Reactive Dyes

Reactive dyes
In the simplest terms ,all reactive dyes are made up of three basic units, a chormophore , a bridge and a reactive group/ groups (either a haloheteocycle or an activated double bond). These dyes are used for dyeing of cellulosic fibers and when these are applied to a cellulosic fiber in an alkaline dye bath ,they form a covalent bond with hydroxyl group of the fiber by chemically reacting with fiber. The covalent bond formed between the dye molecule and fiber make dye molecule a part of the fiber molecule.


Types of the reactive dyes


Basically there are three types of the reactive dyes , which are classified as per the functional group present,

1.mono functional reactive dyes
2.bi functional reactive dyes(homo and hetero bifunctional)

3.poly functional or multifunctional reactive dyes

These dyes are also classified based on their exhaustion properties as well as on their application temperature such as cold brand  and hot brand reactive dyes.

These are water soluble dyes and hence applied from aqueous  bath under neutral to weakly acidic conditions , electrolyte is added to exhaust the dyes on the fiber from the dye bath and then  exhausted dye is fixed on the fiber by adjusting the pH suitable for making a covalent bond between the dye molecule and fiber , with the help of alkali. The unexhausted dye which remains in the liquor is hydrolysed ( hydrolysed dye is formed by the reaction of dye molecule with water under alkaline conditions) and loses its reactivity with cellulosic fiber , that too is absorbed by the fiber . Therefore washing and soaping treatment is done to remove the hydrolysed and excess dye to improve the color fastness of the dyed substrate. The overall colorfastness achieved is good when the material is dyed and washed properly.

Dyeing cycle and Important factors/phases  in reactive dyeing

1. pretreatment of the substrate
2. pH of the substrate prior to dyeing
3. pH of the dyebath
4. solubility of the dyestuff
5. dyeing temperature
6. electrolyte concentarion
7. dyeing time
8. M:L
9. Type of alkali
10. Washing off sequence
11. Quality of water and salt

The  detail of impact of above parameters,

1. Pretreatment of the substrate

The process of removing natural impurities like oils, fats, waxes, pectin, protein, amino acids and hydrophobic impurities from a yarn or fabric is called “Scouring”. Scouring is done to improve absorbency of the textile material by removing the oils and fat in the yarn or fabric by boiling.

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 any 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 brighter 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.

2. pH of the substrate

pH of the substrate prior to dyeing must be controlled and it should be either neutral or slightly acidic  because if the pH is alkaline at the beginning , the dye molecule may form a permanent bond or premature fixation leading to unlevel dyeing.

3. pH of the dye bath

it should be weakly acidic to neutral  in the dyeing bath in migration and exhaustion phase ,before addition of alkali . It must be checked and regulated because presence of bicarbonates in the water may increase the pH with increase in 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 suitable for better shade and color fastness control.

5. Dyeing temperature

The temperature and rate of rise  of the dye bath affects the affinity of the dye molecules towards fiber, rate of hydrolysis , migration and covalent bond formation. Therefore the dyeing temperature selected must be as per the dye sub class. Effect of temperature on the build up and fixation of individual dyes must be studied to form dyes groups having similar characteristics  and then these groups must be selected for making combination shades. 

f. Electrolyte concentration

The dyes must be exhausted by addition of salt or glauber salt before starting the fixation of color. For electrolyte concentration in the dye bath to be used please refer to tables provided by the dye manufacturers. The electrolyte used must be free from 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 depth of the shade. The timings must be optimized to get  maximum exhaustion as well as maximum fixation of the color in the dyebath. Based on the exhaustion and fixation curves of the individual dyestuffs , an optimum time can be selected . There is no advantage of increasing the fixation time than desired 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 ratio

The M:L of dyebath affects the dye or shade performance to a large extent , as for as possible robust dye combinations must be used , which are unaffected by the change in liquor ratio . 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 ,particularly 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 hydrolyesd and unfixed dyes which are present in the dye bath as well as on the fiber after the completion of dyeing cycle, these hydrolyesd 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 acis such as acetic acid is must be used to neutralize the dyebath. The soaping treatment can be done upto a boiling temperature to remove the unfixed and hydrolysed dye effectively. A higher soaping temperature can be selected because  unfixed dye has  practically no affinity for the fiber  and the loosely held dye rapidly diffuse out.  The soaping treatment is recommended in a neutral bath because  under alkaline soaping conditions at higher temperature the dye fiber bond may break and result into loss of color  value due to rupture of dye fiber bond. If the soaping tretment is carried out  efficiently and carefully then there is no need of cationic dyefixing treatment at the end of dyeing cycle

A typical soaping treatment recommended for Procion    XL and XL+ dyes is as shown below

Further Information

Cost reduction in reactive dyeing

A slide show on Reactive dyes click here 

Washing off process by BASF Click Here

Optimizing the process of Reactive Dyeing