By: M. Subramanian Senthil Kannan and R. Nithyanandan


Introduction:

Increasing consideration of ecologic consequences of industrial processes as well as legislation enforcing the avoidance of environmental problems have caused a reorientation of thinking and promoted projects for replacement of conventional technologies. Quality standards to be met by industrial wastewaters will certainly be raised in the future which will in turn cause unpredictable increases in costs, in particular those incurred by having to dispose of dye house effluents. It is, therefore, an important aim of industrial fundamental research to develop new technologies, not only to optimize conventional processes but also to solve respective problems by basically novel concepts. During the last three decades, supercritical fluids, which are characterized by exceptional physical-chemical properties, have to an increasing extent been used in extraction processes. Another field, where it is used is the extraction of natural substances for the production of drugs, cosmetics, spices etc.

A German patent was granted in 1994 for a process in which a dye, free of additives is dissolved in a supercritical fluid and the substrate to be dyed is suffused with this. Research of supercritical fluids as reaction medium and as a solvent medium has seen recent resurgence, driven by needs to satisfy environmental regulations using efficient processing and separation techniques. From solid-fluid to liquid-fluid extractions, polymerization to particle formation, alkylation�s to hydrogenation to nitration, chromatographic separation to chiral separation, polymer cleaning to parts cleaning, potentially, all aspects of chemical processing will be touched by supercritical fluids in future. Carbon dioxide is the most investigated and used supercritical fluid. It is a naturally occurring fluid that is chemically inert, physiologically compatible, and relatively inexpensive and is readily available for industrial consumption.

Supercritical Fluid:

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 such high compressed gases and as such they combine valuable properties of both liquid and gas.

Thus we can say that: -

� A supercritical fluid is a substance under pressure above its critical temperature. Under these described conditions the distinction between gases and liquids does not apply and substance can only be described as fluid.

� Supercritical fluids have properties intermediate between those of gases and liquids, controlled by the pressure.

� They do not condense or evaporate to form a liquid or a gas. Fluids such as supercritical xenon, ethane and carbon dioxide offer a range of unusual chemical possibilities in both synthetic and analytical chemistry.


� Supercritical fluids have solvent power similar to a light hydrocarbon for most solutes. However, fluorinated compounds are often more soluble in carbon dioxide than in hydrocarbons; this increased solubility is important for polymerization

� Solubility increases with increasing density (i.e. with increasing pressure). Rapid expansion of supercritical solutions leads to precipitation of a finely divided solid

� The fluids are completely miscible with permanent gases (e.g. N2 or H2) and this leads too much higher concentrations of dissolved gases than can be achieved in conventional solvents. This effect has been exploited in both organ metallic reactions and hydrogenation

Supercritical fluids offer advantages in textile processing as they combine the valuable properties of both gas and liquid. The solvating power of supercritical fluid is proportional to its density, whereas its viscosity is comparable to that of a normal gas. Such a combination leads to highly remarkable penetration properties. The increased power of solvation with the increase in density is desirable in the dyeing process as it has a decisive effect on the dissolution of disperse dye in the supercritical carbon dioxide medium.

Supercritical Fluid-Carbon Dioxide:

A supercritical fluid may be characterized best by referring to a phase diagram as shown for carbon dioxide in Figure 1. A liquid can be converted to a supercritical fluid by increasing its temperature (T) (and consequently its vapor pressure) and simultaneously increasing pressure (p). A closed system thus reaches critical values where no boundary between the liquid and gaseous state can be distinguished, i.e., the supercritical state.

Further increases in pressure, for example, will greatly increase the dielectric constant of such system, thus imparting dissolving powers even to a system that under normal condition of p and T has almost none (Figure 2).

The critical values of T and p for some selected compounds and typical properties of supercritical fluids are compared to those of gases and liquids in Table 1.


Carbon dioxide is the best choice .It is non-toxic, it is used in the food and beverage industry, it is nonflammable, it is supplied in large amounts either from combustion processes or volcanic sources without the need of producing new gas and it can be recycled in a closed system.

