There is currently no efficient technology that can repurpose a plastics waste stream containing polyethylene (PE) and polypropylene (PP), these two polymers, which account for two-thirds of the world's plastics, are too different. Now, Geoffrey Coates and his colleagues at Cornell University, Ithaca, New York, have collaborated with the Bates group from the University of Minnesota, Minneapolis, USA, to develop a multiblock polymer additive to remedy this situation. A small amount of their additive used in a novel process can bring PE and PP together at last to create a mechanically tough recycled polymer and avoid the need for a costly separation to recycle the two separately. [G Coates et al., Science (2017); DOI: 10.1126/science.aah5744]
Fundamentally, despite being hydrocarbons PE and PP are immiscible. The common grades of PE and isotactic PP in packaging and countless other products do not adhere or blend and so represented an intractable recycling problem. Coates and postdoctoral researcher James Eagan working with Anne LaPointe and former Cornell visiting scientist Rocco DiGirolamo think they may have unraveled an answer ending years of efforts on the part of polymer scientists around the globe. Just a dash of their tetrablock polymer, which contains alternating PE and PP segments, is sufficient to allow the materials to blend.
The team chemically welded together two strips of plastic using different multi-block polymers as adhesives. Mechanical stress tests failed with low molecular weight diblock polymer welds relatively quickly but the group's tetrablock additive made a composite stronger than the individual plastic strips, which themselves broke under stress.
"People have done things like this before," Coates concedes, "but they'll typically use 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," he explains. "What's exciting about this," he adds, "is that 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 adds, 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," Eagan enthuses. "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."
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".