Recent developments in textile wet processing

Textile processing has benefited greatly in both environmental impact and product quality through the use of enzymes, affirms Sarita Sharma.

Wet processing of textiles uses large quantities of water, electrical and thermal energy. The present scenario calls for the conservation of energy or usage of low amount of energy. Water as solvent for chemicals is mostly used because of its abundant availability and low cost. Problems associated with usage of water are effluent generation and additional step is needed to dry the fabrics after each step. The amount of energy spent to remove the water is also huge adding to the woes of processors, making processing the weakest link among the entire textile chain. Various approaches on the several textile substrates have been experimented. None of these methods are commercially viable due to the inherent limitations. Solvent dyeing with different dyes, the use of ultrasonic waves and EM radiations are also one of the sources of getting energy which can be utilised in textile wet processing.

New technologies in textile wet processing

Conventional Textile Coloration and other wet processing with their inherent challenges are about to transform. As, after dyeing, the unspent dyestuffs remain in liquor, thus polluting the effluent. It leads to additional pollution of waste water. To eliminate the disadvantages, various new technologies have been recently developed and now ready to be implemented on a bulk scale. These are some of the revolutionary ways to advance the textile wet processing:

• Ultrasonic waves 
• Microwaves 
• Electrochemical dyeing 
• Pre-treatments using plasma technology 
• Waterless dyeing with Supercritical Carbon Dioxide (scCO2) 
• Biotechnological catalytic bleaching 
• Single stage preparatory process 
• Foam finishing 

1. Ultrasonic waves assisted textile processing

In recent decades, ultrasound has established an important place in different industrial processes and has started to revolutionise environmental protection. Ultrasonic represents a special branch of general acoustics, the science of mechanical oscillations of solids, liquids and gaseous media.

What is Ultrasonic?

Sound waves have been classified into infra-sound (up to 16 Hz), audible sound (16 Hz – 16,000 Hz) and ultrasound, which include sound waves higher than audible sound with a frequency above approximately 16 kHz up to 106 kHz. With reference to the properties of human ear, high frequency inaudible oscillations are ultrasonic or supersonic. In other words, while the normal range of human hearing is in between 16 Hz and 16 kHz, ultrasonic frequencies lie between 20 kHz and 500 MHz.

How ultrasonic radiations work?

Unlike gases and liquid, in solids both longitudinal and transverse waves are transmitted. The effects of ultrasonics actually arise from the way in which sound is propagated through the medium. In liquids, longitudinal vibrations of molecules generate compressions and rare factions, i.e. areas of high and low local pressure. The latter results in the formation of cavities, i.e., very small vapor bubbles of 500 nm in size, which can collapse and cause shock waves through out the medium. The formation of cavitations depends on the frequency and intensity of waves, temperature and vapor pressure of the liquid. Cavitation is the principal physical phenomenon behind all the effects of ultrasound in most of the treatments. Cavitations refer to the formation, growth and collapse of vapour or gas bubbles under the influence of ultrasound.

If the bubbles collapse in the vicinity of a solid surface such as a textile material, it results in the formation of a high velocity micro jet with the velocities as high as 100 m/s – 150 m/s directed towards the solid surface.

The two phenomena attributed to ultrasound are the rapid movement of liquids caused by variation of sonic pressure which subjects the solvent to compression and rarefaction and micro streaming. Simultaneous formation and collapsing of tiny air bubbles result in a large increase in pressure and temperature at microscopic level. Heat induced by the ultrasonic process is adequate for dyeing process and thus eliminates the need for external heating in many cases.

Use of ultrasonic radiation in textile processing

Ultrasonic method has been effectively utilised in various fabric preparation processes including desizing, scouring, bleaching, mercerisation and auxiliary processes like washing and laundering.

I. Desizing: Desizing of cotton and nylon fabrics under ultrasonic treatment results complete removal of oils used in the size recipe while the treatment without ultrasound shows residual oil stains. It was found that the use of degraded starch followed by ultrasonic desizing could lead to considerably energy saving as compared to conventional starch sizing and desizing. Fibre degradation is also reduced and final whiteness and wet ability of the fabric are same as those of without ultrasonic.

