Fillers pyrogenic silicas

Fumed silica, 22 33, 367-368, 374t, 383, 400, 581. See also Pyrogenic silica in synthetic fillers, 11 315-316 Fumed xerogel, powder used in cosmetics, 7 84 It... 

It is advantageous to add silane or tltanate coupling agents to the filler in order to aid the mixing. Increase the adhesion between the resin and filler and to reduce the viscosity of the mortar. Thlxotropes, such as pyrogenic silica may be added to control the flow of the mortar on vertical surfaces. 

By contrast, active fillers, exclusively selected fi om different kinds of pyrogenic silica with BET surfaces of 90-350 mVg, are able to strongly interact with both themselves and the dimethylsiloxane polymers, which is due to their high surface energy and ability to form hydrogen bridges. This interaction results in a marked increase in mechanical strength, mainly abrasion and tear resistance, of the cured rubber. 

The shape and size of particles depend mainly on the mode of preparation of fillers. Most of the amorphous fillers having a spherical shape are produced by fast processes as, for example, precipitated silicas and pyrogenic silicas. Fillers produced by slow processes generally have a crystalline structure, such as kaolin, chalk, zinc oxide, barite, and so on. 

Carbonates, silicates, and silica are the most common fillers. Interaction with the elastomers can be modified greatly by surface treatment with fatty acids or silanes. Pyrogenic silica has an extremely high surface area. 

Synthetic silica types are used in coatings less as filler than as thixotropic, antisettling and matting additives. The different types include pyrogenic silica, precipitated silica and silica aerogels and hydrogels. They consist of coagulated amorphous spherical particles of very fine... 

Silicone sealants of this type are available as colorless (translucent) compositions if desired. Silicone polymer content is in the 60-70% range (40-50% polysiloxane plus up to 20% dimethyl silicone or methylphenylsilicone plasticizer). Pigments and fillers include titanium dioxide, silicas, calcium carbonate, or dried clays. Pyrogenic silicas are used for rheological control to produce nonslump sealants. 

The same increase in the glass transition temperature that can be achieved by increasing the curing temperature can also be realised by adding (nano)fillers Dynamic mechanical analysis can be used to compare an unfilled epoxide film, a film filled with 10% pyrogenic silica, a film filled with io% pyrogenic silica that has been silanised with an exocyclic epoxide and a film filled with io% pyrogenic silica that has been silanised with an endocyclic epoxide. 

Pyrogenic or fumed silica is a finely divided filler which is used as a thixotrope to increase the viscosity of liquid resins. 

The high mechanical strength of natural and organic rubbers as used in tires is due to the incorporation of pyrogenic carbon blacks as active fillers. Elastomers of a more polar polymer backbone, such as polyacrylates, polyurethanes or polysulphides, require fillers of higher polarity. In particular the performance of polydimethylsiloxane elastomers (silicone rubber) is basically related to the addition of fumed silica. 

This group of fillers include pyrogenic, i.e. thermally produced, or wet chemically produced silicas, wet chemically produced silicates, glasses and cristobalite. 

Figure 5.6 shows that pyrogenic manufacturing gives excellent control over particle size distribution and median particle size. These grades of fumed silica differ in properties and require a different technological approaches to their dispersion since small particle size filler is more difficult to disperse. At the same time, smaller particle sizes give more transparent products and better reinforcement. 

In the rubber industry, silica is widely used as a non-black reinforcing filler to improve the mechanical properties of NR composites due to its high specific surface area, particularly tensile strength, tear resistance, abrasion resistance and hardness. Silica can be obtained by various methods, such as precipitated silica by precipitation of an aqueous sodium silicate solution, fumed silica by pyrogenic process, silica from natural resources e.g. rice husk ash and fly ash), and in situ silica or silica sol by the sol-gel process. 

Silicates (e.g., calcium or aluminum silicates) are not as active as fillers as are the silicas. Colloidal silicas can also be prepared by the so called pyrogenic process, wherein sihcon tetrachloride is hydrolyzed at high temperatures as follows ... 

Fillers are added to adhesives primarily for establishing certain consistencies and occasionally for reducing cost. Preferred fillers are pyrogenic and precipitated silicas, chalks, and light and heavy spar. Fibers and metal powders are used in special cases [28]. 

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