The low viscosity of supercritical fluids and the rather high diffusion properties of the dissolved molecules are especially promising aspects for dyeing processes. A supercritical dyeing fluid should easily dissolve solid dyestuffs and should penetrate even the smallest pores without the need of vigorous convection procedures.

Reasons for the Preference of Carbon Dioxide:

Carbon dioxide is frequently used as a solvent because of its special and unique properties: -

� Virtually inexhaustible resources (atmosphere, combustion processes, natural geologic deposits).

� Since carbon dioxide is a constituent of natural metabolic processes occurring in the biosphere it is consumed by assimilation and is returned to the natural circuit by dissimilation .It is not only biodegradable as nutrient promoting the growth of plants, but is an essential element of natural processes.

� Carbon dioxide does not affect the edibility of foodstuffs and will only have toxic effects at extremely high concentrations.

� It is produced on the commercial scale and is readily available together with the necessary logistics.

� No disposal problems. It is recovered from the process in the form of an uncontaminated gas and can be reused.

� Being easy to handle and combustible, carbon dioxide does not cause problems at working places.

� The critical point of the carbon dioxide is within the range which is readily manageable by technical means( 31C and 73 bar )

� It in non-toxic, non-hazardous and has low cost.

� It is nonflammable and non-corrosive

� Processing with supercritical carbon dioxide often generates no waste effluent stream.

� Maintenance and running cost for wastewater treatment and drying process is not necessary.


Description of a Supercritical Dyeing System:

The dyestuff/supercritical carbon dioxide/fiber system will in this respect, represents a three-component/ three-phase system. The three components are the gas, the dyestuff and the fiber polymer. In their solid state, dyestuff and polymer are present in the form of three separate phases besides the supercritical mixture. The dyestuff is dissolved in the supercritical fluid, transferred to, absorbed by and diffused into the fiber. In the first approximation the system is described as the distribution equilibrium of the dyestuff between fluid and fibers. A more exact definition of the thermodynamic processes involved in this system will have to consider the solubility of carbon dioxide in the polymer and in the solid dyestuff as well as the solubility of the polymer in the fluid. For the sake of simplification, the dyestuff will be considered as pure component, whereas the solubility of carbon dioxide and polymer in the solid dyestuff can be neglected. The solubility if the polymer in the fluid is so low that it can be neglected as well. All other mixtures can, however, significantly affect the dyeing process.

Model of the Dyeing Process:

In the following we shall discuss the principle underlying the dyeing by supercritical media drawing on the example of a theoretical dyeing of polyester (PES). In the first instance, the supercritical fluid performs the two essential functions of aqueous liquor, namely the transfer of the dyestuff and of the heat to the fibers. The model is subdivided into four steps:

1. Dissolution of the dyestuff
2. Transfer to the fiber
3. Absorption of the fiber surface
4. Diffusion into the fiber.

The solubilizing power of carbon dioxide in the supercritical state corresponds to that of the weekly polar solvents. In contact to the conditions governing dyeing in an aqueous liquor, the disperse dyestuff is transferred to the fiber out of a molecularly disperse solution and not by micelles which will then allow it�s molecularly disperse liberation. In this respect, there exists a certain similarity to solvent dyeing techniques which, some times ago, were frequently discussed as an option to dye polyester fibers, which, however, did not gain acceptance for environmental and toxicological reasons Other reasons for discarding this option included high prices and unfavorable energy balances in the case of recovery of the solvents. All these shortcomings are avoided in this case of carbon dioxide. On the other hand, potential merits of solvent dyeing techniques are surpassed in many respects.

As will be noted from table 1, the density and thus the dissolvability of the supercritical fluid is more or less similar to relative properties of liquids, whereas the viscosity is similar to that of a gas. This has an impact on the dyestuff transfer.

