Integral-asymmetric cellulose acetate

With the development of the integral asymmetric cellulose acetate membranes for desalination of saline water a few researchers turned again to perva-... 

The membrane can be semipermeable asymmetric membrane made from cellulose acetate or composite membranes made with dense thin polymer coating on a polystyrene support (see Figure 4.18). Asymmetric cellulose acetate membrane consists of a thin rejecting skin about 0.1 -0.5 pm thick integral with a much thicker porous substrate of 50-100 pm thick. The rejecting screen is permeable to water and relatively impermeable to various dissolved impurities, that is, salt ions and other small molecules that cannot be filtered. The permeability of the membrane depends on the construction of the membrane and the solute size. The skin offers hydraulic resistance to the flow. The porous substrate gives the membrane strength but offers... 

Kulprathipanja and coworkers reported the preparation of integrally skinned siUcaUte-1/cellulose acetate flat sheet asymmetric mixed-matrix membranes via phase inversion technique in 1992 [73]. The O2/N2 separation performance of these membranes was investigated. It was demonstrated that the separation factor of... 

Figure 4.1 shows the skins of cellulose acetate reverse osmosis membranes [1] (carbon rephca of the surface). Yeh and Geil [20] reported similar but smaller structures in poly(ethylene terephthalate). Keith [21] called these structures crystalline nodules. Fritzsche et al. [22] also observed nodules in the surface of asymmetric integrally skinned gas separation membranes of polysulfone (Fig. 4.4) by using the SEM technique. They also revealed the presence of micropores on the dense surfaces. 

The first breakthrough came in 1959 when Sourirajan and Loeb discovered a method to make a very thin cellulose acetate (CA) membrane using the phase inversion method [4]. This technique produces homogenous membranes with an asymmetric (or anisotropic) structure. The membranes were subsequently found to be skinned when examined under an electron microscope by Riley in 1964 [3]. The membranes consisted of a very thin, porous salt-rejecting barrier of CA, integrally supported by a fine CA porous substrate. Pictures of asymmetric membranes are shown in Figures 1.2 and 1.3. These early Loeb-Sourirajan (L-S) membranes exhibited water fluxes that were lOtimes higher than those observed by Reid, and with comparable salt rejection [5]. The membrane flux was 8—18 1/m /h (knh) with 0.05% NaCl product water from a 5.25% NaCl feedwater... 

The majority of the commereial gas separation membranes are made by wet phase inversion method whieh results in an integrally skinned asymmetrie membrane. This method was first used by Loeb and Sourirajan to produee cellulose acetate membranes for desalination of sea water. An alternative method for making gas separation membranes uses an ultra-porous skinned asymmetric membrane over which a thin polymer film is deposited by either coating or by interfacial polymerization. This method was developed by Cadotte for the creation of in situ dense skin thin film composite membranes for water desalination. These membrane fabrication techniques were made commercially successful for gas separation membranes by a brilliant empirical discovery for in situ sealing of the tiny pinhole defects on the skin of the membrane. 

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