Compounding Ingredients

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 percent as a percentage on the weight.A technical vulcanisate is made up from the following ingredients.

1.     Base polymer or blend of polymers

2.     Cross linking agents

3.     Accelerators of cross linking agents

4.     Accelerator modifiers (activators and retarders)

5.     Antidegradants (antioxidants, antiozonants, inhibitors of metal catalyzed oxidation, protective waxes)

6.     Reinforcing fillers (black, mineral and organic)

7.     Processing aids (chemical peptisers for polymers, softeners, plasticizers, dispersing aids, tackifiers, factice and lubricants)

8.     Diluents (inert mineral fillers, organic materials, and extending oils)

9.     Colouring materials (organic and inorganic)

10.                        Specific additives (blowing agents, fungicides, fibrous materials)

Crosslinking agents (Vulcanising agents)

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 cross linking reaction and lower the sulphur 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.

Antidegradants

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

Fillers

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

Processing aids

These are chemicals added to the compound that improve the processability. Peptisers are used to increase the efficiency of mastication of rubbers. E.g.: Renacit IV (Zinc salt of pentachlorothiophenol). Factice are added to control die swell, improve surface quality and prevent distortion of shape, particularly in vulcanisation.

Colouring Materials-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.

Specific additives

Blowing agents: DNPT, Sodium Carbonate etc.

Flame retardants: Antimony trioxide, Zinc borate etc.

Antistatic agents: Quaternary ammonium salts, Ethylene oxide condensate etc.