Due to its low viscosity, the fluids will ore readily enter pores and capillaries of fibers and/or fiber bundles. The penetration, for instance of yarn packages, by the supercritical fluid will cause a substantially lower pressure drop. In a practical case, this means high degrees of solved molecules such as for dyestuff are higher by more than three powers of ten compared to those of liquids. This will allow a faster mass transport and, therefore, significantly higher dyeing rates. Due to the favorable diffusion properties of the supercritical fluid, even the times needed for the dissolution of the solid dyestuff will be cut to a negligible minimum.

The state of the dyestuff in a super critical solution can virtually be described as gaseous. This means that it will be absorbed by the fiber at a rate comparable to the high diffusion rates corresponding to that of a gas. In addition, the dissolved dyestuff will be quickly available for diffusion into the boundary layers. This results in high degrees of levelness and low convection in spite of high absorption rates. In addition, the absorption equilibrium will be achieved very quickly, which in turn will favorably influence the degree of levelness. In this connection, we would like to mention that in the case of using dyestuff free carbon dioxide and changing of processing parameters it is possible to extract dyestuff from the fiber.

A crucial difference to dyeing process using a liquid phase is to be seen in the fact that the solubility of the dyestuff in a supercritical fluid can be continuously changed across a wide range. The distribution balance dyestuff-fluid/dyestuff-polymer can in fact 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 percent is reached. In the case of using liquid media this would only be possible by evaporation of the solvent. Spectrophotometric measurements in a supercritical medium during stepwise reduction of the density have shown that short-time over saturated solutions will be formed which accelerate the absorption of the dyestuff molecules lack other condensation nuclei during the gaseous phase.

Supercritical carbon dioxide will be partly dissolved in the polymer. It has a softener-like effect which accelerates the diffusion processes by increasing the chain mobility of the polymeric molecules. This indicates possibilities of cutting dyeing times and/or an option to use lower dyeing temperatures.

As soon as the fluid expanded to the atmospheric pressure again it will completely lose its capacity to dissolve the dyestuff. Any unfixed dyestuff will drop out during the expansion phase in the form of a dry powder and can be disposed of. The textile goods leave the dyeing equipment in a dry state and do not contain any solvent because the carbon dioxide is completely eliminated.

Concepts for Dyeing Equipment Using Supercritical Fluids:

A prospective dyeing apparatus for supercritical liquors, a plant which can be variated to meet special criteria. The machine is an extraction plant modified for processing with the supercritical fluids. In contrasts to conventional extraction plants the dyestuff are applied to the substrate instead of being removed, i.e. the fluid will have to be loaded with dyestuff prior to coming in contact with the goods to be dyed. This can be done in two manners: The dyestuff is filled into the pressure vessel in defined quantities; the dyestuff is filled into an additional small autoclave in the desired (surplus) quantity regulating the carbon dioxide content via pressure, temperature and/or flow control instruments. The absorption of the dyestuff by the fibre, i.e. the diffusion into the inner parts of the fibre, has to meet high levelness standards.

The necessary convection of the liquor can be achieved by an agitator within the dyeing autoclave or by moving the substrate. Another option is to penetrate the goods, either by the circulation of the liquor or by utilizing the current produced by continuous replenishment of carbon dioxide. In the latter case, the flow of replenished carbon dioxide will have to be continuously loaded with dyestuff. Residues of dyestuff or fiber admixtures to be extracted prior to dyeing will be collected in a conventional separator. The separation of phase will in this case be initiated by expansion or by raising the temperature.

Dyeing Apparatus:

An apparatus for dyeing in supercritical carbon dioxide is consists of a temperature controller, a vessel heater which surrounds the vessel, a stainless steel dyeing vessel of 50ml capacity (with a quick release cap), a manometer, a Varex HPLC carbon dioxide pump and a cooler for cooling the head of the carbon dioxide pump. The apparatus was pressure-tested for use up to 350 bars and 100 degree Celsius. A side arm connects the top and the bottom of the cell outside the heater to allow the supercritical carbon dioxide to circulate by thermal convection.

