Breakthrough could transform single-use plastics recycling

Plastics are ubiquitous in many facets of our lives, and the plastics industry is the third-largest manufacturing sector in the United States. But as plastics production develops rapidly, the long-term environmental challenges are globally recognized. Chemically resistant plastic products have extremely long lifetimes before completely decomposing — a single-use coffee pod can last 500 years in a landfill. Plastic waste accumulation has led to pollution that affects land, waterways and oceans; organisms are being harmed by entanglement or ingestion.

Microplastics (small pieces of plastic that retain their original carbon-carbon bond) pose a particular health risk, because of their minute sizes (less than 100 micrometers) and ability to move up the food chain and be ingested by humans. Exposure to microplastics has been associated with respiratory, reproductive, neurological and metabolic diseases, among others.

Yet at present, only around 9 percent of discarded plastics are recycled, approximately 16 percent are combusted for energy recovery, and 75 percent are sent to landfills. Current methods — mechanical, chemical and burning — are not ideal, and recycling single-use plastics remains a daunting scientific and economic challenge.

Mechanical recycling (shredding, heating and remolding) is fairly energy-efficient, but the mechanical properties of plastics degrade significantly after processing. Chemical processes require high temperatures and expensive catalysts, and they often need extensive separations. Burning reduces the value of the plastics, and emits toxic gases and CO2. Even emerging materials like biodegradable polymers degrade too slowly and are too costly.

We’ve got a better idea. Currently, breaking down the existing polymers in plastics is almost impossible, because the carbon-carbon bond connecting the monomers (the small molecular repeating units) is too strong. My research group has discovered a way to make the polymers with a slightly elongated (and therefore weakened) carbon-carbon bond. These polymers are stable under normal conditions and can be broken down via mild heating, providing both application utility, as well as a better recycling solution that advances material circularity and sustainability.

We developed a method to make the polymers in single-crystal form, enabling us to obtain the crystal structures, and to measure the bond length precisely (we want the bond to be slightly weak, but not too weak, so it can be stable in normal use). To create the polymers as single crystals, we crystalize the monomers first, then shine ultraviolet or visible light onto the monomer crystals to convert them into long-chain polymers in the solid state — in a process called topochemical polymerization.

The polymers used in this work are not new. What is new is that we took the polymers and figured out their single crystal structures and measured the carbon-carbon bond lengths. We identified a few cases in which the bonds were elongated, and then we examined and discovered their quantitative depolymerization for the first time.

These polymer crystals are not currently used in industry, but we expect our unique closed-loop recycling feature to draw significant interest, as we have identified a polymer that is compatible with standard industry manufacturing processes (including extrusion and 3D printing). The polymer, once melted, behaves similarly to polypropylene (PP). In principle, with minor modification, it also can be made into something similar to polymethylmethacrylate (PMMA) or acrylonitrile butadiene styrene (ABS) — materials widely used in glass, electronics, and LEGO® and like toys.

Extruded filaments of the novel chemically recyclable polymers with elongated carbon-carbon bonds (highlighted in red in the inset) developed by Purdue’s Letian Dou, the Charles Davidson Associate Professor of Chemical Engineering, and colleagues. (Purdue University photo/Xuyi Luo and Qixuan Hu)

In addition to experimental exploration, we collaborated with Brett Savoie, principal investigator in the Savoie Research Group in Purdue’s Davidson School of Chemical Engineering, to carry out simulations and modeling to understand the polymer physics. His group also uses molecular dynamics and machine learning to predict new structures that can lead to better performance. In addition, we worked with the Mei Group, led by Jianguo Mei in Purdue’s Department of Chemistry, to characterize the polymers we synthesized. We have filed for a patent through the Purdue Office of Technology Commercialization (OTC).

Our research group wants to cover as many applications as possible with these novel polymers — for example, food containers, forks, shampoo containers, medical supplies, water bottles, toys, and air and water purification filters. As we scale up production, we need to make sure the cost of the materials is low enough to be competitive; ideally, in the future, the materials will be used in higher volumes, enabling production costs to lessen. We also will continue searching for new chemistries and new materials with low costs and tunable properties.

I hope our concept will inspire other researchers to design many new materials for different applications. My dream is that, in the future, the majority of the plastics used in every aspect of our daily lives will be completely recyclable, so we will greatly reduce our consumption of petroleum feedstocks and emit almost no pollutants into the environment.

Letian Dou, PhD

Charles Davidson Associate Professor of Chemical Engineering

Davidson School of Chemical Engineering

College of Engineering

Purdue University

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