Natural rubber and synthetic rubbers are important materials for a variety of engineering applications such as tires, medical gloves,rubber fender,rubber pads, gaskets in automobile and machine belts. Natural rubber has excellent dynamic properties with a low hysteresis loss and has high tear and abrasion resistance. Moreover, it can be applied in the presence of low temperature since its glass transition temperature is very low ca. -70°C. However, natural rubber deteriorates when exposed to sunlight, ozone and oxygen. Its applications are also limited because of its low resistance of oil and solvents. Since the quality of natural rubber is not possible to be controlled, synthetic rubbers have been developed to replace natural rubber. Synthetics rubbers such as styrene butadiene rubber (SBR), butadiene rubber (BR), chloroprene rubber (CR), nitrile butadiene rubber (NBR), isobutylene isoprene (butyl) rubber (IIR), ethylene propylene rubber (EPDM or EPR), silicon rubber and chloroprene rubber are synthesized from polymerization. Each rubber has the specific functional group to improve some physical and chemical properties for specified applications. However, these rubbers are more expensive than natural rubber. To achieve the desired properties and increase the value of rubbers, the reinforcement and chemical modification can improve the general properties of rubbers in order to be used in a wider range of applications.
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.
The following list indicates some of the most important synthetic rubbers along with their properties in comparison with natural rubber.
- Styrene butadiene rubber (SBR) – General purpose rubber made up of different types; better abrasion resistance, lower elasticity, poorer low-temperature behavior, better heat and aging resistance, excellent electrical insulation material similar to rubber
- Polybutadiene rubber (BR) – Poor processing properties mean that BR is not used on its own, blended with SBR or NR, abrasion-resistant, good elasticity, flexible at low temperatures.
- Isoprene rubber (IR) – Properties largely comparable with natural rubber, more uniform, cleaner, transparent.
- Acrylonitrile butadiene rubber (NBR) – Oil and fuel resistant, good heat distortion temperature properties, abrasion resistant.
- Chloroprene rubber (CR) – Flame retardant, resistant to grease, oil, weathering and aging, abrasion resistant.
- Butyl rubber (IIR) – Low permeability to gases, resistant to aging, ozone and chemicals, good mechanical properties, abrasion resistant, good electrical insulation properties.
The mechanical properties are improved by adding fillers such as carbon black during vulcanization with sulfur. Temperature resistance, abrasion resistance, aging resistance, resistance to oxygen and chemicals such as acids and petrol are properties which are improved in this way.
Vulcanization of Rubber
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.