How we make it: Better biodegradable plastic
Channel: UC Berkeley
Date: 2021-08-31

Here's how Berkeley researchers make their more "truly" biodegradable plastic.
Also see the accompanying video: https://youtu.be/WxpsHlR_izQ

Biodegradable plastics have been advertised as one solution to the plastic pollution problem bedeviling the world, but today’s “compostable” plastic bags, utensils and cup lids don’t break down during typical composting and contaminate other recyclable plastics, creating headaches for recyclers. Most compostable plastics, made primarily of the polyester known as polylactic acid, or PLA, end up in landfills and last as long as forever plastics.

University of California, Berkeley, scientists have now invented a way to make these compostable plastics break down more easily, with just heat and water, within a few weeks, solving a problem that has flummoxed the plastics industry and environmentalists.

“People are now prepared to move into biodegradable polymers for single-use plastics, but if it turns out that it creates more problems than it’s worth, then the policy might revert back,” said Ting Xu, UC Berkeley professor of materials science and engineering and of chemistry. “We are basically saying that we are on the right track. We can solve this continuing problem of single-use plastics not being biodegradable.”

Xu is the senior author of a paper describing the process that will appear in this week’s issue of the journal Nature.

The new technology should theoretically be applicable to other types of polyester plastics, perhaps allowing the creation of compostable plastic containers, which currently are made of polyethylene, a type of polyolefin that does not degrade. Xu thinks that polyolefin plastics are best turned into higher value products, not compost, and is working on ways to transform recycled polyolefin plastics for reuse.  

The new process involves embedding polyester-eating enzymes in the plastic as it’s made. These enzymes are protected by a simple polymer wrapping that prevents the enzyme from untangling and becoming useless. When exposed to heat and water, the enzyme shrugs off its polymer shroud and starts chomping the plastic polymer into its building blocks — in the case of PLA, reducing it to lactic acid, which can feed the soil microbes in compost. The polymer wrapping also degrades.

The process eliminates microplastics, a byproduct of many chemical degradation processes and a pollutant in its own right. Up to 98% of the plastic made using Xu’s technique degrades into small molecules.

One of the study’s co-authors, former UC Berkeley doctoral student Aaron Hall, has spun off a company to further develop these biodegradable plastics.

Plastics are designed not to break down during normal use, but that also means they don’t break down after they’re discarded. The most durable plastics have an almost crystal-like molecular structure, with polymer fibers aligned so tightly that water can’t penetrate them, let alone microbes that might chew up the polymers, which are organic molecules.

Xu’s idea was to embed nanoscale polymer-eating enzymes directly in a plastic or other material in a way that sequesters and protects them until the right conditions unleash them. In 2018, she showed how this works in practice. She and her UC Berkeley team embedded in a fiber mat an enzyme that degrades toxic organophosphate chemicals, like those in insecticides and chemical warfare agents. When the mat was immersed in the chemical, the embedded enzyme broke down the organophosphate.

Her key innovation was a way to protect the enzyme from falling apart, which proteins typically do outside of their normal environment, such as a living cell. She designed molecules she called random heteropolymers, or RHPs, that wrap around the enzyme and gently hold it together without restricting its natural flexibility. The RHPs are composed of four types of monomer subunits, each with chemical properties designed to interact with chemical groups on the surface of the specific enzyme. They degrade under ultraviolet light and are present at a concentration of less than 1% of the weight of the plastic — low enough not to be a problem.

For the research reported in the Nature paper, Xu and her team used a similar technique, enshrouding the enzyme in RHPs and embedding billions of these nanoparticles throughout plastic resin beads that are the starting point for all plastic manufacturing. CONT'D...
For full story, visit these two Berkeley News webpages: 
https://news.berkeley.edu/2021/08/31/video-how-to-make-plastic-truly-biodegradable/
https://news.berkeley.edu/2021/04/21/new-process-makes-biodegradable-plastics-truly-compostable/.

Video by Roxanne Makasdjian and Jeremy Snowden

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