Reactive dyes and its mechanism

 

Reactive Dye:

•  Reactive dye possesses a reactive group and is capable of reacting chemically with a substrate (cotton, wool, polyamide, etc.) under suitable condition to form covalent dyesubstrate linkages.
•  Here the dye contains a reactive group and this reactive group makes covalent bond with the fibre polymer and act an integral component of the fibre.
• This covalent bond is formed between the dye molecule and the terminal -OH (hydroxyl) group of cellulose fibres or the terminal –NH2 (amino) group of polyamide or wool fibres.
• General Feature of Reactive Dye Molecule:
• Dyes contain anionic functional group, structure of acid, direct or mordant dye with reactive that form covalent bond with fibre.

Ø Reactive system- enabling the dye to react with –OH (Hydroxyl) groups in cellulose.

Ø Chromogen group- contributing the colour and much of the substantivity for cellulose;

Ø Bridging group- link the reactive system to the chromophore.

Ø Water solubilising group– usually sulphonic acid substituents attached to the chromophoric group.

Figure: Chemical Structure of Reactive dye

Reactive dyes are so called Reactive because

• This is the only type of dye which has reactive group and that reactive group reacts chemically with the fibre polymer molecules and form covalent bond.
• This covalent bond is formed between the reactive group and terminal –OH (Hydroxyl) group of cellulose fibre polymer, terminal – NH2 (Amino) group of polyamide and wool fibre.
• The strength of this covalent bond is more than ionic bond, hydrogen bond and vanderwaal’s force of attraction.
• Thus the reactive group becomes an integral part of the fibre.
• For this reason, the dyes are so called reactive dye.
• They are also called fibre reactive group.

Properties of Reactive dyes:

•   Water soluble dyes.
•   Makes covalent bounds with the fabric.
•   A certain amount of dye is hydrolysed during dyeing (10 - 60%)
•   Dyeing is carried out in alkaline condition (pH =11.5).
•   Huge electrolyte is necessary for dyeing with reactive dyes.
•   Fastness (Wash, light, Rubbing, Perspiration) properties are generally good.
•   Easy applicable to cellulosic as well as protein fibres (Wool & Silk)
•   Very popular and wide used in the wet processing industry in Bangladesh.
•   Comparatively cheap.
•   All kinds of shade are achieved.
•   Dyeing method is easy.

History of Reactive dye:

• The concept of attaching a coloured molecule to cellulose by means of a chemical bond is at least a century old, but until the 1950s a commercially viable technique for achieving dyeing of high wet fastness in this way remained vague.
• On the occasion of 100 years celebration of Synthetic dye manufacturing, two chemists of ICI Company (UK) named Stephen and Ratteefirstinventednew dyestuff as a named of Reactive dye in 1956 for dyeing cellulosic fibres. The 1st three Reactive dyes were Procion yellow R, Procion brilliant Red 2B and Procion Blue 3G.
• For this effort, they were awarded gold medal by the Society of Dyes and colourists for the year 1960. Ø These dyes came to our country in Mid 60’s and became very popular during 80’s.

Why Reactive dyes are so popular:

•         Good washing fastness (Rating 4-5)
•         Very good light fastness (Rating 6)
•          Lower cost
•         Simple dyeing method.
•         Good reproducibility.
•         Low dyeing temp (below 100°C).
•        Ability to produce bright shade.
•         Dye molecular composition.
•         Easily applicable to cellulosic fibre as well as protein.
•      All kinds of shade are found.

Importance of Reactive group present in Reactive dye:

•      Reactive group do not contribute colour which is determined by chromogen group.
•      The reactivity of viny1 sulphone group is less then halogen group.
•      If number of reactive group increases, binding also increases depending on dye structure.
•      Reactive dye absorb up to 90%.
•      If the molecular weight of reactive group increases, reactivity also increases.
•      Reactivity of viny1 sulphone group increases with increases of and temperature.
•         Sulphone group has mere solubility but it is not stable.
•      Chlorine imparts medium reactive,but it is cheap.
•      Reactivity of fluorine is the least and its rate of hydrolysis is also less

FIBRES DYED By reactive dyes the following fibres can be dyed successfully:

i.                     Cotton, viscose, flax, and other cellulosic fibres.

ii.                   Polyamide (Nylon) and protein (Wool) fibres.

iii.                  Silk and acetate fibres.

