Toward Closed-loop Circularity of Plastics by Chemical Upcycling

Abstract
Over 400 million tons of solid plastics are globally produced annually, but only ~9% of the plastics used in the United States are currently recycled domestically. That means nearly 90% of our waste plastic is buried in American landfills or incinerated at commercial facilities that emit climate-altering greenhouse gases and airborne toxins. A foundation of a circular economy is closed-loop recycling of waste plastic by reprocessing it into new products of high value, reducing the need for fossil fuels. No new raw materials are needed, and no unusable plastic is discarded. The waste plastic is deconstructed into components that can be recombined chemically to form new products of higher quality or value than the original-a process called upcycling that can sometimes be done by 3D printing (additive manufacturing).
To establish closed-loop circularity of plastics, an ORNL team has focused on upcycling commodity plastics, establishing low-carbon circular manufacturing, and deconstructing and upcycling of engineering plastics. The team’s new catalytic recycling processes have been guided by chemical design, neutron scattering, and high-performance computing. One of our approaches to enable polymer circularity is to upcycle commodity plastics to vitrimers, new recyclable and reprocess able crosslinked polymers, by the presence of dynamic exchangeable groups. In our study, we upcycled commodity thermoplastic elastomers to create exceptionally tough adhesives that widely exceed the adhesive strengths of commercial adhesives (Sci. Adv. 2021; 7: eabk2451). The incorporated dynamic boronic ester enabled reversible adhesion on many different surfaces, which allows debonding and rebonding with recyclability in a stark contrast with conventional single-use, unrecyclable, structural adhesives.
In another system, we have established closed-loop additive manufacturing of upcycled commodity plastics by upcycling acrylonitrile butadiene styrene (the high-quality ABS plastic used to make LEGOs) into a recyclable, robust, and adaptive ABS-vitrimer that is (re)printable via the most common additive manufacturing method called Fused Filament Fabrication (FFF) (Sci. Adv. 2022, 8, eabn6006). The full FFF-processing of ABS-vitrimer overcomes the major challenge of (re)printing crosslinked materials and produces stronger, tougher, solvent-resistant 3D objects that are directly reprint able and separatable from unsorted plastic waste. We also focused on developing robust vitrimer resins for carbon-fiber composite application, which enables facile reprocess ability and multi-cycle recyclability (Cell Reports Physical Science 2022, 3, 101036, and 2023, 4, 101695). Furthermore, we have developed highly efficient organocatalysts that enable low-energy and greener depolymerization pathways for condensation polymers (Mater. Horiz., 2023, 10, 3360-3368). The high efficiency of the organocatalyst enables efficient deconstruction of diverse mixed plastics, opening a new paradigm of mixed plastic recycling. The mixed plastic deconstruction technology led to a start-up company, RE-DU ( www.reduplasti... ). I will discuss our efforts on chemical upcycling of plastics in multiple projects (sponsored by DOE BES, EERE VTO, AMMTO, BTO, and LDRD) and speak on potential impacts of our technology on the local and U.S. economy.
Biographical Sketch
Dr. Tomonori Saito is a distinguished R&D staff member at Oak Ridge National Laboratory and a joint faculty associate professor (graduate advisor) at the University of Tennessee at Knoxville. A synthetic polymer chemist, he has extensive experience in synthesis and various applications of well-defined polymers. He currently leads various polymer science projects at ORNL including polymer upcycling, vitrimer composites, self-healing materials, building materials, polymer electrolytes for lithium-ion batteries, fuel cells, and flow batteries.
Before coming to ORNL in 2010 as a postdoctoral fellow in the Materials Science and Technology Division and joining the Chemical Sciences Division in 2012, Saito was a postdoctoral fellow at Penn State. He earned his doctorate from Virginia Tech and bachelor’s and master’s degrees from Waseda University in Japan.
He has published more than 130 peer-reviewed articles, received 14 issued patents, and won R&D 100 awards in 2012, 2016, 2019, and 2021. He has received several awards including recent two ORNL awards-one on Research Accomplishment and the other on Outstanding Scholarly Output-as well as Battelle’s 2023 ORNL Inventor of the Year.