Page 290 - Handbook of Plastics Technologies

ELASTOMERS

4.82 CHAPTER 4

FIGURE 4.28 Vulcanizate properties as a function of carbon black loading.

Silicates (e.g., calcium or aluminum silicates) are not as active as fillers as are the silicas.
Colloidal silicas can also be prepared by the so called pyrogenic process, wherein silicon
tetrachloride is hydrolyzed at high temperatures as follows:
SiCl + 2 H O → SiO + 4 HCl
4
2
2
This process produces very finely divided silicas, important as fillers for silicone rubbers.
All precipitated silicas and silicate fillers contain some water. Since the water content
can influence processing and vulcanizate properties, it is necessary to control the amount
of water present during processing and packaging.
As with carbon blacks, silica fillers are characterized on the basis of primary particle
size and specific area. The smallest observable single filler particles (primary) have diame-
ters of about 15 nm. The surface forces of the primary filler particles are so high that thou-
sands of them agglomerate to form extremely robust secondary particles that cannot be
broken apart. These secondary particles further agglomerate to form chain-like tertiary
structures, many of which can be more or less degraded by shear forces. Determination of
surface areas is done using the BET nitrogen absorption method.
As with carbon blacks, precipitated silicas are classified with respect to structure by the
degree of oil absorption. Typical values of oil absorption for various silicas are as follows:
• For very high structure silica, >200 ml/100g
• For high structure silica, 175 to 200 ml/g
• For medium structure silica, 125 to 175 ml/g

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