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Applications

The high price of fumed silicas rules them out of most volume polymer applications, and their main use is as a reinforcing filler for silicone rubber, including silicone sealants. Not unexpectedly, they perform well in this polymer, where they are much more effective than carbon blacks. They have the added advantage of matching the polymer refractive index and thus giving a transparent product.

Fumed silicas are difficult to disperse fully in the low-viscosity silicone elastomer gums and hence are mostly supplied premixed by the silicone rubber producer using special methods. Silicone elastomers are exceptionally weak unfilled, and published information on the effect of filler addition to unfilled gum silicone rubber is scarce

Table 8 Illustration of the effect of adding fumed silica to a gum silicone rubber (peroxide cure)

System

Tensile strength (MPa)

Elongation at break%

Gum rubber

0.4

130

With 35 phr. fumed silica

7.0

300

for this reason. Table 8 gives a brief summary of the results to be expected, based on a variety of sources.

Michel (2007) provides information on the use of fumed silica in a variety of other elastomers. This includes hydrogenated nitrile rubber, ethylene acrylate rubber, and fluoroelastomers.

Fumed silicas are able to produce a strong thixotropic effect in many polymer systems, including epoxies and PVC plastisols. This is discussed in more detail later. The thixotropic effect can be achieved at quite low loadings, making their use cost- effective, despite their relatively high price. It is particularly valuable in sealants and coatings, and these have become significant application areas. Not surprisingly, given their high price, they have come under pressure from other types of filler for this, including precipitated silicas and calcium carbonates.

Given the success of precipitated silica in tire treads, it is reasonable to ask whether this is an opportunity for fumed silica. In the absence of any extralarge benefit in rolling resistance, this is felt to be unlikely, due to the higher cost. It is true that the fumed silica will probably require significantly less of the expensive silane, but rough calculations show that, even if no silane is required, the raw material costs with fumed silica will exceed those of precipitated silica/high silane addition level combinations.

Safety

Fumed silicas are amorphous products like the precipitated ones, and the same comments regarding safety issues apply (see section “Safety”).

Sustainability

While the raw material (usually some form of crystalline silica) is extremely abundant, its extraction and conversion are an energy-intensive process. There is also currently little or no recovery or recycling of the material or its composites. Even so, the relatively low volumes involved do not make this of great concern at present.

Future Directions

The production costs of fumed silicas are relatively high for polymer applications, and this means that they are likely to remain niche products and prone to competition from lower-priced products. This has already become apparent in the thixotropic markets, where alternatives such as precipitated calcium carbonate have already made a significant impact. There is also significant effort in upgrading the lower-cost precipitated silicas for use in silicone elastomers.

 
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