Reactive Dyes [ Application, Characteristics, Reactive Groups & Area of Applications, Structure, Properties, Dyeing Method & Experiments]

PART # 2        

4. CHEMICAL STRUCTURE OF REACTIVE DYES:

• General Formula: The general formula representing the chemical structure of reactive dyes are as follows:

S - D - T - X

S: Solubilizing Group (generally, -SO3Na)

D: Chromophore

T: Bridge link (-NH-, -NHCO-, -SO2-, -NHSO2-, -NCH3- and others)

X: Reactive Group

• Chromophore: Many of the chromophore are derived from acid dyes. In chemical structure the following compounds are employed among which azo compounds predominate.

(1) Reactive dye having a chromophore based on azo structure:

- Pyrazoron type (Yellow), H-acid type (Red), H-acid type (Navy-Black)

(2) Reactive dye having a chromophore based on metal complex azo structure:

- H-acid type (Red, Violet-Blue)

(3) Reactive dye having a chromophore based on anthraquinone structure:

- Blue

(4) Reactive dye having a chromophore based on phthalocyanine structure:

- Turquoise Blue

(5) Bi-Functional reactive dye having a chromophore based on azo structure:

- Red

5. GENERAL PROPERTIES OF REACTIVE DYES:

• Reactive dyes are water soluble, and become anionic in aqueous solution.
• Reactive dyes react with -OH groups in cellulosic fibers or with -NH2 groups in polyamide fibers in presence of alkali to form a covalent bond.
• The mechanism of reaction of reactive dyes with cellulose is generally classified into the following two (2) types:

(1) Nucleophilic addition reaction (Vinyl-Sulfone type)

(2) Nucleophilic substitution reaction (Mono-Chloro-Triazine Dyes)

• A reactive dye also react with water in the presence of alkali and hydrolyzed to form a compound having no reactivity. In the dyeing system, a reactive dye reacts not only with fibers but also with water on a competitive basis. However as the rate of reaction with fibers is greater, the proportion of hydrolyzed dye is smaller.
• The substantivity of reactive dyes for cellulose is generally low, and accordingly the unfixed dyes having been hydrolyzed are, after dyeing, rather easily washed off by soaping.

6. DYEING METHOD WITH REACTIVE DYES:

A. Classification of dyeing method :

- Exhaust Dyeing (Yarn dyeing, Piece dyeing, Knit dyeing)

- Continuous Dyeing ( One-bath method, Two-bath method)

- Printing (One-phase method, Two-phase method)

B. Practical Dyeing Method:

• Dissolving Dyes: Dyes are dissolved by pasting with a small amount of water, adding hot water 80-90 Deg. C and stirring thoroughly to complete solution.
• Alternatively, the dyes can be dissolved by sprinkling a small portion with stirring on to hot water filled in a dissolving vessel equipped with a stirrer.

To be continued..................................