Professor Aaron Sadow of Ames Laboratory in Iowa is director of the Institute for Cooperative Upcycling of Plastics (iCOUP). The Institute has developed a chemical process that produces valuable biodegradable chemicals from discarded plastics, which are then used as surfactants and detergents in a range of applications.
Tech Briefs: What is upcycling of plastics?
Professor Aaron Sadow: Basically, you have these materials, in this case plastics, that, once the lifetime for their intended application has been exhausted, you have to decide what to do with them. One approach might be to recycle them, which in principle could be done mechanically. That generally means you melt them down and then re-extrude them for the same application. This conceptually isn't that different from recycling aluminum or glass, or even paper. In most instances, though, the plastics that you get out are of lower quality or almost no value compared to the original material. So, that’s not a very robust approach to waste management.
Obviously, another option is to landfill them, but that's not getting any value out of them. Then there's the waste-to-energy kind of approach, where many polyolefin plastics can be combusted. They're just hydrocarbons, which are similar to fuels, only in solid form, so you could burn them to get heat or electricity, but that is relatively low value.
Our idea is instead of these low value or degraded value methods of addressing what to do with the waste, you could use chemistry and transform them into equal or better higher value materials — that's upcycling. So, instead of recycling where you get back the same material of equal quality, the idea is to produce a material of higher value. The reason for that is to motivate the collection and the processing and cleaning and separation and all the all the activities and processes that need to be associated with recycling. If you could pay for those by the higher value of the product that you're making, that would incentivize all those activities and our carbon balance, or materials balance, would be better. Then you could motivate people to do this rather than just throwing out their plastic bottles.
For example, to get $0.05 per glass bottle in many states, is a similar idea. In the case of bottles, collection is value mandated by the state. As a result, people will do the separation of glass or aluminum and take them to special collection facilities to get the value back. This is kind of the same idea, only the value would come from what we're making, rather than a deposit that you put down on the plastic.
Tech Briefs: So, the upcycling would have to be turned into an industrial process now?
Sadow: The goal would be to innovate new industrial processes that would allow these kinds of chemical transformations. It's a new way of thinking about waste.
I'm sure we could come up with other examples where people have used a product in a chemical conversion to add value. That's really all we're doing — just in this case we're doing it on something that was formerly identified as a waste product.
Tech Briefs: Could you, in not too complicated terms, describe the process?
Sadow: Let me try to do my best on that without going into technical details. The plastics that we're targeting are polyolefin-based polyethylene. An example is the high-density polyethylene that is used in gallon milk containers, in grocery bags, in a lot of packaging materials, because it's strong and has a high melting point.
Other polyethylenes include the low-density polyethylene used in bubble wrap. Linear low- density polyethylene is yet another variant in the structure of the polymer, but in the end they're all hydrocarbons, meaning it they're just composed of hydrogen and carbon — long carbon chains.
The processes we're looking at take those long chains and, using a catalyst, we've figured out how to break selected bonds holding those chains together so that the carbons are no longer extremely long chains of thousands of carbon atoms linked together in a long sequence, but instead 20 to 40 carbons linked together.
Really, the key is figuring out how to turn those very long chains into chains of carbons that can be used for other applications — chains 40 carbons long, plus or minus 10 or something like that might be a lubricant, for example. Or maybe even a larger range, maybe, maybe C25 – C55 chains with various structural features. But in the end, we would have hydrocarbons that could be used as lubricants, lubricating fluids, motor oil, etc.
One of the processes that's been developed by the team, in collaboration with Max Delferro at Argonne National Lab, Ken Poeppelmeier at Northwestern University, as well as Wenyu Huang and Frédéric Perras here in the Ames Lab, has been to understand how to take various kinds of polyethylenes, polyolefins, and make liquids or waxes that have lubricating properties. The way we do that is called hydrogenolysis or catalytic hydrogenolysis. That basically breaks selected carbon-carbon bonds and adds hydrogen to the carbons to make shorter chains.
