Precipitated silica structure

Table 12. Definition of Precipitated Silica Structure Silica Structure Level Oil Absorption (ml/IOOg)...

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]. 

Zinc salt of maleated EPDM rubber in the presence of stearic acid and zinc stearate behaves as a thermoplastic elastomer, which can be reinforced by the incorporation of precipitated silica filler. It is believed that besides the dispersive type of forces operative in the interaction between the backbone chains and the filler particles, the ionic domains in the polymer interact strongly with the polar sites on the filler surface through formation of hydrogen bonded structures. 

The marked difference in the relaxation times for the kaolinite and silica may be attributed to the nature of the surface. Intuitively, the hydrogen bonding which influences the increased structure at the kaolinite surface would be expected to give shorter values for the relaxation time. However this is not observed in the simulations. Instead, shorter values are seen for the silica surface which is a result of water molecules becoming trapped in the cage-like amorphous silica surface. This reflects experimental results where precipitated silica surfaces are microporous and water inclusion in the surface is common. 

Amorphous Silica The term amorphous silica refers to aggregate of smaU particles with high specific surface area. They lack crystal structure and do not form a sharp x-ray diffraction pattern. They are known in several forms such as colloidal silica, precipitated silica, silica gels, and fumed sdica. The surface of such amorphous silica may contain silanol (SiOH) groups or can be anhydrous. 

Keatite. Keatite has been prepared (65) by the crystallization of amorphous precipitated silica in a hydrothermal bomb from dilute alkali hydroxide or carbonate solutions at 380—585°C and 35—120 MPa (345—1180 atm). The structure (66) is tetragonal. There are 12 Si02 units in the unit cell a0 = 745 pm and c0 = 8604 pm the space group is P42. Keatite has a negative volumetric expansion coefficient from 20—550°C. It is unchanged by heating at 1100°C, but is transformed completely to cristobalite in three hours at 1620°C. 

Precipitated silicas are also produced by the addition of sulphuric acid to a solution of sodium silicate but under different conditions, which result in the formation of aggregates of tiny discrete particles rather than the massive structure of a gel. After precipitation the slurry is filtered, washed, dried and deagglomer-ated. 

Differential thermal (Belyankin and Ivanova, 1936) and infrared (Adler et al., 1950) studies prove that allophane is not a fine mechanical mixture of alumina and silica but that these are chemically combined as in co-precipitated silica alumina gels. X-ray patterns usually show one or more diffuse bands, which White (1953) interpreted to mean that the structure was more ordered than glass. 

The precipitated silicas include a wide range of silicas with a variety of structural characteristics. Most of the preparation methods are patented. In general the formation involves a coagulation and precipitation from silica solutions. Properties may therefore be supposed to be similar to those of the gels. For these silicas however, preparation conditions are such as to avoid gel growth and stimulate precipitation. As an overall definition, Barby proposed dry silicas with no long or short distance characteristic structure. 

Periodic nanoporous silicates have been prepared in a wide variety of conditions. Different sources of molecular, and non molecular silica have been used. This includes TEOS, TMOS, fumed, colloidal and precipitated silicas. Depending on the synthesis conditions, particularly on the nature of the silica source, crystallization may take place in seconds at subambient temperatures [82], or at room temperature [60,61,69,72,83]. However, in most cases the crystallization temperature was set in the 80 - 120 °C range. Liu et al. [84,85] found that the use of small amounts of colloidal particles (silica or titania) promotes the formation of ordered structures by providing nucleation seeds. The pH conditions varied from extremely acidic [60,61], to neutral [69,72] to very basic [48,49]. Ryoo and Kim [86]... 

Figure 3 shows mixtures of precipitated silicas in polydimethylsiloxane with different structures, sur ce areas, pore volumes, and pore size distributions. The silica on the right exhibits high transparency in dimethylpolysiloxanes. 

Many solid adsorbents liberate gas as a result of desorption of volatile liquids under the influence of heat. Typical adsorbents with microporous structures such as activated carbons, or precipitated silicas and renewable resources have been used as a coblowing agent in producing low-density extruded polystyrene foam boards. Incorporation of corn cobs or other renewable vegetable matter containing about 10% water together with a primary PBA into polystyrene in the extrusion process produced a low-density polystyrene foam board with bimodal cellular structures. This type of foam with bimodal cell structures has about 10-15% lower K-factor than similar foams without bimodal cellular structures. Similar results were obtained with a precipitated silica for producing a low-density extruded polystyrene foam with bimodal cellular structures. ... 

The proof of reversibility in primary mineral weathering would be instances where primary mineral structures have formed under earth-surface conditions. There are reports that secondary quartz can slowly precipitate at room temperature from solutions supersaturated with monosilicic acid. More typically, however, precipitated silica in soils is structurally disordered, in the form of chalcedony or opal. In fact, as long as alumina is present, silica does not precipitate as a separate phase, reacting instead to form aluminosilicates (layer silicates, imogolite, or allophane). 

Precipitated silicas produced the highest melt elasticity that we have observed from simple Cr/silica catalysts [521]. Silica gels are usually "set (gelled) under acidic or neutral conditions, as the pH from mineral acid is adjusted upward by the addition of sodium silicate. Primary silica particles form a network of chains that occupies the entire volume of the solution as illustrated in Scheme 23. In contrast, precipitated silicas are usually made under basic conditions as the pH of sodium silicate solution is adjusted downward by the addition of acid. Primary silica particles coagulate into strong secondary aggregates that precipitate out of solution as a fine flocculent, but they do not gel. The precipitated silica does not occupy the entire reaction volume as a gel does. The process is illustrated in Scheme 29. After precipitation, the secondary aggregate structure can sometimes be "reinforced" by deposition of a further silica layer. 

The surface chemistry of silica was a subject of intensive study in the period between 1960 and 1970 as a consequence of the widespread industrial use of colloidal, pyrogenic, and precipitated silicas, as well as silica hydrogels and xerogels. Chemical surface reactions and IR spectroscopy were the most-applied methods in surface structure elucidation. Significant contributions to the understanding of the silica surface were made by Fripiat (I), Kiselev and co-workers (2), Hair (3), Little (4), Peri... 

Other Uses. Numerous other uses of silica gels and precipitated silicas are found in the literature. Two relatively new applications that are expected to be significant in the future are battery separators, in which the silica pore structure provides a path for the migration of conductive ions, and low-temperature insulation, in which the low thermal conductivity of dry silica powders makes them useful in consumer products such as refrigerators (Chapter 24). 

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