Thermal-phase-inversion

Thermal Phase Inversion Thermal phase inversion is a technique wFich rnav be used to produce large quantities of MF membrane econornicallv, A solution of polvrner in poor solvent is prepared at an elevated temperature. After being formed into its final shape, a sudden drop in solution temperature causes the polvrner to precipitate, The solvent is then w ashed out. Membranes rnavbe spun at high rates using this technique. 

Lehnert S, Tarabishi H, Leuenberger H. Investigation of thermal phase inversion in emulsions. Colloids Surfaces A Physicochem Eng Aspects 1994 91 227-235. 

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).

Elat sheet membranes with isotropic pore structure in the micron range can be prepared by subjecting the blend to an abrupt change in temperature above the LCST, causing precipitation by addition of antisolvent for the primary polymer, PES. When the quench temperature was below LCST, the pore structure formed was found to be anisotropic. This technique taps into thermal-phase inversion phenomena. 

The nonionic class of alkyl polyglycosides differs from fatty-alcohol ethoxy-lates by its characteristic structure which considerably affects the association of molecules in solution, the phase behavior, and the interfacial activity. The hydrophilic head of the glucose ring, associated with the surrounding water molecules by hydrogen bonds, is rather voluminous as compared to the alkyl chain. However, the hydration is low compared to fatty alcohol ethoxylates. Therefore, basic phenomena like cloud point, thermal phase inversion, or gel formation in medium concentrations of pure solutions cannot be observed in case of the alkyl polyglycosides. 

A.2A Polyolephines Semicrystalline low-density polypropylene (PP) and polyethylene (PE) are used to produce microporous hoUow-fiber membranes by thermal phase inversion in the presence of plasticizers and solvents. Commercial PP and PE membranes feature a symmetric wall with pores whose maximal size is a fraction of a micron... 

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. 

Styrene monomer and a styrene/butadiene copolymer are fed to the first reaction zone. The polymerization is initiated either thermally or chemically. Many chemical initiators are available such as ferf-butyl peroxybenzoate and ferf-butyl peracetate. Conditions are established to prevent a phase inversion or the formation of discrete rubber particles in the first reaction zone. The conversion in the first reaction zone should be 5-12%. An important function of the first reaction zone is to provide an opportunity for grafting of the styrene monomer to the elastomer (8). 

The major trend observed is a modest increase in KIc or GIc by the introduction of initially miscible thermoplastics. This improvement is obtained without any loss in stiffness and thermal properties. Some very high improvements in KIc, claimed by some authors, are due to phase inversion, leading to a thermoplastic matrix with thermoset particles. 

According to Zimmermann [101] and Dewar [102], the allowedness of a concerted pericyclic reaction can be predicted in the following way A cyclic array of orbitals belongs to the Hiickel system if it has zero or an even-number phase inversions. For such a system, a transition state with An+ 2 electrons will be thermally allowed due to aromaticity, while the transition state with An electrons will be thermally forbidden due to antiaromaticity. 

Asymmetric membranes are usually produced by phase inversion techniques. In these techniques, an initially homogeneous polymer solution becomes thermodynamically unstable due to different external effects and the phase separates into polymer-lean and polymer-rich phases. The polymer-rich phase forms the matrix of the membrane, while the polymer-lean phase, rich in solvents and nonsolvents, fills the pores. Four main techniques exist to induce phase inversion and thus to prepare asymmetric porous membranes [85] (a) thermally induced phase separation (TIPS), (b) immersion precipitation (wet casting), (c) vapor-induced phase separation (VIPS), and (d) dry (air) casting. 

A similar thermally-induced inversion of the cholesteric sense was observed for the PBLG liquid crystal in benzyl alcohol. In this solution, a gel-like opaque phase coexists with the cholesteric phase at lower temperatures. The opaque phase disappears around 70 °C, where endothermic peaks are observed in the differential scanning calorimetry curve. The value of S below 70 °C remains constant, and then changes with temperature above 70 °C. The compensation occurs at about 103 °C, and the transition from biphasic phase to the isotropic phase is observed above 150 °C in this case. The results are summarized in Fig. 12, where the reciprocal of the half-pitch is plotted against temperature. The sign of 1/S is taken as positive when the cholesteric sense is the right-handed. 

There are a number of different techniques belonging to the category of phase inversion solvent evaporation, precipitation by controlled evaporation, precipitation from the vapor phase, thermal precipitation, and immersion precipitation (13,34—36). The most commercially available membranes are prepared by the last method. 

Development of W/O/W Emulsion during Phase Inversion. Sherman et al. (2lQ and Dokic et al.(25) emphasized that the development of a W/O/W type dispersion precedes the thermal induced phase inversion of 0/W emulsions. This suggest that the state of multiple structure in emulsions may be generalized as one of the mesophase between 0/W and W/0 emulsions, and also that there is a possibility of more simplifying the method for preparing W/O/W emulsions. 

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). 

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... 

The literature describes numerous manufacturing methods for synthetic membranes. A recent review by Pusch and Walch (1) considers membranes from a number of techniques for manufacturing membranes and discusses applications ranging from microfiltration to desalination to gas separation. In this paper, a thermal phase-separation technique of preparing membranes Is presented. The method Is a development of an Invention described In US Patent 4,247,498 by Anthony J. Castro (,2). This technique Is similar In many respects to the classical phase-inversion methods however, the additional consideration of thermal solubility characteristics of the poly-mer/solvent pair offers new possibilities to membrane production. 

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