Augmented PLA Tiny Additive Has Big Impact on Compostability of Bioplastic

A trace additive could significantly improve the compostability of PLA. Researchers have modified the bioplastic so that it degrades faster under industrial composting conditions without losing strength or transparency. The findings could broaden the use of PLA in packaging, although further testing is needed before home composting becomes a reliable option.

Compostable plastics could be part of a solution to the world’s plastic waste problem. But currently these materials need industrial composting facilities to break down. In a step toward making a home-compostable plastic, researchers reporting in ACS Central Science have augmented polylactide (PLA) — a widely used biobased and compostable polymer— with a small amount of an additive. Tests show it helps the material degrade substantially faster without sacrificing critical qualities like strength or transparency.

PLA is currently found in products such as food packaging, textiles, and biomedical devices, and it accounts for roughly two-thirds of total bio-based and biodegradable plastics production worldwide. “Composting is considered one of the most effective end-of-life strategies for PLA products, especially food-contaminated single-use products, because it eliminates the need for additional sorting and washing processes,” says Marc Hillmyer, a corresponding author of the paper. This process converts organic waste into environmentally innocuous products such as small organic acids. However, PLA “is only industrially compostable in engineered environments where it degrades over a few months,” he adds. The bioplastic usually needs high temperatures and humidity to break down over practical timescales.

Inspired by this limitation, Hillmyer, Christopher Ellison and colleagues wanted to develop a PLA-based material that breaks down faster and under a broader set of conditions. Instead of adding organic acids directly, which can weaken PLA during processing, the team blended PLA with small amounts of organic anhydrides. These compounds are “masked acids,” because once exposed to water, they activate and help catalyze the breakdown of the plastic’s polymer chains.

The researchers created two plastic films containing different organic anhydrides: phthalic anhydride or 2-sulfobenzoic acid cyclic anhydride. They found that PLA blends modified with either substance maintained their mechanical properties, including strength and transparency, compared to pure PLA films.

The 2-sulfobenzoic acid cyclic anhydride additive worked especially well, even at trace levels as low as 100 parts per million. Under industrial composting conditions at 136 degrees Fahrenheit (58 degrees Celsius), PLA containing 0.1% of this additive completely degraded within 21 days, surpassing unmodified PLA, which reached 83% biodegradation after 90 days. The researchers also had success with improving the biodegradability of the modified PLA around 113 degrees Fahrenheit (45 degrees Celsius), a temperature within the range of healthy home composts.

The authors say their approach could expand the practical use of PLA by helping it degrade not only in industrial composting facilities but also in home composting bins. However, more testing is still needed to understand how modified PLA materials behave in various settings and environments.

The authors acknowledge funding from the U.S. Army Corps of Engineers’ Engineer Research and Development Center’s Construction Engineering Research Laboratory, a National Science Foundation Graduate Research Fellowship, and a University of Minnesota Doctoral Dissertation Fellowship.

Original Article: Tiny additive has big impact on compostability of bioplastic; ACS Central Science; DOI:10.1021/acscentsci.6c00395

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