Classification of Reactive dyes:

1. On the Basis of Reactive Group:Two types

i. Hallogen (commonly chlorine) derivatives of nitrogen containing heterocyclic, 3 types

•      Triazine group
•      Pyrimidine dyes
•      Quinoxaline dyes

ii. Activated vinylcompounds: 3 types

•        Vinyl sulphone
•        Vinyl acrylamide
•     Vinyl sulphonamide

2. On the Basis of Reactivity: Reactive dyes are three types

A. Higher Reactive dyes: Reactivity of these dyes is high. So fixations of these dyes are easy and lower alkaline medium is kept. Here pH is maintained 10-11 by using NaHCO3 in dye bath.

Example:ProcionMX,Drimarene K, Levafix E-A.

B. Moderate Reactive Dyes: Reactivity of these dyes is medium. Here pH is maintained 11-12 by using Na2CO3 (soda ash)in dye bath.

Example:Cibacron F, Remazol RR dyes.

C. Lower Reactive Dyes: Reactivity of theses dyes are low. So, highly alkaline environment is required for the fixation of these dyes with substrate. Here pH is maintained 12-12.5 by using strong alkali such as Na2CO3(soda ash) withNaOH (Caustic soda) in dye bath.

Example:Procion H, Levafix E, Drimarene X, Basilen, Cibacron.

3. On the basis of Dyeing temperature and methods:

A. Cold brand: These types of dyes are highly reactive as they possess highly reactive group in their constituent. So, dyeing of these dyes can be done in lower temperature i.e.30-40 0 C.

Example: Porcion MX, Drimarenc K, Levafix E-A.

B.Medium brand: These types of dyes contain reactive group of moderate reactivity. So, dyeing is done in higher temperature than that of cold brand. Here temperature between 40-60 0 C.

Example:Cibacron F, Remazol RR dyes.

C. Hot brand: These types of dyes contain reactive groups least reactivity. So hightemperature is required for dyeing i.e.60-900 C temperature is kept for dyeing.

 Example:Procion H, Levafix E, Drimarene X,BasilenCibacron.

4. On the basis of controlling parameters (Recent classification):

Group 1: Alkali-controllable reactive dyes:

•     Most favourable fixation temperature ranging from 40 – 60°C.
•     Relatively lower exhaustion in neutral salt solution before alkali addition.
•     Care is essential during alkali addition to attain level dyeing because of its high reactivity.

Examples: dichlorotriazine, chlorodifluoropyrimidine, dichloroquinoxaline or vinylsulphone reactive dyes.

Group 2: Salt –controllable reactive dyes:

• The suitable fixation happens at temperatures between 80°C and 100°C.
• Moderately high exhaustion at neutral pH .
• Care is essential during salt dosing to make sure level dyeing due to its low reactivity. It is preferable to add electrolytes portion wise.

Examples : trichloropyrimidine, aminochlorotriazine or bis (aminochlorotriazine). Aminofluorotriazine.

Group 3: Temperature controllable reactive dyes:

• In this dyeing method, the dye-fiber bond is formed at temperatures ranging from 80 ⁰C to more than 100 ⁰C, in the absence of alkali.
• This method has self-leveling characteristics, hence no need any dye bath auxiliary to make possible level dyeing.
• Controlled temperature is necessary to attain good dyeing results.

Examples: bis (aminonicotinotriazine), Kayacelon react (KYK).

Reactive groups used in commercial reactive dyes

Reactive Group	Commercial name	Reactive	Dyeing Temperature °C
DCT	Procion MX	High	25-40
DFTP	Drimarene K	Moderate to High	30-50
DFCP	Levafix E-A	Moderate to High	30-50
MFT	Cibacron F	Moderate	40-60
VS	Remazol	Moderate	40-60
DCQ	Levafix E	Low	50-70
MCT	Procion H	Low	80-85
MCT	Basilen	Low	80-85
MCT	Cibacron	Low	80-85
TCP	DrimareneX	Low	80-95

Hydrolysis of Reactive Dye/Technical Deficiency of Reactive Dye:

• Under alkaline conditions, reactive dye reacts with the terminal –OH group of cellulose or –NH2 group of polyamide fibres.
• If the dye solution is kept for longer timein highly alkaline condition and dyeing is continued for longer time with high temperature, then concentration of the dye bath decreases.
• In that case, reactive dyes react with the –OH group of water. This reaction of dye with water is known as hydrolysis of reactive dye.
• After hydrolysis dye cannot react with fibre.
• The concentration of hydrolysis compound increases continuously. So hydrolysis increases the loss of dye and passes in to waste water.
• The hydrolysis of reactive dye is shown below through scheme 1 and scheme 2.