II. Scouring and bleaching: The scouring of wool in neutral and very light alkaline bath reduces the fibre damage 

and enhance rate of processing for peroxide bleaching of cotton fabric by using 20 KHz frequency and observed an increase in bleaching rate in required time. The degree of whiteness also increases as compared to that of conventionally bleached sample. Effect of ultrasonic in enzymatic scouring has been tested using both acidic pectinases and alkaline pectinases and found to have increased wettability of all treated samples both tests compared to the conventionally treated samples. Ultrasonic treatments help to reduce the processing and temperature required for a result comparable to the normal bleaching and subsequent dyeing processes in terms of absorbency and fastness properties.

III. Mercerisation: Ultrasound is used for mercerising 100 per cent cotton fabrics in the after treatment and speeds up the process up to 2-3 times. Ultrasonic also has been used for evaluating its impact on washing the fabrics and garments under the simulated stain conditions on 100 per cent PES and P/C (65/35) blends using the detergent of 1 g/L.

IV. Dyeing: The possibility of dyeing textile using ultrasound was started in 1941. The dyeing of cotton with direct dyes, wool with acid dyes, polyamide and acetate fibre with disperse dyes can be used. When ultrasound waves are absorbed in the liquid system the phenomenon of cavitation takes place. Cavitation can liberate entrapped gases from liquid or porous materials like textiles, dyebath, etc. The significant increases in rate of dyeing with disperse dyes on polyamide and acetate was obtained. Ultrasound is more beneficial to the application of water insoluble dyes to the hydrophobic fibres. Ultrasound irradiation also produces a greater evenness in color. The dyeing results are affected by frequency of ultrasound used. Frequency of 50 or 100 c/s produces no effect while frequency of 22 to175 Kc/s has been found to be most effective. Enzymatic treatments supplemented with ultrasonic energy resulted in shorter processing times, less consumption of expensive enzymes, less fibre damage, and better uniformity treatment to the fabric.

The influence of ultrasound on dyeing is explained to have three-way effects:

(I)Dispersion: Breaking up of micelles and high molecular weight aggregates in to uniform dispersion in the dyebath.

(ll)Degassing: Removal of dissolved or entrapped gases or air molecules from fibre capillaries and interstices at the cross over points of fibre in to liquid thereby facilitating a dye-fibre contact.

(III)Diffusion: Accelerating the rate of diffusion of dye inside the fibre by piercing the insulating layer covering the fibre and accelerating the interaction between dye and fibre. Effects 1 and II are promoted by the mechanical action of cavitation, while effect III is due to both the mechanical action and the heating of the fibre surface. In case of water soluble dyes, ultrasound constitutes mostly an effective means of mechanical agitation, whereas in case of pigments, which are not soluble in water, ultrasound provides means of pigment dispersion and penetration, which is not provided by the conventional method.

Effect of ultrasonic on fibres: Attempts have been made to analyse the effect of ultrasonic in dyeing processes on almost all types of fibres using direct, reactive, acid and disperse dyes. Ultrasonic waves accelerate the rate of diffusion of the dye inside the fibre with enhanced wetting of fibres. Acoustic irradiation of the liquor results in a higher and more uniform concentration of dyestuff on the fibre surface, making it available for ready diffusion into the fibre interior.

The dyeing results are affected by the frequency of the ultrasound used. Irradiation at very low frequencies of the order of 50 or 100 cps produces no effects. Frequencies in the range between 22 and 175 KHz have been found to be most effective, the latter frequency being preferable for silk, wool and nylon.

Equipment for ultrasound: Generator and converter

or cleaning bath are the two main components of ultrasound equipment. Generator converts 50 to 60 Hz alternate current to electrical energy of high frequency. This electrical energy is fed to the transducer where it is transformed to mechanical vibration. The transducer system vibrates longitudinally transmitting waves into liquid medium. As these waves propagate cavitation occurs. Prototype dyeing machine was designed for continuous dyeing of yam and fabric. The system mainly consists of the tank, transport system and microprocessor, which is used to monitor the process. Ultrasonic tank is of 92 x 60 cm dimensions and capacity up to 200 litres. Temperature can be varied up to 100°C by thermostatic control.

Potential advantages: The use of ultrasound in textile wet processing offers many potential advantages including energy saving by reduced processing temperature, reduced processing time, and lower consumptions of auxiliary chemicals and further processing enhancement by overall cost control. Other benefits include environmental improvements by reduced consumption of auxiliary chemicals, processing enhancement by allowing real-time control, slower overall processing costs, thereby increasing industry competitiveness.

2. Microwaves

Microwaves are electromagnetic waves whose frequency ranges from 1,000 MHz to 10,00,000 MHz. Microwaves are so called since they are defined in terms of their wavelength in the sense that micro refers to tiny. In other words the wavelengths of microwaves are short at the above range of frequency, typically from few centimeters to few millimeters. The higher frequency edge of microwave borders on the infrared and visible light region of the spectrum.

Microwave dyeing: Microwave dyeing takes into account only the dielectric and the thermal properties. The dielectric property refers to the intrinsic electrical properties that affect the dyeing by dipolar rotation of the dye and influences the microwave field upon the dipoles. The aqueous solution of dye has two components which are polar, in the high frequency microwave field oscillating at 2,450 MHz. It influences the vibrational energy in the water molecules and the dye molecules.

The heating mechanism is through ionic conduction, which is a type of resistance heating. Depending on the acceleration of the ions through the dye solution, it results in collision of dye molecules with the molecules of the fibre. The mordant helps and affects the penetration of the dye and also the depth to which the penetration takes place in the fabric. This makes microwave superior to conventional dyeing techniques.

3. Electrochemical dyeing

The vat and sulphur dyes are insoluble in water, therefore for their application, it is necessary to convert them into water-soluble form using suitable reducing agent and alkali. The conventional reducing agents, which reduce the dyestuff, result in non-regenerable oxidised byproducts that remain in the bath. The used dye bath cannot be recycled because the reducing power of these chemicals cannot be regained. The disposal of the dye bath and the washing water cause various problems due to the non ecofriendly nature of the decomposed products. Maximum attention must therefore be paid from the ecological standpoint to the necessary reducing agent for these dyes. Electrochemical dyeing is still in the laboratory stage but could become the dyeing process of the future of the vat, indigo and sulphur according to BASF, a leading dyestuff manufacturing company. Electrons from the electric current replace Electrochemical dyeing in which chemical reducing agents, and effluent contaminating substances can be dispensed with altogether.

Different reducing agents used for vat and shulphur dyes are:

Reducing agent for the vat dyes: Sodium dye thionite is the universal and mainly used reducing agent for the vat dyes. It is also known as sodium hydrosulphite. It reduces the entire vat dye at the temp range 300-600 degree Celsius and above. Sodium dithionite dissociates properly and liberates nascent hydrogen. Sodium dithionite is very unstable and get decomposed (oxidative) and thermally to several byproducts. Some are acidic in nature. The stability of the alkaline solution of sodium dithionite decreased with increased with temperature; increased surface exposed to the air and decreased agitation bath.

Vat dyeing by electrochemical method: Dyestar has patented an electrochemical dyeing process that it developed jointly with the textile machinery manufacturer Thies GmbH & Co and the Institute of Textile Chemistry and Textile Physics at University of Innsbruck in Dornbirn, Austria. According to the company, the process uses an electric current instead of chemical reducing agents, giving it a number of technical, economic and ecological benefits. Dyestar has developed a vat dye, Indanthrane blue E-BC, specifically for this electrochemical dyeing process. The dye liquor used in electrochemical dyeing with Indanthrane blue E-BC can be reused in an unlimited number of times and contamination of dye house effluent is close to zero.

There are two methods by means of which electrochemical dyeing can be carried out:

a.Direct electrochemical dyeing: In case of direct electrochemical dyeing technique, organic dyestuff has been directly reduced by contact between dye and electrode. However in practice, the dyestuff is partially reduced by using conventional reducing agent and then complete dye reduction is achieved by electrochemical process for complete reduction which facilitates the improved stability of the reduced dye. In order to start the process, an initial amount of the leuco dye has to be generated by a conventional reaction, i.e. by adding a small amount of a soluble reducing agent. Once the reaction has set in, it is not needed anymore and further process is self sustaining. The system is found successful in case of sulphur dyes. However, concentration of the dye required to get a specific shade is higher than the conventional reducing process. In such a system, a dyestuff particle must come into contact with the electrode surface in order to get reduced. However, the atmosphere oxygen, present in the dye solution, immediately reoxidises the dyestuff has no protective capacity. Also, since the dye itself must be reduced at the surface of the cathode, cathode area should be large which itself is a constraint.

