Abstract


Textiles have undergone chemical wet processing since time immemorial. Human ingenuity and imagination, craftsmanship and resourcefulness are evident in textile products through out the ages; we are to this day awed by beauty and sophistication of textiles sometimes found in archeological excavations.

Application of ultrasonic waves, microwave dyeing, Plasma technology, supercritical carbon dioxide, and electrochemical dyeing of textiles are some of the revolutionary ways to advance the textile wet processing. . The amount of energy spent to dry the fabric and unspent dyestuffs remaining in liquor are also huge adding to the woes of processors, making processing the weakest link among the entire textile chain. To eliminate the disadvantages, super critical fluids, CO2 is the most versatile and prominently used.


This paper gives a brief idea of working principle of it and its uses for the different classes of dyes and fabrics.


Keywords: ecofriendly, economy, dyeing, energy saving.


Introduction:


Textile wet processing consumes a large amount of energy. These processes involve the use of chemicals for assisting, accelerating or retarding their rates and must be carried out at elevated temperature to transfer mass from processing liquid medium across to the surface of textile substrate in reasonable time. The present day scenario in the textile processing calls for the conservation of energy or usage of low amount of energy. This may be achieved by the various methods such as the use of radiofrequency, Electrochemical dyeing, microwaves, infrared heating etc.


Various approaches like solvent dyeing with different dyes on the several textile substrates have been experimented. None of these methods are commercially viable due to the inherent limitations. The use of ultrasonic waves and EM radiations is also one of the sources of getting energy which can be utilized in textile wet processing. 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 adding to the woes of processors, making processing the weakest link among the entire textile chain. 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.


1. Ultrasonic assisted dyeing14?


1.1.  What are Ultrasonic radiations?


There is a compression or rarefaction during each cycle of wave. When ultrasonic waves are absorbed in liquid system, the phenomenon of cavitation takes place, which is the alternate wave formation, oscillation and collapse of tiny bubbles or cavities. During the rarefaction of the portion of the wave cycle, dissolved gas molecules act as nuclei for the formation of cavities, which may expand relatively slowly up to a diameter as much as 0.1 cm. and then quickly collapse during the compression portion of the cycle.


1.1.1. Source of ultrasonic:


  • Mechanical transducer
  • Piezoelectric transducer
  • Magnetic transducer
  • Cavitization


 

4.6. Disadvantages:


  1. High pressure and high temperature are observed during the process.
  2. The system requires a lot of money.


Conclusion:


It seems apparent from the literatures that ultrasound holds promise in dyeing of variety of substrates. The ultrasonic cavitation accelerates the rate of dyeing and increases the dye uptake on fabric. The typical dyeing process involved the use of chemicals and thermal energy, which can be reduced, by using ultrasound energy. Among the wet processes, application to dyeing seems to be most advantageous, followed by finishing and preparation processes. There may be a possibility of reducing the pollution load on effluent water.


Electrochemical dyeing, in which chemical reducing agents are replaced by electrons from the electric current and effluent contaminating substances can be dispensed with altogether.


The plasma technology is considered to be very interesting future oriented process owing to its environmental acceptability and wide range of applications. Since recently, however, the plasma Technology is being introduced in textile industry as well. The processes do not produce waste waters or chemical effluents, so the methods are economical and reduce the environmental impacts caused by the chemical textile industry.


Dyeing with super critical CO2 is still at its infancy. It has been proved time and again that its successful at laboratory scale. Large amount of research input is needed for system integration. Dyeing with this system has been found successful with synthetic as well as natural fibers. With evolution of time Supercritical CO2 dyeing would be the order of the DAY!!!


