Car tires, replacement hip joints, bowling balls - these and other items are made from a class of plastics called thermosets, known for extreme durability.
Their "crosslinked polymer" chemical structure guarantees longevity but also makes these petrochemical-based materials - which comprise 15%-20% of all polymers produced - impossible to recycle.
"Currently, zero percent of the world's thermoset materials are recycled - they're either incinerated or thrown in landfills," said Brett Fors, professor of chemistry and chemical biology in the College of Arts and Sciences.
The Fors lab has addressed that environmental challenge by creating an alternative made from a bio-sourced material that has crosslinked thermosets' durability and malleability but can be easily recycled and degraded.
"The whole process, from creating to reusing, is more environmentally friendly than current materials," said Reagan Dreiling, a doctoral student in the field of chemistry and first author of "Degradable Thermosets via Orthogonal Polymerizations of a Single Monomer," which published Jan. 29 in Nature.
According to Fors, the corresponding author, it's also remarkable chemistry that initiates two polymerization processes from one monomer base.
The Fors group studies dihydrofuran (DHF), a monomer - or chemical building block - that can be made from biological materials and has the potential to eventually compete with petroleum-based feedstocks.
Dreiling used DHF, a circular monomer with a double bond, as a building block for two successive polymerizations, the second of which results in a crosslinked polymer.
The first polymerization process involves opening the circular monomer and stitching many of them together, creating a long, open chain. Flexible and soft, the resulting material can be completely chemically recycled using heat and degraded by acid, Dreiling said.
Not all DHF is consumed in the first polymerization though, and the remaining DHF is crucial for toughening the material in the subsequent polymerization, Dreiling said. In the second step, DHF monomers are connected to each other and to the first polymer while keeping their rigid, circular structures intact, yielding a material that is strong and tough. This final crosslinked polymer can be recycled through heating and will degrade naturally in the environment.
The first polymer, the flexible one, retains a double bond that is needed to make the second, in a reaction initiated and controlled by light.
"It's so easy," Dreiling said. "Just by changing the amount of time you run each reaction for, the amount of catalyst you put in each reaction, and the intensity of light you use, you can get a wide scope of properties through a simple process."
The more light that reaches the material, Fors said, the more crosslinking and the harder the material. The parts that have received less light will stretch more; the parts of the material not hit by light are completely chemically recyclable.
DHF thermosets show comparable properties to commercial thermosets, including high-density polyurethane (used in electronics instruments, packaging and footwear, for example) and ethylene propylene rubber (used in garden hoses and automotive weatherstripping).
In contrast to current petrochemical thermosets, the DHF-based materials offer a circular economy of use, Fors said. Chemically recyclable, the material can be made back into its building block monomer and used again from scratch. And when some of the material inevitably leaks into the environment, these materials will degrade over time into benign components.
The researchers are working toward applications, including making the DHF-based material useful for 3D printing. They are also experimenting to expand the properties with additional monomers.
"We've spent 100 years trying to make polymers that last forever, and we've realized that's not actually a good thing," Fors said. "Now we're making polymers that don't last forever, that can environmentally degrade."
The study was supported by the National Science Foundation (NSF) and the NSF Graduate Research Fellowship Program, and made use of the Cornell NMR Facility and the Cornell Center for Materials Research, supported by the NSF's Materials Research Science and Engineering Centers program.
Kathleen Huynh, an NSF Research Experience for Undergraduates student from San Jose State University, contributed to the study during a summer visit funded by the Center for Sustainable Polymers.
Kate Blackwood is a writer for the College of Arts and Sciences.
