SAS is produced by either a thermal route (pyrogenic/fumed) or a wet route (precipitated, gel, colloidal) processes. In the initial particle formation step, primary particles with dimensions less than 100 nm are formed by nucleation, coagulation and coalescence. By covalent bonding, these particles form indivisible units, referred to as the aggregates, with external dimensions typically above 100 nm; they are fused together with no clear physical boundaries among them. These aggregates combine by physical attraction forces (van der Waals) and hydrogen bridges to form micron-sized agglomerates.  

 

Wet route

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Precipitated SAS is produced via a wet production route, which is based on a reaction of alkali metal silicate solutions with mineral acids.

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This process description includes synthetic amorphous silica, both precipitated and gel. Raw materials for the production of precipitated silica and silica gels are aqueous alkali metal silicate solutions (e.g. waterglass) and mineral acids, (e.g. sulphuric acid). An alkali metal silicate solution reacts with mineral acids to generate precipitates or gels, depending on the reaction and process conditions. The suspension generated from the precipitation step of the process is subsequently filtered. Precipitated silica is washed and dried and can be further processed by milling.

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Silica gel is produced in a comparable process but not in a water suspension. The process results in a solid form which is then ground before being washed and processed further.

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Thermal route

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Pyrogenic SAS is made by hydrolysis of volatile chlorosilanes or methylchlorosilanes in an air-hydrogen flame at temperature between 1200-1600 °C in a continuous operating process.

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Precursor particles grow by nucleation, condensation and coagulation to yield primary particles.  Under the reaction conditions primary particles collide and stable non-dispersible aggregates are formed. By physical attraction forces between aggregates larger agglomerates are formed.

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The properties of pyrogenic silica products can be controlled by varying process parameter, such as feedstock, flame composition and flame temperature.

 

Surface Treatment to Modify Physical-Chemical Properties

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All types of SAS have silanol groups on their surfaces, which makes them hydrophilic and wettable by water. These silanol groups also provide a means of altering the physical and chemical properties of the silica by treating the surface of the silica with reactive substances. Typically the type treatment is chosen either to make the product hydrophobic (water repellent), or to enable improved blending and bonding properties when the treated silica is used in coatings, adhesives, rubber compounds, and other formulations. These SAS treatments determine the interactions of the silica particles within solids and liquids.

 

Typical treating agents are silanes, siloxanes, silazanes, and polysiloxanes, although other treatment systems including functional organic waxes may be used. These treatments are applied to the silica in varying amounts based on the desired degree of modification, and attach to the particle’s surface to provide the necessary attributes. Surface treatment does not have any significant impact the particle size distribution. Products formed by each type of treatment may or may not be subject to regulation, such as ingredients used in cosmetic or food contact materials, or by chemical identity as described in other country specific regulatory requirements.

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Additional References on Manufacturing Processes: Nanoscale Materials Stewardship Program (NMSP) Voluntary

Submittal Package for Synthetic Amorphous Silica (CAS No. 7631-86-9), The Synthetic Amorphous Silica and Silicates Industry Association

July 25, 2008 (Section 1.1); European Centre for Ecotoxicology and Toxicology of Chemicals (ECETOC) Joint Assessment of Commodity  Chemicals (JACC) No.51 (ISSN-0773-6339-51, 2006)(Section 3)