Synthetic rubber is man made and was first produced by carrying out the polymerisation of dienes (alkenes containing 2 double bonds.) Below is the reaction of a synthetic rubber being polymerised.
H H H H
H2C CH2 CH2 H2C
Butadiene Poly(cis-1,3-butadiene)
There are many similarities between natural and synthetic rubber. Firstly it is evident that both contain cis molecules
2 .Discuss how the structures of natural and vulcanised rubber determine their properties and describe how vulcanising rubber leads to an improvement in its properties for use in car tires
3.Natural rubber has a thermoplastic nature and consequently it is not suitable for being used independently to construct a tyre. Therefore it is necessary that a tyre must be have specific chemicals and synthetic fibres added in order to improve the suitability of rubber being used as car tyres.
Composition, by weight, of a typical tyre rubber formation
The adding of each of these additives has many advantages. Firstly carbon black is added to increase the strength of the tyre and also to increase the abrasion, cut and tear resistance of a tyre, thus extending the lifetime of a tyre. Oils and resins are also used ad additives, and these carry three duties, firstly to improve process during manufacture, secondly to lower the cost and thirdly to improve wet traction in the finished tyre.
An anti ageing chemicals are specifically used to prevent tyre degradation as factors such as heat, weathering and exposure to ultraviolet light lower the life of a tyre.
Curing agents are used to cross-link polymer chains, in order to produce more rigid structures which limits movements between the polymer chains.
4.The disposal of scrap tyres is a growing problem throughout the world, and proposed legislation will prohibit the burying of whole or shredded tyres thus meaning that a new approach must be taken towards old tyres. One way to overcome this predicament and preserve non-renewable resources is through the method of pyrolosis. Pyrolosis is the degradation of the rubber of the tyre using heat in the absence of oxygen. Instead of burning, the tyres break down to form four products, oil, gas, solid carbon steel casing. During pyrolosis the long polymer chains of the rubber decompose at high temperatures to smaller hydrocarbon molecules.
The residual carbon obtained from the process can be upgraded to produce a high value product. This carbon is usually only used for low-grade use such as plastic pipes and plastic shoes. Commercial carbon is highly porous and in order to get activated carbon to the same level it must be processed.
One of the valuable products to be extracted form tyres is benzene. This can be converted into phenol using the cumene process, which happens in three stages.
Firstly, a mixture of Benzene vapour and propene is passed over a catalyst at 250 degrees C and at 30atm pressure. The product is 1-methylethylbenzene or cumene.
Then Cumene is oxidised in air give its peroxide.
The reaction is carried out at temperatures in the range 90 - 130ºC and pressures of 1-10 atmospheres. Careful control of acidity levels, temperature and pressure are vital as at higher temperatures, the hydroperoxide is unstable and can decompose violently.
Finally, hydroperoxide is mixed with dilute sulphuric acid at 60 - 70ºC, to produce both phenol and propanone (acetone) as products. The decomposition of cumene hydroperoxide to phenol and propanone is often described as a cleavage reaction.
Two useful products are therefore obtained, phenol and propanone
The overall process, from benzene to phenol, is:
Benzene + Cyclopropane + Oxygen Phenol + Propanone
C6H6 + C3H6 + O2 C6H5OH + CH3COCH3
Phenol is used to make thermosetting plastics such as electrical equipment, laminate flooring and worktops. Propanone is used as a solvent and is involved in making a vast range of materials such as Perspex.
Bibliography
[1] Article 1 – Getting Tyred With Chemistry, adapted from ‘Getting tyred with chemistry! The chemistry of cars and bicycle tyres’ by Chris Ferguson, Chemistry Review, Volume 11, Number 4, April 20002
[2] Article 2 – Black Magic? High Value Products From Scrap Tyres, adapted from ‘Black Magic? … High Value products from scrap tyres ‘ by Paul Williams, Chemistry Review, Volume 12, Number 2, November 2002
[3] Www.p2pays.org/ref/11/10504/html/biblio/html4/pyh3.htm
[4] Www.uyseg.org/greener_industry/pages/phenol/7PhenolProdMethods2.htm
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