As worldwide production of tyres totals over 800m a year, the potential for recycling the millions of old tyres that are replaced is clearly very considerable. The process of vulcanisation enables the manufacture of high quality tyres that are very strong and resistant to wear, and in technical terms cross-links the hydrocarbon chains that form the rubber to construct an elastic three-dimensional network. The chemically combined sulphur resulting from this process cannot be removed, and attempts to reverse the change in structure brought about by vulcanisation have proved difficult, expensive and energy intensive. Research and development continues, and various techniques exist which show promise of increasing the amount of recycled material that can be employed in the manufacture of new tyres.
A tyre consists of a compound of several materials of which rubber constitutes between 46% and 48%, carbon black 25% to 28%, steel 10% to 12%, oil and vulcanising agents 10% to 12% and textiles 3% to 6%. One type of rubber is used in carcass and another in tread. The elasticity of a tyre case which is so important to its efficient use on a vehicle, presents difficulties in the mechanical processes that will be employed to separate the steel and textile reinforcement from the rubber. Cryogenic treatment has been used to embrittle tyre cases and thus greatly simplify size-reduction and separation.
Rubber powders and granules are used in surfacing children's playgrounds and sports areas where resilience is required. They also find outlets in the manufacture of mats, soles and heels for shoes, to make solid tyres, vehicle fenders, bicycle pedals, tips for walking sticks and crutches. Powdered secondary rubber is prepared for specific uses in the rubber industry, and offers promise as an additive in road surfacing where its elasticity and noise-reduction effect has positive benefits.
Production of rubber powder from used tyre sources has been shown in some cases to modify the characteristics of vulcanisation, and thus widen the range of potential uses, including compounds for the manufacture of new tyres. Development work continues, but resistance to greater use of secondary material by some tyre manufacturers and difficulty in securing sufficient alternative market outlets constitute a handicap in pursuing recycling on a large scale.
Tyres can be treated by pyrolysis, a process of decomposition by heat, which produces oil and other substances, including non-recyclable residues. This route may offer greater scope in the future, but increasing interest is being shown in employing old tyres as a fuel to produce energy, for their calorific value is comparable with that of oil and higher than that of coal. Volume is reduced, steel reinforcement can be recovered as secondary furnace feed, and the remaining product is sterile. Emissions of harmful substances can be controlled to very low levels.
Recycling of rubber, such as that used for tyre inner tubes and conveyor belting, is carried out throughout the world and is a relatively small but well-established business. Used tyres, however, represent a resource that has yet to be tapped on a mass scale, and as public policy is directed increasingly to halting the landfilling of tyre cases, recovery either as an industrial material or for energy values will undoubtedly increase.