Recycling process found for the world’s most common plastic
Shampoo bottles or plastic bags: About a third of plastic production is made up of the plastic called polyethylene. Due to its stability, it is difficult to recycle. Now researchers from California have found a new way of recycling that is also environmentally friendly.
WScientists have developed a new process for recycling the common plastic, polyethylene. The high-quality plastic propylene, the raw material for the production of the plastic polypropylene (PP), is produced in several reaction steps. This in turn can be used for many purposes and – in contrast to polyethylene – also easily recyclable.
Milk packaging, shampoo bottles, plastic bags: about a third of plastic production is made of polyethylene (PE). But because of its stability, this plastic is difficult to recycle. So far, only about 14 percent of this is recycled, creating polymer blends that are made into flooring or other low-grade plastics.
“The mechanical durability and chemical inertness of commercial plastics have led to their widespread use in everyday life,” write John Hartwig and his colleagues at the University of California, Berkeley in the journal Science. “But the same properties make them difficult to recycle.”
Because the inertia prevents the chemical breakdown of the long polymer chains. The Hartwig team has now found a solution for this. The authors of the study describe the conversion into propylene – the basic material of polypropylene (PP), which is used, among other things, for plastic pipes and fittings in vehicles.
The single bonds between the carbon atoms in a long polyethylene chain cannot be efficiently broken chemically. The scientists therefore made a small detour: They first split off some hydrogen atoms from the carbon atoms using platinum-tin and platinum-zinc catalysts. This creates single carbon double bonds in the polyethylene chain, which increases its reactivity. Ethylene molecules, which are present in large quantities in the reaction vessel, couple with double bonds to these carbon atoms. By adding another catalyst, propylene molecules are now split off from the chain.
Another reaction, called isomerization, makes the process even more efficient. This converts more than 80 percent of the polyethylene into propylene, which can then be used to make polypropylene. Until now, shale gas, which is produced through controversial fracking, has often served as the basis for the production of polypropylene.
“We would use less shale gas for this purpose or for the other uses of propylene,” Hartwig said in a statement from his university. This would also make polypropylene production more environmentally friendly.
The same research group recently presented a process for converting polyethylene bags into adhesives. Hartwig admits that both methods are still a long way from commercial use. “But it’s easy to see how this new process would turn the largest amount of plastic waste into a huge chemical feedstock — with a lot of refinement, of course.” The researchers are optimistic because the processes used in the lab are closely related to reactions already underway and can be carried out on an industrial scale with various catalysts.
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