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 rubber Easy to use for manufacturing

Good wear

Good lifetime of tyre

Low cost

Poor building properties

High heat build-up

Butyl rubber Good air retention

Good heat resistance High cost

Fig. 5 Table of the different types of rubber available for use in tyres with their advantages and disadvantages.

Natural rubber will always have a small percentage of impurities in it which will limit the range of properties available, but synthetic rubber can be made from a variety of monomers. This means monomers can be mixed in various ways to get a wider range of physical and chemical properties. These can be easier to control and give more desirable properties.

Carbon black is added to both natural and synthetic rubber to give them their black colour. Advantages to adding carbon black to rubber are an increase in the abrasion, cut and tear resistance, an increase in their tensile strength and an extended lifetime of the tyre. One of the biggest problems with the addition of carbon black is that it makes the processing of the rubber more complicated.

The discovery of vulcanisation was purely accidental. In 1839 Charles Goodyear accidentally heated rubber in the presence of sulphur, this produced a new type of rubber that didn't flow or become sticky at higher temperatures. The process of vulcanisation is also called cross-linking. The cross-links produce a more rigid structure limiting the movement of the polymer chains, this changes the properties of the rubber into a hard, durable material.

Fig. 6 Two diagrams of polymer chains, (a) one with no cross links and (b) one with sulphur cross-links

The process of cross-linking is slow but can be overcome with the addition of accelerators. All accelerators have a sulphur atom in their molecule that will initiate the reaction of the sulphur chains with the rubber (4). Accelerators act as catalysts and increase the rate of the cure. They speed up the addition of sulphur chains to the polymer molecules.

Additives Improvements

Oils - Plasticises the rubber, which means it becomes more flexible, allowing more carbon black to be incorporated It improves the process of manufacturing

Lowers the cost,

Improves traction in wet conditions in the tyre

Reduces the tendency of