Composite 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. 

Compare the flexural stiffness to weight ratios for the following three plastic beams, (a) a solid beam of depth 12 nun, (b) a beam of foamed material 12 mm thick and (c) a composite beam consisting of an 8 mm thick foamed core sandwiched between two solid skin layers 2 mm thick. The ratio of densities of the solid and foamed material is 1.5. (hint consider unit width and unit length of beam). 

It is believed that the thin TLCP-rich skin layer or interlayer may be responsible for a pluglike flow (i.e., a continuous velocity profile), due to a composition-dependent interfacial slippage [9], and, therefore, for the improved fluidity of this binary system. 

Within the conditions, the extremely thick skin layer was produced with a lower T of 280°C combined with a lower y of 116 s . In this case, the major cross-section of these samples was filled with highly oriented TLCP fibers. This led to the highest composite modulus and strength of 4922 MPa and 112 MPa, respectively. In-... 

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. 

The main potential for expansion of UV/EB into aerospace and certain commercial applications is by developing radiation curing of polymeric fiber-reinforced composites. The initial work on composite skin repairs involve applying the UV curing technology with bisacryl phosphine oxide to ensure the cure of relatively thick layers. A total of ten layers were used at a time. The UV cured composites closely matched those produced by heating. ... 

Figure 3.35 Depending on the bore fluid and the composition of the coagulation bath, the selective skin layer can be formed on the inside, the outside or both sides of the hollow fiber membrane...

Polymeric gas separation membranes are produced normally as composites with a selective skin layer made of, for example, poly(dimethyl)siloxane, on a support structure composed of, for instance, polypropylene [146],... 

A special case arises when the "skin" (membrane) layer of a normal composite membrane element is immobilized with a catalyst and not intended for separating reaction species. Consider the example of an enzyme, invertase, for the reaction of sucrose inversion. Enzyme is immobilized within a two<layer alumina membrane element by filtering an invertase solution from the porous support side. After enzyme immobilization, the sucrose solution is pumped to the skin or the support side of the membrane element in a crossflow fashion. By the action of an applied pressure difference across the element, the sucrose solution is forced to flow through the composite porous structure. Nakajima et al. [1988] found that the permeate direction of the sucrose solution has pronounced effects on the reaction rate and the degree of conversion. Higher reaction rates and conversions occur when the sucrose solution is supplied from the skin side. The effect on the reaction rate is consistently shown in Figure 11.6 for two different membrane elements membrane A is immobilized by filtering the enzyme solution from the support layer side while membrane B from the skin layer side. 

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... 

FIGURE 4.5 SEM-picture of a typical composite membrane comprising of support structure of PP and a selective skin layer of PDMS. 

An ideal pervaporation membrane should consist of an ultra thin defect free skin layer (dense layer) supported by a porous support. The skin layer is perm-selective and hence responsible for the selectivity of the membrane. However, the porous support also plays an important role in overall performance of the membrane. The effect of the porous support, of a composite membrane, on the permeation properties of the membrane is discussed in details in the composite membranes... 

For industrial applications, where the membrane sizes are larger, reinforcement of the thin skin membrane by an appropriate support is required to maintain dimensional stability. This type of membrane consisting of a skin layer (perm-selective layer) supported by a suitable support is called a composite membrane. 

A bilayered composite skin equivalent has been developed with a viable dermis and epidermis. The epidermis is composed from cornified differentiated keratinocytes and a dermal matrix composed of a collagen lattice containing viable fibroblasts. Its cellular components assist with wound closure through stimulation of the wound bed. The outer layer of the differentiated bilayered skin equivalent, the stratum corneum, acts as a specialized vapor permeable membrane and protective outer barrier.f ... 

Figure 52. Schematic diagram of reinforced structure of foamed composites. (1) Unidirectional, (2) Continuous-strand mat, (3) Chopped-strand mat, (4) Uni-directional skin layer, (5) Three-dimensional.

Cadotte ( ) presents a comprehensive review of the development of the composite membrane with emphasis on the pros and cons of the four preparation methods mentioned above and on the polymer chemistry Involved. Cadotte points out that while each of the four methods continues to receive some attention, the Interfaclal polymerization method appears to be the most versatile. This method can be used to produce skin layers from polyamines, polylmlnes, polyurethanes, polyesters and other polymers. Elsewhere In this volume, Lee and co-workers (45) discuss the advantages and problems associated with using these composite membranes for ethanol-water separations via counter-current reverse osmosis. Also, Cabasso (44) discusses double-layer composite membranes. 

I) Skin layer of thln-fllm composite see Reference 3. 

Comparison of RO and CCRO. In virtually all RO membranes, a thin, selective skin layer is supported by a much thicker microporous sublayer. During RO operation, the composition of the permeate is determined by the selectivity of the skin layer, the feed solution composition, and the operating pressure. The concentration of the permeate is established as the feed solution flows through the skin layer, and it remains constant inside the sublayer. This concentration profile is shown in Figure 2a. 

---