General Structure of Reactive DyesThe general structure of reactive dye is: D-B-G-X.

Here,

D= dye part or chromogen (color producing part)Dyes may be direct, acid, disperse, premetallised dye etc.

B = bridging part.Bridging part may be –NH- group or –NR- group.

G = reactive group bearing part.

X= reactive group.

Chemical structure of reactive dyes:

Reactive Dyes:

A dye which is capable of reacting chemically with a substrate to form a covalent dye substrate linkage

Only a class of dye that have a reactive Group in their structure

Covalent bond formation with the substrate

Characteristics of Reactive Dye:

Reactive group: Responsible for the reaction between the substrate & dye molecule

Chromophoric group: Coloring agent

Bridging group: That links the reactive system to the chromophore. Also influence the reactivity & degree of fixation.

Solubilizing group: Solubilizing agent

Classification of reactive dyes:

Depending on chemical constitution reactive dyes can be classified as:

Monofunctional Reactive system: Have only one reactive system

Bifunctional Reactive system: Have two reactive system. Homobifunctional reactive system and Heterobifunctional reactive system

Polyfunctional reactive system: May have more then two reactive system.

Depending on application method of temperature reactive dyes can be classified as:

Cold Reactive dyes:

Dyes which have high reactivity need low temperature for dyeing (25-50 0C)

Hot reactive dyes:

Dye which have low reactivity need high temperature for dyeing the textile substrate (60-90 0C)

General properties of reactive dyes:

Reactive dye is anionic in nature.

Reactive dye is a water soluble dye.

They have better wash and light fastness properties.

They have better substantivity.

They form strong covalent bond with the cellulosic fibers.

Alkaline condition is must required for dyeing.

Electrolyte is must required for dye exhaustion in the fiber.

Wide range of color can produce.

Comparatively cheap in price.

Influencing factors

ph

Reactive dyeing is done in alkaline condition for this reason ph of the dye bath should control. ph should be between 11.5-11.

Temperature

Temperature should fix depending on the nature of dye.

Concentration of electrolyte

Concentration of electrolyte depends upon the type of shade.

Substantivity:

Substantivity more depends upon chromophore as compared to reactive system. A high substantivity may results:

Lower dye solubility.

High primary exhaustion.

A high reaction rate.

Defects in Dyeing with Reactive Dyes:

Defects:

Colors are not fast to washing, Abrasion, Staining in the fabric when transporting from place to the other, water marks on the fabric.

Remedy:

Wash the fabric with soap and soda ash at right temperature. Adding sequestering agent will yield good results. Treating with Ammonia will also give good results.

Defect: The fabric has been dyed in darker shade,

uneven dyeing

Remedy:

To take out color from the fabric treat it with caustic for 45-60 min at 70 oC. Thereafter treat the fabric with 10-20 g/l . Acetic Acid for 40-60 min at 80-85 oC.

Defect:

Bleeding in colors during washing, abrasion.

Remedy:

Boil the fabric with caustic, Treat the fabric with Hydrogen Peroxide ( 5-10 g/l, 60-70 oC) to make the color fast.

General steps of dyeing with reactive dyes:

Exhaustion of dyes

Adsorption

Diffusion of dye

Migration of dye molecule

Fixation of dye in the textile substrate

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