Researchers break down and rebuild PET and PP packs in new chemical process

The process effectively breaks down polymers into their basic chemical precursors, paving the way for a more sustainable approach to plastic waste. It is particularly adept at handling polyethylene, found in single-use plastic bags, and polypropylene, used in hard plastics such as microwavable dishes and luggage. This new method also efficiently degrades mixtures of these plastics.

Scaling up this process could revolutionize plastic recycling by converting waste back into monomers for new plastic production, significantly reducing the need for fossil fuels. Although polyethylene terephthalate (PET) was designed in the 1980s for recycling, its volume is small compared to polyethylene and polypropylene, which dominate the plastic waste stream.

John Hartwig, a UC Berkeley chemistry professor leading the research, explained, “We can now return polyethylene and polypropylene to their original monomers using chemical reactions that break stable carbon-carbon bonds. This brings us closer to achieving the same recycling efficiency for polyolefins as we have for polyesters in water bottles.”

Details of the catalytic process were published on August 29 in the journal Science. The new method replaces expensive, soluble metal catalysts with cost-effective solid ones that are commonly used in continuous flow processes. These solid catalysts enhance the scalability of the process.

Graduate student Richard J. “RJ” Conk, working with chemical engineer Alexis Bell, developed a catalyst using sodium on alumina, which efficiently cracks polyolefin chains, and tungsten oxide on silica, which reacts with ethylene to form propylene. This process, known as olefin metathesis, effectively converts the entire polymer chain into propylene or a mix of propylene and isobutylene, a compound used in making polymers and high-octane gasoline additives.

The tungsten catalyst proved even more effective than the sodium catalyst, offering high efficiency in breaking down polypropylene chains. “This combination of tungsten oxide and sodium on alumina outperforms more complex, expensive catalysts in yielding propylene and isobutylene,” Hartwig noted.

The new catalysts avoid the need to remove hydrogen to form carbon-carbon double bonds, a challenging feature of earlier processes. Instead, they break the long carbon chains more effectively, turning them into valuable monomers. The process achieved nearly 90% efficiency in converting polyethylene and polypropylene into gases at room temperature.

Despite challenges with contaminants like PET and PVC, which can reduce efficiency, the process remains promising due to existing recycling methods that separate different plastics.

Hartwig emphasized the importance of addressing the recycling of hard-to-recycle plastics. “While redesigning plastics for better recyclability is ideal, current methods can still make a significant impact. Our work shows the potential for commercial applications of this technology,” he said.

The research was supported by the Department of Energy (DE-AC02-05CH11231) and co-authored by UC Berkeley graduate students and Lawrence Berkeley National Laboratory researchers.