New Polymer Additive Could Revolutionize Plastic Recycling

Novel Polymer Additive Could Transform Plastic Recycling

Geoffrey Coates, a professor of chemistry and chemical biology at Cornell University, typically opens his presentations on plastics and recycling with this question: "Of the 78 million tons of plastic packaging, what percentage of that packaging—such as 2-liter bottles or packaging boxes—is recycled and used in a similar manner?" The answer is just 2 percent. Unfortunately, nearly one-third of waste leaks into the environment, approximately 14 percent goes to incineration and/or energy recovery, while 40 percent remains in landfills and dumpsites.

The most significant problem is that polyethylene (PE) and polypropylene (PP), which make up two-thirds of global plastics, have different chemical structures and therefore could not be recycled together—or at least, no effective technology for collecting these two materials together has existed in the 60 years both have been on the market. This situation may change with a discovery made in Prof. Coates' laboratory. He and his group, in collaboration with a team from the University of Minnesota, developed a multiblock polymer. When small amounts of a blend were added to two normally incompatible materials, it created a new and mechanically tough polymer. The work of the two groups is detailed in an article published online in Science on 23 February titled "Combining polyethylene and polypropylene: Enhanced performance with PE/iPP multiblock polymers." Among the article's authors from Coates' group are postdoctoral researcher James Eagan, researcher Anne LaPointe, and scientist Rocco DiGirolamo. For years, other scientists had attempted to develop a polymer that would achieve the results obtained by Coates, LaPointe and Eagan. The material obtained by adding small amounts of tetrablock (four-block) polymer containing alternating polyethylene and polypropylene segments showed superior strength compared to diblock (two-block) polymers they tested. In their tests, two plastic strips were welded together using different multiblock polymers as adhesive, then mechanically pulled. While welds made with diblock polymers failed immediately, the weld made from the group's tetrablock additive held so well that the plastic strips broke. "People have done things like this before," said Coates, "but if you put in 10 percent of a soft material, you don't get nice plastic properties; you get something that is not as good as the original material. The exciting thing here was 'can we go down to as little as 1 percent of the additive' and we really did get a plastic blend with excellent properties." This tetrablock polymer is extremely important not only for improving recycling but also because it could enable an entirely new class of mechanically tough polymer blends. "Since it's mechanically better, if you could make a milk jug with 30 percent less material, think about the sustainability implications," he said, adding "You're using less plastic, less oil, which means less material needing to be recycled. Because it's lighter, transporting it actually uses less fossil fuel and reduces the carbon footprint." The Center for Sustainable Polymers, a Chemical Innovation Hub of the National Science Foundation (NSF), contributed to this project by providing financial support for the collaboration between Coates' group and the group led by Frank Bates, professor of chemical engineering and materials science at the University of Minnesota.