Fumed silicas applications

Mix D is a typical plastigel. The incorporation of such materials as fumed silicas, certain bentonites or aluminium stearate gives a paste which shows pronounced Bingham Body behaviour (i.e. it only flows on application of a shearing stress above a certain value). Such putty-like materials, which are also... 

Because most latices have low viscosities by compounding, most of the waterborne rubber adhesives are sprayable. Thickeners such as fumed silicas can be added to increase viscosity and thixotropy. This means that even at relatively large viscosities (over 10 Pas) many water-based rubber adhesives can be sprayed. Dip and curtain applications require viscosities between 0.05 and 0.3 Pas, whereas brush application works with viscosities between 1 and 50 Pa s. 

All of the eommereial alkyl eyanoaerylate monomers are low-viseosity liquids, and for some applications this can be an advantage. However, there are instances where a viseous liquid or a gel adhesive would be preferred, sueh as for application to a vertical surface or on porous substrates. A variety of viscosity control agents, depending upon the desired properties, have been added to increase the viscosity of instant adhesives [21]. The materials, which have been utilized, include polymethyl methacrylate, hydrophobic silica, hydrophobic alumina, treated quartz, polyethyl cyanoacrylate, cellulose esters, polycarbonates, and carbon black. For example, the addition of 5-10% of amorphous, non-crystalline, fumed silica to ethyl cyanoacrylate changes the monomer viscosity from a 2-cps liquid to a gelled material [22]. Because of the sensitivity of cyanoacrylate esters to basic materials, some additives require treatment with an acid to prevent premature gelation of the product. 

Barthel, H., Rosch, L., and Weis, J., Fumed Silica-Production, Properties, and Applications, in Organo-silicon Chemistry II From Molecules to Materials, VCH Weinheim, Germany, 1996 Vol. II, 761. 

Silica is the support of choice for catalysts used in processes operated at relatively low temperatures (below about 300 °C), such as hydrogenations, polymerizations or some oxidations. Its properties, such as pore size, particle size and surface area are easy to adjust to meet the specific requirements of particular applications. Compared with alumina, silica possesses lower thermal stability, and its propensity to form volatile hydroxides in steam at elevated temperatures also limits its applicability as a support. Most silica supports are made by one of two different preparation routes sol-gel precipitation to produce silica xerogels and flame hydrolysis to give so-called fumed silica. 

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. 

Fumed silica is prepared by burning volatile silicon compounds such as silicon tetrachloride. This type of silica contains less than 2% combined water and generally no free water. It reacts readily with hydroxyl groups. The particle size is in the region 5-10 nm. Fumed silicas are not generally used in conventional rubber compounding but find application with silicone rubber. The recognised surface area values for best reinforcement of silicone rubber by an amorphous silica lies between 150-400 m2/g. 

For special high strength applications, ie, up to 69 MPa (10,000 psi), special formulations of Pordand cement concretes have been developed. These are based on the use of chemical and mineral admixtures. The typical mineral admixtures are fumed silica and other pozzolanics. The chemical admixtures are generally chemicals termed superplasticizers that allow very low water to cement ratios, ie, between 0.4 and 0.25, and reduce the amount of water needed to provide plasticity or flow to the concrete. Public works applications take just under 32% of the total Pordand cement market streets and highways represent 68% of this usage, and water and waste account for 23%. 

This property is responsible for the fact that silica gels are mainly applied for adsorption while fumed silicas are used in rheological applications. In these the presence of highly dispersed small particles is important. The applications of silica will be discussed in detail furtheron. 

Abstract. Nanocarbon materials and method of their production, developed by TMSpetsmash Ltd. (Kyiv, Ukraine), are reviewed. Multiwall carbon nanotubes with surface area 200-500 m2/g are produced in industrial scale with use of CVD method. Ethylene is used as a source of carbon and Fe-Mo-Al- mixed oxides as catalysts. Fumed silica is used as a pseudo-liquid diluent in order to decrease aggregation of nanotubes and bulk density of the products. Porous carbon nanofibers with surface area near 300-500 m2/g are produced from acetylene with use of (Fe, Co, Sn)/C/Al203-Si02 catalysts prepared mechanochemically. High surface area microporous nanocarbon materials were prepared by activation of carbon nanofibers. Effective surface area of these nanomaterials reaches 4000-6000 m2/g (by argon desorption method). Such materials are prospective for electrochemical applications. Methods of catalysts synthesis for CVD of nanocarbon materials and mechanisms of catalytic CVD are discussed. 

MWNT are produced in form of black powder with bulk density of 15-40 g/dm3. Experiments carried out by us have shown that low bulk density MWNT samples are preferable for application in composite materials, particularly for PTFE-MWNT composites. So, we tried to produce MWNT with the bulk density as low as possible. This was achieved by use of co-precipitated Fe203Mo03-Al303 catalysts containing aerosil (fumed silica) as a pseudo-liquid diluent of growing nanotubes [11, 12],... 

V. M. Gun ko and A. A. Chuiko, Chemical Reactions at Fumed Silica Surfaces, in Colloidal Silica Fundamentals and Applications, edited by H.E. Bergna (Taylor Francis LLC, Salisbury, 2005), pp. 465-497. 

I. V. Laguta, O. M. Stavinskaya, P. O. Kuzema, and O. A. Kazakova, Hydrophilic-hydrophobic fumed silicas as carriers of vitamins for biomedical applications, i Proceedings of IX Polish-Ukrainian Symposium Theoretical and Experimental Studies of Interfacial Phenomena and Their Technological Applications" (Sandomierz, Wolka Milanowska, Poland, September 5-9, 2005) pp. 162-164. 

For commercial processes, formed supports are more useful. Compared with other supports, fumed oxide supports showed new catalytic effects [41]. Some intensively investigated applications for these supports are abstracted in the following. SiC>2 pellets have been successfully introduced in a new generation of precious metal supports in vinylacetate monomer production [42]. This resulted in better selcctivities and an up to 50% higher space-time yield compared with supports based on natural alumo-silicates. In alkene hydration fumed silica pellets serve as a support for phosphoric acid. In this case, an increased catalyst lifetime and a higher space-time yield were observed [43]. Pyrogenic TiC>2 powder can be used as a starting material for the manufacture of monolithic catalysts [44] for the selective reduction of NOv with ammonia. 

Fumed silica is a highly dispersed silicon dioxide of large industrial importance and a wide spectrum of applications. Due to its production in a flame process fumed silica exhibits a smooth and nonporous particle surface. Additionally to its high surface area fumed silica bears isolated and statistically distributed surface silanol groups that render this product hydrophilic. A most important technical reaction, therefore, is the silylation and hydrophobization of the hydrophilic surface. 

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