Coatings, precipitated silicas

Figure 12 shows an optional step at the end of the process in which the silica is chemically reacted with a silicone oil, typically polydimethylsiloxane, to render the product hydrophobic. Other aftertreatments are also commonly employed, such as wax coatings that enhance the performance of precipitated silicas used as flatting agents in paints (78). 

Adsorption of mercury. All chemicals used were Merck or Baker analytical quality reagents, unless stated otherwise. From the commercially available adsorbents the following were used silica gel 60 A porosity, 0.063-0.200 mm particle size (Merck) charcoal 0.3-0.5 mm particle size, gas-chromatographic quality (Merck) alumina R Woelm hydrous zirconium oxide HZ0-1, 100-200 mesh, (Bio-Rad). Except for zirconium phosphate, which was prepared according to Amphlett ( ), all other sorbents were prepared by coating (precipitation) on acid-purified silica gel, as described in (1 ). The Si0 -NH was prepared according to Leyden et al (11). L... 

Figure 2.58. Surface flatting mechanism by precipitated silica, Lo-Vel HSF. Left - distribution of agglomerates, right - surface roughness of coating. Courtesy of PPG Industries, Inc., Pittsburgh, PA, USA.

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. 

Lo-Vel . [PPG Industries] Precipitated silica thiclcener, flatting agent for coatings and lacquers. 

Also here the formulator must carefully select what kind of silica should be used. In the reference coating formulation (Table 6) precipitated silicas perform better than silica gels. 

Precipitated silicas but also fumed silicas are widely used as carrier for liquids and semiliquids and as free flow agent for powder formulations, particularly of hygroscopic and caking substances. Hydrophobic pyrogenic silicas are used in technical products like fire extinguishing powders, pigments or powder coatings, but also in... 

Chem. Descrip. Precipitated silica CAS 1343-98-2 EINECS/ELINCS 215-683-2 Uses Elatting agent for clear and pigmented coatings, coil coatings, air-dried and baking enamels... 

Chem. Desetip. Zinc sulfonate, precipitated silica Uses Dispersant for powd. coatings, in rubber and plastic masterbatching improves disp. of fillers... 

The filled composites containing 5 phr of nano-ZOS and 10-15 phr of SCC exhibited comparable tensile properties, tear strength and hardness as that of the microcomposite containing 5 phr of micro-ZnO and 30 phr of precipitated silica. These observations also support the findings that stearic acid-coated nano-ZnO (ZOS), nano-BIAT and SCC are better dispersed due to the small size of the particles as well as the higher specific surface area as already discussed in this chapter. 

Glass mat rubber Rubber/ glass mat Made by mixing polymeric emulsion, precipitated silica, and rubber. This is then coated on a fiberglass mat and finally cured and dried. Finer pore diameter (<0.2 pm average), high porosity, excellent thermal dimensional stability... 

The choice of antiblocking additive depends on the polymer, the desired film quality, and whether there is a pigment. Several inorganic substances are used, including synthetic amorphous precipitated silica, diatomaceous earths, nepheline syenite, calcined clay, coated calcium carbonate, magnesium carbonate, magnesium sulphate, mica, talc and various zeolites. Calcium carbonate particles are approximately spherical, but silica ones are irregular mica forms sheets and talc is plate-like. 

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