Polyamide asymmetric membrane, skin

Figure 4. Skin structure of a polyamide asymmetric membrane.

Almost all RO membranes are made of polymers, cellulosic acetate and matic polyamide types, and are rated at 96-99% NaCl rejection. RO membranes are generally of two types, asymmetric or skinned membranes and thin film composite membranes. The support material is commonly polysulfones, while the thin film is made from various types of polyamines and polyureas. 

Although the original membranes were asymmetric which are cast by phase inversion, most modern membranes are made in composite form. Asymmetric membranes consist of a skin layer 0.1 - 1 um thick which is permselective with pore size of 10 - 1000 A, supported by a porous layer of the same material with a thickness of 100 - 500 im. Composite membranes are made of two different materials, an ultra thin ( <0.1 pm) skin layer of polyamide, cellulose acetate, polyvinylalcohol, or other materials, supported by an ultrafiltration porous layer such as polysulfone. 

The polymer ntxlules were observed not only on the surface of reverse osmosis membranes, but also on the surface of asymmetric membranes prepared for the separation of gas mixtures by Kazama et al [86]. While preparing hollow fiber membranes from their Cardo-type polyamide polymer, they investigated the microstructure of the membrane by an electron microscope. Skin layers... 

The asymmetric polyamide membranes were prepared using a casting solution consisting of 15 g polymer, 85 g DMAC and 35 wt 2 LiNO, (bop) as the lyotropic salt. Salt concentrations in the casting solution are conveniently expressed as percent based on polymer (BOP). The doped polyamide solution was cast on a Pyrex glass plate using a 25-mil doctor knife. The film was then partially dried to obtain a gel membrane of polymer/solvent/salt. The air side (skin side) was identified and the gel film immersed in ice water. After an hour in ice water, the membrane was transferred to distilled water and stored until it was tested. 

Figure 4 is a micrograph of the skin structure of an asymmetric polyamide and Figure 5 is a micrograph of the skin structure of an asymmetric cellulose acetate membrane. 

Membrane Asymmetric homogeneous or microporous active dense 20 to 50 pm layer of cellulose acetate with total thickness 100 pm composite of a homogeneous polymer film on microporous substructure of polyamide or polysulfone, or asymmetric skin. Usually homogeneous. 

Flat-sheet asymmetric-skinned membranes made from synthetic polymers (also copolymers and blends), track-etched polymer membranes, inorganic membranes with inorganic porous supports and inorganic colloids such as Zr02 or alumina with appropriate binders, and melt-spun thermal inversion membranes (e.g., hollow-fiber membranes) are in current use. The great majority of analytically important UF membranes belong to the first type. They are usually made of polycarbonate, cellulose (esters), polyamide, polysulfone, poly(ethylene terephtha-late), etc. 

Membranes for ultrafiltration are in general similar to those for reverse osmosis and are commonly asymmetric and more porous. The membrane consists of a very thin dense skin supported by a relatively porous layer for strength. Membranes are made from aromatic polyamides, cellulose acetate, cellulose nitrate, polycarbonate, polyimides, polysulfone, etc. (M2, P6, Rl). 

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