GATE (TF) Textile 2010 Question Paper Solution | GATE/2010/TF/29

Acid dyes are water-soluble anionic dyes that are typically used to color protein fibers such as wool, silk, and nylon. The bonding of acid dye with fiber involves a combination of electrostatic attraction and chemical reaction.

When acid dyes are dissolved in water, they form negatively charged ions called anions. Protein fibers, on the other hand, have positively charged amino groups on their surface. The negatively charged anions of the acid dye are attracted to the positively charged amino groups on the fiber surface, forming an electrostatic bond.

However, this electrostatic bond is not strong enough to ensure good dye fastness, so acid dyes also chemically react with the fiber surface to form a covalent bond. This is accomplished by heating the dye and fiber together in an acidic solution, which causes the dye molecules to penetrate the fiber and react with the amino groups on the fiber surface.

Once the covalent bond is formed, the dye becomes highly resistant to washing and fading, resulting in good color fastness. The entire process of bonding acid dye to fiber is known as dyeing, and it is a crucial step in the production of colored textiles made from protein fibers.

Mordant dyes are a class of dyes that require a mordant, or a metal salt, to bond with the fabric. The chemical structure of mordant dyes varies depending on the specific dye, but they all contain groups that can form coordination bonds with metal ions.
The process of dyeing with a mordant involves three main steps: pretreatment, mordanting, and dyeing. In the pretreatment step, the fabric is usually washed and soaked to remove any impurities or sizing agents. In the mordanting step, the fabric is immersed in a solution containing a metal salt, such as aluminum, iron, or chromium. The metal ions in the solution form coordination bonds with the fabric, creating sites where the dye can attach.
The actual dyeing step involves immersing the mordanted fabric into a solution of the dye. The dye molecules contain groups that can form coordination bonds with the metal ions on the fabric surface, resulting in a permanent bond between the dye and the fabric.
The chemical structure of a typical mordant dye contains a chromophore, which is the part of the molecule responsible for its color, and a mordanting group, which is the part of the molecule that forms coordination bonds with metal ions. The mordanting group may be an amino group, a hydroxyl group, a carboxyl group, or a sulfonate group, among others.

For example, the common mordant dye, mordant red 19, has a chemical structure consisting of a chromophore, such as an azo or anthraquinone group, and a mordanting group, such as a hydroxyl or carboxyl group. The mordanting group can form a coordination bond with metal ions, such as aluminum or iron, during the mordanting step, allowing the dye to attach to the fabric and form a permanent bond.

Reactive dyes are a type of dye that form covalent bonds with the fibers they are dyeing, typically cotton, rayon, and other cellulose fibers. These dyes contain reactive groups, such as chlorotriazine, vinyl sulfone, or epoxy groups, that can react chemically with the hydroxyl groups on the fiber’s cellulose molecules.
The reactive dyes typically contain a chromophore, which is the part of the molecule responsible for the dye’s color, and a reactive group, which allows it to react with the fiber. During the dyeing process, the reactive dye is dissolved in an aqueous solution, and the fabric is immersed in the solution.
The reactive group on the dye molecule then reacts with the hydroxyl groups on the cellulose fibers through a chemical reaction called nucleophilic substitution. This reaction creates a covalent bond between the dye and the fiber, resulting in a permanent bond that is highly resistant to washing and fading.
The covalent bond formed between the reactive dye and the fiber is very strong, allowing the dye to remain attached to the fiber even after repeated washing and exposure to sunlight. This makes reactive dyes a popular choice for dyeing cotton and other cellulose fibers, as they provide good color fastness and long-lasting results.

Direct dyes are a class of dyes that are water-soluble and can be directly applied to the fabric without the need for a mordant or other chemicals to fix the dye. The bond present in direct dyes is mainly an ionic bond.
Direct dyes contain polar groups, such as amino, sulfonate, or carboxyl groups, that allow them to dissolve in water. These polar groups can also form ionic bonds with the polar groups on the fiber’s surface, such as hydroxyl and carboxyl groups. The ionic bonds are formed through electrostatic attraction between the positively charged amino group of the dye and the negatively charged groups on the fiber surface.
The strength of the ionic bond between the direct dye and the fiber is relatively weak compared to the covalent bond formed by reactive dyes or the coordination bond formed by mordant dyes. This makes direct dyes less resistant to washing and fading than other types of dyes. However, direct dyes are still commonly used for dyeing cotton, viscose, and other cellulosic fibers, as they are easy to apply and provide vibrant and bright colors.