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Discovery Could Revolutionize Plastics Recycling
July 12, 2017

The next question in our Jeopardy-style game of environmental issues is: What percentage of the 78 million tons of plastic used for packaging actually gets recycled and re-used in a similar way?

Geoffrey Coates, center, in his lab with James Eagan, a postdoctoral researcher in Coates' group and researcher Anne LaPointe. Photo courtesy of Cornell University.

According to the Ellen MacArthur Foundation the answer, sadly, is just 2 percent. Nearly a third is leaked into the environment, around 14 percent is used in incineration and/or energy recovery, and an astounding 40 percent winds up in landfills.

Which leads to the next question, why such abysmal statistics? One of the problems: Polyethylene (PE) and polypropylene (PP), which account for two-thirds of the world’s plastics, have different chemical structures and consequently cannot be recycled together into a single usable product. Or, at least, an efficient technology to meld these two materials into one hasn’t been available in the 60 years they’ve both been on the market.

However, based on information provided by Cornell University, that could change with a discovery by Geoffrey Coates, a Cornell professor of chemistry and chemical biology. In research funded by the Center for Sustainable Polymers, a National Science Foundation (NSF) Center for Chemical Innovation, Prof. Coates and his group have collaborated with a group from the University of Minnesota (UMN) led by Frank Bates, UMN professor of chemical engineering and materials science, to develop a multiblock polymer that, when added in small measure to a mix of the two otherwise incompatible materials, create a new and mechanically tough polymer.

In a paper, “Combining polyethylene and polypropylene: Enhanced performance with PE/iPP multiblock polymers,” published online recently in “Science,” James Eagan, a postdoctoral researcher in Prof. Coates’ group and lead author of the paper, details the two groups’ research.

By adding a minuscule amount of their tetrablock (four-block) polymer – with alternating polyethylene and polypropylene segments – the resultant material has strength superior to diblock (two-block) polymers they tested.

In their test, two strips of plastic were welded together using different multi-block polymers as adhesives, then mechanically pulled apart. While the welds made with diblock polymers failed relatively quickly, the weld made of the group’s tetrablock additive held so well that the plastic strips broke instead.

“People have done things like this before,” Prof. Coates noted, “but they’ll typically put 10 percent of a soft material, so you don’t get the nice plastic properties, you get something that’s not quite as good as the original material. What’s exciting about this [tetrablock polymer] is we can go to as low as 1 percent of our additive, and you get a plastic alloy that really has super-great properties.”

Not only does this tetrablock polymer show promise for improving recycling, Eagan said, it could spawn a whole new class of mechanically tough polymer blends.

“If you could make a milk jug with 30 percent less material because it’s mechanically better, think of the sustainability of that,” he said. “You’re using less plastic, less oil, you have less stuff to recycle, you have a lighter product that uses less fossil fuel to move it.”

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