Thermal-phase-inversion process

Thermal-Phase-Inversion Process. Still another approach to forming porous membranes out of polymers not soluble at room temperature is to elevate the temperature until they dissolve in a selected solvent. The resulting solution is then cooled in a controlled way until the polymer precipitates around the solvent which serves as the "pore-former" at room temperature. The process is called "thermal-phase-inversion."... 

Figure 2.5 Photomicrograph of polypropylene capillary membrane (made by ENKA S thermal-phase-inversion process).

The thermal phase inversion process is also employed for ultra- and microfiltration membranes. Crystalline polymers such as polyethylene and polypropylene are generally preferred as solutions these can be prepared at temperatures above the melting point, but cooling below the melting point will yield rapid crystallization and phase separation. These membranes are often employed in microfiltration and dialysis applications. 

Sol 2 is present either when one phase separates into two phases or when two phases are prevented from recombining into a single phase. It is expedient to entitle this factor inoompatibilityt and to discuss the various phase inversion processes in terms of the reasons for incompatibility. In the sections to follow four phase inversion processes are discussed a dry process, a wet process, a thermal process and a polymer assisted phase inversion process. 

The thermal process is perhaps the most universally applicable of all the phase inversion processes because it can be utilized over the widest range of both polar and nonpolar polymers. However, its commercial use for membrane applications will probably be restricted to polyolefins, particularly polypropylene. A large number of the substances can function as latent solvents (Table X). They usually consist of one or two hydrocarbon chains terminated by a polar hydrophilic end group. Therefore, they exhibit surface activity which may explain their ability to form the emulsion-like Sol 2 micelles at elevated temperatures. One latent solvent which is worthy of special mention because of its broad applicability is N-Tallowdiethanolamlne (TDEA). 

Phase inversion refers to the controlled transformation of a cast polymeric solution from a Hquid into a soHd state. During the phase-inversion process, a thermodynamically stable polymer solution is usually subjected to controlled Hquid-H-quid derabdng. This phase separation of the cast polymer solution into a polymer-rich and a polymer-lean phase can be induced by immersion in a non-solvent bath ( immersion precipitation ), by evaporating the volatile solvent from a polymer that was dissolved in a solvent/non-solvent mixture ( controlled evaporation ), by lowering the temperature ( thermal precipitation ) or by placing the cast film in a vapor phase that consists of a non-solvent saturated with a solvent ( precipitation from vapor phase ) [1]. 

The term phase inversion refers to the process by which a polymer solution inverts into a three-dimensional network. Initially, the solvent system is the continuous phase, and after phase inversion the polymer is the continuous phase." Four phase inversion processes exist (1) the dry process, in which a volatile solvent is lost and phase inversion occurs, (2) the wet process, in which solvent is exchanged for non-solvent and precipitation occurs, (3) the thermal process, where a latent solvent (a substance which is only a solvent at elevated temperatures) is used, involving the cooling of the polymer solution which... 

Although there are many types of membranes employed in various gas separation and water purification processes, similar membrane formation processes are often employed. The phase inversion process promoted by nonsolvent addition and the thermal phase separation process based on the fundamentals discussed above are the most prevalent processes to produce membranes. In addition to References 20-22,References 25-31 " also discuss these processes, and therein applications are covered in more detail than will be attempted in this chapter. 

Recently, PVDF has become a more popular material to produce hydrophobic membranes through phase inversion processes, mainly for membrane contactor and MD applications. It is preferred to other more hydrophobic polymers, such as polypropylene and polytetrafluoroethylene, because of its excellent combination of properties and its solubility in common organic solvents. Furthermore, the excellent thermal stability of PVDF has made it interesting as a membrane material in a wide range of industrial applications. In addition, unlike other crystalline polymers, PVDF exhibits thermodynamic compatibility with other polymers, such as poly(methyl methacrylate) (PMM A), over a wide range of blend compositions, which can be useful in the fabrication of membrane with desired properties. PVDF can be further chemically modified to obtain specific functions. In addition, it can be cross-linked when subjected to electron beam radiation or gamma radiation. 

A cyclic array of orbitals is a Mobius system if it has an odd number of phase inversions. For a Mobius system, a transition state with An electrons will be aromatic and thermally allowed, while that with An+ 2 electrons will be antiaromatic and thermally forbidden. For a concerted photochemical reaction, the rules are exactly the opposite to those for the corresponding thermal process. 

Spherical cells exist in the final gel matrix. Although all phase inversion membranes possess spherical micelles in their nascent Sol 2 condition, only the thermal process retains the spherical mlcellular shape into the final open-cell gel... 

Membranes used in microfiltration, reverse osmosis, dialysis, and gas separation are usually prepared by the wet-extrusion process, since it can be used to produce almost every membrane morphology. In the process, homogeneous solutions of the polymers are made in solvent and nonsolvent mixtures, while phase inversion is achieved by any of the several processes, such as solvent evaporation, exposure to excess nonsolvent, and thermal gelation. In most formulations, polymer solutions of 15-40 wt% concentration are cast or spun and subsequently coagulated in a bath containing a nonsolvent (usually water). 

Another method to prepare membranes utilizes the thermally induced phase inversion (TIP) process. TIP refers to a process whereby the polymer is dissolved in a solvent in which the solubility of the polymer in the solvent is temperature dependent. 

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