References


  1. Ian Holme, International Dyer, 188(5), 7, (2003).
  2. Verschuren J.,Kiekens P., New Cloth Market, 18(4), 35, (2004).
  3. Rouette Hans-Karl, Encyclopedia of Textile Finishing, Springer-Verlag, Berlin, vol no 2, 1613, (2002).
  4. Ramachandran T., Karthik T., and Shetty Guruprasad S., The Indian TextileJournal,114(9), 23, (2004).
  5. Ume vohrer, New Cloth Market, 15(7), 13, (2001).
  6. Zaisheng Cai, Hwang Yoon Joon, Park Yoon-Chewl, Zhang Chuyang, Mccord Marian, Qui Yiping, AATCC Review, 2(12), 18, (2002).
  7. Hildegard Sung-Spitzl, International Dyer, 188(5), 20, (2003).
  8. R. M. A. Malek and Holme Ian, 1st International Conference of Textile Research Division NRC, Cairo, Egpyt, (2004).
  9. Ferrero F.,Tonin C., Peila R., Ramello Pollone F., Coloration Technology, 120(1), 30, (2004).
  10. S. Suh, Shallotte,N .C., AATCC Review, 3(1), 41, (2003).
  11. Joanne Yip,Chan Kwang., Sin Kwang Moon., Lau Kai Shui, Coloration Technology, 118(1), 26, (2002).
  12. Vanlandeghem A., International Dyer, 188(5), 15, (2003).
  13. Saravan D, Ultrasonics assisted textile processing-an update, Colourage, (LIII) (4) (2006) 111-116
  14. Sahoo A and Gupta KK, Electrochemical Dyeing-An overview and techniques, Asian Dyer, April 2007, 65-77
  15. Joshi AS, Malik T. and Parmar S, Supercritical carbon dioxide dyeing of polyester, Asian Dyer, October 2006, 51-54.



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4.2.1. Dyeing process:


The dyeing takes place in following steps

  1. Dissolution of dye in CO2
  2. Transport to the fibers
  3. Adsorption of dye on fiber surface and finally
  4. Diffusion of dye into the fiber 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 mins 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.


4.2.2 Effect of temperature and pressure:


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 fiber 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 fiber.


4.3. Uses of Supercritical CO2:


Supercritical CO2 is used for:

  • Dry cleaning process: Earlier supercritical CO2 was tried for this 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 fibers, yarns and fabrics.
  • Another application is the sterilization and disinfection of textiles and related material in the medical field.


4.4. 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 upto 90% and energy required is about 80% less compared to conventional dyeing. Dyeing is only carried on for 2 hrs compared to 3 or 4 hrs of conventional dyeing.


4.5. Advantages:


  1. elimination of water treatment and water pollution
  2. no need of drying textiles
  3. gives good rubbing fastness
  4. dyeing occurs with high degree of levelness
  5. CO2 is non toxic obtained from natural resources and can be easily recycled in dyeing process
  6. Dyeing houses may be started on sites where there is water scarcity


 

1.1.2. Dyeing:


The use of ultrasound in the dyeing of textile can be explained as: 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 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: Expulsion ( dissolved or entrapped gases or air molecules from fiber capillaries and interstices at the cross over points of fiber in to liquid and removed cavitations.

(Ill) Diffusion: Accelerating the rate of diffusion of dye inside the fiber by piercing the insulating layer covering the fiber and accelerating the interaction between dye and fiber.


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 fiber 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. 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.


1.1.3. Diffusion mechanism:


The diffusion of dye inside the fiber is speeded up in the ultrasonic field. The speed of dye diffusion inside the fiber depends upon the size of the dye molecular and the state of the fiber i.e. the smaller the dye molecule and greater the fiber swelling the higher is the mobility of the dye molecules and the quicker they penetrate inside the fiber.


Another factor, which influences the diffusion of dye inside the fiber, is its activated state. The dye diffuses in the fiber pores, which are full of water and at the same time it is adsorbed by the adjacent macromolecules. Owing to adsorption only a small part of dye can freely move inside.


