Fabric dyeing/coloration process with “Reactive Dye”
What is Reactive Dye?
A dye which is capable of reacting chemically with a substrate to form a covalent dye substrate linkage is known as reactive dye.
Here the dye contains a reactive group and this reactive group makes covalent bond with the fibre polymer and act as an integral part of fibre. This covalen bond is formed between the dye molecules and the terminal -OH(Hydroxyl) group of cellulosic fibres on between the dye molecules and the terminal-NH2 (amino) group of polyamide or wool fibres.
History of Reactive dye
Reactive dyes first appeared commercially in 1956,after their invention in 1954 by Rattee & Stpheness at the Imperial Chemical Industry (ICI). Dyestuffs Divion site in Bleckley,Manchetor,UK.
Uses of Reactive Dye:
By reactive dyes the following fibres can be dyed successfully
1) Cotton,Rayon,Flax and other cellulosic fibres
2) Polyamide and wool fibres
3) Silk and acetate fibres
Trade names of Reactive dyes
General formula of Reactive dye
The general structure of reactive dye is D-B-G-X
D=Dye part or chromogen (Color producing part)
Dyes may be direct,acid,disperse,premetallised dye etc
Bridging part may be -NH-group or -NR-group
G=Reactive group bearing part
Chemical structure of Reactive dyes
Worldwide dye consumption:
- Reactive dye is water soluble
- A certain amound of dye(10-40%) may react with water i.e hydrolysis is occured during dyeing operation.
Reactive dyes quality requirements
- Should be powder or liquid dyes
- Should have excellent stability
- Should have good solubility
- Low to medium substantivity
- Good diffusion and levelling properties
- Good compatibility
- Rapid fixation
- High degree of fixation
- Excellent washing fastness
Properties of Reactive Dyes:
- Water soluble dyes.
- Makes covalent bonds with the fibres
- A certain amount of dye is hydrolyzed during dyeing (10-60%)
- Dyeing is carried out in alkaline condition (PH =11)
- 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 protei fibres(Wool & Silk
- Comparatively cheap
- All kinds of shades is found
- Dyeing method is easy
- Reactive dyes are caionic dyes,which are used for dyeing cellulose,protein and polyamide fibres
- Wide range of colors can be obtained and widely used in Bangladesh
Popularity of Reactive Dye
Reactive dyes are mostly used for dyeing cellulosic fibres.At past cellulosic fibres were dyed with direct and vat dyes, but after the introduction of reactive dyes there utility has become limited.Reactive dyes are superior to direct dye in the following aspects:
1) Ability to procedure bright shades of wide range
2) High levelling quality
3) Good washing fastness
4) Good light fastness
Dyeing mechanism of Reactive Dye
The dyeing mechanism of material with reactive dye takes place 3 stages:
- Exhaustion of dye in presence of electrolyte or dye absorption
- Fixation under the influence of alkali
- Wash off the unfixed dye from material surface
Now they are mentioned bellow:
1) Dye absorption
When fibre is immersed in dye liquor, an electrolyte is added to assist the exhaustion of dye. Here NaCl is used the electolyte. This electrolyte neutralize the negative charge formed in the fibre surface and puts extra energy to increase dye absorption. So when the textile material is introduces to dye
liquor the dye is exhausted on to 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 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 alkali used for this purpose depends on brand of dye and dyeing temperature. Here generally caustic soda, soda ash or NaHCO3 is used as alkali depending upon reactivity of dye.They create proper pH in dye bath and do as the dye-fixing agent.
