Silicas precipitated

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. 

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Kea.tlte, Keatite has been prepared (65) by the crystallisation of amorphous precipitated silica ia a hydrothermal bomb from dilute alkah hydroxide or carbonate solutions at 380—585°C and 35—120 MPa (345—1180 atm). The stmcture (66) is tetragonal. There are 12 Si02 units ia the unit cell ttg = 745 pm and Cg = 8604 pm the space group is P42. Keatite has a negative volumetric expansion coefficient from 20—550°C. It is unchanged by beating at 1100°C, but is transformed completely to cristobahte ia three hours at 1620°C. 

Fig. 5. X-ray diffraction pattern of commercial samples (a) coUoidal silica, DuPont Ludox TM50 (b) silica gel, Sylodent 700 (c) precipitated silica, PPG...

Fig. 9. Transmission electron micrograph of a commercial precipitated silica, PPG Hi-Sd 190. Magnification of 225, OOOx. ...

The typical range of properties for commercial coUoidal silicas, silica gels, precipitated silicas, and pyrogenic silicas is given in Table 1. 

Moisture. The presence of water in a filler is not usually beneficial. Most fillers added to adhesives have a moisture content lower than 1 wt%. Only precipitated silicas and sepiolite contain about 5-10 wt% moisture. For some applications, fillers must be completely dried to exhibit adequate performance. Moisture absorbed on the surface of fillers impacts the rate and extent of curing of rubber base adhesives. 

Although natural quartz, cristobalite and opal are used as fillers, only synthetic products (fumed and precipitated silicas) find use as fillers in rubber base adhesives. 

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). 

Fillers. They are generally added to reinforce NBR adhesives. However, fillers can be added to promote tack, to increase the storage life, to improve heat resistance or to reduce cost. The most common fillers are carbon blacks. Precipitated silica can be used in applications where black colour is not acceptable, but excessive amounts tend to reduce adhesion. Titanium dioxide can be used to impart whiteness, improves tack and extend storage life. 

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]. 

This chapter reports the results of studies on the physical, dynamic mechanical, and rheological behavior of zinc oxide neutralized m-EPDM, particularly in the presence of stearic acid and zinc stearate, with special reference to the effects of precipitated silica filler. 

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. 

Standard solution of silica. Fuse 0.107 g of pure, dry precipitated silica with 1.0 g of anhydrous sodium carbonate in a platinum crucible. Cool the melt, dissolve it in de-ionised water, dilute to 500 mL, and store in a polythene bottle. 1 mL = 0.1 mg Si. Dilute as appropriate, say, to 1 mL = 0.01 mg Si. 

The term polyethylene separator is somewhat misleading, since this separator consists mainly of agglomerates of precipitated silica, held within a network of extremely long-chained, ultrahigh-molecular weight polyethylene molecules. The raw materials, precipitated silica (Si02 — about 60 percent), ultrahigh-... 

A thin layer of a mix of natural rubber, sulfur, precipitated silica, water, and some additives, such as carbon black and vulcanizing agents, is extruded on a paper support belt, calendered, and vulcanized as a roll in an autoclave under elevated pressure and temperature ( 180 °C). A modi-... 

Unfortunately, precipitated silica used in the rubber industry under these conditions does not give similar reinforcement. This is compared in Figure 3.7. 

Fine silica network is visible in the in situ silica-filled rubber composite synthesized from 50 wt% of TEOS, producing almost 15 wt% of sdica. On the other hand, addition of only 10 wt% of precipitated silica externally gives distinct aggregations. 

FIGURE 3.8 Scanning electron microscopic (SEM) pictures of acrylic mbber (ACM)-silica hybrid composites (a) synthesized from 50 wt% tetraethoxysilane (TEOS) and (b) synthesized from 10 parts of precipitated silica. (From Bandyopadhyay, A., Bhowmick, A.K., and De Sarkar, M., J. Appl Polym. Sci., 93, 2579, 2004. Courtesy of Wiley InterScience.)... 

Exposure to fiime and the precipitated silica (amorphous silica) causes fatal bronchial response as these are respirable. Repeated exposure dries the skin and causes mechanical irritation to eyes. Sol-gel silica, as an alternative to this, ceases the floating problem and avoids unnecessary contamination. 

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