The dye molecules spent much of their time in vibrating to and fro before they are adsorbed on the surface. Because of the simultaneous adsorption and diffusion, the diffusion slows down if the rate of adsorption is slow. However, because of the intense cavitation force in the ultrasonic field the dye molecules arrive at the fiber surfaces at a much faster rate as they gain additional kinetic energy. The dye must be in the activated state to diffuse. This activated state to be brought about by ultrasonic energy, which furnishes the vibrating molecules with the critical energy they need to break their static equilibrium and thus to diffuse.


Pressure of the ultrasonic radiation on the surface of the fiber is another factor, which influences the diffusion process. There may be some loosening in the crystalline structure although most transient but of great significance in speeding up the rate of diffusion. Therefore, dyeing carried out at low temperature in the ultrasonic field showed adsorption equivalent to that in dyeing carried out without ultrasound at higher temperature.


1.1.4. 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 liters. Temperature can be varied up to 100C by thermostatic control.


 

1.1.5. Ultrasonic offers many potential advantages in textile wet processing:


  • Energy savings by dyeing at lower temperatures and reduced processing times
  • Environmental improvements by reduced consumption of auxiliary chemicals
  • Processing enhancement by allowing real-time control
  • of color shade
  • Slower overall processing costs, thereby increasing industry competitiveness.


1.2. Microwaves:


Microwaves are electromagnetic waves whose frequency ranges from 1000MHz to10,00,000 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 cms to few mm. The higher frequency edge of microwave borders on the infrared and visible light region of the spectrum.


1.2.1. 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 2450MHz. 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 fiber. 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.


2. Electrochemical dyeing14:


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. Different reducing agents use for vat and shulphur dyes are briefly reviewed with emphasis on the emerging technique of electro chemical reduction.


2.1. 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, which has chemical formula Na2S2O4. It reduces the entire vat dye at the temp range 300-600 C and above. Sodium dithionite dissociates properly and liberates nascent hydrogen.


Na2S2O4 + 4H2O 2NaHSO4 + 6H+

Na2S2O4 + NaOH 2NaSO3 + 2H+


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.


2.2. 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 the 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 have 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.


 

2.3. Reducing agent for sulphur dye:


2.3.1. Conventional reducing agent (sulphide based):


According to the estimate, 90% of sulphur dyes used in world as a whole is still reduced by means of sulphide compounds. The sulphide reducing agent can be sodium sulphide (Na2S), sodium hydro sulphide (NaHS) and sodium polysulphide (Na2SX) where x varies from one to six. The poly sulphide variety is available in aqueous media & others are in both media. Some of the major problems with sulphur dye are contamination of effluent with sulphur. The liberated Hydrogen sulphide is toxic in nature. Attempts are therefore been made replacing sulphide based reducing agents for the dyeing of sulphur dyes. For that ecofriendly reducing agents are introduced in market, such as the Glucose & Mercaptoethanol.


2.3.2 Electrochemical dyeing:


As seen earlier, the conventional reducing agents, which reduce the dyestuff, result in nonregenerable-oxidized 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.


The first attempt although not involving directly the electrochemical dyeing was made by E.H.Daruwala. He tried to reduce the quantity of sodium dithionite needed for the reduction of the vat dyes by the application of a direct voltage this reduction can be traced to the fact that sodium dithionite at the cathode is converted into a form that exhibits increased reducing power. By appropriate cathode reduction under suitable condition (cathode potential, concentration, pH) it is possible to generate a powerful reducing species from sodium dithionite redox potential higher than sodium dithionite itself. So due to this behavior decomposition of hydrosulphide takes place to produce free radical ion SO2


S2O4-2 2(SO2)


However, these products cannot be regenerated at the applied voltage at the cathode, making recycling of the bath liquor impossible. In electrochemical dyeing technique, the same concept is adopted one step ahead and makes the liquor recycling possible.


There are two methods by means of which electrochemical dyeing can be carried out, direct electrochemical dyeing and indirect electrochemical dyeing.


2.4. 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 reoxidizes 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.


 

2.5. 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 oxidized after dye reduction. The oxidized 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.


