Natural rubber is a polymer that is readily synthesised from specific plants
1) Natural rubber is a polymer that is readily synthesised from specific plants. All the chemical reactions have taken place in the plant, and the rubber is extracted, in the form of latex; a sticky substance. Latex, like synthetic rubber, has polymer chains, produced by means of additional polymerisation, which occurred inside the plant. However, natural rubber cannot be used by itself, to make tyres due to its thermoplastic nature. Thermoplastic means materials melting when reheated, as the polymer chains run by each other, as they are not held in place. Synthetic rubber has no such limitations, as they are manufactured and produced with desired thermoset properties, meaning that they will not melt when reheated. A key difference of natural rubber, to synthetic, is that it is restricted, as it can only come from select countries, but synthetic rubber is manmade.
Additional polymerisation is a chemical reaction, which involves the joining of monomers. In order to produce tyres, the monomers must undergo additional polymerisation. This is the main similarity for both rubber types. The major difference is that, the monomer’s identity changes for both natural and synthetic rubber. For natural rubber the monomer is isoprene (2-methylbuta-1, 3-diene), but for synthetic rubber, there are different types of sub units such as, butadiene, copolymers butadiene and phenylethene and 2-methylpropene, producing different qualities of tyres. Synthetic rubber is produced by polymerisation of the chosen monomer. This is because synthetic rubber is evolving to create the highest quality of rubber possible.
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Producing synthetic rubber from isoprene did not render desired results, since it produced a blend of cis and trans polymers, in contrast to the 98% cis polymers, created by the plants, useful for crystalline arrangements. The lab did polymerize butadiene but that to did not render desired results.
Therefore copolymerisation was introduced between butadiene and styrene. Copolymerisation is not present in natural rubber, explaining why natural rubber is not as strong as synthetic rubber. The -A-B-A-B-A- polymer formed is called SBR, another type of rubber:
Another chemical reaction to manufacture synthetic rubber is by using the monomer 2-methylpropene to produce butyl rubber.
2) The structure of natural rubbers are long polymer chains made by repeating monomers. In natural rubber, the majority of the polymers are arranged in cis formation; contributing to its very regular structure, and its ability to form crystalline regions when stretched and this requires a high energy input to break. However, its thermoplastic property is not desirable in tyres.
Vulcanised rubber is an upgrade of natural rubber, founded by Charles Goodyear accidentally, as it has an increased strength. By heating sulphur to the polymers, strong covalent bonds are made between polymer chains by sulphur bridges. This is cross-linking and it increases durability and wear of tyre, as the polymer chain movement is limited, as the chains can not slide past each other, due to the covalent links, disabling it to melt in the summer or to become brittle in winter. Therefore, vulcanising rubber improves overall tyre quality as it is harder than natural and also durable.
* Eliminates cracking
* Good wear
* Low heat build up
* Easy to produce
* Good wear
* Long life span
* Low cost
* Good air maintenance
* Good heat resistance
Adapted from: SALTERS Open Book Paper, Article 1 ‘Get tyred with chemistry’, Table 2 page 5
* Brittle tendency is reduced
* More flexible
* Enables more amounts of carbon black to be added
* Increases lifetime of the tyres, by increasing the overall strength
* Improves tyre manufacture
* Improves wet traction
* Low cost
* Improves durability
* Encourages good wear
“Around 2 million tonnes of tyres are scraped each year”. [4b]
4) A manner of recycling used tyres can be done by the means of pyrolysis, which is the decomposition of tyres using heat, but no oxygen. Pyrolysis is useful, in the sense of its products are valuable. The process produces carbon black, oil, gas and steel, all of which is in demand, for instance, the steel can be reused in tyres and the gases produced, are CH4, H2 and other hydrocarbons which are needed to run the pyrolysis process itself. This process is beneficial to the environment as it produces oil, which is used as fuel, and it is good to conserves this non- renewable energy source, as it is soon to run out.
As for the oil formed, it undergoes another catalytical conversion to be reformed into a new oil, heavy with benzene compounds. This oil is very much a feedstock- a raw material supply- to produce more useful compounds such as phenol. To add an -OH group to straight onto the benzene is expensive due to the requirements such as high temperature, but using the new oil, a longer yet cheaper method is used in order to form phenol. The benzene in the oil is used to form cumene, by reacting it with propene:
C6H6 + C3H6 C6H5CH(CH3)2
Next, the cumene is used to produce peroxide by thermal decomposition:
This is then decomposed more to give phenol and propanone:
There are many uses of phenol, and these include antiseptic properties and to construct thermosetting electrical equipment.
Lastly as the decomposition of tyres produces carbon black, used in ink and paint, but the more preferred applications of carbon black, which is achieved by converting it into activated carbon via acid wash:
C(S) + acid wash Activated carbon.
Activated carbon is a “highly porous carbonaceous with high surface area” [4b] and adsorbs any gas and able to eradicate any pollutants, due to its sulphur compounds left over from vulcanisation. This is extremely useful in industry, to remove mercury- a toxic metal. Normally, to clean up mercury would be expensive, but nowadays it is cheaper by using the result of tyre decomposition. Activated carbon can also be used as decolourizing liquid.
WORD COUNT: 995
1) Transition Metals Quantitive Kinetics and Applied Organic Chemistry, Brian Chapman, Nelson
2) A-Level Chemistry, E.N Ramsden, Stanley Thornes LTD
3) Advanced Chemistry For You, Lawrie Ryan, Stanley Thornes LTD
4) SALTERS Open Book Paper a) Article 1 ‘Get tyred with chemistry’, by Chris Ferguson b) Article 2 ‘Black magic?’ by Paul Williams.
5) Chemical Ideas, Salter’s Advanced Chemistry, Heinemann