A team led by Rafael Auras at Michigan State University's School of Packaging has made a bio-based polymer blend that's compostable in both home and industrial settings.
According to Auras, a professor at the East Lansing, Mich., university and the Amcor Endowed Chair in Packaging Sustainability, biodegradable and compostable materials are a potential tool in the fight against plastic waste.
The school's work is published in the journal ACS Sustainable Chemistry & Engineering.
Less than 10 percent of plastic waste is recycled in the U.S, he said. That means the bulk of plastic waste ends up as trash or litter, creating economic, environmental and even health concerns.
Unlike plastic waste headed for recycling, compostable plastics contaminated with food residues would also not need to undergo the rigorous washing cycles required during the recycling process.
The researchers developed a blend of the bio-based polyester known as polylactic acid, or PLA, and thermoplastic starch. On its own, PLA is industrially compostable, but it cannot be composted at home. And even under industrial conditions, it has been shown that it takes around 20 days before PLA films start to biodegrade, which means that they are only suitable for processing at well-managed industrial composting facilities.
Even there, if the prescribed process of turning and mixing is disturbed, the biodegradation process may not be completed by the end of the cycle. As a result, it produces poorer quality compost containing PLA fragments. An additional challenge are the polymer fragments at the compost pile surface — because they are not exposed to the same conditions, it can take them longer to biodegrade, especially relevant fragments that constantly resurface during pile turning due to their low density.
The upshot is that not all composting facilities are willing to accept PLA.
So in the search to find a way to accelerate the biodegradation process in industrial composting facilities and even to expand PLA's biodegradation efficiency in home and backyard composting settings, the researchers created a blend with the more easily biodegradable TPS.
Starch rapidly degrades to glucose, which, the researchers say, is further assimilated to produce CO2 and water in compost or soil environments. The starch not only can enhance PLA's sensitivity to humid environments, it is an "excellent initial nutrient source for the microbes when introduced into the compost environment," they note.
Different blends of the two materials were tested in simulated composting conditions at moderate (37° C) and warmer temperatures (58° C). The team showed that , in each case, the PLA/starch blends had no lag time — the microorganisms immediately went to work on the TPS component of the blend, which provided enough of a food supply until chemical and enzymatic hydrolysis of the PLA started, leading to its fragmentation and ultimate depolymerization.
Researchers also demonstrated that the PLA component in a PLA/TPS blend underwent a more rapid biodegradation process under moderate temperature conditions, which could lead to the development of home compostable PLA-based films. "Starch," they wrote, "can be used to improve PLA degradation in industrial composting conditions by reducing the lag phase of disintegration of PLA and, even at mild conditions such as those in home composting settings, by inducing biodegradation during the initial days."