A rubber compound is obtained by mixing a base polymer or crude mixture with a series of additives. The choice of the base polymer and the additives is closely linked to the type of properties to be achieved. The resulting product is a non vulcanized compound. The quantity of additives used varies for 20 to 130% as a percentage on the weight. The most common additives are:

Fillers

There are two types of fillers, reinforcing and non reinforcing fillers. Carbon black is commonly used as a reinforcing filler. This is also the reason why most rubbers are black. Calcium carbonate is an example of a non reinforcing filler.

Plasticizers

Besides fillers, plasticizers play the biggest quantitative role in building a rubber compound. The reasons for the use of plasticizers are:

Improvement of flow of the rubber during processing, improved filler dispersion, influence on the physical properties of the vulcanizate at low temperatures. Mineral oils and paraffins are widely used as a plasticizer.

Vulcanization chemicals

Vulcanization is the conversion of rubber molecules into a network by formation of crosslinks. Vulcanizing agents are necessary for the crosslink formation. These vulcanizing agents are mostly sulphur or peroxide and sometimes other special vulcanizing agents or high energy radiation. Since vulcanization is the process of converting the gum-elastic raw material into the rubber-elastic end product, the ultimate properties like hardness and elasticity depend on the course of the vulcanization.

Accelerators

Accelerating agents increase the rate of the crosslinking reaction and lower the sulpher content necessary to achieve optimum vulcanizate properties.

Activators

Like zinc-oxide and stearic acid. They activate the vulcanisation process and help the accelerators to achieve their full potential.

Anti degrading agents

These agents increase the resistance to attacks of ozone, UV light and oxygen.

Process aids

Chemicals which improve the processability.

Pigments

Organic and inorganic pigments are used to colour rubber compounds. The colour pigments are also considered inactive fillers. Only silica’s have a reinforcing effect. Silicone can be coloured easily without loss of properties.