Researchers at the University of Michigan have managed to recycle polyvinyl chloride (PVC), one of the most abundantly produced plastics in the world. The previously unrecyclable plastic usually ends up in landfill, making it crucial to identify productive ways to find new uses for such waste.
Although it is ubiquitous in everyday life, from surgical equipment to household plumbing, window frames, and even the coating on electrical wiring, PVC is difficult to recycle due to its toxicity. Although plastic is usually recycled by melting and then reforming it into lower quality materials, the heating process means plasticizers in PVC leach out easily and can transfer into other plastics that contaminate everything in the recycling stream, as well as releasing hydrochloric acid (HCL) quickly when heated, which can corrode equipment and be a danger to workers.
McNeil's lab have been developing ways to chemically recycle different kinds of plastics by breaking them into constituent parts. As reported in Nature Chemistry [Fagnani et al. Nat. Chem. (2022) DOI: 10.1038/s41557-022-01078-w], here they showed how to chemically recycle PVC into usable material without heat. This was achieved with phthalates, a common but toxic plasticizer, as the mediator for the chemical reaction. The phthalates helped break down PVC while the electrochemical process resolves the issue of HCL, which is still released but at a much slower and more controlled rate.
PVC is a polymer with a hydrocarbon backbone with single carbon–carbon bonds. Attached to every other carbon group is a chlorine group, and heating means the HCL quickly falls off, leaving a carbon–carbon double bond along the backbone. Electrochemistry was used to introduce an electron into the system, breaking the carbon–chloride bond and leaving a negatively charged chloride ion. Electrochemistry allows for metering the rate at which electrons are introduced, thus controlling how quickly HCL is produced.
The approach could lead to new ways to repurpose waste PVC and related polymers using electrosynthetic reactions, including ones that take advantage of existing polymer additives. As principal investigator Anne McNeil told Materials Today, “My hope is that this research will inspire others to consider using electrosynthesis, especially paired electrolysis – where a productive reaction is occurring at each electrode – in polymer upcycling”.
The team also combined the synthetic work with lifecycle assessment to check if generating HCl indirectly through waste PVC offer less than just using purchased HCl. They also hope to work on the process to make it more industrially scalable, and as the polymer byproduct is not a single pure material, to find a way to reuse this still unrecyclable byproduct.
“My hope is that this research will inspire others to consider using electrosynthesis, especially paired electrolysis – where a productive reaction is occurring at each electrode – in polymer upcycling”Anne McNeil
Image of the electrosynthetic reaction with mixed plastics present (credit: Danielle Fagnani)