Americans discard about 250 million tires every year. Researchers have tried, with varying degrees of success, to break down old tires and recycle them into new ones, and they’ve tried adding ground-up tires to asphalt. Still, most tires end up in landfills. Now Kirk Manfredi, a chemist at the University of Northern Iowa, has come up with yet another way to dent this mountain of worn-out rubber: he produces lemon oil from it. Specifically, Manfredi produces limonene, the main ingredient in the oil of lemons and other citrus fruits.
When Manfredi feeds shredded tires into an airless reactor set at about 725 degrees, a thick, dark oil containing a chemical called polyisoprene seeps out. He then heats and vaporizes the polyisoprene, breaking it into individual isoprene molecules. The vaporized isoprenes, rather than reforming into the long, complex polyisoprene chains, naturally tend to bond together in simple pairs to form limonene. The chemical structure matches the oil from lemons, although the source is decidedly artificial.
Manfredi now gets about 2 percent of the weight of the tire as limonene, but he’s working on tweaking the procedure to up the yield. Temperature and pressure are really critical, he says. The size of the tire piece is very important as well; if the piece is too big, stuff that’s in the middle can’t get out into the gas phase as quickly. Manfredi thinks he will ultimately be able to convert about 10 percent of a tire into lemon oil.
He still has to work on the purification process--trace components, for example, now make the limonene smell pretty foul. Once he solves these problems, his tire juice may find its way into lemon-scented soaps and cleaners, and even as a flavoring in sodas. And Manfredi thinks he can isolate other useful compounds from tires as well, such as the main ingredient in bug-repelling citronella candles. He also hopes to find oils that could be used in chewing gum and mouthwash. Says Manfredi, You have to think big.