The Production and Recycling of Car Tyres

Natural rubber is most commonly obtained from the Hevea Braziliensis tree, from South America. The rubber is taken as latex, a colloidal dispersion of nanoscale rubber particles in water (1). In the early 1900s C.D. Harris showed that natural rubber consisted of repeating units, and it was later proven that the units were joined in long chains. Rubber can be made synthetically by joining isoprene units.

Isoprene Poly(isoprene)

(2-methylbuta-1,3-diene) Natural Rubber

Fig. 1 The structure of natural rubber and the monomer isoprene.

Natural rubber has a 98% cis configuration across the double bond. This arrangement gives rubber its property of increasing strength when stretched. This is by allowing the chains to line up and form crystalline regions. In the addition polymerisation of natural rubber, the two double bonds in isoprene open up to form a polymer that contains only one double bond (2). One problem with natural rubber, which prevents it from being used on its own in tyres, is its thermoplastic nature. This problem can be solved by curing the rubber by heating it with sulphur, described later as vulcanisation.

It is impossible to recreate exact conditions for natural rubber synthesis in the laboratory. So when synthetic rubber was first produced, by the polymerisation of dienes, the products were of a poor quality. A better quality rubber was obtained by the polymerisation of butadiene to form poly(butadiene).

Butadiene Poly(cis-1,3-butadiene)

Fig. 2 Structures of butadiene to form poly(butadiene).

One of the best synthetic rubbers is a copolymer. This is obtained by combining a diene and an alkene. To create this specific copolymer, styrene-butadiene rubber (SBR), the polymerisation of butadiene with phenylethene (styrene) takes place in a process called emulsion polymerisation.

H2C=CHCH=CH2

Butadiene

Phenylethene (styrene)

Fig. 3 Below are structures of the monomeric building blocks.

This process involves polymerising butadiene and styrene in water with a surfactant to give a colloidal dispersion of the polymer in water (3). It is usually done at a temperature of 5C to give the desired properties.

Another variety of synthetic rubber is known as butyl rubber and is produced by polymerising 2-methylpropene. The polymer in this case is a saturated hydrocarbon.

Fig. 4 Structure of the saturated hydrocarbon

Rubber type Advantages Disadvantages

Natural rubber High strength

Good adhesion during tyre building

High resilience

Low heat build-up High cost

Thermoplasticity

Butadiene rubber Eliminates cracking

Good wear

Low heat build-up Poor wet traction

Styrene-butadiene