2.5.1. Difficulties to establish 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 oxidized 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.


2.6. Liquor recycling in electrochemical dyeing:


The possibility of restoring the reducing power of a used dye bath is an attractive one in these days of heightened concern over dye house effluents. Naturally if reuse of the mediator system with different dye is intended, the residual dye has to be removed from the dye liquor. This is more straightforward proposition with vat dyes. Because of the insolubility of their oxidized form in aqueous solutions, and their ability to form suspensions, which can be removed by a filtration process form the oxidized dye liquor.


In electrochemical dyeing experiment by Thomas Bechtold the dye liquor recycling loop was repeated nine times. The dyeing experiments showed good reproducibility in the color of the dyed goods, confirming that electrochemical regeneration of the reducing agent can be achieved for many cycles without a measurable loss in the electrochemical activity.


Two process-engineering concepts for continuous electrochemical dyeing, viz. the closed circuit and the mediators concentrate technique have made liquor recycling viable


2.6.1. Close circuit technique:


This technique is called as a close circuit technique because the content of the dye bath are circulated through the electrochemical call in this technique. With this technique, the mediator and the vat dyes like indigo can be recovered from wash water. The washing water is passed trough and ultra filtration unit to remove the insoluble dye. The filtrate of the ultra filtration is then subjected to nano filtration where the concentration of the mediator is increased to a final value of 0.6 mole/liter of Fe (III) complex. The Fe (III) salt concentrate is also metered into the electrochemical cell so that the Fe (II) / Fe (III) ratio in the dye bath is maintained and the prevailing solution potential is maintained. The advantage of this system is that it allows almost any desired amount of reduction equivalent to be admitted into the dye bath at constant concentration ratios.


 

2.6.2. Mediator concentrates technique:


The contents of the dye bath are not circulated through the electrochemical cell in this technique, nor does a dye solution flow through it. The technique is analogous to the metering of dithionite solution in indigo, vat and sulphur dying. As far as the dye bath potential is concerned, metering a similar to that of closed circuit technique. However, the quantity of mediator required in dye bath is correlated with the composition of dye bath liquor, as is the case with conventional reducing agent. Here, an upper limit to the volume of reducing agent that can be metered is imposed. If this limit is exceeded, the dye bath will overflow unless additional technical measures are adopted to prevent it. The great advantage of this technique is that the reduced mediator is stored in the tank. The mediator from this tank can be supplied to several installations with different colouristic settings.


2.7. Future outlook: electro chemical dyeing


Reducing agents should be dispensed with completely on ecological grounds. This is the aim of the most recent development, which is still in the laboratory stage in fact, but could become the dyeing process of the future in BASFs view.

In electrochemical dyeing the chemical reducing agent is replaced by electrons from the electric current introduced into the dye bath via. a special cathode. A distinction drawn here between direct and indirect electrolysis.


In the case of sulphur dyes direct electrolysis is successful i.e. the electrons are transferred directly to the dye, reducing it to the active dyeing species. With vat and indigo, which are present as pigment, and therefore have inadequate interaction with the electrode surface, indirect electrolysis is employed, in which a mediator, which is easily soluble and can be regenerated, transfers the electrons from the cathode to the dye molecules.


3. Plasma technology in textile processing1, 2, 3:

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


3.1. What is plasma? 4


The physical definition of plasma is an ionized gas with an essentially equal density of positive and negative charges. And today the term is recognized 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. 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 fibers placed in the vicinity of the plasma, and this can lead to a variety of fiber-surface treatments. The nature of the gas composition, the type of textile fiber, 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 fiber modification.


 

3.2. Principle of Plasma Application: 5


The plasma atmosphere consists of free electrons, radicals, ions, uv-radiations and lot of different excited particles in dependence of the used gas.

 

 

Different reactive species in plasma chamber interact with the substrate surface cleaning, modification or coating occurs 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 polymerized by a subsequent plasma treatment.


