Used rubber tires have been causing a pollution problem since they replaced the wagon wheel. They are bulky, do not break down on their own, and quickly cause landfills to fill up. On occasion, used tires catch fire and can burn for days or weeks, releasing harmful pollutants both into the atmosphere and into the groundwater.
Some efforts have been made to chop up used tires and reassemble them into a useful product. With glue or heat, chopped tires can be made into floor padding for gyms, machine shops and playgrounds. Chopped tires can be used as mulch or ground cover or they can be used to manufacture sandals, welcome mats and rubber speed bumps. The problem of removing steel belts from tires is difficult to solve, so tires with steel belts may not be suitable for recycling in these applications. Another problem is that making these products from recycled tires costs more than making them from virgin materials, and customers are suspicious that the recycled products may contain carcinogens and other hazardous substances. Therefore, the cost of recycling used tires and public suspicion of them remain issues to be resolved.
Another option for recycling used tires is heat-based pyrolysis. For example, U.S. Pat. No. 8,475,726 for “Reactor and apparatus for pyrolyzing waste, especially tyres” discloses heating used tires to cause them to pyrolize, allowing separation of oils, metal and carbon black. Decades of experimentation with such systems have shown them to be energy-inefficient, resulting in most used tires making their way to landfills. Such systems also have a problem with leaving hazardous materials, including heavy metals, in the recycled material.
An alternative method for recycling used tires is to break down the vulcanized chemical bonds through radio frequency pyrolysis. Such systems in theory hold academic promise but have not proven commercially viable.
Another method for recycling tires employs chemical devulcanization through the addition of diphenyl disulfide and heat to the tires. This system has proven to be dirty, expensive to implement, and creates a substantial pollution problem.
Photo devulcanization processing of used tires is another possibility. In this system, the recycler mixes a photoreactive material with used tires and exposes the photoreactive material to light. The light causes the photoreactive material to build up heat and pyrolize the tire. This method has not been commercially successful due to the problem of chopped used tires being opaque.
Therefore there is a need for a method for recycling used tires which is energy-efficient, does not cause environmental pollution, and deals with the issues of steel in the tires as well as hazardous waste that tire recycling can create.