Microfiltration membrane preparation process

Mikulasek, P., Dolecek, P., Seda, H. and CaH, J., Alumina Ceramic Microfiltration Membranes Preparation, Characterisation and Some Properties , Dev. Chem. Eng. Miner. Process, 2, 115 (1994)... 

Later, Papet [58] presented an alternative process for preparing mbular ceramic cross-fiow filtration membranes. Papet s method consists of the casting of mbular mineral microfiltration membranes with titanium dioxide suspensions. The deposited particles on the porous support were then compressed and finally, the layer was consolidated by firing. 

It will be clear to any ceramic engineer that the support structure requirements for a 20 pm thick microfiltration membrane with 200 nm pores and for a support structure for a 100 nm thick hyperfiltration membrane with 0.7 nm pores are very different. These differences relate to the final properties of the support structure needed as well as the processing route to prepare them. A discussion of these differences is included in this chapter. 

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. 

Today the majority of polymeric porous flat membranes used in microfiltration, ultrafiltration, and dialysis are prepared from a homogenous polymer solution by the wet-phase inversion method [59-66]. This method involves casting of a polymer solution onto an inert support followed by immersion of the support with the cast film into a bath filled with a non-solvent for the polymer. The contact between the solvent and the non-solvent causes the solution to be phase separated. This process involves the use of organic solvents that must be expensively removed from the membrane with posttreatments, since residual solvents can cause potential problems for use in biomedical apphcations (i.e., dialysis). Moreover, long formation times and a limited versatihty (reduced possibUity to modulate cell size and membrane stmcture) characterize this process. 

Polymeric membranes are prepared from a variety of materials using several different production techniques. Table 5 summarizes a partial list of the various polymer materials used in the manufacture of cross-flow filters for both MF and UF applications. For microfiltration applications, typically symmetric membranes are used. Examples include polyethylene, polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) membrane. These can be produced by stretching, molding and sintering finegrained and partially crystalline polymers. Polyester and polycarbonate membranes are made using irradiation and etching processes and polymers such as polypropylene, polyamide, cellulose acetate and polysulfone membranes are produced by the phase inversion process.f Jf f ... 

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). 

The majority of polymer membranes used for microfiltration and ultrafiltration of liquids are prepared by the wet phase inversion process. Such membranes exhibit a typical asymmetric structure characterized by a thin dense surface layer and a thick microporous bulk. Poly(phthalazinone ether sulfone ketone) (PPESK) copolymers, c.f. Figure 7.10, show glass transition temperatures in the range of 263-305°C. The polymers show an outstanding chemical stability. They are soluble only in 98% H2SO4. Concentrated aqueous solutions of sodium chlorate, hydrogen peroxide, acetic acid, and nitric acid show no effect. ... 

For many years, polymeric membranes have been widely utilized in practical appHca-tions without having precise information on their pore size and pore size distribution, despite the fact that most commercial membranes are prepared by the phase inversion technique, and the performance of those membranes is known to be governed by their pore characteristics in a complicated manner [1]. These pore characteristics are influenced both by the molecular characteristics of the polymer and by the preparative method [2]. Crudely, membranes applied for pressure-driven separation processes can be distinguished on the basis of pore diameter as reverse osmosis (RO, < 1 nm), dialysis (2-5 nm), ultrafiltration (UF, 2-100 nm), and microfiltration (MF, 100 nm to 2 J,m). Nanofiltration (NF) membranes are a relatively new class and have applications in a wide range of fields [3]. The pore sizes of NF lie between those of RO and UF membranes. 

Data in this report are generated from both commercial and developmental flat-sheet CA membranes. CA manbranes are prepared by dissolving commercial grades of CA polymers into a solvenl/non-solvent mixture to give a highly viscous dope solution. After microfiltration a knife blade is used to spread the dope onto a woven nylon substrate. The commercial equipment utilized allows for a 1-m width to be cast. The thin dope film is quenched into a water bath to form the microporous structure by the phase inversion process. Membrane is washed with water and post-treated to give finished product in dry state as roll stock. 

When solvents are removed solely by evaporation, the membrane formation is known as a dry phase inversion process (Resting 1985). When the phase separation and structure formation are achieved by immersion of a cast membrane in a quench medium, the process is known as a wet phase inversion process (Heffelfinger 1978). The latter process is used to prepare asymmetric membranes for either microfiltration (Roesink 1989), ultrafiltration (Michaels 1971), reverse... 

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