Recyclable plastic bags in high value raw materials

Sep. 30 () –

Polyethylene plastics, especially plastic bags they are difficult to recycle and when it is obtained they become a polymer for covers and other products of little value.

But a new process developed at Berkeley Lab (United States) could convert plastic waste into high-value raw materials and reduce the need for fossil fuels to produce propylene, as published in the magazine ‘Science’.

The process, which is in the early stages of developmentuses catalysts to break down long polyethylene (PE) polymers into uniform pieces – the three-carbon molecule propylene – that are the raw material for making other types of high-value plastic, such as polypropylene.

This system would convert a waste product -not only plastic bags and containers, but all kinds of PE plastic bottles- into a product in high demand. Previous methods for breaking polyethylene chains required high temperatures and resulted in much less demanding component blends. The new process could not only reduce the need to produce fossil-fueled propylene, often called propene, but also help fill a currently unmet need in the plastics industry for more propylene.

“To the extent that they are recycled, many polyethylene plastics become low-quality materials. You cannot take a plastic bag and make another one with the same properties,” he explains. it’s a statement John Hartwig, the Henry Rapoport Professor of Organic Chemistry at UC Berkeley. But if you can take that bag of polymer down to its monomers, break it down into little pieces, and repolymerize it, instead of extracting more carbon from the ground, it’s used as a carbon source to make other things, for example, polypropylene. We would use less shale gas for that purpose, or for the other uses of propene, and to fill the so-called propylene void”.

Polyethylene plastics account for about a third of the entire global plastics market, with more than 100 million tons produced annually from fossil fuels, including natural gas obtained by hydraulic fracturing, often called shale gas.

Despite recycling programs – recyclable PE products are designated by plastic numbers 2 and 4 – only about 14% of all polyethylene plastic products are recycled. Due to its stability, polyethylene polymers are difficult to break down into their components, or depolymerize, so most recycling involves melting it down and molding it into other products, such as patio furniture, or burning it for fuel.

Depolymerizing polyethylene and converting it to propylene is a way to produce higher value products from essentially zero value waste, while reducing the use of fossil fuels.

Hartwig specializes in the use of metal catalysts to insert unusual and reactive bonds into hydrocarbon chains, most of which are petroleum-based. New chemical groups can then be added to these reactive bonds to form new materials. The hydrocarbon polyethylene, which normally occurs as a polymer chain of about 1,000 ethylene molecules – each ethylene is composed of two carbon atoms and four hydrogen atoms – it was a challenge for his team due to his general lack of reactivity.

With a grant from the US Department of Energy to investigate new catalytic reactions, Hartwig and graduate students Steven Hanna and Richard J. “RJ” Conk came up with the idea of ​​breaking two carbon-hydrogen bonds in polyethylene with a catalyst—initially, an iridium catalyst and, later, with platinum-tin and platinum-zinc catalysts to create a reactive carbon-carbon double bond, which would serve as an Achilles’ heel. With this crack in the armor of the polymer’s carbon-hydrogen bonds, they were able to undo the polymer chain through a reaction with ethylene and two additional catalysts that react cooperatively.

“We take a saturated hydrocarbon – all carbon-carbon single bonds – and remove a few hydrogen molecules from the polymer to make carbon-carbon double bonds, which are more reactive than carbon-carbon single bonds. Some people had studied this process, but no one had done it in a real polymer,” explains Hartwig. “Once the carbon-carbon double bond is achieved, a reaction called olefin metathesis is used, which was the subject of a Nobel Prize in 2005, with ethylene to cleave the carbon-carbon double bond. Now, you have taken this long-chain polymer and you’ve broken it up into smaller pieces that contain a carbon-carbon double bond at the end.”

The addition of a second catalyst, palladium, allowed the propylene molecules (molecules with three carbons) to be repeatedly trimmed from the reactive end. The result was that 80% of the polyethylene was reduced to propylene.

“Once we have a long chain with a carbon-carbon double bond at the end, our catalyst takes that carbon-carbon double bond and isomerizes it, one carbon inside,” he continues. “Ethylene reacts with that initial isomerized product to make propylene and an almost identical polymer, only shorter, with a double bond at the end. And then it does the same thing over and over again. It steps forward, it splits; it goes in, it splits; it goes in and it splits until everything the polymer is cut into three-carbon pieces. From one end of the chain, it just chews up the chain and spits out propylenes until there’s no chain left.”

Reactions were carried out in liquid solution with soluble or “homogeneous” catalysts. The researchers are currently working on a process that uses non-soluble, or ‘heterogeneous’, catalysts to achieve the same result, since solid catalysts can be more easily reused.

The group demonstrated that the process works with a variety of PE plastics, such as translucent milk bottles, opaque shampoo bottles, PE containers, and the hard black plastic lids that join packs of four aluminum cans. All were efficiently reduced to propylene, and only the coloring agents had to be removed.

Hartwig’s lab also recently used innovative catalysis to create a process that turns polyethylene bags into adhesives, another valuable product. Together, these new processes could put a dent in the growing piles of plastic that end up in landfills, rivers, and ultimately the oceans.

“Both are far from commercialization,” he acknowledges, “but it’s easy to see how this new process would turn the largest amount of plastic waste into a huge chemical feedstock, with much more development of course.”