There are lots of techniques for textile coloration. Some of them are conventional and some of them are modern.
According to the conventional technique, it is parted by-
- Electrochemical reaction
- Electrochemical reduction by direct
- Electrochemical reduction by an indirect process
- Electrocatalytic hydrogenation.
According to modern techniques of coloration process of textile fibers are-
- Using nanoparticles
- Using ScCO2 (Supercritical carbon dioxide)
- Using plasma technology
- Using microwave process
- Using ultrasonic system
The conventional process is needed for a large volume of water. It is a big challenge for a wet process engineer to reduce the M:L ratio. Our Environment is getting polluted day by day due to dyeing factories chemicals water, and these techniques are a very lengthy process.
But modern techniques take less time and environment-friendly. To cap it all, these techniques do not require any kind of water. So, it is helpful to reduce the M:L ratio. So, in a broad sense, it is very much helpful for our dyeing sector.
Conventional and modern techniques are discussed below:
Electrochemical reaction: In the coloration of cellulose fibers, vat dying with sulfur dyes still give a relatively maximum part of the dyestuff market. The situation will remain constant also in the upcoming future.
Mainly because vat dyes yield colored fibers of excellent all-round rapidness, particularly to light, washing, and chlorine bleaching. Sulfur dyes are prerequisite for the production of cheap products having average fastness requirements. The dyes are very quick to light and washing, but not to chlorine.
Figure 1: Mechanism of reduction of oxidation of indigo dyes
Below the figure shows these dyes have to be reduced before coloration to be transferred into the water-soluble form which have to toward fibers and which after absorption into the fibers to be dyed, must be re-oxidized to the pure water-insoluble dye pigment make the fibers.
Figure 2: Mechanism of reduction of oxidation of VAT dyes.
In a conventional system, most of the dyes mechanism is used as above. Below I want to show the worldwide dyes consumption of cellulosic fibers.
Figure 3: Mechanism of reduction of oxidation of sulfur dyes.
From this pie chart, we can know that most of the dyeing industries use reactive dyes. But reactive dyes are not environment-friendly. So, we have to move for other dyes due to others issue.
Figure 4: Worldwide consumption for dyes.
Electrochemical reduction by direct process: Generally, the reduction process needs a mediator, but in electrochemical reduction by the direct process, it is not needed. Actually, it does not need any redox mediator. The leuco dye acts as an electron shuttle between the electrode and the surface of the dye pigment. It is generated first to initiate a small amount of reduction which is proceeded by itself.
Figure 5: Mechanism of the direct electrochemical reduction of indigo radical.
Below the mechanism of direct electrochemical reduction of indigo radical is drawn.
Electrochemical reduction by indirect process: This type of reduction process needs a soluble redox mediator to increase the rate of the electron transferring process. Generally, an electrochemical reduction process is an electron transfer process from cathode to the surface of the microcrystals.
Figure 6: Electrocatalytic hydrogenation of indigo dye.
Mediators are regenerated iron complexes with gluconic acid as ligands. These mediators are very expensive and not fully harmless from a toxicological point of view. After the reduction and prior to the coloration process the mediator has to be detached from the soluble leuco dye by ultrafiltration.
Electrocatalytic hydrogenation: Electrocatalytic hydrogenation process is very promising for our textile industries. It is also friendly for ecological aspects. Electrochemical hydrogenation is a process which absorbs hydrogen, produce in-situ by electrolysis of water, reacts with absorbed organic substrates (e.g.: vat dye at the electrode surface).
So, Vat dye is called eco- friendly dye. But it is consumed only 17% worldwide. Below the electrocatalytic hydrogenation process is drawn:
Using nanoparticles: Nanotechnology is essential for textile coloration. Textile fiber composites are basically bound in various types of molecules. After textile coloration, nanoparticles are vibrated and displaced from the actual position. So, dye molecules penetrate the fiber axis and change the phase of the material. Particles which are 1-100 nanometer ranges are called nanoparticles. Nanoparticles are used in two ways in textile coloration. They are:
- Nanosized pigment components in textile coloration
- The durability of nanocomposites.
Below the pictures of nanoparticles are given
Figure 7: Various types of nanoparticles.
Nanoparticles are highly emerging particles which are used for textile coloration. Actually, the nano-sized pigment is used in textile finishing. The advantages of using nanoparticles are nanoparticles can be sized in any shape and they can be dispersed well to avoid aggregation of the nanoparticles in dye baths.
Exhaust dyeing of cationized cotton with nanoscale component dispersion has been shown good result recently and dyeing gives better soft handle and more brilliant shade. So, nanoparticles are very much appropriate for textile coloration.
Using Supercritical Carbon di Oxide (ScC02): The using of ScC02 in dyeing is very much eco- friendly in the textile industry. ScC02 coloration technology has some potential to overcome many technical and environmental issues in many textile applications like Yarn preparation, coloration and finishing.
ScC02 is a unique media for either transporting chemical because super-critical fluids take gas-like viscosities and diffusivities and liquid-like densities. Because C02 is non –toxic, non-flammable, environmentally friendly and chemically inert under some conditions.
This its production is less costly. Supercritical fluids are really produced by the effects of gas and liquid changes in pressure and temperature. In the carbon phase diagram, the triple point occurs wher temperature, volume and pressure remain in the same phase.
The critical point for carbon dioxide occurs at a pressure of 73.8 bar and its temperature is 31.1-degree Celsius.
Using plasma technology: Plasma is a partially ionized gas which contains ions, electrons and neutral particles produced by the interaction of the electromagnetic field with gas under appropriate pressure.
The pretreatment and finishing of textile fabrics can be replaced against the wet chemical method. One of the most expecting and advanced polymer modification techniques is low-temperature plasma treatment, which allows the surface properties to be varied over a wide range and the area of application of polymeric materials to be considerably extended.
This surface modification increases the hydrophilicity of the treated fiber. An important characteristic of plasma treatment is that it affects only the surface of a material subjected to treatment and a very thin near-surface layer whose thickness varies from 100A * to several micrometers, according to different estimates.
The conditions, retaining the mechanical, physio chemicals and electrochemical properties of organic material. The UV photons emitted by plasma have sufficient energy to break chemical bonds (e.g.: C-C, C-H) and to create radicals which can transfer along the chain and regenerate. It is depended on the plasma conditions and on the nature of the polymer.
Using Microwave Process: Microwave promoted organic reactions as well- known as environmental methods that can enhance a great number of chemical processes.
In particular, the reaction time and energy input are supposed to be mostly reduced in the reactions that are run for a long time at high temperatures under conventional conditions. Microwave is volumetric heating fast wheras conventional is a surface heating slowly.
Figure 8: Microwave heating (volumetric) vs conventional heating (surface).
Using of the ultrasonic system: Power ultrasound increases a wide variety of chemical and physical processes, mainly on account of the incident known as cavitation in a liquid medium that is the growth and explosive collapse of microscopic bubbles. The sudden and explosive collapse of these bubbles can make hot spots.
Finally, we can say that the coloration process is essential to our textile industry. The new technology helps the industry to reduce environment pollution and this technology helps to reduce M:L ratio. It is helpful to reduce cost in the dyeing industry. So, all dyeing industry should launch the new technology of dyeing and printing also.