Kevin Whattam

Researchers at the U.S. Department of Energy's Pacific Northwest Laboratory (PNL), Richland, Wash., are developing a method that may cost-effectively find a use for the approximately 2.5 billion waste tires in the U.S.

PNL's laboratory-scale bioprocess reportedly will increase the number of tires that can be recycled by increasing the surface reactivity of ground tire rubber, which will make the processed material easier to mix with virgin rubber.

To accomplish this, PNL scientists put finely ground waste tire rubber into a bioreactor (see chart) that contains microorganisms. The microorganisms selectively attack the carbon-sulfur cross-links in the rubber, or "eat" some of the sulfur. This bioprocess reportedly changes the ground rubber from a nonreactive to a reactive filler; minimizes the damage that occurs to the polymer backbone of the rubber; and allows the rubber particles to maintain many of their original physical properties.

When the bioprocessing is complete, the rubber is rinsed and filtered to remove the microorganisms, and then is allowed to air dry. Once dry, the surface-modified rubber is mixed with virgin rubber. The mixture is cured, which chemically binds the waste tire particles to the new rubber. The resulting compounds have a significant recycle content and reportedly can be used to produce high-performance, moldable goods.

The process is performed at moderate temperatures under one pressure atmosphere and uses no hazardous compounds. Many of the components used in the process reportedly can be recycled.

Not only is PNL's process environmentally-friendly, it also may help to close the loop in tire manufacturing. For example, the surface-modified ground rubber from this technology may be suitable for reuse in manufacturing new and retread automobile and truck tires, one of the largest markets for rubber products.

Processing costs, including the cost of reducing the tire to a micron particle size, average between 55 and 70 cents per pound. This compares favorably to the cost of virgin rubber compounds, which can be more than $1 per pound. The project is funded through the U.S. Department of Energy/U.S. Air Force Memorandum of Understanding Program.

PNL has successfully demonstrated the rubber recycling technology in the laboratory and is planning two large-scale demonstration projects next year in different industrial environments, including a tire retread manufacturing facility and a waste tire processing facility. PNL is currently working with a large tire retreader and plans are under way to enter into a Cooperative Research and Development Agreement in the near future.

Other applications for waste tires include use in asphalt for paving and liner applications, in boat and dock bumpers or to produce low-performance moldable goods.