Researchers at the University of Berkeley have developed a catalytic process capable of recycling polyethylene and polypropylene, two polymers making up two-thirds of the world’s plastic waste. It could open the way to a true circular economy for these plastics, but its industrialization risks encountering several obstacles.
Polyethylene and polypropylene are polymers classified in the polyolefin category and which constitute two-thirds of post-consumer plastic waste worldwide. The first is found in most single-use plastic bags and the second is used to manufacture, among other things, parts for the automobile industry (bumpers) or components for household appliances. Around 80% of these plastics end up in landfills, incinerated or simply thrown into the streets, often ending up as microplastics in waterways and oceans. The rest is recycled into low-value plastic, transformed into decking materials, flower pots and cutlery…
Researchers at the University of Berkeley in California have developed a catalytic process to recycle these two polymers to transform them into reusable hydrocarbon gases for the manufacture of new plastics. Their work, published in the journal Science, paves the way for creating a circular economy for these plastics, reducing the need to make new plastics from oil, which generates greenhouse gases.
The developed process is capable of breaking down polyethylene bags into propylene monomer, which can then be recycled to make polypropylene plastics. For this, two solid catalysts are used. The first, based on sodium on alumina, is capable of efficiently breaking or cracking various types of polyolefin polymer chains, leaving one of the two pieces with a reactive carbon-carbon double bond at the end.
A second catalyst, based on tungsten oxide on silica, then adds a carbon atom at the end of the chain to the ethylene gas, which continuously circulates in the reaction chamber to form a propylene molecule. This latter process, called olefin metathesis, leaves behind a double bond that the catalyst can continuously access until the entire chain is converted to propylene.
The same reaction occurs with polypropylene to form a combination of propylene and a hydrocarbon called isobutylene. The latter is used in the chemical industry to make polymers for products ranging from soccer balls to cosmetics, as well as to make high-octane gasoline additives.
Large-scale recycling represents a real leap forward to take
According to the researchers, the two catalysts were able to transform with almost 90% efficiency an almost equal mixture of polyethylene and polypropylene, into propylene and isobutylene. And when polyethylene or polypropylene is found alone, the efficiency reaches even higher levels. The advantage of the process is that it does not use water and requires less energy than other recycling methods. He is even “more efficient than the manufacture of new, so-called virgin plastics”affirm the authors of this research work.
Interviewed in the Los Angeles Times, Neil Tangri, scientific and policy director of the Global Alliance for Alternatives to Incinerators, an international environmental organization, expresses some reservations about the future industrialization of this process. “My main warning is that going from small batch analysis in the laboratory to large-scale operation in real-world conditions… it’s a huge leap, he explains. So it’s not certain that we’re going to see this move to commercial production within a year or two. »
Additionally, even though the temperature required for catalysis (over 600 degrees) is lower than that of pyrolysis or cracking, the process still requires a lot of energy and “therefore potentially creates a fairly significant carbon footprint”he believes. Furthermore, this temperature is that “which dioxins tend to form. »
Finally, he considers that the sorting and cleaning necessary for the proper functioning of this process when the plastics are contaminated, as well as its energy consumption, will have the effect of increasing the cost of this recycled plastic, which cannot be put on the market. market at a competitive price compared to virgin plastic.
