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Fumed Silica General
Fumed silica is a specialized, high-price, niche product, and its main polymer use is as a reinforcing filler for silicone elastomers. Like the precipitated products, it is amorphous and with a very small particle size. But it is less aggregated and has a more inert surface. This surface has a lower hydroxyl content and much less adsorbed water. Also, unlike the precipitated silicas, it is frequently sold in pre-coated form. In some cases, a trace of another metal oxide (usually alumina) is also incorporated, and the product is then known as a mixed oxide, although the silica content is still usually well over 90%.
This is carried out by “burning” silicon tetrachloride in a hydrogen/oxygen flame. The actual reaction is really hydrolysis rather than oxidation.
Table 6 Major fumed silica producers
There are two broad product types commercially available, hydrophilic and hydrophobic, with the latter being coated postproduction to reduce water absorption.
The starting raw material is sand, which is converted to silicon metal and then to silicon tetrachloride used for combustion. Silicon tetrachloride can be distilled to remove impurities. The combustion process has to be well controlled to produce the desired particle size and degree of aggregation (partial fusion of the particles to produce chain-like structures). The initial fumed product has to be collected and well washed to remove all but the last traces of the hydrogen chloride coproduct. It can be appreciated that the process is costly, and the economics very much depend on being able to recycle the hydrochloric acid (e.g., in making silicon tetrachloride on the same site). There is considerable synergy with the manufacture of silicones, and some companies produce both.
The main producers of fumed silica are given in Table 6.
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