If we broke all the carbon-carbon bonds, we would make methane. But we don't want to do that because methane has much lower value than lubricants and surfactants and other hydrocarbon molecules that are used as chemicals for all the applications of modern society.
Tech Briefs: Do you just mix chemicals together or do you have to add heat or pressure or some other input?
Sadow: We melt the plastic and add a catalyst. We've been mainly studying platinum nanoparticle-based catalysts, but there are a series of kinds of catalysts that you can use. That's really the basic research question: what's the nature of the catalyst? How does the catalyst function to do this kind of process in an efficient manner at the lowest temperature possible, under the mildest conditions, so that it's efficient and low cost?
So, we take the catalyst and melted polymer and then add hydrogen, somewhere around 100 psi or so. This mixture is allowed to react for varying amounts of time, somewhere between six hours and a few days. When you're done, if the catalyst has worked the way you'd like it to, you end up with waxy or liquidy-ish hydrocarbon products. We then analyze them by typical chemical analytical methods, such as gas chromatography, mass spectrometry, or NMR (nuclear magnetic resonance) to learn something about their chemical structure, etc.
Then, we study the lubricating properties of the products —there’s a group at Texas A&M headed by Ali Erdemir in one of our teams. Professor Erdemir is a tribologist who studies the lubricating properties of molecules. There are a lot of ASTM kinds of tests that help understand how the molecular structure relates to lubricating properties and how good the particular sample is, or what are the qualities of it and where did these lie.
Tech Briefs: What has been the scale of your experiments?
Sadow: In my lab and my collaborators labs, we do experiments on the scale of one gram to 20 grams of polymer. That typically makes enough material to analyze chemical properties effectively as well as the physical properties associated with, say, tribology. So, we have projects to try to understand the system, the process for scaling this up for making not a batch process, but a continuous process — integrating separations and isolation into the process to make to make it efficient and to understand how that goes.
Other collaborators at Argonne National Lab are performing techno-economic analysis and life cycle analysis of the overall processes and comparing them to conventional syntheses of related materials to understand whether these could be competitive on an open market. We want to do that research so that one could better assess whether the process could be valuable commercially, to convince companies or investors to think about pursuing it. I’m guessing that's years down the line.
Tech Briefs: Do you care to put an approximate number on the years?
Sadow: The traditional timeline has been expressed in terms of in terms of a decade or more.
Tech Briefs: Can the fact that you’re a government laboratory help move that along since recycling and the environment are high priorities these days?
Sadow: Yes, so as long as government agencies remain interested and concerned, it's true, there will be government support for getting the data that could motivate a private company to do manufacturing or plastic waste processing based on our discoveries or those of other government labs or universities.
Tech Briefs: What I'm thinking is that, to convince a private company to take this on, you'd have to show that the end products that you come up with are economically in line with other processes for creating those same end products. But on the other hand, there's an additional social profit gained in getting rid of all this stuff from landfills.
Sadow: I think you're exactly right. There’s environmental-based benefit, there’s presumably an economic benefit, there's a resource benefit. We take a large amount of our petroleum resources and make them into fuels and then 10% go to chemicals and probably something like half of that goes into plastic — that's a lot of petroleum. So, using that resource more efficiently is important for us for the future. Plastics upcycling is a great first step to thinking about how to make use of our petroleum resources in a circular kind of way. That's why I think there's really a lot of discussion about a circular economy for plastic.
Tech Briefs: If this is based on standard processes, how come no one thought of doing this before?
Sadow: It's not true that no one has thought about doing this before. I think for a long time there has been interest in doing chemical recycling or chemical upcycling, that is, conversion of plastics into new chemicals, and there has been a long interest in learning how to recycle plastics more efficiently than we currently do. I think what recently happened was China stopped accepting American plastic waste a few years ago and it again became a visible problem in the United States — I think that that's the big change.
Tech Briefs: That's interesting, how the scientific and the social and the economic are intertwined.
An edited version of this interview appeared in the October 2021 issue of Tech Briefs.