Lube Tune-Up: Motor oil from recycled plastic could improve automotive-fuel efficiency

Every year, the United States generates 25 million tons of plastic waste, but only about 1 million tons of it gets recycled. To make better use of this vast waste stream, chemists at Chevron and the University of Kentucky in Lexington have developed a technique for transforming plastic into high-performance lubricating oils that could boost the fuel efficiencies of vehicles.

“Automakers have been trying to get as much fuel efficiency out of their cars as they can,” says lead investigator Stephen Miller of ChevronTexaco Energy Technology Company in Richmond, Calif. “You can do that partially with better lubricating oils,” he says. For instance, less-viscous oils lower friction in the engine, thereby reducing fuel consumption.

Toward that goal, Chevron has developed a process that converts wax consisting of hydrocarbon molecules 20 to 50 carbon atoms long into low-viscosity, high-performance oils that last longer than existing engine oils do. “The question is, ‘Where is that wax going to come from?'” says Miller.

For now, chemists derive the wax from natural gas through a conversion process known as Fischer-Tropsch synthesis. Because natural gas is too expensive in the United States to make the gas-to-wax-to-oil sequence cost-effective, Chevron recently announced plans to build a plant in Qatar, where natural gas is less costly.

In the meantime, Miller and his colleagues have been searching for cheaper domestic sources of wax to bypass the Fischer-Tropsch step. The researchers recently turned to polyethylene, one of the major constituents of plastic waste. It’s found in products such as yogurt containers and grocery bags.

In the July 20 Energy & Fuels, Miller’s group describes a method for breaking down polyethylene to make wax with the right molecular properties for conversion into lubricating oil. First, the plastic is melted in a pot, and then it’s pumped into a furnace. There, heat breaks down the molecular chains of polyethylene, which are hundreds of carbon atoms long. The resulting wax is then subjected to a final catalytic process that alters the wax’s molecular structure, yielding oil that’s crystal clear and would flow well even in cold weather.

This plastic-derived oil is just as good as engine oil made from wax via the Fischer-Tropsch process, says Miller.

So far, the researchers have scaled up the method to a small pilot plant. Next, Chevron will have to consider the cost of collecting, sorting, and transporting large volumes of plastic as it assesses the economics of the technology, Miller says. “I think it’s potentially feasible,” he adds.

More challenging than technical issues, says Pete Dinger, director of technology at the American Plastics Council in Arlington, Va., is finding ways to get consumers to divert more of their plastic waste into the recycling pipeline.

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