Use of supercritical fluids in enhancement of polymerization process.

Use of supercritical fluids in enhancement of polymerization process. Prepared by :Chinmay Tiwari Akshay ChothaniChemical EngineeringLD College of EngineeringAhmedabad.

Table of contentsIntroduction of supercritical fluidsProperties and effects of supercritical carbon dioxide on polymerizationIllustrations of enhancement in polymer processing using supercritical fluids 1. Decomposition of waste plastics in supercritical water 2. Polymer clay nano-composites

Introduction of supercritical fluids

Supercritical 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 (toluene, methyl-chloride..etc.)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.

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 behaviour 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, however, that it can still be solidified at extremely high pressures).

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.

Properties and effects of supercritical carbon dioxide on polymerization

CO2 is used in majority as a supercritical fluid as its cheaper, non-poisonous and easily available and its critical properties (critical temperature (304.25 K) and critical pressure (72.9atm or 7.39MPa))are easily attainable with respect to others as seen from the phase diagram of CO2.

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.

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.

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.

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.

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 fluids

Decomposition of Waste Plastics in Super critical Water

Property of super critical water(variation in density linking to dielectric constant values, ion-product Kw value increase at sc state acting as acid solution catalyst)Hydrolysis of plastic in super critical water(condensation polymers having ether , ester and isocyanate bond can be decomposed using sc-water , eg. PET decompose to ter-pthalic acid and ethylene glycol ) Pyrolysis of plastic in super critical water(using sc-water thermal decomposition of plastic takes place giving oil,gas and char as a product)Application off super critical water in waste plastic treatment field

When 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.

Polymer clay nanocompositesResearch 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.

The scCO2-processed samples have been benchmarked with solution blended and melt-compounded PS(polystyrene) 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.

ConclusionThese 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.

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.

Referenceshttp://www.kannangroup.com/nanocomposites.htmlhttp://infohouse.p2ric.org/ref/26/japan/Waste-187.htmlhttp://www3.imperial.ac.uk/vibrationalspectroscopyandchemicalimaging/research/intermolecular/polymerprocessinghttps://workspace.imperial.ac.uk/.../public/reviewscf.pdfeckert.chbe.gatech.edu/pdf/polymer.pdfJournal on supercritical Carbon-Dioxide for Sustainable Polymer Processes - Maartje KemmerePolymer Processing with Supercritical Fluids-S. G. Kazarian

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