Polyethylene plastic bags are difficult to recycle
Robert Sanders/University of California, Berkeley
Plastic bottles and bags can be evaporated into their chemical components and turned into new plastic that has all the properties of virgin material. Though there are still hurdles to overcome, this new process is a big step towards a truly circular economy for plastics.
Since the 1950s, roughly 5 billion tonnes of plastic have been sent to landfills, with recycling efforts covering just 9% of production. With current technology, plastic degrades every time it is recycled, and after just a few cycles of this process it is sent to landfills.
John Hartwig and his colleagues at the University of California, Berkeley, had previously developed a process to break down waste plastic into its component parts, but it relied on expensive metal catalysts iridium, ruthenium, and palladium, which were irretrievably lost in the process. Hartwig says the technology is “good for academic papers and demonstrations, but it’s not anywhere near what could be considered industrialized.”
Now his team has found an improved process that works for both polyethylene, the raw material in most plastic bags, and polypropylene, used to make stiffer objects. The process uses only a catalyst that’s so common it’s considered “earthy” in nature, Hartwig says.
Plastics are made up of large molecules called polymers, which are in turn made up of smaller units called monomers. The catalyst breaks the chemical bonds in the polymers, turning them into gaseous monomers from which new plastic can be made with all the properties of virgin, non-recycled material.
In their experiments, the team used two catalysts — sodium on aluminum oxide and tungsten oxide on silica — to convert a mixture of polyethylene and polypropylene into the monomers propylene and isobutylene with nearly 90 percent efficiency.
Benjamin Ward of Cardiff University in the UK, who was not involved in the study, says thousands of additives, including dyes, flame retardants and plasticizers, make plastics difficult to recycle. These additives can make up a third of the finished product and can also contaminate the end product after it’s recycled. “Additives delay landfills. They delay environmental problems, but they don’t prevent them entirely,” Ward says.
Ward believes the new process solves the additive problem by stripping the material down to its constituent gaseous monomers, which also removes the additives.
Hartwig cautions that there are still many hurdles to overcome, and that the process has only been tested in the presence of a small number of common additives. “There will be additives that poison the catalyst or inhibit it,” he says. “We need to find a way to isolate them, which may not be optimal, or we need to find a different catalyst structure or composition that is more resistant to those additives. It’s definitely a challenge.”
Even if there are processes that can break down waste plastics into their component parts and tolerate additives, concerns remain, says Cressida Bowyer of the University of Portsmouth in the UK. “The toxicity and disposal of the recycled end products (such as catalysts and additives) need to be considered, as these may outweigh the benefits of the recycling technology,” she says. “Recycling should not be seen as a solution or justification for maintaining or increasing the production of single-use or unnecessary plastics and continuing the ‘take, make and throw away’ culture that is currently so prevalent.”
topic:
