What do we mean by the word ‘Rubber’? In this Rubber Technology series we define it as ‘a material that can be stretched or compressed and when the force is removed, will return quickly to its original shape’ without permanent deformation. The main chemical building blocks of rubber are elastomers, or “elastic polymers.” These are large chainlike molecules, which when cured (vulcanized) form chemical crosslinks between the polymer chains. The ability of the chains to stretch and spring back is a measure of the rubber’s elasticity and resilience under load. The first common elastomer was Polyisoprene, from which natural rubber is made. In general, rubber materials are characterised by their low modulus of elasticity. They are resistant to water, alkalis and weak acids. Rubber is also a good electrical insulator, and can be used as a bonding agent. Types of Rubber There are two types of rubber: natural and synthetic. Natural rubber is produced from the latex (milky juice) of Hevea and Ficus tropical plants. It is, therefore, a renewable resource unlike synthetic rubber, which is manufactured from petrochemicals. Natural Rubber Natural rubber is harvested as latex – a natural source of isoprene, called caoutchouc or India rubber – which is then refined for commercial processing. This purified form of natural rubber is the chemical Polyisoprene. Although this has some elasticity, it is normally mixed with chemicals and vulcanized to produce the natural rubber. Natural rubber is both elastic and viscous making it an ideal polymer for dynamic and static engineering applications. Natural rubber properties include: Ease of processing Excellent dynamic performance with a low hysteresis loss Limited high temperature resistance (max. 70C) Good low temperature properties (-50C) Ability to bond strongly with metal parts High resistance to tear and abrasion, can be self-healing Poor resistance to sunlight, oxygen, ozone, solvents and oils.