USE OF SUPERCRITICAL FLUIDS IN ENHANCEMENT OF POLYMERIZATION PROCESS (review).

AbstractOur review paper is about one of the recent advancement in the polymerization process and discussing the methodology of supercritical polymerization using the fluids in their supercritical state for the process to take place. Supercritical fluids are having temperature and pressure higher than the critical temperature. Major examples of fluids used in supercritical state are CO2 and Propane. Use of supercritical fluids in polymer processing increases the rate of reaction and thus decreases the reaction time which is helpful. Supercritical fluids provides higher mass transfer coefficients as compared to liquid phase and hence they provide higher value of overall rate of reaction in polymerization process.KeywordsSupercritical polymerization, fast polymer processing.IntroductionSupercritical fluids (SCFs) have unique properties that may enhance many types of chemical processes. An additional advantage of using SCFs stems from the fact that they may replace many environmentally harmful solvents currently used in industry. In particular, SCFs represent an attractive alternative to organic solvents for use as additives in polymer processing. For example, supercritical carbon dioxide (scCO2), which is by far the most widely used SCF, is relatively cheap, nontoxic, and non-flammable and has zero ozone-depletion potential. Moreover, the fact that CO2 is a gas under ambient conditions makes its removal from the polymeric product very easy, avoiding for example, the costly processes of drying or solvent removal, which is very important in the processing of polymer based materials.But what are the properties of an SCF? A supercritical fluid is defined as a substance above its critical pressure and temperature. However, there is still no apparent distinction between a high-pressure gas and an SCF because, under all circumstances, such a fluid will occupy the full volume of its container, demonstrating the typical behavior of a gas. Nevertheless, such a fluid is usually not called a high-pressure gas but a supercritical fluid. The reason is that one cannot liquefy such a fluid under any pressure once it is heated above its critical temperature (it should be noted, how- ever, that it can still be solidified at extremely high pressures). No phase separation occurs for any substance at pressures or temperatures above its critical values. In other words, the critical point represents the highest temperature and pressure at which gas and liquid can coexist in equilibrium. However, it is very important to note that this definition is for a pure substance.Polymerization process is to react monomer molecules together to form a polymer chain or a 3-D network.

An example ofalkene polymerization, in which eachstyrene monomer's double bond reforms as a single bond plus a bond another styrene monomer. The product ispolystyrene.

Polymers can be synthesized and/or modified in a supercritical medium. Most polymers show some solubility, plasticization, or swelling in supercritical fluid media. Depending on the choice of polymer-SCF medium, the degree of solubilization, plasticization, or swelling varies.Properties of Supercritical CO2 and its effects on the polymerization processCO2 is used in majority as a supercritical fluid as its cheaper, non-poisonous and easily available and its critical properties are easily attainable with respect to others as seen from the phase diagram of CO2. There have been many application of different spectroscopic methods and other new techniques at molecular level to identify for the first time specific molecular interactions between CO2 and polymers that may be responsible for the plasticization of glassy polymers. The changes in IR spectra of CO2 incorporated into various polymers indicate a specific interaction between CO2 and polymer functional groups. Increased polymer segmental mobility has also been observed, indicative of the plasticization phenomenon.

Fig. Visualization of the relative effort required for polymerization and solvent recovery in conventional catalytic polymerization processes based on organic solvents. Thus polymerization with supercritical fluids is preferred. Supercritical fluids, mainly supercritical CO2, have been widely applied in the chemistry and processing of polymers. Elevated pressure CO2 is known to swell and plasticize glassy polymers. The increase in the polymer inter-chain distance upon plasticization by CO2 is accompanied by the enhanced mobility of polymer segments, similar to the plasticizing effect by ordinary solvents. One of the differences between common liquid plasticizers and CO2 is that CO2 is easily removable from the processed polymers, and thus may be used for solvent-free incorporation of additives. It is possible to change the degree of plasticization and swelling of such a polymer, and consequently its free- volume, merely by changing the density of the CO2.The motivation for using SCFs in polymer processing stems not just from the environmental impetus for their use as the benign (not harmful) solvents. As explained above, SCFs have a number of unique properties that could be utilized for polymer synthesis in these media.In addition, it is the molecular structure of some specific fluids, primarily supercritical CO2 that plays a major beneficial role in polymer-processing. The sorption of scCO2 into polymers results in their swelling and changes the mechanical and physical properties of the polymers. The most important effect is the reduction of the glass transition temperature (Tg) of glassy polymers subjected to scCO2, often simply called plasticization. The plasticization of polymers induced by scCO2 has an impact on many polymer-processing operations, These include viscosity reduction for polymer extrusion and blending, enhancement of the diffusion of additives through polymer matrices for impregnation and extraction, enhancement of monomer diffusion for polymer synthesis, foaming of polymers, and changes in polymer morphology due to induced crystallization.