 Controlling Parameters/Factors

a. pH of dye bath:pH range for reactive dyes 10-12.5. Higher PH increases hydrolysis and vice versa.

b. Dyeing time: he exhaustion takes in 20-30 min Higher dyeing time increases hydrolysis and vice –versa.

c. Temperature: 40°C -100°C temperature is applied. Temperature of dyeing hydrolysis and vice-versa.

d. Liquor Ratio:M: L may be 1:5 – 1:10. Longer liquor ratio increases hydrolysis and vice-versa.

e. Concentration of electrolyte: Common salt and Glauber’s salt. Concentration of salt may be 20-100 g/L. Depends on the depth of shade (0.1-8.0%). Higher salt concentration decreases hydrolysis up to a limit and vice-versa.

f. Dye Reactivity: Increase both hydrolysis and fixation rate.

g. Dye substantivity: More substantive dye causes more hydrolysis but if substantivity is reduce to bare minimum or removed altogether, the build-up problem arises during dyeing and colour yield problem (dyeing) of the dye is reduced considerably.

h. Type and number of Reactive groups: Vinyl sulphone group is more prone to hydrolysis than triazinyl group.

For preventing hydrolysis the following precautions are taken:

•   As hydrolysis increases with increasing with increasing temperature during dissolving and application temperature should not be more than 40°C.
•   Dye and alkali solution are prepared separately and mixed just before using.
•   Dye and alkali should not be kept for long time after mixing.

Potential Problem due to Dye hydrolysis:

• Dye wastage occurs up to 40-60% dyes (avg. 50%) due to dyehydrolysis.
• Relatively large amounts of electrolyte are required for exhaust; otherwise dye hydrolysis will occur greatly in dye bath.
•  Longer washing operation for removal of unreacted and hydrolysed dye often costs 50% of total dyeing cost.
• Hydrolysed dye is discharged as colour effluent and effluent cost is risen up.
• Laborious removal of unreacted and hydrolysed dye is require-often a longer operation than the dyeing step itself and not always entirely satisfactory.
•   Fastness properties deteriorate due to dye to hydrolysis.
• Uneven problem for continues dyeing.
• Running shade in batch process.

Dyeing mechanism of reactive dye:

Reactive dyes cover a full range of bright shades, with good to excellent wash fastness, moderate to good light fastness. This class of dye is mostly used to dye the cellulosic fibre; however, suitable functional group of reactive dyes is used for wool and man- made fibre. Dye containing a triazine or vinylesulphone group could form covalent bond with -OH (hydroxyl) group of cellulose fibresor -NH2 (amino group) of protein or woolfibres at proper pH and at reasonable temperature between 30 – 100 ⁰C. The overall dyeing mechanism consists of four stages:

i.                     Adsorption of dye on the fibresurface.

ii.                  Diffusion/exhaustion of dyes into the interior of fibre.

iii.                Fixation of dyes in the fibre.

iv.                 Wash off.

1.Adsorption of dye on the fibresurface:

Adsorption is the movement of the dye from the solution phase to the fibre phase, at the fibre surface. Basically, reactive dyes and cellulose fibres show negative charge in dye bath and repel each other reducing substantivity of dye. Once salt is added in the dye bath to reduce the negative charge by decreasing its zeta potential difference between the fibre phase and dye molecule resulting better absorption. In fact, salt acts as exhaustion agent and assist in movement of dye molecules from dye bath on to the fibre surfaces. The rate of adsorption depends on dye concentration, salt concentration and liquor ration of the dye bath.

2. Diffusion/exhaustion of dyes into the interior of fibre:

The diffusion of the dye means movement of dye molecules from the fibre surface to the interior of the fibre. Moreover, rate of diffusion of the dye molecules depend on temperature of the bath and size of dye molecules. Higher the temperature better is the rate of diffusion for dye molecules of lower size.

3. Fixation of dyes in the fibre:

Fixation of dye means the reaction of reactive group of dye with terminal –OH or –NH2 group of fibre and thus forming strong covalent bond with the fibre. This is an important phase, which is controlled by maintaining proper pH by adding alkali. The good fastness to washing of dyeing with reactive dyes on the cellulose fibres is a result of the stable covalent bond formed between the dye molecules and cellulose polymer. The covalent bond can be formed between the dye and cellulose fibre by substitution reaction with heterocyclic ring of triazine dyes (scheme 1) and by addition reaction with vinyl sulphone dyes (scheme 3). But both dyes are hydrolysed if they react with water (scheme 2 and 4).

4.Wash off:

After dyeing, a good wash must be applied to the material to remove the unfixed and hydrolysed dyes as well as residual salt and alkali from the material surface. This is necessary for level dyeing and good wash fastness. It is done by a series of hot wash, cold wash and soaping wash.