B.Indirect electrochemical dyeing: Thomas Bechtold patented indirect electrochemical dye reduction method in 1993. Here, the dye is not directly reduced at the electrode. Rather, a reducing agent is added that reduces the dye in the conventional manner which in turn gets oxidised after dye reduction. The oxidised reducing agent is subsequently reduced at the cathode surface, which is then further available for dye reduction. This cycle is continuously repeated during the dyeing operation. In electrochemistry, the agent, which under goes reduction and oxidation cycles, is known as reversible redox system and is called a mediator. Thus, in the system, the dye reduction does not take place due to direct contact of dyestuff with the cathode, like in direct electrochemical reduction, but it takes place through the mediator which gets repeatedly reduced due to the contact with the cathode. Therefore this system is known as indirect electrochemical dyeing. The object of the reversible redox system primarily in the first place is to generate a continuous regenerable reduction potential in the dye liquor. Therefore addition of conventional reducing agent is not essential and therefore there is no accumulation of decomposition products of the reducing agents takes place in the indirect electrochemical dyeing.

The electrochemical dyeing appears simple because after dyeing cycle, the unexhausted dye gets precipitated by air oxidation and can be removed by filtration. After the dye removal, the color containing the mediator, ligand and alkali can be recycled for subsequent dyeing operation. This appears to be most important feature in the terms of the cost and the environment friendliness of the process.

Points to be considered in indirect electro chemical dyeing process:

• The actual reduction of the dye should be carried separately into electrochemical cell and the reduced dye is then circulated separately into a conventional dyeing unit.
• To keep the dye in reduced form it is necessary to reduce the oxidised mediator at the cathode.
• The design of the cell should be such that the cathode should have the maximum surface area available for the reduction of mediator.
• A three dimensional electrode with large surface area occupying small place in electrochemical cell should be designed.

4. Plasma technology in textile processing

Plasma has been known from the dawn of mankind from its natural appearance in lightning displays, the solar corona and the northern lights.

What is plasma?

Plasma is the fourth state of matter, after solids, liquids and gases, and this fourth state was first proposed by Sir William Crooke in 1879 as a result of his experiments in the passage of electricity through gases. The word plasma comes originally from a Greek term meaning something formed, fabricated and molded and was first used by Irving Langmuir in 1929.

The physical definition of ‘plasma’ is an ionised gas with an essentially equal density of positive and negative charges. And today the term is recognised as being generated by electrical discharges through a gas and it consists of a mixture of positive and negative ions, electrons, free radicals, ultraviolet radiation and many different electronically excited molecules.

Principle of plasma application: The plasma atmosphere consists of free electrons, radicals, ions, UV-radiations and lot of different excited particles in dependence of the used gas. Thus, gas plasma treatment differs in nature according to the specific gas or gases, e.g. air, ammonia, argon, etc. Any gas plasma contains a complex mixture of species that can interact with textile fibres placed in the vicinity of the plasma, and this can lead to a variety of fibre-surface treatments. The nature of the gas composition, the type of textile fibre, and machine parameters such as the pressure within the plasma chamber, the treatment temperature and time, and the frequency and power of the electrical supply, can be used to vary the type and degree of fibre modification.

Different reactive species in plasma chamber interact with the substrate surface cleaning, modification or coating occur dependent of the used parameter. Furthermore the plasma process can be carried out in different manners. The substrate can be treated directly in the plasma zone. The substrate can be positioned outside the plasma; this process is called remote process. The substrate can be achieved in the plasma followed by a subsequent grafting. The substrate can be treated with a polymer solution or gas which will be fixed or polymerised by a subsequent plasma treatment.

