Traditional “green” plastics have their share of problems, whether it be a compostable food container with sides too flexible to a secure lid or a biodegradable bag that rips on its way to the compost bin. But a new and improved biodegradable polypropylene carbonate film—PPC—may have solve those problems.
By blending the biodegradable carbon polymer with starch, the resulting film is more durable and less leaky compared with other bioplastics or even traditional petrochemical-based plastics, said Fariba Dehghani, a professor at the University of Sydney who helped develop the material.
“Oxygen and humidity are critical,” she said. “If you prevent these two from diffusing into the packaging, you can prevent bacterial growth and increase the shelf life of the food and decrease the spoilage of the food.” Bread packaged in a PPC-coated bag, for example, would take longer to go stale.
Packaging accounted for nearly 40% of the roughly 275,000 million tons of petrochemical-based plastic consumed globally in 2010. Bio-based packaging, or packaging made from primarily renewable resources such as sugar cane, sugar beets, or tapioca, was a small fraction of that at 165,000 tons, according to Gordon Robertson, a professor at the School of Agriculture and Food Sciences at the University of Queensland.
The PPC-starch blend proposed by Dehghani is a petrochemical-based polymer that degrades quickly, making it biodegradable, but with 25% starch, is becomes partially bio-based, too.
While public concern has focused on creating and using biodegradable plastics because they seem “natural” and promise to decompose quickly, freeing space in landfills, he said, the more immediately sustainable solution might be bio-based products, which use less petrochemicals. In order for biodegradable plastics to live up to their promise, they need to be disposed of in municipal composting sites—and few cities have the necessary facilities or the ability to collect biodegradable waste as a separate stream.
Even existing composting facilities vary vastly in their ability to dispense with biodegradable polymers. Mexico, for example, has only two industrial composting sites, and it is unlikely that either could process the current PPC products. “[The polymer] degrades partially, but it doesn’t reach the quality required for fertilizer. In this kind of facility, those compostable plastics won’t fully degrade by the end of the process,” said AlethiaVazquez-Morillas, a professor at the Autonomous Metropolitan University in Mexico City who was not involved with Dehghani’s project. At the end of an incomplete degradation cycle, the remaining plastic solids are sifted out and put into a landfill.
While the results of the study are promising, PPC still has a long way to go before being certified as biodegradable by ASTM International.
Previously, PPC had been restricted to use as an alternative plastic in films, coatings, and adhesives because of the high metal content left over from its manufacturing process. But a new production method washes the polymer with carbon dioxide-laden water to extract residual zinc, making the polymer safe for use with food, Dehghani and her colleagues reported in Green Chemistry earlier this year.
To ensure the safety of the new material, Dehghani mimicked conditions that PPC would encounter as a consumer product—oil, acid to simulate fruit juice, base to simulate a detergent, and alcohol—to see how PPC compared to Ecoflex and other commercially available biodegradable polymers. She found that after six months of incubating in food conditions, PPC had not lost much mass.
“You don’t need to worry that you will contaminate your food with the degradation of this polymer,” Dehghani said.