Illustrations of enhancement in polymer processing using supercritical fluid Decomposition of Waste Plastics in Super critical Water1. Property of super critical waterWater, most important solvent in nature, has fascinating properties as a reaction solvent in its super critical condition. The super critical water is the fluid that is over the critical point of vapour-liquid coexistence state. The critical temperature and critical pressure of water are 647K and 22MPa, respectively. The density of super critical water can continuously be controlled between gas like and liquid like values by varying its pressure and temperature. The value of dielectric constant that is one of the parameter estimating the solvent polarity, increase with increasing density. The relation between dielectric constant and density is shown in figure 1. fig1. Dielectric constant of waterAt super critical condition, the values of the constant between 5 to 25 can be obtained. This corresponds to the dielectric properties of polar organic liquids under normal condition. This property partially explain its ability to dissolve nonpolar organic compounds.At standard conditions, water dissociates slightly into hydrated hydrogen and hydroxyl ions: the "ion product" (Kw) of their concentrations is about 10-14(mol/1)2. The ion product of water is strongly dependent on density and weakly dependent on temperature. This is shown in figure 2fig2. Ion product of water At super critical condition, for example at 573K and 34.5MPa, the value of ion product is 10-11. This value means hydrogen ion concentration of super critical water is almost 3*10-7and this value means that the super critical water at this condition acts the role as acid solutions catalyst.2. Hydrolysis of plastic in super critical waterBy the fascinating properties of super critical water, the substance with ether, ester and isocyanate bond which are the condensation polymerization plastic can easily be decomposed to their monomer when super critical water is used as reaction solvent. It is experienced that the PET (Polyethylene terephthalate) is decomposed to their monomer in the super critical water by the batch-continuous apparatus. Fig.3 shows the relation between the recovery yield of monomer and reaction temperature at 25MPa and 30MPa. The recovery yield of TPA (Ter-phthalic acid) reaches almost 100% over 350K of the temperature. However, yield of EG (Ethylene glycol) is about 30% at the same temperature. It seems that this due to effect of acid catalyst of recovered TPA in super critical water.Fig.3 Relation between recovery yield of TPA from PET and reaction temperature at 25, 30MPa3. Pyrolysis of plastic in super critical waterWhen the super critical water is used as reaction solvent, the temperature of water used as reaction medium is over the pyrolysis condition of plastics. The addition polymerization plastic is converted to the oil in the super critical water.

4. Application off super critical water in waste plastic treatment fieldWhen the waste material which is composed of the condensation polymerization plastic and the addition polymerization plastic, is treated in the super critical water, the former one is selectively decomposed to their monomer in short time, that is the chemicals, and at this time latter one is not decomposed. The addition polymerization polymer, however, is continuously converted to oil following to the monomerization of the condensation polymerization plastic. The technology on this decomposition of plastics in the super critical water is expected as the novel waste material treatment process.fig.4 Polymer clay nanocomposites (illustration 2)

CO2has a readily accessible critical point and is a relatively inexpensive, non-toxic, and environmentally friendly solvent. Our research has shown that soaking commercial clays in supercritical CO2, followed by a rapid depressurization can produce significant clay dispersion without any additional modification of the clays or their modifiers. Clay dispersion has been achieved with a variety of different clays,with and withoutthe presence of polymerThe extent of dispersion is characterized by a wide range of characterization tools, such as WAXD (wide-angle X-ray diffraction), SEM (scanning electron microscope), TEM (Transmission electron microscopy), rheology, tensile and permeability testing.

USE OF SUPERCRITICAL FLUIDS IN ENHANCEMENT OF POLYMERIZATION PROCESS (review).CHINMAY P TIWARI AKSHAY G CHOTHANIDEPARTMENT OF CHEMICAL ENGINEERING DEPARTMENT OF CHEMICAL ENGINEERINGL.D. COLLEGE OF ENGINEERING L.D. COLLEGE OF ENGINEERING AHMEDABAD AHMEDABAD chinmaytiwari15@gmail.com akshaychothani1995@gmail.com

The scCO2-processed samples have been benchmarked with solution blended and melt-compounded PS nanocomposites. The results suggest that the supercritical CO2-processing produces significant dispersion and improves polymer-clay interactions. The low-frequency modulus of scCO2-processed PS/clay melts are more than an order of magnitude better than those prepared by solution blending and melt compounding, for the same clay loading.

Conclusion

These theories shows that conventional processes for polymerization mainly uses liquids as solvents which has its own demerits while using supercritical fluids has its own merits. In using liquid solvents for polymerization their revival from the reaction mixture is costlier whereas it may sometime contaminate the product by its presence but by using non-toxic, non-flammable gas like carbon dioxide easy separations plus higher rate of reaction due to formation of microcellular structure takes place as illustrated in below figure.

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