Phase inversion polymer processing

Most commercially available RO membranes fall into one of two categories asymmetric membranes containing one polymer, or thin-fHm composite membranes consisting of two or more polymer layers. Asymmetric RO membranes have a thin ( 100 nm) permselective skin layer supported on a more porous sublayer of the same polymer. The dense skin layer determines the fluxes and selectivities of these membranes whereas the porous sublayer serves only as a mechanical support for the skin layer and has Httle effect on the membrane separation properties. Asymmetric membranes are most commonly formed by a phase inversion (polymer precipitation) process (16). In this process, a polymer solution is precipitated into a polymer-rich soHd phase that forms the membrane and a polymer-poor Hquid phase that forms the membrane pores or void spaces. 

The Polymer Assisted Phase Inversion (PAPI) Process... 

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. 

The formation of membranes from synthetic polymers is typically achieved by the phase inversion (PI) process, which starts with a stable solution of the polymer which is then subjected to controlled demixing [14]. As a result, a porous structure is obtained where the polymer-rich phase forms the matrix of the membrane. The demixing of a previously formed liquid film (either flat or hollow-fiber cf Scheme 1) can be achieved by two main processes ... 

Asymmetric membranes, which are more important commercially, come in a wide variety of types. I will discuss three types here. The first type of asymmetric membrane is that made by phase inversion. This process, originally developed by Leob and Sourirajan, involves spreading a polymer solution on a moving web. The polymer solution typically contains 20% polymer in a volatile solvent like acetone. Some of the solvent evaporates to form a thin skin of polymer. The moving web then dips into a nonsolvent like water... 

If a linear mbber is used as a feedstock for the mass process (85), the mbber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete mbber particles are formed. This is referred to as phase inversion since the continuous phase shifts from mbber to SAN. Grafting of some of the SAN onto the mbber particles occurs as in the emulsion process. Typically, the mass-produced mbber particles are larger (0.5 to 5 llm) than those of emulsion-based ABS (0.1 to 1 llm) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to faciUtate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extmders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

Phase Inversion (Solution Precipitation). Phase inversion, also known as solution precipitation or polymer precipitation, is the most important asymmetric membrane preparation method. In this process, a clear polymer solution is precipitated into two phases a soHd polymer-rich phase that forms the matrix of the membrane, and a Hquid polymer-poor phase that forms the membrane pores. If precipitation is rapid, the pore-forming Hquid droplets tend to be small and the membranes formed are markedly asymmetric. If precipitation is slow, the pore-forming Hquid droplets tend to agglomerate while the casting solution is stiU fluid, so that the final pores are relatively large and the membrane stmcture is more symmetrical. Polymer precipitation from a solution can be achieved in several ways, such as cooling, solvent evaporation, precipitation by immersion in water, or imbibition of... 

Membranes used for the pressure driven separation processes, microfiltration (MF), ultrafiltration (UF) and reverse osmosis (RO), as well as those used for dialysis, are most commonly made of polymeric materials. Initially most such membranes were cellulosic in nature. These ate now being replaced by polyamide, polysulphone, polycarbonate and several other advanced polymers. These synthetic polymers have improved chemical stability and better resistance to microbial degradation. Membranes have most commonly been produced by a form of phase inversion known as immersion precipitation.11 This process has four main steps ... 

Chemical Phase Inversion Symmetrical phase-inversion membranes (Fig. 20-67) remain the most important commercial MF membranes produced. The process produces tortuous-flow membranes. It involves preparing a concentrated solution of a polymer in a solvent. The solution is spread into a thin film, then precipitated through the slow addition of a nonsolvent, usually water, sometimes from the vapor phase. The technique is impressively versatile, capable of producing fairly uniform membranes whose pore size may be varied within broad limits. 

It has been shown (, , 2.) that a membrane casting dope is a strongly structurlzed polymer solution, and that the morphology of the membrane surface layer can be correlated to the structure of the casting solution. The latter parameter affects the nature and details of the phase inversion process occuring in the upper part of the cast solution, in an incipient skin. Thus the solution structure is one of the factors responsible for the skin properties, and consequently for the performance of the ultimately formed asymmetric membrane. 

Note 1 Phase inversion may be observed during the polymerization or melt processing of polymer blend systems. 

At this point, the phenomenon of phase inversion occurs and the rubber in monomer phase becomes dispersed as discrete particles in a matrix of the polymer in monomer phase. Usually in a mass polymerization process, the rubber will contain occlusions of polymer/monomer, which serve to swell the volume of the rubber particle. In the course of polymerization, monomer is converted to polymer, the viscosity of the mixture increases and greater power is needed to maintain the temperature and the compositional uniformity throughout the polymerized material (8). 

In an alternative approach, MIP membranes can be obtained by generating molec-ularly imprinted sites in a non-specific matrix of a synthetic or natural polymer material during polymer solidification. The recognition cavities are formed by the fixation of a polymer conformation adopted upon interaction with the template molecule. Phase inversion methods have used either the evaporation of polymer solvent (dry phase separation) or the precipitation of the pre-synthesised polymer (wet phase inversion process). The major difficulties of this method lay both in the appropriate process conditions allowing the formation of porous materials and recognition sites and in the stability of these sites after template removal due to the lack of chemical cross-linking. 

Composite MIP membranes can also be obtained by incorporation of MIP particles into the membrane polymer matrix by mixing the particles in an appropriate solvent with the membrane-forming polymer that is then solidified by the phase inversion process. Membranes thus prepared were used for separation, with targets such as tetracycline [257], theophylline [255], methylphosphonic acid [258], bisphenol [259], indole derivatives [260], propanolol [261], luteolin [262] and norfloxacin... 

The first, and currently only, successful solvent-permeable hyperfiltration membrane is the Starmem series of solvent-resistant membranes developed by W.R. Grace [40]. These are asymmetric polyimide phase-inversion membranes prepared from Matrimid (Ciba-Geigy) and related materials. The Matrimid polyimide structure is extremely rigid with a Tg of 305 °C and the polymer remains glassy and unswollen even in aggressive solvents. These membranes found their first large-scale commercial use in Mobil Oil s processes to separate lube oil from methyl ethyl ketone-toluene solvent mixtures [41-43], Scarpello et al. [44] have also achieved rejections of >99 % when using these membranes to separate dissolved phase transfer catalysts (MW 600) from tetrahydrofuran and ethyl acetate solutions. 

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