Plastic bags, bottles, and containers may soon get a second life, thanks to a new chemical process developed by scientists at the University of California, Berkeley. This innovation not only breaks down two of the most common plastics — polyethylene and polypropylene — but does so in a way that allows them to be transformed back into their original building blocks, ready for reuse in new plastic products.
The process, created by a team of chemists and chemical engineers, works by vaporizing polyethylene and polypropylene. These plastics are used in everything from single-use grocery bags to hard plastic containers. The breakthrough in this process involves breaking down the tough carbon-carbon bonds that hold these plastics together, converting them into simpler molecules known as monomers.
This advancement could prove pivotal in tackling one of the world’s most pressing environmental challenges: plastic waste. Traditional recycling methods often degrade plastic into lower-quality materials. So, a plastic bottle can only be recycled two or three times before it becomes unrecyclable. However, the new approach promises to recycle plastic in a way that retains its original value.
Essentially, plastics could become recyclable indefinitely, opening the door to a true circular economy for plastics.
A Path to Circularity
Polyethylene and polypropylene make up roughly two-thirds of all plastic waste. Yet, despite their abundance, recycling rates for these plastics remain dismally low, with only about 9% being effectively recycled. The rest winds up in landfills, gets incinerated, or is left to degrade into harmful microplastics that pollute ecosystems.
Recycling these plastics in a way that retains their value has been a longstanding challenge. But John Hartwig, a professor of chemistry at the University of California, Berkeley, believes their new catalytic process could help create a more circular economy for plastics.
“We have an enormous amount of polyethylene and polypropylene in everyday objects, from lunch bags to laundry soap bottles to milk jugs — so much of what’s around us is made of these polyolefins,” he said.
“What we can now do, in principle, is take those objects and bring them back to the starting monomer by chemical reactions we’ve devised that cleave the typically stable carbon-carbon bonds. By doing so, we’ve come closer than anyone to give the same kind of circularity to polyethylene and polypropylene that you have for polyesters in water bottles.”
Previously, Hartwig and his team had developed a method to break down polyethylene into smaller molecules. But it relied on expensive, short-lived catalysts. The new process uses more durable and cost-effective materials like tungsten and sodium. Improtantly, these catalysts can be reused, making the process more viable for large-scale applications.
“This combination of tungsten oxide on silica and sodium on alumina is like taking two different types of dirt and having them together disassemble the whole polymer chain into even higher yields of propene from ethylene and a combination of propene and isobutylene from polypropylene than we did with those more complex, expensive catalysts,” Hartwig said.
A Promising Process
The first catalyst of sodium on alumina breaks down long plastic chains. This leaves reactive ends that the second catalyst can repeatedly act on. Effectively, it dismantles the entire polymer into its core components — mainly propylene and isobutylene. Both of these are gases at room temperature and valuable commodities in the chemical industry, used to make new plastics and other products like high-octane fuel additives and polymers for cosmetics.
The second catalyst, made from tungsten oxide on silica, adds a carbon atom to the reactive ends of the plastic chains previously cut by the first catalyst. This step converts these reactive ends into propylene, a gas used to make new plastics. The process, known as olefin metathesis, repeats itself as the catalyst continues to break down the polymer chain into smaller and smaller components. Eventually, the entire plastic chain is efficiently converted into reusable monomers.
The team reports that it can convert nearly 90% of mixed polyethylene and polypropylene waste into reusable building blocks. When working with either plastic alone, the yields are even higher.
But the process isn’t without its challenges. Small amounts of impurities, such as additives or other types of plastics like PET and PVC, can reduce the efficiency. However, the current recycling infrastructure already sorts plastics into different categories, so this may not be a major hurdle.
Looking Toward the Future
Polyethylene and polypropylene are incredibly cheap and useful materials. And their widespread use means they aren’t going away anytime soon. In many ways, plastics are too durable for everyone’s good.
The Berkeley team’s research, published in the journal Science, offers a glimpse into a future where common plastics could be recycled into new materials rather than discarded. While challenges remain, particularly with mixed plastic waste, the process could lead to commercial-scale recycling plants that transform the way we think about plastic waste.
“One can argue that we should do away with all polyethylene and polypropylene and use only new circular materials. But the world’s not going to do that for decades and decades. Polyolefins are cheap, and they have good properties, so everybody uses them,” Hartwig said.
“People say if we could figure out a way to make them circular, it would be a big deal, and that’s what we’ve done. One can begin to imagine a commercial plant that would do this.”
