Polyethylene and polypropylene account for two-thirds of the world's plastics. But the polymers' popularity has an equally large downside. Because they have similar densities and physical properties, the polymers are difficult - and expensive - to separate when mechanically recycled together. What results is a weak, degraded material that really isn't good for anything.
Now, Cornell researchers have developed an inexpensive and potentially scalable approach that uses a commercially available peroxide to bind the polymers together, thereby creating a more useful, high-quality plastic recycling additive.
The findings were published May 19 in the Journal of the American Chemical Society. The co-lead authors were postdoctoral researcher Moritz Kränzlein and doctoral student Shilin Cui. The project was led by Geoffrey Coates, the Tisch University Professor of Chemistry and Chemical Biology in the College of Arts and Sciences, the paper's senior author.
In a way, the project is itself a recycled product. In 2017, Coates' lab worked with collaborators from the University of Minnesota to create a multiblock polymer that could combine polyethylene and polypropylene mixtures. The new material was an important scientific achievement, according to Coates, but it proved difficult to scale up. It was also too expensive to be practical.
"We had to go back to the drawing board and think of the Venn diagram of what material would work and what synthetic methods would be affordable," he said. "Why didn't we just do that in 2017? Trust me, if it was easy, somebody else would have done this long ago."
The key to developing their new polyolefin compatibilizer was to look for existing polymers that, with the right processing, could do the job, rather than creating a new one from scratch. Kränzlein began by experimenting with the materials that were already in the lab, always with a few parameters in mind.
"Every second meeting, when I was giving Geoff an update, his first question was, 'What's the cost?' There was always this line of price per pound that I shouldn't cross," Kränzlein said. "We tried to really keep that as a focus of this project. Instead of finding a very elegant or sophisticated solution, we tried to find a real-world solution that works."
A year and a half - and more than 200 experiments - later, the researchers settled on an organic alkyl peroxide that, when heated, essentially plucks hydrogen molecules off high-density polyethylene (HDPE) and isotactic polypropylene (iPP) so they can be grafted together and form a copolymer material that can be added to a mechanical recycling process for HDPE and iPP mixtures, restoring their properties.
The copolymer can basically be thought of as "plastic soap," Kränzlein said.
"When we have mixtures of commodity plastics like HDPE and iPP, those don't mix very well, and those phases separate from each other, which makes the material properties of those mixtures pretty bad," Kränzlein said. "This process adds in a third component that acts like a soap between those two phases, and that soap facilitates a better miscibility between the polymers, and that restores the physical properties of those mixtures, essentially.
"It took us a very, very long time playing around with all the different tuning points," he said, "until we finally achieved something that that we were happy with."
To better understand their compatibilizer's properties and the grafting reaction, the researchers turned to Brett Fors, the Frank and Robert Laughlin Professor of Physical Chemistry (A&S), and doctoral student Jenny Hu, who conducted rheological characterization studies on it.
"So we've got the material," Coates said. "The question is, now can you make larger amounts? We're doing grams-at-a-time scale. Ultimately, you'd want to be doing dozens of kilograms. We're confident it'll scale, but there's going to be a lot of work to get it there."
Coates is hopeful that the compatibilizer could also lead to the creation of new polymer alloys that leverage the respective strengths of different waste plastics. There would be no need for giant million-dollar plants: Just take some preexisting polymers and add the compatibilizer.
"You could make a whole kind of pallet of alloys that might have better properties than either one of the pure polymers alone, just like stainless steel," said Coates, who recently launched a startup that specializes in the compatibilizer technology through Cornell's Praxis Center for Venture Development. "The dream is, if you can make a really rigid polymer that's also really tough, then you can make packaging that uses less material, yet has the same sort of properties. That's one of the other big applications for this technology."
Co-authors include Fors, Hu and research associate Anne LaPointe.
The research was supported the Gerstner Family Foundation, with additional funding from the U.S. Department of Energy through Ames National Laboratory's Institute for Cooperative Upcycling of Plastics, an Energy Frontier Research Center.
