Membrane preparation Loeb-Sourirajan membranes

A typical three-component phase diagram for the components used to prepare Loeb-Sourirajan membranes is shown in Figure 3.11. The comers of the... 

Fig. 11. Schematic of Loeb-Sourirajan membrane casting machine used to prepare reverse osmosis or ultrafiltration membranes. A knife and trough is used to coat the casting solution onto a moving fabric or polyester web which enters the water-filled gel tank. After the membrane has formed, it is washed...

Table 3.2 Historically important examples of conditions for preparation of solution-precipitation (Loeb-Sourirajan) membranes...

Preparation of Asymmetric Loeb-Sourirajan Membranes, Polymer Letters Edition Vol. 11, pp. 201-205 (l9T3) co-authors M. T. So,... 

Interfdci l Composite Membra.nes, A method of making asymmetric membranes involving interfacial polymerization was developed in the 1960s. This technique was used to produce reverse osmosis membranes with dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Sourirajan process (28). In the interfacial polymerization method, an aqueous solution of a reactive prepolymer, such as polyamine, is first deposited in the pores of a microporous support membrane, typically a polysulfone ultrafUtration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, for example, a diacid chloride in hexane. The amine and acid chloride then react at the interface of the two solutions to form a densely cross-linked, extremely thin membrane layer. This preparation method is shown schematically in Figure 15. The first membrane made was based on polyethylenimine cross-linked with toluene-2,4-diisocyanate (28). The process was later refined at FilmTec Corporation (29,30) and at UOP (31) in the United States, and at Nitto (32) in Japan. 

The Loeb-Sourirajan technique is now recognized as a special case of a more general class of membrane preparation process, best called the phase separation... 

The technology to fabricate ultrathin high-performance membranes into high-surface-area membrane modules has steadily improved during the modem membrane era. As a result the inflation-adjusted cost of membrane separation processes has decreased dramatically over the years. The first anisotropic membranes made by Loeb-Sourirajan processes had an effective thickness of 0.2-0.4 xm. Currently, various techniques are used to produce commercial membranes with a thickness of 0.1 i m or less. The permeability and selectivity of membrane materials have also increased two to three fold during the same period. As a result, today s membranes have 5 to 10 times the flux and better selectivity than membranes available 30 years ago. These trends are continuing. Membranes with an effective thickness of less than 0.05 xm have been made in the laboratory using advanced composite membrane preparation techniques or surface treatment methods. 

H. Strathmann, P. Scheible and R.W. Baker, A Rationale for the Preparation of Loeb-Sourirajan-type Cellulose Acetate Membranes, J. Appl. Polym. Sci. 15, 811 (1971). 

Particularly, the nonsolvent immersion, that is, the Loeb-Sourirajan preparation method is an important methodology. In this method, a polymer solution is cast into a film and the polymer precipitated by immersion into water [10,144], The nonsolvent (water) quickly precipitates the polymer on the surface of the cast film, producing an extremely thin, dense-skin layer of the membrane [10,144], The polymer under the skin layer precipitates gradually, ensuing in a more porous polymer sublayer [145], Following polymer precipitation, the membrane is usually annealed in order to improve solute rejection [10,144]. 

Hyperfiltration, particularly RO, was the first membrane process to be run on an industrial scale, as early as the 1960s [1,3]. The great breakthroughs here were the invention in the early sixties by Loeb and Sourirajan [4] of asymmetric membranes prepared via phase inversion and the development of membranes prepared via interfacial polymerization [5]. The membranes appHed are densified even more than those for UF and a Hmit is reached membranes may get so dense that the... 

Based on the pioneering work of Loeb and Sourirajan [4], membranes prepared according to the phase-inversion technique form the most important group of NF/RO-membranes, together with those prepared via interfacial polymerization... 

The membrane is the heart of any membrane-based separation processes. Initial breakthrough in membrane technology came from the phase inversion technique developed by Loeb and Sourirajan. The membrane prepared by adopting... 

The studies of membrane morphology by SEM have produced a large number of cross-sectional pictures for polymeric membranes since the onset of asymmetric cellulose acetate membranes by Loeb and Sourirajan. The contribution of those pictures to the design of novel membranes with improved performance was truly phenomenal. SEM requires cumbersome sample preparation, which may hinder true images. AFM does not need such sample preparation, and the pictures taken by AFM are considered to reflect the true nature of membrane morphology. 

Ultrafiltration. The term ultrafiltration was coined in the 1920s to describe the collodion membranes available at that time. The process was first widely used in the 1960s when Michaels and others at Amicon Corp. adopted the then recently discovered Loeb-Sourirajan asymmetric membrane preparation... 

Cellulose acetate membranes developed by Loeb and Sourirajan for the purpose of seawater desalination continue to be useful in various membrane applications, despite the development of new membrane materials and new membrane preparation techniques. Because of its historical importance, the casting method of the first successful reverse osmosis membrane is described below in detail. 

Ultrafiltration (UF) uses membranes with pore sizes ranging from 10 to 1000 A (0.1 pm). Consequently, large-pore UF membranes will have a similar pore size to smaU-pore MF membranes. The first UF membranes were prepared by Bechhold (1907) from colloidan. By the mid-1920s colloidan UF membranes were available for laboratory use. However, industrial use of UF membranes did not occur till the 1960s after the development of high-flux reverse osmosis (RO) membranes (Loeb and Sourirajan, 1963). Today, ultrafiltration is frequently used in the biotechnology industry for protein concentration and buffer exchange (diafiltration). 

A number of methods has been developed for the manufacture of asymmetric membranes. However, the asymmetric membranes are mostly produced by the method developed by Loeb and Sourirajan (1962), namely the dry and wet method. This method is comprised of the following four steps (1) preparation of the polyimide solution (called dope), (2) molding (cast or spinning) of the dope, (3) coagulation of the dope by contact with nonsolvent of the polyimide to form the asymmetric structure, and (4) drying of the coagulated membrane. 

Strathmann, H., P. Scheible, and R.W. Baker, A rationale for the preparation of Loeb-Sourirajan-type cellulose acetate membranes. Journal of Applied Polymer Science, 1971.15(4) 811-828. 

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