Designer materials to keep plastic out of the lan

image: Two different PDK plastics in acid solution, demonstrating how each polymer easily breaks down into individual monomers during different steps performed at different temperatures, allowing full recycling of both plastics.
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Credit: Jérémy Demarteau/Berkeley Lab

– By Alison Hat

Scientists have designed a new system of materials to overcome one of the biggest challenges in recycling consumer products: recycling mixed plastics. Their achievement will help enable a much wider range of fully recyclable plastic products and establish an effective circular economy for durable goods like automobiles.

We produce staggering amounts of plastic and plastic-containing products every year, but only a tiny fraction of that plastic can be recovered and used to make products of similar quality. This is because most products, from food wraps and single-use bags to sneakers and electronics, are made from mixtures of different plastics, and once they are mixed, these plastics cannot not be salvaged and used to make new bags or sneakers. Instead, most end up in landfills, incinerators or the oceans.

A team of scientists from Lawrence Berkeley National Laboratory (Berkeley Lab) are tackling the challenge of mixed plastics using a custom-designed material called polydiketoenamine (PDK), a new type of plastic they developed to be recycled efficiently and indefinitely , offering a low carbon manufacturing solution for plastic products that should never end up in a landfill.

In a new study published in Scientists progress, the team showed that they could create custom PDKs specifically suited for mixed plastics recycling and could fully recover the constituent plastics of a mixed product made from multiple PDKs and other common manufacturing materials. Brett Helms, from the Berkeley laboratory Molecular foundryled the multidisciplinary team, which also included researchers from Joint Bioenergy Institute (JBEI) and the Berkeley Laboratory Advanced light source, among others. The work is a major validation of a promising material and deepens our knowledge of polymer chemistry.

“We now know how to adapt PDK plastics to recycle complex products that include multiple types of materials,” Helms said. “An example might be a shoe, where a textile is bonded to a rubber by an adhesive. The conventional materials used in these products cannot be recycled for reuse, as they cannot be independently deconstructed. Still, if they were made from different purpose-designed PDK polymers, they could be for the first time.

Creation of a design material

PDKs and other plastics are known as polymers, materials in which the constituent molecules are long chains of small, repeating units called monomers. For this work, the researchers started by making a variety of PDKs with slightly different chemical structures and showed that each could be “depolymerized” or broken down into their respective monomers with high recovery efficiencies. This is basically the plastic recycling process, as these recovered monomers can then be used to create a new batch of PDKs.

The team found that each PDK depolymerizes at a different temperature and rate. To better understand these properties, they used theoretical calculations and computer models (density functional theory) to simulate the different polymers and explore how they form and depolymerize. Using this theoretical knowledge, the team identified the best PDK molecules for the job and further optimized their design.

“A particularly interesting aspect of this work was the tight integration between experiments and calculations,” said Molecular Foundry director Kristin Persson, who led the theoretical work. “By discovering the mechanism underlying circularity, we were able to design new polymers that retain recyclability. We are excited that these design ideas will inform future work. »

“It is through these interactions between theory and experiment that we build the knowledge and framework to establish the design rules governing polymer reactivity,” said Helms. “Otherwise, we would only have observations, rather than an explanation.”

Mixed plastics? no problem

Using these optimized molecules, the researchers demonstrated the success of their materials system by creating blended plastics, each composed of two different PDKs, then depolymerizing and fully recovering the constituent materials. They repeated the demonstration with PDKs of different colors, addressing a particular industry challenge, and showed that with a slightly more complex process, they could again recover PDK monomers in high yields.

The team also showed how PDK can be used to make recyclable flexible plastic packaging from conventional plastics. They formed a multi-layered film from common plastics – polypropylene (PP) and polyethylene terephthalate (PET) – using a “tie layer” of PDK to bind them together. Normally, PP and PET could not be extracted from a multilayer material, but here the researchers leveraged their control over the PDK layer to separate and recover PP and PET films as well.

In a final demonstration of their powerful approach, the researchers constructed an object from a mixture of different PDKs with glass and stainless steel, to simulate the challenges of automotive recycling, and started the recycling process again, demonstrating high yield recovery of PDK monomers as well as glass and metal. These findings could lead to a significant shift in our approach to manufacturing durable goods, enabling a circular economy in which products are designed to be fully recovered and reused.

“Complex consumer products are simply not recycled today; they are either incinerated, landfilled or recycled,” Helms said. “Here we have laid the groundwork for how to recycle these products back to their original monomer building blocks, making it easier to recover materials related to them for reuse, including precious metals or glass. In this way, PDK materials bring more circularity to manufacturing with an inherently low carbon intensity.

This research was funded in part by the U.S. Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy and the laboratory-led research and development program at Berkeley Lab.

The Molecular Foundry and Advanced Light Source are user facilities of the DOE Office of Science at the Berkeley Laboratory. JBEI is a DOE Bioenergy Research Center operated by Berkeley Lab.

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Founded in 1931 on the belief that the greatest scientific challenges are best met by teams, Lawrence Berkeley National Laboratory and its scientists have been awarded 14 Nobel Prizes. Today, Berkeley Lab researchers are developing sustainable energy and environmental solutions, creating useful new materials, pushing the boundaries of computing, and probing the mysteries of life, matter, and the universe. Scientists around the world rely on the laboratory’s facilities for their own scientific discovery. Berkeley Lab is a multi-program national laboratory, operated by the University of California for the US Department of Energy’s Office of Science.

The DOE’s Office of Science is the largest supporter of basic physical science research in the United States and works to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science.


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