OUTER PLANET 78 PETplanet Insider Vol. 23 No. 09/22 www.petpla.net OUTER PLANET Designer polymers for easier mixed-material recycling Acknowledgement: This article is based on a report by Alison Hatt, published on Berkeley Lab’s Newsletter website. Plastics manufacturers and converters have made a lot of noise about increased use of recycled materials but there is another, rather inconvenient reality. Mixed-material products are very difficult to recycle. Trainers (aka sneakers), single-use bags, electronics and even food packaging films are made up of layers of different plastics. Once they are mixed, plastics become very difficult to recycle, especially by conventional, mechanical means. They cannot be recovered and reused, nor even downcycled into insulation or building materials. The likelihood is that they will end up in landfills, incinerators, garbage piles in the developing world or in the oceans. It’s another of the industry’s dirty secrets, one that is well past due to be effectively addressed. Scientists based at the Lawrence Berkeley National Laboratory – the Berkeley Lab, founded in 1931 at the University of Berkeley in San Francisco, California – have been engaged in work that has, they say, led to a new range of polymers that can be mixed, separated and recycled indefinitely. The material, polydiketoenamine (PDK), can be tailored to have different characteristics and performance standards, depending on the solution or application required. A study published in Science Advances demonstrated the Berkeley Lab team’s customised PDKs, tailored for mixedplastic recycling, and the method to fully recover the constituent plastics from products made of multiple materials. “An example might be a shoe, where textile is bonded to rubber. Conventional materials can’t be recycled for reuse, since they can’t be deconstructed independently,” said Brett Helms, of the Molecular Foundry, who headed the Berkeley Lab team. He said that, if they were made from specially designed PDK polymers, then they could be reclaimed, separated and recycled. Like other polymers, PDKs are made up of long-chain molecules, themselves comprised of small monomer units. The Berkeley team’s researchers demonstrated a variety of PDKs, each with a slightly different chemical structure, that could be ‘depolymerised’ – broken down into their monomers, with a high recovery yield. Those recovered monomers can then be used to make new PDKs. The process of breaking down the polymers is a chemical recycling process, using acids. Fig 1: Products made from two different PDK plastics can be deconstructed in acidic solution in stages, allowing for recycling of both plastic materials. (Credits: Brett Helms and Jenny Nuss/ Berkeley Lab) Fig. 1 shows two different PDK plastics in an acidic solution. It illustrates how each polymer can be easily broken down into individual monomers. The researchers demonstrated their system by creating blended plastics using molecules identified and optimised for particular applications. Each was made from two different PDKs, then completely depolymerised and the constituent materials recovered. The demonstration was repeated with PDKs of different colours; a slightly more complex process also delivered high yield levels. An ongoing challenge for recycling, especially in food packaging, is the long-term failure of the industry to produce easily-recyclable packaging film. The Berkeley Lab team showed how PDK can be used to make recyclable, flexible plastic packaging out of conventional plastics. They formed a multilayer film from PP and PET, using a “tie layer” of PDK. The researchers leveraged their control over the PDK layer to separate and fully recover the PP and PET. They also demonstrated an application for automobile recycling, with an object made of glass, steel and different PDKs. Helms said that the Berkeley Lab team has made a great stride in facilitating the recovery of polymers themselves, as well as materials bonded to them, enabling more circularity in manufacturing. The research was supported in part by the U.S. Department of Energy’s Office of Energy Efficiency & Renewable Energy and Berkeley Lab’s Laboratory Directed Research and Development programme. www.lbl.gov Each polymer breaks down into individual monomers in steps conducted at different temperatures.
RkJQdWJsaXNoZXIy NTY0MjI=