Bonds Between Cellulose and Dyestuff:
H-bridges: H-bridges are formed between the functional groups of the dye molecule (-NH2, -OH groups) and the primary alcohol groups in cellulose.

Dipole interaction forces: Dipole interaction forces are formed between the polar groups in the dye molecule and the polar groups in the cellulose molecule (-SO3Na).

Structure of Direct Dyestuff:
Most direct dyes are azo dyes carrying disazo-, polyazo groups. There are also direct dyes in thiazole, phthalocyanine, and anthraquinone structures. In this respect, the chemical structures of direct dyes are similar to acid dyes. In the structure of direct dyes, there is an anionic group that can dissolve the dye molecule in water. Since each of the dyes exhibits different behaviors, direct dyes are examined in three groups.

Class A (Self-leveling dyes): Dyes belonging to this group have good migration properties. Uniform dyeing is achieved without taking special precautions. Wet fastness is low.

Class B (Dyes that can be controlled with salt): The migration capabilities of these dyes are low. Therefore, they do not exhibit uniform dyeing properties. Salt addition is necessary to adjust the drawing and leveling steps. If they are not taken regularly by the fiber at the beginning, it is quite difficult to smooth them later.
Class C (Temperature controlled dyes): These are dyes that are highly sensitive to salt and cannot self-level and have low migration possibilities. Their attraction cannot be sufficiently controlled by adding salt. In addition, proper dyeing is achieved by controlling the temperature.

Recipe Preparation:
Recipe preparation is done in the dye laboratory according to the operating conditions. When creating the recipe, the characteristics of the material to be dyed, the machine to be used in dyeing, the dyestuff and chemicals are taken into consideration. Recipe preparation is done in two ways.

By Finding the Desired Color with an Estimated Recipe:
If the desired color has not been applied before, estimated recipes that can give that color are created. The experience of the laboratory technician plays an important role here. The laboratory technician creates different recipes as an estimate and the product is dyed according to these recipes. The colors obtained at the end of dyeing are compared with the desired color. If the desired color is obtained, that recipe is used, but if the desired color is not obtained, a series of recipes are created according to the closest recipe from the trials and the desired color is tried to be obtained.

Using a Ready Recipe:
If the desired color has been previously studied, the archive is scanned and its recipe is found and dyeing is done according to that recipe.

Dyeing with Direct Dyes:
Dyeing with direct dyes can be done according to exhaustion, HT dyeing, semi-continuous and full-continuous dyeing methods. Direct dyes dye cellulosic fiber in neutral or basic environment, with electrolyte (salt) at boiling temperature. Since regenerated cellulosic fibers swell in basic environment, dyeing in neutral environment is preferred. Factors affecting direct dyeing are as follows: affinity (substantiality), extraction speed, diffusion, migration, bath ratio, temperature, electrolyte addition, auxiliary substance addition, dyeing time.

Dyeing with Direct Dyes by Exhaust Method:

The most commonly applied dyeing method with direct dyes is the exhaust method. In this method, textile auxiliaries, chemicals, alkali and salt are added to the dyeing bath. In some cases, a leveling agent (smoothing agent) is also added. Since the addition of salt and high temperature will increase the speed of dyeing, no salt should be added to the float at the beginning of dyeing, but when the boiling temperature is reached, salt should be added in portions. Otherwise, abrasive dyeing may occur.

Dyeing with Direct Dyes by HT Method:

In dyeing according to the HT method, types of direct dyes that are resistant to high dyeing temperatures should be selected. Since the majority of these dyes are not resistant to high temperatures, protective chemicals are used. Ammonium sulfate can be preferred as a protective chemical. In this dyeing method, increasing the temperature to 100 °C and above may increase the migration degree of direct dyes.

Dyeing with Direct Dyes by Impregnation Method:

In the impregnation method, the fabric, which has been pretreated very well and has good hydrophilicity, is impregnated with dye solution and fixed. Since direct dyes have high substantivity, they are not suitable for the impregnation method. They have found very few areas of application. The most commonly used method in dyeing with direct dyes by impregnation method is the pad-roll dyeing method.

Dyeing Diagram:

Cellulosic material can be dyed with direct dyes according to the diagram below:

PH of the bath;

Direct dyes are applied in solutions that are almost neutral. It is known that adding acid to direct dyes does not provide any advantage to the bath, on the contrary, there is a risk of changing the tone. It is known that weak alkalis delay the adsorption rate and soda, sometimes added at a rate of 3%, removes the hardness of the water and in some exceptional cases increases the solubility of the dye.

Used Chemicals and Their Functions:

Water: For the efficiency of dyeing, the plant water must be soft and its hardness must be removed. Heavy metal ions that give hardness to water negatively affect dyeing.

Salt: Increases the affinity of the dye to the fiber and the amount taken up by the cellulose increases. It provides homogeneous distribution of the dye on the fiber and ensures that the dye molecules are directed to the fabric.

Soda: Soda (Na2CO3), which is a weak alkali; ensures that the direct dye dissolves, remains dissolved in the dye bath and that the dyeing is done properly. By adjusting the pH of the environment, it ensures that the cellulose macromolecules become active in the basic environment and that the bond between the dye and the fiber is formed.

Oxidizing agents: Some direct dyes change their shade when treated at boiling temperature or above pH 7. Some even decompose completely. This means that the dye is easily broken down and reduced. It is possible to prevent this by adding oxidizing agents. For some direct dyes, adding only ammonium sulphate to the bath prevents this reduction risk.

Dissolving Direct Dyes and Preparing the Dye Bath:

Dye bath controls and taking samples, dye baths are usually prepared automatically and can be added to the decreasing float during the impregnation process with the help of dosage pumps. Temperature and pH controls are performed during the process in programmed dyeing machines and process steps are followed. Programs are kept in memory and repeatability is kept at a maximum level in the same tone dyeings. Sample control is performed by taking a piece from the fabric that comes out at the end of the dyeing in dyeings made with the impregnation method, and by taking a piece from the fabric in the boiler at the end of the dyeing temperature in dyeings made with the exhaustion method. After the necessary rinses are made to this fabric, if the desired color is found, the dyeing is terminated.

After-Dyeing After-Processes:

Some additional processes can be performed to increase the durability of direct dyes. However, in recent years, after-treatment with fixators (cationic substances) has become widespread in enterprises. In the process carried out with fixatives (cationic substances), the wet fastness of dyes is increased directly. However, their general principle, which is the formation of large molecule complexes with low solubility on the fiber and therefore low desorption rate, continues to exist with the use of today's resin-type fixative substances. In this regard, the principle of cationic treatment in particular has shown a significant development. After the fastness increasing process, cold and hot rinses are performed to remove dyes physically attached to the fabric according to the color darkness.