Principle - Dyeing Procedure:

The sample to be dyed (usually 10-25 cm) is wrapped around a perforated stainless steel tube and mounted inside the autoclave (1) around the stirrer (as shown in figure). The autoclave is then closed, evacuated and cooled with ice water. Liquid carbon dioxide (8) is filled into the autoclave in condensed form, weighing the filled-in quantity. As soon as the autoclave has reached room temperature again, poly glycol, a heat carrier, is added to the tempering bath. The pressure rises to 250 bars within about 7 minutes, an isochoric process achieved by heating the glycol bath to 130 C. Following a dye time of 10 minutes the pressure within the autoclave is reduced to atmospheric temperature within about 2-3 minutes, the carbon dioxide being routed through a separating vessel in order to recuperate precipitated residual dye stuff. Dyestuff order is placed in the bottom of the vessel; the apparatus is sealed, purged with gaseous carbon dioxide, and preheated. When it reaches working temperature, carbon dioxide is isothermally compressed to the chosen working pressure under constant stirring. Pressure is maintained for a dyeing period of 0-60 minutes and after wards released.

Procedure for SC-CO2 Fabric Dyeing:

The fabric sample to be dyed (size= 10 to 25cm) is wrapped around a perforated stainless steel tube and mounted inside the auto clave around the stirrer. Dyestuff without auxiliary chemicals was placed on the bottom of the vessel and closed & purged with gaseous CO2 and preheated. On reaching working temperature CO2 was compressed to the working pressure under constant stirring. Pressure maintained during the dyeing period of 0 to 60 min and afterwards the fabric is rinsed with acetone to remove residual dyestuff. Technical parameters are given in Table 2.

Procedure for Yarn Packages:

The process developed for the yarn package dyeing as shown in table 3. Dyeing temperatures and volume flow rates are similar with conventional dyeing while actual time required is typically less.


Advantages of Dyeing in Supercritical Carbon Dioxide:

The possible advantages to be claimed of this process are:

� Contaminated waste water streams are not produced.

� Dispersants are not required to solubilise a disperse dye in water.


� Solubilities are controllable by pressure, allowing control of the dyeing intensity and colour.

� Diffusivities in the fluid are higher, making mass transfer in the fluid faster.


� Take up of carbon dioxide by the polymer fibre causes it to swell slightly giving faster diffusion within the polymer.

� Viscosities are lower making the circulation of the dye solutions easier.

� Penetration of voids between fibres is fast because of the absence of the surface tension and the miscibility of air with carbon dioxide under pressure.

� No preparation of processing water (by desalting).

� No effluents.

� Low energy consumption for heating up the liquor.

� Energy preservation because drying processes are no longer required (conventional dyeing processes consume about 3,800 kJ per Kg of water evaporated).

� No air pollution due to recycling of the carbon dioxide (the gas is not contaminated by the processes).

� Substantially shorter dyeing times.

� Environmentally acceptable formulations of dyestuff - no dispersants or adulterants are necessary.

� No chemicals such as leveling agents, pH regulations etc. have to be added.

� Non-exhausted dyestuff is recuperated in the form of a powder-no waste.

� Reductive after treatments can be dispensed with, i.e. a whole processing step consuming water and energy can be eliminated.

� No need for auxiliary agents, disposing agents, adulterants, etc.

� For polyester, no reduction clearing is needed.

� Very less dyeing time.

� Higher diffusion coefficients lead to higher extraction or reaction rates.

� Manipulation of pressure and temperature parameters results in better selectivity.

� Easy separation eliminates multiple processing or post clean-up steps.

Demerits Sc-Co2 for Commercialization:

� Dyeing of multiple packages in the same bath.

� High pressures required for dye solubility

� Impact of dyeing machine weight is related to circulation

� During polyester dyeing ,the trimer is produced .this is removed using aqueous cleaning ,waterless SC-CO2 as a problem to eliminate

� There is little data about dyestuff solubility in SC-CO2.

