Float casting

The first composite reverse osmosis membrane reported in the technical literature was developed by Peter Francis of North Star Research Institute in 1964 (4). This membrane was formed by float-casting an ultrathin film of cellulose acetate (CA) upon a water surface, removing the membrane from the water surface by lamination onto a pre-formed microporous support film and drying to bond the membrane to the support. This float-casting procedure has since been described in the technical literature for both flat sheet and tubular membranes ( 5, 6, T). 

The first composite reverse osmosis membrane to be developed and described consisted of an ultrathin film of secondary cellulose acetate deposited onto a porous Loeb-Sourirajan membrane.3 The ultrathin film of cellulose acetate was fabricated by a water surface float-casting technique. This has been described to some extent in the published technical literature,4 5 and in considerable detail in several reports on government-funded research projects.3 6 Figure 5.2 illustrates this process schematically. 

The free floating film is transferred to a microporous support by bringing a sheet of the support into contact with the underside of the ultrathin film, lifting it from the water surface.9 Also, two layers can be simultaneously cast and laminated to a carrier web.7 8 Numerous patents have appeared in recent years on the fabrication of gas separation membranes by float-casting, even including the double layer membrane technique.10 11... 

Figure 5.2 Schematic diagram of the float-casting of ultrathin cellulose acetate membranes.

Early examples of cellulose acetate composite membranes used cellulose ester sheet materials as the porous underpinnings for the float-cast films. These sheet materials included (a) Loeb-Sourirajan asymmetric cellulose acetate mem-... 

In the fall of 1966, researchers at North Star Research Institute began a search for compression-resistant microporous substrates.19 This effort resulted in the development of microporous sheets of polycarbonate (Lexan) and poly-sulfone (Udel).20 Figure 5.4 shows a graph comparing the flux levels and flux stability for three membranes made at that time (a) float-cast cellulose acetate on microporous polysulfone, (b) float-cast cellulose acetate on a mixed cellulose ester microfilter support and (c) a standard asymmetric cellulose acetate membrane. The improvement in membrane fluxes was readily apparent, when switching from cellulosic substrates to the microporous polysulfone substrate. 

The buttons usually have a film of Pu02 as a result of exposure to glove-box air. Upon casting, this Pu02 floats and remains in the skull along with trapped plutonium metal. This portion of the skull is recycled back into the production sequence. 

Polystyrene can be moulded, cast or extruded in sheets, rods and tubes. It can be easily pigmented. It finds wide, application in making household goods, lenses, plastic moulds, toys, wood laminates, films, battery boxes and electrical components. Expanded polystyrene is used as thermal insulating material and packaging. It is also used in lifejackets and floats. 

Polyacetals and other engineering plastics cost about half that of cast metals, and are therefore used as replacements for cast metal-intense applications. They have been approved by the Food and Drug Administration for contact with foods. Some of the uses of molded polyacetals are as valves, faucets, bearings, appliance parts, springs, automotive window brackets, hose clamps, hinges, video cassettes, tea kettles, chains, flush toilet float arms, gears, shower heads, pipe fittings, pasta machines, desktop staplers, and air gun parts. 

It cannot be that our life is a mere bubble, cast up by eternity to float a moment on its waves and then sink into nothingness. 

The higher the temp of casting and the more fluid the melt, the larger is the fraction of the entrained air that forms into bubbles and floats out of the charge. On the other hand, these conditions maximize cavitation due to shrinkage. The most serious effect of shrinkage is that known to metal founders as piping . 

Once cooled, the metal from a blast furnace is known as a cast metal. (When the ore is an iron ore, the cast metal is known as pig iron) A cast metal is brittle and soft because it still contains impurities, such as phosphorus, sulfur, and carbon. To remove these impurities, oxygen is blown through the molten cast metal in a basic oxygen furnace, shown in Figure 18.25. The oxygen oxidizes the impurities to form additional slag, which floats to the surface and is skimmed off. 

For the production of superpurity aluminum on a large scale, the Hoopes cell is used. This cell involves three layers of material. Impure (99.35 to 99.9% aluminum) metal from conventional electrolytic cells is alloyed with 33% copper (cutcctic composition) which serves as the anode of the cell A middle, fused-salt layer consists of 60% barium chloride and 40% AlF 1.5NaF (chiolite), mp 72(TC. This layer floats above the aluminum-copper alloy. The top layer consists of superpurity aluminum (99.995%). The final product usually is cast in graphite equipment because iron and other container metals readily dissolve in aluminum. For extreme-purity aluminum, zone refining is used. This process is similar to that used for the production of semiconductor chemicals and yields a product that is 99.9996% aluminum and is available in commercial quantities. 

Rotational casting is usually used in making hollow articles such as vessels, dummies, dolls, buoys, floate, etc. The weighed amount of plastisol is given into a metallic sealed mould which then is rotated in three mutually perpendicular directions upon heating in a chamber furnace. Upon gelatination the mould is cooled, stopped, opened, and the article is taken out. 

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