Plastic recycling: how to best reuse carbon atoms

The amount of plastic waste is enormous: worldwide, some 350 million tons are discarded per year, i.e. over 40 kilograms per person per year. Scientists at the University of Groningen’s Faculty of Science and Engineering (FSE) are working on new ways to recycle plastic waste into new, high-quality products. This is the second of four articles on plastic research at FSE.

FSE Science Newsroom | Text René Fransen

Recycling plastics has several advantages: it reduces plastic waste by reusing it and it reduces the need for new feedstock to make plastics. By reusing the carbon atoms that form the backbone of plastic, it also delays the moment when they will be released into the atmosphere as carbon dioxide. However, recycling is a complex process: there are many types of plastics that require different approaches to recycling, explains Professor of Chemical Technology Francesco Picchioni.

A unique position in recycling research

‘At the University of Groningen, we have a unique position when it comes to the research into recycling: we work on recycling many different types of plastics, covering well over 80 percent of all plastic produced. I don’t know any other institutes with a similar wide range.’ Just three types, polyethylene (PE), polypropylene (PP) and polyvinyl chloride (PVC), already make up 75 percent of all plastic produced in the world. ‘And if you add polyethylene terephthalate (PET) that is used in bottles and polyurethane (PUR) that is used in foams, you already pass the 80 percent.’

Picchioni himself works on different polymers. One of his recent achievements is the recycling of rubber. The polymers in rubber are linked together so they form a kind of molecular mesh, and it seemed impossible to break up the mesh without also breaking up the polymers, which would result in shorter polymers that would make a much lower quality rubber. Picchioni found a way to reconnect the shorter polymers in a way that produced a nearly virgin-quality rubber. Startup company New Born Rubber uses the patented method to create new rubber for products such as road plates.

Upcycling and downcycling

‘Another unique feature of our University is that we work on both types of plastic recycling: material recycling and chemical recycling,’ Picchioni adds. Material recycling refers to a process where the material is directly re-processed as such. Usually, this means that the quality of the recycled product is reduced. Chemical recycling means that the polymers of a plastic are broken down into monomers or other molecules, or even into carbon dioxide and water. These can be used as building blocks for new polymers. Turning waste into virgin plastics is called ‘upcycling’. However, it is often easier to break down plastic waste to hydrocarbons that make a low-grade fuel, for example for ships — a process called downcycling.

Picchioni tries to avoid ‘downcycling’, where the recycled product has a lower value than the original. He is currently working on a project using supercritical carbon dioxide to recycle PVC. ‘This type of plastic has many additives. Soft PVC contains up to 30 percent plasticizers, and these additives make it unfit for material recycling,’ he explains. Furthermore, PVC contains chloride, which will turn into hydrochloric acid in most recycling processes.

The supercritical carbon dioxide is able to remove these additives and prevent the generation of hydrochloric acid. ‘We expect to have a small demonstrator reactor ready to clean up 5 kilograms of PVC per hour later this year. And we are patenting a technique that allows us to remove the hydrochloric acid in a controlled way so that PVC can be made suitable for chemical recycling if necessary.'

This is the second part of a series of four articles on plastics. Next week, you can read about using enzymes to recycle plastics.