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Fillers

An important component of elastomers is fillers. A polymer compound cross-linked without the addition of fillers would not produce a viable polymer, and would be useless. This is because the polymer chains do not interact at normal temperatures, since they are chemically inert.

Reinforcing fillers

Only the addition of fillers provides the high level of elasticity. The addition of an active filler to improve the elastomer´s mechanical properties, such as modulus, tensile strength or elongation at break, is known as reinforcement. Reinforcing fillers are needed to fill the gaps between the polymers and support the network from within.

Pyrogenic (fumed) silicas are highly active fillers. The type, quantity and composition of the fumed silica can vary and this determines to a large extent the properties exhibited by the rubber or elastomer.

The excellent reinforcing effect of pyrogenic (fumed) silicas can be directly attributed to the large surface area of these particles. This exists as an external surface, and this is therefore readily available for a large number of interactions with polymer chains of the elastomer network.

Non-Reinforcing fillers

Non-reinforcing fillers are needed for bulking up silicone rubbers, e.g. when compounding. They are also added to attain specific properties, for reducing the price of compounds and for providing better resistance to certain chemicals like oil or gasoline.

Additives

The composition of a finished silicone rubber is far more complex than that of a basic silicone rubber primarily due to the additives used. Such additives are pigments and special additives.

Silicone rubbers are inherently transparent and can be made transparent to opaque in nearly all colours with help of various organic or inorganic pigments.

Special additives are incorporated into silicone rubbers to optimise the properties required. These can include additives for increasing flame retardancy, for stabilising against hot air, or improving chemical and solvent resistance. Oxidative degradation can be retarded by iron oxides.