Paper cups, cartons, and food packaging may appear to be biodegradable, but many contain a plastic liner that cannot be composted. While durable, plastic-free paper products are available, they often allow grease and oil to pass through, weakening the paper and creating a mess. Today, scientists report that they have developed a degradable polymer coating that can block this infiltration and could lead to new biodegradable paper-based materials.
The researchers will present their findings at the American Chemical Society (ACS) fall meeting. ACS Fall 2021 is a hybrid meeting that will be held virtually and in person from August 22-26, and on-demand content will be available from August 30 to September. 30. The meeting includes over 7,000 presentations on a wide range of scientific topics.
“Consumers are pushing for more sustainable products,” says Matthew Carter, Ph.D., principal investigator of the project. “But making a fully degradable paper product for single-use applications, like sandwich wrappers, bowls going to a cafeteria – is a big challenge,” adds Carter, who presented the work at the meeting.
Current oil and grease resistant coatings for disposable paper products often include fluorocarbons or polyolefins, but these polymers are persistent in the environment and cannot be composted. “One of the things that makes conventional polymers useful is their durability,” says fellow senior researcher Andrew Hejl, Ph.D., who works with Carter at Dow. This toughness, which comes from the stable carbon-to-carbon bonds that form during polymerization, is desirable, for example, in a household latex paint that must last for years but problematic in a paper bowl intended for composting.
One way to make polymers less durable is to insert degradable bonds to disrupt their carbon-carbon backbones, Carter explains. To do this, the researchers turned to a radical polymerization technique that had been around for years but had not been widely adopted. Noting a recent surge of interest in universities in using this technique with cyclic ketene and vinyl acetal monomers, the Dow team began to explore how to translate this chemistry in an industrial environment. In academia, the reaction was typically carried out in organic solvents, but Dow wanted to switch to water as a “greener” substitute. This was difficult because cyclic ketene acetal monomers are unstable in water, but the researchers solved this problem by adjusting the reaction conditions, including pH, temperature, and monomer concentration.
Their reaction mixed 2-methylene-1,3-dioxepane cyclic ketene acetal and vinyl acetate to form a polymer with ester linkages in the backbone. The team then coated the paper with a water-based emulsion of this polymer – a synthetic “latex” unrelated to natural latex rubber, and therefore unlikely to trigger an allergic reaction – and left it. to dry. They recently reported that the coating was an effective barrier against oil and grease and that its ester bonds could break down in water, quite slowly at neutral pH but faster at higher pH under laboratory conditions. Today, researchers are starting to assess the degradability of the coating under relevant industrial conditions. So far, they have shown that microorganisms used in industrial composters can help biodegrade the polymer in simulated wastewater. “The concept is to have a long polymer that breaks down into smaller pieces that will eventually be broken down by the microbes into CO2,” Hejl explains.
Other researchers and companies have developed biodegradable restoration products with coatings based on natural materials such as whey or chitosan. “These are great options, but they have limits in terms of getting the performance you want at an appropriate price,” Hejl said. And some coatings have to be applied as a very dilute solution, so a lot of water has to be evaporated to dry them, he adds. “So there are a lot of potential materials, and I don’t think anything has ticked all the boxes yet. But researchers at Dow believe their coating may well fill the bill.
“More broadly, if you look at the concept of disrupting a stable carbon-carbon backbone with degradable bonds, it really opens up a lot of opportunities beyond just paper coatings,” Carter said. “Other applications include personal care or home care formulations, such as shampoo and wax. Many of these materials currently contain synthetic carbon-carbon bond polymers. This approach could therefore be a powerful way to introduce degradability into new classes of materials in the future. “
Presented at the American Chemical Society Fall Meeting, August 22-26, 2021.
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