Plastics have proven to be a revolutionary class of materials, that are durable, lightweight, water resistant and relatively inexpensive and easy to manufacture. It is hard to think of a modern piece of technology that doesn’t include some plastic components, from electronics to plastic bottles.
The same properties that make plastics useful are also behind the creation of a global waste crisis.
Plastics are polymers, long chains of molecules connecting together repeating links called monomers. By changing the chemical structure of the monomers, and the way in which they are connected together, the properties of the plastic can be engineered.
The majority of plastics are durable, which is great for an application (no one wants a leaky bottle of water), but not so great when you need to dispose of the application. Waste plastics can litter the environment for centuries. Larger pieces of plastic, such as bags, straws, bottles, packaging and many everyday objects, are called macroplastics. When present in the environment these pose a threat to wildlife through entanglement or consumption. The problem doesn’t stop there. Macroplastics can break down into smaller and smaller pieces, forming microplastics (<5 mm long), that are becoming increasingly ubiquitous, now found as pollutants in every part of our environment. While these plastics are being broken down into smaller pieces, on the chemical level the strong polymer chains remain strong, making them very hard to get rid of. It is estimated that on our current trajectory, by 2050 there could be more plastic in the ocean than fish (by weight; Ellen MacArthur Foundation, 2016).
Due to a combination of the strength of plastics and poor waste management systems, it is estimated that only 15 % of plastic waste is recycled. Most of the population operates on a linear economy model, a so-called “take-make-discard”, where raw materials are refined, used to manufacture products, which are then simply disposed of at the end of life. It is imperative that we move to a circular economy model for plastic use and recycling, and this work, by two teams of RASEI researchers, provides a step toward this transition.
Current plastic recycling methods are not up to the task. The most globally used method is called mechanical recycling. Plastics, which have been collected and separated, are mechanically broken down through a combination of chopping and grinding to produce a powder that is then melted and extruded into pellets ready for reheating and remolding. This process can only be done a few times. Melting can be destructive to the chemical structure and eventually, through repeated cycles, the plastic loses its strength.
RASEI Fellow Oana Luca and her team, in collaboration with the group of RASEI Fellow Seth Marder, have demonstrated an electrochemical approach to chemical recycling of polyethylene terephthalate (PET) plastics. PET is one of the most common plastics, used in clothing, food containers and bottles. In chemical recycling the work is done at the molecular level. By applying an electric current to a solution of the plastic and a redox catalyst (a molecule capable of capturing and then donating an electron), the polymer chain is broken in a selective manner to produce the original starting materials. Instead of getting a random mixture of chemical structures, as you would through heating in mechanical recycling, you get clean starting materials, which, after separation, can be used to produce new plastics. Using this technique there is no limit to the number of times plastics could be recycled. With the amount of plastic waste in the world, you would never have to use new starting materials!
While this approach holds great promise, there is a lot of work still to be done. The process shown in the lab could break down about 40 milligrams (about 1 /500 of a 16 Oz PET bottle), over a period of several hours.
“Although this is a great start, we believe that lots of work needs to be done to optimize the process as well as scale it up so it can eventually be applied on an industrial scale” said Phuc Pham, a doctoral student in the Luca Group.
The generality of this electrochemical approach offers an exciting opportunity. Different types of plastics could be put into the same chemical reactor and broken down into their respective starting materials, which could then be separated, which could significantly mitigate and streamline some of the waste collection and management issues.
“There are so many polymers and materials out there that people aren’t recycling at all. They’re not being collected. This is the beginning of many, many different kinds of chemistries” said Oana Luca.