Silica, fumed

Fumed silica, or fumed silicon dioxide, is produced by the vapor phase hydrolysis of silicon tetrachloride in an H2/O2 flame. The reactions are shovm in Chapter 19. Hydrophilic fumed silica bearing hydroxyl groups on its surface is produced by this process. Hydrophobic fumed silica is made by processing fumed hydrophilic silica through in-line hydrophobic treatments such as with silanes, siloxanes, silazanes, and so on [1]. Examples of different types of hydrophobic fumed silica coatings include DMDS (dimethyldichlorosilane), TMOS (trimethoxyoctylsilane), HMDS (hexamethyldisilazane). 

So-called fumed silica results from a pyrolitic process, as the product of the hydrolysis of silicon tetrachloride vapor in a flame of hydrogen and oxygen, according to the following set of chemical reactions  

Typical White Minerals Used as Fillers for Polymers 

Filler Chemical Composition Specific Gravity (g/cm ) Surface Area (mVg) Mohs Hardness Specific. Heaf C (at298K)(J/g.K) Thermal Conductivity (W/m.K) 

Dolomite (grinded) Barium sulfate CaCOj.MgCOj 2.40-2.90 5-20 3.5. 0 80-90 4.9-5.S 

Thermal data were obtained from various sources of geological information the actual data of the mineral used as filler in the polymer industry might be slightly different. 

Very finely divided fumed silica (pyrogenic silica) is another important inorganic thickener used in the coating industry that 

Unlike hydrated precipitated silica, fumed silica is used almost exclusively in silicone rubber compounds. Fumed silica is incorporated into silicone gum stock with the use of dough mixers. Fumed silica is unique to silicone rubber because it significantly improves the physical properties of the cured compound (better than hydrated precipitated silica). On the other hand, fumed silica will have a greater tendency than the hydrated precipitated silica to impart crepe hardening to the uncured silicone compound. Many times, the use of fumed silica in silicone rubber is extended with the less expensive precipitated silica. 

Since fumed silica is generally used only in silicone rubber compounds, the rubber industry uses far less fumed silica than hydrated precipitated silica. On the other hand, fumed silica demand is growing significantly faster than the hydrated precipitated silica normally used. 

Fumed silica is produced from the vapor-phase hydrolysis of silicon tetrachloride 

Fumed silica has an ultimate particle size as small as 5 nm, which is finer than hydrated precipitated silica. With a conventional Banbury mixer, a filler this fine would be very difficult to disperse in a general-purpose elastomer compound. Fumed silica can be dispersed using a dough mixer in a gum silicone to achieve the ultimate reinforcement improvement. Fumed silica costs more to produce than hydrated precipitated silica. 

Fumed silica is a better reinforcing agent for silicone compounds than precipitated hydrated silica. While the hydrated precipitated silica does not stiffen the silicone compound as much (crepe hardening), it also does not impart as much true reinforcement (such as better tensile strength, tear strength, and abrasion resistance). 

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The above methods for obtaining D, as well as other ones, are reviewed in Refs. 3-12, and Refs. 7-9 give tables of D values for various adsorbents. For example, D is close to 3 for the highly porous silica gels and close to 2 for nonporous fumed silica and for graphitized carbon black coconut charcoal and alumina were found to have D values of 2.67 and 2.79, respectively [7]. 

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... 

The temperature range of general purpose material is approximately -50 to h-250°C but both ends of the range may be extended by the use of special purpose materials. Whereas the general purpose silicone compounds have a tensile strength of about 1000 Ibf/in (7 MPa) it is possible using fumed silicas to achieve values of up to 20001bf/in (14 MPa). Similarly, whereas the normal cured compounds have a compression set of 20-50% after 24 hours at 150°C, values of as low as 6% may be obtained with the special rubbers. 

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. 

For viscosity or sag control. When the rubber base adhesive is applied on a vertical surface, addition of a filler prevents the adhesive from running down the wall. In solvent-borne formulations, fumed silica can be used as anti-sag filler. In water-borne systems, clays impart yield stress and excellent sag control. 

