Rubber: Natural Rubber, Synthetic Rubber and Vulcanization of Rubber

There is no doubt about the importance of rubber and its wide uses and applications. Rubber was initially made from rubber latex tapped from rubber tree by making incision on standing tree trunks. Rubber is primarily attractive for its elastic properties and its waterproof attributes that are essential for making so many products that have in one way or the other affected our lives. Small wonder, the need to improve on the quality of products made from rubber, to manufacture new products from rubber, to meet the ever increasing demand and to reduce the cost of production; have resulted to the synthesis of artificial rubber products. Small wonder, there are two types of Rubber: natural rubber and synthetic rubber. Both are elastomers with unique chemical features that are unravelled by the chemistry of polymers and polymerization.

There used to be ta ime when the bulk of rubber was made from natural rubber.

Natural rubber is the defining name of rubber as it is the rubber obtained from rubber latex tapped from rubber tree. The latex is made up of isoprene (2-methylbuta-1,3-diene) monomer unit with molecular formula, C₅H₈,and a structural formular CH₂=C(CH₃)CH=CH₂.

The polymerization product of rubber is a long polymeric chain with limited frequency of cross-links. This is what informed of further processing to form materials with larger applications. For example, the need to make inflatable system with strong tensile strength, resulted to the addition of sulphur and heating the rubber to convert it from a soft elastic substance to a hard material by a process known as vulcanization.

Although vulcanization confer a strong tensile strength on the rubber material, it also improve the elasticity and durability of the rubber so that it can maintain its status and withstand changes in temperature and pressure. This brings us to the question of the role of the sulphur atoms in the vulcanized rubber. The sulphur atoms form strong linkages that hold the long rubber chains together.

Synthetic rubber is recently responsible for the bulk of commercial rubber. It produced from crude oil so that the prices of both natural and synthetic rubber is affected by a change in the international market price of crude oil. The first known synthetic rubber was called neoprene poly(2-chlorobuta1,3-diene). There are other synthetic rubbere that includes thiokol, poly(2-methyl propene), poly(buta-1,3-diene) and styrenebutadiene rubber (SBR). But it SBR that fits into various applications so that it is the most useful rubber today.

SBR is formed by a process called copolymerization where styrene (phenylethene) is polymerized with three parts of aqueous buta-1,3-diene. Once formed, it is vulcanized with sulphur (about 3% mass of sulphur) where sulphur atoms are positioned (at the position of the double bonds) between adjacent polymeric chains to give an elastic and hard substance, with resistance to abrasion, so that SBR has become the rubber used for making tyres, footwear, e.t.c.

Rubber is made up of long polymers that can move independently when deformed. It is sticky to tourch, deforms when heated, and brittle when cool. Hence, the addition of sulphur (or other curatives) allows the sulphur atoms to link each of the chains to the other forming an array of cross-linkages that prevent each of the chain to move independently. This is what fortifies tyres of vehicles that deforms when stress is placed on them and restore to their original shapes when stress is removed. While tyre-making from vulcanized rubber is the most popular product of vulcanization, other products such as saxophone mouth piece and bowling ball are made from ebonite (also called vulcanite) which is a hard vulcanized rubber.