What is Reactive Dye? Properties and reaction of Reactive dye, Classification of Reactive Dyes

Content :

Reactive Dye 

Structure & properties of Reactive Dyes

Reaction of Reactive dyes

Dye take up of Reactive dye

Classification of Reactive dyes

Reactive dyes 

• For a long time, chemist had been seeking a method of joining dye with cellulose through covalent bond. 
• At the end of 1940, chemists turned their attension to the dyes containing cyanuric chloride or triazinyl dyes 
• It is possible to make chlorine atom in cyanuric chloride to combine with hydroxyl or amine group of dye molecules

Reactive Dye

An outstandingly important property of cyanuric chloride is that if any chlorine atom left unsubstituted, that can react to cellulose. 

• The reaction of DCT with cellulose is shown below 

reaction of DCT with cellulose

If hydrolysis occurs, that will decrease the color yield and reactive dye converts into a sort of direct dye which is very substantive to the cellulose and also create problem in washing off leading to poor wet fastness 

Exploitation of the dichlorotriazine reactive system soon led to parallel development of the much less reactive mono chlorotriazine dyes, readily made by a substitution reaction between an aryl amine and the dichlorotriazine precursor.

Reaction between an aryl amine and the dichlorotriazine precursor.

 

• More stable padding liquors could be prepared using the aminochlorotriazine types

 

Reaction of Reactive Dyes

   Remazol (HOE) dyes, based on the 2-sulphatoethylsulphone precursor of the vinysulphone reactive system or related species, function by a nucleophilic addition mechanism rather than substitution.

 • Before this can occur, however, alkaline 1,2-elimination of the precursor grouping is necessary to release the reactive vinylsulphone system.

 • In this system the carbon–carbon double bond is polarised by the powerfully electron-attracting sulphone group.

 • This polarisation confers a positive character on the terminal carbon atom, favouring nucleophilic addition of either a cellulosate anion or a hydroxide ion, again leading to either fixation or hydrolysis respectively 

Reactive dyes (Bifunctional) 

• The appearance of two further interesting ranges of bifunctional dyes that are capable of reacting with cellulose via both mechanisms, nucleophilic substitution and nucleophilic addition. 

• In both systems one ring substituent in a halogenotriazine dye carries a 2-sulphatoethylsulphone grouping . 

• The halogeno substituent can be either chlorine or the more reactive fluorine.

The four solubilising groups in the precursor form of CI Reactive Black 5 confer high solubility but unusually low substantivity. 

• It is a nearly symmetrical bis(sulphatoethylsulphone) structure and as these precursor groups lose their ionic charge by 1,2- elimination, the substantivity for cellulose is enhanced and the bis(vinylsulphone) structure formed shows excellent fixation efficiency under alkaline conditions. 

Aminofluorotriazine sulphatoethylsulphone dyes 

• Early in 1988 Ciba-Geigy launched the Cibacron C range of mixed bifunctional dyes. 

• They contain a new aliphatic vinylsulphone system and either a monofluorotriazine bridging group or an arylvinylsulphone function. 

• They are designed mainly for pad applications and appear to be characterised by medium to low affinity, good build-up, easy wash-off and high fixation. 

• Their outstanding bath stability and high fixation make them especially suitable for pad–batch dyeing. • The manufacturing cost of these structures is believed to be relatively high but the purchase cost to the dyer may be offset by enhanced costeffectiveness in use attributable to efficient fixation and easy wash-off, possibly the best approach that time towards environmentally acceptable reactive dyes.

Structure and properties of reactive dye

• The design of reactive dye structures almost always involves one or more compromises between conflicting requirements. 

• There is seldom an ‘ideal’ structure of a desired hue that embodies all possible attractive features with regard to application and fastness properties. 

• The gain in aqueous solubility provided by an extra sulpho group often has to be paid for by a decrease in affinity for cellulose. 

• Enhancement of substantivity is beneficial for high exhaustion but may impair migration or washing-off characteristics. 

• High reactivity offers the possibility of rapid fixation but storage stability may be adversely affected.  

Reactive dyes uptake 

• All conventional reactive dyes for cellulose, irrespective of whether they react by nucleophilic addition, substitution, or both mechanisms rely on the reactivity of the cellulosate anion as the nucleophilic reagent and hence hydrolysis of the dye by reaction with hydroxide ions from water will always compete with the desired fixation reaction. 

• Reaction between the dye and cellulose can occur only when the dye has been absorbed into the cellulose phase. Thus the kinetics of the dye–cellulose reaction are strongly influenced by the rate of absorption of dye. 

• The ratio of the rate constants for reaction of the dye with the fibre and with water is a constant for a given dye over a wide range of alkaline pH values. 

 Factors governing dye uptake 

The efficiency of fixation is a function of: 

1. The substantivity ratio, the relative concentrations of dye absorbed into the substrate and remaining in the dyebath; 

2. The reactivity ratio, the ratio of rate constants for the fixation reaction and hydrolysis; 

3. The diffusion coefficient of the dye in the substrate; 

4. The liquor ratio; and 

5. The surface area of the substrate available for absorption of dye. 

 Factors governing dye uptake 

• The lower the linear density of the fibre, i.e. the greater the surface area per unit weight, the more efficient is the dyeing. 

• The substantivity ratio is the most influential of the factors governing fixation efficiency. Dyes of higher substantivity diffuse more slowly than less substantive dyes. 

• Changes in dyebath conditions that increase substantivity tend to decrease the diffusion coefficient. 

• Lowering the liquor ratio favours increases in the rate and efficiency of fixation. 

 

• An increase of dyeing temperature lowers the substantivity ratio and accelerates the rate of hydrolysis of the dye; both of these effects reduce the fixation efficiency. 

• The rates of diffusion into and reaction with the fibre are also accelerated, however, and these factors both favour fixation of the dye. 

• An increase in electrolyte concentration always enhances substantivity without impairing reactivity providing the dye remains completely dissolved. D

Classification of reactive dyes

 Alkali-controllable reactive dyes: 

• These dyes have optimal temperatures of fixation between 40 and 60°C. 

• They are characterized by relatively low exhaustion in neutral salt solution before alkali is added. 

• They have high reactivity and care in addition of alkali is necessary to achieve level dyeing, preferably at a controlled dosage rate.

• Typical examples of dyes belonging to this group have dichlorotriazine, chlorodifluoropyrimidine, and vinylsulphone reactive systems. 

 Salt-controllable reactive dyes 

• Dyes in this group show optimal fixation at a temperature between 80°C and the boil. 

• Such dyes exhibit comparatively high exhaustion at neutral pH, so it is important to add salt carefully to ensure that dyeing is level. 

• Electrolyte addition is often made portion wise or preferably at a controlled rate of dosage. 

• Dyes with these properties typically have lowreactivity systems such as trichloropyrimidine, aminochlorotriazine or bis(aminochlorotriazine). 

 Temperature-controllable reactive dyes 

• This group is represented by those dyes that react with cellulose at temperatures above the boil in the absence of alkali, although if desired they can be applied under the same conditions as the salt-controllable group with alkaline fixation at a temperature between 80°C and the boil. 

• Dyes in this group have self-levelling characteristics so there is no need to use auxiliary products to facilitate level dyeing. 

• Good results can be achieved by controlling the rate of temperature rise. At present only the Kayacelon React (KYK) range of bis (aminonicotinotriazine) dyes belong to this group.