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The product is an extremely fine powder, containing very small primary particles of silica, fused together into larger aggregates. Particle size and aggregate structure obtainable for both precipitated and fumed silicas are broadly similar. The specific gravity of the fumed product is slightly higher (2.1-2.2) but still low compared with other minerals. The main differences are in purity, water content, surface hydroxyl content, and surface pH. These differences are summarized in the Table 7.
The origin of the low pH of the fumed silica has been the subject of much debate. It is now believed that this is due to the strained silicon to oxygen bonds at the surface.
As mentioned above, there are both hydrophilic (uncoated) and hydrophobic fumed silicas (coated). The hydrophobic ones have various types of surface treatment applied. Where simple hydrophobization is required, then methyl groups are frequently used. These can be introduced by reaction with substances such as dimethyldichlorosilane or hexamethyldisilazane. Silicone fluids can also be used. Organo-silane coupling agents are used when elastomer reinforcement is required, with the principal ones being the vinyl and methacryloxy types.
There are a large number of these hydrophobic products with properties such as particle size and level and type of coating adjusted to suit particular applications (the Evonik web site lists 12 different products). The coated versions are considerably
Table 7 Typical properties for precipitated and fumed silica
more expensive than the uncoated and probably have no more than about 25% of the market.
The general trends in polymer applications with particle size and surface treatment have been summarized by one of the producers (Cabot) as:
Increasing specific surface area leads to:
Increasing surface treatment level (standard non-coupling type) leads to:
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