PLA and a ‘sweet spot’ for starch

They worked with PLA (polylactic acid), which seems like an obvious choice in many ways. It’s been used in packaging for over a decade, and it’s derived from plant sugars rather than petroleum.

When managed properly, PLA’s waste byproducts are all natural: water, carbon dioxide and lactic acid. Plus, researchers know that PLA can biodegrade in industrial composters. These composters create conditions, such as higher temperatures, that are more conducive to breaking down bioplastics than home composters. Yet, the idea of making PLA compostable at home seemed impossible to some people.

“I remember people laughing at the idea of developing PLA home composting as an option”, said Pooja Mayekar, a doctoral student in Auras’ lab group and the first author of the new report. “That’s because microbes can’t attack and consume PLA normally. It has to be broken down to a point where they can utilize it as food”.

Although industrial compost settings can get PLA to that point, that doesn’t mean they do it quickly or entirely.

“In fact, many industrial composters still shy away from accepting bioplastics like PLA,” Auras said.

In its experiments, supported by the U.S. Department of Agriculture and MSU AgBioResearch, the team showed that PLA can sit around for 20 days before microbes start digesting it in industrial composting conditions. To get rid of that lag time and enable the possibility of home composting, Auras and his team integrated thermoplastic starch into PLA. Among other benefits, the starch gives composting’s microbes something they can more easily chow down on while the PLA degrades.

“When we talk about the addition of starch, that doesn’t mean we just keep dumping starch in the PLA matrix”, Mayekar said. “This was about trying to find a sweet spot with starch, so the PLA degrades better without compromising its other properties”.

Fortunately, postdoctoral researcher Anibal Bher had already been formulating different PLA-thermoplastic starch blends to observe how they preserved the strength, clarity, and other desirable features of regular PLA films. Working with doctoral student Wanwarang Limsukon, Bher and Mayekar could observe how those different films broke down throughout the composting process when carried out at different conditions.

“Different materials have different ways of undergoing hydrolysis at the beginning of the process and biodegrading at the end”, Limsukon said. “We’re working on tracking the entire pathway”.

The team ran these experiments using systems that Auras and lab members, past and present, largely built from scratch during his 19 years with MSU. The equipment the researchers have access to outside their own lab in the School of Packaging also makes a difference.

“Working with Dr Auras, the School of Packaging, MSU – it’s great”, Bher said. “Because, at some point, we want to be making actual products. We are using facilities around campus to make materials and test their properties. MSU offers a lot of resources”.

“There’s a reason why this is one of the best schools for packaging”, Mayekar said.

Changing the conversation
Using their expertise and the resources at hand, the researchers have demonstrated that completely compostable biobased plastic packaging is possible. Yet Auras stressed that this alone won’t be enough to guarantee its commercial adoption. The challenges there aren’t solely technical. They’re social and behavioural as well.

“There’s not going to be one solution to the entire problem of plastic waste management”, Mayekar said. “What we’ve developed is one approach from the packaging side”.

Beyond industrial composters’ scepticism about plastics that Auras mentioned earlier, there’s a public misconception that biodegradable and compostable materials can break down relatively quickly anywhere in the environment. However, these materials require certain conditions, like those found in an active compost, to decompose in a timely fashion. Outside of those, biodegradable plastics that are disposed of in the environment are still just litter.

“If people think we develop something biodegradable so it can be littered, that will make the problem worse”, Auras said. “The technology we develop is meant to be introduced into active waste-management scenarios”.

“We need to be conscious of how we manage waste, especially plastics”, Bher said. “Even at home, you’ll need to think about how you’re managing that small composting process”.

“It’s really easy to just blame plastic for its problems, but I think we need to change the conversation about how we manage it”, Mayekar said.

With its work, the team wants to help educate folks and raise awareness around this issue. And they have reason to believe they can change attitudes. After all, nobody’s laughing at the idea of potentially composting PLA at home anymore.

The work is published in the journal ACS Sustainable Chemistry & Engineering. AT

https://pubs.acs.org/doi/10.1021/acssuschemeng.3c01676
https://msutoday.msu.edu/

Image credit: Matt Davenport/MSU