Starch Added to PLA Biopolymer Increases Compostability

Researchers from Michigan State University’s School of Packaging developed a promising, sustainable alternative to make petroleum-based plastics more biodegradable.

A team led by Rafael Auras has made a bio-based polymer blend that’s compostable in both home and industrial settings. The work is published in the journal ACS Sustainable Chemistry & Engineering.

“In the US and globally, there is a large issue with waste and especially plastic waste,” says Auras, MSU professor and the Amcor Endowed Chair in Packaging Sustainability.

“By developing biodegradable and compostable products, we can divert some of that waste,” Auras said. “We can reduce the amount that goes into a landfill.”

Another bonus is that plastics destined for the compost bin wouldn’t need to be cleaned of food contaminants, which is a major obstacle for efficient plastic recycling. Recycling facilities routinely must choose between spending time, water, and energy to clean dirty plastic waste or simply throwing it out.

“Imagine you had a coffee cup or a microwave tray with tomato sauce,” Auras says. “You wouldn’t need to rinse or wash those, you could just compost.”

Starch gives composting microbes something to digest while PLA degrades.

The team worked with polylactic acid, or PLA, which seems like an obvious choice in many ways. Used in packaging for more than a decade, 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.

“I remember people laughing at the idea of developing PLA home composting as an option,” says 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 must be broken down to a point where they can utilize it as food.” In the photo at left, Mayekar is seen working with a bioreactor in the lab.

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 says.

In experiments supported by the U.S. Department of Agriculture and MSU AgBioResearch, the team showed it takes 20 days before microbes start digesting PLA in industrial composting conditions.

To get rid of that lag time and enable the possibility of home composting, Auras and his team integrated a carbohydrate-derived material called thermoplastic starch into PLA. Among other benefits, the starch gives composting’s microbes something they can more easily digest on while the PLA degrades.

Finding a "sweet spot" where PLA degrades better without compromising its other properties.

“When we talk about the addition of starch, that doesn’t mean we just keep dumping starch in the PLA matrix,” Mayekar says. “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 says. “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 says. “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.”

Credit: Matt Davenport