821 The structure and properties of rubber

The outside carcass and tread of a tyre is made from a rubber compound that is a mix of several substances to produce a combination of properties necessary for the tyre to function effectively. Most metallic materials are derived from simple molecules held together by electrostatic bonds which sustain only a limited amount of stretch when subjected to tension (Fig. 8.19). Because of this, the material's elasticity may be restricted to something like 2% of its original length. Rubber itself may be either natural or synthetic in origin. In both cases the material consists of many thousands of long chain molecules all entangled together. When stretched, the giant rubber molecules begin to untangle themselves from their normal coiled state and in the process of straightening out, provide a considerable amount of extension which may be of the order of 300% of the material's original length. Thus it is not the electrostatic bonds being stretched

Fig. 8.18(a-d) Limiting reaction force circle
Fig. 8.19 Metal atomic lattice network

Unstrained state

Fig. 8.20 Raw rubber network of long chain molecules

Unstrained state

Fig. 8.20 Raw rubber network of long chain molecules

Unstrained stale

Fig. 8.21 Vulcanized rubber cross-linked network of long chain molecules

Unstrained stale

Fig. 8.21 Vulcanized rubber cross-linked network of long chain molecules but the uncoiling and aligning of the molecules in the direction of the forces pulling the material apart (Fig. 8.20). Consequently, when the tensile force is removed the molecules revert to their free state and thereby draw themselves into an entangled network again. Hence it is not the bonds being stretched but the uncoiling and aligning of the molecules in the direction of the force pulling the material apart.

Vulcanization To reduce the elasticity and to increase the strength of the rubber, that is to restrict the molecules sliding past each other when the substance is stretched, the rubber is mixed with a small amount of sulphur and then heated, usually under pressure. The chemical reaction produced is known either as curing or more commonly as vulcanization (named after Vulcan, the Roman god of fire). As a result, the sulphur molecules form a network of cross-links between some of the giant rubber molecules (Fig. 8.21). The outcome of the cross-linking between the entangled long chain molecules is that it makes it more difficult for these molecules to slip over each other so that the rubber becomes stronger with a considerable reduction in flexibility.

Initiators and accelerator To start off and speed up the vulcanization process, activators such as a metallic zinc oxide are used to initiate the reaction and an organic accelerator reduces the reaction time and temperature needed for the sulphur to produce a cross-link network.

Carbon black Vulcanized rubber does not have sufficient abrasive resistance and therefore its rate of wear as a tyre tread material would be very high. To improve the rubber's resistance against wear and tear about a quarter of a rubber compound content is made up of a very fine carbon powder known as carbon black. When it is heated to a molten state the carbon combines chemically with the rubber to produce a much harder and tougher wear resistant material.

Oil extension To assist in producing an even dispersion of the rubber compound ingredients and to make processing of the tyre shape easier, an emulsion of hydrocarbon oil is added (up to 8%) to the rubber latex to dilute or extend the rubber. This makes the rubber more plastic as opposed to elastic with the result that it becomes tougher, offers greater wear resistance and increases the rubber's hysteresis characteristics thereby improving its wet grip properties.

Anti-oxidants and -ozonates Other ingredients such as an anti-oxidant and anti-ozonate are added to preserve the desirable properties of the rubber compound over its service life. The addition of anti-oxidants and -ozonates (1 or 2 parts per 100 parts of rubber) prevents heat, light and particularly oxygen ageing the rubber and making it hard and brittle.

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