Overview of Cationic Dyes
▌History
Cationic dyes are the earliest synthetic dyes produced. Aniline violet synthesized by W.H. Perkin of the United States in 1856, and subsequent crystal violet and malachite green, are all cationic dyes. These dyes were previously called salt-based dyes. They can dye protein fibers and cellulose fibers treated with tannins and tartar. They have bright colors but are not lightfast. They were later developed by direct dyes and vat dyes. and acid dyes and other alternatives.
After the industrial production of acrylic fiber in the 1950s, it was discovered that cationic dyes on polyacrylonitrile fiber not only have high directivity and rich color, but also have higher dye fastness than protein fiber and cellulose fiber. It was much higher and thus renewed interest. In order to further adapt to the application of acrylic and other synthetic fibers, many new varieties with high fastness have been synthesized, such as polymethine structure, nitrogen-substituted polymethine structure and pernalide structure, etc., thus making the cationic dye become polyacrylonitrile. A major type of dye for fiber dyeing.
▌Characteristics
Cationic dyes generate positively charged colored ions in solution, which form salts with acid anions such as chloride ions, acetate, phosphate, methyl sulfate, etc. , thereby dyeing polyacrylonitrile fibers. In actual dyeing, several cationic dyes are commonly used to create a specific color. However, mixed dyeing with cationic dyes is often difficult to evenly dye into the same color, resulting in spots and layers. Therefore, when producing cationic dyes, in addition to expanding the variety and quantity, we must also pay attention to the matching of dye varieties; in order to prevent dyeing, we must pay attention to developing varieties with good level dyeing properties. In addition, we must also pay attention to improving the steam fastness of cationic dyes. and light fastness, etc.
Classification of cationic dyes
The positively charged group in the cationic dye molecule is connected to the conjugated system in a certain way, and then forms a salt with the anionic group. According to the position of the positively charged group in the conjugated system, cationic dyes can be divided into two categories: isolation type and conjugated type.
▌Isolated cationic dyes
Isolated cationic dye parent and positively charged groups are connected through an isolating group. The positive charge is localized, similar to the molecules of disperse dyes A quaternary ammonium group is introduced at the end. It can be expressed by the following formula:
Due to the concentration of positive charges, it is easy to combine with fibers. The dyeing percentage and dyeing rate are relatively high, but the leveling property is not good. Generally, the color is dark, the molar absorbance is low, and the color is not rich enough. However, it has excellent heat resistance and light resistance and high fastness. It is often used for dyeing medium and light colors. Typical varieties are:
▌Conjugated cationic dyes
The positively charged groups of conjugated cationic dyes are directly connected to the conjugated system of the dye, and the positive charges are delocalized of. This type of dye has a very bright color and high molar absorbance, but some varieties have poor light resistance and heat resistance. Among the types used, conjugated types account for more than 90%. There are many varieties of conjugated cationic dyes, mainly including triarylmethane, oxazine, polymethine structures, etc.
New cationic dyes
▌Migrating cationic dyes
The so-called migratory cationic dyes refer to a class of cationic dyes that have a relatively simple structure, small molecular weight and molecular volume, and Dyes with good diffusion and leveling properties have now become a major category of cationic dyes. Its advantages are as follows:
① It has good migration and level dyeing properties, has no selectivity for acrylic fiber, and can be applied to different brands of acrylic fiber to better solve the problem of uniform dyeing of acrylic fiber.
② The amount of retarder is small (reduced from the original 2 to 3% to 0.1 to 0.5%), and it is even possible to dye a single color without adding retarder, so its use can reduce dyeing costs.
③ It can simplify the dyeing process and greatly shorten the dyeing time from (original 45-90 minutes to 10-25 minutes).
▌Cationic dyes for modification
In order to adapt to the dyeing of modified synthetic fibers, a batch of cationic dyes were screened and synthesized. The following structures are suitable for modified polyester fibers. The yellow ones are mainly conjugated methane dyes, the red ones are triazole or thiazole azo dyes and isolated azo dyes, and the blue ones are thiazole azo dyes and Oxazine dyes.
▌Dispersed cationic dyes
In order to adapt to the dyeing of modified synthetic fibers, a batch of cationic dyes were screened and synthesized. The following structures are suitable for modified polyester fibers. The yellow ones are mainly conjugated methane dyes, the red ones are triazole or thiazole azo dyes and isolated azo dyes, and the blue ones are thiazole azo dyes and Oxazine dyes.
▌Reactive cationic dyes
Reactive cationic dyes are a new type of cationic dyes. After introducing active groups into conjugated or isolated dye molecules, these dyes are endowed with special properties. Especially on blended fibers, they not only maintain bright colors, but can also dye a variety of fibers.
Properties of cationic dyes
▌Solubility
As mentioned earlier, the salt-forming alkyl group and anionic group in the cationic dye molecule affect the solubility of the dye. . In addition, if there are anionic compounds in the dyeing medium, such as anionic surfactants and anionic dyes, they will also combine with the cationic dyes to form precipitates. Blended fabrics such as wool/nitrile and polyester/nitrile cannot be dyed with ordinary cationic dyes and acidic, reactive, disperse and other dyes in the same bath, otherwise precipitation will occur. Anti-sedimentation agents are generally added to solve such problems.
