Silica processes

Vycor glass (Corning, 7930) is porous silica processed in the same way as CPG except that the last step of base etching is missing. Therefore the surface is rough. Only a 40-A pore is available. When porous silica with a pore size smaller than 75 A is needed, Vycor glass is the choice. Bulk pieces of Vycor glass are available commercially and need to be crushed into small particles before use. 

Carbon dioxide-silica process. Sand is mixed with sodium silicate (3.0 to 3.5% of sand volume), and the mixture is blown or hand-rammed into a core box or around a pattern. Carbon dioxide gas is passed through the compacted sand to harden the binder. The bonding strength eliminates the need for drying or baking the mold and metal can be poured into the mold immediately. Over-gassing should be avoided because it makes the mixture friable. 

An interesting modification of the Stober silica process has been described by Unger et al. (50). By using a mixture of TEOS and an alkyltriethoxysilane they were able to synthesize monodispersed porous silica particles. The porosity is created by the alkyl groups, which act like space holder. After calcination/burnout of the organics, a well-defined porosity is left behind in the silica particles. The materials are used for very fast high-pressure liquid chromatography. 

Figure 2.57. Precipitated silica process. After Bomo F, Meeting of the Rubber Division, ACS, Montreal, May 5-8, 1996, paper E.

Auxiliary agents silane modification of silica, process oils, lubricants... 

Chemical composition of PV-silicas. Processing time VOCI3 - 30 min ... 

Snow FIoss Silica Processed diatomaceous silica JM... 

Silica processing utilizes a chemically inert synthetic amorphous silica adsorbent with an affinity for polar contaminants. The surface area, porosity, and moisture content of the silica adsorbents provide them with the capability of removing soaps, phospholipids, sulfur compounds, and trace metals from edible oils. The function of the moisture is to hold the pores open and aid in the attraction of polar contaminants. Several different options are offered for the use of silica adsorbents. The simplest option adds... 

Guidelines for the construction of viable electrodialysis cells for silica sols process were prepared. This was needed because the commercial one square foot electrodialysis twin cell used in this project develops eternal and internal leaks under the temperature and flow rate conditions specified by our colloidal silica process. 

For example, because of an incomplete hydrolysis of silicate, =Si-0-Na can be observed because of an incomplete washing of silica after filtering, sulfate can be adsorpted onto silica surface. In addition, some salts used for silica processing can also be added and modulate its reactivity (Bomal et al., 1995). 

The in situ sol gel silica process is an alternative method of incorporating silica into rubbers. The in situ sol-gel approach, in preparing silica-reinforced rubber, is considered a novel technique. In principle, a rubber is swollen in a silica precursor, e.g. tetraethyl orthosilicate (TEOS) and the TEOS-swollen rubber is then subjected to hydrolysis and condensation reactions which could be acid- or base-catalysed. Subsequently, silica particles are generated in the rubber or rubber network. The chemical reactions involved in the formation of silica particles are shown in Scheme 7.1 where the overall reaction indicates that 1 mole of TEOS requires 2 moles of water to be converted into anhydrous silica. 

Cruz, J.C., Pfromm, P.H., and Rezac, M.E. 2009. Immobilization of Candida antarctica Lipase B on fumed silica. Process Biochem. 44 62-69. 

Diatomaceous Silica n Si02 7H20. Pigment White 27 (77811). A form of hydrous silica, processed from natural diatomate, a sedimentary rock of varying degrees of consolidation that is composed essentially of the fossilized... 

The gas phase within the flame becomes supersaturated with respect to solid SiOj, which precipitates as molten droplets of transparent soot, approximately 10 nm in diameter. These droplets are subsequently expelled from the flame. (If the expelled soot is given sufficient room to cool to the solid state, it can be collected as a fine white powder, called fumed silica, such as is sold under the trade name Cab-O-SU .) In the Type III silica processes, the silica soot is deposited on a hot (greater than 1600°C) surface where it continuously consolidates to produce a homogeneous boule. 

FIGURE 6.6 Coming Code 7940 synthetic fused silica process. (Illustration courtesy John Rowe, Coming Incorporated, ca. 1993.)... 

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