3.3. Plasma Equipments: 4


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 ionized gas containing ions, electrons, atoms and neutral species. To enable the gas to be ionized 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.


 

3.4. Plasma Application on Textile Substrate:


3.4.1. Pretreatment of Textile Substrate: 6


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 min. in the plasma chamber, however, it increased slowly after the plasma treatment time exceeded 5 min. 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% PVA on cotton.


3.4.2. Plasma Application for Dyeing of Textile Substrate:


3.4.2.1 Dyeability of Cotton Substrate: 7, 8


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% 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.


The contributory factors leading to this increase in dye uptake can be:


  • The change of the fabric surface area per unit volume due to the surface erosion.
  • The etching effect of the plasma effect on the fibred mages the fiber surface and also removes surface fiber impurities (e.g. cotton wax or any remaining warp size, etc.)
  • The chemical changes in the cotton fiber surface (leading to carbonyl and carboxyl groups in the fiber.
  • The possibility of the formation of free radicals on the cellulosic chains of cotton.
  • Thus the action of oxygen and air plasma treatments modifies the surface properties of cotton and leads to an increase in the rate and extent of uptake of direct dye.


3.4.3. Dyeability of Synthetic Fibres: 10


In the synthetic fibres, plasma causes etching of the fibre and the introduction of polar groups. In this case, in situ polymerization of acrylic acid has been applied to polyester, polyamide and polypropylene fabrics in order to evaluate the improvement in dyeability of basic dyes. This procedure could later be extended by using different monomers to improve the affinity of these fibers for other types of dyes. The surface modification induced by grafting of polyacrylic acid has been investigated by scanning electron microscopy and Fourier transform spectroscopy.


3.4.3.1. Microdenier Polyester: 11


Plasma-induced surface modification of microdenier polyester produces cationic dyeable polyester fiber.SiCl4, silicone tetrachloride (ST) and radiofrequency generated (RF) generated plasma are used to create a polysiloxane type surface in polyester and provide sites for basic dyes. The researchers believe that the possibility of using basic dyes. On polyester could lead to a continuous flow system, low energy consumption, and more environmentally friendly consumption, low temperature dyeing technology on polyester substrates.


 

3.4.3.2. Dyeability of Polyamide: 12


Polyamide (nylon6) fabrics have been treated with tetrafluoromethane low temperature plasma and then dyed with commercially available acid and dispersed dyes. The morphology of the treated surfaces was examined by scanning electron microscopy and chemical surface charges characterized by X-ray photoelectron spectroscopy. Dyeing results showed that the plasma treatment slows down the rate of exhaustion but does not reduce the amount of absorption of acid dyes. The dyeing properties of disperse dyes on plasma treated nylon fabric charged markedly when compared with untreated fabric. A slight improvement in colorfastness was seen with the treated sample. The dyeing process had only a minor effect on the water-resistant surface, indicating that a stable surface has been achieved by the plasma treatment.


3.5. Future Growth and Developments: 13


Plasma will play many important roles in the future manufacturing of non-woven and textile products. The first of these will be meeting the need to custom-design products and develop highly technical products, by which the manufacturer must distinguish himself from the competition. Another will be by providing a solution to increasing regulation in the use of process water and in energy consumption. A third in meeting the need of environment ally friendly processes, as well as for a safe operator environment. The fact that new products cab be designed, that quality can be improved and costs can be decreased, will give a further impetus to plasma growth.


4. Supercritical Carbon dioxide (CO2) - the dyeing technique of future: 15


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 adding to the woes of processors, making processing the weakest link among the entire textile chain. 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 fiber. Moreover, by reducing the pressure at the end of the process, dye and CO2 can be recycled.


4.1. Supercritical 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.


4.1.1. Why only CO2:


  1. Abundantly available
  2. Recovery and reuse is easier
  3. Easily Handel able and environment friendly
  4. Non toxic, non hazardous and low cost.
  5. No waste generation
  6. Chemically inert.


4.2. 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 fiber.