Skin layers

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. 

Wool belongs to a family of proteins, the keratins, that also includes hair and other types of animal protective tissues such as horn, nails, feathers, and the outer skin layers. The relative importance of wool as a textile fiber has declined over the decades as synthetic fibers have increa singly been used in textile consumption. Wool is still an important fiber in the middle and upper price ranges of the textile market. It is also an extremely important export for several nations, notably AustraUa, New Zealand, South Africa, and Argentina and commands a price premium over most other fibers because of its outstanding natural properties of soft handle (the feel of the fabric), moisture absorption abiUties (and hence comfort), and superior drape (the way the fabric hangs) (see Fibers Textiles). Table 2 shows wool production and sheep numbers in the world s principal wool-producing countries. 

Skin layer Tie layer Barrier layer Tie layer... 

Example 2.8 A polypropylene sandwich moulding is 12 mm thick and consists of a foamed core sandwiched between solid skin layers 2 mm thick. A beam 12 mm wide is cut from the moulding and is subjected to a point load, IV, at mid-span when it is simply supported over a length of 200 mm. Estimate the depth of a solid beam of the same width which would have the same stiffness when loaded in the same way. Calculate also the weight saving by using the foam moulding. The density of the solid polypropylene is 909 kg/m and the density of the foamed core is 6(X) kg/m. ... 

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). 

Under the steady-state flow conditions, there is an increasing tendency of this fiberlike structure moving toward the capillary wall as shear stress, flow flux, and radial position increase. In fact, we often obtained extru-dates with a very thin TLCP-rich skin layer from the capillary test [8]. 

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. 

If TLCP is well dispersed and fibrillated, Ro/d in Eq, (4) is larger than 10 for most cases. When Rq = 1 mm and - 25%, n will be more than 1,000. These fibrils with their average diameter less than 1 /xm may just form only one interlayer near the capillary wall, or only one thin TLCP-rich skin layer, as mentioned previously. This phenomenon has been observed by us [7,8] and reported by many other researchers [9]. 

Figure 12 shows the dependence of the average aspect ratio and the TLCP volume fraction on the relative sample thickness for the four processing conditions in the core layer, transition layer and skin layer, respectively, by a morphological examination [13]. Generally, the aspect ratio increases from core to skin layer, whereas the situation is reversed for the volume fraction. An average volume fraction about 20% can be clearly seen. 

In all cases of the processing conditions, TLCP domains were well dispersed and deformed to droplets in the core layer, but there was only a narrow distribution of their aspect ratio (about Hd 6) and less orientation. In both transition and skin layers, the domains were also well dispersed, but more oriented and fibrillated in the flow direction. From this reason, we give the distribution of aspect ratio Ud) and fiber number (N) versus fiber length class in Fig. 22, only for skin and transition layers, respectively. 

From the difference of these morphological characteristics, a simplified three-layer model could be set up and the inhomogeneous structure could be classified into these three layers. Figure 23 illustrates the relative thickness of the core, transition, and skin layer for the four sample groups, depending on the processing condition. 

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-... 

Table 5 compares the tensile properties of Vectra A950 in the form of dispersed fibers and droplets in the matrix by injection molding, microfibril by extrusion and drawing [28], injection molded pure thick sample and pure thin sample, and the pure drawn strand [28]. As exhibited, our calculated fiber modulus with its average of 24 GPa is much higher than that of the thick and thin pure TLCP samples injection molded. It can be explained that in cases of pure TLCP samples the material may only be fibrillated in a very thin skin layer owing to the excellent flow behavior in comparison with that in the blends. However, this modulus value is lower than that of the extruded and drawn pure strand. This can be... 

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.

M. S. Determination of the effect of lipophilidty on the in vitro permeability and tissue reservoir charaderistics of topically applied solutes in human skin layers. J. Invest. Dermatol. 2003, 120, 759-764. 

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. 

Modified Eiectronic Structure of Pt Skin Layer and Pt-Ru Aiioy... 

It was found that the intensity of Co2ps/2 decreased significantly (by a factor of 2.5), supporting the concept of Co dissolution from the alloy and formation of the Pt skin layer on the electrode surface during electrochemical stabilization. As shown in Fig. 10.4b, a clear CL shift was stUl observed in the Pt4/7/2 spectmm for the stabilized Pt-Co, in spite of the dissolution of Co, although the CL shift after stabilization was slightly smaller (0.15 eV) than in the as-prepared alloy (0.19 eV). Thus, we... 

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