SAS CHEMISTRY
What is silica?
Silica, chemically silicon dioxide, SiO2, occurs naturally, in several forms, the most obvious being sand.
Silica polymorphs?
Silica, independent of its form and method of preparation (including by-products), is found under CAS Nr. 7631-86-9. However, as the different polymorphs of silica differ in their hazards to human health, it is essential to distinguish carefully between crystalline silica and synthetic amorphous silica (crystalline-free). The situation can be complicated as natural forms of amorphous silica, unlike synthetic versions, often contain crystalline impurities (up to 65 % cristobalite in the case of calcinations). An overview of silica types and CAS numbers is shown in Figure 1:
*Different treating agents may be used such as dichlorodimethylsilane (DDS), polydimethylsiloxane (PDMS) and hexamethyldisilazane (HMDZ).
What is Synthetic Amorphous Silica?
Synthetic Amorphous Silica (SAS), EINECS No. 231-545-4, is a form of silicon dioxide (SiO2) that is intentionally manufactured. SAS has been produced and marketed for almost a century without significant changes in its physicochemical properties. SAS, as white dry powders or as dispersions, is used in a multitude of industrial applications. In addition, it is approved for use in many food, cosmetics and pharmaceutical applications. All historic physicochemical and toxicology data remain valid for SAS manufactured today.
SAS are highly pure, crystalline-free forms of silicon dioxide, SiO2, which may be produced as pyrogenic (also called fumed) silica, precipitated silica and silica gel. SAS products are marketed as dry white powders or dispersions. SAS may sometimes be surface-treated to render it hydrophobic, but these specialised products are produced at relatively low tonnage levels compared to the untreated material.
SAS such as precipitated silica, silica gels and pyrogenic (fumed) silica are produced by the members of SASSI/ASASP in a total volume of 500.000 tons per year.
Chemical structure:
SiO4 tetrahedron is the base unit of the structure of the macromolecular network nSiO2
Surface chemistry:
SAS, as produced by wet or thermal route, has silanol (Si-OH) groups on the surface that render untreated SAS hydrophilic.
SAS may be rendered hydrophobic by surface treatment. The two main categories of surface treated SAS are:
Alkylsilyl treated silica: SAS is treated with silanes and siloxanes resulting in unreactive alkyl groups at the surface to bring hydrophobic properties.
Functionalised alkylsilyl treated silica: functional organic groups are grafted onto SAS surface to improve the cross-linking between the inorganic filler and the organic matrix in a polymer, rubber or coating.