Casting pore

Keywords Porous alumina Mechanical properties freeze casting pore biocompatibility... 

Local stress concentrations are often caused by notches (see chapter 4). Notches may be part of the design (e. g., at bearings or at undercuts), may be caused during manufacture (e.g., tool marks caused by metal cutting), or may be due to imperfections in the material (e.g., casting pores, brittle precipitates). 

In section 5.2.3, we saw that microscopic defects or cracks are usually irrelevant under static loads because they are smaller than the critical crack length from equation (5.27). If loads are cyclic, much smaller defects, like casting pores or inclusions, can initiate fatigue cracks. The fatigue strength of a material is thus much more sensitive to the manufacturing process and material defects than the static strength. 

Phase Inversion (Solution Precipitation). Phase inversion, also known as solution precipitation or polymer precipitation, is the most important asymmetric membrane preparation method. In this process, a clear polymer solution is precipitated into two phases a soHd polymer-rich phase that forms the matrix of the membrane, and a Hquid polymer-poor phase that forms the membrane pores. If precipitation is rapid, the pore-forming Hquid droplets tend to be small and the membranes formed are markedly asymmetric. If precipitation is slow, the pore-forming Hquid droplets tend to agglomerate while the casting solution is stiU fluid, so that the final pores are relatively large and the membrane stmcture is more symmetrical. Polymer precipitation from a solution can be achieved in several ways, such as cooling, solvent evaporation, precipitation by immersion in water, or imbibition of... 

Most solution-cast composite membranes are prepared by a technique pioneered at UOP (35). In this technique, a polymer solution is cast directly onto the microporous support film. The support film must be clean, defect-free, and very finely microporous, to prevent penetration of the coating solution into the pores. If these conditions are met, the support can be coated with a Hquid layer 50—100 p.m thick, which after evaporation leaves a thin permselective film, 0.5—2 pm thick. This technique was used to form the Monsanto Prism gas separation membranes (6) and at Membrane Technology and Research to form pervaporation and organic vapor—air separation membranes (36,37) (Fig. 16). 

Fused-cast refractory is very dense but may contain a system of closed pores and large, highly oriented grains may exist in a particular casting. The size and distribution of the pore and grain phases must be controlled. 

Flow coating Suitable for use on most articles. Gives good penetration into pores of castings Similar to dipping, but the defects not so marked. Tendency for greater solvent loss. 

Lower-density E-plastomers have found alternate use in cast film processes to make elastic film laminates with good breathability which contain laminates of liquid impermeable extensible polymeric films with extensible-thermoplastic-polymer-fiber nonwovens and nonwoven webs of polyethylene-elastomer fibers as the intermediate layers. The development relates to a breathable film including an E-plastomer and filler that contributes to pore formation after fabrication and distension of the film. The method and extent of distension is designed to produce a breathable film by stretching the film to form micropores by separation of the film of the E-plastomer from the particulate solids. This film is useful for manufacture of absorbent personal-care articles, such as disposable diapers and sanitary napkins and medical garments. In detail, these constructions comprise a liquid impermeable extensible film comprising polyolefins. The outer layer contains extensible-thermoplastic-polymer-fiber nonwovens, and an elastic intermediate layer contains nonwoven webs of fiber E-plastomers. The intermediate layer is bonded to the film layer and the outer... 

Porous membranes have been prepared by leaching an additive from films and tubes of PCL (64,72). The procedure involves extrusion or casting blends of PCL and Pluronic F68, the latter being an FDA-approved oxyethylene-co-oxypropylene triblock copolymer. Treatment of the phase-separated blend with aqueous acetone or aqueous alcohols causes both swelling of the polymer and extraction of the Pluronic F68. The induced pore size and void volume may be controlled by the time, temperature, and solvent composition. 

Stellwagen, NC, Apparent Pore Size of Polyacrylamide Gels Comparison of Gels Cast and Run in Tris-acetate-EDTA and Tris-borate-EDTA Buffers, Electrophoresis 19, 1542, 1998. Stellwagen, NC Gelfi, C Righetti, PG, The Free Solution Mobility of DNA, Biopolymers 42, 687, 1997. 

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. 

Procedure. Core floods were carried out in horizontally mounted Berea sandstone cores of length 61 cm and diameter 5 cm. Porosity varied from 18 to 25% and brine permeability from 100 to 800 Jim2. The cores were coated with a thin layer of epoxy and cast in stainless steel core holders using molten Cerrobend alloy (melting point 70°C). The ends of the cores were machined flush with the core holder and flanges were bolted on. Pore volume was determined by vacuum followed by imbibition of brine. Absolute permeability and porosity were determined. The cores were initially saturated with brine (2% NaCl). An oil flood was then started at a rate of lOm/day until an irreducible water saturation (26-38%) was established. 

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