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Chemists at the Department of Energy’s Oak Ridge National Laboratory have discovered a method to transform the polymers found in discarded plastics into new macromolecules with enhanced characteristics compared to the original materials. This technique, known as upcycling, could provide a solution to the approximately 450 million tons of plastic waste generated globally each year, with just 9% being recycled; the majority is either burned or ends up in landfills, oceans, and other areas.
The invention from ORNL has the potential to change how plastics affect the environment by modifying the building blocks of polymers, customizing their properties. By linking molecular subunits, these polymers can interconnect through their backbones and form cross-linked structures, leading to versatile plastics. The structure of these polymer chains dictates the strength, rigidity, and heat resistance of the resulting materials.
Molecular editing holds significant promise; it has been the foundation of two Nobel Prizes in Chemistry. The 2005 prize honored the developers of the metathesis reaction, which creates and breaks double bonds between carbon atoms, allowing for the interchange of subunits to form new molecules bound only by creativity. Similarly, the 2020 prize recognized the creators of CRISPR, a tool for editing DNA, which consists of biopolymers made from nucleotide subunits that encode life itself.
“What we have here is akin to CRISPR for polymers,” stated Jeffrey Foster from ORNL, who led a study published in the Journal of the American Chemical Society. “Instead of modifying gene strands, we focus on altering polymer chains. This is a departure from the typical approach of plastic recycling, which often involves just melting the material and hoping for a positive outcome.”
The ORNL team specifically edited common waste polymers that contribute greatly to plastic pollution. In their experiments, they utilized soft polybutadiene, commonly found in rubber tires, as well as tough acrylonitrile butadiene styrene, found in toys, keyboards, ventilation systems, protective gear, vehicle components, and kitchen appliances.
“This type of waste isn’t being recycled at all,” Foster remarked. “Our technology is tackling a major portion of this waste, meaning a notable benefit from conserving the mass and energy of materials that currently go to landfills.”
The initial step in their process involves dissolving the waste polymers to create drop-in additives for polymer synthesis. The researchers shredded synthetic or commercially sourced polybutadiene along with acrylonitrile butadiene styrene and soaked the materials in a solvent, dichloromethane, to facilitate a chemical reaction at a low temperature (40 degrees Celsius) for less than two hours.
A ruthenium catalyst aided in the polymerization process. This catalyst has been used in the industry to develop durable plastics and to convert biomass, like plant oils, into fuels and other valuable organic substances, showcasing its potential in chemical upcycling.
The molecular frameworks of the polymer’s backbone include functional groups, which are clusters of atoms that act as reactive sites for alterations. Notably, the double bonds between the carbon atoms heighten the likelihood of chemical reactions leading to polymerization. When a carbon ring opens at a double bond, it forms a polymer chain that extends as each functional polymer unit seamlessly integrates, ensuring material conservation. This additive also plays a role in regulating the synthesized material’s molecular weight, thus influencing its properties and performance.
If this approach to material synthesis can be applied to a wider array of industrially significant polymers, it could become a cost-effective method for reusing manufacturing materials that are currently only utilized in single products. The upcycled materials might be softer and more flexible than their predecessors or potentially easier to mold and harden into long-lasting thermoset products.
The researchers upcycled plastic waste by using two complementary processes. Both processes represent forms of metathesis, which refers to a rearrangement of constituents. The double bonds between carbon atoms break and re-establish, allowing polymer subunits to exchange positions.
The first method, called ring-opening metathesis polymerization, unzips carbon rings and elongates them into chains, while the second method, cross metathesis, integrates polymer subunits from one polymer chain into another.
Conventional recycling often overlooks the value within discarded plastics because it reuses polymers that degrade in quality with every melting and reuse cycle. In contrast, ORNL’s pioneering upcycling method leverages existing building blocks to enhance the mass and characteristics of waste materials, adding extra functionality and worth.
“Our new process boasts high atom economy,” Foster noted. “This means we can effectively retrieve nearly all the material we input.”
The ORNL team demonstrated that their process, which requires less energy and emits fewer pollutants than traditional recycling methods, integrates waste materials efficiently without sacrificing the quality of the polymers. Foster, alongside Ilja Popovs and Tomonori Saito, conceptualized the main ideas for the paper, while Nicholas Galan, Isaiah Dishner, and Foster worked on synthesizing monomer subunits and refining their polymerization. Joshua Damron analyzed reaction kinetics using nuclear magnetic resonance spectroscopy, and Jackie Zheng, Chao Guan, and Anisur Rahman assessed the mechanical and thermal properties of the final products.
“We envision that this concept could extend to any polymer that has a functional group in its backbone that can react,” said Foster. If this approach is scaled up and adapted to utilize other additives, it could enable broader classes of waste to be transformed into molecular building blocks, significantly decreasing the environmental effects caused by plastics that are difficult to process. This could bring the idea of a circular economy—repurposing waste instead of discarding it—closer to reality.
In future studies, the researchers aim to alter the types of subunits within the polymer chains and rearrange them to explore the creation of high-performance thermoset materials such as epoxy resins, vulcanized rubber, polyurethane, and silicone. Once cured, thermoset materials’ molecular structures become cross-linked, making them impossible to remelt or reshape, which complicates recycling.
The team is also keen on improving solvents for environmental sustainability in industrial applications.
“Some preprocessing will be necessary for these waste plastics, and we’re still figuring that out,” Foster explained.
This research was funded by the DOE Office of Science (Materials Science and Engineering program) and the ORNL Laboratory Directed Research and Development program.
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