Skin layer, integrally-skinned membranes

Carruthers, S. B., Ramos, G. L., and Kotos, W. J. (2003). Morphology of integral-skin layers in hollow-fiber gas-separation membranes. J. Appl. Polym. Sci. 90, 399. 

One setback for the production of thin-film composite membranes and integrally skinned asymmetric membranes with separating layer thickness of less than 0.2 pm is the defects. A thin coating of a highly permeable polymer can help eliminate the defects. Surface coatings are also applicable in improving the fouling resistance of membranes for UF or NF applications (44). 

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. 

Immediately below the skin layer of integrally-skinned membranes is found a substrate (22) or transition (23) layer with a density between that of the skin and that of the porous... 

The conclusion which can be drawn from the above studies is that the skin layer in integrally-skinned reverse osmosis membranes consists of a single layer of consolidated Sol 2 type micelles. 

The permeability constant of the composite membrane is therefore represented by the harmonic average of the permeability constants of the individual layers, the respective weights being x /Ji., the ratio of layer thickness to the total. Although composite membranes Include layers of dense films or even liquid layers in series with films, in this discussion the term is being limited to those series in which at least one of the members is a phase inversion membrane of either the integrally-skinned or skinless variety. 

In the case of a composite membrane consisting of a skinless porous substrate and a dense film, permeability and permselectivity may be determined solely by the resistance of the denser film. Different membrane polymers may therefore be employed for the thin barrier layer and the thick support structure. This permits a combination of properties which are not available in a single material. Such membranes were initially developed for desalination by reverse osmosis where they are known as thin- or ultrathin-film composites or nonlntegrally-skinned membranes. A second type of composite membrane is utilized for gas separations. It is a composite consisting of an integrally-skinned or asymmetric membrane coated by a second, more permeable skin which is used to fill skin defects. The inventors of the latter have entitled their device a resfstanee model membrane, but the present author prefers the term coated integrally-skinned composites. 

Further approaches to meet the requirement of high selectivity may Include the blending of glassy and rubbery polymers, the chemical alteration of the dense skin-layer of Integral-asymmetric membranes and morphological variations of dense polymer films by proper post-treatment—as exemplified In this paper for CA blend membranes. 

Integrally skinned asymmetric asymmetric membranes used for gas and liquid separations consist of a thin skin layer supported by a porous substructure. The skin layer determines the permeability and selectivity of the membrane, whereas the porous substructure functions primarily as a physical support for the skin. Both layers are composed of the same material and are integrally bonded. The skin layer usually has a thickness on the order of several hundred to several thousand angstroms. 

The membrane can be a solid, a liquid, or a gel, and the bulk phases can be liquid, gas, or vapor. Membranes can be classified according to their structures. Homogeneous or symmetric membranes each have a structure that is the same across the thickness of the membrane. These membranes can be porous or have a rather dense uniform structure. Heterogeneous or asymmetric membranes can be categorized into three basic structures (1) integrally skinned asymmetric membrane with a porous skin layer, (2) integrally skinned asymmetric membrane with a dense skin layer, and (3) thin film composite membranes [13]. Porous asymmetric membranes are made by the phase inversion process [14,15] and are applied in dialysis, ultrafiltration, and microfiltration, whereas integrally skinned asymmetric membranes with a dense skin layer are applied in reverse osmosis and gas separation applications. 

Nanofiltration membranes consist of an active layer and a support, which determine the separation properties and mechanical strength, respectively. The active layer may be integrally connected to the support structure, such as membranes prepared via an immersion precipitation process (Bowen et al, 2001 He et al, 2002). This type of membrane has distinct pores in the nanometer range at the skin layer. The active layer can also be an extra coating layer on to a tailor-made support structure via interfacial polymerization or dip-coating. 

The more important factors from an industrial point of view are a high flux or productivity and a high selectivity or separation eflectiveness. It is here that asymmetric membranes find more application, due to their high flux. When the same material forms two layers differing in their structure, with a thin active dense skin layer associated with another layer that acts as a mechanical support and has no significant resistance to mass flux, the resulting membranes are called integral. 

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