Asymmetric skin layer

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. 

Fig. 16.7. Scanning electron micrograph of a section of an asymmetric polyamine ultrafiltration membrane showing finely porous skin layer on more openly porous supporting matrix.

Membranes with a relatively uniform pore size distribution throughout the thickness of the membrane are referred to as symmetric or homogeneous membranes. Others may be formed with tight skin layers on the top or on both the top and bottom of the membrane surfaces. These are referred to as asymmetric or nonhomogeneous membranes. In addition, membranes can be cast on top of each other to form a composite membrane. 

Zeolite/polymer mixed-matrix membranes can be fabricated into dense film, asymmetric flat sheet, or asymmetric hollow fiber. Similar to commercial polymer membranes, mixed-matrix membranes need to have an asymmetric membrane geometry with a thin selective skin layer on a porous support layer to be commercially viable. The skin layer should be made from a zeohte/polymer mixed-matrix material to provide the membrane high selectivity, but the non-selective porous support layer can be made from the zeohte/polymer mixed-matrix material, a pure polymer membrane material, or an inorganic membrane material. 

Membranes classified under (c) in the introductory section are of considerable importance. Most natural membranes are macroscopically non-homogeneous. In artificial membranes non-homogeneity may be introduced either deliberately (laminates, asymmetric membranes) or spuriously (e.g. skin layers on films made by extrusion). Variation of S and Dx across the membrane, i.e. in the X direction, is of particular interest non-homogeneity along the plane of the membrane is important in certain special cases, e.g. charged mosaic membranes, which are not of immediate interest here. Also asymmetric membranes prepared for the sole purpose of producing an ultrathin active layer to maximize permeation flux are outside the scope of the present discussion. 

An important point to consider about hollow fiber membranes is their morphology. Hollow fiber membranes can be either symmetric or asymmetric.16 Symmetric membranes have continuous pore structure throughout. Asymmetric membranes have a dense upper layer or skin layer that is then supported with a sublayer that is significantly more porous. Figure 6.2 shows SEM images of... 

Effect of Evaporation Condition Previous studies on more traditional applications have investigated the effect of increased air velocity, that is, forced-convection conditions for a combination of dry/wet phase inversion techniques to produce defect-free, ultrahigh flux asymmetric membranes with ultrathin skin layers [115-117]. To investigate the effect of evaporation condition on the release rate of drug, tablets were dip coated with CA solution containing 10% CA, 80% acetone, and 10% water and allowed to dry by blowing air across the surface with a blower (forced convection). As a comparison, tablets coated with the same solution were air dried under natural free-convection conditions. 

Polymeric materials are still the most widely used membranes for gas separation, and for specific apphcations the separation technology is well established (see Section 4.6). Producing the membranes either as composites with a selective skin layer on flat sheets or as asymmetric hollow fibers are well-known techniques. Figure 4.5 shows an SEM picture of a typical composite polymeric membrane with a selective, thin skin layer of poly(dimethyl)siloxane (PDMS) on a support structure of polypropylene (PP). The polymeric membrane development today is clearly into more carefully tailored membranes for specific... 

The choice between the two concepts is mainly based on some parameters such as operation pressure, pressure drop, or type of membrane available. The fiber wall has a structure of the asymmetric membrane, and the active skin layer being placed to the feed side. The hollow-fiber module is featured by a very high packing density, which can reach values of 30,000 vtPlm . 

An integrally skinned asymmetric membrane with a porous skin layer (hereafter called substrate membrane) is prepared from a polymer solution by applying the dry-wet phase inversion method and dried according to the method described later, before being dipped into a bath containing a dilute solution of another polymer. When the membrane is taken out of the bath, a thin layer of coating solution is deposited on top of the substrate membrane. The solvent is then removed by evaporation, leaving a thin layer of the latter polymer on top of the substrate membrane. 

Because it is difficult to make a selective skin layer perfectly defect-free, a method was proposed by Henis and Tripodi to seal defective pores. Their method was applied to asymmetric PS membranes, which led to the production of commercial prism membranes. 

An asymmetric membrane has a very thin dense top layer (or skin) with a thickness of 0.1-0.5 pm. A porous sublayer with a thickness of approximately 50-150 pm supports the dense top layer. The thin dense skin facing the feed solution acts as the selective layer, allowing water passage but rejecting dissolved solids. The resistance to mass transfer across the membrane is also mainly determined by the thin top layer. In asymmetric membranes, the selective top layer and the porous support layer are made of the same polymer material. Asymmetric membranes can be obtained by phase inversion, a technique in which a polymer in solution is transformed in a controlled manner from a liquid into a solid form. The top skin layer and the porous support layer are formed in a single-step process. 

Most UF membranes are asymmetric, having a thin separating layer or skin layer with small pores on one side of the membrane, and a much thicker layer with larger pores below the membrane which provides structural support with minimum flow resistance. Asymmetric membranes are manufactured by wet phase inversion casting. In this process, a casting solution of a polymer in a water-miscible solvent is spread in a thin layer onto a flat surface and then immersed in water. The water causes extraction of solvent and precipitation of the polymer as a porous flat sheet. The skin layer is formed on the upper surface that was in direct contact with water, and the underlying... 

Frequently, only the properties of the skin layer of asymmetric membranes are of Importance, because the skin layer is considered to play a dominant role in determining membrane... 

Cellulosic Membranes. The first asymmetric membrane for gas separation appeared in 1970 (Table II), and It was not surprising that this membrane was a modified CA membrane of the Loeb-Sourirajan type (17). Gelled CA membranes for water desalination must be stored wet In order to maintain their permeation performance. However, In gas permeation, wet, plasticized membranes tend to lose their properties with time due to plastic creep of the soft material under pressure and due to slow drying during which the microporous sublayer may collapse and thus increase the thickness of the dense skin-layer. Gantzel and Merten (17) dried CA membranes with an acetyl-content of 39.4% by quick-freezing and vacuum sublimation at... 

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