Plasma Equipment: Plasma may be generated in the laboratory using non-electrical discharges, e.g. Thermal methods, shock waves, chemical reactions of high specific energy, nuclear radiation or irradiation by high-energy photons, gamma rays or alpha particles. However, for plasma treatment of textiles only electrical-discharge techniques are used. Plasma is a partially ionised gas containing ions, electrons, atoms and neutral species. To enable the gas to be ionised in a controlled and qualitative manner, the process is carried in vacuum conditions. A vacuum vessel is first pumped down via rotary and roots blowers, sometimes in conjunction with high-vacuum pumps, to a low to medium vacuum pressure in the range of 10-2 to 10-3 mbar. The gas is then introduced into the vessel by means of mass flow controllers and valves. Although many gases can be used, commonly selected gases or mixture of gases for plasma treatment of polymers include oxygen, argon, nitrous oxide, tetrafluoromethane and air.

Plasma application on textile substrate

For the pretreatment of textile substrate: The application of sizing agent to warp yarns prior to weaving is essential for high weaving efficiency in the production of most fabrics. Starch-based products carboxymethyl cellulose (CMC) and polyvinyl alcohol (PVA) are most frequently used sizes for cotton yarns. It is very important that these sizes should be removed by wet processing prior to the dyeing and finishing of the woven fabrics. Because of the resulting desizing waste there has recently been great interest in physico-chemicals methods. The weight loss for plasma-treated fabric increased dramatically with the exposure time of less than 5 minutes in the plasma chamber, however, it increased slowly after the plasma treatment time exceeded 5 minutes. The effect of plasma treatment on the removal of PVA was studied. The effect of varying plasma treatment time on the PVA removal was apparent. Even treatment duration of 0.5 minute removed 3.48 per cent PVA on cotton.

Plasma application for dyeing of textile substrate: Dyeability of Cotton Substrate: It has been reported that plasma treatment on cotton in presence of air or argon gas increases its water absorbency. This report was concerned with the effect of air and oxygen plasma on the rate and extent of dye uptake of Chloramine Fast Red K on cotton print cloth. The effect of plasma treatment in two different gas atmospheres (air and oxygen) for different treatment times was studied by applying 2 per cent of Chloramine Fast Red K. The effect of plasma treatment in air and oxygen appears to increase both the rate of dyeing and the direct dye uptake in the absence of electrolyte in the dye bath. Oxygen treatment is more effective than air plasma treatment. This shows that the increase in the rate and extent of dye uptake for the direct dye studied depends more on the oxygen component of the air than on the nitrogen component, which supports an oxidative mechanism of attack on the cotton.

5. Supercritical Carbon dioxide (ScCO2)

Water is a valuable raw material which is not unlimitedly available. It must be protected by appropriate legal measures. Usage of water as solvent for chemicals is mostly because of its abundant availability and low cost. Problems associated with usage of water are effluent generation and additional step is needed to dry the fabrics after each step. The amount of energy spent to remove the water is also huge.The unspent dyestuffs remain in liquor, thus polluting the effluent. It leads to additional pollution of waste water. To eliminate the disadvantages it is proposed that certain gases can replace water as solvating medium. High pressure and temperature are needed to dissolve the dyes. Of all the gases being possible of converted into super critical fluids, CO2 is the most versatile and prominently used. Because of their high diffusion rates and low viscosities that allow the dye to penetrate into the fibre. Moreover, by reducing the pressure at the end of the process, dye and CO2 can be recycled.

Why only CO2?

Prominent substances exhibiting super critical phases are CO2, H2O and Propane, of which CO2 is the second most abundant and second least costly solvent. Low temperature and pressure are needed to convert carbon dioxide gas into super critical fluid. In the supercritical state CO2 exhibits very low viscosity and surface tension properties. Supercritical CO2 is one of the most popular fluids currently used in manufacturing processes.

Following are the benefits which make CO2 most suitable for this purpose.

1.Abundantly available 

2.Recovery and reuse is easier 

3.Easily Handelable and environment friendly 

4.Non toxic, non hazardous and low cost 

5.No waste generation 

6.Chemically inert

Supercritical dye system: It represents the presence of three components: the textile substrate, dye stuff and the super critical fluid. The dyestuff is dissolved in the supercritical fluid, transferred to, absorbed by and diffused into the fibre.