▌Sensitivity to pH value
Generally, cationic dyes are stableThe established pH range is 2.5 to 5.5. When the pH value is low, the amino group in the dye molecule is protonated and changes from an electron donating group to an electron withdrawing group, causing the color of the dye to change; if the pH value is high, the cationic dye may form a quaternary ammonium base, or the structure may be destroyed. The dye precipitates, changes color or fades. For example, oxazine dyes transform into non-cationic dyes in alkaline media, lose their affinity for acrylic fibers and cannot be dyed.
▌Compatibility
Cationic dyes have a relatively high affinity for acrylic fibers, and their migration properties in the fibers are not good, making it difficult to level dye. Different dyes have different affinities for the same fiber, and their diffusion rates inside the fiber are also different. When dyes with greatly different dyeing rates are mixed, color changes and uneven dyeing are prone to occur during the dyeing process; while dyeing When dyes with similar speeds are mixed, their concentration ratio in the dye bath remains basically unchanged, so that the color of the product remains consistent and the dyeing is relatively uniform. The dyeing performance of this dye is called dye compatibility.
For ease of use, people use numerical values to express the compatibility properties of dyes, usually K
Value representation. Use one set of yellow and blue standard dyes. Each set is composed of five dyes with different dyeing rates. There are five compatibility values (1, 2, 3, 4, 5). The dye with the highest dyeing rate has the compatibility value. If the dye is small, the dye migration and leveling properties are poor. The dye with a small dyeing rate has a large compatibility value, and the dye migration and leveling properties are good. The dye to be tested and the standard dye are dyed one by one, and then the dyeing effect is evaluated to determine the compatibility value of the dye to be tested.
There is a certain relationship between the compatibility value of dyes and their molecular structures. When hydrophobic groups are introduced into the dye molecules, the water solubility decreases, the affinity of the dye to the fiber increases, the dyeing rate increases, the compatibility value decreases, the migration and leveling properties on the fiber decrease, and the color amount increases. Some groups in the dye molecules cause steric obstacles due to their geometric configuration, which also reduces the affinity of the dye to the fiber and increases the compatibility value.
▌Lightfastness
The lightfastness of dyes is related to their molecular structure. The cationic group in the conjugated cationic dye molecule is a relatively sensitive part. It is easily activated from the position of the cationic group after being exposed to light energy, and then transferred to the entire color system, causing it to be damaged and fade. Conjugated triarylmethane The light fastness of polymethacone, polymethylmethane and oxazine is not good. The cationic group and the conjugated system in the isolating cationic dye molecule are separated by a linking group. Even if it is activated under the action of light energy, it is not easy to transfer the energy to the color-producing conjugated system, keeping it intact, so it is durable. Light fastness is better than conjugate type.
Cationic fabric
▌What is cationic fabric?
What is cationic fabric? It means that there is a kind of yarn in the fabric that is specially treated cationic yarn, such as cationic polyester yarn or cationic nylon yarn. So why does it need to be made into a cationic yarn? Because the market needs it. Cations are resistant to high temperatures, so during the yarn dyeing process, other yarns will be colored, but cationic yarns will not. In this case, the dyed yarn will show a two-color effect, and yarn with this effect can It is made into various types of clothing, so cationic fabrics are produced.
▌Characteristics of cationic fabrics
① One of the characteristics of cationic fabrics is the two-color effect. Using this feature, some yarn-dyed two-color fabrics can be replaced, thereby reducing the cost of the fabric. , this is a characteristic of cationic fabrics, but it also limits its characteristics. In the face of multi-color yarn-dyed fabrics, cationic fabrics can only be replaced.
② Cationic fabrics have bright colors and are very suitable for use as artificial fibers. However, the color fastness to water washing and light resistance of natural cellulose and protein fabrics is very poor.
③ The wear resistance of cationic fabrics is also very good. After adding some artificial fibers such as polyester and spandex, it has higher strength and better elasticity, and its wear resistance is second only to nylon.
④ Cationic fabrics have some chemical properties, such as resistance to corrosion, dilute alkali, bleach, oxidants, hydrocarbons, ketones, petroleum products and inorganic acids. They also have some physical properties, such as UV resistance.
▌Uses of cationic fabrics
① Cationic fabrics have very good water absorption, and their dyeing vat difference is relatively small, so they are particularly suitable for sports-type clothes and are mainly used. It can be made into sweatshirts, sweatpants, yoga clothes, etc. If the cationic fabric is thicker and its brushing effect is good, it can be used as thermal clothing, thermal pants, etc.
② Cationic polyurethane jersey is also very environmentally friendly and can be used as environmentally friendly polyurethane fabrics.
③ Because cationic fabrics feel soft and comfortable to wear, and are brightly colored and similar to natural fabrics, they have good elasticity and hygienic properties and can be used to make high-end underwear, swimwear and sportswear fabrics.
