LONDON-Incineration with energy recovery is the most promising way to dispose of large volumes of scrap rubber, according to a report from Rubber Consultants. In tires, however, retreading is ``the best method of reducing environmental damage from discarded tires.'' The re-use of tire buffings in recycled products comes close behind.
The economic viability of other methods of recycling is almost always questionable in a free economy, but may be viable with government subsidies or in closed economies, concluded the study.
The report said that, with a few notable exceptions, the deliberate crumbing of tires for use in low-value products is unlikely to be economically justifiable.
Because tires-especially steel reinforced tires-are designed to be both tough and highly resistant to cuts and abrasion, crumbing processes are energy-intensive and relatively expensive ways of creating low-value raw material for use in low-value products.
Cryogenic grinding, where liquid nitrogen is used to cool the rubber to the point of brittleness, is particularly expensive, and should not be considered for general purpose crumbing, unless there are particular circumstances which may make it a sensible choice.
Tire buffings, on the other hand, are created as a by-product of the retreading process. Sale of these buffings improves the cost-efficiency of the retreading process and makes high quality, uncontaminated rubber granules available to a limited aftermarket.
The expense of crumbing tires also may jeopardize the economics of using tire granulate for modifying bitumen in road surfaces. Further, the technical performance of this type of asphalt compared with conventional road surfaces is questionable, according to the report.
There are two reasons why rubber goods are so difficult to recycle: the irreversible reaction which takes place during curing of the rubber component and the fact that few products are made from 100-percent rubber.
Even apparently pure rubber articles such as gloves or adhesives contain chemical additives and modifiers, and most rubber products use significant amounts of plastic and metal in the form of reinforcement or inserts. These non-rubber materials make the recycling process more difficult.
The report highlights the growth in use of thermoplastic elastomers, but concludes that the optimum recycling route for used TPE components is unlikely to be significantly different from the route for other elastomers: incineration with energy recovery.