Dyeing process: Any gas above its critical temperature retains the free mobility of the gaseous state but with increasing pressure its density will increase towards that of a liquid. Supercritical fluids are highly compressed gases and combine valuable properties of both liquid and gas. Supercritical fluids have solvent power similar to a light hydrocarbon for most solutes. Solubility increases with increasing density (i.e., with increasing pressure). However, fluorinated compounds are often more soluble in CO2 than in hydrocarbons, which increased solubility, is important for polymerisation. A liquid can be converted to a supercritical fluid by increasing the temperature and consequently its vapor pressure and simultaneously with increasing pressure. A closed system thus reaches the supercritical state, where no boundary between the liquid and gaseous state can be distinguished.

The dyeing takes place in following steps

 A.Dissolution of dye in CO2 

 B.Transport to the fibres 

 C.Adsorption of dye on fibre surface and finally 

 D.Diffusion of dye into the fibre takes place

The sample to be dyed is wrapped around a perforated stainless steel tube and mounted inside the autoclave around the stirrer. Dyestuff powder is placed at the bottom of the vessel and the apparatus is sealed, purged with gaseous CO2 and preheated. When it reaches the working temperature, CO2 is isothermally compressed to the chosen working pressure under constant stirring. Pressure is maintained for a dyeing period of 60 minutes and afterwards released. The CO2 and excess dyes are separated and recycled. After this dyeing procedure, the dry sample is removed and rinsed with acetone if necessary to remove the adhering residual dye.

Effect of temperature and pressure on supercritical dyeing

The influence of temperature on the dyeing is mainly due to the increase in the diffusion rate of dyes in the polymer and thus affects the dyeing time. Pressure regulates the solubility of the dye stuff. The diffusion coefficients of the dye dissolved in the supercritical medium are higher than in water, leading to generally very short dyeing time. At low temperature, the solubility of the dye stuff in CO2 is high and with low pressure and high temperature the dye content is small but its penetration into the fibre is facilitated. Since dyeing virtually takes place from gaseous phase, whereby the dyestuff is homogenously distributed, a high degree of levelness is achieved. For some fabrics extensive extraction of spinning oils should be avoided due to undesirable hardening of the handle of the fabric. The aim of extraction II with cold CO2 at the end of dyeing process is to remove the unfixed dye and simultaneously decrease the temperature as fast as possible below the glass transition temperature to avoid the extraction of fixed dye from the fibre. 

Any increase in pressure subsequently results, increase in dielectric constant and the dissolving power to a greater extent. Carbon dioxide is frequently used as a solvent because of some inherent advantages associated with the system like, non-toxic, non-corrosive and non-hazardous nature; CO2 is produced commercially and can be transported easily. The critical points of the CO2 can be achieved easily compared to other gases. The dissolved dyestuff available for diffusion into the boundary layers in the supercritical fluid is absorbed and diffuses into the fibres. The state of the dyestuff in a super critical solution can virtually be described as gaseous. Supercritical CO2 has almost a plasticising effect which accelerates the diffusion processes by increasing the chain mobility of the polymeric molecules. This means that it will be absorbed by the fibre at a rate comparable to the high diffusion rates corresponding to that of a gas. The distribution dyestuff-fluid can be continuously shifted in favor of the polymer until after expansion of the gas to standard pressure the solubility in the fluid will be equal to zero, where a theoretical exhaustion level of 100 per cent is achieved.

Uses of Supercritical CO2

Earlier supercritical CO2 was tried for dry cleaning process but due to the damage to the buttons liquid CO2 was preferred.It is used as a medium for extracting materials like natural wax, paraffin wax, knitting oil from fibres, yarns and fabrics. Another application is the sterilisation and disinfection of textiles and related material in the medical field.

Comparison with conventional dyeing process

In conventional method of dyeing, water, dyes, and other auxiliaries are used to enhance the efficiency of dyeing process. The cost of waste water treatment and of arranging water of acceptable quality is becoming serious concerns. Either the water available is too hard or not available in sufficient amount or therefore dyeing plants cannot be set up at some places. Compared to this, use of supercritical CO2 completely avoids the use of water and other auxiliaries, thus creating no effluent. Drying is also not required as CO2 is released in gaseous state. CO2 can also be recycled up to 90 per cent and energy required is about 80 per cent less compared to conventional dyeing. D