Thus research work is under progress to eliminate these demerits .The SC-CO2 dyeing process is also known as �rapid dyeing�.

Other Applications of CO2 in Textiles:

The supercritical system can be used in other process. The UV stabilizer or even perfumes may be transferred to fiber. It has turn out that beside polyester, it can be used for polyolefin�s, protein fiber and cellulose fibers can be dyed with dye stuffs.

The liquid carbon dioxide is non-polar and has a large quadruple moment. Considering new classes of polymeric compounds have been developed with good solubility in liquid CO2. These compounds are applied for non-aqueous sizing.

The use of CO2 has been investigated for desizing operation. Formulation based on fluorinated compounds quantitative desizing using super critical extraction. Supercritical NH3 can be used for mercerization

Future Prospects of Super Critical Carbon Dioxide System:

The investigation to study possibilities of using supercritical system for textile finishing processes have in the first instance been performed with the aim of finding an ecologically acceptable alternative to conventional high-temperature dyeing of polyester, as such media are particularly suited for dyeing with disperse dye stuffs. Moreover, it turned out that, besides polyester, number of other fiber material can be dyed with disperse dye stuffs.

Since autoclaves required for " supercritical dyeing processes ", i.e. equipment permitting operation at the require temperature and pressures with holding capacities up to one cu.m., are considered state of the art and the employed for high pressure extraction processes, many step towards an industrial-scale application in textile plant already being accomplished. The overwhelming international resonance gained by the new method has emphasized the high, existential significance of problem met with the treatment of waste waters in the textile finishing industries. Esp. smaller units allowing short setting-up and dyeing times, i.e. assuring the high degree of flexibility, were in the center of interest.

Other field for application of supercritical fluid system such as extraction processes is at present being considered, for instance preparation plants for the removal of spinning oils etc. In other words the use of carbon dioxide in textile finishing plants is by no means limited to the dyeing of synthetic fibres. It might, for instance be possible to use supercritical ammonia for mercerizing operation, or super critical carbon dioxide could be employed to replace chlorinated hydrocarbons in dry cleaning processes.

Conclusion:

Dyeing in super critical carbon dioxide has been identified as one of the best alternatives to water-based dyeing and the same has been dealt in detail in this paper. But, this favourable concept is waiting for its commercial implementation. The Successful commercialization of the above said concept will definitely improve the economics of dyeing by the way of elimination of wastewater discharges.

References:

� Textile Research-Journal 1993,63 (3) 135-142

� Textile-Praxis-International 1993,48 (1) 32-33 & XXI-XXIV

� Textile-Praxis-International 1991,46 (12) 1322-1323

� Textile Trends 1996 31-40

� JSDC 1998,114(6) 169-173

� International Dyer 1999,184(5) 27-30

� International Dyer 1999,184(9) 33

� Colourage 1999,46(2) 31-32

� Textile-Praxis-International 1992,47(8) 1052 & XXII-XXIII

� Melliand Textilberichte 1995,76(12) 1092-1095

� Melliand Textilberichte 1995,76(9) 676-682 & E175-E176

� Textile research journal 1994,64(1-6) 371

� JSDC, 1997,113(1-6) 159


About the author:

M. Subramanian Senthil Kannan is presently working as a KAM Executive in Consumer Testing Services, SGS India (P) Ltd, Bangalore. He is a Textile Graduate from Bannari Amman Institute of Technology, Tamilnadu. He has obtained his master�s degree in Textile Technology from PSG college of Technology, Coimbatore, Tamilnadu. He is a gold medalist in both of his UG and PG programmes. He has published around 60 articles in various national and international journals. He has also presented many technical papers in various national level symposia and in various national & international conferences. He has won several prizes in paper & poster presentations and quiz competitions in both national and international levels. He has got one and half years of experience in R& D in spinning and weaving and one year in Quality assurance in Spinning.

Email: senthilkannan@gmail.com


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