The reaction takes place in this stage is shown below:
3) Wash off:
As the dyeing is completed, a good wash must be applied to the material to remove extra and unfixed dyes from material surface. This is necessery for level dyeing and good wash fastness. It is done by a series of hot wash, cold wash and soap solution wash
Dyeing cycle and important factors/phases in reactive dyeing
- Pretreatment of the substrate
- pH of the substrate prior to dyeing
- pH of the dye bath
- Solubility of the dyestuff
- Dyeing temperature
- Electrolyte concentration
- Dyeing time
- Type of alkali
- Washing off sequence
- Quality of water and salt
Here we give details description of the above points-
a) Pretreatment of the substrate
The pretreatment includes the scouring and bleaching of the substrate prior to dyeing. The main objective of the scouring treatment is remove the major impurities from the cotton and improving the absorbency.The material shall be free from a contaminants and natural coloring matter in the scouring and bleaching treatment. The extent of the pretreatment such as ground whiteness depends upon the target shade, as bright and whiter ground whiteness is required for light and bright shades and dark and dull shades can be dyed on scoured ground without any difficulty
b) pH of the substrate
pH of the substrate prior to dyeing must be controlled and should be either neutral or slightly acidic because if the pH alkaline at the beginning, the dye molecule may form permanent bond or premature fixation leading to unlevel dyeing
c) pH of the dye bath
It should be weakly acidic to neutral in the dyeing bath in migration and exhastion phase, before addition of alkali. It must be checked and regulated because presence of bicrarbonates in the water may increase the pH with increase temperature, which results into partial fixation of the dye molecules resulting in unlevel dyeing. The pH in the fixation stage must be kept at 10.8-11.0 which shall be achieved by either soda ash alone or a mixture of soda ash and caustic soda
d) Solubility of the dyestuffs
It is better to consider the dye solubility chart of individual colors provided by the manufacturer, the dyes with higher solubility are more sutable for better shade and color fastness control
e) Dyeing temperature
The temperature and rate of rise of the dye bathe affects affinity of the dye molecules towards fiber, rate of hydrolysis, migration and covalent bond formation. Therefore the dye temperature selected must be as per the dye sub class. Effect of temperature on the build up and fixation of individual dye must be studiyed to form dyes groups having similar characteristics and then these groups must be selected making combination shades.
f) Electrolyte concentration
The dyes must be exhausted by addition of salt or glauber salt before starting the fixation of color . The electrolyte used must be free unwanted impurities such as metal salts(iron, copper etc) calcium content, insoluble material, hardness creating salts
g) Dyeing time
The dyeing time must be selected based upon the depthe of shade. The timings must be optimized to get maximum exhaustion as well as maximum fixation of the color in dye bath. Based on the exhaustion and fixation curves of individual dyestuffs.an optimum time can be selected. Ther is no advantage of increasing the fixation time than design because that will not help either in exhaustion or fixation. Generally darker shades need more time in fixation phase than the lighter shades.
h) Material to liquor ration
The chemical concentrations must be changed with the change in liquor ratios, If a dye house is having different capacity machines with different M:L ratio then it shall be taken into consideration from the lab recipe stage
i) Type of alkali for fixation
There are different methods applied to achieve the right pH for the fixation of reactive dyes. Normally soda ash alone is used for the fixation purpose, however a mixed alkali system of soda ash and caustic soda is also used, prticularly in the case of dark shades. A gradual pH change is preferred over the shock change for better dyeing results, therefore a dosing system is strongly recommended in the reactive dyeing to achieve consistent shade reproducibility.
j) Washing off sequence for reactive dyes
The hydrolysed and unfixed dyes which are present in the dye bath as well as on the fiber after the completion of dyeing cycle, these hydrolyzed dyes have no affinity for the fiber but still they act as direct dyes and in the presence of electrolyte penetrates inside the fiber, with the rise in temperature of washing and soaping. If these are not removed before soaping and washing these can severely affect the color fastness properties. Such trouble can be reduced or eliminated by following an optimized washing off sequence after dyeing. The dyed goods must be free from any inorganic salt before going for a soaping treatment. Generally a non ionic soaping agent is used for soaping purpose to get good washing fastness. An organic acid such as acetic acid is must be used to neutralize the dye bath. The soaping treatment can be done up to a boiling temperature to remove the unfixed and hydrolysed dye effectively.