Polyethylene (PE) and polypropylene (PP), which account for two-thirds of the world’s plastics, have different chemical structures and thus cannot be repurposed together. 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.
Geoffrey Coates, a professor of chemistry and chemical biology at Cornell University and his team have collaborated with a group from the University of Minnesota 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. The work is published in Science.
By adding a miniscule 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 and can be easily remoulded, making it better for recycling.
In their test, two strips of plastic were welded together using different multi-block polymers as adhesives, and 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.
Not only does this tetrablock polymer show promise for improving recycling, 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,” James Eagan, a postdoctoral researcher in Coates’ group, is lead author of the paper 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.”