Coatings, fumed silica

Coated fumed silica improves both the flow properties and the charging properties of the toner particle (55). 

AM Alak, T Vo-Dinh. Silver-coated fumed silica as a substrate material for surface-enhanced Raman scattering. Anal Chem 61 656-660, 1989. 

Tetravalent silicon is the only structural feature in all silicon sources in nature, e.g. the silicates and silica even elemental silicon exhibits tetravalency. Tetravalent silicon is considered to be an ana-logon to its group 14 homologue carbon and in fact there are a lot of similarities in the chemistry of both elements. Furthermore, silicon is tetravalent in all industrially used compounds, e.g. silanes, polymers, ceramics, and fumed silica. Also the reactions of subvalent and / or low coordinated silicon compounds normally lead back to tetravalent silicon species. It is therefore not surprising that more than 90% of the relevant literature deals with tetravalent silicon. The following examples illustrate why "ordinary" tetravalent silicon is still an attractive field for research activities Simple and small tetravalent silicon compounds - sometimes very difficult to synthesize - are used by theoreticians and preparative chemists as model compounds for a deeper insight into structural features and the study of the reactivity influenced by different substituents on the silicon center. As an example for industrial applications, the chemical vapor decomposition (CVD) of appropriate silicon precursors to produce thin ceramic coatings on various substrates may be mentioned. 

The hand lay-up or spray-up process, used universally for the production of laminar composites incorporating glass fiber reinforcement, is most efficient for the manufacture of large parts, such as boats, bathtubs, tanks, architectural shapes, and recreational accessories. Resins intended for spray-up processes are usually modified with thixotropic additives, such as fumed silica (1%), to reduce the risk of drainage when applied over large vertical mold surfaces. Molds are also made from FRP for short-run products usually surfaced with a tooling gel coat to provide consistent surface quality and appearance. 

Some materials tend to form granules when sieved. Coating the particles in order to reduce cohesiveness can often reduce this effect. Powders may, for example, be shaken in a container with 1% fatty acid (stearic acid is often used) or fumed silica. Alternatively, the powder may be sieved wet. The addition of 0.1% sub-sieve carbon black has been found useful (although rather messy) for eliminating electrostatic charge. 

INFRARED AND RAMAN SPECTROSCOPY ° ° 21-138 Applications The following use was made of infrared and Raman spectroscopy identification of surface groups on treated and untreated fumed silica, tion of silica functional groups and coatings by Raman spectroscopy. 

Testing procedure In general, standard methods were used, with some improvements to obtain better resolution. A photoacoustic detector was used to obtain spectra of fumed silica. A carbon black background was used. In studies of adhesion of coatings on metal substrates, a gold coated background was used as the reference. 4 Diffuse respectra, and kaolin. 2 A... 

The oxyhydrochlorination catalyst was prepared according to the procedure of Conner et al. [9]. The material was prepared in nonaqueous solvents by successive impregnation of metal chloride salts onto a silica support. A saturated solution of copper (I) chloride in acetonitrile was sorbed into fumed silica (325 mVgram). The acetonitrile was then removed under reduced pressure. A solution of potassium chloride and lanthanum chloride 1n formic acid was added to the cuprous chloride coated silica. The formic acid was removed under reduced pressure to produce the layered oxyhydrochlorination catalyst. The weight composition of the final catalyst was 41.7% CuCI, 37.5% Si02, 11.5% KC1, and 9.4% LaClj. The catalyst could be activated in a stream of hydrogen chloride at 300°C for ten minutes. 

Hydrophobic Silicas. Because foaming is a surface phenomenon, any antifoam used must concentrate at the surface (or gas—liquid interface). Hydrophobic silicas, which are silicas that have been treated with a compound that causes them to float on the top of water, have been used to fulfill this function for almost 30 years. U.S. Patent 3 408 306 (5) discloses the use of a hydrophobic silica dispersed in a hydrocarbon oil. Hydrophobic silica for this composition, which is still in use today, is made either by continuous ( dry roast ) or batch process. In either process, precipitated silicas rather than silica gels or fumed silicas are typically used to make antifoams. During a continuous process, silicone oils, usually poly(dimethylsiloxane), are sprayed onto a bed of hydrophilic silica. The bed is heated to temperatures ranging up to 300 °C, and reaction times are up to 20 h. At these temperatures and reaction times, bond formation between the silica particle and silicone oil may occur in addition to simple coating of the particle. 

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