Fumed silicas (Si02). Fumed silicas are common fillers in polychloroprene [40], natural rubber and styrene-butadiene rubber base adhesives. Fumed silicas are widely used as filler in several polymeric systems to which it confers thixotropy, sag resistance, particle suspension, reinforcement, gloss reduction and flow enhancement. Fumed silica is obtained by gas reaction between metallic silicon and dry HCl to rend silica tetrachloride (SiCU). SiC is mixed with hydrogen and air in a burner (1800°C) where fumed silica is formed ... 

Fumed silicas have an amorphous nature probably caused by the fast cooling during the manufacturing process. It is an important benefit because it does not cause silicosis. 

The thickening mechanism of precipitated silicas is similar to that of the fumed silicas. Precipitated silicas have more silanol groups than fumed silicas, but they have a lower concentration of silica (precipitated silicas generally contain some sodium sulphate). 

Tioxide process. This process is similar to that used to produce fumed silicas. Ultra-low particle size titanium dioxide (15-35 nm) is obtained for use as photocatalyst or UV absorber (for instance in sun protective creams). 

Thickeners. Thickeners increase the viscosity of the polychloroprene latex adhesives. Amounts up to 1% of polyacrylates, methyl cellulose, alginates and polyurethane thickeners can be used. Particular attention should be paid to fluctuations in pH when thickener is added in the formulations. For low-pH (7-10) formulations, fumed silica or some silicates can be used. 

Reinforcing fillers (active) Fumed Silica (Si02) precipitated calcium carbonate (CaCOi) carbon black Thixotropic reinforcing agents (non-slump), adjustment of mechanical properties (cohesion) provide toughness to the elastomer as opposed to brittle materials. 

Various additives and fillers may be employed. Calcium carbonate, talc, carbon black, titanium dioxide, and wollastonite are commonly used as fillers. Plasticizers are often utilized also. Plasticizers may reduce viscosity and may help adhesion to certain substrates. Thixotropes such as fumed silica, structured clays, precipitated silica, PVC powder, etc. can be added. Adhesion promoters, such as silane coupling agents, may also be used in the formulation [69]. 

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. 

Fumed silica/sulfuric acid paste, electrochemical etch 21.7 10.7... 

The nonporous spherical gels for PCHdC are often specially prepared for research purposes. However, nonporous polystyrene/divinylbenzene beads. Solid Bead, can be obtained in various particle sizes from Jordi Associates, Inc. (Bellingham, MA). Columns packed with these gels can be used for HdC of the polymers that are currently analyzed using polystyrene/divinylbenzene SEC columns. Fumed silica nanospheres are offered by Cabot (Tuscola, IL) (17), and nonporous silica (NPS) microspheres are offered by Micra Scientific, Inc. (Northbrook, IL). These nonporous silica gels may also be used for HdC. 

To meet the 2001 U.S. energy standards and the 2003 phase-out of HCFCs, there is a great incentive to develop a significantly better thermal insulation. The most dramatic approach would use vacuum panels for insulating the cabinet. A number of U.S. and Japanese manufacturers have developed such panels and placed these kinds of refrigerators in homes. The panels consist of multilayer plastic envelopes filled with precipitated (fumed) silica. The claimed thermal conductivity is one-fourth that of polyurethane foam. The two major obstacles are cost and the maintenance of vacuum for twenty years. 

The preparation and properties of a novel, commercially viable Li-ion battery based on a gel electrolyte has recently been disclosed by Bellcore (USA) [124]. The technology has, to date, been licensed to six companies and full commercial production is imminent. The polymer membrane is a copolymer based on PVdF copolymerized with hexafluoropropylene (HFP). HFP helps to decrease the crystallinity of the PVdF component, enhancing its ability to absorb liquid. Optimizing the liquid absorption ability, mechanical strength, and processability requires optimized amorphous/crystalline-phase distribution. The PVdF-HFP membrane can absorb plasticizer up to 200 percent of its original volume, especially when a pore former (fumed silica) is added. The liquid electrolyte is typically a solution of LiPF6 in 2 1 ethylene carbonate dimethyl car-... 

The liquid electrolytes used in lithium batteries can be gelled by addition of a polymer [25] or fumed silica [26], or by cross linking of a dissolved monomer [271. Depending on the mechanical properties, gelled electrolytes can be used as separators, or supported by a conventional [27]... 

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